Question 30
During most of their lives, surge glaciers behave like normal glaciers, traveling perhaps
only a couple of inches per day. However, at intervals of 10 to 100 years, these glaciers
move forward up to 100 times faster than usual. The surge often progresses along a glacier
line like a great wave, proceeding from one section to another. Subglacial streams of meltwater
(5) water pressure under the glacier might lift it off its bed, overcoming the friction between ice
and rock, thus freeing the glacier, which rapidly sliders downhill Surge glaciers also might
be influenced by the climate, volcanic heat, or earthquakes. However, many of these
glaciers exist in the same area as normal glaciers, often almost side by side.
(10) Some 800 years ago, Alaska’s Hubbard Glacier advanced toward the sea, retreated, and
advanced again 500 years later. Since 1895, this secentry-mile-long river of ice has been
flowing steadily toward the Gulf of Alaska at a rate of approximately 200 feet per year. In
June 1986, however, the glacier surged ahead as much as 47 feet a day. Meanwhile, a
western tributary, called Valerie Glacier, advanced up to 112 feet a day. Hubbard’s surge
(15) closed off Russell Fiord with a formidable ice dam, some 2,500 feet wide and up to 800
feet high, whose caged waters threatened the town of Yakutat to the south.
About 20 similar glaciers around the Gulf of Alaska are heading toward the sea. If
enough surge glaciers reach the ocean and raise sea levels, West Antarctic ice shelves could
rise off the seafloor and become adrift. A flood of ice would then surge into the Southern
(20) Sea. With the continued rise in sea level, more ice would plunge into the ocean, causing sea
levels to rise even highter, which in turn would release more ice and set in motion a vicious
cycle. The additional sea ice floating toward the tropics would increase Earth’s albedo and
lower global temperatures, perhaps enough to initiate a new ice age. This situation appears
to have occurred at the end of the last warm interglacial (the time between glacations),
(25) called the Sangamon, when sea ice cooled the ocean dramatically, spawning the beginning
of the Ice Age.
【題組】30.What is the main topic of the passage?
(A) The classification of different types of
surge glaciers
(B) The causes and consequences of surge
glaciers
(C) The definition of a surge glacier
(D) The history of a particular surge glacier
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