Glaciers

1
Glacial Landforms and the Ice Age

• Glaciers
• Alpine Glaciers
• Ice Sheets of the Present
• Sea Ice and Icebergs
• The Ice Age
• Landforms Made by Ice Sheets
• Investigating the Ice Age in the PNW

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Glaciers
when snow accumulates to a great thickness, it
can turn into flowing glacial ice alpine
glaciers form in high mountains, while ice
sheets form on continental interiors at high
latitudes Alpine glaciers can flow into ice
sheets and add to overall ice flow
3
Glaciers
glacial ice accumulates when the average snowfall
of the winter exceeds the amount of snow that is
lost in summer by ablation the term ablation
means the loss of snow and ice by evaporation and
melting when winter snowfall exceeds summer
ablation, a layer of snow is added each year to
what has already accumulated as the snow
compacts by surface melting and refreezing, it
turns into a granular ice and is then compressed
by overlying layers into hard crystalline ice
when the ice mass is so thick that the lower
layers become plastic, outward (ice sheet) or
downhill (alpine glacier) flow starts, and the
ice mass is now an active glacier
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Glacial Ice Formation

• Recent snow exposed to freezing and thawing, plus
  metamorphosis
• Granular ice neve forms
• Neve buried, pressure of snow and ice above
  changes it to firn
• Under pressure firn changes to into glacial ice

http//en.wikipedia.org/wiki/ImageGlacial_ice_for
mation_LMB.png
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Alpine Glaciers

• snow collects at the upper end in a bowl-shaped
  depression, the cirque
• the upper end lies in a zone of accumulation
• layers of snow in the process of compaction and
  recrystallization are called firn
• the smooth firn field is slightly bowl-shaped in
  profile
• flowage in the glacial ice beneath the firn
  carries the ice down-valley out of the cirque

Strahler and Strahler, Figure 19.3, p. 632
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Alpine Glaciers

• motion of glacial ice moves most rapidly on the
  glaciers surface at its midline - movement is
  slowest near the bed, where the ice contacts
  bedrock or sediment

Strahler and Strahler Figure 19.4, p. 633
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Alpine Glaciers
(a)

• landforms produced by alpine glaciers
• (a) before glaciation sets in, the region has
  smoothly rounded divides and
• narrow, V-shaped stream valleys
• (b) after glaciation has been in progress for
  thousands of years, new erosional forms are
  developed
• (c) with the disappearance of the ice, a system
  of glacial troughs (U-shaped valleys) is exposed

(b)
(c)
Strahler and Strahler Figure 19.5, p. 634
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Alpine Glaciers

• (a) during glaciation, the U-shaped trough is
  filled by ice to the level of the small
  tributaries (b) after glaciation, the trough
  floor may be occupied by a stream and lakes (c)
  if the main stream is heavily loaded, it may fill
  the trough with alluvium (d) should the glacial
  trough have been deepened below sea level, it
  will be occupied by a fiord

(a)
(b)
(c)
(d)
Strahler and Strahler Figure 19.7, p. 635
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Ice Sheets of the Present

• the Greenland Ice Sheet
• has an area of 1.7 million km2 (about 670,000
  mi2) and
• occupies about seven-eights of the entire island
  of Greenland

Strahler and Strahler Figure 19.9, p. 638
10
http//upload.wikimedia.org/wikipedia/commons/4/4f
/Greenland_ice-sheet_hg.jpg
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Ice Sheets of the Present

• the Antarctic Ice Sheet covers 13 million km2
  (about 5 million mi2)

Strahler and Strahler Figure 19.10, p. 639
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http//upload.wikimedia.org/wikipedia/commons/f/f2
/Antarctic_profile_hg.png
13
Sea Ice and Ice Bergs

• two types - sea ice and icebergs
• sea ice is formed by direct freezing of ocean
  water (lt 5m thick) - pack ice is sea ice that
  completely covers the sea surface
• icebergs are bodies of land ice that have broken
  free from glaciers that terminate in the ocean
  (may be hundreds of meters thick)

Strahler and Strahler Figure 19.11, p. 639
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The Ice Age
an ice age includes cycles of glaciation,
deglaciation, and interglaciation at present, we
are within an interglaciation of the
Late- Cenozoic Ice Age, following a deglaciation
that set in quite rapidly about 15,000 years
ago in the preceding glaciation, called the
Wisconsin Glaciation, ice sheets covered much of
North America and Europe, as well as parts of
northern Asia and southern South America the
maximum ice advance of the Wisconsin Glaciation
was reached about 18,000 years ago
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The Ice Age
the maximum extent to which North America and
Europe were covered during the last advance of
the ice
Strahler and Strahler Figure 19.13, p. 641
Strahler and Strahler Figure 19.14, p. 641
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Why Ice Age Cycles?

• Positive and Negative Feedback
• Atmospheric Composition
• Milankovitch Cycles
• Tectonic Plate Movement
• Supervolcanoes

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Positive Feedback

• Positive Feedback
• Albedo
• Fewer forests (gtAlbedo)
• Ice-free Arctic Ocean?
• Ice ? no evap. or sublimation ? drier earth ?
  less snow in other areas
• Open ? more evap ? more precip ? more snow ?
  higher albedo

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Negative Feedback

• Glacial erosion reduces land surface area
• Lower sea level reduces reflective area (lt
  albedo)
• Glaciers tie up moisture, less precipitation for
  snow accumulation

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Atmospheric Composition

• Carbon dioxide, methane, sulfur, sulfur dioxide
  and particulates affect greenhouse effect
• Variations over time in concentrations affect
  amount of heat as radiation retained from earth
• Greater retention ? warmer
• Less retention ? colder
• Snowball earth

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Snowball Earth

• 850 630 mya
• Cryogenian period
• Equatorial Continent distribution ? high
  weathering of rocks
• Absorbs CO2 and earth cools
• Not a generally accepted Theory (or theory)

http//en.wikipedia.org/wiki/ImageSnowballSimulat
ions.jpg
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Milankovitch Cycles

• Variation in Earths Orbit
• Sometimes cycle reinforce variation, other times
  negate
• Cycles dont match up well to times of ice ages

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Milankovitch Variables
Eccentricity
Precession (wobble)
Axial Tilt (Obliquity)
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Variation in Solar Output

• Solar output varies with time
• Astronomers believe output increases 10/billion
  years
• Shorter term variations would affect
  glaciation/ice ages
• Sunspots and sunspot cycles

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Continental Movement

• Location and distribution of continents affects
  sea and atmospheric circulation
• Heat transfer on earth is affected or controlled
  by sea and atmospheric circulation
• Mountain building, orographic effect also affects
  precipitation patterns and related atmospheric
  effects

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Volcanoes

• Additional particulates might initiate global
  cooling
• However, unlikely particulates would stay
  suspended long enough to initiate on their own
• Undersea volcanoes could release enough methane
  and methane clathrates (methane molecules trapped
  in sedimentary rocks/molecules)

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Landforms Made by Ice Sheets

• ice sheets are highly effective eroding agents
• the slowly moving ice scraped and ground away
  much solid bedrock, leaving behind smoothly
  rounded rock masses
• these bear countless abrasions trending in the
  general direction of ice movement

Strahler and Strahler Figure 19.15, p. 642
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Landforms Made by Ice Sheets

• the term glacial drift describes all types of
  rock debris deposited in close association with
  glaciers
• stratified drift includes sediment laid by water,
  while till is deposited directly by ice

Strahler and Strahler Figure 19.17, p. 643
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Landforms Made by Ice Sheets
moraines are built of rock debris deposited at
the edges of a melting glacier or ice
sheet terminal moraines mark the limits of
glaciation
Strahler and Strahler Figure 19.17, p. 643
30
Landforms Made by Ice Sheets
an outwash plain is formed of stratified drift
left by braided streams issuing from the ice
the plain is built of layer upon layer of
sands and gravels
Strahler and Strahler Figure 19.17, p. 643
31
Landforms Made by Ice Sheets
after the ice has gone, the position of a former
ice tunnel is marked by a long, sinuous ridge of
sediment known as an esker (stream-bed deposit
remains, forming a ridge)
Strahler and Strahler Figure 19.17, p. 643
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Landforms Made by Ice Sheets
a drumlin is a smoothly rounded, oval hill
resembling the bowl of an inverted teaspoon
(consists of till) - lie in a zone behind the
terminal moraine (commonly occur in groups)
Strahler and Strahler Figure 19.17, p. 643
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Landforms Made by Ice Sheets
the long axis of each drumlin parallels the
direction of ice movement drumlins are
typically steeper at the broad end, which faces
oncoming ice
Strahler and Strahler Figure 19.20c, p. 647
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Landforms Made by Ice Sheets
between moraines, the surface overridden by the
ice is covered by glacial till where thick and
smoothly spread, the layer forms a level till
plain
Strahler and Strahler Figure 19.17, p. 643
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Landforms Made by Ice Sheets
marginal glacial lakes formed along the ice
front streams of meltwater from the ice built
glacial deltas into these marginal lakes when
the ice withered away, the lakes drained, leaving
a flat floor exposed
36
Landforms Made by Ice Sheets
former glacial deltas become isolated,
flat-topped landforms known as delta kames,
composed of well-washed and well-sorted sands and
gravels
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Investigating The Ice Age

• several theories have been proposed for the cause
  of the Ice Age change in the position of the
  continents, increased volcanism, decrease in the
  Suns output of energy
• the timing of glaciations and interglaciations is
  determined by variations in insolation produced
  by minor cycles in the Earths orbit and the
  Earths axial rotation (the Milankovitch
  mechanism)

Strahler and Strahler Figure 19.22, p. 649
Milankovitch curve
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Investigating The Ice Age in the PNW

• The elapsed time span of about 10,000 years since
  the Wisconsin Glaciation ended is called the
  Holocene Epoch
• Glaciers in the PNW were a mix of Continental and
  Alpine (of vast extent)
• Alpine glaciers predominated in High Cascades and
  Olympics
• Alpine Glaciers in BC contributed to continental
  ice sheet, the Cordilleran Ice Sheet, to flow
  into Puget Lowlands
• East of Continental Divide was the Laurentide Ice
  Sheet

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http//en.wikipedia.org/wiki/ImageNorthern_iceshe
et_hg.png
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From Orr and Orr, 2005, p. 17
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From Orr and Orr, 2005, p. 17
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http//en.wikipedia.org/wiki/ImageRainiersourdoug
hridge.jpg
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http//upload.wikimedia.org/wikipedia/en/d/d7/Moun
t_Olympus_Washington.jpg
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http//www.nps.gov/archive/olym/glacier.htm
http//www.extremeicesurvey.org/