Page 269 - Fundamentals of Geomorphology

252 PROCESS AND FORM

Plate 10.2 Cirque glacier, Skoltbreen, Okstindan,
northern Norway. Austre Okstindbreen, a crevassed
glacier, is seen in the foreground.
(Photograph by Mike Hambrey)

(Plate 10.2). Valley glaciers sit in rock valleys and are
overlooked by rock cliffs (Plate 10.3; see also Plate 10.4).
They commonly begin as a cirque glacier or an ice sheet.
Tributary valley glaciers may join large valley glaciers to
create a valley-glacier network. Piedmont glaciers form
where valley glaciers leave mountains and spread on to a
flat land as large lobes of spreading ice, an example being
the Malaspina Glacier, Alaska. Tidewater glaciers are Plate 10.3 Valley glacier – the Mer de Glace – in the
valley glaciers that flow into the sea, where they pro- French Alps.
duce many small icebergs that may pose a danger to (Photograph by Mike Hambrey)
shipping.

contributions may come from rainfall freezing on the ice
Glacier mass balance
surface, the condensation and freezing of saturated air,
A glacier mass balance is an account of the inputs and the refreezing of meltwater and slush, and avalanching
outputs of water occurring in a glacier over a speci- from valley sides above the glacier. In temperate regions,
fied time, often a year or more. A glacier balance year ablation results mainly from melting, but it is also accom-
is the time between two consecutive summer surfaces, plished by evaporation, sublimation, wind and stream
where a summer surface is the date when the glacier mass erosion, and calving into lakes and the sea. In Antarctica,
is lowest. Mass balance terms vary with time and may calving is nearly the sole mechanism of ice loss.
be defined seasonally. The winter season begins when The changes in the form of a glacier during an equilib-
the rate of ice gain (accumulation) exceeds the rate of rium balance year are shown in Figure 10.3. The upper
ice loss (ablation), and the summer season begins when part of the glacier is a snow-covered accumulation zone
the ablation rate exceeds the accumulation rate. By these and the lower part is an ablation zone. The firn line is
definitions, the glacier balance year begins and ends in the dividing line between the accumulation and ablation
late summer or autumn for most temperate and subpo- zones. For a glacier that is in equilibrium, the net gains of
lar regions. Most accumulation is caused by snowfall, but water in the accumulation zone will be matched by the