Geology Reference
In-Depth Information
relationship between shear strain and applied stress
in ice:
by the impurity content of the ice. Its effect is that
cold ice flows more slowly than warm ice, because a
20
◦
C change in temperature generates a hundredfold
increase in strain rate for a given shear stress. With an
exponent
n
n
ε
=
A
i
τ
3, a small increase in ice thickness will
have a large effect on the strain rate as it will cube the
shear stress. With no basal sliding, it may be shown
that Glen's flow law dictates that the surface veloc-
ity of a glacier varies with the fourth power of ice
thickness and with the third power of the ice-surface
gradient.
=
ε
where
(eta dot) is the strain rate,
A
i
is an ice hard-
ness 'constant',
τ
(tau) is the shear stress, and
n
is
a constant that depends upon the confining pressure
and the amount of rock debris in the ice - it ranges
from about 1.3 to 4.5 and is often around 3.
A
i
is
controlled by temperature, by crystal orientation, and
Ice may
slip
or
slide
over the glacier bed. Sliding
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
an obstacle and refreeze (a process called
regelation
)as
pressures fall in the lee of the obstacle (Figure 3.14).
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
sediments lying on plains may yield to the force exerted
by the overlying ice.
It would be wrong to suppose that the beds of all
glaciers are passive and rigid layers over which ice moves.
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
stress from the ice and so contribute to glacier motion.
Thus the ice itself creeps and may slide over the till,
ploughing the upper layers of till as it does so. The moving
ice causes shear stress within the body of till, which itself
may move along small fault lines near its base.
()
a
()
b
Regelation ice
Ice surface
Glacier ice
Melting
Refreezing
Melting
Latent
heat
Latent
heat
Bedrock
Figure 3.14
Basal sliding in ice. (a) High stresses upstream
of obstacles in the glacier bed cause the ice to deform and
flow around them. (b) Obstacles are also bypassed by
pressure melting on the upstream side of obstacles and
meltwater refreezing (relegation) on the downstream side.
Sources:
(a) Adapted from Weertman (1957); (b) Adapted
from Kamb (1964)
