A volcanic crater formed by an explosion caused by magma coming into contact with water, usually wider than it is deep.
From German Maar, which comes from the Moselle Valley in Germany where such formations are common. The word entered English through geological terminology in the 19th century.
Maars are explosive evidence of geology's drama—when hot lava meets groundwater, the blast is so violent it creates wide, shallow craters that often fill with lakes.
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