Many mountains are volcanoes. When Earth’s tectonic plates bump into, pull apart from, or slide under each other, magma sometimes pours out. As it seeps through cracks in the crust, it takes the form of lava. Most of the time, this happens on the edges of tectonic plates.
Sometimes, volcanoes form in the middle of plates. That happens as the crust moves across areas in the mantle that are so hot, magma erupts right through the crust. That is called a hot spot. The Hawaiian Islands are sitting on a hot spot.
A volcanic eruption is very violent. There may be earthquakes even before the eruption starts. Clumps of lava and volcanic rock may be sent flying high into the sky. Thick clouds of ash and poisonous gas may fill the air. Fiery lava rivers may flow for miles. A single volcano can cause huge damage. But after a while, the boiling-hot lava and ash harden into rock. A new mountain may become higher, or an old one may change shape.
