Geoscience Reference
In-Depth Information
Australia and then South America broke free 50 Ma and
20 Ma ago respectively. Antarctic glaciation commenced
approximately 40 Ma ago during the Palaeogene and the
continent has supported a polar ice sheet ever since.
Northern hemisphere glaciation, although doubtless
encouraged by Antarctic-driven cooling, had to wait until
the Panama isthmus isolated the Atlantic and Pacific
Oceans just 3 Ma ago and strengthened northern Pacific
and Atlantic circulation. The Quaternary Ice Age com-
menced after 2·6 Ma ago with short, 41 ka cold-temperate
stage cycles operating past the first 1 Ma before settling
into a 100 ka rhythm thereafter.
ICE FLOW AND GLACIER
Ice flow mechanisms
Glacier mass, thermal energy balances and general
thermodynamic character drive ice flow velocity and style.
This, in turn, determines geomorphic activity and
subsequent landsystems. Ice behaves as a plastic material
and is readily deformable under stress. This is shown by
Glen's flow law, defined by:
Plate 15.3
Mount Kenya (5,199 m OD) in January 1960,
Batian and Nelion, lie the Diamond Glacier above the Darwin
Glacier and (left) the Cesar Glacier above the Josef Glacier.
Photo: Malcolm Coe
E
=
A
n
where the rate of deformation or strain rate
E
is
determined by the constant
A
, related to temperature,
shear stress,
, and the exponent
n
, which has a mean value
of 3. The
basal shear stress
,
and two broad correlations are observed. Polar super-
continents, fragmentary oceans and their circulation
systems may induce icehouse conditions. Equatorial
supercontinents, well connected oceans and their
circulation systems may stimulate greenhouse conditions.
Tectonic uplift disturbs atmospheric circulation and
promotes glaciation in mid- to high latitudes. Ice sheet
growth reinforces icehouse conditions through
auto-
catalysis
as the high albedo of snow and ice reflects more
short-wave radiation and thickens boundary inversion
layers. Atmospheric subsidence consequently enhances
polar anticyclonic circulation, blocking advection
warming and 'growing' more ice. The associated fall in sea
level then advances ice shelf grounding lines and the ice
sheet grows further. The reversibility of these effects plays
a major role in glacial-interglacial oscillation but they
undoubtedly assist initial ice sheet formation.
Later stages in the break-up of Pangaea were instru-
mental in initiating the Quaternary Ice Age. Although
'Antarctica' circled the south pole, the southern ocean and
its isolating circumpolar current could not form until
, is given as:
=
gh
sin
a
where
is ice density,
g
is gravitational acceleration,
h
is
ice thickness and
a
is the surface slope of the glacier. Thus
basal shear stress increases with glacier thickness and
surface slope. The rate of deformation is therefore highly
sensitive to an increase in either, and to ice temperature.
In practice the maximum shear stress ice can exert at its
bed before it deforms is about 0·1 MN m
-2
. These
relationships can be appreciated by looking at mass
balance, deformation and ice flow in a
cirque glacier
- the
smallest glacier type, distinguished from snowpacks by
deformation and movement. Annual mass balance adds
an incremental wedge of snow above the ELA and, in
steady state, melts an identical wedge in the ablation
zone (
Figure 15.7
).
The increase in accumulation zone
thickness and overall surface slope exceeds the yield stress
and equilibrium is restored only by ice flow.
