Glaciers are slowly moving masses of ice that have accumulated on land in
areas where more snowfalls during a year than melts. Snowfalls as hexagonal
crystals, but once on the ground, snow is soon transformed into a compacted
mass of smaller, rounded grains. As the air space around them is lessened by
compaction and melting, the grains become denser. With further melting,
refreezing, and increased weight from newer snowfall above, the snow reaches
a granular recrystallized stage intermediate between flakes and ice known as
firn. With additional time, pressure, and refrozen meltwater from above, the
small firn granules become larger, interlocked crystals of blue glacial ice.
When the ice is thick enough, usually over 30 meters, the weight of the snow
and firn will cause the ice crystals toward the bottom to become plastic and
to flow outward or downward from the area of snow accumulation.
Glaciers are open systems, with snow as the system’s input and meltwater as
the system' s 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 ice form for hundreds
or even hundreds of thousands of years before being released again into the
liquid water system as meltwater. In the meantime, however, this ice is not
static. Glaciers move slowly across the land with tremendous energy, carving
into even the hardest rock formations and thereby reshaping the landscape as
they engulf, push, drag, and finally deposit rock debris in places far from
its original location. As a result, glaciers create a great variety of
landforms that remain long after the surface is released from its icy
covering.
Throughout most of Earth’s history, glaciers did not exist, but at the
present time about 10 percent of Earth’s land surface is covered by
glaciers. Present-day glaciers are found in Antarctica, in Greenland, and at
high elevations on all the continents except Australia. In the recent past,
from about 2.4 million to about 10,000 years ago, nearly a third of Earth’s
land area was periodically covered by ice thousands of meters thick. In the
much more distant past, other ice ages have occurred.
14. Glaciers are part of Earth's hydrologic cycle.
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Answer Choices
○Glaciers, which at present contain 80 percent of Earth's freshwater, form
when accumulated snow is compressed and recrystallized into ice over a period
of years.
○When there are glaciers on Earth, water is cycled through the glacier
system, but the cycle period may be hundreds of thousands of years during
periods of ice ages.
○The glacial system is governed by precipitation and temperature in such a
way that glaciers cannot form in temperate latitudes.
○When glacial ice reaches a depth of 30 meters, the weight of the ice causes
ice crystals at the bottom to flow, and the resulting movement of the glacier
carves the landscape.
○If global warming melted the world's glaciers, sea level would rise about
60 meters worldwide.
○Glaciers have had little effect on Earth's surface because only 2 percent
of Earth's water is currently contained in glaciers, and there are fewer
glaciers now than at most times in the past.
各位好，以上這題我百思不得其解。
答案給的是1,2,4 但是我對4很有意見。
因為主題是指glacier也是hydrologic cycle的一部份。但是4指的是glacier會對地
貌造成影響，並不符summary的條件才對。
相較起來5. glacier融化造成海面上升比較符合「glacier為hydrological circle」
這個主題。
請大家不吝賜教！
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