The Earth's crust is made up of huge pieces, called tectonic plates. These plates "float" on the molten magma under the crust. When they collide, fissures form in crust. Through these fissure the magma is pushed up by enormous pressure. When the magma comes out of the fissures it starts cooling and eventually solidifies. The fissure is not sealed very well, so the volcano will erupt many times, each time another layer of solidified magma is added to area around the volcano and, over a very long time the layer build up, and form the caldera of the volcano. cgz.e2bn.net
