Physical Geology

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PowerLecture A Microsoft PowerPoint Link Tool
for
Essentials of Physical Geology 5th Edition Reed
Wicander  James S. Monroe
academic.cengage/com/earthsci
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Chapter 14

• Glaciers
• and Glaciation

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Introduction

• Glaciers are masses of ice which move over land
  by plastic flow and basal slip.
• Glaciers presently contain 2.15 of all water on
  Earth and cover about 10 of the land surface.

Fig. 14.2, p. 359
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Introduction

• The Little Ice Age
• The Little Ice Age was a time of colder winters
  and short, wet summers

• during which glaciers advanced and sea ice
  persisted for long periods of time.
• It occurred from about 1500 to the middle to late
  1800s and had its greatest effect on Northern
  Europe and Iceland.

Fig. 14.1a, p. 358
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The Kinds of Glaciers

• By definition glaciers are moving bodies of ice
  on land, that flows downslope or outward from an
  area of accumulation.
• Sea ice and icebergs is nothing more than frozen
  seawater and are not glaciers because they do not
  form on land.

Fig. 14.4b p. 361
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The Kinds of Glaciers

• Valley Glaciers
• Valley glaciers are
• Long, narrow tongues of ice
• Typically much smaller than continental glaciers
• Flow from higher to lower elevations
• Confined within mountain valleys.
• Create spectacular scenery!

Fig. 14.1b, p. 358
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Valley Glaciers

• Valley glaciers are also called alpine glaciers
  and mountain glaciers
• Valley glaciers that flow into the sea are
  called tidewater glaciers.

Fig. 14.2, p. 359
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The Kinds of Glaciers

• Continental Glaciers
• Continental glaciers flow outward in all
  directions from a zone of accumulation

• Huge - cover vast areas.
• Often develop large ice shelves where they flow
  outward into the sea.

Fig. 14.3 a, p. 360
Fig. 14.3 a-b, p. 360
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The Kinds of Glaciers

• Ice Caps
• Similar to continental glaciers but much
  smaller.
• Some develop from valley glaciers when they grow
  over the top of a divide.

Fig. 14.3 c, p. 360
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GlaciersMoving Bodies of Ice on Land

• GlaciersPart of the Hydrologic Cycle

• Glaciers are a reservoir in the hydrologic cycle
  where water is stored for long periods as it
  moves from the oceans to land and back to the
  oceans.

Geo-Insight 8., p. 371
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GlaciersMoving Bodies of Ice on Land

• How Do Glaciers Originate and Move? Glaciers
  form when winter snowfall exceeds summer melt and
  snow accumulates yearly.
• Ice is a crystalline solid. Fresh snowflakes are
  about 80 air.

• As the snow accumulates it thaws and refreezes,
  becoming a granular type of ice called firn.
• When firn is buried and and recrystallized, it is
  metamorphosed to glacial ice and will flow under
  its own weight.

Fig. 14.4a, p. 361
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Stepped Art
Fig. 14-4a, p. 361
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GlaciersMoving Bodies of Ice on Land

• How Do Glaciers Originate and Move?
• Glaciers move thru Basal Slip and Plastic Flow

• If a slope is present glaciers may slide over
  their underlying surface, a phenomenon called
  basal slip
• Most of their movement is accomplished by plastic
  flow, a type of deformation that takes place in
  response to stress.

Fig. 14.5, p. 362
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GlaciersMoving Bodies of Ice on Land

• Distribution of Glaciers
• Glaciers exist only where there is
• Sufficient precipitation in the form of snow
• Temperatures low enough that they do not melt
• These conditions prevail in
• High mountains (some even near the equator or
• High latitudes (such as in Alaska, the Canadian
  Arctic islands, Greenland, and Antarctica.)

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The Glacial BudgetAccumulation and Wastage

• Glacial budget - A glacier's behavior depends on
  the balance between accumulation and wastage
  (melting).

• The upper part of the glacier, where the snow
  cover is year-round is the zone of accumulation.
• The lower part, where losses exceed gains is the
  zone of wastage.
• The line separating the two is the firn limit.
  It shifts each year.

Fig. 14.7 a-b, p. 366
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The Glacial BudgetAccumulation and Wastage

• Glacial budget - A glacier's behavior depends on
  the balance between accumulation and wastage
  (melting).

• Glaciers having a balanced budget have a
  stationary terminus. The firn limit changes very
  little from year to year.
• Positive and negative budgets result in advance
  and retreat of the terminus, respectively.

Fig. 14.7 a-b, p. 366
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The Glacial BudgetAccumulation and Wastage

• Glacial budget - A glacier's behavior depends on
  the balance between accumulation and wastage
  (melting).

• A valley glacier with a balanced budget will
  deposit a terminal moraine at its base.
• If it has a negative budget a recessional moraine
  may develop.

Fig. 14.7 a-b, p. 366
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The Glacial Budget Accumulation and Wastage

• How Fast Do Glaciers Move?

• The rate of glacial movement depends on the
  slope, discharge and season.
• In general, valley glaciers move more rapidly
  than do continental glaciers

Fig. 14.5, p. 367
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The Glacial BudgetAccumulation and Wastage

• Glacial Surges - During a glacial surge,
  accelerated flow into a glacier causes its
  terminus to advance rapidly

• Its surface breaks into a maze of crevasses.

Fig. 14.6, p. 363
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The Glacial BudgetAccumulation and Wastage

• Theories for the cause of surges

• 1. Water-saturated sediment below a glacier
  allows it to slide
• 2. A glaciers slope increases due to thickening
  in the zone of accumulation and thinning in the
  zone of wastage.

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Erosion and Transport by Glaciers

• Glaciers effectively erode and transport
  significant amounts of sediments because they are
  moving solids.

• They are very effective in eroding soil and
  unconsolidated sediment.
• Glaciers deposit huge amounts of sediment of all
  grain sizes, from boulders the size of a house
  down to rock flour.

Fig. 14.11, p. 369
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Erosion and Transport by Glaciers

• Glaciers
• Push or bulldoze loose materials in their paths
• Erode by abrasion - that is, the movement of
  sediment-laden ice over rock surfaces
• Erode by plucking when ice freezes in or around
  bedrock projections and pulls them loose.

Fig. 14.9, p. 368
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Erosion and Transport by Glaciers

• Glaciers
• Polish rocks as they grind them into a fine
  powder called rock flour.
• Abrasion also results in glacial striations
  scratches made by rocks scraping against one
  another as the glacier moves

Fig. 14.10, p. 368
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Erosion and Transport by Glaciers

• Erosion by Valley Glaciers
• Valley glaciers carve angular peaks and deep
  valleys

• U-Shaped Glacial Troughs
• When mountain valleys are eroded by glaciers they
  are deepened and widened so that they have flat
  or gently rounded (U-shaped) valley floors and
  near-vertical valley walls.

Fig. 14.12 b, p. 369
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Erosion and Transport by Glaciers

• Erosion by Valley Glaciers
• A fiord forms when sea level rises and fills a
  U-shaped glacial valley with sea water.

U-Shaped Glacial Troughs
Geo-inSight 3-5, p. 370
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Erosion and Transport by Glaciers

• Erosion by Valley Glaciers Hanging Valleys
• Create some of the worlds most spectacular
  waterfalls
• Form when a former glacial tributary
  reaches the main valley

Fig. 14.12c, p. 369 Geo-Insight 8., p. 371
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Erosion and Transport by Glaciers

• Erosion by Valley Glaciers
• Cirques

Fig. 14.12c, p. 369

• At the upper end of the glacial trough, a
  scoop-shaped depression, or cirque, eroded into a
  mountain side marks the place where a glacier
  formed and moved out into a trough.

Fig. 14.16a, p. 375
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Erosion and Transport by Glaciers

• Erosion by Valley Glaciers
• Arêtes and Horns

• Both are landforms generated by valley glacier
  erosion.
• An arête is a serrated ridge between U-shaped
  glacial troughs or between adjacent cirques
• A horn is a pyramid-shaped peak left when
  headword erosion takes place by at least three
  glaciers in the same peak.

Geo-inSight 10., p. 370 Fig. 14.12c, p. 369
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Erosion and Transport by Glaciers

• Continental Glaciers
  and Erosional Landforms
• Areas eroded by continental glaciers
• Are smooth and rounded, ice-scoured plains
• Create deranged drainages with swamps and lakes
• Exhibit large areas of polished and striated
  bedrock

Fig. 14.13, p. 372
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Deposits of Glaciers

• Glacial Drift a general term for all glacial
  deposits
• Erratics huge boulders derived from distant
  source areas, transported to their current
  location by glaciers

Fig. 14.14 a-b, p. 373
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Deposits of Glaciers

• Glacial Drift a general term for all glacial
  deposits
• Two types of drift
• 1. Till sediments deposited directly by glacial
  ice. Poorly sorted.
• 2. Stratified drift sediments deposited by
  running water, usually in braided streams.
  Well-sorted.

Fig. 14.15b, p. 374 Fig. 14.18a, p. 377
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Deposits of Glaciers

• Landforms Composed of Till
• End moraines Crescent shaped deposits of till
  that form near the terminus of the glacier.
• Form a pile of rubble at the front of the glacier

Fig. 14.15, p. 374
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Deposits of Glaciers

• Landforms Composed of Till
• Recessional moraine.
• Suppose that a glacier reaches its maximum extent
  and has a balanced budget.
• Accordingly it deposits a terminal moraine.
• If it then has a negative budget,
• Its terminus retreats and perhaps becomes
  stabilized once again if its budget is balanced
• In this case another end moraine is deposited but
  it is called a recessional moraine.

Fig. 14.7b, p. 366
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Deposits of Glaciers

• Landforms Composed of Till
• Lateral and Medial Moraines Ridge shaped
  deposits of till that form within the glacier.
• Created by plucking rock from the valley walls
• Lateral moraines form along the sides of the
  glacier
• Medial moraines form where two lateral moraines
  meet

Fig. 14.16 b, p. 375
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Deposits of Glaciers

• Landforms Composed of Till
• Drumlins
• Streamlined hills of till shaped by continental
  glaciers or by glacial meltwater floods. Form
  drumlin fields, with 100s of drumlins present.

Fig. 14.17, p. 376
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Deposits of Glaciers

• Landforms Composed of Stratified Drift
• Sediments deposited by glacial meltwater. Well
  sorted.
• Outwash Plains vast blankets of sediment,
  usually sand and gravel, that form in front of
  the glacier as it melts
• Valley Trains deposits of braided streams that
  form long, narrow deposits of stratified drift.

Fig. 14.17, p. 376 Fig. 14.18a, p. 377
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Deposits of Glaciers

• Landforms Composed of Stratified Drift
• Kames conical hills created when a stream
  deposits sediment in a depression on the
  glaciers surface.
• Eskers snake-like deposits from sub-glacial
  streams

Fig. 14.17, p. 376 Fig. 14.19, p. 377
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Deposits of Glaciers

• Deposits in Glacial Lakes
• The most distinctive deposits in glacial lakes
  are varves
• Varves consist of couplets of dark and light,
  laminated, fine-grained sediment.
• The dark layers form during the winter when small
  particles of clay and organic matter are
  deposited.
• The light layers are made up of silt and clay
  that form during the warmer months.
• The age of a glacial lake may be determined by
  counting the layers.

Fig. 14.20a, p. 378
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What Causes Ice Ages?

• The Milankovitch Theory
• An explanation for the onset of the glacial
  episodes
• Milankovitch claimed that irregularities in
  Earths rotation and orbit bring about complex
  climatic changes that provide the triggering
  mechanism for glacial episodes.
• The 3 primary factors are
• orbital eccentricity
• changes in axial tilt
• precession of the equinoxes

Fig. 14.21, p. 378
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What Causes Ice Ages?

• Short-Term Climatic Events
• Milankovitch cycles can be measured in 10s of
  thousands of years. They are too long to explain
  events like the Little Ice Age that lasted just a
  few hundreds of years
• Several hypotheses have been proposed
• Variations in solar energy due to solar flares or
  interstellar dust
• Volcanic eruptions are known to cause short term
  climate change. A series of large eruptions
  could produce a prolonged event.

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End of Chapter 14