Glaciers 06.12.18
controls on glacial development: expansion and down-wasting.
Glaciers have a strong link to climate via temperature and precipitation.
Temperature and precipitation unravels into different components: sea , air and temperature variations with elevation.
Climate and energy begins with incoming solar radiation- less radiation coming in at the poles due to the angle of latitude. At poles- energy is less concentrated- less energy per unit area.
Altitude- the higher up mean temperature decreases e.g. Himalayas
Altitude and precipitation effects
Higher elevation= more precipitation (required for glacier building).
Colder ait at the top of mountains.
Colder air holds less moisture, so it is released at altitude.
Greater likelihood of precipitation as snow.
This flags up ice vulnerability at polar regions, as growing ice there is difficult as precipitation is relatively low.
Glacial system
Glacial mass balance- a natural open system- Things can come in and effect it.
Inputs- throughputs- outputs- accumulation- ice flow- ablation.
Glacial system key features 1. Glacial mass
- Glacial mass balance
- Glacial flow.
When talking about inputs and outputs referring to sediment as well as ice.
- Glacial mass accumulation
Snow fall- avalanche etc of snow from other areas to add mass.
wind-blow- Wind blown snow from other areas- areas (deserts) where there's not much precipitation.
Superimposed ice- meltwater coming in and refreezing- slope further up catches the sunlight, ice melts and water refreezes on glacier.
- Glacier mass ablation
Toe of glacier in a slightly warmer area because it is at a lower elevation.
Calving- glaciers that terminate in water, smaller masses of ice break off.
Sublimation- water moving directly from ice on the glacier to vapour in the air.
The higher and warmer the water the more unstable the ice water boundary and the more ice is lost via this mechanism.
How does snow turn to ice?
FIRNIFICATION
As snow is progressively buried by further snowfall it becomes compacted to a firn or a neve, eventually metamorphising into ice.
Key change- air passages are sealed at a density off -830ngm-3 as crystals start to align.
Ice can then deform under gravity.
Glacier mass balance
The system wants to be in a mass balance, from where it is being gained to where it is lost.
So during heavy winter snowfalls, glacier wants to advance to transfer mass.
The total mass of a glacier changes over a mass balance year (Aug- Aug)
If accumulation increases so does mass
Positive net balance- on a whole the glacier has gained mass.
Negative net balance- glacier down-wasting and snout retreating- sometimes the glacier thins rather than retreats.
Tipping point of a glacier- the highest mass. This is important for the geological record as if position changes we can tell if the Glacier is advancing or retreating.
If accumulation = ablation- mass balance = zero.
Net balance gradient
Continental type glacier
Low accumulation, low ablation, glacier is relativity static (not gaining, nor loosing mass). Low dry areas.
Low balance gradients, slow flow e.g. major ice sheets, High Arctic glaciers.
Continental scale large ice mass (impacts on climate change).
Maritime type glacier
Dynamic climate- varies between seasons.
high accumulation, high ablation.
High balance gradient, fast flow. e.g Southern Alps, New Zealand
More mass must be moved through system to keep an equilibrium, meaning higher velocities and flow rates.
Glacial flow
Creep
Creep occurs when firn is turned to ice under pressure - in this situation ice is a fluid.
Ice crystals have parallel cleavage planes and tend to split in 1 particular direction. Under pressure at depth, they align parallel to the shear stress of the ice (i.e. down valley) and begin to slide past each other producing down slope glacial flow. It becomes more organised as it becomes more compressed.
Creep has 2 components
- Basal slip- lubricates and occurs at the ice/ bed interface, favouring easy sliding. (only where there's water- its the lube)
- Internal deformation- under pressure ice deforms easily- major component of flow- even when there is no basal sliding due to lack of water.
Brittle zone- top of glacier where ice isn't under that much confining pressure. So wants to break, crack and fracture.
Plastic zone- Further down into the ice, pressure from above means it flows and behaves plastically.
Fracture
Strain is accommodated by breaking apart.
Fracture- Fracture occurs when the stress placed on the ice cannot be accommodated by deforming and the ice fractures to form crevasses.
Occurs mainly in the brittle zone near the surface- but can extend further into the ice mass.
Glacier shit
Regelation (freezing)- flow and erosion
Ice is not able to erode rock (unless that rock is chalk). So you need particles of rock in the ice to erode the rock.
Pressure melting on up-glacial slide of obstacles. Water re-freezes on the lee-side of the obstacles.
Glacier melts, water surrounds rock particles, these rock particles are then refrozen onto glacier
Striations- caused by englacial debris- scratches from rock being dragged along rock.
Quarrying
Bedrock is under tension, ice is wanting to extend down the valley so its under tension (transfers the tension to the rock).
Weakness is exploited and put under tension, water is forced into the cracks, freezes causing pressure. Material wants to move to low pressure so its not under stress. Some gets frozen and dragged along.
U shaped valleys
What the ice does to the valley in terms of erosion, determines how the glacier glows at a later stage.
ice is flowing the fastest at the middle and bottom middle of the glacier. - where it has the least friction and stress with the valley sides
Where the ice is flowing the fastest is where the most erosion takes place, so the bottom of the glacier becomes very eroded. The valley eventually broadens out, so there's more capacity for the ice to not have friction with the valley sides, so ice movement speeds up over time and becomes more efficent.
Massive amounts of ice covering continents and comprising about 95% of all glacier ice on earth.
Valley or alpine glaciers
Formed in and restricted to mountainous regions.
Comprise a very small amount of the total glacier ice.
Hence we are most concerned about what they do, as they have the possibility to contribute the largest sea level change, due to the volume of water.
Ice types
Warm based ice
Thick ice- high subglacial pressure.
PMP (pressure melting point) reached.
Water and basal sliding present.
Erosion maximised- whole landscape eroded.
Areal scour.
Cold based ice
Thin ice- low subglacial pressure
Ice remains freezing below (PMP).
No water present so no sliding.
Very limited erosion- linear erosion.
Only selected areas eroded- selective erosion.
Trough seperated by low-relief plateau areas/
Plateaus may have cold-based thin ice.
Troughs are ice streams, re-occupying pre-glacial valleys
Valley= thicker ice, hill= thinner ice. So there are different conditions at the hill and valley. valley reaches PMP and so erosion occurs. At the top of the hill it is too cold for melting and so the landscape is preserved.
General glacial knowledge
Only erosion with basal slip.
As the glacier erodes, the basal slip becomes charged with debris until it stops moving= lodgement and deposition occurs.
Crevasses (open because the ice is under tension) allow material to get into contact with the rock and erode.
Glacial-fluvial landforms
Rivers formed by melting of the glacier
Kettle-hole lakes- hole that was left by chunks of ice, left behind, which melted and left a void.
Eskers, kame terraces, kettle-hole, outwash plane, deltas and fans.
Outwash planes- Enormous areas left behind braided rivers- characteristics of outwash that comes out of glacial water.
Deltas- meltwater into lakes or oceans.
Fans- rivers depositing mass amounts of material.
Morainic landforms
(what happens between ice and its bed ;))- erosional features
Drumlins, lodegement tills, moraines, flutes.
Fluted moraines- grooves and ridges, telling of the ice flow.
Rogen morain- sediment under the ice under such tension its ripping apart little chunks.
What do you need to make a glacier?
Precipitation bias to snow.
cold/ mild winters, but mean temperature below 0degrees.
High humidity.
Low temperatures and cool summers.
Maintain snow pack through Summer to accumulate a positive mass balance each year.