Geography Reference
In-Depth Information
or creep (sometimes called plastic flow), sliding (sometimes called
basal slip), and bed deformation. Stress applied under the action of
gravity causes the ice to deform and to creep along. While the
creep rate is much lower for cold ice than warm ice, creep is a slow
process. Once the pressure melting point is reached at the glacier
bed, sliding can occur as water reduces the friction. If the bed of a
glacier is cold, the sliding is restricted. Glacier bases often contain
rock obstacles. Here
regelation
can occur. If ice flows around
obstacles at the bed there will be excess pressure upstream of the
obstacle and lowered pressure on the downstream side. Increased
pressure lowers the ice melting point upstream of the obstacle. The
melted water then flows around the obstacle to the low-pressure
downstream side where it refreezes because the melting point is
higher. This mechanism therefore allows the ice to slide past the
obstacle. Regelation is limited by heat conduction, which is better
for small obstacles. The regelation process is important as it often
causes obstacles to get frozen into the base of the ice and then
moved with the ice mass. Where an ice sheet or glacier sits on soft
sediments then movement of these sediments can also assist ice
movement as the sediments themselves deform.
The rate of movement of glaciers is very variable. Temperate
glaciers often flow at tens of metres per year whereas cold glaciers
flow at rates of two metres per year or less. Some fast flowing ice
streams can flow at several hundred metres per year by fast sliding
or on sediment that is deforming. Around 1 per cent of glaciers
experience sudden phases of surging and then quiet slow moving
periods. It is uncertain why this is the case.
Glacial landforms
Glacial erosion removes enormous volumes of rock and produces
amazing landforms. The mass of ice can crush rock where there are
weaknesses, producing angular sediments. Crushed bedrock can be
taken up into the glacier. The process by which a glacier removes
large chunks of rock from its bed is known as plucking. Plucking
can occur by ice regelation around the rock or by incorporation
into the ice along faults. Glacial abrasion occurs when rocks and
particles at the base of the ice slide over the bedrock thereby
scratching and wearing it down. The sediment resulting from
