第三篇
' f6 `5 Q* f! O# W& U- K" r The Exploding Lakes of Cameroon
% o2 T1 A7 i7 K2 B1 } What comes to mind when you think of a lake? You probably imagine a pretty scene with
6 Z' d$ k2 u5 ]- H I2 A/ L3 p/ O blue water,birds,and fish.For the people in the northwestern Cameroon,however, the image is
" n8 S& M3 F# @3 w3 R very different. For them,lakes may mean terrible disasters. In 1984,poisonous gases exploded
$ O. R7 M2 E. M: N% R o" K out of Lake Monoun and came down into the nearby villages,killing thirty—seven people.Two. C* F8 Y, l0 F
years later,Lake Nyos erupted A cloud of gases rolled down the hills and into the valleys and* I2 M5 U, q3 z+ @- C6 k# S* s
killed 1,700 people.
0 {( L' T8 {; o Lake Nyos and Lake Monoun are crater(火山口)lakes They were formed when water
7 t0 Y) F: N* G; h8 l2 b( i5 ~ collected in the craters of old volcanoes The volcanoes under Lake Nyos and Lake Monoun are
# S4 i) n* B( m8 _( V) a( {7 Q not active anymore. However, poisonous gases from the center of the earth continue to flow up* x0 f7 v% Q( M$ c- d, d9 f4 }
through cracks in the bottom of the lake. This is normal in a crater lake. In most crater lakes,these5 A+ f, ? D8 ?. `# ~
gases are released often because the water‘turns over’regularly.That is,the water from the
+ B4 r7 }6 G( R bottom of the lake rises and mixes with the water at the top,allowing the gases to escape slowly.
, } F. C4 B- o! A; S However,in Lakes Nyos and Monoun,there is no regular turning over.No one knows the
0 V1 s# v& o2 X, t- e reason for this fact,but as a result,these lakes have more gases tapped at the bottom than other L# P2 f5 I. h! i/ _
crater lakes. In fact,scientists who have studied Lakes Nyos and Monoun have found 16,000& F+ f. Q9 @$ ?+ q1 t* j1 q. |
times more gases.When a strong wind,cool weather a storm,or a landslide(滑坡)causes the% `/ {% ?: q- z5 V" _
water to turn over suddenly,the gases escape in a violent explosion.: ^ `! z$ Q1 _" \7 e% R0 c
In the past,no one knew when the gases might explode,so there was no way for the villagers
. v K+ y5 L( {% ~( S* j* m2 p, @ to escape disaster. Now scientists from the United States,France,and Cameroon have found a
2 ^# z' D9 K7 Z y, L way to reduce the gas pressure at the bottom of Lake Nyos.They stood a 672-foot plastic pipe in
" b& C* f, @$ c$ V the middle of the lake,with one end of the pipe near the bottom and the other end in the air.Near
2 ^% V; B. Y& A$ j7 c# f* j the top of the pipe,the team put several holes that could be opened or closed by a computer.Now," z7 @8 i0 B( Z5 M/ q. \" S* p" B- T2 p) [
when the gas pressure gets too high,the holes are opened and some of the gas-filled water shoots* R: Z, \+ d) ~ O" c
up through the pipe into the air like a fountain.With less pressure,a disastrous explosion is much
* t9 _3 m, u6 g; i% `. _ less likely.However, the scientists are not sure that one pipe will be enough to prevent explosions.They hope to put in others soon and they plan to install a similar pipe and a computer system at Lake Monoun as well.2 Y. \) l- R; @/ H
To protect people nearby until all of the pipes are in place.the scientists have installed early9 b! j. \% t I0 O$ J9 d
warning systems at both lakes. If the gas pressure rises to a dangerous level,computers will set/ @& _! Y# K6 u+ N* A
off loud sirens(警报)and bright lights to warn the people in the villages.That way, they will
2 H: Z5 w( u2 e/ _" Q3 ] b have time to escape from the dangerous gases.8 r$ B+ K' |* l# I8 r
41 What will happen when Lake Nyos and Lake Monoun explode?
/ V! J! E8 U% Y V& t' S* w7 [$ e. I A Water will flow down the hills
2 f& r% e. U8 b f3 s B Poisonous gases will be released suddenly." N. U* _( e2 a' L& t2 s! A3 M3 Y1 ^0 i
C A strong wind will rise from the lakes
" _3 b5 a8 W5 G7 n3 p1 Z! \$ O D The volcanoes will come to life.; y2 f) E4 ], y! g: x
42 Which of the following statements about Lake Nyos and Lake Monoun is true?
4 ^9 e4 X5 ~1 \( x! P/ b g Z A They were formed in 1984.
; R$ @( v2 A, V1 @ B They are at the top of two active volcanoes.8 I! A; Y- B$ u9 r8 x! a. r
C They are not like most other crater lakes.$ f7 b; X. U$ q5 x& z* K
D Water in them turns over regularly.
2 {$ V T( ~7 q1 t: A 43 Lake Nyos and Lake Monoun explode because/ `- u3 i6 ]* p8 t, R+ X( u6 t
A the gases rise to the top and mix with air.0 N2 x" b" o) I, o) h5 F
B people from the villages turn over the water.
/ g" `2 ?/ x0 s C scientists have put in a computer system.- n7 b: P; ?- z: Z7 i# \7 y5 p0 e" C
D they have more gases trapped at the bottom than other crater lakes9 j6 K, D0 Y B) E9 e: }
44 A team of scientists has
% ?& P1 M9 d% c$ ~ A erected a pressure-releasing pipe in the lake
3 J- ^" Z5 ?8 P) I3 F. q* g2 B B identified the gases at the bottom of the lake
w) [ ?* Z8 `$ V2 T C built a beautiful fountain near the lakes
6 x3 x9 F, q: n+ c8 I D removed all dangerous gases from the lakes
' ]- H- c; y1 \6 v9 E* V 45 What do we learn from the last paragraph?
3 i4 H6 i3 I! P: Q5 R3 _5 n A Scientists are planning to install pipes in all crater lakes., @7 Z* k; g' N$ v H. F6 x
B Scientists still do not know how to prevent gas explosions
3 J$ l' i% g5 e3 n* N C Explosion disasters could be avoided in the future
8 p& [0 s. m% x" M. g D Warning systems have been set up in the villages nearby. |