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  3. Glaciers consist of fallen snow that compresses over many years into large, thickened ice masses. Most of the world’s glacial ic

Glaciers consist of fallen snow that compresses over many years into large, thickened ice masses. Most of the world’s glacial ic

admin2018-02-17  31
问题    Glaciers consist of fallen snow that compresses over many years into large, thickened ice masses. Most of the world’s glacial ice is found in Antarctica and Greenland, but glaciers are found on nearly every continent, even Africa. Presently, 10% of land area is covered with glaciers. Glacial ice often appears blue because ice absorbs all other colors but reflects blue. Almost 90% of an iceberg is below water; only about 10% shows above water. What makes glaciers unique is their ability to move. Due to sheer mass, glaciers flow like very slow rivers. Some glaciers are as small as football fields, whereas others grow to be over 100 kilometers long.
   Within the past 750, 000 years, scientists know that there have been eight Ice Age cycles, separated by warmer periods called interglacial periods. Currently, the earth is nearing the end of an interglacial, meaning that another Ice Age is due in a few thousand years. This is part of the normal climate variation cycle. Greenhouse warming may delay the onset of another glacial era, but scientists still have many questions to answer about climate change. Although glaciers change very slowly over long periods, they may provide important global climate change signals.
   The girth of the ice, combined with gravity’s influence, causes glaciers to flow very slowly. Once a mass of compressed ice reaches a critical thickness of about 18 meters thick, it becomes so heavy that it begins to deform and move. Ice may flow down mountains and valleys, fan across plains, or spread out to sea. Movement along the underside of a glacier is slower than movement at the top due to the friction created as it slides along the ground’s surface.
   Most glaciers are found in remote mountainous areas. However, some found near cities or towns present a danger to the people living nearby. On land, lakes formed on top of a glacier during the melt season may cause floods. At the narrow part of a valley glacier, ice falling from the glacier presents a hazard to hikers below. When ice breaks off over the ocean, an iceberg is formed.
   Glaciers are a natural resource and contain 75% of the world’s freshwater. People worldwide are trying to harness the power of these frozen streams. Some towns rely on glacial melting from a nearby ice cap to provide drinking water. Some farmers spread soil or ashes over snow to promote melting, hoping that the melting will provide water to irrigate crops in drought-stricken areas. Others have channeled meltwater from glaciers to their fields. Scientists and engineers have worked together to tap into glacial resources, using electricity that has been generated in part by damming glacial meltwater.
According to the passage, what is a negative effect of living too close to a glacier?
选项 A、The mass of the glacier reaches a critical thickness.
B、About 10% of a glacier shows above water.
C、Spreading dark material over snow promotes melting.
D、Lakes formed on top of glaciers may cause floods.
答案D
解析 信息筛选题。选项[A][B][C]在文中均提到，但不符合题目中negative effect的要求，因此只有选项[D]冰川顶部形成的湖泊会导致洪水属于负面影响。故正确答案是[D]。
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