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Glaciers are large masses of ice on land that show evidence of past or present
movement. They grow by the gradual transformation of snow into glacier ice.
A fresh snowfall is a fluffy mass of loosely packed snowflakes, small delicate ice
constals grown in the atmosphere. As the snow ages on the ground for weeks or months,
(5) the crystals shrink and become more compact, and the whole mass becomes squeezed
together into a more dense form, granular snow. As new snow falls and buries the older
snow, the layers of granular snow further compact to form firm, a much denser kind of
snow, usually a year or more old, which has little pore space. Further burial and slow
cementation—a process by which crystals become bound together in a mosaic of
(10) intergrown ice crystals—finally produce solid glacial ice. In this process of
recrystallization, the growth of new crystals at the expense of old ones, the percentage of
air is reduced from about 90 percent for snowflakes to less than 20 percent for glacier ice.
The whole process may take as little as a few years, but more likely ten or twenty years or
longer. The snow is usually many meters deep by the time the lower layers art convened
(15) into ice.
In cold glaciers those formed in the coldest regions of the Earth, the entire mass of ice
is at temperatures below the melting point and no free water exists. In temperate glaciers,
the ice is at the melting point at every pressure level within the glacier, and free water is
present as small drops or as larger accumulations in tunnels within or beneath the ice.
(20) Formation of a glacier is complete when ice has accumulated to a thickness (and thus
weight) sufficient to make it move slowly under pressure, in much the same way that solid
rock deep within the Earth can change shape without breaking. Once that point is reached,
the ice flows downhill, either as a tongue of ice filling a valley or as thick ice cap that
flows out in directions from the highest central area where the most snow accumulates.
The up down leads to the eventual melting of ice.
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วันพฤหัสบดีที่ 29 กรกฎาคม พ.ศ. 2553 เวลา 20:43:01