The glacial system Flashcards
What is a glacier?
A large body of ice formed from compressed snow
What are the inputs into a glacier?
- precipitation (eg snow or rain)
- rock debris and sediment
- avalanches
- solar and kinetic energy (melting makes movement)
What are the processes of a glacier?
- transportation
- erosion
- deposition
- ice movement
- weathering
What are the outputs of a glacier?
- calving
- sublimation
- ablation
- deposition
- moraine
What is calving?
When chunks of ice break off the end of a glacier
What is sublimation?
When ice turns directly into water vapour and is carried away by the wind
solid –> straight to gas
What is deposition? (output)
When the glacier retreats, it dumps the moraine or erratics
What is moraine?
The accumulation of debris (till) deposited directly by a glacier as it advances and retreats
What is till?
Unstratified (no layers) and unsorted sediment containing a mix of rock particles (clay, sand, gravel, boulders) deposited by a glacier
What is ablation (output)?
All the ways that mass can be lost from a glacier
What is transportation?
What are the different types?
The movement of rock fragments within a glacier. Includes
- supraglacial transport (over the top)
- subglacial transport (along the bed)
- englacial transport (through the middle)
- proglacial
(in front of the glacier)
Give an example of supraglacial transport
- Freeze-thaw action breaks rocks off the cliff above the glacier/where glaciers meet and drops it onto the glacier
Give an example of subglacial transport
- plucking and entrainment (capturing of material) cause loose sediment to get lodged in the glacier as it moves
Give an example of englacial transport
- when sediment falls into crevasses or gets buried by snowfall
Give an example of proglacial transport
- bulldozing at the snout pushed sediment in front of the glacier
How do we know if sediment has been transported by a glacier?
- ANGULAR
- little attrition so there is minimal rounding vs rivers with lots of attrition - UNSORTED
- glaciers deposits sediment all at once whereas rivers have capacity and deposit larger ones first
Does transport happen in mostly cold or warm based glaciers? Why?
- warm based
- meltwater
- moves faster
What is deposition? (process)
When a glacier drops/leaves behind sediments while moving
What are the four different types of till?
(What different types of deposition created different types of till)
Ablation
Flow
Lodgement
Deformation
What is ablation till?
The dumping of sediment as the glacier thaws
Also known as MELT OUT
What are the characteristics of ablation/melt out till?
Till is angular, unsorted and less compact
Often forms hummocks of moraine left round the ice margins (e.g. lateral)
What is lodgement till?
When transported material gets lodged/stuck in the glacier bed
Occurs when friction between the subglacial debris and bed becomes greater than the drag of ice
What are the characteristics of lodgement till?
Rounded clasts (due to abrasion occurring between the ice and bed of glacier) in the form of till sheets
What is flow till?
When meltwater modifies sediment/till deposits by causing them to creep, slide or flow
What are the characteristics of flow till?
The till will be layered and sorted
What is deformation till?
Where weak underlying bedrock is deformed (broken up) by the glacier moving above it
What are the characteristics of deformation till?
The sediment/till is aligned in the direction of ice flow, generally in a flat layer
What is erosion?
What are the different types?
The curving and shaping of the land by/underneath the glacier as rocks and ice rub against the surrounding landscape. Includes
- plucking
- abrasion
- subglacial fluvial erosion
What is plucking?
When the glacier freezes to the landscape, and as it moves it pulls material away
How does plucking work?
- as glacier moves, it exerts pressure on valley sides and bottom
- this causes slight melting due to generated friction and heat (PMP)
- meltwater freezes round rocks and stones at the sides and bottom of the glacier
- as the glacier moves, it pulls the rock away (it ‘plucks’ it)
What is abrasion?
The process of rocks and sediment embedded in the glacier wearing away the surrounding landscape over time
How does abrasion work?
- rocks and ice embedded in a glacier rub against the surrounding rock, eroding the landscape and wearing it away
- can leave rock flour
- can leave marks called striations
What factors affect the rate and amount of abrasion?
- meltwater
- debris
- mass of glacier
- ice thickness
- speed
- gradient
- type of rock (relative hardness)
- bedrock permeability
- jointing (weakness within rocks)
How does ice melt when the temperature is below freezing?
- PMP
- Friction
- Geothermal heat flux
What glacial landforms does erosion (abrasion) create?
Corries
Roche Moutonnée
Glacial trough/U-shaped valley
Hanging valley
Truncated spurs
Which glacier has more erosive force: warm based or cold based? Why?
Warm based have more erosive force because
- move faster
- extra glacial material
What is subglacial fluvial erosion?
When the meltwater stream beneath the glacier carrying large amounts of sediment exert large forces on the bedrock, and wear it away
How does subglacial fluvial erosion work?
- pressure melting at the base of the glacier/water percolating form the top produces a meltwater stream
- the stream carries sediment which scrapes along the bedrock - ABRASION
- high pressure water exerts forces and weakens the rock - HYDRAULIC ACTION
- meltwater freezes on jagged rock - PLUCKING
What is ice movement?
When the glacier moves very slowly under the force of gravity
- slower in the upper zone: brittle ice cracks and forms crevasses
- faster in the lower zone: steady pressure, meltwater and frictional heat make it move easier
What are the different types of ice movement?
Basal slippage
Internal deformation
Rotational flow
Compressional flow
Extensional flow
Subglacial deformation
Glacial surges
What is basal slippage?
When friction, pressure and heat from ice moving over bedrock causes meltwater. lubricates rock = slides along
- 2-3m per day
What are the 2 types of basal slippage?
Regelation slippage
(Enhanced) Basal Creep
What is regelation slippage?
When the melting of ice under pressure, and then refreezing, allows the glacier to move over obstacles
How does regelation slippage work?
- Glacier meets rocky outcrop
- pressure increases on the upside of the obstacle
- If pressure is sufficient, it causes melting = meltwater
- Meltwater flows around sides of outcrop, refreezes on other side where the pressure is lower
What is (enhanced) basal creep?
When ice deforms plastically to flow around larger obstacles that cannot be melted over
How does (enhanced) basal creep work?
- The glacier meets an obstacle too large to be melted over
- Pressure increases on the upside of the obstacle
- Ice crystals deform plastically around it
- The larger the obstacle the larger the pressure needed to deform around it
What is internal deformation?
The very slow movement of ice due to the ice crystals inside the glacier deforming, and gravity allowing the crystals to slide past each other downhill. The base remains stationary
What are the 2 types of internal deformation?
intergranular flow
intragranular flow
What is intergranular flow?
When ice crystals align themselves in the direction of the glacial movement due to pressure (influencing gravity) and sliding past each other
What is intragranular flow?
When individual ice crystals crack/fracture and deform due to stresses in the ice
Does internal deformation occur in warm or cold based glaciers? Why?
Cold based because basal temperatures are too cold for significant melting, so the only movement is due to internal deformation
What is rotational flow?
When the glacial ice forming in hollows/depressions rotates/pivots as it starts to move down the slope
What is compressional flow?
As the slope gets less steep, the ice mass thickens and movement slows.
- this extra pressure increases erosional potential, which increases the gradient and extensional flow
What is extensional flow?
As the slope steepens, the ice mass thins and movement quickens
- decreases erosional potential
What is sub glacial deformation?
Where material at the base of a glacier is deformed due to the combination of the weight of ice, the high pressure, and the water at the base of the glacier allowing the material to move more easily
What is a glacial surge?
rare, short lived event where ice races forwards at 10-100x the usual pace
Only 4% world glaciers prone to surge
Normally occur at temperature latitude mountains
Give an example of a glacial surge
Hubbard Glacier, Alaska
- 7-8m snowfall in mountains above Valerie Glacier initiated the surge
- normal rate = 15cm a day
- can reach 34m/day
- no longer access village Yakutat because Russel Fjord is blocked by ice
What causes glacial surges?
abrupt collapse of glacier in the accumulation zone
climate change
What are some effects of glacial surges?
Destroyed settlements
Block rivers
Floods/meltwater
What is weathering?
The breakdown of rocks in situ
What is freeze-thaw action?
When the freezing and expansion of water in rocks eventually breaks them apart
How does freeze-thaw weathering work?
- Water enters crack in rock in day
- Water freezes at night, exerts huge forces on rock, crack expands by 9%
- Next day, melts and process continues
How does freeze-thaw action help the development of glacial landforms?
- provides debris that will be used as an abrasive tool (more erosive force)
- loosens rocks on the cliff faces, making plucking easier (more abrasive tools, more erosive force)
What features of the landscape does freeze-thaw action create?
- scree slope
- protalus rampart
- jagged cliff face
What is carbonation?
Chemical weathering primarily affecting carbonate rocks (e.g. limestone) occurring when weak carbonic acid (formed from the absorption of CO2 by rainwater) reacts with the calcium carbonate in the rocks, gradually dissolving them
Usually in temperate glacial areas - sufficient rainfall
How does chemical weathering work?
- Rainwater absorbs CO2 from atmosphere as it falls, forming weak carbonic acid
- When the carbonic acid comes into contact with calcium carbonate in the rocks a reaction occurs, where the calcium carbonate dissolves
- This forms calcium bicarbonate which can easily be washed away
- Over time, small amounts of rock are removed as calcium bicarbonate dissolves into groundwater, leading to gradual breakdown and weakening of rock structure
What landforms can chemical weathering influence?
- caves and caverns
- sinkholes
- gorges
- limestone pavements
- Karst landscapes
What is nivation?
A blanket term for active processes at edges of snow patches
- chemical/physical weathering under patches of snow
Temperatures and meltwater promote freeze-thaw.
- makes debris, meltwater transports it away
- repeated cycles melting/freezing/transport forms nivation hollows
- soil saturation = destabilises slopes, slumping may occur
What factors control ice flow?
Gravity - steeper means more
Mass - heavier means more movement
Meltwater - lubricant
Friction - less means more
Temperature - cold ice frozen to bedrock
