This preview has intentionally blurred parts. Sign up to view the full document

View Full Document

Unformatted Document Excerpt

19: Chapter Glacial Modification of Terrain TOPICS The Impact of Glaciers on the Landscape Glaciations Past and Present Pleistocene Glaciation Contemporary Glaciation Types of Glaciers Continental Ice Sheets Mountain Glaciers Glacier Formation and Movement Changing Snow to Ice Glacial Movement The Effects of Glaciers Erosion by Glaciers Transportation by Glaciers Deposition by Glaciers Deposition by Meltwater Continental Ice Sheets Development and Flow Erosion by Ice Sheets Deposition by Ice Sheets Glaciofluvial Features Mountain Glaciers Development and Flow Erosion by Mountain Glaciers Deposition by Mountain Glaciers The Periglacial Environment Causes of the Pleistocene Are We Still in an Ice Age? People and the Environment: Disintegration of Antarctic Ice Shelves Focus: Antarctica KEY TERMS ablation (of glacial ice) (p. 563) ablation zone (p. 564) accumulation (of glacial ice) (p. 563) accumulation zone (p. 563) arte (p. 576) basal slip (p. 564) cirque (p. 575) cirque glacier (p. 562) col (p. 576) continental ice sheet (p. 561) drift (glacial drift) (p. 567) drumlin (p. 571) equilibrium line (p. 564) erratic (glacial erratic) (p. 567) esker (p. 573) glacial flour (p. 566) glacial plucking (p. 565) glacial steps (p. 579) glacial trough (p. 579) glaciofluvial deposition (p. 567) ground moraine (p. 571) hanging valley (p. 580) 1 highland ice field (p. 562) horn (p. 576) kame (p. 573) kettle (p. 571) lateral moraine (p. 580) medial moraine (p. 582) moraine (p. 571) nv (firn) (p. 563) outwash plain (p. 572) paternoster lakes (p. 579) patterned ground (p. 583) periglacial zone (p. 557) piedmont glacier (p. 562) CHAPTER OUTLINE Pleistocene epoch (p. 555) pluvial effects (p. 557) proglacial lake (p. 584) recessional moraine (p. 571) roche moutonne (p. 568) stratified drift (p. 572) tarn (p. 577) terminal moraine (p. 571) till (p. 567) till plain (p. 571) valley glacier (p. 562) valley train (p. 573) I. The Impact of Glaciers on the Landscape A. Glacier occurs when there is a net year-to-year accumulation of snow over a period of years. B. Glaciers have had overwhelming impact on landscape because moving ice grinds away almost anything in its path. 1. Significantly reshape the topography. C. About 7% of all contemporary erosion is accomplished by glaciers. II. Glaciations Past and Present A. Record is incomplete and often approximate. B. Pleistocene Glaciation 1. Only most recent ice age has influenced contemporary topography, as most changes made by previous ice ages have been eradicated by other geomorphic events. a) Ice Age, when capitalized, refers to Pleistocene because of its impact. (1) Many parts of the continental terrain have been imprinted by Pleistocene events. b) Pleistocene Epoch began at least 1.8 million years ago, but almost every year evidence is found that it began even earlier. (1) Most recent findings show that the amplitude of climate fluctuations from a glacial period to an interglacial period had increased around 2.5 million years ago. (2) Consisted of an alternation of glacial and interglacial periods. (a) Glacialtimes of ice accumulation. (b) Interglacialtimes of ice retreat. (3) Evidence showing that the last major ice retreat took place more recently than 9,000 years ago. (a) Ice Age may not have ended yet at all. (i) Period since last glacial stage is known as the Holocene Epoch. (a) May either be a postglacial epoch or just the latest in a series of interglacial interludes. (b) Given that current evidence points to as many as 18 or 19 glacial episodes (glacial and interglacial periods) taking place during the Pleistocene, it is no surprise that we may experience another glacial period. c) Refrigerated high-latitude and high-altitude areas. (1) At its maximum, covered one-third of the total land area of Earth. 2 d) Pleistocene had indirect effects on landscape through periglacial processes, sea-level changes, crustal depression, and pluvial developments. (1) Periglacial processes of erosion and deposition from glacier meltwater and of frost weathering affected more than 20% of Earths land area. (a) Periglacial zonean area of indefinite size beyond the outermost extent of ice advance that was indirectly influenced by glaciation. (2) Sea-level changesWorldwide lowering of sea level occurred during every episode of Pleistocene glacial advance. (3) Crustal depressionWeight of ice caused portion of Earths crust to sink, sometimes to depths of 4,000 feet. Portions of Canada and northern Europe are still in isostatic rebound. (4) Pluvial developments increased rainfall, which in turn created lakes where they never existed. C. Contemporary Glaciation 1. Ice covers about 10% of Earth now. a) More than 96% of that is in Antarctica and Greenland (Fig. 192 shows distribution of glacial ice). 2. Antarctic Ice Cap a) Glacial ice covers more than 98% of this continents surface. b) Bedrock floor is irregular and deeply buried, and in some places well-below sea level. (1) If West Antarctica lost its ice, it would appear as a considerable number of scattered islands. c) Continent is two unequal sections separated by the wide upland belt of the Transantarctic Mountains. d) A few interior valleys, called the Dry Valleys, are curiously ice free because winds blast away snow and keep precipitation out. These three parallel valleys contain several large lakes, ponds, and a river that flows for one or two months each year. 3. Greenland Ice Cap a) Much smaller than Antarctica. (1) 1,740,000 square kilometers. 4. North American Glaciers a) Most glaciers are in the Pacific Northwest, with greater than 50% in the North Cascade Mountains. b) Sizable glaciers also present in Alaska. III. Types of Glaciers A. Glaciers pattern of movement and its effect on topographic shaping can vary considerably depending on quantity of ice and particularly on the environment. B. Two different types of glaciers: ice sheets and mountain glaciers. C. Continental Ice Sheets 1. Ice sheetan immense blanket of ice that completely inundates the underlying terrain to depths of hundreds or thousands of feet. a) Formed in nonmountainous areas of continents. 2. Only two true ice sheets currently, in Antarctica and Greenland. 3. Outlet glaciera tongue of ice around the margin of an ice sheet that extends between rimming hills to the sea. a) Icebergs form from chunks of ice that break off ice shelves and outlet glaciers. D. Mountain Glaciers 1. Two types of mountain glaciers: ice fields and alpine glaciers. 3 a) Icefieldan unconfined sheet of ice in high-mountain areas, and which can develop into valley glaciers and piedmont glaciers. (1) Valley glaciera long, narrow feature resembling a river of ice, which spills out of its originating basin and flows down-valley. (2) Piedmont glaciera valley glacier that extends to the mouth of the valley and spreads out broadly over the flat land beyond. b) Alpine glacierindividual glacier that develops near a mountain crest line and normally moves down-valley for some distance. (1) Cirque glaciera small glacier confined to its cirque and not moving down-valley. (2) An alpine glacier typically breaks out of its basin and forms a valley glacier, and can extend to mouth of valley to create a piedmont glacier. IV. Glacier Formation and Movement A. Glaciers require certain circumstances to form and then depend on just the right combination of temperature and moisture to survive. B. Balance of accumulation and ablation is critical for persistence of glacier. 1. Accumulationaddition of ice into a glacier by incorporation of snow. 2. Ablation wastage of glacial ice through melting and sublimation. C. Changing Snow to Ice 1. Firn (Nv)snow granules that have become packed and begin to coalesce due to compression, achieving a density about half as great as that of water. 2. Equilibrium linea theoretical line separating the ablation zone and accumulation zone of a glacier along which accumulation exactly balances ablation. 3. Every glacier can be divided into two portions. a) Upper portion is the accumulation zone, because accumulation exceeds amount lost by melting and sublimation. b) Lower portion is ablation zone, because more is lost than is added each year. D. Glacial Movement 1. Very little similarity between glacial movement, which is orderly, and liquid flow, which is disordered. 2. Because ice under glacier is under considerable pressure, it deforms rather than breaks. 3. Flow is often erratic and all parts of glaciers do not move at the same rate. a) Fastest moving is at and near the surface. b) If glacier is confined, as in valley glacier, the center moves faster than sides (as in streamflow). 4. Glacier Flow Versus Glacier Advance a) Flow is the continual movement of the ice toward the edge(s) of the glacier. b) Advance means the forward movement of the outer margins of the glacial body. E. Erosion by Glaciers 1. Volume and speed determine the effectiveness of glacial erosion. 2. Erode by plucking and abrasion. 3. Glacial Plucking a) Pluckingquarrying action in which rock particles beneath the ice are grasped by the freezing of meltwater in joints and fractures and pried out and dragged along in the general flow of a glacier. (1) Probably accomplishes a glaciers most significant erosive work. (2) Particularly effective on leeward slopes (those facing away from the direction of movement). 4. Glacial Abrasion 4 a) Abrasion tends to polish when bedrock is highly resistant and dig striations and grooves in less resistant. 5. Glacial erosion effects are more notable in hilly areas; making entire landscape becomes more angular and rugged. F. Transportation by Glaciers 1. Glaciers are extremely competent in their ability to transport rock debris. 2. Glacier flourrock material that has been ground to the texture of very fine talcum powder by glacial action. a) Perhaps most typical component of glacial load. b) Most of load is picked up from bottom, and so carried along there in a narrow zone. c) Alpine glaciers also carry some material on top of ice, where mass wasting from surrounding slopes placed debris. d) Transportation occurs at variable speeds outward or down-valley. (1) Rate depends on season, variations in ice accumulation, and gradient of underlying slopes. e) Flowing water transports water to many glaciers. G. Deposition by Glaciers 1. Transportation and deposition are probably the major roles of glaciers in landscape modification. 2. Gave U.S. Midwest one of the worlds most productive soils (at the expense of central Canada, where the soil, regolith, and even some bedrock was scoured, transported, and later deposited. a) Driftall material carried and deposited by glaciers. (1) Name comes from thought that the material had drifted from biblical floods. b) Tillrock debris that is deposited directly by moving or melting ice, with no meltwater flow or redeposition involved. c) Glacial erraticoutsized boulder included in the glacial till, which may be very different from the local bedrock. H. Deposition by Meltwater 1. Glaciofluvial deposition, through meltwater, occurs around margins of all glaciers and can continue far out into periglacial zones. a) Meltwater actually deposits or redeposits much of the debris carried by glaciers. b) Can occur by (1) Subglacial streams issuing from ice, depositing debris; (2) Meltwater from glaciers, picking up material already deposited and redepositing it elsewhere. (a) Most of meltwater deposition actually involves redeposition. V. Continental Ice Sheets A. Most extensive features to appear on face of planet. 1. Pleistocene ice sheets reshaped the terrain and drainage of nearly one-fifth of Earths land surface. B. Development and Flow 1. Pleistocene ice sheets, except for that in Antarctica, developed in subpolar and midlatitude locations then spread in all directions. 2. Preexisting terrain channeled the initial flow, but then ice accumulation overrode most of the preglacial topography. a) Eventually, various ice sheets coalesced into one, two, or three massive sheets on each continent. C. Erosion by Ice Sheets 5 1. Ice sheets generally result in a gently undulating surface: low relief but not absolute flatness. a) Ice-scoured rock knobs and scooped-out depressions, bare rock and lakes, dominate. b) Results in erratic and inadequately developed stream patterns. c) Creates most conspicuous features of U-shaped valley bottoms. (1) Ice sheet in central New York reshaped parallel stream valleys into the long, narrow, deep Finger Lakes. 2. Roche moutonnea characteristic landform produced when a bedrock hill or knob is overridden by moving ice. The stoss side is smoothly rounded and streamlined by grinding abrasion as the ice rides up the slope; but the lee side is shaped largely by plucking, which produces a steeper and more irregular slope. 3. Erosional effects, however, are modified by depositional debris. D. Deposition by Ice Sheets 1. Till plainan irregularly undulating surface of broad, low rises and shallow depressions produced by the uneven deposition of glacial till. 2. Morainethe largest and generally most conspicuous landform feature produced by glacial deposition, which consists of irregular rolling topography that rises somewhat above the level of the surrounding terrain. a) Terminal morainesmark the farthest advance of the glacier. b) Recessional morainesmark positions where the ice front had temporarily stabilized. c) Ground morainemark where large quantities of till were laid down from underneath the glacier. 3. Kettlean irregular depression in a morainal surface created when blocks of stagnant ice eventually melt. 4. Drumlina low, elongated hill formed by ice-sheet deposition. The long axis is aligned parallel with the direction of ice movement, and the end of the drumlin that faces the direction from which the ice came is blunt and slightly steeper than the narrower and more gently sloping end that faces in the opposite direction. a) Depositional features subsequently shaped by erosion. b) Usually occur in groups, sometimes in hundreds. (1) Central New York and eastern Wisconsin have the greatest concentrations. E. Glaciofluvial Features 1. Meltwater is incapable of moving larger material, so glaciofluvial features are composed largely or entirely of gravel, sand, and silt. a) Stratified driftdrift that was sorted as it was carried along by the flowing glacial meltwater. 2. Outwash plainrelatively smooth, flattish alluvial apron deposited beyond recessional or terminal moraines by streams issuing from ice. a) Most extensive glaciofluvial features. 3. Valley traina lengthy deposit of glaciofluvial alluvium confined to a valley bottom beyond the outwash plain. 4. Eskerlong, sinuous ridges of stratified drift composed largely of glaciofluvial gravel and formed by the choking of subglacial streams during a time of glacial stagnation. 5. Kamea relatively steep-sided mound or conical hill composed of stratified drift found in areas of ice-sheet deposition and associated with meltwater deposition in close association with stagnant ice. 6. Lakes are also very common on these landscapes. a) Old stream systems were obliterated by ice sheets, and water remains ponded in many erosional basins and kettles. 6 VI. Mountain Glaciers A. Dont reshape the terrain as much as ice sheets did. 1. Mountains protrude above mountain glacier ice. 2. Mountains channel the movement of mountain glaciers. 3. Produce a rugged landscape as opposed to the smoothing and rounding of terrain accomplished by ice sheets. B. Development and Flow 1. Both highland icefields and alpine glaciers advance downslope, usually finding path of least resistance down preexisting stream valley. a) Highland icefields can extend broadly, submerge all but uppermost peaks, and extend into a series of lobes that move down adjacent channels. b) Alpine glaciers usually form in sheltered depressions near heads of stream valleys. C. Erosion by Mountain Glaciers 1. Highland icefields and alpine glaciers can dramatically reshape topography. a) Cirquea broad amphitheater hollowed out at the head of a glacial valley by ice erosion. (1) Basic landform in glaciated mountains, marking place where alpine glacier originated, being quarried out of mountainside, though precise mechanics of formation are unknown. b) Artea narrow, jagged, serrated spine of rock; remainder of a ridge crest after several cirques have been cut back into an interfluve from opposite sides of a divide. c) Cola pass or saddle through a ridge produced when two adjacent cirques on opposite sides of a divide are cut back enough to remove part of the arte between them. d) Horna steep-sided, pyramidal rock pinnacle formed by expansive quarrying of the headwalls where three or more cirques intersect. e) Tarnsmall lake in the shallow excavated depression of rock benches of a glacial trough or cirque. f) Glacial trougha valley reshaped by an alpine glacier, usually with a relatively straight course with a fluctuating gradient. g) Paternoster lakesa sequence of small lakes found in the shallow excavated depressions of a glacial trough. h) Hanging valleya tributary glacial trough, the bottom of which is considerably higher than the bottom of the principal trough that it joins. (1) Typically, streams that drain from tributary valleys must plunge over waterfalls to reach the floor of the main trough. D. Deposition by Mountain Glaciers 1. Drift occurs in the middle and lower courses of glacial valleys, only rarely in high country. 2. Moraines are the principal depositional landform, but are smaller and less conspicuous than those produced by ice sheets. a) Lateral moraines are the largest depositional features, being well-defined ridges of unsorted debris built up along the sides of valley glaciers. b) Medial moraines are formed by the trapping of lateral moraine debris between two valley glaciers that have joined. VII. The Periglacial Environment A. More than 20% of worlds land area is presently periglacial, but mostly from Pleistocene Epoch. B. Periglacial lands are either in high latitudes or high altitudes. 1. Patterned groundvarious geometric patterns that repeatedly appear over large areas in the Arctic, with unknown origins. a) Most unique and eye-catching periglacial terrain. 7 b) Widely accepted hypothesis is that frost action is instrumental in formation. c) Emphasizes role of soil ice in producing geomorphic activities that usually dont occur in warmer regions. 2. Proglacial lakea lake formed when ice flows across or against the general slope of the land and the natural drainage is impeded or completely blocked so that meltwater from the ice becomes impounded against the ice front. a) Most are small and temporary, but some are large and long-lived. VIII. Causes of the Pleistocene A. Scientists have postulated many theories and constructed many models to try to explain the sporadic glaciation and deglaciation of Earth. 1. Some theories are based on variations in intensity of solar radiation Earth has received. 2. Some look at shifting of Earths axis or variation in eccentricity of Earths orbit. 3. Some focus on changes in the amount of carbon dioxide in atmosphere. 4. Some founded on changes in the position of continents and ocean circulation patterns. 5. Some rooted in increased altitude occurring after period of tectonic upheaval. 6. Some combine elements of the above. a) None of the theories are widely acceptedstill looking for a convincing explanation. B. Are We Still in an Ice Age? 1. Still question if living in postglacial period or interglacial period. a) Based on the climate pattern of the last two million years, it is possible that Earth will enter another period of glaciation. b) Question if human-enhanced greenhouse effect will postpone the onset. IX. People and The Environment: Disintegration of Antarctic Ice Shelves A. Antarctica exerts a prominent influence on the worlds environment, particularly in terms of sea level, oceanic temperatures, circulation patterns, nutrient content of the oceans, and atmospheric circulation. a) If it melted, sea level would rise around the world by about 73 meters. b) Normal process of calving of icebergs from Antarctic ice sheets that flow toward the sea. c) Increase in the rate of flow and disintegration. (1) Since 1993 about 8000 square kilometers of shelf ice has disappeared. d) This will not cause sea-level rise, but it can trigger in the rate of flow of land-based ice off of the continent. X. Focus: Shrinking Alaskan Glaciers A. In Alaska, and throughout most of the world, glaciers are shrinking. 1. Much of the evidence is generated by comparing old photographs with contemporary images of the glaciers. 2. These photos also reveal a change in vegetation patterns, with increased vegetative cover in once relatively barren areas. 3. These causes are certainly linked to global climatic change. Study Questions for Key Concepts Glaciations Past and Present (p. 555) 1. Why is the Pleistocene epoch so important to physical geography, whereas other ice ages are not? 8 The Pleistocene Epoch is so important to physical geography in comparison to other ice ages because of its influence on current topography. It is known as the Ice Age, with capital letters, because it is the ice age that molded the glaciated landscapes we know. The other ice ages also no doubt had altered the landscape, or so we can assume by the concept of uniformitarianism, but that influence has been since eradicated by other geomorphic events, so nearly all the evidence of past glaciation we see is from the Pleistocene Epoch. 2. Briefly describe the worldwide extent of ice cover during the peak of the Pleistocene glaciations. During the Pleistocene Epoch, landscapes at high latitudes and high altitudes were virtually refrigerated, so we had glaciation, albeit to a limited extent, even in high mountain areas of central Africa, New Guinea, and Hawaii. More extensive glaciation occurred everywhere else in the world, including southernmost South America, much of the South Island of New Zealand, and more than half of Europe. Asia was less affected, presumably because its subarctic portion did not receive enough precipitation for the ice to last. But by far the greatest extent of glaciation occurred in North America. Table 192 offers a comparison of area covered. The most extensive Pleistocene ice mass, the Laurentide ice, covered most of Canada and a great portion of the northeastern United States. A small area in northwestern Canada as well as extensive portions of northern and western Alaska were never glaciated, for reasons not yet understood. Likewise, there was a pocket in southwestern Wisconsin and parts of three adjoining states that the Ice Age never covered. 3. Explain how and why global sea level fluctuated during the Pleistocene. The buildup of ice on the continents meant that less water was available to drain from the continents into the oceans, a condition that resulted in a worldwide lowering of sea level during every episode of glacial advance; the when glaciers retreated, sea level would again rise as meltwater returned to the oceans. At the peak of the Pleistocene glaciations, global sea level was about 130 meters (430 feet) lower than it is today. These fluctuations in the amount of drainage water caused a significant difference in drainage patterns and topographic development on seashores and coastal plains. 4. What is the relationship of large continental ice sheets to the crustal depression associated with isostatic adjustment (isostasy)? The enormous weight of accumulated ice on the continents caused portions of Earths crust to sink, in some cases by as much as 1200 meters (4000 feet). After the ice melted, the crust slowly began to rebound. This isostatic adjustment has not yet been completed, and some portions of Canada and northern Europe are still rising as much as 20 centimeters (8 inches) per decade. 5. What is meant by the pluvial effects of the Pleistocene? During the Pleistocene glaciations, there was, on almost all areas of the continents, a considerable increase in the amount of moisture available. This increase was caused by a combination of meltwater runoff, increased precipitation, and decreased evaporation. A prominent result of these pluvial effects was the creation of many lakes in areas where none had previously existed. Most of these lakes have subsequently been drained or significantly reduced in size, but they have left lasting imprints on the landscape. Many of these can be seen in the western part of the United States. The present-day Great Salt Lake in Utah is a tiny remnant of a much larger Pleistocene lake known as Lake Bonneville, and todays Bonneville Salt Flats were once the floor of this enormous lake. 6. Describe and explain the formation of large Pleistocene lakes in western North America. 9 See question 5. 7. Describe the global extent of glaciers today. About 10 percent of Earths land surfacesome 15 million square kilometers (6 million square miles)is covered with ice today, but more than 96 percent of that total is in Antarctica and Greenland. Antarctic ice is by far the most extensive ice cap on Earth. At present, about 98 percent of its surface is covered with glacial ice, representing about 85 percent of the worlds landice total. This ice is more than 4000 meters (13,000 feet) thick in some places and more than 1500 meters (5000 feet) thick over most of the continent. Greenland ice is much less extensive1,740,000 square kilometers (670,000 square miles)but still of impressive size. Elsewhere there are only relatively small ice masses on certain islands in the Canadian Arctic, Iceland, and some of the islands north of Europe. In the conterminous United States, most glaciers are in the Pacific Northwest, and more than half of these are in the North Cascade Mountains of Washington (Figure 19-6). In Alaska, there are 75,000 square kilometers (29,000 square miles) of glacial ice, amounting to about 4 percent of the total area of the state. The largest Alaskan glacier is the Bering Glacier, near Cordova, which covers 5830 square kilometers (2250 square miles) and is more than twice the size of Rhode Island. Types of Glaciers (p. 560) 8. Describe and contrast continental ice sheets, highland ice fields, valley glaciers, and cirque glaciers. Continental ice sheets are glaciers that formed in non-mountainous areas of the continents. During the Pleistocene these were vast blankets of ice that completely inundated the underlying terrain to depths of hundreds or thousands of meters. Because of their immense size, ice sheets have been the most significant agents of glaciation across the land surface. Only two true ones exist today, in Antarctica and Greenland. The ice in an ice sheet accumulates to great depths in the interior of the sheet but is much thinner at the outer edges. Around the margin of the sheet, some long tongues of ice, called outlet glaciers, extend between rimming hills to the sea. In other places, the ice reaches the ocean along a massive front, where it sometimes projects out over the sea as an ice shelf. Highland icefields are unconfined sheets of ice accumulation that occur in a few high-mountain areas; they may cover a few hundred or few thousand square kilometers, submerging all the underlying topography except perhaps for some protruding pinnacles (called nunataks). Such highland icefields are notable in parts of the high country of western Canada and southern Alaska and on various Arctic islands (particularly Iceland). Their outlets are often tongues of ice that travel down valleys in the mountains and so are called valley glaciers. If the leading edge of a valley glacier reaches a flat area and so escapes from the confines of its valley walls, it is called a piedmont glacier. Alpine glaciers are those that develop individually high in the mountains rather than as part of a broad icefield, usually at the heads of valleys. Very small alpine glaciers confined to the basins where they originate are called cirque glaciers. Normally, however, alpine glaciers spill out of their originating basins and flow down-valley as long, narrow valley glaciers. Glacier Formation and Movement (p. 563) 10 9. Describe and contrast the processes of glacial ice accumulation and ablation. Every glacier can be divided into two portions on the basis of the balance between accumulation and ablation. The upper portion is called the accumulation zone because here the amount of new ice from snowfall added each year exceeds the amount lost by melting and sublimation. The lower portion is called the ablation zone because here the amount of new ice added each year is less than the amount lost. Separating the two zones is a theoretical equilibrium line, along which accumulation exactly balances ablation. 10. Describe the metamorphosis from snow to glacial ice. Snow does not come from water freezing, but instead from water vapor crystallizing. So rather than being frozen water, snow is made up of lacy crystals that are about 1/10th as dense as water. When snow is compacted by another layer of snow, it will become progressively denser the more compression is added. Eventually it will become about half as dense as water (and is called firn or nv). With time, air will be squeezed out from the pore spaces because of the weight of overlying snow, and the density will approach 90 percent that of water and become glacial ice, with a characteristic bluish tinge. Glacial ice will become increasingly denser if compression continues and more air is forced out. 11. Explain how the balance between ice accumulation and ablation influences the advance or retreat of a glacier. (In other words, explain what causes glaciers to grow larger or to become smaller.) The advance or retreat of a glacier is completely dependent on the rate of accumulation vs. the rate of ablation. If the rate of accumulation exceeds the rate of ablation, the glacier experiences net gain and advances. However, if the rate of ablation exceeds the rate of accumulation, the glacier experiences net loss, and the glacier retreats. 12. Discuss the different components of glacial movement. While the flowing of water produces a disordered, pell-mell movement of molecules, the sliding of glacial movement is very orderly. Nevertheless, all parts of a glacier do not move at the same time, and the flow is often erratic. There are three types of movement within the glacier: laminar flow along internal planes, which causes different portions of the glacier to move with different speeds; basal slip at the bottom of the glacier, over which the entire mass slides on the slippery layer that sunken meltwater provides on the bottom; and an oozing (which in ice sheets occurs outward from around the edge, and in alpine glaciers occurs down-valley from the toe). 13. Why can a glacier continue to erode and transport rock even while it is retreating? Glacialflowandglacieradvancearedifferent.Aslongastheglacierexists,icecontinuestofloweither laterallyoutwardordownhill,buteitherdirectionisconsideredforward.Thisforwardflowcanbe consideredanadvance,eveninaretreatingglacier.Aretreatingglacierisonethatmerelyhasagreater rateofablationthanaccumulation.Therefore,thematerialaglacierpicksupandcarriescontinuestobe transporteddownstreamevenwhiletheglacierisinretreat. The Effects of Glaciers (p. 565) 14. Contrast the erosional processes of glacial plucking and glacial abrasion. 11 Plucking tends to roughen the underlying surface while abrasion tends to polish it and dig striation and grooves. Most erosion by glaciers occurs by plucking, while abrasion produces mostly minor features. 15. Describe the characteristics of glacial till. Rock debris deposited directly by moving or melting ice, with no meltwater flow or redeposition involved, is called till. Direct deposition by ice is usually the result of melting around the margin of an ice sheet or near the lower end of an alpine glacier, but it is also accomplished whenever debris is dropped on the ground beneath the ice, especially in the ablation area. In either case, the result is an unsorted and unstratified agglomeration of fragmented rock material. Most of the fragments are angular because they have been held in position while carried in the ice and consequently have had little opportunity to become rounded by frequent impact the way pebbles in a stream would. 16. How is a deposit of till likely to look different from a deposit of alluvium? Till is an unsorted and unstratified agglomeration of fragmented rock material. Most of the fragments are angular because they have been held in position while carried in the ice and consequently have had little opportunity to become rounded by frequent impact the way pebbles in a stream would. The latter would be characteristic of alluvium. 17. What is a glacial erratic? Sometimes outsized boulders are included in the glacial till; such enormous fragments, which may be very different from the local bedrock, are called glacial erratics. 18. How does glaciofluvial deposition differ from deposition directly by glacial ice? Glacial deposition is usually the result of melting around the margin of an ice sheet or the lower end of an alpine glacier, but it also occurs when the glacier drops debris on the ground beneath it. Such glacial deposition is unsorted and unstratified, and the fragments tend to be angular because they have been held in position rather than being free to collide with other fragments and thus become rounded. In glacial fluvial deposition, the debris is deposited or redeposited by meltwater. Because meltwater is incapable of moving larger material, glaciofluvial features are composed largely or entirely of gravel, sand, and silt, and their deposition tends to follow the patterns of water deposition, providing more sorting and rounding of particles. Continental Ice Sheets (p. 568) 19. Describe and explain the formation of a roche moutonne. As a glacier moves, hills are generally sheared off and rounded by the moving ice. A characteristic shape produced by both continental ice sheets and mountain glaciers is the roche moutonne, which is often produced when a bedrock hill is overridden by moving ice. (The origin of this French term is unclear. It is often translated as sheeps back, but some authorities believe it is based on a fancied resemblance to wavy wigs that were fashionable in France in the late 1700s and were known as moutonnes because they were pomaded with mutton tallow.) The stoss side (facing in the direction from which the ice came) of a roche moutonne is smoothly rounded and streamlined by grinding abrasion as the ice rides up the slope, but the lee side (facing away from the direction from which the ice came) is shaped largely by plucking, which produces a steeper and more irregular slope. 20. Explain the formation of terminal moraines and recessional moraines. 12 A terminal moraine is a ridge of till that marks the outermost limit of a glacial advance, occurring where a glacier reaches its equilibrium point. Recessional moraines are ridges that mark positions where the ice front was temporarily stabilized during the final retreat of the glacier. They both are in the form of concave arcs that bulge outward in the direction of ice movement. They differ, however, because the glacier drops the majority of its load at the terminal moriane, so they are larger, while the leftovers get dropped in portions among the recessional moraines, so they are smaller. 21. Describe and explain the formation of a kettle. Kettles are associated with continental glaciation and form when large blocks of ice left by a retreating glacier become surrounded or even covered by glacial drift; after the ice block melts, the morainal surface collapses, leaving an irregular depression. 22. What is meant by stratified drift? The deposition or redeposition of debris by ice-sheet meltwater produces certain features both where the sheet covered the ground and in the periglacial region. These features are composed of stratified drift, which means that there has been some sorting of the debris as it was carried along by the meltwater. Glaciofluvial features, then, are composed largely or entirely of gravel, sand, and silt because meltwater is incapable of moving larger material. 23. Describe the formation of a glacial outwash plain. The most extensive glaciofluvial features are outwash plains, which are smooth, flat alluvial aprons deposited beyond recessional or terminal moraines by streams issuing from the ice. Streams of water, heavily loaded with reworked till or with debris washed directly from the ice, issue from the melting glacier to form a braided pattern of channels across the area beyond the glacial front. As they flow away from the ice, these braided streams, choked with debris, rapidly lose their speed and deposit their load. Such outwash deposits sometimes cover many hundreds of square kilometers. They are occasionally pitted by kettles that often become ponds or small lakes. 24. Why are there so many lakes in areas that were glaciated by continental ice sheets during the Pleistocene? Lakes are very common in areas that were glaciated during the Pleistocene. The old stream systems were obliterated by the ice sheets, and water remains ponded in the many erosional basins and kettles, and behind morainal dams. One has only to compare the northern and southern parts of the United States to recognize this fact. Most of Europe and the northern part of Asia demonstrate a similar correlation between past glaciation and present-day lakes. Mountain Glaciers (p. 575) 25. Why are mountainous areas that have experienced glaciation usually quite rugged? In comparison to continental glaciation, mountain glaciation has a considerably less obliterating effect on the landscape. The mountains are steep to begin with, but the glacial action, particularly by the plucking and abrading of erosion, creates steeper slopes and greater relief than that experienced by the pre-glacial terrain. 13 26. Contrast the general cross-sectional shape of a stream valley with that of a glacial trough (glacial valley). A glacier moves down a mountain valley with much greater erosive effectiveness than a stream. The glacier is denser, carries more abrasive tools, and has an enormously greater volume. It erodes by both abrasion and plucking. The lower layers of the ice can even flow uphill for some distance if blocked by resistant rock on the valley floor, permitting rock fragments to be dragged out of depressions in the valley floor. The principal erosive work of a valley glacier is to deepen, steepen, and widen its valley. Abrasion and plucking take place not only on the valley floor but along the sides as well. The cross-sectional profile is changed from its stream-cut V shape to an ice-eroded U shape flared at the top. Moreover, the general course of the valley is straightened because the ice does not meander like a stream; rather it tends to grind away the protruding spurs that separate side canyons, creating what are called truncated spurs, and thereby replacing the sinuous course of the stream with a straight, U-shaped glacial trough. 27. Describe the general downvalley profile of a glacial trough. As might be expected, a glacier grinding along the floor of a glacial trough does not produce a very smooth surface. Valley glaciers do not erode a continuously sloping channel because differential erosion works with ice as well as with water. Therefore, resistant rock on the valley floor is gouged less deeply than weaker or more fractured rock. As a result, the long profile of the glaciated valley floor is irregular, with parts that are gently sloping, flat, or steep, and with some excavated depressions alternating in erratic sequencelandforms known as glacial steps. The resulting landscape of the glacial trough, after the ice has melted away, usually shows an irregular series of rock steps or benches, with steep (although usually short) cliffs on the down-valley side and small lakes in the shallow excavated depressions of the benches. The postglacial stream that flows downvalley out of the cirque has a relatively straight course but a fluctuating gradient. Rapids and waterfalls are common, particularly on the cliffs below the benches. The various shallow lakes occur in a sequence called paternoster lakes, after a fancied resemblance to beads on a rosary. 28. Describe and explain the formation of cirques, horns, and hanging valleys. The basic landform feature in glaciated mountains is the cirque, a broad amphitheater hollowed out at the head of a glacial valley. It has very steep, often perpendicular, head and side walls and a floor that is either flat or gently sloping or else gouged enough to form a basin. A cirque marks the place where an alpine glacier originated. It is the first landform feature produced by alpine glaciation, essentially being quarried out of the mountainside. The shifting of the equilibrium line back and forth as a result of minor climatic changes may generate much of this quarrying action, abetted by plucking, mass wasting, and frost wedging. By the middle of summer, a large crevice known as the bergschrund opens at the top of the glacier, exposing part of the headwall to frost wedging. The shattered rock from the headwall is eventually incorporated into the glacial ice. As the glacier grows, its erosive effectiveness within the cirque increases, and when the glacier begins to extend itself down-valley out of the cirque, quarried fragments from the cirque are carried away with the flowing ice. Cirques vary considerably in size, ranging from a few hectares to a few square kilometers in extent. Many large cirques apparently owe their development to repeated episodes of glaciation. A cirque grows steeper as its glacier plucks rock from the head and side walls. Where cirques are close together, the upland interfluve between neighboring cirques is reduced to little more than a steep rock wall. Where several cirques have been cut back into an interfluve from opposite sides of a divide, a narrow, jagged, serrated spine of rock may be all that is left of the ridge crest; this is called an arte (French for fishbone; derived from the Latin arista, spine). If two adjacent cirques on opposite sides of a divide are being cut back enough to remove part of the arte between them, the sharp-edged pass or 14 saddle through the ridge is referred to as a col (collum is Latin for neck). An even more prominent feature of glaciated highland summits is a horn, a steep-sided, pyramidal rock pinnacle formed by expansive quarrying of the headwalls where three or more cirques intersect. The name is derived from Switzerlands Matterhorn, the most famous example of such a glaciated spire. When occupied by glaciers and thus covered by a relatively level field of ice, main and tributary valleys may appear equally deep. When the ice melts, however, that valleys are of different depths becomes obvious; the mouths of the tributary valleys are characteristically perched high along the sides of the major troughs, forming hanging valleys. Typically, streams that drain the tributary valleys must plunge over waterfalls to reach the floor of the main trough. 29. What are paternoster lakes and why do they form? See question 27. 30. What is a lateral moraine? The largest depositional features produced by mountain glaciation are lateral moraines; these are welldefined ridges of unsorted debris built up along the sides of valley glaciers. The debris is partly material deposited by the glacier and partly rock that falls or is washed down the valley walls. 31. How does a medial moraine form? Where a tributary glacier joins a trunk glacier, their lateral moraines become united at the intersection and often continue together down the middle of the combined glacier as a dark band of rocky debris known as a medial moraine. Medial moraines are sometimes found in groups of three or four running together, indicating that several glaciers have joined to produce a candy-cane effect of black (moraine) and white (ice) bands extending down the valley. The debris left by meltwater below mountain glaciers is similar to that bordering ice sheets because similar outwash is produced. The Periglacial Environment (p. 583) 32. Briefly describe patterned ground in periglacial areas. The most unique and eye-catching periglacial terrain is patterned ground, the generic name applied to various geometric patterns that repeatedly appear over large areas in the Arctic. The patterns are varied, with their formation related to frost action. The principal significance of patterned ground is that it demonstrates the mobility of periglacial surfaces, emphasizing the role of soil ice in producing geomorphic activities largely unknown in warmer regions. 33. What is a proglacial lake? Another sometimes conspicuous development in periglacial regions is proglacial lakes (pro here means marginal to or in advance of). Where ice flows across a land surface, the natural drainage is either impeded or blocked, and meltwater from the ice can become impounded against the ice front, forming a proglacial lake. Such an event sometimes occurs in alpine glaciation but is much more common along the margin of continental ice sheets, particularly when the ice stagnates. Most proglacial lakes are small and quite temporary because subsequent ice movements cause drainage changes and because normal fluvial processes, accelerated by the growing accumulation of meltwater in the lake, cut spillways or channels to drain the impounded waters. Sometimes, however, proglacial lakes are large and relatively long lived. Such major lakes are characterized by considerable fluctuations in size due to the changing location of the 15 receding or advancing ice front. Several huge proglacial lakes were impounded along the margins of the ice sheets as they advanced and retreated during the Pleistocene epoch in North America, Europe, and Siberia. 16 ... View Full Document

End of Preview

Sign up now to access the rest of the document