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17_glaciers_09_post - 17 Surface Processes 3-Glaciers and...

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Unformatted text preview: 17: Surface Processes 3-Glaciers and Ice Ages Glaciers in Luane National Park, Yukon, Canada. Photo: S.J. Krasemann ure 17: 1 Why this is important • Glaciers are extremely effective agents of erosion, transport, and deposition. erosion, Fig. 22.20a ure 17: 2 Yosemite National Park, California. Photo: R.W. Schlische Why this is important • During glacial periods, worldwide sea level falls, which then rises during interglacial periods. which Fig. 22.35 ure 17: 3 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. would Fig. 22.35 ure 17: 4 Types of glaciers • Glacier: accumulation of ice that flows under its own weight • Alpine (valley) glacier: occupies former stream valleys Fig. 22.4 ure 17: 5 Types of glaciers • Glacier: accumulation of ice that flows under its own weight • Alpine (valley) glacier: occupies former stream valleys Fig. 22.4 Herbert Glacier, a valley glacier near Juneau, Alaska. Photo: G. Dimijian ure 17: 6 Types of glaciers • Ice sheet: broad accumulation of ice of continental proportions; called an ice shelf if it extends out to sea Fig. 22.5 ure 17: 7 Types of glaciers • Ice sheet: broad accumulation of ice of continental proportions Fig. 22.5 ure 17: 8 Types of glaciers • Ice cap: smaller than ice sheet ure 17: 9 Satellite view of Iceland Types of glaciers Sentinel Range rises above the Antarctic ice sheet. Photo: B. Crowell ure 17: 10 10 Review Questions 17­1. A. True / B. False: During the last glacial age, it was possible to walk from Alaska to Russia. 17­2. Which feature is generally the smallest? A. alpine glacier B. ice cap C. ice sheet 17­3. Which feature is generally the largest? A. alpine glacier B. ice cap C. ice sheet 17­4. At the present day, continental glaciers (ice sheets) are limited to Antarctica and ____________. A. Alaska B. Greenland C. Canada D. Siberia 17­5. The maximum thickness of ice on Antarctica is approximately ___. A. 3.5 miles B. 350 meters C. 3500 meters D. 3500 feet 17­6. 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 ure 17: 11 11 Accumulation vs. loss Glaciers form where winter snow… The Great Gorge, Ruth Glacier, Alaska Range. Photo: P. L. Kresan ure 17: 12 12 Formation of glacial ice • Involves pressureInvolves induced compaction induced (expulsion of air) and partial melting & repartial freezing. Fig. 22.8 ure 17: 13 13 Movement of glaciers • ~30-m-thick ice flows under its own weight ~30-m-thick Q1. Crevasses (open cracks) are present ___. A. only in the upper A. brittle part brittle B. only in the lower B. ductile part ductile C. in both the C. upper brittle part and lower ductile part. part. Ductile Fig. 22.12 ure 17: 14 14 Movement of glaciers • ~30-m-thick ice flows under its own weight ~30-m-thick Original position Original position of markers placed on ice Final position of markers placed on ice Q2. Where does a 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 ure 17: 15 15 Ductile Final position Movement of glaciers • Valley glaciers: Valley Fig. 22.11 ure 17: 16 16 Movement of glaciers • Ice caps & sheets: flow outward in all directions Ice flow Fig. 22.11 ure 17: 17 17 Glacial budget Accumulation (falling snow) vs ablation (=loss) (melting, evaporation, calving) Fig. 22.13 ure 17: 18 18 Glacial budget Iceberg calving, Glacier Bay National Park, Alaska. Photo: T. Bean ure 17: 19 19 Glacial budget Fig. 22.14 Accumulation = loss: end of glacier is… ure 17: 20 20 Glacial budget Fig. 22.14 Accumulation > loss: end of glacier moves… ure 17: 21 21 Glacial budget Fig. 22.14 Accumulation < loss: even though ice still flows downslope, end of glacier… ure 17: 22 22 Glacial erosion • Plucking: water seeps in cracks, freezes, and breaks off rock fragments. • Abrasion: ice and rocky debris scour bedrock surfaces, forming glacial striations and grooves that indicate the direction of ice flow. Fig. 22.22b ure 17: 23 23 Glacial erosion • Abrasion: ice and rocky debris scour bedrock surfaces, forming glacial striations and grooves that indicate the direction of ice flow. See Fig. 22.21c Glacial polish, striations, and grooves, Glacier Bay National Park, Alaska. Photo: C. Clifton ure 17: 24 24 Glacial erosion (alpine) 1. Before glaciation ure 17: 25 25 Fig. 22.23a Glacial erosion (alpine) 2. During glaciation Fig. 22.22b ure 17: 26 26 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) angular and triangular peaks (horns) and Fig. 22.23c 3. After glaciation ure 17: 27 27 Glacial erosion (alpine) • V-shaped mountain stream valleys are eroded to UUshaped shaped valleys. valleys Fig. 22.22d U­shaped valley, Glacier National Park, Montana. Photo: T. Bean ure 17: 28 28 Glacial erosion • Fjords: glacial valleys that were flooded following a rise in sea level. Fig. 22.24 McCarthy fjord, Kenai Fjords National Park, Alaska. Photo: P.L. Kresan ure 17: 29 29 Glacial erosion (alpine) Fig. 22.22e • Rounded hills become more angular with sharp ridges (arete) and and triangular peaks (horns) Horns of Grand Teton Mountains, Wyoming Photo: P. Anderson ure 17: 30 30 Glacial deposition • Till: unsorted, unstratified sediments deposited near the margins of a glacier to form mounds called moraines. Fig. 22.26c ure 17: 31 31 Glacial deposition • Sorting (uniformity of grain sizes): Sorting • Highly viscous ice transports clasts… Highly ure 17: 32 32 Glacial deposition End moraine: ridge of glacially deposited sediment that forms at terminus of glacier Plate ure 17: 33 33 Glacial deposition • Erratic: glacially deposited rock with a lithology different from the underlying bedrock [See Fig. 22.34] Glacial erratic, South Bubble, Acadia, Maine. Photo: P. L. Kresan ure 17: 34 34 Review Questions 17­7. A. True / B. False: Because glacial advance is driven by gravity, it is impossible for glaciers to advance over perfectly flat terrain. 17­8. A. True / B. False: If ablation (loss of glacial ice from melting, evaporation, and calving) exceeds accumulation, the terminus of the glacier retreats. 17­9. A. True / B. False: Valleys before glaciation are typically U­shaped and are typically V­shaped after glaciation. 17­10. A. True / B. False: Glacial striations and grooves are evidence of the glacial erosion mechanism of plucking. 17­11. When sea level rises, causing the ocean to fill a glacially carved valley, a(n) ___ results. A. smorgasbord B. tarn C. fjord D. ford 17­12. 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 17­13. Which term does not belong with the others? A. aréte B. fjord C. horn ure 17: 35 35 D. moraine Glacial ages • Extent of ice: 30% of land surface surface Fig. 22.40 ure 17: 36 36 Glacial ages • Sea level: lower than present Sea Fig. 22.35 ure 17: 37 37 Glacial ages • Sea level: lower than present Sea Fig. 22.35 ure 17: 38 38 Glacial ages • Weight of ice sheets caused crust to… Weight • Melting of ice sheets caused crust to… Melting Fig. 22.33 ure 17: 39 39 Glacial ages •Timing: based on oxygen isotopes Timing: (O16 & heavier O18) preserved in (O preserved shells of marine organisms. shells • During glacial ages, O16, During which is preferentially evaporated, becomes concentrated in ________ & depleted from _________. _________ Fig. 22.44 ure 17: 40 40 Causes of glacial ages • Tectonics: movement of continents from warm Tectonics: latitudes to colder latitudes. latitudes • Milankovitch cycles: variation in sunlight reaching the Milankovitch cycles Earth's surface driven by variations in the Earth's orbit around the sun ure 17: 41 41 Causes of glacial ages: orbital changes • Eccentricity: cycle over which the Earth's orbit changes from circular to slightly elliptical (100 kyr and 400 kyr periods) Fig. 22.46 ure 17: 42 42 Causes of glacial ages: orbital changes • Tilt: tilt of the Earth's axis varies from 21.5° to 24.5°; 41 kyr period Fig. 22.46 ure 17: 43 43 Causes of glacial ages • Precession: wobble of the Earth's axis, like a spinning top; 23 kyr period Fig. 22.46 ure 17: 44 44 Causes of glacial ages • Milankovitch cycles: Milankovitch cycles variation in amount of sunlight (insolation) reaching the Earth's surface driven by variations in the Earth's orbit around the sun ure 17: 45 45 Fig. 22.46 Causes of glacial ages • Lower CO2 & CH4 concentrations favor colder climates. Lower • Both were lower during glacial ages...but cause & effect? Both ure 17: 46 46 Causes of glacial ages • Albedo: amount of sunlight reflected back into space from Earth's surface • During glacial ages, snow & During ice increase albedo, leading to additional cooling (positive feedback). (positive ure 17: 47 47 Review Questions 17­14. 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% 17­15. Sea level was about ___ km lower during the last glacial age than at present. A. 0.01 B. 0.1 C. 1 D. 10 17­16. A. True / B. False: The weight of glacial ice during the last glacial age caused the crust to be depressed. 17­17. A. True / B. False: During glacial times, the oceans become enriched in O18. 17­18. 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 17­19. Which Milankovitch cycle is related to the shape of the Earth’s orbit around the sun? A. precession B. tilt C. eccentricity ure 17: 48 48 Review Questions 17­20. Which Milankovitch cycle is related to the wobble of the Earth’s axis? A. precession B. tilt C. eccentricity 17­21. The most recent glacial age began about ___ years ago. A. 2500 B. 25,000 C. 250,000 D. 2,500,000 17­22. A. True / B. False: During glacial times, the carbon dioxide concentration in the atmosphere was higher. 17­23. A. True / B. False: Ice sheets cause more sunlight to be reflected out into space, resulting in additional cooling of the atmosphere. 17­24. 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 ure 17: 49 49 ...
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This note was uploaded on 09/25/2011 for the course GEOLOGY 100 taught by Professor Lepre during the Fall '11 term at Rutgers.

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