Physical Geography: Glacial Processes, Patterns and Landforms
Glaciation
Glaciation the spread of large masses of ice over land and sea
Glaciers are rivers of ice that move slowly
Move downslope under the influence of gravity and the pressure of own weight
Form where rate of accumulation of snow and ice is greater than rate of melting
Largest reservoir of fresh water on Earth
Zone of accumulation
Compressed snow is called firn
When all the air is forced out it is called glacial ice
Zone of ablation
Formation of glacial ice
Valley glaciers: form in mountainous locations and move down valleys
Continental ice sheets/glaciers: enormous areas of glacial ice and snow
How ice moves
Basal sliding
Friction between the base of the glacier and the valley floor
Meltwater acts as a lubricant
Glacier slides downslope under the influence of gravity
Plastic flow
Ice crystals react to pressure and gravity
Melting and refreezing
Ice changes internally over time without completely melting or breaking
Processes of glacial erosion
Glaciers erode the landscape they travel over in two ways:
Plucking
Abrasion
Rock is well jointed
Rock has already been weakened by freeze-thaw action
Bottom of glaciers scrape along valley floors – creates friction causing melting around the base of the glacier
Meltwater refreezes
Freezes around the rocks on the valley floor and these become part of the glacier
Glacier advances
Newly trapped rock is plucked out of the valley floor
New material is then used in process of abrasion
Occurs when the bedrock beneath the glacier is eroded by the debris/material embedded in the sides and bottom of the glacier
‘Sandpaper effect’ – scrapes the rock over which it is travelling and leaves scratches or grooves in the rock
Striations – show the direction of the ice flow
Factors affecting the rate of glacial erosion
Thickness of ice
Geology
Gradient
Landforms of glacial deposition (Cirque, corrie, coom)
Example: Coumshingaun, Co. Waterford
Processes: plucking, abrasion, rotational slip, freeze-thaw action, nivation
Birthplace of a glacier
Depression in a mountain
Three steep sides, north-facing slope
Glacial lake called a tarn
Alternative freezing and thawing, nivation
Bergschrund forms, freeze-thaw action
Rotational slip
Arête, narrow ridge
Formed when two cirques form side-by-side or back-to-back
Pyramidal peak formed when three or more cirques are eroded back-to-back or side-by-side around the sides of a mountain
Isolated peak in the centre
Landforms of glacial erosion (U-shaped valley)
Example: Glendalough, Co.Wicklow
Processes: abrasion, plucking, freeze-thaw action
Glacial troughs
Glaciers take the easiest route as they move from upland areas down through their valleys
Route is often a pre-existing V-shaped river valley
Glacier moves through this valley, changing the shape from a V-shape to a U-shape
Erode vertically and laterally
Valleys have steep sides and flat floors
Glacier cuts off interlocking spurs of the V-shaped valley, leaving truncated spurs
Hanging valleys, tributary valley
Ribbon lakes are long, narrow lakes found in glaciated U-shaped valleys
As a glacier advances through its valley it abrades the landscape
Differential erosion
Within the rock basin meltwater accumulates, on either side hard resistant rock called rock bars
If the lakes are joined by a river they are called paternoster lakes
Fjords are drowned U-shaped valleys
Result of melting glaciers
How the ice transports and deposits material
Deposited debris, glacier’s load
Various sizes ranging from large boulders to fine rock flour
Material may be angular or rounded in shape
Onglacial
Englacial
Subglacial
Dropping or laying down of sediment that was once transported by a glacier
Lowland areas
Deposited material is called glacial drift
Material deposited directly by ice is called till or boulder clay
Material deposited by glacial meltwater is known as fluvio-glacial deposits
Landforms of glacial deposition (Moraine)
Example: Glengesh, Co. Donegal
Processes: deposition, melting, mass movement, freeze-thaw action
Moraine: long ridge of unsorted boulder clay
Lateral moraine
Medial moraine
End/terminal moraine
Recessional moraine
Ground moraine
Landforms of glacial deposition (Drumlins)
Example: Clew Bay, Co. Mayo
Processes: deposition, melting, plucking, friction
Oval-shaped hills consisting of boulder clay
Unstratified material and impermeable
Show direction of glacier movement
Occur in swarms or clusters, drumlin belt
‘Basket of eggs’ topography
Drowned drumlins – as the ice melted sea levels rose and the drumlins appear as islands in the sea
Landforms of glacial deposition (Erratics)
Example: granite from Connemara found in the Burren, Co. Clare
Processes: deposition, plucking, melting
Material transported by the glacier and are said to be ‘out of place’ when deposited
Range from huge boulders to smaller rocks and pebbles
Landforms of fluvio-glacial processes
Glacial spillway (erosion)
Example: Glen of the Downs, Co. Wicklow
Processes: melting, hydraulic action, abrasion
Proglacial lake, overflows
Hydraulic action and abrasion cut a deep V-shaped gash
Eskers (deposition)
Example: Eiscir Riada
Processes: melting, deposition, sorting
Ridges of stratified sand and gravel
Lowland areas
Up to 30 m high
Exploited for building materials
Outwash plains (deposition)
Outwash plains (deposition)
Example: Curragh, Co. Kildare
Processes: deposition, sorting, melting
Glaciers melt
Release vast amounts of water
Spreads outwards beyond the end/terminal moraine
Carries large volumes of rock, gravels and sand
Infertile, not suitable for arable farming
Kames and kettle holes (deposition)
Example: Westmeath
Processes: melting, deposition, ice calving
Piles of sediment consisting of gravels and sand
Deposited along the front of a retreating glacier
Kettle holes are blocks of ice separate from the main glacier
Buried partly in meltwater sediments
Blocks of ice melt leaving depressions or holes
Fill with water – form kettle hole lakes
Irish glaciations
The Munsterian
Began 175,000 years ago
Lasted 20,000 years
Originated in Scotland and mainland Europe
Only highest peaks called nunataks escaped
The Midlandian
Took place 70,000 years ago
Lasted for 10,000 years
Ice came from local sources