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Ch19
1.
Chapter 19: Glacial Modification
of Terrain McKnight’s Physical Geography: A Landscape Appreciation, Tenth Edition, Hess
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 – Exist in nonmountainous areas – Antarctica and Greenland only two – Outlet glaciers • Mountain glaciers – Highland icefields: ice sheets that submerge most underlying topography; valley and piedmont glaciers – Alpine glaciers: develop individually instead of part of ice field, cirque glaciers 4© 2011 Pearson Education, Inc. Figure 19-2
5.
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 and interglacial periods – Wisconsin glacial stage marked end – At peak, 1/3 of total land covered in ice 5© 2011 Pearson Education, Inc.
6.
Glaciations Past and
Present • Pleistocene glaciation (cont.) – Laurentide ice and the Driftless Area • 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) 6© 2011 Pearson Education, Inc.
7.
Glaciations Past and
Present • Maximum extent of the Pleistocene glaciation 7© 2011 Pearson Education, Inc.Figure 19-5a
8.
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 8© 2011 Pearson Education, Inc. Figure 19-8
9.
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 9© 2011 Pearson Education, Inc. Figure 19-10
10.
Glacier Formation and
Movement • Require balance between accumulation and ablation • Snow begins as crystallized water vapor • Compressed to granular form • More compression causes granules to coalesce, névé/firn • Further compression results in glacial ice • Ablation and accumulation zones 10© 2011 Pearson Education, Inc. Figure 19-11
11.
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 • Plastic flow and basal slip • Glacier flow versus glacier advance 11© 2011 Pearson Education, Inc. Figure 19-12
12.
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 – Glacial abrasion—bedrock worn down by rock debris embedded in glacier – Subglacial meltwater erosion 12© 2011 Pearson Education, Inc. Figure 19-15
13.
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 13© 2011 Pearson Education, Inc. Figure 19-16
14.
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 – Large boulders that are different from surrounding local bedrock, glacial erratics 14© 2011 Pearson Education, Inc. Figure 19-18
15.
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 – Glaciofluvial deposition 15© 2011 Pearson Education, Inc. Figure 19-17
16.
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 16© 2011 Pearson Education, Inc. Figure 19-19
17.
Continental Ice Sheets •
Erosion by ice sheets – Principal topography from ice sheet is gently undulating surface – Valley bottoms created from moving ice – Roche mountonnée, stoss side versus lee side – Postglacial landscape has low relief but is not absolutely flat 17© 2011 Pearson Education, Inc. Figure 19-20
18.
Continental Ice Sheets •
Deposition by Ice Sheets – Irregular, uneven surface of deposition, till plain – 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 – Drumlins 18© 2011 Pearson Education, Inc. Figure 19-21 Figure 19-24
19.
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 19© 2011 Pearson Education, Inc. Figure 19-26
20.
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 20© 2011 Pearson Education, Inc. Figure 19-29
21.
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 arête – Cols and horns 21© 2011 Pearson Education, Inc. Figure 19-31
22.
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 22© 2011 Pearson Education, Inc. Figure 19-38
23.
Mountain Glaciers • Deposition
by mountain glaciers – Continental ice sheets more responsible for deposition than mountain glaciation – Moraines primary deposition mechanism – Lateral moraines – Medial moraines 23© 2011 Pearson Education, Inc. Figure 19-41
24.
Mountain Glaciers • Distribution
of moraines around a valley glacier 24© 2011 Pearson Education, Inc.Figure 19-42
25.
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 25© 2011 Pearson Education, Inc. Figure 19-45
26.
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 26© 2011 Pearson Education, Inc. Figure 19-46
27.
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? 27© 2011 Pearson Education, Inc. Figure 19-47
28.
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 28© 2011 Pearson Education, Inc.
29.
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 29© 2011 Pearson Education, Inc.
30.
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 30© 2011 Pearson Education, Inc.
31.
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 31© 2011 Pearson Education, Inc.
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