Topic 2 EQ 2 Flashcards
What is accumulation ?
The input of snow and ice to a glacier - by precipitation, avalanches and wind deposition.
What is ablation?
Output of snow and ice from a glacier - by melting, calving, sublimation and evaporation
When do accumulation rates increase?
When there are high levels of precipitation, low average temperatures , low levels of insolation and low wind speeds.
Where is accumulation greatest?
At higher altitudes on slopes with a poleward aspect. The upper part of glacier where accumulation is greater than ablation is called the accumulation zone.
What the the boundary between the ablation and accumulation zone called?
The equilibrium line
How is the glacier mass balance calculated?
By subtracting total ablation for the year from the total accumulation. When total accumulation exceeds ablation there is a net positive mass balance and the glacier will advance. When total annual ablation exceeds accumulation the glacier will retreat.
What is dynamic equilibrium?
When accumulation equals ablation, the glacier snout will be stationary.
What are negative feedback cycles?
Act to minimise the effect of the new inputs in order to re -establish stability; thus a system with negative feedback cycles can self-regulate. For example, if a stationary glacier is affected by an increase in snowfall the glacier will advance which will cause an increase in volume in the ablation zone which will increase outputs so overtime the inputs will increase outputs
What are positive feedback cycles?
Amplify the initial change and may cause a shift in the system to a new state of equilibrium. For example if a glacier has a positive mass balance and the glacier surface increase, there will be an increase in albedo which will cause decreases in temperature.
What does the rate of glacier movement depend on?
The temperature and whether the pressure melting point is reached. Most temperate glaciers reach PMP but in many polar glaciers it is too cold to reach PMP
What is Pressure melting point ?
At the surface of the glacier the melting point is 0°c but with increased ice depth the melting point is fractionally lowered by the pressure of the overlying ice.
What processes do temperate glaciers move by ?
Basal slip and internal deformation
What process does polar glaciers move by?
Internal deformation
What is basal slip?
Occurs when the base of the glacier is at the pressure melting point , which means that meltwater is present and acts as a lubricant, enabling the glacier to slide over the bedrock.
What is 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.
What is inter-granular movement ?
Where individual ice crystals slip and slide over each other.
What is intra-granular movement?
Where individual ice crystal become deformed or fractured due to the intense stresses with in the ice.
What can basal slip be subdivided into?
Creep and regelation
Extending and compressing flow
Surges
What is creep?
Creep a large obstacle greater than 1 m wide causes an increase in pressure which make the ice plastically deform around the feature.
What is regelation?
Smaller obstacles less than 1 m wide will cause pressure melting, increasing ice movement by basal slip. The ice refreshes on the down glacier side of the obstacle.
What is extending flow?
Over steep slopes, the rate of basal slip will increase and the ice will accelerate and thin.
What is compressing flow?
Over shallower slopes, basal slip slows and the ice decelerates and thickens
What are surges?
Short lived events when a glacier can advance substantially, moving up to 100 times faster than normal. Various causes such as earthquakes but the most common is enhanced basal sliding triggered by the build up of meltwater at the ice-rock interface.
How does ice thickness control the rate of glacier movement?
When ice is 50m thick plastic flow begins, so greater thickness creates more pressure in the ice which causes faster movement.
How does precipitation level control rate of glacier movement?
High snowfall leads to greater accumulation and faster movement.
How does the rate of ablation control the rate of glacier movement?
The more meltwater there is the faster the movement as basal slippage increases.
How does bedrock permeability control the rate of glacier movement?
If bedrock is permeable then meltwater May be able to percolate slowing the movement of a glacier. Movement may be more rapid over deformable rocks such as clay.
How does ice and slope gradient control the rate of glacier movement?
The steeper the angle the faster the movement due to gravity
How does ice temperature control rate of glacier movement ?
Colder ice moves more slowly as it does not deform as easily and it stays stuck to bedrock more.
Jakobshavn, Greenland average rate of movement per year and year
12,600 m
2003
Trapridge, glacier , Canada
8.5 m per year
2005
What is erosion?
The removal of weathered materials by glacial ice includes plucking, abrasion, crushing and basal melting.
What is entrainment ?
Small rock fragments are trapped by basal ice freezing around them and applying sufficient drags to pull them along.
What is transport?
Rock debris transported on the ice surface, with in the ice and at the base of the ice.
What are micro-scale landforms?
Small scale landforms up to 1m long
Eg striation
What are meso scale landforms?
Medium scale landforms, eg roches moutonées, ribbon lakes and drumlins.
What are macro-scale landforms?
Large scale landforms eg glacial troughs, cirques and pyramidal peaks.
What are subglacial environments ?
Environment beneath the glacier ice subject to immense pressure from the overlying weight of ice
What is an ice marginal environment?
Environments at the edge of the glacial ice where a combination of glacial and fluvioglacial processes occur.
What is a proglacial environment?
Environments located at the front of a glacier , ice cap or ice sheet and dominated by fluvio-glacial processes.
What are periglacial environments?
Environments near glaciers and dominated by freeze-thaw processes, but not characterised by moving ice.
What are upland glacial landscapes?
Those at higher altitudes in hills and mountains
What are lowland glacial landscapes?
Those at lower altitudes on valley floors and coastal plains.
What are active glacial landscapes ?
Currently experiences glaciation, active glacial processes and landform development.
What are relict glacial landscapes?
Not currently characterised by glaciers but feature fossilised glacial landforms due to past glaciation.
What do the rates of accumulation and ablation vary with?
Climate
What is the glacier driven by?
Energy from the sun - evaporates water from the ocean to create air masses
