Tectonic activity
Table of compositon.
World map
The Earth's crust is divided into seven large plates and several smaller ones. A plate is a
rigid section of the crust which 'floats' on the semi molten mantle. These plates are
moved by convection currents that occur within the mantle. Plates are made of either
oceanic crust or oceanic and continental crust. No plate is made of only continenental
crust. The plates have recognized boundaries, where either plates are moving towards
one another, moving away or moving past one another.
Desturctive or collision margins
Diagarm ww 212
Destructive margins
To the west of South America, the Nazca Plate (oceanic crust, basalt 3.0) is moving
towards the American Plate (continental crust, granite 2.7). Where the plates meet, the
Nazca Plate is forced downwards to form a subduction zone with an associated deep-sea
trench (the Peru-Chile trench). The increase in pressure, where the plate is forced
downwards, can trigger off severe earthquakes. As the crust continues to descend, it
melts, partly due to heat resulting from friction caused by contact with the American
Plate, and partly due to the increase in temperature as it re-enters the mantle. The newlyformed magma, from the destroyed oceanic crust, is lighter than the mantle. Some of it
will rise to the surface to form volcanoes and long chains of fold mountains (Andes).
The chemistry of the magma changes to andesite which has a higher viscosity than
basalt. This stops the gases getting out and causes the magma to solidify quickly. Thus
giving cone shaped volcanoes which will explode dangerously.
Usually they appear at destructive margins where volcanic eruptions are at their most
violent. Melting snow, caused by the eruption of Nevado del Ruiz in Colombia in 1985,
created mudflows which killed 21,000 inhabitants of the town of Armero.
Diagram
Composite vol
Collision margins
The Indian Plate is moving into the Eurasian Plate at a rate of 5 em a year. As both
plates consist of continental crust, neither can sink. The land between them has been
buckled and pushed upwards to form the Himalaya. This movement, which is still taking
place, accounts for massive earthquakes which periodically occur where the plates meet.
Evidence suggests that Mount Everest is increasing in height. Less obvious, however, is
the effect of the collision felt by the whole of India. In September 1993 an earthquake in
central India caused the death of 22,000 people and left another 150,000 homeless.
Diagram 212
Constructive margins
These occur when two plates are moving away from each other. Generally they occur at
mod ocean ridges where the movement enables basalt to move to the surface. These
eruptions of basalt are 'quite' as the magma has a low viscosity and allows the gas to
escape. Earthquakes occur but they tend to be shallow and small.
Diagram ww 211
Volcanoes associated with basalt.
Basalt volcanoes have a very wide base and so are called shield volcanoes. This is
because of the low viscoisity and the fact that the basalt magma flows easily.
Shield vol
Conservative margins
Diagram ww 212
These margins are found where two plates are forced to slide past one another. Although
crust is neither formed nor destroyed at this point, earthquakes can occur if the plates
`stick'. The San Andreas Fault, which passes through California, marks the junction of
the Pacific and North American Plates. The American Plate moves more slowly than,
and at a slight angle into, the Pacific Plate. Instead of the plates slipping past each other,
they tend to stick. When sufficient pressure builds up, one plate is jerked forward,
sending shockwaves to the surface. These shockwaves caused earthquakes in San
Francisco in i989 and in 1906, when the ground moved by 6 metres. So an earthquake is
simply a release in pressure as two plates move.
Case study of a volcanic eruption Pinatubo
Earthquakes
Earthquakes occur where two areas of the earth's crust try to move in different
directions. If friction prevents movement between these two areas then pressure will
build up. When pressure is eventually released by a sudden earth movement, the result is
an earthquake. Earthquakes are measured on the Richter scale. Each level of magnitude
on the scale is ten times greater than the level below it. The Mercalli scale is another
method of measuring, this measures the 'damage'.
The point at which the earthquake happens below the ground surface is called the Focus.
The epicentre is the point on the ground surface directly above the focus. If the epicentre
is in, or close to, large urban areas then the effects can be devastating, as in Kobe in
Japan in 1995.
Table of quakes Bowen
The effects of earthquakes
The destructive effects of an earthquake can be classified into primary and secondary
effects.
Primary effects are the immediate damage caused by the quake, such as collapsing
buildings, roads and bridges, which may kill many people. Those lucky enough to
survive can suffer badly from shock and panic.
Secondary effects are the after-effects of the earthquake, such as fires, tidal waves,
landslides and disease.
 Fire - earthquakes destroy gas pipes and electric cables, causing fires to spread.
Broken water mains prevent the Fires being extinguished. Fires spread very quickly
in cities, especially in poor-quality housing areas where wooden buildings are
common
 Tsunamis (tidal waves) - an earthquake on the sea floor or close to the coast may
cause huge tidal waves.
 Landslides - earthquakes often cause landslides, especially in steep river valleys and
areas of weak rocks
 Disease and famine - fresh water supplies are often cut off causing typhoid and
cholera. Lack of shelter and food causes much suffering.
The impact of tectonic activity (earthquakes and volcanoes) is often much more severe
in LEDCs than in MEDCs. Many LEDCs cannot afford early warning systems or
evacuation plans.
Housing is often overcrowded and poorly constructed so that it collapses or catches fire
easily. Communications are frequently poor so that emergency services take a long time
to reach a disaster area.
An earthquake or volcano usually has less impact in the countryside than in towns and
cities, where there are higher population densities. The urban infrastructure means there
is very much more damage to property, industry and services. Collapsed power lines and
fractured gas mains add to the casualties and increase the chance of fires and explosions.
Disease may also spread more quickly in more populated areas.
Case study of eq LA and India
Fold mountain
Fold mountains are found in many parts of the world; they form along the plate
boundaries where great Earth movements have taken place.
Map Bowen 8
There were long periods of quiet between Earth movements during which sedimentary
rocks, thousands of metres thick, formed in huge depressions called geosynclines.
Diagram Bowen 8
Rivers carried sediments and deposited them into the depressions. Over millions of years
the sediments were compressed into sedimentary rocks such as sandstone and limestone.
These sedimentary rocks were then Forced upwards into a series of folds by the
movement of the tectonic plates. Sometimes the folds were simple upfolds (anticlines)
and downfolds (synclines), as in the Jura mountains in France. In some places the folds
were pushed over on one side giving overFolds, while in some of the highest fold
mountains, like the Alps, the rocks have been severely folded and faulted into nappes
(Figure 3).
Fold mountains have been formed at times in the Earth's geological history called
mountain-building periods. Recent mountain-building movements have created the Alps,
the Himalayas, the Rockies and the Andes, some of which are still rising. For this reason
many of these ranges are called young fold mountains.
Living in the fold mountains
The main activities in the Alps are connected with tourism, dairy farming, forestry, HEP
and industry.
Farming
The valley floor is the ideal location for farming. It is sheltered, the land is flatter and
the soils are deeper and more fertile. The valley floor also has better access and
communications,
Traditional farming was dairying, using a system called transhumance, the seasonal
movement of animals. This was the seasonal movement of cattle to the high pasture
where butter and cheese were made. Mow quads and cable cars allow the milk to be
brought straight back to the farm.
Tourism
There are many important tourist centres in the Alps, such as Chamonix and St Moritz.
Many new resorts have developed since the 1 950s as tourism, especially skiing, has
expanded. The tourist industry is so important that large amounts of money have been
spent on roads, railways, tunnels. avalanche shelters, cable cars, hotels and restaurants.
Some of these developments have also benefited the farmers and other local people.
The area attracts skiers, climbers and walkers, as well as people who simply want to
admire the spectacular scenery. Alpine features attractive to visitors include:
· beautiful winter scenery with glaciated mountains,snow-capped peaks and forests
· lakeside summer resorts, such as Lucerne and Garda
· extensive winter sports facilities, both indoor and outdoor, such as ski slopes. ice rinks
and toboggan runs
· good communications, close to international road and raii routes using Alpine passes
such as the Simplon and St Gotthard, as well as international airports at Geneva and
Zurich
. close to affluent parts of Europe, such as Germany and France
· the Alpine climate, with good snowfall but also crisp, clear days for winter sports, and
summers, that are warm with showers.
Hydroelectric power (HEP) and industry
The steep slopes, high precipitation and summer melting of the glaciers produce fastflowing rivers that are ideal for generating HEP The narrow valleys are easy to dam and
there are lakes in which to store water Some of the cheap HEP is used by industries
which require a high input of electricity, such as sawmills, electrochemicals, fertilizer
manufacture and aluminium smelting. Some of the electricity is also exported to other
regions to supply towns and cities. Traditional industries in the Alps include
papermaking, textiles, leatherworking, furniture and cuckoo clocks.
Forestry
Up to a height of about 1 800m the slopes are covered in coniferous forests. giving rise
to a timber industry. The soft wood is felled for use as fuel, for building and in the
manufacture of wood pulp and paper In Switzerland, small craft industries use the
timber in woodcarving and to manufacture the famous cuckoo clocks.
Problems of fold mountain areas
Fold mountains like the Alps tend to have low population densities. The high altitude
and steep slopes make it difficult to build houses and communication links, and to locate
industries. Roads and railways need expensive tunnels, passes, avalanche shelters and
hairpin bends. There is little flat land for farming and the use of machinery is difficult.
The climate is also cold and wet with heavy snowfall and strong winds, especially at
high altitudes. The growing season is therefore short and travel can be difficult,
especially in the winter Avalanches and rock falls can block roads. and some settlements
and roads need avalanche protection measures, e.g. shelters, wooden fencing, wire
cages. However. the population density in the Alps is higher than in many fold mountain
areas because the soil in the valley floors is reasonably fertile and the region is attractive
to tourists from nearby wealthy countries.