SLCSS/GEOGRAPHY/CE/LANDFORM/81
Denudation
 From the last two chapters, we learn that compression and folding of sediments along
destructive plate margin will give rise to the formation of fold mountains. Also, the intrusion of
lava will supply volcanic rocks on the mountains.
 We know that the Earth movement is continuous, so if there are continuous supply of sediments
and lava, these mountains should keep on rising and never ends.
 However, the fact tells us that the heights of mountains are quite constant over time. Why?
 Because of denudation.
 In fact, no landforms on the Earth’s surface are permanent. All are being changed very slowly
and continuously by various processes as shown below:
Weathering
Mass movement
Denudation
Erosion by
Processes changing the
shape of earth surface
Transportation by
Deposition
River
Marine (wave)
Wind
Glacier
Physical
Chemical
Biological
Creep
Flow
Fall
Slide
River
Marine (wave)
Wind
Glacier
River
Marine (wave)
Wind
Glacier
Definitions:
Denudation
The wearing away of land surface by natural agents, including the process
of
_____________________,
_________________________________,
_______________________ and _______________________________.
Weathering
The actual loosening, ________________ and _________________ of
rocks largely by various climatic agents, such as temperature change,
rainwater, snow and ice. The weathered rock materials can become the
inputs for the processes of mass wasting, erosion, transportation and
deposition.
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Mass wasting
The ________________ movement of weathered materials in mass mainly
due to the force of ____________________.
The breaking down of rocks and the removal of the weathered materials by
Erosion
various agents such as running ______________, moving __________,
_____________, ________________ and currents.
Transportation
The process which _____________ the loose materials from one place to
another place where it is laid down as new deposit.
The setting down of transported materials. This takes place when the
Deposition
transportation agent does not have enough _________________ or
________________ to carry the load.
A. Weathering
1. Physical (mechanical) weathering
-
_____________________________ of rocks into smaller and angular fragments without
change in their chemical composition
-
mainly caused by __________________________ changes which cause repeated
expansion and contraction of rock minerals
I. Types of physical weathering
-
Four types of physical weathering are studied below, their resultant landscapes are
more commonly found in Hong Kong.
a. Block disintegration
 Favourable conditions:
– barren areas with large ________________________________ range, e.g.
desserts
– in __________________________ rocks
 Process and resultant landscape :
– in daytime, intense solar heating causes rocks to _________________
– at night, the temperature falls so rocks cool and ___________________
– repeated expansion and contraction produces stress along _______________
 joints are then widened and deepened and finally break down the rocks block
by block
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Differential rates of
expansion and
contraction of
different mineral
grains causing them
to break away from
the parent rock.
Accumulation
of sand grains
derived from
the rock block
b. Granular disintegration
 Favourable conditions:
– barren areas with large daily temperature range, e.g. ________________
– in ____________________________ rocks with different structure, e.g.
granite and sandstone
 Process and resultant landscape :
– individual minerals __________________ and ________________ at
different rates due to their different structure
– repeated expansion and contraction causes different minerals to loosen grain
by grain
 sand grains are accumulated at the base of rocks
c. Exfoliation (Onion weathering)
 Favourable conditions:
– barren areas with large daily temperature range, e.g. desserts

– in fine-grained rocks with ______________________________, e.g. rhyolite
Process and resultant landscape :
– in daytime, intense solar heating causes the ___________________________
of rocks to expand more than the interior part
– at night, temperature falls and the outer layers cool and _________________
– repeated expansion and contraction creates stress in rocks and produces radial
and concentric cracks in the outer layers

in time, the ______________________________ peel off
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d. Frost shattering
 Favourable conditions:
– areas with temperature fluctuating __________ and ____________ freezing
point, e.g. on mountain areas, or temperate latitudes in winter
 Process and resultant landscape :
– water in the cracks or joints of rocks may _______________ at night and
expand, this will ___________________ the cracks
– in daytime, ice ______________, water fills the cracks more deeply and
more water can be held
– repeated freeze and thaw action causes the cracks to widen and deepen, and
finally rock break down into fragments
 rock fragments accumulate at the foothills to form screes
II. Other processes responsible for physical weathering
a. Wetting and drying
– commonly found in coarse-grained granite and other permeable rocks
– certain minerals in rocks absorb rain water and expand
– these minerals contract when they dry up
– alternate expansion and contraction of minerals causes rocks to break down
b. Growth of salt crystals
– commonly found in desserts
– dissolved minerals (e.g. salt solution) in rocks re-crystallize when water
evaporates
– growth of crystals produces stress in rocks
– cracks and joints in rocks are widened, then rocks break down
2. Chemical weathering
-
________________________ of rocks through various chemical processes
it changes the original minerals of rocks into new minerals which could be easily
removed, so the rocks become less resistant against weathering and erosion
active in hot and wet regions, e.g. equatorial rain-forest
I. Processes of chemical weathering
a. Solution / Carbonation
- dissolved carbon dioxide in rain water acts as a mild carbonic acid to dissolve
minerals in rocks
- e.g. carbonation often occurs on limestone region, by which can dissolve and
remove limestone minerals along joints by rain water combined with carbon
dioxide, joints are then deepened to form karst landscape.
CaCO3+H2O+CO2
Ca(Hco3)2
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b. Hydration
- minerals in rocks absorb water and then expand to become hydrated minerals,
which are soluble in water and are more easily affected by other processes
- the expansion of minerals will weaken the rock structure
c. Hydrolysis
- combination of water with minerals brings chemical changes which weaken rock
structure
d. Oxidation
- some rocks contain iron compounds which can be oxidized to form iron oxide
which may be soluble
II. Types of chemical weathering
-
Chemical weathering is most common in Hong Kong, comparing with physical and
biological weathering.
- It is due to its heavy ____________________, high __________________ and the
presence of granite, which occupies one-third of the total land area in Hong Kong.
- All these factors facilitate chemical weathering. (will be explained later)
- There are two common types of chemical weathering in Hong Kong:
a. Honeycomb weathering
 Favourable conditions:
– in __________________ region where sea water sprays onto rock surface
– in rocks with different structure which contain _________________ minerals
 Process and resultant landscape :
– ___________ rain water or __________________________ falling on rocks
will dissolve some minerals by solution
– dissolved minerals are then removed, leaving __________________on rocks
 acidic water and chemical weathering concentrate at the hollows and make
them larger, rocks with honeycombed surface are formed
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b. Spheroidal weathering
 Favourable conditions:
– on land with _____________________ and exposed rocks, e.g. granite land
in Hong Kong
– in area with __________ temperature and ______________ rainfall
 Process and resultant landscape :
– acidic ______________________ goes into _______________ of granite,
______________________________starts
– rocks decay along the joints while the inner parts of blocks remain
____________________
– the inner parts become rounded ____________________, which are
surrounded by _____________________ materials

when weathered materials are ______________________ by agents of mass
wasting or erosion, e.g. rainfall or wind, _______________are exposed on
the ground called ________________
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3. Biological (Organic) weathering
-
weathering by _____________, _________________, man and decomposers (e.g.
bacteria) by mechanical and chemical means
I. Mechanical
-
plant ___________________ grow deeply into rocks, then widen their cracks and
joints, and rocks break down
-
burrowing animals like _________________ will loosen the soil, allowing acidic
rain water enter the rocks and speed up chemical weathering
II. Chemical
-
decay of organic matter by ______________________ produces organic acid which
weakens rock surface
4. Factors affecting weathering
Factors
Humidity
Physical weathering
Chemical weathering
Alternate wetting and drying Various chemical processes
cause stress on rocks
requires the presence of water
Temperature Large temperature range and High temperature speeds up
freeze-thaw changes cause chemical reaction
alternate expansion and
contraction of rocks and
finally split rocks along
widened joints
Rock
Well-jointed or coarse-grained rocks are easily weathered
structure
Rock
Limestone contains soluble
composition
minerals while granite contains
felspar which is easily
decomposed
Vegetation
Sparse vegetation cover causes land surface expose to
various agents of weathering
Relief
Biological weathering
High temperature speeds
up biological activity
which produces organic
acid to decay rocks
Plant roots grow into joints
and break up rocks
mechanically
Plants give out organic
acid which help to decay
rocks
Lowland helps accumulation of weathered material, which protect the underlying rock
from further weathering
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5. Weathering in different climatic regions
Regions
Types of weathering
Equatorial
regions, e.g. TRF ________________ and
Explanations
___________ temperature and
_________________
_____________________ rainfall
weathering is active
Dense vegetation provide large amount
of organic acid
_____________ weathering is _____________ temperature range due
less significant
to continuous hot seasons and cloud
cover
Tropical regions,
______________ weathering ________ and _________ summer
e.g. HK
is active in summer
Arid regions, e.g.
______________ weathering _______________ daily temperature
dessert
is much significant, however, range
_____________ weathering is
active when water is available
Temperate and
polar regions
______________ weathering Freeze-thaw actions is important
is much significant
Mass movement (Mass wasting)
6. Factors affecting mass movement
I. Slope steepness
-
slow movement is commonly found on gentle slopes while rapid movement is found
-
on slopes with _______________________
hilly relief or slope undergone slope cutting usually facilitate mass movement
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vegetation
rain
weathered material
gravity
relief/slope
gradient
II. Degree of weathering
-
the higher the degree of weathering, the more the ____________________________
provide for mass movement
III. Amount of rainfall
-
rain water acts as a _______________________, which reduces the cohesion and
friction between soil particles and speeds up mass movement
-
rain water add ______________________ to weathered materials and increase the
force of gravity
IV. Presence of vegetation cover
-
plants can absorb rainwater and decrease the pressure of downslope movement
-
_____________________ can bind loose weathered materials together
V. Human impact
-
digging at the base of slopes like mining and tunneling results in unstable slope, road
cutting will cause _______________________ slope
-
_____________________ cover may be cleared for construction
VI. Climate
-
climate is an important factor, it affects some of the factors shown above:
-
e.g. hot and humid climate facilitates ____________________ weathering, large
-
daily temperature range favours __________________ weathering, all these provide
-
large amount of ________________________materials for mass movement
-
e.g. regions with prolonged ___________________________supply large amount of
rain water
7. Types and processes of mass movement
I. Slow movement
a. Soil creep
 Process:
– alternate _____________________ and __________________ of soil
particles in response to changing ___________________ and
____________________ contents
– these lead to the loosening of soil particles and they move dowslope slowly
due to the force of ____________________
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
Evidence of soil creep:
– ____________ trees and fences, _____________ walls with accumulation of
soil, terracettes
Solifluction (Solifluxion)
 Process:
– occurs in arctic regions with great depth of frozen ground
– in warmer summer, only the top soil thaws while the subsoil remains frozen
and known as permafrost
– impermeable permafrost hinders meltwater from infiltration
– the top soil becomes saturated with water and increases in weight, then
moves downslope slowly under the force of gravity
 Evidence of Solifluction:
– Solifluction lobes and terraces
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II. Rapid movement
a. Rock fall
- rocks suddenly fall vertically
from a cliff
b. Mudflow and earthflow
- rapid and sudden downslope movement of waterlogged soil particles in the form
of a stream after sporadic intense downpours, e.g. desert
c. Landslide
- landslide is common in Hong Kong
 Types of landslide:
– includes _______________ and __________________
– slide is the movement of rock materials downslope along a slide plane, while
slump occurs along a curved slide plane

Causes of landslide in Hong Kong:
– Presence of deep weathered profile for mass movement because:
 __________ and _____________ climate favours chemical weathering
 _______________ is common in Hong Kong, it is easily weathered by
physical and chemical processes
– High downslope force for weathered materials because:
 ____________ relief with many steep ______________
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 abundant __________________ in summer reduces cohesion and friction
between soil(_________________ effect), and add ___________ to soil
thus increase the force of gravity
 ____________________ and _____________________ cause bare slope
and remove the support from plant roots
 __________________ or unstable slopes are caused by road cutting and
constructions.

Effects of landslide:
–
–
–
–
–

roads blocked by rock materials / affects traffic
loss of life and property
destroy of natural landscape
spoon-shaped scar on hill slope is left
top soil is removed, so regrowth of vegetation is difficult
Measures to prevent mass movement, especially landslide:
– reduces water in soil by:
 ____________________ (planting of trees and grass)
 spreading ______________ or ______________ on steep slopes
 construction of _________________ channels and _____________
– reduce ________________ of steep slopes
– _________________ unstable rock materials
– construct ______________________ to hold back possible landslides
– install instruments on slopes to provide early ____________________
– carry out regular _____________________ on danger slopes
– restrict ____________________ and _________________ on steep slopes
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Landscape formed by weathering, erosion and mass movement
– badland on granite slope in Hong Kong
Formations:
1. Removal of __________________ cover by human or hill fires cause the topsoil expose to
various agents of weathering and erosion.
2.
_________________ weathering takes place under sunlight and enlarges ______________
of granite.
3.
__________________weathering takes place after rainfall when _____________ rain water
concentrate at joints, joints are then enlarged into ___________________.
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4.
As rills are widened by _________________ erosion and __________________________
of weathered materials, __________________ are formed.
5.
When an area is full of gullies and become bare surface without vegetation cover,
________________ is formed.