Glacitation - Unit 4 Flashcards
What is weathering?
The breakdown of rock where it’s located. This involves:
- Physical processes, e.g. freeze thaw
- Chemical processes, e.g. carbonation
What is erosion?
The removal of rock by ice, water or wind
Explain the 4 steps in freeze-thaw weathering
- In glacial environments, temperatures may fluctuate either side of 0°C
- When water freezes it expands by 9% in volume
- When water freezes it exerts pressure on the crack
- Repeated freeze thaw cycle fractures the rock
What’s dilatation?
- Pressure release
- Rocks can fracture when the overlying pressure is released
- Can occur when a glacier is melting, losing weight and hence exerting less downward pressure
What are the 3 erosional processes?
- Abrasion
- Plucking
- Sub-glacial meltwater erosion
Describe abrasion
- As a glacier moves the weathered rock debris embedded in its base (subglacial debris) it scrapes against the bedrock and wears it away
What factors will result in the most abrasion?
- Thick ice (stronger/more pressure, create more meltwater)
- Lots of subglacial meltwater (increases debris as there’s more thawing)
- Debris angular/ a lot of it (more powerful/ easier to break down and more likely to scratch the rock)
- Faster movement of glacier (covers a larger distance in a smaller time)
Describe plucking (quarrying)
- Large downwards pressure from weight of glacier
- Causes melting
- Water refreshes to obstacles
- Plucked from bedrock when glacier moves
What type of rocks are susceptible to plucking?
Highly jointed or fractured rocks
Describe sub-glacial meltwater erosion
- Meltwater channelled beneath glacier
- High velocity meltwater erodes material at glacier’s bed
- Forms large subglacial meltwater channels
Explain process of cirque formation
Stage 1:
- FTW occurs in small hollows on north-facing slopes = rock disintegrates
- Summer meltwater removes rock debris, which enlarges hollow
- Process = nivation
- Snow lasting more than 1 year becomes compacted & eventually turns into ice, known as neve
Stage 2:
- Further accumulation of ice adds weight = rotational movement of ice
- Movement aided by meltwater from pressure melting
- Rock fragments from back wall of hollow removed through plucking = steep, jagged & angular
- Rock fragments transported by ice causes abrasion
- Meltwater enters system through a crevasse (bergschrund)
- Pressure release causes dilatation cracks
- May start to move over lip & out of rock basin it has created
- Exhibit abrasion of upglacier & plucking on downglacier side
What is a cirque?
Example?
- A large amphitheater shaped hollow
- Cwm Idwal
What is an arête?
Example?
- A sharp ‘knife-edge’ ridge which has been produced by 2 cirques eroding back towards each other
- Striding edge, Lake District
What are pyramidal peaks?
Example?
- Where 3 or more cirques around a mountain have been formed
- As they continue to develop back to back the remaining central area of the mountain becomes sharper and steepened
- Glyder Fawr above Cwm Idwal
How are arêtes and pyramidal peaks formed?
- Snow gathers and forms ice on a rounded summit
- Frost-shattering makes peaks jagged
How do glacial troughs form?
Example?
- Glacier moves through non-resistant rock and erodes the valley into a U-shape
- Yosemite Valley
How do truncated Spurs form?
Example?
- As a glacier moves down a river valley it erodes each spur
- Over time the Interlocking Spurs erode back to become a truncated spur, which often have a steep cliff face
- Half Dome - Yosemite
How do hanging valleys form?
Example?
- Formed by tributary glaciers which enter a glacial trough
- Smaller valleys meet the main glaciated valley
- They lie suspended above in the valley side
- They may often possess waterfalls
- Yosemite
What is a ribbon lake?
A long and narrow, finger-shaped lake, usually found in a glacial trough
What is a roche moutonnee?
A piece of resistant rock on the floor of a glacial trough that has undergone abrasion of the upvalley side and plucking on the downside valley
- Typically 1-5m high and 5-20m long
How are Roche moutonnees formed?
Example?
- Pressure of obstacle produces meltwater as PMP is reached
- Glacier moves over obstacle (basal sliding) as it lubricated the base and reduces friction
- Abrasion on upvalley side leaves striations
- Pressure meltwater seeps into cracks in bedrock
- Pressure is released and meltwater refreezes, widening joints
- As glacier moves, pieces of rock are ‘plucked’, leaving it steep and jagged
- Pen Y Benglog
What is a crag and tail?
A meso-scale landform consisting of a rock hill and tapering ridge
How is a crag and tail formed?
Example?
- The more resistant rock is not eroded as easily as the surrounding softer rock
- This leaves a steep upglacier side and leads to the formation of the crag
- Soft rock is eroded as glacier moves over it
- The soft rock on down glacier side is protected by crag so less susceptible to erosion
- With distance from the crag, the softer rock is less protected and erosion can occur freely, this leads to the formation of a tapered down-glacier side
- Edinburgh Castle
What are striations?
Scratches on the hard bedrock produced by the process of glacial abrasion
How are striations formed?
Example?
- Debris frozen in the basal ice is dragged across the bedrock
- Isle of Skye
What are chatter marks?
Small intermittent fractures in the bedrock
How are chatter marks formed?
Example?
- When the basal debris within the ice is not in continuous contact with the bedrock
- Yosemite National Park
How is polished rock formed?
Example?
- The ice can polish the bedrock if sand and silt is found in the basal ice layer as opposed to angular rock fragments
- Central Park, New York
How are ribbon lakes formed?
Example?
- Glacier moves over an area containing alternate bands of resistant and non-resistant bedrock
- Glacier erodes the non-resistant rock more quickly by abrasion, creating a hollow = rock basin
- On either side of the rock basin, the more resistant rock is eroded less (rock bars)
- Rock bars act as dams between which rainwater may accumulate after the retreat of the ice age, filling up the rock basin and creating a river lake
- Llyn Ogwen
