 Photograph by D.A. Swanson
on 4 April 1980 |
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 bombs. |
| Phreatic eruption at the summit of Mount St. Helens, Washington. Hundreds of these steam-driven explosive eruptions occurred as magma steadily rose into the cone and boiled groundwater. These phreatic eruptions preceded the volcano's plinian eruption on 18 May 1980. | |
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