2014年4月27日托福考前冲刺题(三)_第5页

　　In Death Valley, California, one of the hottest, most arid places in North America, there is much salt, and salt can damage rocks impressively. Inhabitants of areas elsewhere, where streets and highways are salted to control ice, are familiar with the resulting rust and deterioration on cars. That attests to the chemically corrosive nature of salt, but it is not the way salt destroys rocks. Salt breaks rocks apart principally by a process called crystal prying and wedging. This happens not by soaking the rocks in salt water, but by moistening their bottoms with salt water. Such conditions exist in many areas along the eastern edge of central Death Valley. There, salty water rises from the groundwater table by capillary action through tiny spaces in sediment until it reaches the surface.

　　Most stones have capillary passages that suck salt water from the wet ground. Death Valley provides an ultra-dry atmosphere and high daily temperatures, which promote evaporation and the formation of salt crystals along the cracks or other openings within stones. These crystals grow as long as salt water is available. Like tree roots breaking up a sidewalk, the growing crystals exert pressure on the rock and eventually pry the rock apart along planes of weakness, such as banding in metamorphic rocks, bedding in sedimentary rocks, or preexisting or incipient fractions, and along boundaries between individual mineral crystals or grains. Besides crystal growth, the expansion of halite crystals(the same as everyday table salt) by heating and of sulfates and similar salts by hydration can contribute additional stresses. A rock durable enough to have withstood natural conditions for a very long time in other areas could probably be shattered into small pieces by salt weathering within a few generations.

　　The dominant salt in Death Valley is halite, or sodium chloride, but other salts, mostly carbonates and sulfates, also cause prying and wedging, as does ordinary ice. Weathering by a variety of salts, though often subtle, is a worldwide phenomenon. Not restricted to arid regions, intense salt weathering occurs mostly in salt-rich places like the seashore, near the large saline lakes in the Dry Valleys of Antarctica, and in desert sections of Australia, New Zealand, and central Asia.

　　41. What is the passage mainly about?

　　A. The destructive effects of salt on rocks.

　　B. The impressive salt rocks in Death Valley.

　　C. The amount of salt produced in Death Valley.

　　D. The damaging effects of salt on roads and highways.

　　42. The word "it" in line 9 refers to

　　A. salty water

　　B. groundwater table

　　C. capillary action

　　D. sediment

　　43. The word "exert" in line 14 is closest in meaning to

　　A. put

　　B. reduce

　　C. replace

　　D. control

　　44. In lines 13-17, why does the author compare tree roots with　growing salt crystals?

　　A. They both force hard surfaces to crack.

　　B. They both grow as long as water is available.

　　C. They both react quickly to a rise in temperature.

　　D. They both cause salty water to rise from the groundwater table.

　　45. In lines 17-18, the author mentions the "expansion of halite　crystals...by heating and of sulfates and similar salts by hydration　in order to

　　A. present an alternative theory about crystal growth

　　B. explain how some rocks are not affected by salt

　　C. simplify the explanation of crystal prying and wedging

　　D. introduce additional means by which crystals destroy rocks

　　46. The word "durable" in line 19 is closest in meaning to

　　A. large

　　B. strong

　　C. flexible

　　D. pressured

　　47. The word "shattered" in line 20 is closest in meaning to

　　A. arranged

　　B. dissolved

　　C. broken apart

　　D. gathered together

　　48. The word "dominant" in line 22 is closest in meaning to

　　A. most recent

　　B. most common

　　C. least available

　　D. least damaging

　　49. According to the passage, which of the following is true about the

　　effects of salts on rocks?

　　A. Only two types of salts cause prying and wedging.

　　B. Salts usually cause damage only in combination with ice.

　　C. A variety of salts in all kinds of environments can cause weathering.

　　D. Salt damage at the seashore is more severe than salt damage in Death Valley,

　　50. Which of the following can be inferred from the passage about rocks that are found in areas where ice is common?

　　A. They are protected from weathering.

　　B. They do not allow capillary action of water.

　　C. They show similar kinds of damage as rocks in Death Valley.

　　D. They contain more carbonates than sulfates.