Topic 3: Coastal Landscapes and Change Flashcards
How can we classify coastlines?
- Geology: the type of rock influences the resulting type of coast, e.g. rocky cliffs or coastal plains.
- Energy levels: creates high or low energy coastlines .
- Balance: deposition and erosion have a balance which either leads to coastlines being built or eroded away.
- Sea level: changes in sea levels results in submergent or emergent coastlines.
What are the types of rock?
- sedimentary
- igneous
- metamorphic
Sedimentary rock:
Form from the build up, compacting, and hardening if sediments into layer over time by lithification.
- Older sedimentary rock can be very resistant.
- Younger sedimentary can be more easily eroded, e.g. clays
- the average rate of erosion of sedimentary rocks is 2 - 6cm per year.
Metamorphic rock:
Highly resistant rock formed from the change in the structure of sedimentary and igneous rocks, caused by variations in heat and pressure. e .g. slate, marble and schist.
- the process of change is referred to as the rocks having been metamorphosed..
- marble is a np example of a M rock that has been formed from the changing structure of limestone caused by the re-crystallisation of calcite.
- M rocks are resistant to erosion
Igneous rock:
Highly resistant rock produced by volcanic activity, e.g basalt and granite. Form from the crystallisation of magma.
- they here more resistant to erosion and weathering as they contain crystals which increase the strength of the rocks and reduce the number of lines of weaknesses that would be exploited by physical processes.
- they can either be intrusive (formed from magma inside the Earth) or extrusive (formed when magma escapes through vents on the Earth’s surface).
- average rate of erosion of igneous rocks is 0.1 - 0.5cm per year
What is the littoral zone?
- the littoral zone is a series of sub-zones to represent the features of the wider coastline from sea to land.
- it includes four key sub-zones: offshore, foreshore, backshore and nearshore.
- this zine reaches dynamic equilibrium where there is a balance between inputs and outputs.
- short term factors: individual waves, daily tide and seasonal storms.
- long term factors: climate chnage and sea level changes.
The littoral zone inputs and outputs
Sediment is the key input and output of the system and is determined by:
- inputs - the action is waves, currents and wind.
- outputs - washed out into the sea by erosion.
Coastal system inputs:
- Marine: waves, tides, storm surges
- Atmospheric: weather/ climate, climate change, solar energy
- Land: rock type and structures, tectonic activity
- People: human activity, coastal management
Coastal system processes:
- weathering
- mass movement
- erosion
- transport
- deposition
Coastal system outputs:
- erosional landforms
- depositional landforms
- different types of coasts
High energy coastlines:
Rocky coastlines are generally found in high-energy environments.
High energy coasts are characterised by:
- destructive waves
- long fetches
- high rates of erosion
- caves, arches, stacks and stumps
- cliffs and wave-cut platforms
High energy coastlines in the UK:
- stretches if the Atlantic - facing coast, where the waves are powerful for much of the year (such as Cornwall or northern Scotland)
- the rate of erosion is higher than the rate of deposition
- erosional landforms, such as headlands, cliffs and wave-cut platforms tend to be found here
Low energy coastlines:
Sandy coasts are generally found in low-energy environments.
Low energy coasts are characterised by:
- constructive waves
- shorter fetches
- higher rates if deposition
- spits and bars, beaches, sand dunes and salt marshes.
Low- energy coastlines in the UK:
- stretches of the coast where the waves are less powerful, or where the coast is sheltered from large waves (such as Lincolnshire and Northumberland)
- the rate of deposition is higher than the rate of erosion
- landforms such as beaches, spits and coastal plains tend to be found here.
Lithology definition:
Geological structures and characteristics
Joints definition:
(Vertical cracks) - these are fractures caused either by contraction as sediments dry hit, or by earth movements during uplift.
Dips definition:
Refer to the angle at which rock strate lie (horizontally, vertically p, dipping towards the sea, dipping inland)
- bedding planes that dip towards the sea create a gentler cliff profile, but these cliffs are vulnerable to mass movement processes, like rockfalls.
- bedding planes that dip towards land tend to create a steeper cliff profile, but these cliffs are more vulnerable to erosion processes, like hydraulic action and abrasion.
Faults definition:
Formed when the stress or pressure to which a rock is subjected, exceeds its internal strength (causing it to fracture). The faults then slip or move along fault planes.
- rocks with the presence of more joints and faults, like sedimentary rocks, are more susceptible to the processes of erosion and weathering. This is because these processes exert forces on the weaknesses found in the layers of the rocks.
Folding definition:
Formed by pressure during tectonic activity, which makes rock buckle and crumple (e.g. the Lulworth Crumple)
Strata definition:
Layers of rock
Concordant coastlines - what are they?
Concordant coastlines have alternating bands of hard (more resistant) and soft (less resistant) rock parallel to the coast.
A resistant rock is eventually eroded - allowing the sea to break through to the less resistant rocks behind. Erosion follows quickly.
E.g. this has led to a formation of a small bay or cove at Lulworth.
Discordant coastlines:
- Discordant coastlines have alternating bands of hard and soft rock at 90 degrees to the coast.
- Erosional landforms are more common on discordant coastlines because erosion happens at different rates along their length.
- more resistant rock leads to headlands
- less resistant rock forms bays
Discordant coast - Isle of Purbeck:
- Bagshot and Tertiary beds consist of unconsolidated clays which are less resistant and have formed a large bay at Studland.
- chalk is strong and resistant therefore has led to the Foreland headland.
- Wealden bed also consists of unconsolidated material and has led to the formation of Swanage Bay.
- Purbeck and Portland beds consist mainly of limestone . This is resistant and has led to headlands at Peveril point and Durlston Head.
What are Dalmatian coasts?
- another type of concordant coastlines
- they have formed as a result of a rise in sea level. Valleys and ridges run parallel to each other and when the valleys flood because of a rise in sea level, the tops of the ridges remained above the surface if the sea - as a series of offshore islands that run parallel to the coast.
Examples of Dalmatian coasts:
- e.g. the Dalmatian coast, Croatia
- e.g. Lulworth Cove, south-west Dorset coastline. Here, the more-resistant rock (e.g. limestone) acts as a barrier to protect the less-resistant rock.
Haff coats:
- also consist of concordant features - long spits of sand and lagoons - aligned parallel to the coast.
- these are named after the Haffs, or lagoons of the southern shore of the Baltic Sea, which are enclosed by sand spits or dunes.
Bays and headlands:
On discordant coastlines, the retreating, less resistant rock and the exposed resistant rocks cause a change in the shape of the coastline. This leads to wave refraction.
This changes in the way in which waves approach the coastline can cause an increase in the rate of erosion on the headlands.
This leads to the formation of headland features like caves, arches, stumps and stacks.
Bays are formed when weaker rocks (e.g. shale and clays) are eroded.
How do headlands and bays affect incoming waves in different waves?
- Headlands: force the incoming waves to refract or bend - contracting their energy at the headlands. This increases the waves’ erosive power, which leads to a steeping of the cliffs and their eventual erosion into arches and stacks
- Bays: when waves enter a bay, their energy is dissipated and reduced. This leads to the disposition of sediment (sand or shingle) - forming a beach.
Why is vegetation important?
Supports the development of sandy coastlines and so plays an important role in the rate of coastal recession.
Sand dunes:
- Sand dunes need a continuous supply of sand, powerful winds to transport sand and obstacles to trap the sand, like plants.
- a typical sand dune transect goes from the mobile dunes nearest the sea (embryo, fore and yellow) to the fixed dunes nearest the backshore (grey dunes and dune slack)
Sand dunes - plants:
- Xerophytes, which can withstand periods of dry weather.
- Halophytes, plants that can withstand high concentrations of salt from seawater.
Vegetation succession in dunes:
- typical plants that colonise embryo and foredunes are sea rocket and sea crouch. These plants have deep roots and can tolerate high concentrations of salt.
- yellow dunes tend to be colonised by marram grass. These plants also have deep roots and are salt tolerant.
- heathers are commonly found in grey dunes because of the higher rates of humus.
What are the two types of waves?
- destructive
- constructive
Factors affecting waves:
- fetch: distance the wave has travelled
- duration of the wind: how long the wind has been blowing for
- speed of the wind: how fast the wind is blowing.
Destructive waves:
- also referred to as winter waves
- responsible for erosional processes
- happen at a high frequency (10-15 waves per minute)
- steep and hug, with a circular motion so waves break at a greater height. This causes the wave to ‘plunge’ a shorter distance along the beach.
- destructive waves remove material from coasts because the swash is less powerful than the backwash.
Constructive waves:
- happen at a low frequency (6-9 waves per minute)
- long and low
- over time, constructive waves will form gently sloping beaches
- constructive waves deposit material on coasts as the backwash is less powerful than the swash.
- they move in an elliptical motion, with waves breaking with little height. This results in the ‘spilling’ and breaking out further along the beach.
Swash and backwash definitions:
Swash: the movement of the wave up the beach - constructs the beach
Backwash: the movement of the wave back down the beach - destructs the beach
Beach profiles - summer:
- steeper in summer, when constructive waves are more common than destructive.
- constrictive are less frequent, so wave energy dissipates and deposits over a wide area.
- swash deposits larger material at the top of the beach - creating a berm.
- as berm builds up the backwash becomes weaker and only had enough energy to move smaller material so beach material becomes smaller towards the shoreline.
Beach profile - winter:
- destructive waves occur at a higher frequency.
- berms are eroded by plunging waves and high-energy swash.
- strong backwash transports sediment offshore (depositing it as offshore bars)
- sometimes backwash exerts a current known as a rip, or undertow - dragging sediment back as the next wave arrives over the top.
Processes of erosion:
- abrasion
- corrosion
- attrition
- hydraulic power
What is erosion?
Erosion is the wearing away of rock.
Several erosional processes happen at coastlines, with their ability to alter the shape of coasts influenced by the wave type, the coastline shape and its lithology.
What is abrasion?
- where pieces of rock are picked up by waves and hit against the bed, beach or cliffs. This wears them away over time.
What is corrosion?
Corrosion happens when there is a chemical reaction between the seawater (which contains a weak acid) ans susceptible rocks like limestone.
What is attrition?
Attrition is where pieces of bedload (material carried in the water) are hit against one another. This causes them to break apart and become smaller and more rounded.
What is hydraulic power?
- Hydraulic power causes the breakdown of cliffs due to the force of the water being compressed until the cracks of the rock.
- the repeated action of the water forces in and out of the cracks in the rock leads to the breakdown if the surrounding cliff.
Wave-cut platforms and cliffs:
Over time, cliffs are eroded by destructive waves, resulting in the marination of wave-cut platforms.
Wave cut platforms and cliffs process:
- when waves break against the foot of a cliff, erosion is concentrated near the high-tide line. This creates a wave cut notch which undercuts the cliff.
- as the wave-cut hitch gets bigger the rock above becomes unstable and eventually the upper part collapses.
- the repeated action of erosional processes means that the notch migrates inland and the cliff retreats - leaving a shoreline or wave-cut platform.
- they usually have a gentle slop of less than 4 degrees and are only exposed during low tide.
- they rarely extend further than a few hundred metres as their width means a wave will break and disspaite its energy before hitting the cliff. This reduces the rate of erosion and prevents further growth of the platform.
Erosion of headlands:
1) corrosion and hydraulic action - destructive waves can erode weaknesses in headlands by the processes of corrosion and hydraulic action.
2) when the weaknesses in the rock widen, abrasion becomes more important.
3) formation of coastal landforms - over time, these erosional processes lead to the formation of coastal landforms such as caves, arches, stacks and stumps.
Coastal landforms caused by headland erosion
- caves
- arches
- stacks
- stumps
What are caves?
- erosion attacks lines of weaknesses in the headland
- when the crack widens into a small hollowed out area, a cave has formed.
- when joints and faults are eroded by hydraulic action and abrasion, creating caves
What are arches?
- when two caves are joined up or a single cave is eroded right the way through the headland, an arch is created.
- e.g. Durdle Door on the Dorset Coast.
- the gap is then further enlarged by erosion and weathering - becoming wider at the base.
What are stacks?
When the top of an arch collapses because of gravity, a column called a stack is left behind.
What is longshore drift (LSD)?
Longshore drift transports transport material along coastlines when waves approach the beach at angle.
Longshore drift process:
- Wind approaches the coast at an angle because of a prevailing wind direction.
- waves are controlled by the wind and so this angle will be the direction the swash moves up the beach.
- gravity is the only force that acts on the backwash, so it falls back to the sea at right angles to the coastline.
- because of the difference between the angle of the swash and the angle of the backwash, sediment repeatedly moves in the shape of a right-angled triangle.
- the net effect of the movement of sediment up and down the beach is a lateral shift - and the process is known as longshore drift.
- over time, sediment is carried along the beach.
- where the removal of sediment is greater than the supply of new sediment, the beach is eroded.
Types of despositional landforms:
- spits
- bars
- tombolo
- cuspate forelands
- beaches
- barrier beaches
What are spits?
- when the coast changes direction at an estuary, longshore drift continues to move sediment across the inlet.
- the river doesn’t let the spit completely join to the coast on the other side because the river has the energy to move the sediment.
- spits are the long fingers of sand sticking out from one side in a coastline that have been curved by secondary winds. They often have salt marshes behind them.
What are bars?
- A bar is formed when a spit grows across a bay.
- Lagoons often form behind bars.
- they are also known as sandbars snd are submerged (or partly exposed) ridges of sand or coarse sediment - created by waves offshore from the coast.
destructive waves erode sand from the beach with their strong backwash and deposit it offshore in bars. - e.g. Moria Harlech, Wales
What are tombolos?
- tombolo: a beach (or ridge of sand and shingle) that has formed between a small island and the mainland.
- deposition occurs where waves lose their energy and the tombolo begins to build up.
- tombolos may be covered at high tides, e.g. at St Ninian’s in the Shetland Islands, and the Lindisfarne in Northumberland .
What are cuspate forelands?
- Cuspate forelands form when sediment is deposited across a bay caused by longshore drift transporting sediment in two directions.
- This leads to the formation of two spits which eventually meet and this then results in the trapping of sediment until eventually new land is formed.
- cuspate foreland - a triangular-shaped headland that extends out from the main coastline.
- occurs where a coast is exposed to LSD from opposite directions.
- sediment is deposited at the point where the two meet, which forms a natural triangular shape as it builds up.
- as vegetation begins to grow on the deposited sediment, it helps to stabilise the landform and protect it from storms that could erode it.
- cuspate forelands can be small - extending out from the coast for just a few metres or they can extend to up to 3 miles.
- e.g, Dungeness in Kent.
What are beaches?
- Beaches are made by constructive waves moving and depositing sand or shingle inland.
- Generally, a more gently sloping beach tends to be formed from sand, whereas a steeper sloping beach is formed from pebbles.
- beaches are commonly found in bays
- wave refraction creates a low-energy environment, which then leads to deposition.
- a beach could consist of either sand or shingle - depending on factors like the nature of the sediment and the power of waves.
- beaches can also be swash-aligned or drift-aligned.
- sediment may be graded asking a drift-aligned beach. Finer shingle particles are likely to be carried further by LSD and become increasingly rounded as they move.
What are barrier beaches?
- where a beach or spit extends across a bay to join two headlands, it forms a barrier beach or bar. For example, Loe Bar in Cornwall and Start Bay in Devon.
- barrier beaches and bard can also trap water behind them to form a lagoon, such as Slapton Ley, Devon.
- where a beach becomes separated from the mainland, it is referred to as a barrier island. They vary in scale and form - usually sand or shingle features - are common in areas with low tidal ranges, where the offshore coastline is gently sloping.
Large-scale barrier islands can be found along the Dutch coast, and in Nirth America along the south Texas coast.
Deposition definition:
Waves lose their energy and sediment is deposited (dropped off)
Methods of transportation:
- traction
- saltation
- suspension
- solution
Traction:
- sediment rolls along, pushed by waves and currents
- the sound of rolling pebbles and shingles can often be heard clearly at the beach.
- e.g. pebbles, cobbles and boulders
Saltation:
- sediment bounces along, either due to the force of water or wind.
- on a dry, windy days a layer of saltating sand is often seen 2-10cm along the beach surface.
- e.g. sand-sized particles
Suspension:
- sediment is carried in the water column
- on soft-rock coasts such as Holderness, the sea is often muddy brown in colour due to suspended sediment.
- e.g. silt and clay particles.
Solution:
- dissolved material is carried in the water as a solution.
- this type of sediment transport is of limited importance on coasts.
- e.g. chemical compounds in solution.
What are sediment cells?
- sediment moves along the coast in sediment cells.
- within each cell, the sediment moves between the beach, cliffs and sea through the process of erosion, transport and deposition.
- any action that takes place in one place has an impact elsewhere in the cell.
- each cell operates between physical barriers that prevent the sediment from moving any further along the coast (e.g. major headlands or river estuaries.
- the coastline of England and Wales is divided up into 11 major sediment cells.
The sediment cell as a system:
- sediment cells act as systems - with sources, transfers and sinks.
- larger sediment is not transferred between cells, but finer sediment in suspension out at sea (e.g. some of the finer boulder clay eroded from Holderness) can be be transferred.
- keywords: sediment budget, positive feedback, negative feedback, dynamic equilibrium.
Sediment cells - sources:
- The sources are subaerial processes, erosional processes (breaking down cliffs) and sediments brought to the coastline by rivers.
Sediment cells - transfers:
- Transfers are longshore drift, onshore and offshore winds and tides.
Sediment cells - sinks:
The sinks are depositional landforms (spits, bars, beaches and sand dunes).
Sediment cells - sediment budget definition:
- the amount of sediment available within a sediment cell.
- within each cell, depositional features build up which are in line (equilibrium) with the amount of sediment available.
Sediment cells - positive feedback definition:
- if the sediment budget falls, waves continue to transport sediment (erosion may therefore increase in some areas, because the sea has surplus energy). One change has led to another.
- enhances and speeds up processes, promoting rapid change.
Sediment cells - negative feedback definition:
- if the sediment budget increases, more deposition is likely.
- the sea corrects itself, because it can only carry so much - and any surplus is deposited.
- NF: the sea returns to a situation where it can handle the sediment supply.
- the regulation and reduction of a natural process.
Sediment cells - dynamic equilibrium definition:
- where landforms and processes are in a state of balance.
Weathering definition:
- the wearing away of rock in situ (where it is) by various processes.
Types of weathering:
- mechanical: when rocks are broken down without their chemical composition being changed.
- chemical: when rocks are broken down because of their chemical composition being changed.
- biological: when rocks are broken down by living things.
What are the processes of mechanical weathering?
- freeze-thaw
- salt crystallisation
- wetting and drying
What is freeze-thaw (mechanical weathering)?
- dominant in cold climates, such as the UK.
- occurs when water enters crack or joints when it rains. Thus water then freezes in the cold weather.
- when the water freezes, it expands in volume by around 10%. This puts pressure on the rocks which causes the cracks to widen.
- this repeated action causes material (scree) to break off and collect at the bottom of the cliff face.
What is salt-crystallisation? (mechanical weathering)
- when salt water evaporates, it leaves salt crystals behind.
- these can grow over time and exert stresses in the rock (just like ice does) which causes it to break up.
- ## salt can also corrode rock, particularly if it contains traces of iron.
What is wetting and drying? (mechanical weathering)
- frequency cycles of wetting and drying are common on the coast.
- rocks are rich in clay (e.g. shale) expand when they get wet and contract as they dry. This can cause them to crack and break up.
What are the processes of chemical weathering?
- carbonation
- hydrolysis
- oxidation
What is carbonation? (chemical weathering)
- the slow dissolution of limestone due to rainfall (weak carbonic acid pH5-6) producing calcium bicarbonate in solution.
- this affects limestone and other carbonate rocks.
What is hydrolysis? (chemical weathering)
- the breakdown of minerals to form new clay minerals, plus minerals in solution, due to the effect of water and dissolved CO2.
- igneous and metamorphic rocks containing feldspar and other silicate minerals.
What is oxidation? (chemical weathering)
- the addition of oxygen to minerals, especially iron compounds.
- this produces iron oxides and increases volume contributing to mechanical breakdown.
- Sandstones, silt stones and shales often contain iron compounds that can be oxidised.
What are the processes of biological weathering?
- plant roots
- rock boring
- animals
Biological weathering - plant roots:
- tree and plant roots grow into small cracks and fissures in the rock faces with cause the rocks to break apart.
- this happens more as the plants grow.
Biological weathering - rock boring:
- many species and molluscs bore into the rock face and may also secrète chemicals that dissolve rock.
- this can especially affect sedimentary rocks such as limestone in the inter-tidal zone.
Biological weathering - animals:
- different birds (e.g. puffins) and animals (e.g. rabbits) dig burrows into cliffs and cause them to break.
What is mass movement?
- Mass movement describes when rocks and loose material shift down slopes.
- This happens when gravity overcomes the force supporting the material.
- Mass movements can cause rapid coastal retreat and are common when the material is saturated.
- the downslope movement of materials under the force of gravity.
