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