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Coastal processes G3.2
Marine erosion (G3.2.Ai)
There are four main types of erosion that happen on the coast:
Hydraulic Action:
Hydraulic action is the name given to erosion which is caused by the force of the waves against rocks
and cliffs. This process takes the form of the sea's waves crashing against rocks and cliffs.
Water drives air into cracks in the rock; pressure builds up, exploding the crack and forcing bits of
rock break off.
The stronger the wave, the greater its explosive power.
A primary example of hydraulic action is a wave striking a cliff face which compresses the air in
cracks of the rocks. This exerts pressure on the surrounding rock which can progressively crack,
break, splinter and detach rock particles. This is followed by the decompression of the air as the
wave retreats which can often occur suddenly with explosive force which additionally weakens the
rock. Cracks are gradually widened so the amount of compressed air increases, and hence the
explosive force of its release increases. Thus, the effect intensifies in a 'positive feedback' system.
Over time, as the cracks may grow they sometimes form a sea cave.
Abrasion / Corrasion:
When waves approach the coastline they are carrying material such as sand, shingle, pebbles and
boulders. Abrasion occurs when this material is hurled against cliffs as waves hit them, wearing the
cliff away. Waves carry debris (such as sand, shingle and pebbles) which is hurled against the
coastline. The debris rubs against the rock and eventually causes pieces to break off. These broken
fragments are in turn thrown back at the rock face by the waves and the process repeats itself. As
with other types of erosion, the extent of Corrasion depends on the force and height of the waves,
the amount of debris in the water and the strength of the rock affected.
Corrasion is also known as abrasion.
Corrosion / Solution:
Corrosion (or solution) is an example of a chemical process. Certain chemicals in sea water, such as
salt (sodium chloride) dissolve minerals in the rocks, causing them to slowly break up. Sea water can
also contain mild acids which have a similar effect. Chemicals released into the sea as a result of
pollution can speed up the rate of corrosion.
Attrition:
Fragments of rock which have broken off the cliff face fall down and can collect on the seabed. Here,
moved by the water, they knock against each other and the bedrock, becoming ever smaller,
rounder and smoother.
Weathering (G3.2.Bi)
Weathering is the gradual breakdown of rocks.
In this section you will cover the main types of weathering:
Freeze thaw – this involves water percolating into cracks and pores in the cliff. When this water
freezes it expands (by about 9%) this applies pressure on the cliff, which can lead to the breakup of
the cliff. Coastal locations encourage freeze thaw weathering. This is because cliffs provide
extensive bare rocky outcrops that are often cracked and weathered. Coastal areas also have a good
supply of water to seep into the cracks. However due to the milder climate at the coast freezing
temperatures are often rare. It is only in exceptional winters that this process is really effective.
However despite fewer freezing periods at the coast freeze thaw wreathing still has a part to play
here. A wet autumn and a cold February can easily lead to major rockfalls, this happen in 2001 along
the south coast of the UK. Chalk, which is a porous and permeable rock, this means that it absorbed
the water well, was the main type of rock to be affected by these conditions.
Freeze thaw wreathing creates angular rocks which the sea uses very effectively through erosion.
Freeze thaw shattering: Occurs in rocks that contain crevices and joints (e.g. joints formed in granite
as it cooled, bedding planes found in sedimentary rocks, and pore spaces in porous rocks), where
there is limited vegetation cover and where temperature fluctuates around 0 0C. In the daytime,
when it is warmer water enters the joints, but during cold nights it freezes. The process of shattering
of rock is due to frost cycles i.e. fluctuating above and below 00C. The process occurs with climates
with rapid frost cycles, rocks with joints and rainfall. Frost leads to mechanical breakdown in two
ways:
1.
As ice occupies 9% more volume than water, it exerts pressure within the joints.
2. When water freezes within the rock it attracts small particles of water, creating increasingly
large ice crystals.
In either case the process slowly widens the joints and, in time, causes process of rock to shatter (or
disintegrate) from the main rock. Where the block disintegration occurs on steep slopes large
angular rocks collect at the foot of the slope as scree; if the slopes are gentle large blockfields tend
to develop.
Salt Weathering: salt water is of course readily available at the coast, when the water has
evaporated it leaves behind salt crystals. These crystals can grow larger over time and exert stresses
in the rock in much the same way as ice does; this causes the rock to break apart. Salt is also capable
of corroding rocks with iron traces.
-when saltwater enters pore spaces or joints in rocks, it evaporates; salt crystals are likely to form.
As the crystals become larger, they exert stresses upon the rock, causing it to disintegrate. During
the day water enters the rock and is heated, water evaporates leaving salt crystals. These are large
in volume and put pressure on rocks by expansion and eventually will disintegrate.
Wetting and drying: frequent cycles of wetting and drying are common on the coast. Clay rich rocks
such as shale will expand as they get wet. As they dry they will contract this can cause them to crack,
and contribute to their break up.
Wetting and drying: Affects less resistant rocks such as clays. The clay is porous and has the ability
to absorb. When these rocks are wet they expand and when dry they contract. Over time they
disintegrate the rocks.
Carbonation: involves the slow dissolving calcium carbonate from rocks such as limestone and chalk.
When it absorbs carbon dioxide from the air water forms a weak and carbonic acid. This acid reacts
with the calcium carbonate in certain rocks to form calcium bicarbonate. This in turn is easily
dissolved. The cooler the temperature of the water, the more CO2 will be absorbed. This increases
the effectiveness of the carbonation.
Biological weathering: several types of biological weathering take place at the coast:
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Plant roots prise apart rocks on cliffs when they grow into small cracks and then thicken and
deepen with age.
Water passing through decaying vegetation becomes acidic- leading to enhanced chemical
weathering.
When birds (puffins) and animals (rabbits) dig burrows in the cliffs.
Marine organisms are also capable of burrowing into rocks (piddocks)or secreting acid
(limpets)
Mass Movement:
Rotational slip:
when softer materials overlies a much more resistant rock, the cliffs are subject to slumping or slip.
With excessive lubrication, whole sections of the cliff face may move downwards with a slide plane
that is concave this produces a rotational movement. Slumps are a common feature of the British
coast particularly where glacial deposits form the coastal areas like in east Yorkshire and north
Norfolk.
Characteristics:
-Fairly wet, and relatively fast movements
-Material does not travel far from source
-typical on soft rocks, often those that are unconsolidated or weakly consolidated
-most common on muds and clays on the coast
-Triggers = Earthquakes, heavy rain, eruptions, ash fall, saturation, snow melt
-Anthropogenic Influences = terminal groyne syndrome, climate change, building on cliff tops.
Rotational Slip at Barton on sea: CASE STUDY
Erosion within Christchurch bay is
between 1 and 3m per year.
The rocks that the cliffs consist of are made up of gravels, sands and clays. This means that they are easily eroded and
have little strength to resist collapse. (gravels and sands are permeable and they will absorb any water content that
enters it. Clay is however impermeable and though this is the case, it becomes extremely slippery when it comes into
contact with water, this causes land mass movements such as slides and slumping)
-The arrangement of the rocks will cause the water to pool up within the cliffs and then the water pressure within the
cliffs will encourage cliff collapse.
-The area of the coast has a south-westerly prevailing wind, this means that long shore drift will occur and pass the sand
towards the Hurst castle spit.
-In the same continuum, a large groyne or barrier has been built at Bournemouth, this means that any long shore drift
that should bring a replenishment of sand, is being caught up at Bournemouth beach. This causes the destructive waves
to have more effect on the cliff faces because there is less friction on the beach to slow the waves down.
-There are several small streams (bunnys) that run towards that area of the coast, however they disappear into the
permeable sands before they can reach the sea, this causes a larger build-up of water in the cliffs.
-The large number of buildings that are situated on top of the cliffs will add weight to them, this will make them more
vulnerable to collapse, it can also interfere with any natural drainage systems inside the cliffs.