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How do Glaciers Move?
•Internal deformation
•Extending Flow
•Compressive Flow
•Basal Sliding
•Regelation
•Surging
•Lateral Shearing
•Creep
•Rotational Flow
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1. Processes of Glacial Erosion
1. Abrasion
2. Plucking
3. Pressure Release (Dilitation)
4. Subglacial Water Erosion
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1. ABRASION
With it's load of abrasive rock
fragments, the base of the glacier acts
like a belt sander, scraping across the
rock, eroding it, producing characteristic
erosional features, and creating a supply
of material that leads eventually to the
formation of depositional features as
well. This scraping process is called
Abrasion.
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Factors affecting Abrasion
•Hardness of particles and bedrock
•Ice thickness
•Basal Water Pressure
•Sliding of Basal Ice
•Movement of debris towards glacier base
•Efficient removal of fine debris
•Debris particle size and shape
•Presence of debris in basal ice
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STRIATIONS
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Striations
When a glacier moves across the underlying rock, the process of
abrasion wears it away. It is the fragments of rock held in the ice that
do the abrading, scraping across the rock surface like nails across a
wooden desk top. Larger rock fragments leave deep scratch marks
behind them. These scratch marks are straight parallel lines that reveal
the direction of ice movement.
Freshly exposed striations have a preferred orientation of rock grains.
By lightly running a finger along the striation it is possible to discover
that when moving one way along it, the rock feels smooth, but when
moving the other way it feels more coarse. The moving ice leaves the
rock grains aligned with the direction of movement, so when the
striation feels smooth, your finger is moving in the direction of ice
flow.
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Striation Mt. Sirius, Antarctica, 1986
.
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The Grooves on Kelley's Island have been the source of debate for
over 100 years. Some say they were cut by glacier ice, others say by jets of
subglacial water. Note the curved forms suggesting fluid flow.
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Chattermarks: Mt. Sirius, Antarctica, 1986
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Molded
Wall, New
Zealand
Lateral
glacial
abrasion
has
smoothed
this
metamorphic rock
along the valley
side.
The
flowlines indicate
a
downward
direction.
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2. Plucking
Occasionally, a moving glacier may become
stuck on its bed. This occurs when for some
reason a reduction in pressure causes liquid
water to freeze, attaching the moving ice to the
bedrock. As the ice continues to move an
immense pulling force is applied to the attached
rock which may then fracture and be plucked
from its position. It involves the removal of
much larger fragments of rock than abrasion.
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The Growth of a Corrie
•Snow falls on a north-facing slope and tends to stay (ie does not
melt) as in the northern hemisphere it is colder here. Hence there
is minimal (generally in summer) ablation and accumulation
continues. Therefore, the Net Balance is positive - hence the
glacier grows - and increases in size.
•The snow eventually forms FIRN - and eventually a glacier is
born (I.e. the accumulation of years of compressed snow).
•At the ice-rock interface freeze-thaw starts to operate attacking the bedrock and loosening rock particles, which break
off and are used as tools of abrasion. These form striations,
chattermarks and grooves in the bedrock - and SCOUR out the
bedrock - hollowing it out.
•The glacier “flows” downhill under gravity and ROTATIONAL
FLOW exentuates this process
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The Growth of a Corrie cont.
•Over time, the arm-chair shaped hollow is formed - I.e the classical
shape of a typical corrie (cirque, cwm).
•Eventually, due to Milankovitch, the Ice Ages ebbed away and a
corrie is left over - which has the typical shape of:
•Upper cliff section
•Scree slope
•Armchair hollow (typically with a small lake left over)
•Rock Lip (where the Ice Fall once would have been)
•Example = Cwm Idwal, Nant Francon Valley, North Wales
•Where 2 corries erode backwards an Arete is formed, and where 3 or
more erode backwards a Pyramidal Peak is formed - eg Matterhorn,
Switzerland
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Plucking - eg Roche Mountonnees
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Plucking
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Roche Mountonnee
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Stoss and Lee outcrop, Mount Desert Island, Maine
The various steps and controlling fractures are evident here. Notice that the
general shape of the outcrop suggests ice diversion slightly upward.
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Plucked
Face, The
Beehive,
Mount
Desert
Island,
Maine
Jointing patterns in
the granite bedrock
define the size and
shape of blocks
that can be removed.
Ice flow from left to
right
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3. Pressure Release (Dilitation)
When a glacier erodes, the replacement of a certain volume of rock
by ICE (one-third of its density), causes dilitation and the
separation of the rock along sheet joints.
For example after the Last Glacial Maximum in Europe, in the
warmer interglacial period, the Laurentide Ice Sheet began to
retreat. Consequently, a huge amount of ice melted from the land,
releasing trillons of tons of pressure on the earths surface. As a
result of this pressure release, rocks buckled along existing cracks
and joints - making them vulnerable to processes of weathering
and erosion in the Periglacial environment that followed.
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•4. Sub-Glacial Water Erosion
Temperate glaciers, especially in summer, have many
meltwater streams on their surface, which plunge down
crevasses (called moulins) into the base of the glacier.
Many such streams descend right to the valley floor,
where they are another cause of erosion of the rock
surface. This is particularly true of those which carry a
large load of sediment, rock flour or material of
morainic origin.
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2. Factors Affecting Erosion
•Flow Types
•Glacier Size
•Temperature
•Gradient
•Weathering
•Regime
•Periglacial Processes
•Rock Types - Geology
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Velocity of Glacier Flow depends on:
•Gradient
•Thickness of Ice
•Internal Ice Temperatures
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3. Landforms
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Landforms of Glacial Erosion
•Small Scale
Features
•Striations
•Chattermarks
•Grooves
•Friction Cracks
•Large Scale Features
•U-Shaped Glaciated Valley
•Aretes
•Pyramidal Peak
•Corries
•Fjords
•Glacial Troughs
•Truncated Spurs
•Hanging Valleys
•Ribbon Lakes
•Drumlins
•Roche Moutonnee
•Cols
•Bergschrund
•Knock and Lochan
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Case Study 1.
•Nant Francon Valley,
Snowdonia
Case Study 2.
•Saas Fee, Switzerland
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U Shaped Valley
When a glacier erodes its valley a classic U shape is formed, the side walls
tending to be steep and possibly curving inwards at the base, and the valley
floor almost flat.
U shaped valleys often start life as river valleys that existed before glaciation
occurred. The glaciers then followed the existing V shaped valleys, eroding
and deepening them as the ice moved. Over time the valleys became
straightened, widened and deepened, keeping the steep sides and acquiring a
flat base. U shaped valleys are also known as Glacial
Troughs.
The flat floor is roughly shaped by the ice which tends to cut down more
evenly than flowing water. A thick layer of glacial debris (ground moraine) is
deposited as the ice retreats, smothering any minor irregularities, and creating
a well drained and fertile soil.
In mountainous areas, the low lying flat valley floors are frequently used for
farming, transport routes and habitation. They offer the easiest routes through
the mountains, are warmer than the higher ground, and have good water
supplies. The flatness of the ground is particularly advantageous for rail and
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road systems where steep inclines are best avoided.
•U-Shaped Valley
The glacial-shaped
valley trends north
through the
Brooks Range. For
scale, look at the
road on the floor
of the valley.
Atigun Glacier
and one of its
moraines are in the
foreground.
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•Cirques / Corrie / Cwm
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Ice Sheet
Nunatak
Pyramidal Peak
Fjord
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•Cirques / Corrie / Cwm - eg Cwm Idwal, Snowdonia
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•Arete - eg Striding Edge, Helvellyn, Cumbria
When a corrie is formed, its
back and side walls tend to
be steep and jagged,
perhaps almost vertical.
When two corries form next
to each other, and their
adjacent walls are eroded
backwards until they meet,
a narrow and pointed rock
ridge is formed. This is
often likened to a knife
edge, with near vertical
sides and a sharp top edge.
This feature is called an
arete
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Pyramidal Peaks
•eg Matterhorn, Switz.
When three or more
corries erode backwards
and meet they cannot
form an arete; it has steep
sides but doesn't have the
length to make a ridge.
Imagine three corries at
the corners of a triangle,
eventually all eroding
back and meeting in the
middle. A sharp pointed
pyramid shape is created.
This is called a Pyramidal
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Peak, or Horn,
Fjord
- A glacial trough whose floor is occupied by the
sea.Common in uplifted mid-latitudes coasts, in Norway.
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Fjords: Characterized
by: steep sides, overdeepened rock
basins,shallow thresholds at the coast. Glaciers exloit: preexisting river valleys and underlying weaknesses in bedrock
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•Truncated Spurs
Spurs that projecting into the original river
valley are cut short, their lower ends being
destroyed by the moving ice. They may be
cut back right to the edges of the new
valley, or still project slightly into the
valley. This shortens the spurs, or truncates
them. They are then known as Truncated
Spurs
eg Lauterbrunnen, Switzerland
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Hanging Valley
They are the product of different
rates of erosion between the main
valley and the valleys that enter it
along its sides. The floors of the
tributary valleys are eroded and
deepened at a slower rate than the
floor of the main valley, so the
difference between the depths of
the two valleys steadily increases
over time. The tributaries are left
high above the main valley,
hanging on the edges, their rivers
and streams entering the main
valley by either a series of small
waterfalls or a single impressive
fall.
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Ribbon Lakes
When a glacier moves along
its valley, changes in the rate
of flow caused by extension
or compression may lead to
increased deepening of
sections of the valley floor.
Areas of softer rock may
also experience increased
deepening. When the glacier
retreats, the deepened
sections fill with melt water
and become lakes. These
lakes remain long after
glaciation has ended,
supplied by rainfall and
subsequent streams and
rivers. The English Lake
District owes its character
to these narrow ribbon lakes
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along its valley floors
Drumlins
Drumlins are formed of till. They are elongated features that can reach a kilometer or more
in length, 500m or so in width and over 50m in height. The Stoss end is the steeper of the
two ends and used to face into the ice flow.
The Lee slope is the more gentle slope and becomes lower as you move away from the
source of the ice. This means that the highest point will always be at the Stoss end of the
drumlin, and the lowest point will be the end of the Lee slope. It is common to find several
drumlins grouped together. The collection of drumlins is called a swarm.
There is still some debate about how drumlins are formed, but the most widely accepted
idea is that they were formed when the ice became overloaded with sediment. When the
competence of the glacier was reduced, material was deposited, in the same way that a
river overloaded with sediment deposits the excess material. The glacier may have
experienced a reduction in its competence for several reasons, including melting of the ice
and changes in velocity.
It is difficult to understand how the material could have been directly deposited in the
characteristic shape of a drumlin unless the ice was still moving at the time, but it may also
have been reshaped by further ice movements after it was deposited
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Drumlin
In the diagram above, the ice was flowing from left to right. The long axis of the
drumlin is the line A-B, the point of maximum width is the line C-D, and the highest
point on the landform is at E. Not all drumlins will show such a distinct difference in
slope angle between the stoss end and lee slope, but the stoss end will always be the
steeper of the two.
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Roche Moutonnee
Roche Moutonnee are outcrops of resistant bed rock with a
gentle abraded slope on what would have been the upstream
side of the ice (stoss slope) and a steep rougher slope on the
downstream side (lee slope). The name is French and
translates into English as 'sheep rocks', a good description
of them when seen from a distance. The smooth upstream
slope is probably caused by abrasion as the ice advances
over the rock, and the rough 'tail' is due to the action of
plucking where ice has attached to the rock and literally
pulled rock fragments away. Plucking could occur because
as the ice moved up the stoss slope there was a reduction in
pressure, allowing liquid water to re-freeze and attach the
ice to the underlying rocks.
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Roche Moutonnee
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Col
A pass or
saddle
between
2 mountain
peaks
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Bergschrund
Explorer on
Skillet Glacier in
1936.
Bergschrund is
visible as the
dark band of ice
in the
background.
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Knock and Lochan
A landscape of ice-moulded
rock knobs with intervening
lochans which have been
eroded along lines of structural
weakness. This type aite is
found in NW Scotland (eg
Barra) and also is Canadian and
Scandanavian Shields
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Conclusions
•The effectiveness of glacial erosion depends much on the
movement of ice, the load the glacier carries and the
hardness of the rocks it transverses
•Abrasion and Plucking are the two most important processes
of glacial erosion
•Pressure Release and Subglacial meltwater erosion
undoubtedly make some contributions as well
•Glacial troughs with truncated spurs, hanging valleys,
ribbon lakes (often drained with Cirques and Aretes at their
heads) are characteristic landforms resulting from erosion
•Ice Erosion affects both the long and transverse profiles of
the valleys
•A parabola is a useful way to describe the shape of a glacial
trough in conjunction with a form ratio
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