Page 575 - Geosystems An Introduction to Physical Geography 4th Canadian Edition

Chapter 17 glacial and Periglacial Landscapes 539
Grimsvötn Volcano
Ash
Vatnajökull
Iceland
Atlantic Ocean
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(a)
◀Figure 17.5 Ice caps and ice fields. (a) The Vatnajökull ice Cap in southeastern iceland is the largest of four ice caps on the island (jökull means “ice cap” in Danish). note the ash on the ice cap from the 2004 grímsvötn eruption. (b) The Patagonian ice fields
of argentina and Chile. [(a) and (b) naSa/gSFC.]
the elevation above which the winter snow and ice remained intact throughout the summer melting season but below which melting occurs. At the lower end of the glacier, far below the firn line, the glacier undergoes wasting (reduction) through several pro- cesses: melting on the surface, internally, and at the base; ice removal by deflation from wind; the calv-
ing of ice blocks; and sublima- tion (recall from Chapter 7 that this is the phase change of solid ice directly into water vapour). Collectively, these processes cause losses to the glacier’s mass, known as ablation.
These gains (accumulation) and losses (ablation) of glacial ice determine the glacier’s mass balance, the property that de- cides whether the glacier will advance (grow larger) or retreat (grow smaller). During cold pe- riods with adequate precipita- tion, a glacier has a positive net mass balance, and advances. In warmer times, a glacier has a negative net mass balance, and retreats. Internally, gravity con- tinues to move a glacier forward even though its lower termi- nus might be in retreat owing to ablation. Within the glacier
is a zone where accumulation balances ablation; this is known as the equilibrium line, and it generally coin- cides with the firn line (Figure GIA 17.2).
Illustrating the global trend, the net mass balance of the South Cascade Glacier in Washington State dem- onstrated significant losses between 1955 and 2010. As a result of this negative mass balance, in some years the terminus retreated tens of metres, and it has retreated every year in the record except 1972. Figure GIA 17.3 presents a photo comparison of the South Cascade Glacier between 1979 and 2010. See also Figure 11.19 for a photographic record of change at Athabasca Glacier, Alberta, between 1917 and 2005.
A comparison of the trend of this glacier’s mass bal- ance with that of others in the world shows that tem- perature changes apparently are causing widespread reductions in middle- and lower-elevation glacial ice. The present wastage (ice loss) from alpine glaciers world- wide is thought to contribute over 25% to the measured
North Patagonia Ice Field
CHILE
South Patagonia Ice Field
Lago Buenos Aires
ARGENTINA
Lago Cardiel Lago San Martin
Lago Viedma Lago Argentino
ATLANTIC OCEAN
100 KILOMETRES
0 50
(b)
the systems described in this text, glacial processes are linked to the concept of equilibrium. A glacier at equi- librium maintains its size because the incoming snow is approximately equal to the melt rate. In a state of disequi- librium, the glacier either expands (causing its terminus to move downslope) or retreats (causing its terminus to move upslope).
Glacial Mass Balance
A glacier is an open system, with inputs of snow and out- puts of ice, meltwater, and water vapour as illustrated in this chapter’s Geosystems in Action, Figure GIA 17. Gla- ciers acquire snow in their accumulation zone, a snow- field at the highest elevation of an ice sheet or ice cap or at the head of a valley glacier, usually in a cirque (Fig- ure GIA 17.1). Snow avalanches from surrounding steep mountain slopes can add to the snowfield depth. The accumulation zone ends at the firn line, which marks