Chapter 19: Glacial Modification of Terrain

1. Chapter 19: Glacial Modification of Terrain

2. Glacial Modification of Terrain The Impact of Glaciers on the Landscape Glaciations of the Past and Present Types of Glaciers Glacier Movement and Formation The Effects of Glaciers Continental Ice Sheets Mountain Glaciers The Periglacial Environment Causes of the Pleistocene 2 � 2011 Pearson Education, Inc.

3. The Impact of Glaciers on the Landscape Snowpack over years turns to ice Ice mass motion under gravity grinds anything in its path Glaciation increases erosion rate on a mountain by at least 10 times to an unglaciated mountain Modifies flat landscapes as well 3 � 2011 Pearson Education, Inc.

4. Types of Glaciers Continental ice sheets Ice Sheet- an immense blanket of ice that completely inundates the underlying terrain to depths of hundreds or thousands of feet. Formed in non-mountainous areas of continents. Only two true ice sheets currently, in Antarctica and Greenland. Outlet glaciers-a tongue of ice around the margin of an ice sheet that extends between rimming hills to the sea. Icebergs form from chunks of ice that break off ice shelves and outlet glaciers. 4 � 2011 Pearson Education, Inc.

5. Types of Glaciers Mountain glaciers Highland icefields: ice sheets that submerge most underlying topography Valley-a long, narrow feature resembling a river of ice, which spills out of its originating basin and flows down-valley. Piedmont glaciers-a valley glacier that extends to the mouth of the valley and spreads out broadly over the flat land beyond. Alpine glaciers: develop individually instead of part of ice field Cirque glacier�a small glacier confined to its cirque and not moving down-valley. An alpine glacier typically breaks out of its basin and forms a valley glacier, and can extend to mouth of valley to create a piedmont glacier. � 2011 Pearson Education, Inc. 5

6. Glaciations Past and Present Glacial ice volume has varied considerably over last few million years Evidence left behind allows scientists to determine the chronology of past glaciations Pleistocene glaciation Began at least 2.59 million years ago Last major ice retreat occurred only 9000 years ago Dominant environmental characteristic was refrigeration of high-latitude and high-elevation areas Consistent alterations of glacial(times of ice accumulation) and interglacial periods (times of ice retreat.) At peak, 1/3 of total land covered in ice 6 � 2011 Pearson Education, Inc.

7. Glaciations Past and Present Indirect effects of Pleistocene glaciation Periglacial processes Periglacial zone�zone where ice never existed but glacial factors affected the landscape such as erosion from ice melt, solifluction Sea-level changes�buildup of ice on continents led to less drainwater on continents and brought about a lowering of sea levels Crustal depression�the weight of the ice on the continents caused continents to sink, ice melt allowed for continental rebound Pluvial developments�considerable runoff results in increased moisture, leading to increased precipitation and less evaporation. Developed many lakes, including the Great Salt Lake (formed from Lake Bonneville) 7 � 2011 Pearson Education, Inc.

8. Glaciations Past and Present Maximum extent of the Pleistocene glaciation 8 � 2011 Pearson Education, Inc.

9. Glaciations Past and Present Contemporary glaciation Limited ice cover today (about 10% of total land surface) 96% of the total ice cover is Greenland and Antarctica Antarctic ice cap Consists of two unequal sections separated by Transantarctic mountains West Antarctica has a few �dry valleys� Greenland ice cap North American glaciers 9 � 2011 Pearson Education, Inc.

10. Glaciations Past and Present Climate change related to contemporary glaciation Retreating of polar ice caps Shrinking ice caps an indicator of a warming climate Antarctic ice shelves breaking Higher flow rates of outlet glaciers 10 � 2011 Pearson Education, Inc.

11. Glacier Formation and Movement Require balance between accumulation (addition of ice into a glacier by incorporation of snow) and ablation(wastage of glacial ice through melting and sublimation) Snow begins as crystallized water vapor Compressed to granular form More compression causes granules to coalesce, n�v�/firn snow (granules that have become packed and begin to coalesce due to compression, achieving a density about half as great as that of water) Further compression results in glacial ice 11 � 2011 Pearson Education, Inc.

12. Glacier Formation and Movement Every glacier can be divided into two portions. Upper portion is the accumulation zone, because accumulation exceeds amount lost by melting and sublimation. Lower portion is ablation zone, because more is lost than is added each year. � 2011 Pearson Education, Inc. 12

13. Glacier Formation and Movement Glacier �flow� is orderly sliding of ice molecules Ice under extreme pressure deforms instead of slipping Meltwater contributes surface for glacier to slide on Flow in response to overlying weight Glacier flow versus glacier advance Flow is the continual movement of the ice toward the edge(s) of the glacier. Advance means the forward movement of the outer margins of the glacial body. 13 � 2011 Pearson Education, Inc.

14. The Effects of Glaciers Transportation by glaciers Glaciers effective to move large rock pieces Typically move glacial flour Most rock material transported along base of the ice Remaining glacial ice free of rock debris Role of flowing water on moving ice, melt streams Cracks in ice in which streams run�moulins 14 � 2011 Pearson Education, Inc.

15. The Effects of Glaciers Erosion by glaciers Volume and speed determine success of glacial erosion Erosive power of moving ice slightly larger than that of water Glacial plucking�picking up of rock material through refreezing of meltwater Probably accomplishes a glacier�s most significant erosive work. Particularly effective on leeward slopes (those facing away from the direction of movement). 15 � 2011 Pearson Education, Inc.

16. The Effects on Glaciers Glacial abrasion�bedrock worn down by rock debris embedded in glacier Subglacial meltwater erosion Meltwater streams flowing below the glacier not only transport rock; they can also erode smooth grooves and channels into the bedrock. � 2011 Pearson Education, Inc. 16

17. The Effects of Glaciers Deposition by glaciers Glaciers move lithospheric material from one region to another in a vastly different form Material moved by glaciers�drift Till�rock debris deposited by moving or melting ice Glacial erratics- Large boulders that are different from surrounding local bedrock 17 � 2011 Pearson Education, Inc.

18. The Effects of Glaciers Deposition of meltwater Large portion of debris carried by glaciers deposited or redeposited by meltwater Subglacial streams from glaciers carry sedimentary material 18 � 2011 Pearson Education, Inc.

19. Continental Ice Sheets Ice sheets third most extensive feature on the planet Development and flow of ice sheets Pleistocene ice sheets originated in midlatitudes and subpolar regions Ice flowed outward from center of accumulation Ice sheets ebbed and flowed with changing climate 19 � 2011 Pearson Education, Inc.

20. Continental Ice Sheets Erosion by ice sheets Principal topography from ice sheet is gently undulating surface Valley bottoms created from moving ice 20 � 2011 Pearson Education, Inc.

21. Continental Ice Sheets Deposition by Ice Sheets Till plain�an irregularly undulating surface of broad, low rises and shallow depressions produced by the uneven deposition of glacial till 21 � 2011 Pearson Education, Inc.

22. Continental Ice Sheets Moraines�land consisting primarily of till Three types of moraines Terminal moraine�marks outermost limit of glacial advance Recessional moraine�positions where ice front is stabilized Ground moraine�large quantities of till laid down from under a glacier instead of from its edge, kettles � 2011 Pearson Education, Inc. 22

23. Continental Ice Sheets Kettle�an irregular depression in a morainal surface created when blocks of stagnant ice eventually melted Drumlin- a low, elongated hill formed by ice-sheet deposition. Drumlin- The long axis is aligned parallel with the direction of ice movement, and the end of the drumlin that faces the direction from which the ice came is blunt and slightly steeper than the narrower and more gently sloping end that faces in the opposite direction. � 2011 Pearson Education, Inc. 23

24. Continental Ice Sheets Glaciofluvial features Deposition of debris by ice-sheet meltwater produces features, composed of stratified drift Composed of gravel, sand, silt since meltwater is incapable of moving larger material Outwash plains Valley trains Eskers Kames Lakes very common 24 � 2011 Pearson Education, Inc.

25. Mountain Glaciers Mountain glacier development and flow Usually form in sheltered depressions near heads of stream valleys Erosion by mountain glaciers Basic landform in glaciated mountains is the cirque Marks the location where an alpine glacier originated Shifting equilibrium line generate quarrying action, bergschrund formation 25 � 2011 Pearson Education, Inc.

26. Mountain Glaciers Erosion by mountain glaciers (cont.) Quarried fragments from cirque carried away when ice flows out of cirque Cirque ice melts away, depression that holds water is a tarn Several cirques cut back into interfluve result in spine of rock, an ar�te Cols and horns 26 � 2011 Pearson Education, Inc.

27. Mountain Glaciers Erosion in the valleys Some glaciers never leave cirques Principle erosive work is to deepen, steepen, and widen valley U-shaped glacial troughs Glacial steps result from differences in rock resistance Small cliffs and small lakes, paternoster lakes Hanging valleys 27 � 2011 Pearson Education, Inc.

28. Mountain Glaciers Deposition by mountain glaciers Continental ice sheets more responsible for deposition than mountain glaciation Moraines primary deposition mechanism Lateral moraines Medial moraines 28 � 2011 Pearson Education, Inc.

29. Mountain Glaciers Distribution of moraines around a valley glacier 29 � 2011 Pearson Education, Inc.

30. The Periglacial Environment Periglacial�on the perimeter of glaciation Permafrost presence Frozen ground exists in Alaska, Canada, Russia Extends to great depths Patterned ground Proglacial lakes 30 � 2011 Pearson Education, Inc.

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32. Causes of the Pleistocene Glaciations What initiates ice ages? Any plausible theory must account for four main characteristics Ice accumulation is in both hemispheres but is non-uniform Concurrent development of pluvial conditions in dryland areas Multiple ice advance and retreat cycles Eventual total deglaciation 32 � 2011 Pearson Education, Inc.

33. Causes of the Pleistocene Glaciations Cold versus warm climate for glaciation Role of Milankovitch cycles Variations in solar output Variations in carbon dioxide in atmosphere Changes in continental positions Atmospheric circulations Tectonic upheaval Are we still in an ice age? 33 � 2011 Pearson Education, Inc.

34. Summary Glaciers impact the landscape through ice mass motion and associated erosion There are two primary well known eras for glaciation, the Pleistocene and contemporary glaciation During the Pleistocene, ice occupied a third of the total land mass of the Earth There were four indirect effects of the Pleistocene glaciation Antarctica and Greenland make up a large percentage of the contemporary glaciation 34 � 2011 Pearson Education, Inc.

35. Summary There are two primary types of glaciers, continental ice sheets and mountain glaciers Glacier formation involves the process of converting snow to ice through intense pressure and snow accumulation Glaciers move via sliding along a land surface; meltwater helps enhance the ability of glaciers to move Glaciers have two primary erosive effects Glaciers are capable of transporting large rock material as well as glacial flour 35 � 2011 Pearson Education, Inc.

36. Summary Glaciers deposit material through their transport as well as meltwater Continental ice sheets have a unique set of erosive and depositional characteristics Moraines are glacier-deposited landforms that consist entirely or largely of till Glaciofluvial features play an important role in the distribution of deposited glacier material Mountain glaciers have limited erosive and depositional characteristics 36 � 2011 Pearson Education, Inc.

37. Summary Valley effects of mountain glaciers can drastically alter the landscape in these regions The region surrounding a glacier that is modified by the glacier but not under it is called the periglacial There are numerous unique characteristics of the periglacial environment The exact causes of ice ages, including the Pleistocene, are unknown, though many theories hypothesize about the different effects that could have contributed It is unknown if we are still in an ice age 37 � 2011 Pearson Education, Inc.