glacial erosion Flashcards
sub aerial weathering
-freeze thaw weathering occurs when water finds its way into small cracks in the rock
-when temperature falls below 0, the water in the ice freezes forming ice, this causes pressure on the surrounding rock
-these forces make the cracks in the rock bigger
-when temps rise again the frozen water thaws
-this cycle of freezing and thawing occurs many times until eventually fragments of the rocks break away completely
nivation
-Fluctuating temperatures and meltwater promote chemical weathering and freeze-thaw action
-Weathered material is transported with the summer meltwater
-Repeated cycles of melting, freezing, and transportation form nivation hollows
-Saturated debris (due to summer meltwater), destabilises the slope and slumping may occur
plucking
-Movement of the ice mass generates friction and heat, causing the base of the glacier to slightly melt
-This meltwater freezes around rocks and stones under the glacier
-As the glacier moves forward, it ‘plucks’ this ice, pulling the rock away
quarrying
-Similar to plucking in that pieces of bedrock are transported and eroded within the glacier
-As a glacier moves through a valley, pressure is exerted on the sides and bottom of the valley
-Friction causes melting, allowing meltwater to surround the rocks in the valley
-As the meltwater refreezes, it pulls on the ice and quarry’s the sides of the valley away
abrasion
-Abrasion occurs as bits of rocks, stones, and boulders stuck in the ice, grind against the rock below the glacier wearing it away and producing rock flour
-Striation (scratch) marks arise when rocks beneath the glacier are transported across the bedrock
crushing
This happens when pressure exerted by the ice mass and its debris, crushes the bedrock surface leaving chattermarks fractures as it moves over the bedrock
basal melting
-As pressure increases, the melting point of water decreases
-The thicker the glacier, the greater the pressure; the lower the temperature at which water melts
-As temperate glaciers move down the valley, friction melts the glacier’s base
-This layer of meltwater acts as a lubricant and allows the glacier to ‘float’ allowing basal sliding and the glacier can move faster
mass movement
Can occur quickly with the sudden movement of large ice masses, usually due to basal slipping - ice sheet calving is a good example of mass movement
corrie/cirque
-Deep, pre-glacial hollows of accumulated snow and ice -As the accumulated ice begins to flow; basal/rotational sliding along with plucking and abrasion, hollows the mountain into a bowl-shape
-Debris is pushed to the edges of the corrie, which acts as a dam (corrie lip) to the accumulating snow
-As the ice thickens within the hollow, it flows over the corrie lip and downhill as a glacier
-Plucking, abrasion and freeze-thaw weathering, steepen the back wall of the corrie, into the familiar armchair shaped landform
-Examples include Helvellyn Corrie in the Lake District and Cwm Idwal in Eryri National Park (Snowdonia)
Arête
-Arêtes are knife-edge, steep-sided ridges
-Formed when two corries cut back into the mountainside
-As each corrie glacier erode either side of the ridge, the edges become steeper and the ridge narrower
-This gives the arête it’s a jagged profile
-Examples include Crib Goch in Eryri National Park, and Striding Edge in Lake District England
Pyramidal peak
-Three-sided, pointed mountain peak
-Formed when three or more back-to-back corrie glaciers carve away at the top of a mountain
-This creates a sharply pointed mountain summit
-Examples include Yr Wyddfa (Snowdon) in Wales and Buachaille Etive Mòr, Glencoe, Scotland
Tarn or cirque lakes
-Tarn or cirque lakes form when the ice within the corrie melts
-Because of the corrie lip at the bottom end, the meltwater is held in place and a circular body of water is formed
-Examples include Red Tarn, Helvellyn in the Lake District and Cadair Idris in Eryri National Park (Snowdonia)
truncated spur
-Truncated spurs are past interlocking spur edges of past river action that have been cut-off forming cliff-like edges on the valley side
-Found between hanging valleys and are an inverted ‘V’ shape
-Formed when past ridges/spurs are cut off by the lower valley glacier as it moves past
-An example is Nant Ffrancon Valley in Eryri National Park
Hanging valley
-These are small tributary glaciers found ‘hanging’ above the main valley floor
-When melting occurs, there are waterfalls onto the valley floor
-An example is Cwm Dyli in Eryri National Park
ribbon lake
-As a glacier flows it travels over hard and softer rock
Softer rock is less resistant to erosion, so a glacier will carve a deeper trough over this type of rock
-When the glacier has melted, water collects in these deeper areas
-This creates a long, thin lake called a ribbon lake
-Examples include Lake Windermere in the Lake District and Llyn Ogwen in Eryri National Park
-The areas of harder rock left behind are called rock steps
glacial trough/u shaped valley
-Glacial troughs are steep-sided valleys with a flat floor
-They start as V-shaped river valleys but due to the size and weight of the glacial ice it changes to a U shape as the glacier erodes the sides and bottom making the valley deeper and wider
-Examples are found all over the UK, but Nant Ffrancon and Nant Gwynant in Eryri National Park are good examples
-At the end of the last Ice Age, many coastal troughs (glacial) flooded to form fjords (Norway) or sea lochs (Scotland)
Knock and lochan
-These are macro-scale, scoured rugged, lowland features
-Consist of ‘knocks’, small rounded hills with no vegetation, and ‘lochs’ of small lakes
-Created by the intense erosion of the surface by an ice sheet
-The knocks are resistant bedrock hills with the lochs being formed by over deepening hollows
-Examples are found in the Outer Hebrides, Scotland
crag and tail
-A macro-scale, tapered ridge of glacial deposits, extended to one side, found in both upland and lowland areas
-Formed when a ‘crag’, a large section of hard, resistant bedrock e.g. volcanic rock, protects a section of softer, unconsolidated material, the ‘tail’
-The advancing ice sheet is forced to go around the upstream or stoss section of resistant rock (crag), leading to melting and refreezing around loose rocks and boulders, which are plucked as the ice mass moves forward
-The leeward or downstream side of less resistant material is protected directly behind the crag
-Velocity and pressure is reduced and deposition occurs
-As the ice mass continues, abrasion of the softer rock occurs and the tail is drawn and smoothed by abrasion with the plucked crag debris
-Example is Castle Rock (crag) and Royal Mile in Edinburgh (tail)
Roche moutonnée
-A meso-scale, resistant, bare mass of rock, on the valley floor, that has been sculpted by flowing ice
-Reversed crag and tail, with plucking occurring on the leeward side and smoothing on the upstream, stoss side
-The upstream or stoss side of the outcrop, is smoothed due to abrasion by the glacier
-The moving ice leads to localised pressure melting
-This eases basal sliding and increases erosion over the rock, creating striations across the top of the rock
-On the leeward or downstream side, the pressure reduction refreezes the meltwater
-This bonds the base of the glacier to the outcrop
-As the glacier continues to flow, loose rocks/boulders are plucked out and a jagged, steep surface is left behind
