phreatic eruption

^{ALASKA VOLCANO OBSERVATORY GLOSSARY}

An explosive volcanic eruption caused when water and heated volcanic rocks interact to produce a violent expulsion of steam and pulverized rocks. Magma is not involved.

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^{GLOSSARY OF VOLCANIC TERMS}

A steam eruption, commonly associated with water, mud, and other earth materials, that is caused when groundwater, heated by a magma, flashes (and explosively expands) into steam (Harris, 2000, p. 1301). Phreatic eruptions expel no juvenile (magmatic) material, and are commonly the precursor to magmatic eruptive activity.

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^{USGS PHOTO GLOSSARY OF VOLCANIC TERMS}

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.

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Photograph by D.A. Swanson on 4 April 1980

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.