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Volcanic crater
From Wikipedia, the free encyclopedia
A volcanic crater is a conspicuous feature of many volcanoes, especially those currently or recently active. It is typically 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 great depth. During certain types of climactic eruptions, the volcano's magma chamber may empty enough for an area above it to subside, forming what may appear to be a crater but is actually known as a caldera.
In the majority of typical volcanoes, the crater is situated atop the mountain formed from the erupted volcanic deposits such as lava flows and tephra. Volcanoes that terminate in a principal crater are usually of a conical form. Other volcanic craters may be found on the flanks of volcanoes. Some volcanic craters form by collapse with little or no explosive activity.
Some volcanoes, such as maars, consist of a crater alone, with scarcely any mountain at all. These volcanic explosion craters are formed when magma rises through water-saturated rocks and causes a phreatic eruption. Volcanic craters from phreatic eruptions often occur on plains away from other obvious volcanoes.
Volcanic craters have also been identified on the Moon, Mars, Venus, and Io.
