Floods from natural rock-material dams

Breached dams formed naturally of rock or rock debris have produced many of the largest floods of Earth history. Two broad classes of natural impoundments are (1) valley-blocking accumulations of mass movements and volcaniclastic debris, and (2) closed basins rimmed by moraines, tectonic depressions, and calderas and craters formed during volcanic eruptions. Each type is restricted to particular geological and geographical environments, making their incidence non-uniform in time and space.