Plastic deformation occurs below a depth of 50 meters feet from the surface of the glacier. Thick glacial ice is quite heavy, and the great weight of the glacier may cause the ice along the base of the glacier to melt. Melting occurs because the temperature at which ice melts is reduced due to the pressure exerted by the weight of the overlying glacial ice.
The meltwater functions as a lubricant allowing the glacier to slide more readily over bedrock and sediments. If a great deal of slippery meltwater accumulates under the ice, the glacier may begin to advance very rapidly as a surge.
Sometimes known as a galloping glacier, a surging glacier flows at a very rapid rate. Muir Glacier has retreated 50 kilometers 31 miles. Both images by the United States Geological Survey. The area of glacial ice formation is called the zone of accumulation.
In this zone more snow accumulates each winter than that which melts away during the summer. Buried accumulations of snow turn into firn and eventually recrystallize into glacial ice. Glacial ice flows away from the zone of accumulation when the thick ice deforms plastically under its own weight. In a valley glacier the ice flows downslope from the zone of accumulation, while for a continental glacier the ice flows laterally outward and away from the zone of accumulation.
The area of a glacier that experiences a greater amount of melting than glacial ice formation is called the zone of wastage zone of ablation. In this zone, as the ice melts away, bits of sand and gravel on the surface of the glacier are left behind.
It is important to note that glacial ice is always replenishing this zone as glacial ice continues to flow from the zone of accumulation. The line that separates the zone of accumulation from the zone of wastage is called the snow line equilibrium line. The snow line may be visible at the end of summer between the clean icy surface of the zone of accumulation and the dirty, sediment-covered surface of the zone of wastage.
The upper 50 meters of the surface of the glacier, where the ice does not undergo plastic deformation, is referred to as the zone of fracture. In this zone the ice is brittle and only deforms by cracking, breaking, and fracturing. Crevasses are fractures or breaks in the ice that may be hundreds of meters long and up to 50 meters deep. The end or toe of the glacier is called the terminus and is part of the zone of wastage. When the terminus of the glacier flows into a body of water, the ice at the toe calves or breaks off to form floating chunks of ice called icebergs.
John Muir wrote about one of his adventures in Alaska, when he and the camp dog, Stickeen, went on a lengthy hike up a valley glacier [6]. On the return trip their way was barred by crevasses, and John had to walk a considerable distance until he discovered a precarious, narrow ice bridge spanning a deep crevasse.
Understandably, Stickeen was quite reluctant to traverse the dangerous bridge of ice and John spent considerable time and effort coaxing the fearful dog to cross. In continental glaciers like Antarctica and Greenland, the thickest parts 4, m and 3, m respectively are the areas where the rate of snowfall and therefore of ice accumulation are highest. The flow of alpine glaciers is primarily controlled by the slope of the land beneath the ice Figure In the zone of accumulation , the rate of snowfall is greater than the rate of melting.
In other words, not all of the snow that falls each winter melts during the following summer, and the ice surface is always covered with snow. In the zone of ablation , more ice melts than accumulates as snow. The equilibrium line marks the boundary between the zones of accumulation above and ablation below. Above the equilibrium line of a glacier, not all of the winter snow melts in the following summer, so snow gradually accumulates.
The snow layer from each year is covered and compacted by subsequent snow, and it is gradually compressed and turned into firn within which the snowflakes lose their delicate shapes and become granules. With more compression, the granules are pushed together and air is squeezed out.
Downward percolation of water from melting taking place at the surface contributes to the process of ice formation. The equilibrium line of a glacier near Whistler, B. The position of the equilibrium line changes from year to year as a function of the balance between snow accumulation in the winter and snowmelt during the summer.
Cool summers promote glacial advance and warm summers promote glacial retreat. Glaciers move because the surface of the ice is sloped. Exercise: Connect the Words with Definitions Draw lines to connect the words to their definitions. Roll some Silly Putty into a cigar shape to make it look like a glacier. Then grab the ends and pull it slowly apart. See it sag and still stay as one piece.
This is like ice. When ice moves slowly, it flows and deforms. Distributed Active Archive Center. Facebook Twitter. Glaciers Are Solid Rivers. A glacier is a large accumulation of many years of snow, transformed into ice. This solid crystalline material deforms changes and moves. Gravity is the cause of glacier motion; the ice slowly flows and deforms changes in response to gravity. A glacier molds itself to the land and also molds the land as it creeps down the valley. Many glaciers slide on their beds, which enables them to move faster.
The glacier and its load of rock debris flow down-valley. A glacier discharges snow from its accumulation area in the same way a stream discharges water from its watershed. Sometimes, in cold climates with a lot of snow, like Alaska, glaciers flow all the way down to sea level. These glaciers carve fjords and make icebergs. Muir Glacier, Alaska.
Photo by Austin Post. Glacier Advance and Retreat Glaciers advance and retreat. This is the upper region of the glacier. The ice flows like a conveyor belt driven by gravity and ever mounting snows. This is the lower region of the glacier. Matt Chernos. The road to a PhD Save my name, email, and website in this browser for the next time I comment. This site uses Akismet to reduce spam.
Learn how your comment data is processed. Glacier mass balance Glacier flow Internal deformation Basal sliding Subglacial deformation Different types of glacier flow References Comments Glacier mass balance Components of mass balance of a glacier. Boulder pavement at Whitburn Bay, County Durham.
The boulders represent an erosional surface between two glacial tills. Carboniferous limestone boulder in till at Whitburn Bay. The boulder is shaped by glacial erosion and scratched by contact with other rocks and ice. Channel of sand and gravel deposited at the ice-bed interface.
Two tills rest on top of Magnesian Limestone bedrock at Whitburn Bay, overlain by deformed glaciofluvial sands sands deposited by a proglacial river. Note the large, faceted boulders at the boundary between the two tills. Folded and deformed sands overlying glacial till at Whitburn Bay, County Durham.
This sand was deposited in front of the glacier proglacial , but then overridden by the ice. Complexly interbedded, folded and deformed sands in Middle Pleistocene sediments at Warren House Gill. Deformed and faulted chalk clasts in Middle Pleistocene glacial tills in North Norfolk, indicating brittle deformation. Soft chalk clasts have been strectched and folded here in north Norfolk by glacial processes. Rafts of chalk have been thrust up on top of one another within glaciotectonised sediments in north Norfolk.
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