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