GCSE Geography Natural Hazards
Structure of the Earth
Structure of the Earth
1. The inner core is the most central part of the earth and is the hottest, with temperatures reaching 5,500 degrees Celsius. It is solid and made up of iron and nickel. With its extreme temperatures the inner core acts like the engine room for the earth.
2. The outer core is the layer which surrounds the inner core, also made up of iron and nickel but being liquid rather than solid. It is still extremely hot with temperatures reaching similar numbers to the inner core.
3. The mantle is the widest section of the earth with a thickness of approximately 2,900km. The mantle is made up of semi-molten rock called magma. In the upper part of the mantle, the rock is solid. However, as you get further in and the temperatures increase, the rock begins to melt and become more liquid like.
4. The crust is the final, and most outer layer of the earth. It is very thin with only an approximate depth of 0-60km. Thiis is the sold rock layer upon which we live.
There are two different types of crust, the oceanic crust, which carries water and the continental crust which carries the land.
The earth's crust is broken up into pieces called plates. Convection currents are created by the heat in the mantle layer rising and falling. Where these diverge near the earth's crust, plates move apart from each other, but where they converge, plates move towards each other. The movement of the plates is called plate tectonics.
Tectonic plates cause volcanoes and earthquakes. The point where two plates meet is called aplate boundary, and this is often where an earthquake or volcano would form.
At constructive plate boundaries the plates move away from each other.
At destructive plate boundaries the plates move towards each other.
At conservative plate boundaries, the plates slide past each other.
Constructive Plate Boundaries
At constructive boundaries, the plates are moving apart from each other due to the convection currents inside the mantle. As the plates move away from each other (very slowly) magma rises up from the mantle and erupts to the earth's surface.
When the magma reaches the surface, it cools and solidifies, forming a new layer of igneous rock. As this process repeats millions of times, eventually it builds up to form a volcano. Constructive boundaries tend to be under the sea, e.g. the Mid Atlantic Ridge. Here, chains of volcanoes have formed under the sea.
At constructive boundaries, both earthquakes and volcanoes can form.
Destructive Plate Boundaries
At a destructive plate boundary, the continental and oceanic plates are moving towards each other. The oceanic plate is denser and more heavy than the continental plate, so as they move together, the oceanic plate is forced under the continental plate. The point at which this happens is called the subduction zone. As the oceanic plate is forced under the continental one, it melts to from magma and earthquakes are triggered. Magma collects to form a magma chamber and then this magma rises up out of the cracks in the continental crust. As pressure builds up, a volcanic eruption can occur.
As the plates are squeezed together, the continental plate is forced upwards. This is called folding and the process causes fold mountains. These can also form where two continental plates push together, e.g. the Himalayas and the Alps.
At a destructive plate boundary, volcanoes and earthquakes can form.
An earthquake is the shaking and vibrating caused by the movement of tectonic plates. Earthquakes can happen on any type of plate boundary. Plates do not always move smoothly along and sometimes build up tension which is released in the form of an earthquake. The point inside the crust where this pressure is released is called the focus. The point above this, on the earth's surface, is called the epicentre. The earthquake can travel for miles away from this.
Earthquake energy is released in seismic waves. These spread away from the epicentre, although they are felt the strongest at the epicentre and become less strong the further you travel away. The most damage will also tend to be done at the epicentre.
Measurement and effects
The power of an earthquake is measured using a seismometer which detects the vibrations caused by an earthquake and plots them on a seismograph. The magnitude of the earthquake is measured using the Richter Scale, which is a measuremennt from zero to ten.
Earthquakes that measure just one or two on the Richter scale happe very often and sometimes without you even noticing. However, earthquakes measuring above seven can be very devastating.
Social effect: Short term- many people injured and killed, homes destroyed, transport and communication links disrupted, water pipes burst and water is contaminated. Long term- diseases spread and people have to be rehomed.
Economic effects: Short term- shops and businesses can be destroyed and damaged communication systems can make trade a difficult. Long term- the cost of rebuilding is extremely high, income can be lost, and investment in the area might be focused only on one area, so the other things that have been destroyed might not be fixed for a while.
Environmental effects: Short term- the built landscape might be destroyed, fires can spread, fires can also damage areas of woodland, landslides may occur, tsunamis, may cause flooding in coastal areas. Long term- important natural and human landscapes might be destroyed.
Factors affecting the impact
Factors affecting the imapct of an eartquake
- Distance from the epicentre
- Richter Scale measurement
- LEDC or MEDC
- Population density
- Time of day and year
Earthquakes and volcanoes usually have a worse effect in an LEDC. This is for many reasons, communication systems may be underdeveloped, so the population may not be well educated on what to do if there was an earthquake. Also, construction standards tend to be poorer so houses collapse more easily. Furthermore, the collapse of building can cause a high death toll, and evacuation and emergency plans can be difficult to put into place due to lack of resources. Clearing up can be difficult because they do not have the money to do it quickly. This may force people to live in temporary housing for a long time.
Formation and inside a volcano
The formation of a volcano
1. Magma rises through the cracks and weaknesses of the earth's crust.
2. Pressure builds up inside the earth and released (e.g. as a result of plate movement) in an explosion of magma.
3. Lava from the explosion cools and solidifies, building up gradfually with each explosion.
4. Overtime, after several explosions, the rock builds up to form a volcano.
Inside a volcano
The magma chamber sits below the volcano and is where the magma is held. The main vent is the middle and main outlet for the magma to escape from.Secondary vents are smaller outlets through which magma can also escape. The crater is created after the explosion blows the top off the volcano.
Types of volcano
Active volcanoes erupt frequently, dormant volcanoes do not erupt frequently but are not extinct. Extinct volcanoes are ones which will never erupt again.
Shield volcanoes are normally found at constructive plate boundaries, they are low with gently sloping sides and formed by eruptions of thin, runny lava. Eruptions from these volcanoes tend to be frequent but relatively gentle.
Composite volcanoes are made up of alternating layers of lava and ash. They are usually found at destructive boundaries and the eruption tend to run down the sides at high speeds and high temperatures.
A volcanic eruption can have positive and negative effects.
- The dramatic scenery attracts tourists and brings income to the area.
- The lava and ash breaks down to provide valuable nutrients for the soil. This creates very fertile soil which is good for agriculture.
- The high level of heat inside the earth and out from the volcano can provide a good opportunity for geothermal energy.
- Many lives can be lost due to a volcanic eruption.
- Mudflows can be created by the mixing of ash and mud with rainwater.
- Lava and mud flows can destroy or severly damage settlements.
- Human and natural landscapes can be destroyed.