"Cascade Range Summary"
Mount St. Helens, Washington |
Mount St. Helens Volcano:
Mount St. Helens (2,549 meters - 8,364 feet ((9,677 feet before May 18, 1980)), located in southwestern Washington about 50 miles northeast of Portland, Oregon, is one of several lofty volcanic peaks that dominate the Cascade Range of the Pacific Northwest. Geologists call Mount St. Helens a composite volcano (or stratovolcano), a term for steep-sided, often symmetrical cones constructed of alternating layers of lava flows, ash, and other volcanic debris. Composite volcanoes tend to erupt explosively and pose considerable danger to nearby life and property. In contrast, the gently sloping shield volcanoes, such as those in Hawaii, typically erupt nonexplosively, producing fluid lavas that can flow great distances from the active vents. Although Hawaiian-type eruptions may destroy property, they rarely cause death or injury. Before 1980, snow-capped, gracefully symmetrical Mount St. Helens was known as the "Fujiyama of America." Mount St. Helens, other active Cascade volcanoes, and those of Alaska comprise the North American segment of the circum-Pacific "Ring of Fire," a notorious zone that produces frequent, often destructive, earthquake and volcanic activity. -- Tilling, et.al., 1990 Baron St. Helens: Some Indians of the Pacific Northwest variously called Mount St. Helens "Louwala-Clough," or "smoking mountain." The modern name, Mount St. Helens, was given to the volcanic peak in 1792 by Captain George Vancouver of the British Royal Navy, a seafarer and explorer. He named it in honor of a fellow countryman, Alleyne Fitzherbert, who held the title Baron St. Helens and who was at the time the British Ambassador to Spain. Vancouver also named three other volcanoes in the Cascades - Mounts Baker, Hood, and Rainier - for British naval officers. -- Tilling, et.al., 1990 Eruptive Background: Ancestral Mount St. Helens began to grow before the last major glaciation of the Ice Age had ended about 10,000 years ago. The oldest ash deposits were erupted at least 40,000 years ago onto an eroded surface of still older volcanic and sedimentary rocks. Intermittent volcanism continued after the glaciers disappeared, and nine main pulses of pre-1980 volcanic activity have been recognized. These pulses lasted from about 5,000 years to less than 100 years each and were separated by dormant intervals of about 15,000 years to only 200 years. A forerunner of Spirit Lake was born about 3,500 years ago, or possibly earlier, when eruption debris formed a natural dam across the valley of the North Fork of the Toutle River. The most recent of the pre-1980 eruptive activity began in A.D. 1800 with an explosive eruption, followed by several additional minor explosions and extrusions of lava, and ended with the formation of the Goat Rocks lava dome by 1857. Mount St. Helens is the youngest of the major Cascade volcanoes, in the sense that its visible cone was entirely formed during the past 2,200 years, well after the melting of the last of the Ice Age glaciers about 10,000 years ago. Mount St. Helens' smooth, symmetrical slopes are little affected by erosion as compared with its older, more glacially scarred neighbors - Mount Rainier and Mount Adams in Washington, and Mount Hood in Oregon. The local Indians and early settlers in the then sparsely populated region witnessed the occasional violent outbursts of Mount St. Helens. The volcano was particularly restless in the mid-19th century, when it was intermittently active for at least a 26-year span from 1831 to 1857. Some scientists suspect that Mount St. Helens also was active sporadically during the three decades before 1831, including a major explosive eruption in 1800. Although minor steam explosions may have occurred in 1898, 1903, and 1921, the mountain gave little or no evidence of being a volcanic hazard for more than a century after 1857. Consequently, the majority of 20th-century residents and visitors thought of Mount St. Helens not as a menace, but as a serene, beautiful mountain playground teeming with wildlife and available for leisure activities throughout the year. At the base of the volcano's northern flank, Spirit Lake, with its clear, refreshing water and wooded shores, was especially popular as a recreational area for hiking, camping, fishing, swimming and boating. The tranquility of the Mount St. Helens region was shattered in the spring of 1980, however, when the volcano stirred from its long repose, shook, swelled, and exploded back to life. -- Tilling, et.al., 1990 Mount St. Helens Awakens: The 1980 eruptions of Mount St. Helens in southwestern Washington marked the re-awakening of a relatively young (40,000 years) volcano that had been dormant since 1857. Frequent dacitic eruptions during the previous 2,500 years had produced pyroclastic flows, ash falls, debris flows, lava domes, and lava flows of andesite and basalt. Pyroclastic flows and lahars accompanied most eruptive periods and were largely responsible for forming fans around the base of the volcano, some of which dammed the North Fork Toutle River to form Spirit Lake between 3,300 and 4,000 years ago. The magnitudes of the 1980 eruptions were not exceptional by worldwide historical standards; however, they were the first volcanic eruptions in the conterminous United States since 1914 (Lassen Peak) and focused national attention on events leading up to the climactic eruption of May 18, 1980. That eruption led to exceptional opportunities for scientific observations, data collection, and the study of infrequent and often inaccessible geologic events and processes. -- Simon, 1999 May 18, 1980: 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. 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 19 kilometers into the sky, where the prevailing wind carried about 490 tons of ash across 57,000 square kilometers of the Western United States. -- Brantley, 1994 Following May 18, 1980: Following the most recent major eruption, on May 18, 1980, there were 5 smaller explosive eruptions over a period of 5 months. Thereafter, a series of 16 dome-building eruptions through October 1986 constructed the new, 270-meter- (-880- feet) high, lava dome in the crater formed by the May 18, 1980 eruption. -- Wolfe and Pierson, 1995 MORE about Mount St. Helens:
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