A crater is the circular surface depression caused by volcanic activity, usually on the summit or flank of a volcano. Craters are built by the collapse of the summit, accumulation of lava and pyroclastic material around an open vent or pipe, or explosion of gas, lava, and pyroclastics from a volcano.

Fissures may erupt from the walls or base of craters. During periods of volcanic activity, the crater floor can be a molten, seething lake of liquid lava. Mark Twain described the Kilauea crater in this way in the late 1800s.

Craters may naturally fill with water, creating a crater lake. Some crater lakes can have high temperatures and be acidic, due to continued volcanic activity.