Why do volcanoes erupt?
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.
Sometimes the plates collide. When that happens, one plate can slide under the other. This also causes the mantle underneath to melt. The magma pushes upward, melting more rock. The hot magma collects under the surface of the Earth. If the pressure is high enough or a crack opens in the crust, the magma spews out. Now it is called lava.