Page 576 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

540 part III The earth–atmosphere interface
rise in sea level. The Visual Analysis at the end of this chapter shows retreating glaciers and the landscapes left behind. The World Glacier Monitoring Service (www .wgms.ch) collects data on glacier changes from a scientific collaboration network in more than 30 countries. The Glaciology Section of Natural Resources Canada contrib- utes data on mass balance changes for six glaciers, three in the Western Cordillera (Helm, Peyto, and Place) and three in the High Arctic (Devon, Meighen, and White). All of these glaciers, like South Cascade Glacier, exhibit a negative trend in mass balance in the period of mea- surement from the early 1960s to the present (www.statcan .gc.ca/pub/16-002-x/2010003/part-partie2-eng.htm). An accel- eration in the rate of losses is seen in the past three de- cades. The Western Cordillera glaciers are losing mass at a faster rate than the High Arctic glaciers.
the basal ice by compression at one moment, only to have it refreeze later. This process is ice regelation, meaning to refreeze, or re-gel. Such melting/refreezing action incorpo- rates rock debris into the glacier. Consequently, the basal ice layer, which can extend tens of metres above the base of the glacier, has a much higher debris content than the ice above.
A flowing alpine glacier or ice stream can develop vertical cracks known as crevasses (Figure 17.6). Cre- vasses result from friction with valley walls, from ten- sion due to stretching as the glacier passes over convex slopes, or from compression as the glacier passes over concave slopes. Traversing a glacier, whether an alpine glacier or an ice sheet, is dangerous because a thin veneer of snow sometimes masks the presence of a crevasse.
Glacial Movement
Glacial ice is quite different from the small, brittle cubes of ice we find in our freezer. In particular, glacial ice behaves in a plastic (pliable) manner; it distorts and flows in its under- lying portions in response to the weight and pressure of overly- ing snow and the degree of slope below. In contrast, the glacier’s upper-surface portions are quite brittle. Rates of flow range from almost no movement to a kilo- metre or two of movement per year on a steep slope. The rate of accumulation of snow in the gla- cier’s formation area is critical to the speed of forward motion.
Glaciers, then, are not rigid blocks that simply slide downhill. The greatest movement within a valley glacier occurs internally, below the rigid surface layer, which fractures as the underly- ing plastic zone moves forward (Figure 17.6a). At the same time, the base creeps and slides along, varying its speed with tempera- ture and the presence of any lubri- cating water beneath the ice. This basal slip usually is much less rapid than the internal plastic flow of the glacier, so the upper portion of the glacier flows ahead of the lower portion.
Unevenness in the landscape beneath the ice may cause the pressure to vary, melting some of
Distance of basal slip
Crevasses
Distance of internal flow
Ice regelation
(a) Cross section of a glacier, showing its forward motion, brittle cracking at the surface, and flow along its basal layer.
(b) Surface crevasses are evidence of forward movement on the Fox Glacier, South Island, New Zealand.
▲Figure 17.6 Glacial movement. [(b) David Wall/alamy.]
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