Desert Formation
The deserts, which already occupy approximately a fourth of the Earth's land surface, have in recent decades been increasing at an alarming pace. The expansion of desert like conditions into areas where they did not previously exist is called desertification. It has been estimated that an additional one-fourth of the Earth's land surface is threatened by this process.
Desertification is accomplished primarily through the loss of stabilizing natural vegetation and the subsequent accelerated erosion of the soil by wind and water. In some cases the loose soil is blown completely away, leaving a stony surface. In other cases, the finer particles may be removed, while the sand-sized particles are accumulated to form mobile hills or ridges of sand.
Even in the areas that retain a soil cover, the reduction of vegetation typically results in the loss of the soil's ability to absorb substantial quantities of water. The impact of raindrops on the loose soil tends to transfer fine clay particles into the tiniest soil spaces, sealing them and producing a surface that allows very little water penetration. Water absorption is greatly reduced; consequently runoff is increased, resulting in accelerated erosion rates. The gradual drying of the soil caused by its diminished ability to absorb water results in the further loss of vegetation, so that a cycle of progressive surface deterioration is established.
In some regions, the increase in desert areas is occurring largely as the result of a trend toward drier climatic conditions. Continued gradual global warming has produced an increase in aridity for some areas over the past few thousand years. The process may be accelerated in subsequent decades if global warming resulting from air pollution seriously increases.
There is little doubt, however, that desertification in most areas results primarily from human activities rather than natural processes. The semiarid lands bordering the deserts exist in a delicate ecological balance and are limited in their potential to adjust to increased environmental pressures. Expanding populations are subjecting the land to increasing pressures to provide them with food and fuel. In wet periods, the land may be able to respond to these stresses. During the dry periods that are common phenomena along the desert margins, though, the pressure on the land is often far in excess of its diminished capacity, and desertification results.
Four specific activities have been identified as major contributors to the desertification processes: overcultivation, overgrazing, firewood gathering, and overirrigation. 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.
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.
Firewood is the chief fuel used for cooking and heating in many countries. The increased pressures of expanding populations have led to the removal of woody plants so that many cities and towns are surrounded by large areas completely lacking in trees and shrubs. The increasing use of dried animal waste as a substitute fuel has also hurt the soil because this valuable soil conditioner and source of plant nutrients is no longer being returned to the land.
The final major human cause of desertification is soil salinization resulting from overirrigation. Excess water from irrigation sinks down into the water table. If no drainage system exists, the water table rises, bringing dissolved salts to the surface. The water evaporates and the salts are left behind, creating a white crustal layer that prevents air and water from reaching the underlying soil.
The extreme seriousness of desertification results from the vast areas of land and the tremendous numbers of people affected, as well as from the great difficulty of reversing or even slowing the process. Once the soil has been removed by erosion, only the passage of centuries or millennia will enable new soil to form. In areas where considerable soil still remains, though, a rigorously enforced program of land protection and cover-crop planting may make it possible to reverse the present deterioration of the surface.
Paragraph 1: The deserts, which already occupy approximately a fourth of the Earth's land surface, have in recent decades been increasing at an alarming pace. The expansion of desertlike conditions into areas where they did not previously exist is called desertification. It has been estimated that an additional one-fourth of the Earth's land surface is threatened by this process.
1. The word threatened in the passage is closest in meaning to
Restricted
Endangered
Prevented
Rejected
Paragraph 3: Even in the areas that retain a soil cover, the reduction of vegetation typically results in the loss of the soil's ability to absorb substantial quantities of water. The impact of raindrops on the loose soil tends to transfer fine clay particles into the tiniest soil spaces, sealing them and producing a surface that allows very little water penetration. Water absorption is greatly reduced; consequently runoff is increased, resulting in accelerated erosion rates. The gradual drying of the soil caused by its diminished ability to absorb water results in the further loss of vegetation, so that a cycle of progressive surface deterioration is established.
2. According to paragraph 3, the loss of natural vegetation has which of the following consequences for soil?
Increased stony content
Reduced water absorption
Increased numbers of spaces in the soil
Reduced water runoff
Paragraph 5: There is little doubt, however, that desertification in most areas results primarily from human activities rather than natural processes. The semiarid lands bordering the deserts exist in a delicate ecological balance and are limited in their potential to adjust to increased environmental pressures. Expanding populations are subjecting the land to increasing pressures to provide them with food and fuel. In wet periods, the land may be able to respond to these stresses. During the dry periods that are common phenomena along the desert margins, though, the pressure on the land is often far in excess of its diminished capacity, and desertification results.
3. The word delicate in the passage is closest in meaning to
Fragile
Predictable
Complex
Valuable
4. According to paragraph 5, in dry periods, border areas have difficulty
Adjusting to stresses created by settlement
Retaining their fertility after desertification
Providing water for irrigating crops
Attracting populations in search of food and fuel
Paragraph 6: Four specific activities have been identified as major contributors to the desertification processes: overcultivation, overgrazing, firewood gathering, and overirrigation. 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.
5. The word progressively in the passage is closest in meaning to
Openly
Impressively
Objectively
Increasingly
6. According to paragraph 6, which of the following is often associated with raising crops?
Lack of proper irrigation techniques
Failure to plant crops suited to the particular area
Removal of the original vegetation
Excessive use of dried animal waste，
7. The phrase devoid of in the passage is closest in meaning to
Consisting of
Hidden by
Except for
Lacking in
Paragraph 9: The final major human cause of desertification is soil salinization resulting from over irrigation. Excess water from irrigation sinks down into the water table. If no drainage system exists, the water table rises, bringing dissolved salts to the surface. The water evaporates and the salts are left behind, creating a white crustal layer that prevents air and water from reaching the underlying soil.
8. According to paragraph 9, the ground’s absorption of excess water is a factor in desertification because it can
Interfere with the irrigation of land
Limit the evaporation of water
Require more absorption of air by the soil
Bring salts to the surface
9. All of the following are mentioned in the passage as contributing to desertification EXCEPT
Soil erosion
Global warming
Insufficient irrigation
The raising of livestock
Paragraph 10: The extreme seriousness of desertification results from the vast areas of land and the tremendous numbers of people affected, as well as from the great difficulty of reversing or even slowing the process. Once the soil has been removed by erosion, only the passage of centuries or millennia will enable new soil to form. In areas where considerable soil still remains, though, a rigorously enforced program of land protection and cover-crop planting may make it possible to reverse the present deterioration of the surface.
10. Which of the sentences below best expresses the essential information in the highlighted sentence in the passage?
Incorrect choices change the meaning in important ways or leave out essential information.
Desertification is a significant problem because it is so hard to reverse and affects large areas of land and great numbers of people.
Slowing down the process of desertification is difficult because of population growth that has spread over large areas of land.
The spread of deserts is considered a very serious problem that can be solved only if large numbers of people in various countries are involved in the effort.
Desertification is extremely hard to reverse unless the population is reduced in the vast areas affected.
11. It can be inferred from the passage that the author most likely believes which of the following about the future of desertification?
Governments will act quickly to control further desertification.
The factors influencing desertification occur in cycles and will change in the future.
Desertification will continue to increase.
Desertification will soon occur in all areas of the world.
Paragraph 7:■ 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.■
12. Look at the four squares [■] that indicate where the following sentence can be added to the passage.
This economic reliance on livestock in certain regions makes large tracts of land susceptible to overgrazing.
Where would the sentence best fit?
13-14. Directions: An introductory sentence for a brief summary of the passage is provided below. Complete the summary by selecting the THREE answer choices that express the most important ideas in the passage. Some answer choices do not belong in the summary because they express ideas that are not presented in the passage or are minor ideas in the passage. This question is worth 2 points.
Many factors have contributed to the great increase in desertification in recent decades.
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Answer Choices
1. Growing human populations and the agricultural demands that come with such growth have upset the ecological balance in some areas and led to the spread of deserts.
2. As periods of severe dryness have become more common, failures of a number of different crops have increased.
3. Excessive numbers of cattle and the need for firewood for fuel have reduced grasses and trees, leaving the land unprotected and vulnerable.
4. Extensive irrigation with poor drainage brings salt to the surface of the soil, a process that reduces water and air absorption.
5. Animal dung enriches the soil by providing nutrients for plant growth.
6. Grasses are generally the dominant type of natural vegetation in semiarid lands.#p#副标题#e#
参考答案：
1. 2
2. 2
3. 1
4. 1
5. 4
6. 3
7. 4
8. 4
9. 3
10. 1
11. 3
12. 2
13-14.1 3 4
参考翻译：
众所周知，鲸类动物是哺乳动物,如鲸鱼、鼠海豚和海豚。它们用肺呼吸，而不是鳃，属于胎生。鲸类动物呈流线型的身体，后腿的消失，尾片和气孔的出现，这些特征都不能掩饰它们和陆生哺乳动物的相似之处。然而，想知道世上第一只鲸长什么样并非易事，不像还原海獭及鳍足类动物(四肢水陆两用如海豹，海狮，海象)的原貌那么简单。一些完全水生的鲸类动物虽然已经灭绝，但仍可通过化石来对它们进行考察。陆栖哺乳动物和海洋鲸类之间有何联系?近期发现的化石已经可以很清晰地帮助人们了解这个问题，以及他们之间的过渡关系。
科学家们通过一些令人振奋的发现重现了鲸类动物几近真实的起源。1979 年，在巴基斯坦北部，一个寻找化石的考察队发掘到了最古老的鲸鱼化石。这块化石被官方命名为Pakicifus，以纪念人们发现它的地方。这块化石是在一条河的沉积岩中发现的，这条河有 5200 万年的历史，离古地中海不远。
Pakicifus 包括一个完整原始动物的头盖骨，它的主人是现代鲸类的祖先。尽管只是个头盖骨，但它却提供了研究原始鲸类动物起源的珍贵信息。这个头盖骨和鲸类动物的很像，但它的下颌骨和现代鲸类略有不同，现代鲸类动物的下颌骨中含有额外的空间储存脂肪或者油脂来吸收水下的声音。Pakicifus 的主人可能会像陆生哺乳动物那样通过张开的耳朵来探测声音。另外，这个头盖骨没有呼吸孔，而鲸类动物有，这便是鲸类动物为了适应水生环境的另一种适应性表现。然而，专家认为 Pakicifus 的其它特征表明它们是已灭绝的食肉哺乳动物(中兽科动物)和鲸类动物的过渡型。有人认为 Pakicifus 靠吃浅水的鱼类为生，未能适应在辽阔的大海里生活。它们很有可能在陆地进行生育繁殖。
1989年，在埃及有了另一个重大发现。人们在古地中海残留的沉积物中发现了另一类早期鲸鱼 Basilosaurus 的一些骨骸，这些骨骸如今暴露在撒哈拉大沙漠上。Basilosaurus 生活在大约 4000 万年前，比 Pakicifus 鲸鱼晚了 1200 万年。尽管发现的这些骨骼并不完整，但这是专家们第一次在原始动物身上发现完整的后肢，它有三个小脚趾作为的足部特征。可这些后肢还太小，远无法支撑 50 英尺长的Basilosaurus 在陆地行走。因此，Basilosaurus 必定是完全水生的鲸鱼，它们的后肢已经不起任何作用，或者说已经退化。
1994年，巴基斯坦报道了一个更令人兴奋的发现。目前已经灭绝的鲸鱼Ambulocetus natans(可以步行的鲸类)4900 万年前曾在古地中海生活过。比Pakicetus 晚大约300 万年，比 Basilosaurus 早900 万年左右。幸运的是，被发现的 Ambulocetus natans 保留着完整的后肢。它的后肢很强壮，底部有长足，非常像现在的鳍足类动物。这些后肢使得他们既能在陆地行走又能在海里游行。虽然 Ambulocetus natans 保留了尾巴，但它们缺少现代水生鲸类动物用于行动的主要身体部位——尾片。不过，从 Ambulocetus 的脊椎结构上可以看出，即使缺少尾片，它们也能像现代鲸鱼那样通过身体背部的上下摆动来游走。大的后肢通常被当作是水中前行的发动机。在它们可能交配繁殖的陆地上，Ambulocetus 行动起来非常像现代海狮。毫无疑问，鲸鱼是连接着陆地生命和海洋生命的物种。
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