Many mountains are volcanoes. When tectonic plates hit each other, pull apart, or slide under one another, magma may pour out. It comes through cracks in the crust in the form of lava.
Usually, that happens on the edges of tectonic plates. Sometimes, volcanoes form in the middle of plates. Parts of the mantle are so hot, magma blasts through the crust. This is called a hot spot. The Hawaiian Islands sit on a hot spot.
A volcanic eruption is really violent. There may be earthquakes before it starts. Chunks of lava and rock may fly high into the sky. Thick, choking clouds of gas and ash may fill the air. Burning lava rivers may travel for miles. One volcano can cause huge damage. But soon, the boiling-hot lava and ash harden into rock. A new mountain may rise, or an old one may take on a new shape.
