Arise with
Geography
Students’ Book 3
Steven Chanyenga
CLAIM Limited
7 Glyn Jones Road
P.O. Box 503
Blantyre
Malawi
© CLAIM 2014
All rights reserved. No part of this book may be reproduced, stored in a
retrieval system or transmitted in any form, electronic, photocopying,
recording, mechanical, or otherwise except with prior written permission of
the publisher.
Editor :
Yakosa Nyekanyeka
Designers
:
Chilungamo Lipenga
Joe Kima Phulusa
Proofreader :
Patrick Mapondo
ISBN : 978-99960-35-81-4
ii
Acknowledgements
The period I spent writing this book would not have gone so smoothly if it was
not for the guidance and patience of so many. I am using this opportunity to
express my gratitude to everyone who supported me throughout the course of
writing this book.
I would like to express my sincere gratitude to the people at CLAIM who were
most helpful in steering me toward a particular author for writing this book.
I specifically want to thank Andrew Chisamba, John Yohane Milanzi and Ron
Muphuwa for lending their expertise in overseeing the many details of the
production process at CLAIM. They made sure that I was aware of all the
resources at hand and that I had a good time. One simply could not wish for
better or friendlier production managers.
Furthermore I would also like to acknowledge with much appreciation the
crucial role of the editors and designers who made many helpful suggestions
in producing this book whilst allowing me the room to work in my own way.
I am thankful for their inspiring guidance, invaluably constructive criticism
and friendly advice during the course of writing. I am sincerely grateful to
them for sharing their truthful and illuminating views on a number of issues
related to the book.
I also owe my gratitude to the authors of the books, articles, websites and
illustrations listed in the reference section. During the course of writing, I
gathered a tremendous amount of information and relied heavily on their
work.
Special thanks go to my lovely wife, Katija, and the rest of my family for their
everlasting supply of support, advice, and encouragement. It was not easy for
them to endure the loneliness all the days I kept my self away from them.
Nothing worthwhile ever is.
Last but not least, I would like to thank my workmates at Malosa Secondary
School: Andreya Chipoya, Daniel Hussein and Ellina Mnyenyembe, who
were always willing to evaluate my work and cheerfully offered constructive
feedback. They took the time to teach me what I did not understand and
invested their effort in answering my countless questions every day. Thanks
to their comments and advice.
iii
iv
Table of Contents
Unit Title Page
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
Land use..........................................................................................
Landforms........................................................................................
Riverine features..............................................................................
Costal features................................................................................
Map work.........................................................................................
Statistical methods in Geography..................................................
The theory of Continental Drift......................................................
The theory of plate tectonics..........................................................
Mountain building..........................................................................
Volcanism.........................................................................................
Earthquakes.....................................................................................
Rocks................................................................................................
Riverine landforms.........................................................................
Coastal landforms...........................................................................
Relief features of the ocean basins.................................................
World pressure belts......................................................................
Prevailing winds.............................................................................
Air masses.......................................................................................
Fronts...............................................................................................
Local winds.....................................................................................
Cyclones and anticyclones..............................................................
Clouds...............................................................................................
Precipitation....................................................................................
Rainfall............................................................................................
Climatic regions and world vegetation (Biomes).............................
Environmental issues........................................................................
Desertification.................................................................................
Climate change...............................................................................
World fishing....................................................................................
Regional and international trade blocs .........................................
v
1
13
33
43
53
79
107
117
135
161
179
193
203
219
233
251
261
274
282
294
306
320
332
342
354
376
386
396
408
422
vi
Land use
Unit
1
Human beings make use
of the land they inhabit to
a greater degree than any
other species. People have
modified the landscape to
suit their own purposes
and they choose areas
that generate the greatest
payoff
or
economic
returns.
Information
on existing land use
patterns and changes
in land use through
time is one of the prime
prerequisites for better
use of land. Learning this
unit will help you not
only identify the past and
present land use patterns
in
your
community,
but
also
understand
how development has
occurred, and the policies
that are in place to
control and guide future
land use decisions. In this
unit, you will learn how
to interpret map symbols
in relation to land use.
You will also learn the
factors that affect land
use patterns.
Land use
Land use is commonly defined as a series of
operations on land, carried out by humans, with
the intention to obtain products and/or benefits
through using land resources. Human land use
even extends to the oceans. People appropriate
the surface for maritime traffic lanes, and mine
the seabed for petroleum and metals. Humans
use land for agriculture, settlements and forestry.
Another, more subtle(slight and not obvious)
form of land use, is leaving land in its natural
state. Parks and nature reserves may still satisfy
human demands for recreation and for ecosystem
services such as water catchment, and the control
of land erosion.
Activity 1
Brainstorming prior knowledge about
land uses
1. What do you already know (or think you
know) about land uses?
2. What specifically do you want to learn
about land uses?
3. List these on a chart paper.
4. You will come back to the chart at the end
of the unit.
The following sections outline some of the land
uses you will find on topographic maps:
Urban land use
Urban land use relates to what activities are
taking place in a city or town, indicating their
intensity and concentration. Generally, central
areas of cities have a higher level of intensity and
1
concentration of economic, social and cultural activities than the peripheral.
Agricultural land use
Agricultural land use consists of land used to grow food crops, and to graze or
feed livestock to yield meat, dairy and poultry products. The type of farming
practiced tends to vary with the distance from large centers of population.
Close to cities, where land is most valuable, farms tend to occupy comparatively
small plots and intensive crops are grown. The farms become larger with
increasing distance from the center, and the crops, such as wheat, corn, and
other grains, as well as livestock, are more extensive. Evidence for agriculture
on a map includes the following:
a. general cultivation or estate symbols
b. orchards
c. produce market such as ADMARC
d. agricultural extension offices or research stations
e. dip tanks
Forestry land use
Forestland is land, which is stocked with trees capable of producing timber or
other wood products, and exerts an influence on climate. A symbol for forest
reserve provides clear evidence for forestry land use on a map. Presence of
saw mills for timber harvesting may also provide clues for forestry land use
on the map.
Settlement or residential land use
This is where people live (houses, apartment buildings). Always printed in
black, it can be seen on a map through its site, location pattern, alignment and
density. Some settlements are dispersed, others nucleated while others are
linear. The nature and causes of various settlement patterns may be clearly
understood by comparing the settlement map with the contour map.
Institutional land use
This land use category contains public or government related structures such
as schools, town hall, police station, churches and public buildings.
2
Transport and communication land use
On a map, this land use category is evidenced by the presence of the following:
a. national or state highways, district roads, cart tracks, camel tracks,
and footpaths
b. railways and railway stations
c. waterways (lakes, perennial rivers and canals)
d. shipyards (places where ships are built or repaired), docks or jetties
(floating platforms jutting out into a body of water, which boats are tied
to), harbours or ports
e. airports, airstrips and aerodromes
f. post offices, internet cafés, etc
However, the land uses included in the transportation and communication
category occur to some degree within all of the other urban or built-up
categories and actually can be found within many other categories. They are
usually considered an integral part of the land use within which they occur,
unless they can be mapped separately at whatever scale is being employed.
Recreational land use
In this land use type, land is devoted specifically for fun or entertainment
purposes. Land uses included in this category are public parks, campgrounds
and golf courses.
Wildlife conservation
In this land use, development for settlement, industry and mining is banned,
and in which wildlife is strictly protected. Wildlife reserves are dedicated to
the upkeep and preservation of outstanding national features and wildlife.
Facilities for recreation are also provided.
Commercial land use
Commercial land use is predominantly concerned with the sale of products
and services. Components of this land use category are urban central business
districts, shopping centers, resorts, warehouses, driveways and parking lots.
Commercial land uses may also include some noncommercial uses such as
churches, schools, and some residential units.
3
Industrial land use
This land use category includes transportation, oil and gas, communication,
utility facilities and extractive development. Industrial land uses are extremely
varied, depending on the nature of the industry being considered. Urbanindustrial land usage generally refers to the location of factories and other
utilities to a particular site. Industrial land use in rural areas can include
mines, and smelters.
Mining land use
Topographic maps often show mining pits with a distinctive shading pattern.
Gravel pits and quarries also receive a point symbol. Oftentimes other clues
also reveal mining, especially past mining include:
a. irregular contours that do not match the general trend of the landscape,
b. unexpected depressions shown by contours, and rail lines or roads
dead-ending in the area.
Map symbols in relation to land use
Symbols are used on topographical maps to show features which are prominent
because of size, location, or usage. Such features include government or public
buildings, colleges, schools, churches, hospitals, railroad stations, markets,
factories, mines and buildings of historical or cultural interest. However, all
land use features cannot be shown individually on a map. Therefore different
land use areas are indicated by shades of different colours. For example, green
is used for forest reserves, estates, wildlife reserves and recreational areas.
Table 1 below shows symbols of some of the most common land uses.
Activity
2
Interpreting map symbols in relation to land uses
1. In groups, move to different stations designated around the classroom
and use a topographical map to complete the following table:
Table 1: Symbols used to represent land use features
Symbol
Feature Name
Symbol
P
4
Feature Name
Dam wall
C
Huts
Marsh
Pipeline
Police
F
2. What other land uses do you find on the map, but are not shown in this
table?
3. What evidence is there on the map to show the presence of the land
uses you have named in 2 above?
4. What occupation could be linked to each land use?
5. After you have worked on the task for an interval of perhaps 2 minutes,
you should all, except one member of your group, move to different
stations.
6. When you visit each station, interview the remaining member from the
original group to find out how that group completed the table.
7. Take notes on what you learn from the other stations and take them
back to your original station for discussion.
8. Report your findings to the class for discussion.
Please note! Land use should not be confused with occupation: the job by
which somebody earns a living, although there is some overlap between some
land uses and occupation. For example, agriculture or farming as a land
use can also be an occupation. So, you should be able to determine human
occupation on a map area by relating to land use activities on that map.
Factors that influence location of land uses
Some regions are more advantageous than others for particular activities.
The following factors explain why some activities are found in some places
but not others:
5
a. Topography: Lowlands offer the most opportunities for a variety of uses.
They are the easiest to build houses, industries and communications on.
Flat land can also be important for grazing and cultivation purposes.
Highlands or steep lands are very difficult to use. They have poor and
thin soils to support agriculture as the soils creep down the steep slopes
and water runs quickly off the land. Mountains can make road and
rail communications difficult to construct when they form a barrier to
route-way; hence, roads and rails may avoid them. However, mountain
passes and gaps can provide land for railway lines and roads.
b. Climate: Cool, wet climates always favour settlements and agricultural
land uses.
c. Soil characteristics: Fertile soils are usually favoured for agricultural
land use. As people are attracted to settle in or near these areas,
settlement becomes another land use.
d. Accessibility: Some functions need to link with others, and therefore
they locate where there are good transport and communication
networks.
e. Land value: Generally the most accessible areas such as city centres
and along major transportation routes, especially at intersections have
the highest land values. Such expensive land areas are occupied land
uses that produce the highest income per unit of land e.g. tall office
buildings. Cheap land, usually found outside the towns is ideal for more
spacious activities such as industries and low density housing.
f. Government policy: In order to control development and to avoid
land use conflicts, governments pass laws which restrict the use of land
in some areas.
g. Water supply sources: Cities, towns and nucleated villages can
develop in river confluences. In some cases, linear settlements develop
along rivers with buildings forming a long line along the river. The rivers
provide the settlements with water for domestic use and also for farming
purposes. Rivers can also be important for fishing, communication and
tourism.
h. Land tenure and land inheritance: Land tenure is the rights a
person has to the land. If the person owns the land, he/she has a greater
freedom of choice because the person can make long-term investments
such as the growing of tree crops like cocoa and coconuts, the putting
up of farm buildings and the making of farm roads. On the other hand,
there is no guarantee of continued land use if the land is not owned by
the person. Therefore he/she only plants short-term crops such as corn
and peas and have no incentive to improve the land.
6
Activity
3
Analysing factors that affect land use patterns
Study the land use map of Lilongwe in Figure 1 below, and use it to answer
questions that follow.
Lilongwe
Scale/maBstab/Echelle 1:30 000
1/2
1 kilometre
0
0
1/2
mile
Figure 1: Land use map of Lilongwe
1. Locate the open spaces and leisure areas such as parks or forests.
7
2. Suggest why these open spaces are located there.
3. Now, be in groups to compare and discuss your reasons.
4. Present your work to the class for discussion.
Land use changes
Land use never stays still; it is in a constant state of change as a consequence
of human actions to secure essential resources. It can also change due to
environmental processes. These changes have both desirable and undesirable
impacts. The latter are the chief causes of concern as they impinge variously
on human well-being and welfare.
As you walk around in your community, the change in land use is sometimes
very visible. Fields that last year had crops growing or cattle grazing may have
sprouted new homes. Forest lands may have been turned into agricultural
lands (see Figure 2 below).
3 years ago
At present
Figure 2: Land use changes like this can have undesirable environmental impacts
The following activity will help you understand some of the changes that may
have occurred in your community.
Activity
4
Investigating land use changes in your area
Visit a place you had last visited some 3 to 5 years ago. Alternatively, go
around your community and interview at least four people to find out the
following:
8
1. What are some of the biggest land use changes that have occurred in
this community during your lifetime?
2. What is causing these changes?
3. Are these changes good or bad? Justify your answer.
4. Is there need to control land use in your community? Why or why not?
5. What do you suggest in order to make land use in your community
sustainable?
6. Prepare a summary of how your community has been changing through
the eyes and memories of those you interviewed.
7. Present your findings to the class for discussion.
Causes of land use changes
a. Population change: An increase in local populations will increase the
demand for food and other economic opportunities. This will eventually
force people to migrate and open new land for farming, settlements and
other land uses.
b. Technological changes: Technological innovations, such as tractors,
irrigation equipment or techniques, hybrid seeds and chemical fertilizers
have contributed to an expansion of cultivation into the less fertile and
sloping areas of natural forests.
c. Land policy and development programs: Governments may
declare some areas as protected landscapes for nature conservation in
a country or for road building. This may displace population and land
use within that part of the country.
d. Climate change: Changes in climate may result in prolonged
droughts, and this would make some agricultural lands less productive.
If irrigation is not possible in such areas, agriculture may eventually be
displaced by other land uses that suit the conditions.
e. Changingincomes and food preferences: Changes in food
consumption towards more affluent patterns are stimulating a rapid
increase in demand for meat, milk and eggs. This may displace valuable
forest lands into farms to increase production.
f. Loss of soil fertility: Diminishing soil fertility may result in low food
production, and this may force people to turn the land into other uses
or abandon the land completely for other productive areas.
9
Undesirable impacts of land use changes
Much as we appreciate that land use change is necessary and essential for
economic development and social progress, it brings the following undesirable
effects:
a. Loss of biodiversity: When land is transformed from a primary forest
to a farm, habitats that support biodiversity are destroyed. Draining
wetlands for crop production and irrigation water diversions also has
had a negative impact on many wildlife species.
b. Climate change: Conversion of forests into urban land uses disrupts
the hydrological cycle and adds to the greenhouse effect.
c. Pollution: Land clearing for modern agricultural practices, which
include intensive inputs of chemical fertilizers, herbicides, pesticides
and the concentration of livestock and their manures within small areas,
have substantially increased the pollution of surface and groundwater.
In some cases these agricultural chemicals remain as contaminants in
the soil.
d. Low production of food and other essentials: Conversion of
farmland and forests to residential use reduces not only the amount of
land available for food and timber production, but also the amount of
open space and environmental amenities for local residents.
e. Loss of culture and family structure: Conversion of farmland
and forests to urban developments has encroached upon some rural
communities. This expansion of urban life has degraded the rural
community’s culture and identity.
f. Interruption of the water cycle: Conversion of less productive
lands into irrigated agricultural lands has changed the water cycle and
caused groundwater levels to decline in many parts of the world.
Activity
5
Reflecting on the topic
1. What major issues have you learned in this unit?
2. Why was it necessary for you to learn these issues you have mentioned?
3. Return to the chart you prepared at the beginning and discuss whether
what you thought you knew was accurate.
4. What questions do you have about what you have learned?
5. Report your questions to the class for discussion.
10
Summary
Land is an important resource on the earth’s surface and is put to many uses.
Some of the land uses include residential, industrial, urban, agricultural,
commercial, transport, forestry and recreation. On a land use map, different
symbols and colours are used to show features which are prominent because of
size, location, or usage. The uses, to which this land is put, vary considerably
from place to place, depending on topography, climate, soil characteristics,
accessibility, water supply, land value, government policy, land tenure/
inheritance and other factors. Land use is constantly changing due to population
changes, technological changes, land policy and development programs,
climate change, loss of soil fertility, and changes in consumption patterns.
However, the undesirable effects that result from these land use changes,
such as biodiversity loss, climate change, pollution, loss of traditional family
structure, low food production and interruption of the water cycle are among
the chief causes of concern.
Glossary
Land use: the arrangements, activities and inputs people undertake in a
certain land cover type to produce, change or maintain it.
Zoning: the process of planning for land use by a community to allocate
certain kinds of structures and activities in certain areas.
Review questions
1. Explain the term ‘land use’.
2. Describe any three types of land uses.
3. Why is the land in your community put to different uses? Give any two
reasons.
4. Explain three effects of developing areas without proper land use
plans.
5. What are the potential environmental impacts of land use changes?
6. How does land use change affect agriculture and rural communities?
References
Erle Ellis (2010) http://www.eoearth.org/view/article/154143/ Published: April
18, 2010, 3:06 pm. Updated: March 20, 2013, 11:49 pm.
Obara, D. A. (1991). Gold Medal Geography. Nairobi: MacMillan Kenya
Limited.
11
Ryan et al. (2010). A Synthesis of the Science on Forests and Carbon for U.S.
Forests. Issues in Ecology, Report Number 13, Spring 2010. Accessed
on 19/06/14 from http://www.extension.org/pages/58382/other-factorsinfluencing-forest-carbon-storage#.U6MJ9EBzBdg
Waugh, D. (1990). Geography: An Integrated Approach. Hong Kong: Thomas
Nelson Limited.
White, R. (1998). Africa in Focus: A Physical, Human and Economic Geography.
Oxford: MacMillan education Limited.
http://www.canmaps.com/>Etopo/help/05-topographic-map-symbols-mapkeys.htm)
http://www.mah.gov.on.ca/Page1758.aspx 19/06/14
http://prezi.com/dcgpsgprxfol/factors-affecting-land-use-physical-factorseconomic-factors-and-human-factors/
12
Landforms
Unit
2
Landforms
Landforms are features that make up the earth’s
surface. Figure 3 below shows some of the
landforms. They are created because of the various
forces of nature such as wind, water and ice and
Our planet is blessed also by the movement of the earth’s tectonic
with a wide range of plates. Some landforms are created in a matter of
topographical
features. few hours; others take millions of years to form.
Most of the time, people
pay little attention to their
surroundings and have a
difficult time interpreting
and visualizing landforms
on topographical maps.
Being aware of your
surroundings is a very
important skill to learn,
which you will then be
able to apply in to the
rest of your life and build
on
your
observation
skills. By learning about Figure 3: Landforms
landforms,
you
can
correctly interpret surface
features from contour line Activity 1
patterns, and understand Sharing basic knowledge and
the diversity of our anticipations about landforms
physical
environment
1. What do you already know (or think you
as well as the processes
know) about landforms?
that shape it. In this
unit, you will identify
2. What specifically do you want to learn
various landforms on
about landforms?
topographical maps and
3. Draw a table like the one below on a chart.
draw cross-sections of
selected landforms from
Know
What to know Learn
topographical maps.
13
4. Write down what you already know in the first column and what you
want to know in the middle column. You will fill in what you will have
learned in the third column at the end of the unit.
5. Display the chart in front of the class for reference.
There are many types of landforms on the earth’s surface. They include
mountains, hills, valleys, spurs, cliffs, escarpments, gaps, saddles, islands,
archipelagos, peninsulas, etc. Landforms go on different shapes and sizes.
Activity
2
Identifying landforms in your area
In groups,
1. Go to a point where you can have a better view of the surrounding.
2. Describe how the land looks like around where you are. Are there places
that look different? How?
3. Name and describe some common landforms around where you are.
4. Sketch simple line drawings of each landform on a piece of paper.
5. How do you think such features would be shown on a map?
6. Compare and discuss your work with other groups.
You need to be familiar with the shape and contour patterns of landform
features so you can interpret a topographic map. The patterns created by
contour lines make some landforms easily identifiable because each landform
feature has quite a distinctive look.
Mountain
A mountain is often a rocky area of a land mass that rises steeply above
the surrounding terrain. Mountains, plateaus and hills fall under the same
general definition. The difference between a mountain and a hill is mostly an
issue of size and height (mountains are considerably larger, taller and more
complex than hills). Mountains usually have a limited summit area and this
distinguishes them from plateaus (mountains are generally much narrower at
the top than at the base).
Mount Everest is the highest mountain in the world. The peak of Mount
Everest is 8,850 meters (29,035 feet) above sea level. However, Mount Everest
14
is NOT the tallest mountain despite being the highest peak on earth. At 8,850
m, Everest is the highest mountain on earth – in that it reaches the highest
altitude – but the tallest is actually Mount Mauna Kea in Hawaii, USA. Only
4,205 m of it can be seen (the rest is underwater), but from its submarine base
in the Hawaiian Trough, it reaches up to a total of 10,205 m.
Kilimanjaro Mountain is the highest in Africa while Mulanje Mountain is
Malawi’s tallest mountain. On a topographic map a mountain or hill will
appear as a series of irregularly shaped, successively smaller concentric
circles or ovals, with the smallest, inner circle representing the highest point
(see Figure 4 below).
Figure 4: Mountain
Conical hill
A conical hill is an elevated hill with steep slopes on each side and a narrow
top or summit. It is shown on a map by contour lines in a series of concentric
rings (see Figure 5 below).
500
350
Figure 5: Conical hill
15
Plateau
A plateau is a large fairly flat piece of uplifted land surrounded by steep
slopes. On a topographic map, a landform can be identified as a plateau when
the contour lines are quite close to each other all around while on the upper
slopes there are few lines or hardly any lines signifying that the top is rather
flat (see Figure 6 below). The Tibetan Plateau of Tibet in Asia (also called
“roof of the world”) is the world’s largest and highest plateau. It covers nearly
2 300 000 km2 and rises to a height of about 5 000 m. In the Victoria district
of Australia, a famous plateau is the Lord Stanley Plateau. There are many
plateaus in the Grand Canyon region of the USA called the Colorado plateaus.
Plateaus in Malawi include Zomba, Nyika, Viphya and Dedza.
Figure 6: A Plateau
Mesa
A mesa is a broad, flat-topped elevation with one or more cliff-like sides (Figure
7). A mesa is a smaller landform than a plateau, though many mistakenly
refer to a mesa as a plateau. The steep outer edge of the mesa is portrayed
by closely spaced contours, which come together in places where there are
vertical cliffs. Toward the foot of the hill
the contour spacing is wider, indicating
a generally concave slope.
Figure 7: Mesa
16
Butte
A butte is an isolated hill with steep, often
vertical sides and a small flat top, smaller
than mesas and plateaus. Unlike mesas
which have tops that are wider than they
are tall, buttes are taller than they are wide.
Buttes are the remains of eroding plateaus
or mesas. They are created when hard rock
overlies a layer of less resistant rock that
is eventually worn away as streams slowly
cut through a mesa or plateau. The hard
top layers of buttes resist weathering and
erosion. As a result, the formations stay
about the same height as the original
plateau or mesa (see Figure 8).
Figure 8: Butte
Please note! Plateaus, mesas and buttes have the following in common:
• they have a flat top surface.
• they have steep sides.
• they suddenly arise in the midst of surrounding plain areas.
The reason why they are differently named is because of size differences. Of
the three similar geological landforms, a butte is the smallest while a plateau
is the largest.
Figure 9: Mesa, Butte and Plateau
Ridge
A ridge is a long narrow line of high ground, with the land dropping away on
either side. On a topographic map, the contour lines of a ridge tend to be more
or less oval in shape; with the closed end of the contour line pointing away
from high ground (see Figure 10 below). The world’s longest ridge is under
water, the Mid-Atlantic Ridge, which runs across the Atlantic Ocean from
South to North.
17
350
250
Figure 10: Ridge
Knoll
A knoll is a small rounded hill which occurs often on the side of larger hills
or mountains (see Figure 11). It is shown on a map by contour lines forming
concentric circles. The inside of the smallest closed circle is the hill top or
summit.
Knoll
Knoll
650
400
250
Figure 11: Knoll
Cliff
A cliff is a steep face of rock and soil. On a topographic map it is shown by
converging contours. Sometimes the “carrying” contour line has tick marks
pointing toward low ground (Figure 12). An example of a famous cliff is the
White Cliffs of Dover in Kent, England. Many famous cliffs are also found in
the Grand Canyon in the U.S.A.
18
{
Cliff
converging
contours
forming a cliff
500
350
Figure 12: Cliff
Escarpment
An escarpment is a long, steep slope separating two flat or slightly sloped areas
that are at different heights. In other words, it is a long cliff. An escarpment
results from erosion or faulting. The Niagara Escarpment in North America
is famous around the world for its vast size. It runs a distance of 725 km and
rises to a height of 110 m above the surrounding land. Malawi’s Rift Valley
has a series of escarpments, which include the Livingstonia and Golomoti.
Figure 13 below shows an escarpment.
2500
2150
19
Figure 13: An escarpment
Gap
A gap is an opening between hills or in a ridge or mountain chain. On maps,
the innermost contour lines of gaps look like an hourglass, which indicates a
low spot between two higher points (see Figure 14).
Pass
A pass is similar to, but generally narrower than, a gap and is usually found
at higher altitudes (see Figure 14 below). There are thousands of named
passes around the world, some of which are well known, such as the Great
St. Bernard Pass at 2,473 meters (8,114 feet) in the Alps. A steep and narrow
mountain pass is known as a col.
Saddle
A saddle or col is a low area in a ridge of hills. It is rather similar to a gap or a
pass but much smaller. On a topographic map, it appears like two knolls next
Saddleon a larger hill (see Figure 14 below).
to each other
Pass
Gap
Saddle
Pass
Gap
650
500
500
Figure 14: Saddle, pass and gap
20
250
Spur
A spur is a long, gently sloping ridge of land that runs down from a hill to
lower ground. A series of spurs that jut out from alternating sides of a river
valley, are described as interlocking spurs (see Figure 15 below). Spurs
often provide access to and from the high ground, for walkers, for roads, etc.
A spur on a map looks like a long, narrow tongue of contour lines, dropping
away from a mountain top or a ridge. Usually its sides will be quite steep, but
its top will slope gently downwards. Contour lines depicting a spur on a map
bend towards low ground.
Interlocking Spurs
Valley
350
Spur
200
River
150
Figure 15: Interlocking spurs
Valley
A valley is a long low area of land that is surrounded by higher ground, often
with a river or stream running through it. Through erosion and deposition,
rivers gradually shape their valleys. The rate at which a river deepens or
widens its valley depends on the following factors:
• Speed of water flow down the river channel: This will generally
reach a maximum where the volume of water flowing through the river
is large and the slope of the river channel is steep. If a river flows very
fast, it cuts more deeply into its bed and increase the steepness of its
sides.
• Resistance of the material through which the river channel is
cutting: If a river flows through less resistant rocks, it easily erodes
the rock material and cuts more deeply into its bed, thereby increasing
the steepness of its sides.
Valleys are classified as U-shaped or V-shaped depending upon the shape.
V-shaped valleys are formed due to active vertical erosion, whereas U-shaped
valleys are formed due to lateral erosion. Contours indicating a valley are
either U-shaped or V-shaped and tend to parallel a major stream before
crossing it (see Figure 16 below). To determine the direction in which water
is flowing, look at the contour lines. The closed end of the contour line (the tip
21
of the V or U) always
points
ground.
V-shaped
valley upstream or toward high
U-shaped
valley
650
650
400
400
Figure 16: V – and U – shaped river valleys
Valleys are of special interest and importance to humans:
• they have rich deposits of alluvial soil, making them ideal for agriculture.
• the sides of the valleys act as natural walls for creation of dams that
could be used to generate electricity.
As a result, many human civilizations have settled in valleys, taking advantage
of the rivers as a source of water.
Dune
A dune is a hill or a ridge made of sand (see Figure 17 below). Dunes are
shaped by the wind, and they change all the time.
Figure 17: Dunes
22
Alluvial fan
An alluvial fan is a fan-shaped deposit of sediment formed at the point where
a stream exits in a narrow, deep and steep valley (canyon) and enters a flatter
land (Figure 18). Here, the fast flowing stream slows and spreads, thus,
depositing the sediments.
Canyon
Scarp
Alluvial fan
Figure 18: Alluvial fan
Drawing cross-sections
A cross-section is a side view of a landscape drawn through a portion of a
topographic map. When you draw a cross-section, you are actually showing
the shape of a feature (such as a mountain) viewed from the side, as if cut
through with a knife. Cross sections are very useful in understanding the
relief of a map area. They are drawn using the contour lines on a topographic
map.
Work through the following steps to draw the cross-section:
Step1: Place a blank piece of paper along the line where you have to make a
cross-section.
Step2: On the blank paper, mark clearly the starting and ending points of
section on the blank piece. Below these marks, write down the elevation of the
starting and ending points of your section.
Step3: Mark the points where the contour lines cross the paper and write the
height of each contour beside it (see Figure 19).
23
670
0
3
00
68 800
6
00
68
00
Spring
68
00
00
69
69
00
68
80
67
60
67
2
Contour Interval 20 Feet
Figure 19: Mark the contours on a piece of paper, indicating their values
Step4: Once you are certain you have all of the appropriate tic marks and
elevations, remove your paper from the map and place it on a graph paper to
create a line of section. Mark the starting and ending points of your line of
section on the graph paper and then draw vertical lines above your starting
and ending points. These lines will be the boundaries of the section.
Step5: Use the maximum and minimum elevations along your line of section
to determine a vertical scale that can fit in the heights you have marked on
your cross-section.
Step6: Put a small mark on the graph paper directly above each dot at a
corresponding elevation as shown in Figure 20.
24
6760
6780
6800
6900
6900
6800
6800
6800
6800
Figure 20 Draw a baseline and horizontal lines to a suitable scale
6760
6780
6800
6900
6900
6800
6800
6800
6800
Step7: Smoothly join all the marks on the graph paper to show the cross section
(see Figure 21). You can shade the cross-section and label any landscape
features identified on the map.
Figure 21: Cross-section
Activity
3
Identifying landforms and drawing cross-sections from a
topographic map
Get a topographic map from your teacher or school library. Use it to complete
the following task:
1. Identify at least five landforms on the map.
2. Choose a place on the map with a different shape of valley and draw a
cross-section to represent it. Label your drawing and add a heading.
3. Which features do you think are inter-visible and which are not along
25
the cross section?
4. Why do you think it is necessary to draw a profile of the landscape?
5. Present your work to the class for discussion.
Cross-sections are important because of the following:
a. They give a clear shape of a topographic feature.
b. They enable people to determine intervisibility – the concept of
whether one place on a map can be seen from another.
Intervisibility
Intervisibility is a theoretical concept largely interested in determining
whether there is any higher ground between points that may cut off the view
of each other. It is decided upon by studying the heights between the two
places. Any ground which cannot be seen behind a higher height is known as
dead ground (see Figure 22 below).
Dead ground
Visible
Viewpoint
Visible
Dead ground
Figure 22: Intervisibility
Activity
4
Determining intervisibility
Look at Figure 23 below and use it to complete the following task:
1. State whether both cars are intervisible or not from the man on the hill.
Give a reason for your answer.
26
2. Why do you think it is necessary to determine intervisibility of points
on a landscape?
3. Report your work to the class for discussion.
A
B
Figure 23: Intervisibility
The best way of testing intervisibility on a topographic map is to see if,
a. there is any higher ground between two points OR
b. the slopes are concave or convex.
i. A concave slope declines in steepness with movement down-slope.
This provides low ground between the lowest and highest points of the
slope, hence, creating intervisibility. On a topographic map, a concave
slope is shown by contour lines that are closely spaced at the top and
widely spaced at the bottom. Figure 24 below shows a concave slope.
Figure 24: Concave slope and intervisibility
ii. A convex slope gets progressively steeper downhill. Contour lines widely
spaced at the top and closely spaced at the bottom indicate a convex slope
(see Figure 25). If a convex slope is between the two places, the second
cannot be seen. In a convex slope the line of sight is blocked, hence, there is
no intervisibility.
27
Figure 25: Convex slope and intervisibility
Intervisibility is particularly important in:
a. planning the establishment of radio communications from one point to
another.
b. assessing areas of observation from a fire lookout or security tower.
Activity
5
Drawing cross-section
Elevation (m)
Use Figure 26 to base your answer to the questions that follow. Points A
through F represent locations on the map.
a.
re
St
D
F
120
120
b.
B
c.
E
1
NORTH
2 km
Elevation (m)
LAKE
0
A
F
70
A
F
A
F
am
100
C
Elevation (m)
A
70
Elevation (m)
River
100
120
d.
Contour Interval + 10 meters
70
120
70
A
Figure 26: Contour line patterns and cross-sections
28
F
1. Which of the diagrams (labeled a, b, c, or d) best represents the
topographic profile between point A and point F?
2. Is there any intervisibility between points A and F on the map? Give a
reason for your answer.
3. Draw a cross-section between points A and E, and determine their
intervisibility.
4. Report your work to the class for discussion.
Activity
6
Reflecting on the topic
1. What have you learned in this unit?
2. Return to the chart you prepared at the beginning.
3. Do you think what you thought you knew was accurate?
4. What questions do you have about what you have learned?
5. Report your findings to the class for discussion.
Summary
Landforms are natural physical features of the earth’s surface, for example,
mountains, plateaus, mesas, buttes, ridges, knolls, conical hills, cliffs,
escarpments, valleys, spurs, gaps, passes, saddles, alluvial fans, dunes, and
deserts. Identifying these features on a map is an important skill to master.
Drawing cross-sections and determining intervisibility through these features
makes it even more interesting and useful in our daily lives.
Glossary
Landform: a natural physical feature of the earth’s surface.
Cross-section: a slice through a particular feature.
Intervisibility: whether or not observers at two different points on a map
can see one another, assuming that there are no tall trees, poor weather
conditions or other obstructions in the way.
Concave slope: a slope that declines in steepness with movement downslope.
Convex slope: a slope that gets progressively steeper downhill.
29
Review questions
1. Use Figure 27 below to answer the questions that follow.
Figure 27: Topographic maps
a. Looking at the overall pattern of the contour lines on map A, what
do you think is the general direction of flow of water when it rains?
b. Identify the features to which the arrows point in both maps.
c. Name and describe the features labeled x, y, z.
d. Draw a cross-section between features y and z and state their
intervisibility.
2. Describe how contour lines show the following on a map:
a. Concave slope
b. Convex slope
c. V-shaped valley
3. State any two ways in which buttes are different from mesas.
References
Bowen, A. D. et al. (1997). Map Reading for Southern Africa. Cape Town:
Maskew Miller Longman.
Bunnett, R. B. (1973). General Geography in Diagrams. England: Longman.
Bunnett R. B. (1984). Physical Geography in Diagrams for Africa. England:
Longman Group Limited.
Harte, J and Dunbar, C. (1994). Skills in Geography. Cambridge: Cambridge
University Press.
30
Kalaluka, L. (1978). Map Reading for Central Africa. England: Longman.
White, R. (1998).. Africa in Focus: A Physical, Human and Economic
Geography. Oxford: MacMillan Education Limited.
http://www.lrrpublic.cli.det.nsw.edu.au/>Using a map to identify landforms
– 28/09/2013
http://www.guinnessworldrecords.com/world-records/size/highest-mountain
http://www.justtrails.com/wp-content/uploads/2012/12/South-Oregon-ButteComposite1.jpg 28/09/2013
http://peakery.com/mitchell-butte-arizona/ 05/01/14
http://4.bp.blogspot.com/-D4OXuSmjBqc/URm0waqAdI/AAAAAAAACtw/B_
Q769Qt6so/s1600/A+mountain+ridge+in+Japan.jpg
www.mulgrave.com 03/01/14
http://www.environmentalgraffiti.com/news-bandiagara-escarpment
http://www.gutenberg.org/files/36463/36463-h/images/i-39-f.png 12/01/14
http://clasfaculty.ucdenver.edu/callen/1202/Landscapes/Arid/
AlluivalFanDiag.jpg
www.mulgrave.com/IB GEOGRAPHY 11 Topographic Mapping WORKSHEET
15 ... 02/01/14
http://easymapwork.blogspot.com/p/cross-section.html
www.regentsearth.com/ Interactive MappingReview 14/01/14
31
32
Unit
Riverine features
Riverine features
Rivers begin in upland areas and flow downhill
towards the sea. The start of a river is called
the source and the end is called the mouth. The
path taken by a river from the source to the mouth is
its course. From its source, a river flows downhill,
joined by small streams (tributaries). Eventually
the river becomes bigger, draining water to the
For millions of years,
sea or lake. The point where tributaries join the
water
has
travelled
main river is called the confluence.
across
landscapes,
shaping
the
surface All the land that supplies a river and its
of the earth. Rivers tributaries with water is called a drainage basin
have produced some of or catchment area. The division that separates
the most spectacular one drainage area from another drainage area is
landforms on earth. known as a watershed (see Figure 28 below).
These features have Rivers create a variety of features along their
influenced the course course by means of erosion and deposition.
of human agriculture
and the distributions of
human populations since Neibouring Ridges
Neibouring
Hill top
ancient times. Learning drained basin
drained basin
this unit will help you
become familiar with
various riverine features.
In this unit, you will
identify riverine features
on a topographic map.
3
Sea
Spur
Watershed around the drainage
basin in the centre of the diagram
33
Diagram of Watershed
Sea
Watershed
Areas which contribute
to the neighbouring
dranage basins
Figure 28: Watershed and catchment area
(Source: RB Bunnett (1973) General Geography in diagrams)
When studying river we often divide then into three main sections: the upper
course; middle course and lower course. Each part of the river has distinctive
features which form the characteristics of the river and its surrounding valley
change.
Upper course
This is the section of the river nearest the source, and is usually found in
the mountains or hills. Here, water flows quickly through a narrow channel
with a steep gradient; as it does so it cuts downwards. This vertical erosion
results in a number of distinctive features, which include interlocking spurs,
waterfalls, rapids, V-shaped valley and gorges.
Middle course
This is the section where the river leaves the mountains and enters a less
hilly environment. In the middle course, the angle at which the river flows is
less steep. Here, the river channel has become much wider and deeper as the
river has been fed by many tributaries upstream. The river begins to meander
and the valley sides are also less steep. The features commonly found here are
meanders, ox-bow lakes and river cliffs.
34
However, the middle course of some rivers (as is the case with Shire River)
has features that are normally found in the upper course. The middle course
of Shire River is steeper than its upper course, with a series of rapids and
waterfalls. The reason for this is the nature of the source. The source of Shire
River is a lake, which is located in a lowland region (rift valley) and not a
mountain as is the case with other rivers.
Lower course
This is the section closest to the mouth. Here the river travels over much
flatter land. The landforms found in the lower course include meanders, oxbow
lakes, braided rivers, levees and deltas.
Activity
1
Describing the course of a river
Study the contour map of a river course below and use it to answer the
questions that follow.
650
900
Figure 29: A river course
1. In which direction is the river flowing?
2. Identify the upper and lower courses of the river.
3. Write a description of contour patterns following the river course from
the source to mouth, and the main changes that you would expect to see
in the river and its valley.
4. Present your work to the class for discussion.
35
Waterfalls
A waterfall is a place in the course of a stream or river where water flows over
a vertical drop (see Figure 30 below). Unlike other riverine features which
are represented by contour lines on topographic maps, waterfalls are usually
represented on topographic maps by symbols. A small blue line across the
river/stream represents a waterfall on a topographic map (see Figure 31).
Hard rock topples over
Source
River
waterfalls are often
formed where hard rock
lies on top of softer rock
Pebbles, stones and boulders
Plunge
Pool
Rapids
Hard rock
Soft Rock
Mouth
MIDDLE COURSE
UPPER COURSE
LOWER COURSE
Figure 30: Waterfall and rapids
Rapids
A rapid is that part of a river, often with rugged rocks, where water flows
very fast and turbulently because of a relatively steep gradient of the riverbed
at that place. Like waterfalls, rapids are usually represented on topographic
maps by symbols. Two lines coloured blue are drawn across the river or stream
to represent rapids (see Figure 31 below).
Please note: the following common mistakes of locating features from the
legend: Candidates choose the wrong feature(s) because there is more than
one feature attached in one symbol. For example;
.............water, waterfall, rapids, dam
Figure 31: Watercourse waterfall, rapids and dam
From this symbol, Candidates are supposed to deduce that,
…is a watercourse
…is a waterfall
…is a rapid
…is a dam
36
Gorges
A gorge is a narrow valley between hills or mountains, typically with steep
rocky walls and a stream running through it. Since a gorge is a valley with
very steep sides and is both narrow and deep, contour lines are close together
or converge into one carrying line on both sides of a stream to represent the
steepness (see Figure 32).
200
River
200
300
300
Figure 32: Gorge
Meanders
Meanders are bends or curves along a river’s course. Rivers meander when
they are traveling on top of a relatively flat surface or through a series of
interlocking spurs. On a topographic map, contours will be nearly parallel to
the meandering river.
Activity 2
Drawing a river course on a contour
map of a river valley
The map below shows the contours of a river
valley. Use it to answer the questions that follow.
1. Insert a river that correctly matches the
contours.
2. In which direction is the river you have
inserted flowing?
3. What section of a river course is displayed
in the diagram?
Figure 33: Contour map of a river section
37
4. Identify at least three other landforms displayed by contour patterns in
this diagram.
5. Report your work to the class for discussion.
Ox-bow lakes
An ox-bow lake is a U-shaped body of water formed when a wide meander
from the main stem of a river is cut off to create a lake. Figure 34 below
shows an ox-bow lake.
Figure 34: Ox-bow lake
Activity
3
Investigating features of a river meander
1. Study the enquiry questions below and decide which one you are going
to investigate. Then discuss what data you need and how you are going
to collect it.
2. Do river valleys become wider further downstream?
3. Does the volume of water in a river increase further downstream?
4. On which side of a meander bend does the river flow fastest?
5. Is there any evidence of erosion and deposition on a meander section?
6. Report your findings to the class for discussion.
Floodplains
A floodplain is the wide, flat area of land on either side of the river in its middle
and lower courses (Figure 35). On a topographic map, the contour lines are
spread out widely, with ox-bow lakes, levees and marshes lying adjacent to a
wide and meandering stream.
38
Floodplain
Floodplain
River
Figure 35: Floodplain
Levees
Levees are natural walls of silt along the banks of a river channel, which are
often higher than the flood plain (Figure 36).
Backswamp
Natural levees
Backswamp
River
Figure 36: Levees
Deltas
A delta is a flat area of sand and silt built into the sea at the mouth of a
river (see Figure 36 below). It results from the accumulation and deposit of
sediment transported by rivers flowing to the sea.
Figure 37: Delta
39
Activity
4
Completing a spider diagram
1. In this unit you have learned about riverine landforms. Complete a
spider diagram like the one below to make a summary of the facts you
have learned while studying this section.
2. Add any further relevant facts, including details from your fieldwork.
3. Report your work to the class for discussion.
The main features of a lower
course of a river are........
There is more erosion on the outside
bend of meander because.....
Vertical erosion means....
Deposition is more common
on the inside bend of a mender
because.......
The source of a river is where....
and the main features are....
Riverine Features
The main features of the
upper course of a river are......
Lateral erosion means.....
As a river flows from its source
to its mouth, it changes in the
following ways......
Figure 38: Spider diagram
Summary
Rivers are a key player in shaping the landscape. Rivers often originate in
the mountains either as glacial streams or as a result of snowmelt. These
water bodies often start as trickles and quickly move to become steep fastflowing narrow rivers. As a river makes its way down and across the earth
on its way to the sea or ocean, it often carves its path into gorges, waterfalls,
rapids, meanders, ox-bow lakes, floodplains, levees, deltas and river braids.
It is important therefore that you are able to identify these features on a
topographic map.
Glossary
Confluence: a point at which two or more streams meet
Drainage basin or catchment area: the area of land that drains rainfall
into a river or lake
Watershed: the division that separates one drainage area from another
drainage area
40
Review questions
1. Describe the following riverine features:
a. waterfall
b. levee
c. floodplain
d. delta
2. Briefly describe how a gorge is shown on a contour map.
3. Describe two processes by which rivers transport material.
4. What are the negative impacts on human developments on a floodplain
can cause? Give any two.
References
Bunnett, R. B. (1973). General Geography in Diagrams. London: Longman.
Bunnett R. B. (1984). Physical Geography in Diagrams for Africa. London:
Longman Group Limited.
White, R. (1998). Africa in Focus: A Physical, Human and Economic Geography.
Oxford: MacMillan Education Limited.
http://geographyfieldwork.com/ValleyContours.htm
www.quia.com
http://earthintegral.files.wordpress.com/2011/11/1-pond-with-reeds.jpg
clasfaculty.ucdenver.edu
http://www.ikonet.com/en/visualdictionary/static/us/the_shoreline 25/01/14
41
42
Unit
4
Coastal
areas
are
naturally dynamic, as
they
are
constantly
shaped by natural forces.
Graced by some of the
world’s most spectacular
features, they capture
the interest of millions
of people. In fact coastal
areas are the home for the
majority of the world’s
population.
However,
human activities along
the coast often jeopardize
opportunities for coasts
to fulfill their socioeconomic and ecological
roles.
Learning
this
topic will help you gain
knowledge of coastal
processes
and
the
protective function of
coastal systems. This
knowledge will enable
you to make efforts to
manage coastal problems
and to restore coastal
capacity to accommodate
short
and
long-term
changes
induced
by
human activities and
extreme events. In this
unit, you will identify
coastal features on a
topographical map.
Coastal features
Coastal features
Coastal features are landforms that are formed
next to the sea or lake as a result of a number of
activities such as wave erosion. There are many
features along the coastal areas, but the following
sections will briefly outline a few that are more
common:
Headlands, promontories and capes
A headland is a piece of land jutting out into the
sea from the mainland, usually with steep high
cliffs (see Figure 38 below). Long, narrow and
high headlands are called promontories. A
headland or promontory of large size extending
into a body of water, usually the sea is called a
cape.
Headland
SEA
Figure 39: Headland
Caves, arches, stacks and stumps
A sea cave is a hollow opening in the base of a
sea cliff, usually at sea level, formed by waves
acting on weak parts of the weathered rock. The
43
cave slowly enlarges to form an arch. A sea arch is an opening through a
headland, which leaves a bridge of rock over the water.When the roof of the
arch collapses, a stack is formed.Stacks are steep-sided pillars of rock that
have been isolated from nearby cliffs at the shoreline by the erosion of the
waves. Stumps are low outcrops of rock left after the coastal stacks have been
removed. Figure 40 shows a sea cave, an arch, a stack and a stamp.
site of arch
collapse
blowhole
original shape of
headland
headland
stack
arch
stump
lines of weakness
sea cave
undercutting
wave-cut platform
exposed at low tide
Figure 40: Caves, arches, stacks and stumps
Bay
A bay is a body of water that is partly enclosed by land (see Figure 41).
Bays are found between headlands where there are alternating outcrops of
resistant rock and less resistant rock.
Bay
Figure 41: Bay
44
Spit
A spit is a sandy extension of a beach, stretching partway across the mouth of
a bay (see Figure 42 below).
Spit
Figure 42: Spit
Lagoon
A lagoon is a shallow, landlocked body of water along the coast which is partly
or completely separated from the sea by a narrow stretch of land, usually a
sand bar or a spit. Figure 43 below shows a lagoon.
Lagoon
Sand bar
Figure 43: Lagoon
Gulf
A gulf is an arm of a sea or ocean that extends into land. It is larger than a
bay. Most gulfs are connected with the sea by one or more straits. The Gulf of
45
Mexico in Figure 44 below is a good example.
United States of America
Antlantic
Ocean
Gulf of Mexico
Cuba
Mexico
Figure 44: A gulf
Fjord
A fjord is a long, narrow sea inlet that is bordered by steep cliffs (see Figure
45 below). Many fjords are remarkably deep because the huge glaciers that
formed in these valleys were so heavy that they eroded the bottoms of the
valleys far below sea level. The waters of the sea invaded the valleys after the
glaciers melted. Fjords are very common along the western coast of Norway.
Figure 45: Fjord
Peninsula
A peninsula is a strip of land that is almost surrounded by water and connected
to a larger landmass by a narrow strip of land called isthmus (see Figure
46).
46
Peninsula
Isthmus
Figure 46: A small peninsula in Croatia
Strait
A strait is a narrow body of water that connects two larger bodies of water
(Figure 47). If the passage of water connecting the two larger bodies of water
is long and wide, it is called a sound.
Strait
Figure 47: Strait
Estuary
An estuary is a v-shaped opening at the mouth of a river where freshwater
from rivers and streams meets and mixes with salt water from the ocean (see
Figure 48).
47
Estuary
Figure 48: An estuary
Activity
1
Identifying erosional and depositional features of the coast
1. Working in groups, collect pictures of a coastal environment.
2. Record on separate cards the coastal features you see in the pictures.
3. Within your groups, sort the cards into two groups, those which are a
result of erosion and those which are a result of deposition.
4. Compare your work with that of other groups.
5. Brainstorm a range of human activities within the coastal environments.
6. Present your ideas to the class for discussion.
Identifying coastal landforms on topographic maps
You may be expected to identify a number of important coastal features on a
topographic map. On a topographic map, you need to think about what the
features would look like if you were looking down onto them from above.
Activity
2
Identifying coastal landforms on topographic maps
1. Copy Figure 49 overleaf into your book. Label your sketch map to show
the features marked with numbers on the map.
2. Underneath, write a few clues on how you were able to identify them
(e.g. the arch can be identified due to the narrowing in the headland to
form a ‘neck’).
3. Report your work to the class for discussion.
48
SEA
a
b
c
e
j
f
h
i
k
d
g
100 m
0m
5150
m
0m
m
LAND
m
150
10
200 m
m
150
100
Figure 49: Sketch-map of an imaginary coastal area
You should be able to easily identify some of the features such as spits and
sand bars from their distinctive shapes on maps. You should also be able to
identify some of these features from the map key.
Stacks and stumps are the most obvious identifiable of these features. They
appear as small islands surrounded by water a few metres away from the rest
of the headland. Arches are usually identifiable by a narrowing in the width
of the land to form a ‘neck’ through which the sea passes.
However, you should remember to use contour lines to provide information on
the height of the land. If there is a high contour line or spot height near to the
coastline, you can tell that sheer cliffs will be present.
You may also be expected to identify an area of coastline as being either an
erosional or depositional coastline. Look for the following clues:
• Erosional coastlines will feature a number of headlands and rocky bays
with caves, outcrops of loose rock (scree) on the coastline due to mass
movement from cliffs. They are more likely to be found in higher areas
(identifiable by spot heights and tightly-packed or high value contour lines).
• Depositional coastlines will feature sandy or shingle beaches, salt
marshes, estuaries, and spits. Identifiable by a lack of contour lines, these
features are also more likely to be found in lower areas.
Activity
3
Reflecting on important issues in the topic
1. In groups of four, locate an important issue that you feel the topic has
covered.
2. Formulate a problem or question about it for another group to answer.
49
3. Write the problem down on a sheet of paper, and hand that piece of
paper to another group.
4. Once your group is handed a problem statement, think of a solution to
the problem. Each group has a fixed amount of time.
5. Present your problem and its solutions to the class for discussion.
Summary
The coastal environment of the world is made up of a wide variety of landforms
manifested in a spectrum of sizes and shapes. Features in the coastal systems
are associated with erosion and deposition. Erosional features include
headlands, bays, capes, peninsulas, caves, arches, stacks, and stumps.
Depositional features include beaches, spits, lagoons, sand bars, estuaries,
and tombolos. These features can easily be identified on a topographic map
using contour lines, map key and their shapes.
Glossary
Promontory: a long, narrow and high headland.
Sea cave: a hollow opening in the base of a sea cliff, usually at sea level
Arch: an opening through a headland, which leaves a bridge of rock over the
water
Stack: a steep-sided pillar of rock that has been isolated from nearby cliffs at
the shoreline.
Stump: a low outcrop of rock left after a coastal stack has been eroded.
Fjord: a long, narrow sea inlet that is bordered by steep cliffs
Review questions
1. Describe the following coastal landforms:
a. headlands
b. fjords
c. arches
d. stacks
e. lagoon
50
2. Briefly describe how you would identify headlands and bays on a
topographic map.
3. Draw and label a diagram to explain the formation of a spit.
4. Explain the difference between a headland and a cape.
5. Give three ways in which coastal features are important.
References
Bunnett, R. B. (1973). General Geography in Diagrams. London: Longman.
Bunnett R. B. (1984). Physical Geography in Diagrams for Africa. London:
Longman Group Limited.
Pallister, J. et. al. (2001). Longman Geography for GCSE. Longman: Essex
White, R. (1998). Africa in Focus: A Physical, Human and Economic Geography.
Oxford: MacMillan Education Limited.
http://media.tiscali.co.uk/images/feeds/hutchinson/ency/c00575.jpg
http://www.answers.com/topic/how-are-arches-and-sea-stacks-formed
13/12/13
http://www.prestfeldeyear6geography.blogspot.com13/12/13
http://www.prettyusefulmaps.com/whitepark-bay-walks/ 03/01/14
http://www.gulfmex.org/archive/map.htm 12/01/14
http://www.merriam-webster.com/art/dict/fjord.htm
http://en.wikipedia.org/wiki/Peninsula 13/12/13
http://www.firefly.morehouse.org.uk/>IDENTIFYING COASTAL
LANDFORMS FROM MAPS
51
52
Map work
Unit
5
Measurement
is
an
important
element
in
geography.
You
should become familiar
with
using
different
measurement tools such
as rulers, pieces of string
and scale. With these
tools, you will measure
distance between points,
calculate area on a map,
draw cross-sections and
river profiles, reduce
and enlarge maps and
calculate
gradient.
Learning about these
concepts will give you a
chance to use map skills
that people need and use
outside of school. Many
careers like architecture,
aeronautical and graphics
design, engineering, and
many
others
include
the use of distance,
area, cross-section and
gradient on a regular
basis. With some of these
careers, it is crucial to
know and understand
the importance of these
topics.
Measuring distance between points
on a map
When we talk about distance, we mean the
measurement of how far we travel between
two places. On a map, it is the measurement
between any two points. The two points may be
the distance along a route such as a road, railway
line, a footpath or a river and distance between
settlements.
Maps are made to scale, that is, there is a direct
relationship between a unit of measurement on
the map and the actual distance that the same
unit of measurement represents on the ground.
Activity
1
Interpreting map scale
Get a topographic map from your teacher or
school library and use it to complete the following
activity:
1. Check the scale of the map.
2. What does the scale of the map mean? Write
down your interpretation of the scale.
3. How does it compare with that of your
partner?
4. Report your interpretation to the class for
discussion.
Sometimes you may need to measure only straight
distances but at other times, you may also need to
measure curved ones.
53
Measuring straight-line distance
When the line is straight, the distance can be measured with the help of the
following:
Straight edge piece of paper
Measuring straight line distance is simple using a straight-edge piece of paper.
Example
Measure the distance between A and B in Figure 50 below using the scale
given.
A
B
Scale 1: 50 000
m 1000
500
0
1
2
3 km
Figure 50: Straight line distance
Step 1: Draw a straight line joining A and B as done in Figure 51 below.
A
B
Scale 1: 50 000
m 1000
500
0
1
2
3 km
Figure 51: Straight line joining points whose distance is to be measured
Step 2: Get a piece of paper of a suitable length, which has a straight edge.
Step 3: Place the straight edge of the piece of paper next to the line joining
54
the two points. Mark off point A on paper at the point where A is and point B
where B is (see Figure 52).
B
A
A
B
Pencil marks on
piece of paper
Scale 1: 50 000
m 1000
500
0
1
2
Figure 52: Measuring straight line distance using a straight-edged piece of paper
Step 4: Take the marked straight edge of the piece of paper and put it against
the linear scale given at the bottom of the map as shown in Figure 53 below.
You can see that the distance between A and B is 4 kilometres.
Figure 53: Read off the distance on the linear scale
Please note! If the distance is less than a kilometre, it is measured in terms
of meters using the part of the scale to the left of zero. This part is divided into
ten equal portions, each representing 100 meters. Put the start mark against
the zero while the end mark lies on the left. Read off the distance in meters.
Using a pair of dividers
With a pair of dividers, follow the procedure below:
55
Step 1: Draw a pencil line joining the places on the map; for example, points
P and Q in Figure 54.
Step 2: Open the pair of dividers to obtain the distance between the two places.
P
Q
Figure 54: Open the pair of dividers to obtain the distance
Step 3: Use the dividers to read the distance from the linear scale. Note that
if you use a pair of dividers, do not change the span until you have read the
distance in the linear scale. Figure 55 below shows that the distance between
P and Q is 1 km.
m 1000
500
0
1
2
3 km
Figure 55: Do not change the span until you have read the distance in the linear scale
56
Measuring curved distance
Roads, railway lines, rivers and distances between settlements are not always
straight. These may follow irregular patterns and great care needs to be taken
when measuring them. There are many ways of measuring these irregular
distances but the most convenient include the following:
Using a piece of string
With this method, a fine piece of twine or heavy thread, preferably white
in colour, works best (being both flexible and manageable). Stretchy string
should not be used.
Example
Measure the distance of the road from the shop to the School in Figure 56
below.
Figure 56: Winding distance
Step 1: Take a length of string one which is longer than the distance to be
measured and place one end on your starting point.
Step 2: Now lay the string along the road or path carefully, following the
curves as closely as you can (see the illustration below). When you reach your
finishing point, mark it on your string with a pen.
57
Figure 57: String Method
Step 3: Now that you have your distance from the map, you can straighten out
your string and place it against the scale bar to find out the actual distance.
Figure 58: Taking true distance from a linear scale
Using a straight edge piece of paper
Measuring curved distance on a map can also be done with the help of a strip
of paper.
58
Step 1: Put the edge of a piece of paper along the first straight section of the
line to be measured. The corner of the straight edge should be on your starting
point.
Figure 59: Measuring distance using a straight edge paper
Step 2: Now pivot the paper along the curves or bends, making sure the edge
follows the route that you want to take. Every time the route bends, make a
small mark on the edge and pivot the paper so the edge is back on course (see
Figure 60). Repeat this process until you reach your destination.
Figure 60: Marks on a piece of paper where the path bends
Step 3: Now you can place the sheet against the scale bar on your map to
59
get the distance on the ground. The last mark you made will tell you the real
distance you need to travel.
Figure 61: Reading distance from a linear scale
Using a pair of dividers
Winding distance can also be measured with the help of a plain divider, but
the measurement may be less accurate.
Step 1: Divide the line by pencil marks into sections that are almost straight.
Step 2: Measure each of these sections with a pair of dividers as shown in
Figure 62 and write down each measurement.
Figure 62: Using pair of dividers for measuring winding distance
60
Step 3: Add the length of each of the straight sections.
Step 4: Then use the linear scale to find the actual distance on the land.
Using a rotameter
A Rotameter is an instrument having a route measuring wheel. Distance
between two points is measured by allowing the wheel of the rotameter to
move along the route (see Figure 63 below).
Figure 63: Measuring distance on a map using a rotameter
Activity
2
Measuring distance on a topographic map
Your teacher will give you a topographic map. Use it to do the following:
1. measure the distance covered by road, railway or river on the map,
following the steps outlined above.
2. use the scale to convert the distance covered on the map into real world
distance.
3. report your work to the class for discussion.
Calculating area on a map
Regular shapes
Topographic maps have grid squares. A square has four equal sides. To
calculate the area of a square on a map, first measure the length of one side
of the map. Use the scale of the map to calculate the actual distance on the
ground. Then multiply the length of the side by itself.
61
Example
Calculate the area covered by the swimming pool in Figure 64 below:
4 cm
10
Scale 1: 50 000
21
22
24
SWIMMING
POOL
09
08
23
21
10
09
22
23
4 cm
24
08
Figure 64: Square map
Using the scale 1:50 000 (1cm represents 0.5 km).
One side is 2 kilometres
Area of the swimming pool
=2x2
= 4 square kilometres
In a rectangular map, the opposite sides are equal. Measure the width and
length of the rectangle. Use the scale of the map to calculate the actual ground
distances of the measured sides. Then multiply the actual ground length by
the actual ground width of the map.
Example
Calculate the area covered by the
garden in Figure 65 below:
4 cm
To calculate the area of the garden
using a scale of 1cm representing 0.5
km we get the following:
10
Scale 1: 50 000
22
23
24
10
09
09
GARDEN
08
Length 2 km
Width 1 km
21
21
22
4 cm
Figure 65: Rectangular map
62
23
24
08
Area = (length x width)
= (2 x 1) km
= 2 square kilometres
Irregular shapes
Measuring the area of irregular shaped features like lakes, farms and forest
reserves involves a number of steps. For example, to calculate the area of the
wetland in Figure 66, the procedure that follow should be involved:
11
21
22
22
Scale
1: 1:50
50 000
Scale
000
2323
24 24
25 25 26 26
10
11
10
WETLAND
09
09
2 cm
08
21
22
22
2 cm
2323
24 24
25 25 26 26
08
Figure 66: Area of an irregular shaped feature
Step 1: Count the number of grid squares that fall within the drawn boundaries
of the feature. Any grid square that is partly enclosed is counted as a half
square. It is clear from Figure 67 below that there are three whole squares
and twelve half squares covered by the Wetland.
11
21
21
2222
1
2
2 cm
08
21
2 cm
24 24
1
2
2222
25 25 26 26
1
2
1 WETLAND1
1
2
09
23 23
1
2
1
2
10
Scale 1: 50 000
11
1
2
10
1
2
09
1
2
1
2
1
2323
24 24
1
2
25 25 26 26
08
Figure 67: Calculating the area of an irregular shaped feature
63
The area of the wetland can therefore be calculated as follows:
Number of full squares = 3
Number of half squares =12
Step 2: Change the number of half squares into full squares and then add the
total number of full squares.
To change 12 half squares into complete squares we divide 12 by 2 = 6
Total number of full squares = 3+ 6 = 9
Step 3: Find the area of one grid square using the scale of the map and then
multiply it by the total number of whole grid squares.
Using the scale of the map, the area of each square is 1 km2
The total area is therefore 9 x 1 km2 = 9 square kilometre
Activity
3
Calculating area of irregular shapes
Look at the different fields shown in Figure 68 below.
A
1. Which field appears to
have the greatest area?
E
B
2. Which field appears to
have the least area?
3. Which fields might
equal in area?
c
D
F
be
4. Explain how you worked
out the area of the fields.
5. Report your work to the
class for discussion.
Scale 0 1km
Figure 68: Areas of different fields
64
How to draw a river profile
The long profile of a river is a section drawn along the river gradient from
source to mouth. You will need a topographic map of the section of river, a
straight-edge piece of paper and a graph paper.
Step 1: Use a piece of paper to measure the length of the section of the river
you want to draw on the map. This will be the length of the river profile for
the horizontal axis on the graph paper.
Step 2: Mark clearly the starting and ending points of the profile on the piece
of paper. Below these marks, write down the elevation of the starting and
ending points of the river profile using evidence of spot heights or contour
lines on the map. The difference between these two heights will give you the
height of your profile for the vertical axis.
Step 3: Place a strip of paper along the river bed on the map where you want
to take your profile. Mark the points where the contour lines cross the paper
and write the height of each contour beside it (just like you did before when
drawing cross-sections).
Figure 69: Place
a strip of paper along the river bed
Step 4: Take the paper off the map and use graph paper to construct the river
profile. The length of the profile forms the length of the bottom axis. Put sea
level (0 metres) at the bottom of the side axis, and then draw a scale that can
fit in the heights you have marked on your river profile.
Figure 70: Mark the points on a graph paper
65
Figure 71: Join the points
Step 5: Join up the points to draw the river profile and label the profile with
the landscape features that you may have identified on the map (such as
waterfalls and rapids).
Clearly, when contours are closer together across a river bed, the river is
going down a steep stretch (as is usually the case with the upper section), but
when they are spread apart the river is going down a gentle stretch (e.g. the
lower section).
Activity
4
Drawing cross-sections and river profiles
Your teacher will give you a topographic map.
1. Draw a line across a section of the topographic map. For creating
an interesting cross section, drawing the line across a peak on the
topographic map is most beneficial.
2. Which features do you think are inter-visible and which are not along
the cross section?
3. Why do you think it is necessary to draw a profile of the landscape?
4. Report your work to the class for discussion.
Enlarging and reducing maps
Map enlargement is the process of making the size of a map larger than its
original size using scale.
Map reduction refers to the process of making the size of a map smaller
than its original size using scale.
• Large-scale maps show a small area in detail. They are called largescale maps because the features on them appear relatively large.
66
• Small-scale maps show a larger area in less detail making features on
them appear smaller.
Activity
5
Discussing large scale and small scale maps
1. If one map has a scale of 1:50 000 and another has 1:100 000, which
map covers more area?
2. Which map can show more detail?
3. Is a map with more detail always better?
4. Give situations where it may be necessary to change the dimensions of
a map.
5. Present your work to the class for discussion.
A large map scale has a larger fraction (1:25 000) than a small scale (1:100
000). The smaller the number on the scale, the smaller the area covered by the
map, resulting in greater detail; the larger the number, the larger the area
covered by the map, resulting in less detail.
For every map enlargement or reduction, a scale factor (enlargement/
reduction factor) must be specified. The scale factor is how many times the
new map is larger or smaller than the original map. If the scale factor is
greater than 1, the new map is larger than the original map. However, if
the scale factor is less than 1, the new map will be smaller than the original
map. Enlargements with scale factor 2 have been doubled. If a map has been
reduced by half, the scale factor is 1/2.
Steps for enlarging maps
Example
Enlarge the map in Figure 68 to twice its original size. On the enlarged map,
show the new scale and the forest reserve.
67
Scale 1: 100 000
13
21
22
23
24
25
26
13
12
12
11
11
10
10
09
09
08
21
1 cm
1 cm
22
23
24
25
26
08
Figure 72: Small-scale map
Step 1. Work out the new scale. In doing this, the dimensions of the original
map and the reproduced map can be kept in proportion. The new scale could
be worked out as follows:
Since the new map will be twice as big as in the original map, we multiply the
current scale by 2; in this case, 1:100,000 × 2.
=
=
=
1
100 000
x
2
1
2
100 000
1
50 000
or
1:50 000
The new map will have the scale of 1:50 000. This means that 1cm measured
on the map will represent half a kilometer on the actual ground so the new
map will have the original dimensions increased in proportion. In this case,
one kilometer will be represented by 2 cm on the new map, unlike in the
original map where 1cm represents one kilometer.
Step 2. On a separate piece of paper, draw grid boxes for the new map, making
sure that the side of each grid box is twice as big as in the original map; in this
case, 2cm instead of 1cm. They should be the same number of grid boxes as in
the original map (see Figure 73).
68
13
21
22
23
24
25
26
13
12
12
11
11
10
10
09
09
2 cm
08
21
2 cm
22
23
24
25
26
08
Figure 73: Grid boxes twice as big as in the original map
Step 3. The next step is to sketch the outline of the forest reserve following
the grid boxes. The new map is now twice as big as the original map. See
Figure 74 below.
13
21
22
23
24
25
26
13
12
12
11
11
FOREST
RESERVE
10
10
09
09
08
08
21
2 cm
22
23
24
25
26
Figure 74: This map is now twice the size of the original map
69
Steps for reducing maps
The reverse process could be used to reduce the map. For example, reduce the
map in Figure 75 below by half its original size. On the reduced map, show
the new scale and the tea estate.
Step 1. Work out the new scale as follows: Divide the original scale by 2 to
decrease the proportion of the map dimensions by half; in this case, 1:50 000
÷ 2.
=
=
=
1
50 000
1
50 000
1
100 000
2
÷
1
2
x
1
or
1:100 000
The new map will be drawn to a scale of 1:100 000.
13
21
22
23
24
25
26
13
12
12
11
11
TEA ESTATE
10
10
09
09
08
21
2 cm
22
23
24
25
26
08
Figure 75: A large-scale map
Step 2. Draw grid boxes for the new map on a separate piece of paper, making
sure that the side of each grid box is half the original map; in this case, 1cm
as shown in Figure 76.
70
13
21
22
23
24
25
26
13
12
12
11
11
10
10
09
09
08
21
1 cm
22
23
24
25
26
08
Figure 76: Grid boxes half the original dimensions
Step 3. Sketch the outline of the estate following the grid boxes as done in
Figure 77.
Scale 1: 100 000
13
21
22
23
24
25
26
12
13
12
11
11
TEA ESTATE
10
10
09
09
08
21
22
23
24
25
26
08
Figure 77: The dimensions of this map are half the scale of the original map.
71
Activity
6
Enlarging/reducing maps
Study the topographic map in Figure 78 below and use it to complete the task
that follows.
Scale
0
1
2 km
Figure 78: Topographic map
1. Reduce the map by half its original size. On the new map, show the
lake, the road and the new scale.
2. Present your work to the class for discussion.
Calculating gradient
Gradient is a measure of how steep or gentle the slope is. The gradient
between two places is often expressed as a ratio. It is a ratio of a vertical
distance to a horizontal distance covered between two points of reference (see
Figure 79 below).
Figure 79: Slope and gradient
72
The average gradient of a terrain feature can conveniently be calculated from
contour lines on a topographic map. On a topographic map, the steepness
of the ground is related to how the contour lines are spaced. Areas of a map
where contours are close together indicate steep slopes, while areas with
widely spaced contour lines are gentle slopes. The difference in elevation
between adjacent contour lines is called contour interval.
To calculate gradient using a topographic map, you will need to determine the
following:
Vertical Interval
This is the difference in elevation between two points; it is calculated by
subtracting the elevation of one point which is on the lowest contour line from
the elevation of the other point at the higher contour line.
For example, if the point at the lowest contour lies 2650 meters above sea
level, and the other point at 3050 metres above sea level, the vertical interval
would be (3050m–2650m) 400 meters.
Horizontal Equivalent
This is the distance from one point to the other and is calculated by measuring
distance with a ruler and applying the map scale.
For example, if the map scale is 1:50,000 and the distance between the two
points when measured with a ruler is 4 cm, the horizontal equivalent would
be worked out as follows: 1:50,000 means that 1cm on the map is equivalent to
50,000cm on actual ground. So the 4cm map distance given will be equivalent
to (4 × 50,000) cm on actual ground. Since 100cm make 1m, we divide the
product by 100 to give the answer in meters.
=
4 x 50,000
= 2000m
100
Gradient is calculated using the following formula:
Gradient =
Vertical interval(VI)
Holizontal Equivalent (HE)
Example
Based on the topographic map in Figure 80 below, what is the gradient
between trigonometrical beacon (∆301) and spot height (•201) if the map
distance between these two points is 6cm and the map scale is 1:50 000?
73
Scale 1:50, 000
Ocean
250
0
30301
A
50 0
100
150
200
201
B
D
C
N
Figure 80: Topographic map
Step 1: Subtract the bigger height from the smaller height to get Vertical
Interval
VI = 301m-201m
= 100m
Step 2 : To get Horizontal Equivalent, calculate the distance in metres:
H =
(6 x 50,000 )
100
= 3000m
Step 3: Bring down the formula for gradient to replace values above:
VI
G = HE
m
1
G = 100
simplify this fraction by 100 to get 30
3000 m
=
1:30 (expressed as a ratio)
The gradient of 1:30 means that for every 30 meters of horizontal travel, there
will be 1 meter of altitude gain.
Gradient can be expressed as a percentage by multiplying the ratio by 100. In
this case, it will be;
1
30
x 100 = 3.3 %
This small percentage means that it is a slightly gentle slope that one would
comfortably climb.
74
Please note! When calculating gradient, the units for vertical interval and
the units for horizontal equivalent must be the same.
Activity
7
Discussing slopes and calculating gradient on a map
Basing your answers on the topographic map in Figure 80 above;
1. Which part of the map is uphill and which one is downhill?
2. Where is the land steep and where is it gentle? How do you tell?
3. What is the contour interval of this map?
4. What is the highest elevation? Lowest elevation?
5. Measure the distance, in centimeters, from point A to point B.
6. Convert this distance into metres using the scale provided on the map.
7. Repeat numbers 5 and 6 for point C to point D.
8. Determine the elevation at point C and point D.
9. Calculate the elevation gradient between point C and point D.
10. Present your work to the class for discussion.
Gradients with larger fractions are steeper than those with smaller fractions.
For example, a gradient of 1: 5 is steeper than the gradient of 1:10. The reason
is that a gradient of 1:5 has to cover less distance on the actual ground for 1
unit increase in height. Thus, it is steeper (see Figure 81 below).
1:5
1:10
10
Figure 81: Gradient (Not drawn to scale)
75
1
5
1
Activity
8
Comparing gradient
Look at the photographs in Figure 82 below and answer the questions that
follow.
Figure 82: Road signs
1. What does each of the road signs mean?
2. Which road sign shows a steeper gradient?
3. Why do you think these road signs were put there?
4. Present your work to the class for discussion.
Summary
Measuring distance between points, calculating area on a map, drawing
cross-sections and river profiles, reducing and enlarging maps and calculating
gradient are important elements in geography that you should become familiar
with. With these map skills people excel in many careers like architecture,
aeronautical and graphics design, engineering, and many others.
Glossary
Scale factor: the number of times a new figure is larger or smaller than the
original figure
Contour interval: the difference in elevation between adjacent contour lines
Horizontal Equivalent: the horizontal distance between two points on two
consecutive contour lines for a given slope
Rev
76
Review questions
Base your answers to questions 1 to 7 on the topographic map below. Points X,
Y, and Z are locations on the map. Elevations are expressed in meters.
X
Lake
Z
0
1
SCALE
2 km
Contour interval = 20 meters
Y
N
Figure 83: Topographic map
1. Draw a cross-section between points X and Y, and determine their
intervisibility.
2. What is the elevation of point Z?
3. State the direction in which the river is flowing.
4. How long is the river from the railway to the lake?
5. Calculate the area covered by the lake.
6. Reduce the map by half its size, and on the new map, show the railway,
the river and the lake.
7. Calculate the gradient of the river from the railway to its mouth.
References
Bowen, A. D. et al. (1997). Map Reading for Southern Africa. Cape Town:
Maskew Miller Longman.
Harte, J and Dunbar, C. (1994). Skills in Geography. Cambridge: Cambridge
University Press.
Kalaluka, L. (1978). Map Reading for Central Africa. London: Longman.
http://www.bmouthyexplorers.org.uk/resources/measuring-distance.aspx
02/01/14
77
http://weather.gladstonefamily.net/topoweb/guide.html 13/01/14
http://passyworldofmathematics.com/gradient-and-slope/
http://www.sir-ray.com/Topographic%20Map%20Review%20Lab.htm
02/01/14
www.thutong.doe.gov.za/GEOGRAPHY GRADE 10 - Thutong 02/01/14
http://www.sir-ray.com/Topographic%20Map%20Review%20Lab.htm
02/01/14
http://www.dreamstime.com/02/01/14
78
Unit
6
Statistical methods
in Geography
Statistical methods in Geography
Statistics refers to the science of creating
new knowledge from a set of data. It involves
the collection, analysis and interpretation of
numerical facts. The term statistics may also refer
to the figures, which result from manipulating the
Statistics plays a vital numerical data, for instance mean, mode, median,
role in every field of standard deviation, and others.
human activity. Statistics Data are a collection of facts, often numerical, from
has an important role in which new knowledge may be derived. This means
determining the existing that data are raw numbers, which by themselves
position of per capita are of limited value to decision makers unless
income, unemployment, they are organized, processed and interpreted
population growth rate, into meaningful and useful knowledge. The data
housing,
schooling, that has been processed or transformed for use is
medical facilities, and called information.
other aspects in a country.
This topic is therefore
important to help you Nature of geographical data
acquire inquiry skills Geographical data have variables and attributes.
that would enable you to Variables are often described as quantitative
understand and control data whereas attributes are best known as
situations that affect qualitative data.
your life. In this unit, you
will identify various ways
of collecting geographical Variable (quantitative) data
data,
design
data
collection
instruments, These are data whose characteristics are
and collect data using measurable and can be observed. Variables are
appropriate instruments. either discrete or continuous.
You will also analyze
a. Discrete variables are distinct and
data using appropriate
separate values involving only whole
procedures.
numbers, that is; they do not include
fractions, for instance number of children.
It is impossible to have one and a half
children.
79
b. Continuous variables are unbroken range values, which include
both whole numbers and fractions e.g. height, weight, age, distance
and area.
Attribute (qualitative) data
These are data whose values are not measurable but can be observed or
identified and described as present or absent e.g. sex (males/females) religion
(Christianity, Islam, etc.).
The role of statistics
a. It helps increase objectivity and precision in explaining the spatial
patterns of geographical distributions and relationships. In practical
terms, this describes the reality of people’s everyday lives, which helps
determine where the poor are and where the resources are most needed.
b. Statistics enables scientists to handle large quantities of figures and
summarise them in a way that can easily be described and explained.
This helps people to monitor and assess effectiveness of government
policies towards achieving development goals.
c. It helps to precisely establish the relationship between variables, for
example, the relationship between number of people and number of
health facilities in an area. This enables policy makers to formulate
good development plans.
Types of data sources
There are two types of data sources:
a. Primary data: Primary data is collected directly from first hand
experiences or observations. The sources are interviews, observations,
questionnaires, measurements, experiments, etc.
b. Secondary data: This is material collected by other people and
made available as published or unpublished information. The sources
include books, reports, magazines, articles, directories, photographs,
newspapers, television and radio programmes.
Activity
1
Identifying types of data
1. State whether each of the following represents discrete data or
continuous data:
80
a. Number of candidates arrested for cheating during national
examinations in Malawi per year.
b. Number of babies born at Queen Elizabeth Hospital in one year.
c. Volume of water consumed each day.
d. Temperature of the classroom.
2. Report your work to the class for discussion.
Data collection methods
The most common methods of data collection are the following:
Observation
Observation is the act of watching with scrutiny what is happening so as
to record the desired information. For instance, one can observe behaviour,
traffic, etc. The observer usually has a checklist of what is to be recorded when
observed. The observer can be either non-participant or participant. He is a
participant when he is involved in what is happening in order to detect the
existence or presence of some important facts.
Advantages of observation
a. There is high possibility of getting accurate data since the observer
records from first hand experiences.
b. It enables the researcher to get information that is more detailed.
Disadvantages of observation
a. Respondents may not act like their true self when they realise that
they are being observed. This is called Hawthorne effect.
b. Time and cost are very high.
c. It is based on the observer’s opinion and therefore can be biased.
d. It may get the researcher into dangerous situations.
Interview
This is a conversation between two or more people where questions are asked
by the interviewer to obtain information from the one being interviewed.
Sometimes tape recordings are used to facilitate free flow of information.
81
Advantages of interview
a. There is direct interaction between the interviewer and the respondent,
allowing the former to observe and record some behaviour in the
respondent.
b. The interviewer has great control over the research setting in which he
is able to modify questions that appear to be misunderstood.
c. It is not restricted to the literate.
Disadvantages of interview
a. It is costly to train interviewers and to meet their travelling as well as
living expenses.
b. It is time consuming to conduct interviews.
c. It may bring nervousness to the respondent and this may increase the
possibility of low quality data.
Questionnaire
A questionnaire is a piece of paper (form) on which is a set of questions
requiring written answers, which ask for information about a particular
problem. Usually the respondent (informant) records the answers; the
questionnaire is self-administered, and is either posted to the respondent or
directly distributed to him. The questionnaires are sent or given back to the
data collector when completed.
Advantages of questionnaires
a. Use of questionnaires is cost effective since it does not need trained
people to distribute forms to respondents.
b. It is relatively quick to collect information even from beyond the physical
reach of the researcher.
c. It has standardised questions whose responses can easily be processed
into usable information.
Disadvantages of questionnaires
a. Use of questionnaires is restricted to literate people.
b. It does not provide room for clarifying difficult questions.
c. It has great chances of low returns of completed questionnaires.
d. Standardisation of questions may leave out important facts.
82
Activity
2
Collecting data
Conduct the following data collection activity individually:
1. Make a record of how much water you use each day for one week.
To do this, make a data collection sheet on a paper (like the one in
Table 2 below).
2. Figure out how much water each activity took by checking the item
used to draw water (e.g. bucket, cup, dish, pot, etc.), and enter into the
data collection sheet.
3. At the end of the sheet in the total column, add up the number of
toilet flushes, dish washers, clothes washers, hands washers, drinking,
cooking, bathing and teeth brushing.
Table 2: Data Collection Sheet
Frequency
Water-use Activity
Item
used
Day
1
Day
2
Total
Day
3
Day
4
Day
5
Day
6
Day
7
Toilet flushes
Clothes
washing
Hands washing
Drinking
Cooking
Bathing
Teeth brushing
Dish washing
4. How much water do you use in a week?
5. What method of collecting data is this one you used?
6. Present your findings using any visual illustration.
Analyzing data using appropriate procedures
Data analysis is a practice in which, unorganised or unfinished data is ordered
and organised, so that useful information can be highlighted. It involves
processing and working on data, in order to understand that all is present in
the data. Data analysis methods help us to understand facts, observe patterns,
formulate explanations, and try out hypotheses. Data analysis is not only used
in all kinds of science and business processes, but also in administration and
policy-making.
83
Quantitative data analysis
A quantitative approach is often concerned with finding evidence to either
support or contradict an idea or hypothesis you might have. This is also
called a deductive approach. A hypothesis is where a predicted answer to a
research question is proposed.
Types of quantitative data analysis
There are two specific types of quantitative data analysis, descriptive and
inferential.
Descriptive analysis
Descriptive statistical data analysis is used to construct simple descriptions
about the characteristics of a set of quantitative data and to summarize the
information in the data. Descriptive statistics do not, however, allow us to
make conclusions beyond the data we have analysed or reach conclusions
regarding any hypotheses we might have made. They are simply a way to
describe our data.
Approaches to descriptive data analysis
There are two basic approaches to descriptive data analysis: measures of
central tendency and measures of dispersion.
Measures of central tendency
A measure of central tendency is a single value that attempts to describe
a set of data by identifying the central position within that set of data. As
such, measures of central tendency are sometimes called measures of central
location. There are three main measures of central tendency: the mean, the
median, and the mode. In the following sections, we will learn how to calculate
the mean, mode and median and look at the conditions under which they are
most appropriate to be used.
Mean
The mean, also known as the arithmetic average, is the sum of all the values
in the data set divided by the number of values in the data set.
It is calculated using the following equation: X
84
= ∑x
n
Where
X = “the mean of x.” It is pronounced as x bar.
x
= the values in the data set
∑
= pronounced “sigma”, which means “sum of...”
n
= the total number of values in the data set.
For example, take the following set of data: {1,2,3,4,5}. The mean of this data
would be:
1+2+3+4+5
X = ∑x
n =
5
= 15
= 3
5
Here is a more complicated data set: (10, 14,86,2,68,99,1). The mean would be
calculated like this:
X
+ 68 + 99 + 1
= ∑x
= 10 + 14 + 86 + 27
= 280
7 = 40
n
The advantage of the mean is that it can be used for both continuous and
discrete numeric data.
Disadvantages of the mean
a. The mean cannot be calculated for categorical data, as the values cannot
be summed.
b. As the mean includes every value in the distribution the mean is
influenced by outliers (extreme values) and skewed (unbalanced)
distributions.
For example, consider the wages of staff at a factory below:
Staff
Salary
1
2
3
4
5
6
7
8
9
10
$15 $18 $16 $14 $15 $15 $12 $17 $90 $95
The mean salary for these ten staff is $30.7.
However, inspecting the raw data suggests that this mean value might
not be the best way to accurately reflect the typical salary of a worker,
as most workers have salaries in the $12 to $18 range. The mean is
being skewed by the two large salaries.
85
Therefore, in this situation, we would like to have a better measure of
central tendency. As we will find out later, taking the median would be
a better measure of central tendency in this situation.
Median
The median is the middle value in distribution when the values are arranged in
ascending or descending order. The median divides the distribution in half (there
are 50% of observations on either side of the median value). In a distribution
with an odd number of observations, the median value is the middle value.
Looking at the retirement age distribution below (which has 11 observations),
the median is the middle value, which is 57 years. It is the middle mark
because there are 5 scores before it and 5 scores after it:
54, 54, 54, 55, 56, 57, 57, 58, 58, 60, 60
This works well when you have an odd number of scores, but what happens
when you have an even number of scores? What if you had only 10 scores?
Well, you simply have to take the middle two scores and average the result.
So, if we look at the example below: the two middle values are 56 and 57,
therefore the median equals 56.5 years:
52, 54, 54, 54, 55, 56, 57, 57, 58, 58, 60, 60
Advantages of the median
a. The median is less affected by outliers and skewed data than the mean,
and is usually the preferred measure of central tendency when the
distribution is not symmetrical (unbalanced).
b. It can be used for determining qualities and factors to which
mathematical measurements cannot be given.
c. Even when the values of all items are not known the median can be
obtained.
Disadvantages of the median
a. It is often tedious to arrange the data in the necessary form.
b. It cannot be used to determine the total value of all the cases or items.
c. The median cannot be identified for categorical nominal data, as it
cannot be logically ordered.
86
Mode
The mode is the most commonly occurring value in a distribution.
Consider this data set showing the retirement age of 11 people, in whole years:
54, 54, 54, 55, 56, 57, 57, 58, 58, 60, 60
This table shows a simple frequency distribution of the retirement age data.
Age
54
55
56
57
58
60
Frequency
3
1
1
2
2
2
The most commonly occurring value is 54; therefore the mode of this
distribution is 54 years.
On a histogram or bar chart, the mode represents the highest bar. You can,
therefore, sometimes consider the mode as being the most popular option.
Normally, the mode is used for categorical data to determine the most
common category, as illustrated below:
15
Mode
10
5
0
Car
Train
Bus
Bicycle
Figure 84: Mode
We can see above that the most common form of transport, in this particular
data set, is the bus.
87
Advantages of the mode
a. The mode can be found for both numerical and categorical (nonnumerical) data.
b. Its value is not affected by extreme items
Disadvantages of the mode
a. It is tedious to arrange the data.
b. The mode may not reflect the centre of the distribution very well in
some distributions. In the example above, when the distribution of
retirement age is ordered from lowest to highest value, it is easy to see
that the centre of the distribution is 57 years, but the mode is lower, at
54 years (see below).
Median
}
Mode
54, 54, 54, 55, 56, 57, 57, 58, 58, 60, 60
c. It is also possible for there to be more than one mode for the same
distribution of data, (bi-modal, or multi-modal). The presence of more
than one mode can limit the ability of the mode in describing the centre
or typical value of the distribution because a single value to describe
the centre cannot be identified.
d. In some cases, particularly where the data are continuous, the
distribution may have no mode at all (i.e. if all values are different).
In cases such as these, it may be better to consider using the median or
mean, or group the data in to appropriate intervals, and find the modal
class.
How does the shape of a distribution influence the measures
of central tendency?
We often test whether our data is normally distributed because this is a
common assumption underlying many statistical tests.
Symmetrical distributions
When a distribution is normal, we say it is symmetrical. When you have a
normally distributed sample you can legitimately use the mean, the median
88
or the mode as your measure of central tendency. In fact, in any symmetrical
distribution the mean, median and mode are equal and are all in the middle of
the distribution. The following graph shows an age data set with a distribution
which is symmetrical. The mode, median and mean all equal.
Mean
Median
Mode
120
Frequency
100
80
60
40
20
0
0
30
40
}
}
10
20
Age (years)
Balanced left and right tails
Figure 85: Normal distribution
However, in this situation, the mean is widely preferred as the best measure
of central tendency because it is the measure that includes all the values in
the data set for its calculation, and the value of the mean will be affected by
any change in any of the scores. This is not the case with the median or mode.
Skewed distributions
When a distribution is skewed the mode remains the most commonly occurring
value, the median remains the middle value in the distribution, but the mean
is generally ‘pulled’ in the direction of the tails. In a skewed distribution, the
median is often a preferred measure of central tendency, as the mean is not
usually in the middle of the distribution. The more skewed the distribution,
the greater the difference between the median and mean, and the greater
emphasis should be placed on using the median as opposed to the mean.
A distribution is said to be positively or right skewed when the tail on the
right side of the distribution is longer than the left side. Although there are
exceptions to this rule, generally, most of the values, including the median
value, tend to be less than the mean value.
89
The following graph shows an age data set with a distribution which is right
skewed.
120
Mode
Frequency
100
80
Median
60
Mean
40
20
0
10
0
20
30
40
}
}
Age (years)
long right tail
short left tail
Figure 86: Positive distribution
100
Median
80
Mean
60
40
20
0
0
10
20
30
}
Age (years)
}
The following graph
shows an age data set
with a distribution
which is left skewed.
Mode
120
Frequency
A distribution is said to
be negatively or left
skewed when the tail
on the left side of the
distribution is longer
than the right side.
Although there are
exceptions to this rule,
generally, most of the
values, including the
median value, tend to
be greater than the
mean value.
Long left tail
Figure 87: Negative distribution
90
Short right tail
40
How do outliers influence the measures of central tendency?
Outliers are data value(s) that are notably extreme or different from the rest
of the other values in a set of data.
It is important to detect outliers within a distribution, because they can alter
the results of the data analysis. The mean is more sensitive to the existence of
outliers than the median or mode.
Consider the initial retirement age data set again, with one difference; the last
observation of 60 years has been replaced with a retirement age of 81 years.
This value is much higher than the other values, and could be considered
an outlier. However, it has not changed the middle of the distribution, and
therefore the median value is still 57 years.
54, 54, 54, 55, 56, 57, 57, 58, 58, 60, 81
However, in some situations, despite the existence of outliers in a distribution,
the mean can still be an appropriate measure of central tendency, especially
if the rest of the data is normally distributed. If the outlier is confirmed as a
valid extreme value, it should not be removed from the data set.
Activity
3
Calculating measures of central tendency and analysing
data
1. Find the mean, median and mode for the following data: 5, 15, 10, 15,
5, 10, 10, 20, 25, 15.
2. State whether each of the following data sets is symmetric, skewed
right or skewed left.
a.
b.
c.
Figure 88: Distribution histograms
3. Present your work to the class for discussion.
91
Measures of dispersion (variability)
Measures of dispersion describe the spread of values in a set of data. There
are several measures of dispersion, and a few of the more common ones are
described briefly in the sections below.
Range
The range is the simplest measure of variability to calculate, and one you
have probably encountered many times in your life. The range is simply the
highest score minus the lowest score. For example, here is a dataset with 10
numbers: 99, 45, 23, 67, 45, 91, 82, 78, 62, 51. The range will be calculated as
follows:
The highest number is 99 and the lowest number is 23, so 99 – 23 equals 76;
the range is 76.
Interquartile range
The interquartile range is the spread of the middle 50% of scores. For example,
suppose that we have the following set of scores:
4, 5, 6, 6, 7, 8, 8, 9, 11, 11, 14, 15, 17, 18, 18, 19
There are 16 scores, which can be divided into the bottom 25% (4), the middle
50% (8), and the top 25% (4). The middle 50% of scores start with 7 and run
through to 15. Between 15 and 17 lies the upper boundary of the interquartile
range, and is given by the mean of these two values, i.e. 16. The lower boundary
of the interquartile range lies between 6 and 7, and is the mean of these two
values, i.e. 6.5.
The interquartile range is the difference between the upper and lower
boundaries, i.e. 16 – 6.5 = 9.5.
Standard deviation
This measure indicates to what degree the individual observations of a data
set are dispersed or ‘spread out’ around their mean. The more widely the
values are spread out, the larger the standard deviation. For example, say we
have two separate lists of exam results from a class of 30 students; one ranges
from 31% to 98%, the other from 82% to 93%, then the standard deviation
would be larger for the results of the first exam.
92
Inferential data analysis
Inferential data analysis often relies on a small subset of a larger set of data
called a sample to draw inferences (conclusions) about the larger set. The
larger set is known as the population from which the sample is drawn. The
purpose of inferential data analysis is to;
• estimate the characteristics of a population from data gathered on a
sample.
• determine whether the difference between groups or relationships
between variables in a sample is large enough to be able to say that the
findings are significant. If the findings are indeed significant, then the
conclusions can be applied, or generalised, to the entire population
For example, imagine that you have been hired by the Malawi Electoral
Commission to examine how Malawians feel about the fairness of the voting
procedures in the country. Whom will you ask?
It is not practical to ask every single Malawian how he or she feels about
the fairness of the voting procedures. Instead, you ask a relatively small
number of Malawians, and draw inferences about the entire country from
their responses. The people actually asked constitute our sample of the larger
population of all Malawians.
Inferential statistics are based on the assumption that sampling is random,
where members of the population are chosen without an identifiable pattern.
This is to ensure that the sample does not over-represent one kind of citizens
at the expense of others. For example, it would be wrong if the sample is made
up entirely of Blantyre residents because it could not be used to infer the
attitudes of other Malawians.
Activity
4
Determining the significance of research findings
A substitute teacher wants to know how students in the class did on their last
test. The teacher asks the 10 students sitting in the front row to state their
latest test score. He concludes from their report that the class did extremely
well.
1. What is the sample?
2. What is the population?
3. Can you identify any problems with choosing the sample in the way
that the teacher did?
4. Present your answers to the class for discussion.
93
Qualitative data analysis
Qualitative data analysis is an inductive approach where researchers
are concerned with generalising to produce a universal claim or principle
from observed instances. It involves the identification, examination, and
interpretation of patterns and themes in textual data and recordings, and
determines how these patterns and themes help answer the research questions
at hand.
Approaches to qualitative data analysis
There are many approaches to qualitative data analysis techniques, including
the following:
Content analysis
Content analysis is a form of analysis that counts and reports the frequency
of concepts, words, and behaviours held within the data. The researcher
develops brief descriptions of the themes and meanings, called codes. Similar
codes may be grouped together to form categories. Content analysis is often a
time-consuming process, because it requires in-depth reading and rereading
of material.
Narrative analysis
In this technique, the researcher listens to the stories of the research subjects,
attempting to understand the relationships between the experiences of the
individuals and their social framework. Narrative analysis is a basic human
way of making sense of the world, so many social scientists are interested in
studying it. Narrative analysis mainly focuses on written or oral texts, but
can also be used to analyse photographs, films or even dance performances.
Discourse analysis
This is the study of the ways in which language is used in texts and contexts. In
discourse analysis, researchers study larger chunks of language as they flow
together in order to understand how it affects the meaning of the sentence. Two
sentences taken together as a single discourse can have meanings different
from each one taken separately. Imagine two independent signs at a beach
resort: one says “Please use the toilet, not the beach.” The other says, “Beach
for members only.” The signs seem quite reasonable if you regard each one
independently. But taking them together as a single discourse makes you go
back and revise your interpretation of the first sentence after you have read
the second.
94
Semiotics analysis
This is the study of signs, gestures and symbols of all kinds, what they mean,
and how they relate to the things or ideas they refer to.
Logical analysis
This is an outline of generalised causation, logical reasoning process, etc.
It uses flow charts, diagrams, etc. to pictorially represent these, as well as
written descriptions.
Domain analysis
This analysis involves a search for the larger units of cultural knowledge. It is
concerned with the structures and rituals, which serve to support, maintain
and provide uniqueness to a particular culture under study. Similarities
of what is considered culturally important by a group will closely relate to
the language of people in that particular cultural context. Domain analysis
describes the meanings of the social situation to participants. It also relates
the social situation and cultural meanings.
Data visualization
Data visualization is the presentation of data in a pictorial or graphical
format. The concept of using pictures to understand data has been around
for many years. Because of the way the human brain processes information,
using charts or graphs to visualize large amounts of complex data is easier
and quicker than focusing on spreadsheets or reports.
Statistical tables
These are visual representation of numerical information, which is arranged
in labelled columns and rows. Tables are a great way to display a great deal
of information in an orderly, clear and easy to read format. They allow easy
comparison between sets of data. Tables also summarise data and therefore
save space and time. For example, Table 3 clearly shows how enrolment in
form one increased each year for Mapundi Secondary School.
Table 3: Student enrolment in form one at Mapundi Secondary School
Year
2000
2001
Number of students
40
50
95
2002
2003
2004
2005
2006
70
120
130
130
140
Frequency tables
A frequency table is a record of how often each value (or set of values) of the
variable in question occurs. It may be enhanced by the addition of percentages
that fall into each category.
A frequency table is used to summarise categorical, nominal, and ordinal
data. It may also be used to summarise continuous data once the data set has
been divided up into sensible groups.
Example
Suppose that in a class of thirty students, the following scores were given
after a short test:
5, 2, 2, 3, 4, 4, 3, 2, 0, 3, 0, 3, 2, 1, 5, 1, 3, 1, 5, 5, 2, 4, 0, 0, 4, 5, 4, 4, 5, 5
The frequencies of the different scores can be summarised as:
Score
0
1
2
3
4
5
Frequency
4
3
5
5
6
7
Frequency (%)
13%
10%
17%
17%
20%
23%
Graphs
A graph is a visual display of amounts or data. There are different kinds of
graphs, each having special features.
Line graphs
These compare two variables, each plotted along an axis. The vertical axis
shows the values of a dependent variable and horizontal axis is always the
independent variable. The resulting points are joined with a continuous
line (as in Figure 89).
96
Student enrolment at Mapundi Secondary School
Number of Students
160
140
120
100
80
60
40
2000
2001
2002
2003
Years
2004
2005
2006
Figure 89: Line Graph
Bar graphs
Bar graphs use vertical or horizontal rectangular blocks that level off at the
appropriate point proportional to the values that they represent. Bar graphs
are often used when dealing with discrete data; hence, the bars are separated
from each other.
Example
Table 4 below shows the birthdays of students by month. Draw a bar graph
to show the data presented in the table.
Table 4: Birthdays of students by month
Month of birthday
January
February
March
April
May
June
July
August
September
October
November
December
Number of students
1
1
2
4
6
8
7
6
3
2
1
2
97
The following steps should be followed:
Step 1: Put the months in order along the horizontal axis of the bar graph
starting with January. The months will be the independent variable.
Step 2: On the vertical axis, make a scale that starts with zero and goes to
the largest value of the number of students; in this case, eight (8). This will be
the dependent variable.
Step 3: Draw a bar for each of the months listed on the horizontal axis. The
height of each bar should correspond with its value (number of students) on
the vertical axis. Space between the bars should be equal. The width of the
bars should also be equal (see Figure 90).
10
Birthday of students by month
9
Number of students
8
7
6
5
4
3
2
1
0
J
F M A M J J A S
Month
O N D
Figure 90: Bar Graph
Histograms
A histogram is a visual representation of a continuous variable. Unlike a bar
graph, a histogram has no space between bars. When drawing a histogram,
the steps for drawing a bar graph should be followed except that each bar
should touch the other bars on its sides (Figure 91).
98
10
Birthdays of students by month
Number of Students
8
6
4
2
0
J F M A M J J A S O N D
Month
Figure 91: Histogram
Pie charts
A pie chart is a circle graph used to compare portions of the same whole. It
does not use a set of axes to plot points as the other graphs do. In addition, a
pie chart does not work with the same type of data that the other graphs work
with. The circle of a pie chart represents 100%, and each portion that takes up
space within the circle stands for a part of that 100%.
When drawing a pie chart a number of steps should be followed. Consider the
following example: a group of students were suspended from Malaga Boarding
Secondary School for committing various offences as follows:
Table 5: Offences committed by students
Offence
Absconding classes
Going out of bounds
Defiance of authority
Use of abusive language
Number of students
05
20
25
10
Step 1: Find the total number of students that were suspended from school:
05+20+25+10 = 60. The result (60) represents the whole circle, which is 100%
or 3600.
Step 2: For each offence, calculate the size of the portion or sector, and the
following formula can be used to work out the size of the sector:
99
Value of factor to be in this sector
size of sector =
x Number of degrees in a circle
Sum of the values of all the factors
Absconding classes:
5
60
x 360o = 30o
Going out of bounds:
20
60
x 360o = 120o
Defiance of authority:
25
60
x 360o = 150o
Use of abusive language:
10
60
x 360o = 60o
If it is worked out in percentages: 100% is equal to 360°and therefore each 1%
is equal to 3.6°on the chart, so that the first case would be;
5
60
x 100 x 3.6 = 30o
This should be done with the rest of the cases.
Step 3: Draw a circle of a suitable radius, and then using a protractor divide
the circle into sectors bearing the angles above accurately. Begin with the
largest sector and move clockwise to the smallest sector (see Figure 92).
150o
First sector
Figure 92: Begin with the largest sector
100
Step 4: Label the sectors properly, as shown in Figure 93 below.
Absconding
classes
Use of abusive
language
30o
60
o
150
o
Defiance of
authority
120o
Going out of bounds
Figure 93: Pie chart
Always bear in mind that the number of sectors should not exceed ten to avoid
crowding the chart.
Statistical maps
Statistical maps are special types of maps in which the variation in quantity of
a factor such as rainfall, population, or crops in a geographic area is indicated.
Dot maps and isoline maps are some of the types of statistical maps (Figure
94).
The dot maps show the
number of occurrences
of some geographical
phenomena, mostly
population distribution.
Isoline maps use lines to join
points of equal values e.g.
contour maps join places of
equal height.
1004
1004
L
1000
Windhoek
Mozambique
Channel
Johannesburg
Durban
1016
H
1008
L
Cape Town
1008
1012
1012
Figure 94: Isoline map
101
1016
1020
H
Age-sex pyramids
These are used to quickly examine the age and gender of a given human
population. They let viewers get an overview of the country’s population and
find instances where certain events have led to a significant change in the
population (see Figure 95 below).
Malawi-2012
Male
Female
100+
95-99
90-94
85-89
80-84
75-79
70-74
65-69
60-64
55-59
50-54
45-49
40-44
35-39
30-34
25-29
20-24
15-19
10-14
5-9
0-4
2
1.6
1.2
0.8
Population (in millions)
0.4
0
0
0.4
Age group
0.8
1.2
1.6
2
Population (in millions)
Figure 95: Age-sex pyramid
Flow diagrams or flow charts
Flow diagrams show a sequence of activities or a process. The steps are shown
as boxes of various kinds, connected with arrows (Figure 96).
Flow
diagrams
are
used in designing and
documenting or managing
complex processes. Like
other types of diagrams,
they help the viewer
understand a process, and
perhaps find hidden and
other less-obvious features
within it.
Farmers
COLLECTION TRADER
INDUSTRY
FOOD
INDUSTRY
BIG TRADER
RETAILERS
(MARKETS AND SHOPS)
CONSUMERS
Figure 96: Flow process of
marketing macadamia nuts
102
EXPORTER
Activity
5
Reflecting on important issues in the topic
1. In groups of four, locate an important issue that you feel the topic has
covered.
2. Formulate a problem or question about it for another group to answer.
3. Write the problem down on a sheet of paper, and hand that piece of
paper to another group.
4. Once your group is handed a problem statement, think of a solution to
the problem. Each group has a fixed amount of time.
5. Present your problem and its solutions to the class for discussion.
Summary
Statistics involves the collection, analysis and interpretation of numerical
facts. Geographical data have variables and attributes. Data is collected
through direct observation, interviews and use of questionnaires. Data can be
analysed through quantitative approaches such as descriptive and inferential
statistics. There are two basic approaches to descriptive data analysis:
measures of central tendency (e.g. median, mean, and mode) and measures
of dispersion (e.g. range, interquartile range and standard deviation). Data
can also be analysed through qualitative approaches such as content analysis,
narrative analysis, discourse analysis, semiotics analysis, logical analysis,
domain analysis and event analysis, among others. The analysed data is
usually presented using universal illustrations such as tables, line graphs,
bar graphs, histograms, pie charts, flow diagrams, isoline maps, age-sex
pyramids, among others.
Glossary
Data: information, often in the form of facts or figures obtained from
experiments or surveys, used as a basis for making calculations or drawing
conclusions
Variable: Data whose characteristics are measurable and can be observed.
Attribute: Data whose values are not measurable but can be observed or
identified and described as present or absent.
Hawthorne effect: An effect in social research in which findings are
103
attributable to the attention of researchers to the subjects of their research
rather than to factors significant to the research topic.
Hypothesis: A tentative explanation for a phenomenon, used as a basis for
further investigation.
Central tendency: An average value of any distribution of data that best
represents the middle.
Outlier: An observation in a data set which is unusually large or an unusually
small in value compared to the others in the data set.
Skewness: A lack of symmetry in the distribution of the sample data values.
Review questions
1. What is statistics?
2. Explain three ways in which statistics is important.
3. What is the difference between a variable and an attribute?
4. Describe any three methods of collecting data.
5. Using the data shown in the table below:
a. draw a histogram.
b. state whether the histogram is symmetrical or skewed.
Table 6: Scores of a Geography quiz
Score
0
1
2
3
4
5
6
7
8
9
10
Frequency
8
10
15
22
18
10
6
5
2
3
1
6. What method do researchers use to infer meaning from data and to
determine what conclusions are justified?
7. Explain the difference between descriptive data analysis and inferential
data analysis.
104
8. Describe the three measures of central tendency.
9. State the two purposes of inferential statistics.
10. What are the five methods for graphically representing a frequency
distribution? When should each be used?
References
Obara, D. A. (1991). Gold Medal Geography. Nairobi: MacMillan Kenya
Limited.
https://statistics.laerd.com/statistical-guides/measures-central-tendencymean-mode-median.php 11/01/14
http://www.abs.gov.au/websitedbs/a3121120.nsf/home/statistical+language++measures+of+central+tendency 11/01/14
http://onlinestatbook.com/2/introduction/inferential.html 10/01/14
http://www.stats.gla.ac.uk/steps/glossary/presenting_data.html 11/01/14
Malawi Age structure - Demographics - Mundiwww.indexmundi.com
105
106
Unit
7
Since it was formed, the
earth has undergone
many changes, including
landform
creation.
Geologists
have
put
forward many theories
to
explain
earth
surface
changes
but
none have succeeded
in providing universal
explanation.
However,
the concept of continental
drift gave clues that
helped
scientists
to
make discoveries that
led to the theory of
plate
tectonics,which
successfully
explains
many of earth’s major
geologic
features.
Therefore, it is very
important to learn the
concept of continental
drift before moving on
to plate tectonics. In this
unit, you will explain
the theory of continental
drift and the evidence
supporting
it.
You
will also examine the
weaknesses of the theory
of continental drift.
The theory of
continental drift
Continental drift theory
A theory is a supposition or a set of ideas intended
to explain facts or events.
Continental drift is the gradual movement of the
continents across the earth’s surface through
geological time.
Continental drift theory is a set of ideas that
explained how continents shift position on Earth’s
surface.
Though first proposed by American geologist
Frank Bursley Taylor in a lecture in 1908, the
first detailed theory of continental drift was put
forth by German meteorologist and geophysicist
Alfred Wegener in 1915. Wegener hypothesized
that there was once a gigantic supercontinent
called Pangaea, meaning, “all the lands”.
About 200 million years ago, the Pangaea broke
into two large continental landmasses, Laurasia
in the northern hemisphere and Gondwanaland
The supercontinent of Pangea
The breakup of Pangea
Laurasia
future india
future
Australia
200 million years ago
Gondwanaland
future india
future
Australia
180 million years ago
future india
future Australia
65 million years ago
Present
Figure 97: Drifting of continents
107
in the southern hemisphere. Laurasia and Gondwanaland then continued to
break apart into the various smaller continents that exist today (Figure 93).
It is believed that the continents are still drifting and will continue to drift to
different positions.
Activity
1
Brainstorming the effects of continental drift
In groups of four, discuss the following questions:
1. What changes might have occurred as the continents moved from their
previous locations to their present-day locations?
2. Do you think that the breakup of Pangaea into Gondwanaland and
Laurasia affected organisms originally living on Pangaea?
3. Do you think that the breakup of Gondwanaland into the southern
continents affected the organisms living in Gondwanaland?
4. Give evidence to support your ideas.
5. What types of ‘clues’ could exist in our modern world that might suggest
the existence of the supercontinents?
6. Report your findings to the class for discussion.
Evidence that support the theory of continental drift
The following pieces of evidence support and give credence to the theory of
continental drift:
a. The jig-saw fit of the continents: The shapes of some continents
(most notably, Africa and South America) seem to fit together like a
jigsaw puzzle, if they were to be moved
closer. This suggests that at some point
in time the continents were once joined
together. Figure 98 below illustrates the
jig-saw fit of the continents.
b. Similarity in plant/animal fossils
of the continents: Fossil plants and
animals (remains of dead plants and
animals) in India, South Africa, Australia,
South America and Antarctica are
similar to each other. It was impossible
for an animal to move from one continent
to another because the distance between
continents is too long for any animal to
swim or a bird to fly.
108
Activity
2
Mapping the location of different types of fossils in
continents that broke apart from Gondwanaland
1. Get copies of Figure 99 below and cut out the shapes of the following
continents and countries: South America, Antarctica, Australia, Africa,
Madagascar and India.
Africa
aga
scar
South America
Antarctica
Mad
Australia
2. Using an atlas and the
key code below, mark
the locations of the 4
different fossils and
the places listed in the
chart or table below.
3. Arrange the continents
into what you think
Gondwanaland
may
have looked like.
4. Present your work to
the class for discussion.
India
Figure 99: Southern continents (source:www.imaxmelbourne.com.au/.../Gondwanaland.pdf)
Glossopteris = Green ‘G’ Cynognathus = Orange ‘C’
Lystrosaurus = Red ‘L’ Mesosaurus = Blue ‘M’
Table 7: Fossils and their locations (source:www.imaxmelbourne.com.au/.../Gondwanaland.
pdf)
Fossil Name
Glossopteris
Description
A fern
Present day location
- Southern tip of India near Madurai
- Prince Harald Coast, Antarctica
- Southern tip of Madagascar
- Oates Coast, Antarctica
- Southeaster Australia (near
Melbourne)
109
Cynognathus
A land reptile
- South eastern Argentina (near Bahia Blanca)
- Southwestern South Africa (near
Cape Town)
Lystrosaurus
A land reptile
- Wilhelm II Coast, Antarctica
- Madagascar, north of Antananarivo
- Central India (between Bangalore
and Hyderabad)
Mesosaurus
A freshwater
reptile
- Eastern Tanzania (near Dar es
Salaam)
- Eastern Brazil (near Salvadore)
- Cameroon, West Africa
c. Similarity in rock sequence of the continents: There are striking
similarities in rock layers between eastern South America and Western
Africa.
d. Climatological anomalies: A number of climatic anomalies have
been discovered which suggest that continents must once have been in
a different position and therefore have experienced a different climate.
For example;
i.
the presence of coal seams underneath the cold and dry Antarctic
ice cap. Coal can only form in warm and wet conditions. This
suggests that the Antarctic ice cup was previously in the warm
and wet latitudes and that it drifted to its present location.
ii.
there is evidence of glacial deposits in Congo (a country which
now has an equatorial climate). Glacial deposits are heaps of
rocks and sediments left behind by a large slow moving mass of
ice. Equatorial climate does not allow glaciations, so it simply
suggests that Africa drifted from the cold latitudes to its present
location.
e. Magnetic variations on the continents (Palaeomagnetism) When igneous rocks form they cool from a molten magma until they
solidify. As they do so, iron minerals in them become orientated in line
with the Earth’s magnetic field. Once the rocks are solid they do not
110
change their orientation. However, the orientation of these minerals in
rocks of differing ages from the same location in North America and in
Europe is consistently different. This strongly suggests that continents
have drifted relative to each other since these rocks were formed. If the
continents had not moved, the magnetic orientations should have been
the same. They are not.
Activity
3
Analysing evidence: continental drift
1. In groups of four, carefully read the information in Table 8.
2. Look at each statement in the table carefully and then mark whether
you think it is or is not evidence. If you think it is evidence check “yes”
and if you think it is not evidence check “no”.
3. Cross out each statement that you have decided was not evidence. You
will no longer consider these statements.
4. Do you think each piece of evidence supports the idea that continents
have moved? Check “yes” if you think it supports it, and “no” if you
think it does not.
5. With your group, discuss:
a. whether you identified a statement as evidence or not
b. how each statement you checked as evidence either supports or
contradicts the idea of continental movement
6. Report your work to the class for discussion
Table 8: Analysing evidence: continental drift (source:https://www.teachingchannel.
org/.../p/.../continental-drift-lesson-plan.pdf)
Is it evidence?
Yes
Statements
No
Does it support
the idea that the
continents have
moved?
Yes
1. The fossils of the Glossopteris plant are found
in southern Africa, South America, Australia,
Antarctica,and India.
111
No
2. After examining the location of tiny rocks
and the direction of grooves formed by large
glaciers scraping across southern areas of
Africa, South America, Australia, Antarctica,
and India,Wegener concludes that if all these
places were fitted together, they would form a
continuous ice sheet expanding outward in all
directions.
3. German scientist Alfred Wegener spoke at the
Geological Association meeting in 1912.
4. “Continents must always have been where they
are because they are so large.”
5. When Wegener placed South America and Africa together he observes that a South American mountain range in Argentina lines up with
an ancient African mountain range in South
Africa. He then wrote: “It is just as if we were to
refit the torn pieces of a newspaper by matching
their edges and then check whether the lines of
print ran smoothly across. If they do, there is
nothing left but to conclude that the pieces were
in fact joined in this way.”
6. On the western coast of Africa rock layers were
observed in the following sequence: basalt rock,
shale containing fossil reptiles, coal layers
containing Glossopteris fossils, rocks containing
Mesosaurus fossils, and shale. An almost identical sequence of rock layers was discovered on
the eastern coast of South America.
7. It seems impossible that the continents could
move. Wegener’s theory must be a fairy tale.
8. When geologists used computers to match
coasts of South America and Africa, they match
extremely well at an ocean depth of 1,000 meters.
9. When satellites and lasers were used to measure the movement of continents in the 1980s,
it was established that they continue to move at
an average of about 2 cm per year.
10. Fossils of Megascolecina earthworms are found
in South America, Africa, India, and Australia,
as well as the islands of Madagascar and New
Guinea.
The following activity will probably help you to understand some of the
strengths and weaknesses of Wegner’s theory of continental drift.
112
Activity
4
Examining strengths and weaknesses of Alfred Wegner’s
theory of continental drift
1. In pairs, draw a full page chart that looks like the one below to argue for
or against the following statement:
‘The continents were once joined and they have been drifting apart.’
I agree! I disagree!
__________________________________
_____________________________
__________________________________
_____________________________
__________________________________
__________________________________
_____________________________
_____________________________
2. Think of reasons why you agree with the statement and reasons why you
disagree. You should list your reasons under the appropriate column.
3. Now join another pair to make a group of four.
4. Share your list of reasons with each other.
5. Debate the issue in your groups. Each person should argue for one
position or another.
6. Report your points to the class for discussion.
Weaknesses of the continental drift theory
Wegener’s theory of continental drift was not widely accepted by scientists
because of the following:
a. The theory could not suggest the means by which the continents might
drift; that is, it was unable to explain the cause of the drifting of the
continents.
b. The theory only focussed on the continental crust, and not on the oceanic
crust, i.e. it could not suggest what happens to the oceanic crust as the
continents move.
113
Activity
5
Reflecting on important issues in the topic
1. In groups of four, locate an important issue that you feel the topic has
covered.
2. Formulate a problem or question about it for another group to answer.
3. Write the problem down on a sheet of paper, and hand that piece of
paper to another group.
4. Once your group is handed a problem statement, think of a solution to
the problem. Each group has a fixed amount of time.
5. Present your problem and its solutions to the class for discussion.
Summary
Continental drift theory states that the relative positions of the continents on
the earth’s surface have changed considerably through geologic time. The theory
of continental drift was put forth by German meteorologist and geophysicist
Alfred Wegener in 1915. He believed that all the continents were united into a
vast supercontinent, which he called Pangaea. Later, Pangaea broke into two
super-continental masses—Laurasia to the north, and Gondwanaland to the
south. The present continents began to split apart, drifting to their present
positions. As evidence Wegener cited the unusual presence of coal deposits in
the South Polar regions, glacial features in present-day equatorial regions,
similarity of rocks in west Africa and eastern South America, similarity of
fossils in South America, South Africa, Australia and Antarctica, and the
jigsaw fit of the opposing Atlantic continental shelves. However, Wegener’s
theory stirred considerable controversy during the 1920s. The theory was not
generally accepted, particularly by American geologists, because it did not
explain the causes of the drifting continents and the means by which they
drift.
Glossary
Laurasia: An ancient landmass, consisting of the northern part of the ancient
supercontinent of Pangaea, thought to include what would become North
America, Greenland, northern and central Europe, and most of Asia.
Gondwanaland: An ancient landmass, consisting of the southern part of the
supercontinent of Pangaea, comprising South America, Africa, peninsular
South Asia, Australia, and Antarctica.
Fossil: The remains of an animal or plant preserved from an earlier era inside
a rock or other geologic deposit.
114
Review questions
1. What is Pangaea?
2. What does the theory of continental drift state?
3. Who came up with the idea of continental drift?
4. What evidence found in tropical regions of Africa and South America
most strongly supports the theory that the continents were once joined?
5. Why was the theory of continental drift originally rejected by the
scientific community? Give two reasons.
References
Bradberry, J. (1985). Introducing Earth Science: A Practical Approach to
Geology. Oxford: Basil Blackwell Limited.
Bunnett R. B. (1984). Physical Geography in Diagrams for Africa. London:
Longman Group Limited.
Gardner, J. et al. (2011). CK-12 Earth Science Honors for Middle School
Teacher’s Edition; http://www.ck12.org
Waugh, D. (1990). Geography: An Integrated Approach. Hong Kong: Thomas
Nelson Limited.
http://bc.outcrop.org/images/tectonics/lutge8e/FG16_03.JPG 26/01/14
www.imaxmelbourne.com.au/.../Gondwanaland.pdf29/05/14
https://www.teachingchannel.org/.../p/.../continental-drift-lesson-plan.pdf
29/05/14
115
116
Unit
8
The theory of
plate tectonics
The theory of plate tectonics
Plate tectonics is possibly the most important
geological theory ever developed. It incorporated
the earlier theory of continental drift that was
proposed by Alfred Wegener in the early 20th
In geology there are century.
processes and features
that people have observed Activity 1
for centuries but never Sharing basic knowledge and
really understood until
anticipations about plate tectonics
the advent of plate
1. What do you already know (or think you
tectonics. With plate
know) about plate tectonics?
tectonics it is now
possible to explain the
2. What specifically do you want to learn
location of the world’s
about plate tectonics?
basic physical features
3. Draw a table like the one below on a chart.
like continents, mountain
chains,
and
ocean
Know
Want to know
Learned
trenches. The theory can
also be used to explain
the location of natural
hazards like earthquakes
4. Write down what you already know in the
and volcanoes, so it is an
first column and what you want to know
important topic to learn.
in the middle column. You will fill in what
In this unit, you will
you will have learned in the third column
explain the theory of plate
at the end of the unit.
tectonics and examine
5. Display the chart in front of the class for
the causes of plate
reference.
tectonics. You will also
explain different types of
plate boundaries, identify
Plate tectonics is the movement, destruction and
features and how they
renewal of the earth’s crustal plates. Crustal
are formed along plate
or tectonic plates are large blocks of rock on
boundaries. Finally, you
which the continents and oceans sit. The theory
will explain the effects of
of Plate Tectonics states that the earth’s crust is
geological features on the
broken into several of these rigid plates that move
environment and human
independently of one another.
activities.
117
What causes plate tectonics?
The answer lies in the convection currents in the mantle. Tectonic plates
rest upon a layer of heated, pliable rock called asthenosphere. Due to extreme
heat deep down the Earth’s interior the mantle flows slowly and circulates,
much as water boiling in a pot. The heated asthenosphere’s molten material,
or magma, rises, while cooler, hardened matter sinks deeper into the mantle.
Sinking rock eventually reaches the extremely hot temperatures of the lower
mantle, heats up, and begins to rise again.
This continual, roughly circular motion is called convection.The flow of the
asthenosphere due to convection currents provides horizontal forces on the
plates of the lithosphere causing them to move in different directions. Where
the mantle is rising, the plates are torn apart and molten material wells up to
the surface, forming a new crust. Where the mantle is sinking, the plates are
pulled together and wrinkled or subducted (drawn under other plates, down
into the mantle) and partially melted – see Figure 100.
Trench
Oceanic ridge
Plate
Plate
Asthenosphere
Asthenosphere
Hot rock rises
Cool rock sinks
Figure 100: Convection in the Mantle
Activity
2
Experiment – Investigating the force of convection currents
You will need water in a glass dish, potassium permanganate (you can use dye
if the potassium permanganate is not available), Bunsen burner, matches, 2
mugs and 2 small pieces of thin wood (about 2 mm thick), each 4x10 cm to
conduct an experiment.
1. Arrange the dish and other materials as shown in Figure 97. Let the
water stand without heat until the water is not moving.
2. Light the candle at the bottom of the dish and let the liquid heat up for
a couple of minutes.
118
3. As the water heats and begins to flow, drop a single large crystal of
potassium permanganate to the bottom of the water, just above the
heat source.
4. Observe the pattern of fluid flow by noting the location of the dye or
potassium permanganate over time. Be sure to view the model several
times during the experiment, both from above the dish and from the
side of the dish.
5. Draw a sketch of the circulation using arrows.
a. Is the pattern approximately the same on the two sides of the heated
area?
b. Where do you observe upward flow?
c. Where do you see downward flow?
d. Where do you observe horizontal flow?
Glass
dish
Water
Candle
Figure 101: Thermal convection experiment
6. Now, place the thin pieces of wood on the surface of the liquid (directly
above the heated area) as shown in Figure 102.
7. What happens to the pieces of wood?
8. How do you relate
this experiment to
plate tectonics? What
do the boiling liquid,
pieces of wood and
the candle represent?
9. Present you findings
to the class for
discussion.
Boiling water
Glass
dish
Wood
Wood
Figure 102: Arrangement of 2 pieces of wood on the
surface of boiling water (view from above the dish)
119
From continental drift to plate tectonics
Alfred Wegener’s failure to address the two most fundamental problems
against his theory of continental drift gave rise to the widely accepted theory of
Plate Tectonics. The discovery of seafloor spreading in the early 1960s was
particularly important. It helped the majority of scientists in the geophysics
community accept Wegener’s theory of Continental Drift and improved upon
it.
Seafloor spreading is the process in which the ocean floor is widened when
new crust is formed along the ocean ridges.
Evidence supporting the concept of seafloor spreading
a. Variations in thickness of sediments on the ocean floor: it was
also discovered that the thickness of sediments deposited on the ocean
floor increases away from the mid-ocean ridge. The possible explanation
to this is that the older sea floor has had more time to accumulate
sediments.
b. Variations in age of sediments on the ocean floor: it was also
discovered that the age of sediments at the bottom of the pile increases
as distance from the mid-ocean ridges increases.
c. Variations in age of rocks on the ocean floor: it was found that
the volcanic rocks on ocean floor gets progressively older away from the
ridge, clearly suggesting that the ocean floor is indeed spreading.
d. Magnetic variations on the ocean floor (Palaeomagnetism):
during cooling, minerals in the basaltic rocks align themselves along
the earth’s magnetic field forming a permanent record of magnetic field
in the rocks. It was discovered that the magnetic fields in the rocks of
the ocean floor switch positions as distance from the mid-ocean ridge
increases.
Activity
3
Demonstrating the seafloor spreading
Subduction zone
Mid-Ocean ridge
Subduction zone
1. Align four desks with their edges just touching so that you create three
gaps. The center gap is your midocean ridge. The gap to the right of the
center gap represents a subduction
zone. The gap to the left of the center
gap represents another subduction
zone. When you are finished, your
setup should look like the diagram in Figure 99: Gaps between desks
Figure 99.
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2. Use a marker to draw stripes in alternating colours on a long piece of
paper.
3. Cut the paper in half parallel to the long edge to get two strips of paper
as shown in Figure 103 below.
Figure 103: Strips of paper
4. Place two pieces of paper vertically into the center gap between the
desks. Leave just enough of the papers sticking out so that there is
something to hold onto.
5. Slowly pull the papers out from the gap, spreading the papers apart
towards and into the subduction zones. Make sure that both papers are
pulled at the same rate (see Figure 104).
Subduction zone
Subduction zone
Figure 104: Seafloor spreading model
6. What do the alternating stripes of colours in your model represent on
the real ocean floor?
7. The earth is about 4.6 billion years old. Based on observations of your
sea-floor spreading model, why do you think that the oldest ocean floor
is only about 200 million years old?
8. Present your work to the class for discussion.
The theory of plate tectonics acknowledges that the continents move about the
surface of the earth, but they do not slide through the oceanic crust. Rather,
they ride as passive passengers on great plates of lithosphere that originate
at ocean ridges and are destroyed at the deep ocean trenches.
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Types of tectonic plates
A plate may be made up entirely of oceanic or continental lithosphere, but
most are partly oceanic and partly continental.
Continental plates
These contain continental material, and have the following characteristics:
• They are composed mostly of light coloured granite rocks, which principally
contain silica and alumina. For this reason, the upper layer of the
continental plates is often termed the sial (from silica and alumina).
• They have a lower average density (less weight) than do oceanic plates.
• They have the oldest land areas on earth because their low density allows
the continents to float permanently on the upper mantle, persisting more
or less intact for billions of years.
• They are thicker than the oceanic plates (35 to 70 km thick).
Examples of continental plates include the African plate, Eurasian plate,
Australian plate, Iran plate, Arabian plate, North and South American plates.
Oceanic plates
These are made up entirely of oceanic material, and have the following
characteristics:
• They consist of much younger, dark-coloured volcanic lava rocks (basalt),
forming directly from the molten mantle material. Two of the principle
components of the basalt layer are silica and magnesium; hence, the
oceanic plates are given the name sima.
• They are denser than the continental plates. As an oceanic plate ages, it
accumulates a heavy under-layer of cooled mantle rock; the resulting twolayer structure eventually sinks of its own weight into the mantle, where
it is melted down and recycled.
• They are much newer than the continental plates because of the recycling
process; no oceanic plate older than about 200 million years exists on the
surface of the earth.
• They are thinner than the continental plates (5 to 10 km thick).
Examples of oceanic plates are Pacific, Nazca and Cocos plates (Figure 105).
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Pacific
Plate
North
American
Plate
Caribbean
plate
Cocos
Plate
Nazca
Plate
Eurasian Plate
Philippine
Pacific
African
Plate
South
American
Plate
Pacific
Plate
Indo-Austraria
Plate
Antarctic Plate
KEY
Plate boundary
Direction of plate movement
Figure 105: Major tectonic plates of the earth
Types of plate boundaries
Plate boundaries are points between two or more plates. There are three
primary types of tectonic plate boundaries characterized by their distinct
movements: convergent, divergent and shearing.
Divergent or constructive boundaries
Divergent plate boundaries are locations where plates are moving away from
one another. This occurs above rising convection currents. The rising current
pushes up on the bottom of the lithosphere, lifting it and spreading laterally
beneath it. This lateral flow causes the plate material above to be dragged
along in the direction of flow. At the crest of the uplift, the overlying plate is
stretched thin, breaks and pulls apart. Divergent boundaries may occur in
two ways:
a. Oceanic from oceanic crust: As two oceanic plates diverge, magma
from the mantle erupts through the crack to form new crust in form of
a ridge (see Figure 106). When ocean plates diverge, the ocean floor
expands (seafloor spreading) as more crust is formed at the ridge. The
Mid-Atlantic ridge is a result of the moving apart of American Plates
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and Africa-Europe, causing the Atlantic Ocean to expand (Figure
104). As the Atlantic Ocean is enlarging slowly, the Pacific Ocean is
shrinking.
Mid-ocean ridges are vast mountain chains in the ocean and are as tall
if not taller than mountain chains on land. The peaks of some of these
mountains rise above the surface of the ocean to form islands. Iceland
and Azores are good examples.
Figure 106: Oceanic plate divergence
b. Continental from continental crust: At diverging continental from
continental crust boundaries, rift valleys are created. This is because
there is no magma to fill the crack, so it is a down-warp in earth’s
crust. An example of a rift valley is the Great Rift Valley in East Africa,
where Africa will eventually split into two pieces.
Figure 107: Continental plate divergence
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Geological features that form along divergent plate
boundaries
• Volcanic mountains: As the plates pull apart, the lithosphere thins and
molten magma from the earth’s mantle erupts onto the surface, forming
new lithosphere. Over time, the lava from these volcanoes can build up
and form volcanic mountains. You learn about such mountains in Unit 9.
• Oceanic ridges: When a divergent boundary occurs under the ocean,
the rising convection current below lifts and stretches the lithosphere,
producing deep cracks. Magma squeezes and erupts through the widening
cracks, producing a long chain of large underwater mountains - mid-ocean
ridge.
• Volcanic islands: Sometimes, the underwater volcanic mountains formed
may grow so tall and emerge on the surface of the water, producing volcanic
islands e.g. Iceland.
• Rift valleys: When the continental plate stretches beyond its limits, tension
cracks or fractures begin to appear on the Earth’s surface. Eventually,
huge blocks of the earth’s surface may sink relative to the neighboring
blocks, producing rift valleys, e.g. the Great East African Rift Valley.
• Block mountains: When part of the fractured land block is lowered so that
the remaining blocks stand high above the surroundings, block mountains
are formed.
• Earthquakes: As the plates pull apart, faulting may occur, and as the land
blocks slide against each other’s rough edges, sudden vibrations known as
earthquakes are produced.
Convergent or destructive boundaries
Convergent boundaries occur when plates move toward each other and collide.
The converging lateral current beneath the lithosphere causes the plate
materials above to be dragged along in the direction of flow. This drives plates
into collision, hence convergent boundary.
a. Continental to continental crust: These plate boundaries form when
two pieces of continental crust converge. At these boundaries, neither
piece of continental crust is dense enough to subduct into the mantle,
so they collide and wrinkle, forming fold mountain ranges. Figure 108
below shows the Himalayas as an example of mountains created by a
continental to continental crust convergent boundary (Indo-Australian
and Eurasian Plates).
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Himalaya
Tibetan Plateau
Figure 108: Continental and continental crust collide
(Source: http://bc.outcrop.org/images/tectonics/press4e/figure-02-09c.jpg 26/01/14)
b. Oceanic to continental crust: Continental to oceanic crust plate
boundaries occur when the oceanic and continental plates converge.
When this happens, the oceanic plate always subducts (goes
underneath) under the continental plate, forming Ocean Trenches
(see Figure 109 below). Rocks in the ocean crust are constantly being
consumed at the subduction zones and created at the mid-ocean ridges;
hence, they never have a chance to get very old. The oldest rocks in the
ocean are no older than 200 million years old. The continental plate
may buckle, forming mountains e.g., the Andes of South America. The
oldest rocks in the world are on the continents because continents are
too light to get subducted and have been floating around on the surface
of the earth ever since the first one was formed.
Andes mountains
Figure 109: Continental and oceanic crust collide
(Source: http://bc.outcrop.org/images/tectonics/press4e/figure-02-09b.jpg 26/01/14)
c. Oceanic to oceanic crust: Oceanic to oceanic crust boundaries form
when two pieces of oceanic crust converge. When two oceanic plates
converge, one is denser and will subduct under the other, forming very
deep trenches (Figure 110).
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Japanese Islands
(island arc)
Figure 110: Oceanic and oceanic crust collide
(Source: http://bc.outcrop.org/images/tectonics/press4e/figure-02-09a.jpg 26/01/14)
Geological features that form along convergent plate
boundaries
• Trenches: When continental and oceanic plates collide, the less dense
continental plate usually rides up over the oceanic plate, which goes down
into the mantle. This subduction of the oceanic plate creates a long steepsided depression in the ocean floor (known as trench).
• Volcanic mountains: During plate collision, the crustal plate which goes
down into the mantle is destroyed and melted, resulting in volcanic
activities, which may produce volcanic mountains.
• Fold mountains: When two sections of continental lithosphere collide,
the lithosphere tends to crumple and be pushed upward, forming fold
mountains.
• Earthquakes: The collision of plates may lead to sudden vibrations in the
earth’s crust, causing earthquakes.
Shearing or conservative or transform boundaries
These occur when two plates slide against each other (Figure 111). They are
called conservative because plate material is neither created nor destroyed
at the boundaries. Earthquakes occur here very frequently because of the
immense amount of friction. Volcanoes are rare because crust is neither being
formed nor subducted into the mantle. Faults are created here; specifically
strike-slip faults, where there is little or no vertical motion of the plates
involved. The San Andreas Fault in California is a classic example.
127
Continental
lithosphere
Figure 111: Shearing plate movement (Source: http://bc.outcrop.org/images/tectonics/lutge8e/FG01_11C.
JPG 26/01/14)
Geological features that form along transform plate
boundaries
• Earthquakes occur here very frequently because of the immense amount
of friction.
• Faults are created here; specifically strike-slip faults, where there is little
or no vertical motion of the plates involved. The San Andreas Fault in
California is a classic example.
Volcanoes are rare here because crust is neither being formed nor subducted
into the mantle.
Activity
4
Demonstrating plate motions
1. In groups of four mold some portions of clay.
2. Put two portions of clay together horizontally, long end to long end.
3. Push them toward each other.
a. What happens? Record observations relative to the changes in the
clay and include a sketch of the observations.
b. Why do you think this happened?
c. Name the type of boundary you have just created.
128
4. Now, put the portions of clay together, long end to long end, and slide
them along each other in the opposite directions.
a. What happens?
b. Based on what happened to your clay, what do you think would
happen if two crustal plates moved past each other?
5. Pull the two pieces of clay in the opposite directions to simulate a
divergent boundary.
6. Identify and label these plate boundaries on a topographic map of the
world.
7. Report your findings to the class for discussion.
Effects of geological features on the environment
Positive:
• Volcanic rock features create very fertile soils when they weather,
which give rise to a wide range of vegetation. The vegetation in turn
supports some natural ecosystems
• Mountains formed by plate tectonics (e.g. fold mountains) easily
influence rain formation due to their great heights and this makes them
ideal places for growth of natural vegetation. The vegetation supports
multiple species of animals, hence creating a beautiful ecosystem.
• Mountains influence rain or snow formation. The heavy rain or snow
received in mountain areas also gives rise to important rivers, which
support a diverse aquatic ecosystem.
Negative:
• Earthquakes can tear apart ground surface, destroy forests and
permanently displace some rocks; hence, damaging ecosystems in the
environment.
• The lava in volcanic eruption melts out and destroys the original
landforms biodiversity that existed on the surface.
• There is severe soil erosion and rock fall on the steep slopes of rift
valleys which result in the destruction of soil surface.
• When these tectonic plates shift, the locations of continents and oceans
in relation to latitudes also changes. This is known to have caused some
of the major climatic changes in history that may have contributed to the
extinction of some species. The volcanic eruptions increase the degree
of carbon dioxide (CO2) and sulfur dioxide (SO2) in the atmosphere.
This is what leads to a rise in the temperature levels globally.
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Impact of of geological features on human activity
Positive:
• Geological features such as volcanoes have provided people with
opportunities for
i. extracting precious minerals:
ii. attractions for tourists:
iii. harnessing geothermal power:
iv. agriculture:
v. settlements:
• The occurrence of earthquakes and volcanoes has helped people to learn a
lot about the interior structure of the earth.
• Mountain ranges are aesthetically appealing formations and have some
effects on us. The Himalayas, the Swiss Alps, and the Andes are some
spectacular examples. They are tourism sites, sources of rivers for
harnessing hydro-electric power and irrigation.
Negative:
• Volcanic activities related to plate tectonics are known to have brought
about massive loss of lives, property, livestock, crops and disruption to
communications and transport.
• Earthquakes related to plate tectonics have caused terrible catastrophes
throughout history. As a result of violent earthquakes, infrastructure and
property have been damaged and millions have died around the world. For
example, the 7.7 magnitude earthquake that struck the Chinese province
of Hebei in 1976 killed as many as 800,000 people.
• Escarpments and mountains related to plate tectonics have confronted
people with a barrier to communication. Fault scarps form waterfalls
and rapids which hinder navigation of rivers. Mountainous regions are
particularly difficult to build in due to the steep sided valleys
Activity
5
Reflecting on the topic
1. What new things have you learned in this unit?
2. Why is it important that you have learned these things?
3. Return to the chart you prepared at the beginning.
130
4. Do you think what you thought you knew was accurate?
5. What questions do you have about what you have learned?
6. Report your findings and questions to the class for discussion.
Summary
Plate tectonics theory was developed to explain the phenomenon of continental
drift and is currently the theory accepted by the vast majority of scientists
working in this area. Based on the evidence of seafloor spreading, formation of
mountain belts and seismic activities, the theory of plate tectonics divides the
outermost part of the earth’s interior into two layers: the outer lithosphere and
the inner asthenosphere. The lithosphere is broken-up into ten major plates:
African, Antarctic, Australian, Eurasian, North American, South American,
Pacific, Cocos, Nazca, and the Indian plates, which essentially “float” on the
asthenosphere. The areas on the margins of tectonic plates are called plate
boundaries and it is where seismic, volcanic, and tectonic activity takes place
as a consequence of the relative motion of the plates. There are three types of
plate boundaries, namely, convergent, divergent and shearing. The tectonic
activities that occur in plate margins have influenced lives of people in a
variety of forms such as tourism, mining, geothermal energy, hydropower,
among others.
Glossary
Plate tectonics: a theory that gives moving plates of the Earth’s crust
supported on less rigid mantle rocks as the cause of volcanic and seismic
activity, and the formation of mountain belts.
Tectonic plate: a segment of the earth’s crust that moves relative to other
segments, carrying continents and oceans.
Seafloor spreading: the process in which the ocean-floor or seafloor is
widened when molten material from the earth’s mantle rises up at ocean
ridges to create new crust.
Plate boundary: an area on the margins of tectonic plates where seismic,
volcanic, and tectonic activity takes place as a consequence of the relative
motion of the plates.
Tectonic activity: movement and deformation of the Earth’s crust.
Subduct: to be carried under the edge of an adjoining continental or oceanic
plate.
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Review questions
1. What does the theory of plate tectonics state?
2. Describe the discoveries that led to the theory of plate tectonics.
3. Name two plates composed entirely of oceanic material.
4. What are the different kinds of motion that occur at the edges of tectonic
plates?
5. What kind of plate movement adds new crustal surface area to the
earth?
6. Explain why the margins of subduction of crustal plates are destructive.
7. Based on the geologic past, we can assume that earth is always
changing. What modern-day evidence supports this idea? Hint: think
about natural disasters. Where do they often occur?
8. Match the letters A, B, C,D and E in the following figure to the labels
below:
North
America
Plate
Cocos
Plate
Pacific
Plate
Anatolian Eurasian Plate
Plate
Iran
Plate
Caribbean
Plate
Philippine
Plate
Pacific
Plate
African
Plate
Nazca
Plate
Indo-Australian
Plate
South
American
Plate
Antarctic Plate
Scotre
Plate
Figure 112: Tectonic plate movement
ocean trench, transform plate boundary, Mid­Atlantic Ridge, subduction zone,
shearing plate boundary
132
References
Bradberry, J. (1985). Introducing Earth Science: A Practical Approach to
Geology. Oxford: Basil Blackwell Limited.
Bunnett R. B. (1984). Physical Geography in Diagrams for Africa. London:
Longman Group Limited.
Gardner, J. et al. (2011). CK-12 Earth Science Honors for Middle School
Teacher’s Editionhttp://www.ck12.org
Waugh, D. (1990). Geography: An Integrated Approach. Hong Kong: Thomas
Nelson Limited.
White, R. (1998). Africa in Focus: A Physical, Human and Economic Geography.
Oxford: MacMillan education Limited.
http://science.taskermilward.org.uk/mod1/KS4Physics/AQA/P1%20
Part%201/P1_4.htm
http://bc.outcrop.org/images/tectonics/press4e/figure-02-06a.jpg 26/01/14
http://bc.outcrop.org/images/tectonics/press4e/figure-02-09b.jpg 26/01/14
http://bc.outcrop.org/images/tectonics/press4e/figure-02-09c.jpg 26/01/14
http://bc.outcrop.org/images/tectonics/lutge8e/FG01_11C.JPG 26/01/14
http://csep10.phys.utk.edu/astr161/lect/earth/consequences.html 10/12/13
http://library.thinkquest.org/17701/high/effects/ 11/12/13
www.mml.co.za/...exam.../Platinum-Geography-Grade-10-ExamPractic...‎Grade 10 Grade 10 - Maskew Miller Longman 12/02/14
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134
Unit
9
Mountain ranges are one
of the most spectacular
features of the landscape.
Behind each mountain
we see today, there are
geologic events that took
place millions of years
ago. These events are still
happening today. The
mountains, the rocks and
minerals, of which they
are made, affect climate,
soils, living things and
subsequently the lives
of people. Studying this
unit will increase your
understanding on how
mountain ranges form
and how they impact
human activities. In this
unit, you will explain
the main processes of
mountain building in
relation to plate tectonics,
and the different features
formed. Finally you will
analyse the effects of
mountain building on life
and human activity.
Mountain building
Mountain building
Mountain building (orogeny) refers to events
that lead to the uplift of earth’s crust to produce
mountains. Mountain building occurs mainly
because of the movements of earth’s tectonic
plates. Most mountains exist as a group called
mountain range. The longest mountain range
on earth is entirely underwater (the Mid-Atlantic
Ridge), which extends about 16 000 km from
Iceland to near the Antarctic Circle.
Activity
1
Viewing and discussing mountains in
your local area
In small groups, look around the surrounding
mountains in your area and discuss the following
questions:
1. What are mountains?
2. What do they consist of?
3. Are they uniform or mixed in composition?
4. Where do they come from?
5. Have mountains just always been there?
6. Do mountains stay the same or do they
change?
7. Report your answers to the class for
discussion.
Processes of mountain building in
relation to plate tectonics
• Folding
Folding is the bending of the rock masses, which
135
were originally flat horizontal surfaces. The forces that cause folding range
from slight differences in pressure in the earth’s crust, to large collisions of
the crust’s tectonic plates. As a result, a fold may be only a few centimeters in
width, or it may cover several kilometers.
Features formed from folding
Folding can produce the following landscape features:
a. fold mountains
b. anticlines or domes
a. synclines or basins
b. volcanoes
Fold mountains
Fold mountains are the most common type of mountains on earth. They
are formed when two plates collide head on, and their edges are bent into a
series of folds. Fold Mountains usually occur along the edges of continents
where an area of sea separates two plates. It is here that sediments eroded
from the neighbouring continent settle on the sea floor in depressions called
geosynclines (e.g. trenches). These sediments gradually become compressed
into layers of sedimentary rocks. When these layers of sedimentary rock are
subjected to forces of
compression by the
converging plates,
they crumple and
fold. Eventually the
folded sedimentary
rock layers form
a range of Fold
Mountains (see
Figure113).
Where the layers
of rock are folded
upwards, they
form an anticline.
Those that are
folded downwards
form a syncline
(Figure114).
Figure 113: Formation of fold mountains
136
Anticline
Syncline
Figure 114: Anticlines and synclines
Activity 2
Demonstrating folding
You can simulate this geologic process using towels.
1. In groups of four, lay four towels of different colours on top of each other
on a table to make a layered portion.
2. Draw a side view of this model of unfolded layers.
3. Now, gently push the long ends together to create folds as shown below.
4. Why do you think this
happened?
5. How does this model
illustrate processes at
work in the Earth’s
crust?
6. What is the name of
the force at work in the
earth’s crust that makes
folded mountains?
Figure 115: Folded towels
7. What do you think would happen if you had rocks that were less squishy
and more brittle than the towels?
8. Report your findings to the class for discussion.
Types of folds
The intensity of compression forces determines the shape and size of the folds.
Simple (symmetrical) fold
When the compression is too intensive the crustal rock layers get folded into
137
gentle, simple folds called symmetrical folds. Here both sides are equally bent
and are of equal steepness (Figure 116).
Figure 116: Simple fold
Over fold (asymmetrical fold)
When the compression force is more on one end, a fold is overturned and one
side of the anticline becomes steeper than the other (Figure 117).
Figure 117: An over fold
Recumbent fold
When one side is pushed so much that it lies positioned over the other side,
both sides of the fold become largely horizontal (Figure 118).
Figure 118: Recumbent fold
138
Overthrust fold
Due to excessive folding, a fracture is formed, and one side of the fold slides
forward over this fault. The portion which slides forward is called Nappe
(Figure 119).
Nappe
F
re
tu
c
ra
Figure 119: Overthrust fold
Types of Fold Mountains
There are two types of fold mountains:
Young fold mountains
These have been formed recently in the earth’s history (about 10 to 50 million
years of age) e.g. the Rocky in North America, the Alps in Europe, the Andes
in South America, the Atlas in North Africa and the Himalayas in Asia.
However, some, especially the Himalayas, continue to increase in elevation
even today. The Himalayas are the largest fold mountains on earth and have
the twenty-eight tallest peaks in the world.
Characteristics of young fold mountains
a. They have greater length and height than any other mountain ranges.
b. They are associated with volcanic activity and have many active
volcanoes.
c. Most fold mountains, except the Himalayas, are found on the margins
of continents.
d. They have pointed peaks and rugged features like steep slopes and
deep valleys.
e. They are mostly made up of sedimentary rocks formed due to deposition
and consolidation of sediments in depressions.
139
f. They are rich in minerals such as petroleum, gold, tin, copper and
aluminium.
Old fold mountains
These were formed in very ancient times and are over 200 million years of age,
e.g. Aravallis of India, Appalachians of USA and the Urals between Europe
and Asia. Figure 120 below shows the location of fold mountain ranges.
Urals
Rocky
Appalachians
Alps
Himalayas
Atlas
Aravallis
Andes
Key
0
Fold Mountains
4000km
Figure 120: Fold mountain ranges
Characteristics of old fold mountains
a. They have relatively low heights because they have been subjected to
weathering and erosion for so long.
b. They have rounded peaks and gentle slopes.
c. They are also rich in minerals like aluminium, gold and copper.
Importance of folding
a. Agriculture: folding creates mountain areas that have fertile soil with
lots of natural minerals ideal for crop growing; so farming is the most
primary activity in all fold mountains. However farming is restricted
because of the height and steepness of the slopes. The higher mountain
slopes are mostly used for grazing livestock.
140
b. Forestry: Fold mountains easily influence rain formation due to their
great heights and this makes them ideal places for growing trees. The
trees provide wood for fuel, building materials, furniture and paper.
c. Tourism: Fold mountains have spectacular scenery, which attracts
tourists. People visit to do sports like skiing, climbing and snowboarding.
d. Hydroelectric power schemes: The heavy rain or snow received in
fold mountain areas, gives rise to important rivers such as the Indus
and Ganges in the Himalayas. The steep gradient makes the rivers
flow swiftly, making the areas ideal for generating hydro-electricity.
e. Mining: Anticlines are good sites for oil accumulation forming oil
reservoirs.
f. Water supply: Folding results into the formation of synclines, which
are suitable for water accumulation forming aquifers or groundwater
basins and aluminium smelting due to cheap and plentiful supply of
electricity.
The Alps is the most densely populated mountain area in the world, with
many resources that drive the economy of the region.
Problems of living in fold mountain areas
a. Steep terrain: Mountainous regions are particularly difficult to build
in due to the steep sided valleys. Roads and other communications
links have to snake their way up wherever they can, and often these
roads are not big enough to adequately service a large community. For
farmers, it is difficult to use machinery on the steep slopes
b. Cold climate: The climate is very cold and wet, meaning that most
industrial and agricultural activity is difficult. For farmers they have a
very short growing season.
c. Avalanches and rock falls: Avalanches and rock falls are a constant
threat, blocking roads and destroying settlements. Some settlements
and roads need avalanche protection measures, e.g. shelters, wooden
fencing and wire cages, so huge amounts of money are spent each year
to combat the avalanche threat, especially with the large number of
tourists using the mountains.
• Faulting
Faulting is the formation of cracks or lines of weakness in the earth’s crust,
resulting from forces of tension and compression acting on hard, brittle rocks.
141
Features formed from faulting
The lines of weakness formed during faulting initiate the formation of several
features including;
a. block mountains (horsts)
b. tilt blocks
c. escarpments
d. fault scarps
e. rift valleys
f. rift valley lakes
g. earthquakes
h. volcanoes
Block mountains
Block Mountains are formed by the sinking or rising of huge blocks of the
earth’s surface relative to the neighboring blocks. The forces that rage inside
the earth have fractured the earth’s crust. The line of crack or fracture in
earth’s crust along which rock on one side has moved relative to rock on the
other is called a fault. (When no movement has occurred, the fracture is
known as a joint).
Parts of a fault
a. Fault line: The intersection of a fault plane with the earth’s surface,
along which a crack takes place (see Figure 121).
b. Fault plane: The area where crustal blocks meet and move along a
fault from the fault line down into the crust. The fault plane may be
vertical in relation to the earth’s surface. If so, the fault is known as a
vertical fault. If the fault
Fault scarp
plane is slanted, the fault
is known as an inclined
fault.
Fault line
c. Fault scarp: The total
distance that the two sides
of a fault have moved
relative to each other.
Fault plane
Figure 121: Parts of a fault
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Types of faults
There are three basic types of faults: normal, reverse, and tear
Normal faults
Normal faults, shown in Figure 122, occur when underground pressure
causes the crust to stretch or pull apart (tension).
tension
Figure 122: Normal fault
Reverse faults
These occur when underground pressure causes the crust to compress, pushing
blocks together (see Figure 123).
compression
Figure 123: Reverse or thrust fault
Tear faults
In a tear fault, crustal blocks slide past one another, traveling in opposite
directions (horizontal movement) – see Figure 124. Tear faults are also
known as wrench or transform or strike-slip faults.
Figure 124: Tear or transform fault
143
Activity
3
Demonstrating faulting
You will need about 5 to 6 hardback textbooks for this activity. This works best
if the books are identical and have horizontal bands (lines) on the binding.
1. Hold 5 to 6 similar textbooks upright on a desk to represent the earth’s
crustal blocks. Make sure the books create a flat surface on top.
2. Measure and record the length of the surface by placing a ruler across
the top of the books, beginning with the first book on the left and ending
with the last book on the right.
3. What do the spaces in between the books represent in the earth’s crust?
4. Now, move one hand so that the books tilt to one side at a 30-45 degree
angle, as shown below.
5. What happens to the flat surface
when the books (blocks) tilt?
6. How did the measurements
change? Why do you think this
happened?
7. Draw diagrams to illustrate your
observations.
8. What do you think would happen
if the earth’s crust is pulled
apart?
Figure 125: Tilted books
9. Do you think this could make mountains?
10. Report your findings to the class for discussion.
Formation of block mountains by tension
When there are two parallel faults and plates are
moving away from each other due to tension, part
of the land block is lowered so that the remaining
blocks stand high above the surroundings to
produce block mountains. The fallen block, called
graben, produces a rift valley (Figure 126).
Tension fource
Faults
Block mountains
Rift valley
Faults
Figure 126: Formation of block mountains by tension
144
Formation of block mountains by compression
Forces of compression exist when plates move towards each other. The force
of compression may produce block mountains in two ways:
• If the fault planes of the parallel faults are angled downward towards
each other, a crustal block between them may be squeezed upwards (see
Figure 127). This uplifted block is called a horst. A large horst that is
lifted high can form a fault-block mountain. Lifted block mountains
have a flat top and extremely steep slopes.
Faults
Compression force
Block mountain
• If there are two parallel faults,
and the fault planes are angled
downward away from each other,
compression may squeeze down the
crustal block between the faults to
produce a rift valley (see Figure
128). This is sometimes accompanied
by tilting of the surrounding blocks.
Block mountains formed in this way
have one steep side contrasted by a
gentle slope on the other side.
Figure 127: Formation of block
mountain by compression
Faults
The tallest fault block mountain in the
world is the Sierra Nevada of California
in USA. It is about 4,421 meters tall,
with a length of over 600 km and nearly
120 km wide. Other examples of block
mountains include the Black Forest
of Europe, the Rwenzori Mountain
ranges of Zaire, the Mathew ranges and
NyiruNdoto in Northern Kenya.
Compression force
Block mountains
Compression force
Figure 128: When the middle block is
squeezed down, the adjacent blocks stand out
as host mountains
Characteristics of block mountains
a. They have a steep and small slope on the fault scarp but the slope on
the other side is long and gentle.
145
b. They have nearly flat or gently sloping tops.
c. They are usually long.
d. They are associated with rift valleys.
Tilt blocks
Tilt blocks are formed when one side of the middle block is uplifted higher
than the other side. The top of the middle block will not be flat but will be
tilted. e.g., west Kenya tilt block, which rises to about 1900 metres towards
Lake Victoria.
Escarpments
Escarpments are steep cliff-like slopes. Some escarpments may extend
several hundreds of kilometres. When escarpments are eroded, they become
fault scarps. Examples of such escarpments in East Africa include: Mandi
(Kenya), Butiaba (Uganda), Kikuyu (Kenya), Mau (Kenya) and Lake Manyara
(Tanzania).
Rift valleys
A rift valley is a flat-bottomed valley formed by the sinking of the ground between
two nearly parallel faults. Rift valleys have the following characteristics:
a. They have steep and nearly parallel walls following the fault lines.
b. They are long and deep.
c. Their floors are nearly
flat, with several in
land water basins
which may contain
lakes.
The most well-known rift
valley is that of East Africa,
the Great Rift Valley, which
stretches from Mozambique
in the south to Syria in the
north (Figure 129). It has a
total length of 6,440 km.
Figure 129: The Great East-African Rift Valley
146
Another example of a rift valley is the Rhine Valley between Vosges and Black
Forest Mountains in Europe.
Benefits of the rift valley to the people of East Africa
a. Tourism: the rift valley presents beautiful scenery that attracts
tourists.
b. Fishing: rift valleys are suitable for water accumulation forming rift
valley lakes. The largest fresh water lakes in the world are all found in
rift valleys, for example, Lake Tanganyika, Lake Albert, Lake Malawi
etc. These lakes are home for wide varieties of fish species, hence
supporting the fishing industry.
c. Navigation: the rift valley lakes help in navigation (water transport).
d. Lumbering: forests on the slopes are sources of timber.
e. Agriculture: gentle slopes are used for crop farming and settlement
due to fertile soils. The rift valley lakes are also used for irrigation.
Areas of little rainfall (rain shadow) provide pasture for grazing.
f. Wildlife conservation: for example game parks in rift valley areas.
g. Mining: the rift valley lakes provide mineral resources. For example,
Lake Magadi contains vast deposits of soda ash, which is one of the
most important minerals in Kenya.
h. Heating: the rift valley has many hot springs along the fault lines
because of the following:
i.
Water collecting in these valleys sinks deep enough beneath the
surface in order to be heated by the hot rocks that lie underground.
ii.
The valleys are geologically active and the fractures provide
weak points for hot rocks to force into the crust where it gets in
contact with groundwater.
iii.
The fissures or cracks allow permeability through rocks so water
can flow in the subsurface and rise to or near the land’s surface.
The hot springs are harnessed to heat houses, swimming pools and for
other domestic purposes.
Problems faced by the people living in the rift valley areas
of East Africa
a. Earthquakes (tremors): rift valleys lie in plate boundaries where
crustal movements are most active. These movements unleash
earthquakes that may destroy property.
147
b. Little rainfall or drought: most rift valley areas are in the rain
shadow as a result they are very hot and only suitable for grazing
unless irrigation is practiced.
c. Poor means of transport and communication: The steep
escarpments hinder development of transport and communication
lines.
d. High temperatures and high evaporation rates: These conditions
create salty lakes such as Natron, and Magadi in Kenya.
e. Landslides and soil erosion: There is severe soil erosion and rock
fall on the steep slopes which result in the destruction of soil surface,
crops and people’s property.
Importance of faulting
a. Agriculture: faulting has resulted into the formation of high mountains
in East Africa. For example the Rwenzori in western Uganda, the
southern highlands and the Usambara Mountains in Tanzania and
the Mathew Ranges in Kenya. These are the most productive areas,
where both cash crops and subsistence crops are grown. Besides, these
highlands receive abundant and reliable rainfall, which supports the
production of both cash and subsistence crops in the fertile gentle
slopes.
b. Fishing: Rift valley lakes for example Lake Tanganyika, Lake Turkana,
Lake Naivasha and Lake Baringo are fishing grounds.
c. Mining: Faulting has resulted into the formation of mineral-rich lakes.
For example, Lake Magadi contains vast deposits of soda ash, which is
one of the most important minerals in Kenya.
d. Tourism: Faulting presents impressive scenery which can be used for
tourism. The Block Mountains offer spectacular views, and suitable
terrain for skiing and mountaineering. The rift valley lakes have
millions of colourful flamingos and other birds.
e. Wildlife conservation: Faulting has resulted into the formation
of highlands, which have been made into National parks and game
reserves e.g. the slopes of the Nyandarua and Rwenzori Mountains.
These parks also attract many tourists.
f. Hydropower generation: The fault scraps of the mountains created
during faulting, and the heavy rains they receive create waterfalls,
such as the Karuma falls and Murchison Falls, which may be developed
into hydropower schemes.
148
Problems caused by faulting
a. Communication barrier: Escarpments and mountains hinder
transport development. Fault scarps form waterfalls and rapids which
hinder navigation of rivers. It is also difficult to settle on the steep
areas on the rift valley escarpments.
b. Drought: Rift valleys are very hot and only suitable for grazing because
they are in the rain shadow unless irrigation is practiced as with the
case of mubuku.
c. Soil erosion and rock fall: There is severe soil erosion and movement
of loose rocks down the steep slopes which result in the destruction of
soil surface, crops and at times people’s property.
d. Earthquakes: Faulting produces earthquakes, which are often
destructive, especially when they occur in areas of dense human
settlement.
• Volcanicity
Volcanicity is the eruption of magma and volatile material to the surface from
the interior of the earth.
Features formed from volcanicity
When volcanoes erupt, several features are formed, including the following:
a. volcanic mountains
b. lava plateaus
c. crater lakes
d. hot springs
e. geysers
f. fumaroles
Volcanic mountains (mountains of accumulation)
Volcanic mountains are formed when molten rock ejected from fissures in the
earth’s crust piles on the earth’s surface. The molten rock is in form of lava,
and is accompanied by ash, dust, mud, cinders and chunks of rock solidified
from lava.
Volcanic mountains are common in the Circum-Pacific belt and include Mt.
Fuji in Japan, Mt. Mayon in Philippine and Mt. Merapi in Sumatra. In Africa,
some volcanic mountains are found along the Great East African Rift Valley,
149
e.g. Mt. Kilimanjaro and Mt. Kenya. Mt. Cameroon in West Africa is also a
result of the accumulation of volcanic material.
Characteristics of volcanic mountains
a. They usually occur in isolation.
b. They are conical in shape.
c. They are symmetrical. This means that their sides are of equal
steepness. Depending on the thickness of the lava from which they are
formed, their sides are either steep or gently sloping.
Importance of volcanicity
a. Agriculture: Volcanicity results into the formation of volcanic
mountains and lava plateaus, which provide fertile volcanic soils for
agriculture. Volcanic mountains also influence formation of relief
rainfall which is important for agriculture.
b. Settlement: The highland areas created through volcanicity are
densely settled. This is due to the fertile soils and cool climate, e.g.
Bugishu and Kigezi in Kenya and Kilimanjaro Highlands in Tanzania.
c. Tourism: The volcanic features especially mountains are tourist
attractions. They provide sporting activities like mountain climbing. As
a result, they generate income in form of foreign exchange and provide
employment to local people.
d. Electricity generation: The volcanic mountains so formed are a source
of many rivers, which provide water for domestic use and generate
hydroelectric power. Hot springs or Geysers are potential source of
geothermal power (electricity). In Kenya, the Olkaria Geothermal
Power Station near Lake Naivasha generates electricity.
e. Lumbering: There are forest reserves on the slopes of mountains like
Elgon, Mufumbiro Kenya, and Kilimanjaro which are valuable source
of timber and firewood.
f. Wildlife conservation: The forests also act as wildlife conservation
areas e.g. Bwindi impenetrable forests, has the largest population of
gorillas, which promote tourism.
g. Mining: Lava or magma is rich in minerals e.g. the Kimberlite volcanic
rock in Tanzania is centre for gold mining. Minerals provide revenue
and employment.
h. Fishing: Volcanicity result into the formation of crater or caldera lakes
and lava dammed lakes. These lakes support fishing, which provides
food and employment.
150
Problems caused by volcanicity
a. Volcanic features especially mountains are communication barriers
due to steepness.
b. It is very expensive and risky to construct roads and railways in the
hilly areas.
c. Volcanic eruption leads to loss of lives and property.
d. Heavy rainfall and steepness lead to soil erosion and landslides.
e. Mountains act as barriers to rainfall especially on the leeward side
(rain shadow areas), causing drought.
Activity
4
Locating mountain ranges on a plate boundary map of the
world
You will need a pencil and a copy of the map in Figure 130 for this activity.
1. Use the information in Table 9 to locate each mountain on the map.
Since the map is small, you can just use the number of each mountain
for labelling.
Table 9: Latitudes and longitudes of mountains
1. Kilimanjaro
2. Mulanje
3. Atlas
4. Flat Irons
5. Franklin Mountains
6. Hopi Butte
7. Iliamna
8. Mauna Loa
9. Mitten Buttes
10. Ruby Mountains
11. Torres Del Paine
12. Zagros Mountains
Latitude
3.04o S
16.03o S
32.0o N
39.99o N
31.9 N
35.50 N
60.03 N
19.48 N
36.92 N
45.31 N
53.0 S
27.3 N
151
Longitude
37.21o E
35.5° E
7.54o W
105.29 W
106.49 W
111.00 W
153.09 W
155.6 W
110.07 W
122.23 W
72.5 W
54.5 W
180
150
120
90
60
30
0
30
60
60
120
90
150
180
60
Eurasian Plate
North American
Plate
30
Pacific
Plate
0
Carabbean
African Plate
30
Equator
0
Nazca
Plate
30
60
Pacific
Plate
Arabian
South American
Plate
Indo-Australian
Plate
Antarctic Plate
Destructive Margin
Collision Margin
30
60
Constructive Margin
Conservation Margin
Direction of Plate Movement
Figure 130: World map showing plate boundaries, longitudes and latitudes
2. Based on your completed map, answer the following questions:
a. Name the mountains (or their numbers) that are located on a
convergent plate boundary.
b. What types of mountains are created at convergent plate boundaries?
c. Name the mountains that are located on a divergent plate boundary.
d. What types of mountains form at divergent boundaries?
e. Name the mountains that are not located near a plate boundary.
f. What could explain the presence of mountains that are far away
from plate boundaries?
g. What do you notice about the relationship between plate boundaries,
volcanoes, and mountains?
3. Report your findings to the class for discussion.
Denudation
Denudation is the collective processes of erosion, weathering and mass
wasting (the downward movement of loose rock and soil along a slope),
which causes the wearing away of the earth’s surface leading to a reduction in
elevation and relief of landforms and landscapes.
152
Features formed from denudation
The process of denudation leads to the formation of a wide range of features
including;
a. residual mountains
b. inselbergs
c. dissected plateaus
d. valleys
e. gorges
f. waterfalls
Residual (relict or remnant) mountains
Residual mountains are formed when the general level of the land has been
lowered by weathering and erosion, leaving behind some very hard and
resistant areas. The hard areas stand up as residual mountains (see Figure
131 below). Monadnock Mountain in USA and Mulanje Mountain of Malawi
belong to this type.
Figure 128: Formation of residual mountains
Figure 131: Formation of residual mountains
153
Activity
5
Modelling the formation of residual mountains
You will need sand, water in a watering can and a large piece of rock for this
activity. Prepare the materials as follows:
1. Make two raised areas of the same height with the sand. However, one
of the two areas should have the piece of rock beneath the sand.
2. Draw each area in side view (not bird’s eye view) before continuing to
the next step.
3. Gently pour water from a watering can over each area separately to
make them erode. Make sure that you use a gentle flow of water to
prevent the erosion from occurring too rapidly.
4. Observe and draw (side view not bird’s eye view) the same two areas
after erosion.
5. How does this experiment relate to the formation of residual mountains?
6. Report your findings to the class for discussion.
Characteristics of residual mountains
a. They have bare and very rocky walls due to erosion of loose material.
b. Their peaks may be sharp or rounded depending on the rock
characteristic.
c. They are highly dissected with narrow valleys.
d. Most residual mountains have rich deposits of bauxite from which
aluminium is extracted.
• Inselbergs
An inselberg is an isolated steep-sided hill of solid rock that rises abruptly
from a gently sloping or virtually level surrounding plain. It is a residual
relief feature created by the eroding action of water and wind over time. The
rock making up the inselberg is more resistant to erosion than the rocks that
once made up the surrounding plain.
In Malawi the best known inselbergs include Hora Mountains in Mzimba,
Elephant Rock (Figure 129) in the Chikangawa Forest on Viphya Plateau,
Ntchisi Hill in Ntchisi, Mlanda and Bunda Hills in Lilongwe, Sakata Hill in
Zomba, and Chiradzulu Hills in Chiradzulu.
154
Figure 132: Elephant Rock on Viphya Plateau
Importance of denudation
• Mining: Denudation results into the formation of residual mountains,
which are a storehouse of mineral ores especially bauxite from which
aluminium is extracted.
• Tourism: Residual mountains, plateaus, gorges and valleys attract
many people for skiing, climbing mountains, and taking photos.
• Forestry: The forests in these mountains provide valuable wood and
herbs and are also a natural home to many kinds of birds and animals.
• Hydropower generation: These mountain areas influence rain
formation, and this leads to formation of rivers. Rivers originating from
the mountains help in generating hydroelectricity.
• Agriculture: Through weathering process, denudation creates
nutrient-rich soil that allows plants and trees to grow, and ultimately
makes life on earth possible. If weathering never occurred, the earth
would have a surface of bare rock, and no plant or animal life could
exist. The rainfall received in the residual mountain areas supports a
wide range of crops and livestock. Rivers also carry fertile soil from the
mountains to the flood plains, where they help in agriculture.
• Conservation of wildlife: Since most mountains are not easily
accessible to man, residual mountains help in the conservation of
wildlife on earth.
• Stability of land: Denudation processes decrease the steepness of
slopes, leaving them more stable.
155
Problems caused by denudation
• Rock falls or rock slides: rock falls and rock slides are very dangerous
because they can occur without warning. They occur when weathered
rocks fall from cliffs or steep hillsides. As the massive rock fall contacts
the base of the mountain, it breaks into thousands of fragments that
continue tumbling down slope at high velocity. The great energy of such
rocks can destroy and bury man-made structures and can even kill.
Figures 133 and 134 illustrate rock fall and rock slide, respectively.
mountain
mountain
massive
rock fall
town
rock avalanche
town site
Figure 133: Rock fall
potential rock slide
rock slide
rock layer prone
to sliding
slide
block debris flow
Figure 134: Rock slide
Mudflow, commonly referred to as mudslides, is perhaps the most
dangerous and destructive form of mass wasting. Mudslides commonly
bury homes, as well as any person unlucky enough to be caught in their
path.
• Damage to farms and shores: Denudation processes carry away top
soil. This is of particular concern to farmers, because it can reduce the
fertility of the soil and thereby decrease the productivity of farmland.
The actions of ocean waves can also lead to the loss of beaches and
coastal property by carrying weathered sand out to sea.
156
Activity
6
Examining types of mountains and their formation
In groups of four, study the mountain images shown in Figure 135 below to
do the following activity:
1
2
3
Figure 135: Types of mountains
1. What are some of the ways that these mountains differ?
2. What do you think might be responsible for these different shapes?
3. How do you think each type of mountain might have formed?
4. Record your ideas on a piece of paper.
5. Draw illustrations of mountain formation for your presentation based on
each photograph.
6. Report your work to the class for discussion.
Summary
While mountain belts can form in several different ways, they are almost
always the result of interactions between tectonic plates. Convergence and
divergence of tectonic plates bring about folding and faulting in the layers of
crustal rocks. Plate tectonics also results in volcanism. All these processes,
together with denudation, create mountains. Mountains can be classified
into different basic types based on the cause that formed the mountain, type
of rock, shape and displacement of land: fold mountains, block mountains,
volcanic mountains and residual mountains. Mountains throughout the
world are primarily used for recreation, logging, mining, grazing, wildlife
conservation and other activities. People living in mountain regions face
communication problems, drought, earthquakes, landslides, avalanches, rock
fall, cold climate, steep terrain, volcanic eruptions, among other challenges.
Glossary
Geosyncline: a long broad depression in the earth’s crust containing very
thick deposits
157
Anticline: an arch-shaped formation of upwardly folded layers of sedimentary
rock
Syncline: a downward fold in a rock formation that is shaped like a basin or
trough
Nappe: a large mass of rock that has been forced over other rocks.
Fault: a break in the rock layers that make up the earth’s crust, along which
rocks on either side have moved past each other
Graben: a broad valley, especially a rift valley
Mass wasting: the downward movement of loose rock and soil along a slope
Review questions
1. Identify the major mountain belts and determine which of the following
tectonic scenarios led to their formation:
a. on-going ocean – continent convergence.
b. on-going continent – continent convergence.
2. Which two plates collided in the building of the Andes?
3. Mention any three characteristics of young fold mountains.
4. Give two human activities supported by young fold mountains.
5. Will Mount Everest always be the highest mountain on Earth? Give a
reason for your answer.
6. With the aid of well-labelled diagrams, describe how tension forces
form block mountains.
7. Mention any two denudation processes which reduce the size of Block
mountains.
8. With the aid of well-labelled diagrams, describe how residual mountains
are formed.
9. What happens to a mountain range after the tectonic forces that caused
its uplift are no longer operating at that site?
10. Draw a fully-labelled diagram of an asymmetrical fold.
References
Bradberry, J. (1985). Introducing Earth Science: A Practical Approach to
Geology. Oxford: Basil Blackwell Limited.
Bunnett R. B. (1984). Physical Geography in Diagrams for Africa. London:
Longman Group Limited.
158
Gardner, J. et al. (2011). CK-12 Earth Science Honors for Middle School
Teacher’s Edition http://www.ck12.org
Leong, G. C. (1983). Certificate Physical and Human Geography. Oxford:
Oxford University Press.
Waugh, D. (1990) Geography: An Integrated Approach. Hong Kong: Thomas
Nelson Limited.
http://www.elateafrica.org/elate/geography/earthmovements/faultiing.html
11/12/13
http://www.yourdictionary.com/fold 31/05/14
http://www.elateafrica.org/elate/geography/earthmovements/faultiing.html
11/12/13
http://www.elateafrica.org/elate/geography/earthmovements/
earthmovementsexercise.html 11/12/13
http://en.wikipedia.org/wiki/Denudation 11/12/13
http://geology.campus.ad.csulb.edu/people/bperry/Mass%20Wasting/Types_
of_Mass_Wasting.htm
http://little-blossoms-childminding.blogspot.com/2013/03/earth-sciencestructure-of-earth-and.html 21/06/14
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160
Volcanism
Unit
10
Volcanism
Volcanism is a process through which material
from the mantle in form of molten rock and
gases are forced into the earth’s crust and onto
its surface. Pressure that builds up in the mantle
may be released in weak crustal rocks by folding,
Volcanism has played faulting and other plate movements, forcing the
a key role in shaping molten (liquid) rock onto the earth’s surface.
the world around us:
whatever processes you
look at, from the shaping Activity 1
of the continents to the Investigating underlying causes of
creation of the earth’s
volcanism
oceans and atmosphere,
volcanism has had a The following activity should help simulate
role to play. Learning volcanism.
this topic will help you
1. Get a warm bottle containing Coca-Cola
understand the question
and shake it vigorously before you open it.
of how humanity lives
2. Now, open the bottle immediately.
with volcanoes: the risks
they pose, the benefits
3. Record your observations.
they offer. In this unit,
4. Why do you think this happened?
you will explain the
5. How does this demonstration relate to the
term volcanism and the
concept of volcanism?
formation of a volcano.
You will also explain
6. Present your findings to the class for
extrusive and intrusive
discussion.
features formed from
a volcano. Finally, you
will assess the effects of When the molten material is beneath the earth’s
volcanic activity.
crust it is called magma, and it contains gases
and mineral crystals. However, once the molten
rock breaks through the earth’s surface all the
gases are released and the remaining material is
called lava (see Figure 136).
161
Figure 136: Red-hot lava flowing from a volcano
Types of lava
Basic lava
Basic lava has the following characteristics:
a. It is highly fluid and flows for several kilometres before it solidifies
to form wide based and gentle sloping mountains, lava plains and
plateaus.
b. It is extremely hot.
c. It is rich in iron and magnesium.
d. It erupts quietly.
Acidic lava
Acid lava has the following characteristics:
a. It has relatively low temperatures compared to basic lava.
b. It is very thick, sticky and traps a lot of gas and water.
c. It is rich in silica.
d. It does not flow much before it solidifies to form narrow based and steep
sided mountain features.
e. It erupts violently. The reason is that its viscosity and stickiness choke
the vent and block the passageway causing a build-up of pressure.
When the pressure is so great, the blockage is exploded out of the way,
hence violent eruption.
162
Activity
2
Simulating the concept of viscosity of lava
This activity will help you simulate the concept of viscosity of lava and the
shape of mountain features formed. You will need two plastic cups, two plastic
plates, a table spoon, a ruler, maize flour and water. Prepare the materials as
follows:
1. Label one cup high viscosity and one cup low viscosity.
2. Label one plate high viscosity and one plate low viscosity.
3. Measure about 60 ml. (1/4 cup) of flour into each cup.
4. Add 2 tablespoons of water to cup of flour labeled high viscosity and stir
until mixed.
5. Add 3 1/2 tablespoons of water to cup of flour labeled low viscosity and
stir until mixed.
6. Now, slowly pour the contents of the high viscosity cup of flour onto the
plate labeled high viscosity and observe the speed of flow.
7. Repeat step number 6 with the low viscosity content and its plate.
8. Allow the two contents to rest for 2 minutes before measuring their
base diameter and height with a ruler.
9. How do they compare? Illustrate your observation with diagrams.
10. Report you findings to the class for discussion.
What is a volcano?
A volcano is actually an opening or a fissure, in the earth’s crust, through which
lava or molten rocks, ash and toxic gases from below the surface of Earth are
discharged by a sudden, violent eruption. Sometimes, it can be a mountainlike structure with a bowl-shaped depression at the top that opens downward
into the crust through which molten rock and gases from the interior of the
earth are ejected.
The formation of a volcano
A volcano is formed when hot, molten rock, called magma, along with
some gases and hot ash from the interior of the earth, manages to rise up
to the surface via a vent or a fissure. While the gases get thrown into the
air, the magma and ash cool down forming distinctive volcanic landforms.
163
Parts of a volcano
a.Vent: A fissure in the earth’s crust (or in the surface of some other planet)
through which molten lava and gases erupt (see Figure 137).
b.Crater: this is a bowlshaped geological formation
Ash and gas
at the top of a volcano.
c.Volcanic ash: refers to
very small solid particles of
Crater
rock ejected high into the
atmosphere from a volcano
Cone
Vent
Layers ash of
during an eruption
Lava flow
ash and lava
d.Volcanic cone: this is the
pile of lava, dust, ashes, and
rock around the vent. It can
be found in different shapes’
Magma
Figure 137: Parts of a Volcano
Activity
3
Examining the relationship between population density
and areas of increased risk for volcanic eruption
1. Look at the world map with distribution of frequent volcanoes in Figure
138 on page 165, and compare it to the world map that shows plate
movements in Figure 112 on page 131.
2. What do you think is the relationship between plate boundaries and
volcanic areas?
3. Now, compare this with the map of the world that shows population
density in your atlas.
4. Discus whether there is a relationship between population density and
areas of increased risk for volcanic eruption.
5. Have people avoided populating areas that are at risk of volcanic
eruption? Why do you think this is the case?
6. Why do you think anyone would want to live in a region that is at risk
for volcanic activity?
7. Report your findings to the class for discussion.
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Stages of a volcano’s life cycle
Active volcanoes
These erupt frequently and may have been in continuous eruption for
thousands of years. There are over 500 active volcanoes on the earth’s surface.
Many of these are found at the edges of plates, where new crust is formed
and old crust is destroyed. One of the most famous examples is Mauna Loa of
Hawaii. It is the world’s largest active volcano, which has been erupting for
almost 100,000 years. It has erupted 33 times since 1843 and its most recent
eruption occurred in 1984. This mountain, together with other island chains
around the edges of the Pacific Ocean, forms the so-called Ring of Fire (see
Figure 138).
Dormant volcanoes
Dormant volcanoes are those that are historically active but they have been
quiet for an extended period of time. Some volcanoes lie dormant for thousands
of years before erupting again. The best example of a dormant volcano is
Mauna Kea, one of the five volcanoes that make up the Islands of Hawaii. The
volcano last erupted in 2460 BC.
Iceland
Mauna Loa
EQUATOR
KEY
ATLANTIC OCEAN
PACIFIC OCEAN
Areas with active
volcanoes
Figure 135: The Pacific Ring of Fire
0
4000km
Figure 138: Pacific ring of fire
Extinct volcanoes
These are those that are historically inactive and no longer have lava supply
to erupt in future; i.e. they have ceased erupting. Kilimanjaro Mountain in
Tanzania and Elbrus Mountain in Russia are examples of extinct volcanoes.
Volcanoes become dormant and eventually extinct because the earth’s plates
are constantly shifting above volcanic hotspots. Therefore, the magma finds a
new source to the surface, creating a new active volcano.
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Tectonic plates and volcano formation
Volcanic structures are usually formed at places where the tectonic plates are
either converging or diverging. These areas are so weak that the molten rock,
which is under pressure below the surface easily breaks through. When two
plates diverge, or pull away from each other, the underlying magma emerges
from the resultant crack or fault, forming a volcano. Divergent boundaries
are usually found in oceanic plates, and are primary sources of newly formed
ocean floors. The emergent lava may go on to form islands.
Volcanic hot spots
Whilst most volcanoes occur along plate boundaries, there are exceptions.
For example the volcanic Hawaiian Islands which can be found in the middle
of the Pacific Plate are formed due to a hotspot. Hotspots are fixed points
that are situated beneath the tectonic plates – away from plate boundaries –
Heat at the location of a hot spot is more in comparison to other areas of the
magma chamber and so, the process of melting of rock and subsequent rising
of the magma is comparatively quicker at this point, which result in volcanic
activity.
A hot spot is fixed, but the volcano is constantly in motion because it is formed
over the tectonic plates. As the volcano passes the location of the hot spot, it
gets cut off from its source of magma, and becomes extinct in the future. As
new tectonic plates keep on coming into contact with the hot spot, it goes on
giving rise to volcanoes. This ever-continuing cycle of volcano formation and
extinction, thus results in the formation of a chain of volcanic landforms.
Types of volcanoes
Volcanoes come in different shapes and sizes, depending on the makeup of the
magma, the style of the eruption, and how often they erupt.
Shield volcanoes
Shield volcanoes get their name from their shape that resembles a warrior’s
shield.
a. They are wide at the base and have gentle slopes.
b. Lava flows out quietly and easily for great distances. The spreading
lava creates the shield shape.
c. Shield volcanoes are built by many layers over time and the layers are
made of lava only.
d. They have regular and frequent eruptions.
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e. Shield volcanic eruptions are non-explosive because they form from low
viscous (basic) lava.
Figure 139 shows a shield volcano. Although shield volcanoes are not steep,
they may be very large. Mauna Loa of Hawaii Islands is the world’s largest
shield volcano, making up half of the entire island.
Lava flow
Crust
Figure 139: Shield volcano
Cinder cones
Cinder cones are the simplest and smallest kind of volcanoes. They are
formed when eruptions are very violent. The lava blows furiously into the
air and breaks up into small pieces called cinders (see Figure 140). When
these fragments land to the ground, they build up and harden into a cone, e.g.
Paricutin Volcano in Mexico.
a. Cinder cone volcanoes have a bowl
shaped crater.
b. They have tall and very steep
sides.
c. They have explosive eruptions,
which produce a lot of cinder and
ash.
Cinder
Composite volcanoes
Composite volcanoes are formed from
alternating eruptions of thick sticky
lava and fluid lava as well as ash. These
Crust
eruptions may sometimes recur over
Figure 140: Cinder cone
thousands of years, building up tall
mountains. Composite volcanoes
a. are tall with narrow base and steep sides. Most of the earth’s tallest
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volcanoes are composite volcanoes, e.g. Kilimanjaro Mountain in
Tanzania.
b. have both a central vent and a number of side vents where lava comes
out.
c. are made of alternate layers of lava and ash.
d. have irregular eruptions with long dormant periods.
e. build from both explosive and effusive eruptions.
Figure 141 below is an illustration of a composite volcano.
Figure 141: Composite volcano
Extrusive and intrusive features formed from a volcano
When molten rock breaks through the earth’s crust, several features are
formed. These features are grouped into two types: intrusive and extrusive
features.
Intrusive features
Intrusive features are formed
under the ground when the
molten rock cools and solidifies
before reaching the surface.
Examples include batholiths,
laccoliths, lapoliths, phacolith,
plutons, stocks, sills and dykes
(see Figure 142).
Figure 142: Intrusive volcanic features
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Batholith
A batholith is a very large body of igneous rock formed beneath the earth’s
surface by the intrusion and solidification of magma. It is typically several
kilometres in depth and extends over hundreds of square kilometres. Most
batholiths intrude across mountain folds, forming the root of the mountain.
Examples of batholiths include Dartmoor, Devon and Mourne Mountains in
Northern Ireland.
Laccolith
A laccolith is a mass of igneous rock that intruded between layers of
sedimentary rock; the overlying layers of the sedimentary rock are notably
pushed upward by the intrusion to form dome-shaped mountains, e.g. Navajo
Mountain in Utah.
Sill
A sill is a tabular body of igneous rock formed by horizontal intrusion of
magma along bedding plane, between two rock layers.
Dyke
When a mass of igneous rock from magma intrusion cuts across the layers of
sedimentary rock and forms a wall-like structure.
Lopolith
This is a large saucer-shaped body of igneous rock intruded between layers
of sedimentary rock. It is similar to a laccolith but concave downward rather
than upward.
Phacolith
A phacolith is a lens-shaped igneous body located near the top of an anticlinal
fold or the bottom of a syncline. It is also formed much like a laccolith but, is
much shallower. Its structure is due to the folding of crustal rock layers.
Extrusive features
Extrusive features are formed when lava cools and solidifies on the surface
of the earth. Examples include volcanic mountains, volcanic islands, lava
plateaus, Mid-Ocean Ridges, Calderas, hot springs, geysers, fumaroles, etc.
169
Volcanic islands
Some volcanoes are also found in the oceans. Most of the volcanoes are
invisible from our naked eyes,
Kauai
3.8 to 5.6
since they are hidden under
million
Oahu
Years old
2.2 to 3.4 Molokal
the water. However, sometimes
1.3 to 1.8 Maul
0.8 to 1.3
due to repeated eruptions, tall
Hawall
0.7
mounds are formed that rise
Direction of plate movement
above the surface of the water,
thus giving rise to ‘oceanic
TE
islands’ (see Figure 143).
IC PLA
t
PACIF
Hotspo
Iceland in Northern Atlantic
TLE
Ocean and Hawaii in the
MAN
Pacific Ocean were formed in
Figure 143: Volcanic islands
this way.
Lava plateaus
Lava plateaus are formed by the large outpourings of fluid lava from long
narrow openings in the crust. During each eruption, the lava flows out from
these openings, solidifies and builds up layer upon layer each time (see Figure
144). In many cases, repeated eruptions build lava plateaus of varying sizes.
Some of the most notable are the Columbia in the USA, the Deccan in India
and the Siberian in Russia.
New lava layer
Fissures
Lava Plateu
A lava plateau is made up of
many layers of thin, runny
lava that erup from long
cracks in the ground.
Lava layers
Figure 144: Lava plateau
Calderas
A caldera is a large crater in a volcano. It is formed when the top of a volcano
gets blown off in a
2. Violet eruption blows off 3.
violent eruption or 1.
the top of cone to create
collapses into the
a caldera
magma
chamber
Crater
Caldera
(see Figure 145).
Violet
In either way, the
eruption
crater gets enlarged
and
may
later
contain a lake.
Figure 145; Caldera formation
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Hot spring
Hot springs are areas of natural hot water that bubbles to the surface from
the ground (Figure 146). They occur when underground water is heated up
by hot rocks beneath the surface. The heated water rises through cracks in
the ground. The water in hot springs often exceeds temperatures of 60° C.
Figure 146: Hot spring
Hot springs are common along the rift valleys. The fault lines in the rift
valleys provide areas where water sinks deep enough beneath the surface to
be heated by hot volcanic rocks. Iceland and New Zealand have thousands
of hot springs. Due to Malawi’s geological setting within the rift valley, the
country has about twenty one (21) major hot springs along the rift valley from
the northern region to the southern region (see the map in Figure 147).
Temperatures of these hot springs vary from lukewarm to boiling with the
highest recorded temperature at 79.3°C. The Chiweta springs were found to
be the hottest of all the springs. The source of heat is the high heat fluxes from
the crustal rocks due to conduction.
Activity
4
Examining the potential benefits of Malawi’s hot springs
A number of hot spring resources exist in Malawi from north to south which
currently are only utilised for domestic purposes and tourism. Use this
information to answer the following questions:
1. Name any three hot springs and the districts in which they are found –
one in each region.
2. Why do you think hot springs are common along the rift valley?
3. If properly explored, how do you think the hot springs could become
important for the country’s economic development?
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4. What do you think are the potential
benefits of these hot springs to the
environment if utilised fully?
5. Report your work to the class for
discussion.
lake Malawi
Geysers
Geysers are fountains of hot water and
superheated steam periodically shooting
out of the ground in a spectacular eruption
(see Figure 147). When water is both
superheated by magma and flows through a
narrow passageway underground, pressure
can build. Eventually, the pressure grows
so great that the superheated water bursts
out onto the surface to create a geyser. They
are caused when underground chambers
of water are heated to the boiling point by
volcanic rock. When heat causes the water
to boil, pressure forces a superheated column
of steam and water to the surface. Iceland,
the Rotorua District of New Zealand and
Yellowstone Park of USA are the three major
areas to which most of the world’s geysers are
confined.
Figure 147:
Malawi
Places of hot springs in
Fumaroles
Fumaroles are holes in
volcanic areas from which
steam and hot gases such
as sulphur dioxide, carbon
dioxide hydrogen sulphide
are emitted. Fumaroles are
similar to geysers, but release
bursts of hot gases instead of
water (see Figure 149).
Figure 148: Geyser
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Figure 149: Fumarole
Benefits of volcanoes to human activity
The benefits humans can reap from volcanoes are the following:
a. when a volcano erupts it throws out a lot of mineral-rich ashes, which
enrich the soil with nutrients, and this helps to boost crop production.
b. volcanoes form new landmasses and islands for settlement and other
activities.
c. they provide resources for geothermal energy extraction such as
geysers and hot springs. The type of energy is very clean and almost
inexhaustible.
d. molten rocks give rise to beautiful landscape, which attracts tourists.
This in turn creates more jobs.
e. volcanic areas are rich in mineral resources such as diamond, gold,
copper and other industrial resources.
f. volcanoes create lava dammed lakes, which are important for fishing.
g. volcanic mountains lead to formation of relief rainfall or ice caps which
are a source of many rivers. The rivers may provide water for domestic
use and generate hydroelectric power.
Problems caused by volcanoes
Volcanic eruptions can have a devastating effect on people and the environment.
a. Ash particles continually get deposited on the roofs of the dwellings. If
the weight increases beyond what a roof can endure, it buckles, causing
injury and even death by roof collapse. When inhaled, particles of
volcanic ashes can cause death by choking the lungs and causing burns.
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Many people are also killed by explosions of extremely hot lava and
toxic gases.
b. Pyroclastic flows (the burning gases that descend from the volcano at
high speeds of over 200 kilometres an hour) engulf and burn everything
in their path. They are the main killer of people and destroyer of wildlife
in volcanic eruptions.
c. Lava flows can destroy settlements and clear areas of woodland or
agriculture and a large number of people are forced to desert their
homes and land forever.
d. Carbon dioxide emitted from volcanoes adds to the natural greenhouse
effect. Sulphur dioxides cause environmental problems, because they
are converted to sulphuric acid in the stratosphere; the main cause of
acid rain.
e. Volcanic eruptions can inject massive quantities of ash into the
atmosphere, greatly reducing the solar heating of the earth and
potentially interrupting the global food supply for several years.
f. Volcanic ash clouds can disrupt aircraft travel, such as the incident in
1989 when ash from Alaska’s Redoubt volcano temporarily disabled a
passenger airplane.
g. Intrusive features like sills and dykes form waterfalls and rapids which
hinder navigation of rivers.
h. Volcanic features especially mountains act as barriers to communication
and rainfall especially on the leeward side (rain shadow areas), causing
aridity.
Activity
5
Making a dictionary of volcanic terms
1. As a class, brainstorm to put together a list of volcanic terms.
2. Then, be in groups of at least five students and each group should
choose 5 terms to research.
3. Put your work together as a class and paste it on the wall in your
classroom. You can also distribute it to lower grades or younger siblings.
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Activity
Reflecting on important issues in the topic
1. In groups of three, to locate an important issue that you feel the topic
has covered.
2. In your group formulate a problem or question about your issue for
another group to answer.
3. Write the problem down on a sheet of paper, and hand that piece of
paper to another group.
4. Once your group is handed a problem statement, you should think of a
solution to the problem. Each group has a fixed amount of time.
5. Report your work to the class for discussion.
Summary
Volcanism occurs at plate boundaries and at hot spots. Volcanoes are classified
into extinct, dormant and active depending on the frequency of their eruption.
Many active volcanoes circle the shores of the Pacific Ocean in what is called the
Ring of Fire. There are three kinds of volcanoes: composite, shield, and cinder
cone. Features formed from a volcano are grouped into intrusive and extrusive.
Intrusive features include batholiths, laccoliths, sills and dykes. Extrusive
features include volcanic islands, lava plateaus, calderas, hot springs, geysers,
fumaroles. Volcanic areas are important for mining, agriculture, generation of
geothermal power, tourism and settlement. However, volcanic activities are
known to have caused loss of human life, wildlife, property, and disruption of
air travel.
Glossary
Magma:is a liquefied mixture of molten rock, crystals and dissolved gases
found beneath the earth’s surface
Lava: is what magma is called when it reaches the surface and loses its gases.
Hot spot: an area in the mantle from which hot magma up wells deep in the
Earth.
Cinder: loose fragments of porous solidified lava.
Ring of fire: an area where a large number of earthquakes and volcanic
eruptions occur in the basin of the Pacific Ocean.
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Review questions
1. Copy the diagram of a volcano shown below in your notebook, and on
your diagram, label the following features:
a. magma chamber
b. main vent
c. crater
d. gas and dust
e. layers of cooled lava
Figure 150; Parts of a volcano
2. What is the difference between basic lava and acid lava? Give two
points.
3. Describe any three landforms of intrusive volcanic activity.
4. Explain two differences between volcanoes along constructive and
destructive plate margins.
5. Describe the three shapes of volcanoes and explain how each is formed.
6. Give a reason why some volcanic eruptions are more violent than
others.
7. Describe any three ways in which people have taken advantage of living
in volcanic areas.
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References
Bradberry, J. (1985). Introducing Earth Science: A Practical Approach to
Geology. Oxford: Basil Blackwell Limited.
Bunnett, R. B. (1973). General Geography in Diagrams. London: Longman.
Bunnett R. B. (1984). Physical Geography in Diagrams for Africa. London:
Longman Group Limited.
Desonie et al. (2011)CK-12 Earth Science Honors for Middle School Teacher’s
Edition, CK-12 Foundation, www.ck12.org
Dulanya Zuze (2006). GEOTHERMAL RESOURCES OF MALAWI - AN
OVERVIEWwww.geothermal-energy.org/pdf/IGAstandard/SGW/2006/
dulanya.pdf- accessed on 22/06/14.
Gardner, J. et al. (2011). CK-12 Earth Science Honors for Middle School
Teacher’s Editionhttp://www.ck12.org
Leong, G. C. (1983). Certificate Physical and Human Geography. Oxford:
Oxford University Press.
Waugh, D. (1990). Geography: An Integrated Approach. Hong Kong: Thomas
Nelson Limited.
White, R. (1998). Africa in Focus: A Physical, Human and Economic Geography.
Oxford: MacMillan Education Limited.
http://farm5.staticflickr.com/4026/4539624939_092a34be72_z.jpg 16/01/14
http://www.buzzle.com/articles/how-are-volcanoes-formed.html 12/12/13
http://www.smithsonianmag.com/travel/What-Were-Still-Learning-AboutHawaii.html# 12/12/13
http://www.studyblue.com 12/12/13
http://mayhem-chaos.net/photoblog/archives/001081.html 12/12/13
www.studentsoftheworld.info 12/12/13
http://www.bestourism.com/items/di/7814?title=Sol-de-Manana-GeyserBolivia&b=337 12/12/13
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Unit
11
Earthquakes
Earthquakes
Earthquakes refer to the sudden, sometimes,
violent shaking of the earth’s surface. There are
hundreds of thousands of earthquakes every year
but most of these pass unnoticed because they
are minor tremors and can only be detected using
Earthquakes
happen sensitive instruments.
all the time all over the
world. Wherever you live,
you will experience an Causes of earthquakes
earthquake at some point.
Earthquakes are caused by the following:
Anyone who experiences
a. Plate collision: when two plates run into
an earthquake would be
each other friction is produced and this
happy to have learned
causes the ground to shake.
not just what to do in the
event of seismic activity,
b. Faulting: the crustal rocks grind against
but the ways in which
each other as the plates slip past one
buildings around them
another, hence, causing the ground to
have been engineered
vibrate. Besides, rocks along a fracture
to keep them safe.
or fault in the earth may lock together,
Knowledge gained in
causing pressure to build up over time.
studying earthquakes can
When the pressure is intense, the rocks
be applied to engineering,
may suddenly jerk free, thereby releasing
architecture, and other
shock waves.
scientific
fields.
The
c. Explosive volcanic eruptions: these
future needs scientists,
may also cause the ground to shake though
engineers, and officials
not severe.
who understand and
take an interest in these
phenomena.
In
this
unit, you will explain Activity 1
the term earthquake Investigating
the
causes
of
and its causes. You will earthquakes
also explain the effects
1. Stand around a table in groups, with both
of earthquakes and the
hands resting on the table. Ask one member
relationship among fold
of your group to let a heavy book fall on the
mountains,
volcanoes
middle of the table.
and earthquakes.
179
2. What do you all feel?
3. Repeat this activity until everyone has experienced the feeling.
4. What do you think is happening in the table so that you could get that
feeling?
5. Now, what do you think happens when two crustal plates collide?
6. Report your findings to the class for discussion.
Nature of earthquakes
The focus (sometimes referred to as the hypocenter) is the point within a
geological fault that is rupturing where the earthquake begins. Earthquakes
occurring at a depth of less than 70 km are classified as ‘shallow-focus’
earthquakes, while those with a focal-depth of between 70 and 300 km
are commonly termed ‘mid-focus’ or ‘intermediate-depth’ earthquakes. In
subduction zones, where older and colder oceanic crust descends beneath
another tectonic plate, deep-focus earthquakes may occur at much greater
depths (ranging from 300 up to 700 kilometers). Shallow earthquakes cause
the most damage because the focus is near the Earth’s surface where people
live. The epicenter is the point on the surface of the earth that is directly
above the focus.
Figure 151: Nature of an earthquake
The slippage of rocks at the focus emits large amounts of energy in form of
waves that radiate outwards in circles through the interior of the earth and
across the surface. The energy becomes gradually weaker with distance from
the point of origin. Earthquakes are therefore strongest at the epicentre
because it is where the waves first reach the surface. This explains why an
earthquake’s destructiveness is more extensive at the epicentre than at any
point away from it.
180
Activity
2
Demonstrating waves
1. Go to a nearby pond as a class. Alternatively, put water in a large and
wide-mouthed basin. Make sure the water is still.
2. Drop a small object into the water at the centre of the pond or basin.
3. What happens when the object hits the water?
4. How do you relate this experiment with earthquakes?
5. Report your findings to the class for discussion.
Types of seismic waves
Seismic waves are vibrations that travel through the Earth carrying energy
released during an earthquake. In the broadest sense, seismic waves can be
categorized into two major groups: body and surface waves.
Body waves
These traverse through the interior of the Earth. They include two
different types according to the relative direction of disturbance with respect
to direction of propagation: P- and S-waves.
a. P waves or primary waves
are the fastest kind of
seismic wave. They occur
before the others, in
other words they are the
first waves that happen.
These P waves are able to
travel through both solid
rock, such as granite,
and liquid material, such
as volcanic magma and
the water of the oceans.
They push and pull the
rock they move through,
causing the ground to
buckle and fracture (see
Figure 152 ).
Particle motion
Expand
Wave propagation
Figure 152: Primary waves
Direction of wave propagation
181
Compress
b. S waves or secondary waves follow P waves. They are the second
waves felt in an earthquake. S waves move the ground up and down,
and side-to-side (see Figure 153). An S wave is slower than a P wave
and only moves through solid rock. A liquid is not rigid enough to
transmit an S wave. If a liquid is sheared sideways or twisted, it will
not spring back; hence S waves cannot propagate in the liquid parts of
the earth, such as oceans and lakes.
Particle motion
Wave propagation
Direction of wave propagation
Figure 153: Secondary waves
Surface waves
These are restricted to near the ground surface. Such waves correspond to
ripples of water that travel across a lake. Surface waves in earthquakes can
be divided into two types: Love and Rayleigh waves.
a. Love wave moves the ground from side to side in a horizontal plane
but at right angles to the direction of propagation (Figure 154). The
horizontal shaking of Love waves is particularly damaging to the
foundations of structures.
182
Direction of wave propagation
Figure 154: Love waves
b. Rayleigh waves move both vertically and horizontally just like rolling
ocean waves (see Figure 155).
Direction of wave propagation
Figure 155: Rayleigh waves
Surface waves travel more slowly than body waves (P and S); and of the
two surface waves, Love waves generally travel faster than Rayleigh
waves. Love waves do not propagate through water, whereas Rayleigh
waves, because of the vertical component of their motion, can affect the
bodies of water such as lakes.
Measurement of earthquakes
Earthquakes are measured using an instrument called a seismograph. The
seismograph has its base set firmly in the ground. When an earthquake shakes
the ground, the base of the seismograph shakes too, causing a pen/needle to
183
scribble zigzags on a rotating dram (Figure 156). If the pen/needle prints a
straight line, it means no vibrations in the ground. Seismographs can detect
movements as small as 0.00001mm to movements as large as about 1 m.
Spring
Weight
Rotating drum
Pen
Vertical ground movement
Figure 156: Vertical seismograph
Earthquakes are measured in terms of magnitude and intensity.
Magnitude
Magnitude is a measure of the amount of energy released during an earthquake.
It is measured using the Richter scale, devised in 1935 by Charles Richter,
an American geologist. The scale depends on the amount of ground shaking.
It ranks earthquakes from 0 to 9, based on how much the ground shakes.
The greater the amount of vibrations are, the stronger the earthquake and
therefore the higher the value on the scale.
Intensity
Intensity of an earthquake is a measure of the degree of damage to the surface
and the effects on humans. Intensity largely depends on observations of
effects on the crust, not actual ground motions recorded by seismographs. The
Mercalli scale, invented by Giuseppe Mercalli in 1902, uses the observations
of the people who experienced the earthquake to estimate its intensity. While
intensity helps to determine how much of an area was affected, the amount
of damage caused by the earthquake may not accurately record how strong it
was because of the following reasons:
a.The way in which seismic waves travel varies as they pass through the crust
and surface material (rock or dirt) the buildings rest on. Solid rock usually
shakes less than sand, so a structure built on top of solid rock should not be
as damaged as it might if it was sitting on a sandy lot.
184
b.Different building designs hold up differently in an earthquake, some
buildings can withstand the violent shaking of an earthquake while others
cannot.
c.Some witnesses of the earthquake might exaggerate just how bad things
were during the earthquake.
This implies that earthquakes can have one magnitude but different intensities
because intensity varies with location.
Activity
3
Case studies
Read the following newspaper articles and use the information to answer the
questions that follow.
A. Disaster at Killari: the 1993 earthquake
At 4 o’clock in the morning on 30 September 1993, the village of Killari
in Central India was hit by a severe earthquake. Killari is 450 km east
of Bombay. Several houses were destroyed, 30,000 people lost their
lives and many more were left homeless. The quake measured 6.4 on
the Richter scale….
B. The Kobe earthquake, 1995
Early in the morning on Tuesday 17 January 1995 the shock waves
of a huge earthquake roared through the city of Kobe. Measuring 7.2
on the Richter scale, it was the worst earthquake to hit Japan in 50
years. More than 3,500 people were killed. Some 20,000 houses were
destroyed and about 250,000 people were left homeless. Fire broke out
from power lines brought down, consuming many destroyed buildings.
Operations at Japan’s largest port (Kobe) ceased….
(Source: Pallister, J. et. al. (2001). Longman Geography for GCSE. : Essex
Longman)
1. Make a list of the damage caused by earthquakes in both cases.
2. Why do you think the smaller Killari earthquake in India caused many
more deaths than the larger Kobe earthquake in Japan?
3. The main cause of loss of life in an earthquake is the collapse of
buildings. Describe how town planners can reduce earthquake damage
to buildings.
4. Present your findings to the class for discussion.
185
Earthquakes are much more damaging in less economically developed
countries than in more economically developed countries because of the
following reasons:
a. Buildings are poorly built and often cannot resist the shock waves.
b. buildings are often made of heavy, local material or rock and when they
collapse, the people inside have little chance to survive.
c. most of the poor countries are unable to mount a quick rescue operation
because their emergency services are usually few in number and not
well trained.
World earthquake zones
The most important earthquake belt is the Circum-Pacific Belt (the Ring of
Fire), which affects many populated coastal regions around the Pacific Ocean.
For example, those of New Zealand, New Guinea, Japan, the Aleutian Islands,
Alaska, and the Western coasts of North and South America (Figure 157).
Iceland
EQUATOR
ANTLANTIC OCEAN
PACIFIC OCEAN
KEY
0
Earthquake zones
4000km
Figure 157: Major earthquake zones of the world
Effects of earthquakes
a. Destruction of buildings and loss of property: earthquakes
usually lead to the destruction of structures such as buildings, bridges
and dams. Depending on the severity of the earthquake, power lines
may get damaged or gas pipes broken, starting dangerous fires.
b. Landslides: earthquakes can also trigger landslides, the falling of
186
unstable regions of hillsides or mountains. This can block rivers thereby
causing floods.
c. Damage to the environment: earthquakes can tear apart ground
surface, destroy forests and permanently displace some rocks; hence,
damaging ecosystems in the environment.
d. Severe injuries and loss of life: People who are near to collapsing
buildings or mudslides may be killed instantly or become trapped under
falling debris.
e. Spread of diseases: Earthquakes may lead to prevalence of some
diseases due to poor sanitary conditions and lack of proper medical care
because hospitals, dams and water lines may have been destroyed.
f. Tsunamis: Earthquakes that occur along coastlines or anywhere
beneath the oceans can generate tsunamis. Tsunami is a Japanese word
which means “harbor wave” (“tsu” means harbor, while “nami” means
“wave.”). The shaking of the seafloor during an earthquake displaces a
large water mass from its equilibrium position, producing a wave in the
water. The size of the tsunami wave is usually related to the size of the
earthquake, with larger tsunamis generated by larger earthquakes.
Tsunami waves can be as high as 30 meters or more. But the sense of
displacement is very important. Tsunamis are generally only formed
when an earthquake causes vertical displacement of the seafloor. The
wave grows bigger when it gets nearer to the coast causing the coastal
areas because the water is shallower here (see Figure 158).
Figure 158: How a tsunami occurs
187
The relationship among fold mountains, volcanoes and
earthquakes
Fold mountains, volcanoes and earthquakes are seemingly very different
geological events, yet they are actually closely related;
a. they all result from movements of the earth’s crust.
b. they all occur at the boundaries of tectonic plates.
c. they have a causal connection with each other i.e. the formation of
each is associated with the other For instance, fold mountains are
usually associated with volcanic activity. A volcanic eruption is usually
accompanied by earthquakes. As molten rock forces its way through
the upper parts of a volcano’s interior the ground shakes, causing
minor earthquakes. Similarly, earthquakes can lead to volcanic
eruptions. Violent shaking of the ground during an earthquake can
create fractures and other ground disturbances that can affect shallow
magma reservoirs; hence causing volcanic eruption.
The close relationship between fold mountains, earthquakes and volcanic
outbursts is evident from the maps depicting the locations prone to these
phenomena (see Figure 159). If you compare the maps that illustrate fold
mountain zones, earthquak9 zones and volcanic zones, you will find them
matching to each other.
Activity
4
Examining earthquake and other tectonic hazard zones
The map below shows the distribution of the earthquake, active volcanoes and
young fold mountain zones. Study it carefully and answer the questions that
follow.
Caucasus Himalayas
Mount
St Helens
Mauna Loa
Kilauea
Atlas
Mountains
N
s
Krakatoa
PACIFIC
OCEAN
de
INDIAN
OCEAN
Mont Pelée
An
ANTLANTIC
OCEAN
es
ki
oc
R
Alps
KEY
Young fold mountains
Major volcanoes
Earthquake zones
Plate boundary
Figure 159: Distribution of young fold mountains, active volcanoes and earthquake zones
188
1. Which ocean has a ring of volcanoes around it?
2. Where are most of the volcanoes located in relationship to the crustal
plates?
3. Are there any volcanoes not located on the edge of a crustal plate? What
might account for the location of these volcanoes?
4. Where are most of the major earthquakes in relationship to the crustal
plates? Why do some locations receive so many earthquakes?
5. What is the relationship between the locations of the major volcanoes
with the location of the major earthquakes?
6. Are most mountain ranges located in an area of major earthquake and
or volcanic areas? Why?
7. Are there any major earthquakes not located on the edge of crustal
plates? Explain the location of these earthquakes?
8. Report your findings to the class for discussion.
Shield areas
Shield areas are large ancient and tectonically stable regions of the crust.
They are made up of some of the planet’s oldest rocks, largely igneous and
metamorphic. They are normally the heart or centre of continents. Examples
include the Canadian Shield, Brazilian Shield, African Shield, Australian
Shield, Arabian Shield, Indian (Deccan) Shield and others (see Figure 160).
Figure 160: Shield areas
189
Activity
5
Examining shield areas
1. Study the world map in Figure 157, showing the world’s main shields.
2. Relate the map with that of the tectonic plate boundaries in Figure
112 on page 133
3. Why do you think shield areas are tectonically stable?
4. In groups, discuss the economic activities likely to take place in shield
areas.
5. Report your findings to the class for discussion.
Shield areas are quite stable, free from volcanoes, earthquakes and other
crustal deformations because they are away from plate boundaries. However,
their margins are subjected to tectonic forces that have been destroying and
rebuilding the margins for over millions of years. The folding and uplifting of
marine sediments, for example, have produced long mountain chains, which
have been added to these shield margins.
Economic importance of shields
a. Mining: Shields are richly endowed with valuable minerals such as
nickel, gold, uranium, silver, aluminium, zinc and copper. This has led
to the development of mining towns throughout the shields to extract
the minerals.
b. Electricity generation: These extensive bare rocks absorb plenty of
rainwater and hence they become important watersheds for a multitude
of rivers for hydroelectric power generation.
c. Tourism: Shields contain some of the oldest volcanoes, which may be
centres of attraction for tourists.
d. Constructions: The rocks that form the surface of the shields provide
construction materials like concrete.
e. Agriculture and forestry: Weathering of these ancient rocks create
very dense soils, especially in the lowlands, suitable for agriculture.
The rich soils also give rise to a wide range of vegetation that supports
some major logging industries as well as natural ecosystems.
190
Summary
Earthquakes occur in plate boundaries as the plates slide past or collide into
one another. They also result from violent volcanic explosions. From the point
of origin, the energy released spreads out in rings moving across and below
the surface. The ground movements during an earthquake are detected and
recorded accurately by a seismograph. The Richter scale measures the energy
released by an earthquake. The impacts of earthquakes vary based on their
energy and intensity. The strongest earthquakes that occur can result in
ground rupture, causing damage to bridges, dams, roads, railroad tracks, and
the foundations of buildings. They can also cause landslides and avalanches
as a result of the shaking. All these may injure and kill people. Some areas
of the crust away from plate margins (shields) are very stable and therefore
unaffected by earthquake or volcanic activity. These areas are particularly
important for agriculture, forestry, hydropower generation, tourism, and
mining.
Glossary
Focus: the point of origin of an earthquake.
Epicentre: the exact location on the earth’s surface directly above the focus
of an earthquake.
Seismic wave: a vibration that travels through the earth carrying the energy
released during an earthquake.
Seismograph: an instrument used to detect and record motions of the ground
during an earthquake.
Richter scale: a numerical scale from used to measure the severity of
earthquakes according to the amount of energy released.
Shield: an ancient, stable area of crust away from plate margins and therefore
unaffected by volcanic or earthquake activity.
Review questions
1. Give three causes of earthquakes.
2. Outline any three effects of earthquakes.
3. Why is an earthquake’s destructiveness more extensive at the epicentre
than away from it?
4. Explain why earthquake zones and volcanic areas often occur close
together.
5. When an earthquake occurs on the sea floor, coastlines may be affected
by a tsunami. List three consequences of a tsunami.
191
6. Explain the ways in which developed countries are less likely to suffer
severely from earthquakes than developing countries.
7. Name any three shields.
8. Describe two characteristics of shield areas.
9. With examples, explain any three ways in which shield areas are
economically important.
References
Bradberry, J. (1985). Introducing Earth Science: A Practical Approach to
Geology. Oxford: Basil Blackwell Limited.
Bunnett R. B. (1984). Physical Geography in Diagrams for Africa. London:
Longman Group Limited.
Gardner, J. et al. (2011). CK-12 Earth Science Honors for Middle School
Teacher’s Editionhttp://www.ck12.org
Leong, G. C. (1983). Certificate Physical and Human Geography. Oxford:
Oxford University Press.
Pallister, J. et. al. (2001). Longman Geography for GCSE. Essex: Longman
Waugh, D. (1990). Geography: An Integrated Approach. Hong Kong: Thomas
Nelson Limited.
White, R. (1998). Africa in Focus: A Physical, Human and Economic Geography.
Oxford: MacMillan Education Limited.
http://bc.outcrop.org/images/earthquakes/lutge8e/FG15_08A.JPG 26/01/14
http://bc.outcrop.org/images/earthquakes/lutge8e/FG15_08B.JPG
http://bc.outcrop.org/images/earthquakes/lutge8e/FG15_08C.JPG
http://bc.outcrop.org/images/earthquakes/lutge8e/FG15_08D.JPG
192
Rocks
Unit
12
The identification of
rocks is fundamental
to the Earth Sciences
and their study is the
key to understanding
the processes that have
shaped
the
earth’s
surface. Knowledge of
rocks helps engineers
or architects to identify
suitable areas to develop
buildings, tunnels and
bridges, and determine
if the rocks in that area
may break if exposed to
new forces, such as the
pressure from vehicle
traffic
and
people.
In this unit, you will
describe
main
types
of rocks and explain
how they are formed.
You will also identify
the characteristics and
samples of each type of
rock. Finally, you will
examine the importance
of rocks to life and human
activities.
Rocks
A rock is a naturally occurring mass of solid matter
that contains minerals. The entire lithosphere or
crust of the earth is made up of rocks.
Activity
1
Examining rock samples
1. Collect a sample of rocks from the
schoolyard.
2. When you have collected the rocks and
returned to the classroom, break into small
groups.
3. Share and compare your rocks within your
group.
4. Put the rocks into groups according to
colour, hardness and texture.
5. What do you think is the cause of all these
differences in rock properties?
6. How do you think each type of rock might
have formed?
7. How are the three types of rocks related?
8. Draw a diagram to illustrate your answer.
9. Present your findings to the class for
discussion.
Types of rocks
There are three types of rocks based on how
they were formed; igneous, sedimentary and
metamorphic rocks.
193
Igneous rocks
Igneous rocks are formed through the cooling and solidification of magma or
lava. There are two groups of igneous rocks based on where they are formed:
intrusive (plutonic) rocks or as extrusive (volcanic) rocks.
Intrusive (plutonic) igneous rocks
Intrusive rocks are formed below the surface and have large crystals. Examples
of plutonic rocks include granite, diorite, gabbro, peridotite and pegmatite.
Magma that forms intrusive igneous rocks cools slowly deep under the earth’s
surface because temperatures are relatively high. The slow cooling allows
crystals to grow.
Extrusive (volcanic) igneous rocks
Extrusive rocks are formed on the surface and have extremely small crystals.
Examples of these rocks include basalt (Figure 161), rhyolite, andesite,
komatite, obsidian, pumice, scoria and tuff. The liquid rock that reaches the
surface cools very quickly due to low temperatures. Because of this immediate
cooling, crystals do not have time to grow.
Granite
Basalt
Figure 161: Igneous rock samples
Characteristics of igneous rocks
a. They usually have crystals.
b. They have no layers.
c. They normally contain no fossils due to high heat when forming; so all
fossils get burnt.
194
Sedimentary rocks
Sedimentary rocks are formed from sediments deposited on the bottom of
rivers, lakes and oceans. The sediments are pieces of earth that have eroded
or worn away and washed downstream into water bodies and then settled out.
As subsequent layers of sediment are laid over previous layers, the pressure
from the weight causes the underlying sediments to harden into a rock. There
are two major classes of sedimentary rocks, clastic and non-clastic.
Clastic sedimentary rocks
These are made from broken particles of other rocks. Examples include
sandstone and shale.
Non-clastic sedimentary rocks
These do not come from broken rock fragments, but rather from organic and
chemical substances.
a. Organic sedimentary rocks: are made from the accumulation of the
remains of living things such as plants and skeletons, e.g. coal and
limestone.
b. Chemical sedimentary rocks: are formed through the deposition
and crystallization of dissolved chemical substances from solutions.
Examples include rock salt (evaporate) and gypsum (precipitate).
Characteristics of sedimentary rocks
a. They are often rich in fossils of plants and animals.
b. The rocks usually display many layers in them due to cyclical deposition
(see Figure 162).
c. They rarely contain crystals.
d. The rocks are porous because they are composed of all sizes of particlesfine, small and big; so the water can easily penetrate through the pores
between the particles.
Coal
Sandstone
Figure 162: Sedimentary rock samples
195
Activity
2
Experiment – Sedimentary rock formation
In groups of five,
1. Put sand, fine soil, and pebbles into a glass jar.
2. Fill the jar with water, put the top on, and shake it up.
3. Set the jar down to allow the materials to settle.
4. If you can leave the jar for several days, most of the materials will
settle and the water will become almost clear.
5. How many layers can be seen from your model?
6. How does this model relate to the formation of sedimentary rocks?
7. Report your findings to the class for discussion.
Metamorphic rocks
Metamorphic rocks are formed when the existing rocks are altered by either
excessive heat or pressure, or through the chemical action of fluids. These
factors cause chemical changes or structural modification to the minerals
making up the rock.
The characteristics of original rock have profound influence on the nature of
metamorphic rocks. However, when the rock metamorphosed under extreme
conditions, the original characteristics are lost or badly distorted.
Some examples of metamorphic rocks are marble (from limestone), quartzite
(from sandstone), gneiss (from granite) and graphite (from coal). Figure
163 shows examples of metamorphic rock samples.
Gneiss
Marble
Figure 163: Metamorphic rock samples
196
Types of metamorphism
• Thermal metamorphism
In this type of metamorphism, there is a structural and chemical change in a
rock due to heating. Rocks begin to change chemically at temperatures above
2000c. At these temperatures, the crystalline structures of the minerals in
the rock are broken down and rearranged or transformed into new mineral
alignment. Thermal metamorphism has two sub-categories:
a. Regional metamorphism: This is large-scale heating and modification
of large volumes of existing rock through tectonic subduction at
convergent boundaries. However, temperatures here may be high
enough to cause complete melting of the rock to become magma.
b. Contact metamorphism: This is small-scale heating and alteration
of rock by way of a localized igneous intrusion.
• Dynamic metamorphism
Dynamic metamorphism involves change in structure of rock due to pressure.
The minerals in the rocks under pressure do not change chemically but
structurally. Rocks beneath the ground are subjected to pressure because of the
weight of overlying materials. The effect of this pressure is the reorientation
of mineral crystals in the rocks, hence, changing their structure. However,
pressure almost never acts in isolation, as temperatures do get higher with
increasing depth below the earth’s surface.
• Metasomatic metamorphism
This one involves the chemical replacement of elements in rock minerals
when gases and liquids penetrate into rocks. Water and carbon dioxide can
enhance metamorphism by dissolving some particles and by causing chemical
reactions in the rock. Usually, the result of this process is the creation of new
minerals, which change the chemical composition of the rock.
The rock cycle
The rock cycle is the process in which rocks transform from one rock type into
another. Each of the three types of rocks, that is, igneous, sedimentary and
metamorphic, may form at the expense of another.
Sedimentary rocks can come from the weathering, erosion, deposition and
lithification of other rock material, either igneous, metamorphic, or other
197
sedimentary rocks. Metamorphic rocks are formed when intense heat and/or
pressure are applied to sedimentary or igneous rocks. Any of the three types
of rocks can melt to form magma, which later solidifies into igneous rocks.
This relationship of rocks continues on and on, creating a type of natural
recycling of rocks (Figure 164).
Figure 164: The rock cycle
Activity
3
Role playing the rock cycle
1. As a class, walk out and create three (3) stations on the playground.
2. Write the following names on large pieces of papers and put them in the
stations: igneous rock, sedimentary rock, and metamorphic rock.
3. Break into three teams and each team should stand at a different
station to represent a type of rock.
4. Identify the different places water can go from your station in the rock
cycle.
198
5. What conditions are necessary for your type of rock to change from your
station to the next? You should describe the process as the rock changes
from your station to the next station.
6. Keep track of your movements by recording each move you make,
including stops at each station.
7. Has any group returned to the same station they started from?
8. Draw a diagram to illustrate your movements and the processes
involved in the rock cycle.
9. Present your work to class for discussion.
Economic importance of rocks
a. Crashed rock is used for constructing road surfaces, buildings and other
structures.
b. Rocks such as coal are important sources of energy.
c. Rocks are reserves of valuable minerals such as gold, silver, aluminium,
petroleum, etc.
d. When they weather, rocks give rise to soil that supports various forms
of agriculture.
e. Rocks are large reservoirs of water in the mountain ranges that give
rise to rivers, which may be used for generating hydro-electricity and
for irrigation.
Activity
4
Examining the importance of rocks to life and human
activities
1. In groups of four, walk around your schoolyard and record human and
natural features that were made from rocks.
2. How important are these features to the lives of humans?
3. Report your findings to the class for discussion.
Summary
Rocks form from several basic processes. They are grouped into three main
groups: igneous, sedimentary and metamorphic. Igneous rocks form when
199
hot, liquid rock material cools and hardens into a solid. Sedimentary rocks
form from pieces of other rocks that are squeezed or cemented together.
Metamorphic rocks form from either igneous or sedimentary rocks that are
changed by heat, pressure, or by chemical reaction. Each of these rocks gets
its properties from the way it forms. Rocks are continually changing from one
kind into another in a never ending process called the rock cycle. Rocks are
important for constructions, as energy sources, as sources of minerals and
soil. They are also large reservoirs of water.
Glossary
Clastic: describes rock that is composed of fragments of other rocks.
Metamorphism: a process of change in the physical structure of rock
Rock cycle: the dynamic transitions through geologic time among the three
main rock types: sedimentary, metamorphic, and igneous.
Review questions
1. Explain why igneous rocks do not contain fossils.
2. State two main groups of igneous rocks and in each case give an
example.
3. Igneous rocks begin their lifecycle in the mantle. Describe the state of
the material in the mantle.
4. Briefly describe how sedimentary rocks can be formed from igneous
rocks.
5. One of your classmates has brought some rock samples to school. You
have been asked to identify the sedimentary rock. List two of the
characteristics you would look for.
6. Using examples from Malawi, give any two economic uses of sedimentary
rocks.
7. Igneous and sedimentary rocks may be changed into metamorphic rock.
State the conditions under which this change may occur.
8. With aid of a clearly labelled diagram, describe the rock cycle.
References
Bradberry, J. (1985). Introducing Earth Science: A Practical Approach to
Geology. Oxford: Basil Blackwell Limited.
200
Bunnett R. B. (1984). Physical Geography in Diagrams for Africa. London:
Longman Group Limited.
Gardner, J. et al. (2011). CK-12 Earth Science Honors for Middle School
Teacher’s Editionhttp://www.ck12.org
Leong, G. C. (1983). Certificate Physical and Human Geography. Oxford:
Oxford University Press.
Waugh, D. (1990). Geography: An Integrated Approach. Hong Kong: Thomas
Nelson Limited.
White, R. (1998). Africa in Focus: A Physical, Human and Economic Geography.
Oxford: MacMillan education Limited.
http://www.cotf.edu/ete/modules/msese/earthsysflr/rock.html 01/06/14
http://questgarden.com/90/08/9/091106105545/ 01/06/14
201
202
Riverine landforms
Unit
13
Rivers are constantly
modifying their channel
and
surrounding
landscape, creating some
of the most spectacular
landforms on the planet.
A large percentage of the
world population lives
on or near these riverine
landforms due to their high
agricultural productivity
and proximity to river
resources. In unit 3, you
learned how to identify
various riverine features
on topographic maps.
Studying about riverine
features
will
reward
you with the ability to
understand more about
the forces that shape our
world and their impact on
life. In this unit, you will
learn how these riverine
landforms are formed.
You will also learn the
importance of riverine
landforms.
Riverine landforms
Riverine landforms are features produced by
the rivers as they interact with the geology and
topography of the land. They include waterfalls,
rapids, gorges, valleys, meanders, ox-bow lakes,
levees, floodplains, deltas and estuaries.
Waterfalls
A waterfall is a steep drop in the course of a river.
Waterfalls form when a band of hard resistant
rock (cap rock) lies over softer and less resistant
rock. The softer rock is quickly eroded, causing
a step in the riverbed. The great force of falling
water and the rocks carried by the river cut into
the land at the foot of the waterfall, creating a
depression (known as a plunge pool) at the foot
of the waterfall (see Figure 165). The Angel Falls in Venezuela is the world’s highest
waterfall at 979 metres. Other well-known falls
include the Victoria Falls of the Zambezi River,
Niagara Falls in North America, Inga Falls of the
Democratic Republic of Congo, Nkula and Tedzani
Falls in Malawi.
1. Waterfalls are often formed where a layer
of harder rock overlays a layer of softer
rock.
2. As the river passes over the softer rock,
it is able to erode it at a faster rate,
forming a step in the river bed.
Hard rock
Hard rock
Soft rock
Soft rock
3. As the notch grows, eventually there isn’t
enough support
under the harder
Hard rock
rock and it
collapses into
Soft rock
the plunge pool.
Plunge pool
4. This adds rocks and boulders to the plunge
pool, and so the
process of corrasion
Hard rock
works with hydraulic
action to further
Soft rock
erode the plunge
pool and notch.
Rocks and boulders
Figure 165: Formation of waterfall http://www.geocaching.com/
geocache/GC2MAZT_hilton-falls-waterfall-earthcache 16/01/14
203
Rapids
Rapids are a series of very short and fast falls. At the rapids, the river is
shallow and flowing very quickly over rocks, boulders and stones sticking out
above the water level.
Rapids are formed when a river flows through an area of alternating bands
of resistant and less resistant rocks.The less resistant rocks are eroded more
quickly, resulting in the more resistant rocks are at a higher level compared
to the less resistant rocks.Therefore, this results in the river falling in a series
of steps along the bands of resistant rocks to form rapids (see Figure 166
below). A series of rapids is known as a cataract.
Resistant rock
Less resistant rock
Resistant rock
Less resistant rock
Figure 166: Formation of rapids
(Source: http://www.angelfire.com/hero/gerald_koh_s9029362a/rapids.htm)
Activity
1
Identifying waterfalls and rapids on a topographic map
You will need a topographic map for this activity.
1. Look in the key of the map. What symbols have been used to represent
waterfalls and rapids on the map?
2. Identify the features on the map.
3. Present your work to the class for discussion.
Activity
2
Matching letters on a diagram with statements
Waterfalls are usually found in the upper course of a river and are linked to
changes in rock type along the course of a river. The diagram below is of a
waterfall on a river.
204
Whinestone
A
F
B
Limestone and shale
Former
position of
waterfall
C
D
E
Figure 167: Features of a waterfall
(Source:https://www.pearsonschoolsandfecolleges.co.uk/.../Chapter1RiversandCo... 01/06/14)
1. Match the letters on the diagram with the statements below.
River flows over more resistant rock
Plunge pool at base of waterfall
Overhang eventually collapses
Softer rocks behind waterfall eroded
Boulders from previous rock fall
Waterfall retreats, forming a gorge
2. Explain how the waterfall retreats back up the valley.
3. Explain how spectacular physical features like waterfalls can be an
advantage to an area. Use a spider diagram to organise and plan your
ideas.
4. Present your work to the class for discussion.
Gorges
A gorge is a narrow valley between hills or mountains, typically with steep
rocky walls and a stream running through it. A gorge and a canyon are just
the same thing. A gorge may be formed as a waterfall retreats upstream,
eroding away the rock at the base of a river valley (see Figure 168); or it
may be caused by rejuvenation, when a river begins to cut downwards into
its channel. The Grand Canyon of the Colorado River in Arizona, USA, is the
world’s largest gorge. It is about 446 km long and 1.6 km deep. In Malawi,
there is Mpatamanga Gorge on Shire River (a few kilometres south of Tedzani
Falls) and the steep Ruo Gorge at Minunu on the Mulanje Massif.
205
Gorge
Waterfall
Figure 168: Gorge
Meanders
Meanders are bends in a river’s course. Meanders form when areas of
alternating pools (deep water) and riffles (shallow water) develop at equally
spaced intervals along a stretch of river (see Figure 169). Water is deeper in
pools, so the river is more efficient when passing over them. Therefore, the
energy and erosive power is increased when passing over these areas. On the
other hand, the river is less efficient when passing over riffles as there is more
friction causing the river to lose energy. This combination of the river gaining
and losing efficiency
Riffle
Straight
Pool
at different intervals
causes the river’s flow
to become uneven,
and maximum flow
Thalweg line
Sinuous
is concentrated on
one side of the river.
Pool
Riffle or cross over
Figure 169: Formation
of a meander
As the water speeds up, turbulence increases in and around pools, causing more
lateral erosion (abrasion and hydraulic action) and deepening of the pools –
river cliff. This leads to the increased amount of eroded material being deposited
on the inside of the next bend where the river loses energy – slip off slope.
Combination of erosion and deposition exaggerates bend until large meanders
are formed (see
KEY
Inside of bend
Land lost to the
Figure 170).
river (eroded)
Outside
of bend
New land gained from
the river (deposited)
Fastest current
Lateral Erosion
Figure 170: Features
of a meander
Deposition
Collapsed section
of river cliff
(Source:http://www.bbc.co.uk/bitesize/higher/geography/physical/hydrosphere revision/3/)
206
Activity
3
Identifying and describing features of a meander
The diagram shows a river meander. Use it to answer questions that follow.
1. What is shown by the arrow on the
diagram?
2. Shade in and label the areas where
you would expect:
a. lateral erosion
A
b. deposition
3. Explain why erosion and deposition
might occur in the areas you have
shaded.
Figure 171: River meander
4. Describe and explain what is likely
to happen at point A on the diagram.
5. A group of students investigating
meanders on a river collected the
Chapter1RiversandCo... 01/06/14)
following information about the
width and depth of a river on a
meander.
a. Use the students’ data to complete Distance Depth
the cross-section of the river shown in from
of river
Figure 172.
left-hand (cm)
riverbank
(m)
Surface of river
0
0.5
100
10
20
1.0
90
30
40
2.0
80
50
60
70
3.0
65
80
90
4.0
45
100
5.0
20
0
1
2
3
4
5
6
Distance from left-hand riverbank(m)
6.0
5
River depth (cm)
(Source:https://www.pearsonschoolsandfecolleges.co.uk/.../
Figure 172: Cross-section of a meander
(Source:https://www.pearsonschoolsandfecolleges.co.uk/.../Chapter1RiversandCo... 01/06/14)
b. Put the correct letter on the cross-section to
show the following features.
6. Present your work to the class for discussion.
207
A
B
C
D
E
Fastest current
Slowest current
Erosion
Deposition
River cliff
Ox-bow lakes
An ox-bow lake is a U-shaped body of water formed when a wide meander from
the main stem of a river is cut off to create a lake. The neck of the meander
becomes narrow and narrower as the outer banks of a meander continue to be
eroded through processes such as hydraulic action
Eventually, the two outer bends meet due to the narrowing of the neck and
the river cuts through the neck of the meander usually during a flood event
when the energy in the river is at its highest. Rather than flowing around
the bend the water now takes its shortest route.
Deposition gradually seals off the old meander bend forming a new straighter
river channel. The old meander bend is left isolated from the main channel
as an ox-bow lake due to deposition (see Figure 173 below). Over time this
feature may fill up with sediment and may gradually dry up (except for periods
of heavy rain). The feature left behind when the water dries up is known as a
meander scar.
There are many ox-bow lakes alongside the Mississippi River, which include
the Reel-foot Lake and Lake Chicot.
(a)
Current
strongest
on outside
of bend
Sediments deposited
on inside of bend
River
breaks
through
narrow
gap when
in flood
Banks liable to erosion
Newer deposits of sediment
Gap between
two arms of
river narrowed
by erosion by
erosion
Rapid
erosion of
banks on
outside of
bends
(c)
(b)
River
still flows
around
meander
(d)
Older deposits of sediment
Strongest current
Old path of river
now dry
Current
along
straighter
path
becomes
dominant
Abandoned
meander or
oxbow lake
Figure 173: Formation of an ox-bow lake
Importance of ox-bow lakes
a. Animal habitats: the still, freshwater in an oxbow lake creates a
significant aquatic habitat for wetland and marshland. Tadpoles
and young frogs, fish, young turtles, some types of snails and certain
208
aquatic plants favor a setting with calmer waters than a moving river
can provide.
b. Water retention: oxbow lakes may behave similar to wetlands in that
they act as a sponge for retaining water that is dispersed slowly during
dry seasons, thereby providing water to plants and animals at a time
when other sources have dried up.
c. Flood control: oxbow lakes also capture floodwaters during rainy
season, slowing the land erosion and property destruction of an
overflowing river.
Please note! Sometimes ox-bow lakes are formed when a river channel is
straightened artificially to improve navigation or to control floods.
Floodplains
A floodplain is the wide, flat area of land on either side of the river in its middle
and lower courses. A flood plain forms through both erosion and deposition.
The river’s lateral erosion as it meanders widens the valley. When the river
overflows, water pours onto its banks and as it drains away, fine material
(alluvium) is deposited. This creates a floodplain. The valley widening process
is illustrated in the diagrams below.
(a)
(b)
(c)
Floodplain
Floodplain
Floodplain
Floodplain
Figure 174: Stream meandering and floodplain development
(Source:http://web.gccaz.edu/~lnewman/gph111/topic_units/fluvial/fluvial2.html 19/01/14)
The Pantanal that extends over Brazil, Bolivia and Paraguay is the world’s
largest floodplain. It covers approximately 168 000 km2. The Lower Shire
Floodplain is notably the largest and most significant floodplain in Malawi.
The following are some of the importance of floodplains:
209
a. They are often the most productive parts of river systems in terms of
agriculture due to deposition of nutrients from upstream.
b. They accommodate floodwaters, thereby making the floods less severe
and delaying the onset of flooding further downstream.
c. They often include wetlands, which have high wildlife value.
Levees
Levees are natural walls of silt along the banks of a river channel, which are
often several metres higher than the flood plain (Figure 175). They provide
a natural protection against flooding. Notable levees are found on the lower
reaches of the Mississippi in the USA and along the Po in Italy. The Hwang
Ho in China also has well developed levees.
During normal conditions the river stays
within its channel
Smaller sediment is carried The largest sediment is
carried further away and
deposited close to the channel
then deposited
as the river starts to close
Layers of sediment cover the
flooplain
This creates LEVEES or
large natural embankments
close to the channel
Figure 175: Formation of levees
Deltas
A delta is a low-lying landform consisting mainly of mud, silt, sand, and gravel
laid down by a river at its mouth. A river collects sediments as it travels along its
course. The flow of the river’s water is slowed when it enters a large, relatively
quiet body of water—such as a bay or a gulf. As a result, the sediments settle
to the bottom at the river’s mouth. Eventually these sediments build up and
extend seaward as a landform (delta) barely above sea level (see Figure 176).
The delta is often cut by river channels called distributaries.
210
1. Deposition of sediments takes place at the mouth of the stream/river
Stre
am
Mouth
flow
Sea
Sediments
2. Over time, layers of sediments raise the seafloor at the mouth
Stre
am
flow
Sea
Sediments
3. A delta is eventually formed
Stre
am
flow
Delta
Sea
Sediments
Figure 176: Formation of a delta
(Source:http://www.thisoldearth.net/Geology_Online1_Subchapters.cfm?Chapter=5&Row=4)
Conditions necessary for the formation of a delta
a. The load carried by the river must be large and heavy enough to be
deposited at the mouth.
b. The river should flow slowly as it enters the sea. This allows suspended
material and other sediments to be deposited.
c. The river should deposit sediments faster than the sea is able to remove
them, so the delta grows outward into the sea.
Types of deltas
Deltas vary in shape, depending on the conditions that created them. The
following are the most common types of deltas:
Arcuate delta
It is fan-shaped with many active, short distributaries taking sediments to
their mouths- e.g. Nile River (Figure 177). The receiving (ambient) waters are
rather shallow and have relatively even wave action arriving perpendicular to
211
the shore with minimal long-shore current. As the sediments exit the many
distributary mouths, the waves push them back, so the coastline is rather
smooth.
Miditerranean Sea
Port
Said
Suez
Canal
Cairo
Nile
River
Suez
Egypt
Figure 177: Arcuate delta
Bird-foot delta
Shaped like a foot of a bird, this delta tends to have one or very few major
distributaries near their mouths (see Figure 178). A broad, shallow shelf
deepens abruptly, so the tributaries grow long and thin like a bird’s toes. The
Mississippi River has this type of delta.
iss
iss
M
Mississippi
pi
ip
Louisiana
Baton Rouge
ve
Ri
r
New Orleans
Gult of Mexico
Figure 178: Bird-foot delta
212
Estuarine delta
This type of delta has a river that empties into a long, narrow estuary that
eventually becomes filled with sediment (inside the coastline) e.g. Seine River
of France. Figure 179 shows an estuarine delta.
SEA
Figure 179: Estuarine delta
Cuspate (tooth-shaped)delta
This usually has one distributary emptying into a flat coastline with regular
wave action hitting it head-on. It is shaped like a tooth (Figure 180) -e.g.,
Tiber River of Italy.
SEA
Figure 180Cuspate delta
Why do people live in deltas?
Deltas have always been attractive settlement sites. Some of the main reasons
why people settle in deltas are the following:
a. Agriculture: The soil in delta areas is generally fertile and easy to
cultivate. The freshwater of the river can be used for irrigation (see
Figure 178).
213
b. Fishing: There are often rich fishing grounds where the sea and river
meet. Deposition of sediments provides nutrients for the growth of
planktons, which attract large shoals of fish. This has set precedence
for fishing communities to naturally migrate towards delta areas.
c. Transportation: Many of the world’s major ports are situated at the
mouths of delta-forming rivers, including Rotterdam harbour in the
Rhine delta of Netherlands and Shanghai in China. The proximity
of transport routes over the water, where the sea and the river are
directly connected with each other, makes such a site ideal for trade
and industry.
d. Mining: Oil and gas reserves are often related to delta regions, for
example in the Niger Delta (Nigeria). Here, the delta sediment has
been deposited on a much older oil-bearing sedimentary basin. The
Mississippi and the Rhine also discharge into oil and gas-bearing deltas.
e. Tourism: Delta areas inspire great interest to many tourists, so
tourism is one of their most important economic uses.
Figure 181 Uses of the delta region: Agriculture
(Source:www.loicz.org/DELTAS - Coastal Vulnerability and Management - of LOICZ)
The Ganges Delta in Asia is one of the most densely populated regions on
earth. Other examples of densely populated deltas are the Fraser delta in
Canada and the Chao Praya in Thailand.
Problems of living in delta areas
a. Flooding: deltas are increasingly becoming vulnerable to flooding due
to sinking of deltas due to a variety of reasons such as the following:
214
i.
ii.
Sediment compaction due to removal of gas, oil and water from
the delta’s underlying sediments.
Upstream trapping of sediments in dams and reservoirs.
iii.
Rising sea level due to melting of glaciers and ice caps due to
global warming.
iv.
Increased frequency of storms and tsunamis.
b. Pollution: Deltas being areas of intensive agriculture, settlements
and industrialization also see large amounts of nutrient inputs
from fertilizers or sewage which reach coastal waters resulting in
eutrophication (depletion of oxygen in water) and pollution related
problems.
c. Ecosystem collapse: Dams, barrages,water diversion into irrigation
canals, and exploitation of groundwater aquifers have reduced the flow
of water and sediments into the delta and this has resulted in increased
loss of estuarine flora and fauna. Construction of ports and coastal
structures has also disrupted ecosystems in delta regions.
Activity
4
Reflecting on important issues in the topic
1. What did you find most interesting or important ideas did you find in
this unit?
2. Formulate a problem or question about your issue for the person sitting
next to you to answer.
3. Write the problem down on a sheet of paper, and hand that piece of
paper to your friend.
4. Once you are handed a problem statement, you should think of a
solution to the problem.
5. Report your work to the class for discussion.
Summary
Rivers create various landforms along their course by means of erosion and
deposition. Upper-course river features include gorges, interlocking spurs,
waterfalls and rapids. Middle-course river features include meanders and
oxbow lakes. Lower-course river features include floodplains, levees and
deltas. Many of these river landforms have supported a wide range of human
activities such as tourism, hydro-electric power generation, agriculture,
transportation, fishing and mining.
215
Glossary
Plunge pool: a depression at the foot of a waterfall excavated by the action
of the falling water
Cataract: a series of river rapids and small waterfalls with only moderate
vertical drop
Riffle: a rocky or shallow part of a stream or river with rough water
Pool: a deep place in a river or stream where the water runs more slowly
Meander scar: a crescent shaped geological feature formed when an ox-bow
lake dries up
Distributary: a stream channel leading water away from a main single
stream channel
Review questions
1. What is the difference between a drainage basin and a watershed?
2. Identify 3 features of the lower course of a river.
3. Explain one disadvantage of the area of the upper course of a river for
human activity.
4. Explain the differences in the size of the material on the river bed in the
upper and lower courses of a river.
5. Using well-labeled diagrams, explain the formation of the following
riverine landforms:
a. waterfalls
b. ox-bow lakes
c. delta
6. What features are associated with waterfalls? Name any two.
References
Bunnett, R. B. (1973). General Geography in Diagrams. England: Longman.
Bunnett R. B. (1984). Physical Geography in Diagrams for Africa. London:
Longman Group Limited.
Leong, G. C. (1983). Certificate Physical and Human Geography. Oxford:
Oxford University Press.
http://www.geocaching.com/geocache/GC2MAZT_hilton-falls-waterfallearthcache 16/01/14
216
http://www.acegeography.com/landforms-resulting-from-erosion-anddeposition---middle-course.html
http://www.thisoldearth.net/Geology_Online1_Subchapters.
cfm?Chapter=5&Row=4
http://www.angelfire.com/hero/gerald_koh_s9029362a/rapids.htm
https://www.pearsonschoolsandfecolleges.co.uk/.../Chapter1RiversandCo...
01/06/14
http://www.thestudentroom.co.uk/showthread.php?t=869146
http://www.bbc.co.uk/bitesize/higher/geography/physical/hydrosphere/
revision/3/
http://geoquest-nsew.blogspot.com/2011/12/oxbow-oxbow-lake.html 07/01/14
http://web.gccaz.edu/~lnewman/gph111/topic_units/fluvial/fluvial2.html
19/01/14
http://www.coolgeography.co.uk/GCSE/AQA/Water%20on%20the%20Land/
Meanders/Landforms%20Meanders.htm
http://geography.howstuffworks.com/terms-and-associations/delta.htm
20/01/14
www.loicz.org/DELTAS - Coastal Vulnerability and Management - of LOICZ
217
218
Coastal
landforms
Unit
14
Coastal Landforms
A coast is a broad area of land that borders the sea
or lake. Erosion and deposition constantly shape
this area of land into various spectacular features.
Activity
1
The coastal environment
basic
knowledge
and
of the world is made Sharing
up of a wide variety of anticipations
landforms manifested in
a spectrum of sizes and
shapes. The coast and Instructions
its adjacent areas on and
1. Write down what you already know and
off shore is an important
what you want to know about coastal
part of a local ecosystem.
landforms.
Coasts also face many
2. Now, get into groups and share your ideas
environmental challenges
and anticipations.
relating
to
humaninduced impacts.
In
3. Draw a table like the one below on a chart.
unit 4, you learned about
how to identify various
Know
Want to know
Learned
features of the coast
on topographic maps.
Learning about coastal
landforms is really a root
to understanding the
physical processes that
4. Write down what you already know in the
create these features and
first column and what you want to know
their impact on life. In
in the middle column. You will fill in what
this unit, you will learn
you will have learned in the third column
about how these coastal
at the end of the unit.
landforms are formed.
5. Display the chart in front of the class for
You will also learn about
reference.
the importance of coastal
landforms.
219
Headlands
A headland is a piece of land that juts into the sea from the mainland coastline.
Headlands are shaped by erosion. They are formed when the sea attacks a
section of the coast consisting of alternating bands of hard and soft rock. The
bands of soft rock such as sand and clay erode more quickly than those of more
resistant hard rock such as chalk. The resistant rock would form a headland
(see Figure 182). A headland of large size is called a cape. Some famous
capes around the world are the Cape of Good Hope, south of Cape Town in
South Africa; Cape Morris Jessup in Greenland; Cape Cod in south-eastern
Massachusetts; and Cape Maclear, a spectacular sand beach in Mangochi,
Malawi.
Soft rock
Hard rock
Soft rock
SEA
Headland
Bay
Bay
SEA
Figure 182:Formation of headland and bay
Bays
A bay is a body of water that is partly enclosed by land. Bays are found
between headlands where there are alternating outcrops of resistant rock and
less resistant rock. Waves erode the areas of softer rock more rapidly than the
220
hard rock to form bays. Malawi bays include Koko Bay, Mazinzi bay, Monkey
Bay, Nkhata Bay, Nkope Bay and Senga Bay.
Lagoons
A lagoon is shallow coastal body of water, which is partly or completely
separated from the lake or sea by a narrow stretch of land, usually a sandbar.
When long shore drift occurs on a coastline, across the entrance of a bay it
creates a bar of sand or spit, which may eventually seal off the entrance to
form a lagoon (see Figure 183). Lagoons are shallow due to the gentle slope
of the coast. They are sensitive to changes in sea level. A relative drop in sea
level may leave a lagoon largely dry. Lagoons can also be fragile ecosystems
susceptible to pollution effects from municipal, industrial and agricultural
runoff. Some of the most famous lagoons in the world include Ria de Aveiro Lagoon in
Portugal, Dean’s Blue Hole in Bahamas and Marovo Lagoon in New Georgia.
Chia Lagoon in Nkhotakota is the most spectacular lagoon in Malawi.
Spit
Lagoon
Longshore drift
Sand bar
Longshore drift
Figure 183: A lagoon
Spit
A spit is a long, narrow stretch of pebbles and sand which is attached to
the land at one end, with the other end tapering into the sea. It forms when
longshore drift occurs on a coastline, across the entrance of a bay. Longshore
drift is the slow movement of sand material along the coast, parallel to the
shoreline. The drift’s sediment is then deposited where the water deepens;
forming a spit. Figure 184 illustrates how a spit is formed.
221
Longshore drift
Sand bar
builds up
Sand
As deposition of
sand continues,
the sand bar
grows into a spit
Spit
Figure 184: Formation of a spit
The longest spit in the world is the Arabat Spit in the Sea of Azov in Ukraine
and Crimea, which is approximately 110 km long. Sungu Spit is the most
famous in Malawi.
Island
An island is any comparatively small body of land surrounded by water. There
are many types of islands but the following are more prominent:
a. Continental islands are formed as the earth’s shifting continents break
apart. The continental islands still sit on the continental shelf. When
the breakup occurs, some large chunks of land split. These fragments
of land become islands. Greenland the largest island, Australia, and
Madagascar are these types of continental islands. Other continental
islands are formed because of changes in sea level. The water levels
gradually rise due to a rise in temperatures and melting of glaciers.
The ocean floods many low-lying areas, creating islands such as the
British Isles, which were once part of mainland Europe.
b. Volcanic islands, also known as oceanic islands are formed by
volcanic activity on the seabed, often near the boundaries of the tectonic
plates that form the earth’s crust. Where two plates pull apart, lava
erupts to form an undersea ridge. Layers of lava build up until a ridge
breaks the sea’s surface to form an island (see Figure 185).
Volcanic islands often occur in groups or chains. A chain of islands that
222
are close to one another is called archipelago. The Aleutian Islands
of Alaska (shown in Figure 186) and the Hawaiian Islands are both
archipelagos.
Figure 185: Formation of volcanic island
Figure 186: The Aleutian Archipelago
c. Coral islands are low islands formed in warm waters by tiny sea
animals called corals. Corals build up hard external skeletons of calcium
carbonate or limestone. Colonies of corals may form huge reefs. Some
coral reefs may grow up in thick layers from the seafloor, until they
break the water’s surface, creating coral islands. Other organic and
inorganic material, like rock and sand, helps create coral islands. The
223
islands of the Bahamas, in the Atlantic Ocean and Caribbean Sea, are
coral islands.
d. Barrier islands are narrow and lie parallel to coastlines. Some are a
part of the continental shelf (continental islands) and made of sediment—
sand, silt, and gravel. They are called barrier islands because they act
as barriers between the ocean and the mainland. They protect the coast
from being directly battered by storm waves and winds.
There are countless islands in the ocean, lakes, and rivers around the world.
In Malawi, the most well-known islands are Likoma and Chizumulu of Lake
Malawi.
Tombolo
A tombolo is a sand bar that connects an island to the mainland. Once attached,
the island is then known as a tied island.Tombolos are often formed where
a spit continues to grow by long shore drift, joining land to an offshore island
(see Figure 187 below). A well
known example of a tombolo is
Chesil Beach in Dorset, South
Offshore island
West England.
Spit
Figure 187: Formation of a tombolo
Longshore drift
Tombolo
Peninsula
A peninsula is a strip of land
Longshore drift
that is almost surrounded by
water and connected to a larger landmass by a narrow strip of land called
isthmus. The term “peninsula” is from the Latin words for “almost island.”
Most commonly, peninsulas are formed through a gradual rise in water level,
Low-lying land
due to increased temperatures
and typically where the land is at
a low elevation. Gradual rise in
the water level leads the land to
be surrounded by water on three
Isthmus
sides, and develop into a peninsula
(see Figure 188).
Peninsula
The Arabian Peninsula is the
Rise in sea level
world’s largest. Most of the state of
Figure 188: Formation of a peninsula
224
Florida in the southeastern United States, bordering the Atlantic Ocean and
the Gulf of Mexico, is also a peninsula, just as most of the country of Italy is. In
Malawi, there is Luromo Peninsula at Chilumba, in Karonga and Nankumba
Peninsula near Cape Maclear in Mangochi.
Caves, arches, stacks and stumps
Arches and sea stacks are coastal rock formations that are created by the action
of waves breaking against a headland cliff. The ocean waves differentially
erode the headland (in other words, the erosion is irregular rather than
uniform) because sea cliffs are made of various types of rock. Over time, the
crashing of the water can erode the rock so much that it creates holes in the
rock; these holes can eventually develop into caves. If water breaks through
the cave, it can eventually form an arch, around which the water rushes.
As the roof of the arch is continually undercut, it may fall, leaving behind
columns of rock not attached to the cliff, known as stacks. Continued erosion
and weathering of stacks will lead to the formation of a stump that is visible
only at low tide. Figure 189 below illustrates how caves, arches, stacks and
stumps are formed.
1.
2.
Cave widened and deepened
by erosion to form an ARCH
Weak areas are attacked
by waves and opened
to form a CAVE (due to
crosion)
4.
3.
As the roof of the arch is continually
undercut it envetually collapses
leaving an isolated STACK
Headland Retreating
Stack is continually croded
eventually forming a STUMP
Figure 189: Caves, arches, stacks and stumps
(Source: http://www.worldlywise.pbworks.com 13/12/13)
Fjords
Fjord is a Norwegian word that means “long arms of the sea”. Basically, fjords
are valleys that were carved out by glaciers long ago and then flooded by
water (Figure 190). They are found in abundance along the Norwegian coast.
225
Fjords
Figure 190: Fjords
(Source: http://www.ikonet.com/en/visualdictionary/static/us/the_shoreline 25/01/14)
Strait
A strait is a narrow body of water that connects two larger bodies of water.
A fracture in an isthmus due to tectonic shifts can lead to the formation of
a strait. The Strait of Gibraltar (Figure 191), the only link between the
Mediterranean Sea and the Atlantic Ocean, is one strait that was formed by
tectonic activity. If fractures in an isthmus are created by human activity,
the straits are usually called canals, e.g. the Suez Canal. A strait can also
be formed by a body of water overflowing land that has subsided or has been
eroded. The Bosporus, which links the Black Sea and the Aegean Sea, was
formed this way.
STRAIGHT OF GIBRALTAR
EUROPE
MEDITERRANEAN
Spain
Algeciras
it
a
Str
UK
braltar
of Gi
7m
Tangier
Ksar es
Sehir
Morocco
AFRICA
7 km
Mediterranean
Sea
Atlantic
Ocean
NORTH AFRICA
EUROPE
Ceuta
Spain
Black Sea
ASIA
Atlantic
Ocean
Canary Morocco
Islands
(Sp.)
Tunisia
Mediterranean Sea
Algeria
Libya
Westen
Sahara
(Mor.)
WEST AFRICA
300 Miles
0
300 Kilometer
CENTRAL
AFRICA
Egypt
Red
Sea
EAST AFRICA
Figure 191: Strait of Gibraltar
Estuaries
An estuary is a body of water at the mouth of a river where freshwater from
the river mixes with saltwater from the sea (see Figure 192). Estuaries form
226
a transition zone between river environments and ocean environments and
are subject to both marine influences, such as tides, waves, and the influx
of saline water; and riverine influences, such as flows of fresh water and
sediment. The inflow of both seawater and freshwater provides high levels of
nutrients in both the water column and sediment, making estuaries be among
the most productive natural habitats in the world. Examples of estuaries
include bays, sounds, salt marshes, mangrove forests, mud flats, swamps,
inlets, and sloughs.
Estuary
SEA
Figure 192: An estuary
Estuaries are full of decaying plants and animals. This makes the soil of
estuaries rich in nutrients. Because the soil is so rich, many different plants
grow in estuaries. The plants attract many different animals to the estuary
and those animals attract other animals to the estuary. Common animals
include shore and sea birds, fish, crabs, lobsters, clams and other shellfish,
marine worms, raccoons, opossums, skunks and many reptiles.
Activity
2
Identifying
and discussing
coastal features
1. Study Figure
190 and use it
to answer the
questions that
follow.
River estuary
Lagoon
J
K
J
B
A
C
Figure 193: Coastal features
I
E
F
D
G
(Source:http://www.ikonet.com/en/visualdictionary/earth/geology/commoncoastal-features/common-coastal-features.php 25/01/14)
227
a. Name the features A, B, C, D, E, F, G, H, I, J and K.
b. Describe how feature F has been formed.
2. How do you think each of the coastal landforms may affect people’s
lives?
3. How may human activities be a threat to the landscape and wildlife in
the coastal area?
4. Report your work to the class for discussion.
The importance of coastal landforms
a. Tourism: Tourism activities thrive on the beautiful scenery of coastal
landforms and the great attractions they provide. Millions of people
visit coastal landforms each year to boat, swim, watch birds and other
wildlife, and fish.
b. Transportation: The coastal landforms support important public
infrastructure, serving as harbours and ports vital for shipping,
transportation, and industry. Many of the products we use every day
pass through one or more coastal landforms on a commercial shipping
vessel before ever reaching our home.
c. Fishing: Coastal landforms such as estuaries, bays and lagoons are
important habitats for fish and ecosystems.
d. Settlement: Favourable biophysical and climatic conditions, together
with the ease of communication and navigation frequently offered
by coastal landforms have encouraged human settlement in coastal
landforms since prehistoric times.
e. Wildlife habitats: Coastal landforms provide critical habitat for
species that are valued commercially, recreationally, and culturally.
Birds, fish, amphibians, insects, and other wildlife depend on these
landforms to live, feed, nest, and reproduce.
f. Electricity generation: Some coastal landforms such as lagoons and
estuaries are dammed to generate tidal energy.
Activity
3
Case study
A river estuary in a coastal community (shown in Figure 194) was once a
striving fishing harbour. Pollution and overfishing have resulted in recent
decline in fish stocks and little work for most of the fishers. The town council
wants to revive the fortunes of the area. Council members are holding talks
228
with the directors of a leisure company who want to build a holiday camp and
recreation centre. Fearing that it would change the nature of the coastal area
for the worse, some council members are greatly opposed to the scheme.
Figure 194: Coastal area
1. In groups of five, present a case for building a camp and recreation
centre. In your proposals you should:
a. suggest a site for the centre.
b. locate routes for the access roads leading to the centre.
c. describe how this coastline can be used for leisure and recreation.
d. identify any parts of the coast which you would expect to be protected
by developers.
2. Look critically at your proposals and put the case for the opponents of
the scheme. Make detailed reference to the expected damaging impact
on specific sites which you think are worthy of conservation.
3. Present your work to the class for discussion.
Source: Pallister, J.et. al. (2001). Longman Geography for GCSE: New Edition
Activity
4
Reflecting on the topic
1. What new things have you learned in this unit?
2. Why is it important that you have learned these things?
3. Return to the chart you prepared at the beginning.
4. Do you think what you thought you knew was accurate?
5. What questions do you have about what you have learned?
6. Report your findings and questions to the class for discussion.
229
Summary
The extent to which the shape of a beach or coast is altered depends largely
on the action of waves upon it. Along the coastline there are features created
by erosion. These include cliffs, headlands and bays, caves, arches, stacks and
stumps. Along a coastline you will also find features created by deposition.
These include beaches, spits and tombolos. These coastal features are
important for tourism, transportation, fishing, settlement, wildlife habitat,
and electricity generation.
Glossary
Longshore drift: the gradual movement of material along a coast caused by
the action of waves
Archipelago: a group or chain of islands close to one another
Tied island: an island connected to the mainland by a tombolo
Isthmus: a narrow strip of land that joins two larger areas of land
Review questions
1. The following diagram shows a depositional feature on a constructive
coastline. Use it to answer the questions that follow.
Figure 195: Coastal features (Source:https://www.pearsonschoolsandfecolleges.co.uk/.../
Chapter1RiversandCo... 01/06/14)
a. Name the feature labeled A shown on the diagram.
b. Complete the diagram by putting the following terms in the correct
boxes:
230
i. longshore drift
ii. mudflats (muddy beach)
iii. ocean current
iv. recurved end
v. salt marsh
vi. tidal lagoon.
c. Explain how feature A was formed. Use diagrams if you wish.
2. With the aid of well-labelled diagrams, explain the various stages and
processes involved in the formation of a stack.
3. Refer to the diagram below showing an irregular coastline and answer
the questions that follow.
Wind
Bay
Headland
Bay
N
W
E
S
Headland
Figure 196: Coastal features
a. Based on the wind direction, in which direction are the longshore
currents likely to travel along the coast?
b. Name the feature formed when longshore drift carries sediment
into the open water of the bays.
References
Bowen, A. D. et al. (1997). Map Reading for Southern Africa. Cape Town:
Maskew Miller Longman.
Bunnett, R. B. (1973). General Geography in Diagrams. London: Longman.
231
Kalaluka, L. (1978). Map Reading for Central Africa. London: Longman.
Pallister, J. et. al. (2001). Longman Geography for GCSE: New Edition
http://media.tiscali.co.uk/images/feeds/hutchinson/ency/c00575.jpg 21/01/14
http://faculty.scf.edu/rizkf/OCE1001/OCEnotes/chap10.htm 20/01/14
http://www.geocaching.com/geocache/GC29XRY_mierzeja-wislana-frischenehrung-vistula-spit?guid=1a67492b-7cba-4b94-9d1d-a20f39d13279
http://gizzardstone.com/submarine-volcanic-island-formation/ 21/01/14
http://www.navy.mil/navydata/cno/n87/usw/issue_18/forgotten.htm 13/12/13
http://education.nationalgeographic.com/education/encyclopedia/island/?ar_
a=1
http://img.geocaching.com/cache/log/91616715-cc37-4fa0-910e-4a35f6ddff0a.
jpg 21/01/14
http://www.answers.com/topic/how-are-arches-and-sea-stacks-formed
13/12/13
http://www.worldlywise.pbworks.com 13/12/13
http://www.ikonet.com/en/visualdictionary/static/us/the_shoreline 25/01/14
http://education.nationalgeographic.com/education/encyclopedia/strait/?ar_
a=1 13/12/13
http://ocyaniqueprofessionals.blogspot.com/2012/06/strait-of-gibraltar.html
13/12/13
http://www.ikonet.com/en/visualdictionary/earth/geology/common-coastalfeatures/common-coastal-features.php 25/01/14
http://ian.umces.edu/imagelibrary/displayimage-6882.html
https://www.pearsonschoolsandfecolleges.co.uk/.../Chapter1RiversandCo...
01/06/14
http://home.comcast.net/~rhaberlin/csquiz.htm 10/02/14
232
Relief features of
the ocean basins
Unit
15
The
ocean’s
surface
has been a forbidding
boundary
for
most
of
human
history,
separating the known
from the unknown. People
had no idea what lay
beneath the waves except
for the tiny amount of
the ocean floor visible in
shallow water. Today, we
have the ability to gather
detailed
information
about the ocean floor. This
knowledge is critical for
understanding the ways
in which the different
features of the ocean
basin may impact ocean
circulation. In this unit,
you will explain relief
features of the ocean
basins and the terms
ocean currents, drift and
streams. You will then
identify major ocean
currents of the world,
explain their causes and
factors that influence
their direction. Finally,
you will suggest the
effects of ocean currents.
Relief features of ocean basins
An ocean basin is a depression of the earth’s surface
in which an ocean lies. Over 70 percent of earth’s
surface is covered by a single, interconnected body
of water that is somewhat arbitrarily divided into
four ocean basins, the Atlantic, Pacific, Indian,
and Arctic. These ocean basins differ from each
other in many respects. Yet, they all contain
certain common features such as continental
shelf, continental slopes, continental rise, abyssal
plains, oceanic ridges, trenches, seamounts and
guyots (see Figure 197 below).
Continental
rise
Abyssal
plain
Continental
shelf
Continental
slope
Shoreline
Coastal
plain
Submarine
gorge
Figure 197: Relief features of the ocean basin
(Source:http://ocean.fsu.edu/courses/sp04earthsys/AA/depenv/depenv.html)
Continental shelf
A continental shelf is the shallow, relatively
flat and submerged part of the continent that
stretches from the coastline towards the ocean
basin. Continental shelves are important grounds
for;
a.Fishing: The shallow waters allow sunlight
to reach the ocean floor, enabling growth of
planktons. Rivers discharge in this zone bring
in a lot of nutrients required by fish. These
233
attract large shoals of fish.
b. Petroleum drilling: Almost all oil and gas is found in deep underground
reservoirs on land and in the seabed (on the continental shelf). Continental
shelves account for much of the offshore oil drilling that take place around
the world.
Continental slope
The seafloor drops away suddenly at the edge of the continental shelf to form
the continental slope. As you can see, this is formed of land-derived sediment
which has piled up at the foot of the continental crust. Its angle of slope rarely
exceeds ten degrees, and is more typically around four degrees. The slope
here is much steeper than on the shelf (usually around 30º), and is the site
of submarine landslides, fast moving currents and sediment slumps. The
continental slope is often cut by massive underwater canyons and gorges,
created by currents carrying sediment from the continental slope down to the
deep sea.
Continental rise
This has an average inclination of about half a degree from vertical, and
flattens out into the sea. Here, sediment which has moved down from the
continental shelf piles up at the base of the slope.
Abyssal plain
The abyssal plains are the deepest regions of the ocean basins (with the
exception of subduction trenches), and they form vast expanses of flat, cold,
dark terrain.
Please note! The shelf, slope, and rise are known collectively as the continental
margin.
Oceanic ridges
An oceanic ridge is a narrow, largely continuous range of underwater
mountains found in all major oceans. It is created by the rise of magma from
the earth’s interior. Typically, an oceanic ridge has a valley known as a rift
running along its spine. The mid-ocean ridges of the world are connected and
form a single global mid-oceanic
Rift
Ocean
Mid-ocean ridge
ridge system that is part of
every ocean, making the midoceanic ridge system the longest
mountain range in the world.
Rising magma
Figure 198: Oceanic ridge
(Source: http://www.kidsgen.com/school_projects/broken_earth.htm)
234
Trenches
A trench is a long, narrow valley on the ocean floor, most often found adjacent
to a continental margin. Trenches form at sites where one lithospheric plate
is forced beneath another, or subducted, as a result of seafloor spreading
elsewhere. Figure 196 shows a trench. Trenches occur much more commonly
in the Pacific than in any of the other oceans. The Marianas Trench, which
runs from the coast of Japan south and then west toward the Philippine
Islands, is the deepest trench on Earth. Its deepest spot is 11,022 m below sea
level and it runs a distance of about 2,550 km. The longest trench is located
along the coast of Peru and Chile. Its total length is 5,900 km and it has a
maximum depth of 8,055 m.
Friction between the two converging plates is responsible for the earthquakes
and volcanic activity commonly associated with trenches.
Japanese Islands (island arc)
Japan Trench
Eurasian Plate
Pacific Plate
Figure 199: Oceanic trench
(Source:http://bc.outcrop.org/images/tectonics/press4e/figure-02-09a.jpg26/01/14)
Seamounts and guyots
A seamount is an isolated undersea mountain of volcanic origin that rises
from the seabed to a height of up to 1,000 m, usually 1,000 m to 2,000 m below
the surface of the sea. A guyot is a flat-topped underwater mountain of a type
commonly found in the Pacific Ocean and considered to be an extinct volcano.
Figure 200 shows seamounts and guyots.
Seamount
Volcanic
island
Erosion
Figure 200: Seamounts and guyots
235
Guyot
Activity
1
Identifying features of the ocean basin on a globe
1. Hold a globe, turn it around and look at the oceans.
2. Are they connected to form one world ocean?
3. Imagine you travel in a submarine from New York to Spain.
4. Trace with your finger and describe your journey.
5. What seafloor features do you encounter?
6. On your next trip, you travel by submarine from Japan to Baja,
California. Again, use your finger to trace your route and describe your
journey, including all the major seafloor features you encounter.
7. How do you think water in the ocean flows?
8. How do you think the different features you found on your routes may
impact ocean circulation and biological processes?
9. Report your work to the class for discussion.
Ocean currents, drifts and streams
An ocean current is a continuous flow of ocean water in a directed and regular
pattern. Currents are like rivers in the oceans. However, while the motion
of rivers can easily be seen, that of oceans and seas is more difficult to see
because the water bodies are extremely large and deep. A wide, slow-moving
ocean current principally caused by winds is known as a drift. The continuous
flow of the current in a specified direction is called a stream. Stream currents
occur where an ocean current flows through a constriction between two land
masses as is the case in the area between Florida and Cuba. The velocity
of the current increases greatly as it leaves the constriction area. The Gulf
Stream is a good example of such current.
Major ocean currents of the world
The circulation of water in the world’s oceans is so complex, with currents
flowing at different depths in different directions. Like the atmosphere, the
general circulation of the oceans follows a specific pattern. Water of varying
characteristics, such as temperature and salinity is exchanged within the
interconnected network of oceans by the ocean circulation pattern.
236
Types of ocean currents based on depth
• Surface currents
These are horizontal circulations of surface water to a depth of about 400
meters from the surface of the ocean. They usually travel over long distances
and make up about 10% of all the water in the ocean.
• Deep currents
The world’s oceans also have significant currents that flow beneath the surface
below 400 meters and make up about 90% of the ocean (Figure 201). Deep
currents generally travel at a much slower speed when compared to surface
flows. Deep ocean currents are caused by differences in water temperature
and salinity.
Surface
Currents
deep
currents
Figure 201: Surface currents and deep currents
Types of ocean currents based on temperature
Currents are classified as either warm or cold currents based on the water
temperatures transferred into a region. Where do you think water in the
ocean would be warmer – near the equator or near the poles? Why?
• Warm currents
A warm current brings warm water into cold water. Warm currents originate
in warm areas (the tropics) and flow to the polar latitude areas. Generally,
they are found along the east cost of most continents. Examples of warm
currents include Gulf Stream, Mozambique, North Atlantic Drift, Brazilian,
Kurosiwo and East Australian (Figure 202).
237
• Cold currents
A cold current brings cold water into warm water. Cold currents operate
a little differently. They come from cold areas in the polar and temperate
latitudes and tend to flow towards the equator. They are found along the west
costs of most continents. Examples are the Peruvian, Labrador, Canaries,
Kamchatka, Benguela, West Wind Drift, East Greenland and West Australian.
Figure 202: Pattern of ocean currents around the world’s oceans
Causes of ocean currents
The primary causes of ocean currents are combined forces of wind and
differences in density of the water.
Prevailing winds
Wind blowing with great persistence across the ocean causes the surface water
to move due to friction. The global winds that blow in different directions
across the earth can influence and create surface currents in the oceans by
setting the surface waters into motion. In the following activity, students will
have the chance to observe how water moves as wind blows across it.
Activity
2
Investigating how wind causes ocean currents
You will need water in a clear rectangular dish, food colouring, a cereal bowl
and a petri dish.
238
1. Carefully fill the clear tray with water. Do not fill it completely to the
top. Let the water settle.
2. Place a drop of food colouring at one end of the tray and gently blow
across the tray. Observe and sketch what you see happening at the
surface of the water and along the bottom of the dish.
a. Are your sketches different from each other? If so, how are they
different?
b. Where do the currents move most rapidly?
c. What happens to the water as it moves away from the wind source?
3. Gently place the cereal bowl upside down in the center of the glass tray.
Make sure that the bowl sticks out of the water. If it does not, lower the
water level in the tray and try again. The bowl represents an island.
4. Add a drop of food colouring in front of the island and gently blow across
the tray.
5. Observe and sketch what happens to the food colouring in front and
back of the island.
a. What effect does the island have on the current?
b. Is the current stronger in front of or behind the island? How can you
tell?
6. Remove the cereal bowl. Change the water if the food colouring added
during step 3 makes it difficult to see additional drops. Add a petri dish
that is completely below the water line. The petri dish represents a
submarine island.
7. Add a drop of food colouring between you and the Submarine Island
and blow across the tray.
8. Observe and sketch what happens to the food colouring. How are these
results different from those obtained for the island in step 3?
9. Repeat the procedure but use objects of irregular shapes.
a. Are the currents more or less complex with the irregular-shaped
objects? Why?
b. Do the currents always move in the direction of the wind? If not,
what factors might influence the direction of movement?
c. How do bottom currents differ from top currents?
10. Report your findings to the class for discussion.
239
Activity
3
Comparing global wind patterns and surface currents
Figure 200 and Figure 201 below show the global wind patterns and surface
currents respectively. Study them carefully and answer the questions that
follow.
60o N
40o
20o
0o
20o
40o
60o S
40o E
60o
80o
100o 120o 140o 160o 180o 160o 140o 120o 100o
80o
60o
Figure 203: Global wind patterns
Figure 204: Global wind-driven surface currents
240
40o 20o W
0o 20o E
1. How do the map of wind patterns and the map of surface currents
compare?
2. Do these patterns look similar to one another? Discuss why or why
not?
3. Report your answers to the class for discussion.
Temperature
Temperature difference causes the density of water to vary in the oceans. Warm
water is less dense than cold water. Equatorial surface waters are warm due
to intense heating, hence their density is low, whereas polar surface waters
are very cold due to permanent low temperatures in the Polar Regions and
their density is high. As a result the heavier cold waters of the Polar Regions
sink and creep slowly along the bottom of the oceans towards the Equator.
The warm equatorial surface waters on the other hand, move on the surface
towards the poles to replace the sinking cold water, thus creating horizontal
movement on the surface and the bottom.
Activity
4
Investigating the effect of temperature on water density
You will need a clear rectangular glass dish, cold water, hot water, ice, food
colouring – red and blue, two strong plastic bags and a stone.
1. Fill the clear dish to half with cold water.
2. Place a stone in a plastic bag, fill the bag with hot water, and drop it to
one corner of the basin. Be sure to exercise proper safety precautions in
handling the hot water.Wear heat resistant gloves.
3. Add a bag of ice water to the opposite side of the basin, and hold it in
place.
4. Place a few drops of blue food colouring by the ice, and a few drops of
red coloring by the rock.
5. Observe what happens to the dyes over the course of several minutes.
6. Write a sentence to summarise your observations about how the cold
dye and warm dye behave in the basin.
7. Report your findings to the class for discussion.
Salinity
The density of water also varies with differences in salinity of water in the
ocean. Waters of high salinity are denser than waters of low salinity. As a
result the heavier and more saline waters sink and flow at the bottom towards
241
waters of low salinity. On the other hand, waters of low salinity flow on the
surface towards the sinking high saline waters, thus forming ocean currents.
Activity
5
Investigating the effect of salinity on water density
You will need salt, two hardboiled eggs, two large beakers filled with cool
water and a spoon.
1. Add a table spoon of salt around 200 g to one beaker and stir the
mixture.
2. Place the hardboiled egg in the freshwater beaker and observe what
happens.
3. Gently place the other egg in the saltwater beaker and watch what
happens.
4. Now, remove the egg from the freshwater beaker and then gently pour
the freshwater on top of the seawater with its egg still in place. Observe
what happens to the egg.
5. Write in your own words how salinity affects density, and how this
affects the world’s oceans.
6. From this experiment, do you think fresh water from nearby landmasses
can affect the salinity in the oceans?
7. How might the influx of fresh water affect the global circulation of
ocean currents?
8. Present your findings to the class
Factors that influence the direction of ocean currents
The direction taken by ocean currents is influenced by:
a. Wind: Prevailing winds such as the Trade Winds push the surface
currents towards the direction which they blow.
b. Rotation of the earth: As the earth turns, it curves the ocean
currents, to the right in northern hemisphere and to the left in southern
hemisphere due to the Coriolis Effect. The Coriolis Effect is a deflection
of moving objects when they are viewed in a rotating reference frame,
such as the earth. Without the spinning of the earth from its rotation,
the ocean currents would move in straight lines towards the poles or
the equator. The curved moving surface water flows in large, almost
enclosed, circulating loops called gyres.
Gyres rotate clockwise in the northern hemisphere and anticlockwise
in the southern hemisphere (Figure 205). Ferrell’s Law states
242
this phenomenon: Any object or fluid moving horizontally in the
Northern Hemisphere tends to be deflected to the right of its path of
motion, regardless of the compass direction of the path. In the Southern
Hemisphere a similar deflection is toward the left of the path of motion.
Clockwise
Gyre
Equator
Anticlockwise
Gyre
Figure 205: Gyres
Activity
6
Discussing Coriolis Effect
A missile is launched from Point 1 and targeted to hit Point 2 in Figure
206 below. However, the Coriolis Effect is not factored into the missile’s
coordinates.
1. Given this oversight, which point would the missile most likely strike?
243
North pole
Equator
A
Point 2
B
C
Point 1
South pole
Figure 206: Coriolis Effect
2. What is the Coriolis Effect attributed to?
3. Present your work to the class for discussion.
c. Shape of continents and underwater topography: Continents act
as natural walls that divert a current. For example, the tip of Southern
Chile in Latin America diverts part of the West Wind Drift northwards
as the Peruvian Current. Similarly, tall features on the ocean floors,
such as oceanic islands and ridges, provide barriers that turn the
currents into certain directions.
Importance of ocean currents
a. Shipping: Circulation of warm currents helps keep ports free of ice in
the cold polar latitudes that would otherwise be frozen during the cold
winters. This makes shipping possible, thereby facilitating the flow
of goods. In addition, large ocean liners and tankers sailing along the
ocean currents speed up travel time and save fuel consumption since
the currents provide an extra force to move the ships.
b. Fishing: Currents help to move food and nutrients, making them
available for photosynthesis, metabolic requirements and/or
consumption for fish. Areas where warm and cold currents meet tend
to have rich deposition of nutrients due to the mixing of the water
244
currents. Some of the world’s major fishing grounds are located in the
regions where warm and cold currents meet, e.g. the West Pacific
fishing ground is where the cold Kamchatka and the warm Kurosiwo
currents meet, the North Western Atlantic fishing ground is due to
the meeting of cold Labrador Current and warm North Atlantic Drift
Current.
Where cold currents prevail, there is upwelling of cold deeper waters,
which uplift dissolved mineral salts (e.g. phosphates and nitrates).
These encourage the growth of planktons, hence attracting large
shoals of fish. This is common in coastline affected by such currents
as Peruvian (Humboldt), Californian, West Australian, Benguela and
Canaries.
c. Climate: Warm currents bring significant warmth and precipitation
to the adjacent landmasses. Sunlight causes warm water to evaporate
more easily than cold water, and thereby produce the atmospheric
vapour that results in rain. Therefore, lands that are affected by warm
currents tend to support a wide range of agricultural activities since
they have abundant rainfall and a comparatively warm climate.
In contrast, lands impacted by cold currents tend to experience a
comparatively cool and dry climate. The cool air brought to the adjacent
landmasses by the cold currents is dense, and therefore does not rise
to cause rainfall. This has eventually developed desert conditions along
the margins of the landmasses, e.g. Atacama Desert due to the Peruvian
Cold Current, Mohave Desert due to the Californian Cold Current and
the Namib Desert due to the cold Benguela Current.
d. Alternative energy: Ocean currents are also gaining significance as
a possible form of alternative energy. The currents carry an enormous
amount of energy that is captured and converted into a usable form
through the use of water turbines.
What happens when circulation of ocean currents is disturbed?
Over the past several decades, it has been observed that some phenomena
disrupt the normal pattern of ocean currents, resulting in undesirable
consequences. For example;
a.Global Warming: As the Earth continues to warm and Arctic sea ice melts,
the influx of freshwater from the melting ice is making seawater at high
latitudes less salty and hence less dense. This less dense water will not be
able to sink and circulate through the deep ocean as it does normally. The
disruption of the flow of ocean currents may have serious consequences on
marine life, shipping and fishing.
245
b.El Nino: This is the unusual warming of surface waters in the eastern
tropical Pacific Ocean. In order to understand the development of El Niño,
it is important to be familiar with non-El Niño conditions in the Pacific
Ocean. Normally, strong trade winds blow westward across the tropical
Pacific. These winds push warm surface water towards the western Pacific,
causing it to pile up in the western part of the ocean. In the process, cooler
waters from below rise up towards the surface on the coasts of Ecuador,
Peru, and Chile to replace the sun-warmed surface water pushed to the
west (Figure 204). This process is known as upwelling.
During an El Niño event, the winds pushing the warm water towards the west
get weaker. The weakening of westward-blowing trade winds causes some of
the water piled up in the west to slump back down to the east, repressing
the cold Peruvian current (see Figure 207). Basically therefore an area which
would have normally been affected by a cold current is now all under the
influence of a warm current. El Niño is Spanish for “the little boy” and it
refers to infant Jesus. It was named by Mexican fishermen who noticed that
the climate pattern formed around Christmas time almost every five years.
Figure 207: Normal situation (La Niña)
246
Figure 208: During El Niño
Effects of El Nino
El Niño has both positive and negative effects on the surrounding regions and
wildlife;
Positive effects:
a. The warm water in an El Niño evaporates easily bringing more rainfall,
and this is important for agriculture.
b. El Nino disrupts the development of hurricanes on the Atlantic Ocean
due to the change in the airflow over the ocean.
Negative effects:
a. The encroaching warm water along the Peruvian coast during an El
Niño displaces the nutrient-rich north-flowing cold ocean current,
disrupting food chain of fish, birds, and sea mammals. This wreaks
havoc on the Peruvian economy.
247
c. El Niño is often accompanied by copious rains, which are uncommon in
this part of the world, leading to floods and landslides.
d. Elsewhere, over Northern Australia, Indonesia and the Philippines, El
Niño conditions are associated with drought, which results in increased
risk of wild fires.
e. The unusual warm weather results in more tornadoes and thunderstorms
in southern USA.
Activity
7
Demonstrating the El Niño effect, trade winds, and
upwelling
1. Fill the tray with water to within 1 inch of the top.
2. Add blue food colouring to the water until a nice “ocean blue”. (Some
of the food colouring will settle to the bottom which is fine because this
will show the upwelling.)
3. Gently pour the oil over the surface of the water. The oil will separate
out and float on the surface of the water. The liquids in the plastic
container represent a slice across the Pacific Ocean in the vicinity of
the equator. The oil represents the warm layer of surface water that
has been heated by the sun. The blue water represents the colder water
below the surface warm layer.
4. Put the container on the paper and mark East and West at either end,
Indonesia and South America.
5. Plug in the hair dryer or fan, being careful to keep it away from any
water spills. The hairdryer or fan represents the trade winds. Turn it
on and direct the ‘wind’ across the surface of the oil-topped water from
the East to the West, as shown below.
Clear plastic
container
Oil
Water
Figure 209: El Niño experiment
248
6. Describe what effect this has on the the “warm” and “cold” water.
7. What do you notice about the sediment of the blue food dye?
8. What process does this represent in the Pacific Ocean and how is it
important?
9. Now, turn off the fan (trade winds) and describe what happens when
the winds stop.
10. Report your findings to the class for discussion.
Summary
Oceanic currents are found all over the globe and are caused by wind,
temperature and salinity differences. Ocean currents vary in size, importance,
strength and in type. They can be either surface or deep water. Ocean currents
are either warm or cold based on where they originate from. Warm currents
originate from the hot equatorial waters while cold currents originate from
the cold polar oceans. Some of the more prominent warm currents include
the Gulf Stream, Mozambique, North Atlantic Drift, Brazilian, Kurosiwo and
East Australian. Examples of cold currents include the Peruvian, Labrador,
Canaries, Kamchatka, Benguela, West Wind Drift, East Greenland and West
Australian. Ocean currents are important to the world’s weather, navigation
and the distribution of the world’s sea life.
Glossary
Rift: a linear zone where the earth’s crust is being pulled apart
Seamount: an isolated undersea mountain of volcanic origin
Guyot: an isolated underwater volcanic mountain (seamount), with a flat top
Drift: a wide, slow-moving ocean current principally caused by winds
Stream: the continuous flow of the current in a specified direction
Coriolis Effect: a deflection of moving objects when they are viewed in a
rotating reference frame, such as the earth
Gyre: a large system of spinning ocean currents, generally those involved
with large movements of wind
Review questions
1. Name any three features of the ocean floor.
249
2. What is the difference between a continental shelf and a continental
slope?
3. How are mid-ocean ridges and trenches similar, and how do they differ?
4. Why do you think some ocean currents are warm while others are cold?
5. Explain any two beneficial effects of ocean currents on human activities.
6. With specific examples, describe how ocean currents lead to the
formation of deserts.
7. Explain any three effects of El Niño.
8. Why is the Coast of Peru one of the most productive fishing areas in the
world?
References
Bradberry, J. (1985). Introducing Earth Science: A Practical Approach to
Geology. Oxford: Basil Blackwell Limited.
Bunnett, R. B. (1973). General Geography in Diagrams. London: Longman.
Bunnett R. B. (1984). Physical Geography in Diagrams for Africa. London:
Longman Group Limited.
Waugh, D. (1990). Geography: An Integrated Approach. Hong Kong: Thomas
Nelson Limited.
White, R. (1998). Africa in Focus: A Physical, Human and Economic Geography.
Oxford: MacMillan Education Limited.
http://wps.prenhall.com/esm_mcknight_physgeo_9/84/21603/5530553.cw/
content/index.html 02/06/14
http://ocean.fsu.edu/courses/sp04earthsys/AA/depenv/depenv.html
http://www.classroomatsea.net/general_science/ocean_basins.html 13/12/13
http://www.kidsgen.com/school_projects/broken_earth.htm
http://bc.outcrop.org/images/tectonics/press4e/figure-02-09a.jpg 26/01/14
http://er.jsc.nasa.gov/seh/Ocean_Planet/activities/ts2siac1.pdf 02/06/14
http://er.jsc.nasa.gov/seh/Ocean_Planet/activities/ts2siac1.pdf 02/06/14
http://wps.prenhall.com/esm_mcknight_physgeo_9/84/21603/5530553.cw/
content/index.html 02/06/14
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World pressure
belts
Unit
16
An important characteristic
of
the
earth’s atmosphere is
its pressure as it often
determines wind and
weather patterns across
the globe. By learning
about world pressure
belts, you will be able to
understand the earth’s
circulation patterns and
predict weather for use
in daily life, navigation,
shipping,
and
other
important activities. In
this unit, you will explain
the term air pressure and
factors that influence it.
You will also locate the
main air pressure belts
on world maps. Finally,
you will account for the
distribution of pressure
belts in the world.
Air pressure
Air pressure, sometimes also called barometric or
atmospheric pressure, is the weight of air in the
atmosphere pressing down on the earth surface at
any given place and time. Air has weight though
it is invisible. The force of gravity continuously
acts upon it.
Air pressure is measured in Pascal (Pa), but this
is usually given in millibars (mb). One millibar
is equal to 100 Pascal. Lines called isobars join
places with equal atmospheric pressure on a map.
Activity
1
Investigating the weight of air
For this activity, you will need a deflated
basketball, a pump, and a balance that measures
in grams.
1. Place the deflated ball on the balance to get
the initial weight.
2. Now, inflate the ball and weigh it as shown
below.
3.
4.
5.
6.
7.
8.
Figure 210: The weight of air
(Source: http://www.middleschoolchemistry.com/lessonplans/
chapter1/lesson5 29/01/14)
251
9. Did the ball weigh more, less or the same after you pumped air into it?
Why?
10. Report your findings to the class for discussion.
Measuring air pressure
A barometer measures air pressure. There are two types of barometers;
Mercury barometer and Aneroid barometer.
Mercury barometer
A mercury barometer measures the pressure by noting the height of mercury
in a glass tube, which is supported by the weight of the column of air over the
barometer. One centimetre of mercury is equal to 13.33 mb. At sea level, the
normal atmospheric pressure can support a column of mercury about 76 cm
high, and this is equal to 1013 mb.
A mercury barometer has a section of mercury exposed to the atmosphere.
The atmosphere pushes downward on the mercury (see Figure 211). If there
is an increase in pressure, it forces the mercury to rise inside the glass tube
and a higher measurement is shown.
If atmospheric pressure drops, downward force on the mercury lessens
and the height of the mercury inside the tube lowers. In this case, a lower
measurement would be shown. The mercury barometer is one of the most
accurate instruments used in weather stations but it is difficult to move since
it has separate parts.
Vacuum
Glass tube
76 cm
Air pressure
Mercury
Figure 211: Simple mercury barometer
252
Aneroid barometer
An aneroid barometer is a collapsible metal box that has been sealed after
removing some of its air (a partial vacuum). The metal box is mechanically
connected through a lever system to a dial needle, which points to a scale
indicating pressure readings.
A higher atmospheric pressure will squeeze the metal box, and this will in
turn pull the dial needle to the right to show high measurement. However,
lower pressure will allow the metal box to expand, causing the needle to swing
to the left to show a low measurement (Figure 212). The aneroid barometer
is the most widely used because it is more compact and portable, though less
accurate than the mercury barometer.
Dial
29
LOW
HIGH
28
30
Chain
31
Spring
Levers
Collapsible Box
Figure 212: Aneroid barometer
Activity
2
Determining atmospheric pressure
1. Fill a clear plastic cup half full with drinkable water.
2. Put a straw straight down into the cup of water.
3. Discuss what is happening with atmospheric pressure on the cup, the
straw and the water.
4. In small groups discuss ways you can make the environment unstable
to get the water to move up the straw; you should record your ideas on
paper.
253
5. Once you have an idea or two written down, you should test your ideas
to see what the results will be.
6. Discuss with your groups the results of the experiments you just
completed.
7. On your paper, write a results summary explaining what happened
and why you think it happened.
8. Report your results summary to the class for discussion.
Factors that affect air pressure
Air pressure is not the same all over the planet. It varies from place to place
and from time to time according to its relationship with several environmental
factors, including;
a. Temperature: High temperature causes the surrounding air to be
heated. As air gets heated, it expands and becomes less dense. Such air
tends to rise, causing air pressure to fall.
b. Altitude: A high altitude area has thin column of air above it. Thin
air means less number of air molecules. Such air weighs less, hence
low pressure. Thick air column in lower altitudes contains a greater
number of molecules, which exert more pressure.
However, high altitudes may sometimes have a higher pressure than
lower altitudes because of temperature. Mountaintops do not retain
much heat since the air above them is thin and the reflected heat easily
escapes, keeping the temperatures cool. Cool air is denser and hence,
exerts more pressure.
c. Amount of water vapour in the air: Water vapour is less dense than
other gases in the air, so as water vapour increases, the density of air
decreases, causing low pressure.
d. The volume that the air takes up: If two different containers, one
greater than the other, are filled with the same amount of air under the
same temperature, the air molecules in the greater volume will be more
spread out. This means that the air is thin. Since thin air weighs less,
there will be less pressure in a large volume as compared to that in a
smaller volume where air particles are closely packed. On the Earth’s
surface, air that flows from the poles towards the equator crosses
latitudes that are getting longer, and hence it spreads out to occupy
much greater space. This would give a comparatively low pressure.
Distribution of pressure belts in the world
A pressure belt is an area that has consistently high or low pressure, and lies
254
parallel to latitudes. Low pressure belts exist over the Equator or Doldrums
(0o) and the sub-polar latitudes (at 60o in both hemispheres). High pressure
belts are found at horse latitudes (30o north and south of the equator), and in
the polar latitudes. Figures 213 and 214 show the distribution of global air
pressure belts.
90o N
60o N
HIGH PRESSURE
Polar Latitudes
Temperate Latitudes
LOW PRESSURE BELT
30o N
HIGH PRESSURE BELT
Horse Latitudes
0o
LOW PRESSURE BELT
Doldrums
30o S
HIGH PRESSURE BELT
Horse Latitudes
LOW PRESSURE BELT
60o S
90o S
HIGH PRESSURE
Temperate Latitudes
Polar Latitudes
Figure 213: World pressure belts
Figure 214: Cross section of pressure belts and air flow
Equatorial Low Pressure Belt
The Equatorial Low Pressure Belt or ‘Doldrums’ lies along the equator,
between 10°N and 10°S latitudes. The sun shines almost vertically on the
equator throughout the year. As a result, the air gets warmed up and rises
over the region, creating low pressure at the surface. Since the larger part of
this low pressure belt passes along the oceans, the vertical winds obtain huge
amount of moisture, which lead to formation of cumulonimbus clouds and
255
thunderstorms (convectional rainfall). Surface winds are generally absent
since winds approaching this belt begin to rise near its margin. Thus, only
vertical currents are found. Therefore this belt is called doldrums (the zone of
calm) due to virtual absence of surface winds. This belt also coincides with the
Inter-Tropical Convergence Zone (ITCZ) i.e. the zone of convergence of trade
winds from two hemispheres – from sub-tropical high pressure belts.
Sub-Tropical High Pressure Belts
These belts are located at about 30°North and South of Equator. The major
cause of the high pressure in these regions is the descending movement of
air. The air rising in the equatorial latitudes is deflected towards poles due
to the earth’s rotation and eventually gets cooled and descends in the subtropical latitudes. The descending air is dry causing clear skies and little or
no precipitation. This is what is responsible for formation of deserts in these
regions. Calm conditions with feeble and variable winds are found here. In
olden days vessels with cargo of horses passing through these belts found
difficulty in sailing under these calm conditions. They used to throw the
horses in the sea in order to make the vessels lighter. Henceforth these belts
or latitudes are also called ‘horse latitudes’.
Sub-polar low-pressure belts
These belts are located between 60° and 70° in each hemisphere. Winds
coming from the sub-tropical and the polar high belts converge here to
produce cyclonic storms or low pressure conditions. Despite being located in
regions that are much colder than the subtropical high pressure belts near the
equator, the sub-polar belts are persistently low pressure areas. The reason
behind this is the meeting of two contrasting air masses. Warm humid winds
(the westerlies) coming from subtropical latitudes meet the denser, dry, cold
polar winds. Such different masses of air never mix. The lighter tropical air is
forced to rise over the cold polar winds, causing pressure to drop.
Polar High Pressure Belts
These are found at the North and South Poles, between 70° to 90° North and
South. In Polar Regions, the sun never shines vertically. Sun rays are always
slanting here resulting in extremely low temperatures. The low temperatures
that prevail here cause the air to compress and its density to increase. Hence,
high pressure is found here.
In reality, the pressure belts are not continuous or steady. There are a number
of reasons for this, including the following:
a. The surface of the earth is not uniform or smooth. There is uneven
heating due to land/water contrasts.
b. The sun does not remain over the equator, but moves from 231/2 degrees
256
north to 231/2 degrees south of the Equator and back over the course of
a year. This variation in the position of the overhead sun causes the
pressure belts to shift north and south through about 5 to 10 degrees of
latitude.
Activity
3
Locating air pressure belts on a world map
1. Look at the temperature profiles in Figure 215 from the climate graphs
for the following cities: Cape Town (South Africa), Kampala (Uganda),
McMurdo Station (Antarctica) and London (England).
2. Locate each city on the map in Figure 216.
Figure 215: Climate graphs
Figure 216: World map
3. For each of the cities listed above, show and locate on the map which
pressure belt they are closest to.
257
4. How does this explain the variation in rainfall among these cities?
5. Which pressure belt is Malawi closest to?
6. Report your work to the class for discussion.
Activity
4
Reflecting on important issues in the topic
1. In groups of four, locate an important issue that you feel the topic has
covered.
2. Formulate a problem or question about it for another group to answer.
3. Write the problem down on a sheet of paper, and hand that piece of
paper to another group.
4. Once your group is handed a problem statement, think of a solution to
the problem. Each group has a fixed amount of time.
5. Present your problem and its solutions to the class for discussion.
Summary
Atmospheric pressure is measured using an instrument called a barometer.
Depending on temperature of the air, altitude, humidity of the air and the
volume the air takes up, atmospheric pressure varies from place to place and
from time to time. Low pressure belts exist over the Equator (0o) and the subpolar latitudes (at 60o in both hemispheres). High pressure belts are found at
horse latitudes (30o north and south of the equator), and in the polar latitudes.
The distribution of these pressure belts is responsible for the global wind
systems and climate patterns.
Glossary
Isobar: a line drawn on a weather map that connects places with equal
atmospheric pressure.
Barometer: an instrument used in weather forecasting for measuring
changes in atmospheric pressure.
Pressure belt: an area of consistently high or low pressure parallel to
latitudes.
258
Review questions
1. What are air pressure belts?
2. Explain any three factors that affect air pressure.
3. Why does Thyolo experience higher pressure at about 500 metres above
sea level than Nsanje at 100 metres below sea level?
4. Identify pressure systems that drive global wind circulation.
5. Explain why global energy distribution varies across the earth’s surface.
References
Bunnett, R. B. (1973). General Geography in Diagrams. London: Longman.
Bunnett R. B. (1984). Physical Geography in Diagrams for Africa. London:
Longman Group Limited.
Leong, G. C. (1983). Certificate Physical and Human Geography. Oxford:
Oxford University Press.
Obara, D. A. (1991). Gold Medal Geography. Nairobi: MacMillan Kenya
Limited.
Waugh, D. (1990). Geography: An Integrated Approach. Hong Kong: Thomas
Nelson Limited.
White, R. (1998). Africa in Focus: A Physical, Human and Economic Geography.
Oxford: MacMillan education Limited.
http://www.middleschoolchemistry.com/lessonplans/chapter1/lesson5
29/01/14
http://cwx.prenhall.com/bookbind/pubbooks/lutgens3/medialib/abcontrol/
pages/question.htm 29/01/14
259
260
Unit
17
Prevailing winds
Prevailing winds
Prevailing winds, also known as planetary or
permanent winds are winds that blow frequently
from a single general direction. The prevailing
wind systems of the earth blow from the highpressure belts towards adjacent low-pressure
You most likely observe belts. They occur over large sections of the earth’s
winds on a regular surface, causing global circulation of air.
basis. Learning about
prevailing winds will
help you to understand Activity 1
its role in climate Investigating the causes of prevailing
modeling, and consider
winds
why different parts of the
1. Inflate a balloon. Do not tie it. Hold the
world experience a range
neck of the balloon closed.
of climates. Knowledge
of prevailing winds is
2. Describe how the inflated balloon feels.
also a prerequisite for
3. What caused the inflated balloon surface
studying other weather
to feel the way it did when the neck was
phenomena
such
as
closed?
cyclones,
tornadoes,
anticyclones, air masses,
4. Open the neck of the balloon without letting
etc. In this unit, you
go of the balloon.
will explain the term
5. Record your observations of what happens.
prevailing winds, their
6. What caused the air to leave the balloon
types and how they are
when the neck was opened?
influenced by air pressure
belts. You will then
7. Why didn’t outside air move into the balloon
explain how the Ferrel’s
when the neck was opened?
law of deflection (Coriolis
8. How does this experiment relate to the
force) affects prevailing
occurrence of prevailing winds?
winds. Finally, you will
interpret isobars on air
9. Report your findings to the class for
pressure map.
discussion.
261
Types of prevailing winds
The prevailing winds of each hemisphere are divided into three wind belts;
the trade winds belt, the westerlies belt and the polar easterlies belt (see
Figure 217 below).
90oN
60oN
Polar easterlies
Westerlies
30oN
Horse latitutes
Northeast trade winds
0o
Doldrums
Southeast trade winds
30oS
Horse latitutes
Westerlies
Polar easterlies
60oS
90oS
Figure 217: Prevailing winds (Source: http://www.embroidela.com 14/12/13)
Trade winds
Trade winds blow from the 30 degree latitudes towards the equator. Sailors
on trade ships named them centuries ago because the winds helped them
sail their trade ships across the oceans. In the Northern Hemisphere, the
trade winds blow mostly from the northeast, so they are called the Northeast
tradewinds. In the Southern Hemisphere, the winds blow from the southeast
and are known as the Southeast trade winds (see Figure 214). Trade winds
have the following characteristics:
a. They are warm winds, carrying enough moisture to produce heavy
precipitation.
b. They are steady winds blowing about 17 to 20 km per hour.
c. They converge at the equator producing convectional storms that produce
some of the world’s most dense clouds and heaviest precipitation. Since
air rises, there are no surface winds over the region. The stillness of
the Equator made sailors give the region the name ‘doldrums’, which
means, calm.
d. They blow from an easterly direction.
262
Importance of the trade winds
a. Sailors utilise the trade winds as a sort of oceanic fast lane to cut down
fuel consumption and travel time across the Atlantic and Pacific oceans.
b. Steadiness of the winds generally brings clear weather, making it
favourable for mariners.
c. They help in generating wind power. The force of these steady winds
is harnessed to generate wind power, which may be used to electrify
homes, water pumping stations, and other operations.
d. The trade winds also play a greater role in global weather, bringing
tropical storms (known as hurricanes in the Americas and typhoons in
Asia). Weakened trade winds would result in numerous consequences,
including the disruption of normal weather and climate patterns.
e. They are responsible for persistence of surface ocean currents and
oceanic upwelling. This is economically important, particularly for the
Pacific fishing industries.
Westerlies
The westerlies generally blow from the subtropical high-pressure areas
(horse latitudes) towards the 600 latitudes in both the Northern and Southern
Hemispheres. The horse latitudes experience long periods of calmness, and
early ships, relying on wind power, stalled. This caused the sailors to run
out of food supplies, and so many horses that were carried on board starved
to death. The sailors threw their dead horses into the water, hence the name
horse latitudes.
The westerlies have the following characteristics:
a. They predominantly blow from a westerly direction due to the deflecting
effect of the earth’s rotation; hence they are called the westerlies.
b. They are warm winds.
c. They are strongest in the winter when the pressure is lower over the
poles and weakest during the summer and when pressures are higher
over the poles.
d. They are moist winds since they originate from wet surface regions.
Importance of the westerlies
a. The westerlies play an important role in bringing warm weather and
warm equatorial waters to the western coasts of continents, more so
in the southern hemisphere because there are fewer land masses in
263
the southern oceans to slow these winds down. This may result in
precipitation in these areas, which helps in agriculture.
b. They are useful in shipping, yacht races and aviation. Trans-oceanic
and trans-continental flying in the easterly direction is assisted by the
winds, hence, it requires less fuel and shorter time.
Polar easterlies
These winds blow persistently from the poles towards the equator. They are
formed as cold air at the poles sinks and begins to move towards the equator.
The polar winds eventually meet with the prevailing westerlies at around 600
latitudes in both hemispheres, causing violent storms and various weather
changes.
Polar easterlies have the following characteristics:
a. They are generally winds of intensely cold air.
b. They are dry since the surface moisture in areas they originate from is
frozen due to cold temperatures.
c. They blow from an easterly direction. The deflection of the winds to the
west in each hemisphere causes the polar winds to assume an easterly
direction.
d. Unlike the westerlies in the middle latitudes, the polar easterlies are
often weak and irregular.
Importance of the polar easterlies
a. When the cold polar easterlies clash with the warm westerly winds they
create a polar front, a region of mild and moist winds that brings about
precipitation. This is known as a temperate climate which is found in
areas at the mid-latitudes.
Problems caused by prevailing winds
a. Soil erosion: prevailing winds blowing across soil surfaces that are
loose and dry lead to soil erosion, which has serious effect on crop
production.
b. Disruption of transport: flying or sailing against strong head on
prevailing winds may reduce speed and increase fuel consumption.
This necessitates reduced carrying capacity (pay load) of an aircraft in
terms of weight.
c. Wind chills: a wind chill is the cooling factor caused by the combination
264
of wind and temperature to produce conditions that would be much
colder than the temperature alone and is the most debilitating factor
of Antarctic expeditions. Cattle and sheep are prone to wind chill,
rendering their hair and wool coverings ineffective.
d. Localized blizzards: a blizzard is a severe snowstorm with strong
winds and poor visibility. Blizzards are caused when the surface wind
sweeps up any loose snow, even if the skies above are clear and no snow
is falling. These conditions make travel and outdoor activities virtually
impossible.
How prevailing winds are influenced by air pressure belts
The prevailing wind system of the world accompanies the presence of the High
and Low-Pressure Belts, which are largely the result of unequal heating of the
planet’s surface by the sun. The Earth’s curved surface causes some parts of
it to receive the Sun’s rays more directly than other parts. For example, the
Sun shines more directly on the surface at the equator than at the poles. The
heated air over the equator rises and creates low pressure, whereas the colder
air over the poles drops and creates high pressure. We know that winds tend to
blow from the high-pressure centres to the low-pressure centres (Figure 218).
Any difference in pressure can cause wind, but greater differences produce
stronger winds. The colder air from the poles rushes toward the equator to
take the place of the rising warmer air. The warm air that rises over the
Equator tends to drift pole-wards to replace the sinking cold air. This steady
exchange of warm and cold air that occurs between the equator and the poles
produces global wind belts.
1028
1012
1024
1016
1020
A high pressure and low pressure center with wind directions
(northen hemisphere). Circles are isobars with pressure in mb
Figure 218: Pressure and wind (Source: http://myweb.cwpost.liu.edu/vdivener/ers_1/chap_6.htm
14/12/13)
265
Activity
2
Examining prevailing winds
1. Copy the diagram in Figure 219 and use it to do this activity.
N
Equator
S
Figure 219: Major latitudes of the earth
2. In that diagram, label the doldrums, horse latitudes, and the latitudes
at which each occurs
3. Name the 3 wind belts: Latitudes at which they occur:
Direction:
a. ______________
_____________________
___________
b. ______________
_____________________
___________
c. ______________
_____________________
___________
4. Label each wind belt and the direction the wind flows.
5. What winds would Columbus have used to travel from Spain to the
Caribbean?
6. Which winds would he have needed to return to Europe?
7. Would winds have favored European explorers seeking to travel east
around the tip of Africa?
8. Report your work to the class for discussion.
266
How Ferrel’s law of deflection (Coriolis force) affects
prevailing winds
There are a number of forces that control wind, which include the following:
a. Pressure Gradient Force (PGF)
b. Ferrel’s Law or Coriolis Force (CF)
c. Friction (F)
Pressure Gradient Force (PGF)
Pressure Gradient Force (PGF) is the initial force that caused air to move. Air
moves directly away from higher pressure and directly toward lower pressure
in order for the atmosphere to achieve a pressure balance. The greater the
difference in pressure, the faster the air flows. The change in air pressure
with distance is what is called Pressure Gradient. The steeper the pressure
gradient, the stronger the wind blows. Assuming that there were no other
forces acting on wind, Pressure Gradient Force would move the air directly
from a high pressure to a low pressure at 90° to the isobars in a straight path.
However, the real wind direction is influenced by more than just Pressure
Gradient Force. For instance, because of the earth’s rotation, there is a second
force known as the Coriolis force (CF) that affects the direction of wind flow.
Coriolis force (CF) or Ferrel’s law of deflection
Ferrel’s Law states that “air is deflected from its original straight path to
the left (standing with your back to the wind) in the Southern Hemisphere
and to the right in the Northern Hemisphere”. Once an air parcel moves out
of the regions of higher pressure and into the regions of lower pressure, the
Coriolis force acts at right angles to the wind, causing it to follow a curved
path instead of a straight line. Ferrel’s law applies to every particle or body
that is set in motion upon the surface of the rotating earth (e.g. water, air, ice,
airplanes, missiles or the like). As with any force that causes an object to be
deflected, the amount of deflection will be a function of the following:
a.Distance from the Equator: The Coriolis force significantly affects all freemoving bodies that travel over great distances (i.e. travels a long time.).
An example would be air that is moving over hundreds of kilometers to
the Equator from the poles which takes many hours to do so. Therefore,
the greater the distance to the equator the wind covers, the greater the
deflection.
b.Speed of the moving particle: The amount of deflection increases with
increasing speed. The faster the wind speed the greater the deflection.
This explains why there is need to consider the Coriolis Effect for objects that
either travel at high speeds or remain airborne for a long time; otherwise they
will go off track. For example, people need to make corrections for the Coriolis
267
Effect when firing long–range missiles in order to hit their targets. Modern
aircraft on long distance flights also have to compensate for the Coriolis Effect.
In other words, they aim for where they expect the landing site to be, at the
time they reach their destination. Figure 217 illustrates the Coriolis Effect.
Imagine the path of a rocket launched from the North Pole toward a target
located on the equator. The true path of this rocket is straight, and the path
would appear to be straight to someone out in space looking down at Earth.
However, to someone standing on Earth, it would look as if the rocket swerved
off its path and landed 15 degrees to the west of its target.
Figure 220: Coriolis Effect
In the same way, if wind blows toward the Equator from the North Pole, it
would end up reaching the right of its true path. This variation would occur
because the target area would have moved eastward before the wind reached
it because of the greater eastward velocity at the equator. If wind blows
northward from the equator, it would again land to the east of its intended
north path. In this case, the wind was moving eastward faster at the Equator
than was its target farther north. An exactly similar displacement occurs if
the wind blows in any direction.
268
Friction of the earth
As has been discussed so far, the amount of deflection by Coriolis force is a
function of wind speed. Therefore, friction along the earth’s surface due to
mountains and the heavily forested areas substantially decreases the speed of
the wind. A slower wind reduces the strength of the Coriolis force. A weaker
Coriolis force cannot deflect air the full 90°. Instead, the weaker Coriolis force
only deflects air approximately 60° away from the Pressure Gradient Force
direction. Now wind will flow at an angle across isobars toward lower pressure.
Activity
3
Modelling the Coriolis Effect
1. Draw dot A in the center of a piece of foam board.
2. Draw dot B along the outer edge of the foam board. Roll a table-tennis
ball from dot A to dot B. Record your observations.
3. Center the foam board on a turntable. Have your partner rotate the
foam board at a medium speed.
4. Roll the ball along the same path. Record your observations.
5. Contrast the path of the ball when the foam board was not moving to
when it was spinning.
6. How might air, moving from the North Pole to the equator, travel due
to earth’s rotation?
7. Report your findings to the class for discussion.
Interpreting isobars on an air pressure map
All weather maps use a standard set of symbols to portray features of the
weather. Figure 221 below shows some of the more commonly used symbols.
Isobar
1012
High pressure cell
Low pressure cell
Figure 221: Weather symbols
An isobar is a line on a weather map that joins locations of equal air pressure.
The number written on an isobaric line is the atmospheric pressure for that
isobar. By plotting isobars at intervals based on pressure readings, areas of
high and low pressure can be depicted on a map, just like hills and valleys on
a contour map of a landscape.
269
We can determine the likely weather of a place from the isobars on a map.
For example, low-pressure areas have inflowing air that rises at the center.
The rising air generates clouds and precipitation. High-pressure areas are
associated with descending, out flowing air and usually bring dry, clear
weather.
Wind direction and relative speed can also be determined from the isobars.
Isobars are spaced closer together in areas where the wind is strong and
further apart where the wind is light. In the illustration below thicker arrows
represent relatively faster winds.
b
0m
m
b
102
b
10
36
1012 mb
m
28
10
le
a
gr
nt
Ge
1004 mb
di
en
996 mb
t
Ste
ep
gra
die
nt
b
8m
98
L
Weak winds
Strong winds
Figure 222: Spacingof isobars and wind speed
(Source: www.cengage.com/.../0495555061_137182.pdfAtmospheric Pressure, Winds, and Circulation Patterns - Cengage ...29/12/13
Activity
4
Drawing and interpreting isobars on an air pressure map
1. Figure 223 below is a map of Southern Africa showing pressure
readings. Copy the map into your exercise book and use it to complete
the activity that follows.
2. Using a line, join the places with the same pressure readings.
3. What name is given to these lines in 1 (a) above?
4. On the map itself, insert H on an area of high pressure and L on an
area of low pressure.
270
1024
1024
1026
1028
1026
1030
1030
1024
1030
1026
1028
1034
1032
1026
1026
1028
1032
1028
1024
1024
1026
1028
1024
1032
1034
1034
1030
1028
1034
1034
1028
1028
1030
1026
1032
Figure 223: Air pressure map
5. Indicate the direction of wind on the map. Detach the map and attach
it to your answer sheet.
6. What instrument was used to arrive at the readings shown on the map?
7. Report your work to the class for discussion.
Please note! It is not possible to measure atmospheric pressure at every
point within an area covered by a weather map, so isobars are based on air
pressure readings taken at weather stations.
Summary
The world is a continuous cycle of prevailing winds. Different regions on
earth have different prevailing wind directions. Variations arise due to the
positions and differential heating rates of the continents and oceans, and the
earth’s Coriolis Effect. The following prevailing winds across the earth may be
identified: the trade winds, which blow from the 30 degree latitudes towards
the equator; the westerlies, which blow from the subtropical high-pressure
areas (horse latitudes) towards the 600 latitudes in both the Northern and
Southern Hemispheres; and the polar easterlies, which blow persistently from
the poles towards the equator. The prevailing winds enabled a round-trip
trade route for sailing ships crossing the Atlantic and Pacific oceans. They
also play an important role in global weather patterns, persistence of ocean
currents and generation of wind power.
271
Glossary
Planetary wind: any wind system of the earth’s atmosphere, which owes its
existence and direction to solar radiation and the rotation of the earth.
Doldrums: a low-pressure area around the equator where the prevailing
winds are calm.
Horse latitude: either of two belts or regions near 30 degrees north or 30
degrees south; characterised by calms and light-baffling winds
Blizzards: a severe snowstorm with strong winds and poor visibility.
Review questions
1. Name the three major prevailing winds of the world.
2. Draw the map of atmospheric circulation and wind belts (label the
name of each wind belt) of the world.
3. What consequence does the Coriolis Effect have, relative to the earth’s
surface, on winds that are changing latitude?
4. At what latitude are prevailing westerly winds typically found?
5. How are winds deflected in the Northern Hemisphere and Southern
Hemisphere as a result of the Coriolis Effect?
References
Bunnett, R. B. (1973). General Geography in Diagrams. London: Longman.
Bunnett R. B. (1984). Physical Geography in Diagrams for Africa. London:
Longman Group Limited.
Leong, G. C. (1983). Certificate Physical and Human Geography. Oxford:
Oxford University Press.
Obara, D. A. (1991). Gold Medal Geography. Nairobi: MacMillan Kenya
Limited.
Waugh, D. (1990). Geography: An Integrated Approach. Hong Kong: Thomas
Nelson Limited.
White, R. (1998). Africa in Focus: A Physical, Human and Economic Geography.
Oxford: MacMillan education Limited.
http://www.embroidela.com 14/12/13
http://curiosity.discovery.com/question/what-are-wind-belts 14/12/13
http://myweb.cwpost.liu.edu/vdivener/ers_1/chap_6.htm 14/12/13
http://abyss.uoregon.edu/~js/glossary/coriolis_effect.html 14/12/13
http://www.kish.in/winds_and_its_types/ 14/12/13
272
www.cengage.com/.../0495555061_137182.pdf Atmospheric Pressure, Winds,
and Circulation Patterns - Cengage ...29/12/13
http://www.wisegeek.com/in-meteorology-what-is-an-isobar.htm 29/12/13
273
Unit
18
Air Masses
Air masses
An air mass is a large body of air that has basically
the same temperature and humidity through its
whole length, width, and depth. Air masses form
over areas of the earth’s surface where conditions
remain the same for long periods of time. Air
over these regions can be stagnant long enough
to eventually take on the characteristics of the
Weather
is
always
surface below.
changing. One day might
be cold and cloudy. The For example, the Arctic is frigid and parched – so
next day might be warm the air which remains over it becomes cold and
and sunny. Even on the dry as well. The tropical oceans are warm and
same day, the weather wet, steaming beneath the fierce equatorial sun –
can change a lot. A so hot, soggy air masses form over the seas near
beautiful morning may the equator. The arctic and the tropics, in fact,
be followed by a stormy are two of the major places where air masses are
afternoon. Air masses are created. Areas in which air masses develop are
one of the key components called source regions.
to understanding these
Activity 1
weather changes. In this
unit, you will explain the Identifying air masses
term air mass. You will The diagram below depicts two types of air
also explain the main masses that commonly influence weather in the
types of air masses and United States of America. Air mass #1 lies over
consider their associated the continent while Air mass #2 lies largely over
weather.
the Atlantic Ocean.
Air mass #1
Air mass #2
Figure 224: Air masses
274
1. For each air mass, identify the following characteristics:
a. type of air mass:
b. source region:
c. relative temperature:
d. wind direction:
e. moisture content:
2. Present your work to the class for discussion.
Factors that affect the characteristics of air masses
a. The nature of the underlying surface: This determines the moisture
content of the air mass. Air masses that develop over land and are
relatively dry while air masses that develop over sea surfaces have
higher moisture content. Air masses that develop over sea surfaces are
called maritime air masses, and those over land surfaces are called
continental air masses.
b. Latitudinal position of the source area: This determines the
temperature characteristics of the air mass. For example, tropical air
masses formed in the tropical latitudes are warmer than the polar
air masses formed in the polar latitudes.
Classification of air masses
Air masses are classified and named on the basis of the location and
characteristics of their source regions.
Classification based on latitudinal position of the source
region:
a. Polar (P): Polar air masses originate over cold regions.
b. Arctic (A) or Antarctic (AA): These air masses originate over the cold
Arctic and Antarctic Regions.
c. Tropical (T): Tropical air masses originate over the warm tropics.
Classifications based on the surface of the source region:
a. continental (c): Continental air masses originate over land.
b. maritime (m): Maritime air masses originate over water.
275
The main types of air masses and their associated weather
By combining the characteristics of latitude and underlying surface, we have
the following major types of air masses, each with its own characteristic
weather conditions:
Continental polar (cP) or continental arctic (cA)
These air masses start over large expanses of northern land such as the
wintery regions of northern Canada and Alaska. These air masses are cold
and dry, with high pressure and stable air. The fact that they are usually dry
means that there are few clouds to hold in the heat, and they can be especially
bitter in winter. Those searing winter nights when the stars seem more vivid
than at any other time are continental polar or continental arctic nights. In
the summer, these masses produce dry, sunny, mild weather -- often the best
outdoor days of summer.
Maritime polar (mP)
Maritime polar air masses are those which begin their life over cold polar seas
or perhaps over cold continental regions before passing over ocean waters and
picking up moisture. They tend to be cold, although not as cold as continental
polar air masses, moist, and unstable, meaning that precipitation such as
rain, sleet, and snow can occur within the air mass as well as along the frontal
boundary.
Arctic (A) or Antarctic (AA)
Arctic or Antarctic air masses form in the Arctic and Antarctic regions. These
air masses are very similar to continental polar air masses, except that they
originate over the permanent ice cap near the north or south poles and are
bitterly cold.
Continental tropical (cT)
Continental tropical air masses start over land in the tropical regions, where
solar heat is intense and there is little water to draw from to make the air
mass more humid. They are characterized by hot, dry weather with clear
skies. Deserts and high plains are the usual breeding grounds for these air
masses. These air masses enter the temperate regions mostly in the summer
months. If a continental tropical air mass moves in an area and stagnates, a
severe drought can result.
276
Maritime tropical (mT)
Maritime tropical air masses have the tropical oceans as their source region,
and tend to be warm, moist, and usually unstable. Summer weather may
turn hot and muggy, with hazy sunlight and scattered thunderstorms in the
afternoon. Winter weather turns warmer and damper - perhaps resulting in a
mild thaw if the air mass is strong enough.
Maritime Equatorial (mE)
The equator does not have a large land area; as a result the equatorial air is
not dry. This means the continental Equatorial (cE) air mass is not found;
there is only mE air, which is moist and hot.
Activity
2
Locating and describing air masses
1. Figure 225 below illustrates the abbreviations used to identify large
air masses of differing temperature and humidity. Use it to answer
questions that follow.
1. Identify the air masses that form over the areas shown on the map by
their full names.
Figure 225: World distribution of air masses
2. Work with your group to examine how these air masses differ.
3. Describe the likely weather conditions in the Midlands and along the
south west coast of Africa.
4. Which of these air masses do you think could be stable or unstable?
5. Present your work to the class for discussion.
277
An air mass does not remain permanently in its area of origin, fortunately. If
it did, most of the earth’s surface would be without rain and would be a lifeless
desert. The atmosphere is very unstable place due to temperature differences,
the action of the sun, and the earth’s rotation. It is these that move air masses
north and south from their point of origin. When an air mass moves out of
its source region, it is exposed to new surface conditions, which change its
temperature, humidity and stability. For example, as northern polar air mass
moves southward, it encounters warmer land surfaces and consequently, is
heated by the ground below.
Almost all of the weather we experience is a result of air masses. When an
air mass meets another in our vicinity, we see various effects such as rain,
thunderstorms, fog, and similar phenomena. These effects occur at the edge of
air masses, but can be many hundreds of miles deep; the stronger the impact
between the air masses, the more lasting and impressive the effects.
Stable and unstable air masses
Stable air mass: Stable air mass is one, which is relatively cooler and denser
than the air above it. It is caused by lack of surface warming. Because of this,
it resists rising and remains in place. This makes stable air masses relatively
calm within their lower layers. They are free from convection and other
disturbances, and consequently have the following characteristics:
a. Poor surface visibility due to smoke, dust and other particles trapped
near the surface. These particles cannot rise out of the air mass because
there are no convection currents.
b. Low stratus clouds which settle on the ground as fog.
Unstable air mass: Unstable air mass is air that is warmer than its
surroundings and tends to rise easily; thereby encouraging convection currents
(see Figure 226). It is caused by surface warming. Unstable air can rise to
great heights, where it can condense large quantities of water vapour and so
forming showers or even thunderstorms. Unstable air mass is characterised
by;
a.vertical cumulus clouds
that produce heavy
showers or thundershowers
b.severe turbulence or
convective activity
c.good surface visibility
Figure 226: Stable and unstable air masses
278
Activity
3
Determining stability of the air
Figure 227 below shows three ways in which temperature affects atmospheric
stability.
10o 5o
10o 13o
10o 10o
20o 20o
20o 20o
20o 20o
Elevation
1 km
A
B
C
Figure 227: Relationship of air stability to air temperature
1. In each situation assume that the balloon is filled at ground level with
air at 20oC, and then lifted manually to a height of 1 km. The air in the
balloon will expand and cool to about 10oC. Whether the balloon rises
or falls upon release depends on the surrounding air temperature and
density.
2. Describe what would happen to the balloon in situation “A,” “B,” and
“C.” Give a reason for your answer in each case.
3. Report your answers to the class for discussion.
Activity
4
Reflecting on the topic
1. Summarise the most important ideas you have just discussed about the
topic.
2. Why is this knowledge worth having?
3. What can you do about the issues you have been discussing in the unit?
4. Report your answers to the class for discussion.
279
Summary
The day-to-day weather we experience depends on the temperature, stability,
and moisture content of the air mass we are experiencing. Areas in which
air masses originate are called source regions. Classification of an air mass
depends on – The latitude of the source region and the nature of the surface
in the area of the origin (ocean or continent). The following air masses are
identified: cA continental Arctic, cP continental Polar, cT continental Tropical,
mT maritime Tropical, mP maritime Polar. Once an air mass moves from its
source region, it is gradually modified by the surface over which it is moving.
Air masses can be classified as stable or unstable. Stable air does not possess
a tendency to rise. It is denser than the air above it, thus it remains in place.
Unstable air is less dense than the surrounding air and thus it rises.
Glossary
Source region: an extensive region of the earth’s surface where an air mass
originates.
Maritime air mass: an air mass which originates over water.
Continental air mass: an air mass that originates over land.
Review questions
1. Explain the meaning of the following terms
a. air mass
b. source region
2. Give two factors that affect air masses.
3. What are the two characteristics of an air mass that you need to know
in order to classify it?
4. What air mass category means that the air mass formed over water?
5. How is a maritime tropical air mass different from a continental tropical
air mass?
6. With the aid of a labelled diagram, describe what happens when two
air masses of different temperatures meet.
7. How does stable air differ from unstable air? Describe the general
nature of the clouds and precipitation expected with each.
280
References
Bunnett, R. B. (1973). General Geography in Diagrams. London: Longman.
Bunnett R. B. (1984). Physical Geography in Diagrams for Africa. London:
Longman Group Limited.
Leong, G. C. (1983). Certificate Physical and Human Geography. Oxford:
Oxford University Press.
Obara, D. A. (1991). Gold Medal Geography. Nairobi: MacMillan Kenya
Limited.
Waugh, D. (1990). Geography: An Integrated Approach. Hong Kong: Thomas
Nelson Limited.
White, R. (1998). Africa in Focus: A Physical, Human and Economic Geography.
Oxford: MacMillan education Limited.
http://ww2010.atmos.uiuc.edu/%28Gh%29/guides/crclm/act/arms.rxml
02/02/14
281
Fronts
Unit
Fronts
A front is a zone (boundary) in the atmosphere
that separates two masses of air of different
characteristics in terms of temperature or
moisture content. When two different air masses
meet, they do not mix readily because they have
different properties in terms of temperature and
One of the first things density. So a front is a boundary separating these
you probably do every air masses.
morning is look out the
Activity 1
window to see what the
weather is like or listen Sharing
basic
knowledge
and
to the day’s forecast to anticipations
decide what clothes you
1. Write down what you already know and
will wear. As we saw
what you want to know about fronts.
in Unit 18, air masses
2. Now, get into groups and share your ideas
bring distinctive weather
and anticipations.
features to a region.
However, there is more
3. Draw a table like the one below on a chart.
that you need to learn
Know
What to know
Learned
about air masses. When
different air masses come
together, they create
fronts, which produce
4. Write down what you already know in the
some of the most dramatic
first column and what you want to know
weather
changes
on
in the middle column. You will fill in what
earth. In this unit, you
you will have learned in the third column
will explain the term
at the end of the unit.
front and how it is formed.
5. Display the chart in front of the class for
You will also identify
reference.
the characteristics of
different types of fronts. Types of fronts
You will then describe The type of front depends on both the direction
the type of weather in which the air mass is moving and the
associated with each type characteristics of the air mass. There are five
of front. Finally, you will types of fronts that will be described below: cold
explain the ITCZ and its front, warm front, stationary front, occluded front
impact on weather.
and dryline.
19
282
Cold front
A cold front is a zone separating two air masses, where the cold air mass is
pushing into a warmer air mass. The air behind the cold front is colder and
typically drier than the air ahead of it, which is generally warm and moist.
Cold fronts have the following characteristics:
a. They have steep slopes, which cause the warm moist air to quickly rise
along the leading edge.
b. They move faster than all other types of fronts and they tend to move
the farthest while maintaining their intensity.
c. They have extremely tall clouds produced by the rapidly rising warm
moist air.
d. They have the most violent weather, among all types of fronts,
accompanied by thunder and lightning. A line of thunderstorms that
can form along or ahead of a cold frontis known as a squall line.
e. They have heavy rainfall which only lasts for short periods of time.
On a weather map, the cold front is drawn as a line with triangles. The
triangles are like arrowheads pointing in the direction that the front is moving
(see Figure 228).
Warm Air
Warm Air
Cold Air
Warm Air
Warm Air
COLD FRONT
Cold Air
COLD FRONT
Figure 228: Cold front
Warm front
A warm front is where a warm air mass is pushing into a cold air mass.
However, it is difficult for warm air to move against the cold air because the
warm air is less dense, so it slides up and over the cold air. The air behind a
warm front is warm and moist, while that ahead of it is cooler and less moist.
Warm fronts have the following characteristics:
a. Gentle slopes, which often leads to a gradual rise of warm air.
b. Warm fronts tend to move slowly.
283
c. Warm fronts are typically less violent than cold fronts.
d. Widespread and continuous precipitation, which often occurs along and
ahead of the front.
On a weather map, a warm front is drawn as a line with semicircles. The
semicircles are on the side of the line where the front is moving (see Figure
229 below).
Warm Air
Warm Air
Cold Air
Cold Air
WARM FRONT
Figure 229: Warm Front
Occluded front
An occluded front is when a fast moving cold front catches up and overtakes a
slow moving warm front. It is a composite of two fronts. An occluded front is
formed when the cold front surrounds the warm air and eventually lifts it up
completely from the ground. As a result, the warm air between the cold and
warm front is shut off, thereby causing air pressure to stop decreasing.
When two different air masses meet, a wave develops and spins due to the
rotational effect of the earth. One part of the wave bulges into the cold air
causing the cold air to wrap up and push against the warm air; this forms a
cold front. The other part of the wave bulges into the warm air causing the
warm air to push into the cold air, creating a warm front (Figure 230 a). The
cold front moves faster than the warm front because cold air is comparatively
dense. At the cold front, the colder air sinks under the warm air and eventually
lifts all the warm air off the ground. This causes the cold front to merge with
the warm front to form an occluded front (Figure 230 c).
Occluded fronts are indicated on weather maps by a line with alternating
semicircles and triangles on the same side pointing in direction of travel.
284
Figure 230: Formation of an occluded front
The occluded front has two main types: Cold and warm occluded fronts. In
both cases a cold front overtakes a warm front.
a. Cold occluded front: In a cold occluded front, a very cold front
overtakes a warm front. The cold front lifts the warm air above the
cooler air and creeps underneath the cool air on the ground. So the
warm front becomes an upper level front, and the occluded front is an
extension of a cold front (see Figure 231).
285
Warm air
Cold air
Cool air
Figure 231: Cold occluded front
b. Warm occluded front: In a warm occluded front a cold front of
moderate cool air overtakes a warm front. But in the warm front the
temperature difference is high, because the air on the ground is very
cold. So the whole cold front with cooler air, together with the warm air
runs over the very cold air mass. The cold front becomes the upper level
front, and the occluded front is an extension of the warm front (Figure
232).
Warm air
Cool air
Cold air
Figure 232: Warm occluded front
Occluded fronts have weather characteristics typical of both cold and
warm fronts. The weather is very unsettled, marked by heavy rains and
thunderstorms potentially over a prolonged period.
Stationary front
A stationary front is a zone where air masses are not moving against each
other; neither of the air masses is strong enough to push into the other.
However, the air masses may move parallel to the boundary, producing weak
winds and prolonged rainfall.
286
Stationary fronts are represented on a weather map by alternating lines
with triangles and semicircles facing opposite directions. The triangles point
towards the warmer air and the semicircles point towards the cold air (see
Figure 233 below).
Figure 233: A stationary front
Dryline
A dryline is a boundary that separates a warm moist air from a hot dry air.
It is a special type of front in which the temperature usually does not change,
but humidity changes as the front moves through. Drylines are common over
the southern Great Plains of North America when dry, continental tropical
air (cT) from the southwest meets maritime tropical (mT) from the Gulf of
Mexico. The result is a line of intense thunderstorms, rain and tornadoes. A
dry line is represented on weather maps by a dashed line (see Figure 234).
cP
cT
Dryline
mT
Figure 234: Dryline
287
Activity
2
Interpreting weather maps on which fronts are already
identified
10
08
10
00
10
04
99
6
99
6
10
00
Use the map below to complete the task that follows.
y
x
Figure 235: Weather map
Imagine that you are located at point x on the map.
1. What changes in the sky conditions do you expect as the cold front
approaches?
2. As the front passes, what changes in wind direction will occur?
3. What change in temperature will occur at the time the front passes?
Now imagine that you are located at point y on the map.
4. Prior to the arrival of the warm front, what changes will occur in the
sky conditions?
5. What changes do you expect in wind direction as the warm front passes?
6. What changes in weather do you predict after the front passes?
7. Over which point, x or y, are thunderstorms more likely to occur? Why?
8. Report your work to the class for discussion.
The ITCZ and its impact on weather
ITCZ is an acronym for the Inter Tropical Convergence Zone.The ITCZ, known
for centuries by sailors as the Doldrums, is a belt of low pressure which circles
the earth near the equator where the trade winds from the Northern and
Southern Hemispheres clash.
288
Weather in the ITCZ
While the region can be quiet and calm, it is also the birthplace of some of the
world’s strongest storms. It is characterised by strong convective activity which
generates violent thunder storms that can tower up to 18,000 km, far higher
than any commercial airliner could fly over. The ITCZ is also characterised
by massive rain-bearing clouds (Cumulonimbus) over large areas, presenting
a formidable obstacle to aircraft transit. If conditions are favorable, some
clusters of thunderstorms that form along the ITCZ can grow into hurricanes.
Aircraft flying through the ITCZ will encounter all the hazards associated with
Cumulonimbus clouds such as turbulence, heavy rain, icing, hail, thunder
and lightning (see Figure 236). Aircraft often must fly around, rather than
over, thunderstorms. An aircraft that flies into a huge thunderstorm may not
make it out.
Inter-tropical Convergence
Zone a breeding ground for
thunderstorms
Equatorial regions receive more direct
heating from the sun than other parts of
the globe, resulting in a nearly-continuous
band of thunderstorms, known as the
Inter-tropical Convergence Zone (ITCZ).
Aircraft that cross the ITCZ face all the
hazards of flight near thunderstorms,
including turbulence and lighting.
Antlantic Ocean
Tropical of Cancer
Converging air
Heavy
rainfall
Africa
Trade
winds
Equator
South America
Tropic of Capricorn
Tropical air forms thunderstorms
as trade winds from Northen and
Southern Hermspheres converge
near the Equator.
Figure 236: The Inter Tropical Convergence Zone
(Source: http://usatoday30.usatoday.com/weather/news/2009-06-03-inter-tropical-convergence-zonethunderstorms_N.htm)
Average position of the ITCZ
The ITCZ follows the sun, shifting slightly to the north during Northern
Hemisphere summer and slightly to the south during Northern Hemisphere
289
winter (see Figures 237 below). However, the mean or average position of the
ITCZ is located north of the equator because there is much more landmass in
the Northern Hemisphere as compared to the Southern Hemisphere. Since
land heats more than water, when the land surface is warmer than the water
it promotes a large scale sea breeze (airflow from ocean toward the land). This
produces a convergence of air over the land. Due to this circulation, areas such
as South East Asia have heavy rain in the summer. The ITCZ has its most
northerly displacement in the summer over Asia due to the large landmass of
Asia to the north of the equator.
December Solstice
H
90oS
L
H
Equator
30oS
60oS
L
H
30oN
H
60oN
90oN
Equinox
Polar
H Cell
90 S
o
L
Ferrel
Cell
60 S
Hadley
Cell
H
L
H
30 S
Equator
30oN
o
o
Hadley
Cell
Ferrel
Cell
Polar
Cell
H
L
60oN
90oN
June Solstice
L
H
90 S
o
60 S
o
H
30 S
o
L
Equator
H
30 N
o
L
60 N
o
H
90oN
Figure 237: Variation in the position of the Inter Tropical Convergence Zone
(Source: http://www.eoearth.org/view/article/156717/ 29/12/13)
Variation in the location of the ITCZ results in the alternation of wet and
dry seasons in the tropics. Most important, the movement of the ITCZ affects
rainfall in southern Africa and the Sahel of western Africa, particularly
vulnerable regions because they have only one rainfall season each year.
When the ITCZ does not migrate as far south as usual, droughts can occur in
Southern Africa; when it migrates further north than usual it brings heavy
rain and floods to the Sahel Region (as happened in 2007). Figure 235 shows
the position of the ITCZ in July and January.
290
July ICTZ
January ICTZ
Figure 238: Shifting of the Inter Tropical Convergence Zone
(Source: http://www.ice-age-ahead-iaa.ca/unauthorized_hurricane.html29/12/13)
Activity
3
Discussing the ITCZ and its impact on weather
Work in groups of four to complete the following activity:
1. Using Figure 238, draw lines to identify and then label the equator,
the Tropic of Cancer, and the Tropic of Capricorn.
2. What latitude receives direct sunlight all year?
3. What impact will this heating have on the surface air?
4. What will happen to this air?
5. Is this air moist or dry? Explain why.
6. What cloud patterns do you expect to observe in the area of the ITCZ?
7. Why does the position of the ITCZ change over a year?
8. Within which latitudes is it usually located?
9. What does the seasonal motion of the ITCZ mean for January and July
climates in India and Malawi?
10. Choose a spokesperson from your group of four to present your work to
the class for discussion.
Activity
4
Reflecting on the topic
1. Summarise the most important ideas you have just discussed about the
topic.
291
2. Why is this knowledge worth having?
3. What can you do about the issues you have been discussing in the unit?
4. Report your answers to the class for discussion.
Summary
Air masses with different temperatures and densities are always colliding
in the atmosphere, but do not mix, thus creating fronts. The collision often
causes storms and changeable weather. The types of fronts discussed in this
module include: cold front, warm front, occluded front, stationary front and
dryline. Each of these has its own weather characteristics. The ITCZ, located
around the Equator, is one of the most notable front regions. It is the birthplace
of some of the world’s strongest storms, characterised by strong convective
activity which generates violent thunderstorms.
Glossary
Front: A line along which one mass of air meets another that is different in
temperature or density.
Squall line: A line of thunderstorms that occur along or ahead of a cold front.
Dryline: A boundary that separates a warm moist air from a hot dry air.
Inter Tropical Convergence Zone: A belt of low pressure which circles
the Earth near the equator where the trade winds from the Northern and
Southern Hemispheres clash.
Review questions
1. Describe any three types of weather fronts.
2. Name the type of front along which the intensity of precipitation is
greater, but the duration shorter.
3. How does the lifting of air compare between a cold front and a warm
front?
4. How is a cold, warm and stationary front depicted on a weather map?
5. Briefly describe what happens along Inter Tropical Convergence Zone
(ITCZ).
6. Describe the climate region which is produced by the Inter Tropical
Convergence Zone.
7. How does the ITCZ help bring precipitation in Southern Africa?
292
References
Bunnett, R. B. (1973). General Geography in Diagrams. London: Longman.
Bunnett R. B. (1984). Physical Geography in Diagrams for Africa. London:
Longman Group Limited.
Leong, G. C. (1983). Certificate Physical and Human Geography. Oxford:
Oxford University Press.
Obara, D. A. (1991). Gold Medal Geography. Nairobi: MacMillan Kenya
Limited.
Waugh, D. (1990). Geography: An Integrated Approach. Hong Kong: Thomas
Nelson Limited.
White, R. (1998). Africa in Focus: A Physical, Human and Economic Geography.
Oxford: MacMillan Education Limited.
http://www.skybrary.aero/index.php/Inter_Tropical_Convergence_
Zone_%28ITCZ%29 29/12/13
http://usatoday30.usatoday.com/weather/news/2009-06-03-inter-tropicalconvergence-zone-thunderstorms_N.htm
http://www.theweatherprediction.com/habyhints2/453/ 29/12/13
http://www.eoearth.org/view/article/156717/ 29/12/13
www.elsenburg.comThe Science of Climate Change in Africa: Impacts and
Adaptation 29/12/13
http://www.ice-age-ahead-iaa.ca/unauthorized_hurricane.html29/12/13
293
Unit
20
Local winds
Local winds
Local winds, also known as periodic winds, are
small scale convective winds of local origin arising
from small-scale pressure differences. Local
terrain has a very strong influence on local winds,
and the more varied the terrain, the greater the
You most likely have influence.
direct
experiences General characteristics of local winds
with winds and their
a.They blow over a much smaller area, covering a
associated effects. In
few hundreds of kilometers.
Unit 17, you learned
how winds form and b.They tend to be short-lived lasting typically
several hours to a day.
why different types of
winds exist. Learning c.They change direction and speed over a short
about local winds will
period of time.
make you more aware
of what influences these There are many such winds around the world, some
winds, and what can of them cold, some warm, some wet, some dry. In
be expected in various some, areas local winds have been identified by
topographic situations. the inhabitants as having a special effect on the
In this unit, you will lives of people. The following are some of the most
explain the meaning of well-known local winds in the world:
the term local winds. Land and sea breezes
You will also explain the
characteristics of local Land and sea breezes are two related local winds
winds and account for common in around locations near large bodies.
their occurrence. You Sea breeze
will then explain the
A sea breeze is wind blowing from the sea towards
occurrence of land and
the coast. During the day, land heats up much
sea breezes. Finally, you
quicker than water. As the land heats up, it warms
will explain the influence
the air close to the ground. The warm air becomes
of land and sea breezes on
less dense and begins to rise. Nevertheless, over
local weather and human
the water, air remains cool and thus denser than
activities.
the warm air over the land. The cool air flows
towards the land to replace the rising air, hence,
sea breeze (see Figure 239).
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Warm air
Cool air
Sea breeze
Land
Sea
Figure 239: Sea breeze
Effects of sea breezes
Sea breeze is important to coastal communities because it keeps these locations
cooler than other locations farther inland during the day and during summer.
Many tourists choose these areas as travel destinations during summer for
heat comfort.
At certain locations, converging sea breezes can cause air to rise, forming
clouds, precipitation, and potentially, thunderstorms because the air mass is
moving in from over water. The precipitation may be crucial to coastal farms
during dry summer seasons.
Land breeze
A land breeze is the type of wind that blows from the land to the sea, and
usually occurs at night.
Warm air
During the night, land cools
Cool air
off much quicker than water.
The water will then be warmer
than the land. The air over the
water slowly begins to rise,
Land breeze
creating a net movement of
cool air from the land surface
Land
towards the water to replace
Sea
the rising warm air (see
Figure 237 below).
Figure 240: Land breeze
Effects of land breeze
Land breezes assist fishermen to go to the sea at night in their small fishing
boats. The land breeze can also help move pollutants out to sea.
295
Activity
1
Investigating differential heating of land and water from
solar radiation
1. Fill one container about half full with soil, and another with water to
the same level.
2. Place thermometers upright into the soil and water, submerging the
ball of the thermometer into the material.
3. Before you put them out in the sun, take the initial temperature,
adjusting the water’s temperature if needed to match the soil’s
temperature.
4. Thereafter, place the containers in the sun so that they all obtain equal
amounts of heat from it. Make sure the thermometers are upright and
not receiving direct sunlight.
5. Observe the temperature of each material in Celsius every 5 minutes
and record the temperature readings for each in a data table like the
one below.
6. After 40 minutes, move the containers into a shade and record the
falling temperatures for about 30 minutes.
Time (in
00 05 10 15 20 25 30 35 40 45 50
Minutes)
Water
Temperature
Soil
Temperature
7. Which material heats and cools faster/slower? Why?
8. How can you relate this experiment to land-sea breezes?
9. What do you think would happen if the earth were covered with over
70% land instead of water?
10. Report your findings to the class for discussion.
The monsoon winds of Asia are giant sea and land breezes produced by
seasonal changes in pressure systems. During summer, the continent of
Asia heats up more than the surrounding oceans; this creates an intense low
pressure area over North-central Asia and India, and a comparatively high
pressure zone over the cool ocean surfaces. Winds therefore blow from the
oceans onto land (Figure 241). During winter, the flow of air reverses. The
continent cools rapidly forming a high-pressure area over land. Now the drier
cold air of the continent blows toward the ocean.
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Low
pressure
S.E.monsoon
High
pressure
Himalayas
N.W.monsoon
Himalayas
S.W.monsoon
N.E.monsoon
Summer
Winter
Figure 241: Monsoon winds of asia
Mountain and valley breezes
Mountain and valley breezes are two related local winds common in regions
with great topographic relief. They occur one after the other on a daily cycle
through a process similar to sea and land breezes.
Valley breeze
A valley breeze (anabatic wind) is a gentle wind blowing up a mountain
slope during the day. When the
Warm air
Warm air
sun rises, it is the top of the
Cool air
mountain peaks that receive
first light, and as the day
progresses, the mountain slopes
take on a greater heat load
than the valleys. This results
in rapid warming of the air over
the mountain slopes, causing it
Figure 242: Valley breeze
to rise. The cool air from the valleys gently moves upslope to replace the rising
warm air, hence, valley breeze (see Figure 243). The air current that moves
uphill is called anabatic wind; from the Greek word “anabatos”,verbal of
meaning “moving upward”.
Mountain breeze
A mountain breeze develops
during the night when air
along the mountain slopes
begins to cool quickly. As the
air cools, it becomes denser
and begins to flow down-slope
into the valley. However,
if the mountain slopes are
covered with ice and snow,
the surrounding air becomes
Cool air
Warm air
Night
Figure 243: Mountain breeze
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Cool air
cold and dense, causing the mountain winds to blow during the day into the
warmer valleys (see Figure 243). The air current moving down a slope by
gravity is called katabatic wind; from the Greek word katabatikos meaning,
“going downhill.”
Drainage/ Descending winds
Drainage winds are local to mountainous regions and can occur only under
calm, clear conditions. Most often, the term refers to winds, which form when
cool air over a high cold mountain is set in motion and descends under the
influence of gravity. Drainage winds are more generally known as katabatic
winds. Examples of drainage winds include the Bora in Northern Adriatic
Sea Coast, the Mistral in Southern France, and the Santa Ana in Southern
California. Mountain breezes discussed above are also an example of drainage
winds.
However, not all down-slope winds are Katabatic. For instance, drier and
warmer rain shadow winds such as the Chinook and Foehn develop when
moist air is driven over a
Condensation and
mountain range and drops its
precipitation
Warm, dry air descends
moisture to form clouds and
on the leeward side
rain on the windward side of
the mountain range. Warm
dry air then descends on the Warm moist Air rises
on the windward side
other side of the mountain
(rain shadow area). For this
reason the windward areas
are greener than the leeward OCEAN
areas(see Figure 244).
Figure 244: Descending winds
Figure 245: Distribution of local winds
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Chinook winds
The Chinook is a wind that flows down the eastern side of the Rocky Mountains
in the Canadian province of Alberta and the State of Montana (Figure 246).
The wind develops when warm moist air from the Pacific Ocean blows into
the western side of the Rocky Mountains. The mountains force the air to rise
causing its moisture to condense in order to form clouds and precipitation.
The condensation of water vapour in this way releases to the surroundings
the latent heat energy that warmed the risen moist air. So the Chinook has
the following characteristics:
a. It is so warm that it causes rapid and large temperature changes in a
short time. This causes the snow to evaporate or melt away, leaving the
ground dry in a space of few hours. It is for this reason that the wind
earned the name Chinook, which means, “snow eater”.
b. It is extremely dry since it results from air that has dropped most of its
moisture on the windward slopes.
c. The wind often moves at a high speed.
d. It is accompanied by a band of flat rainless cloud up high in the sky.
Figure 246: Chinook Winds
Effects of the Chinook
a. This warm, dry wind has a modifying effect on the severity of winter in
the region of its occurrence east of the Rocky Mountains.
b. The wind blows snow across roadways, making driving dangerous.
Trains have been known to be derailed by Chinook winds in the region.
c. Melting of snow allows the nutritious grasses on the plains to sprout,
and this provides winter grazing for livestock. The advent of the Chinook
at a critical period is also the means of saving the herds from freezing.
d. Abrupt temperature changes can cause pneumonia, septicemia and
shipping fever in cattle.
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Foehn winds
The Foehn is a type of down-slope wind that occurs in the leeward side of
the Alps Mountains of Europe (see Figure 247). It occurs when moist winds
from the Mediterranean Sea blow over the mountain. The Foehn wind has the
following characteristics:
a. It is very warm, such that it can raise temperatures by as much as 300c
in just a matter of hours.
b. It is dry since it results from air that has dropped most of its moisture
on the windward slopes.
c. It is associated with serious natural disasters such as droughts, which
can dry up plants and farmlands, worsen forest fires, and melt snow,
causing avalanches and floods.
Figure 247: Foehn winds
Effects of Foehn winds
a. They bring warmer and drier weather.
b. They bring droughts, dry up plants and farmlands, and exacerbate
forest fires.
c. They also melt snow, causing avalanche and floods.
d. Melting of snow makes pasture land ready for animal grazing.
e. The high temperatures also help the grapes to ripe early.
Harmattan winds
The Harmattan is a wind that blows south into the Gulf of Guinea from the
Sahara Desert (Figure 248). It has the following characteristics:
a. It is hot since it originates from a hot desert region. However, the wind
gets cooler with distance, bringing some relief from the oppressive heat
in some areas; this is why the Harmattan has earned the name “doctor
wind”.
b. It is intensely dry, but gets moist with distance because it causes
evaporation of moisture from the ground surface.
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c. It is dusty. The wind usually carries large amounts of dust, which
it transports hundreds of kilometres out over the Atlantic Ocean;
dust from the Sahara has been reported in America. The dust often
interferes with aircraft operations and settles on the decks of ships.
The interruption of transport can bring trade and other activities to a
halt.
Condensation and
Precipitation
Harmattan wind
Evaporation of
moisture
ANTLANTIC
OCEAN
GUINEA
MALI
Sahara desert
NIGER
Figure 248: Harmattan winds
Effects of the Harmattan winds
a. The heavy amount of dust in the air (caused by the wind) can severely
limit visibility and block the sun for several days, comparable to heavy
fog. This costs airlines millions of dollars in cancelled and diverted
flights each year.
b. The wind can cause severe crop damage since it is very hot and intensely
dry.
c. It is a serious health hazard, particularly eye infection due to the dust.
d. It accumulates some amount of moisture and gives rainfall in West
African region, providing relief to the inhabitants of the area.
Chiperoni winds
Chiperoni is a kind of wind experienced in the Shire Highlands of Southern
Malawi. The name is derived from Mount Chiperone, an isolated mountain
peak in northern Mozambique. When winds blow inland from the Indian
Ocean, they are forced to rise over Chiperone Mountain (see Figure 249). It
has the following characteristics:
a. It is accompanied by cold rainy conditions over the Shire Highlands
and the surrounding areas.
b. It is associated with Mwera on Lake Malawi, which disrupts fishing
activities in the lake.
c. It occurs during the cold dry season from May through August.
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Chiperoni wind
Condensation and
Precipitation
Chiperoni
mountain
Shire highlands
MOZAMBIQUE
MALAWI
INDIAN
OCEAN
Figure 249: Chiperoni winds
Activity
2
Case study
Read the following case study and answer the questions that follow:
Nkhata Bay fishermen tragedy
Twelve bodies of fishermen were recovered in Nkhata Bay after they drowned
in Lake Malawi over the weekend of June 7th to 8th 2014. The fishermen were
reported missing after their boats capsized following strong Mwera winds.
Following the tragedy, it was sad to see relations of people whose canoes
were found without people collapsing in shock while others were mourning.
It was feared that more other fishermen had died because people found eight
canoes whose owners were still missing. People only prayed that the bodies
of those that were dead should be washed ashore and identified for proper
burial. The police spokesperson asked people in the district to avoid going for
fishing when there is stormy weather.
1. What was the probable cause of the Mwera winds on the Lake?
2. Suggest the socio-economic impacts of the winds on the affected
communities in Nkhata Bay.
3. What do you think local authorities should do in order to avoid such
incidences in future?
4. Get into groups to compare and discuss your answers.
5. Report your answers to the class for discussion.
Effects of Chiperoni winds
a. The dry conditions have generally provided an environment conducive
to harvesting of various crops.
b. The rains brought by the wind over Shire Highlands are generally
good for the growing of winter crops.
c. The Mwera it brings on Lake Malawi disrupts fishing activities.
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Activity
3
Researching local wind types
1. Choose a local wind type from the following list: Chinook, Foehn,
Harmattan, and Chiperoni.
2. Research your wind type and prepare an oral presentation. The
following information should be included:
a. a map of where the wind occurs
b. factors that cause the wind pattern
c. beneficial effects the wind has in the area it occurs
d. problems the wind brings to the area it occurs
3. Present your work to the class for discussion.
Activity
4
Reflecting on the topic
1. Summarise the most important ideas you have just discussed about the
topic.
2. Why is this knowledge worth having?
3. What can you do about the issues you have been discussing in the unit?
4. Report your answers to the class for discussion.
Summary
There are various types of local winds around the world. Some of the most
prominent examples are land-sea breezes, mountain-valley breezes, Southern
California’s warm and dry Santa Ana Winds, the cold and dry mistral wind
of France’s Rhône Valley, the very cold, usually dry bora wind on the eastern
coast of the Adriatic Sea, the Chinook winds in North America, the Foehn winds
in Europe, the Harmattan winds in North-west Africa and the Chiperoni in
Central-East Africa.Whether winds are local or global, they are an important
component to atmospheric circulation and play an important role in human
life on earth as their flow across vast areas is capable of moving weather,
pollutants, and other airborne items worldwide.
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Glossary
Monsoon wind: a seasonal shift in the prevailing wind direction that usually
brings with it a different kind of weather
Anabatic wind: an air current that moves uphill due to heating
Katabatic wind: an air current moving down a slope by gravity
Avalanche: a mass of snow, ice, and rocks falling rapidly down a mountainside
Review questions
1. What physical mechanism generates a sea-land breeze system?
2. With the aid of well-labelled diagrams, explain how land and sea
breezes occur.
3. Describe two characteristics of Chinook Wind.
4. What is the difference between prevailing and local winds? Give two
points.
5. Why are the windward areas of most mountains and islands greener
than the leeward areas?
References
Bunnett, R. B. (1973). General Geography in Diagrams. London: Longman.
Bunnett R. B. (1984). Physical Geography in Diagrams for Africa. London:
Longman Group Limited.
Leong, G. C. (1983). Certificate Physical and Human Geography. Oxford:
Oxford University Press.
Obara, D. A. (1991). Gold Medal Geography. Nairobi: MacMillan Kenya
Limited.
Waugh, D. (1990). Geography: An Integrated Approach. Hong Kong: Thomas
Nelson Limited.
White, R. (1998) Africa in Focus: A Physical, Human and Economic Geography.
Oxford: MacMillan education Limited.
http://www.weatherwizkids.com/weather-wind.htm 04/06/14
www.cengage.com/.../0495555061_137182.pdf Atmospheric Pressure, Winds,
and Circulation Patterns - Cengage ...29/12/13
304
305
Cyclones and
anticyclones
Unit
21
Cyclones
Cyclones are severe rotating storms caused
by winds blowing around a central area of
low atmospheric pressure. Based on previous
knowledge of major wind patterns and your life
experiences, you should know by now that storms
have caused some of the most devastating effects
on both human and natural environment. The
following activity will help you reflect on some of
the catastrophes caused by such extreme weather
events.
Cyclones are one of
nature’s most destructive
forces, and the study of
these storms will reward
you with the ability to
understand more about
the forces that shape our
world.
Understanding Activity 1
what cyclones are and
Researching catastrophic weather
how they form, will
also help you realise events
the kinds of dangers
1. In groups, go to the library or internet and
cyclone pose to people
research an historic weather event. Good
and property. In this
sources of information include websites,
unit, you will explain the
magazine and newspaper articles, and
development of cyclones
books, if available. During your research
and anticyclones. You
look for
will also explain the
a. What the event was
weather associated with
b. When it occurred
tropical and temperate
cyclones.
c. The economic and human (death) toll
d. The science behind the event
e. The lessons learned in terms of safety
measures.
2. Report your findings to the class for
discussion.
Development of cyclones
Cyclones are formed when warm air rises, creating
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a low-pressure zone. Air with higher pressure tends to move in to replace
the rising warm air. Due to the turning effect of the earth, the air does not
move straight into the low-pressure zone but swirls in around and towards
it. The storms spin in a clockwise direction in the Southern Hemisphere and
anticlockwise in the Northern Hemisphere (see Figure 250). On a weather
map they can be identified by a closed isobar in the central area of low pressure.
L
L
Northen Hemisphere
Southern Hemisphere
Pressure gradient
Surface winds
Generalized wind flow
Figure 250: The flow of air in a cyclone for Northern and Southern Hemispheres
(Source: www.cengage.com/.../0495555061_137182.pdf Atmospheric Pressure, Winds, and Circulation Patterns Cengage ...29/12/13)
Cyclones can and do occur at any latitude and in any climate. There are
generally two types of cyclones based on their geographical origin: temperate
cyclones and tropical cyclones.
Temperate cyclones
Temperate cyclones are often called depressions or extra-tropical cyclones
and are caused by the meeting and imperfect mixing of the warm tropical air
and the cold polar air at the temperate latitudes (60° North and South of the
Equator).
Once it has formed, a temperate cyclone moves across the planet’s surface as a
large storm system. The warm and cold fronts each produce weather systems
that they would ordinarily generate. However, the cold front almost always
moves faster than the warm front. This is because cold air is denser than
warm air. The warm front pushes more slowly, since it is moving a denser
mass out of the way, while the cold front, with its greater mass, moves quickly,
displacing the lighter warm air ahead of it with ease.
Sooner or later, the cold front catches up with the warm front, since they are
rotating around a common centre, but at different speeds. When this happens,
they blend together into an occluded front. Occluded fronts usually signal
the end to the low pressure system (cyclone) driving it.
307
Characteristics of temperate cyclones
a. They have weather fronts as the basis for their whole structure and
existence.
b. They are very large, since they contain two fronts and several air
masses, which also help to make their weather effects last for a long
time.
c. They have a bent oval shape which is often described as a comma.
d. They are associated with unsettled weather conditions, which can
be both long-lasting and various, because they contain air masses of
contrasting temperature and humidity.
e. They usually move over land rather than water, and this limits the
moisture they can pick up. However, they can still produce very intense
thunderstorms, squall lines, powerful winds, hail, and tornadoes.
Tropical cyclones
Tropical cyclones originate exclusively over warm tropical oceans. They are
smaller than the depressions but very violent. Severe heating of the oceans in
the tropical regions causes moisture to evaporate rapidly and rise higher into
the atmosphere. As warm air rises, pressure over the ocean drops dramatically,
causing cooler air from the surrounding high pressure areas to rash in to fill
the space left by the rising warm air. Then that “new” air becomes warm and
moist and rises, too. As the warmed, moist air rises, it cools off and the water
in the air forms towering cumulonimbus thunderclouds and heavy rains.
Fed by the ocean’s heat and water evaporating from the surface, airflow begins
to pick up speed and eventually builds up an enormous storm system which
spins due to the earth’s rotational effect.
The structure of a tropical cyclone
The central part of the tropical cyclone is known as the eye. The eye is usually
30 to 50 km across. It is an area of calm, with light winds and no rain. The low
pressure in the eye lifts water surface in the centre (Figure 251). The rising
water may be as high as 3–12 meters, and it appears like a water-wall moving
towards the shore.
Surrounding the eye is a region of high-speed winds (150–250 km per hour)
and thick cumulonimbus clouds with heavy rain – the vortex. These weather
conditions in the vortex are caused by moist air condensing as it rises. This
region is the most violent part of the storm. Away from this region the wind
speed gradually decreases.
308
Diverging airflow in
upper atmosphere
Eye
Thunderclouds
Never
surface
convergence
of moist
warm air
Vortex
Ocean
Vortex
Figure 251: Cross section of a cyclone
(Source: http://worldlywise.pbworks.com/ 30/12/13)
Conditions necessary for the formation of a tropical cyclone
Tropical cyclones develop under the following conditions:
a. There must be warm water surface. The ocean water must be warmer
than 27°C. The heat and moisture from this warm water is ultimately
the source of energy for the cyclones.
b. There must be low-pressure to cause the warm moist air to rise.
c. There must be sufficient amount of Coriolis force. This would cause the
storm to spin as cool air flows into the low pressure centre. Cyclones
cannot occur along the equator, as there is insufficient Coriolis force to
deflect the air moving towards the low-pressure centre.
d. The vertical wind speed (shear) must be low. Under this condition
the heat and moisture are retained rather than being exchanged and
diluted with the surrounding air.
Characteristics of tropical cyclones
a. They occur only on warm water surfaces. Tropical cyclones weaken
rapidly when they hit land or colder ocean water because these locations
no longer have a stream of warm moist air to maintain them.
b. They are almost perfectly round.
c. They have very little temperature difference from one part of the storm
to another, though they are generally warmer at the centre.
d. They have no fronts.
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e. They come with strong winds, heavy rainfall and severe thunderstorms
that can often last for a long period (as much as two to three weeks).
Figure 252 shows a satellite image of a tropical cyclone just before it made
landfall.
Figure 252: Satellite image of a tropical cyclone just before it made landfall
(Source: http://www.physicalgeography.net/fundamentals/7u.html 30/12/13)
Activity
2
Modelling a cyclone
1. Remove the labels and the lids from both bottles.
2. Join the lids of the bottles together (flat sides together) using the glue
and strong tape.
3. Using the nail and the hammer, carefully punch a large hole through
the middle of the joined lids. You will need a hole large enough to let
water flow through. You may need to experiment to find the best width.
(Hint: at least 1/2 cm in diameter.)
4. Fill the plastic bottle with water until it reaches around three quarters
full.
5. Add a few drops of dish washing liquid.
6. Sprinkle in a few pinches of glitter (this will make your tornado easier
to see).
7. Screw the joined lids onto the full bottle and then screw the empty
bottle onto the top of the full bottle. Make sure the seal is tight.
8. Turn the bottles upside down so that the one full of water is on the top
and hold it by the neck (see Figure 253). Spin the bottle in a circular
motion really fast for a few seconds. Do not shake it up and down or it
won’t work.
310
9. Describe what you see happening to the water in
the bottle.
10. Describe the shape that forms when the water
spins.
11. How does the ‘cyclone in a bottle’ model a real
cyclone? How is it different?
12. Get into small groups to share your responses
and then meet together as a class to discuss
the similarities and differences between the
simulation cyclone and a real cyclone.
Figure 253: Cyclone in a bottle
Distribution of tropical cyclones
Tropical cyclones are called by different names in different places: they are
known as cyclones in the Indian Ocean, hurricanes in the Caribbean Islands,
willy-willies in the North West Australia, and typhoons in the China Sea
(see Figure 254). All of these terms refer to the same weather event.
Figure 254: Distribution of tropical cyclones
Hazards associated to cyclones
a. Flooding: since it develops over ocean surfaces, the rotating storm
may be pushed toward the shore. This advancing surge, combined with
the normal tides, can increase the average water level, thereby causing
severe flooding in coastal areas (see Figure 255). The thunderstorm
activity in a tropical cyclone produces intense rainfall, potentially
resulting in flooding, mudslides, and landslides.
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b. Destruction of structures: many homes are damaged or destroyed
when the high wind simply lifts the roof off of the dwellings. The debris
picked up by the storms (i.e. wood, metal siding, toys, trash cans, tree
branches, etc.) is thrown at high speeds into other structures, causing
widespread damage.
c. Physical injuries and deaths: injuries and deaths result from
building collapse, wind-strewn debris, drowning, fires and electrocution
due to destruction of main electricity lines.
d. Water borne diseases and vector transmitted diseases: human
exposure to disease vectors can be increased due to changes in the
physical environment.
e. Interruption of land transport, air travel and shipping: tropical
cyclones often destroy key bridges, overpasses, and roads, complicating
efforts to transport food, clean water, and medicine to the areas that
need it. Tropical cyclones on the open sea cause large waves, heavy
rain, and high winds, disrupting international shipping and, at times,
causing shipwrecks.
f. Crop damage: tropical cyclones destroy crops, farm machinery,
sheds, fences, and livestock, which in turn lead to food shortages and
unemployment.
g. Disruption of economies: cyclones lead to the loss of investments
and jobs; for example, destruction of factories, closure of shops, small
businesses, and industrial production units, etc. During the emergency,
people must leave their jobs and devote their time to disaster related
activities such as search-and-rescue or caring for survivors. During
this period, normal economic activities are severely curtailed. In
addition increased expenditures for relief and repair or replacement of
infrastructure at a time when there is an overall decrease in economic
activity create a financial burden on the government.
h. Stress: people may suffer from stress due to loss of possessions and
housing.
i. Environmental damage: sensitive ecosystems may be destroyed
and plant and animal habitats lost. Sea fish are often killed because of
silting, and freshwater fish may be killed in storm surges.
Surge 15 ft
17 ft Storm tide
2 ft Normal high tide
Mean sea level
Figure 255: Storm surge (Source: http://worldlywise.pbworks.com/ 30/12/13)
312
Tornadoes
Tornados are rotating columns of air which extend from the base of a cloud
to the ground (see Figure 256). They rotate counterclockwise in the northern
hemisphere and clockwise south of the equator, although some (usually very
weak tornados) rotate in the opposite direction.Tornadoes are the most violent
storms on earth. They differ from cyclones in that they generally develop over
land, and are relatively small, with a short lifespan. However, they are more
destructive than cyclones as the speed of winds is very high, exceeding 480
km per hour.
Figure 256: Tornado
(source: http://mytornadoalley.files.wordpress.com/2009/10/tornadoes.jpg 30/12/13)
Tornadoes have been observed on every land surface except Antarctica.
However, the United States is subject to far more tornadoes than other regions
of the planet. Between 800 and 1,200 tornadoes are being seen annually in
the country. They are especially feared in the Mississippi Valley, east of the
Rocky Mountains and here they are called twisters.
Why is the USA subject to far more tornadoes than other
regions on the planet?
The answer lies in the following:
a. The shape of the North American continent. Unlike other continents,
North America has no mountain ranges that extend from east to west,
only North to South. Arctic air and tropical air can therefore clash over
the plains of the central USA without any natural barrier to mitigate
their contact.
b. The Rocky Mountains and the hot dry Chinook winds. This dry air
descending from the Rockies on the eastern side presses down on the
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hot, wet air flowing in at ground level from the Gulf of Mexico, creating
regular low pressure areas with intense instability. This provides the
USA with a record-breaking supply of tornadoes.
Activity
3
Case study - Cyclone Tracy
Read an extract from a news report below and answer the questions that
follow:
Cyclone Tracy was a devastating cyclone that struck Darwin on Christmas
morning 1974. Cyclone Tracy is known as one of Australia’s worst cyclone
because it occurred on Christmas day.
Cyclone Tracy was small and was first detected on the December 20th 1974
in the Arafura Sea 370 km north east of Darwin. It was first announced as a
tropical cyclone on the 21st December, before then it was considered a tropical
low. Although a warning was issued no one paid attention to it because they
had had many false alarms before. As the cyclone occurred on Christmas day
the resident were not prepared.
The cyclone lasted two days and ended on the 26th of December. 71 lives
were lost, 64 on land and 7 out at sea. More than 30,000 people were left
without a home. All power lines, communications, water and sewage were
also destroyed. 90% of all buildings were destroyed and had to be rebuilt.
Darwin was almost entirely rebuilt according to new cyclone codes to make the
buildings stronger and safer in the event of a cyclone striking again. Many of
the evacuated citizens never returned to Darwin and found homes elsewhere.
Darwin’s population has slowly increased and now has increased to 127,532
people.
1. Based on the report, why do people in general pay little attention to the
weather? Do you think this is a good idea? Why or why not?
2. What effects do you think cyclones have on the natural and human
environment?
3. Why do you think Cyclone Tracy was so devastating even though it was
a small cyclone?
4. Suggest how people can prevent or reduce the damage caused by tropical
cyclones.
5. Present your work to the class for discussion.
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Anticyclones
Anticyclones are opposite to cyclones in all respects. They are the centres of
high pressure with gentle outward flow of air. Anticyclones are created by dry
air masses. Dry air is heavier than a similar volume of wet air, so it tends to
sink and compress, forming an area of high pressure. The fact that that air is
sinking means that winds flow outwards from the high pressure area’s center
at ground level, where the earth’s surface itself prevents the heavy, dry air
from sinking any more.
These winds spiral outwards in a clockwise direction in the Northern
Hemisphere and in a counterclockwise pattern in the Southern Hemisphere,
due to the effects of the earth’s rotation (see Figure 257). Anticyclones can
be very large, typically at least 3,000 km wide which is much larger than
depressions. Once they become established, they can give several days of
settled weather.
H
Northern Hemisphere
H
Southern Hemisphere
Pressure gradient
Surface winds
Generalised winds flow
Figure 257: The flow of air in an anticyclone for Northern and Southern Hemispheres
Characteristics of anticyclones
a. They have cloudless and rainless weather conditions since air descends
at the centre due to high pressure. There are two bands of permanent
high pressure, one north and one south of the equator at about 30
degrees. It is on these latitudes that permanent anticyclones form,
ensuring everlasting clear weather and creating some of the world’s
mightiest deserts.
b. They only involve one type of air mass which usually covers large areas
and do not have any fronts.
c. Their centres are cooler than their surroundings.
d. Anticyclones have very light winds blowing outwards from the centre.
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Hazards of anticyclones
While most high pressure weather is fair, anticyclones may sometimes produce
some adverse weather conditions. The following hazards can result:
a. Prolonged heat waves or drought. During the summer, a persistent
warm core anticyclone that becomes stationary over a region may
produce drought conditions (due to lack of rainfall and evaporation
from the earth’s surface) causing death, crop failures, and wild fires.
b. Ice caps and glaciers melt at a faster rate in mountainous regions,
causing floods. Rivers and lakes that are not glacially fed can dry up.
Activity 4
Matching weather condition to the cause.
The following statements and weather conditions apply to anticyclones.
1. Match the weather condition to the cause.
Weather condition
Cause of weather condition
Dry weather
Hot sunny days
High pressure
Long days
high pressure, high sun angle, descending air, long hours of daylight
2. Present your work to the class for discussion.
Activity 5
Developing a storm safety plan
1. In groups, create a safety plan and kit for your school in the case of a
hurricane or other likely storm for your area. In your plan, consider the
following:
a. What first aid items should be in the kit?
b. What other items should be included? Why?
c. Where should a person go to escape the storm?
d. If a person can’t escape the storm, what is the next best thing to do?
e. After the storm, what should a person do to keep safe and also help
others who may have been harmed in the storm?
2. Present your ideas to the class for discussion.
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Summary
Cyclones and anticyclones are two types of pressure centers. Cyclones are
centers of low pressure, whilst anticyclones are high-pressure centers. There
are two types of cyclones, namely, depressions or extratropical cyclones and
tropical cyclones. Depressions, just like tornadoes, are caused by the meeting
and imperfect mixing of contrasting air masses. Tropical cyclones originate
exclusively over warm tropical oceans and are caused by severe heating. The
Coriolis Effect causes cyclones to have a counterclockwise wind circulation in
the Northern Hemisphere and a clockwise one in the Southern Hemisphere.
Depressions or cyclones are associated with cloudier, wetter, and windier
conditions. Anticyclones, as atmospheric systems opposite to cyclones, have
a clockwise circulation in the Northern Hemisphere and a counterclockwise
one in the Southern Hemisphere. Anticyclones typically result in stable, fine
weather, with clear skies.
Glossary
Cyclone: a large-scale storm system with heavy rain and winds that rotate
around and toward a low-pressure center
Eye: a calm area at the center of a storm
Vortex: a region within a storm where a mass of air swirls violently as it rises
Anticyclone: a large system of atmospheric high pressure marked by
circulating winds, bringing generally settled weather.
Tornado: an extremely destructive funnel-shaped rotating column of air that
passes in a narrow path over land
Twister : a tornado
Review questions
1. State the two types of cyclones.
2. Briefly describe how a cyclone is formed.
3. Using a well-labelled diagram, describe the flow of air in a cyclone in
the Southern Hemisphere.
4. State two favourable conditions for the formation of a tropical cyclone.
5. Describe any three characteristics of a tropical cyclone.
6. Briefly describe the weather associated with anticyclones.
7. Figure 258 indicated features of an idealized middle-latitude cyclone.
Use it to match the following responses with the correct letter: warm
front, region of lowest pressure, cold front, mT air mass, cP air mass.
317
B
L
E
A
D
C
Figure 258: Cyclone
(Source: http://wps.prenhall.com/wps/media/tmp/labeling/2131732_dyn.gif 11/02/14)
References
Bunnett, R. B. (1973). General Geography in Diagrams. London: Longman.
Bunnett R. B. (1984). Physical Geography in Diagrams for Africa. London:
Longman Group Limited.
Leong, G. C. (1983). Certificate Physical and Human Geography. Oxford:
Oxford University Press.
Obara, D. A. (1991). Gold Medal Geography. Nairobi: MacMillan Kenya
Limited.
Waugh, D. (1990). Geography: An Integrated Approach. Hong Kong: Thomas
Nelson Limited.
White, R. (1998). Africa in Focus: A Physical, Human and Economic Geography.
Oxford: MacMillan Education Limited.
http://scienceweb.asta.edu.au/years-5-6/unit3/lesson-three/yr56-unit3-lessonthree.html 03/02/14
http://blog.metservice.com/tag/depression/
www.cengage.com/.../0495555061_137182.pdf‎Atmospheric Pressure, Winds,
and Circulation Patterns - Cengage ...29/12/13
http://worldlywise.pbworks.com/ 30/12/13
http://www.physicalgeography.net/fundamentals/7u.html 30/12/13
http://scienceweb.asta.edu.au/years-5-6/unit3/lesson-three/yr56-unit3-lessonthree.html 03/02/14
318
http://worldlywise.pbworks.com/ 30/12/13
http://mytornadoalley.files.wordpress.com/2009/10/tornadoes.jpg 30/12/13
http://wps.prenhall.com/wps/media/tmp/labeling/2131732_dyn.gif 11/02/14
319
Clouds
Unit
22
You most likely observe
clouds on a regular
basis. Clouds are the
atmosphere’s way of
moving water from one
place to another. Clouds
have a huge and complex
effect on the flow of energy
in the earth’s atmosphere.
It is important therefore
that you understand
what clouds are and
the relevance clouds
have to our daily lives.
Knowledge about clouds
will also enable you to
predict with confidence
how cloud cover will be
affected by changing
atmospheric conditions.
In this unit, you will
explain how clouds are
formed and identify the
main types of clouds.
Clouds
A cloud is a visible collection of a large number of
tiny water droplets or ice particles being carried
by current of air. Almost all the air around us is
moist. That means that it contains water in the
form of vapour. Most of the time, water vapour
in the air cannot be seen unless it collects and
condenses to form a cloud. When you look into the
sky and see a cloud, it is actually moisture you are
seeing.
How do clouds form?
Clouds form when the invisible water vapour in
the air condenses into visible water droplets or ice
crystals. There is water around us all the time in
the form of tiny gas particles, also known as water
vapour. There are also tiny solid particles of dust
and soot floating around in the air these are called
aerosols. The water vapour and the aerosols are
constantly bumping into each other. When the air
is cooled to dew point, some of the water vapour
turns into liquid water droplets and sticks to the
aerosols when they collide (condensation). Dew
point is the temperature at which the air cannot
hold all the moisture in it and dew begins to form.
Bigger water droplets eventually form around the
aerosol particles. When billions of these droplets
come together they become a visible cloud.
Clouds form when the air is saturated and cannot
hold any more water vapour, this can happen
under the following conditions:
a. Increased amount of water in the air:
For example, through evaporation to the
point that the air cannot hold any more
water.
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b. Low atmospheric temperature: The air should be cooled to its dew
point so that it contracts and is unable to hold any more water. This
turns the water vapour into liquid.
c. Soot and dust particles (aerosols or condensation nuclei): These
provide a surface on which water molecules should collect. This dust
comes from the earth’s surface.
Activity
1
Observing clouds
1. In groups of five, go outside and observe clouds.
2. What makes one cloud different from another?
3. What do you think the clouds are made up of?
4. Discuss what you think happened that allowed the clouds to form.
5. Report your findings to class for discussion.
Types of clouds
There are many different types of clouds. The type of cloud depends on how
high up in the atmosphere the water condenses and its appearance (texture)
from the ground.
Classification based on appearance
Latin words are used to describe the appearance of clouds as seen by an observer
on the ground. Table 10 below summarises the four principal components of
this classification system.
Table 10: Classification of clouds
Latin Word
cumulus
stratus
cirrus
nimbus
Translation
heap
layer
curl of hair
rain-bearing
Example
fair weather cumulus
altostratus
cirrus
cumulonimbus
The word ‘nimbus’ in front of any type of word or cloud name means a cloud
that produces precipitation.
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Activity
2
Identifying clouds
1. In groups of five, get a copy of a cloud chat like the one in Figure 259
below.
Figure 259: Cloud chat
(Source: http://www.rmets.org/weather-and-climate/resources/cloudwheel-cloud-identification03/02/14 – date
accessed)
2. Go outside and observe the clouds.
3. Identify cloud types in the sky and find them on their cloud wheel.
4. Give words or phrases that describe the clouds (e.g. round, puffy, flat,
thin, etc.).
5. Compare your work with the other groups.
6. Present your work to the class for discussion.
Classification of clouds based on height
Clouds can occur at any level of the atmosphere wherever there is sufficient
moisture to allow condensation to take place. Clouds are classified into three
main groups: low, middle and high level clouds.
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Low level clouds
Low level clouds are closer to the ground; so when observed from the ground,
they appear to move faster than other clouds. The clouds generally move in
the direction of the surface wind. Examples of the low level clouds are: stratus,
cumulus and stratocumulus.
Stratus clouds
Stratus clouds are the lowest clouds and sometimes appear at ground level
in the form of mist or fog.
Stratus clouds look like flat
sheets are a fairly uniform
grey or white colour and
usually cover most of the
sky (see Figure 260). The
sun or moon may shine
through if there are no
other clouds above the
layer of stratus cloud.These
clouds may be accompanied
by drizzle, snow or snow
Figure 260: Stratus clouds
grains.
Cumulus clouds
Cumulus clouds look like big fluffy balls of cotton usually spotted in fair
weather. If they get bigger they can sometimes produce showers. Although
their base is usually relatively dark, the top of these clouds are mostly
brilliant white when lit by
the sun. Cumulus clouds
usually form alone, and
there is a lot of blue sky
between different clouds.
Figure 261 is an example
of cumulus clouds.
Figure 261: Cumulus clouds
323
Stratocumulus clouds
Stratocumulus clouds tend to spread more horizontally rather than vertically.
They usually form in rows
or patches, with blue sky in
between (see Figure 262) .
The color of stratocumulus
clouds can be from white to
dark gray. Stratocumulus
clouds can be present in all
types of weather conditions,
from dry settled weather to
light rain and snow.
Figure 262: Stratocumulus clouds
Middle level clouds
Middle level clouds develop in the middle layers of the atmosphere. These
clouds are brighter and less fragmented in appearance due to their distance
from the ground and the higher composition of ice crystals. Middle level clouds
tend to, apparently, move slower than the lower level clouds. Examples of
middle level clouds are: altocumulus, altostratus and nimbostratus.
Altocumulus clouds
These clouds look like puffy gray balls called cloudlets, and sometimes appear
in rows. The sides away from the sun are shaded, so they are usually darker
than the rest, and this helps
to set them apart from
higher cirrocumulus clouds.
If you see these clouds on
a hot summer morning
it often means that there
will be unstable conditions
with thunderstorms in the
afternoon. Figure 263 is
an example of altocumulus
clouds.
Figure 263: Altocumulus
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Altostratus clouds
Altostratus cloud appears as a flat, smooth dark grey sheet. Altostratus clouds
cover the entire sky over a large
area, and usually produce steady
precipitation ahead of a storm(see
Figure 264). Even though you can
see the sun through altostratus
clouds, the clouds do not let enough
sunlight through to produce
shadows, which is how you can
differentiate between altostratus
and nimbostratus.
Figure 264: Altostratus
Nimbostratus clouds
Nimbostratus can be described as a widespread light grey or white sheet of cloud
thick enough to block out the sun(see Figure 265). It is difficult to determine
the apparent speed and direction of
nimbostratus because of its lack of
contrast. Nimbostratus clouds are
often accompanied by continuous
heavy rain or snow and cover most
of the sky. If there is hail, thunder or
lightning it is a cumulonimbus cloud
rather than nimbostratus.
Figure 265: Nimbostratus
High level clouds
High level clouds are in the highest and coldest levels of the troposphere. They
mostly appear brilliant white because the water drops have turned into ice
crystals at that level. In most cases, the direction of movement of the higher
level clouds does not necessarily represent the wind direction at the ground
level. In fact, the wind at upper and ground levels often differ. Examples of
high level clouds are: cirrus, cirrostratus and cirrocumulus.
325
Cirrus clouds
Cirrus clouds are usually short, detached, hair-like and quite thin. Generally
they do not produce precipitation because there is very little water vapour
at the height at which they form.
In the day time, cirrus clouds are
whiter than any other cloud in the
sky. However, they may take on the
colours of the sunset or sunrise while
the sun is setting or rising. Figure
266 is an example of cirrus clouds.
Cirrostratus clouds
Figure 266: Cirrus clouds
Cirrostratus clouds, as shown in Figure 267 below, are transparent and cover
large areas of the sky. They sometimes produce white or coloured rings, spots
or arcs of light around the sun or moon.
When shining through cirrostratus
clouds, the sun will normally cast
shadows, which can help distinguish
the clouds from similar nimbostratus
clouds. Presence of cirrostratus clouds
usually mean that there will be rain
or snow within 24 hours.
Figure 267: Cirrostratus clouds
Cirrocumulus clouds
Cirrocumulus clouds are brilliant white but with uneven appearance. They
can look like small rounded puffs or
cotton balls (cloudlets) either alone or in
rows and are relatively rare. Composed
almost entirely from ice crystals, the
little cloudlets are regularly spaced,
often arranged as ripples in the sky,
and this is how you can tell that they
are cirrocumulus clouds, and not cirrus
or cirrostratus clouds (see Figure 268 ).
Figure 268: Cirrocumulus clouds
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Clouds of great vertical extent
The cumulonimbus cloud: Cumulonimbus clouds are the tallest of all
clouds that can span all cloud layers
(see Figure 269). Commonly known
as thunderclouds, the base is often flat
and very dark. Cumulonimbus clouds
are associated with extreme weather
such as heavy torrential downpours,
hail storms, lightning and tornados.
It is a cumulonimbus cloud rather
than nimbostratus if there is thunder,
lightning or hail.
Figure 269: Cumulonimbus clouds
(Source: http://carramar-natural-disasters.
wikispaces.com/ 30/12/13)
Activity
3
Creating a cloudscape
1. In groups of four, use construction paper, cotton balls, glue and crayons
to create a scene that incorporates all the types of cloud discussed
above.
2. Make sure your cloudscapes show the clouds in relation to ground level
and should depict the attributes discussed.
3. Place the clouds in order from high-level clouds to surface clouds.
4. Come back together as a class to present your cloudscapes for discussion.
Measuring cloud cover
Cloud cover refers to the fraction of the sky obscured or covered by clouds
when observed from a particular location. An okta is a unit of measurement
used to describe the amount of cloud cover at any given location such as a
weather station. Sky conditions are estimated in terms of how many eighths
of the sky are covered in cloud, ranging from 0 oktas (completely clear sky)
through to 8 oktas (completely overcast). In addition, there is an extra cloud
cover indicator ‘9’ indicating that the sky is totally obscured (i.e. hidden from
view); usually due to dense fog or heavy snow (see Table 11 below.)
327
Table 11: Oktas – the amount of cloud is shown by the amount of shading in the circle.
Symbol
Scale
Cloud cover
0 Oktas
Clear sky
1 Oktas
12.5% (sky almost clear)
2 Oktas
Activity
25% cloud cover (scattered clouds)
3 Oktas
37.5% (sky partly cloudy)
4 Oktas
50% (sky half cloudy)
5 Oktas
62.5% cloud cover
6 Oktas
75% (sky mostly cloudy)
7 Oktas
87.5% cloud cover
8 Oktas
100% (sky completely cloudy)
9 Oktas
Sky obscured from view
4
Estimating cloud cover
1. Choose a spot in your schoolyard from where you can observe the sky.
2. Imagine that the visible sky is a circle.
3. Draw a circle and estimate how much of it is covered by clouds.
4. Shed in the circle to show the amount of cloud in the sky.
5. Get into small groups to compare your diagrams and then meet together
as a class to discuss the cloud cover.
Importance of clouds
Clouds complete the following important functions to the earth-atmosphere
system:
a. They help regulate the earth’s energy balance by reflecting and
scattering solar radiation and by absorbing some of the energy emitted
by the earth’s surface. As clouds move in the atmosphere, they carry
with them the heat they have absorbed in hot regions and re-radiate it
back down toward the surface of other regions of lower temperatures.
328
This helps to redistribute extra heat, for instance, from the equator
toward the poles.b. Clouds are required for precipitation to occur and,
hence are an essential part of the hydrologic cycle.
c. Clouds indicate what type of atmospheric processes are occurring (e.g.,
cumulus clouds indicate surface heating and atmospheric turbulence).
How do clouds cause rain?
To get rain, the water condensing in the clouds has to become heavy enough
to fall to earth. The tiny droplets just aren’t heavy enough to fall. To become
heavier, the droplets need to acquire more water and become larger. Some
will collide with other droplets and become larger, and others will grow as
water condenses out of the air directly into the droplet. This process will be
happening to millions of tiny droplets in the cloud, all growing at the same
time, but at different speeds.
Eventually, if the droplets keep growing, they will reach a mass where they
cannot stay floating in the cloud because they are too heavy and will start to
fall to earth as rain!
Activity
5
Reflecting on the topic
1. Summarise the most important ideas you have just discussed in the
topic.
2. Why is this knowledge worth having?
3. What can you do about the issues you have been discussing in the unit?
4. Report your answers to the class for discussion.
Summary
Clouds form when the atmosphere can no longer hold all the invisible water
vapour. Any more water vapour condenses into very small visible water drops
that collect into clouds.There are three families of clouds based on how they
look. They are Cirrus (curl of hair), Cumulus (heap) and Stratus (layers).
Clouds are also grouped by their height above the ground. Low-level clouds
include: Stratocumulus, Stratus, and cumulus. Medium-level clouds include:
Altocumulus, Altostratus, and Nimbostratus. High-level clouds include:
Cirrus, Cirrocumulus, and Cirrostratus. Cumulonimbus clouds are the biggest
clouds of all, and span all 3 altitude ranges. Each cloud carries a message
about the weather to come, so meteorologists use clouds to help them make
forecasts.
329
Glossary
Aerosols: extremely-fine solid or liquid particles suspended in air
Condensation: the conversion of a vapor or gas to a liquid
Dew point: the temperature at which the air cannot hold all the moisture in
it and dew begins to form
Okta: a unit of measure used to specify the amount of cloud cover, equivalent
to enough clouds to cover one eighth of the sky
Review questions
1. Briefly describe how clouds are formed.
2. What is the function of each of the following in cloud formation?
a. Condensation nuclei
b. Dew point.
3. What is the basis of cloud classification?
4. Why are high clouds always thin?
5. What does the word “stratus” mean?
6. Which clouds look like heaps of cotton?
7. Violent thunderstorms are most likely to form from what type of clouds?
8. Describe how cloud cover is measured.
9. Explain two reasons why clouds are important.
References
Bunnett, R. B. (1973). General Geography in Diagrams. London: Longman.
Bunnett R. B. (1984). Physical Geography in Diagrams for Africa. London:
Longman Group Limited.
Leong, G. C. (1983). Certificate Physical and Human Geography. Oxford:
Oxford University Press.
Obara, D. A. (1991). Gold Medal Geography. Nairobi: MacMillan Kenya
Limited.
Waugh, D. (1990). Geography: An Integrated Approach. Hong Kong: Thomas
Nelson Limited.
White, R. (1998). Africa in Focus: A Physical, Human and Economic Geography.
Oxford: MacMillan Education Limited.
330
http://www.rmets.org/weather-and-climate/resources/cloudwheel-cloudidentification03/02/14 – date accessed‎
http://www.metoffice.gov.uk/learning/clouds/high-clouds
http://www.metoffice.gov.uk/learning/clouds/high-clouds
http://www.metoffice.gov.uk/learning/clouds/high-clouds
http://carramar-natural-disasters.wikispaces.com/ 30/12/13
331
Unit
23
Precipitation
Precipitation
Precipitation is any form of water (liquid or solid)
falling from the atmosphere. Water evaporates
from the bodies of water on the earth’s surface and
rises into the atmosphere. As air rises, it cools and
cannot hold as much moisture as it could when
it was warmer. As a result it condenses into tiny
liquid water droplets, some of which may freeze
into ice particles depending on the atmospheric
temperature and altitude at which they form.
These droplets come together and produce clouds.
When cloud particles become too heavy to remain
suspended in the air, they fall to the earth as
precipitation.
There are several types
of
precipitation
that
place experience around
the world. Although all
of the different types of
precipitation come from
one source the clouds,
each type of precipitation
is different and forms
under different weather
conditions. It is very Activity 1
helpful to distinguish Sharing
basic
knowledge
and
what the difference is anticipations
and why certain types
1. Write down what you already know and
of precipitation fall from
what you want to know about precipitation.
the clouds. In this unit,
you will explain forms
2. Now, get into groups and share your ideas
of
precipitation.
You
and anticipations.
will also explain how
3. Draw a table like the one below on a chart.
precipitation is formed.
Know
Want to know
Learned
4. Write down what you already know in the
first column and what you want to know
in the middle column. You will fill in what
you will have learned in the third column
at the end of the unit.
5. Display the chart in front of the class for
reference.
332
Precipitation occurs either in liquid form (e.g. rain, drizzle and freezing rain)
or in solid form (e.g. hail, sleet andsnow).
Liquid precipitation
Rain: Rain is liquid water falling visibly in separate drops from the atmosphere.
In order for the raindrops to become heavy enough to fall, droplets of water
in the cloud collide together with other droplets and other particles in the
air - like soot and dust to become larger. Once the drops become too heavy to
stay in the cloud, we get rain. Technically, rain is not just any liquid that falls
out of the sky. Rain can have water droplets of up to 6 mm in diameter, but
anything less than 0.5 mm in diameter is classed as drizzle.
Necessary conditions for formation of rain
The following conditions are necessary for rain to occur:
a. There should be sufficient amount of evaporation from the water bodies
to supply water vapour into the atmosphere.
b. There should be wind to carry the water vapour from one place to
another.
c. There should be cooling of the rising moist air to condense into clouds.
d. There must be sufficient nuclei (solid particles of dust, smoke, etc) in
the atmosphere to aid condensation.
e. Temperatures near and above the earth’s surface should be above the
melting point of water.
Sometimes rain evaporates before it reaches the ground, resulting in virga.
If you look into the distance and see
gray streaks below a cloud that do
not reach the ground, you are seeing
virga (see Figure 300 below).
Another reason rain may not reach
the ground is updrafts. If the wind
is blowing upward faster than the
rain is falling, the rain cannot reach
the ground.
Figure 300: Virga
(Source: http://www.publicdomainpictures.net/view-image.
php?picture=black-cloud-no-rain&image=1171&large=1
30/12/13)
333
Please note! When rain becomes mixed with pollutants such as sulfur oxides
and nitrous oxides, acid rain occurs. Acid rain kills plants and pollutes water
sources. Acid rain is one of the reasons the Appalachian Mountains are littered
with dead trees. Because of industrialised areas to the west, the Appalachian
Mountains experience a large amount of acid rain.
Drizzle
A drizzle is liquid precipitation that reaches the surface in the form of drops
that are less than 0.5 millimeters in diameter larger than the droplets in
clouds, but smaller than raindrops.
Rain and drizzle are generally beneficial for plants. However, excessive rain
can cause significant runoff and erosion that can damage fields and wash out
plants or drown their roots.
Freezing rain
Freezing rain is rain droplets which fall in supercooled liquid form, but freeze
on impact with the ground or another object to form clear ice. Freezing rain
develops as falling snow encounters a layer of warm air deep enough for the
snow to completely melt and become rain. As the rain continues to fall, it
passes through a thin layer of cold air just above the surface and is super
cooled. Because they are supercooled, they instantly refreeze upon contact
with anything that is at or below 0oC, creating a glaze of ice on the ground,
trees, power lines, or other objects, hence freezing rain. Figure 301 illustrates
how freezing rain is formed.
Tempereture
3 km
Snow
2
deep warm layer
Snow melts completely
1
rain drops become “supercooled” in cold
air and freeze on contact with the surface
causing FREEZING RAIN
Shallow cold layer
0oC
0oC
0oC
Figure 301: Formation of freezing rain
(Source: http://www.nssl.noaa.gov/education/svrwx101/winter/types/ 30/12/13)
Freezing rain coats the roads, sidewalks, and runways with ice making
travel not only difficult, but often deadly.Heavy freezing rain can also cause
334
a lot of damage to trees, plants and also power lines when the accumulated
precipitation weighs down branches and snaps them off.
Solid precipitation
Hail: Hail is solid precipitation in the form of balls or pieces of ice known as
hailstones. Hail only forms in cumulonimbus clouds - more commonly known
as thunderclouds. In thunderclouds, drops of water are continuously taken
up and down though the cloud. When they go to the top of the cloud, it is very
cold and they freeze into ice
and are continually bounced
Hail too large
up and down inside the cloud.
for cloud to hold
Hail growing in circulating
falls to earth
As they rise and fall like this,
convection currents
causing strong
more ice builds up in layers
cold downdraft
around them until they reach
such a size that gravity takes
over and pulls them to the
ground (see Figure 303). By
this time they are big balls of
ice, and so do not have time
to melt before they reach the
Freezing Level
ground. Hailstones can vary
in size from 5 mm to 150 mm
in diameter, however most
hailstones are smaller than
25 mm.
Rain drops being sucked
into the updraft
Figure 302: Hail formation
(Source: http://bsmearthscience.blogspot.com/ 30/12/13)
Hail is most common in areas with warm summers where there is enough
heat to cause the uplift of air. The hail stone reaches the ground as ice since
it is not in the warm air below the thunderstorm long enough to melt before
reaching the ground.
Hailstones have been known to cause extensive damage to buildings, car
windscreens and crops, and have caused fatalities of animals and even humans
caught in the open.
Snow: Snow is tiny ice crystals stuck together to become snowflakes. If
enough ice crystals stick together, they will become heavy enough to fall to
the ground. For snow to reach the earth’s surface the temperature in the air
needs to be at or below freezing. Figure 302 shows how snow is formed.
335
Temperature
Snow
3 km
2
Precipitation falls as SNOW when air
temperature remains bolow freezing
throughout the atmosphere
1
0oC
0oC
0oC
Figure 303: Formation of snow
Cold, snowy weather can be fun! Skiing on snow is one of the popular activities
in the winter. However, snow can also be dangerous. When the weather gets
warmer and snow starts to melt, it can slide down the mountainside as an
avalanche. Tonnes of snow and ice crash down into the valley below and bury
anything in their path. Trees are uprooted by the snow, buildings flattened
and many people are killed by avalanches. As snow piles up on streets, cars
or buildings, it often traps people inside and disrupts transportation. Figure
304 shows snow fall.
Figure 304: Driving in snow
Sleet: Sleet is frozen raindrops that form when a partially melted snowflake
or raindrop turns back into ice as it falls through the air. Sleet begins as
snow high in the atmosphere and then partially melts when it falls through
a shallow layer of warm air. These slushy drops refreeze as they next fall
through a deep layer of freezing air above the surface, and eventually reach
the ground as frozen rain drops that bounce on impact (see Figure 305 below).
336
Temperature
3 km
Snow
Shallow warm layer
Partly smelted snow
Deep cold layer
Partly frozen drops refreeze
and become SLEET
0oC
2
1
0oC
0oC
Figure 305: Formation of sleet
(Source: http://www.nssl.noaa.gov/education/svrwx101/winter/types/ 30/12/13)
Sleet, shown in Figure 306, can also be beneficial for its water content, but the
freezing temperatures that accompany it can cause significant plant damage.
Figure 306: Sleet fall
Activity
2
Discussing forms of precipitation
1. Get into five groups and choose one type of precipitation (rain, hail,
sleet, freezing rain or snow) to research on.
2. Obtain pictures of your type of precipitation from the internet or
geography magazine.
3. Discuss how it is formed.
4. How do people respond to your type of precipitation?
5. Let students suggest the characteristics of your type of precipitation.
337
6. Suggest some effects of your type of precipitation.
7. Report your findings to the class for discussion.
8. Discuss the following questions as a class:
a. Which type of precipitation is water in its liquid form?
b. Which types of precipitation are made of water in its solid form?
c. How is sleet different from hail?
d. How is rain different from snow?
e. How are rain, sleet, snow, and hail similar?
Activity
3
Reflecting on the topic
1. Summarise the most important ideas you have just discussed about the
topic.
2. Why is this knowledge worth having?
3. What can you do about the issues you have been discussing in the unit?
4. Report your answers to the class for discussion.
Summary
Ice crystals and water droplets forming after water vapour condenses, can
take on a variety of forms as it falls to the earth as precipitation. Each of
these forms of precipitation is unique with it’s own important characteristics.
The main types of precipitation are rain, snow, sleet, freezing rain, and hail.
Rain takes place when drops of liquid water fall all the way to the surface of
the earth. Snow forms when water vapor turns directly into ice without ever
passing through a liquid state. Sleet forms as raindrops freeze on their way
down. Freezing rain takes place when water droplets freeze the instant they
strike an object on the ground. Hail occurs when frozen balls of ice fall to the
ground.
Glossary
Drizzle: Light steady rain
Virga: Vertical trails of rain, snow, or ice from the underside of a cloud that
evaporate before reaching the ground
338
Thundercloud: A large dark cumulonimbus cloud that produces thunder
and lightning
Review questions
1. List the forms of precipitation and the circumstances of their formation.
2. Which of these forms of precipitation do you think is the most important?
Give two reasons.
3. Why does air cool when it rises through the atmosphere?
4. Tione and Tadala are copilots with Air Malawi and they are flying
passengers to Nairobi. On their way, they see virga ahead of them.
What sort of conditions do the pilots expect to encounter?
5. What type of precipitation, other than rain, could we expect from
cumulonimbus clouds?
References
Bunnett R. B. (1984). Physical Geography in Diagrams for Africa. London:
Longman Group Limited.
Leong, G. C. (1983). Certificate Physical and Human Geography. Oxford:
Oxford University Press.
Obara, D. A. (1991). Gold Medal Geography. Nairobi: MacMillan Kenya
Limited.
Waugh, D. (1990). Geography: An Integrated Approach. Hong Kong: Thomas
Nelson Limited.
White, R. (1998). Africa in Focus: A Physical, Human and Economic Geography.
Oxford: MacMillan Education Limited.
http://www.publicdomainpictures.net/view-image.php?picture=black-cloudno-rain&image=1171&large=1 30/12/13
http://www.metoffice.gov.uk/learning/rain/what-is-precipitation 30/12/13
http://www.nssl.noaa.gov/education/svrwx101/winter/types/ 30/12/13
http://ww2010.atmos.uiuc.edu/%28Gh%29/guides/mtr/cld/prcp/zr/frz.rxml
30/12/13
http://bsmearthscience.blogspot.com/ 30/12/13
http://www.metoffice.gov.uk/learning/rain/what-is-precipitation
http://www.nssl.noaa.gov/education/svrwx101/winter/types/ 30/12/13
339
http://www2.ljworld.com/photos/galleries/2011/feb/24/rain-snow-sleet-falllawrence-feb-24/
https://www.nc-climate.ncsu.edu/edu/k12/.preciptypes 30/12/13
340
341
Rainfall
Unit
24
Rainfall is by far the
most common type of
precipitation
and
a
major
component
of
the water cycle in our
atmosphere,
and
is
needed everywhere for
life. In the previous unit,
you learned how rainfall
is
formed.
Different
types of rainfall exist in
specific areas across the
earth.
Understanding
the mechanism behind
each type of rainfall can
influence the choices we
make about where to live,
and how we generally
conduct our lives. In this
unit, you will explain the
formation of different
types of rainfall. You will
also identify areas in the
world receiving different
types of rainfall, and
interpret rainfall data
from different sources.
Types of rainfall
There are three main types of rainfall - convection
rain, frontal rain and orographic rain. All have
the common theme of air being forced to rise.
Activity
1
Discussing types of rainfall
1. In groups of five, research on different types
of rainfall such as convectional, cyclonic
and orographic rainfall.
2. In your research, you should consider the
following:
a. The characteristics of each type of
rainfall
b. The type of rainfall that commonly
occurs in your area
3. Produce a diagrammatic representation of
each type of rainfall process.
4. Report your findings to class for discussion.
Convection rainfall
This is very common in areas where the ground
is heated by the hot sun, such as the tropics. The
sun heats up the ground, which heats the air
above it. This heated air then rises and cools, and
the water vapour condenses into water droplets,
forming clouds, which produce rain. The colder
air in the sky slowly sinks to take the place of
rising warm air. This creates convection –
circulatory movement in the air, hence, the term
convection rainfall. Figure 307 is an example
of how convection rainfall is formed.
342
Convection rainfall has the following characteristics:
a. It is usually accompanied by lightning and thunderstorms i.e. tropical
depression (typhoons).
b. It falls heavily, but it is short-lived (it lasts for a short time).
c. It covers a small area.
d. It commonly occurs in the late afternoon after the maximum heating,
and in hot areas such as equatorial/tropical regions.
Clouds
Cool air depleted of
moisture descends
Warm air rises
Figure 307: Convection rainfall
(Source: http://www.geogrify.net/GEO1/Lectures/Weather/Cloud.html 31/12/13)
Frontal (cyclonic) rainfall
Frontal rainfall happens when two air masses (a warm one and a cold one)
meet. When contrasting bodies of air meet, an abrupt zone or boundary is
formed. This boundary is called a front and is accompanied by rainfall, hence,
the term frontal rainfall (see Figure 309). Because the warm air is less
dense it is pushed upwards over the cold air. As the warm air rises, it cools
and the water vapour within it condenses to form clouds, which give rain.
Frontal rainfall has the following characteristics:
a. It produces cumulonimbus clouds.
b. Rainfall is heavy but for a short time.
c. It is associated with storms (cyclones).
d. It usually occurs in winter, but can occur any time of the year.
343
Clouds
Warm air is forced
to rice over cold air,
cools and condenses
to give clouds and
rain
Rain
Cold
Warm air
Figure 308: Frontal rainfall
Orographic (relief) rainfall
With relief rainfall, it is the presence of hills or mountains that leads to the
warm air rising. The winds pick up moisture from the sea as they pass over
it, and this makes the air moist. As the air rises to pass over the higher land,
it cools and the water vapour condenses, forming clouds. Depending on the
atmospheric conditions, the droplets then fall as rain, sleet, hail, or snow on
the side of the hill facing the wind (windward side).
It is much drier on the leeward side of the mountain (the side facing away
from the wind). This area is said to be in the ‘rain shadow’. There is little
rain here as the air is descending and warming up (see Figure 309 below).
Relief rainfall has the following characteristics:
a. It is widespread and is of a longer duration.
b. It occurs in the mountains on the side that faces the direction of wind
(windward).
Clouds
Moisture
condenses
as air cools,
forming clouds
and rain
ar
in
dw
W
Warm,
moist air
rises
Ocean
d
Rain
Cool air
depleted
of moisture
sinks and
warms
Mountain
Figure 309: Relief rainfall
344
Similarities and differences
All the three types of rainfall are similar in that they involve the following:
a. Warm moist air rising
b. Warm air cooling
c. Water vapour condensing to form clouds
d. Further cooling leading to precipitation
The difference is the reason why the air is rising:
a. In convectional rainfall, air rises because it is being heated.
b. In orographic rainfall, air is forced to rise over mountains.
c. In frontal rainfall, warm air is rising over cold air.
Activity
2
Identifying areas in the world receiving different types of
rainfall
Study a physical map and temperature distribution map of the world in your
atlas. Relate the two maps to the rainfall distribution map shown below.
Tropic of Cancer
Equator
Tropic of Capricon
KEY mm per annum
›1000 mm
500 - 1000 mm
250 - 500 mm
‹250 mm
Figure 310: World distribution of rainfall
1. From the world map, name the areas that you think receive;
a. Orographic rainfall
345
b. Convection rainfall
c. Frontal rainfall
2. Account for the type of rainfall in each area you have identified.
3. Which areas receive;
a. The highest amount of rainfall? Why?
b. The least amount of rainfall? Why?
4. Why does it rain more on the coast than the inland?
5. Present your work to the class for discussion.
How is rainfall measured?
The instrument used to measure rainfall is known as a rain gauge. All forms
of precipitation are measured on the basis of the vertical depth of water that
would accumulate on a plain surface, if the precipitation remains where it
falls. It is measured in millimetres over a set period, usually 24 hours.
305 mm
The rain gauge is composed of three parts: a funnel, a collecting bucket and
a measuring cylinder. The funnel directs the precipitation into the collecting
bucket/glass bottle. The rain accumulated in the bucket of the rain gauge is
poured into a specially calibrated measuring glass cylinder and the millimetre
reading is recorded. Thereafter the water is discarded. The measuring cylinder
has a tapered or narrowed end to allow accurate recordings up to 0.1mm.
Figure 311 below shows parts of a rain gauge and how it is set.
Funnel
Tapered measuring
cylinder (mm)
Inner can
Ground level
Glass
bottle
Outer case
Figure 311: Rain gauge
346
Is placement of a rain gauge that important?
The placement of rain gauges is extremely important to record accurate
readings. Rain gauges should be placed in areas free of obstacles whenever
possible. For instance, they
a. Should be placed away from buildings or trees to avoid blocking the
rain. This also prevents the water collected on the roofs of buildings
or the leaves of trees from dripping into the rain gauge, resulting in
inaccurate readings.
b. Should be placed 30 cm above the ground to prevent splashes of water
from entering the rain gauge when the rain drops hit the ground. This
also prevents the evaporation of water due to the reflected heat from
the ground.
Interpreting rainfall data from graphs
Rainfall data collected at different locations over a specific period of time are
used to make graphs and maps. Interpreting a series of these graphs would
help to compare the rainfall patterns of different locations.
The average rainfall typically experienced in a particular location is shown
on a bar graph. It is usually presented alongside with temperature, but the
temperature is shown on a line graph. The two are usually represented on the
same set of axes with the months of the year along the base (see Figure 278).
When interpreting rainfall or temperature data;
a. Look at the overall shape of the graph. Is the temperature line steep or
gentle? Does it change throughout the year and/or look almost flat?
b. Look for extremes quote the highest and lowest temperature and
rainfall and the month in which it occurs. Remember to quote units,
e.g. celsius or millimetres.
c. Can you identify the seasons when most rain or least rain falls? Or
when the highest and lowest temperatures are experienced?
d. Work out the temperature range by subtracting the lowest figure
from the highest figure.
e. Add the rainfall totals for each month together to work out the total
annual rainfall.
347
Activity
3
Interpreting climate graphs
The climate graph below shows average annual rainfall and temperature
throughout the year for a particular area. Look at the information in the
graph and answer the questions that follow.
250
25
200
20
150
15
100
10
50
5
Average rainfall(mm)
30
0
Jan Feb Mar Apr May Jun Jul
Aug Sept Oct Nov Dec
0
Average temperature(oC)
300
Indicates average temperature
Figure 312: Climate graph
(Source: http://www.bbc.co.uk/schools/gcsebitesize/geography/geographical_skills/graphs_rev4.shtml 31/12/13)
1. Is the temperature the same all year round? If it is different, how many
seasons does the location experience?
2. Which season is the warmest? Is it warm (10 to 20°C), hot (20 to 30°C)
or very hot (above 30°C)?
3. Which season is the coolest? Is it mild (0 to 10°C), cold (-10 to 0°C) or
very cold (below -10°C)?
4. What is the range of temperature? (Subtract the minimum temperature
from the maximum temperature).
5. Does the rainfall occur all year round?
6. What is the pattern of the rainfall? Check which season(s) is/are drier
or wetter than others.
7. What is the total annual rainfall? Add each month’s total together to
get the annual total.
8. Then put the rainfall and temperature information together what does
it tell you about this area?
9. Describe the patterns in temperature and rainfall, including how they
relate to each other.
10. Report your work to the class for discussion.
348
Factors that affect the amount of rainfall in a location
The factors affecting rainfall at a specific location include the following:
Latitude: It rains more in the areas near the equator than in the temperature zones
and polar regions. The temperature is higher near the Equator so there is
more evaporation.
Altitude: It rains more in high areas than in low areas. Mountains force
air masses passing through to rise. As this happens, the air cools causing
condensation, cloud formation and eventually rainfall – a phenomenon known
as orographic precipitation. Rainfall tends to increase as elevation increases.
Because orographic rainfall drains the moisture from the air, it is responsible
for the rain shadow effect described above.
Nearness of large lakes or oceans: Closeness to large bodies of water,
particularly oceans, provides plenty of water for evaporation. So it rains more
on the coast than inland.
Prevalent wind direction: Winds can greatly affect the amount of rainfall
an area receives depending on the amount of moisture they are carrying.
Prevailing winds (winds that blow more often in one direction) can move air
masses from the ocean onto a continent bringing moisture onto the continent.
The moisture condenses into clouds, which produce more rainfall.
Vegetation: Vegetation adds water to the atmosphere through the process
of transpiration (where they release water from their leaves during
photosynthesis). This moisture contributes to the formation of rain clouds
which release the water back on the ground. When the forests are cut down,
less moisture goes into the atmosphere and rainfall declines, and this decline
sometimes leads to drought.
Global warming: Global warming refers to an average increase in the earth’s
temperature, which in turn causes changes in rainfall pattern. A warmer
earth may cause air to expand, increasing its capacity to hold more water
vapour. So there will be no water vapour to condense out of the air into liquid
water droplets. Condensation occurs when air cools and contracts so that it is
no longer able to hold all of the water vapour it was able to hold when it was
warm. This extra water vapour begins to condense out of the air into liquid
water droplets.
The lower the temperature of the air, the greater the condensation of water
vapour to raindrops. Air temperature also determines the type of precipitation
that might occur (whether it will rain, snow, or sleet).
Advantages of rain
Rain is probably the most important form of precipitation because it is liquid,
as opposed to non-liquid kinds of precipitation such as snow, hail and sleet.
349
In liquid state, rain can do the following:
a. Dissolve soil nutrients for plant growth since it easily soaks into the
ground. This helps farmers to grow crops.
b. Recharge groundwater, rivers, wetlands and lakes through infiltration
and runoffs. This is important for our survival and the survival of
animals.
c. Clean up our surroundings by washing away dirt.
Disadvantages of rain
During the dry season, people lookout for the first drops of rainfall to grow
crops, graze livestock and to have the high temperatures and great amounts
of dust reduced. If the rain is just adequate there is usually no cause of worry,
but once it falls in large amounts then people start looking out. Too much rain
can cause a lot of havoc due to:
a. Flooding: Several days of steady rain can result in severe flooding,
which destroys homes, crops, roads businesses, and, in worst cases
lives.
b. Mudslides: In the event of excessive rain following a dry period,
mudslides can occur, with devastating effects. Not only do they erode
valuable soil, they also can destroy homes and can be life-threatening.
Usually, they occur on hills, and even if there aren’t homes on the
hillside, there is the danger of potentially fatal carnage to roads,
businesses and houses at the bottom of the hill.
c. Erosion: Too much rainfall can wash away top soil and nutrients
causing the land to become unproductive.
d. Water borne diseases: Rain washes dirt and untreated sewage into
streams and other water sources, spreading diseases such as cholera
and diarrhoea. Rain also creates breeding grounds for mosquitoes that
spread malaria.
Activity
4
Discussing the importance of rain
1. Hold a panel discussion on the advantages and disadvantages of
rainfall.
2. In your discussion, consider the following questions:
a. Why do we need rain?
b. What would happen if it did not rain?
350
c. To what extent does your community depend on rain?
d. What other things exist because of rain in your community?
e. What problems does your community face because of rain?
Summary
There are three different types of rainfall: convectional, frontal and relief.
Convectional rainfall is rain that is formed when the ground is heated by the
sun. Frontal rainfall is formed when warm air meets colder air which rises
over colder heavier air. Relief rainfall is formed when warm, moist air rises
over mountains. Rainfall is measured using an instrument called a rain gauge.
The amount of rainfall received in a location depends on latitude, altitude,
vegetation, prevalent wind direction, nearness to large water bodies and
global warming. Rainfall is important for dissolving soil nutrients, recharging
ground water and cleaning up dirt. However, too much rainfall can cause
flooding, mudslides, erosion and water borne diseases.
Glossary
Convection: a circulatory movement in a liquid or gas
Front: a line along which one mass of air meets another that is different in
temperature or density
Windward: the side of a mountain that is facing the wind
Leeward: side of a mountain feature that is away or sheltered from the wind
Rain shadow: an area on the side of a mountain barrier that is sheltered
from prevailing winds and rain-bearing clouds, resulting in relatively dry
conditions
Review questions
1. Figure 313 is a diagram showing a type of rainfall. Study it and answer
questions that follow.
Condensation
Air
cools
B
Sun’s
heat
Rainfall
Figure 313: Formation of rainfall
(Source:http://www.tads.co.uk/files/test/page_09.
htm 13/02/14)
A
351
a. Identify the type of rainfall.
b. Describe what is happening at points A,B.
c. List any three characteristics of this type of rainfall.
2. Explain with the aid of a diagram how relief rainfall occurs.
3. Why does the leeward side of a mountain receive very little or no
rainfall?
4. Explain any three factors that affect the amount of rainfall in an area.
5. In what three ways is rainfall important?
6. Describe any two problems brought by rainfall.
References
Bunnett R. B. (1984). Physical Geography in Diagrams for Africa. London:
Longman Group Limited.
Leong, G. C. (1983). Certificate Physical and Human Geography. Oxford:
Oxford University Press.
Obara, D. A. (1991). Gold Medal Geography. Nairobi: MacMillan Kenya
Limited.
Waugh, D. (1990). Geography: An Integrated Approach. Hong Kong: Thomas
Nelson Limited.
White, R. (1998). Africa in Focus: A Physical, Human and Economic Geography.
Oxford: MacMillan Education Limited.
http://www.bbc.co.uk/schools/gcsebitesize/geography/geographical_skills/
graphs_rev4.shtml 31/12/13
http://www.fs.fed.us/pnw/olympia/silv/local-resources/glossary/index.shtml
http://www.geogrify.net/GEO1/Lectures/Weather/Cloud.html 31/12/13
http://worldlywise.blogspot.com/2008/01/types-of-rainfall.html 31/12/13
http://www.logan.qld.gov.au/>Year 7 Lesson 2 Rainfall processes10/05/13 –
date accessed.‎
http://www.bbc.co.uk/schools/gcsebitesize/geography/weather_climate/
climate_rev5.shtml 31/12/13
http://www.bbc.co.uk/schools/gcsebitesize/geography/geographical_skills/
graphs_rev4.shtml 31/12/13
352
http://www.profedesociales.com/>5. What factors affect precipitation?11/05/13
– date accessed‎
http://www.tads.co.uk/files/test/page_09.htm 13/02/14
353
Unit
25
Climatic regions and World
vegetation (Biomes)
Climatic regions and world vegetation
(Biomes)
Climate refers to the average weather conditions
observed over a long period of time for a given
area. Temperature and precipitation are two
of the most important factors that determine the
There
is
increasing climate of a region.
recognition that climate The earth’s surface has been divided into several
has a central role in global climatic regions based on the annual and monthly
economic
and
social averages of temperature and precipitation.
sectors. Climate directly A climatic region is an area experiencing a
affects the prosperity uniform pattern of temperature and precipitation
of many sectors, such over a long period of time. Some regions are hot
as agriculture, tourism, and wet while others are hot and dry. Some are
energy
and
health. cool and wet where as others are cold and dry.
Similarly, unfavourable
climatic conditions have
negative and prolonged Activity 1
impacts. It is important
therefore
that
you Locating climatic regions on a world
understand climate and map
all the components that
1. Draw an outline map of the world.
are involved. In this
2. On the map, mark and label three climatic
unit, you will explain
regions.
climatic
region
and
identify world climatic
3. Which climatic regions would people be
regions. You will also
more likely to go swimming? Why?
explain characteristics of
4. Which climatic regions would the following
climates and associated
activities be available: skiing, ice skating,
vegetation. Finally, you
winter activities? Why?
will explain the influence
5. Present your work to the class for discussion.
of climate and vegetation
on economic activities.
The climate of a region will determine what plants
will grow there, and what animals will inhabit it. A
division of the world’s vegetation that corresponds
to a defined climate, characterised by specific types
354
of plants and animals is called a biome. The following sections describe some
of the world’s major climate regions or types, and their associated biomes and
human activities.
Equatorial climate
Equatorial climate zone is largely situated at latitudes within 50 north and
south of the Equator; covering wide areas in South America, Central Africa
and South-East Asia (see Figure 314). It is dominantly found in the lowlands
of the Amazon, the Congo, Malaysia and the East Indies. However, not all
places along the equator have an Equatorial climate. The high mountain
areas like the Andes in South America experience cool climate despite lying
within the equator.
EQUATOR
N
Hot, wet Equatorial regions
0
4000 km
Figure 314: Equatorial climate regions
Distinguishing features of the equatorial climate
Equatorial climate is a simple climate type to identify. The line on the
temperature graph is almost straight to show uniformly high temperatures all
year round. The rainfall graph shows heavy rains falling all year round, but
with two peaks of maximum rainfall, which coincides with the direct overhead
sun during the two equinoxes.
Temperature
The average monthly temperatures are around 270C with very little variation
or temperature range, usually of less than 30C (Figure 315).
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Precipitation
Precipitation in the Equatorial Region is heavy and is usually convectional,
exceeding 2000 mm per annum. It is well distributed throughout the year.
In fact, there is no month without rain. There are two periods of maximum
rainfall, July and September, when the sun is directly overhead at the equator.
Extensive cloud cover and heavy rainfall in the equatorial climate prevent
temperatures from rising to the extremes even though the regions receive the
greatest amount of sunshine. So the highest daily temperatures are recorded
outside the equatorial regions.
40
30
20
10
Rainfall
(mm)
0
Temp.
(oC)
320
380
340
200
160
120
180
40
0
J
F
M
A
M
J
J
Month
A
S
O
N
D
Temperature range: 2oC
Annual total rainfall: 2 500 mm
Figure 315: Equatorial climate
Natural vegetation
Equatorial regions support a luxuriant type of dense, evergreen vegetation
(the equatorial rain forest) because of the very heavy rainfall and uniformly
high temperatures. The trees are mostly huge, tall, and are arranged in layers,
forming a leafy canopy over the forest floor (see Figure 316).
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The tropical rainforest has four layers:
a. Emergent layer: the uppermost layer where the tallest trees rise
above the rest. The tree grows between 40 and 48 m to capture direct
sunlight.
b. Canopy layer: tall trees in this layer are called canopy trees. They
grow so close together that their crowns interlock to form a continuous
canopy (a cover or ceiling for the rainforest). The canopy later shuts
out most of the sunlight from the forest.
c. Understory layer: short trees are found in this layer. Some epiphytes
and lianas are also found here.
d. Ground or forest floor layer: shrubs, ferns, mosses, fungi and other
small plants are found here because they do not need much sunlight to
grow.
Activity
2
Identifying layers of the Equatorial rainforest
Study the diagram of an Equatorial rainforest in Figure 282 below. Use it to
answer questions that follow.
40mA
30m-
20m-
10m-
B
Climbing plants
called lianas grow up
to the sunlight.
The forest floor is very
dark and groomy so few
plants can grow here.
0m-
At ground level there
is a mass of rotting
vegetation and fungus.
Figure 316: Layers of an Equatorial rainforest
(Source: http://geography.parkfieldprimary.com/climate-types/equatorial-regions/plants)
357
1. How tall are the tallest trees?
2. What is the correct name of the parts of the forest shown at A and B?
3. Based on what you see in the diagram, how is the rainforest adapted to
the hot, wet Equatorial climate?
4. Present your work to the class for discussion.
Figure 317: shows part of the South American Rainforest.
In order to survive in the hot, wet tropics, plants of the tropical rainforest
have had to develop the following special
features (adaptations):
a. Thin and smooth barks because there
is no need for protection against harsh
weathers. The smoothness helps
water drain off easily and prevents
bacteria from growing on it.
b. The trees have branches on the
topmost portion of the trunk to get as
much sunlight as possible.
c. Roots are shallow; they do not need to
reach deep since nutrients in the soil are
near the surface. However, tall trees have buttress roots which grow one
to five metres above the ground
to support weight of the trees
(see Figure 284 below).
Figure 317: South American rainforest
d. Thousands of flowering plants
grow onto trees so they get
sunshine. There are also many
climbing plants into the canopy
so their leaves get more sunlight.
e. The leaves of rainforest trees are
large and broad to maximize the
surface area for photosynthesis.
f. The leaves are also waxy, and Figure 318: Buttress roots
have ‘drip tips’ to let the rain
drain off quickly (see
Figure 319). The wax also helps prevent harmful bacteria from growing on
the leaf.
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Economic activities in
equatorial climate region
the
Some of the world’s most developed
areas with higher population totals lie
within the Equatorial Rainforests, e.g.
Singapore, Malaysia, Nigeria, Ivory
Coast and Ghana. The region is endowed
with abundant resources that support a
wide range of human activities;
a.
Figure 319: Drip tips on the leaves of
plants
Agriculture: in the equatorial
regions, large forest areas have been
cleared for plantation agriculture
such as rubber and oil palm
plantations in West Malaysia. Some
parts of the region e.g., the Amazon
Basin in Brazil are also used for
commercial livestock ranching/ beef
cattle rearing.
b. Pharmaceuticals: tropical rain forests are called “the world’s largest
pharmacy” because of the large amount of natural medicines discovered
in rainforests that are derived from rainforest plants. For example,
rain forests contain the “basic ingredients of hormonal contraception
methods, cocaine, stimulants, and tranquilizing drugs”, Curare (a
paralysing drug) and quinine (a malaria cure) are also found there.The
luxuriant trees provide timber resources.
c. Tourism: currently one of the largest economic values of tropical
rainforests comes in the form of tourism. People travel both nationally
and internationally to experience rain forests firsthand. Climbing
through the canopies, camping, biking and animal/insect watching are
all also common forms of tourism done in the forest.
d. Mining: many rainforests are rich in oil deposits and mineral reserves
such as bauxite, coal, copper, diamonds, gold, iron ore, kaolin, nickel,
tin and uranium.
e. Lumbering: the luxuriant and diverse vegetation provides a wide
variety of woods for making furniture, pulp, and shipboard or as
cellulose for the production of plastics.
f. Fishing: there are numerous rivers due to the heavy rainfall totals,
and this has made fishing one of the most important occupations in the
region.
g. Electricity: some of the rivers have been developed for hydroelectric
power generation, and this has attracted some industries.
h. Hunting and gathering: in the forests, most primitive people live as
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hunters and collectors and the more advanced ones practice shifting
cultivation.
Factors that limit development in the equatorial climate
region
Large parts of the Equatorial Rainforests are less developed and thinly
populated, e.g. the Amazon of South America and Congo of West Africa. The
region is less developed because of the following:
a. Inaccessibility: The dense forests, numerous rivers and swamps
hinder easy access to resources and makes construction of roads and
railways difficult. In this case, transporting the heavy logs to the desired
destinations tends to be difficult. For this reason, lumbering has until
recently been a difficult occupation despite the luxuriant trees. It is
also difficult and expensive to clear part of the forest for agriculture or
constructions.
b. Soil erosion and flooding: Heavy rainfall results in flooding and
washing away of soil nutrients, causing farming almost impossible.
Moreover, the luxuriant trees render clearing a piece of land for farming
a difficulty.
c. Excessive heat and high humidity: make the climate uncomfortable
for human settlement.
d. Diseases and pests prevail in the hot humid conditions, and these
reduce crop and animal production.
Tropical continental (Sudan type) climate
Areas with a tropical continental climate are mainly located between latitudes
50 and 150 north and south of the equator within central parts of continents.
The most characteristic areas of savanna climate include the Llanos of Orinoco
valley, the Campos
of Brazil, hilly areas
of Central America,
southern Zaire or
Congo, and parts of
Northern Australia
(see Figure 320).
The savanna climate
EQUATOR
is confined within
N
the tropics and is
best developed in
Sudan, hence it
is also called the
Hot, wet Equatorial regions
0
4000 km
Sudan Climate.
Figure 320: Savanna climate regions
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Distinguishing features of the savanna climate
It is easy to distinguish the savanna climate from the other types of climate. It
has a hot wet and a cooler dry season. The savanna climate graphs in Figure
321 show a general pattern of rainfall and temperature for the Southern and
Northern Hemispheres.
Southern Hemisphere
Northern Hemisphere
40
30
20
20
10
Rainfall
(mm)
Temp.
(oC)
280
200
200
160
160
120
120
80
80
40
40
F
M
A
M
J
J
A
S
O
N
0
D
Temp.
(oC)
280
240
J
0
320
240
0
10
Rainfall
(mm)
0
320
40
30
J
F
M
A
M
J
Temperature range: 9 C
Temperature range: 8 C
Annual total rainfall: 865 mm
Annual total rainfall: 815 mm
o
J
A
S
O
N
D
o
Figure 321: Savanna climate
Temperature
Temperatures are high during one part of the year, and cool during the
other. The climate has the highest temperatures just before the onset of the
rainy season. Great distance from the sea, less cloud and vegetative cover
also contribute to the high temperatures. The annual temperature range is
slightly greater than that of the equatorial climate due to the sun being at a
slightly lower angle in the sky for part of the year.
Precipitation
There is seasonality of rainfall just like the monsoon climate. However, its
rainy season is much shorter and receives far less rainfall than the monsoon
climate. The rainfall generally arrives in heavy bursts from thunderstorms.
Malawi’s climate is generally subtropical, with the characteristics of the
savanna climate. A hot, rainy season runs from November through April.
From May to November, it is cool and dry throughout the country. The two
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climate graphs shown below were taken from stations with savanna climate,
one in the Northern Hemisphere and the other in the Southern Hemisphere.
Natural vegetation
The type of vegetation is mostly savanna or tropical grassland, which has the
following characteristics:
a. Trees are always short and scattered amongst the luxuriant tall grasses.
The vegetation found in the tropical continental climate has a seasonal
pattern of growth, shedding leaves in the cool dry season to prevent
excessive loss of water through transpiration.
b. They lie dormant during the long drought.
c. Plants have long roots, which reach deep down to the moist rock layers
in search of ground water.
d. Many of the trees are umbrella-shaped (see Figure 322) to shield their
roots from the scorching heat and to expose only a narrow edge to the
strong trade winds that blow all year round.
e. They also have broad trunks (e.g. baobab in Figure 323) to store
excessive water.
Figure 322: Savanna vegetation Tanzania: EcoLibrary.org
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Figure 323: Baobab
Economic activities in the tropical continental climate
a. Agriculture: Savanna lands support a wide range of tropical crops
such as sugar cane, cotton, coffee, oil palm, groundnuts and fruits. The
natural vegetation in the savanna climate does not require a tremendous
effort in clearing land. Livestock production is also widely practised
because natural pasture is readily available. However, agriculture is
not much developed. Some tribes live as pastoralists like the Masai and
others as settled cultivators like the Hausa of northern Nigeria.
b. Tourism: The grasslands provide pasture and shelter for wild animals,
making tourism an increasingly important economic activity.
c. Mining: Many Savanna climate regions are also rich in mineral
reserves such as bauxite, coal, copper, diamonds, gold, nickel, tin and
uranium.
d. Fishing: The heavy rainfall received during the short rainy season
gives rise to permanent water bodies. This has made fishing one of the
most important occupations in the region.
e. Electricity: the seasonal heavy rains also made some of the rivers
support hydroelectric power generation.
However, the following problems persist:
a. Droughts are common since rainfall is often unreliable.
b. There is prevalence of tropical diseases and tsetse flies, which poses a
hazard to crop and animal production.
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c. There are poor soils due to leaching of nutrients during torrential rains.
d. The dry season makes much of the region prone to wildfires.
Hot desert climate
The hot desert climate is found around the tropics of Capricorn and Cancer,
usually on the west side of continents in the trade winds belt. Examples are
the Thar Desert in Pakistan and the Atacama Desert in Chile. The map in
Figure 324 below shows the regions that experience hot desert climate.
EQUATOR
N
Hot deserts
Mid-latitude deserts
0
4000 km
Figure 324: Hot desert climate regions
Distinguishing features of the hot desert climate
It is easy to identify the hot desert climate; its climate graph will show very
high temperature but very little rainfall, even in the wettest months.
Temperature
Desert temperatures are hot in winter and very hot in summer. The annual
temperature range is often 200Cto 300C, and the diurnal temperature range
is over 500C.
Why is there a big daily (diurnal) temperature range?
During the day the sun is high in the sky. Cloudless skies let the intense solar
radiation reach the bare sand surfaces on ground. The bare ground heats up
and air temperatures close to the ground may reach over 40°C. At night the
364
sun has set, so the ground receives no radiation. Cloudless skies allow the
heat of the ground to radiate into the atmosphere. Air temperatures near the
ground can fall to below 00C.
Precipitation
The amount of precipitation in hot deserts is extremely low and unreliable
(see Figure 325); some desert areas may receive rain only once every two to
three years. However, no deserts are truly dry even though they suffer from
extreme water shortages.
40
30
20
10
Rainfall
(mm)
0
Temp.
(oC)
320
280
240
200
160
120
80
40
0
J
F
M
A
M
J
J
A
S
Month
Temperature range: 9 C
Annual total rainfall: 150 mm
o
Figure 325: Hot desert climate
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O
N
D
Natural vegetation
Vegetation in the hot desert climate is scanty due to lack of rain. Plants have
adapted to this very hot and dry environment in a number of ways:a. Desert plants usually have small leaves. This conserves water by
reducing surface area from which transpiration can take place.
b. Other plants have thorns instead of leaves to reduce loss of moisture.
c. Some plants have thick waxy layers on the outside of their stems and
leaves. This adaptation helps to reflect some of the heat and reduce
loss of water.
d. A number of desert plants
are succulents, storing water
in leaves, fleshy stems, and
root bulbs. The Cactus in
Figure 326 shows these
features.
e. Woody desert plants either
have long root systems that
reach deep water sources
or have spreading shallow
roots that are able to take
up surface moisture quickly
from heavy dews and
occasional rains.
f. Some flowering desert plants
are ephemeral; they live for
a few days at most. But their
seeds lie dormant in the soil,
sometimes for years, until a
soaking rain enables them to
germinate and quickly bloom. Figure 326: Desert vegetation (cactus)
Economic activities in the hot desert climate
Economic development in the hot desert climate regions is, with few exceptions,
limited by the availability of water. The following are the most important
economic activities in the hot desert climate regions:
a. Hunting and gathering: The Bushmen of the Kalahari Desert region
remain so primitive in their mode of living that they barely survive.
These people are nomadic hunters and food gatherers, growing no crops
and domesticating no animals.
366
b. Agriculture: The crops are grown mainly by irrigation using the oasis.
Wheat, barley, dates, figs and a variety of vegetables are grown for both
commercial and local use. There are also nomads who live chiefly by
herding sheep, goats and camels in mountains and around the oases.
c. Mining: Hot deserts are a source of enormous wealth in natural
resources like petroleum and copper. Therefore, extracting industries
have settled down in hot deserts, building plants for processing these
resources.
d. Tourism: Hot deserts are attractive places for adventurous tourists
in various countries, providing different tours and sports such as
motorcycling, hiking and camel riding.
e. Solar power generation: Intense sun’s heat and extreme temperatures
in deserts make them ideal places for producing solar energy.
However, the hot desert climate poses the following challenges:
a. Droughts and extreme temperatures: Hot deserts receive the
largest amount of sunshine of any environment on the planet, and
it is immensely dry. This results in sweltering heat, which could be
dangerous to life.
b. Lack of significant water bodies: Due to the lack of substantial
precipitation, hot deserts can only obtain their scant amount of
freshwater from two sources: oases and exotic streams. Oases occur in
places either where a spring is present or the groundwater table lies
very close to the surface. Exotic streams are streams or rivers that
begin in a more humid region and flow into the desert. Many exotic
streams dry up before ever reaching the sea.
c. Intense dusty winds: The desert weather has tendencies to have
intense winds, which cause sand and other objects to be tossed around.
This could be dangerous to driving, flying and walking.
Mediterranean climate
The largest area with a Mediterranean climate is around the Mediterranean
Sea, which has given the climate its name. The Mediterranean climate is also
found in the coast of California; southern parts of Australia, Western Cape of
South Africa, and central Chile (see Figure 327).
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Mediterranean
Basin
California
EQUATOR
N
Central Chile
Western cape,
South africa
South Western
Australia
0
Mediterranean climate regions
4000km
Figure 327: Mediterranean climate regions
Distinguishing features of the Mediterranean climate
The distinctive feature of this climate that makes it easy to identify is summer
drought and winter rains.
Temperature
Winter temperatures normally do
not fall below freezing point (00)
while summer temperatures are
around 210C to 270C.
40
30
20
10
Rainfall
(mm)
0
Temp.
(oC)
320
Precipitation
280
240
The
Mediterranean
climate
has frequent winter rains; with
annual totals of over 420 mm
(Figure 328). The summer is dry
because it experiences cold ocean
currents that bring dry air and
no precipitation. During the winter
the currents shift and warmer,
moist air brings rain to these areas.
200
160
120
80
40
0
J
F
M
A
M
Temperature range: 8 C
o
J
Month
J
A
S
Annual total rainfall: 635 mm
Figure 328: Mediterranean climate
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O
N
D
Natural vegetation
Vegetation in Mediterranean areas is varied. It ranges from scrub to grassland
to woodlands. Some people call this kind of vegetation “chaparral”, a Spanish
word that means “an area of small evergreen oak trees.”Figure 329 shows
Mediterranean vegetation.
Figure 329: Mediterranean vegetation
Many plants have adapted themselves to the demands of Mediterranean
temperatures;
a. The olive tree is a distinctive feature of the landscape, with tough, waxy
leaves and thick bark, which enable it to cope with the excessive heat
and dryness of the summer months. Some plants remain evergreen,
e.g. the oak.
b. The vegetation is short, dense, and scrubby. The reason it looks like
this is because short, dense and scrubby vegetation can survive very
well in dry habitats. This is called a drought-resistant strategy for
survival.
c. Many of these plants have the ability to lose their leaves when times get
tough. Unlike normal deciduous plants, which lose their leaves in the
winter, drought deciduous plants lose their leaves in the summer.
This strategy reduces the energy and water demand of the plant and
helps to conserve water during the summer drought.
d. They have both a long deep taproot, and a dense network of lateral
roots close to the surface to absorb as much moisture as possible.
e. They produce thick, woody tubers called burls, which are found at the
base of the plant. These burls are so thick they can even resist being
369
burned all the way through in a fire. The fire-resistant burl at the
base of the trunk can re-sprout, enabling the shrubs to regrow to their
original size in just a few seasons.
Economic activities in the Mediterranean climate region
a. Agriculture is more developed in the Mediterranean region. It is in
the Mediterranean climate that much of the world’s citrus, vines and
olives are grown for wine and olive oil production.
b. Tourism: the long hot, dry summers attract many tourists from the
cold regions.
c. Mining: the Mediterranean climate region has abundant mineral
resources such as coal, gold, diamond, platinum, etc.
However, the following problems are common:
a. The hot, dry summers make much of the region prone to frequent
wildfires.
Cool temperate interior (Siberian) climate
This type of climate is experienced only in the northern hemisphere between 45°
and 75° latitudes. It extends from Alaska to New found land in North America
and From Norway through Finland and Siberia (Russia) to Kamchatka in
Eurasia. The world map in Figure 330 shows areas that have this type of
climate.
EQUATOR
N
Cool temperature continental
0
4000 km
Figure 330: Cool temperate interior (Siberian) climate regions
370
Distinguishing features of cool temperate interior (Siberian)
climate
The greatest annual temperature range of over 55°C is found here. This
climate is also distinct for its evergreen coniferous (boreal) forest biome
Temperature
The climate is characterised by long and bitterly cold winters, with
temperatures dropping to as low as −34°C, with occasionally strong dry winds
such as the blizzards of northern Canada. Summers are short (4 – 5 months),
with cool temperatures of around 15°C to 20°C.
Precipitation
Precipitation is very little, about 380 – 635 mm a year, but it is quite well
distributed throughout the year. In winter, the precipitation is in form of
snow because the average temperature is always below freezing. In summer,
the precipitation is in form of rain of convection type as the land is heated.
Figure 331 below shows a cool temperate interior or continental climate.
30
20
10
-0
-10
-20
-30
Temp.
(oC)
Rainfall
(mm)
160
120
80
40
0
J
F
M
A
M
J
J
A
S
O
N
D
Month
Temperature range: 30oC
Annual total rainfall:533 mm
Figure 331: Cool temperate interior (Siberian) climate
371
Natural vegetation
Plants in the Siberian Climate have adapted in many ways for survival
purposes, which include the following:
a. They have thick barks to protect the trunks from excessive cold.
b. They have conical shape or sloping tree branches to prevent snow from
accumulating, which may snap the branches.
c. They have small, thick, leathery and needle-shaped leaves to check
excessive transpiration in the warm summer due to intense continental
heating.
d. They are evergreen, mostly conifers because temperatures are very
low for more than half
the year. A conifer is any
tree that has thin leaves
needles and produces
cones e.g. pines, firs,
junipers, larches, spruces,
and yews. Figure 332
shows a coniferous forest
in the Siberian Climate.
e. The vegetation in this
climate is widely space
and has little under
growth due to poor soils.
Being evergreen forest
leaf fall is very little for
humus formation, and the
rate of decomposition of
the fallen leaves in such a
region of low temperature
Figure 332: Siberian climate vegetation
is slow.
Economic activities in the cool temperate interior climate
region
Due to the harsh climate, most of the Siberian climate regions have seen little
human activity, even though they are rich in natural resources. The following
are some of the major economic activities in the region:
a. Lumbering: the large reserves of the coniferous forests provide soft
wood for timber, plywood, paper and pulp, fuel, matches, furniture and
other products.
372
b. Tourism: despite the cold, the climate draws adventurous travellers
from many parts of the world, so tourism is one of the most notable
economic activities in the region.
c. Mining: the region is a vast storehouse of natural resources such as oil
and gas, copper, nickel, coal, uranium and other valuable minerals; so
many nations have turned their attention to these areas for mining or
oil drilling operations.
d. Agriculture: in this climate region, reindeer and sheep herding is an
especially popular method of survival, both as a direct food source, and
as an economic export (meat and fur). However, it is virtually impossible
to grow typical food crops due to long, cold winter, frozen soils and scant
rainfall.
e. Fishing: the north Pacific, Arctic and north Atlantic provide fishing
opportunities to coastal regions in the Siberian climate.
Activity
3
True or false questions
1. Are the following statements true or false? Correct the false ones in
your exercise book.
a. An equatorial climate is always humid and hot.
b. The tropical climate is a type of temperate climate.
c. The polar and alpine climates have the lowest temperatures.
d. The Mediterranean climate has dry summers.
e. The maritime climate is hot in summer and cold in winter.
2. Report your answers to the class for discussion.
Activity
4
Reflecting on the topic
1. Summarise the most important ideas you have just discussed in the
topic.
2. Why is this knowledge worth having?
3. What can you do about the issues you have been discussing in the unit?
4. Report your answers to the class for discussion.
373
Summary
The world has several climatic regions. The classification is based on maximum
and minimum temperatures and the temperature range as well as the total
and seasonal distribution of precipitation. A simple summary of climatic
regions is as follows: Equatorial – hot and wet all year; Monsoon – hot, wet
summers and cool, dry winters; Savanna – hot, wet summers and cool, dry
winters; Hot desert – dry, hot all year; Mediterranean – mild, wet winters, dry
hot summers; Siberian – very cold all year. Each of these climatic regions has
its own vegetation adapted to prevailing conditions. Human activities have
also responded differently to the demands of each climatic region.
Glossary
Climatic region: an area of the earth’s surface that possesses a distinct type
of climate
Canopy: the uppermost layer of vegetation in a forest, consisting of the tops
of trees forming a kind of ceiling
Buttress roots: large roots on all sides of a shallowly rooted tree, projecting
from the trunk into the soil to support the tree
Oasis: fertile ground in a desert where the level of underground water rises
to or near ground level, and where plants grow and travelers can replenish
water supplies
Exotic: introduced from another place or region
Chaparral: a dense thicket of bushes or small trees, especially of evergreen
oaks in southern California
Deciduous: describes trees and bushes that shed their leaves in the fall
Burl: a knotty growth on a tree trunk
Review questions
1. The table below shows climatic data for a station. Use the data to
answer the questions that follow.
Months
J
F
M
A
M
J
J
A
S
O
N
D
Temperature (ºC)
-20
-20
-16
-10
8
10
16
12
3
-8
-12
-18
Rainfall (mm)
10
8
8
12
10
60
80
75
50
25
20
18
a. Using the figures above draw a climate graph.
b. Identify the type of climate for the station.
c. Why do you think few people live in areas with this climate? Give
two reasons.
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2. Explain three ways by which plants have become adapted to hot, dry
climatic conditions.
3. Differentiate between the concepts annual temperature range and
diurnal temperature range.
4. Suggest three reasons why places along the equator are warmer than
those closer to the poles.
5. Provide a reason for the average temperatures on the east coast of
South Africa being warmer than those on the west coast.
References
Bunnett, R. B. (1973). General Geography in Diagrams. London: Longman.
Bunnett R. B. (1984). Physical Geography in Diagrams for Africa. London:
Longman Group Limited.
Leong, G. C. (1983). Certificate Physical and Human Geography. Oxford:
Oxford University Press.
Obara, D. A. (1991). Gold Medal Geography. Nairobi: MacMillan Kenya
Limited.
Waugh, D. (1990). Geography: An Integrated Approach. Hong Kong: Thomas
Nelson Limited.
White, R. (1998). Africa in Focus: A Physical, Human and Economic Geography.
Oxford: MacMillan Education Limited.
http://www.clipart.dk.co.uk/1109/az/Weather/Climate_zones 07/06/14
http://geography.parkfieldprimary.com/climate-types/equatorial-regions/
plants
375
Environmental
issues
Unit
26
As
the
population
continues to grow, the
need for food, clean
water, fuel, and space
will increase. Changes
to the natural and
built
environments
will continue to have
significant economic and
other social impacts.
Learning this topic will
prepare you to understand
and address a wide
range of environmental
issues
affecting
our
communities. Only an
environmentally literate
public will be able to
find workable, evidencebased
solutions
for
these challenges. In this
unit, you will explain
environmental
issues.
You will also explain
the meaning of the term
pollution and describe
causes and effects of
pollution. Finally, you
will suggest ways of
controlling pollution.
Environmental issues
Environmental issues or environmental problems
are the known aspects of human activity that
have negative effects on the sustainability of the
environmental quality necessary for the wellbeing
of the organisms living in it. Pollution, climate
change, global warming, desertification, resource
depletion, ozone depletion, overpopulation,
hazardous wastes and deforestation are some of
the major current environmental issues affecting
the entire world.
Every environmental problem has causes,
numerous effects, and most importantly, a
solution. The next sections focus on different types
of pollution, their causes, effects and solutions.
Activity
1
Discussing
environmental
affecting your area
issues
1. Get into groups and brainstorm to list the
environmental issues in your area.
2. Research one of the factors you have
listed. Focus your research on how your
environmental issue affects ecosystems,
particularly those in your area.
3. When your research is complete, choose one
ecosystem in your area that has been affected
by the environmental issue you have been
assigned and prepare an environmentalimpact statement about it. Your statement
should include four elements:
a. a description of the current environmental
status of the ecosystem
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b. a description of the way or ways in which the problem affects the
ecosystem
c. a description of the existing methods that are being used to combat
the environmental issue
d. suggestions for future methods of combating the factor
4. Share your findings with the class for discussion.
Pollution
Pollution is the introduction of waste materials into the natural environment
that renders part of the environment unfit for intended or desired use. These
uses include all wildlife and ecological requirements to sustain life in all its
natural forms. Pollution could be a result of natural causes, but the vast
majority is attributed to human activities.
Types of pollution
Pollution exists in many forms and affects many different aspects of the
earth’s environment. The three main types of pollution are water, air and
land pollution.
Activity
2
Exploring and cleaning pollution in your school
1. In groups of four, take a walk around the school grounds.
2. Observe and record any type of pollution found in the surrounding land,
water or air.
3. Discuss your feelings about the trash and the effects it would have on
the environment.
4. What services are offered by your community to help citizens dispose of
wastes?
5. Can you do something to clean up the pollution you have observed? Get
a broom and clean up the school grounds.
6. Make posters to inform others about the dangers of pollution. (Display
them around the school.)
7. If possible, your teacher should give you boxes and paint to make “litter
boxes. Decorate and write on the boxes reasons why it is imperative not
to litter. (Place the boxes around the school).
8. Get involved in cleanup activities in your community?
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Air pollution
Air pollution is the accumulation in the atmosphere of substances that, in
sufficient concentrations, endanger human health or produce other measurable
effects on living matter and other materials. It is probably the worst and most
widespread form of pollution in the world. Air pollution tends to be of greatest
concern for locations in industrialised urban areas because these areas have
the most abundant waste products of fuels used in transportation, industrial
processes and general combustion (see Figure 333).
Figure 333: Industrial emissions are one of the major causes of air pollution
Causes of air pollution
a. Combustion of fossil fuels: The burning of fuels in factories and
automobiles in big cities releases several primary pollutants, especially
carbon monoxide, nitrogen and sulphur oxides into the air.
b. Deforestation: When forests are cleared or burned, the carbon dioxide
they store escapes back into the air, causing air pollution.
c. Volcanic eruptions and other natural processes release harmful
gases into the atmosphere.
d. Increased urbanisation: This leads to more construction, transport
and industrial activities, which cause fine particles of dust to rise into
the atmosphere.
Activity
3
Discussing effects of air pollution
1. In groups of five, hold a round table discussion on the effects of air
pollution.
378
2. Pass an object (a stone, a small ball) from speaker to speaker. Only the
speaker holding the object may speak.
3. Each speaker marks his or her contribution by placing a pen or pencil on
the table in the middle of the group. That individual may not contribute
again until every other student has placed his or her pen in the middle.
No one should dominate.
4. At any time, your teacher will come to your group, select a pen on the
table, and ask what contribution its owner made as the whole class
listens.
Did your discussion come up with something close to what is outlined below?
Harmful effects of air pollution
a. Diseases such as bronchitis, lung cancer, and heart disease may all
eventually appear in people exposed to air pollution.
b. Acid rain: This is precipitation containing harmful amounts of nitric
and sulfuric acids. These acids are formed primarily by nitrogen
and sulfur oxides released into the atmosphere when fossil fuels are
burned. In the environment, acid rain damages trees and causes soils
and water bodies to acidify, making the water unsuitable for some fish
and other wildlife. It also speeds up the decay of buildings, statues, and
sculptures that are part of our national heritage.
c. Poor visibility or haze: This occurs when sunlight encounters tiny
pollution particles in the air. Haze obscures the clarity, color, texture,
and form of what we see, and this causes serious problems in the
transport industry, especially aviation and shipping.
d. Depletion of the protective ozone layer due to harmful substances,
including chlorofluorocarbons. Depletion of the ozone layer can cause
increased amounts of Ultra-violet radiation to reach the earth, which
can lead to more cases of skin cancer and impaired immune systems.
Ultra-Violet radiation can also damage sensitive crops, such as soy
beans, and hence, reduce crop yields.
e. Global climate change: production of large amounts of greenhouse
gases, including carbon dioxide and methane has caused the Earth’s
atmosphere to trap more of the sun’s heat. This in turn has caused the
Earth’s average temperature to rise; a phenomenon known as global
warming. Global warming could have significant impacts on human
health, agriculture, water resources, forests, wildlife, and coastal areas.
379
Water pollution
Water pollution is the introduction into fresh or ocean waters of chemical,
physical and other material that degrades the quality of the water and affects
the organisms living in it. Water
pollution is a serious global
problem and is the major cause
of the deteriorating eco-system
of rivers, streams, lakes, seas
and oceans. Figure 334 shows
liquid wastes being discharged
into a stream.
Figure 334: Liquid wastes from industries
Causes of water pollution
a. Dumping of industrial wastes, containing heavy metals, harmful
chemicals, by-products, organic toxins and oils into the nearby source
of water.
b. Oil spills from ruptured oil rigs, refineries or oil tankers can produce
widespread and potentially hazardous substances into water bodies.
c. Improper disposal of human and animal wastes.
d. The residue of agricultural practices, including fertilizers and
pesticides enter the groundwater systems through rainwater causing
water pollution.
e. Natural disasters like hurricanes can destroy nuclear power plants
and automobiles into water bodies causing widespread damage to
marine life.
Harmful effects of water pollution
a. Waterborne diseases such as cholera and diarrhea are produced by
the pathogens present in polluted water, affecting humans and animals
alike.
b. Scarcity of safe and portable water for humans, animals and
plants.
c. Water pollution affects the chemistry of water. The pollutants,
including toxic chemicals, can alter the acidity, conductivity and
temperature of water. This would in turn affect the pattern of ocean
currents as well as the climate of the surrounding regions.
d. Water pollution kills life that inhabits water-based ecosystems
because the pollutants reduce the availability of oxygen in the water
body.
380
Land pollution
Land pollution is the degradation of the earth’s land surface by introducing
chemicals, waste products, or similarly damaging or poisonous substances
(see Figure 335). Land pollution has some of the most devastating effects
on both nature and living beings. It is characterised by the contamination of
earth’s surface, where humans and other creatures live.
Causes of land pollution
a. Industrial wastes are major contributors of land pollution. Dumping
of toxic materials such as chemicals and paints makes the areas
surrounding the industries look very filthy.
b. Increase in urbanization: This causes large amounts of rubbish
to be left untreated and then dumped along streets. This makes the
streets unhealthy, unfit and dirty to reside in. The waste matter
usually consists of leftover food, fruit and vegetable peels and other
non-decomposable solid materials such as glass, cloth, plastic, wood,
paper, etc.
c. Improper treatment of sewage leads to the accumulation of solids,
such as biomass sludge. These solid wastes overflow through the
sewage, making the entire area look dirty.
d. Agricultural wastes including the waste matters produced by crop,
animal manure and residues of the farmland are one of the major causes
of land pollution. The pesticides and fertilizers used by farmers to
increase the crop yield, leaches into the nearby land areas and pollutes
them.
e. The disposal of non-biodegradable wastes, including nuclear
wastes, containers, bottles and cans made of plastic, used cars and
electronic goods, also leads to the pollution of land.
f. Burning of solid fuels
leads to the formation of
ashes, which is yet another
cause of land pollution.
g. Mining
leads
to
the
formation of piles of coal and
slag. When these wastes
are not disposed through
proper channel, they are
accumulated and contaminate
the land.
381
Figure 335: Land Pollution
Harmful effects of land pollution
a. Land pollution has serious effect on wildlife. Vegetation, which provides
food and shelter to wildlife, is destroyed. This often disrupts the balance
of nature, causing human fatalities.
b. It leaves places dirty and makes them unhealthy. Skin problems and
other diseases are often diagnosed due to land pollution.
c. The toxic chemicals disposed of on land may leach into ground water,
posing serious long-term hazards to the environment.
d. Land pollution releases airborne chemicals and smell, which endanger
health and lower property values in an area.
Activity 4
Researching ways of reducing pollution
There are a number of ways to reduce pollution: using renewable energy,
recycling products, reusing products, planting trees, reducing industrial and
vehicle emissions.
1. Choose one of the above topics for homework. You should read the
section in the book on the topic, as well as do additional internet or
library research.
2. If you are doing the same topic with some friends, you should come
together the next day, and teach each other new information learned.
3. Later in the class period, make new groups; each group having 4
members, one from each of the subgroups.
4. Within these smaller groups, each member will teach the rest of the 3
members about his/ her topic.
5. Take notes on each topic and ask questions to assist each other in
understanding.
6. Your teacher will give you a quiz, a game or an assignment to check
your understanding.
Possible control measures to pollution
a. Using renewable and clean sources of energy such as solar and wind.
b. Using recycled products.
c. Reusing things such as paper and plastic bags.
d. Public awareness campaigns on the causes and dangers of various
forms of pollution.
382
e. Legislating and enforcing laws that protect the environment against all
forms of pollution.
f. Planting more trees to capture and store carbon dioxide.
g. Encouraging the use of public transport to reduce the number of vehicles
on roads. This may lower the level of exhaust gases from automobiles.
Summary
There are several environmental issues that require urgent attention to make
the ecology friendly. Pollution is one of the most serious of these environmental
problems. There are different types of pollution, but the following are more
prominent: air pollution, water pollution and land pollution. Human activities
such as burning of fossil fuels, deforestation, agricultural wastes, industrial
wastes, urbanisation, and others cause pollution. Natural processes such as
volcanic eruptions, earthquakes and floods also cause pollution. All types
of pollution cause some damage to living creatures and the environment.
Different types of pollution cause different types of harm. Apart from causing
harm to the natural environment, pollution can lead to serious health effects,
such as respiratory diseases, including asthma and lung cancer. In view of the
ill effects of pollution, many governments are encouraging public transport,
solar energy, recycling of wastes to check pollution. Various awareness
programmes are being undertaken to make the public aware about the evil
effects of pollution.
Glossary
Pollution: contamination of the natural environment, usually by introducing
chemicals, waste products, or similarly damaging or poisonous substances
Haze: mist, cloud, or smoke suspended in the atmosphere and obscuring or
obstructing the view
Global warming: an increase in the world’s temperatures, believed to be
caused in part by the greenhouse effect
Review questions
1. State three types of pollution, and for each, give any two causes.
2. Explain any three harmful effects of pollution on the environment.
3. Explain three reasons why environmental degradation is a gender
issue.
4. What is global warming?
383
5. Give one cause of global warming.
6. Suggest three control measures against pollution.
References
Bunnett, R. B. (1973). General Geography in Diagrams. London: Longman.
Bunnett R. B. (1984). Physical Geography in Diagrams for Africa. London:
Longman Group Limited.
Leong, G. C. (1983). Certificate Physical and Human Geography. Oxford:
Oxford University Press.
Obara, D. A. (1991). Gold Medal Geography. Nairobi: MacMillan Kenya
Limited.
Raw, M (1989). Resources and Environment. London: UNWIN Hyman Limited.
Simbeye, E. K and Munthali, M. Y. (2010). Target in Human and Economic
Geography: Senior Secondary School Geography. Blantyre: Bookland
International.
Waugh, D. (1990). Geography: An Integrated Approach. Hong Kong: Thomas
Nelson Limited.
White, R. (1998). Africa in Focus: A Physical, Human and Economic Geography.
Oxford: MacMillan Education Limited.
http://library.thinkquest.org/26026/Environmental_Problems/
environmental_problems.html 31/12/13
384
385
Unit
27
Desertification is one
of the most important
global
environmental
changes affecting human
populations. Most likely,
you are visually familiar
with deserts even if
you have never visited
one. You are probably
conscious that life is a
struggle in deserts for
people and wildlife. In
Malawi,
desertification
is increasingly becoming
a threat to development.
Knowledge
about
desertification will give
you an opportunity to
become better informed
and help you understand
and organises projects that
combat desertification. In
this unit, you will explain
the meaning of the term
desertification. You will
then describe causes of
desertification and assess
its effects.
Desertification
Desertification
Desertification is a process in which habitable
land is gradually transformed into a desert, as
shown in Figure 336. The process may involve the
expansion of an existing desert or the formation
of a new desert. Desertification slowly spreads
outward from anywhere it is induced, and the
rate of expansion may vary from place to place.
For this reason, it is best described as a creeping
desert.
Figure 336: Desertification
Desertification became well known in the 1930s,
when parts of the Great Plains in the United
States turned into the “dust bowl” because of
drought and poor practices in farming. However,
the term itself was not used until 1950. Currently,
desertification is a worldwide problem occurring
in many places but is most severe in dry grassland
regions of North Africa (the Sahel Region), Asia,
Central Australia and portions of South and North
America (see Figure 303). It is known to occur
386
at the expense of large areas of agricultural lands and unique ecosystems
adapted for life in arid conditions.
Activity
1
Examining areas of desertification on a world map
Study the world map in Figure 337 below, showing areas of desertification.
Use it to answer questions that follow.
Figure 337: Desertification regions
1. Locate the Sahara Desert and name the countries that are along its
southern border.
2. How is the expansion of the desert affecting these countries?
3. Find pictures and accounts from magazines or newspapers to investigate
how life is a struggle for people and wildlife in desert regions.
4. From your knowledge about the causes and effects of desertification,
why do you think people do so many things that threaten their survival?
5. Present your answers to the class for discussion.
Causes of desertification
Desertification is largely induced by human activities. Some of the more
predominant ones are:
a. Deforestation: Destruction of forest cover exposes land to erosion,
which decreases soil depth and soil fertility. In turn, reduced soil
387
fertility restricts future plant growth. This eventually results in failures
of expected rainfall, hence producing desert conditions.
b. Overgrazing by livestock and wild animals: Overgrazing
destroys
vegetation
and
causes compaction of soil
under trampling hoofs. This
renders the earth’s surface
bare, hard, infertile, and thus
unproductive. Figure 338
shows an area affected by
overgrazing.
c. Shifting cultivation: Here,
as the original land becomes
less productive people often Figure 338: Overgrazing
respond by converting more
rangeland into cultivated land or by converting more forested dry lands
into croplands. The new lands often have to be “created” by slashing
and burning of preexisting vegetation, hence, increasing the area of
land which is of poor quality.
d. Incorrect irrigation in arid regions: Pumping water from
underground to water crops has reduced the level of the water table.
This causes plants to dry since their roots can no longer reach water
supplies. Poor irrigation methods have also led to the accumulation of
salts in the surface soil causing massive death of plants.
e. Pollution: Some pollutants like sulphur dioxide combine with water to
form acids in the atmosphere. This leads to acid rains, which kill many
forests. Disappearance of forest cover causes the atmosphere to contain
less water vapour, hence leading to less rain.
f. Industrialisation or urbanisation: Urbanisation is also encroaching
upon agricultural lands causing immense damage to our natural
resources.
g. Mining: Mineral exploitation also takes up vast areas of productive
land, destroying vegetative cover. This triggers soil erosion and siltation
of water bodies, resulting in land degradation and water scarcity.
Effects of desertification
Desertification is a silent disaster. Its effects are often felt far beyond the
regions where it is occurring. Some of the known effects include:
a. Reduced food production: Soil degradation decreases the land’s
ability to support plant life, so crop yields become unpredictable. As
388
the land is degraded there is also less food for livestock populations,
causing them to decrease in number. With the increasing human
population, desertification affects the food security of the people living
in the affected areas, weakening their economies, particularly when
they have no other resources than their agriculture.
b. Climate change: Loss of vegetation reduces the rate of transpiration,
and this may disturb the hydrological cycle. It may also lead to the
accumulation of carbon dioxide in the air, which traps heat and raise
atmospheric temperatures. This may interfere with condensation
process during rain formation, hence causing erratic rains.
c. Increased frequency of sand and dust storms: Degradation of
lands destroys ground cover and enhances warming of the earth’s
surface. This leads to an increase in the frequency of sand and dust
storms. Increased number of dust storms contributes to air pollution
and causes eye infections, respiratory problems, and allergies.
d. Soil erosion and degradation: The reduction in plant cover that
accompanies desertification leads to accelerated soil erosion, which
in turn causes siltation of water bodies. As protective plant cover
disappears, floods become more frequent and more severe.
e. Forced migration: Desertification leads to forced mass exodus of people
from affected areas to urban areas. This gives rise to overcrowding,
increased unemployment, and stress on social services.
f. Increased over-exploitation of accessible natural resources:
This is often accompanied by a breakdown in solidarity within the
community, causing conflicts.
g. Loss of biodiversity: Desertification has led to loss of wide variety of
plant and animal species. Loss of vegetation cover destroys habitats of
wild animals, causing many to die. The disappearance of these plants
can also affect the possibility of producing plant-based medicines to
combat specific diseases or epidemics.
h. Depletion of ground and surface water resources. Desertification
reduces infiltration and this has a direct impact on river flow rates and
the level of groundwater tables. The reduction of river flow rates and
the lowering of groundwater levels lead to the drying up of lakes.
Activity
2
Community action against desertification
1. Organise an excursion close to your school. Look for an area damaged
by erosion.
389
2. Examine the leading causes of erosion in your area.
3. As a class, decide on the major erosion problems facing your local
community.
4. Create posters that can be hung in community areas (with permission)
to increase awareness about local erosion problems and why this can
hurt the entire area.
Why is desertification becoming a threat to Malawi?
a. Reduced agricultural land: Malawi’s economy is agricultural based
and the majority of the country’s population works in agricultural
industries. Therefore, the loss of agricultural land to desertification is
extremely costly not only to individual farmers but for the economy of
the country as a whole. Food and water shortages lead to malnutrition,
famine, disease and high death rates.
b. Poverty and underdevelopment: Malawi is importing more food
supplies than it is exporting to offset the increasingly serious effects of
desertification. This lowers the country’s reserves of foreign currency;
as a result it obtains huge loans from other nations to pay for imports.
This increases the debt burden that is reducing the possibility of making
productive investment in order to break the spiral of underdevelopment.
c. Depletion of water resources: Loss of vegetation to desertification
results in little rainfall. If there is low rainfall groundwater reserves
do not refill, water sources become depleted, and wells run dry. Lake
Malawi is the country’s major tourist site, so shrinking of the lake
would seriously affect tourism.
d. Loss of biodiversity: Desertification contributes to the destruction of
the habitats of animal and vegetable species and micro-organisms. This
would deprive tourists of game viewing and reduce tourism activities.
e. Increased flooding: As protective plant cover disappears due to
desertification, rainwater does not soak into the ground but rather
flows straight into rivers. This makes the rivers swell and floods become
more frequent and more severe. The Lower Shire Valley is always hit
by floods almost every year.
f. Increased illiteracy rates among women: Due to desertification,
women walk long distances to fetch water and to collect firewood. The
time they spend seeking new water and firewood sources could be spent
in school, job training, on paid work, or even on leisure and health
promoting activities.
g. Increased urbanisation rate: Land degradation due to desertification
390
is forcing many Malawians in rural areas to abandon their land because
it can no longer sustain them. These people migrate to other regions or
to urban slums in search of other opportunities.
h. Climate change: Increase in average temperatures and unpredictable
rains are seriously affecting the country’s agriculture sector and this is
affecting many lives.
Activity
3
Discussing the causes and effects of desertification
1. Get into groups and draw a tree with roots, a trunk and branches.
2. Label ‘desertification’ on the trunk of the tree.
3. Write the causes of desertification on the roots.
4. Now, brainstorm the effects of desertification and write them on the
branches of the tree.
5. Discuss the existing methods that are being used to combat
desertification in your area.
6. Display and present your work to the class for discussion.
Measures to control desertification
a. Restoring and protecting forest cover: Planting trees even at
the margins of deserts creates shelterbelts for checking the spread of
deserts. Plant cover such as grasses can help stabilise the soil and cut
down on erosion by wind and rain.
b. Rotational grazing, which is the process of limiting the grazing
pressure of livestock in a given area. Livestock are frequently moved to
new grazing areas before they cause permanent damage to the plants
and soil of any one area.
c. Proper water management: Applying advanced techniques of water
management is crucial to prevent desertification. This includes rain
harvesting and water recycling. Capturing water during heavy rain
falls would help prevent fertile topsoil from running off with it. This
would also be able to provide a source of water during droughts.
d. Proper land management; by developing appropriate farming
practices suited to the fragile semi-arid regions. For example, proper
crop rotation and the use of manure as a fertilizer.
e. Controlling population growth to reduce pressure on the natural
resources.
391
f. Providing alternatives to firewood and charcoal as sources of
energy.
g. Environmental education to increase general awareness of the
problem of desertification, and therefore encourage local communities
to regain a sense of respect for, and understanding of their environment.
h. Irrigation improvements, which can inhibit water loss from
evaporation and prevent salt accumulation. This technique involves
changes in the design of irrigation systems to prevent water from
pooling or evaporating easily from the soil. For example, use of drip
irrigation helps to save water.
i. Creating other opportunities for people to earn a living would help
to relieve dependence on land and, in turn, the pressures that are
causing desertification.
Activity
4
Designing a sustainable farm
1. Break into small groups and get a large sheet of paper.
2. Using the information that you have gained in this unit, design a farm
that is sustainable by considering the following:
a. How can soil erosion be avoided?
b. How will the crops be given what they need water, nutrients,
protection from pests, soil preparation, etc. without long term
damage to the farm and the greater environment?
3. Present your work to the class for discussion.
Summary
Desertification ranks among the greatest development challenges of our time
because of its toll on human well-being and on the environment. There are
many factors that contribute to desertification. Prolonged periods of drought
can take a severe toll on the land. Rapid population growth can force people
to move into environmentally fragile areas, putting undue pressure on the
land. Mining, pollution, industrialisation or urbanisation, shifting cultivation,
incorrect irrigation systems, overgrazing and deforestation can cause land
degradation, mainly, but not exclusively, in dryland regions. Desertification
has serious effects on the natural environment and the lives of humans,
such as reduced food production, increased frequency of dust storms, soil
erosion, forced migration, loss of biodiversity, scarcity of water, among others.
Improvements in irrigation, afforestation, water management, population
392
control, alternative sources of energy, and environmental education can
effectively help to combat the problem.
Glossary
Creeping desert: Developing or advancing of a desert gradually over a period
of time
Shifting cultivation: a form of agriculture in which an area of ground is
cleared of vegetation and cultivated for a few years and then abandoned for a
new area until its fertility has been naturally restored
Biodiversity: The range of organisms present in a particular ecological
community or system
Review questions
1. Define the term desertification.
2. Explain three causes of desertification.
3. Describe two effects of desertification.
4. Name two countries in West Africa where desertification is a serious
problem.
5. Why is desertification also known as ‘creeping desert’?
6. Suggest any three control-measures to desertification.
7. Why is desertification becoming a threat to Malawi? Give any three
points.
References
Bunnett, R. B. (1973). General Geography in Diagrams. London: Longman.
Bunnett R. B. (1984). Physical Geography in Diagrams for Africa. London:
Longman Group Limited.
Leong, G. C. (1983). Certificate Physical and Human Geography. Oxford:
Oxford University Press.
Obara, D. A. (1991). Gold Medal Geography. Nairobi: MacMillan Kenya
Limited.
Raw, M (1989). Resources and Environment. London: UNWIN Hyman Limited.
Waugh, D. (1990). Geography: An Integrated Approach. Hong Kong: Thomas
393
Nelson Limited.
White, R. (1998). Africa in Focus: A Physical, Human and Economic Geography.
Oxford: MacMillan Education Limited.
394
395
Unit
28
The earth’s climate is
dynamic
and
always
changing
through
a
natural cycle. The profile
of climate change has
risen from being an
environmental issue to
a major development
issue.
Learning
this
topic will help you to
better understand the
vulnerability of human
societies to the impacts
of both current climate
and
future
climate
change. This knowledge
will help you and your
community to prepare for
and respond effectively
to changes in climate. In
this unit, you will explain
the meaning of the term
climate change. You will
then examine causes and
effects of climate change.
Finally, you will explain
climate change mitigation
and adaptation measures.
Climate change
Climate change
Climate change is a shift in long-term patterns
of climatic factors such as temperature and
precipitation. The earth’s climate is constantly
changing at all scales. Since the early 20thcentury,
earth’s mean surface temperature has increased by
about 0.8°C. The rise in the average temperature
of the earth’s atmosphere is called global
warming. Precipitation patterns have also shifted
dramatically in response to the rise in global
temperatures. Figure 339 gives an impression of
the manifestations of climate change.
Figure 339: Climate change
Activity 1
Discussing climate change
Work in small groups
1. Do you think our climate is changing?
2. What have you heard about climate change?
396
3. What evidence is there that the earth is actually warming up?
4. What might be causing any global warming?
5. What effect might global warming have on the earth’s inhabitants?
6. Report your answers to the class for discussion.
Causes of climate change
For hundreds of millions of years life on earth has flourished and
evolved. However, this does not mean that the climate has been stable
throughout this time. Geological data shows evidence of large-scale climate
changes in the past. The climate changes that have occurred are caused by
both natural and human factors.
Natural causes
There are a number of natural factors responsible for climate change, but the
more prominent ones include the following:
a. Volcanic eruptions: Large and explosive volcanoes release large
volumes of sulphur dioxide, water vapour, dust and ash into the
atmosphere where they may stay for years. The particles (aerosols) of
these materials partially block the incoming rays of the sun, leading
to a reduced amount of solar radiation reaching the earth’s surface.
For instance, the eruption of Mount Pinatubo of Philippines in 1991
caused a 0.5oC drop in global temperature. With reduced amount of
solar energy, water body surfaces may not be warm enough to allow
rain formation causing droughts, hence climate change.
b. Plate tectonics and continental drift: The movement of crustal
plates builds up mountain ranges. It also causes earthquakes which
deform other physical features of the landmass. During these geologic
movements, the positions of landmasses and water bodies change in
relation to latitudes, and this may modify the flow of winds and ocean
currents, causing climate change.
c. Shifts in the earth’s orbit: The earth’s orbit around the sun is an
ellipse (oval shape – like a stretched circle with slightly longer flatter
sides), not a circle. However, the ellipse changes shape. Sometimes it
is almost circular and the earth stays approximately the same distance
from the sun as it progresses around its orbit. At other times the
ellipse is more pronounced so that the earth moves closer and further
away from the sun as it orbits (see Figure 340). However, this is only
noticeable over thousands of years. These changes in the earth’s orbit
can trigger changes in climate. Studies have shown that for the past 1.5
397
million years, the earth has gone into and out of an ice-age climate. Ice
age is a period of time
Ear
or
th’s
th’s bit
or
during which the earth’s
Ear
bi
t
average temperature is
Sun
reduced, causing polar
ice caps and glaciers
Sun
to grow in size, and
global sea levels to fall
dramatically.
d. Changes
in
the
orientation of the
Less elliptical orbit
More elliptical orbit
earth’s
axis
of
rotation: The earth Figure 340: Changes in the earth’s orbit
rotates around an axis
but the earth’s axis is not upright, it leans at an angle. As the earth
spins on its axis, it does not achieve perfect rotation. The angle of tilt
changes slightly with time, moving from 22.1 degrees to 24.5 degrees
and back again in about 41 000
years (Figure 341). When the
24.5
Axis of
angle increases the summers
22.1
rotation
become warmer and the winters
become colder, but when the angle
decreases summers and winters
become mild.
o
o
e. Changes in the amount of sun’s
heat: The sun’s energy output
is not constant, but changes
over time. Scientists track these
changes using observations of
sunspots (dark areas on the
surface of the sun) and more
recently by using satellites to
measure solar energy. Sometimes
the sun has many of these spots;
and at other times they disappear.
More spots mean more solar
Figure 341: Changes in the angle of the
energy being fired out from the
earth’s axis of rotation
sun towards earth, causing slight
changes in the earth’s climate.
Human causes
Human activities such as the following have caused substantial acceleration
to the current changes in the earth’s climate.
398
a. Burning fossil fuels: Since the Industrial Revolution fossil fuels (such
as coal, natural gas and oil) have been widely used to power factories
and automobiles, and to heat houses. The burning of these fossil fuels
leads to production of large amounts of carbon dioxide and other gases,
which accumulate in the atmosphere. The rich countries, such as USA,
Japan, China, Russia and UK are the biggest emitters of carbon dioxide
(see Figure 342). These gases trap heat that is otherwise supposed to
be lost into space from the earth and consequently lead to a sustained
increase in the average temperature of the earth’s atmosphere (global
warming). This causes the climate to change.
b. Deforestation: Trees help to regulate the climate by taking up carbon
dioxide (CO2) from the atmosphere through the process of photosynthesis.
So, large amount of carbon is stored in the world’s forests. When forests
are cleared or burned, the carbon stored in the trees is released into
the atmosphere as carbon dioxide, adding to the greenhouse effect. On
top of that, when a forest is destroyed, it can no longer absorb carbon
dioxide from the atmosphere, thereby leading to climate change.
c. Increased agricultural activities: The expansion of farming
activities alters the earth’s land cover, which can change its ability to
absorb or reflect heat and light. Raising livestock (e.g. cattle) also creates
large quantities of carbon dioxide and methane emissions. Methane is
produced as part of the normal digestive processes in animals which
can be emitted by the exhaling and belching of the animal. If beef and
dairy cattle numbers increase, methane emissions will also increase.
All these may result in climate change
d. Urbanisation: Urbanization is linked with development and has been
quite rapid in recent years. Construction of tall concrete buildings,
tarmac roads and other modern infrastructure modify the drainage and
heat conductivity of the earth’s surface. The tall buildings towers and
flyovers may also reduce the free flow of air, causing climate change.
e. Rapid population growth: Rapid increase in population leads to
greater consumption of food, water and energy; putting a great stress
on environment. This may also lead to climate change.
399
Annual carbon dioxide emissions
(expressed in metric tons of carbon)
U.S. 1,637
China* 1,631
Russia 441
India 412
Japan 342
Top 10 countries
Germany 218
Annual emmissions
Canada 179 (in million metric tones**)
U.K 163
S.Korea 129
Iran 128
Annual emissions by country
(in million metric* tons)
200 and greater
100-199
50-99
29-49
less than 20
no data
ˇ2006 data
*China surpassed the United States in emmissions
on a monthly basis in summer 2006
**2006 data
Figure 342: Annual carbon dioxide emissions
(Source: http://www.britannica.com/EBchecked/media/84923 18/02/14)
Activity
2
Greenhouse experiment
Greenhouse gases act as a layer that heat cannot pass through to radiate
back into space, much like the glass walls of a greenhouse. If the walls of a
greenhouse actually make a temperature difference, find out by how much?
1. Using thermometers and 2 boxes, one open on the top, and the
other covered with a clear plastic, monitor temperature in the boxes
throughout the day on a sunny day to determine the answer.
2. Report your findings to the class for discussion.
3. How do you relate this to what happens in real life?
Effects of climate change
There is growing evidence that climate change, particularly increasing
temperatures, has significant impact on the world’s physical, biological and
human systems, and these impacts could become more severe.
a. Shrinking of the Antarctic sea ice and mountain glaciers: The
Antarctic ice cap and glaciers are frozen fresh-water reservoirs. When
the ice or glacier melts into the oceans the salinity of the ocean water is
greatly affected, thereby changing the flow of ocean currents. This has
serious consequences on marine life, shipping and fishing.
400
b. Increased flooding due to rising of sea levels: Melting of mountain
ice, increased rainfall and expansion of seawater from increased
warming (thermal expansion) cause sea levels to rise. Rising sea levels
could put some small, low-lying island states and coastal communities
at a greater risk of flooding.
c. Declining food production due to severe droughts: Climate
change creates increasingly dry conditions that could cause permanent
drought, causing crop failure. Figure 343 shows two extreme results of
climate change.
d. Increased global warming: As the ice melts it exposes the land
beneath and forms liquid water lakes, both land and water are less
reflective than ice so they absorb more solar heat radiation. This further
adds to global warming.
e. Loss of biodiversity: Climate change alters habitats, causing
changes in the abundance and composition of plant and animal species.
Animals may either move to cooler habitats or die due to increasing
temperatures.
f. Increased frequency and severity of storms: Warm ocean
surfaces are conducive to formation of storms, and as the temperatures
increasingly rise, so does the frequency of severe storms.
g. Spread of diseases: With climate change, disease causing organism
multiply rapidly, as a result, diseases like malaria will spread to
different areas, affecting the health of many people.
h. Shifting water resources: Water is essential to life and it is essential
to human civilisation. Either too much or too little is a problem. Climate
change may, ironically, give us both. As atmospheric circulation and
precipitation patterns shift, one thing is for certain--water resources
will be impacted. In some regions, climate change threatens to reduce
fresh water availability due to decreased rainfall that ultimately feeds
major reservoirs. In other regions, there is increased flooding from the
intense rainfalls due to warmer, more moisture-laden atmosphere.
Figure 343: Examples of flooding (left) and drought (right)
(Source: https://www.e-education.psu.edu/meteo469/node/16701/01/14)
401
Activity
3
Discussing ways of controlling climate change
Global warming is currently occurring because certain ‘human’ activities are
causing a net increase in the atmospheric concentration of several ‘greenhouse
gases’ (GHGs)
1. List the sources of greenhouse gases.
2. If you wished to reduce the amount of greenhouse gas increase in the
atmosphere, which sources would be most important to control? Why?
3. Would there be problems with such controls? If so, what might they be?
4. Present your work to the class for discussion.
Climate change mitigation and adaptation measures
Mitigation measures to climate change
Mitigation involves actions that seek to limit the magnitude of climate change
itself by reducing greenhouse gas emissions. These efforts may include the
following:
a. Afforestation to remove greater amounts of carbon dioxide from the
atmosphere and to reduce flooding and drought.
b. Providing alternatives to fossil fuels (e.g. solar, wind, hydro and
geothermal power) and increasing use of environmental friendly
technologies to reduce or entirely eliminate greenhouse gas emissions.
c. Protecting threatened forests and wetlands, and planting more trees to
capture and store carbon dioxide.
d. Proper disposal of waste products.
e. Recycling and reusing some products.
f. National and international support (funding) of actions in the
environmental protection domain.
g. Establishing strict laws and heavy penalties for non-environmental
friendly actions by companies and industries through international
agreements.
h. Encouraging local food production through local agriculture, community
gardening, etc. to decrease the distance food must be transported. This
would help reduce emissions.
402
i. Promotion of more fuel efficient vehicles, hybrid vehicles, cleaner diesel
vehicles, biofuels, modal shifts from road transport to rail and public
transport systems, non-motorised transport (cycling, walking).
j. Integrating climate change into education systems of all parties to the
Convention.
k. Raising public awareness of the importance of climate change and its
implications to our lives. This may be achieved through the media,
school quiz competition, poetry, music and drama.
Activity
4
Debate
Use the following information to complete the task that follows:
Much of the carbon dioxide emissions responsible for the current climate
change have arisen from the use of fossil fuels in the industrialized countries.
These countries have benefited from the use of relatively inexpensive fossil
fuels in enlarging their economies and achieving a high standard of living.
Based on this background, conduct a class debate on the following question:
Should governments of poor countries take the responsibility of
mitigating climate change?
1. Create two groups and each group should take one side.
2. Write down your thoughts and arguments for your assigned side.
3. Thereafter you should debate the two sides of the issue.
4. Summarise the most important points to draw conclusions.
The industrialised countries need to take responsibility and assume leadership
to,not only reduce their own emissions, but also assist developing countries in
reducing their emissions.
Adaptation measures to climate change
Adaptation, by contrast, involves efforts to reduce the impacts of climate
change on vulnerable communities and their livelihoods through various
measures, while not necessarily dealing with the underlying cause of those
impacts. The measures may include:
a. Enhanced financial and technical support to the agricultural
communities so that indigenous and more drought tolerant food crops
like cassava, millet, sorghum sweet potatoes can be re-introduced into
403
the farming systems. This could ensure food security of households
affected by disasters like drought/dry spells.
b. Promoting irrigated agriculture by developing irrigation schemes
along river basins, construction of water basins and pans, but also
reconfiguring irrigated production systems to use water more efficiently
and to accommodate the use of marginal quality water.
c. Addressing land degradation by building soil and stone bunds, creating
grass strips and contour levelling as well as incorporating trees or
hedgerows. These measures will increase rain-water infiltration, reduce
run-off during floods, reduce soil erosion, and help trap sediments
including dead plant matter.
d. Creating functional linkages with development partners for technology
enterprise initiatives.
e. Diversifying rural economies, e.g. through value addition to agricultural
products and financial support for apiculture (bee keeping) with the
aim of reducing reliance on climate-sensitive agricultural practices.
f. Reducing reliance on centralised food system where commodity
production is concentrated in a few locations that may be vulnerable to
climate disruptions such as storm damage, pest outbreaks, etc.
g. Switching to new crops, seeds or agricultural practices can moderate
the impacts on agriculture of changes in temperature and water
availability.
h. Improving weather and flood forecasting and communications can
assist evacuation, relief and rehabilitation.
Activity
5
Climate change survival game
In groups of three or four, study the following climate change scenarios and
complete the task that follows.
Climate change survival activity cards
Scenario one
Scenario two
Over the past 20 years, the average winter
snowfall has increased in England by 6%.
Over the past 20 years, average
temperatures have increased in Southern
Africa by 3 °C.
A whopping 53 centimetres of snow fell in
the Region of Peel during the 2013/2014
winter season. A Regional record!
News headline: “Summer Heat wave hits
Central Africa. Temperatures reach all
time seasonal high!”
404
Due to increased run-off, a flood warning was On July 10th, ESCOM announced rolling
issued on April 22nd, 2014 for Chikhwawa
blackouts due to energy demand for air
and Nsanje
conditioning and refrigeration.
On the night of April 24th, Sheena called
the emergency department: “My basement
is filling with water. There is already two
metres on the floor, can you help me!”
The town of Mangochi designates a local
library as a “cooling centre”. This is a place
for people to visit when there is a heat
wave in the city. The cooling centre is airconditioned, free of charge and provides
free drinking water.
The city of Blantyre spent $1.2 million on
upgrading the storm sewer systems.
On July 13th, Elliya’s neighbour called the
emergency department. “I have a 75 year
old mother suffering from heat stroke, I
need an ambulance”
School children are planting trees around
their school.
Kelvin decides to walk to work today.
Ali bought a bike.
Douglas bought solar power panels today.
1. Prepare cards and write one statement on each card. Put the climate
change scenario cards in an envelope and pass it to another group.
2. Sort through the cards you have received and place them in logical
sequence. Most cards will have natural links to other statements.
There are climate events leading to impacts, and the eventual need to
mitigate and/or adapt to the situation.
3. What were the impacts?
4. What were the factors that led to the impact?
5. How did you decide to mitigate and/or adapt to the impact?
6. Discuss barriers you might face in real life in your efforts to mitigate
and/or adapt to the impacts and consider ways to overcome them.
7. Report your work to the class for discussion.
Summary
Climate change represents one of the greatest environmental, social and
economic threats facing the planet. Global climate has been changing
throughout the earth’s history due to natural causes such as volcanic
eruptions, plate tectonics and continental drift, shifting of the earth’s axis of
rotation, shift in the earth’s orbit and changes in the amount of the sun’s heat.
However, natural causes alone cannot explain all of the changes that we have
observed over the past few years. Human activities such as burning fossil fuels
for heat and energy, and clearing forests are contributing to climate change,
405
primarily by releasing billions of tons of carbon dioxide (CO2) and other heattrapping gases, known as greenhouse gases, into the atmosphere every year.
Changes in the climate system have led to higher temperatures, rising sea
levels, increased frequency and severity of storms, loss of biodiversity, drought
and ice melt. The impacts of changing climate are often most felt in fragile
and degraded regions in the developing world, adding an additional level of
complexity to the challenges of sustainable development. We can prepare for
and mitigate some of the likely climate change impacts to reduce their effect on
ecosystem and human well-being. Afforestation, public awareness campaigns,
strengthening water conservation programs, upgrading storm-water systems,
developing early warning systems, developing emergency preparation and
response strategies and international agreement on emissions reductions can
help reduce climate change impacts.
Glossary
Ellipse: a shape resembling an oval – like a stretched circle with slightly
longer flatter sides
Ice age: a period in the earth’s history when temperatures fell worldwide and
large areas of the earth’s surface were covered with glaciers
Sunspots: any of the relatively cool dark patches that appear in cycles on the
sun’s surface and possess a powerful magnetic field
Review questions
1. Describe the process of climate change.
2. Explain three points why global climates have changed throughout
earth’s history.
3. Explain how human activities have accelerated global climate change.
4. Explain three ways in which climate change negatively affects the
earth as a planet.
5. Describe any three climate change adaptation measures advocated in
your community.
6. Describe some of the mechanisms governments are using to mitigate
the effects of climate change. Give any three.
References
Obara, D. A. (1991). Gold Medal Geography. Nairobi: MacMillan Kenya
Limited.
406
Phiri, F. R. (2006). Senior Certificate Physical and Human Geography.
Blantyre: Dzuka Publishing Company.
Raw, M (1989). Resources and Environment. London: UNWIN Hyman Limited.
Waugh, D. (1990). Geography: An Integrated Approach. Hong Kong: Thomas
Nelson Limited.
White, R. (1998). Africa in Focus: A Physical, Human and Economic Geography.
Oxford: MacMillan education Limited.
https://publichealthmatters.blog.gov.uk/category/priority3/climate-change/
http://www.britannica.com/EBchecked/media/84923 18/02/14
https://www.e-education.psu.edu/meteo469/node/167 01/01/14
http://edugreen.teri.res.in/explore/climate/adapt_miti.htm 01/01/14
407
Unit
29
World fishing
World fishing
Fishing is the activity of catching fish carried
out in a very wide range of aquatic environments
including the open seas and oceans, rivers, lakes,
swamps and man-made ponds. Fish are abundant
where food supplies are most plentiful. They feed
Fish is a significant source on very small marine organisms, collectively
of protein and income called plankton.
as an export product for
millions of people around
the
world.
However, Factors that influence the distribution
human
population of fishing areas
growth has placed a
a. Shallowness of water: Plenty of sunlight
significant
pressure
is able to penetrate to the seabed in
on fish resources. It is
shallow water adjacent to land masses.
meaningful to study this
This encourages rapid growth of plankton,
unit because it will enable
which support many fish species.
you to gain knowledge
about
world
fishing
b. Continuous deposition of nutrients:
and
better
influence
Large quantities of nutrients from upstream
sustainable management
are deposited in river mouths as the rivers
of the available fish
enter lakes and seas. The nutrients support
resources. In this unit,
a rich collection of plankton and fish.
you will explain the
c. Meeting of cold and warm currents:
development
of
the
The meeting of cold and warm currents
world fishing industry
results in the precipitation of nutrients
and
identify
major
that are important for the rapid growth of
fishing grounds. You will
plankton, hence, attracting large shoals of
also explain the main
fish.
fishing methods and the
d. Upwelling of nutrient-rich cold water:
importance of the fishing
When cold water rises from the deep ocean
industry,
and
other
towards the surface, it brings with it
resources from the sea.
nutrients from sediments that accumulated
Finally, you will examine
on the sea floor. This also encourages the
the challenges faced by
growth of plankton and multiplication of
the fishing industry and
fish near the surface.
suggest possible solutions
to the challenges.
408
Major fishing grounds of the world
Almost all the major fishing grounds are located in the Northern Hemisphere
(see Figure 310 below).
Activity
1
Examining features of the world’s major fishing regions
Study the major fishing region of the world in Figure 344. Use the information
to complete the following task:
1. Name the countries found in each of the fishing regions.
2. Compare the major fisheries of the world map with the physical map of
the world in your atlas.
3. What do you think are the geographic and ecological features that make
the world’s major fishing regions such good fishing areas?
4. Report your findings to the class for discussion.
North Easten
Atlantic
North Easten
Pacific
N
North Western
Pacific
North Westen
Atlantic
Peru
Important fishing grounds
0
Potential fishing grounds
Figure 344: Major fishing grounds of the world
409
4000k
The following sections describe the world’s major fishing grounds.
North-East Atlantic
This is in the coastal waters of North-Western Europe. The area has many
rivers draining into the sea, depositing large quantities of nutrients, hence,
making the area rich in planktons and fish. Norway is the leading fishing
country in the region. The harsh climate and absence of mineral and forest
resources has forced the country to turn to the sea for fishing.
North-western Atlantic
This is the coastal waters of Eastern Canada where the North Atlantic Drift
and cold Labrador Currents meet, causing rapid growth of plankton and
plenty of fish. Harsh climate in the region could not support agriculture as
a result people turned to fishing for a livelihood, making Canada one of the
three major fishing countries in the world.
North-western Pacific
This is the shallow waters of North-East Asia where fishing is dominated by
Japan, another major fishing country in the world. The meeting of the warm
Kurosiwo and cold Kamchatka Currents make planktons and fish thrive in
the region.
The mountainous nature of Japan and severe cold climate makes almost 80%
of its land non-agricultural, with little pasture for livestock to provide enough
meat. This has driven many people to seek a livelihood and animal protein
food from the sea. The indented coastline has even made the area ideal for
fishing by providing calm waters, safe landing places and sheltered fishing
ports.
North-eastern Pacific
This is the region adjoining the western shores of North America from Alaska
to California. Many fish inhabit the seas along its highly irregular and
indented coastline. Salmon is the most valuable fish caught and is mostly
exported in canned form.
South-eastern Pacific
These are the fishing grounds of Peru, the only significant fishing region in
Southern Hemisphere. The upwelling of nutrient-rich cold water along the
410
coast of Peru and Chile, gives rise to abundant plankton and fish in the region.
Other fishing grounds include the coasts of South Africa, Brazil, Morocco,
India and some inland water bodies.
Types of fishing
There are two principal types of fishing, and these are:
a. Inshore fishing: This is carried out along the coastal waters, stretching
to only 70 kilometres from the shore. Small fishing vessels are used,
and these usually stay at sea for only one or two days.
b. Offshore fishing: This is done beyond the 70 kilometres limit into the
sea, and involves much larger fishing vessels that are well refrigerated.
Since they are equipped with refrigerated holds, the vessels may remain
at sea for several weeks at a time without the fish going bad.
Types of fish
Fish is generally divided into two depending on their feeding habits: pelagic
and demersal fish.
Pelagic fish
The word pelagic is derived from Ancient Greek pélagos, meaning ‘open sea’.
When referring to fish, the term refers to those that spend much of their lives
swimming and feeding primarily in the surface layers or a short distance
below the surface. Pelagic
fish are fast moving; they
are small in size and often
swim in shoals and tend to
be nomadic, migrating over
long distances (see Figure
345). Examples include
usipa, tuna, menhaden and
anchovies.
Figure 345: Pelagic fish
411
Demersal fish
The word demersal comes from the Latin word ‘demergere’, which means ‘to
sink’. When referring to fish, the term
refers to those that live and feed on or
near the bottom of seas or lakes (the
demersal zone).They occupy the sea
floors and lake beds, which usually
consist of mud, sand, gravel or rocks,
but not in the deepest waters (see
Figure 346). Examples of demersal
fish include codfish, mudfish, flatfish,
etc.
Figure 346: Demersal fish
(Source: http://nw08.american.edu/~vconn/seafood/West.
html 01/01/14)
Methods of fishing
Different kinds of fishing techniques are used to obtain maximum harvests
from the sea, and the most effective ones include the following:
Drift netting
Drift netting is used in surface waters to catch pelagic fish. The nets hang
vertically in the water, supported along the top edge by floats and weights
below. They are made of very thin
Floats
twine such that they are nearly
invisible in the water. Fish are
trapped by their gills in the meshes
of the net when they try to swim
across it. Their bodies are too big to
pass through and the mesh gets into
their gills when they try to move
backwards (Figure 347).
Figure 347: Drift Netting
Seine netting
This is also used to catch pelagic fish close to the surface. The nets used are
similar to drift nets but they are not left hanging in the water. Instead, they
are pulled by their ends using boats called seiners to surround a shoal of fish
412
(see Figure 348). This is the only
technique that provides the largest
catches of fish.
Trawl netting
Trawl netting involves a coneshaped net with its mouth kept
open by otter boards, and dragged
along the sea bed by boats or
vessels known as trawlers to Figure 348: Seine Netting
catch Demersal fish (Figure 315).
Lining
Otter boards
weights
headline
codend
Figure 349: Trawl Netting
This is used where the sea floor
is rugged and likely to damage
nets. The lines, which carry
hundreds of baited hooks, can
be up to two kilometres long
trailed by fishing vessels (see
Figure 349below).
Float
Fish traps
These are in form of skilfully weaved
baskets containing baits inside. These
traps are lowered into shallow coastal
waters and left for one or two days
before they are hauled up. Where
water moves swiftly, supporting poles
Baited hook
are erected to prevent the traps from
being washed away (see Figure 351). Figure 350: Lining
Figure 351: Fish trap in action
(Source:http://livingprimitively.com/wp-content/
fishtrap1.jpg 01/01/14)
413
Harpooning
Harpooning is used to catch large, near surface swimming fish such as
swordfish and whales. Fishermen fire barbed spears (harpoons), attached to
the fishing vessel by
a line, using guns.
When the harpoon
is fired into the
target, the barbed
points become firmly
anchored in its flesh,
thereby enabling the
people to drag the
fish to the ship (see
Figure 352).
Figure 352: Harpooning
Importance of the fishing industry
a. Food: Fish provides a cheap source of animal protein and essential
minerals such as iron, calcium, iodine, copper, magnesium and
phosphorous.
b. Employment: The industry provides a wide range of employment
opportunities in fishing, net making, boat building, fish processing,
canning, transport and marketing around the world.
c. Raw materials: The remains or wastes from processed fish are
important raw materials for manufacturing fertilizers, livestock feed,
glue, soap, cosmetics, and other products.
d. Foreign exchange: It helps foreign exchange earnings through
exports of fish items.
e. Tourism: Fishing supports a thriving tourism industry through
angling (game fishing) for the tourists.
Other resources from the sea
There are several other resources that can be obtained from the sea other than
fish, some of which are; whales, seals, shellfish, minerals (petroleum, common
salt, magnesium, potassium bromide, etc.), seaweed, fresh water, etc.
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Activity
2
Field trip to a local fish market
1. Visit a nearby fish market.
2. What are the types of fish sold at the market?
3. Find out from the traders where the fish come from.
4. Do they catch the fish themselves? If yes, how do they do it?
5. What is the most popular fish sold in the market?
6. Which of these types of fish is consumed the most? Why?
7. What do you think are the implications of the amount and type of fish
consumed in your area on the sustainability of the species?
8. Come back together as class and discuss your findings.
Challenges faced in the fishing industry
a. Overfishing: Too many fish are caught, including the young fish in
order to meet the ever-increasing demand for more food for the world’s
growing population. People are taking far more fish out of the ocean
than can be replaced by those remaining in many parts of the world.
For this reason, fishing is generally described as a robber economy.
Overfishing is a global problem with many serious implications, such
as the following:
i.
The balance of the food chain is disturbed (through changing the
relative abundance of predators and prey) when certain species
are removed. As a result, many other ocean species like seabirds
and sea mammals are vulnerable to the lack of food.
ii.
The economic welfare of millions of people dependent on marine
products is put at risk.
iii.
Income generated by tourism could be lost if fisheries are depleted
and marine biodiversity is lost (the vibrant aquatic life attracts
divers, sports fisherman and other visitors).
iv.
The collapse of fisheries can thus have devastating economic
impacts for developing countries, as well as for countries whose
trade in fishery products makes up a large percentage of their
total merchandise exports.
v.
Evolutionary effects and changes in fish behaviour, for example
migration patterns, due to loss of learning from older fish, which
have basically been removed from the population.
b. Water pollution: Industrial wastes discharged into water bodies
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contain poisonous chemicals that are potentially dangerous to all
aquatic life. Oil spills may cause oil slicks on the surface of water,
depriving the fish of oxygen, and this they can kill fish and underwater
plant life.
c. Climate change: Rising global temperatures have seriously disrupted
the pattern of ocean currents and upwelling of nutrient-rich cold waters.
In some regions, climate change has dried up fresh water sources due to
decreased rainfall. All these have resulted in the shrinking or loss of
the once flourishing fishing grounds around the world.
d. Deforestation: Loss of forest cover has led to an increase in erosion
and subsequent siltation of water bodies and degradation of fishing
grounds.
Possible solutions to the challenges faced by the fishing
industry
a. Monitoring and controlling fishing effort and destructive fishing
practices. This can be achieved through;
i.
Laying down strict rules about mesh size of the nets so that small
immature fish are not caught
ii.
Limiting the fishing season or restricting fishing for specific
species only during certain times of the year to allow the fish to
spawn and multiply
iii.
Using quota system to limit the quantity of fish caught per season
b. Enacting and enforcing laws against water pollution
c. Restocking over-fished waters
d. Promoting fish farming
e. Promoting sustainable utilisation of fisheries resources, such as setting
aside certain areas as protected spots in which fishing is prohibited
f. Promoting research in world fisheries to help understand various fish
species, their feeding habits, population, habitat, life expectancy and
migratory behavior
g. Promoting regional and international cooperation and collaboration
in fisheries development, management, security and access to shared
resources
h. Promoting social responsibility and good governance in the fisheries
sector
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Activity
3
Fishing game
(adapted from:www.aroundtheamericas.org/LESSON 6 Sustainable Fisheries - Around the Americas 04/02/14)
As a class, you will all be “fishers” whose livelihoods depend on catching fish.
Each fisher must catch at least two fish (large or small) in each round to
survive (i.e., get enough fish to either eat or sell).
Beans represent the largest and most valuable fish (tuna, swordfish, etc.).
Maize grains represent the next most-valuable fish (cod, salmon, etc.).
There are three fishing seasons, each using different methods. Each season
lasts for 20 seconds. Choose a time keeper to declare the fishing season either
“open” (“start fishing”) or “close” (“stop fishing”).
Season 1: (Method – fishing rods)
1. When the fishing season is declared open, you must hold your hands
behind your backs and use the “fishing rod” (straw) to suck “fish” beans/
maize from the “ocean” (bowl) and deposit them into your “boat” (cup).
Alternatively, close your eyes or put on blindfolds to simulate locating
and catching fish without using technologies such as sonar.
2. At the end of 20 seconds, the time keeper should declare the fishing
season “closed”.
3. Each fisher should count his or her catch (beans/maize in their boat)
and record the data.
4. Fishers who did not catch the two-fish minimum must sit out for the
following round.
5. The fish remaining in the ocean after each fishing season represent the
breeding population. Add one new fish for every fish left in the ocean
(bowl).
Season 2: (Method – trawl nets or long-line fishing)
6. In order to fish with your “newer technology,” you may use your hands
on the straws.
7. Repeat steps 2–5 in Season 1.
Season 3: (Method – the latest “fish finder” sonar)
8. In order to fish with the “latest technology,” fishers may use a spoon.
Give only one student per group the use of a spoon. Sometimes there is
a technological disparity among competitors.
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9. Repeat steps 2–5 in Season 1.
Season 4 : Expand fishing areas
10. How many groups have their “ocean basin” ran out of fish?
11. Brainstorm how the fishers are going to survive now when their ocean
is depleted?
Reflection
12. As a fisher, how did you feel when you realised that you had depleted
your fish stock?
13. As a fisher, how did you feel when other fishers joined your ocean group?
14. How does this activity relate to real ocean and fishery issues?
15. How do you think the depletion of fish stocks may impact other animals
(e.g., birds and marine mammals?
16. What happens to a resource when you have rapid human population
growth, growing technology, and a finite resource?
17. Decide on a plan to make the fisheries more sustainable.
18. Present your work to the class for discussion.
Activity
4
Debate
Use the following information to complete the task that follows:
In poor nations it is often difficult for people to have enough resources to
survive and their quest for sustenance places challenges on ecosystems and
the environment. People in wealthier countries take resources unsustainably
from their region and from around the globe.
Based on this background, conduct a class debate on the following question:
Should governments of poor countries create and enforce laws that
limit people from exploiting natural resources like fish?
1. Create two groups and take one side.
2. Write down your thoughts and arguments for your assigned side.
3. Thereafter you should debate the two sides of the issue.
4. Summarise the most important points to draw conclusions.
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Summary
Fishing is one of the largest industrial sectors in the world. Almost half of the
world’s catch is caught in less than 10% of the ocean. The distribution of fish
in the oceans is patchy and much localised in response to shallowness of the
water, deposition of nutrients in river mouths, meeting of ocean currents and
upwelling of nutrient-rich cold waters. There are five great and important
fishing grounds in the world. These are: the North East Atlantic, the North
West Atlantic, the North West Pacific Region, the North East Pacific and the
South East Pacific. The methods used for catching fish are very diverse, but
the following are more prominent: drift netting, seine netting, trawl netting,
lining, harpooning and use of fish traps. The fishing industry is important for
food, employment, raw materials, foreign exchange earnings and recreation.
The industry if facing serious challenges such as overfishing, water pollution,
climate change and degradation of fishing grounds due to deforestation.
Governments are making efforts to sustain the industry by, among other
things, monitoring and controlling fishing activities, enacting and enforcing
laws against pollution, promoting fish farming, restocking overfished waters,
promoting research in world fisheries and promoting international cooperation
in management of shared fisheries resources.
Glossary
Plankton: a mass of tiny animals and plants floating in the sea or in lakes,
usually near the surface, and eaten by fish and other water animals
Upwelling: a process in which cold nutrient-rich water rises to the surface
from the ocean depths
Pelagic fish: fish that spend much of their lives swimming and feeding
primarily in the surface layers or a short distance below the surface
Demersal fish: fish that live and feed on or near the bottom of seas or lakes
Review questions
1. State the two main types of fish and give any one example of each.
2. Describe the factors that have led to the abundance of planktons in
each of the following fishing grounds:
i.
North east Atlantic
ii.
South eastern Pacific
3. Briefly explain how fish is caught using trawl netting method.
4. Give any two resources that can be collected from the sea other than
fish.
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5. Explain why the fishing industry is called a robber economy.
6. Explain any two strategies that can be used to overcome the problems
faced in the fishing industry.
References
Bunnett, R. B. (1973). General Geography in Diagrams. London: Longman.
Leong, G. C. (1983). Certificate Physical and Human Geography. Oxford:
Oxford University Press.
Obara, D. A. (1991). Gold Medal Geography. Nairobi: MacMillan Kenya
Limited.
Phiri, F. R. (2006). Senior Certificate Physical and Human Geography.
Blantyre: Dzuka Publishing Company.
Raw, M (1989). Resources and Environment. London: UNWIN Hyman Limited.
Simbeye, E. K and Munthali, M. Y. (2010) Target in Human and Economic
Geography: Senior Secondary School Geography. Blantyre: Bookland
International
White, R. (1998). Africa in Focus: A Physical, Human and Economic Geography.
Oxford: MacMillan Education Limited.
http://www.stette.no/?c=18581 01/01/14
http://nw08.american.edu/~vconn/seafood/West.html 01/01/14
http://livingprimitively.com/wp-content/fishtrap1.jpg 01/01/14
www.aroundtheamericas.org/LESSON 6 Sustainable Fisheries - Around the
Americas 04/02/14
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Regional and
international trade blocks
Unit
30
Regional
blocs
and
international
trade
A trade bloc is voluntary grouping of countries of
a specific region where barriers to trade, (tariffs
and non-tariff barriers) are reduced or eliminated
among the participating countries. Trading blocks
are in different levels depending on the stages of
economic integration. From simple to complex,
the trade blocs include the following: free trade
area, customs union, common market, economic
union and political union (see Figure 353).
Level of intergration
The growth of regional
trading blocs has been one
of the major developments
in international relations
in recent years. Virtually
all countries are members
Political
of a block and many
Union Common government
belong to more than
Economic
Common currency, harmonized tax rates
one. In fact, more than
common monetary and fiscal policy
Union
one third of world trade
Common
Factors of production move freely
takes place within such
Market
between members
agreements.
Learning
Customs
this topic will help you to
Common external tariffs
Union
understand why there are
Free
regional agreements and
Free trade between members
Trade
why they vary widely. In
Complexity
this unit, you will explain
the term trade block. Figure 353: Levels of economic integration
You will also explain
the aims of regional
and international trade Activity 1
blocks. You will then
explain
the
benefits Researching trade blocs
and challenges of trade
1. Get into groups of five to research any one
agreements. Finally, you
of the following: free trade area, customs
will explain the role of
union, common market, economic union
customs in international
and political union. In your research, you
trade.
should focus on;
a. The purpose of your trade bloc.
b. Examples of your trade bloc.
422
c. Draw an outline map showing member countries of your trade bloc.
2. Report your findings to the class for discussion.
Free trade area
This is the first level of formal economic integration. When a group of countries
agree to eliminate tariffs, quotas and preferences on most goods and services
that flow between them, they create what is called a free trade area, e.g.,
Southern Africa Development Community (SADC) and Caribbean Free Trade
Area (CARIFTA). However, each of the members may impose its own tariffs
on goods from the non-member countries.
Customs union
A customs union (CU) builds on a free trade area by setting up common tariffs
on goods of the non-member countries, while conducting free trade among
them. This helps increase efficiency as well as establishes closer political and
cultural ties between the member countries. Examples include East African
Community (EAC), Southern African Customs Union, Customs Union of
Belarus, Kazakhstan, and Russia.
Common market
A common market represents a major step towards significant economic
integration. In addition to containing the provisions of a customs union, a
common market (CM) requires that factors of production, such as labour and
capital, are free to move within member countries, expanding scale economies
and comparative advantages. Thus, a worker in a member country is able to
move and work in another member country. In a common market, the factors of
production become more efficiently allocated, thereby increasing productivity
further. Examples include the Common Market for Eastern and Southern
Africa (COMESA) and the Central American Common Market (CACM).
Economic union
An economic union adds to a common market the need to harmonise a number
of key policy areas, including the use of a common currency. The participant
countries have both common policies on product regulation, freedom of
movement of goods, services and the factors of production (capital and labour)
and a common external trade policy. The European Union (EU) and the Union
State of Russia and Belarus are examples of economic unions.
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Political union
A political union is when a group of nations or states share a joint government
that is internationally acknowledged. It is the most consolidated form of
economic integration. The United States is an example of even closer political
union.
The aims of regional and international trade blocs
a. To promote trade within the block and defend its members against
global competition
b. To remove trade restrictions among member nations
c. To improve social, political, economic and cultural relations among
member nations by easing travel restrictions, sharing electrical power
and currency, and ultimately, promoting investment and a common
market
d. To encourage free transfer of resources such as labour, capital, goods
and services among member countries
e. To establish collective bargaining
f. To enhance economic growth through the promotion of cross-border
investments, promotion of research and adaptation of science and
technology in development
g. To raise the living standards of its people
SADC
The Southern Africa Development Community (SADC) is an intergovernmental organisation headquartered in Gaborone, Botswana. In 2008,
SADC established a free trade zone with the East African Community (EAC)
and the Common Market for Eastern and Southern Africa (COMESA). It was
during this time that Malawi, Zambia, Tanzania and Mozambique became
members of the SADC Free Trade Area. The other members include Mauritius,
Zimbabwe, Madagascar, South Africa, Angola, Democratic Republic of Congo,
Seychelles, Lesotho, Swaziland, Namibia and Botswana.
ECOWAS
The Economic Community of West African States (ECOWAS) is a regional
group of 16 countries founded in 1975. The headquarters of ECOWAS are
located in Lome, Togo. Members include Togo, Benin, Liberia, Burkina Faso,
Mali, Cape Verde, Mauritania, Cote D’Ivoire, Niger, Gambia, Nigeria, Ghana,
Senegal, Guinea, Sierra Leone and Guinea Bissau.
424
EAC
The East African Community (EAC) was first established in 1967 but it
collapsed in 1977 due to political differences. It was re-established in 1999
with its headquarters in Arusha, Tanzania. EAC’s member states include
Kenya, Uganda, Tanzania, Rwanda and Burundi.
Figure 354 shows SADC, EAC and ECOWAS member countries.
Senegal
Gambia
Guinea-Bissau
Guinea
Mali
Niger
Benin
Sierra Leon
Ivory
Liberia Coast
Ghana
Nigeria
Togo
Democratic
Republic of
Congo
SADC
EAC
ECOWAS
Angola
Namibia
0
0
1000 ml
1000 km
Zambia
Uganda
Kenya
Rwanda
Burundi
Tanzania
Malawi
Zambia
Seychelles
Mozambique
Mauritius
Madagascar
Botswana
Swaziland
Lesotho
South Africa
Figure 354: SADC, EAC and ECOWAS member countries
COMESA
The Common Market for Eastern and Southern Africa (COMESA) was
established in 1994. Its headquarters are located in Lusaka, Zambia. Currently,
COMESA has 19 member states, which include Malawi, Burundi, Comoros,
Democratic Republic of Congo, Djibouti, Egypt, Eritrea, Ethiopia, Kenya,
Libya, Madagascar, Mauritius, Rwanda, Seychelles, Sudan, Swaziland,
Uganda, Zambia, Zimbabwe and Angola. The map in Figure 355 shows
COMESA member countries.
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Please note! Countries, which were previously members but now have
withdrawn from COMESA, include Tanzania, Lesotho, Mozambique and
Namibia.
COMESA was intended to be a purely trade and investment oriented
organisation, as such it does not have any organs dealing with politics.
Figure 355: COMESA member countries
CARIFTA
The Caribbean Free Trade Area (CARIFTA) was fully established in 1968.
Some of its member states are Jamaica, Antigua, Barbados, Dominica,
Guyana, Tobago and Trinidad (see Figure 356).
426
CACM
The Central American Common Market (CACM) is an association of five
Central American nations (Guatemala, Honduras, El Salvador, Nicaragua
and
Costa
Rica).
It was established
in 1960. However,
due
to
internal
political
instability
and mounting debt
pressures in some
member
countries,
the CACM suspended
its activities in the
mid-1980s,
but
later renewed them
in the 1990s. Its
headquarters
are Figure 356: CACM and CARIFTA member countries
located in Guatemala.
EU
The European Union (EU) is currently a 27-member country organisation,
with its headquarters
in Brussels, Belgium.
Its member countries
include Belgium,
Germany, France,
Denmark, Finland, Italy,
Poland, Netherlands,
Portugal, Spain,
Romania and Sweden
(Figure 357). The
UK, which had been
a member for nearly
43 years, left the EU
in 2016 with the hope
to protect jobs for
its citizens, reduce
immigration into the
UK and to preserve its
sovereignty.
Figure 357: EU member countries
427
Benefits and challenges of trade agreements
Benefits
a. Competition: Trade agreements bring together manufacturers from
different countries, resulting in greater competition. Accordingly, the
increased competition spurs companies to innovate and develop better
products keeping prices low and quality high.
b. Economic development: Trade agreements enable larger countries
to take advantage of increased market size, and have their economies
grow. This growth overflows into smaller countries that are economically
unstable or mired in poverty but are open to trade.
c. International cooperation: Trade agreements force countries to
support the rule of law. The World Trade Organisation requires members
to honor all agreements and abide by all World Trade Organisation
rulings. They must obey the rules if they want to retain the benefits
of free trade. If a country does not enforce contracts it loses business
and investors move their money elsewhere. Since international trade
relies on traders keeping their agreements, countries and companies
are more accountable to each other and therefore more stable.
d. Resource allocation: Trade agreements improve the allocation of
global resources. If countries or people can trade for the items they
need, they can focus on making the ones they do best to export to other
countries.
e. Business incentives: Trade agreements open markets and offer
business incentives and protections. They include commitments to
protect intellectual property rights and labour rights and open regions
to competition.
Challenges
a. Political instability: Serious wars and conflicts have resulted in
insufficient diversification of national economies in many African
countries. This posed a great challenge for trade blocks in the process
of regionally integrating some African countries.
b. Fierce competition among trade blocks: A number of regional trade
blocks compete against each other, and if this scenario materialises,
the gains from free trade within blocs could be offset by a decline in
trade between blocs. Besides, some member states participate in more
than one regional grouping and this may undermine the aims of one
trade group.
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c. Diversion of trade: The introduction of a common external tariff by
a regional bloc on non-member countries can divert trade from more
efficient external exporters to less efficient ones. For example, should
the introduction of a common external tariff by a regional bloc result in
a relative increase in the import tariff for country X outside the region
compared with that for country Y inside the region, one would expect
an increase in imports from country Y and a drop in imports from
country X. As a result, however, consumers must buy goods from the
less efficient producer.
d. Disparity in the economic size of member states: The economic size
of some member countries is perhaps many times greater than that of
others in one regional economic grouping. As a result, member countries
with poor quality products find it difficult to compete effectively with
those that have a wider and better economic base.
Activity
2
Debate
1. Divide into small groups and get a sheet of paper.
2. Draw a large shape with the same number of sides as there are people
in the group. For example, a group of three would draw a triangle, a
group of four a square.
3. Write the topic inside the shape (see Figure 358 below)
Topic: Regional
trade blocs
are leading to
fragmented
world
economy.
Figure 358 : Consensus diagram for a group of four people
4. Draw a large margin inside this shape and mark off a section for each
person.
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5. Each person in the group should take a turn at making a statement or
giving an opinion on the topic.
6. If everyone in the group agrees with the statement/opinion then the
person should write it in the centre of the shape.
7. Any statement that is not agreed on by the whole group should be
written in the person’s individual section. You should use different
coloured markers.
8. Share and discuss statements with the whole class.
The role of customs in international trade
Every country’s wish is to have a favourable balance of trade, but this is not
usually achieved in many countries due to foreign competition. All nations
utilise some assortment of customs or tariffs for their own benefit. Tariffs in
form of customs duty nearly always are placed on goods that are brought into
a country (imports), as opposed to excise duty charged on goods produced
within the country.
Advantages of customs
a. Protecting consumers - A government may levy a tariff on products
that it feels could endanger its population. For example, Malawi may
place a tariff on imported beef from Mozambique if it thinks that the
beef could be tainted with disease.
b. Protecting local industries and employment - The government of
a developing economy will levy tariffs on imported goods in industries
in which it wants to foster growth. This increases the prices of imported
goods and creates a domestic market for domestically produced goods,
while protecting those industries from being forced out by more
competitive pricing. Ultimately, it decreases unemployment and allows
developing countries to shift from low value primary products to refined
goods.
c. Retaliation - Countries may also set tariffs as a retaliation technique
if they think that a trading partner has not played by the rules or has
gone against the foreign policy objectives of the government.
d. Revenue collection - Customs are usually a major budget contributor,
and sometimes the most important source of revenue for a country.
Money collected from tariffs on imported goods helps the domestic
government to run various development programmes.
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Disadvantages of customs
a. Poor quality products: If an industry develops without competition, it
could end up producing lower quality goods, and the subsidies required
to keep the state-backed industry active could lower economic growth.
b. Reduced trade: Trade restrictions limit world trade. As a result, global
resources are less efficiently allocated and the level of world production
and income is reduced, causing widespread unemployment.
c. Trade war: Increasing prices of imported products encourages
retaliation, which may result in trade war.
Activity
3
Simulation game–examining effects of trade restrictions
on trading activities (adapted from:www.sceconomics.org/Lesson 1.2: Why Do People Trade? - SC
Economics 04/02/14)
1. Bring from home one or more small items to trade that you no longer
want. Do not show the items to anyone.
2. Next, divide your class in half, with one group on each side of your
room.
Trading Round 1
3. Begin to trade your items only with people of your side of the room for
at least 5 minutes. At the end of this first trading round;
a. How many students on your side made a trade?
b. If you are one of those who made trade, why did you decide to
make trade?
c. How many students in your group decided not to make trade?
Why?
d. How many students had the most to trade?
e. How many students had the least to trade?
f. Were there any restrictions placed on your trade?
g. How did this trade restriction affect your trading decisions?
h. In a real world situation, what trade restrictions would have
accompanied your trade?
431
Trading round 2
4. This time there won’t be any restrictions. You may trade any items
with any other students in the room if you want to, again for 5 minutes.
a. How many students made a trade in this round?
b. Of those who made trade, how many are better off as a result of the
trade?
c. How did the elimination of trade restrictions affect your trading?
5. In groups of four, discuss how the trading sessions resembled trading
in the real world.
6. Report your conclusions to the class for discussion.
Elements of international trade
Balance of payment
This is a record of all monetary transactions between a country and the rest
of the world for a specific period, usually a year. Funds for a nation obtained
from exports, tourism, loans and investments by foreigners are recorded as
positive or surplus items, while uses of such funds by the nation for imports
or investment in foreign countries are recorded as negative or deficit items.
When positive and negative items balance there can be no overall surplus or
deficit; hence balance of payment.
In economic terms, a positive or surplus balance of payments means a nation
has more funds from trade and investments coming in than it pays out to other
countries, resulting in the appreciation in the value of its national currency
against currencies of other nations.
Balance of trade
It is a similar record but registers only visible exports and imports. It is the
difference between the values of a country’ exports and imports over a certain
period of time, normally a year. When the value of a country’s exports exceeds
its imports, it has a favourable balance of trade (trade surplus). However,
when the value of imports exceeds the exports, the country registers a trade
deficit.
Trade deficits can occur in both developing and advanced countries. The
United States, for example, has been running a trade deficit for many years.
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Causes of trade deficit
a. Population growth: As population increases, imports become essential
and the quantity of imports may increase to meet the needs of the
people. This may result in a trade deficit.
b. Development programmes: In developing countries, these programmes
require import of capital goods, technology, some raw materials, which
are not available at home, and highly skilled and specialised work
force. If import of these items continues for a long time, the countries
may land in a balance of trade deficit.
c. Demonstration effect: When people like to flaunt imported goods, and
imitate the consumption pattern of the developed countries, their
import will increase. This may cause disequilibrium in the balance of
trade.
d. Natural disasters: Natural calamities such as droughts and floods may
adversely affect agriculture and industrial production in a country. The
exports may therefore decline while the imports may go up causing a
discrepancy in the country’s balance of payments.
e. Dependence on primary products: Developing countries export low
value primary products like minerals and crops to developed countries.
The developed countries refine the primary goods into high value
usable products, which they export at very high prices to poor countries.
This causes trade deficit. While there is some manufacturing in most
developing countries, it is often operated by multinational companies
from the developed countries taking advantage of cheap industrial land
and labour rates.
f. Political instability: Conflicts in a country cause low production.
Ultimately, balance of trade remains unfavourable.
g. Import of non-essential goods
Implication of a trade deficit
The long-term trade deficit in a country may result in unstable economy where
the following problems are serious:
a. Foreign debts: When a country buys more goods and services than
it sells, it must finance the difference by borrowing money from other
countries.
b. Shortage of foreign currency: Trade deficits are paid for out of
foreign exchange reserves, and may continue until such reserves are
depleted.
433
c. Dependence on foreign aid: In poorer countries, foreign aid may
be an important finance to their national budget, as is the case with
Malawi. This forces such countries to adopt undesirable policies dictated
by the donor countries as terms and conditions for their financial aid.
d. Unemployment: Increased imports deprive the domestic industries of
markets; as a result they lower their production which in turn leads to
widespread unemployment.
The difference between balance of payment and balance of
trade
Balance of trade is only a part of the balance of payments. Deficit balance of
trade does not necessarily mean that the balance of payment is also deficit.
Conversely, surplus balance of trade also does not mean that the balance of
payment is surplus, because trade balance may be covered by other surplus
investments.
Activity
4
Reflecting on the topic
1. Summarise the most important ideas they have just discussed about
the topic.
2. Why is this knowledge worth having?
3. Write down what you can do about the issues you have been discussing
in the unit.
4. Report your answers to the class for discussion.
Summary
Trade blocks are increasingly shaping the pattern of international trade. There
are several types of trade blocks. Some of these are: free trade area, customs
union, common market, economic union, and political union. Each of these
levels of economic integration has its specific goals and objectives. These trade
agreements are particularly important for increased production, economic
development, international cooperation, resource allocation and business
incentives. However, the success of many of these regional trade groupings
is facing many challenges, such as political instability in some countries,
fierce competition among trade blocks, diversion of trade, and disparity in the
economic size of member states. The customs utilised by countries in trade
serve to protect consumers from hazardous goods, to protect local industries
434
and employment, to provide means of retaliation if a trading partner has not
played by the rules and to provide revenue. However, customs encourage poor
quality products due to lack of competition, reduces trade, and encourages
trade war.
Glossary
Trade block: A voluntary grouping of countries of a specific region where
barriers to trade are reduced or eliminated among the participating countries.
Trade barrier: A tariff or boycott that a nation imposes to limit or burden
trade.
Tariff: A duty levied by a government on imported or exported goods.
Free trade area: A group of countries that eliminate trade barriers between
themselves while remaining free to pursue independent policies with regard
to trade barriers with nonmember countries.
Customs union: An association of countries that enjoy free trade among
themselves and agree on tariffs for nonmembers.
Common market: An association of countries that enjoy free movement of
factors of production, such a labour and capital, within member countries, in
addition to the provisions of a customs union.
Economic union: A merging of the economies of two or more states to function
as a unit that shares a common financial policy and currency.
Political union: A group of nations or states that share a joint government
that is internationally acknowledged
Customs duty: Tariffs placed on goods that are brought into a country
(imports).
Excise duty: Tariffs charged on goods produced within the country.
Trade deficit: The difference, measured in monetary value, between a
nation’s imports and its exports when the imports exceed the exports.
Review questions
1. Define the term trade bloc.
2. How have trade agreements affected world trade? Explain two points.
3. Give any three reasons why countries form regional groupings in trade.
4. Explain two advantages of trade barriers in international trade.
5. Table 7 shows trade statistics for Malawi with COMESA and SADC in
2005. Study it and answer the questions that follow.
435
Table 12: Malawi’s 2005 regional trade with COMESA and SADC (Source: NSO)
SADC region
country
Angola
Botswana
DRC
Lesotho
Madagascar
Mauritius
Mozambique
Namibia
South Africa
Swaziland
Tanzania
Zambia
Zimbabwe
TOTAL
Imports in Malawi
Kwacha
616,522
165,413,430
159,973
296,545
72,110,620
17,595,700,738
15,311,287
44,767,204,306
550,559,550
4,454,920,717
7,821,746,051
10,690,028,908
86,134,068,647
Exports in Malawi
Kwacha
3,909,248
202,602,339
35,000,133
58,960,108
4,460,249
2,192,021,255
0
11,329,221,267
2,629,894
489,124,722
913,596,325
1,330,826,939
16,562,352,479
a. From which country did Malawi import most?
b. What do you think were the goods mostly imported from this country
you have mentioned above?
c. By how much did South Africa’s imports of Malawian products
surpass the rest of the countries in SADC and COMESA regions?
d. Calculate the balance of trade for Malawi in 2005.
e. Based on the balance of trade you have calculated above, how do
you assess Malawi’s economic status in 2005?
6. Distinguish between balance of trade and balance of payments.
7. Explain three effects of unfavourable balance of payments on an
economy.
8. Suggest any two corrective measures that a government can take in
order to improve an unfavourable balance of payments.
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