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8/8/2019 Glacial Systems- Lecture Compatibility Mode
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25/08/20
GLACIAL DEPOSITIONAL
ENVIRONMENTby
AP Dr Chow Weng Sum
Glacial Formation & Growth
• Glacier – a moving body of ice that formson land from the accumulation andcompaction of snow - moves downslopeunder influence of gravity
• Glaciers typically form at or above thesnowline –i.e. the lowest topographic limit
of year-round snow cover • Snow that falls below the snowline
typically melts in summer
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Glacial Budget• Budget consists of difference between the
glacier’s annual gain of snow & ice and itsannual loss (or ablation) of snow & ice by
melting & sublimation
• If budget is positive, glacier increases in
thickness and area; its outer margin (terminus)
advances downslope in confined glaciers and
outwards in unconfined glaciers
• If budget is negative,glacier decreases in size &
terminus retreats (by melting)• If budget is balanced, terminus remain stationary
Glacier budget• All glaciers have: zone of accumulation
: zone of ablation
: equilibrium line
Position of equilibrium line changes every year,depending on budget
Zone of accumulation can be identified by ablanket of snow that survives summer melting, isnourished by snowfall and sometimes from
nearby avalanchesZone of ablation is caused by summertime melting
of snow in middle and low altitudes; in highaltitudes, sublimation causes ablation
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GLACIAL MOVEMENT
Glacier Flow
• Accumulation zone: as snow is compacted
it flows at an angle downward toward the
glacier’s bed
• Ablation zone: ice flows at an angle
upward toward the surface and outward
toward the glacier’s edges
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From Steven Dutch, University of Wisconsin Green Bay,
http://www.uwgb.edu/dutchs/EarthSC202Notes/GLACIAL.HTM
Snowfall vs Melting & Evaporation (Ablation)
Glacial Flow• Glaciers flow by: a) Internal deformation
b) Basal sliding
Internal deformation: pressure from overlying snow
& ice deforms a glacier’s ice crystals, fracturing
them along planes of weakness, changing their
shapes, and causing them to slip past one
another
Basal sliding: In warmer glaciers, thawing occurs
at the bases, producing a film of water thatenables the glacier to slide across its bed
Ice flows when a glacier’s thickness exceeds about
60m
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Velocity of glacial Flow• Cold-based non sliding ice sheets
advances at only a few metres per year
• Warm-based alpine glaciers on steepslopes can flow 300m or more annually
• Some glaciers periodically ‘surge’ or accelerate in brief episodes lasting severalmonths to a few years at speeds of up to100 times the normal velocity
• Periods of surge are separated by longer
periods (10 to 100 years) of normal flow
Types of glaciers
CONFINED GLACIERS
• Alpine glaciers are confined by surroundingbedrock highlands
• a) Cirque glaciers- which create & occupy semi-circular basins or mountainsides, usually at headof valleys
• b) Valley glaciers- flow through and modify pre-existing stream valleys
• c) Ice caps – formed at tops of mountains
UNCONFINED GLACIERS
• The only unconfined form of glacier is acontinental ice sheet- covering the continent
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CIRQUE GLACIER
CIRQUE(Amphitheatre-like valley formed at head of valley)
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GLACIAL VALLEY
Glacial Erosion• Abrasion- occurs when rock fragments
embedded in the base of a glacier scrape the
surface of the underlying bedrock- giving rise to
striations or grooves on the polished bedrock.
• The abrading stones themselves become
abraded and eventually become pulverised into
a silky rock powder called glacial flour
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GLACIAL STRIATIONS
Glacial grooves- Kelly's island, Ohio:
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Polished rock surface due to glacial
abrasion in Aland
Polished floor of former glacier in Eckeroe
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Polished bedrock, Eckeroe
Glacial deposits• Glacial deposits are typically poorly sorted, often
ranging in size from fine clays to house-sized
boulders
• a) Glacial erratic : an exceptionally large,
glacially transported boulder that has been
eroded from one type of bedrock and then
transported atop another
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ERRATIC (A boulder that differs from the surrounding rock and is
believed to have been brought from a distance by glacial action )
Glacial deposits
• b) Glacial till- glacial drift deposited directly from
glacial ice ; till is exclusively deposited in the
ablation zone; it is generally unstratified, and
commonly consists of large rock fragments
surrounded by a finer-grained matrix of sand, silt
and clay
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GLACIAL TILL
GLACIAL TILL
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Glacial landform
• Glacial moraine: glacial till often forms a
moraine, a landform that typically accumulates
at the margin of a glacier.
• Moraines occur as bands of hills marking the
various advances and retreats of a glacier
Figure 8.29 From Boggs (2006), Principles of Sedimentology and Stratigraphy, 4th ed., p. 279
Glacial moraine:
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GLACIAL MORAINE
ICEBERGS IN ANTARTICA
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Figure 8.30 From Boggs (2006), Principles of Sedimentology and Stratigraphy, 4th ed., p. 280
Glaciomarine sedimentation:
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