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Crater lakes (maar) and lava dam lakes formed by lava flows. At the summit of
the cones, or even at the surface of a plateau, a section
of land may collapse into a magma chamber thus forming collapse
craters (or caldera).
If the base is impermeable, a lake may then fill the crater.
This is the origin of both Lake Pavin and the Gour de Tazenat.
Another type of lake may be produced from volcanic activity,
when a lava flow dams up a river. A lake is then formed behind
the cooled lava, as was the case for Lake Aydat and Lake Guéry.
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