Proglacial Lakes - Freezing of Lakes and the Evolution of their Ice Cover - page 178

6.1 Ice Sheets and Glaciers

Ice sheets and glaciers form during long periods from annual snow accumulation
exceeding ablation (see Paterson 1999). Ice sheets are large (area more than 50,000 km 2 )
continental ice masses, and in the present Earth such are only the Antarctic and Greenland
ice massifs. Smaller long-term accumulations of ice on land are counted as glaciers. The
largest of them is the Southern Patagonia ice field in Argentina and Chile, covering an

area of 13,000 km 2 (Rignot et al. 2003), which is just 0.75 % of the area of Greenland ice
sheet. In Europe the largest glaciers are Austfonna, Svalbard (8,500 km 2 ), and Vat-
naj kull, Iceland (8,100 km 2 ) (Fig. 6.1 ). In high-polar regions, glaciers exist at all altitudes

but only in mountains in lower latitudes. At the Arctic Circle in northern Europe the
glaciation level is close to 1 km altitude, and in tropical regions this altitude is 4

ö

- 5 km.

The areal distribution of the surface mass balance of ice sheets and glaciers is

dM
dt ¼

þ ð : Þ

P E R Y 6 1

where M M(x, y; t) is the ice mass per unit area, P is the precipitation, E is the

evaporation, R is runoff, and Y is the net transport due to snowdrift. Each ice sheet and
glacier possesses an accumulation and an ablation zone, where the annual surface mass

¼

Fig. 6.1 Landscape photograph showing Vatnaj ö kull in summer