Glacier

• a body of land ice that moves
• forms from the metamorphosis of snow: new snow (.01-.3 gm/cc) to firn, snow that has survived a melt season, (.55 gm/cc) to ice (.89-.92 gm/cc)
• rate of transformation from firn to ice depends on temperature, from about 50 years in humid temperate climates (e.g. coastal mountains) to 100s of years in cold dry climates

Distribution of glacier ice

• about 10% of the earth's land surface
• 96% currently is in two glaciers, the Greenland and Antarctic ice sheets
• during the Pleistocene glaciation, up to 30% of the earth's land surface was covered with glacier ice
• thus many of the mid- to high-latitudes landforms of the world, especially in the northern hemisphere, are glacial in origin

Classification of glaciers, according to

1. morphology
   
   
   1. niche
      
      
      • smallest glaciers
      • from in hollows, benches or from avalanche snow
      • often hanging on a cliff
      
      
   2. cirque
      
      
      • cirque have a morphology that favours the accumulation and persistence of snow and ice
      • the source of alpine valley glaciers
      
      
   3. alpine
      
      
      • valley glaciers that form when cirque glaciers advance into the valley and coalesce
      
      
   4. outlet
      
      
      • valley glaciers that radiate from an ice field
      
      
   5. transection
      
      
      • glaciers that transect drainage divides
      • ice fields, caps or sheets
      
      
   6. peidmont
      
      
      • "foot of the mountain"
      • valley glaciers which have advanced beyond the mountain front
      
      
   7. floating
      
      
      • terminus in deep water
      • e.g. margins of the Greenland and Antarctic ice sheet
      
      
   8. ice cap
      
      
      • forms a cap on a mountain range or upland, i.e. most of the peaks are under the ice
      
      
   9. ice sheet
      
      
      • continental glacier
   
   
2. temperature: melting point decreases by 0.7^o per km of ice
   
   
   1. warm- (wet-) based or temperate
      
      
      • ice is at the pressure melting point throughout the glacier
      • thus the mean annual temperature of the ice is about 0^o
      • geothermal heat and melt are concentrated at the glacier bed
      • abundant meltwater and high glacier velocities from sliding
      
      
   2. cold-based or polar
      
      
      • the ice is below the pressure melting point throughout the glacier
      • that is the glacier is frozen to its bed (permafrost) and thus can move only by internal creep
      
      
   3. intermediate or subpolar
      
      
      • pressure melting point occurs only at the bed, otherwise the glacier is cold
      
      
   
   
   • note: parts of one glacier can fall into different temperature classes; glaciers usually are cold based at the margins where the ice is thin and thus there is a higher thermal gradient (= higher heat loss) between the air and the bed of the glacier
   
   
3. activity
   
   
   1. active
      
      
      • continuous supply of ice from the zone of accumulation
      • thus active glaciers are moving even as they waste away and retreat
      • passive
      
      
   2. undernourished
      
      
      • low velocity transfer of ice from the zone of accumulation
      
      
   3. dead
      
      
      • detached from zone of accumulation
      • usually because ice has been buried in debris or thinned out, for example, as an ice sheet advances over a topographic high
      • dead ice wastes downward as opposed to retreating from the terminus

Glacier mass balance

• inputs - outputs = glacier mass balance
  
  
  • solid precipitation (mostly snowfall) + blown snow + snow avalanches - melt - evaporation/sublimation - calving [floating glaciers] = glacier mass balance
  • inputs > outputs = +ve mass balance, the glacier expands
  • outputs > inputs = -ve mass balance, the glacier retreats, i.e. the rate of ablation exceeds the rate of flow
  • inputs = outputs = balance, the glacier is at a standstill, but still moving
  
  
• equilibrium (firn) line
  
  
  • the boundary between the zone of accumulation (inputs) and the zone of wastage (outputs) where the glacier is balanced
  • moves up the glacier during the melt season until the highest annual position is reached in late summer or early fall
  
  
• the larger the glaciers the more delayed is the response of the terminus to a change in mass balance, up to 1000s of years for a continental ice sheet

Glacier flow

Glen's flow law

• strain rate is proportional to temperature cubed (T³)
• the behavior of glacier ice is mostly plastic; beyond a yield stress of about 1000 N/m², it deforms continuously, i.e. flows under its own weight with no increase in stress
• but its not perfectly plastic, because it creep under low stress, i.e. like a viscous fluid

basal shear stress

• shear stress = W sin(slope) and the density of ice is fairly constant, therefore bed shear stress is a function of the thickness and slope of the ice
• so glacier flow is driven by slope (mountains) or thickness (ice sheets)

mechanisms of glacier flow

creep

internal plastic deformation by slippage within ice crystals and recrystalization

basal sliding of temperate (wet-based) glaciers

includes regelation (melt and refreezing) around small obstacles and enhanced creep (local increase in stress and flow rate) around larger obstacles

variations in velocity

• high velocity in thickest ice, i.e. in the middle of the glacier and near the equilibrium line
• extending flow
  
  
  • acceleration with increasing slope or thickness
  • flow lines are directed towards the bed promoting erosion
  • causes crevasses and, on steep slopes, ice falls
  
  
• compressive flow
  
  
  • deceleration with decreasing slope or thickness (e.g. the terminus) >flow lines are directed upward, carrying debris to the ice surface

Glacial erosion and transport

• the low yield stress and plastic deformation of ice around rock obstacles lessens the effectiveness of glacial erosion, but
• the highest regional denudation rates, estimated from river sediment loads, are for glaciated mountain landscapes, although high relief, steep slopes, and periglacial processes (especially mass wasting) all contribute to high denudation rates
• besides erosion, glaciers contribute to denudation by exporting debris, over steepening slopes, and serving as heat sinks and reservoirs of meltwater

erosional processes

• abrasion
  • scouring by rock fragments embedded in the sliding (wet-based) ice
  • directions of glacier flow are reconstructed from striations
  • produces rock flour, fine sediment that becomes suspended in glacial lakes giving them a blue-green colour
  
  
• plucking (quarrying)
  • freezing of rock fragments to the glacier
  • produces chatter marks with removal of the fragments

sediment load

• subglacial
  • mostly debris plucked from the bed and embedded in the ice
  
  
• englacial
  • subglacial debris carried up from the bed along flow lines or supraglacial debris buried by snow and ice in the accumulation zone
  
  
• supraglacial
  • transported on the surface
  • mass wasted from adjacent cliffs
  • major source of lateral and medial moraines, including ice-cored lateral moraines