http:/bailiedu.com GRE 阅读素材之沙漠的形成阅读素材之沙漠的形成备考 GRE 的小伙伴们是不是很为 GRE 阅读的出题范围感到头疼呢？如果自己的课外科学、文化或者常识的知识不全面的话，很容易就掉入 GRE 阅读的陷阱里面。再者 GRE 阅读中充斥的很多的长难句，如果对阅读的背景知识不了解的话，会更加深做题的难度。举例说明，大家知道沙漠是怎么形成的吗？很多小伙伴，会说水土流失，干旱等等，但是对于 GRE 阅读的出题者来说，绝非这么简单的，那么究竟沙漠是怎么形成,那么和前程百利小编一起来学习下吧！Desert FormationThe deserts, which already occupy approximately a fourth of the Earths 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（沙漠化）. It has been estimated that an additional one-fourth of the Earths land surface is threatened by this process. 沙漠已经占据了地球陆地面积约四分之一，而且最近几十年正以惊人的速度扩张。沙漠化是指类似沙漠的环境漫延到原本并非沙漠的区域。据估计，地球表面另外四分之一的地方正面临沙漠化威胁。http:/bailiedu.com Desertification is accomplished primarily through the loss of stabilizing natural vegetation and the subsequent（随后） accelerated erosion（侵蚀） 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. 沙漠化主要通过以下过程实现：首先自然植被不断减少，随后风力和雨水加速了土壤的腐蚀。有的时候松散的土壤全部被风刮走，留下石质化的表层；其它情况下细小的沙粒可能会被吹走，而正常沙粒大小的砂子不断堆积，从而形成移动的沙丘或者沙脊。Even in the areas that retain a soil cover, the reduction of vegetation typically results in the loss of the soils ability to absorb substantial（大数量） 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（漏水）. Water absorption（吸水性） 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 http:/bailiedu.com established. 即便是在保留了土壤表层的区域，植被减少也已成为土壤大量吸取地下水的能力下降的典型因素。雨水对松散土壤的冲击会把细小的粘土颗粒冲到土壤空隙中，封闭了土壤并降低土地表层水的渗透率。地表对水的吸收急剧减少，大量水资源流失，因此土壤的腐蚀率也随即增加。地表吸收水分的能力进一步弱化使得土壤越发干燥，导致植被的进一步流失，于是便形成了土壤沙漠化的恶性循环。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. 在一些地方，沙漠面积的扩大很大程度上归因于干燥的气候条件。在过去的几千年里，不断增加的温室效应使得一些地方干旱问题愈发严重。倘若空气污染带来的温室效应继续恶化，沙漠化进程会在未来数十年内加速实现。There is little doubt, however, that desertification in most areas results primarily from human activities rather than natural processes. The semiarid（半干旱） lands bordering the deserts exist in a http:/bailiedu.com 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. 然而，可以肯定的是，大部分地区沙漠化主要都是由于人类活动造成，而非自然条件导致。沙漠边缘的半干旱土地所处的生态平衡环境非常脆弱，环境压力持续增加，而这些半干旱区域适应环境压力的能力极其有限。人口数量的增加使得人们不断向土地施压，依其提供食物和燃料。在湿润的季节里，土地兴许能够应付这些压力。但是在干旱的季节里，在沙漠周边的土地上，存在着这样一个十分普遍的现象：人类对土地施加的压力远远超过了土地自身减压的能力，因此最终形成了沙漠。Four specific activities have been identified as major contributors to the desertification processes: overcultivation（耕种过度）, overgrazing（过度放牧）, firewood gathering（过分砍伐）, and overirrigation（过量灌溉）. The cultivation of crops has expanded into progressively drier regions as population densities have grown. These regions are especially likely to have periods of http:/bailiedu.com 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. 导致沙漠化的主要因素有四个：过度种植，过度放牧，过分砍伐，过度灌溉。由于人口密度增加，人们对粮食作物的种植已经扩展到日益干燥的区域进行。这些区域很有可能经常会发生干旱，所以农作物种植失败是很正常的事情。大多数农作物的种植需要事先移除天然植被，而农作物歉收后又会留下大面积荒地，非常容易被风力和雨水侵蚀。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（践踏） and pulverization （粉碎）of the soil. This is usually followed by the drying of the soil and accelerated erosion（加速侵蚀）. 在半干旱地区，草坪是主要的天然植被，家畜饲养是当地的一项主要经济活动。在一个地区过量饲养家畜会导致植被覆盖面积减少，土地被大量践踏和碾碎。通常，随之而来的就是土地硬化和加速侵蚀。http:/bailiedu.com Firewood is the chief fuel used for cooking and heating in many countries. The increased pressures of expanding populations have led to the removal（除去） of woody plants so that many cities and t