Glaciers - Erosion + Deposition Flashcards
What is the difference between weathering and erosion
Erosion = the wearing away and removal of rock material by a MOVING force (moving ice)
Weathering = the breakdown and decay of rock in situ woth NO MOVEMENT or removal involved
Give two processes of glacial erosion
Plucking (also known as Quarrying)
Abrasion
What is plucking
- Meltwater seeps into joints in the rocks of the valley floor and sides
- This water then freezes and becomes attached to the glacier
- As the glacier advances, it pulls pieces of rock away
- Plucking is particularly effective under temperate glaciers where basal ice is at the pressure melting point and when the bedrock is highly jointed
What is abrasion (in relation to glaciation)
1) Debris entrained in the ice at the base and sides of the glacier rubs against surface rocks, wearing them away like sandpaper
2) Abrasion can have two effects depending on the nature of the debris in the ice:
- Coarse and angular debris scrapes and scratches rock to form striations which are long and thin grooves in the bedrock
- Finer debris is polished and smoothes the bedrock
3) Very fine material is produced by abrasion called rock flour. This is washed out in meltwater streams making the water a milky colour
Give 3 processes of weathering in glacial environments
Freeze-thaw (frost shattering)
Dilation
Nivation
Describe freeze-thaw weathering
1) Water seeps into cracks and joints in the rock
2) when it freezes it expands by 9-10% in volume
3) Water in the cracks melts when the temperature rises above 0ºC and is common between day and night
4) This process is most active on the valley sides and the mountains surrounding the glaciers above the ice, but less so under the glacier where the temperature is more constant
What is Dilation (pressure release weathering)
1) Glaciers exert considerable downward pressure on the bedrock bellow
2) When a glacier retreats or the ice thickness is reduced, the pressure is reduced
3) The rock on the valley floor, and to a lesser extent the sides, expand and fracture as the weight is removed
4) This makes the rock fracture in layers parallel to the surface
What is nivation
1) Snow patches often form in hollows on north-facing slopes
2) In the summer, as the snow patches melt, some of the meltwater percolates into cracks in the rock below the snow patch which then leads to freeze-thaw weathering
3) When there is intense melting, meltwater streams transport the weathered material from bellow the snow patch
4) A process is known as solidification also transports weathering material downslope by the force of gravity
5) This series of processes are responsible for enlarging small hollows which can then form cirques or corries
What is a corrie/cirque
Oval-circle shaped valley which is formed through erosion of ice or glaciers
Formed through nitration - Mt Blanc
What is an arête
An enlarged ridge - formed when neighbouring corries run back to back as each glacier erodes the other side of the ridge - edge becomes steeper and narrower
What is a pyramidal peak
Angular, sharpy pointed mountain peak
Formed by glaciation
What is a glacial trough
U shape, a steep-sided, broadly flat bottomed valley formed by the erosional process of a glacier
What is a truncated spur
Steep cliffs in a glacial trough
Caused when a glacier has cut-off the tips of rocky spurs as it moves downhill
What is a hanging valley
A small valley hanging above the main valley
Formed when small tributary glacier has not been able to erode down as the large glacier is present
What is a Roche moutonnee
A bare outcrop of rock on the valley floor that has been sculpted by lots of ice
What are the different sources of debris transported by a glacier
Nivation
Avalanche
Erosion - plucking + abrasion
Scree - freeze-thaw weathering
What is drift
All the material deposited during glaciation
What is till
Material deposited directly by glacial ice
What is outwash
Material deposited by meltwater
30% of Europe is covered by………mainly deposited in past glacial periods
drift
What are the characteristics of till
Jagged and angular - due to less erosion, particularly by meltwater and attrition
Unsorted because glaciers do not deposit particles on order of size
Unstratified as glacial till is dropped in mounds and ridges, not in layers
The orientation of the long axis is parallel with the direction of ice flow, offers less resistance to the ice
What is Till Fabric Analysis
Fieldwork technique to determine the orientation and shape of deposits
What factors affect rates in erosion, transportation and deposition of glaciers
Discharge in meltwater streams Warm based glacier Gradient Type of sediment, large rock = erode more Less accumulation = less erosion
What is Laminar flow
Movement of individual layers within the glacier, often the layers on the annual accumulation
What is Intergranular
When individual ice crystals re-orientate in relation to each other
What is basal sliding
Ice slides over the valley floor because meltwater has reduced friction between the base of the glacier and the valley floor. Friction can lead to the formation of meltwater
Give 3 basal movements
Sliding
Creep
Sub-glacial deformation
What is basal creep
Occurs as ice deforms under pressure when encountering obstructions on the valley floor. This enables the ice to spread around and over the obstruction, before re-freezing again when the pressure is reduced
WHat is sub-glacial deformation
When ice is carried by saturated base sediments moving beneath it on gentle gradients
What is extending
Ice moving over a steep slope is unable to deform quickly enough and so it fractures, forming crevaces.
What is compression
When the gradient is reduced, the ice thickens and the following ice pushes the slower moving, leading ice
