Environmental Engineering Reference
In-Depth Information
Plate 15.3
Iceflow styles. Foliation (internal deformation)
parallel to iceflow in the Rhone glacier (left) contrasts with
extending flow (due to basal sliding) and associated
crevassing in the 0·5 km wide Breithorn glacier icefall (right)
in Switzerland.
Photos: Ken Addison.
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 a
−1
, with cold-based glaciers in the low range of
10
1-2
m a
−1
and warm-based glaciers in the high range 10
2-3
m a
−1
. Outlet glacier
velocities are among the highest at 10
3
m a
−1
, and the fastest known stable glacier is
Jakobshavn Isbrae in west Greenland, moving at 7·5 km a
−1
or 20 m
per day
. Extreme
velocities encountered in unstable
surging glaciers
(see below) may exceed 50 m day
−1
or 10 m a
−1
but cannot be sustained for long.
Flow mechanism and velocity are inconstant, as
rotational sliding
in the simple glacier
illustrates (Figure 15.5). 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 by
