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托福阅读――地质地貌类(六)
智 课 网 托 福 备 考 资 料
托福阅读――地质地貌类(六)
Glaciers are open systems, with snow as the system’s input
and meltwater as the systems main output. The glacial system is
governed by two basic climatic variables: precipitation and
temperature. For a glacier to grow or maintain its mass, there
must be sufficient snowfall to match or exceed the annual loss
through melting, evaporation, and calving, which occurs when
the glacier loses solid chunks as icebergs to the sea or to large
lakes. If summer temperatures are high for too long, then all the
snowfall from the previous winter will melt. Surplus snowfall is
essential for a glacier to develop. A surplus allows snow to
accumulate and for the pressure of snow accumulated over the
years to transform buried snow into glacial ice with a depth great
enough for the ice to flow. Glaciers are sometimes classified by
temperature as faster-flowing temperate glaciers or as
slower-flowing polar glaciers.
Glaciers are part of Earth’s hydrologic cycle and are second
only to the oceans in the total amount of water contained. About
2 percent of Earth’s water is currently frozen as ice. Two percent
may be a deceiving figure, however, since over 80 percent of the
world’s freshwater is locked up as ice in glaciers, with the
majority of it in Antarctica. The total amount of ice is even more
awesome if we estimate the water released upon the
hypothetical melting of the world’s glaciers. Sea level would rise
about 60 meters. This would change the geography of the planet
considerably. In contrast, should another ice age occur, sea level
would drop drastically. During the last ice age, sea level dropped
about 120 meters.
When snowfalls on high mountains or in polar regions, it
may become part of the glacial system. Unlike rain, which returns
rapidly to the sea or atmosphere, the snow that becomes part of
a glacier is involved in a much more slowly cycling system. Here
water may be stored in
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