Page 98 - Fundamentals of Geomorphology

GEOMORPHIC MATERIALS AND PROCESSES 81

relationship between shear strain and applied stress by the impurity content of the ice. Its effect is that
in ice: cold ice flows more slowly than warm ice, because a
20 C change in temperature generates a hundredfold
◦
˙ ε = A i τ n increase in strain rate for a given shear stress. With an
exponent n = 3, a small increase in ice thickness will
where ˙ε (eta dot) is the strain rate, A i is an ice hard- have a large effect on the strain rate as it will cube the
ness ‘constant’, τ (tau) is the shear stress, and n is shear stress. With no basal sliding, it may be shown
a constant that depends upon the confining pressure that Glen’s flow law dictates that the surface veloc-
and the amount of rock debris in the ice – it ranges ity of a glacier varies with the fourth power of ice
from about 1.3 to 4.5 and is often around 3. A i is thickness and with the third power of the ice-surface
controlled by temperature, by crystal orientation, and gradient.

Ice may slip or slide over the glacier bed. Sliding
a
cannot take place in a cold-ice glacier, because the ()
glacier bottom is frozen to its bed. In a warm-ice glacier,
sliding is common and is aided by lubricating melt-
water, which if under pressure will also help to bear
the weight of the overlying ice. Enhanced basal creep,
whereby increased stress on the stoss-side of obstacles
raises the strain rate and allows ice to flow around the
obstacle, assists the slippage of ice over irregular beds in
warm-based and cold-based glaciers. Also, under warm-
based glaciers, water may melt as pressures rise on striking
b
an obstacle and refreeze (a process called regelation)as ()
surface
Regelation ice
ce
pressures fall in the lee of the obstacle (Figure 3.14). Regelation ice I Ice surface
Such pressure melting appears to work best for obstacles
smaller than about 1 m. In some situations, glaciers may
also move forward by deforming their beds: soft and wet Glacier ice
Glacier ice
Melting
sediments lying on plains may yield to the force exerted Melting
Refreezing
by the overlying ice. Refreezing Melting
Melting
Latent
Latent
It would be wrong to suppose that the beds of all heat
heat
Latent
glaciers are passive and rigid layers over which ice moves. Bedrock Latent
Bedrock
heat
heat
Where the bed consists of soft material (till), rather than
solid bedrock, the ice and bed form a coupled system in
which the bed materials deform in response to applied Figure 3.14 Basal sliding in ice. (a) High stresses upstream
of obstacles in the glacier bed cause the ice to deform and
stress from the ice and so contribute to glacier motion. flow around them. (b) Obstacles are also bypassed by
Thus the ice itself creeps and may slide over the till, pressure melting on the upstream side of obstacles and
ploughingtheupperlayersoftillasitdoesso.Themoving meltwater refreezing (relegation) on the downstream side.
ice causes shear stress within the body of till, which itself Sources: (a) Adapted from Weertman (1957); (b) Adapted
may move along small fault lines near its base. from Kamb (1964)