A phreatic eruption, also called a phreatic explosion, ultravulcanian eruption or steam-blast eruption, occurs when magma heats ground water or surface water. The extreme temperature of the magma causes near-instantaneous evaporation of water to steam, resulting in an explosion of steam, water, ash, rock, and volcanic bombs. At Mount St. Helens in Washington state, hundreds of steam explosions preceded the 1980 Plinian eruption of the volcano. A less intense geothermal event may result in a mud volcano.
Phreatic eruption at the summit of Mount St. Helens, Washington, in the spring of 1980
Mount St. Helens is an active stratovolcano located in Skamania County, Washington, in the Pacific Northwest region of the United States. It lies 52 miles (83 km) northeast of Portland, Oregon, and 98 miles (158 km) south of Seattle. Mount St. Helens takes its English name from that of the British diplomat Alleyne Fitzherbert, 1st Baron St Helens, a friend of explorer George Vancouver who surveyed the area in the late 18th century. The volcano is part of the Cascade Volcanic Arc, a segment of the Pacific Ring of Fire.
3,000 ft (0.9 km) high steam plume on May 19, 1982, two years after the 1980 major eruption
Mount St. Helens pictured the day before the 1980 eruption, which removed much of the northern face of the mountain, leaving a large crater
East Dome on the east flank of Mount St. Helens in 2013.
The symmetrical appearance of St. Helens prior to the 1980 eruption earned it the nickname "Mount Fuji of America". The once-familiar shape was formed out of the Kalama and Goat Rocks eruptive periods.