Glacial Erosion

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Glacial Erosion
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The power of a glacier to move material is a
function of its thickness and its speed
The rate of erosion is greatest near the margins
of glaciers, and is greater in temperate glaciers
than in polar glaciers.
Cold-based glaciers, however, often have longer
lifespans
Erosional processes 1. Abrasion 2. Plucking and
Quarrying 3. Moving meltwater abrasion and
dissolution
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Abrasion Glacier ice cannot abrade most rock due
to softness (even cold glaciers).
Rock fragments act as abrasive elements
Ice is simply a power source and the matrix
within which rock abrades
Where do the rocks come from ? Free rocks,
subglacial freezing and thawing, quarrying
or valley walls
Significant abrasion may only occur when large
clasts or a large number of particles exist at
the base
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Quarrying and Bulldozing
Glaciers exert compressive forces on obstructing
rock and tensile forces when parts of the glacier
freeze to the bottom
Glaciers are capable of removing fractured
segments of rock
Loose or fractured substrate can be bulldozed
Thrust-faulting can move basal material to the
surface
Repeated advancing and retreating or changes in
applied force load and unload the substrate,
causing bending and fracturing. This is
exacerbated by freeze-thaw weathering.
Pressure melting point varies with snow
accumulation, surface melting and crevassing
(freeze-thaw zones change). If glacier is frozen
to surface and rock is fractured, it may be
plucked by the glacier above and incorporated
into the ice.
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Plucking Mechanism
A. Glacier frozen to bed where PMP below
surface
B. Frozen bed may expand (eg. due to thinning)
C. Glacier advances, plucking some of the
substrate frozen to the ice
D. After several cycles
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Subglacial Meltwater Erosion
Large amount of water generated at base of
temperate glaciers
Meltwater may flow through fractures, tunnels and
thin sheets.
Subglacial lakes form under thick polar
glaciers. Sudden release generates powerful
subglacial floods.
Water flows abrade the substrate because they
carry sediment. The water itself may dissolve
carbonates.
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Erosional features
Large-scale features
Small-scale features
Roches moutonnées Crag and tail Drumlins Flutes Ci
rque Snow hollows Glaciated valley features
Striae Grooves P-forms Channels Potholes
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Striae
Scratches produced by abrasion
Preserved best in fine-grained, brittle rock (eg.
limestone, quartzite)
Form parallel to flow direction as rocks within
the ice matrix abrade the underlying substrate
The form of striae provide a clue to the size,
concentration and hardness of clasts
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A. Multiple sets (deeper ones survive) B.
Wedge-shaped C. Nailhead D. Rat-tail E.
Polished Surface
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Simple striae Scratches of various length
Wedge-shaped Clasts abrade bedrock progressively
deeply and nailhead striae until they are
retracted back into the ice (triangular or
ellipsoidal)
Rat tail striae Ridges formed downstream from an
obstruction due to abrasion
Polished surfaces Moving mass of silt or sand
finely abrades or fine scratches underlying
substrate
Crescentic marks Presence of moving clast under
pressure causes tensional stresses upstream
and compressional forces downstream. Gouges
or fractures form if bedrock strength
exceeded.
Crescentic gouges Semilunate scours, concave
upstream formed after a rock fragment is
removed from between fractures
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Rat-tail
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Crescentic Gouges
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Grooves Linear erosional features formed in
solid bedrock Less than 2m deep and about
50-100 m long. Striae are visible
inside. Likely formation mechanism Large
boulders or bands of debris gouge the substrate.
Followed by further abrasion by sediments in ice
or subglacial water
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Multiple grooves, Sperry Glacier, Montana
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Potholes
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Potholes Round (often deep) bedrock scours
formed when small cavities are enlarged and
deepened by rock clasts caught in turbulent
vortices. The original clast is often still in
the (now dry) pothole.
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Large-scale Erosional Features
Formed by glacial plucking, often accompanied by
abrasion and flowing water.
Roche moutonnée
Streamlined forms with a smooth, gentle
upslope portion and a steep, jagged downslope
portion.
Formed by both ice sheets and valley glaciers
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Formation of Roche Moutonnée
1. Pre-existing morphological irregularity of
some sort (eg. small outcrop of relatively hard,
especially igneous rock) 2. High stresses form
upstream causing basal melting and the glacier
slides 3. Embedded clasts abrade the bedrock
upslope 4. Downslope, there is a pressure drop,
so the pressure melting point rises. The
glacier freezes to the base. 5. As glacier
pulls away, tension causes quarrying or plucking
of fragmented rocks downslope.
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Roche moutonnée, Yosemite national Park
Roche moutonnée, north of Ottawa, Ontario
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Crag and tail Consists of a resistant bedrock
knob and a streamlined remnant of bedrock or
sediments on the tail (lee side).
Crag and tail, Princess Mary Lake, Nunavut
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Flutes Sub-parallel grooves with ridges of
variable size They form in flat areas, parallel
to the direction of glacier movement Form on
bedrock or sediment-covered terrain. Mostly
erosional, but also depositional as basal
sediment is squeezed into fractures at the base
of the glacier.
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Fluted terrain, Peterborough, Ontario
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Cirque How are they formed ? Small, thin
glaciers near the snowline respond to rapidly
changing climatic conditions. Rotational mass
movements of the glacier carry ice and
sediments toward the lip of the hollow Erosion
is efficient because of frequent freeze-thaw
weathering Sculpts mountains into steep arêtes
(ridges) and horns (pyramidal mountains). The
same process may sculpt nunataks
Nivation Hollows Small niches cut into the sides
of mountains through freeze-thaw cycles that
break up local rocks and the movement of the
resulting sediment downslope
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Pyramidal form (horn) caused by cirque
erosion (Matterhorn, Swiss Alps)
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Nivation Hollows, Ellef Ringnes Island, Nunavut
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Glaciated Valleys Scoured by streams, then
modified by glaciers Traverse Shape U-shape
in cross section (glacial modification of
V-shaped fluvial valley) How does it change
to a U-shape ? 1. Velocity of glacier higher
at mid-sections of V-shaped valley walls than
at base or upper sections of wall 2. Velocity
may reach zero at the base and upper
sections of valley walls. A U-shape is most
efficient for glacier flow.
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Longitudinal Profile Generally, glaciers help
to straighten and deepen valleys. Erodability
usually varies along longitudinal profile as a
result of lithological and structural
characteristics eg. shale eroded preferentially
to granite. This leads to steps.
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Hanging valleys form where small glaciers meet
larger ones due to their weaker erosive
capability
Glaciated valleys can be carved and then flooded
during and/or after ice retreat, resulting in
fjords.
Sognefjord, Norway
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Glacial Transportation,
and Depositional Landforms
DRUMLIN FIELD
TERMINAL MORAINE
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Glacial Transportation Types of Glacial
Drift Supraglacial Drift Subglacial (Basal)
Drift Englacial Drift Sediment added to a
glacier by (a) plucking and abrasion of the
substrate (b) falling from side or head walls of
valleys and nunataks (c) wind transportation of
material onto glacier surface
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Ice sheets get most of their sediment load from
the surface Valley glaciers get their sediment
from both the bed and side Sediments are
transported (a) above the glacier (supraglacial
drift) (b) within the glacier (englacial
drift) (c) at the base (subglacial or basal
drift) Particles tend to concentrate in patches
called moraines (a) lateral moraines are derived
from the valley walls (b) medial moraines form
from the joining of lateral moraines (c) basal
moraines form from the material eroded at the
base (d) internal moraines form when sediments
fall into crevasses, where lateral moraines
coalesce at the confluence of glaciers or when
basal drift is thrust upward at the terminus
(thrust- faulting)
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Subglacial drift - composed of material derived
from the local substrate (some clasts may be
added from other parts of the glacier or from
previously-deposited glacial sediments) - subgla
cial drift, where there is basal melting, forms a
water- saturated moving carpet, facilitating
basal sliding - clasts abrade against bedrock and
may also be crushed - fine powder or silt can
also develop as a by-product of abrasion
Supraglacial drift - Important in valley glaciers
in which the confining walls provide the
material (largely angular particles) - In ice
sheets, from nunataks, upward thrusting of basal
material and windblown sediment
Glacial Deposition Till Material deposited
directly by a glacier
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Glacial Landforms Formed by Glacial Sediments
Drumlin Shape Oval, streamlined, hills,
shaped like inverted spoons or tear-drops (blunt,
rounded heads and long, pointed tails along a
straight axis). Lemniscate loop shape. Simple or
composite Generally 1-2 km long, 400 to 600 m
wide and 15 to 30 m in height (rock drumlins
can be larger) Vary in size and shape,
especially in different fields Often occur in
staggered pattern associated with small end
moraines, and eskers
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Drumlin Composition Composed of till,
sometimes stratified Drumlin Origin Erosional
Hypotheses Depositional Hypotheses Meltwater
Hypothesis
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Esker A sinuous low ridge composed of sand and
gravel formed by deposition from meltwater
running through a channel beneath or within
glacier ice.
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Moraines Accumulations of glacial sediment that
form under moving parts of glaciers and under
stagnant ice at glacier margins 1. Ground
Moraines 2. Terminal Moraines 3. Recessional
Moraine 4. Interlobate Moraines 5. Push
Moraines 6. Ice-thrust Ridges 7. Lateral
Moraines 8. Prairie Mounds 9. Moraine
Plateaus 10. Till Ridges
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Ground Moraines Basal lodgement till, often
draped by ablation till Deposited by rapidly
retreating glaciers Usually less than 3m in
thickness Corrugated surface with irregular
ridges transverse to flow direction and fluting
parallel to ice flow
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Terminal Moraines One or more subparallel ridges
of accumulated glacial drift at the front of a
glacier Similar in shape to the glacier terminus
Formed because glacier terminus remains
stationary while the rest of the glacier
continues to carry sediment to the
landform Often have a hummocky topography (knobs
and kettles) Knobs and kettles are the result of
differential ice melting and sediment release
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Topographic map of hummocky topography of a
terminal moraine
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Cross-sectional diagram of ground and end
moraines.
Recessional Moraines Moraines formed in the same
way as end moraines, during short-lived
interruptions in glacier retreat (upslope from
the main end moraine features)
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Landforms left at the lower end of a valley, by a
retreating glacier. Quarrying and abrasion is
more severe higher in the valley, while drift
thickens downvalley
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Interlobate Moraines
Form when a large volume of sediment- laden
meltwater is funneled between receding glacier
lobes (eg. Oak Ridges Moraine, Ontario) Up to
50m high and 10 to 100s of kilometres
long Consist of stratified sand and gravel
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Push Moraine
Glacier bulldozes and deforms glacial
drift Occurs at margin of the glacier Usually
less than 10m in height
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Ice-thrust Ridges Deformed bedrock with folds
(often over basal till) and faults Usually
covered in ablation till, especially in
depressions Fields of ridges up to 30m
high Spaced 200 to 300 metres apart, traced for
100s of km Most common where bedrock is of
varying strength Eg. Milk River Ridge
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Ice-thrust moraine, Saskatchewan. Ridges in
foreground are composed of deformed glacial
sediments and uplifted sandstone bedrock.
Depressions can be seen in the background.
Ice-thrust ridges in Saskatchewan. Ridges on
horizon are cored by bedrock masses uplifted by
ice pushing. Note also the spillway.
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Lateral Moraines Ridges of till along edges of
glaciated valleys Debris is from the glacier and
rocks fallen from the valley walls Deposits are
often reworked by meltwater streams
(terracing) Since they are ice-cored, there is
differential melting leading to deformation as
well as some slumping.