2.2 How are glacial landforms developed?? Flashcards
2.2 How are glacial landforms developed?
Key idea ⮕ Glaciated landforms develop due to a variety of interconnected climatic and geomorphic processes.
Geomorphic processes
-Weathering (Physical or mechanical, Chemical, Biological)
-Mass movement (Rock fall, Slides)
-Glacial processes (Erosion, Nivation, Transportation, Deposition, Till)
Weathering
A ubiquitous process in that it happens everywhere. In glacial areas some types of weathering are particularly significant and therefore influence the formation of glacial landforms.
Physical or mechanical (Weathering)
The breakdown of rock is largely achieved by physical weathering processes that produce smaller fragments of the same rock. No chemical alteration takes place during physical weathering. By increasing the exposed surface area of the rock, physical weathering allows further weathering to take place.
Freeze-thaw (Physical or mechanical Weathering)
Water enters cracks/joints and expands by nearly 10% when it freezes. In confined spaces this exerts pressure on the rock causing it to split or pieces to break off, even in very resistant rocks. The more frequent and regular the fluctuations of temperature around zero, the more effective this process will be.
Frost shattering (Physical or mechanical Weathering)
At extremely low temperatures, water trapped in rock pores freezes and expands. This creates stress which disintegrates rock to small particles.
Pressure release (Physical or mechanical Weathering)
When the weight of overlying ice in a glacier is lost due to melting, the underlying rock expands and fractures parallel to the surface. This is significant in the exposure of sub-surface rocks such as granite and is also known as dilatation. The parallel fractures are sometimes called pseudo-bedding planes.
Chemical (Weathering)
Decay of rock is the result of chemical weathering, which involves chemical reaction between the elements of the weather and some minerals within the rock. It may reduce the rock to its chemical constituents or alter the chemical and mineral composition of the rock. Chemical weathering processes produce weak residues of different material that may then be easily removed by erosion or transportation processes.
The rate of most chemical reactions is faster when temperature is higher - see Van’t-Hoff’s Law.
Van’t-Hoff’s Law
States that a 10°C increase in temperature leads to a 2.5 times increase in the rate of chemical reaction (up to 600°C), so most chemical weathering processes are most effective in warm or hot climatic regions. This is why warm, moist tropical environments experience the fastest rates of chemical weathering and cold, dry ay ocesses spued locations the slowest.
Oxidation (Chemical Weathering)
Some minerals in rocks react with oxygen, either in the air or in water. Iron is especially susceptible to this process. It becomes soluble under extremely acidic conditions and the original structure is destroyed. It often attacks the iron-rich cements that bind sand grains together in sandstone.
Carbonation (Chemical Weathering)
Rainwater combines with dissolved carbon dioxide from the atmosphere to produce a weak carbonic acid. This reacts with calcium carbonate in rocks such as limestone to produce calcium bicarbonate, which is soluble. This process is reversible and precipitation of calcite happens during evaporation of calcium rich water in caves to form stalactites and stalagmites.
Solution (Chemical Weathering)
Some salts are soluble in water. Other minerals, such as iron, are only soluble in very acidic water, with a pH of about 3. Any process by which a mineral dissolves in water is known as solution, although mineral specific processes, such as carbonation, can be identified.
Hydrolysis (Chemical Weathering)
This is a chemical reaction between rock minerals and water. Silicates combine with water producing secondary minerals such as clays. Feldspar in granite reacts with hydrogen in water to produce kaolin (china clay).
Hydration (Chemical Weathering)
Water molecules added to rock minerals create new minerals of a larger volume. This happens to anhydrite forming gypsum. Hydration causes surface flaking in many rocks, partly because some minerals also expand by about 0.5% during the chemical change as well because they absorb water.
Biological (Weathering)
Biological weathering may consist of physical actions such as the growth of plant roots or chemical processes such as chelation by organic acids. Although this, arguably, does not fit with the precise definition of weathering, biological processes are usually classed as a type of weathering. Certainly the effects are very similar to some of the physical and chemical processes even if it may be difficult to directly relate them to the weather.
In glacial environments, plant and animal activity may be severely limited by the low temperatures and so these mechanisms may be of very little significance.
Tree roots (Biological Weathering)
Tree roots grow into cracks or joints in rocks and exert outward pressure. This operates in a very similar way and with similar effects to freeze-thaw. When trees topple, their roots can also exert leverage on rock and soil, bringing them to the surface and exposing them to further weathering. Burrowing animals may have a sìmilar effect.
Organic acids (Biological Weathering)
Organic acids produced during decomposition of plant and animal litter cause soil water to become more acidic and react with some minerals in a process called chelation. Blue-green algae can have a weathering effect, producing a shiny film of iron and manganese oxides on rocks.
Mass movement
Mass movement occurs when the forces acting on slope material (mainly the resultant force of gravity) exceed the forces trying to keep the material on the slope (predominantly friction). In glacial landscape systems, the most significant mass movement processes are those acting on steep slopes, which lead to the addition of material to the glacier below, loading it with debris and providing the tools for abrasion.
Rock fall (Mass movement)
On slopes of 40° or more, especially if the surface is bare, rocks may become detached from the slope by physical weathering processes. These then fall to the foot of the slope under gravity. Transport processes may then remove this material, or it may accumulate as a relatively straight, lower angled scree slope.
Slides (Mass movement)
These may be linear, with movement along a straight line slip plane, such as a fault or a bedding plane between layers of rock, or rotational, with movement taking place along a curved slip plane. Rotational slides are also known as slumps. In glaciated landscape systems, slides may occur due to steepening or undercutting of valley sides by erosion at the base of the slope, adding to the downslope forces. Slumps are common in weak rocks, such as clay, which also become heavier when wet, adding to the downslope force.
Glacial processes
Moving ice in a glacier is a source of energy in glaciated landscape systems, and the energy can be expended through geomorphic processes to shape landforms. These processes can also supply material in the form of sediment, which can be deposited in, or transported within, the glacial system.
Erosion (Glacial processes)
Glacial erosion occurs as glaciers advance and this mainly occurs in upland areas. There are two main processes of erosion by glaciers:
-Plucking
-Abrasion.
Plucking (Erosion (Glacial processes))
This mainly happens when meltwater seeps into joints in the rocks of the valley floorl sides. This then freezes and becomes attached to the glacier. As the glacier advances it pulls pieces of rock away. A similar mechanism takes place when ice re- freezes on the down-valley side of rock obstructions. Plucking is particularly effective at the base of the glacier as the weight of the ice mass above may produce meltwater due to pressure melting. It will also be significant when the bedrock is highly jointed which allows meltwater to penetrate. Plucking is also known as quarrying.
Abrasion (Erosion (Glacial processes))
the debris embedded in its base/sides scours surface rocks, wearing them away. The process is often likened to the action of sandpapering. The coarse material will scrape, scratch and groove the rock. The finer material will tend to smooth and polish the rock. The glacial debris itself is also worn down ially if the ed from These vity. material, lower and therefor if the waten is confined o Bupis * determines long Suipp ne. pappaqua as a glacier moves across the surface, greater the due to osion per unit pe Movemen not only the basal gacial en
Factors that affect glacial abrasion (Erosion (Glacial processes))
-Presence of basal debris
-Debris size and shape
-Relative hardness of particles and bedrock
-Ice thickness
-Basal water pressure
-Sliding of basal ice
-Movement of debris to the base
-Removal of fine debris
Estimates of rates of erosion (Erosion (Glacial processes))
-Embleton and King (1968) suggest that mean annual erosion for active valley glaciers is between 1000 and 5000 m³.
-Boulton (1974) measured erosion on rock plates placed beneath the Breiðamerkurjökull glacier in Iceland and found that under ice 40 m thick, basalt eroded at 1 mm per year and marble at 3 mm per year. The ice had a velocity of 9.6 m per year. However, if the velocity increased to 15.4 m/year, the rate of erosion of marble increased to 3.75 mm, even though the ice was 8 m thinner. In this instance it would suggest that velocity is more important than ice thickness.
-In comparison, ice 100 m thick flowing at 250 m per year in the Glacier d’Argentière eroded a marble plate at up to 36 mm/year.
Nivation (Glacial processes)
A glacial process that is not easily classified as erosion or weathering. This complex process is thought to include a combination of freeze-thaw action, solifluction, transport by running water and, possibly, chemical weathering. Nivation is thought to be responsible for the initial enlargement of hillside hollows and the incipient development of carries.
Sources of Transportation (Glacial processes)
-Rockfall - weathered debris falls under gravity from the exposed rock above the ice down onto the edge of the glacier.
-Avalanches - these often contain rock debris within the snow and ice that moves under gravity.
-Debris flows - in areas of high precipitation and occasional warmer periods, melting snow or ice can combine with scree, soil and mud.
-Aeolian deposits - fine material carried and deposited by wind, often blowing across outwash deposits.
-Plucking - large rocks plucked from the side and base of valleys
-Abrasion - smaller material worn away from valley floors and sides
Transportation (Glacial processes)
Moving ice is capable of carrying huge amounts of debris. This material comes from a wide range of sources.
How material in Transportation may be classified (Glacial processes)
-Superglacial is debris being carried on the surface of a glacier. This will most often come from weathering and rockfall.
-Englacial is debris within the ice. This may have been supra glacial material that has been covered by further snowfall, fallen into crevasses or sunk into the ice due to localised pressure melting.
-Subglacial is debris embedded in the base of the glacier which may have been derived from plucking and abrasion, or that has continued to move down through the ice as former englacial debris.
Deposition (Glacial processes)
Glaciers deposit their load when their capacity to transport material is reduced. This usually occurs as a direct result of ablation during seasonal periods of retreat or during de-glaciation. However, material can also be deposited during advance or when the glacier become overloaded with debris.
Till (Deposition (Glacial processes))
-Lodgment till - material deposited by advancing ice. Due to downward pressure exerted by thick ice, subglacial debris may be pressed and pushed into existing valley floor material and left behind as the ice moves forward. This could be enhance by localised pressure melting around individual particles that are under significant weight and pressure. Drumlins are the main example of landforms of this type.
-Ablation till - material deposited by melting ice from glaciers that are stagnant or in retreat, either temporarily during a warm period or at the end of the glacial event. Most glacial depositional landforms are of this type
Glacial outwash
Sediment deposited by meltwater streams emanating from a glacier.
Glacio-fluvial
Relating to meltwater from a glacier.
Glacial drift
The general term for all of the materials carried and deposited by a glacier
Three distinctive characteristics of till (Deposition (Glacial processes))
-Angular or sub-angular in shape because it has been embedded in the ice and has not been further eroded
-Unsorted - all sizes are deposited en masse, together. When water deposits material, it loses energy progressively and deposits material in a size-based sequence.
-Unstratified is where glacial till is dropped in mounds and ridges rather than in layers, which is typical of water-bourne deposits.
Glacial landforms
-Erosional (Corries, Arêtes and pyramidal peaks, Troughs, Roche mountonnées and striations, Ellipsoidal basins)
-Depositional (Moraines, Erratics, Drumlins, Till sheets)
Erosional (Glacial landforms)
-Corries
-Arêtes and pyramidal peaks
-Troughs
-Roche mountonnées and striations
-Ellipsoidal basins
Corries (Erosional (Glacial landforms))
Armchair-shaped hollows found on upland hills or mountainsides. They have a steep back wall, an over-deepened basin and often have a lip at the front which may be solid rock or made of morainic deposits.
Arêtes and pyramidal peaks (Erosional (Glacial landforms))
An arête is a narrow, steep-sided ridge found between two carries. The ridge is often so narrow that it is described as kinfe-edged. Arêtes form glacial erosion, with the steepening of slopes and the retreat of corries that are back to back or alongside each other.
Where three or more corries develop around a hill or mountain top and their back walls retreat, the remaining mass will be itself steepened to form a pyramidal peak. Weathering of the peak may further sharpen its shape. The Matterhorn in the Swiss Alps is an excellent example and is over 1,200 m high.
Arêtes and pyramidal peaks (Erosional (Glacial landforms)) (Located example)
Striding Edge in the Lake District which has steep slopes either side that are 200-300 m high and almost vertical in places. Striding Edge itself is so narrow that it is just wide enough for one person to walk along the footpath that runs along the crest towards the summit of Helvellyn.
Troughs (Erosional (Glacial landforms))
Glaciers flow down pre-existing river valleys under gravity. As they move they erode the sides and floor of the valley, causing the shape to become deeper, wider and straighter. The mass of ice has far more erosive power than the river that originally cut the valley. Although they are usually described as being U-shaped, they seldom are. Rather, they are parabolic, partly due to the weathering and mass movement of the upper part of the valley sides that goes on both during the glacial period and in the subsequent periglacial period as the glacier retreats.
Roche mountonnées and striations (Erosional (Glacial landforms))
Projections of resistant rocks are sometimes found on the floor of glacial troughs.
As advancing ice passes over them, there is localised pressure melting on the up-valley side. This area is smoothed and streamlined by abrasion and often has striations which are scratches or grooves made by debris embedded in the base of the glacier.
On the down-valley side pressure is reduced and meltwater re-freezes, resulting in plucking and steepening. Roche mountonnées can indicate the direction the ice moved through an area. They vary in size but in the Coniston area of the Lake District they are typically 1-5 m high and 5-20 m long.
Ellipsoidal basins (Erosional (Glacial landforms))
All of the erosional glacial landforms are formed by the action of valley glaciers and their tributaries. This is known as Alpine glaciation. Significant contrasts can be seen when looking at the impact of large ice sheets on the landscape. Ellipsoidal basins are major erosional landforms created by ice sheets. The Laurentide ice sheet covered much of North America between about 95,000 and 12,000 years ago.
Depositional (Glacial landforms)
A number of different landforms are produced by sediment being directly deposited by ice:
-Moraines
-Erratics
-Drumlins
-Till sheets
Moraines (Depositional (Glacial landforms))
A terminal moraine is a ridge of till extending across a glacial trough. They are usually steeper on the up-valley side and tend to be crescent shaped, reaching further down-valley in the centre. These landforms mark the position of the maximum advance of the ice and were deposited at the glacier snout. Their crescent shape is die to the position of the snout; further advance occurs in the centre of the glacier, as there is no friction with the valley sides.
Lateral moraine (Depositional (Glacial landforms))
A ridge of till along the edge of a glacial valley composed primarily of debris that fell to the glacier from the valley walls.
Recessional moraines (Depositional (Glacial landforms))
A series of ridges running transversely across a glacial trough.
Terminal moraine
A cross-valley, ridge-like accumulation of glacial sediment that forms at the farthest point reached by the terminus of an advancing glacier.
Erratics (Depositional (Glacial landforms))
An individual piece of rock, varying in size from a small pebble to a large boulder. These are distinctive because of a different geological composition from that of the area in which they have been deposited. They were eroded, most likely by plucking, or added to the supraglacial debris by weathering and rockfall, in an area of one type of geology and then transported and deposited into an area of differing rock type.
Drumlins (Depositional (Glacial landforms))
A mound of glacial debris that has been streamlines into an elongated hill. Often they are prominent landforms, sometimes more than 1 km in length and 100 m high. In plan they are typically pear-shaped and aligned in the direction of the ice flow. The higher and wider stoss, or blunt, end faces the ice flow, while the lee side is more gently tapered.
Why Drumlins may be formed (Depositional (Glacial landforms))
-Lodgement of subglacial debris as it melts out of the basal ice layers.
-Reshaping of previously deposited material during a subsequent re-advance.
-Accumulation of material around a bedrock obstruction - these are known as rock-cored drumlins.
-Thinning of ice as it spread out over a lowland area, reducing its ability to carry debris.
Till sheets (Depositional (Glacial landforms))
Formed when a large mass of unstratified drift is deposited at the end of a period of ice sheet advanced, which smooths the underlying surface. They may not be very conspicuous in terms of relief, but they are significant landforms because of their extent.
