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
