Plate Tectonics
WHAT DO I NEED TO BE ABLE TO DO:
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LESSONS
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Is the world becoming a more hazardous place?
We are living in a world which is experiencing more natural disasters than ever before, however, not all disasters are increasing in frequency. As more people are faced with the prospect of falling victim to a natural hazard we must try to find ways to prepare for and manage the threats we face.
The theory of plate tectonics
The idea of plate tectonics is largely based upon Wegner's theory of continental drift in 1912. He proposed that the world's continents were moving, as the shapes of the continents appeared to fit together like a jigsaw suggesting they had moved apart over time. This theory was not accepted until the 1950s after the discovery of seafloor spreading in the Atlantic Ocean. The movement of the earth's crust is now generally know as plate tectonics.
Structure of the earth
The Earth can be divided up into four main layers.
CRUST This is the solid layer on which we live. It varies in thickness, from 10km to over 60km. The crust is broken into pieces which are called tectonic plates.
MANTLE This is the thickest section of the earth, with a diameter of around 2 900km. This layer is normally referred to as being semi-molten, but this is an oversimplification. Instead the mantle varies in its viscosity, but can flow very slowly. Within the mantle are convection currents, which in turn move the tectonic plates.
OUTER CORE This layer is generally believed to consist of liquid nickel and iron. It is extremely hot with temperatures exceeding 5500°C.
INNER CORE This layer is believed to be solid due to the immense pressure placed upon it. It is made up of iron and some radioactive elements. It is also extremely hot, around 5500°C.
Although the Earth can be divided into four layers, the boundaries between these are often blurred and difficult to determine. Therefore, many referred to the crust at the upper sections of the mantle as the lithosphere. Below the lithosphere lies the asthenosphere, which is the section of the mantle which lies between 100km and 200km below the surface.
- Crust
- Mantle
- Outer Core
- Inner Core
CRUST This is the solid layer on which we live. It varies in thickness, from 10km to over 60km. The crust is broken into pieces which are called tectonic plates.
MANTLE This is the thickest section of the earth, with a diameter of around 2 900km. This layer is normally referred to as being semi-molten, but this is an oversimplification. Instead the mantle varies in its viscosity, but can flow very slowly. Within the mantle are convection currents, which in turn move the tectonic plates.
OUTER CORE This layer is generally believed to consist of liquid nickel and iron. It is extremely hot with temperatures exceeding 5500°C.
INNER CORE This layer is believed to be solid due to the immense pressure placed upon it. It is made up of iron and some radioactive elements. It is also extremely hot, around 5500°C.
Although the Earth can be divided into four layers, the boundaries between these are often blurred and difficult to determine. Therefore, many referred to the crust at the upper sections of the mantle as the lithosphere. Below the lithosphere lies the asthenosphere, which is the section of the mantle which lies between 100km and 200km below the surface.
Tectonic plates
The Earth's crust is divided into pieces called tectonic plates, or plates. There are seven primary plates (African, Antarctica, Eurasian, Indo-Australian, North American, Pacific, South American) and seven smaller secondary plates (Nazca, Scotia, Arabian, Philippine, Juan de Fuca). These plates can be classified as either oceanic or continental. Generally oceanic crust is found beneath the oceans and continental crust is found under land. Although plates are usually a combination of the two. There are some key differences between oceanic and continental curst.
OCEANIC CRUST
CONTINENTAL CRUST
- Normally thinner
- Denser (heavier)
- Can be destroyed and made
- Younger
CONTINENTAL CRUST
- Normally thicker
- Less dense (lighter)
- Cannot be destroyed or made
- Older
Tectonic plates move due to convection currents. These occur due to the immense amounts of heat which is given out from reactions within core. This then heats the magma within the lower sections of the mantle, causing it to expand and become less dense. The less dense magma then begins to rise within the mantle towards the crust. As it moves towards the crust it begins to cool and become denser until it eventually sinks, creating a convection current. The friction between the magma and the crust results in the tectonic plates being dragged and moved.
Plate Boundaries
Destructive plate boundaries, also known as convergent plate boundaries, occur when an oceanic crust is moving towards the continental plate. As the oceanic plate is denser it sinks below the continental plate, which is known as subduction. Friction between the two plates builds up and eventually results in earthquakes. The continental crust is folded up to create a range of fold mountains, such as the Andes. Meanwhile the subducted oceanic plate melts due to the heat within the mantle, resulting in an increase in pressure within the mantle. This pressure builds overtime until it is eventually released in the form of a volcanic eruption.
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Constructive plate boundaries, also know as divergent plate boundaries, occur when two plates are moving apart from each other. This results in sea-floor spreading, which results in the formation of new land, such as Iceland. The two plates move apart leaving a gap which the magma from within the mantle can then rise to fill. This results in volcanic eruptions as magma moves onto the surface in the form of lava. This process is repeated resulting in the formation of new land.
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Conservative plate boundaries, also known as transform boundaries, occur when two plates are moving past each other. They may be moving in opposite directions (as shown above) or in the same direction but at different speeds, such as the San Andreas fault in California. Friction between the two plates builds overtime, until eventually the pressure is too great and an earthquake is triggered. At these boundaries land is neither destroyed or created.
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Collision plate boundaries, also known as convergent plate boundaries, occur when two continental crusts are moving towards each other. As they are of similar densities neither can be subducted. Instead they are folded up creating fold mountain ranges, such as the Himalayas.
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VOLCANIC ACTIVITY
A volcano is essentially an opening in the Earth’s crust through which magma from within the mantle is erupted onto the land. The vast majority of volcanoes are found at plate boundaries although there are some exceptions, such as Hawaii and Mount Teide, which are located on hotspots. Magma refers to the molten rock inside the Earth;s interior. When this molten rock moves onto the Earth’s reface it is called lava.
Not all volcanoes are capable of erupting, instead we classifying them by their state of activity.
There are several types of volcano with their physical appearance differing dramatically. Their shape depends on the the type of lava which is ejected from the volcano. Shield volcanoes are the product of very hot and runny lava, whilst thick (viscous) lava produces cone volcanoes.
Not all volcanoes are capable of erupting, instead we classifying them by their state of activity.
- ACTIVE VOLCANO - a volcano that has erupted recently and is likely to erupt again in the near future
- DORMANT VOLCANO - a volcano that has not erupted in recent history but it has the potential to erupt again at some point in the future
- EXTINCT VOLCANO - a volcano that is extremely unlikely to erupt again as no magma is being produce within its magma chamber.
There are several types of volcano with their physical appearance differing dramatically. Their shape depends on the the type of lava which is ejected from the volcano. Shield volcanoes are the product of very hot and runny lava, whilst thick (viscous) lava produces cone volcanoes.
SHEILD VOLCANO
Shield volcanoes having very gently sloping slides but cover a large area. The lava is extremely runny so is able to cover large distances before cooling and turning to rock. Therefore the volcano does not increase much in height following each eruption, instead the sides maintain a low gradient. The low viscosity of the lava also means that any gases are able to escape so that pressure does not build up. This means that eruptions are calm but frequent.
e.g. Mauna Loa, Hawaii |
COMPOSITE CONE VOLCANO
Cone volcanoes have steeply sloping sides making them triangular in appearance. They are made up of layers of ash and lava from previous eruptions which builds up over time. The lava is extremely viscous, this means that it does not travel far, so when it cools the volcano increases in height and the sides become steeper. The viscous nature to the lava also means that gas within the lava is unable to escape creating violent and explosive eruptions.
e.g. Mount Etna, Italy |
VOLCANIC HAZARDS
Volcanoes can create a number of primary hazards as well as some secondary hazards. Primary hazards include lava flows, pyroclastic flows and volcanic/lava bombs. Secondary hazards include tsunami, acid rain and lahars.
Volcanoes can create a number of primary hazards as well as some secondary hazards. Primary hazards include lava flows, pyroclastic flows and volcanic/lava bombs. Secondary hazards include tsunami, acid rain and lahars.
- LAVA FLOWS - these are the most well known volcanic hazard but actually pose very little danger to humans. They are slow moving so can be easily avoided. However, they can bury and incinerate any land or property in their path.
- PYROCLASTIC FLOWS - These are giant clouds of superheated gas and ash. They are extremely dangerous and are responsible for the vast majority of deaths due to the volcanic eruptions They can move at speeds up to 500 km/hr and reach temperatures over 700°C. They will destroy anything in their path.
- LAHARS - These are a secondary hazard and normally occur on snow covered volcanoes. The hot ash and gas erupted by the volcano melts the snow. They then mix to create a fast moving mudflow which flows down the flanks of the volcano burying anything in its path.
- ASH CLOUDS - Whilst they are not as 'deadly' as pyroclastic flows they can still cause significant amounts of damage and disruption. The weight of ash which falls on building can cause them to collapse. Ash can also destroy crops and contaminate water supplies. They can reduce sunlight and disrupt air travel.
- LAVA OR VOLCANIC BOMBS - Semi molten pieces of rock are thrown out during an eruption and can cause fires. If there are no settlements in close proximity to the volcano then these pose little threat to people.
- VOLCANIC GASES - Poisonous gases, such as carbon monoxide or sulphur dioxide can be released during eruptions. These can kill people and livestock if concentrations are high enough. They also contribute to the greenhouse effect.
MAGNITUDE OF ERUPTIONS
Not all volcanic eruptions are of the same magnitude (size) so the USGS developed a scale to measure the size of an eruption. This scale is called the Volcanic Explosivity Index or VEI scale. It is based on the volume of material erupted, height of the eruption cloud and observations to determine the explosivity value. The scale is open-ended, however, the largest eruptions in history have been given a score of 8 (super volcano) so this is believed by many to be its upper limits. The scale is logarithmic, with each interval on the scale being ten times larger than the previous.
Not all volcanic eruptions are of the same magnitude (size) so the USGS developed a scale to measure the size of an eruption. This scale is called the Volcanic Explosivity Index or VEI scale. It is based on the volume of material erupted, height of the eruption cloud and observations to determine the explosivity value. The scale is open-ended, however, the largest eruptions in history have been given a score of 8 (super volcano) so this is believed by many to be its upper limits. The scale is logarithmic, with each interval on the scale being ten times larger than the previous.
LIVING NEAR VOLCANOES
Our understanding of volcanoes has improved dramatically, yet people still live on their slopes. There are a number of reasons people live in areas which are potentially very dangerous.
Our understanding of volcanoes has improved dramatically, yet people still live on their slopes. There are a number of reasons people live in areas which are potentially very dangerous.
- They are considered by many to be areas of natural beauty, this makes them a popular tourist destination.
- Land surrounding volcanoes is very fertile due to minerals which are present in the area.
- Mineral are often produced in volcanic regions, which in turn can then be mined.
- There is the potential of producing geothermal energy, which is both clean and cheap.
- People are often left with no other alternatives as the land is very cheap. Also population growth has left many with no other alternative due to the lack of available land.
- People may become complacent if the volcano has not erupted for a long period.
VOLCANIC ERUPTION IN AN MEDC - ICeland
Iceland lies on the Mid-Atlantic Ridge, a constructive plate margin where the Eurasian plate and the North American plate are moving away from each other. As the plates move apart magma rises to the surface creating several active volcanoes, which run in a band (SW-NE) through the centre of Iceland. One of these active volcanoes is Eyjafjallajokull which is located beneath an idea cap in southern Iceland, 125km SE of Reykjavik (Iceland’s capital city).
In March 2010, magma broke through the crust beneath the Eyjafjallajokull glacier. The eruptions in March were mostly lava and presented very little threat to local communities. However, the eruptions entered a new phase in mid-April, this phase was much more explosive. For several days these violent eruptions released huge quantities of ash into the atmosphere.
The main primary effect of this eruption was the volcanic ash which was produced. Large areas of agricultural land was left coated in ash and many people had to be evacuated to prevent them choking on the ash fall. The secondary effect of the eruption was flooding. As the eruption occurred beneath a glacier large amounts of meltwater were produced. This could have resulted in serious problems but the Icelandic government responded quickly and efficiently to prevent serious damage.
In March 2010, magma broke through the crust beneath the Eyjafjallajokull glacier. The eruptions in March were mostly lava and presented very little threat to local communities. However, the eruptions entered a new phase in mid-April, this phase was much more explosive. For several days these violent eruptions released huge quantities of ash into the atmosphere.
The main primary effect of this eruption was the volcanic ash which was produced. Large areas of agricultural land was left coated in ash and many people had to be evacuated to prevent them choking on the ash fall. The secondary effect of the eruption was flooding. As the eruption occurred beneath a glacier large amounts of meltwater were produced. This could have resulted in serious problems but the Icelandic government responded quickly and efficiently to prevent serious damage.
LOCAL EFFECTS
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NATIONAL EFFECTS
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INTERNATIONAL EFFECTS
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LOCAL RESPONSE
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INTERNATIONAL RESPONSE
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VOLCANIC ERUPTION IN AN LEDC - Indonesia
Mount Merapi is located in SE Asia in Indonesia. It is located on the island of Java and has erupted on a regular basis since the 1500s. This is due to the fact that Indonesia lies on a destructive plate boundary where the Indo-Australian Plate is being subducted beneath the Eurasian Plate. This subduction zone forms part of the Pacific Ring of Fire.
PRIMARY EFFECTS
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SECONDARY EFFECTS
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SHORT-TERM RESPONSES
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LONG-TERM RESPONSES
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SEISMIC ACTIVITY
Earthquakes are defined as any movement in the earth's crust. Earthquakes are the result of two plates moving past each other quickly. Friction builds up between the two plates, causing the plates to 'stick'. When the pressure then becomes too great they will 'jolt' past each other causing seismic waves, which we know as an earthquake. The point at which the earthquake takes place is called the focus and this is always beneath the surface. The point on the earth's surface directly above the focus is called the epicentre.
Thousands of earthquakes happen everyday, but the vast majority of them are so small that they are not felt by humans. The strength of an earthquake is referred to as its magnitude and is most commonly measured on the Richter Scale. This scale was devised by C. Richter and is based upon the strength of the seismic waves, which can be measured with a seismometer. The Richter Scale is a log scale, where each number is 10x larger than the last, this means that the scale can be easily compressed down to a reasonable amount, so although the scale is effectively endless many regard it as going up to 10.
Earthquakes can cause several secondary hazards, that can often be more damaging than the primary hazard. Secondary hazards caused by earthquakes include:
Earthquakes are extremely hard to predict because they give no warning and the first earthquake is normally the strongest. Scientists (seismologists) look at earthquake history an attempt to identify plate pressure points to ascertain where earthquakes are likely to happen, but they can not predict how strong they will be or where they will happen.
Thousands of earthquakes happen everyday, but the vast majority of them are so small that they are not felt by humans. The strength of an earthquake is referred to as its magnitude and is most commonly measured on the Richter Scale. This scale was devised by C. Richter and is based upon the strength of the seismic waves, which can be measured with a seismometer. The Richter Scale is a log scale, where each number is 10x larger than the last, this means that the scale can be easily compressed down to a reasonable amount, so although the scale is effectively endless many regard it as going up to 10.
Earthquakes can cause several secondary hazards, that can often be more damaging than the primary hazard. Secondary hazards caused by earthquakes include:
- Tsunamis
- Dam failure
- Landslides
- Fires
- Disease
- Exposure
- Liquefaction
Earthquakes are extremely hard to predict because they give no warning and the first earthquake is normally the strongest. Scientists (seismologists) look at earthquake history an attempt to identify plate pressure points to ascertain where earthquakes are likely to happen, but they can not predict how strong they will be or where they will happen.
EARTHQUAKE IN AN MEDC - NEW ZEALAND
New Zealand is a group of islands which are located in the Pacific Ocean within the continent of Oceania. It is to the south east of Australia and consists of a North Island and a South Island. New Zealand is a developed country with a GDP per capita of $27,7000.
On 22nd February 2011 Christchurch suffered an earthquake measuring 6.3 on the Richter Scale. The epicentre of the earthquake was 6 miles SE of Christchurch and the focus was very shallow at 3.1 miles.
The earthquake was the result of the conservative plate margin which runs beneath the city where the Pacific Plate and Australian Plate move in opposite directions.
On 22nd February 2011 Christchurch suffered an earthquake measuring 6.3 on the Richter Scale. The epicentre of the earthquake was 6 miles SE of Christchurch and the focus was very shallow at 3.1 miles.
The earthquake was the result of the conservative plate margin which runs beneath the city where the Pacific Plate and Australian Plate move in opposite directions.
PRIMARY EFFECTS (direct result of the earthquake)
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SECONDARY EFFECTS (knock-on effects)
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SHORT-TERM RESPONSES
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LONG-TERM RESPONSES
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EARTHQUAKE IN AN LEDC - HAITI
Haiti is a small island located in the Caribbean Sea and lives to the east of Cuba. It’s capital city is Port-au-Prince. It is considered by many as the poorest country in the western hemisphere with a GDP per capita of $1,200. On 12th January 2010 16:53 an earthquake struck 16 miles to the west of Port-au-Prince with a focus depth of 5 miles. It measured 7 on the Richter Scale and was the result of friction between the North American Plate and the Caribbean Plate which move in the same direction but at different rates (conservative plate boundary).
PRIMARY EFFECTS (direct result of the earthquake)
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SECONDARY EFFECTS (knock-on effects)
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SHORT-TERM RESPONSES
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LONG-TERM RESPONSES
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