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上次复习的时候读了一篇关于 Ice age, 由于不是太懂就找了更多的资料。 以下两篇应该会有用的`` 1。 An ice age is a time when thick ice sheets called glaciers cover huge areas of land. An ice age may last for millions of years and can cause big changes to Earth's surface. Ice ages have happened a number of times throughout Earth's history. 2。 A glacier is a large mass of thick ice that remains frozen from one year to the next and slowly flows over land. During the Ice Age, between 1.6 million and 10,000 years ago, large parts of the world were covered with glaciers that advanced and retreated (grew and shrank). At times during the Ice Age, glaciers covered about 30 percent of the Earth's surface. Almost all of Canada, the northern third of the United States, much of Europe and all of Scandinavia, and large parts of Siberia, in Russia, were covered. At other times during the Ice Age there were fewer glaciers than there are today.
The continent-size glaciers of the Ice Age have retreated, or disappeared, but glaciers do exist all over the world today. Most of Greenland is covered by glacier ice, as is Antarctica. There are smaller glaciers in the Rocky Mountains of North America, in the European Alps, and even at the equator. Countries along the equator generally have warm climates, but very high up in the mountains the snow never melts fully and glaciers have formed.
Glaciers form when snow is packed down into ice. Snow crystals accumulate year by year in areas where the temperatures never become high enough to completely melt the snow and ice.
Snow builds up in several ways. Snowfall is the major contributor. Other ways include the freezing of water from rivers, rain, and other sources and snow avalanches from nearby slopes. As snow builds up, the crystals become packed down. When a large amount has accumulated, the snow is under great pressure from the weight. Eventually the snow becomes packed so tightly together that it turns into ice. The rate at which packed snow becomes glacier ice depends on the wetness and temperature of the climate.
The Earth's glaciers make up more than 90 percent of the world's ice and hold about three quarters of the world's freshwater. Because glaciers hold so much water, many people believe that if temperatures around the world increased enough that the glaciers melted, sea levels would increase greatly and flood coastal areas. The world's sea levels are already rising about 0.08 inch (2 millimeters) every year, and scientists believe that melting glaciers and ice sheets on land cause much of this rise.
Despite this activity, it is unclear how much sea levels might eventually rise because of melting glaciers. Some scientists believe that if temperatures on Earth were to rise dramatically, the warming of the air would cause moisture to form in the atmosphere. This moisture would fall as rain and snow, which could balance out any glacial melting that might occur.
25号就要考了，一定要加油咯！！！！
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发表于 2007-8-24 02:23:00
