Glacial Processes (Edexcel A Level Geography)

• 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

Warm and cold based glacial profile - meltwater is the important factor in movement 

• 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

• 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