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Glaciers and Glacial Geology Comp. by: PG2693 Stage : Proof ChapterID: 9780521519786c08 Date:18/1/10 Time:20:56:26 Filepath:H:/ 01_CUP/3B2/Anderson&Anderson_9780521519786/Applications/3B2/Proof/9780521519786c08.3d CHAPTER 8 Glaciers and glacial geology Fire and Ice Some say the world will end in fire Some say in ice. From what I’ve tasted of desire I hold with those who favor fire. But if I had to perish twice, I think I know enough of hate To say that for destruction ice Is also great And would suffice. Robert Frost Figure 8.0 Kennicott Glacier, Wrangell–St. Elias National Park, Alaska. Mt. Blackburn graces the left skyline. Little Ice Age moraines bound the ides of the glacier roughly 16 km from its present terminus near the town of McCarthy. (photograph by R. S. Anderson) 212 Comp. by: PG2693 Stage : Proof ChapterID: 9780521519786c08 Date:18/1/10 Time:20:56:26 Filepath:H:/ 01_CUP/3B2/Anderson&Anderson_9780521519786/Applications/3B2/Proof/9780521519786c08.3d we address the processes that modify landscapes once occupied by glaciers. These In this chapter large mobile chunks of ice are very effective agents of change in the landscape, sculpting distinctive landforms, and generating prodigious amounts of sediment. Our task is broken into two parts, which form the major divisions of the chapter: (1) understanding the physics of how glaciers work, the discipline of glaciology, which is prerequisite for (2) understanding how glaciers erode the landscape, the discipline of glacial geology. 213 Comp. by: PG2693 Stage : Proof ChapterID: 9780521519786c08 Date:18/1/10 Time:20:56:27 Filepath:H:/ 01_CUP/3B2/Anderson&Anderson_9780521519786/Applications/3B2/Proof/9780521519786c08.3d Glaciers and glacial geology 214 Although glaciers are interesting in their own right, How does the water discharge from a glacial outlet and lend to alpine environments an element of beauty stream vary through a day, and through a year? of their own, they are also important geomorphic actors. Occupation of alpine valleys by glaciers leads to the generation of such classic glacial signatures as Glaciology: what are glaciers and how U-shaped valleys, steps and overdeepenings now do they work? occupied by lakes in the long valley profiles, and hanging valleys that now spout waterfalls. Even A glacier is a natural accumulation of ice that is coastlines have been greatly affected by glacial pro- in motion due to its own weight and the slope of its cesses. Major fjords, some of them extending to water surface. The ice is derived from snow, which slowly depths of over 1 km, punctuate the coastlines of west- loses porosity to approach a density of pure ice. This ern North America, New Zealand and Norway. We evolution is shown in Figure 8.1. Consider a small will discuss these features and the glacial erosion alpine valley. In general, it snows more at high alti- processes that lead to their formation. tudes than it does at low altitudes. And it melts more It is the variation in the extent of continental scale at low altitudes than it does at high. If there is some ice sheets in the northern hemisphere that has driven place in the valley where it snows more in the winter the 120–150 m fluctuations in sea level over the last than it melts in the summer, there will be a net accu- three million years. On tectonically rising coastlines, mulation of snow there. The down-valley limit of this this has resulted in the generation of marine terraces, accumulation is the snowline, the first place where each carved at a sea level highstand corresponding to you would encounter snow on a climb of the valley an interglacial period. in the late fall. If this happened year after year, a Ice sheets and glaciers also contain high-resolution wedge of snow would accumulate each year, com- records of climate change. Extraction and analysis of pressing the previous years’ accumulations. The snow cores of ice reveal detailed layering, chemistry, and air slowly compacts to produce firn in a process akin to bubble contents that are our best terrestrial paleocli- the metamorphic reactions in a mono-mineralic rock mate archive for comparison with the deep sea near its pressure melting point. Once thick enough, records obtained through ocean drilling programs. The interest in glaciers is not limited to Earth, either. As we learn more about other planets in the solar system, attention has begun to focus on the 0 potential that Martian ice caps could also contain paleoclimate information. And further out into the 20 Upper Seward Glacier solar system are bodies whose surfaces are at mostly 40 water ice. Just how these surfaces deform upon impacts of bolides, and the potential for unfrozen 60 water at depth, are topics of considerable interest in Depth (m) the planetary sciences community. 80 Finally, glaciers are worth understanding in their site 2, Greenland own right, as they are pathways for hikers, sup- 100 pliers of water to downstream communities, and sources of catastrophic floods and ice avalanches. 120 The surfaces of glaciers are littered with cracks and 400 500 600 700 800 900 3 holes, some of which are dangerous. But these Density (kg/m ) hazards can either be avoided or lessened if we Figure 8.1 Density profiles in two very different glaciers, approach them with some knowledge of their the upper Seward Glacier in coastal Alaska being very wet, the origin. How deep are crevasses? How are crevasses Greenland site being very dry. The metamorphism of snow is much typically oriented, and why? What is a moulin? more rapid in the wetter case; firn achieves full ice densities by 20 m on the upper Seward and takes 100 m in Greenland. Ice with no What is a medial moraine, what is a lateral pore space has a density of 917 kg/m3. (after Paterson, 1994, moraine? Are they mostly ice or mostly rock? Figure 2.2, reproduced with permission from Elsevier) Comp. by: PG2693 Stage : Proof ChapterID: 9780521519786c08 Date:18/1/10 Time:20:56:28 Filepath:H:/ 01_CUP/3B2/Anderson&Anderson_9780521519786/Applications/3B2/Proof/9780521519786c08.3d Types of glaciers: a bestiary of ice 215 and all the small glaciers and ice caps of the world roughly 2 m. We note, however, that the small ice accumulation area bodies of the globe are contributing disproportio- ELA snowline, ELA nately to the present sea level rise. ablation area Elevation, z Elevation, b(z) Types of glaciers: a bestiary of ice – 0+ + local mass balance, b 0 First of all, note that sea ice is fundamentally different – b(x) from glacier ice. Sea ice is frozen seawater; it is not born of snow. It is usually a few meters thick at the best, with Qmax Q(x) pressure ridges and their associated much deeper keels x being a few tens of meters thick. Icebreakers can plow Ice Discharge, Q Q(x) = W(x)b(x) dx 0 through sea ice. They cannot plow through icebergs, 0 which are calved from the fronts of tidewater glaciers, Distance Downvalley, x and can be more than a hundred meters thick. It is icebergs that pose a threat to shipping. Figure 8.2 Schematic diagrams of a glacier (white) in mountainous topography (gray) showing accumulation and Glaciers can be classified in several ways, using size, ablation areas on either side of the equilibrium line. Mapped into the thermal regime, the location in the landscape, and the vertical, z (left-hand diagram), the net mass balance profile, even the steadiness of a glacier’s speed. Some of these b(z), is negative at elevations below the ELA and positive above it. We also show the net balance mapped onto the valley- parallel classifications overlap, as we will see. We will start axis, x (follow dashed line downward), generating the net with the thermal distinctions, as they play perhaps the balance profile b(x). At steady state the ice discharge of the most important role in determining the degree to glacier must reflect the integral of this net balance profile (bottom diagram). The maximum discharge should occur which a glacier can modify the landscape. at roughly the down-valley position of the ELA. Where The temperatures of polar glaciers are well below the discharge goes again to zero determines the the freezing point of water throughout except, in terminus position. some cases, at the bed. They are found at both very high latitudes and very high altitudes, reflecting the very cold mean annual temperatures there. As is seen this growing wedge of snow-ice can begin to deform in Figure 8.3, to first order, a thermal profile in these under its own weight, and to move downhill. At this glaciers would look like one in rock, increasing with point, we would call the object a glacier. It is only by depth in a geothermal profile that differs from one in the motion of the ice that ice can be found further rock only in that the conductivity and density of ice is down the valley than the annual snowline. different from that of rock. A glacier can be broken into two parts, as summa- In contrast to these glaciers, temperate glaciers are rized in Figure 8.2: the accumulation area, where there those in which the mean annual temperature is very is net accumulation of ice over the course of a year, and close to the pressure-melting point of ice, all the way the ablation area, where there is net loss of ice. The two to the bed. The distinction is clearly seen in Figure 8.3. are separated by the equilibrium line, at which a bal- They derive their name from their location in temperate ance (or equilibrium) exists between accumulation and climates whose mean annual temperatures are closer ablation.
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