Phreatic 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 1,170°C) 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