What is a volcano?

Deep underground it is hot enough to melt rock. This melted rock is called magma. It is lighter than the solid rock around it, so it rises and collects in a magma chamber. When pressure builds up, magma pushes through a crack, or vent, and erupts. Once it is outside, it is called lava.

Some eruptions are gentle, with runny lava flowing out like hot syrup, as in Hawaii. Others are explosive, blasting out gas, ash and rock high into the sky. Each eruption adds a new layer, and over time the volcano grows into a mountain with a crater at the top.

Scientists describe volcanoes as:

• Active: has erupted recently or is erupting now.
• Dormant: "sleeping"; no eruption for a long time, but it could erupt again.
• Extinct: not expected to erupt ever again.

Most volcanoes are found along the edges of tectonic plates, the giant slabs of Earth's crust. A huge horseshoe of volcanoes and earthquakes around the Pacific Ocean is called the Ring of Fire. Some volcanoes, like those in Hawaii, form over "hot spots" in the middle of a plate.

Volcanoes can be dangerous, so scientists called volcanologists watch them closely for warning signs like small earthquakes and gas. But volcanoes also help people: volcanic soil is fertile, and heat underground can make electricity. Cooled lava becomes new rock, such as basalt; see rocks.