Geography- The Restless Earth

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Tectonic Plates

Plates

-The Earth's crust is made up of seven principal tectonic plates and numerous other smaller plates. The plates are sections of the crust that "float" on the mantle, which is made up of molten rock. Where the plate's meet, huge forces mean that they can form features such as volcanoes, fold mountains, deep-sea trenches and earthquakes.

-There are two main types of tectonic plate. Oceanic crust is often only about 5km thick, but is very dense. Continental crust is considerably thicker, often being approximately 30km deep, but is less dense

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Convection Currents

The Earth's Tectonic Plates all move very slowly on the mantle, meeting along the four main boundaries that can be found in the next section. The plates move due to convection currents in the mantle. These are hot currents of molten rock that slowly move within the mantle and cause the plates above them to move, usually by as little as one or two centimetres each year.

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Plate Boundries

Destructive Plate Boundries 

 Also known as convergent boundaries or compressional boundaries.

  • These cause violent volcanoes and earthquakes, as well as deep-ocean trenches and fold mountains.
  • An oceanic plate and continental plate move towards each other.
  • The denser oceanic plate dives under the lighter continental one, creating a deep ocean trench.
  • As the oceanic plate goes deeper into mantle it melts in the subduction zone, due to friction and the increased temperature.
  • The newly molten rock is lighter that that which surrounds it, so it will rise towards the surface and cause volcanoes on the earth's surface.
  • The continental crust is crumpled by the collision of the two plates creating Fold Mountains.
  • If the magma rises offshore it will form an Island Arc, like the West Indies and Japan.

A good example of a destructive plate boundary is where the Nazca plate dives underneath the South American plate. This has caused volcanoes, earthquakes and the formation of the Andes Mountain Range.

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Constructive Plate Boundries

Also known as divergent or tensional boundaries.

  • Although often not as violent as those on destructive plate boundaries, volcanoes and earthquakes do occur on constructive plate boundaries. They also cause mid-ocean ridges to form.
  • Two plates move away from each other.
  • Molten rock (magma) rises from the mantle to fill the gap between the two plates. This forms a mid-ocean ridge.
  • Volcanoes can also form here, along the edges of the plate boundary, due to the rising magma. These volcanoes are called shield volcanoes.

A good example of a constructive plate boundary can be found where the NorthAmerican plate is moving away from the Eurasian plate. This has caused the Mid-Atlantic ridge to form and has created Iceland through volcanic activity.

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Conservative Plate Boundries

Also known as passive plate boundaries.

  • The main effects of a conservative plate boundary are earthquakes, which can be fairly violent and frequent.
  • Two plates slide past each other, without creating or destroying any land.
  • As they move past each other they often get stuck, building up great pressure until finally they jolt past each other. This sudden movement is what causes earthquakes.

The best-known example of a conservative plate boundary is the San Andreas Fault, where the North American and Pacific plates are actually moving in the same direction, but at a different speed.

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Collision Margins

  • Where two continental crusts collide neither can sink.
  • Instead they push into each other forcing material to be folded up into huge mountain ranges.
  • Often this movement and pressure can cause earthquakes, but no volcanoes will occur on these boundaries.

The best example is found where the Indian plate collided with the Eurasian plate to form the Himalayas.

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Volcanoes

Types of Volcanoes

Volcanoes are formed along two types of plate boundary: destructive and constructive boundaries. The basic shape of a volcano is similar throughout the world, however there are many factors which influence how the volcano is built.

Volcanoes occur where molten rock (magma) is allowed to escape to the surfaceo f the earth. This usually occurs at plate boundaries through cracks in thecrust called vents.

Once it has reached the surface, the magma becomes known as lava. The composition of the lava determines the shape of the final volcano.

Volcanoes also throw out ash, steam, dust, pumice, and gases, which can be poisonous. However it is the lava that mainly helps to shape the volcano.There are three main volcanic cones: acid lava cones, composite cones and basic lava cones.

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Acid cone volcanoes are steep sided due to the fact that the lava is thick and acidic, meaning that it doesn't flow far before solidifying, for example Mt. Pelee.

Shield cone volcanoes are wide-based, with gentle slopes. Their lava is runny and thin, which means that it can travel a long way before cooling and solidifying. Often these eruptions are non-violent and can last for years, such as the one at Kilaueain Hawaii.

Composite Cone volcanoes are steep-sided,and made of alternate layers of ash and lava. Often the lava cools to create a plug in the vent, meaning that a huge explosion is needed to remove it.The best example is Mt. St. Helens.

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Example- Mt. St. Helens

Mt. St. Helen's, May 1980 (The U.S.A)

  • Erupted on the morning of May 18th 1980, after being inactive for over 120 years.
  • Caused by the subduction of the oceanic Juan de Fuca plate under the continental North American plate, on the West Coast of the United States. The volcano forms part of the Rockies Range of mountains. 
  • After minor earthquakes and a small eruption during March, scientists carefully monitored the mountain. By the beginning of May a bulge had developed on the North side of the mountain. This activity forced the Local Authorities to create a "Red Zone" around the volcano, from where any residents were evacuated, and no one was allowed to re-enter. This saved a lot of people from the disaster.
  • The mountain managed to reduce its height by nearly 400 metres and blew an amphitheatre shape hole in its side, 3km long and 500 metres wide.
  • 61 people were killed by the eruption, mainly by the poisonous gases, but also by being caught up in the massively swollen rivers.
  • No wildlife, either plants or trees, survived within the 25km blast zone. However vegetation has already made great strides to re-colonise the area.
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  • At approximately 8.30 a.m. on the 18th May, the volcano erupted a small amount of ash and stem. Then, a couple of minutes later, something occurred that nobody could have predicted. An earthquake caused the bulge to collapse down the side of the mountain. This released the full power of the explosive gases behind as the mountain literally blew its own side off.
  • The material from the massive landslide sped down the mountain, filling Spirit Lake and then racing on as a mud flow (it had mixed with the lake water) down the Toutle River and Kalama River.
  • The explosions from the side of the mountain sent a speeding cloud of gas, steam and dust across the ground to the North of the volcano, flattening everything in its path. The trees of the forests around the mountain were toppled as if they were matchsticks, with the majority of them facing in the same direction.
  • These explosions also instantly melted all of the snow on the mountain, adding to the mudflows that were racing down the mountain and into the rivers. These flows took all with them, including huge trees and even bridges.
  • During the remainder of the day huge gas and ash eruptions occurred, spreading a massive cloud of ash that took 7 days to entirely circle the world. The town of Yakima, 120 miles away, was so badly affected by the dust and ash that residents could only go outside if they wore facemasks.
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Mt. Pinatubo, June 1991 (The Philippines)

  • Mt. Pinatubo had not erupted for over 600 years. Its slopes had become fertile, well-cultivated paddy fields. People did not expect it to erupt.
  • On 12th June the mountain erupted. Measurements and predictions by scientists had meant that over 200,000 people had been evacuated by the time that the mountain erupted.
  • The eruption sent a huge cloud of gas and ash up into the atmosphere. Torrential rain then caused much of the ash to be deposited back on the ground as mud.
  • An area of over 600km in radius had ash falls from the volcano, with nearly 50cm falling near the mountain itself.
  • Most terrifying of all were the lahar's that was produced. These are huge, speeding mudslides, formed by the ash and the torrential rain that swept down covering entire villages in a think layer of mud, often up to 10 feet deep. They destroyed over 200,000 homes and covered 50,000 hectares of farmland.
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  • Although a relatively small number of people were killed (350), the effects of the eruption were devastating. Diseases such as malaria and cholera spread quickly in the refugee camps set up to help the evacuee's. Over the next few years, heavy rains caused ash and dust from the eruption to create more devastating lahars.
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Earthquakes

Main Concepts

Earthquakes occur along faults, which are large cracks in the earth's crust. Most of these are associated with the larger plate boundaries, along which the largest earthquakes usually occur.

-They are caused by the sudden jerking movements of the fault, either laterally or vertically, and are almost impossible to predict.

Earthquakes are measured in two ways:

  • The Richter scale measures the magnitude of an earthquake using an instrument called a seismograph. The Richter scale is logarithmic, meaning that an earthquake measuring 7 is 10 times more powerful than one measuring 6, and 100 times more powerful than one measuring 5.
  • The mercalli scale measures the damage caused by an earthquake. It rates each quake from I to XII, depending on how much damage was done, and is dependent not only on the magnitude of depth of the earthquake.
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The point at which an earthquake actually begins, deep below the earth's surface is called the focus. If the focus is deep then the effects of the earthquake may be less as the shockwaves have more rock to move through. Obviously this also depends on what type of rock it is. The point directly above the focus, on the earth's surface, is called the epicentre. The effects of the earthquake are usually worst here, and then radiate out from this spot.

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Effects of an Earthquake

The effects of an earthquake can be easily split up into two sections. Primary effects are those that occur immediately as the earthquake happens. These include buildings collapsing, roads and bridges being destroyed and railway lines being buckled. All occur due to the shaking of the ground.

Secondary effects are the subsequent effects of the quake, and can be even more devastating then the primary ones. The main secondary effects are:

  • Fires: usually from ruptured gas lines. This was the main cause of death and damage after the San Francisco earthquake in 1906.
  • Tidal waves: A tidal waves caused by an earthquake is called a tsunami. They can travel very quickly across entire oceans, before engulfing land 1000's of miles away. The 1964 Alaskan earthquake caused considerable damage in several Californian coastal areas. Although Los Angeles has escaped so far, its is still considered to be a tsunami hazard prone area.
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  • Landslides can often be triggered by earthquakes, causing huge amounts of material to be moved very quickly. This is actually what occurred just before the volcanic eruption on Mt. St. Helens. They are most likely to occur where the land is steep, saturated or weak.
  • Diseases can spread very quickly in the unsanitary conditions often left behind by massive earthquakes. Water becomes contaminated very quickly, and in Less Economically Developed Countries (LEDC's) especially; access for the medical services can be badly hampered by the damage caused by the quake. The most common diseases to be associated with earthquakes are therefore water-borne ones like cholera and typhoid.
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Examples

Turkey. August 1999

  • Centred around Izmit in Northwest Turkey, the earthquake occurred on Tuesday 17th August 1999. It affected an area over a radius of 80km.
  • Primary effects included a death toll of approximately 5,500, with another 30,000 injured and 250,000 made homeless. Over 100,000 buildings collapsed. Infrastructure damage included a 1km stretch of elevated road, numerous railway bridges, and 120 of the city's 150 quays.
  • Of 1200 people rescued on the day of the quake, only 300 survived because of the totally inadequate hospital facilities.
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  • Secondary effects included outbreaks of cholera and typhoid, due to the polluted water supply. People made homeless by the quake had no escape from the intense heat, dust and stench of rotting bodies. A few days after the quake there was no water, electricity or sewage systems working.The threat of disease was so great that rescue workers had to wear protective gloves and masks, and the cities Gocuk, Yalova and Izmit had large areas quarantined.
  • It took the Government in Ankara 48 hours to organise emergency troops to go to the area. Locals were totally unprepared for the disaster, despite it being a common earthquake area.
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Kobe, Japan. January 1995

  • The earthquake occurred at 5.46am on the 17th January 1995. It measured 7.2 on the Richter Scale and lasted 20 seconds.
  • Kobe lies on the Nojima fault, a destructive boundary, where the Philippine plate dives below the Eurasian plate. This plate boundary is the reason for Japan's existence but also means that there is a constant earthquake threat.
  • Kobe was unlucky in the sense that the focus of the earthquake was very close to the surface and the epicentre was right beside the city.
  • Primary effects included a death toll of approximately 5,500, with another 30,000 injured and 250,000 made homeless. Over 100,000 buildings collapsed. Infrastructure damage included a 1km stretch of elevated road, numerous railway bridges, and 120 of the city's 150 quays.
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  • Secondary effects included the fact that electricity, gas, water and sewage systems were all hugely disrupted. Emergency services found it very difficult to get into the city due to the massive destruction of the roads. Many temporary shelters were required, as well as food and medicines. Cold weather meant that diseases spread quickly.
  • A week after the earthquake fires still were burning, 2 million homes still were without power and 1 million were without water. The fires destroyed over 7,000 more homes. Hundreds of aftershocks, 74 strong enough for people tofeel, meant people were too afraid to return to their homes for weeks after the event.
  • Tough new laws, building codes and emergency plans were brought in after criticism of the Japanese Government. Work is continuing to try to predict future earthquakes, but as yet there is very little way of giving any significant warning time.
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Fold Mountains

The Formation of Fold Mountains

Form along both destructive and collision plate boundaries, in other words where two plates are pushing towards each other.

The best examples are the Himalayas, the Rockies, the Andes and the Alps, all of which are huge fold mountain ranges caused by the collision of two plates.

The general theory is that as two plates, with land masses on them, move towards each other they push layers of accumulated sediment in the sea between them up into folds. Thus most fold mountains will continue to grow, as the plates constantly move towards each other.

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The Formation of Fold Mountains at Destructive Plate Boundries:

  • As already seen, at a destructive plate boundary the oceanic plate is subducted beneath the continental one. The molten material then rises to the surface to form volcanoes, either in an island arc (e.g. the West Indies) or on the continental land mass (e.g. the volcanoes of the Andes). In both cases Fold Mountains can be formed.
  • When the Nazca plate dives under the South American one, their motion forward also has been pushing sediment together. This, over millions of years, has been pushed up into huge fold mountains: The Andes. Within them there are also volcanoes as the mountains are above the subduction zone.
  • If an island arc has been formed, the same idea occurs. Over millions of years the movement of the two plates together will push the island arc nearer to the continent. As this occurs the sediments on the seabed are folded up to become huge mountains.

 

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The Formation of Fold Mountains at Collison Margins:

  • These occur less frequently, but two excellent examples are the Himalayas, where the Indian plate is moving North and East towards the stationary European plate, and the Alps, formed by the collision between the African and Eurasian plates.
  • In these examples both plates are Continental ones, and so can neither sink nor be destroyed. The material between them is therefore forced upwards to form the mountains.
  • For the Himalayas the material that now forms the mountains was originally on the bottom of the non-existent Tethy's Sea. As the Indian plate pushed towards the Eurasian one, the sediments were folded up to form the Himalayas, leaving the only trace of the sea to be the fossilised shells that you can find high up in the mountains.
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Human uses of Fold Mountains

Humans use Fold Mountains for a wide variety of purposes:

Farming is a primary activity in all of the fold mountain ranges around the world. Mainly, due to the height and steepness of many of the slopes, this is restricted to cattle and sheep farming. However in the foothills of the Himalayas the Nepalese people use terraces in the mountainside to help them grow crops, and some southern facing Alpine slopes are used for vines and fruits.                                     -In the Alps a system called transhumance was used. This basically is the seasonal movement of grazing animals between the high ground in the warmer summer months and the valley floors in the colder autumn months. Nowadays transhumance is a little outdated as modern technology has meant that farmers can stay in one place all year.                                                                                  Tourism is another major use of the Fold Mountains of the world. Because they are in more economically developed countries,the Alps and the Rockies are perhaps the best examples of the impact of tourism.However, it is an increasing industry in both the Andes and the Himalayas,as people look for less crowded places to go to.

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  • The main tourist attraction in the Rockies and the Alps is skiing. Hundreds of thousands of people ski each year and this has brought great changes and problems to the main areas.
  • The increase in tourism has meant much-improved infrastructure, a huge increase in hotels and restaurants and the development of entire resorts. It has brought a large amount of much needed money into these areas and allowed local people to diversify from farming into many other jobs.
  • Fold Mountains have a lot of other things to attract visitors. These include hill walking, the attractive scenery, river rafting, and climbing. All these have contributed to areas in the Alps and the Rockies becoming all year round holiday resorts.

Forestry is another big business in these mountainous regions. Examples of cultivated coniferous forests can be seen in the Alps, where the trees have been deliberately planted as crops. However in the foothills of the Himalayas large-scale deforestation is also taking place, with logging companies cutting down vast tracts of the deciduous rainforest there.                                                Many of the Fold Mountain regions of the world are prime spots for the generation of hydroelectric power (HEP). They have a plentiful supply of water; deep, narrow valleys with quick flowing rivers, and they are sparsely populated, meaning that few people are displaced when a reservoir is created.The HEP is then used either for electricity in cities some distance away, or as a power source for local industries, such as saw and paper mills.

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The Problems living in Fold Mountains:

  • Mountainous regions are particularly difficult to build in due to the steep sided valleys and cold climate. 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.
  • 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, and it is difficult to use machinery on the steep slopes.
  • Avalanches are a constant threat, as was seen to devastating effect in Ranrahirca, Peru, in 1962. Huge amounts of money are spent each year to try and combat the avalanche threat, especially with the large amount of tourists using the mountains.
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The Impact of Natural Hazards

MEDC's v LEDC's

Natural hazards will affect More Economically Developed Countries (MEDC's)in a differing way to those that occur in Less Economically Developed Countries(LEDC's).

  • Health Care: MEDC's have the medical resources and money to quickly get appropriate aid to areas after a natural disaster. LEDC's often have to rely on aid from overseas as their health system, which is inadequate. This overseas aid takes time to arrive, which could mean far more casualties.
  •  Emergency Services: In MEDC's who have a volcanic or earthquake risk, such as Japan and New Zealand,there are well thought out emergency procedures. Practices in schools and places of work mean that people know what to do it the event of a natural disaster. The Government's and military have special emergency plans to help with the situation.

Often LEDC's do not have these emergency plans, and so (as seen in TURKEY)far more damage can be done before the emergency services reach the stricken area.

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Building Technology: Countries such as Japan and the United States have been at the fore front of developing buildings that have more chance of resisting an earthquake. Most houses in San Francisco are made of wood, to make them more flexible and allow them to move with the quake. Larger skyscrapers are built with flexible foundations, which literally allow them to sway during a quake, rather than being rigid and falling down. Many countries in areas prone to natural hazards have building codes to say where they can and cannot build, and how high the buildings can be. New Zealand is a good example of where this occurs. LEDC's don't tend to have the technology available or money to pay for it, and sooften their buildings are very susceptible to earthquakes. One example was the Armenian earthquake in 1988, which was 0.1 less on the Richter scale than Kobe, but killed 20,000 more people. Most of the Armenian houses were built of stone and so collapsed instantly.

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Scientific Prediction: Scientists work throughout the world, trying to predict earthquakes and volcanoes. So far they have found it very difficult to predict earthquakes, although scientists monitoring the San Andreas Fault in California have planted a huge number of seismographs in the ground to try to detect even the faintest of tremors. Volcanoes generally are easier to predict, although the specific time of the eruption is not so easy to do. Scientists can measure changes within the mountain that helps them to predict that the volcano is going to erupt. This usually allows the Local Authorities sufficient time to evacuate people from the danger area (as seen at both Mt. St. Helens and Mt. Pinatubo). However they still find it very difficult to accurately predict the size of the eruption.MEDC's do tend to have more investment for this type of research and development than LEDC's.

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Recovery: MEDC's tend to be able to recover quickly from a natural disaster, due to having the investment and technology needed to return the area to as good as new as soon as possible. Because LEDC's often have to rely on aid from overseas, this quick recovery is often impossible for them.

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Comments

Rebecca

Love these, perfect for the Edexcel Spec :) thank you ***

BeckyXX

Thanks. This was really helpfull :)

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