Outer Earth processes and systems - Physical Processes in Earth and Environmental Sciences - page 290

Boundary layer zone of
maximum shear and velocity
gradient located largely within
weak zone of basal till

Boundary layer zone of
maximum shear and velocity
gradient located within
shearing zone of basal ice

(a) (b)

Plastic
material
velocity
profile

“Two-step”
velocity
profile

Ice

Ice

t o

t o

Till
Bedrock

t s

Bedrock

τ

Appropriate for valley glaciers
with cold and/or dry
ice beds well below
their pressure melting
point

Appropriate for Antarctic ice
streams with warm and/or wet
ice beds close to pressure
melting point

Fig. 6.79 (a) Predominant till deformation with minor internal ice deformation; (b) Basal ice sliding and internal ice deformation.

made manifest at shallow levels by awesome crevasse
fractures (Fig. 6.80). The concept of effective stress is rel-
evant again here, for the shear strength,

s , of subglacial
sediment must be exceeded by that of the driving bed
shear stress,

o , for ice flow if deformation is to occur at all.
Ignoring cohesive strength, assuming that resistance is due
to solid friction ( ) and that strength is much reduced by

high porewater pressures, we may write o s , or g h

sin P is the excess of lithostatic pres-

sure above porewater pressure. The reduced strength
allows the driving force provided by the tractive force of
the glacier, actually quite small for most glaciers due to the
low slopes involved, and about 20 kPa for the Antarctic ice
cap, to cause deformation and steady forward motion.
Direct subglacial measurements of rates of till deformation
indicate values of viscosity for deforming till of between
3

P tan where

10 10 Pa s, with yield stresses of about
50-60 kPa. Despite knowing little about the in situ prop-
erties of deforming subglacial sediment beds ( till ) it seems
clear that both the glacier ice and the till must move along
and be deformed during transit.
The process of basal sliding must also involve enhanced
creep around drag-creating obstacles, pressure melting
around obstacles, and direct lubrication by abundant basal
meltwater. The latter comes from surface meltwaters let
into the sole by crevasses and ice tunnels, ice melted by
geothermal heat (e.g. spectacular Lake Vostok under the
Antarctic ice cap), and ice melted by pressure at the glacier
sole. The water under ice streams is often modelled as a
thin (few centimeters) film but is more realistically
thought to occur in a network of very shallow subice chan-
nels cut into the deforming till. It may be possible to char-
acterize a glacier bed by some roughness coefficient or

10 9

and 3

Fig. 6.80 A glacier crevasse: product of shallow level brittle tensile
deformation analogous to tension gashes in deformed rock
(Section 4.14).

at depth. A condition of no-slip must exist at the ice-bed
interface, with a general absence of englacial or subglacial
drainage. Forward motion of such ice is therefore by inter-
nal ice creep alone, the basal ice defining a plastic flow
boundary layer of differential velocity. Glacial debris is
transported within the ice, with substrate erosion due to
plucking and grinding effective only at the summits of
protuberances on the bed.
It seems that up to 90 percent of total glacier movement
may occur by basal sliding, the rest by internal deformation