Glaciers - Enquiry Question 2 and 3 Flashcards
Positive glacier regime
When the glacier is increasing in mass (when accumulation exceeds ablation), e.g. during the winter months
Negative glacier regime
When the glacier is decreasing in mass (ablation exceeds accumulation), e.g. in the summer months
What percent (estimated) of the world’s ice masses currently reducing in size?
75%
Negative feedback cycles
- Acts to minimise the effect of new inputs in order to regain stability and equilibrium
- Increased inputs = increased outputs
Positive feedback cycles
- Amplifies the effects of an input which would cause a shift in the system
- Growing glacier - increased albedo effect = more ice = glacier advancing
- Retreating glacier - decreased albedo effect = more melting/less reflection = glacier retreating
Describe the difference between positive and negative feedback (4)
Negative feedback is when the cycle acts to minimise any changes and regains an equilibrium. For example, the implications of a glacier receiving an increase in snowfall provides the foundation for it to advance. Subsequently, this would cause the glacier to advance further down the valley with the consequence of more ablation and displacement of ice, balancing out the system.
Conversely, positive feedback is when the cycle amplifies the effects of a change resulting in an imbalance. For example when a glacier has a negative mass balance, there is persistent ablation. The implications of this is that the albedo effect will decline, fundamentally causing temperatures to increase thereby contributing to further ablation reinforcing the change.
Greenland Ice Sheet
- One of the world’s 2 remaining ice sheets
- Contains more than 2.5 million km³ of ice
- Has an area of 1.7 million km²
- 3km thick at the centre
Inputs and mass balance of the Greenland ice sheet
- +520 accumulation of snowfall in central areas
- -290 ablation of melting edges
- -200 ablation by calving icebergs
- -60 ablation by summation
- Mass Balance = -30 (the Greenland ice sheet is retreating)
Positive feedback loops occurring at the Greenland ice sheet
- Snow/ice melt (greenhouse effect) → melting reveals bare ground → Albedo effect reduced, acceleration of land warming up → less reflection of solar radiation → increased global warming
- Snow/ice melt (greenhouse effect) → melting reveals bare ground → methane released into the atmosphere → increased global warming
Positive feedback loops occurring at the Greenland ice sheet
- Snow/ice melt (greenhouse effect) → melting reveals bare ground → Albedo effect reduced, acceleration of land warming up → less reflection of solar radiation → increased global warming
- Snow/ice melt (greenhouse effect) → melting reveals bare ground → methane released into the atmosphere → increased global warming
Pressure Melting Point (PMP)
- Temperature at which the ice is on the verge of melting
- Glacier surface = 0°C
- Can be lower within a glacier due to increased pressure, so ice can melt below 0°C
How does altitude affect glacier movement?
Affects precipitation and temperature. Greater precipitation and lower temperatures increase the supply of snow and ice, and so its mass balance
How do gravity and gradient slope affect glacier movement?
Gravity causes ice to move; the steeper the gradient, the faster it flows
How does ice mass/thickness affect glacier movement?
The heavier/greater the mass, the greater the pressure in the ice, which causes faster movement
How does rock type affect glacier movement?
If rock is permeable, then meltwater may percolate through, slowing the movement of the glacier. If rocks are impermeable, there will be more meltwater, causing the glacier to move quicker
How does ice temperature affect glacier movement?
Colder ice moves slowly as it does not deform as easily, and it stays stuck to the bedrock
How does meltwater affect glacier movement?
The more meltwater there is, the faster the movement as basal slippage increases
Inter-granular movement
Individual ice crystals slip and slide over each other
Intra-granular movement
Ice crystals deform due to stress within the ice and eventually moves downhill under the influence of gravity
Basal slip
This occurs when the base of the glacier is at the pressure melting point, which means they meltwater is present and acts as a lubricant, enabling the glacier to slide more rapidly over the bedrock. Basal slip can be further subdivided into several processes: creep and regelation, extending and compressing flow, and surges
Creep and regelation
Basal slip is enhanced by obstacle on the valley floor. A large bedrock obstacle (>1m wide) causes an increase in pressure, which makes the ice plastically deform around the feature (creep). Smaller obstacles (<1m wide) will cause pressure-melting, increasing ice movement by basal slip. The ice refreezes on the downglacier (lee) side of the obstacle. The process of melting under pressure and refreezing is known as regelation
Extending and compressing flow
Over steep slopes, the rate of basal slip will increase and the ice will accelerate and thin. This is known as extending flow. Over shallower slopes, basal slop shows and the ice decelerates and thickens. This is known as compressing flow
Surges
In these short-lived events a glacier can advance substantially, moving up to 100 times faster than normal. They have various causes (e.g. earthquakes) but the most common is enhanced basal sliding triggered by the build-up of meltwater at the ice-rock interface
Internal deformation
This occurs when the weight of glacier ice and gravity causes the ice crystals to deform, so that the glacier moves downslope very slowly