Coasts 1b Flashcards
(71 cards)
what is geomorphology
study of nature and origin of landforms, particularly weathering and erosion that occur in an atmosphere and hydrosphere.
These processes continually shape the Earth’s surface and generate the sediments that circulate in the rock cycle.
what is weathering
the alteration and breakdown of rock minerals and rock masses when they are exposed to the atmosphere.
Weathering processes occur in situ, with no movement of rock materials involved
how does weathering change rocks
changes them from a hard state to become much softer and weaker
what are the 3 main types of weathering
- physical or mechanical
- chemical
- biological
what is physical weathering?
a physical force exerted on a rock that breaks intact rock into smaller fragments of the same rock.
no chemical alteration occurs but by increasing the exposed surface area of the rock it allows further weathering to take place (e.g. it increases the surface area for chemical weathering).
what is free-thaw weathering
Caused by the expansion of freezing water in fractures, which forces rocks apart. Water enters cracks/joints in rocks and freezes, expanding by about 10 percent. This exerts pressure on a rock and over repeated freeze thaw cycles cracks/joints are widened eventually resulting in fragments of rocks breaking off. This will occur even in very resistant rocks.
thermal expansion
Repeated expansion and contraction due to heating and cooling, causes stresses which break rock. The stress is strongest on the outside of the rock; thin curved sheets peel away leaving an ‘onion skin’ appearance. Common in hot areas where there is a marked difference between daytime and night-time temperatures.
Pressure release
Causes physical weathering due to unloading. Removal of surface rocks, by weathering and erosion, brings the rocks below to surface and they become subjected to less and less pressure. This unloading of pressure causes the rocks to expand and fracture horizontally, with an increasing number of fractures as the rock approaches the Earth’s surface.
Salt crystalisation
Saline solutions seep into cracks and evaporate laving salt crystals behind. Heating results in expansion of the crystals, exerting pressure on rock and resulting in it breaking apart.
what is chemical weathering
Chemical weathering is the breakdown of rock by chemicals from the atmosphere and soil; It is mainly caused by the chemical actions of water and the substances dissolved in it.
hydrolysis
Silicate minerals will readily react with water and break down to clay minerals (termed hydrolysis), which is then washed away. Feldspar in granite, for example, will react with hydrogen in water to produce kaolinite (china clay).
Hydration
Some minerals may absorb water resulting in new minerals of a larger volume. This process can cause surface flaking in rocks, where minerals expand by around 0.5 percent. An example of hydration would be where anhydrate takes up water to form gypsum.
Oxidation
Oxidation is the reaction with oxygen, usually to form an oxide. Oxygen, in air or water, can react with certain minerals present in rock. Iron is particularity vulnerable to oxidation and becomes soluble under acidic conditions. In rich cements that are present in many sandstones are weakened by this process. Oxidation normally involves a colour change to red or browny-yellow.
Solution
Some minerals will readily dissolve in water and will be carried away in solution. Some salts are soluble and so too is iron (in acidic water- pH of 3).
carbonation
A special kind of solution. Carbon dioxide in the atmosphere dissolves in rainwater and pore water in the soil to form carbonic acid. Reaction between carbonic acid and minerals that contain metallic ions to form carbonates (or hydrogen carbonates). Calcium carbonate reacts with acidic rain water to form calcium hydrogen carbonate (calcium bicarbonate); the latter dissolves in water. When hard water evaporates and loses water/and or carbon dioxide, calcium carbonate is formed e.g. stalactites and stalagmites in limestone caves. Carbonation can also remove cement (e.g. calcite cement) that holds sand particles together leading to their disintegration.
what is biological weathering
Biological weathering comprises physical pressure and chemical reactions as a result of biological processes. Examples include root action, burrowing and organic acids.
Root action and burrowing
Tree roots can grow along areas of weakness e.g. bedding planes, joints and cracks forcing them apart mechanically. Roots also keep surfaces open so that water can enter, making chemical weathering easier. When trees sway in the wind, their roots can prise open fractures in rocks. Burrowing animals may have a similar effect.
Organic acids
Decomposition of plant and animal litter can lower the pH of soils, making them more acidic; this increased acidity can react with susceptible minerals. On shore coastal platforms burrowing bivalves may secrete acid to bore through the rock resulting in hollows.
Q. Explain how physical, chemical and biological weathering can work together to disintegrate rock.
As physical and biological weathering breaks rocks apart, the surface area increases, which in turn increases the effectiveness of chemical weathering.
Alternatively, a rock that is chemically weathered will be easier to break up physically.
weathering links to climate
Physical weathering: mechanical processes dominate in cold and dry climates
Chemical weathering: processes of mineral decay dominate in warm and humid climates
Biological weathering: vegetation, and animals, tend to be more active in warm and humid climates
what is mass movement
the downslope movement of soil, sediment and rock. Mass movement processes are occurring continuously on all slopes; some act very slowly others occur very suddenly, often with disastrous results. It involves significant quantities of material being released and falling under gravity, usually onto the shore
Rock fall
extended
Rapid free-fall of rock from a steep cliff face. Rock fragments fall from the face of the cliff due to the action of gravity. This is made worse by freeze-thaw. Well joined rock is particularly vulnerable to rock fall.
As rock falls it may hit cliff face with enough force to dislodge further rock sometimes resulting in a rock avalanche
Exceptionally large rockfalls composed of rocks and debris that move rapidly as dry flows are termed rock avalanches.
Translation Slide
extended
Translational slides arise when rocks and unconsolidated material on the cliff face are saturated with water. Eventually the material slips down the slope.
Translational slides occur more often on soft rock coastlines where there are natural joints in the rock, into which water can get. They occur on steep gradient slopes and are very similar to slumps, although slumps occur on shallower, concave slopes.
Rotational Slide (slump) (extended)
Occasional, rapid movements of a mass of earth or rock sliding along a concave plane. They can occur after periods of heavy rain, when the water saturates the overlying rock, making it heavy and liable to slide. Water also lubricates the lower layers which then gives way.
Undercutting of a steep slope by a river or sea erosion weakens the rock above, also making slumping likely. Material at the slump foot (toe) has to be removed by wave action before more slumping can replace it.
Slumps are common in weak rocks, such as clay which become heavier when wet.
