Volcanoes occur in two kinds of places. They may burst out of the ground (erupt) in the middle of an ocean. Hawaii is like this and has the world's biggest volcanoes. More commonly they occur along a line of weakness in the Earth's crust, the same kind of place where you find earthquakes. Then you get long lines of volcanoes, such as occur all around the Pacific Ocean. These are called the Pacific Ring of Fire volcanoes.

Volcanoes sometimes erupt violently with ash and gas. These volcanoes do not erupt very often. You can tell them by their steep cones. The volcanoes which erupt frequently tend to erupt lava. They often have much gentler-sided cones.