《托福阅读――地质地貌类(六)》由会员分享，可在线阅读，更多相关《托福阅读――地质地貌类(六)（11页珍藏版）》请在金锄头文库上搜索。
1、智 课 网 托 福 备 考 资 料托福阅读地质地貌类（六）Glaciers are open systems, with snow as the systems input and meltwater as the systems main output. The glacial system is governed by two basic climatic variables: precipitation and temperature. For a glacier to grow or maintain its mass, there must be sufficient snowfal
2、l to match or exceed the annual loss through melting, evaporation, and calving, which occurs when the glacier loses solid chunks as icebergs to the sea or to large lakes. If summer temperatures are high for too long, then all the snowfall from the previous winter will melt. Surplus snowfall is essen
3、tial for a glacier to develop. A surplus allows snow to accumulate and for the pressure of snow accumulated over the years to transform buried snow into glacial ice with a depth great enough for the ice to flow. Glaciers are sometimes classified by temperature as faster-flowing temperate glaciers or
4、 as slower-flowing polar glaciers.Glaciers are part of Earths hydrologic cycle and are second only to the oceans in the total amount of water contained. About 2 percent of Earths water is currently frozen as ice. Two percent may be a deceiving figure, however, since over 80 percent of the worlds fre
5、shwater is locked up as ice in glaciers, with the majority of it in Antarctica. The total amount of ice is even more awesome if we estimate the water released upon the hypothetical melting of the worlds glaciers. Sea level would rise about 60 meters. This would change the geography of the planet con
6、siderably. In contrast, should another ice age occur, sea level would drop drastically. During the last ice age, sea level dropped about 120 meters. When snowfalls on high mountains or in polar regions, it may become part of the glacial system. Unlike rain, which returns rapidly to the sea or atmosp
7、here, the snow that becomes part ofa glacier is involved in a much more slowly cycling system. Here water may be stored in ice form for hundreds or even hundreds of thousands of years before being released again into the liquid water system as meltwater. In the meantime, however, this ice is not sta
8、tic. Glaciers move slowly across the land with tremendous energy, carving into even the hardest rock formations and thereby reshaping the landscape as they engulf, push, drag, and finally deposit rock debris in places far from its original location. As a result, glaciers create a great variety of la
9、ndforms that remain long after the surface is released from its icy covering. Throughout most of Earths history, glaciers did not exist, but at the present time about 10 percent of Earths land surface is covered by glaciers. Present-day glaciers are found in Antarctica, in Greenland, and at high ele
10、vations on all the continents except Australia. In the recent past, from about 2.4 million to about 10,000 years ago, nearly a third of Earths land area was periodically covered by ice thousands of meters thick. In the much more distant past, other ice ages have occurred.Paragraph1: Glaciers are slo
11、wly moving masses of ice that have accumulated on land in areas where more snowfalls during a year than melts. Snow falls as hexagonal crystals, but once on the ground, snow is soon transformed into a compacted mass of smaller, rounded grains. As the air space around them is lessened by compaction a
12、nd melting, the grains become denser. With further melting, refreezing, and increased weight from newer snowfall above, the snow reaches a granular recrystallized stage intermediate between flakes and ice known as firn. With additional time, pressure, and refrozen meltwater from above, the small fir
13、n granules become larger, interlocked crystals of blue glacial ice. When the ice is thick enough, usually over 30 meters, the weight of the snow and firn will cause the ice crystals toward the bottom to become plastic and to flow outward or downwardfrom the area of snow accumulation. 1. The word “ i
14、nterlocked ” in the passage is closest in meaning to intermediate linked frozen fully developed 2. According to paragraph 1, which of the following does NOT describe a stage in the development of firn? Hexagonal crystals become larger and interlock to form a thick layer. Snow crystals become compact
15、ed into grains. Granules recrystallize after melting, refreezing, and further compaction. Grains become denser owing to reduced air space around them.Paragraph2: Glaciers are open systems, with snow as the systems input and meltwater as the systems main output. The glacial system is governed by two
16、basic climatic variables: precipitation and temperature. For a glacier to grow or maintain its mass, there must be sufficient snowfall to match or exceed the annual loss through melting, evaporation, and calving, which occurs when the glacier loses solid chunks as icebergs to the sea or to large lakes. If summer temperatures are high for too long, then all the snowfall from the previous winter will melt. Surplus snowfall is essential for a glacier to develop. A surplu
