Geologically speaking, what natural events lead up to a volcano eruption?

Question

Geologically speaking, what natural events lead up to a volcano eruption?

Answer 1

Melting of rocks within the crust or the mantle, with the result that magmas rise through the crust and erupt at the surface.  Melting is caused by several factors:

1: Reductions of pressure by thinning of the crust in rifts - incipient spreading ridges

2: Introduction of volatiles which facilitate melting by means of subduction at subduction zones.

3: Presence of mantle hot spots.  These are thoght tobe due to anomalously hot regions at the core mantle boundary which are long lived.

On an individual basis, volcanic eruptions are usually preceded by small earthquakes caused by the rising magma, by bulging of the volcano slopes, and by increased gas emissions.