Page 268 - Fundamentals of Geomorphology

GLACIAL AND GLACIOFLUVIAL LANDSCAPES 251

separates the western inlandsis from the eastern shorter extending into Wilkes Land and Queen Maud
2
inlandsis. It covers some 1,970,000 km , and the Ross Land respectively (Figure 10.2).
Sea, the Weddell Sea, and the Antarctic Peninsula An ice shelf is a floating ice cap or part of an ice
bound it. sheet attached to a terrestrial glacier that supplies it with
Ice at the base of an ice sheet is generally warmer ice. It is loosely constrained by the coastal configuration
than ice at the cold surface, and in places it may be and it deforms under its own weight. Ice is less dense
warm enough to melt. Meltwater so created lubricates than water and, because near the coast ice sheets gener-
the ice sheet, helping it to flow more speedily. The result ally rest on a bed below sea level, there comes a point
is fast-flowing currents – ice streams – in the ice sheet. where it begins to float. It floats in hydrostatic equilib-
Ice streams are characteristically hundreds of kilometres rium and either it stays attached to the ice sheet as an
long, tens of kilometres wide (with a maximum of around ice shelf, or it breaks away (calves) as an iceberg. Being
50 km), and up to 2,000 m thick; some flow at speeds afloat, ice shelves experience no friction under them, so
of over 1,000 m/yr (Colour Plate 11, inserted between they tend to flow even more rapidly than ice streams,
pages 208 and 209). They account for about 10 per cent up to 3 km/year. Ice shelves fringe much of Antarctica
of the ice volume in any ice sheet, but most of the ice leav- (Box 10.1). The Ross and Ronne–Filchner ice shelves
ing an ice sheet goes through them. Ice streams tend to each have areas greater than the British Isles. Antarctic
form within an ice sheet near its margin, usually in places ice shelves comprise about 11 per cent of the Antarc-
where water is present and ice flow converges strongly. tic Ice Sheet and discharge most of its ice. They average
The nature of the bed material – hard rock or soft and about 500 m thick, compared with an average of 2,000 m
deformable sediments – is important in controlling their for grounded Antarctic ice. All current ice shelves in
velocity. At ice stream edges, streams deformation causes Antarcticas are probably floating leftovers of collapsed
ice to recrystallize, so rendering it softer and concentrat- marine portions of the larger grounded Antarctic Ice
ing the deformation into narrow bands or shear margins. Sheet that existed at the height of the last glaciation
Crevasses, produced by rapid deformation, are com- around 18,000 years ago.
mon in shear margins. The fastest-moving ice streams
have the heaviest crevassing. Terrestrial and marine ice Ice fields and other types of glacier
streams exist. Terrestrial ice streams lie on a bed that
slopes uphill inland. Marine ice streams ground far- Several types of glacier are constrained by topography
ther below sea level on a bed that slopes downhill into including ice fields, niche glaciers, cirque glaciers, valley
marine subglacial basins. In Antarctica (Box 10.1), ice glaciers, and other small glaciers. Ice fields are roughly
streams are the most dynamic part of the ice sheet, level areas of ice in which underlying topography controls
and drain most of the ice. Ice streams may play two flow. Colour Plate 12 (inserted between pages 208 and
major roles in the global climate systems. First, they 209) shows the North Patagonian Ice Field and the glacial
determine the response of their parent ice sheet to cli- landforms associated with it. Mountain glaciers form in
mate change. Second, they determine global sea level by high mountainous regions, often flowing out of ice fields
regulating the amount of fresh water stored in the ice spanning several mountain peaks or a mountain range.
sheets. Hanging glaciers,or ice aprons, cling to steep moun-
Ice divides separate ice moving down opposite flanks tainsides.They are common in the Alps, where they often
of an ice sheet, so partitioning the ice sheet into several trigger avalanches, owing to their association with steep
ice drainage basins. Interior domes and saddles are high slopes. Niche glaciers are very small, occupying gul-
and low points along ice divides. The chief ice divide lies and hollows on north-facing slopes (in the northern
on Antarctica is Y-shaped, with a central dome – Dome hemisphere) and looking like large snowfields.They may
Argus – at the centre of the Y and branching ice dives develop into a cirque glacier under favourable conditions.
at each extremity, the longest passing near the South Cirque or corrie glaciers are small ice masses occu-
Pole and extending into West Antarctica and the two pying armchair-shaped bedrock hollows in mountains