261010机考雅思阅读How deserts are formed

A

A desert is a barren area of landscape where little precipitation occurs and consequently living conditions are hostile for plant and animal life. Deserts have been defined and classified in a number of ways,generally combining total precipitation,number of days on which this falls, temperature,and humidity and sometimes additional factors.In some areas,the deserts are separated by rivers,mountains or other landforms and in other areas,the borders of deserts cannot be determined.

B

The raising of livestock is a major economic activity in semiarid lands,where grasses are generally the dominant type of natural vegetation.The consequences of an excessive number of livestock grazing in an area are the reduction of the vegetation cover and the trampling and pulverization of the soil.This is usually followed by the drying of the soil and accelerated erosion.

C

Sand and dust storms are natural events that occur in arid regions where the land is not protected by a covering of vegetation.Dust storms usually start in desert margins rather than the deserts themselves where the finer materials have already been blown away.As a steady wind begins to blow,fine particles lying on the exposed ground begin to vibrate.At greater wind speeds, some particles are lifted into the air stream.When they land,they strike other particles which may be jerked into the air in their turn, starting a chain reaction. Once ejected, these particles move in one of three possible ways, depending on their size, shape and density.

D

The cultivation of crops has expanded into progressively drier regions as population densities have grown.These regions are especially likely to have periods of severe dryness so that crop failures are common. Since the raising of most crops necessitates the prior removal of the natural vegetation, crop failures leave extensive tracts of land devoid of a plant cover and susceptible to wind and water erosion.

E

In a state of America in 20th century,a large area of farmland had become desert.And measures have been taken to prevent the reoccurring of desertification.

F

People have struggled to live in deserts and the surrounding semi-arid lands for millennia. Nomads have moved their flocks and herds to wherever grazing is available and oases have provided opportunities for a more settled way of life.The cultivation of semi-arid regions encourages erosion of soil and is one of the causes of increased desertification. Some nomads take the desert wherever they go.

G

It is a misconception that the lack of rainfall causes the desertification.Even in some arid areas, the land will recover as soon as rain comes.More desertification is caused by human activities. In the West Africa,a serious desertification causes a large number of people to lose their lives.

H

Although there is tremendous publicity on the severity of desertification,the researchers have never reached a consensus on the reasons and process of desertification.

I

Nowadays,new technologies are being applied to help solve desertification problems.Satellites have been put into use to study the impact of people and livestock on earth.However,other types of technology about the problems and process of desertification are still needed.

Questions 14-20

The Reading Passage has seven paragraphs A-I. Which paragraph contains the following information? NB You may use any letter more than once.

14 the reference of irregular movement of particles 15 an account that a productive land turned into a desert in 20th century 16 the types of deserts 17 the method of tackling the problems of deserts 18 the effect of migration on desertification 19 the lack of agreement among the scientists about the causes of desertification 20 the example of the fatal effects of farming practice