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LITHOSPHERE GLACIATION
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Glaciation
Learning Intentions
Be able to understand the processes which occurred during the last ice age.
Be able to describe and explain the formation of features formed during glacial periods.
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Rock breaks up
over time
Processes Of Weathering
Freeze-Thaw
Water enters crack
Ice freezes and exerts pressure from inside rock
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Conditions and processes which encourage the formation of scree slopes.
• Steep and bare rock faces with lines of weakness/well-jointed carboniferous
limestone.
• Cold climate where temperatures often fall below freezing point at night.
• The two factors above allow physical weathering to take place in the form of
freeze-thaw action/frost shattering, where water collects in the rock fractures,
freezes and expands by about 9% exerting great pressure on even the hardest
rock.
• Repeated freeze-thaw action splits the rock into large sharp fragments which
break off and are moved downhill by gravity to accumulate at the base of steep
slopes as a scree or talus slope as large heaps of rock debris.
Biological weathering
• Root wedging
• Animals burrows
4
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Processes Of Erosion PluckingWater can enter cracks in the rock and freeze, as it does so it freezes the glacier and rock together.
As the glacier moves it literally pulls the rock out and is likely to occur more when temperatures fluctuate around 0ºC.
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Processes Of Erosion Plucking
The water from this process often comes from the friction between ice moving over rock.
Another source of the water is a result of melting of the ice; during the day it melts, but
refreezes at night.
Plucked Rock
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Processes Of Erosion AbrasionThis is when rock under the ice or stuck in the bottom of it, scrape away at the surface of the bed rock surface.
The rock comes from the debris produced by Freeze Thaw and Plucking.
The bedrock is scratched, polished, smoothed and eventually worn away by the scouring action.
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Processes Of Erosion Abrasion
Striations or scratches can be left on the rock
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Snow collects in hollows, especially on the less sunny north and east facing slopes, turns to glacial ice and moves downwards under the force of gravity
Rocks are plucked out and the hollow is widened by abrasion to become a corrie.
A corrie is a deep, rounded hollow with a steep back wall.
CORRIES
Features of erosion
LITHOSPHERE GLACIATIONCorries Task 3 Q2
Match the letters to the descriptions.
a
b
c
d
Flat Base
Horse shoe shape
Steep sides
Cliffs/ Outcrops/Scree
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Corrie Development 1
S N
Hollow
Layers
Ice
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Corrie FormationStage 1
Snow collects in hollows high up in the mountains, usually in north facing slopes.Pressure causes the lower layers of iceto turn to ice.Gravity and reduced friction cause the iceto move downhill.The hollow in which the snow and ice collected is eroded by the ice to form a much deeper, steeper hollow called a Corrie.
LITHOSPHERE GLACIATION
FT
PL
AB
Corrie Development 2
RS
L
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Corrie Formation
Stage 2As meltwater under the ice seeps into cracks the rock in the hollow is weathered by freeze thaw action. Then when the meltwater freezes onto the rock plucking takes place.The backwall and sidewalls of the hollow are eroded quickly by these two processes.Abrasion due to rocks taken in by frost shattering and plucking deepens the hollow.The glacier slips in a rotating movement, which means that it is eroding the most at the back wall and base. So with less erosion at the base so a lip starts to form, which has deposits of rocks left on it by the glacier
LITHOSPHERE GLACIATIONCorrie Development 3
Lip
Lochan
Scree
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Corrie FormationStage 3
When the ice melts a corrie can clearly be seen.
It is an armchair shaped hollow in the mountainside with steep sidewalls and steep backwall and a flat base.
Sometimes after glaciation, corries fill with meltwater to form corrie lochs or tarns.
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Formation of a corrie
Features of erosion
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Once the glacier retreats, the corrie may be filled with water. A small, generally circular loch is formed. This is known as a tarn or corrie lochan.
lip
Back wall
scree
Features of erosion
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Red Tarn
Features of erosion
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An arete is a narrow, sharp-edged ridge which forms the side walls of corries or separates different glacial valleys.
ARETE
Features of erosion
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Arete Formation
Aretes are formed when two corries are found back to back or side by side in the mountain area.
Plucking and freeze thaw action erode the backwalls of both corries towards each other.
The land between them gets narrower and narrower until all that remains is a steep knife-edged ridge of rock.
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Striding Edge arete onHelvellyn,Lake District
Striding Edge
Features of erosion
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MATTERHORN
Pyramidal peaks are also called horns.
Features of erosion
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Diagram Of Pyramidal PeakPyramidal Peak
Erosion
Pyramidal Peaks
a
b
cc
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Pyramidal Peak FormationFound where 3 or more corries are located in the same mountainside.Plucking and freeze-thaw action of the backwalls of each corries occurs to such an extent that the rounded summit is eroded into a sharp peak or point.
The rock erodes back so that the mountain becomes steeper and the three corrie walls come close enough to converge
This leaves a pointed and jagged (freeze thaw) peak
eg. The Eiger (The Alps)
LITHOSPHERE GLACIATIONU Shaped Valley
a
b
c
d Steep sides
Straight valley
Deep valley
Loch in wide valley
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U-shaped Valley – Misfit Steam
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U-shaped Valley – Ribbon Lake
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A VALLEY GLACIER
The next few slides will help to explain the formation of this feature.
Features of erosion
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U Shaped Valleys
It flows down V shaped river valleys
Ice starts to flow out of corries or
off ice caps
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Ice moves down river valley
Usual erosion occurs
Abrasion deepens
Plucking erodes sides back
Glacier also bulldozes rock out of the way
U shaped valley develops
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U-shaped valley formationWhen a glacier erodes its valley, a classic U shape is formed, the side walls tend to be steep and possibly curving inwards at the base, and the valley floor almost flat.
U shaped valleys start life as V shaped river valleys that existed before glaciation. Ice moves down the V shaped valley and glaciers erode it and change its shape as the ice moves.
Plucking and freeze-thaw action truncate spurs of rock and steepen the valley sides and abrasion deepens and flattens the valley floor. Over time the valley also becomes straighter.
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Misfit Stream
After glaciation when the ice melts, the river which originally flowed on the floor of the V-shaped valley may once again begin to flow through the U-shaped valley.
It is now called a Misfit Stream. It is so called because it looks out of place in the now large valley and also because it did not contribute to its formation.
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Ribbon LakeAt some points in the U-shaped valley, glaciers erode more deeply than elsewhere.
This might have been because the rock was softer and more easily eroded or because the ice was thicker at this point and therefore more powerful.
Where ice did this, it would create an overdeepened hollow which after glaciation filled with meltwater to become a Ribbon Lake.
The lake takes on the same shape as the valley in which it was formed – so tends to be long and quite narrow.
LITHOSPHERE GLACIATIONTruncated Spurs
Truncated Spur
Steep Cliffs Overlooking U shaped valleys
Often Scree collects underneath
LITHOSPHERE GLACIATIONTruncated Spurs
Bulldozed by glaciers
Abrasion & plucking help with erosion
Post glaciation freeze-thaw has created more scree below
LITHOSPHERE GLACIATIONTruncated Spurs
l
mk
Cliff Face
Scree
Steep Sides
U Shaped Valley
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These are deep U shaped valleys formed by the erosion of valley glaciersAs glaciers emerge from the lip of their corries they merge and form a more powerful glacier which erodes more readilyThey follow the course of former V shaped river valleysAs they progress down the valley through their own weight (gravity) they erode like a corrie glacierThe top of the valley sides are eroded by freeze thaw weathering, mid height of the valley by plucking and the base by abrasionThis means that the former V shaped valley will widen, steepen and deepensWhen a U shaped valley cuts across a water shed it is called a glacial breech The interlocking spurs of the river valley literally have their noses cut off by the ice, leaving a ridge descending into the valley which suddenly steepens, sometimes into an outcrop or a cliff faceThe steepest part of the valley is often at the trough head (start) where the descending corrie glaciers move quickly under gravity. Other more eroded points tend to be a result of the glacier moving over weaker rock which abrades readily, deepening the baseThis over deepening can fill in with water leaving a ribbon loch eg, Loch Avon (Cairngorms)
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Stages in the formation of a U-shaped valley
Features of erosion
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A U-shaped valley in Canada.
Features of erosion
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The sea lochs of western Scotland are the best examples of fiords in the British Isles.
When a glaciated valley by the coast is submerged or drowned by a rise in sea level, a fiord is formed.
Fiord/fjord
Features of erosion
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Vertical erosion in the main glacier is far greater than in the tributary glaciers. So the valleys are not the same depth.
After the glacier has retreated, rivers flowing down the tributary join the main valley via a waterfall
A hanging valley
Can you spot the river delta,too?
Features of erosion
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U-shaped valleys have few contours on their floors.
Note the very steep sides.
There is a hanging valley here.
Truncated spur
Misfit stream
waterfall
Features of erosion
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The English Lake District owes its character to these narrow ribbon lakes along its valley floors.
When the glacier retreats, the deepest parts fill with water and become lakes.
When a glacier moves along the valley, some parts are deepened more than others.
Ribbon lakes
Features of erosion
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alluvium = silt deposited by a river
An alluvial fanCaused when a stream falling from a side valley reaches flatter ground on the valley floor.
Material is dropped at the ‘break of slope’ to form this fan shape.
(This is really a depositional feature.)
Erosion/ deposition
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These are partly erosional, partly depositional features.The rock face facing the ice is steepened by glacial erosion. Softer rock on the other slope is protected from erosion to form a tail of boulder clay.
Edinburgh CastleA crag and tail
Plug of volcanic rock
tail
Erosion/ deposition