Page 117 - Sedimentology and Stratigraphy

104 Glacial Environments

7.2 GLACIAL ICE
Ice is a solid, but under pressure it will behave in a
ductile manner and flow by moving away from the
point of higher pressure. Pressure is provided by the
weight of ice above any particular point and the ice
will flow slowly as an extremely viscous fluid (4.2.1).
Glacier ice moves at rates which vary from as little as
a few metres per year to hundreds of metres a year.
Different parts of a body of ice move at different rates
because of different pressure gradients, resulting in
movement by internal deformation within the ice
mass. Typically the flow rate is greatest at the surface
of the ice decreasing downwards towards the base of
Fig. 7.3 Floating ice, including icebergs, is formed by
calving of ice from a glacier. the glacier, and valley-confined glaciers have greatest
flow in the middle of the valley, decreasing towards
the margins.
shelves, areas of floating ice extending out into the
shallow marine realm. At the freezing point of pure
water (08C) ice has a density of 0.92 g cm 3 and 7.2.1 Thermal regimes of glaciers
therefore floats on both fresh water (density 1.0 g
cm 3 ) and seawater (density 1.025 g cm 3 ). At the In cold, polar regions glaciers and ice caps lie on
front of these ice shelves the ice breaks up to form ground that is permanently frozen (Fig. 7.4). The ice
floating masses, icebergs (Fig. 7.3), which drift in the is therefore frozen to the ground and these cold
ocean currents and wind for hundreds or thousands glaciers move entirely by internal deformation,
of kilometres before completely melting. with the upper layers of the ice body shearing over

Fig. 7.4 The thermal regimes of glaciers
are determined by the climatic setting:

glaciers frozen to bedrock tend to occur in
polar regions, while temperate glaciers
occur in mountains in lower latitudes.