红DeltaTest2听力The Formation and Movement of Glacier原文文本答案解析+音频录音mp3

[00:00.00]You might expect that heavy snowfall is the main requirement for the formation of a glacier. [00:06.17]But if you stop and think about it, you’ll realize there are lots of areas of the world that receive huge amounts of snow but have no glaciers. [00:14.49]Snowfall alone isn’t enough. [00:17.32]For a glacier to form, the snow can’t melt. [00:20.27]It has to be conserved. [00:22.08]In the extremely cold Arctic and Antarctic, there are large areas that receive minimal precipitation and actually qualify as desert. [00:30.77]Although there’s little snow, there are massive ice sheets because the snow that does fall is conserved and transformed into ice.
[00:39.08]Of course, lots of places do get large amounts of snow, and they do have glaciers ... high mountains, for example. [00:46.29]In high mountains where the climate is humid and cold, snowfall is frequent. [00:51.35]Above the snowline, the annual snowfall exceeds the annual melting, so snow builds up. [00:57.41]The accumulation is thickest in the hollows at the heads of valleys, because these locations are relatively high and cool, and are protected from sun and wind.
[01:07.43]As snow accumulates in a hollow, it’s gradually converted to ice. [01:12.36]First, the fragile snow crystals break as they’re compressed by the weight of more snow settling on top of them. [01:19.45]There’s some melting and refreezing because of compaction, earth heat, and seasonal temperature fluctuations. [01:27.43]So, the snow crystals are broken, then they’re wetted by meltwater, and refrozen, over and over again.
[01:35.23]Gradually, over time, the snowflakes change into grains. [01:40.79]They become rounded and granular, like the grains of coarse sugar. [01:45.55]There are pockets of air between the grains, connecting the grains to one another. [01:50.33]This old snow, called “firn,” is generally created after one complete winter-summer cycle.
[01:59.02]Firn is actually bits of ice. [02:01.66]The firn begins to re-crystallize, and eventually, it combines and crystallizes into solid ice—a glacier. [02:09.66]What happens is, the small grains coalesce to form large interlocking crystals of ice, with air trapped as bubbles inside the crystals. [02:20.03]In the end, it’s pure ice, with all the air squeezed out. [02:26.45]The flow of the glacier down the mountain contributes to crystal growth, as the movement helps to compress the air out.
[02:33.89]As the hollow in the valley head fills with snow turning to ice, the hollow enlarges, and the rock walls are carved out by shifting ice. [02:44.79]As new snow is added, the lower part of the snow-and-ice mass bulges out, kind of like a mud pie. [02:53.20]As the mass continues to bulge, part of the ice moves over the edge of the hollow and starts moving down the valley. [03:00.68]Large glaciers usually move faster than small ones. [03:04.81]Also, the movement is faster in the summer, when more meltwater is present beneath and around the ice mass to lubricate it and buoy it up.
[03:14.44]Most valley glaciers move at a rate of... oh ... between a few inches and a few feet a day. [03:21.44]But some glaciers—called surging glaciers—can travel as much as 300 feet a day. [03:29.68]There are at least 200 of these surging glaciers in Alaska and the Northwest Territories. [03:35.24]The surging is caused by a number of conditions, like ... oh ... sudden adjustment to an increase in the snow load on top, or, more likely, an increase in the production of meltwater due to a rise in temperature. [03:49.45]Glaciers that have more meltwater are better lubricated and tend to move faster than drier ones. [03:56.03]In very cold climates, glaciers are quite dry because of the lack of melting. [04:01.66]The amount of water is slight, so the glacier does not slide as quickly.
[04:06.72]In warmer climates, glaciers are better lubricated with meltwater. [04:12.09]They also cause more erosion, more carving out of the valley floor. [04:16.44]This is because during the melt-freeze cycle, parts of the glacier freeze to the bottom and sides of the valley, and then, as the ice moves on, large chunks of glacier pluck out rock. [04:28.64]So you can see why glaciers in warmer climates have a greater impact on the landscape than those in very cold climates.