Geography Reference
In-Depth Information
As the melting point of ice reduces with increasing pressure
there is an important difference to be made between 'warm' ice
and 'cold' ice. At a depth of 1 kilometre within an ice sheet the
melting point of ice occurs at the cooler temperature of -0.7°C,
rather than 0°C. Therefore, the thicker the ice mass the more
likely it is to produce water at depth at the
pressure-melting
point
. Warm ice is at the pressure-melting point and contains
liquid water, whereas cold ice occurs at temperatures below the
pressure-melting point and does not contain liquid water. The
pressure-melting point concept is important for understanding
what might be happening within the ice and at the base of the ice
mass, because water can help glaciers move over the bed. Warm
ice occurs throughout temperate glaciers except near the surface of
the glacier which becomes cold in winter. Cold ice occurs
throughout cold glaciers. If a glacier bed is cold flowing water will
not occur and so there will be less sliding and deformation of sedi-
ments than under a warm ice base. In some places the heat released
from the Earth produced by tectonic activity can melt ice at the
base of an ice mass. There are several lakes below the Antarctic ice
mass, the largest being 4 kilometres below the ice surface, and cov-
ering around 14,000 square kilometres. One of the theories behind
why these large lakes exist is that they are warmed by tectonic
activity occurring below them. However, little is known about
these lakes and they are difficult to study because of their location.
Water is produced at the snow surface of many glaciers in
summer. Unless it refreezes, meltwater may percolate downwards
through any snow and run across the surface of the glacier ice.
Water will flow downslope along the ice surface and will emerge
as a series of small streams many of which will have created smooth
channels on the ice surface. The water may flow to the outlet of
the glacier or some of it may flow within the glacier in tunnels.
Water that descends from the glacier surface may arrive at the bed
at discrete locations flowing in channels. Two types of channels
exist beneath glaciers: Nye (N) channels and Röthlisberger (R)
channels. N-channels are formed in the bedrock, and R-channels
are formed upwards into the ice.
To move ice from the accumulation zone downslope to the
ablation zone the glacier must physically flow downslope. There
are three mechanisms by which glaciers flow: internal deformation
