Glaciology L4: Processes of glacier erosion and deposition

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What are some of the arguements for studying glacier erosion?
Over time have formed spectacular landscapes- able to remove and transport huge volumes of rock
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Define: erosion
The process by which materials are removed from the Earth's surface
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Why is it difficult to study glacial erosion?
Hard to make direct measurements (glaciers are inaccessible and processes are slow). Glaciers= complex systems- the interactions between ice dynamics, ice temperature and hydrology all strongly affect the erosive processes.
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List the main erosive processes that occur in the glacier environment
Bedrock crushing; plucking and quarrying; abrasion; mechanical erosion by meltwater; chemical erosion by meltwater
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How does bedrock crushing occur?
When there is a failure of the bedrock through direct fracturing by direct loading from the weight of overlying ice. Rock is broken up into large angular rocks
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What factors affect bedrock crushing?
Extent of normal stress (how hard a material is being pushed down in the perpendicular direction) (force per unit area); strength of bedrocks
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How is normal stress calculated?
Product of ice density, action of gravity and ice thickness (pigh). Density of ice varies from 830-910 kg m3. Gravity= 9.81 m s-2. Thickness= maximum 5km (Antarctica). Measured in MPa.
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How able are rock types to resist normal stress?
Most rock types= strong enough to resist. Only the thickest glaciers on the weakest rock are likely to be able to exert enough pressure to cause bedrock crushing e.g. Shale, Tuff
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If most rock types are resist to high normal stress, why are effects of bedrock crushing observed?
Bedrock= not smooth. Stress is concentrated at weaknesses in bedrock- fractures formed by pre-glacial weathering. Repeated loading/unloading cycles. Stress may be concentrated at clasts (big lumps of rock stuck in bedrock).
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Describe the process of plucking and quarrying
Crushed bedrock= picked up. Process by which a glacier removes large blocks from its bed. Entrainment by: freezing (regelation((melted ice refreezes)) around rock) or incorporation into ice along faults
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What determines whether or not plucking/quarrying takes place?
Balance between tractive forces (from glacier) and frictional forces (from bedrock). High basal water pressure may reduce frictional forces
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Define: abrasion
The process whereby bedrock is scoured by debris carried in the basal layers of the glacier (Sugden & John, 1976)- rock particles in basal ice are dragged over the bed
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What features are produced by abrasion?
Striation (bedrock scoring); polishing (reducing roughness of brittle protuberances); roundness of particles
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What is the rate of erosion by abrasion controlled by?
Contact pressure between ice and bed; rate of glacier sliding; concentration of hardness of particles in ice
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When is abrasion most effective?
Significant basal ice debris content; clasts=harder than bed; significant sliding; thick ice (high normal pressure) but high effective pressure (want low water pressure- otherwise reduce friction); meltwater flushing removes finer product(get in way)
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What types of material are found in glaciers?
Basal material- striation and rounded larger clasts and fine rock flour
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How does mechanical erosion by subglacial meltwater occur?
Erosion by flowing water. Corrasion (mechanical weathering/grinding) or cavitation (pressure changes in turbulent flow) controlled by the nature of the drainage system, rate of flow, turbulence and the load of suspended loading
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Why is glacial meltwater usually errosively efficient?
High velocities; high sediment load; cold temperatures
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What are the products of mechanical erosion by water?
N (Nye) channels; smooth bedrock
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List some of the processes that occur as part of chemical erosion by meltwater
(Anything to do with the decomposition or alteration of minerals)- Dissolution, hydrolysis, hydration, oxidation, reduction, carbonation, cation exchange.
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What are erosion rates by meltwater enhanced by?
Availabilty of fresh surface water, freshly abraded surfaces, dissolves CO2 from dissolution of CaC03. Dissolved H+ acidity. Long contact time.
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What is the total amount of erosion the sum of?
Bedrock crushing + Plucking/Quarrying + Abrasion + Mechanical erosion by water + Chemical erosion by water
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What are the two broad controls on the rate at which glaciers erode bedrock?
Basal temperature(sliding rate, production of meltwater, chemical processes) and geotechnical properties of bed (hard brittle rocks-susceptible to crushing, soft=readily abraded, very soft- ploughing may occur).
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How can erosion in glaciers be measured? What are the drawbacks of these methods?
Direct- e.g. place materials beneath ice- marble/metal plates- found 0.9-36 mm /yr. Boulton, 1974 (doesn't measure plucking or chemical erosion). Suspended sediment load in streams and basal ice (0.073-0.165 mm / yr)
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How does glacial erosion vary globally?
In a cold polar glacier- 0.01 mm /yr - little water. Small alpine glacier- 1 mm / yr (dynamic-temperate). Large Alaskan glacier- 10/100 mm /yr (v.dynamic).
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How significant is the rate of glacial erosion?
Over time: cumulative effect. 1 mm/yr = 1 km over 1 million years
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Aside from erosion of the bedrock/valley sides where else may sediment originate from?
Freeze-thaw, rockfall, avalanches- material falls onto glacier
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What influences whether sediment stays on the surface or is incorporated into a glacier?
Low level path-operated upon: Deposition in accumulation zone- buried and transported en- or subglacially. High level path: Deposited in ablation zone - remains @ surface (passive transport)- released at snout by melt
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Contrast the sediment transported via high level and low level paths
High level- angular, coarse, passive transport; Low level-striated and rounded, characteristic size distribution= bimodal or multimodal (altered during transport)
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What is till?
Sediment/debris directly deposited by a glacier
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Describe diamicton
Poorly sorted unconsolidated sediment
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Where is sediment deposited by land-based glaciers?
Directly in the immediate vicinity of glacier or far beyond via meltwater
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List the different types of till
Lodgement till, melt-out till, flow till
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What is lodgement till?
Occurs when force from the ice flow= insufficient to keep the particle in motion. Originates from the base of ice so sediment= bi or multi modal distribution. Either fine particles or clasts which are rounded and striated
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How does melt out till occur?
Material =released from melting debris-rich ice. Supraglacial- solar radiation melts ice- material= angular then reworked by flowing water. Subglacial melt-out till-ice=melted by geothermal heat. Modification is water dependent- rounded/striated
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When is flow till produced?
If debris on the ice becomes saturated, creep, slide or flow can occur which sorts and aligns particles. As sediment is largely supraglacial, material is angular.
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Why is till important?
Covers 70-80% mid-latitudinal land areas. Engineering resouce
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Where has the most focussed glacial erosion occurred historically in the UK?
In the central locations of the largest glaciers e.g. In the UK- N. Scotland
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Card 2

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Define: erosion

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The process by which materials are removed from the Earth's surface

Card 3

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Why is it difficult to study glacial erosion?

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Card 4

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List the main erosive processes that occur in the glacier environment

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Card 5

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How does bedrock crushing occur?

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