Perhaps the most conspicuous part of a volcano is the crater, a basin of a roughly circular form within which occurs a vent (or vents) from which magma erupts as gases, lava, and ejecta. A crater can be of large dimensions, and sometimes of vast depth. Very large features of this sort are termed calderas. Some volcanoes consist of a crater alone, with scarcely any mountain at all; but in the majority of cases the crater is situated on top of a mountain (the volcano), which can tower to an enormous height. Volcanoes that terminate in a principal crater are usually of a conical form.

On Earth, volcanic explosion craters are often formed when magma rises through water-saturated rocks and causes a phreatic, or steam explosion. Volcanic explosion craters from phreatic eruption often occur on plains away from other obvious volcanoes. Other volcanic eruptions can cause mild explosions, often in a series of events. Some volcanic craters form by collapse with little or no explosive activity. Volcanic craters are typically seen either on the summits or on the flanks of volcanoes. Volcanic craters have also been identified on the Moon, Mars, Venus, and Io.