Phreatic eruption

Phreatic eruptions are steam-driven explosions that occur when water beneath the ground or on the surface is heated by magma, lava, tephra or pyroclastic-flow deposits. The intense heat of such material causes water to flash to steam, thereby generating an explosive eruption.

Craters formed by phreatic explosions when lava flows entered Lake Myvatn, northern Iceland. Photo: J. Alean