A volcano is an opening in Earth's surface where hot melted rock can escape from deep underground. This melted rock is called magma when it's still underground and lava when it flows out onto the surface. Volcanoes form in places where Earth's rocky outer shell, called the crust, is weak or cracked.
Deep beneath the surface, temperatures are so hot that rock melts into a thick, glowing liquid. When pressure builds up beneath the Earth's crust, it pushes the magma upward through cracks and weak spots until it erupts out of the volcano. When a volcano erupts, it can shoot out lava, clouds of ash, chunks of rock, poisonous gases, and even lightning!
Some volcanic eruptions are quiet and gentle, with lava slowly oozing down the sides of the mountain like thick syrup. Other eruptions are explosive and very dangerous, sending ash miles into the sky and destroying everything nearby. Not all volcanoes erupt all the time. Scientists put volcanoes into three groups. Active volcanoes erupt regularly or have erupted recently and could erupt again soon. Dormant volcanoes haven't erupted in a long time but still could erupt again someday. Extinct volcanoes will probably never erupt again because they no longer have magma beneath them.
Despite their dangers, volcanoes also create benefits. Volcanic soil is very rich and good for growing crops, which is why people sometimes live near volcanoes.