Magma

Magma is melted rock, when it reaches the surface it's called lava. Magma comes from deep underground where it's much hotter, so the rock down there is melted, isn't it? Well not really. Beneath the earth's crust is the mantle and it's by and large solid too. The difference between the crust and the mantle is mainly chemical, they are both solid.
So then, where does magma come from, how does all that rock melt and create such appalling displays at volcanic eruptions?

As it turns out, magma forms by decompression melting. Not only is the temperature higher deep underground in the mantle, the pressure is also terrifically greater. We usually think of solids melting when the temperature rises, but they can also melt when the pressure decreases. Peridotite the mantle rock, is solid at the high pressures and temperatures found down in the mantle, but it will melt when the pressure is released by an opening to the surface - a volcano.