Question: why do volcanos erupt?

James Hickey answered on 11 Nov 2013:

Hi Han22511! That’s a really awesome question, and something that volcanologists like me are working towards answering. We have quite a good understanding of what makes volcanoes erupt (as I will explain below) but we’re still working on improving our knowledge of this.

Magma, which is a mixture of partially melted rocks, is less dense than surrounding rocks. Because of this it begins to rise up due to a force called buoyancy, like how the coloured blobs in a lava lamp rise up. It is this initial density difference that brings magma from great depths in the Earth to beneath volcanoes.

Underneath volcanoes the magma may get ‘stuck’ when the density difference and buoyancy force is no longer big enough to make is rise on its own. This creates ‘magma chambers’ where magma can be stored before an eruption. How the magma is erupted from a chamber is still being researched.

One way is that a new batch of magma rises from deep and enters the magma chamber. But because the magma chamber is already full it bursts and magma is forced upwards towards the surface (a bit like when you burst a balloon if you blow too much air into it). Another way is that gases dissolved in the magma start to come out (like when you open a bottle of coke) and the pressure inside the chamber gets too much, and again, the chamber bursts.

Once the magma chamber has broken apart, the pressure in the magma pushes the rocks apart as it makes its way up towards the Earth’s surface (a bit like a nail being hammered into a piece of wood). This is what causes small earthquakes around volcanoes too! When the magma gets close enough to the surface it can then erupt ‘explosively’ or ‘effusively’ (an effusive eruption just means that lava pours out onto the surface).