The Earth consists of four concentric layers: inner core, outer core, mantle and crust. The crust is made up of tectonic plates, which are in constant motion. Earthquakes and volcanoes are most likely to occur at plate boundaries.
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.
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.
Two different types of crust: continental crust, which carries land, and oceanic crust, which carries water. (BBC BITESIZE, 2014)
The Earth's crust is divided up into pieces, these pieces are 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.
The image above shows where the different plate boundaries are located around the world. The arrows show which direction the plates are moving.
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.
The picture above shows how many of the volcanoes and earthquakes are on the plate boundaries.
Different plate boundaries
At a tensional, constructive or divergent boundary the plates move apart. Constructive boundaries tend to be found under the sea, eg the Mid Atlantic Ridge.
At a compressional, destructive or convergent boundary the plates move 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.
At a conservative or transform boundary the plates slide past each other.
(BBC BITESIZE, 2014)
Earthquakes
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 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.
The impacts can vary depending on the settlement and how built up it is etc. The impacts can be measured by:
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)
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, eg 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
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 effects of a volcanic eruption may eventually be beneficial, despite perhaps causes a lot of devastation immediately.
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.
How are volcanoes formed?
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.
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.
Volcanoes can be active,dormant or extinct!
Shield volcanoes
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.
Composite volcanoes
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.
Hurricanes
The strongest tropical storms are called hurricanes, typhoons or tropical cyclones. How hurricanes formHurricanes need a lot of heat to form and a sea surface temperature of at least 26°C, which is why they usually occur over tropical seas. They also need to be between 5 and 20° north or south of the equator. It works like this:
When this warm and wet air rises, it condenses to form towering clouds, heavy rainfall. It also creates a low pressure zone near the surface of the water.
Rising warm air causes the pressure to decrease at higher altitudes. Warm air is under a higher pressure than cold air, so moves towards the ‘space’ occupied by the colder, lower pressure, air. So the low pressure ‘sucks in’ air from the warm surroundings, which then also rises. A continuous upflow of warm and wet air continues to create clouds and rain.
Air that surrounds the low pressure zone at the centre flows in a spiral at very high speeds - anti-clockwise in the northern hemisphere - at speeds of around 120 km/h (75 mph).
Air is ejected at the top of the storm – which can be 15km high – and falls to the outside of the storm, out and over the top, away from the eye of the storm. As this happens, it reduces the mass of air over the ‘eye of the storm’ - causing the wind speed to increase further. Some ejected air also cools and dries, and sinks through the eye of the storm, adding to the low pressure at the centre.
The faster the winds blow, the lower the air pressure in the centre, and so the cycle continues. The hurricane grows stronger and stronger.
Seen from above, hurricanes are huge circular bodies of thick cloud around 450 km (300 miles) wide. The cloud brings heavy rain, thunder and lightning.
In the centre is the eye of the hurricane, about 45 km across (30 miles) across. Often there will be no clouds in the eye. Seen from below it will seem calmer, with a circle of blue sky above. The eye is formed because this is the only part of the hurricane where cold air is descending.
In the northern hemisphere, the prevailing easterly tropical winds tend to steer hurricanes toward land - although their course is unpredictable. As hurricanes move inshore, their power gradually reduces because their energy comes from sucking up moist sea air.