Snow, Ice, Avalanches, and Glaciers - Mountain Geography: Physical and Human Dimensions - page 195

Glaciers contain a number of interesting features resulting from the transformation
of snow to ice and from downslope movement. Most of these are beyond our present
scope, but three of them—crevasses, ogives, and moraines—require mention. A cre-
vasse is a crack in the ice that may range up to 15 m (50 ft) in width, 35 m (115 ft) in
depth, and several tens to hundreds of meters in length. Most are smaller than this, es-
pecially in temperate mountain glaciers, where the average crevasse is only 1-2 m (3-7
ft) wide and 5-10 m (16-33 ft) deep (Fig. 4.36). Crevasses are among the first struc-
tural features to appear on a glacier and may develop anywhere from the head to the
terminus. Crevasse formation is primarily a response to tensional stress, so their distri-
bution, size, and arrangement provide useful information on the flow behavior of the ice
(Sharp 1988; Benn and Evans 2010). Crevasses occur most often where the middle and
the sides of the glacier move at different rates, or where the ice curves around a bend,
or where the slope steepens and the rate of movement increases (Fig. 4.33). Crevasses
are most often transverse to the direction of flow, but they can be oriented in any dir-
ection. Radial crevasses often occur where a glacier extends beyond its valley walls to
spread out as a piedmont lobe. They are also largely restricted to the surface, where
the ice is more brittle and fractures easily; the greater pressure at depth results in clos-
ure by plastic flow. A special type of crevasse develops at the upslope end of the glacier
where the ice pulls away from the rocky headwall. This is known as the bergschrund.
Rock debris from the headwall and valley sides falls into the bergschrund and other
crevasses and becomes incorporated into the glacier, often not to be seen again until
it is released by glacial melting at the terminus. The presence of crevasses, therefore,
increases the efficiency of rock transport. Crevasses also hasten ablation by increasing
the glacier's surface area, by the pooling of meltwater, and by disaggregating the ice
near the terminus. Crevasses pose great danger to travel across glaciers. This is partic-
ularly true after fresh snow has bridged the surface, hiding the underlying chasms from
view.