Structure of the earth
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 diagram below shows the structure of the earth. In geography, taking a slice through a structure to see inside is called a cross section.
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 tensional, constructive or divergent boundary the plates move apart.
- At a compressional, destructive or convergent boundary the plates move towards each other.
- At a conservative or transform boundary the plates slide past each other.
Different exam boards and textbooks may use different names for each of the boundary types. For example, a destructive boundary may also be called a collision boundary. Use any term so long as you use it correctly, but it is best to stick to the terms you have been taught.
At a tensional or 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 of igneous 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.
At a compressional or 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 the Himalayas and the Alps were formed.causing mountains and possibly volcanoes to form along the destructive plate boundary.
Earthquakes are caused by the release of built-up pressure inside the Earth's crust. An earthquake's power is measured on the Richter scale using an instrument called a 'seismometer'.
The effects of an earthquake can be devastating - they can destroy settlements, change landscapes, and cause many deaths.
Causes of Earthquakes
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
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.
Earthquakes measuring just one or two on the scale are very common and can happen everyday in places like San Francisco. These earthquakes are so small that people cannot feel them, they can only be picked up by a seismometer.
Earthquakes measuring around 7 or 8 on the Richter scale can be devastating. The earthquake in China's south-western Sichuan province in May 2008 measured 7.8 on the Richter scale.
The World’s largest earthquake with a instrumentally documented magnitude occurred on May 22, 1960 near Valdivia, in southern Chile. It has been assigned a magnitude of 9.5 by the United States Geological Survey. It is referred to as the "Great Chilean Earthquake" and the "1960 Valdivia Earthquake.
Effects of a 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)
The short term effects are 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.
The long term effects are Disease may spread. People may have to be re-housed, sometimes in refugee camps.
The short effects are shops and business may be destroyed. Looting may take place. The damage to transport and communication links can make trade difficult.
The long term effects are 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.
The short term effects are the built landscape may be destroyed. Fires can spread due to gas pipe explosions. Fires can damage areas of woodland. Landslides may occur. Tsunamis may cause flooding in coastal areas.
The long term effects are Important natural and human landmarks may be lost.
Effects are often classified as primary and secondary impacts. Primary effects occur as a direct result of the ground shaking, eg buildings collapsing. Secondary effects occur as a result of the primary effects, for example tsunamis or fires due to ruptured gas mains.
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.
LEDCs often suffer more from the effects of volcanoes and earthquakes than MEDCs.
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.
Volcanoes and Volcanic Eruptions
Volcanoes form when magma reaches the Earth's surface, causing eruptions of lava and ash. They occur at destructive (compressional) and constructive (tensional) plate boundaries.
The immediate effects of volcanic eruptions can be devastating, but they may be beneficial in the long term.
Key Facts of a Volcano
- A volcano is formed by eruptions of lava and ash.
- Volcanoes are usually cone shaped mountains or hills.
- When magma reaches the Earth's surface it is called lava. When the lava cools, it forms rock.
- Volcanic eruptions can happen at destructive and constructive boundaries, but not at conservative boundaries.
- Some volcanoes happen underwater, along the seabed or ocean floor.
The Formation of a volcano
- Magma rises through cracks or weaknesses in the Earth's crust.
- Pressure builds up inside the Earth.
- When this pressure is released, eg as a result of plate movement, magma explodes to the surface causing a volcanic eruption.
- The lava from the eruption cools to form new crust.
- Over time, after several eruptions, the rock builds up and a volcano forms
Inside a Volcano
- The magma chamber is a collection of magma inside the Earth, below the volcano.
- The main vent is the main outlet for the magma to escape.
- Secondary vents are smaller outlets through which magma escapes.
- The crater is created after an eruption blows the top off the volcano.
An eruption occurs when pressure in the magma chamber forces magma up the main vent, towards the crater at the top of the volcano. Some magma will also be forced out of the secondary vent at the side of the volcano.
Types of Volcanoes
Volcanoes can be described in terms of activity and can be:
- still active and erupt frequently
- dormant (temporarily inactive but not fully extinct)
- extinct (never likely to erupt again)
Volcanoes can also be described by their shape or type - shield or composite.
- Shield volcanoes are usually found at constructive or tensional boundaries.
- They are low, with gently sloping sides.
- They are formed by eruptions of thin, runny lava.
- Eruptions tend to be frequent but relatively gentle.
Types of Volcanoes (continued)
- Composite volcanoes are made up of alternating layers of lava and ash (other volcanoes just consist of lava).
- They are usually found at destructive or compressional boundaries.
- The eruptions from these volcanoes may be a pyroclastic flow rather than a lava flow. A pyroclastic flow is a mixture of hot steam, ash, rock and dust.
- A pyroclastic flow can roll down the sides of a volcano at very high speeds and with temperatures of over 400°C.
A supervolcano is a volcano on a massive scale. It is different from a volcano because:
- it erupts at least 1,000 km3 of material (a large volcano erupts around 1 km3)
- it forms a depression, called a caldera (a volcano forms a cone shape)
- a supervolcano often has a ridge of higher land around it
- a supervolcano erupts less frequently - eruptions are hundreds of thousan
Yellowstone is one example of a supervolcano. Three huge eruptions have happened in the last 3 million years. the last eruption was 630,000 years ago, and was 1,000 times bigger than the Mount St Helens eruption in 1980.
The large volume of material from the last Yellowstone eruption caused the ground to collapse, creating a depression called a caldera. The caldera is 55 km by 80 km wide. The next eruption is predicted to have catastrophic worldwide effects.
The supervolcano at Yellowstone is formed because of a volcanic hotspot.
Every year millions of visitors come to see the related features, such as geysers and hot springs. Old Faithful is one example of a geyser.
Effets of a Volcano
The dramatic scenery created by volcanic eruptions attracts tourists. This brings income to an area.
The lava and ash deposited during an eruption breaks down to provide valuable nutrients for the soil. This creates very fertile soil which is good for agriculture.
The high level of heat and activity inside the Earth, close to a volcano, can provide opportunities for generating geothermal energy.
Many lives can be lost as a result of a volcanic eruption.
If the ash and mud from a volcanic eruption mix with rain water or melting snow, fast moving mudflows are created. These flows are called lahars.
Lava flows and lahars can destroy settlements and clear areas of woodland or agriculture.
Human and natural landscapes can be destroyed and changed forever.
Formation of Fold Mountains
Fold mountains occur near convergent or compressional plate boundaries. Examples of fold mountains include the Alps, Rockies, Andes and Himalayas.
- 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
- 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.
Managing tectonic hazards
It's not possible to prevent earthquakes and volcanic eruptions. However, careful management of these hazards can minimise the damage that they cause. Prediction is the most important aspect of this, as this gives people time to evacuate the area and make preparations for the event.
Predicting and Preparing for Volcanoes
Unfortunately volcanic eruptions and earthquakes cannot be prevented.
Managing hazards such as earthquakes and volcanoes can be done by: prediction and preparation.
As a volcano becomes active, it gives off a number of warning signs. These warning signs are picked up by volcanologists (experts who study volcanoes) and the volcano is monitored.
Managing Tectonic Hazards 1
- Hundreds of small earthquakes are caused as magma rises up through cracks in the Earth's crust.
- Temperatures around the volcano rise as activity increases.
- When a volcano is close to erupting it starts to release gases. The higher the sulfur content of these gases, the closer the volcano is to erupting.
- Seismometers are used to detect earthquakes.
- Thermal imaging techniques and satellite cameras can be used to detect heat around a volcano.
- Gas samples may be taken and chemical sensors used to measure sulfur levels.
The techniques available for predicting and monitoring volcanic activity are becoming increasingly accurate. Volcanoes such as Mount St Helens in the USA and Mount Etna in Italy are closely monitored at all times. This is because they have been active in recent years and people who live nearby would benefit from early-warning signs of an eruption. However, as well as prediction, people need to be prepared for an eruption.
Preparing For An Eruption
A detailed plan is needed for dealing with a possible eruption. Everyone who could be affected needs to know the plan and what they should do if it needs to be put into action. Planning for a volcanic eruption includes:
- creating an exclusion zone around the volcano
- being ready and able to evacuate residents
- having an emergency supply of basic provisions, such as food
- funds need to be available to deal with the emergency and a good communication system needs to be in place
Preparing and Predicting for earthquakes
Earthquakes are not as easy to predict as volcanic eruptions. However, there are still some ways of monitoring the chances of an earthquake:
- Laser beams can be used to detect plate movement.
- A seismometer is used to pick up the vibrations in the Earth's crust. An increase in vibrations may indicate a possible earthquake.
- Radon gas escapes from cracks in the Earth's crust. Levels of radon gas can be monitored - a sudden increase may suggest an earthquake.
Many of the prediction techniques used to monitor earthquakes are not 100 per cent reliable. Planning and preparing for an earthquake is therefore very important.
People living in earthquake zones need to know what they should do in the event of a quake. Training people may involve holding earthquake drills and educating people via TV or radio.
Preparing and Predicting for Earthquakes (2)
People may put together emergency kits and store them in their homes. An emergency kit may include first-aid items, blankets and tinned food.
Earthquake-proof buildings have been constructed in many major cities, eg the Transamerica Pyramid in San Francisco. Buildings such as this are designed to absorb the energy of an earthquake and to withstand the movement of the Earth.
Roads and bridges can also be designed to withstand the power of earthquakes
These cards were made By Theo Fadayiro (ManLikeTheo)