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Glacial Erosion. How do Glaciers Move?. Internal deformation Extending Flow Compressive Flow Basal Sliding Regelation Surging Lateral Shearing Creep Rotational Flow. 1. Processes of Glacial Erosion 1. Abrasion 2. Plucking 3. Pressure Release (Dilitation) 4. Subglacial Water Erosion.

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slide3

How do Glaciers Move?

  • Internal deformation
  • Extending Flow
  • Compressive Flow
  • Basal Sliding
  • Regelation
  • Surging
  • Lateral Shearing
  • Creep
  • Rotational Flow
slide5

1. Processes of Glacial Erosion

1. Abrasion

2. Plucking

3. Pressure Release (Dilitation)

4. Subglacial Water Erosion

slide6

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.

slide7

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
slide10

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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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.

slide15

Molded Wall, New Zealand

  • Lateral glacial abrasion has smoothed this metamorphic rock along the valley side. The flowlines indicate a downward direction.
slide16

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
slide18

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
slide45

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.
slide46

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
slide47

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.
slide48

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.
slide50

2. Factors Affecting Erosion

  • Flow Types
  • Glacier Size
  • Temperature
  • Gradient
  • Weathering
  • Regime
  • Periglacial Processes
  • Rock Types - Geology
slide51

Velocity of Glacier Flow depends on:

  • Gradient
  • Thickness of Ice
  • Internal Ice Temperatures
slide53

Landforms of Glacial Erosion

  • 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
  • Small Scale Features
  • Striations
  • Chattermarks
  • Grooves
  • Friction Cracks
slide54

Case Study 1.

  • Nant Francon Valley, Snowdonia
  • Case Study 2.
  • Saas Fee, Switzerland
slide56

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 road systems where steep inclines are best avoided.

slide57

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.

slide61

Ice Sheet

Nunatak

Pyramidal Peak

Fjord

slide63

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

slide64

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 Peak, or Horn,
slide65

Fjord - A glacial trough whose floor is occupied by the sea.Common in uplifted mid-latitudes coasts, in Norway.

slide66

Fjords:Characterized by: steep sides, overdeepened rock basins,shallow thresholds at the coast. Glaciers exloit: pre-existing river valleys and underlying weaknesses in bedrock

slide67

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
slide70

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.

slide71

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 along its valley floors
slide72

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
slide73

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.

slide74

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.

slide76

Col

A pass or saddle between

2 mountain peaks

slide78

Bergschrund Explorer on Skillet Glacier in 1936. Bergschrund is visible as the dark band of ice in the background.

slide79

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

slide85

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