Knowt
The challenge of natural hazards
Types of natural hazard
Natural hazards can be placed into two categories - tectonic hazards and climatic hazards.
Tectonic hazards occur when the Earth's crust moves. For example, when the plates move, friction can cause them to become stuck. Tension builds until the plates release, which leads to an earthquake.
Climatic hazards occur when a region has certain weather conditions, for example heavy rainfall can lead to flooding.
Hazards can have economic, social and environmental consequences. For each hazard event the risks, or probability, of a particular consequence occurring can vary greatly.
This depends on certain factors. For example in a developing country, the death toll tends to be high but the short-term economic costs are often relatively low, whereas in a developed country, the death toll tends to be low but the short-term economic costs can be extremely high.
The long-term situation is more complex. Developing countries can be slower to repair damage to roads and buildings. This can lead to a reduction in tourists and therefore a long-term loss of valuable income.
Hazard risks are increasing due to population growth, urbanisation, pressure on marginal land and changes to the natural environment.
The Earth is made up of different 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.
The outer core is the layer surrounding the inner core. It is a liquid layer, also made up of iron and nickel.
The mantle is the thickest section of the Earth at approximately 2,900 km. The mantle is made up of semi-molten rock called magma.
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. It is either continental or oceanic.
The Earth's crust is broken into tectonic plates. It was once believed that convection currents in the mantle caused the plates to move. However, it is now recognised to be more complicated than this. Mechanisms called slab pull and ridge push are believed to move the tectonic plates. Ridge push is where the new crust formed at divergent plate margins is less dense than the surrounding crust and so it rises to form oceanic ridges. The older seafloor either side of the ridge slides away and this moves the seafloor apart – moving the tectonic plates.
Therefore, instead of tectonic plates moving because of the convection currents, evidence suggests it is the plates that drive the convection. Slab pull occurs where older, denser tectonic plates sink into the mantle at subduction zones. As these older sections of plates sink, newer and less dense sections of plate are pulled along behind. Sinking in one place leads to plates moving apart in other places.
The movement of the plates and the activity inside the Earth, is called the theory of plate tectonics.
Global distribution of earthquakes and volcanoes
The earth's surface is made up of two types of crust:
Oceanic crust - found underneath the oceans. It is denser than continental crust and can be subducted.
Continental crust - found under land masses or continents. It is generally older than oceanic crust and is less often destroyed.
Earthquakes are found along all types of plate margins as shown on this map. Volcanoes however, only occur at constructive and destructive plate margins.
Key fact
A lot of volcanic activity occurs in the 'ring of fire'. The 'ring of fire' is a group of volcanoes that are located along the plate margin of the Pacific plate.
Hawaii is an arc of volcanic islands, found above a hot spot.
Destructive, constructive and conservative plate margins
Destructive plate margins
A destructive plate margin usually involves an oceanic plate and a continental plate. The plates move towards one another and this movement can cause earthquakes.
As the plates collide, the oceanic plate is forced beneath the continental plate. This is known as subduction. This happens because the oceanic plate is denser (heavier) than the continental plate.
When the plate sinks into the mantle it melts to form magma. The pressure of the magma builds up beneath the Earth's surface. The magma escapes through weaknesses in the rock and rises up through a composite volcano. The volcanic eruptions are often violent, with lots of steam, gas and ash.
If two continental plates collide, neither can sink and so the land buckles upwards to form fold mountains. This is called a collision margin. Earthquakes can occur at collision margins.
Constructive plate margins
At a constructive plate margin the plates move apart from one another. When this happens the magma from the mantle rises up to make (or construct) new land in the form of a shield volcano. The movement of the plates over the mantle can cause earthquakes.
Conservative plate margins
At a conservative plate margin, the plates move past each other or are side by side moving at different speeds. As the plates move, friction occurs and plates become stuck. Pressure builds up because the plates are still trying to move. When the pressure is released, it sends out huge amounts of energy, causing an earthquake. The earthquakes at a conservative plate boundary can be very destructive as they occur close to the Earth's surface. There are no volcanoes at a conservative plate margin.
Causes of earthquakes
What causes earthquakes?
Earthquakes are the sudden violent shaking of the ground. This happens because the Earth's plates are constantly moving. Sometimes, because of friction, plates try to move and become stuck. Pressure builds up because the plates are still trying to move. When the pressure is released, it sends out huge amounts of energy causing the Earth's surface to shake violently.
The point inside the Earth's crust where the earthquake originates from is known as the focus. The earthquake's energy is released in seismic waves and they spread out from the focus. The seismic waves are most powerful at the epicentre. The epicentre is the point on the Earth's surface directly above the focus.
Key fact
Earthquakes are found at all three plate boundaries: constructive, destructive and conservative plate boundaries.
Measuring earthquakes
Figure caption,
A Willmore seismometer measures earthquakes
Earthquakes, until recently, have been measured on the Richter scale. The Richter scale measures the magnitude of an earthquake (how powerful it is). It is measured using a machine called a seismometer which produces a seismograph. A Richter scale is normally numbered 1-10, though there is no upper limit. It is logarithmic which means, for example, that an earthquake measuring magnitude 5 is ten times more powerful than an earthquake measuring 4. Earthquakes measuring 1-2 on the scale happen regularly, and they are so small that people cannot feel them. Earthquakes measuring upwards of 7 are less frequent but very powerful, and can cause a lot of destruction.
The largest earthquake ever recorded was in Chile in 1960, which measured 9.5 on the Richter scale. The Richter scale is not very accurate in measuring these larger earthquakes and today scientists use the Moment Magnitude Scale which uses the same logarithmic scale but which more accurately measures the strength of larger earthquakes.
Figure caption,
Different magnitudes of earthquake on the Richter scale
Effects and responses of earthquakes
Effects
The effect of an earthquake is the damage which happens as a result of the earthquake. The effects of an earthquake can vary depending on:
The size of the earthquake on the Richter scale - the higher it is on the scale, the more destruction it can cause.
Level of development - whether it occurs in a rich or a poor country. Richer countries will be more likely to be able to predict, protect and prepare themselves from the effects of an earthquake.
The depth of the focus - if it's shallow, it can be more destructive.
Distance from epicentre - the effects of an earthquake are more severe at its centre.
Population density - the more people living in an area, the more likely that more deaths and casualties may arise.
The time of day - whether people are in their homes, work or travelling.
Key fact
We can classify the effects of an earthquake into the following categories:
primary effects - things that happen immediately as a result of an earthquake
secondary effects - things that happen in the hours, days and weeks after the initial earthquake
Responses
Responses are how countries react to an earthquake. They are categorised as follows:
Short-term or immediate - a response in the days and weeks immediately after a disaster has happened. Short-term responses mainly involve search and rescue and helping the injured.
Long-term - responses that go on for months and years after a disaster. It involves rebuilding destroyed houses, schools, hospitals, etc. It also involves kick-starting the local economy.
Prediction, protection and preparation
Prediction
Prediction involves using seismometers to monitor earth tremors. Experts know where earthquakes are likely to happen. However, it is very difficult to predict when they will happen. Even looking at the timescale between earthquakes doesn't seem to work.
Protection
Protection involves constructing buildings so that they are safe to live in and will not collapse. Some examples of building improvements are:
rubber shock absorbers in the foundations to absorb the Earth tremors
steel frames that can sway during Earth movements
open areas outside of the buildings where people can assemble during an evacuation
Figure caption,
Example of an earthquake-proof building
Preparation
In earthquake-prone countries, hospitals, emergency services and residents practise for an earthquake. They have drills in all public buildings so that people know what to do in the event of an earthquake. This helps to reduce the impact and increases their chance of survival.
The challenge of natural hazards
Types of natural hazard
Natural hazards can be placed into two categories - tectonic hazards and climatic hazards.
Tectonic hazards occur when the Earth's crust moves. For example, when the plates move, friction can cause them to become stuck. Tension builds until the plates release, which leads to an earthquake.
Climatic hazards occur when a region has certain weather conditions, for example heavy rainfall can lead to flooding.
Hazards can have economic, social and environmental consequences. For each hazard event the risks, or probability, of a particular consequence occurring can vary greatly.
This depends on certain factors. For example in a developing country, the death toll tends to be high but the short-term economic costs are often relatively low, whereas in a developed country, the death toll tends to be low but the short-term economic costs can be extremely high.
The long-term situation is more complex. Developing countries can be slower to repair damage to roads and buildings. This can lead to a reduction in tourists and therefore a long-term loss of valuable income.
Hazard risks are increasing due to population growth, urbanisation, pressure on marginal land and changes to the natural environment.
The Earth is made up of different 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.
The outer core is the layer surrounding the inner core. It is a liquid layer, also made up of iron and nickel.
The mantle is the thickest section of the Earth at approximately 2,900 km. The mantle is made up of semi-molten rock called magma.
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. It is either continental or oceanic.
The Earth's crust is broken into tectonic plates. It was once believed that convection currents in the mantle caused the plates to move. However, it is now recognised to be more complicated than this. Mechanisms called slab pull and ridge push are believed to move the tectonic plates. Ridge push is where the new crust formed at divergent plate margins is less dense than the surrounding crust and so it rises to form oceanic ridges. The older seafloor either side of the ridge slides away and this moves the seafloor apart – moving the tectonic plates.
Therefore, instead of tectonic plates moving because of the convection currents, evidence suggests it is the plates that drive the convection. Slab pull occurs where older, denser tectonic plates sink into the mantle at subduction zones. As these older sections of plates sink, newer and less dense sections of plate are pulled along behind. Sinking in one place leads to plates moving apart in other places.
The movement of the plates and the activity inside the Earth, is called the theory of plate tectonics.
Global distribution of earthquakes and volcanoes
The earth's surface is made up of two types of crust:
Oceanic crust - found underneath the oceans. It is denser than continental crust and can be subducted.
Continental crust - found under land masses or continents. It is generally older than oceanic crust and is less often destroyed.
Earthquakes are found along all types of plate margins as shown on this map. Volcanoes however, only occur at constructive and destructive plate margins.
Key fact
A lot of volcanic activity occurs in the 'ring of fire'. The 'ring of fire' is a group of volcanoes that are located along the plate margin of the Pacific plate.
Hawaii is an arc of volcanic islands, found above a hot spot.
Destructive, constructive and conservative plate margins
Destructive plate margins
A destructive plate margin usually involves an oceanic plate and a continental plate. The plates move towards one another and this movement can cause earthquakes.
As the plates collide, the oceanic plate is forced beneath the continental plate. This is known as subduction. This happens because the oceanic plate is denser (heavier) than the continental plate.
When the plate sinks into the mantle it melts to form magma. The pressure of the magma builds up beneath the Earth's surface. The magma escapes through weaknesses in the rock and rises up through a composite volcano. The volcanic eruptions are often violent, with lots of steam, gas and ash.
If two continental plates collide, neither can sink and so the land buckles upwards to form fold mountains. This is called a collision margin. Earthquakes can occur at collision margins.
Constructive plate margins
At a constructive plate margin the plates move apart from one another. When this happens the magma from the mantle rises up to make (or construct) new land in the form of a shield volcano. The movement of the plates over the mantle can cause earthquakes.
Conservative plate margins
At a conservative plate margin, the plates move past each other or are side by side moving at different speeds. As the plates move, friction occurs and plates become stuck. Pressure builds up because the plates are still trying to move. When the pressure is released, it sends out huge amounts of energy, causing an earthquake. The earthquakes at a conservative plate boundary can be very destructive as they occur close to the Earth's surface. There are no volcanoes at a conservative plate margin.
Causes of earthquakes
What causes earthquakes?
Earthquakes are the sudden violent shaking of the ground. This happens because the Earth's plates are constantly moving. Sometimes, because of friction, plates try to move and become stuck. Pressure builds up because the plates are still trying to move. When the pressure is released, it sends out huge amounts of energy causing the Earth's surface to shake violently.
The point inside the Earth's crust where the earthquake originates from is known as the focus. The earthquake's energy is released in seismic waves and they spread out from the focus. The seismic waves are most powerful at the epicentre. The epicentre is the point on the Earth's surface directly above the focus.
Key fact
Earthquakes are found at all three plate boundaries: constructive, destructive and conservative plate boundaries.
Measuring earthquakes
Figure caption,
A Willmore seismometer measures earthquakes
Earthquakes, until recently, have been measured on the Richter scale. The Richter scale measures the magnitude of an earthquake (how powerful it is). It is measured using a machine called a seismometer which produces a seismograph. A Richter scale is normally numbered 1-10, though there is no upper limit. It is logarithmic which means, for example, that an earthquake measuring magnitude 5 is ten times more powerful than an earthquake measuring 4. Earthquakes measuring 1-2 on the scale happen regularly, and they are so small that people cannot feel them. Earthquakes measuring upwards of 7 are less frequent but very powerful, and can cause a lot of destruction.
The largest earthquake ever recorded was in Chile in 1960, which measured 9.5 on the Richter scale. The Richter scale is not very accurate in measuring these larger earthquakes and today scientists use the Moment Magnitude Scale which uses the same logarithmic scale but which more accurately measures the strength of larger earthquakes.
Figure caption,
Different magnitudes of earthquake on the Richter scale
Effects and responses of earthquakes
Effects
The effect of an earthquake is the damage which happens as a result of the earthquake. The effects of an earthquake can vary depending on:
The size of the earthquake on the Richter scale - the higher it is on the scale, the more destruction it can cause.
Level of development - whether it occurs in a rich or a poor country. Richer countries will be more likely to be able to predict, protect and prepare themselves from the effects of an earthquake.
The depth of the focus - if it's shallow, it can be more destructive.
Distance from epicentre - the effects of an earthquake are more severe at its centre.
Population density - the more people living in an area, the more likely that more deaths and casualties may arise.
The time of day - whether people are in their homes, work or travelling.
Key fact
We can classify the effects of an earthquake into the following categories:
primary effects - things that happen immediately as a result of an earthquake
secondary effects - things that happen in the hours, days and weeks after the initial earthquake
Responses
Responses are how countries react to an earthquake. They are categorised as follows:
Short-term or immediate - a response in the days and weeks immediately after a disaster has happened. Short-term responses mainly involve search and rescue and helping the injured.
Long-term - responses that go on for months and years after a disaster. It involves rebuilding destroyed houses, schools, hospitals, etc. It also involves kick-starting the local economy.
Prediction, protection and preparation
Prediction
Prediction involves using seismometers to monitor earth tremors. Experts know where earthquakes are likely to happen. However, it is very difficult to predict when they will happen. Even looking at the timescale between earthquakes doesn't seem to work.
Protection
Protection involves constructing buildings so that they are safe to live in and will not collapse. Some examples of building improvements are:
rubber shock absorbers in the foundations to absorb the Earth tremors
steel frames that can sway during Earth movements
open areas outside of the buildings where people can assemble during an evacuation
Figure caption,
Example of an earthquake-proof building
Preparation
In earthquake-prone countries, hospitals, emergency services and residents practise for an earthquake. They have drills in all public buildings so that people know what to do in the event of an earthquake. This helps to reduce the impact and increases their chance of survival.
