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Phreatic EruptionPhreatic eruptions occur when rising hot lava or magma makes contact with either ground water or surface water. The extreme temperature of the magma (600 to as high as 1,170C) causes the water to rapidly flash to steam resulting in an explosion of steam, water, ash, blocks, and volcanic bombs. At Mount St. Helens hundreds of steam explosions preceded a 1980 plinian eruption of the volcano. Phreatic eruptions typically do not include molten lava, just steam and solid rock fragments. The eruptions may leave a crater called a maar, typically a shallow crater. In cases where magma or lava is erupted along with the steam the eruption is called phreato-magmatic. Phreatic explosions are at times accompanied by CO2 and or H2S gas emissions which can be deadly. A phreatic eruption in Java in 1979 killed 149 people, most of whom were overcome by poisonous gases. Kilauea volcano in Hawaii has a long record of phreatic explosions. The 1924 phreatic eruption hurled eight ton blocks of rock a distance of one kilometer. Less intense geothermal situations create some types of mud volcanoes. Related Site USGS Photo Glossary Entry for Phreatic Eruption
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