Plate margins and collision zones

Destructive plate margins

At a , the plates move towards one another. This usually involves an oceanic plate and a continental plate.

The collision process is known as . The oceanic plate is denser (heavier) than the continental plate. As the plates move towards each other, the oceanic plate is forced beneath the continental plate. When the plate sinks into the mantle it melts to form magma. The magma escapes under pressure through cracks in the rock and rises up through a volcano. The volcanic eruptions can be violent, with lots of steam, gas and ash.

Subduction can cause volcanic activity and/or earthquakes. Many of these margins are found around the Pacific plate where it subducts under the Eurasian plate near Japan.

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.

Constructive plate margins

At a the plates move apart from each other.

When the plates part the magma rises up to make (construct) new crust in the form of either a shield volcano or a fissure volcano. The movement of the plates over the mantle can also 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 are moving past each other or move side by side at different speeds, like cars on a road.

As the plates move, friction occurs and they become stuck. The plates are still trying to move so pressures and stresses build up in the crust. When the pressure is released suddenly, waves of energy move through the crust, causing an earthquake.

The earthquakes at a conservative plate margin can be very destructive as the focus is 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.

Collision zones

Collision zones form when two continental plates move towards each other and collide. The land between the plates is forced upwards to form fold mountains, eg The Alps and Himalayas.