1. Causes of Volcanoes

2. Plate Boundaries
Subduction volcanoes occur where plates move towards each other and collide.
One plate, usually the one that is heavier or more dense, is forced to dive (subduct) beneath the other into the mantle, causing it to heat up and melt.
This melted material pushes its way back to the earth’s surface under great pressure, and often erupts explosively.

3. Hot Spots Some volcanoes are found away from the edges of the plates.
They are formed where rising plumes of magma force their way to the surface from deep within the mantle, a ‘hot spot’.
As the plate moves, the existing volcano is carried away from the hot spot and becomes extinct. The next time the pressure builds up, new magma erupts to form another volcano in its place.
The best-known hot-spot volcanoes are in the Hawaiian Islands.

4. Rift Valleys
Where plates are moving apart, magma rises through the gap between the separating plates and creates new rock.
Here the volcanoes are known as rift volcanoes.
There are more rift volcanoes than subduction volcanoes
Most of them are hidden from view on the ocean floor, and they do not often threaten human life or property.
They erupt more gently than other types of volcano, with lava that oozes rather than explodes from inside them.