tailieunhanh - The Complete Guide to the TOEFL IBT part 7

Tham khảo tài liệu 'the complete guide to the toefl ibt part 7', ngoại ngữ, toefl - ielts - toeic phục vụ nhu cầu học tập, nghiên cứu và làm việc hiệu quả | 82 Practice Test 2 tributary glaciers that form in smaller valleys that lead into the main valley. And sometimes you get one or more valley glaciers that flow together forming what are called piedmont glaciers. Now uh the second major type of glacier is called the continental glacier. It s a lot larger than a valley glacier. The average continental glacier is about the size of the state of West Virginia. Today continental glaciers are found only on the island of Greenland and on the continent of Antarctica but still they cover almost 10 of the world s land area. During the Ice Ages and remember we said the last one of those was only about eleven thousand years ago an additional 20 of the world was buried under these giant continental glaciers. Most of North America most of the northern hemisphere for that matter was covered by continental glaciers. Now a continental glacier moves too but not down a slope the way a valley glacier does. In fact most continental glaciers were on relatively flat land. Still they move at a . . . uh well you can measure their movement. As ice piles up to a greater and greater thickness it can be 1 000 meters deep or more you get a tremendous amount of pressure inside the ice sheet. This force is so powerful that it causes the interior ice to practically liquefy and so a continental glacier moves out in all directions from the glacier s central point. At some point glaciers all types of glaciers become stationary. In other words they appear to stop growing. That s because they re melting at the same rate at which new ice is being added. Then they begin to recede. When they recede valley glaciers seem to be moving uphill. Continental glaciers seem to be retreating towards their central point. What s really happening is that they are melting faster than they are adding new materials. A lot of glaciers around the world these days are receding the glaciers in the high mountains of Africa Mt. Kenya Mt. Kilimanjaro for example are noticeably .