DEFINITION OF MOVEMENT OF GLACIERS

When glacial ice gets thick enough, it begins to flow and

deform under the influence of gravity. The thickness of the

ice must be great enough to overcome the internal forces that

resist movement, which depend on the temperature of the

glacier. The thickness at which a glacier starts flowing also

depends on how steep the slope it is on is. Thin glaciers can

move on steep slopes, whereas to move across flat surfaces,

glaciers must become very thick. The flow is by the process of

creep, or the deformation of individual mineral grains. This

creep leads to the preferential orientation of mineral (ice)

grains, forming foliations and lineations, much the same way

as in other metamorphic rocks.

Some glaciers develop a layer of meltwater at their base,

allowing basal sliding and surging to occur. Where glaciers

flow over ridges, cliffs, or steep slopes, their upper surface

fails by cracking, forming large deep crevasses (these can be

up to 200 feet deep). A thin blanket of snow can cover these

crevasses, making for very dangerous conditions.

Ice in the central parts of valley glaciers moves faster

than it does at the sides, because of frictional drag against the

valley walls on the side of the glacier. Similarly, a profile with

depth of the glacier would show that it moves the slowest

along its base, and faster internally and along its upper surface.

When a glacier surges, it may temporarily move as fast

along its base as it does in the center and top. This is because

during surges, the glacier is essentially riding on a cushion of

meltwater along the glacial base, and frictional resistance is

reduced during surge events. During meltwater-enhanced

surges, glaciers may advance by as much as several kilometers

in a year. Events like this may happen in response to climate

changes.

Calving refers to a process in which icebergs break off

from the fronts of tidewater glaciers or ice shelves. Typically,

the glacier will crack with a loud noise that sounds like an

explosion, and then a large chunk of ice will splash into the

water, detaching from the glacier. This process allows glaciers

to retreat rapidly.

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