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Bryan Gartland and Daniel Smith  last modified on 5/5/99

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Glaciers across the globe exist in many different shapes and sizes.  The implications based on a glaciers size go far beyond just their appearance.  Besides the spatial differences, glaciers also vary in temperature.  An understanding of the different shapes glaciers take is essential for any study in glaciology.  Similarly, the temperature of the ice is one of the major factors contributing to glacial erosion.
 

Crevasses are fractures in glacial ice that can be up to several meters wide and tens of meters deep.  They are formed by extensional and shear stresses built up in moving ice.  Drag on either slower moving ice or bedrock produces velocity differences within a glacier and leads to crevasse formation.  Where ice is flowing over uneven bedrock the ice is forced to bend and this causes fracturing.  Crevasses are very significant to glaciers because they provide a pathway for surficial water to penetrate to the base of a glacier.  The water that reaches the base of a glacier can lubricate the bedrock and reduce drag, thus increasing glacier velocity.

Transverse and Splaying crevasses on Nisqually Glacier Mount Rainier, Washington (Bryan Gartland)

Glaciers are made of frozen water, or ice.  Glacial ice obviously must remain at or below its freezing point (0° C at the surface) to remain solid, but the exact temperature of the ice controls the behavior of the glacier in many ways.  Subglacial erosion, transport, and deposition are all dependent on ice temperature (Benn and Evans 1998).  A glacier with a higher temperature will have water at its base, greatly lubricating the bed on which it slides; while a cooler glacier will often be frozen to its bed, making it much less erosive.  While the temperature within a glacier fluctuates throughout the ice, cold-based glaciers are usually located at high latitudes and warm-based glaciers are usually found at lower latitudes.

There are three major factors controlling the temperature of glacial ice: solar radiation, geothermal heat flux, and internal friction (Ritter 1995).  Solar radiation warming the glacier surface is by far the most influential factor.  On a global scale, atmospheric temperature (created by solar radiation) is a major control of the distribution of glaciers across time and space; locally, solar radiation fluctuation can alter the ice temp on an annual, seasonal, and daily basis.  Glacial temperatures vary the most at the surface.  Surficial heat flux results mostly on the conduction of heat from the atmosphere, the temperature of firn lying on the ice, and the transfer of latent heat by the freezing of meltwater (Sugden and John 1976). 
Geothermal heat flux is also a common mechanism that raises ice temperature.  Basal ice temperatures are affected by geothermal heat only in select areas (e.g. the Cascade Volcanos and Iceland).  On a smaller scale, geothermal heat may only warm a glacier at specific points or hotspots.  The presence of geothermal heat will often melt basal ice and increase sliding on the bed.  In cases of rapid geothermal heat flux, glaciers have been known to surge at unusually fast rates.
The third major controlling factor is the internal friction of ice.  In active glaciers, there can be enough frictional force created by the flowing ice to create heat, raising the ice to relatively warmer temperatures.
A simple temperature profile of a stationary glacier with consistent thickness is shown in this equation from Drewry (1986):

An important influence on glacial temperature is the pressure melting point, or PMP.  The PMP refers to the decreasing of ice's melting temperature with depth beneath the surface.  Pressure lowers the melting point at a rate of 0.072° C per million Pascals (Mpa), or ice 2000 m beneath the surface of a glacier would melt at -1.27° C instead of 0° C (Benn and Evans 1998).  Due to the PMP and the insulating capabilities of ice and snow, temperatures generally increase at an exponential rate with depth in a glacier.
 

Figure 1.  Selected temperature profiles from the Greenland and Antarctic Ice Sheets (modified from Paterson 1994).

Glacial ice falls into two temperature categories, based on its closeness to the PMP.  Warm ice is at or near the PMP, while cold ice is below it.  The other major difference between warm ice and cold ice is that warm ice contains water.  Warm ice forms whenever there is sufficient heat to bring the ice to its melting point (Sugden and John 1976).  The addition of water to the base of the ice facilitates slippage and flow.  The soft, moving ice allows the glacier to more effectively pluck and erode underlying bedrock.
Since cold ice remains well below its melting point, it is free of water.  Cold ice forms in two main situations (Sugden and John 1976).  Either firn accumulates at temps so low that little or no surface melt occurs, or the top 5-20 m of the glacier are kept frozen year round by frigid temperatures.  Since cold ice is constantly frozen to its bed, no basal sliding can occur.  Glaciers in ancient polar environments left little evidence of plucking or abrasion on their beds.  This is because cold glaciers flow with an internal deformation and slippage of the ice.  In comparison to the sliding motion of warm glaciers, cold glaciers create relatively little erosion.
 

Figure 2.  Schematic diagram of the subdivisions of the accumulation zone of a glacier (NASA from Paterson, 1981).

Due to snow's excellent insulating characteristic, warm ice often exists below heavy snow coverage, even if the glacier is located in a polar region.  Snow and firn easily absorb meltwater (figure 2), which in turn transfers latent heat to the ice upon freezing.  Cold ice, on the other hand, is common on glaciers, or parts of glaciers, that have exposed ice.  The hard, smooth "blue ice" is an impermeable shield, causing any water to run off without penetrating beneath the surface.  In polar and subpolar glaciers, which make up most of the world's current ice mass, it is common to find warm ice beneath snow or firn in the accumulation zone and cold, bare ice at the margins, where it is exposed to chilly atmospheric temperatures (Sugden and John 1976).

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