What happens immediately before a volcano erupts

Expanding gases pushes magma through the pipe

Before a volcanic eruption, magma must travel upwards through rock. The travelling magma produces these precursors to an eruption: an increase in earthquakes beneath a volcano, volcanic tremors, disturbance of ground features, increased release of steam from the volcano, increased temperatures and increase of gases. Additionally, different catalysts occur beneath the earth's surface prior to an eruption.