In a volcanic eruption, hot lava spews out from beneath the Earth's crust up to the Earth's surface. This lava is actually hot melted rock called magma. Just underneath the Earth's crust is a layer called the mantle, which is made of up plates that are always moving and shifting. Sometimes the plates separate. That creates heat and causes the mantle to melt into magma. The magma comes up through the crack between the plates. It spreads out, cools down, and becomes rock again. The solid magma stays beneath the Earth's surface.