Phreatic eruption

Phreatic eruptions are steam-driven explosions that occur when water beneath the ground or on the surface is heated by magma, lava, hot rocks, or new volcanic deposits (for example, tephra and pyroclastic flow deposits). The intense heat of such material (as high as 1,170° C for basaltic lava) may cause water to boil and flash to steam, thereby generating an explosion of steam, water, ash, blocks, and volcanic bombs.

Related Sites

USGS Photo Glossary Entry for Phreatic Eruption (http://volcanoes.usgs.gov/Products/Pglossary/HydroVolcEruption.html)