Glaciers are large masses of ice on land that show evidence of past or present<br><br>movement. They grow by the gradual transformation of snow into glacier ice.<br><br>A fresh snowfall is a fluffy mass of loosely packed snowflakes, small delicate ice<br><br>constals grown in the atmosphere. As the snow ages on the ground for weeks or months,<br><br>(5) the crystals shrink and become more compact, and the whole mass becomes squeezed<br><br>together into a more dense form, granular snow. As new snow falls and buries the older<br><br>snow, the layers of granular snow further compact to form. firm, a much denser kind of<br><br>snow, usually a year or more old, which has little pore space. Further burial and slow<br><br>cementation―a process by which crystals become bound together in a mosaic of<br><br>(10) intergrown ice crystals―finally produce solid glacial ice. In this process of<br><br>recrystallization, the growth of new crystals at the expense of old ones, the percentage of<br><br>air is reduced from about 90 percent for snowflakes to less than 20 percent for glacier ice.<br><br>The whole process may take as little as a few years, but more likely ten or twenty years or<br><br>longer. The snow is usually many meters deep by the time the lower layers art convened<br><br>(15) into ice.<br><br>In cold glaciers those formed in the coldest regions of the Earth, the entire mass of ice<br><br>is at temperatures below the melting point and no free water exists. In temperate glaciers,<br><br>the ice is at the melting point at every pressure level within the glacier, and free water is<br><br>present as small drops or as larger accumulations in tunnels within or beneath the ice.<br><br>(20) Formation of a glacier is complete when ice has accumulated to a thickness (and thus<br><br>weight) sufficient to make it move slowly under pressure, in much the same way that solid<br><br>rock deep within the Earth can change shape without breaking. Once that point is reached,<br><br>the ice flows downhill, either as a tongue of ice filling a valley or as thick ice cap that<br><br>flows out in directions from the highest central area where the most snow accumulates.<br><br>The up down leads to the eventual melting of ice.
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