How do volcanic eruptions tend to occur?

Answer 1

Often, eruptions occur due to "tectonic activity" which is the movement of tectonic plates that make up the earths crust at boundaries between plates (usually destructive tectonic boundaries). Beneath the earths crust is the mantle - a huge layer of molten rock and magma. When tectonic plates move suddenly e.g. when there is an earthquake, gaps and cracks created in the tectonic plates allow magma to rise up to the surface and erupt from volcanoes. It rises to the surface as the hot magma is less dense than the rock in the crust. These types of eruption are usually slow.

Violent volcanic eruptions are often caused when the magma that has risen to the surface is unable to erupt from a volcano due to a particularly hard/dense layer of rock or it losing so much heat up it begins to harden and become solid rock its self. this creates pockets of magma called magma chambers. As more and more magma builds up there over time the pressure builds up and up until it is sufficient to overcome the tougher rock around it and explodes out violently all at once in a much more violent volcanic eruption.