The Spectacular Eruption of Mount St. Helens
A The eruption in May 1980 of Mount St. Helens, Washington State, astounded the world with
its violence. A gigantic explosion tore much of the volcano's summit to fragments; the energy
relead was equal to that of 500 of the nuclear bombs that destroyed Hiroshima in 1945.展览的意思
B The event occurred along the boundary of two of the moving plates that make up the Earth's
crust. They meet at the junction of the North American continent and the Pacific Ocean. One
edge of the continental North American plate over-rides the oceanic Juan de Fuca micro-plate,
producing the volcanic Cascade range that includes Mounts Baker, Rainier and Hood, and Lasn
Peak as well as Mount St. Helens.
C Until Mount St. Helens began to stir, only Mount Baker and Lasn Peak had shown signs of
life during the 20th century. According to geological evidence found by the United States
slack
Geological Survey, there had been two major eruptions of Mount St. Helens in the recent
(geologically speaking)past: around 1900 B.C., and about A.D. 1500. Since the arrival of
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Europeans in the region, it had experienced a single period of spasmodic activity, between 1831
and 1857. Then, for more than a century, Mount St. Helens lay dormant.
D By 1979, the Geological Survey, alerted by signs of renewed activity, had been monitoring
the volcano for 18 months. It warned the local population against being deceived by the
mountain's outward calm, and forecast that an eruption would take place before the end of the
century. The inhabitants of the area did not have to wait that long. On March 27, 1980,a few
belong
clouds of smoke formed above the summit , and slight tremors were felt. On the 28th, larger and
darker clouds,. consisting of gas and ashes,. emerged and climbed as high as 20,000 feet. In April
a slight lull ensued, but the volcanologists remained pessimistic. The, in early May, the northern
flank of the mountain bulged, and the summit ro by 500 feet.
i miss you什么意思E Steps were taken to evacuate the population. Most- campers, hikers, timbercuttersleft the
slopes of the mountain. Eighty-four-year-old Harry Truman, a holiday lodge owner who had lived
there for more than 50 years, refud to be evacuated, in spite of official and public, including an
entire class of school children, wrote to him, begging him to leave. He never did.
F On May 18, at 8.32 in the morning, Mount St. Helens blew its top. literally. Suddenly, it was
1300 feet shorter than it had been before its growth had begun. Over half a cubic mile of rock had
disintegrated . At the same moment, an earthquake with an intensity of 5 on the Richter scale was
recorded. It triggered an avalanche of snow and ice. mixed with hot rock-the entire north face of
the mountain had fallen away. A wave of scorching volcanic gas and rock fragments shot
horizontally from the volcano's riven flank, at an inescapable 200 miles per hour. As the sliding
ice and snow melted, it touched off devastating torrents of mud and debris,
which destroyed all
life in their path. Pulverid, which destroyed all life in their path. Pulverid rock climbed as a
dust cloud into the atmosphere. Finally, viscous lava, accompanied by burning clouds of ash and
gas, welled out of volcano's new crater, and from lesr vents and cracks in its flanks.
G Afterwards, scientists were able to analy the quence of events. First, magmamolten
rock-at temperatures above 2000oF. had surged into the volcano from the Earth's mantle. The
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build-up was accompanied by an accumulation of gas, which incread as the mass of magma
grew. It was the pressure inside the mountain that made it swell. Next, the ri in gas pressure
caud a violent decompression. Which ejected the shattered summit like a cork from a shaken
soda bottle. With the summit gone, the molten rock within was relead in a jet of gas and
fragmented magma, and lava welled from the crater.
雅然培训H The effects of the Mount St. Helens eruption were catastrophic. Almost卫衣
all the trees of the
surrounding forest, mainly Douglas firs. were flattened. and their branches and bark ripped off by
the shock wave of the explosion. Ash and mud spread over nearly 200 square miles of country. All
the towns and ttlements in the area were smothered in an even coating of ash. V olcanic ash silted
up the Columbia River 35 miles away, reducing the debris that accumulated at the foot of the
volcano reached a depth. in places, of 200 feet.
I The eruption of Mount St. Helens was one of the most cloly obrved and analyd in
history. Becau geologists had been expecting the event, they were able to amass vast amounts of
technical data when it happened. Study of atmospheric particles formed as a result of the
explosion showed that droplets of sulphuric acid, acting as a screen between the Sun and the敌特
Earth's surface, caud a distinct drop in temperature. There is no doubt that the activity of Mount
St. Helens and other volcanoes since 1980 has influenced our climate . Even so, it has been
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