Question: how do volcanoes erupt

Katy Mee answered on 24 Jun 2010:

For a volcano to erupt there needs to be a number of factors in place – firstly you have to have magma (molten rock). Magma forms in various different settings around the Earth, mainly where an oceanic plates and continental plates meet. Since oceanic plates are less dense than continental ones, the oceanic plates are forced down beneath the continental plates (subduction). This causes the tip of the oceanic plate to melt at a certain depth and this molten rock becomes buoyant and rises to the surface through the more dense rock above it.

The reason that you get an eruption is because quite often this magma is stored just below the surface in a magma reservoir where the gases in the liquid start to build up, causing a huge build up of pressure. At some critical point this pressure becomes too much for the surrounding solid rock to contain and an explosion occurs.