Phreatic eruption
Phreatic eruptions occur when rising magma makes contact with ground or surface water. The extreme temperature of the magma (anywhere from 600°C to 1,170°C) causes near-instantaneous evaporation to steam resulting in an explosion of steam, water, ash, rock, and volcanic bombs. At Mount St. Helens hundreds of steam explosions preceded a 1980 plinian eruption of the volcano. A less intense geothermal event may result in a mud volcano.
Phreatic eruptions typically include steam and rock fragments; the inclusion of lava is unusual. 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 dioxide and or hydrogen sulfide gas emissions; both gases are deadly. 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.