plate margin

The meeting place of one plate (plates make up the top layer of the Earth's structure) with another plate. There are four types of plate margin – destructive, constructive, collision, and conservative. A volcano may be found along two of the types of plate margin, and an earthquake may occur at all four plate margins.

Destructive or convergent plate margins
At a destructive margin an oceanic plate moves towards (and disappears into the mantle of) a continental plate or another oceanic plate. This is the subduction zone. As it is forced downwards, pressure at the margins increases, and this can result in violent earthquakes. The heat produced by friction turns the crust into magma (liquid rock). The magma tries to rise to the surface and, if it succeeds, violent volcanic eruptions occur.

Constructive or divergent plate margins
At a constructive margin the Earth's crust is forced apart. Magma rises and solidifies to create a new oceanic crust and forms a mid-ocean ridge. This ridge is made from igneous rock; such ridges usually form below sea level on the sea bed (an exception to this is in Iceland).

Collision plate margins
A collision margin occurs when two plates moving together are both made from continental crust. Continental crust cannot sink or be destroyed, and as a result the land between them is pushed upwards to form high ‘fold’ mountains like the Himalayas. Earthquakes are common along collision margins but there are no volcanic eruptions.

Conservative plate margins
At a conservative margin two plates try to slide past each other slowly. Quite often, the two plates stick and pressure builds up; the release of this pressure creates a severe earthquake. There are no volcanic eruptions along conservative plate margins because the crust is neither being created nor destroyed. The San Andreas Fault in California lies above the North American and Pacific plates, and is an example of a conservative plate margin.