2-3-1-glacial-erosion-landforms-and-landscapes

Glacial Erosion

Sub-aerial weathering

• Freeze-thaw or frost shattering, happens when rocks contain cracks and where temperatures regularly dip below the freezing point

• Any water in the cracks will freeze as the temperature drops, which expands as it freezes, exerting pressure on the crack

• Repeated freezing and thawing of water will eventually break the rock apart and it will pile up as scree at the foot of the slope

• When trapped under the ice, the sharp, angular rocks are an effects abrasive tool

Carbonation

• Is an important process in cold environments and occurs in rocks with calcium carbonate, such as chalk and limestone

• Rainfall (pH of 5.6 ) combines with dissolved carbon dioxide or organic acid to form a weak carbonic acid solution

• Calcium carbonate (calcite) in rocks, reacts with the acidic water and forms calcium bicarbonate, which is soluble and removed in solution by meltwater

• The effectiveness of the solution is related to the pH of the water as carbon dioxide is more soluble at lower temperatures 

Nivation

• A blanket term for active processes that occur at the edges of snow patches 

• The processes include the physical and chemical weathering that occur underneath patches of snow

• 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 

Glacial erosion

• After glaciers break down the rock through freeze-thaw action, erosion continues the process by plucking and abrasion

• 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 

• It is the weight of the ice in a glacier that forces it to advance downhill, eroding the landscape as it moves

• Ice advances in a circular motion called a rotational slip, which hollows and deepens the landscape

• 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 

• Processes do not work in isolation or at the same rate, there are continuous adjustments, particularly after glacial retreat; where landforms are not only shaped but reshaped by by the combined action of mass movement, weathering, erosion, and fluvial action

Glacial Erosion Landforms

Glacial cycle of erosion

• There are 3 stages to glacial erosion

  • Youthful

  • Mature

  • Aged

• Youthful

  • This marks the beginning of erosional landforms

  • The shaping and hollowing of a corrie by ice

  • The beginnings of aretes and horns

• Mature

  • Corries are well-formed and begin to meet

  • The glacial valley takes on its ribbon-shaped with a regular, stepped graded contour

  • Hanging valleys are visible

  • The valley floor begins to deepen and takes on the shape of a trough

• Aged

  • ‘U’-shaped valley is clearly defined

  • Development of the outwash plain, including features of drumlins, eskers, kettle holes, etc. 

  • Corries converge, mountain summit heights decrease and their peaks become rounded

• Erosional landforms are created when moving masses of glacial ice slide and grind over bedrock

• Glacial ice contains large quantities of unsorted sand, gravel, and rock that was plucked out of the bedrock

• Ice sliding across the bedrock, grind the debris into a fine, but gritty powder called rock flour

• Rock flour polishes the surface of the bedrock to a smooth finish called glacial polish

• The remaining trapped debris and larger rocks, create long grooves, called glacial striations, as they flow over the bedrock

• These striations indicate the direction of ice flow

• Corrie/cwm/cirque

  • Corrie, cwm and cirques are all the same feature and are deep, pre-glacial hollows of accumulated snow and ice 

  • In Wales corries are called cwms and in France they are called cirques 

  • Found at the apex of a glacial valley, on the coldest aspect of the mountain, with the greatest accumulation of 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 

  • As the name suggests, this is a 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 

• Corrie, tarn or cirque lakes

  • Corrie, 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)

Scouring & Differential Geology

• Scouring is the ability of the ice mass to erode large areas of pre-existing landscape

• Occurs when the glacier overspills its containing valley

• Ice sheet scouring produces the following landscapes:

• 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