Crater lake
This page describes the geological feature. For the lake of 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.
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 craters formed by impact events. Lacking ongoing volcanic processes, these do not tend to develop acidity or geothermal activity.