Plate Tectonics - Processes and Landforms at Plate Margins

There are three types of plate margin: constructive, destructive and conservative. At each one there are distinctive landforms and events that characterise them.

Constructive

This margin can be found on land as well as in the centre of oceans. The processes occurring here are that the plates are being pulled apart in a pattern of divergence. Most commonly found in oceans, the plate type is oceanic on both sides and upon moving apart a gap is left, which is immediately filled with magma that rises up from the mantle. Due to a lack of pressure, the lava that is produced from the gap has a low viscosity and a low silica content which makes it runny. The lava can then travel over a large area before it solidifies. As this process occurs along the ridge for hundreds or thousands of miles, a chain of submarine volcanoes are formed. This continual process creates shield volcanoes. Transform faults are formed at right angles to the margin and earthquakes can occur here. They are usually of low magnitude.

Where a constructive margin occurs on land it creates a rift valley. As the crust tears apart, sections of land drop and create deep rift valleys. Thinner crust allows gas and steam eruptions to occur.

Destructive

This margin is associated with converging plates, which are can involve two oceanic plates colliding, one continental and one oceanic, or two continental plates in collision.

Where two oceanic plates converge a volcanic island arc will result parallel to the plate margin - examples are Japan, and the Aleutian Islands; both curved lines of volcanoes. The volcanoes are very explosive, because the resulting magma from subduction is silica-rich andesitic magma with high levels of gas present and therefore very explosive.

Where continental crust and oceanic plate converge, the oceanic crust is subducted underneath the continental crust as it is denser. This area of subduction creates a deep section of the ocean known as an oceanic trench. The crust is pushed/pulled down at various angles. As the plate descends there is an increase in pressure, heat and friction and this causes the plate to melt. This molten material will then force its way back up to the surface where it will emerge through the crust as an explosive composite cone volcano. Fold mountain chains are also found along the edge of the continental plate, as the subduction of the oceanic plate crumples up the continental plate. Earthquakes at this margin can be of high magnitude due to the pressure that is built up between the two plates.

When two continental plates converge, neither plate can be subducted due to the plates having a similar density. This margin is also known as a collision zone. The plates collide into each other and crumple upwards to form fold mountains. Due to the lack of subduction there is no volcanic activity, but earthquakes are violent due to the constant friction that builds up between the two plates trying to override each other.

Conservative

This margin is associated with plates that slide alongside each other. They can either slide in the same direction but at different speeds or in opposite directions. There are no volcanoes found at this margin due to the lack of material being destroyed. As they move past each other, friction builds up and energy is released as an earthquake. They are usually shallow focus earthquakes due to the lack of subduction. The means they can produce deadly, high magnitude earthquakes as friction is released close to the surface