The structure of the Earth
The Earth is made up of four distinct layers:
- The inner core is in the centre and is the hottest part of the Earth. It is solid and made up of iron and nickel with temperatures of up to 5,500°C. With its immense heat energy, the inner core is like the engine room of the Earth.
- The outer core is the layer surrounding the inner core. It is a liquid layer, also made up of iron and nickel. It is still extremely hot, with temperatures similar to the inner core.
- The mantle is the widest section of the Earth. It has a thickness of approximately 2,900 km. The mantle is made up of semi-molten rock called magma. In the upper parts of the mantle the rock is hard, but lower down the rock is soft and beginning to melt.
- The crust is the outer layer of the earth. It is a thin layer between 0-60 km thick. The crust is the solid rock layer upon which we live.
There are two different types of crust: continental crust, which carries land, and oceanic crust, which carries water.
The Earth's crust is broken up into pieces called plates. Heat rising and falling inside the mantle creates convection currents generated by radioactive decay in the core. The convection currents move the plates. Where convection currents diverge near the Earth's crust, plates move apart. Where convection currents converge, plates move towards each other. The movement of the plates, and the activity inside the Earth, is called plate tectonics.
Plate tectonics cause earthquakes and volcanoes. The point where two plates meet is called a plate boundary. Earthquakes and volcanoes are most likely to occur either on or near plate boundaries.
At a Constructive boundary the plates are moving apart. The plates move apart due to convection currents inside the Earth. As the plates move apart (very slowly),magma rises from the mantle. The magma erupts to the surface of the Earth. This is also accompanied by earthquakes.
When the magma reaches the surface, it cools and solidifies to form a new crust ofigneous rock. This process is repeated many times, over a long period of time.
Eventually the new rock builds up to form a volcano. Constructive boundaries tend to be found under the sea, eg the Mid Atlantic Ridge. Here, chains of underwater volcanoes have formed along the plate boundary. One of these volcanoes may become so large that it erupts out of the sea to form a volcanic island, eg Surtsey and the Westman Islands near Iceland.
The diagram below to see how magma pushes up between the two plates, causing a chain of volcanoes along the constructive plate boundary.
At a Destructive boundary the plates are moving towards each other. This usually involves a continental plate and an oceanic plate.
The oceanic plate is denser than the continental plate so, as they move together, the oceanic plate is forced underneath the continental plate. The point at which this happens is called the subduction zone. As the oceanic plate is forced below the continental plate it melts to form magma and earthquakes are triggered. The magma collects to form a magma chamber. This magma then rises up through cracks in the continental crust. As pressure builds up, a volcanic eruption may occur.
As the plates push together, the continental crust is squashed together and forced upwards. This is called folding. The process of folding creates fold mountains. Fold mountains can also be formed where two continental plates push towards each other. This is how mountain ranges such as theHimalayas and the Alps were formed.
The formation of fold mountains
- Where an area of sea separates two plates, sediments settle on the sea floor in depressions called geosynclines. These sediments gradually become compressed into sedimentary rock.
- When the two plates move towards each other again, the layers of sedimentary rock on the sea floor become crumpled and folded.
- Eventually the sedimentary rock appears above sea level as a range of fold mountains.
Where the rocks are folded upwards, they are called anticlines. Where the rocks are folded downwards, they are called synclines. Severely folded and faulted rocks are called nappes.
Characteristics of the Alps
- High mountain ranges, eg Mont Blanc, which is 4,810 m above sea level.
- Glaciated valleys, eg the Rhone Valley.
- Pyramidal peaks, eg the Matterhorn.
- Ribbon lakes, eg Lake Como.
- Fast-flowing rivers.
- Contrasting microclimates on north facing (ubac) and south facing (adret) slopes.
- Geologically young (30-40 million years old).
Human activity surrounding fold mountains
Human activity surrounding fold mountains
- Winter sports such as skiing in resorts such as Chamonix.
- Climbing and hiking in the summer months.
- Summer lakeside holidays, eg Lake Garda.
- Agriculture - takes place mainly on south facing slopes and includes cereals, sugar beet, vines and fruits.
- Forestry - coniferous forests for fuel and building.
- Communications - roads and railways follow valleys.
- Hydroelectric power (HEP) - steep slopes and glacial meltwater are ideal for generating HEP. Hydroelectric accounts for 60 per cent of Switzerland's electricity production.
Earthquakes are caused by the release of built-up pressure inside the Earth's crust. An earthquake's power is measured on the Richter scaleusing an instrument called a 'seismometer'. An earthquake is the shaking and vibration of the Earth's crust due to movement of the Earth's plates (plate tectonics). Earthquakes can happen along any type of plate boundary. Earthquakes occur when tension is released from inside the crust. Plates do not always move smoothly alongside each other and sometimes get stuck. When this happens pressure builds up. When this pressure is eventually released, an earthquake tends to occur. The point inside the crust where the pressure is released is called the focus. The point on the Earth's surface above the focus is called the epicentre. Earthquake energy is released in seismic waves. These waves spread out from the focus. The waves are felt most strongly at the epicentre, becoming less strong as they travel further away. The most severe damage caused by an earthquake will happen close to the epicentre.
Measurement of earthquakes
Measurement of earthquakes
The power of an earthquake is measured using a seismometer. A seismometer detects the vibrations caused by an earthquake. It plots these vibrations on a seismograph.
The strength, or magnitude, of an earthquake is measured using the Richter scale. The Richter scale is numbered 0-10.
Effects of an earthquake
Earthquakes can destroy settlements and kill many people. Aftershocks can cause even more damage to an area. It is possible to classify the impacts of an earthquake, by taking the following factors into account:
- short-term (immediate) impacts
- long-term impacts
- social impacts (the impact on people)
- economic impacts (the impact on the wealth of an area)
- environmental impacts (the impact on the landscape)
Short-term (immediate) impacts
Social impacts -People may be killed or injured. Homes may be destroyed. Transport and communication links may be disrupted. Water pipes may burst and water supplies may be contaminated.
Economic impacts- Shops and business may be destroyed.Looting may take place. The damage to transport and communication links can make trade difficult.
Environmental impacts-The built landscape may be destroyed. Fires can spread due to gas pipe explosions. Fires can damage areas of woodland. Landslides may occur. Tsunamismay cause flooding in coastal areas.
Social impacts-Disease may spread. People may have to be re-housed, sometimes in refugee camps.
Economic impacts-The cost of rebuilding a settlement is high. Investment in the area may be focused only on repairing the damage caused by the earthquake. Income could be lost.
Environmental impacts-Important natural and human landmarks may be lost.
Factors affecting the impact of an earthquake
- Distance from the epicentre - the effects of an earthquake are more severe at its centre.
- The higher on the Richter scale, the more severe the earthquake is.
- Level of development (MEDC or LEDC) - MEDCs are more likely to have the resources and technology for monitoring, prediction and response.
- Population density (rural or urban area). The more densely populated an area, the more likely there are to be deaths and casualties.
- Communication - accessibility for rescue teams.
- Time of day influences whether people are in their homes, at work or travelling. A severe earthquake at rush hour in a densely populated urban area could have devastating effects.
- The time of year and climate will influence survival rates and the rate at which disease can spread.
Effects of an earthquake
Earthquakes and volcanoes in LEDCs
LEDCs often suffer more from the effects of volcanoes and earthquakes thanMEDCs.
The effects of an earthquake or a volcano in LEDCs
- Communication systems may be underdeveloped, so the population may not be well educated about what to do in the event of a volcanic eruption or an earthquake.
- Construction standards tend to be poor in LEDCs. Homes and other buildings may suffer serious damage when a disaster occurs.
- Buildings collapsing can cause high death tolls.
- Evacuation and other emergency plans can be difficult to put into action due to limited funds and resources.
- Clearing up can be difficult. There may not be enough money to rebuild homes quickly and safely. Many people could be forced to live in emergency housing or refugee camps.