Tectonics

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The Earth’s structure and plate movement

The Earth has four main layers – the inner core, the outer core, the mantle and the crust.

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Inner Core

The inner core is 5,500°C - extremely hot. It is a very dense solid made from iron and nickel.

(http://www.bbc.co.uk/staticarchive/c8d3da857120b5167d106b33dca53abbf3bbf425.png)

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Outer Core

The outer core is 2,000 km thick and is a liquid.(http://www.bbc.co.uk/staticarchive/c8d3da857120b5167d106b33dca53abbf3bbf425.png)

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The mantle

The mantle is semi-molten and about 3,000 km thick(http://www.bbc.co.uk/staticarchive/c8d3da857120b5167d106b33dca53abbf3bbf425.png).

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The Crust

(http://www.bbc.co.uk/staticarchive/c8d3da857120b5167d106b33dca53abbf3bbf425.png)The crust is the rocky outer layer. It is thin compared to the other sections, approximately 5 to 70 km thick. If the Earth was scaled down to the size of an apple, the crust would be about the thickness of the apple skin. The crust is made up of pieces called plates. There are two types of crust: oceanic and continental crust. The oceanic crust is found under the sea and is thinner and less dense than the continental crust.

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Plate movement

Heat from the core causes convection currents in the mantle. These currents slowly move the crust around. In some places the crust is destroyed. In other places new crust is formed.

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Plate movement

Earthquakes and volcanoes are primarily found at plate boundaries. The plates are like giant rafts that slowly move around. Their movement is driven byconvection currents in the mantle. The mantle is much hotter than the crust and its rock is molten. At the boundaries between plates, molten magma is able to force its way to the surface and escape as lava.

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Destructive plate boundary

A destructive plate boundary is sometimes called a convergent or tensionalplate margin. This occurs when oceanic and continental plates move together. The oceanic plate is forced under the lighter continental plate. Friction causes melting of the oceanic plate and may trigger earthquakes. Magma rises up through cracks and erupts onto the surface.

An example of a destructive plate boundary is where the Nazca plate is forced under the South American Plate.

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Collision zones

Collision zones form when two continental plates collide. Neither plate is forced under the other, and so both are forced up and form fold mountains.

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Constructive plate boundary

A constructive plate boundary, sometimes called a divergent plate margin, occurs when plates move apart. Volcanoes are formed as magma wells up to fill the gap, and eventually new crust is formed.

An example of a constructive plate boundary is the mid-Atlantic Ridge

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Conservative plate boundary

A conservative plate boundary, sometimes called a transform plate margin, occurs where plates slide past each other in opposite directions, or in the same direction but at different speeds.

Friction is eventually overcome and the plates slip past in a sudden movement. The shockwaves created produce an earthquake.

This occurs at the San Andreas Fault in California.

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Volcanoes

volcano is an opening in the Earth’s crust. It allows hot magma, ash and gases to escape from below the surface.

  • Magma chamber – large underground pool of magma

  • Lava – magma, once it reaches the surface

  • Crater – bowl-shaped basin in the top of the volcano

  • Vent – central tube which magma travels through

  • Secondary cones – eruptions from other vents may build up secondary cones on the flanks

  • Ash, steam and gas – material thrown out by the volcano

  • Volcanic bombs – larger material thrown out by the force of eruption

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Types of volcano

There are two types of volcano, composite and shield.

Composite volcanoes are steep-sided and cone-shaped, made up of layers of ash and lava and containing sticky lava which doesn’t flow very far.

Mount Etna in Italy is a composite volcano.

Shield volcanoes have gently sloping sides and runny lava that covers a wide area. Gases escape very easily from shield volcanoes.

Mauna Loa in Hawaii is a shield volcano.

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Earthquakes

Large earthquakes are usually connected with plate boundaries. Earthquakes happen often but most are too small for us to notice. Seismometers record earth movements.

An earthquake is a sudden shockwave caused by rocks being under stress from the movements of plates at plate boundaries. Eventually the stress in the rock builds up enough to deform and reach breaking point. At that point, the stored up energy is released in the form of shockwaves.

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Measuring an earthquake

In the past, the Richter scale was used to measure the power of earthquakes. Earthquakes are now measured using the Moment Magnitude Scale (or simply Magnitude scale). This measures the size of the seismic waves during the earthquake. Each step in the scale is ten times greater than the previous number. This is a logarithmic scale.

The amount of damage caused by an earthquake is measured by the Mercalli Scale. This is a measure of intensity, and changes according to which area you are measuring – damage nearer the epicentre would usually be greater than further away.

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Measuring an earthquake

In the past, the Richter scale was used to measure the power of earthquakes. Earthquakes are now measured using the Moment Magnitude Scale (or simply Magnitude scale). This measures the size of the seismic waves during the earthquake. Each step in the scale is ten times greater than the previous number. This is a logarithmic scale.

The amount of damage caused by an earthquake is measured by the Mercalli Scale. This is a measure of intensity, and changes according to which area you are measuring – damage nearer the epicentre would usually be greater than further away.

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Case study: volcanic eruption

Nyiragongo is a steep-sided, active volcano. It is located in the Democratic Republic of Congo. Nyiragongo and the shield volcano of Nyamuragira, 40 km away, are part of the East African Rift Valley. This is an area of many faults where the plates are being stretched as they move away from each other.

Cause of the eruption

The eruption happened on 17 January 2002. The volcano has a lava lake in its crater. Fissures opened up to the south side of the volcano and three streams of lava from the lake drained through the fissures. The lava reached speeds of 60 km/h. There was little warning as the lava reached the city of Goma. The unrest in the country has made it difficult to monitor the volcano and put emergency responses in place.

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Social impacts of the eruption (effects on people)

  • Homes were destroyed by ash and lava.
  • 45 people died in the first 24 hours.
  • The lava flow made it difficult to travel around Goma as it filled the roads. Aid agencies were unable to access some areas of Goma.
  • The lava took a long time to cool and it burnt people as they tried to return to their homes.
  • Cholera spread because of lack of sanitation in areas that people fled to.
  • Around 50 people were killed when fuel exploded while they were trying to siphon it off at a petrol station.
  • 14 nearby villages were destroyed in the lava flow.
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Economic impacts of the eruption (effects on money

  • People returned to Goma hoping to find aid. One month after the eruption, 350,000 people were dependant on aid.
  • People lost their businesses and jobs.
  • Aviation fuel stores exploded as the lava flow damaged Goma airport.

Environmental impacts of the eruption

  • Lava covered 15 per cent of the city of Goma and destroyed 30 per cent of the city.
  • If the lava was to reach Lake Kivu, or seismic activity disrupted the lake, then dangerous gases of carbon dioxide and methane could be released from the floor of the lake.
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