Question: What makes the magma come out of the chamber and how does magma form?

Kate Dobson answered on 20 Nov 2014:

Generally magma “forms” when molten material work its way out of the mantle and up into the crust. Most of the upper part of the mantle is a mixture of crystals and very small volumes of melt, and the little pockets of melt are slightly less dense than the crystals and so can rise. Once in the crust the magma begins to evolve depending on it’s composition, temperature, and what rocks it has to pass through, and starts to cool and for it’s own crystals.

Magma usualy erupts becasue because of a change in pressure. Either more magma is pumped into the bottom of the system forcing it out the top, or pressure changes cause gas bubble to expand and the total volume of the magma to increase. Sometimes water comes into contact with the magma and as it turns to steam the magma is fragmented.