![]() ![]() Glacial retreat of the Laurentide ice sheet created such features as the Great Lakes. The Laurentide ice sheet was almost three kilometers (two miles) thick and covered North America from the Canadian Arctic all the way to the modern U.S. Today, about one-tenth of Earth’s land is covered by glacial ice. During the Pleistocene Ice Age, nearly one-third of Earth’s land was covered by glaciers. ![]() Ice sheets reached their greatest size about 18,000 years ago. This was the last glacial period, or ice age. Huge ice sheets covered much of North America, Eurasia, and South America during the Pleistocene era. While the ice sheet has advanced and retreated with climate change, it has been a constant feature of the landscape the entire time. Ice Sheets in History Antarctica has been covered by an ice sheet for 40 million years. Greenland, the largest island in the world, would become an archipelago, a chain of small islands only connected by waterways. Antarctica would shrink by more than a quarter as the western part shrunk into the sea. If the continental glaciers were to suddenly disappear, the landscape of Antarctica and Greenland would change drastically. ![]() Eastern Antarctica is about 2.5 kilometers (1.6 miles) below sea level because of the colossal weight of the ice sheet above it. Both the Antarctic and Greenland ice sheets have caused the land under them to sink. Greenland's ice shelves break up much faster than those surrounding Antarctica. Glacial melt happens across about half of the Greenland ice sheet, whereas it is much more isolated on the far western part of Antarctica. The annual snow accumulation rate is more than double that of Antarctica. The Greenland ice sheet interacts much more dynamically with the ocean than the Antarctic ice sheet. It is still the second largest body of ice on the planet. The Greenland ice sheet is much smaller than the Antarctic ice sheet, only about 1.7 million square kilometers (656,000 square miles). If it melted, sea level would rise by about 60 meters (200 feet). The Antarctic ice sheet is about two kilometers (1.2 miles) thick. It covers more than 14 million square kilometers (5.4 million square miles) and contains about 30 million cubic kilometers (7.2 million cubic miles) of water. Continental Glaciers The Antarctic ice sheet is the largest block of ice on Earth. It is more than 400 kilometers (249 miles) long and 2,500 meters (1.5 miles) thick. The Lambert Glacier moves as quickly as 1,200 meters (0.7 mile) every year. The largest glacier in the world is an ice stream, the Lambert Glacier in Antarctica. The slightly warmer, softer ice of the ice stream is where most of the ice sheet's crevasses are located. Ice streams can move as quickly as 1,000 meters (0.6 mile) every year. These fast-moving glaciers are called ice streams. The bottom of the ice sheet melts, causing the ice above it to move at a faster rate than the rest of the ice sheet. Compression and geothermal energy sometimes cause the bottom of an ice sheet to be slightly warmer than the ice above it. An ice sheet flows, oozes, and slides over uneven surfaces until it covers everything in its path, including entire valleys, mountains, and plains. They behave plastically, or like a liquid. Ice sheets tend to be slightly dome-shaped and spread out from their center. At this point, the glacier begins to move under its own weight. When the ice grows thick enough-about 50 meters (165 feet)-the firn grains fuse into a huge mass of solid ice. As years go by, layers of firn build on top of each other. ![]() The hard snow underneath gets even denser. New snow falls and buries the grainy snow. It slowly changes texture from fluffy powder to a block of hard, round ice pellets. The slightly melted snow gets harder and compresses. Snow accumulates year after year, then melts. How Ice Sheets Form Ice sheets formed like other glaciers. During the last glacial period, however, much of Earth was covered by ice sheets. Today, there are only two ice sheets in the world: the Antarctic ice sheet and the Greenland ice sheet. Making up ice fields, ice caps, and eventually ice sheets are individual glaciers. A series of connected ice caps is called an ice field. A mass of glacial ice covering less area than an ice sheet is called an ice cap. As ice sheets extend to the coast and over the ocean, they become ice shelves. Ice sheets contain about 99% of the fresh water on Earth, and are sometimes called continental glaciers. An ice sheet is a mass of glacial ice more than 50,000 square kilometers (19,000 square miles). ![]()
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