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Mount St. Helens

Position of center of photo (Lat/Long): [46.20009/-122.18819]

Mount St. Helens

Map: Mount St. HelensOn May 18, 1980, the Mount St. Helens volcano erupted. A series of earthquakes preceded the eruption and triggered a collapse of the north side of the mountain into a massive landslide. This avalanche coincided with a huge explosion that destroyed more than 700 square kilometers (270 square miles) of forest in a few seconds, and sent a billowing cloud of ash and smoke approximately 24 kilometers (15 miles) into the atmosphere. The height of Mt. St. Helens was reduced from about 2950 m (9677 ft) to about 2550 m (8364 ft) as a result of the explosive eruption. Because the eruption occurred in an easily accessible region of the United States, Mount St. Helens has provided unprecedented opportunities for U.S. researchers to collect scientific observations of the geology of an active volcano and document the regional ecological impact and recovery from an eruption.

Mount St. Helens is an example of a composite or stratovolcano. These are explosive volcanoes that are generally steep-sided, symmetrical cones built up by the accumulation of debris from previous eruptions and consist of alternating layers of lava flows, volcanic ash and cinder. Some of the most photographed mountains in the world are stratovolcanoes, including Mount Fuji in Japan, Mount Cotopaxi in Ecuador, Mount Hood in Oregon, and Mount Rainier in Washington. The recently erupting Mount Usu on the island of Hokkaido in Japan is also a stratovolcano. Stratovolcanoes are characterized by having plumbing systems that move magma from a chamber deep within the Earth's crust to vents at the surface.

During an International Space Station expedition, astronauts observed and captured this detailed image of the volcano's summit caldera. In the center of the crater sits a lava dome that is 267 meters (876 feet) above the crater floor and is about 1,067 meters (3,500 feet) in diameter. The dome began to form after the 1980 eruption, but there have been no dome-building eruptions for more than a decade.

The upper slopes of the 1980 blast zone begin at the gray-colored region that extends north from the summit of the volcano. The volcanic mud and debris from the eruption choked all of the drainages in the region. The deeply incised valley to the left is the uppermost reach of the South Fork of the Toutle River. Devastating mudslides buried the original Toutle River Valley to an average depth of 46 meters (150 feet), but in places up to 183 meters (600 feet). Even today, heavy precipitation can send unconsolidated volcanic debris downstream. A special dam was constructed on the North Fork of the Toutle River to catch the sediments from moving further downstream. Levees and dredging also help stem further mudslides. The dark green area south of the blast zone is the thickly forested region of the Gifford Pinchot National Forest.

Source of material: NASA

Further information: WikiPedia article on Mount St. Helens

Last Update: 2011-03-30