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Dec 30, 2015
How do Glaciers Move?Internal deformationExtending FlowCompressive FlowBasal SlidingRegelationSurgingLateral ShearingCreepRotational Flow
1. Processes of Glacial Erosion
1. Abrasion2. Plucking3. Pressure Release (Dilitation)4. Subglacial Water Erosion
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.
Factors affecting AbrasionHardness of particles and bedrockIce thicknessBasal Water PressureSliding of Basal IceMovement of debris towards glacier baseEfficient removal of fine debrisDebris particle size and shapePresence of debris in basal ice
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.
Striation Mt. Sirius, Antarctica, 1986.
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.
Chattermarks: Mt. Sirius, Antarctica, 1986
Molded Wall, New ZealandLateral glacial abrasion has smoothed this metamorphic rock along the valley side. The flowlines indicate a downward direction.
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.
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
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 sectionScree slopeArmchair 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
Plucking - eg Roche Mountonnees
Stoss and Lee outcrop, Mount Desert Island, MaineThe various steps and controlling fractures are evident here. Notice that the general shape of the outcrop suggests ice diversion slightly upward.
Plucked Face, The Beehive, Mount Desert Island, MaineJointing patterns in the granite bedrock define the size and shape of blocksthat can be removed. Ice flow from left to right
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.
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.
2. Factors Affecting ErosionFlow Types Glacier SizeTemperatureGradientWeathering Regime Periglacial Processes Rock Types - Geology
Velocity of Glacier Flow depends on:GradientThickness of IceInternal Ice Temperatures
Small Scale FeaturesStriationsChattermarksGroovesFriction Cracks
Landforms of Glacial ErosionLarge Scale FeaturesU-Shaped Glaciated ValleyAretesPyramidal PeakCorriesFjordsGlacial TroughsTruncated SpursHanging ValleysRibbon LakesDrumlinsRoche MoutonneeColsBergschrundKnock and Lochan
Case Study 1.Nant Francon Valley, Snowdonia
Case Study 2.Saas Fee, Switzerland
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.
U-Shaped ValleyThe 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.
Cirques / Corrie / Cwm
Ice SheetNunatakPyramidal PeakFjord
Cirques / Corrie / Cwm - eg Cwm Idwal, Snowdonia
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
Arete - eg Striding Edge, Helvellyn, Cumbria
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,
Fjord - A glacial trough whose floor is occupied by the sea.Common in uplifted mid-latitudes coasts, in Norway.
Fjords: Characterized by: steep sides, overdeepened rock basins,shallow thresholds at the coast. Glaciers exloit: pre-existing river valleys and underlying weaknesses in bedrock
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
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