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