The ash and rock crumble to the ground, but the lava is either moving down the volcano side very slowly or at a high speed. The lava burns down almost everything in its way, and it sometimes leaves bits of things burning. The lava from the volcano can cool fast, or sometimes the lava will slowly cool down from its intense heat. Lava that cools slowly forms igneous rocks. There are many types of igneous rocks.
Volcanoes can damage themselves in the explosion.
A volcano literally blows its top off. One of the volcanoes that has blown its top from an explosion is Mt. St. Helens. Mt. St. Helens has erupted more than once. Volcanoes can be under water or on land. It just depends on the environment how fast the volcano can make the magma the magma makes lava and the volcano makes an explosion.