Comparing Earthquakes

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Map of recent earthquakes and volcanic eruptions
The distribution of recent earthquakes and volcanic eruptions
Recorded earthquakes between 1901 and 2000 and the Earth's tectonic plates
Recorded earthquakes between 1901 and 2000 and the Earth's tectonic plates

A plate margin is the boundary / location where two slabs of Earth’s crust meet. This may be the meeting of crust of the same type or different types – such as oceanic and continental.

There are three main types of plate margins. These are constructive, destructive and conservative plate margins. Processes that occur at these boundaries lead to the formation of earthquakes and volcanoes. 

Constructive Plate Margins

At a constructive plate margin, the tectonic plates move away from each other. This is caused by convection currents in the mantle and slab pull/ridge push. Where two plates move apart, magma rises from the mantle to construct new land either through a volcano or a fissure (large crack through which magma rises). If this occurs in the middle of an ocean T a mid-oceanic ridge is formed. Rift valleys are formed where two plates move apart on land.

The movement of the plates over the mantle can cause earthquakes.

Iceland was formed due to the North American plate and Eurasian Plate separating at a constructive plate margin.

A constructive plate margin

Destructive Plate Margins

Destructive plate margins typically form when an oceanic plate moves towards a continental plate. The oceanic plate usually subducts (sinks under) the continental plate. Earthquakes can occur when friction between the two plates is released.

The oceanic plate subducts the continental plate because it is thinner and denser (heavier). An oceanic trench is formed where the oceanic plate subducts the continental plate. 

When the oceanic plate sinks into the mantle, it melts to form magma. Pressure then builds below the surface of the Earth and magma rises through weaknesses in the crust and rises to the surface through a composite volcano. Eruptions from composite volcanoes tend to be violent, with large volumes of steam, ash and gas.

If two continental plates move towards each other, neither sinks, so the land folds and rises upwards, forming fold mountains. This type of margin is known as a collision margin. Earthquakes can occur at collision margins; however, they are free from volcanoes.

Conservative Plate Margins

At a conservative plate margin, two plates move past each other, either in opposite directions or the same direction but at different speeds.
As the plates move, friction occurs, and the plates get stuck. Over time pressure builds up until it is released, sending out vast amounts of energy, causing an earthquake.

Earthquakes along conservative plate margins can be very destructive as they often occur close to the surface of the Earth.

Volcanoes do not occur at conservative plate margins.

What might the examiner ask?

A plate margin is the boundary / location where two slabs of Earth’s crust meet. This may be the meeting of crust of the same type or different types – such as oceanic and continental.

A constructive plate margin occurs where two plates move apart, whereas plates move together at a destructive plate margin. New crust is created at a constructive plate margin where it can be melted and destroyed at a destructive plate margin. Contrasting landforms occur at the different margins with fold mountains being found at destructive margins together with ocean trenches and composite volcanoes instead of mid-ocean ridges and shield volcanoes and rift valleys at constructive margins. Eruptions are more violent and earthquakes more severe at destructive plate margins.

Two tectonic plates move apart due to convection currents in the Earth’s crust that determine the direction of movement. As they pull apart, a ‘gap’ is created between the plates. This is filled by magma rising up out of the mantle to plug the gap and make the crust complete.
As this occurs again and again, layers of lava solidify and build up to create volcanoes. This often happens under the oceans.

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