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Glacial Systems
and Mass Balance The Glacial System Glaciers gain mass through the transformation of snow into ice and then flow downhill (in response to gravity) and eventually lose mass due to melting. (Photo) Hence glaciers can be viewed as systems with inputs, stores, transfers and outputs. (Fig) Glaciers accumulate mass from snow falling onto its surface, snow avalanching from the valley sides and by the accretion of rime ice by the direct freezing of atmospheric moisture onto the glacier. (Fig) Glaciers lose or ablate mass by melting as a result of warm air temperatures or applied pressure, evaporation, wind erosion or by calving into icebergs along a floating ice front. (Photo) In extremely cold and arid areas, such as the interior of Antarctica ice, mass can also be lost by sublimation, i.e. the process thereby a solid (ice) changes directly into a gas (water vapour). Sequence of photographs showing the collapse of ice (calving) at the front of the San Rasael Glacier, Chile
Glacier Mass Balance Glaciers gain mass in the accumulation zone, i.e. the upper part of the glacier where input (winter snowfall etc) exceeds output (summer melting etc). In contrast, mass is lost in the lower ablation zone where outputs exceed inputs (Photo). At the transition between the two zones accumulation equals ablation. (Fig) Hence, this boundary is called the equilibrium line, which in turn approximately coincides with the position of the snowline. The gains and loses
of ice experienced by a glacier constitute its mass balance or glacial
budget. (Fig)
Overtime if a glacier gains more mass than it loses (i.e.
accumulation > ablation) then the mass balance of a glacier is positive
and this causes the snout of the glacier to advance. Alternatively,
if
the mass balance is negative (accumulation < ablation) the reverse
will occur and the position of the snout will retreat. Finally, if the
glacier neither gains nor loses mass (accumulation = ablation) the balance is zero
and the snout will remain stationary.
Glaciers are extremely sensitive to climate change, which is currently being demonstrated by the gradual and sustained retreat of nearly all the world’s glaciers over the last 150 years in response to global warming
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