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.

One plate subducts (goes under) the other. A trench forms at the subduction zone betwen the oceanic crust and land (continental crust). Volcanoes and new mountains may form nearby.

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.

Magma rises through the gap between the two plates, pushing them apart. A volcano forms on the Earth's crust at this point.

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.

At a conservative plate margin, plates slide past each other.
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