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Lakes are formed by many different geologic processes, but one of the most dramatic is a maar. A maar is a low-relief crater caused by a phreatomagmatic eruption. This is a situation where ground water comes in contact with lava or magma and the resulting steam causes an explosion. This digs out a hole in the ground, creating the maar crater. And then, it usually fills back in to create a lake. The word “maar” comes from the German word, which is derived from the Latin word “mare” (sea).
Maars can occur anywhere in the world where magma comes in contact with ground water, and can range in size from 10 meters to 8 km across. They’re surrounded by a low rim composed of loose fragments of volcanic rocks and rocks torn from the ground when the explosive eruption happened. They can be 10 to 200 meters deep.
The largest known maar is found on the Seward Peninsula in northwest Alaska, and range in size from 4-8 km across. These maars are so large because the magma encountered large regions of permafrost, creating huge explosions.
A maar is related to a tuff ring. In the case of a tuff ring, the crater edge is raised above ground level. An even more dramatic tuff cone can rise up 300 meters above the surroundings.
Meteor crater in Arizona was once thought to be a maar, but geologists now know that it was created by a meteor impact about 50,000 years ago.
We have also recorded an episode of Astronomy Cast about Earth, as part of our tour through the Solar System – Episode 51: Earth.