Types of flow: internal deformation, rotational, compressional,
extensional and basal sliding; warm and cold based glaciers.
It is hard to understand that Ice moves given that it is a solid,
but it can and does. Ice can move at
extraordinary speeds, and glaciers in surge conditions are known to move at up
to
300m a day.
The Franz Joseph glacier in New Zealand has been known to surge in the
past. Ice can move in many ways and
this is determined by the glacial mass balance and the temperature and
precipitation regime of the area the glacier is found within.
Where accumulation and ablation are equal the glacier is said to be in a
steady state. This rarely happens,
and 2 other states exist;
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Accumulation is greater than ablation leading to growth in ice
mass and potential glacial advance down the valley
Ablation is greater than accumulation leading to a loss of ice mass
and the potential retreat of the glacier up valley.
Over a year
accumulation tends to be within the colder months and ablation within the warmer
months. |
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There are 2 basic types of glaciers and these types massively
determine the type of movement that takes place within the ice.
Cold based glaciers (POLAR) occur in the high Latitudes where the
temperature of the snow fall is far below zero degrees and the ice of the
glacier remains at below zero throughout the year. These glaciers therefore stay
frozen to the bedrock all year and there fore there is little ice movement and
therefore little erosion. Greenland and the Antarctic have cold based glaciers.
Warm based glaciers or TEMPERATE glaciers have water present
throughout the ice mass and this water acts as a lubricant.
This may be for a period of the year or all year, and allows for much
greater rates of movement and thus more erosion.
These glaciers are often found in mountain glaciers at lower latitudes
but higher altitudes than polar glaciers.
Pressure melting point;
Think about it!
For the 7 factors Listed here at the base of the page discuss how the factor could influence the rate of movement by completing the Diamond rank (ignore the last 2 squares!).
Cold based glaciers move mainly by
INTERNAL DEFORMATION.
These glaciers are frozen to the bed and therefore only move 1-2cm a day.
The ice crystals within the glacier orientate themselves in the direction
of ice movement. This allows ice
crystals to slide past one another.
Where the ice movement is fast enough crevasses may develop (this process could
also occur in warm based glaciers).
Temperate glaciers move mainly through BASAL SLIPPAGE. If the glacier moves, this can raise the temperature of the base ice through pressure and friction. The basal ice can then melt, and this water helps to allow the ice to slip more easily over its bed. This could move at 2-3m per day and pick up material with which it can use to erode its bed. This is related to Regelation, which is the phenomenon of melting under pressure and freezing again when the pressure is reduced.
Gif animation on how regelation works
Where an obstacle is encountered on the bed, pressure will
increase. As the stress on the ice
builds up as it tries to overcome the obstacle, the ice can behave like plastic
and flows round or over the obstacle.
The lower the temperature the greater the pressure that is needed.
In addition, the bed over which the ice flows is not of even
gradient. J.F.Nye suggested that
where ice flows over steeply sloping gradient the ice movement would speed up,
resulting in a thinner ice sheet with crevasses – an area of EXTENDING flow.
In contrast, where the ice flows over a gentler gradient the ice
decelerates and backs up, creating an area of COMPRESSING flow.
Rates of movement are determined by many things;
The amount of precipitation
The amount of ablation
The steepness of the ice
The thickness of the ice
The permeability of the surface upon which the ice sits.
Where in the glacial long profile you are
Proximity to the equilibrium or firn line
Click here for full screen version