Page 97 - Fundamentals of Geomorphology

80 INTRODUCING LANDFORMS AND LANDSCAPES

It is important to distinguish between active ice rates tend to be swiftest in warm ice. Warm ice is
and stagnant ice. Active ice moves downslope and at the pressure melting point and contrasts with cold
is replenished by snow accumulation in its source ice, which is below the pressure melting point. For a
region. Stagnant ice is unmoving, no longer replen- given stress, ice at 0 C deforms a hundred times faster
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ished from its former source region, and decays where than ice at −20 C. These thermal differences have
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it stands. led to a distinction between warm and cold glaciers,
even though cold and warm ice may occur in the
Ice flow same glacier (p. 79). Details of glacier flow are given
in Box 3.5.
Ice moves by three processes: flow or creep, fracture Ice fractures or breaks when it cannot accommodate
or break, and sliding or slipping. Ice flows or creeps the applied stresses. Crevasses are tensional fractures that
because individual planes of hydrogen atoms slide on occur on the surface. They are normally around 30 m
their basal surfaces. In addition, crystals move rela- deep in warm ice, but may be much deeper in cold ice.
tive to one another owing to recrystallization, crystal Shear fractures, which result from ice moving along slip
growth, and the migration of crystal boundaries. Flow planes, are common in thin ice near the glacier snout.
rates are speeded by thicker ice, higher water con- Fractures tend not to occur under very thick ice where
tents, and higher temperatures. For this reason, flow creep is operative.

Box 3.5

GLACIER FLOW

Glaciers flow because gravity produces compressive on ice thickness and ice-surface slope. The shear
stresses within the ice. The compressive stress depends stress at the base of glaciers lies between 50 and
2
on the weight of the overlying ice and has two 150 kN/m .
components: the hydrostatic pressure and the shear Under stress, ice crystals deform by basal glide,
stress. Hydrostatic pressure depends on the weight which process occurs in layers running parallel to the
of the overlying ice and is spread equally in all crystals’ basal planes. In glaciers, higher stresses are
directions. Shear stress depends upon the weight required to produce basal glide because the ice crystals
of the ice and the slope of the ice surface. At any are not usually orientated for basal glide in the direction
point at the base of the ice, the shear stress, τ 0 , of the applied stress. Ice responds to applied stress as
is defined as a pseudoplastic body (see Figure 3.4). Deformation of
ice crystals begins as soon as a shear stress is applied, but
τ 0 = ρ i gh sin β the response is at first elastic and the ice returns to its
original form if the stress is removed. With increasing
Where ρ i is ice density, g is the acceleration of stress, however, the ice deforms plastically and attains
gravity, h is ice thickness, and β is the ice-surface a nearly steady value beyond the elastic limit or yield
slope. The product of ice density and the gravita- strength. In this condition, the ice continues to deform
3
tional acceleration is roughly constant at 9.0 kN/m , without an increase in stress and is able to creep or flow
so that the shear stress at the ice base depends under its own weight. Glen’s power flow law gives the