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17_glaciers_10_post - 17: Surface Processes 4 / Glaciers...

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Unformatted text preview: 17: Surface Processes 4 / Glaciers and Ice Ages Glaciers in Luane National Park, Yukon, Canada. Photo: S.J. Krasemann ture 17: 1 ure Why this is important • Glaciers are extremely effective agents of erosion, transport, and deposition. transport, Fig. 22.20a Yosemite National Park, California. Photo: R.W. Schlische ture 17: 2 ure Why this is important • During glacial periods, worldwide sea level falls, which then rises during interglacial periods. rises Fig. 22.35 ture 17: 3 ure Why this is important • Melting of all ice locked up in glaciers and ice sheets would result in a worldwide rise in sea-level of ~75 m. in Fig. 22.35 ture 17: 4 ure Types of glaciers • Glacier: accumulation of ice that flows under its own weight Glacier • Alpine (valley) glacier: occupies former stream valleys Alpine occupies Fig. 22.4 ture 17: 5 ure Types of glaciers • Glacier: accumulation of ice that flows under its own weight Glacier • Alpine (valley) glacier: occupies former stream valleys Alpine occupies Fig. 22.4 Herbert Glacier, a valley glacier near Juneau, Alaska. Photo: G. Dimijian ture 17: 6 ure Types of glaciers • Ice sheet: broad Ice broad accumulation of ice of continental proportions; called an ice shelf if it extends out to sea extends Fig. 22.5 ture 17: 7 ure Types of glaciers • Ice sheet: broad accumulation Ice broad of ice of continental proportions proportions Fig. 22.5 ture 17: 8 ure Types of glaciers • Ice cap: smaller than ice sheet Ice Satellite view of Iceland ture 17: 9 ure Types of glaciers Sentinel Range rises above the Antarctic ice sheet. Photo: B. Crowell ture 17: 10 ure 10 Formation of glacial ice • Involves pressureInvolves induced Compaction induced Compaction (expulsion of air) and partial melting& partial re-freezing Fig. 22.8 ture 17: 11 ure 11 Movement of glaciers • ~30-m-thick ice flows under its own weight ~30-m-thick Q1. Crevasses (open Q1. cracks) are present ___. ___. A. only in the A. upper brittle part upper B. only in the B. lower ductile part lower C. in both the C. upper brittle part and lower ductile part. part. Crevasse Ductile Fig. 22.12 Final position ture 17: 12 ure 12 Movement of glaciers • ~30-m-thick ice flows under its own weight ~30-m-thick Original position of markers placed on ice Final position of markers placed on ice Original position Q2. Where does a Q2. glacier flow the fastest? A. near the central part of the base B. near the sides of the base C. near the central part of the top D. near the sides of the top Fig. 22.12 ture 17: 13 ure 13 Ductile Final position Movement of glaciers • Valley glaciers: move downhill Valley Fig. 22.11 ture 17: 14 ure 14 Movement of glaciers • Ice caps & sheets: flow outward in all directions Ice flow Fig. 22.11 ture 17: 15 ure 15 Glacial budget Accumulation (falling snow) vs ablation (=loss) (melting, evaporation, calving) evaporation, Fig. 22.13 ture 17: 16 ure 16 Glacial budget Iceberg calving, Glacier Bay National Park, Alaska. Photo: T. Bean ture 17: 17 ure 17 Glacial budget Fig. 22.14 Accumulation = loss: end of glacier is… stationary ture 17: 18 ure 18 Glacial budget Fig. 22.14 Accumulation > loss: end of glacier moves… downslope ture 17: 19 ure 19 Glacial budget Fig. 22.14 Accumulation < loss: even though ice still flows downslope end of glacier… retreats upslope_ ture 17: 20 ure 20 Glacial erosion • Plucking: water seeps in cracks, freezes, and breaks off rock fragments. Plucking • Abrasion: ice and rocky debris scour bedrock surfaces, forming glacial striations Abrasion ice and grooves that indicate the direction of ice flow. and Fig. 22.22b ture 17: 21 ure 21 Glacial erosion • Abrasion: ice and rocky Abrasion ice debris scour bedrock surfaces, forming glacial striations and grooves that indicate the direction of ice flow. direction See Fig. 22.21c Glacial polish, striations, and grooves, Glacier Bay National Park, Alaska. Photo: C. Clifton ture 17: 22 ure 22 Glacial erosion (alpine) 1. Before glaciation ture 17: 23 ure 23 Fig. 22.23a Glacial erosion (alpine) 2. During glaciation Fig. 22.22b ture 17: 24 ure 24 Glacial erosion (alpine) • V-shaped mountain stream valleys are eroded to U-shaped valleys. U-shaped • Rounded hills become more angular with sharp ridges (arete) and triangular peaks (horns) ridges Fig. 22.23c 3. After glaciation ture 17: 25 ure 25 Glacial erosion (alpine) • V-shaped mountain stream valleys are eroded to U-shaped valleys. valleys Fig. 22.22d U-shaped valley, Glacier National Park, Montana. Photo: T. Bean ture 17: 26 ure 26 Glacial erosion • Fjords: glacial Fjords glacial valleys that were flooded following a rise in sea level. rise Fig. 22.24 McCarthy fjord, Kenai Fjords National Park, Alaska. Photo: P.L. Kresan ture 17: 27 ure 27 Glacial erosion (alpine) Fig. 22.22e • Rounded hills become more angular with sharp ridges (arete) and ridges and triangular peaks (horns) Horns of Grand Teton Mountains, Wyoming Photo: P. Anderson ture 17: 28 ure 28 Glacial deposition • Till: unsorted, Till unsorted, unstratified sediments deposited near the margins of a glacier to form mounds called moraines. moraines Fig. 22.26c ture 17: 29 ure 29 Glacial deposition • Sorting (uniformity of grain sizes): very poor Sorting • Highly viscous ice transports clasts of… Highly all sizes (clay to boulders) ture 17: 30 ure 30 Glacial deposition End moraine: ridge of glacially deposited sediment that forms at terminus of glacier Plate ture 17: 31 ure 31 Glacial deposition • Erratic: glacially deposited rock with a lithology different from the Erratic glacially underlying bedrock underlying [See Fig. 22.34] Glacial erratic, South Bubble, Acadia, Maine. Photo: P. L. Kresan ture 17: 32 ure 32 Glacial ages • Extent of ice: 30% of land surface 30 (10% now) Fig. 22.40 ture 17: 33 ure 33 Glacial ages • Sea level: _____ than present _____ Fig. 22.35 ture 17: 34 ure 34 Glacial ages • Sea level: lower than present Sea Fig. 22.35 ture 17: 35 ure 35 Glacial ages • Weight of ice sheets caused crust to… subside Weight • Melting of ice sheets caused crust to… rebound Melting Fig. 22.33 ture 17: 36 ure 36 Glacial ages •Timing: based on oxygen isotopes (O16 & heavier O18) preserved in shells of heavier preserved marine organisms. marine • During glacial ages, O16, which is During which preferentially evaporated, becomes concentrated in becomes glacial ice & depleted from seawater. Up to 30 ice ages seawater Up occurred in earths past – according to micro organisms according Fig. 22.44 QuickTime™ and a GIF decompressor are needed to see this picture. ture 17: 37 ure 37 Causes of glacial ages • Tectonics: movement of continents from warm latitudes Tectonics: to colder latitudes. to • Milankovitch cycles: variation in sunlight reaching the Milankovitch variation Earth's surface driven by variations in the Earth's orbit around the sun around ture 17: 38 ure 38 Causes of glacial ages: orbital changes • Eccentricity: cycle over which the Earth's orbit changes from Eccentricity cycle circular to slightly elliptical (100 kyr and 400 kyr periods) circular Fig. 22.46 ture 17: 39 ure 39 Causes of glacial ages: orbital changes • Tilt: tilt of the Earth's axis varies from 21.5° to 24.5°; 41 kyr Tilt tilt period period Fig. 22.46 ture 17: 40 ure 40 Causes of glacial ages • Precession: wobble of the Earth's axis, like a spinning top; 23 kyr Precession wobble period period Fig. 22.46 ture 17: 41 ure 41 Causes of glacial ages • Milankovitch cycles: Milankovitch variation in amount of sunlight (insolation) reaching the Earth's surface driven by variations in the Earth's orbit around the sun the Fig. 22.46 ture 17: 42 ure 42 Causes of glacial ages • Lower CO2 & CH4 concentrations favor colder climates. Lower • Both were lower during glacial ages...but cause & effect? Both ture 17: 43 ure 43 Causes of glacial ages • Albedo: amount of sunlight Albedo amount reflected back into space from Earth's surface Earth's • During glacial ages, snow & ice During increase albedo, leading to additional cooling additional (positive feedback). ture 17: 44 ure 44 Review Questions 17-1. Which statement below is false? A. During the last glacial age, it was possible to walk from Alaska to Russia. B. The smallest glacial feature is an alpine (valley) glacier. C. The largest glacial feature is an ice cap. 17-2. At the present day, continental glaciers (ice sheets) are limited to Antarctica and ____________. A. Alaska B. Greenland C. Canada D. Siberia 17-3. The maximum thickness of ice on Antarctica is approximately ___. A. 3.5 miles B. 350 meters C. 3500 meters D. 3500 feet 17-4. If all ice on Earth were to melt, global sea level would rise by approximately ___. A. 7.5 feet B. 7.5 meters C. 75 feet D. 75 meters E. 3500 m 17-5. When sea level rises, causing the ocean to fill a glacially carved valley, a(n) ___ results. A. smorgasbord B. tarn C. fjord D. ford ture 17: 45 ure 45 Review Questions 17-6. Which statement below is false? A. Because glacial advance is driven by gravity, it is impossible for glaciers to advance over perfectly flat terrain. B. Valleys before glaciation are typically V-shaped and are typically U-shaped after glaciation. C. Glacial striations and grooves are evidence of the glacial erosion mechanism of abrasion. 17-7. A. True / B. False: If ablation (loss of glacial ice from melting, evaporation, and calving) exceeds accumulation, the terminus of the glacier retreats. 17-8. Sediments deposited by glaciers as they melt are characterized by ___. A. uniformly large grains B. uniformly small grains C. a mix of large and small grains 17-9. Which term does not belong with the others? A. aréte B. fjord C. horn D. moraine 17-10. A. True / B. False: During glacial times, the oceans become enriched in the heavier isotope O18 because the lighter isotope O16 preferentially evaporates and is incorporated in glacial ice. ture 17: 46 ure 46 Review Questions 17-11. The percentage of the Earth's surface covered by ice during the most recent glacial age is ___. A. 1% B. 10% C. 30% D. 50% 17-12. Sea level was about ___ km lower during the last glacial age than at present. A. 0.01 B. 0.1 C. 1 D. 10 17-13. Match the Milankovitch cycle to its correct period. A. eccentricity, 1.0x106 years; precession, 2.3x105 years; tilt, 4.1x105 years B. eccentricity, 1.0x105 years; precession, 2.3x104 years; tilt, 4.1x104 years C. eccentricity, 1.0x105 years; precession, 2.3x103 years; tilt, 4.1x104 years 17-14. Which Milankovitch cycle is related to the shape of the Earth’s orbit around the sun? A. precession B. tilt C. eccentricity 17-15. Which Milankovitch cycle is related to the wobble of the Earth’s axis? A. precession B. tilt C. eccentricity ture 17: 47 ure 47 Review Questions 17-16. The most recent glacial age began about ___ years ago. A. 2500 B. 25,000 C. 250,000 D. 2,500,000 17-17. Which statement below is false? A. During glacial times, the carbon dioxide concentration in the atmosphere was lower. B. Ice sheets cause more sunlight to be reflected out into space, resulting in warming of the atmosphere. C. The weight of glacial ice during the last glacial age caused the crust to subside. 17-18. Which North American city would not be flooded if all the ice contained in ice sheets were to melt? A. Boston, MA B. Chicago, IL C. Miami, FL D. Memphis, TN E. New York City, NY ture 17: 48 ure 48 ...
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