How do volcanic eruptions occur?

Heat emanating from the hot/radioactive core of our planet can cause partial melting of some components of the upper mantle and crust of Earth. Where this happens the melt slowly aggregates/pools to form magma. As magma is a fluid, it can exert pressure in all directions as compared to a solid which when compressed in one direction resists deformation. This difference allows the fluid magma to force aside overlying solid rock and make its way upwards as structures called "Dykes".

The Dykes will rise to the surface and stop when the weight of the column of magma in the dyke equals the weight of and equal column of the crustal rocks (through which it is forcing itself). Thus depending on the density difference between the magma and crustal rocks, the Dyke could break through to the surface (pouring magma over the land) or stop at some position before the surface (both conditions are observable in the geological record).

However, most magmas contain a lot of dissolved volatile components (gasses) and as the Dyke nears the surface (and the pressure reduces), these come out of solution and the magma foams - like opening a bottle of Champagne.

The presence of the foam means that the overall density of the magma column is reduced and the magma often breaks through to the surface and beyond, allowing the magma to build a volcano (which is higher than the land around it) above the point of crustal exit.

The escaping gas is therefore the main cause of a volcanic eruption and is responsible for the explosions, eruption columns and lava fountains.