Some volcanoes erupt explosively, like opening a shaken soda bottle, because their magma is full of gases. Other volcanoes erupt gently, like boiling milk spilling over the sides of a pot, because their magma has fewer gases. An "active" volcano erupted recently or might erupt soon. A "dormant" or sleeping volcano has been quiet for a long time, but may erupt again someday. An "extinct" volcano has not erupted for over 10,000 years.

Definition: a crack in the Earth's crust where magma rises from below the Earth's surface
Materials released: lava, gases, ash, rocks, and minerals
Types: active, dormant, extinct
Frequency of eruption: On land, about 50 to 70 volcanoes erupt each year.
No. of active named volcanoes: about 1550
Cool fact: Most magma erupts underwater.

Image credits: main image, courtesy of USGS; Jim Webster: courtesy of AMNH.