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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.
The most mysterious part about Alaska’s lakes is______.
选项 A、their fast-growing speed
B、their variety in size
C、their elongated-egg shape
D、their uniform orientation
答案D
解析 本题考查推理引申题。四个选项在第一段都有涉及，而且也都是阿拉斯加湖泊群的特点。但是第二段末句指出，湖泊的形状和大小不一是正常现象，但走向相同却很少见。因此[D]才是其最神秘的一点。
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