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  3. Growth Secrets Of Alaska’s Mysterious Field of Lakes The thousands of oval lakes that dot Alaska’s North Slope are some of t

Growth Secrets Of Alaska’s Mysterious Field of Lakes The thousands of oval lakes that dot Alaska’s North Slope are some of t

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问题             Growth Secrets Of Alaska’s Mysterious Field of Lakes
    The thousands of oval lakes that dot Alaska’s North Slope are some of the fastest-growing lakes on the planet. Ranging in size from puddles to more than 15 miles in length, the lakes have expanded at rates up to 15 feet per year, year in and year out for thousands of years. The lakes are shaped like elongated eggs with the skinny ends pointing northwest.
    How the lakes grow so fast, why they’re oriented in the same direction and what gives them their odd shape have puzzled geologists for decades. The field of lakes covers an area twice the size of Massachusetts, and the lakes are unusual enough to have their own name: oriented thaw lakes. "Lakes come in all sizes and shapes, but they’re rarely oriented in the same direction," said Jon Pelletier, an assistant professor of geosciences at The University of Arizona in Tucson.
    Now Pelletier has proposed a new expla-nation for the orientation, shape and speed of growth of oriented thaw lakes. The lakes’ unusual characteristics result from seasonal slumping of the banks when the permafrost thaws abruptly, he said. The lakes grow when rapid warming melts a lake’s frozen bank, and the soggy soil loses its strength and slides into the water. Such lakes are found in the permafrost zone in Alaska, northern Canada and northern Russia.
    Previous explanations for the water bodies’ shape and orientation invoked wind-driven lake circulation and erosion by waves. On Alaska’s North Slope, the prevailing winds blow perpendicular to the long axis of the lakes. According to the traditional explanation, such winds set up currents within the lakes that erode the banks, particularly at the lakes’ ends. Such currents would erode coarse grained, sandy soils faster than fine-grained clay soils.
    According to Pelletier, one key ingredient for oriented thaw lakes is permafrost the special mixture of soil and ice that forms the surface of the land in the Far North. On the north coast of Alaska and at similar latitudes throughout the world, the top, or active, layer of the permafrost melts at some point in the summer and refreezes again in the fall.
    If the temperature warms gradually, the ice portion of the permafrost melts slowly, allowing the water to drain out of the soil and leave relatively firm sand or sediment behind. However, if an early heat wave melts the permafrost’s ice rapidly, the result is a soggy, unstable soil. When such rapidly thawed permafrost is part of the vertical bank of a lake, the bank slumps into the water, enlarging the lake. More of the bank collapses if the soil is fine-grained, rather than sandy.
    Another ingredient in Pelletier’s explanation is a long, gentle slope. Because Alaska’s oriented lakes are embedded in a gently sloping landscape, the downhill end of a lake always has a shorter bank. According to Pelletier’s Computer model, shorter banks melt more and have bigger slumps. Therefore when the lake experiences thaw slumping, Pelletier’s model says the lake grows more in the downhill direction than it does uphill, generating the lakes’ characteristic elongated egg shape.
Which of the following statement will support Pelletier’s theory?
选项 A、More big lakes occurred on coarse-grained, sandy soils.
B、The lakes are oriented perpendicular to the wind.
C、Larger lakes generally have lower banks.
D、The lakes have indeed grown more in the uphill direction.
答案C
解析 本题考查具体细节题。第四段阐述的足传统的理论。末句提到粗糙的沙质土壤受到侵蚀的速度更快，即，这样的土壤更容易引起湖泊的扩张。因此[A]支持的是传统的理论。第四段第二、三句提到，盛行风垂直吹向湖泊的长轴，激起的水流侵蚀湖岸，从而引发湖泊的扩张。因此[B]支持的也是传统的理论。最后一段中，佩尔蒂埃提出了湖泊扩张形成的另一因素，即，长且平缓的斜坡。这种斜坡促使稍矮些的下坡方向的湖岸出现更大程度的塌陷，因此湖泊从稍矮些的湖岸处开始扩张。换言之，更大的湖泊群应该有更低的湖岸。[C]与此意相符，为正确项。同理，根据末段最后一句可知，[D]错在“上坡”，应该是“下坡”。
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