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Lesson 2: Creators and Destroyers
On May 18, 1980, Mount St. Helens erupted violently. At 8:32 a.m. Pacific Daylight Time, a magnitude 5.1 earthquake occurred about a mile beneath the volcano, triggering a catastrophic series of events that transformed Mount St. Helens' picturesque mountain landscape into a gray wasteland.
The Catastrophic Eruption
The earthquake shook the walls of the volcano's summit crater and triggered many small rock avalanches.Within seconds, a huge slab of the volcano's north flank began to slide, and small dark clouds billowed out of the base of the slide. Plumes of steam and ash also rose from the volcano's crater. As the avalanche of rock and ice raced down the mountain's north flank at more than 250 kilometers per hour (155 miles per hour), a massive explosion blasted out of the north side of the volcano. This lateral blast became a fearsome torrent of ash and rock that outraced the avalanche. Probably no more than 20 to 30 seconds had elapsed since the triggering earthquake!
The Eruption Was No Surprise
The eruption of Mount St. Helens was not a surprise. For nearly 2 months, scientists had been monitoring changes at Mount St. Helens. For a volcano to erupt, magma must move to the Earth's surface. Increased earthquake activity, eruptions of steam and ash, and changes in the shape of the surface of the volcano all signal that magma is on the move toward the surface.
Inside the volcano, the solid rock that surrounds the molten rock often cracks from the increased pressure and causes earthquakes. Between March 20 and May 18, more than 10,000 earthquakes were recorded beneath Mount St. Helens. The largest of these were felt by people living near the volcano. In addition to recording the discrete jolts characteristic of earthquakes, seismographs also detected continuous rhythmic vibrations called harmonic tremors. These numerous small earthquakes were further evidence that magma was moving within the volcano.
As magma made room for itself inside the volcano's cone, the surface of the volcano swelled, or inflated. By early April, Mount St. Helens' north flank began to visibly bulge and crack. The bulge grew 2 to 3 meters (7 to 9 feet) a day and it moved outwards about 150 meters (450 feet) in 2 months.
When the 5.1 magnitude earthquake shook Mount St. Helens on May 18, 1980, the bulge collapsed. The resulting avalanche was the largest volcanic avalanche recorded in historical times. In turn, the sudden removal of masses of rock and ice by the avalanches triggered an explosive eruption of steam trapped in cracks and voids in the volcano and of gases dissolved in the magma. Unleashed by the abrupt release of pressure, magma, rock, ash, aerosols, and gases exploded from within the volcano's north flank.
The Mountain is Transformed
In a few minutes, Mount St. Helens' symmetrical cone was transformed. It was 400 meters (1,312 feet) shorter and a gaping crater was gouged into its north side. An avalanche of rock, ash, ice, water, and fallen trees flowed as far as 9 kilometers (15 miles) down the valley of the North Fork Toutle River. Debris dumped into Spirit Lake raised the lakebed by more than 940 meters (295 feet). The lake's cool, crystal-clear waters became a black stew of rocks, mud, and floating trees. Gone were 70 percent of the glaciers that had crowned the volcano, melted by the heat of the eruption or carried away by the fast-moving avalanche. Towering forests with trees up to 45 meters (150 feet) were flattened and strewn like match sticks in the wake of the lateral blast and debris-laden avalanche.
Eruptions Continue
Between May 18, 1980, and October 1995, Mount St. Helens has had at least 21 eruptions of magma and dozens of smaller gas explosions. All of the volcanic activity has taken place in the bottom of the crater created by the May 18, 1980, eruption. There Mount St. Helens is rebuilding itself. During each eruption, new lava squeezes up and pushes aside old material from the surface of the dome. The volcanic activity that began in 1980 is not yet over.
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