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Ice Movement

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;

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.

Mass balance

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;

Get the lesson activities here

Think about it!

Revise your knowledge of key terms here at Glacial movement terminology

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!). 

The melting point of ice is 0C at the surface of the ice, but this can vary within the ice profile because of the pressure that the mass of ice exerts.  This means that at the base of a glacier ice can melt at below 0C, allowing the ice to move with the help of melt water even if the air temperature is below freezing point.  This is particularly relevant to TEMPERATE glaciers, but has no impact on POLAR glaciers.  
Basal Slippage
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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.

Glacial systems diagram

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

 


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