GEOL: CHAPTER 14

1. GEOL: CHAPTER 14 Glaciers and Glaciation

3. Ice Ages • Ice Age: 1.8 million to 10,000 years ago • Warming: Holocene Maximum 6,000 years ago • Medieval Warm Period: 1000-1300 A.D. • Little Ice Age: 1500 to mid-1800s • Glacier: a mass of ice that moves by plastic flow and basal slip

4. Valley Glaciers • Confined to mountain valleys • Flow from higher to lower elevations • Also called “alpine glaciers” and “mountain glaciers” • Tidewater glaciers flow into the ocean • Can be several kilometers across, 200 km long, and hundreds of meters thick

5. Valley Glaciers A valley glacier in Alaska. Notice the tributaries that unite to form a larger glacier.

6. Continental Glaciers • Cover at least 50,000 km2 • Also called ice sheets • Not confined by topography • Flow outward in all directions from central area • 3000m thick; Greenland and Antarctica • Ice cap: less than 50,000 km2

7. The West and East Antarctic ice sheets merge to form a nearly continuous ice cover that averages 2,130m thick.

8. View of part of the Antarctic ice sheet. Notice the nunatak, which is a peak extending out of the glacial ice.

9. Glaciers and the Hydrologic Cycle • Can flow into oceans and calve icebergs • Remote from oceans: glaciers flow to lower elevations and melt into streams or groundwater • Sublimation: ice changes to water vapor

10. Origin of Glacial Ice • Glaciers form where more snow falls than melts in the warm season: net accumulation of snow • Snow converts to firn: granular type of snow • Buried firn is compacted and recrystallized to glacial ice

11. Snowflakes Glacial ice Granular snow Firn Stepped Art Fig. 14-3, p. 286

12. How Glaciers Move • Plastic flow: stress and strain induce permanent deformation and movement • Occurs continuously • Basal slip: sliding over the underlying surface • Occurs occasionally • Crevasses: upper 40 m behaves like a solid and fractures

15. Distribution of Glaciers • Need adequate snowfall • Need cold enough temperatures • Valley glaciers on highest mountains in western United States • Common in Canada and other northern countries

16. Glacial Budget • Glacial budget: balance between expansion and contraction of glacier in response to accumulation and wastage • Zone of accumulation: where additions exceed losses and surface always covered with snow • Zone of wastage: losses exceed additions, from melting, sublimation, calving

17. Glacial Budget, cont. • Firn limit: separates the zone of accumulation from the zone of wastage • If moves up the glacier, the glacier is receding • If moves down the glacier, the glacier is advancing

18. Winter Accumulation (B) and summer snow and ice melt (A) are equal. That is, additions and losses are equal so the glacier’s terminus remains stationary. The terminal moraine is deposited at the terminus of a glacier.

19. Summer snow and ice melt (A) are much greater than winter accumulation (B), and the glacier’s terminus retreats, although the glacier continues to move by plastic flow and basal slip. The recessional moraine is deposited at the glacier’s new terminus.

20. Winter accumulation (B) is much greater than summer snow and ice melt (A), so the glacier’s terminus advances. As it does so, it overrides and modifies its previously deposited moraines.

21. Glacial Movement Rates • Valley glaciers typically move faster than continental glaciers • Rates from centimeters to tens of meters per day • More rapid flow on steeper slopes • Valley glaciers move faster in warmer months because melt increases basal slip

22. Glacial Movement Rates, cont. • Friction from slow ice flow near the sides and bottom of glacier • Rates are fastest in center and near top • Glacial surge: greatly accelerated flow, usually from a valley glacier • Tens of meters per day, for months

23. Flow velocity in a valley glacier varies both horizontally and vertically. Velocity is greatest at the top center of the glacier, because friction with the walls and floor of the trough slows the flow adjacent to these boundaries. The lengths of the arrows in the figure are proportional to velocity.

24. Erosion and Transport • Glaciers push unconsolidated materials • Plucking: glacial ice pulls rock from bedrock • Bedrock eroded through abrasion: • Glacial polish: smooth surface • Glacial striations: scratches • Glacial flour: pulverized rocks

26. Glacial Striations and Polish Abrasion

27. Glacial Sediments • Continental glacier sediments come mostly from abrasion of bedrock • Valley glaciers get sediments from: • Bedrock • Mass wasting

28. Sediment Transport by Valley Glaciers Debris on the surface of the Mendenhall Glacier in Alaska. The largest boulder is about 2 m across. Notice the icefall in the background. The person left of center provides scale.

29. Valley Glacier Erosional Features • U-shaped glacial troughs • Hanging valleys • Cirques • Aretes • Horns

33. Landforms Produced by Valley Glacier Erosion: U-shaped glacial troughs, cirques, and arêtes are visible in this view of the Chugach Mountains in Alaska.

34. U-Shaped Glacial Troughs • Valley with steep walls and broad, flat floor formed by glacier movement through a stream valley • Fiords: drowned glacial troughs • High latitudes where glaciers exist near sea level • As glaciers melt, sea level rises

35. Classic U-Shaped Valley at Milford Sound, New Zealand

36. Hanging Valleys • Tributary valley whose floor is above the main valley • Often have spectacular waterfalls, like Nevada Falls in Yosemite National Park • Often formed by glaciers

37. Nevada Falls and Brideveil Falls from hanging valleys in Yosemite National Park

38. Cirques, Aretes, Horns • At upper ends of glacial troughs and at ridges that separate them • Cirque: steep-walled, bowl-shaped depression • From frost wedging, glacial plucking, and glacial erosion • Tarn lakes

39. Cirques, Arêtes, Horns, cont. • Arête: a narrow serrated ridge between two glacial valleys or cirques • From headward erosion • Horn: Steep-walled pyramid-shaped peak formed by headward erosion of at least three cirques

40. The Matterhorn in Switzerland

41. Erosional Landforms of Continental Glaciers • Areas eroded by continental glaciers are often smooth and rounded • Ice-scoured plains: large areas where continental glaciers removed soil and sediment • Deranged drainage • Many lakes and swamps • Low relief

42. This low-relief surface is an ice-scoured plain in the Northwest Territories of Canada. Numerous lakes, little or no soil, and extensive bedrock exposures are typical of these areas eroded by continental glaciers.

43. Glacial Drift • Sediment deposited by glaciers • Vast sheet covers northern U.S. • Glacial erratics: rock fragments carried from source by glaciers • Till: all sediment deposited by glacial ice • Stratified drift: layered, with some sorting

44. A glacial erratic in the making. This boulder on the surface of the Mendenhall Glacier in Alaska will eventually be deposited far from its source.

45. House and Barn Rocks, glacial erratics found in Westford, MA

46. This glacial drift from the Matanuska Glacier, near Palmer, AK, is till, because it is unsorted and shows no stratification.

47. End Moraines • Till deposited at terminus of glacier that was stationary for a few years or decades • Flow continues in stationary glacier, which results in continued sediment deposits at terminus

48. An end moraine (specifically—terminal moraine) deposited by a valley glacier.

49. Closeup of till found in an end moraine.

50. Ground and Recessional Moraines • Ground moraine: layer of sediment released from melting ice as a glacier’s terminus retreats • Recessional moraine: an end moraine that forms when glacier’s terminus retreats and then stabilizes