Coasts 1b Flashcards
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.
Debris & Mud Flows
extended
Mudflows occur on very steep slopes along the coastline. Where there is limited vegetation to bind the soil together and the ground is very saturated heavy rain can produce sheet flow over the upper cliff surface. The soil continues to be lubricated and it eventually flows over the cliff face and down onto the shore at a fast speed.
A mudflow is a slurry of water and fine sediment. A debris flow is a mudflow with many large rocks.
Soil creep (or downhill creep) (extended)
Very slow movement, occurring on very gentle slopes because of the way soil particles repeatedly expand and contract in wet and dry periods. When wet, soil particles increase in size and mass, and expand at right angles. When the soil dries out, it contracts vertically. As a result, the soil slowly moves downslope.
transitional slide
when blocks of one rock moves over another along a line of weakness, such as a bedding surface. Tends to be linear.
Rock fall
Rocks undercut by the sea or slopes affected by physical weathering
Mudflow
Heavy rain causes fine material to flow rapidly downhill
Rotational Slide
excessive lubrication causes soft material to move along a concave plane
soil creep
very slow movement of soil particles down slope
the factors that may trigger a mass movement
Angle of slope (steeper is faster) Nature of regolith (material above bedrock) Amount and type of vegetation Water Type and structure of rock Human activity Climate
define erosion
The wearing away of the land surface and fragments by movement (water, air, ice and gravity).
Erosion sorts the insoluble and soluble raw materials of weathering by a variety of physical and chemical processes and further breaks down the insoluble fraction.
hydraulic action
the force of moving turbulent water can disturb and remove loose deposits/dislodge fragments of rock.
Wave quarrying/pounding
breaking waves exert pressure on the rock. Air is trapped in cracks in a cliff face; as the water pulls back air is released under pressure widening the crack. The rock face is weakened over time.
corrosion (solution)
the wearing away of the bedrock and particles in the river by chemical action. The water dissolves soluble minerals (solution) and reacts with chemically active minerals (especially if the river is acidic).
Abrasion
the wearing away of the rock surfaces by mechanical effects- particles rubbing, grinding, scratching, smoothing effects.
Attrition
the wearing away of the particles involved in abrasion. They bang into each other and against the floor and sides of the river.
effects of erosion on the particles
- Reduction in size
- Reduces angularity of fragments
- Makes it easier to transport them further
transportation- traction
the rolling or scraping of particles across the bed
transportation- saltation
hopping skipping motion of particles on the bed
transportation- suspension
very small particles carried in moving water. Small particles held/floating in the water are part of the suspended load.
transportation- Solution
Dissolved material. Dissolved material in the water are part of the dissolved load.
Longshore Drift
Waves approach the shore at an angle; swash moves material up the beach in the same direction as the wave. Backwash moves the material back down the steepest gradient- usually perpendicular to where it is picked up by the next incoming wave.
Deposition occurs when velocity and/or volume of water decreases and energy levels are reduced. Deposition takes place in coastal environments when:
- Sediment accumulation exceeds removal
- Waves slow after breaking
- Backwash water percolates into beach material
- There is a sheltered area such as an estuary
Q. Describe and explain the relationship between particle size and settling velocity
Positive correlation- as grain size increases so too does velocity required for deposition
Larger and heavier particles require more energy to transport them. As flow velocity decreases, the largest particles are deposited first and so on, sequentially until finest particles are deposited
Q. How would you expect particle sizes to differ from the cliff to the shore? Explain your answer.
Particle sizes will be bigger by cliffs as they will have recently fallen from the cliffs so will have had less time to be subjected to erosion. Particles near the shore would also be smaller as they will have been moved by longshore drift (insufficient energy to move the larger particles)
what are the modes of transport in which a river can carry material
saltation
solution
suspension
traction
Is dry or wet sand easier for the wind to pick up?
dry sand is easier
wet sand- the moisture increases cohesion between particles, helping them to stick together
is attrition more effective on land or in water
Attrition on land is particularly effective in wind as particles tend to be carried for much greater distances than in water, and the particles are not protected from collisions by the film of water around them
why might wind speed fall in a certain area?
surface friction.
In coastal areas this will occur inland, where friction from vegetation is much greater than on the open sea
salt spray
Coats the cliff face above the waves and infiltrates into pores. When the water evaporates, salt crystals grow and push apart the grains, thereby weakening the rock.
The formation of erosional landforms:
There are several important erosional landforms
- Headlands and bays
- Cliffs and platforms
- Geos and blowholes
- Caves, arches, stacks and stumps
Q. Explain how Geology can lead to the formation of headlands and bays
Cliffs along the coastline do not erode at the same pace. When a stretch of coastline is formed from different types of rock, headlands and bays can form.
Bands of soft rock such as clay and sand are weaker therefore they can be eroded quickly. This process forms bays. A bay is an inlet of the sea where the land curves inwards, usually with a beach. Hard rock such as chalk is more resistant to the processes of erosion. When the softer rock is eroded inwards, the hard rock sticks out into the sea, forming a headland.
Erosional features such as wave-cut platforms and cliffs can be found on headlands, since they are more open to the waves. Bays are more sheltered with constructive waves which deposit sediment to form a beach.
wave cut notch
the point of maximum impact of destructive waves at the base of the cliff
when a cliff collapses in cliff retreat, what type of process is it?
mass movement
cliff retreat steps
- wave cut notch
- cliff face and structure eroded
- cliff face succumb to gravity and collapse
- material carried away by LSD
2 chemical processes that may weaken the structure of the rock
oxidation
carbonation
explain/describe the formation of a shore platform
- (cliff retreat)
- shore platform exposed- cut into by abrasion
- eroded by hydraulic and chemical action
- when exposed, weathered by sub aerial processes
- friction from platform slows approaching waves for them to break on the platform rather than the base of the cliff
how do marine organisms such as algae accelerate weathering?
at low tide
at night
algae release CO2 bc not photosynthesising
CO2 mixes with water making it more acidic
increasing chemical weathering
where is wave erosion greatest on a shore platform
high and low tide as it is there for the longest time
Geo
narrow, steep-sided inlets on a coastline where there is a weakness (joint or fault) in the rock which is exposed by erosion processes.If a narrow cave becomes enlarged by continual erosion and the roof collapses then it becomes a geo.
Blowhole
if part of the roof collapses resulting in a vertical shaft that reaches the cliff top it forms a blowhole. In storm conditions large waves may force spray out of the blowholes.
caves, arches, stacks and stumps
- geo- narrow steep sided inlets in the side of a cliff. They begin to form in lines of weakness such as joints or faults-
- caves- wave refraction focuses wave energy on headlands. wave erosion is concentrated forming a wave
- arches- cave erodes through to other side
- arch collapse- arch becomes unstable and collapses under its own weight- through eg. freeze thaw, hydraulic action
- stack- steep vertical sided column of rock standing apart from the headland. Protects the adjacent shore from waves
- sand may collect on on the lee side of the stack, slowly building a tombolo
- stump- only visible at low tide
define wave refraction
the depth of water around a coast varies and as a wave approaches a coast its progress is modified due to friction from the seabed, halting the motion of waves
explain wave refraction
As wave crests come in straight to the beach at the headland the water is shallow
Whereas it is deeper in the bays
This causes the waves to slow down more around the tip of the headlands, which causes the waves to bend around the headland (this change in direction is refraction).
The refraction causes the energy of the waves to become concentrated in the shallow water near the headlands resulting in more erosion there.
In contrast the energy of the waves gets spread out in the deep bays, on both sides of the headland, so that the energy is dissipated and deposition occurs.
As the waves break on the sides of the headland at an angle there is longshore movement of eroded material into the bays, adding to the build-up of beach sediment.
the formation of depositional landforms:
- Beaches
- Spits
- Bars
- Tombolos
- Salt marshes
- Dunes
percentages of where sediment on beaches comes from
cliff erosion- 5%
offshore- 5%
rivers- 90%
what determines the colour and size of sand on a beach?
the local rocks it came from
eg. the cliffs or rivers that transport material to the sea
features of eaches (list)
berm cusp ridge runnel storm ridge
berm
a sand ridge that marks the end of a swash zone
