게시일 : 2007.05.20

김재응 강사의 배경지식 7


알아두면 좋은 배경지식들을 모아서 올려드립니다.
토플시험은 배경지식도 충분히 알고 있어야 합니다.
Cheer up!!!

Glacier

Glaciers, perennial accumulations of ice, snow, sediment, rock and water, respond to changes in temperature, snowfall and geologic forces. Several components make up a glacial system: the ice and sediment contained in the glacier; the valleys, fiords and rock features it flows over, on, or around; and the deposits left by its retreat or advance. New snow layers create pressure on existing layers of snow and ice. This process, ″firnification″, changes snow to firn, a dense granular snow (like corn snow). After the first season′s melt, snow becomes firn. As it is compressed further, firn becomes ice.As the snow collects over many years, an ice field forms. Ice flows down the valleys and slopes of the mountains to the lower elevations, and glaciers are born.


Glaciers form where more snow falls than melts. A glacier′s accumulation area, located at higher elevations, accrues a wealth of snow and ice. The ablation area, located at lower elevations, loses ice through melting (downwasting) or calving. A glacier′s terminus or face advances when more snow and ice amass than melt, and it retreats when melt exceeds accumulation. When melt equals accumulation, a glacier achieves equilibrium and its face remains stationary. Whether the glacier′s face is advancing or retreating, glacial ice persistently glides down-valley.

Coerced by gravity, ice pursues the path of least resistance. Ice depth and bedrock angle influence the rate of glacial flow. Glaciers contain two zones of ice flow. The zone of plastic flow, ice closest to the bedrock, experiences extreme pressure from the weight of the ice above and conforms to the anomalies in the bedrock. The zone of brittle flow, the upper 150 feet of glacial ice, lacks this pressure and reacts inelastically to the bedrock features, forming elongated cracks called crevasses which fluctuate with the glacier′s flow. Tubular chutes or moulins drain surface meltwater, and formidable spires of ice called seracs reach skyward. Ice plummets over particularly steep terrain creating ice falls. One　 theory suggests that differences in seasonal flow rates over an icefall create the convex bands called ogives at the base of the falls, which undulate down glacier. The erosive power of glacial flow changes the landscape and scrapes much of the soil and rock from the valley walls that channel its irrepressible flow.


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