Crater lake
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This page describes the geological feature. To see the lake that goes by the same name, see Crater Lake.
A crater lake is a lake that forms in a volcanic calderas or craters after the volcano has been inactive for some time. The best-known crater lake, which bears the same name as the geological feature, is Crater Lake in Oregon state's Mount Mazama. Some crater lakes are acidic, however, some lakes' water remains fresh. Some also have geothermal activity, especially if the volcano is merely dormant rather than extinct.
Maar.jpg
A maar is a broad, low relief crater that is caused by a phreatic eruption or explosion caused by groundwater contact with hot lava or magma. The maar typically fills with water to form a relatively shallow crater lake.
Some crater lakes form in the impact craters left by impact events. These do not tend to develop acidity or geothermal activity, lacking ongoing volcanic processes.