- Phreatic eruption
A Phreatic eruption, also called an ultravulcanian eruption, occurs when rising
magma makes 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 tosteam resulting in an explosion of steam, water, ash, rock, andvolcanic bomb s. AtMount St. Helens hundreds of steam explosions preceded a 1980plinian eruption of thevolcano . A less intense geothermal event may result in amud volcano . In 1949,Thomas Jaggar described this kind of activity as steam-blast eruptions.Phreatic eruptions typically include steam and rock fragments; the inclusion of lava is unusual. The temperature of the fragments can range from cold toincandescent . 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 bycarbon dioxide orhydrogen sulfide gas emissions. The former canasphyxiate at 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 , inHawaii , 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 ofSurtsey , the 1965 eruption ofTaal Volcano , and the 1982Mount Tarumae eruption.References
* [http://volcanoes.usgs.gov/Products/Pglossary/HydroVolcEruption.html USGS Photo Glossary Entry for "Phreatic Eruption"]
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