Page 577 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Wilkins Ice Shelf
Pine Island Glacier
Thwaites Glacier
0 500
Ronne Ice Shelf
West Antarctica
1000 KILOMETRES
East Antarctica
Larsen C Ice Shelf
Amery Ice Shelf
Jakobshavn Glacier on the western Greenland coast, for example, is one of the fastest moving, at between 7 and 12 km per year. In 2012 scientists reported that although the overall trend in Greenland was toward glacial surging, in some regions glaciers slowed be- tween 2005 and 2010, pointing to the complexity of gla- cial behaviour.
Scientists are investigating the exact causes of gla- cier surges. Some surge events result from a buildup of water pressure under the glacier—sometimes enough to actually float the glacier slightly, detaching it from its bed during the surge. Another cause of gla- cier surges is the presence of a water-saturated layer of sediment, a soft bed, beneath the glacier. This is a deformable layer that cannot resist the tremendous sheer stress produced by the moving ice of the glacier. In Antarctica, scientists examining cores taken from several ice streams now accelerating through the West Antarctic Ice Sheet think they have identified the oc- currence there of this kind of glacial surge—although water pressure is still important. As any type of surge begins, ice quakes are detectable, and ice faults are visible.
Scientists think that glacial surges in Greenland are related to the meltwater that works its way to the basal layer, lubricating the interface between the glacier and the underlying bedrock. In addition, the warmer surface waters draining beneath the glacier deliver heat that in- creases basal melt rates. However, a 2013 study reported that the meltwater flows in distinct channels under the ice, affecting the channels but not necessarily lubricating wide areas of the ice sheet basal layer.
Glacial Erosion The process by which a glacier erodes the landscape is similar to a large excavation project, with the glacier hauling debris from one site to another for deposition. The passing glacier mechanically picks up rock material and carries it away in a process known as glacial plucking. Debris is carried on its surface and is also transported internally, or englacially, embedded within the glacier itself.
When a glacier retreats, it can leave behind cobbles or boulders (sometimes house-sized) that are “foreign” in composition and origin to the ground on which they are deposited. These glacial erratics, lying in strange lo- cations with no obvious sign of how they arrived there, were an early clue to the glacial plucking that occurred during times when blankets of ice covered the land (pictured ahead in Figure 17.11).
The rock pieces frozen to the basal layers of the gla- cier enable the ice mass to scour the landscape like sand- paper as it moves. This process, called abrasion, produces a smooth surface on exposed rock, which shines with glacial polish when the glacier retreats (Figure 17.8). Larger rocks in the glacier act much like chisels, gouging the underlying surface and producing glacial striations parallel to the flow direction.
Ross
Ice Shelf
Speed (metres per year)
<1.5 10 100 1000
▲Figure 17.7 First complete map of ice movement speed in Antarctica. The black lines show ice divides, similar to drainage divides. Colours indicate the speed of ice movement; fastest movement is in red and purple. note that the fast-flowing ice channels extend far inland, a surprise to scientists. The tributaries shown in blue are moving faster than the ice sheet around them but do not move as quickly as an ice stream. This map was created using data from a group of international satellites from the Canada, Japan, and european Space agencies. [RADARSAT–SAR, naSa/JPL.]
Scientists recently completed a map of ice move- ment on the Antarctic continent based on satellite radar measurements from 1996 to 2009 (Figure 17.7). The map reveals that many of the tributaries around ice shelves extend surprisingly far inland and are moving by basal slip (sliding on the ground) rather than slowly deform- ing under the weight of the ice. Identifying the areal ex- tent of this type of motion on the ice sheet is important because a loss of ice at the coasts as ice shelves break up may open the tap for massive amounts of ice to flow more quickly from the interior, with implications for sea- level rise.
Glacier Surges Although glaciers flow plastically and predictably most of the time, some will advance rap- idly at speeds much faster than normal, in a glacier surge. Research in the St. Elias Mountains in Yukon and Alaska has shown that 136 valley glaciers in the re- gion are surge-type glaciers. Characteristics of surging can be interpreted from glacier surface patterns visible on satellite images, and detailed field studies have been conducted on a number of these glaciers.
A surge is not quite as abrupt as it sounds; in gla- cial terms, a surge can be tens of metres per day. The
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