Environmental Engineering Reference
In-Depth Information
conditions lead to surging. This may be catastrophic for the glacier if the snout itself
advances rapidly. The glacier is drawn down faster than new ice accumulates, leading to
early downwasting. Surging can also be triggered by earthquakes and landsliding in
glaciated orogens and is normally restricted to warm-based glaciers.
GLACIER EROSION AND ENTRAINMENT
The low yield stress of ice is scarcely promising for its erosive power, since the shear
strength of intact bedrock is two to four orders of magnitude larger. Yet glaciers are
undoubtedly one of Earth's most powerful erosion agents. Glaciers commonly excavate
troughs 1-2 km deep through alpine orogens and beneath ice sheets, where the
mechanisms responsible are inaccessible. This has tested glaciologists' ingenuity, and a
variety of both more and less acceptable processes have been proposed.
ABRASION, CRUSHING AND ENTRAINMENT
Bedrock
striations
provide abundant evidence that debrischarged basal ice can abrade a
rigid bed provided that the abrading tools are harder than the substrate. Without the
supply of larger rock fragments, however, this process is limited by the progressive
comminution of debris and smoothing of bedrock. This is unlikely to inflict more than
surface ornament (Plate 15.5) and does not occur without first overcoming the technical
difficulty posed by the low yield stress of ice. Debris is not moved by traction at the ice-
debris-bedrock contact, since the ice readily deforms when shear stress is applied.
Instead, pressure-melting occurs until the particle is almost wholly absorbed by the ice,
equalizing bedrock-debris and debris-ice stresses. Gripped by regelation ice in the way
that glue grips sand on sandpaper or tarmac grips chippings on road surfaces, the tool is
now ready to abrade.
DEFORMATION OF THE GLACIER BED
applications
Glacier velocity is linked with erosive power but not in the simple relationship applicable
to flowing water or wind. The deformability of ice and its bed is worth exploring as the
prelude to glacier erosion and deposition, which could be seen as discrete events in a
continuum of processes. Glaciers were formerly thought to move over a rigid bed but we
now appreciate that the glacier bed is a composite interface between ice, rock, water,
debris (sediment) and even air. The bed can therefore also be fluid, not only in the
presence of atmospheric or supercooled meltwater but also as fluidized debris. Any one
of these materials may locally form the 'bed' over which all or part of the ice moves. Not
only is the boundary deformable but the
zone of deformation
can move from one locus to
another and is susceptible to changes in any of the material properties. Changes in ice
thickness or velocity alters basal shear stress. Pressure-melting inevitably pumps up pore
water and rock discontinuity water pressures. Changes in the character of granular debris,
pressure-melting conditions and pore water press ure alter the strength and deformability
of basally lodged or entrained sediments. General relationships between the zone of
deformation and glacial geomorphic processes are shown in Figure 1.
