Glacial Processes (Edexcel A Level Geography)
Revision Note
Written by: Jacque Cartwright
Reviewed by: Bridgette Barrett
Polar & Temperate Glaciers
All ice moves under the influence of gravity downslope from high mountains to low outwash plains
Weight, mass, and slope angle increase the shear stress point
The momentum of movement builds towards the zone of ablation, which maintains dynamic equilibrium with the slope angle
Movement towards the margins and snout of glacial ice continues, regardless of overall glacial retreat or advance
Temperature and pressure melting point differentiates polar and temperate glacial movement
Pressure melting point (PMP)
The temperature at which ice melts at a given pressure is the pressure melting point (PMP)
The melting point of water depends on air pressure above the ice
As air pressure increases, the temperature at which ice melts lowers
At 1 atmosphere pressure, the melting point of ice is 0°C
At 200 atmospheres, the melting point decreases to -1.85°C
Warm-based glaciers
Occur in temperate regions such as southern Iceland and western Norway
They are relatively small and range in width from hundreds of meters to a few kilometres
Melting occurs during the summer months
It is this meltwater that 'lubricates' the base and sides of the glacier, which assists movement (called basal sliding) and increases rates of erosion, transportation, and deposition
As such, all ice in these glaciers is at, or close to, the melting point of ice
Temperatures at the base are, therefore, at or just above the pressure melting point
Cold-based glaciers
Occur in polar regions such as central Greenland and Antarctica
They are large, vast sheets and caps of ice covering hundreds of km²
Temperatures remain below melting point, with low rates of precipitation, resulting in low levels of accumulation
Basal temperatures remain below the PMP, therefore, basal sliding does not happen
This results in little erosion, transportation, and deposition
Any movement is by internal deformation
The ice stays frozen to the bedrock and moves slowly at 1-2cm a day
Orientation of the ice crystals in the glacier, to the direction of movement, allows the crystals to slide over each other
Examiner Tips and Tricks
Ensure that you can write clear definitions of pressure melting point, warm and cold-based glaciers for the exam. Practice drawing diagrams to help support your answer. A well labelled or annotated diagram will gain you credit in the exam.
Movement of Glaciers
Glaciers move very slowly, under the force of gravity
Movement in the upper zone of the glacier forms crevasses, as the ice is relatively brittle and cracks
The lower zone has a steady pressure which along with meltwater and frictional heat, leads to easier/faster movement
Ice Movement | Explanation |
|---|---|
Basal sliding | Friction, pressure and heat from ice moving over bedrock, leads to melting. The meltwater then acts as a lubricant, assisting further glacial flow Enhanced basal creep - basal ice bends/deforms around bedrock outcrops, PMP is not reached and ice crystals deform around the outcrop Regelation creep/slip - basal ice undergoes increased resistance and pressure on the upslope of bedrock outcrop. Pressure melting occurs and this eases the flow over the outcrop. Pressure is reduced on the downslope of the outcrop, and the meltwater refreezes Movement is between 2-3m a day |
Internal deformation | Occurs in both cold and warm-based glaciers Intergranular flow - where individual ice crystals orientated themselves in the direction of glacial movement and slide past one another Laminar flow - movement of individual layers within the glacier Movement is between 1-2cm a day |
Rotational flow | Occurs in depressions/hollows where glacial ice forms - the ice rotates/pivots as it starts to move downslope |
Compressional flow | Ice mass thickens as slope gradient reduces and movement slows Erosional potential increases, which could lead to a steeper gradient and extensional flow |
Extensional flow | Ice mass thins and movement increases when slope gradient steepens Erosional potential decreases |
Diagram showing basal sliding and internal deformation as ice flows
Diagram showing compressional, rotational and extensional ice flow
Rate of Glacier Movement
No two glaciers move at the same rate, due to differences in processes and environment in which they exist
Most glaciers move anywhere between 3m and 300m per year
The highest rates of movement occurs during a glacier surge, where ice moves forward 10-100 times normal speed
There are a number of factors controlling the rate of glacial movement
Table of Factors Controlling Glacier Movement
Factor | Effect |
Lithology | Permeable bedrock allows meltwater to percolate through, slowing glacier movement. Rock such as clay, allows deformation and therefore, increased movement |
Altitude | Rates of snowfall and accumulation increases with altitude, therefore, alpine glaciers have higher rates of movement. Temperature increases with lower altitudes, increasing rates of meltwater and therefore, basal slip increases |
Slope Angle | Steeper slopes lead to increased glacial movement |
Size/Thickness | The greater the thickness, the greater the pressure within the ice, creating faster movement |
Mass Balance | Higher rates of accumulation increases thickness and gradient of the ice, which increases velocity. Similarly, higher rates of ablation increases basal meltwater which leads to increased basal slippage |
Ice Temperature | Cold based glaciers move slower than warm based glaciers, as the ice does not deform readily and tends to freeze to the bedrock |
Examiner Tips and Tricks
Refresh your geographical skills as you may need to compare rates of glacial movement in the exam. Make sure you understand and can use 'standard deviation' and 'measures of central tendency'.
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