E Steps were taken to evacuate the population. Most- campers, hikers, timbercuttersleft the</p>slopes of the mountain. Eighty-four-year-old Harry Truman, a holiday lodge owner who had lived d2 |. q6 s! E9 z
there for more than 50 years, refused to be evacuated, in spite of official and public, including an8 A- b) w1 k' |
entire class of school children, wrote to him, begging him to leave. He never did.; b" E n4 _2 E1 S
* h: K: _! E4 E9 [
F On May 18, at 8.32 in the morning, Mount St. Helens blew its top. literally. Suddenly, it was0 }7 g% B" P- ?* u
1300 feet shorter than it had been before its growth had begun. Over half a cubic mile of rock had& d, J. F+ \$ I% F; }, I
disintegrated . At the same moment, an earthquake with an intensity of 5 on the Richter scale was
+ W+ i2 J2 b& D3 x8 Urecorded. It triggered an avalanche of snow and ice. mixed with hot rock-the entire north face of
R" P5 W0 W+ H1 M" E9 _the mountain had fallen away. A wave of scorching volcanic gas and rock fragments shot& L, L# R0 j. H( `* b: @: d
horizontally from the volcano's riven flank, at an inescapable 200 miles per hour. As the sliding
( ^, E0 i& j2 vice and snow melted, it touched off devastating torrents of mud and debris, which destroyed all
1 ^( P4 F3 `8 W0 D/ [( A$ qlife in their path. Pulverised, which destroyed all life in their path. Pulverised rock climbed as a
* M& s. ~9 x3 @- T, \' Ydust cloud into the atmosphere. Finally, viscous lava, accompanied by burning clouds of ash and
2 c: p6 M a, |- \, A% N+ T& dgas, welled out of volcano's new crater, and from lesser vents and cracks in its flanks.7 _- K# K& o9 l o+ H+ @, `
2 Z5 i) z0 q; l* ]6 EG Afterwards, scientists were able to analyse the sequence of events. First, magmamolten
) L% S) h/ y3 s x* S r' }rock-at temperatures above 2000oF. had surged into the volcano from the Earth's mantle. The
! h8 h, E- J. h; p: J. b, B# q! Zbuild-up was accompanied by an accumulation of gas, which increased as the mass of magma7 c* ~/ O- d- S7 G1 N4 K- ]
grew. It was the pressure inside the mountain that made it swell. Next, the rise in gas pressure
" O8 `; Z9 @; Y6 E5 Qcaused a violent decompression. Which ejected the shattered summit like a cork from a shaken( s! e* \( Z- a# O) p$ Z" V! f
soda bottle. With the summit gone, the molten rock within was released in a jet of gas and
9 W3 Z# p! K( p* I( Z) S7 efragmented magma, and lava welled from the crater.
1 N9 I! U9 ^! x' k
; q7 R* A& E, ]: kH The effects of the Mount St. Helens eruption were catastrophic. Almost all the trees of the
+ z* v! [6 i X! m2 S* Z. Rsurrounding forest, mainly Douglas firs. were flattened. and their branches and bark ripped off by6 F6 W' C+ S4 A0 Y. T f
the shock wave of the explosion. Ash and mud spread over nearly 200 square miles of country. All" s$ b' b5 s1 _' K, c2 [
the towns and settlements in the area were smothered in an even coating of ash. Volcanic ash silted
a/ X( A. w3 T( P; I/ R& j" E0 dup the Columbia River 35 miles away, reducing the debris that accumulated at the foot of the
- O) M9 ^* f9 o+ Fvolcano reached a depth. in places, of 200 feet.! f5 j# J' G6 r8 T, u# S, ~
- V" c' s3 Z% S* y5 ~: O: }0 t( yI The eruption of Mount St. Helens was one of the most closely observed and analysed in D) E8 y. G$ i& N
history. Because geologists had been expecting the event, they were able to amass vast amounts of8 k- l8 _1 Y2 w1 Z4 n
technical data when it happened. Study of atmospheric particles formed as a result of the
- Q5 U& ^. L" m: Uexplosion showed that droplets of sulphuric acid, acting as a screen between the Sun and the
4 v2 p6 {6 c' a' ^* eEarth's surface, caused a distinct drop in temperature. There is no doubt that the activity of Mount
( Z; Q/ V$ M5 r: \* X- \St. Helens and other volcanoes since 1980 has influenced our climate . Even so, it has been) S& }9 A0 B8 m8 }+ U( M& b
calculated that the quantity of dust ejected by Mount St. Helens - a quarter of a cubic mile- was
+ f8 q- Y7 O: X% T$ l' R7 rnegligible in comparison with that thrown out by earlier eruptions, such as that of Mount Katmai! \! ]$ ~3 }: \9 S- r
in Alaska in 1912 (three cubic miles). The volcano is still active. Lava domes have formed inside
/ c5 B+ R( c/ h( Gthe new crater, and have periodically burst. The threat of Mount St Helens lives on.( h# K- [. i9 g/ K
.Questions 1 and 2
1 b. S) D: Z" q- m% pAnswer questions 1 and 2 by writing the appropriate letter A-I inboxes 1 and 2 on your answer& x3 W ^+ U" l8 N- }( r+ R- J6 c* W
sheet.6 e) P* ` c7 R' F
# T+ M4 o. e7 w( i
首页1 2 3尾页 |