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On May 18, 1980 a major volcanic eruption occurred at Mount St. Helens, a volcano located in the state of Washington, United States. The eruption (a VEI 5 event) was the only significant one to occur in the contiguous 48 U.S. states since the 1915 eruption of Lassen Peak in California.〔 The eruption was preceded by a two-month series of earthquakes and steam-venting episodes, caused by an injection of magma at shallow depth below the volcano that created a huge bulge and a fracture system on the mountain's north slope. An earthquake at 8:32:17 a.m. PDT (UTC−7) on Sunday, May 18, 1980 caused the entire weakened north face to slide away, creating the largest landslide ever recorded. This suddenly exposed the partly molten, gas- and steam-rich rock in the volcano to lower pressures. The rock responded by exploding a hot mix of lava and pulverized older rock toward Spirit Lake so fast that it overtook the avalanching north face. An eruption column rose into the atmosphere and deposited ash in 11 U.S. states.〔 At the same time, snow, ice and several entire glaciers on the volcano melted, forming a series of large lahars (volcanic mudslides) that reached as far as the Columbia River, nearly to the southwest. Less severe outbursts continued into the next day, only to be followed by other large, but not as destructive, eruptions later that year. Approximately fifty-seven people were killed directly, including innkeeper Harry R. Truman, photographer Reid Blackburn and geologist David A. Johnston. Hundreds of square miles were reduced to wasteland, causing over a billion U.S. dollars in damage ($2.88 billion in 2014 dollars〔As given by (【引用サイトリンク】title=Inflation Calculator )〕), thousands of game animals were killed, and Mount St. Helens was left with a crater on its north side. At the time of the eruption, the summit of the volcano was owned by the Burlington Northern Railroad, but afterward the land passed to the United States Forest Service.〔 The area was later preserved, as it was, in the Mount St. Helens National Volcanic Monument. == Build-up to the eruption == Mount St. Helens remained dormant from its last period of activity in the 1840s and 1850s until March 1980. Several small earthquakes, apparently beginning on March 15, indicated that magma may have begun moving below the volcano.〔 Then on March 18, at 3:45 p.m. Pacific Standard Time (all times will be in PST or PDT), a shallow magnitude 4.2 earthquake (the initial reading was 4.1), centered below the volcano's north flank,〔 signaled the volcano's violent return from 123 years of hibernation.〔Harris, ''Fire Mountains of the West'' (1988), p. 202〕 A gradually building earthquake swarm saturated area seismographs and started to climax at about noon on March 25, reaching peak levels in the next two days, including an earthquake registering 5.1 on the Richter scale. A total of 174 shocks of magnitude 2.6 or greater were recorded during those two days. Shocks of magnitude 3.2 or greater occurred at a slightly increasing rate during April and May with five earthquakes of magnitude 4 or above per day in early April, and eight per day the week before May 18.〔 Initially there was no direct sign of eruption, but small earthquake-induced avalanches of snow and ice were reported from aerial observations. At 12:36 p.m. on March 27, phreatic eruptions (explosions of steam caused by magma suddenly heating groundwater) ejected and smashed rock from within the old summit crater, excavating a new crater wide〔 and sending an ash column about into the air.〔 By this date a eastward-trending fracture system had also developed across the summit area.〔Harris, ''Fire Mountains of the West'' (1988), p. 204〕 This was followed by more earthquake swarms and a series of steam explosions that sent ash above their vent.〔 Most of this ash fell between from its vent, but some was carried south to Bend, Oregon or east to Spokane, Washington.〔Harris, ''Fire Mountains of the West'' (1988), p. 203〕 A second, new crater and a blue flame were observed on March 29.〔 The flame was visibly emitted from both craters and was probably created by burning gases. Static electricity generated from ash clouds rolling down the volcano sent out lightning bolts that were up to long.〔 Ninety-three separate outbursts were reported on March 30,〔 and increasingly strong harmonic tremors were first detected on April 1, alarming geologists and prompting Governor Dixy Lee Ray to declare a state of emergency on April 3.〔 Governor Ray issued an executive order on April 30 creating a "red zone" around the volcano; anyone caught in this zone without a pass faced a $500 fine or six months in jail. This precluded many cabin owners from visiting their property. By April 7 the combined crater was 1,700 feet long by 1,200 ft wide and 500 ft deep (). A USGS team determined in the last week of April that a section of St. Helens' north face was displaced outward by at least .〔 For the rest of April and early May this bulge grew by per day, and by mid-May it extended more than north.〔 As the bulge moved northward, the summit area behind it progressively sank, forming a complex, down-dropped block called a graben. Geologists announced on April 30 that sliding of the bulge area was the greatest immediate danger and that such a landslide might spark an eruption. These changes in the volcano's shape were related to the overall deformation that increased the volume of the volcano by by mid-May. This volume increase presumably corresponded to the volume of magma that pushed into the volcano and deformed its surface. Because the intruded magma remained below ground and was not directly visible, it was called a cryptodome, in contrast to a true lava dome exposed at the surface. On May 7, eruptions similar to those in March and April resumed, and over the next days the bulge approached its maximum size.〔 All activity had been confined to the 350-year-old summit dome and did not involve any new magma. A total of about 10,000 earthquakes were recorded prior to the May 18 event, with most concentrated in a small zone less than directly below the bulge.〔 Visible eruptions ceased on May 16, reducing public interest and consequently the number of spectators in the area. Mounting public pressure then forced officials to allow 50 carloads of property owners to enter the danger zone on May 17 to gather whatever property they could carry.〔 Another trip was scheduled for 10 a.m. the next day.〔 Because that next day was Sunday, more than 300 loggers would not be working in the area. By the time of the climactic eruption, dacite magma intruding into the volcano had forced the north flank outward nearly and heated the volcano's groundwater system, causing many steam-driven explosions (phreatic eruptions). 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「1980 eruption of Mount St. Helens」の詳細全文を読む スポンサード リンク
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