{"id":181,"date":"2024-03-08T22:59:29","date_gmt":"2024-03-08T22:59:29","guid":{"rendered":"https:\/\/pressbooks.ccconline.org\/ppscgey1108geologyofnationalparks\/chapter\/glaciers\/"},"modified":"2024-03-28T22:34:08","modified_gmt":"2024-03-28T22:34:08","slug":"glaciers","status":"publish","type":"chapter","link":"https:\/\/pressbooks.ccconline.org\/ppscgey1108geologyofnationalparks\/chapter\/glaciers\/","title":{"raw":"Glaciers!","rendered":"Glaciers!"},"content":{"raw":"
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How Glaciers Work<\/h2>\r\n[caption id=\"\" align=\"aligncenter\" width=\"623\"]\"Part Figure 16.2.1 Part of the continental ice sheet in Greenland, with some outflow alpine glaciers in the foreground.[\/caption]\r\n\r\n
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There are two main types of glaciers.<\/p>\r\n

Continental glaciers<\/strong> cover vast areas of land in extreme polar regions, including Antarctica and Greenland (Figure 16.2.1<\/strong>).<\/p>\r\n

Alpine glaciers<\/strong> (a.k.a. valley glaciers) originate on mountains, mostly in temperate and polar regions (Figure 16.0.1<\/strong>), but even in tropical regions if the mountains are high enough.<\/p>\r\n\r\n<\/div>\r\n

Earth\u2019s two great continental glaciers, on Antarctica and Greenland, comprise about 99% of all of the world\u2019s glacial ice, and approximately 68% of all of Earth\u2019s fresh water. The Antarctic Ice Sheet is vastly bigger than the Greenland Ice Sheet; it contains about 17 times as much ice. If the entire Antarctic Ice Sheet were to melt, sea level would rise by about 80 m and most of Earth\u2019s major cities would be completely submerged.<\/p>\r\n\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"552\"]\"Schematic Figure 16.2.3 Schematic ice-flow diagram for the Antarctic Ice Sheet.[\/caption]\r\n

Continental glaciers do not flow \u201cdownhill\u201d because the large areas that they cover are generally flat. Instead, ice flows from the region where it is thickest toward the edges where it is thinner, as shown in Figure 16.2.3<\/strong>. This means that in the central thickest parts, the ice flows almost vertically down toward the base, while in the peripheral parts, it flows out toward the margins. In continental glaciers like Antarctica and Greenland, the thickest parts (4,000 m and 3,000 m respectively) are the areas where the rate of snowfall and therefore the rate of ice accumulation are highest.<\/p>\r\n\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"547\"]\"Schematic Figure 16.2.4 Schematic ice-flow diagram for an alpine glacier.[\/caption]\r\n

The flow of alpine glaciers is primarily controlled by the slope of the land beneath the ice (Figure 16.2.4<\/strong>). 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).<\/p>\r\n

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. Eventually the granules are \u201cwelded\u201d together to create glacial ice (Figure 16.2.5<\/strong>). Downward percolation of water from melting taking place at the surface contributes to the process of ice formation.<\/p>\r\n\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"334\"]\"Steps Figure 16.2.5 Steps in the process of formation of glacial ice from snow, granules, and firn. (Image Description below.)[\/caption]\r\n\r\n<\/div>\r\n \r\n

\r\n\r\nFigure 16.2.5 image description: The comparative density of snowflakes, ice granules, firn, and ice.\u200b<\/strong>\r\n\r\n\r\n\r\n\r\n\r\n\r\n\r\n
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Shape<\/strong><\/p>\r\n<\/th>\r\n

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Amount of air<\/strong><\/p>\r\n<\/th>\r\n

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Density (grams per <\/strong>centimetres<\/strong> cubed)<\/strong><\/p>\r\n<\/th>\r\n<\/tr>\r\n

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Snowflake<\/p>\r\n<\/td>\r\n

\r\n

90%<\/p>\r\n<\/td>\r\n

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Less than 0.1<\/p>\r\n<\/td>\r\n<\/tr>\r\n

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Ice Granules<\/p>\r\n<\/td>\r\n

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50%<\/p>\r\n<\/td>\r\n

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From around 0.3 to 0.5<\/p>\r\n<\/td>\r\n<\/tr>\r\n

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Firn<\/p>\r\n<\/td>\r\n

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30%<\/p>\r\n<\/td>\r\n

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From around 0.5 to 0.7<\/p>\r\n<\/td>\r\n<\/tr>\r\n

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Ice<\/p>\r\n<\/td>\r\n

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20% (as bubbles)<\/p>\r\n<\/td>\r\n

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Greater than 0.7<\/p>\r\n<\/td>\r\n<\/tr>\r\n<\/tbody>\r\n<\/table>\r\n

The equilibrium line of a glacier near Whistler, B.C., is shown in Figure 16.2.6<\/strong>. Below that line, in the zone of ablation, bare ice is exposed because last winter\u2019s snow has all melted; above that line, the ice is still mostly covered with snow from last winter. 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. More winter snow and less summer melting obviously favours the advance of the equilibrium line (and of the glacier\u2019s leading edge), but of these two variables, it is the summer melt that matters most to a glacier\u2019s budget. Cool summers promote glacial advance and warm summers promote glacial retreat.<\/p>\r\n\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"623\"]\"The Figure 16.2.6 The approximate location of the equilibrium line (red) in September 2013 on the Overlord Glacier, near Whistler, B.C.[\/caption]\r\n

Glaciers move because the surface of the ice is sloped. This generates a stress on the ice, which is proportional to the slope and to the depth below the surface. The stresses are quite small near the ice surface but much larger at depth, and also greater in areas where the ice surface is relatively steep. Ice will deform, meaning that it will behave in a plastic manner, at stress levels of around 100 kilopascals; therefore, in the upper 50 m to 100 m of the ice (above the dashed red line), flow is not plastic (the ice is rigid), while below that depth, ice is plastic and will flow.<\/span><\/p>\r\n

When the lower areas of ice of a glacier flows, it moves the upper ice along with it, so although it might seem from the stress that the part underneath moves the most; in fact while the lower part deforms (and flows) and the upper part doesn\u2019t deform at all; the upper part moves the fastest because it is pushed along by the ice below it.<\/p>\r\n

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 16.2.8<\/strong>, for example, the glacier is speeding up over the steep terrain, and the rigid surface ice has to crack to account for the change in velocity.<\/p>\r\n\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"466\"]\"Crevasses Figure 16.2.8 Crevasses on Overlord Glacier in the Whistler area, B.C.[\/caption]\r\n

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. If the base is cold, the ice will be frozen to the material underneath and it will be stuck\u2014unable to slide along its base. In this case, all of the movement of the ice will be by internal flow.<\/p>\r\n

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. The slow transfer of heat from Earth\u2019s interior provides enough heat to warm up the base if the ice is thick, but not enough if it is thin and that heat can escape. It is typical for the leading edge of an alpine glacier to be relatively thin.<\/p>\r\n

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.<\/p>\r\n

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).<\/p>\r\n

Calving of icebergs is an important process for glaciers that terminate in lakes or the ocean.<\/p>\r\n\r\n

Glacial Erosion<\/h2>\r\n

Glaciers are effective agents of erosion, especially in situations where the ice is not frozen to its base and can therefore slide over the bedrock or other sediment. In fact, the ice itself is not particularly effective at erosion because it is relatively soft (Mohs hardness 1.5 at 0\u00b0C); instead, it is the rock fragments embedded in the ice and pushed down onto the underlying surfaces that do most of the erosion. A useful analogy would be to compare the effect of a piece of paper being rubbed against a wooden surface, as opposed to a piece of sandpaper that has embedded angular fragments of garnet.<\/p>\r\n

The results of glacial erosion are different in areas with continental glaciation versus alpine glaciation. Continental glaciation tends to produce relatively flat bedrock surfaces, especially where the rock beneath is uniform in strength. In areas where there are differences in the strength of rocks, a glacier obviously tends to erode the softer and weaker rock more effectively than the harder and stronger rock. In many cases the existing relief is due the presence of glacial deposits\u2014such as drumlins, eskers, and moraines (all discussed below)\u2014rather than to differential erosion (Figure 16.3.1<\/strong>).<\/p>\r\n\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"751\"]\"Drumlins Figure 16.3.1 Drumlins \u2014 streamlined hills formed beneath a glacier, here made up of sediment \u2014 in the Amundsun Gulf region of Nunavut. The drumlins are tens of metres high, a few hundred metres across, and a few kilometres long. One of them is highlighted with a dashed white line.[\/caption]\r\n

Alpine glaciers produce very different topography than continental glaciers, and much of the topographic variability of western Canada can be attributed to glacial erosion. In general, glaciers are much wider than rivers of similar length, and since they tend to erode more at their bases than their sides, they produce wide valleys with relatively flat bottoms and steep sides\u2014known as U-shaped valleys (Figure 16.3.2<\/strong>).<\/p>\r\n\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"592\"]\"A Figure 16.3.2 A depiction of a U-shaped valley occupied by a large glacier.[\/caption]\r\n

U-shaped valleys and their tributaries provide the basis for a wide range of alpine glacial topographic features, examples of which are visible on the International Space Station view of the Swiss Alps shown in Figure 16.3.4<\/strong>. This area was much more intensely glaciated during the past glacial maximum. At that time, the large U-shaped valley in the lower right was occupied by glacial ice, and all of the other glaciers shown here were longer and much thicker than they are now. But even at the peak of the Pleistocene Glaciation, some of the higher peaks and ridges would have been exposed and not directly affected by glacial erosion. A peak that extends above the surrounding glacier is called a nunatuk. In these areas, and in the areas above the glaciers today, most of the erosion is related to freeze-thaw effects.<\/p>\r\n\r\n\r\n[caption id=\"\" align=\"aligncenter\" width=\"562\"]\"A Figure 16.3.4 A view from the International Space Station of the Swiss Alps in the area of the Aletsch Glacier. The prominent peaks labelled \u201cHorn\u201d are the Eiger (left) and Wetterhorn (right). A variety of alpine glacial erosion features are labelled.[\/caption]\r\n\r\n

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Figure 16.3.4<\/strong> is a Space Station view of a glaciated terrain in the Swiss Alps.\u00a0 Some of the important features visible are<\/p>\r\n\r\n