Condensed_Notes_Lecture%20Final_F07 - LECTURE 18 GLACIERS...

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LECTURE 18: GLACIERS Glacier ice is a metamorphic rock. As a glacier is nourished by snowfall (deposition of “sediment”), the older buried snow metamorphoses as the delicate snowflakes become more compact and rounded, transforming into “old snow” or firn . With further compaction the trapped air is mostly driven out and the ice crystals continue to coarsen and develop interlocking projections, all of these changes occurring as the material remains solid, a metamorphic process. Whether a glacier is confined by valley walls (an Alpine glacier), or whether it is an overspreading ice sheet, its internal processes are the same. At high elevations there is a zone of net accumulation , and a glacier in an ideally steady state maintains a constant size and shape as the ice flows to lower elevations, ultimately to disappear in a zone of net wastage . Wastage can occur by melting or by discharge of icebergs. A water molecule in a snowflake deposited high upon a glacier travels down into the body of the ice (it gets buried by later snowfalls), to emerge again to the surface in the zone of net wastage. Under most circumstances, internal flow is streamline : the pathways of individual particles, whether H 2 O molecules or embedded debris, are parallel. Streamline flow is in contrast with turbulent (chaotic) flow within a river. Under short-term stress, ice acts as a brittle material. It shatters and can develop deep crevasses (open fractures). Under high confining pressure equivalent to a depth in the glacier greater than ~100 meters, and under sustained long-term stress, the ice acts as a ductile material. Deep ice deforms by flowing. Typically, a glacier flows partly as an entire body of ice sliding across the bedrock, but there is also internal deformation, the upper part flowing more rapidly while motion at the base of the glacier, or where it is in contact with valley walls, is retarded by friction against the bedrock. In a Polar ice sheet, which is a “cold” glacier, the atmospheric temperature remains below freezing throughout the year; a snowstorm can occur in winter or summer. The surface temperature of the ice may be -10° or -15°C, and the deeper ice is even colder. (Older ice was precipitated during a colder Ice Age episode.) The temperature increases toward the base of a Polar ice sheet due to geothermal heat that continually rises from the bedrock. In a temperate or “warm” glacier enough melting and transport of heat energy occurs to bring the entire glacier exactly to the melting (= freezing) point. A temperate glacier is 0°C at the surface, and slightly colder at the base. Ice floats on water; therefore ice is less dense than water. Application of pressure tends to shift H 2 O into the denser state, which is the liquid state. Therefore the base of the glacier, at higher pressure, must be colder than 0°C to remain solid. Pure ice is too weak and easily deformed to be an effective agent of erosion.
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This note was uploaded on 04/07/2008 for the course GEO 303 taught by Professor Leon during the Spring '08 term at University of Texas.

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Condensed_Notes_Lecture%20Final_F07 - LECTURE 18 GLACIERS...

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