UNIT 37 LANDFORMS AND LANDSCAPES OF CONTINENTAL ICE SHEETS

UNIT 37: LANDFORMS AND LANDSCAPES OF CONTINENTAL ICE SHEETS AND MOUNTAIN GLACIERS Unit Overview This unit examines continental glaciers. The main sections cover:           Contemporary ice cover Age of the present ice sheets North America’s glaciation: the final four (and probably many more?) Landscapes of continental ice sheets Aggradational landforms of ice sheets Basal and ablation tills Moraines Mountain glaciers Degradational landforms of mountain glaciers Aggradational landforms of mountain glaciers Continental glaciers are important sculptors of the Earth’s surface, and they tend to lower the overall relief. The two present-day ice sheets are located over Antarctica and Greenland. Those ice sheets are the two Holocene survivors of a larger group of Pleistocene ice sheets. Ice sheets are thickest in their interior regions and thinner at their margins. Extensions of ice sheets over the ocean are known as ice shelves. Continental glaciers can produce degradational and aggradational landforms. Basal and ablation tills play an important role in the particle size and fabric (stone orientation, layering etc) of glacial tills. Mountain glaciers are distributed throughout the world on mountain tops, except for Australia, which has a lover elevation. Different landforms are associated with different portions of a mountain glacier. In the upper portion of the glacier’s zone of accumulation, landforms known as cirques are formed. A cirque is a bowl-shaped, steep-sided depression with a gently sloping floor. Other related glacial landforms are horns and aretes. Lakes, such as tarns and finger lakes, are associated with continental glaciers. Fjords—narrow, steep-sided, elongated estuaries—are among the most spectacular landforms associated with glacial erosion. The dominant degradational landforms of mountain glaciers are glacial valleys, high mountain landforms, lakes, and fjords. The dominant aggradational landscape of continental glaciation is a flat or gently undulating plain underlain by heterogeneous material. Unit Objectives     To delineate contemporary continental glaciers and define their former extent during the Late Cenozoic Ice Age To identify typical landforms produced by continental glaciers To examine the current distribution of mountain glaciers and to comment on the Pleistocene extent of these glaciers To discuss the landforms produced by mountain glacier erosion and deposition Glossary of Key Terms Ablation till A surface deposit of loose, permeable, somewhat stratified, sandy, and stony till overlying denser till. Alp Topography reminiscent of preglacial times. Antarctic A mass of ice that formed on Antarctica beginning about 40 million icecap years ago and that has existed through the late Cenozoic Era and is unlikely to melt in the Holocene Era (present). Arête A sharp ridge of erosion-resistant rock formed between adjacent cirque glaciers. Basal A compact mixture of sediment, ranging in size from clay to boulders, (lodgement) deposited directly from the bottom of a glacier. till Calving When an ice sheet enters the sea, the repeated breaking away of the leading edge of that glacier into huge, flat-topped, tabular icebergs. A steep-walled semicircular basin in a mountain; may contain a lake. A general term for unsorted, unstratified rock debris composed of a wide range of particle sizes; use of this term carries no suggestion about how such debris was formed or deposited. Cirque Diamicton Drumlin A smooth elliptical mound created when an icesheet overrides and reshapes preexisting glacial till; long axis lies parallel to the direction of ice movement. Esker A glacial outwash landform that appears as a long ribbon-like ridge in the landscape because it was formed by the clogging of a river course within a glacier, the debris from which remains after the ice melts. A geographical area in central New York State that is named for a series of narrow glacial lakes that lie parallel in a north–south direction Finger lakes Fjord A deep, steep-walled, elongated estuary (drowned river mouth) formed by erosion by a glacier and submerged with seawater. A discrete region of outward ice flow in a continental glacier; possesses its own rates of snow accumulation, ice formation, and velocity. Large area, such as those in the Alps, carved to depths of hundreds or thousands of metres by the erosive action of glaciers. Flow regime Glacial trough Greenland icecap Greenland’s icecap is about 1/8th of Antarctica, covering 1.7 million square km—most of Greenland. Ground moraine Blanket of unsorted glacial till that was laid down at the base of a melting glacier. A tributary valley that is undercut by a large valley glacier so that the hanging valley empties above the main valley floor (often accompanied by hanging waterfalls). A steep-sided, pyramid-shaped mountain carved out by the erosive action of several cirque glaciers A regional mass of ice smaller than a continent-sized ice sheet. Smaller ice sheet that is floating; seaward extension of a continental glacier, such as Antarctica’s Ross Ice Shelf. A low ridge-like mass of loose rock pieces carried on, or deposited at, the side of a mountain glacier. Hanging valley Horn Icecap Ice shelf Lateral moraine Laurentide ice An icecap that covered Canada and the United States during the sheet Pleistocene ice age. Medial moraine The moraine that occurs when the lateral moraines of two glaciers meet and merge. Moraine A ridge or mound of glacial debris deposited during the melting phase of a glacier. A mountain peak of a buried landscape that protrudes through the overlying glacier. Floating sea ice that forms from the freezing of ocean water. Nunatak Pack ice Recessional A moraine ridge marking a place where glacial retreat was moraine temporarily halted. Rock flour Clay sized particles of rock generated from glacial erosion that become suspended in river water, making the water appear cloudy. The small depression formed over rocks that are susceptible to erosion. Flow from a glacier creates an erosion differential over rocks that are more resistant to erosion versus rocks that are susceptible. Relating to the bottom of, or below, a glacier. Relating to the middle of, or above, a glacier A deep, typically circular lake that forms when a cirque glacier melts. Rock steps Subglacial Supraglacial Tarn Terminal moraine The rock debris carried in and just ahead of the leading front of a glacier, which is deposited as an irregular ridge when the ice’s forward progress stops; these ridges are important to Earth scientists because they mark the farthest extent of an ice lobe. Till One of the two types of glacial drift; the solid material (ranging in size from boulders to clay particles) carried at the base of a glacier that is deposited as an unsorted mass when the ice melts back. Truncated spur A former river valley spur that has been sliced off by a valley glacier. Wisconsinan glaciation The youngest glacier in North America, based on the four major advances of the laurentide ice sheet. Unit Outline  Contemporary ice cover o The Antarctic Icecap  Continental landmass lies below glacier, with mountain ranges and a huge plateau  Nunataks are exposed mountain peaks that protrude through the ice and snow  Volume and weight of the ice sheet  about 65 per cent of fresh water on Earth is locked in this ice sheet  landmass below is depressed isostatically from weight of ice o Volume and weight of the icecap About 65 per cent of the freshwater on Earth is contained within the Antarctic icecap.  The weight of the icecap has caused the continent of Antarctica to sink by 600 m. o Features of the Antarctic Ice Sheet  Ice dome is divided into flow regimes  Ice tongues are glaciers that move out into the sea  Ice shelves are floating extensions of the main glacier, attached to continent  Ice shelves break off into icebergs at their outer edges, a process called calving  Icebergs float because they have lower density than the cold seawater  Zone of floating pack ice surrounds zone of icebergs o The Greenland Icecap  Much smaller than Antarctic Ice Sheet  Greenland is only about 11 per cent free of ice cover  Northern hemisphere has no polar ice sheets  Age of the present ice sheets o Analyze microorganisms of sediment to measure age of ice sheets o There have been more than 30 glaciations since the Late Cenozoic began o Antarctic Ice Sheet appears older, possibly 15 million years old North America’s glaciation: the final four o Last four glaciations actually represent the last dozen of the Late Cenozoic  Nebraskan (most distant)  Illinoisan  Kansan Complex  Wisconsinan (most recent) Landscapes of continental ice sheets o Sheet glaciers create massive, erosional landscapes Aggradational landforms of ice sheets  Glacial processes  A complex mix of transportation and deposition of glacial deposits Basal and ablation tills o Deglaciation begins with the deposition of material  Various sized solid material carried at the base of glacier (basal till)  Various sized solid material carried at the top of glacier (ablation till).  Basal till + ablation till = diamiton (sediment composed of variously sized particles) o Basal till  Compacted due to the weight of the glacier      The composition of a till reflects the geology of the drainage basin of the immediate area.  Repeated ice advances and recession can deposit a very thick layer of glacial drift.  The orientation of the basal till pebbles is important because they melt out in situ and are undisturbed, giving clues to the movement of the depositing ice. o Ablation till  Supraglacial till which is not compacted.  Thermal regime important to the deposition of ablation till  Basal temperature favouring a dominance of melting  Basal temperature favouring both freezing and melting  Basal temperature favouring freezing to the subglacial surface  Temperatures favouring melting throughout the ice o Moraines  A terminal moraine is the outermost ridge of debris left by a glacier  A recessional moraine is differentiated as a ridge where an already receding glacier temporarily stops and deposits additional material o Mountain Glaciers  Many glacier are currently enduring the interglacial period however many have a decreasing mass balance.  Global distribution  Every major landmass on Earth except Australia contains alpine glaciers on mountain tops.  North America o Athabasca glacier (Canada) o South Cascade glacier (US)  South America o Andes mountain glaciers  Africa o Mount Kilimanjaro o Mount Kenya  New Zealand o The Southern Alps  Europe o The European Alps o The south-central Asian zone  Asia o The European Alps, extend from northeastern Afghanistan to southwestern China  Isolated remnant glaciers o East Africa o Ecuador o Peru o Norway   The Mass Balance o The accumulation zone  Sometimes called neve o Losses o The Victoria Glacier  Net accumulation  Net ablation Degradational landforms of mountain glaciers o Glacial valleys  Typically a U-shaped valley, widened into a U-shaped trough by glacier  Truncated spurs are mountain edges that have been sheared off by glaciers  Hanging valleys, formed by large and small glaciers joining together that have different base levels o High-mountain landforms  Initial landscape has ridges and peaks  Glacial erosion begins, and ice hollows out cirques, steep-sided depressions  As cirques intersect over time, their edges form knifelike ridges (aretes) that eventually are eroded away to form a single jagged peak (horn)  Rock steps show local effects of frost wedging and jointing o Lakes  In warmer areas, cirque basins filled with water during interglacials; called tams  Lakes also form in glacial troughs; many are elongated finger lakes o Fjords  A narrow, steep-sided estuary formed by a glacial trough that filled with ocean water Aggradational landforms of mountain glaciers o Finely ground particles of glacial debris called rock flour o Moraines  Larger fragments deposited in terminal moraines, the stalled edge of a moving glacier  Recessional moraines and associated debris formed during glacial retreat  Both terminal and recessional moraines form dams to create lakes  Material that falls first from valley wall becomes lateral moraine  When a trunk glacier is jointed by a tributary glacier, their lateral moraines join to become a medial moraine  Erosion at the base of the valley forms a ground moraine, which is thickened by deposits when the glacier recedes o Postglacial landscape change  River action modifies glacial action when glaciers retreat  A valley train is left when meltwater fills the valley floor  Glacial lakes often drained when glaciers recede, and deposits redistributed    Areas modified by glaciers at risk for mass movements Review Questions 1. Name five landforms associated with a receding ice sheet. 2. What were the most recent glaciations in North America? 3. Describe the global distribution of mountain glaciers? Why would glaciers be found in tropical locations? 4. Using figure 37.19 describe the degradational landforms of mountain glaciers. 5. Compare and contrast the difference between basal till and ablation till. Self-Testing Questions Multiple-Choice Questions 1. Which of the following was the most recent of North America’s glaciations? a. b. c. d. e. Wisconsinan Kansan complex Illinoisan Idahoan All were recent glaciations. 2. Which of the following is not one of North America’s most recent glaciations? a. b. c. d. e. Wisconsinan Kansan complex Illinoisan Idahoan All were recent glaciations. 3. Which type of material is deposited first when a deglaciation period begins? a. b. c. d. e. Stratified drift Till Drumlins Eskers Moraines 4. ________ till is compacted because of the pressure from the overlying ice mass. a. Basal b. Ablation c. Nuvee d. Arête e. Ogive 5. Most of the world's glacial ice is in ________. a. b. c. d. mountain glaciers alpine glaciers the Antarctic icecap the Greenland ice sheet 6. The Wisconsin, Illinoisan, Kansan, and Nebraskan ice advances ________. a. b. c. d. account for all known Pleistocene ice advances account for almost all Pleistocene ice advances are only the last of many more Pleistocene ice advances. The Wisconsin glaciations was not a part of the Pleistocene ice advance. 7. Icebergs are formed by ________. a. b. c. d. the freezing of sea water river ice which is carried out to sea calving water freezing on the sea floor and floating to the surface 8. The Pleistocene ice advances ________. a. b. c. d. are known to have ended are the only ones known in geologic history Both of the above. None of the above. 9. Hanging valleys ________. a. form when a glacier in a valley cuts deeper than the tributary glacier b. are a common location for waterfalls c. are among the few cases of tributary valleys that do not meet the main at the level of the valley floor. d. All of the above. 10. Which of these is till? a. Fine clay b. Well-sorted outwash sand c. A polished rock outcrop valley d. Unsorted sand, gravel, and clay in a moraine 11. Which of the following is not a feature of mountain glaciations? a. b. c. d. e. Horn Arête Cirque Drumlin Rock steps 12. Which of these is true? a. b. c. d. Only continental glaciers leave moraines. Only mountain glaciers produce striations. Only continental glaciers produce cirques. None of the above. 13. The distinguishing feature of valleys formed by glaciers, which differentiates them from valleys formed by streams, is ________. a. b. c. d. e. the presence of rock steps the presence of cirques their characteristic V-shape their characteristics U-shape None of the above. 14. A lateral moraine is developed ________. a. b. c. d. on the side of a glacier at the top of the glacier at the end of the glacier None of the above. 15. The bowl-shaped depression at the upper end of a glacial trough is a(an) ________. a. b. c. d. e. inselberg drumlin cirque till lateral moraine True-or-False Questions 1. Glaciers can only exist where average temperatures are below freezing. True _____ False _____ 2. Nunataks are exposed mountain peaks that rise above an icecap. True _____ False _____ 3. It is theorized that there were approximately five glaciations in the Late Cenozoic Ice Age. True _____ False _____ 4. The Greenland Icecap is much larger than the Antarctic Icecap. True _____ False _____ 5. Australia is the only continent that does not have a mountain glacier. True _____ False _____ 6. Glacial deposits include tills, loess, coversands, varved lake sediments, outwash sand, and gravels. True _____ False _____ Answer Key Multiple-Choice Questions 1. a 2. d 3. b 4. a 5. c 6. a 7. c 8. a 9. d 10. d 11. d 12. d 13. d 14. a 15. c True-or-False Questions 1. 2. 3. 4. 5. 6. False True False False True True