The transformation is accompanied by a steady decrease in porosity (a), increase in crystal size, and increase in density, represented by specific gravity (sg). The transformation typically progresses in the following steps.

1. snow (a=95%; sg.±.05)
2. firn (a=50%; sg±.4±.8)
3. pure ice (a=0%; sg>.9) - isolated air bubbles

 On the surface of warm glaciers snow may melt and rapidly freeze to the surface of the glacier forming superimposed ice, thereby bypassing the firn stage.

Read: Ice formation from Secrets of the Ice written by Lynn Rosentrater and designed by Planet Interactive for the Museum of Science, Boston

The rate of ice formation is determined by:

• Accumulation rate: The faster the rate of accumulation the faster the transformation
  • Annual accumulation of snow at GISP2 Summit, Greenland
• Thermal regime: Transformation is most rapid at near freezing temperatures.

  Ice formation may take thousands of years (e.g. Antarctica) to a few hours (e.g. superimposed ice)

Zone of accumulation

• The region of net accumulation, where the accumulation of ice is greater than melting.
• Sources: snow, rain frozen onto the surface of the glacier, rime (freezing of super-cooled water droplets), avalanches, drifting snow transported by wind.
• In alpline regions the zone of accumulation the surface across the glacier has a concave upward profile.
• Snow fields Photos from Tom Lowell (15-GEOL-574),University of Cincinnati

Snowline and equilibrium line

The snowline is the line above which there is a net accumulation of snow. The snowline is easily visible and is often used as a rough estimate of the equilibrium line. 

The equilibrium line marks the cross-section where total accumulation equals total ablation. The amount of ice entering into the section equals the amount of ice flowing out. On temperate glaciers the equilibrium line may be lower than the snowline if superimposed ice dominates the lower portion of the zone of accumulation.

• The middle of a glacier Photos from Tom Lowell (15-GEOL-574),University of Cincinnati

Zone of ablation--region of net ice loss

The region of net ablation, where melting or removal of ice is greater than accumulation

Processes of ablation included melting, calving sublimation, calving, melting, wind erosion, etc.

• In alpline regions the zone of accumulation the surface across the glacier has a convex upward profile.

 

• Terminal margins of glaciers Photos from Tom Lowell (15-GEOL-574),University of Cincinnati
• Iceberg A-3 calved from Antarctica's Ronne Ice Shelf
  • Calving of the Antarctic Ice Sheet/Shelf

Zone of stagnant ice

Region of stagnant ice within the zone of ablation

Stagnation = the inability of ice to internally deform

Stagnation is primarily controlled by

• ice thickness
• volume of internal debris
  

The Response of glaciers

Response (relaxation) time: The time it takes for the system to respond to a particular change. The larger the glacier the longer the response time.

Once a glacier is formed it will begin to influence the surrounding climate

• Glaciers will increase albedo thereby inforcing climatic cooling.
•  Large ice sheets may result in a redistribution of precipitation. The equilibrium line of large ice sheets, ice shelves and maritime domes is ofter near the glacier's margin rather than the center.

Glaciers in equilibrium will rapidly respond to dampen the effect of short term changes

•  High snowfall results in higher rate of flow which transfers more ice to the zone of ablation (fairly high response time)
•  A high ablation rate results in thinning, reduction of ice flow and less ice transfered to the zone of ablation (low response time) 

Effect of changes in snowline elevation

• Ice sheet: large areal expanses may be placed in the zone of ablation if the snowline is raised a few meters.

• Snow Crystals, 1999, Kenneth G. Libbrecht

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Lindley Hanson (email) Last Modified 2/10/03