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Mount St. Helens Volcanic Activity
 

Mount St. Helens is 34 miles almost due west of Mount Adams, which is in the eastern part of the Cascade Range. These "sister and brother" volcanic mountains are each about 50 miles from Mount Ranier, the giant of Cascade volcanoes. Mount Hood, the nearest major volcanic peak in Oregon, is about 60 miles southeast of Mount St. Helens.

Mount St. Helens was named for British diplomat Alleyne Fitzherbert (1753-1839), whose title was Baron St. Helens. The mountain was named by Commander George Vancouver and the officers of H.M.S.Discovery while they were surveying the northern Pacific coast from 1792 to 1794.

Mount St. Helens was known as "the Fuji of America" because its symmetrical beauty was similar to that of the famous Japanese volcano. The graceful cone top, whose glistening cap of perennial snow and ice dazzled the viewer, is now largely gone. On May 18, 1980, the missing mountaintop was transformed in a few hours into the extensive volcanic ash that blanketed much of the Northwestern United States and into various other deposits closer to the mountain.

Even before its recent loss of height, Mount St. Helens was not one of the highest peaks in the Cascade Range. Its summit altitude of 9,677 feet made it only the fifth highest peak in Washington. It stood out handsomely, however, from surrounding hills because it rose thousands of feet above them and had a perennial cover of ice and snow. The mountain is about 6 miles across at its base, which is at an altitude of about 4,400 feet on the northeastern side and about 4,000 feet elsewhere. At the pre-eruption timberline (upper limit of trees), the width of the cone was about 4 miles.

The pre-eruption landscape was dominated by dense coniferous forests and clear streams and lakes. Streams and lakes were cold and contained low levels of nutrients and organisms. Productive resident and anadromous fisheries made these fast-flowing streams and clear lakes popular recreational areas.

The catastrophic eruption on May 18, 1980 was preceded by 2 months of intense activity that included more than 10,000 earthquakes, hundreds of small phreatic (steam blast) explosions, and the outward growth of the volcano's entire north flank by more than 80 meters. A magnitude 5.1 earthquake struck beneath the volcano at 8:32 a.m. on May 18, setting in motion the devastating eruption.

Within seconds of the earthquake, the volcano's bulging north flank slid away in the largest landslide in recorded history, triggering a destructive, lethal lateral blast of hot gas, steam, and rock debris that swept across the landscape as fast as 1,100 kilometers per hour. The lateral blast, which lasted only the first few minutes of a 9-hour continuous eruption, devastated more than 150 square miles of forest and recreation area, killed countless animals, and left about 60 persons dead or missing.

Temperatures within the blast reached as high as 300 degrees Celsius. Snow and ice on the volcano melted, forming torrents of water and rock debris that swept down river valleys leading from the volcano. Within minutes, a massive plume of ash thrust 15 miles into the sky, where the prevailing wind carried about 490 tons of ash across 57,000 square kilometers of the Western United States.

The 9-hour eruption, the huge debris avalanche that immediately preceded it, and intermittent eruptions during the following 3 days removed about 4 billion cubic yards (0.7 cubic mile) of new magmatic material and of the upper and northern parts of the mountain, including about 170 million cubic yards (0.03 cubic mile) of glacial snow and ice. The eruption caused pyroclastic flows and mudflows, the largest of which produced deposits so extensive and voluminous that they reached and blocked the shipping channel of the Columbia River about 70 river miles from the volcano.

Following the 1980 explosive eruption, more than a dozen extrusions of thick, pasty lava built a mound-shaped lava dome in the new crater. The dome is about 1,100 meters in diameter and 250 meters tall.


 
 
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