- Phreatic eruption
A Phreatic eruption, also called an ultravulcanian eruption, occurs when rising
magmamakes contact with ground or surface water. The extreme temperature of the magma (anywhere from 600 °C to 1,170 °C (1110–2140 °F)) causes near-instantaneous evaporation to steamresulting in an explosion of steam, water, ash, rock, and volcanic bombs. At Mount St. Helenshundreds of steam explosions preceded a 1980 plinianeruption of the volcano. A less intense geothermal event may result in a mud volcano. In 1949, Thomas Jaggardescribed this kind of activity as steam-blast eruptions. Phreaticeruptions typically include steam and rock fragments; the inclusion of lava is unusual. The temperature of the fragments can range from cold to incandescent. If molten material is included, the term phreato-magmatic may be used. These eruptions occasionally create broad, low-relief craters called " maar". Phreatic explosions can be accompanied by carbon dioxideor hydrogen sulfidegas emissions. The former can asphyxiateat sufficient concentration; the latter is a broad spectrum poison. A 1979 phreatic eruption on the island of Java killed 149 people, most of whom were overcome by poisonous gases.
It is believed the 1883 eruption of
Krakatoa, which obliterated most of the volcanic island and created the loudest sound in recorded human history, was a phreatic event. Kilauea, in Hawaii, has a long record of phreatic explosions; a 1924 phreatic eruption hurled rocks estimated at eight tons up to a distance of one kilometer. Additional examples are the 1963–65 eruption of Surtsey, the 1965 eruption of Taal Volcano, and the 1982 Mount Tarumaeeruption.
* [http://volcanoes.usgs.gov/Products/Pglossary/HydroVolcEruption.html USGS Photo Glossary Entry for "Phreatic Eruption"]
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