Environmental Engineering Reference
In-Depth Information
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
autocatalysis
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 instrumental in initiating the Quaternary
Ice Age. Although 'Antarctica' circled the South Pole, the southern ocean and its
isolating circumpolar current could not form until Australia and then South America
broke free 50 Ma and 20 Ma ago respectively. Antarctic glaciation commenced
approximately 40 Ma ago during the Eocene 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 commenced after 2·4 Ma ago with short, 41 ka cold- temperate
stage cycles operating up to the first 1 Ma before settling into a 100 ka rhythm thereafter.
ICE FLOW AND GLACIER GEOMORPHIC PROCESSES
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:
where the rate of deformation or strain rate
E
is determined by the constant
A
, related to
temperature, shear stress, τ, and the exponent
n
, which has a mean value of 3. The
basal
shear stress
, τ, is given as:
where ρ 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 15.5). The increase in accumulation zone
