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Glacial Processes and Landforms: Lecture Notes, Lecture notes of Mechanics

Glaciers affected landscapes directly, through the movement of ice & associated erosion ... erosion by cirque glaciers: move by rotational sliding as ice.

Typology: Lecture notes

2021/2022

Uploaded on 09/27/2022

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454 lecture 10
GLACIAL PROCESSES AND LANDFORMS
Glaciers affected landscapes directly, through the movement of
ice & associated erosion and deposition, and indirectly through
changes in sealevel (marine terraces, river gradients, climate)
climatic changes associated with changes in atmospheric &
oceanic circulation patterns
resultant changes in vegetation, weathering, & erosion
changes in river discharge and sediment load
Many high-latitude regions are dominated by glacially-produced
landforms
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GLACIAL PROCESSES AND LANDFORMS

Glaciers affected landscapes directly, through the movement of ice & associated erosion and deposition, and indirectly through

  • changes in sealevel (marine terraces, river gradients, climate)
  • climatic changes associated with changes in atmospheric & oceanic circulation patterns
  • resultant changes in vegetation, weathering, & erosion
  • changes in river discharge and sediment load Many high-latitude regions are dominated by glacially-produced landforms

Glacial origin Glacier: body of flowing ice formed on land by compaction & recrystallization of snow Accreting snow changes to glacier ice as snowflake points preferentially melt & spherical grains pack together, decreasing porosity & increasing density (0.05 g/cm 3 0.55 g/cm 3 ): becomes firn after a year, but is still permeable to percolating water Over the next 50 to several hundred years, firn recrystallizes to larger grains, eliminating pore space (to 0.8 g/cm 3 ), to become glacial ice

Glacial mechanics Creep: internal deformation of ice creep is facilitated by continuous deformation; ice begins to deform as soon as it is subjected to stress, & this allows the ice to flow under its own weight Cavell Glacier, Jasper National Park, Canada Sliding along base & sides is particularly important in temperate glaciers two components of slide are regelation slip – melting & refreezing of ice due to fluctuating pressure conditions enhanced creep

Velocity along glacier increases to the equilibrium line as discharge increases (Q = w d v), & decreases after this line as ablation becomes active decreases from the surface to the bedrock & from the center to the edges as a result of boundary resistance and internal mechanics As ice flows, it thickens through compressive flow when the bed is concave upward or addition of ice is low, or thins through extending flow

Fairweather Range, Alaska 454 lecture 10 Glacier Bay, Alaska Mendenhall Glacier, Alaska Glacier Bay, Alaska

Glacier Bay, Alaska

Surging glacier: movement may have characteristics similar to kinematic wave, but does not require external stimuli such as mass addition

  • sudden, brief, large-scale ice displacements periodically occur
  • move 10-100 times faster than normal
  • periodicity at 15-100 years
  • probably due to unique conditions creating cyclic instability within the glacier
  • fairly common phenomenon
  • surge chaotically breaks surface
  • key may lie in mechanics of basal sliding (eg. meltwater lowers basal shear stress)

Generally, valley glaciers are active, & temperate ice sheets are polar and passive firn line (above here snow remains on ground perennially) equilibrium line (annual volumes of accumulation & ablation are equal) ablation (-) accumulation (+) Glacial budget: mass balance of glacier; budgeting of gains & losses of mass on glacier during specific time interval accumulation ablation snow melting sublimation rain & other water evaporation calving avalanches wind erosion Usually consider one budget year (time between two successive stages) when ablation attains maximum yearly value – usually end of summer

  • mass balance: advance; steep or vertical front -- mass balance: recede; gently sloping, partially buried snout rates of accumulation & ablation matter, as well as overall balance temperate glaciers – high accumulation & ablation rates, move rapidly polar glaciers – low accumulation & ablation rates, passive both may have 0 net balance

marginal or chevron (^) transverse splaying (^) radial splaying

Taku Glacier, Alaska

Erosional Processes and Features Glacial erosion occurs via abrasion: scraping from debris carried in ice, depends on relative hardness of debris & bedrock; amount of debris; and velocity of flow; about 0.06-5 mm/yr quarrying/plucking: ice exerts shear force on rock loosened by fractures & meltwater freezes to rock as glacier thickness/ velocity/meltwater fluctuate Abrasion features striations: mm deep, continuous only for short distances grooves: 1-2 m deep, 50-100 m long, boulders carried along crescentic marks: chipping of underlying rock

striations, Athabaska Glacier, Canada glacial polish, Yosemite, CA