The plastic lower ice of a glacier can flow like a very viscous fluid, and can therefore flow over irregularities in the base of the ice and around corners. However, the upper rigid ice cannot flow in this way, and because it is being carried along by the lower ice, it tends to crack where the lower ice has to flex. This leads to the development of crevasses in areas where the rate of flow of the plastic ice is changing. In the area shown in Figure The base of a glacier can be cold below the freezing point of water or warm above the freezing point.
If it is warm, there will likely be a film of water between the ice and the material underneath, and the ice will be able to slide over that surface. This is known as basal sliding Figure If the base is cold, the ice will be frozen to the material underneath and it will be stuck — unable to slide along its base.
In this case, all of the movement of the ice will be by internal flow. One of the factors that affects the temperature at the base of a glacier is the thickness of the ice.
Ice is a good insulator. It is typical for the leading edge of an alpine glacier to be relatively thin see Figure This is illustrated in Figure Because the leading edge of the glacier is stuck to its frozen base, while the rest continues to slide, the ice coming from behind has pushed or thrust itself over top of the part that is stuck fast. Just as the base of a glacier moves more slowly than the surface, the edges, which are more affected by friction along the sides, move more slowly than the middle.
If we were to place a series of markers across an alpine glacier and come back a year later, we would see that the ones in the middle had moved farther forward than the ones near the edges Figure Glacial ice always moves downhill, in response to gravity, but the front edge of a glacier is always either melting or calving into water shedding icebergs.
If the rate of forward motion of the glacier is faster than the rate of ablation melting , the leading edge of the glacier advances moves forward. If the rate of forward motion is about the same as the rate of ablation, the leading edge remains stationary, and if the rate of forward motion is slower than the rate of ablation, the leading edge retreats moves backward. Meltwater flows out to the terminus through hidden channels and tunnels. Oldest ice is the deepest.
If the balance is tipped, the glacier shifts and either advances or retreats. Motion and Movement Mass Balance : The difference between the amount of material that a glacier accumulates and the amount lost during ablation is called its mass balance. The ice in the middle of a glacier flows faster than the ice along the sides of the glacier. Illustration by Erica Herbert.
Revealed by Satellite Radar. Interferogram of Bagley Ice Field. There are many types of moraines: Terminal or toehold — The advancing ice scrapes and grinds the bedrock boulders and gravel beneath it and pushes ahead of itself a ridge or terminal moraine of rock and earth. Lateral — their rock material comes from the valley walls. Medial — When two lateral moraines combine, or a tributary glacier joins the main flow, they form a single medial moraine, which extends as a long, dark stripe down the middle of the glacier towards the snout.
When medial moraines come close to one another near the terminus, a glacier may look multicolored or striped. Medial moraines can create interesting swirls and loops. Ablation — an accumulation of melted-out rocks sometimes just sparse collections of glacial till. End and Push — created near the margin of a glacier, at the terminus. Ground and Dump — glaciers often dump out their supply of rocks as they retreat. Moraines from tributaries. Barnard Glacier, Alaska, Looping medial moraines.
Photos by James Roush. Hubbard Glacier, Alaska. Photo by Alean. Glacier flour. Photo by Dr. Bruce Molnia, Alaska Geographic Society. Photo by Alaska Geographic Society. Stakes measuring glacier movement at the Bering Glacier in Alaska. Photo by James Roush. Scientist measures the movement of the Bering Glacier in Alaska. How do Glaciers Move? Vocabulary Plus!
Review Questions some of the answers may come from the vocabulary list What causes the glacier to be in motion? True or False: Glaciers slide on their beds and this enables them to move faster. When this happens, the glacier can ride on the deforming sediments in its bed. Explore This Park. How Glaciers Move. Evidence of the flowing ice can be found in glacier's heavily crevassed surface. NPS Photo Glaciers move by a combination of 1 deformation of the ice itself and 2 motion at the glacier base.
One component of glacier flow is the deformation of the ice itself. This water can come from a number of different sources, including: Meltwater from the surface of the glacier that makes its way through crevasses, moulins, and weaknesses in the ice to the glacier bed.
Ice at the base of the glacier melting due to friction between the moving glacier and its bed. Heat emitted from the Earth surface due to radioactive decay and leftover heat from Earth's formation that melts the ice.
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