8.2 Characteristics and formation of coastal landforms Flashcards
Factors that affect cliff profiles
Bedding and jointing
Rock type
Dip of bedding planes
Composite rock
Relative position of weaker rock
Permeability of rock
Latitude affecting climate
Influence of bedding and jointing in influencing cliff profiles
Well-developed jointing and bedding of certain harder limestones creates geometric cliff profiles with steep, angular faces and a flat top bedding plane. Wave erosion at the base opens up these lines of weakness to create wave-cut notches and whole blocks of rock fall away to create angular overhangs and cave shapes. If cliffs are formed in tectonically active areas, faulting may have occurred which creates areas of weakness to be exploited by weathering and erosion
If at a headland, refraction will focus on the sides of the headland and areas of weakness to create geos, caves, arches, stacks and stumps
Cliff rock types erosion rates
Granitic = <0.001m/yr
Limestone = 0.01-0.1m/yr
Shale = 0.01-0.1m/yr
Chalk = 0.1-1m/yr
Glacial till = 0.1-10m/yr
Volcanc deposits = 10m/yr
Influence of dip of bedding planes in affecting cliff profiles
If bedding planes dip vertically then a sheer cliff is formed.
If bedding planes dip steeply seaward, steep, shelving cliffs with landslips result.
If bedding planes dip landward, sliding is unlikely.
Cliffs where the strata and planes dip seaward present the most challenges in terms of management as they are more unstable than those that dip landwards
Influence of composite rocks in affecting cliff profiles
Many cliffs are composed of more than one rock type
The exact shape and form of the cliff depends on factors such as strength (granite – relative slow retreat whilst glacial till cliffs will be rapid) and structure of rock, relative hardness and nature of waves.
Influence of relative position of the weaker rock in affecting cliff profiles
If the weaker rock is at the base of the cliff underlying more resistant rock: undercutting and cliff collapse will occur.
If the weaker rock is at the top – subaerial processes will operate from above whilst waves will also attack the base.
If rock type is uniform – more uniform retreat with sheer cliff faces if bedding planes are vertical.
Influence of permeability of rock in affecting cliff profiles
If impermeable rock overlies permeable rock – limited percolation and cliff is more stable
If permeable overlies impermeable – water passes through into the underlying rock and slope failure is more likely where the water builds up at the junction between the two rock types and sub-aerial processes maybe more important than wave erosion.
Influence of latitude affecting climate in affecting cliff profiles
Tropics: low wave energy levels and high rates of chemical weathering as climate warm and moist so cliffs are low gradient.
High Latitudes: past climate periglacial processes have produced large amounts of cliff-base materials which have deposited at the base of cliffs which creates relatively low gradient cliff profiles
Temperate climates of the Mid Latitudes: tend to have the steepest cliffs. Rapid removal of debris by high-energy waves prevents the build-up of material on the base whilst active cliff development occurs as a result of undercutting.
Bevelled cliffs
Formed at a number of stages (pre-glacial, glacial and post glacial)
1. Cliff formed due to marine processes during warm (inter-glacial) period when sea levels were higher than they were today
2. During glacial periods (colder), sea levels dropped as water locked on land as ice – periglacial processes removed the edge of cliff creating a bevelled edge whilst solifluction carries the material to the bottom of the slope and creates a sloping cliff profile
3. Post glaciation, sea levels rose again and marine action removed the soliflucted glacial deposits from the base of the cliff, steepening the cliffs at the base whilst leaving the bevelled cliff face above and a lower angle.
Bevelled cliff diagram
Wave cut platforms
These are remnants of the previous cliff line. They form as a ledge of bedrock left behind as the cliff retreats. The platform slopes at at 4-5 degree angle down to the sea. Traditionally it was thought that these form as waves erode the base of the cliff in the inter-tidal zone – hence the name. However, there is some controversy over the importance of other agents of weathering and erosion.
Some geomorphologists argue these are relict or ancient features originally cut long ago when sea level was more constant. They think that in post-glacial times, sea level has not remained sufficiently constant to erode such platforms and that they are today simply modifying these platforms slightly.
In addition to being modified by wave action, they are also being affected by weathering processes. In high latitudes, following glaciation, the land is rising (isostatic change) so arguably marine erosion is having less effect and the platforms are now equally weathered as a result of frost action as they are eroded by marine processes. In other areas, solution weathering through salt crystallisation and slaking may also support marine erosion especially in the tidal zone. Marine organisms (notably algae) may also weather the platform – they accelerate the weathering process at low tide. At night algae releases CO2 which combines with cool sea water to create an acidic environment causing ‘rotting’. Other organisms like limpets secret organic acids that slowly dissolve the rock and molluscs and sea urchins can actually ‘bore’ holes into rock surfaces especially chalk and limestone.
Wave cut platform anomaly/case study
In the case study of Kaikoura peninsula in NZ, the wavecut platform is now more modified by subaerial processes than wave action – waves are breaking early despite being high energy as they travel over the wavecut platform and orbits become more elliptical due to friction until they break (when wave reaches height to length ratio of 1:7 and usually happens in water that is 1.3 x the height of the wave)
Wave cut platforms diagram
Describe the marine erosion processes of hydraulic action, wave quarrying and corrasion/abrasion. Explain the factors that make these processes effective in the development of cliffs (10)
(mark scheme)
Hydraulic action is wave pounding, although often confused with quarrying; most effective when storm waves break against the base of a cliff releasing great energy (up to 30 tonnes/sq.m). Wave quarrying is the compression of air within cracks in rocks by wave impact followed by the sudden decompression as the water recedes. This creates an explosive effect which can in time open up cracks. Corrasion/abrasion is where breaking waves use available sediments such as pebbles and cobbles to grind, wear away, i.e. abrade or corrade, the cliff base. The factors are the resistance of rocks such as granite or less resistance of incoherent rocks such as weakly cemented sandstones or clays. Also important will be the degree of jointing or faulting making quarrying effective.
Explain how rock type and structure influence the development of different cliff profiles (cross sections)
(mark scheme)
Three different cliff profiles could be:
* vertical profiles are characterised by horizontal rock strata where marine erosion and removal of material keep pace with sub-aerial weathering/mass movements;
* seaward dipping profiles often reflect seaward dipping rock strata or cliff decline where marine erosion has ceased and sub-aerial processes have a greater influence;
* slope over wall profiles with less resistant strata overlying resistant strata or where sea level rise has allowed marine processes to erode a more gently sloping previous profile. However, other profiles could be a blocky profile, such as in well jointed granite; stepped profiles or profiles shaped by rotational slumping might also be considered.
Explain the importance of rock type, structure and erosional history on the evolution of cliff profiles.
(mark scheme)
Cliff profiles are influenced by a range of factors. Rock types, such as limestone and granite, and structure (jointing and bedding) control the cliff profile. Well-developed jointing and bedding can create steep, angular cliff faces with flat tops. Some cliffs are formed of a mixture of rock types – the exact shape of the profile being dependant on strength and structure of rock, relative hardness and the nature of the waves. Lines of weakness are opened up by erosion and complete blocks fall away leaving overhangs and caves. The dip of the bedding planes will create different cliff profiles – bedding dipping steeply seaward forms shelving cliffs with landslips. Beds dipping vertically create a sheer cliff face. Beds dipping landward tends to result in steep, possibly jagged, profiles with rockfalls. Erosional history may involve discussion of sub-aerial processes and changes in sea level. Cliff retreat will be slower in resistant rocks such as granite, and faster in glacial till.
Type of rock affecting cliff profile - resistant rock e.g. granite
Produces a vertical or very steep cliff face. Rock falls main type of mass movement. Cliff retreat is slow and cliffs are high
Type of rock affecting cliff profile - Less resistant or unconsolidated rock eg glacial till, sand and clay
Produces more gently sloping, stepped cliff profile. Rotational slumping is main type of mass movement with mudflows developing at base of cliff. Retreat is rapid and cliffs will be lower
How does Rock structure (including bedding/jointing and dip) affect susceptibility to erosion and weathering and mass movement
Rock types, such as limestone and granite, and rock structure (jointing and bedding) control the rates of erosion and hence shape the cliff profile. Well-jointed or faulted rocks are more susceptible to erosion – especially which types of erosion?
Dip of the bedding plane
Dip is the angle of inclination of the rock strata from the horizontal. It’s a tectonic feature from when the rocks were formed. Sedimentary rocks are deposited horizontally, but can be tilted by folding and faulting by tectonic forces. It can have dramatic effects on cliff profiles.
Effect of a low angel of seaward dip (<45*)
It produces a steep profile, that may even exceed 90⁰, creating areas of overhanging rock; very vulnerable to rock falls. Frequent small-scale mass movement of material weathered from cliff face. Major cliff collapse when undercutting by marine erosion makes overhang unsustainable
Effect of a landward dipping strata
Produces steep profiles on 70-80’ as downslope gravitational force pulls loosened blocks into place. Very stable profile with small rock falls. Produces very jagged and uneven profile but retreat by erosion is slow
Effect of bedding planes dipping steeply seaward
Cliff decline is occurring where marine erosion has ceased and sub-aerial processes have a greater influence as slides of large slabs occur relatively easily
Effect of beds dipping vertically or horizontally
The creation of a sheer vertical cliff face – here marine erosion keeps pace with subaerial processes of weathering and mass-movement. Geometric, angular and flat topped cliffs here
Bays and cliffs at different types of coastlines
Coastlines, that are discordant in geology help create typical headland and bay features that also erode over time to form distinctive erosional features like wave-cut platforms and arches and stacks.
Bays are sheltered, low energy zones that form in bands of weak geology, e.g. clays. Here the cliff erodes at a faster rate. Bays are flanked by headlands which are exposed rocky outcrops positioned at 90 perpendicular to the bay. They consist of more resistant rock, e.g. limestone. Due to the way waves refract around headlands, destructive waves concentrate their energy on their sides and over time develop unique coastal features, such as caves, arches and stacks.
At a discordant coastline headlands and bays may be formed because the cliff is subject to differential rates of erosion, due to bands of varying resistant geology. Wave refraction is the process by which waves become distorted by differentiated rates of friction caused by shallower water ahead of coastal features. In deep water waves are unaffected but in shallow water waves slow down. On approaching the shoreline, wave orthogonals will bend and concentrate around the sides of headlands; waves approaching headlands slow down and build height creating destructive waves. The waves become refracted around the headland and so wave energy becomes concentrated on the sides of the headland. In the bays, the waves run parallel with the coastline and spread out as they are slower and sheltered by headlands. This reduces drift and allows bays to build up beaches
Using a labelled diagram, describe the landforms shown in Photograph A and explain how they developed (10)
With the aid of a labelled diagram, describe the main physical features of the landscape shown in Fig. 4.1 (4)
Suggest how geology has influenced the landscape shown in Fig. 4.1 (6)
What processes are responsible for creating wave cut notches, cliffs and wave-cut platforms?
Cliffs overview + stat
Cliffs occur along approximately 80% of the world’s coastline.
There is a huge variety of cliff profiles and associated landforms due to the different factors and controls acting upon them, such as sea level, history, geology, climate, waves and tides. All rocky coasts are eroding coasts, and cliff erosion is the process by which they attempt to reach equilibrium with the dynamic driving conditions of waves, wind, tides and currents.
Caves, arches and stacks formation
Cracks can sometimes form in resistant rocks which steadily erode to become a cave.
When prominent bays and headlands are created waves begin to refract around the headlands, concentrating most of the energy on the sides.
This high concentration of energy further erodes cracks and caves. It is likely that a second cave may form on the opposite side of the headland.
The water erodes the caves simultaneously mainly through hydraulic action, until they eventually meet.
The resulting iconic land form is then considered an arch.
The roof of the arch has no support and is highly susceptible to weathering including salt crystallisation and biological weathering.
As the weathering continues, the arch will eventually collapse under its own weight. This leaves a stack, a tall, lone, column of rock sticking out in the sea.
This stack is now exposed to the full force of the sea, coming under heavy erosion. The stack will eventually collapse to form a stump.
Blow holes formation
When a cave is formed and erosion continues, the roof of the cave will become weakened. As the waves crash into the cave, they can be reflected upwards further eroding the top of the cave.
At the same time, weathering of the cliff, for example through carbonation, can weaken the rock above until eventually a hole appears.
As the waves continue to crash into the cave and are reflected upwards, water rushes through the hole and creates a blowhole.
Wave cut platform diagram
Wave cut notches and wave cut platforms formation
A wave-cut notch is formed at the base of the cliff simply where the majority of the waves force is concentrated.
As the waves erode the base of the cliff, an indentation at the base of the cliff is formed. This is a wave-cut notch.
As the notch enlarges, the cliff face becomes undermined until it can no longer hold its own weight and collapses.
Processes of attrition and transportation then clear the fallen debris washing it out to sea, leaving a small bedrock ledge.
This process is repeated many times leading to the formation of a wave-cut platform. This action of falling rock to create the platform is known as cliff-retreat.
Wave-cut platforms are characterised by gently sloping angles, hard bedrock and rock pools.
Over time as the platform grows the waves have further to travel to reach the cliff, by which time they will have lost the majority of their energy. The waves will no longer be able to undercut the cliff.
Caves, stacks, stumps etc diagram
Sediment input from rivers
Rivers transport a variety of materials - usually fine-grained silts and clays, but also larger particles of sand and gravel; this is known as alluvium.
Sediment deposited by rivers at the coastline may be intermittent, mostly occurring during floods when high rates of erosion have occurred along the river bed and banks.
Responsible for 70% of sediment at the coastline but in some locations may be as high as 90% This sediment may help shape the coastline since when deposited in low energy environments, it may result in formation of salt marshes and deltas.
Rivers erode the upland areas and bring sediment to the coast, either at estuaries or deltas where the sediment gets deposited or becomes entrained in suspension to be then moved by waves or currents.
Again, the nature of the sediment will reflect the local lithology and geography; in mountain environments in Scotland and Wales, coarse sediments are brought down by steep gradient rivers with more energy whereas in lowland Britain, rivers carry mostly clay and silt Sandy beaches of Caribbean are largely river sediment that has been re- worked by waves.
Example of sediment input from glacial deposits
GLACIAL DEPOSITS – shingle beaches of southern Britain are made of shingle derived from glacial and periglacial processes that happened 10000 years ago – showing importance of climate change and se level rise since this has caused these deposits to be rolled onshore
Sediment input from cliffs
Usually generate coarse sand and shingle (small rounded pebbles) which is produced as waves and longshore currents rework these through attrition and abrasion. These are common in temperate latitudes made up of quartz sand grains. This source from eroding cliffs is very important for maintaining beaches downdrift of the eroding cliff sections
Influence of subaerial processes on cliffs at higher latitudes
Physical weathering here may well produce coarser rock fragments and gravel eg frost action + salt weathering
Input of sediment from the sea
Huge volumes of sand and clay were deposited here in the ice age and may be brought onshore by waves and tides – in post-glacial times, offshore gravel deposits have been brought onshore creating significant build up at Chesil Beach in Dorset and Blakeney Point in Norfolk. This source is now considered exhausted but offshore sandbanks still provide a source of sediment that is brought by waves onshore
Accretion
The accumulation of sediment, deposited by fluid flow processes
Input of sediment from wind
This aeolian material is typically fine sand, as wind has less energy than water and so cannot transport very large particles
Aeolian
Relating to wind action
Inputs of sediment relating to humans
In recent decades, large-scale human intervention (including beach nourishment and large-scale coastal defences) has disrupted natural systems and affected sediment supply to the coast. This is why both spatially and temporally, it is important to calculate the sediment budget for any part of the coast as the amount and transfer of sediment is critical in coastal management decisions.
Flocculation importance specifics
In some cases, sediment will flocculate (eg clay which is cohesive in structure) and so become heavier and fall out of deposition – it is especially important in the formation of estuaries, salt marshes where only a small drop in velocity will lead to the sediment falling out of suspension and getting deposited
Describe the sources of coastal sediments and explain how waves transport and deposit sediments (10)
Describe the characteristics of breaking waves and explain the processes by which waves transport sediment on beaches (10)
Describe the nature of coastal sediment cells. How do processes of sediment movement contribute to the formation of coastal landforms? (10)
Describe the changes to the sediment cell shown in Fig 1B. (4)
Candidates should interpret Fig. 1B to identify the changes that have occurred. The main changes are:
* no input from cliff erosion
* no input of river sediment
* addition of groynes * construction of sea wall
* reduced input from offshore bar
* sand dunes starting to erode.
* 1 mark for each valid point.
Explain how the changes you have identified in (a) have affected the operation of the sediment cell shown in Fig. 1A (6)
- Candidates require an understanding of the nature of a sediment cell and how it operates as a coastal system.
- The following could be identified:
- reduction in input of river sediment will make the marine erosive processes more active and may lead to erosion of the coast in a downdrift direction
- the groynes will help to build up the beach and protect the town but may lead to erosion in the sediment starved areas downdrift especially the spit
- the sea wall will protect the town but will also starve downdrift areas of sediment
- there is reduction of input from offshore bars which will also make the marine erosional processes more active and beaches may be depleted.
- Level 3 5–6
- Response applies knowledge and understanding of the operation of sediment cells and convincingly explains both the major processes at work and the effect they have on the sediment cell. Response is well-founded in detailed knowledge and strong conceptual understanding of the topic. Any examples used are appropriate and integrated effectively into the response.
With the aid of diagrams, describe and explain the formation of beach profiles (10)
Explain how the interaction of winds, different types of breaking waves and longshore drift can lead to the development of spits and dunes along a stretch of coast (10)
Fig 2 shows the operation of some marine processes along a coastal area. Describe the processes shown in fig 2 and explain how these processes contribute to the formation of coastal landforms (10)
Explain how sedimentation and salt marshes develop in tidal estuaries along a depositional coast (10)
Describe the sources of coastal sediment and explain how sediment is transported along coasts (10)
Beaches in summer vs winter diagram
Explain how differences in wave enregy affect the cross section of beaches (6)
With the aid of diagrams, describe the characteristics and explain the formation of offshore bars, barrier beaches and barrier islands (10)
Explain how the profile and plan form of beaches are affected by the action of constructive and destructive waves (10)
Compare the characteristics of constructive and destructive waves. Explain the effects of waves on the development of beach profiles (10)
With the aid of diagrams, describe the characteristics and explain the formation of offshore bars, spits and tombolos (10)
Describe the characteristics of breaking waves and explain the processes by which waves transport sediment on beaches (10)
Describe the characteristics of the spit shown in Fig 4.1. (4)
Suggest how the spit shown in Fig. 4.1 has formed
.
Describe the characteristics from A to B shown in Fig. 4.1. [4]
Explain how embryo dunes are formed and develop into fixed dunes. [6]
Compare the global pattern of coastal dunes and coastal saltmarshes shown in Fig. 4.1.
Dune evolution diagram + facts
