Cryospheric systems - Fundamentals of the Physical Environment - page 355

+ WEDGE
of Net annual
accumulation

+ Deflation

- WEDGE
of Net annual
ablation

Nivation
processes

ELA ELA

Dirt cone

Moraine

0

or

50m

Meltwater

Abrasion Quarrying

Cirque
threshold

Quarrying 100 0

m

Internal
flow line

Ice velocity

Accumulation Ablation

Figure 15.7 Model mass balance of a cirque glacier, showing net accumulation and ablation wedges, ice flow lines, relative
velocities and associated geomorphic processes.

Ice flow style in this or any other glacier is determined
by the snow-ice transformation process, basal ice
temperature (warm- or cold-based) and the nature and
slope of the glacier bed as well as its surface. Intergranular
or regelation creep, already identified as initiating ice flow,
is one component of internal deformation where parts of
the glacier move relative to others. At a larger scale, ice
behaves like a viscous fluid in its boundary layer - with
lower velocities close to the bed or valley sides - and like
a brittle solid where velocities exceed plastic deformation
rates, forming tell-tale crevasses and shear planes ( Figure
15.8 , Plate 15.4 ). Cold-based glaciers may be frozen to
their bed, while ice higher in the glacier shears past the
stationary basal layer. Warm-based glaciers, and zones of
pressure-melting in cold-based glaciers, also slide past
their bed and valley sides. Basal sliding is facilitated by a
thin water film between ice and substrate or by a
deformable bed (see box, p. 361). Surface water (from
rainfall or melt) may reach the bed in thin glaciers and
average geothermal heat flux is capable of melting some
6 mm yr -1 of basal ice. The principal source of water,
however, is pressure-melting. This occurs quite readily in
isothermal ice. It is also induced here, and in colder ice,
where basal stress increases on the upstream side of
bedrock obstacles. The resultant supercooled water
reduces bed friction, encouraging the glacier to slide
past or around the obstacle, and regelates as stress falls
downstream.

Ice flow patterns and velocities

All glaciers flow through a combination of internal
deformation and basal sliding. The proportion of both
varies according to thermodynamic character and
according to mass balance trends, season and location
within the overall ice stream. Cold-based glaciers move
primarily by internal deformation, whereas basal sliding
is a major component in warm-based glaciers, reflected
in their respective velocities. 'Average' velocity lies in the
broad range 3-300 m yr -1 , with cold-based glaciers in
the low range of 10 1-2 m yr -1 and warm-based glaciers
in the high range 10 2-3 m yr -1 . Outlet glacier velocities are
among the highest at 10 3 m yr -1 , and the fastest known
stable glacier is Jakobshavn Isbrae in west Greenland,
moving at 10 km yr -1 or 30 m per day . Extreme velocities
encountered in unstable surging glaciers (see below) may
exceed 50 m day -1 or 10 m yr -1 but cannot be sustained

for long.
Flow mechanism and velocity are inconstant, as
rotational sliding in the simple glacier illustrates ( Figure
15.7 ). Ice accelerates through the accumulation zone from
zero velocity at the ice divide to a velocity maximum
beneath the ELA, before decelerating to zero again at the
snout. These patterns are accompanied by internal vectors
moving towards and then away from the bed respectively.
Ice therefore experiences divergent or extending flow
upstream of the ELA, whereas downstream ice converges