Coasts Flashcards
Different ways of classifying a coast
EMERGENT/SUBMERGENT
- Emergent = When the coast is rising relative to sea level due to tectonic uplift or elastic rebound theory
- Submergent = These coastlines are being flooded by the sea, due to a rise in relative sea level on a global scale. it could also be a result of subsiding land on a local level.
Different ways of classifying a coast
PRIMARY/SECONDARY
- Primary = Formed by more land-driven rather than ocean-driven processes like plate tectonics, land erosion and sedimentation.
- Secondary = Formed by more ocean driven processes like wave erosion or growth of a coral reef.
Different ways of classifying a coast
TIDAL RANGE
- Microtidal = Tidal range does not exceed 2m
- Mesotidal = Tidal range is between 2-4m
- Macrotidal = Tidal range exceeds 4m
what is geological structure?
Geological structure = the arrangement of rocks in three dimensions - Strata which is the different layers of rock and how they relate to one another. Deformation which is the degree to which rock units have been deformed by tectonic activity and faulting which is the presence of major fractures that have moved rocks from their original position.
Explain how geological structure influences the coastal landscape.
POINT 1
Firstly, geological structure influences coastal type. On the one hand, concordant, or Pacific coasts are created when rock strata runs parallel to the coastline or alternatively discordant or Atlantic coasts, form when different rock strata intersect the coast at an angle, so geology varies.Concordant = Lulworth Cove, The hard Portland limestone and fairly resistant Purbeck beds protect much softer rock landward. At Lulworth Cove, marine erosion has broken through the resistant beds and rapidly eroded forming caves
On the other hand, discordant coastlines are dominated by headlands and bays. Less-resistant rocks are eroded to form bays whereas more resistant geology remains as headlands. Furthermore, geological structure influences erosion rates. On a discordant coastline, alternating layers of hard and soft rock are perpendicular to the coast. Because the soft rock is exposed, it is eroded faster than the hard rock. This differential erosion creates headlands and bays along discordant coastlines whereas Concordant coasts have alternating layers of hard and soft rock that run parallel to the coast. The hard rock acts as a protective barrier to the softer rock behind it preventing erosion. If the hard rock is breached though, the softer rock is exposed and a cove can form.
Explain how geological structure influences the coastal landscape.
POINT 2
Secondly, geological structure influences cliff profiles. Cliff profiles are influenced by two different aspects of geology. The resistance erosion of the rock and the dip of rock strata in relation to the coastline. Dip, meaning the angle of rock strata in relation to the horizontal is important. Sedimentary rocks are formed in horizontal layers but can be tilted by plate tectonic forces. In particular, this is significant when the profile is more vertical such as with ‘horizontal dip’ which is a vertical or near vertical profile with notches reflecting strata that are more easily eroded. In addition to dip, a ‘fault’ represents a major weakness within rock layers, either side of a fault rocks are easily eroded by marine erosion. Furthermore, fissures are much smaller cracks exploited by erosion which along with faults can influence structure leading to wave-cut notches or jointing of rocks caused by fissures.
Explain how lithology (rock type) can influence rates of erosion at the coast.
point 1 rock type
- Hard rocks e.g. Gabbro are resistant to weathering & erosion so a coastline made of granite (e.g., Land’s End) will change slowly.
- Soft rocks (e.g Limestone) are more susceptible to weathering & erosion so a coastline made of chalk (e.g. Dorset) will change relatively quickly.
Explain how lithology (rock type) can influence rates of erosion at the coast.
point 2 type of coastline
DISCORDANT/CONCORDANT
- Concordant coasts have alternating layers of hard and soft rock that run parallel to the coast. The hard rock acts as a protective barrier to the softer rock behind it preventing erosion. If the hard rock is breached though, the softer rock is exposed and a cove can form (e.g. Lulworth Cove).
- On a discordant coastline, alternating layers of hard and soft rock are perpendicular to the coast. Because the soft rock is exposed, it is eroded faster than the hard rock. This differential erosion creates headlands and bays along discordant coastlines.
How can the permeability of rocks influence the stability of cliffs? (probably 6)
Permeability is the amount of water a rock allows to flow through it. Permeable rocks allow water to flow through them, and include many sandstones and limestones. Impermeable rocks do not allow groundwater flow and include clays, mudstones and most igneous and metamorphic rocks. Permeability is important because groundwater flow through rock layers can weaken rocks by removing the cement that binds sediment in the rock together. It can also create high pore water pressure within cliffs, which affects their stability. Water emerging from below ground on to a cliff face at a spring can run down the cliff face and cause surface runoff erosion, weakening the cliff. Differences in the erosion resistance of rock including unconsolidated material on the cliff top causes slumping. Furthermore, resistant rock creates a ‘bench’ feature at the cliff base.
Outline the value of ecosystems, such as salt marshes and sand dunes, that are found on some coastlines.
POINT 1 - STABILISING PLANTS
Vegetation protects coastlines from erosion of unconsolidated sediment by stabilising the influence of plants. Firstly, dune successional development which occurs at sandy coastlines stabilises sediment by binding the roots of plants together making them harder to erode. When submerged, plants growing in sediment provide a protective layer so the surface of the sediment is not directly exposed to moving water and therefore erosion. Finally, plants protect sediment from wind erosion by reducing wind speed at the surface due to friction with the vegetation. Embryo dunes stabilise the mobile sand with their root system by reducing wind speeds at the sand surface, allowing more sand to be deposited. This includes adding dead organic matter to the sand, beginning the process of soil formation. Furthermore, embryo dunes alter the environmental conditions from harsh salty sand to an environment that other plants can tolerate. New plant species therefore colonise the embryo dunes creating a foredune.
Outline the value of ecosystems, such as salt marshes and sand dunes, that are found on some coastlines.
POINT 2 SALT MARSH SUCCESSION
Secondly, salt marsh succession occurs in estuarine areas. This begins in the ‘algal’ stage where salt marshes occur on and within bare mud, binding it together. Next, roots stabilising the mud in the pioneer stage being stabilis
ed in the establishment stage with salt marsh height increases. With height increases occurring due to sediment being trapped.
Outline the factors that influence the size of breaking waves at the coast.
WIND SPEED
The harder and faster the winds blow the bigger the wave. Normal winds rarely produce waves bigger than 15 m.
WIND DURATION
FETCH
This is why the UK’s largest waves are generally experienced in Cornwall as the fetch is over 4000 km from Florida.
DEPTH OF WATER
If water depth near the shore is shallow this means that waves will break further from their shore and limiting their size. However, if depth of water is more near the coast this will mean less waves break and size is increased.
DIRECTION AND SPEED OF TIDE
Describe how waves change as they approach a coastline
Waves are affected by water depth. At a water depth of about half the wavelength, the internal orbital motion of the water touches the seabed. This creates friction and slows down the wave. As waves approach a shoreline the water shallows and the shape of waves changes, wavelength decreases and wave height increases, so waves ‘bunch’ together.
Waves break in shallow water because the crest of the wave begins to move forward much faster than the wave trough. The trough experiences significant friction with the sediment and rock of the shore. Eventually, the wave crest outruns the trough and the wave topples forward - a breaker.
Briefly explain how constructive and destructive waves influence beach morphology.
- Beach morphology = Characteristics and make up of beach.
- Constructive waves have a long period, a long wavelength and a low amplitude. When they break on a beach, they have a strong swash and a weak backwash. This means they deposit more material on a beach than they remove. Over time, they build up gentle beaches. The repeated action of pushing material up a beach eventually leads to the development of berms.
- Destructive waves have a short period, short wavelength and a high amplitude. They tend to be steep and form during storms. When they break on a beach, they have a weak swash but powerful backwash. As a result, they remove material from a beach and produce a steep beach with breakpoint bars. In particularly stormy weather, destructive waves can be powerful enough to throw material to the back of a beach producing a storm beach—a ridge of coarse material.
How does the presence of structural features, such as faults, influence coastal erosional landforms?
If structural features (faults,fissures and joints) are more common in a coast this will mean it is more open to erosion with the weaknesses in the rock in particular by hydraulic action which air gaps trapped in rocks compressed by waves crashing against the cliff face.
The location of microfeatures found within cliffs, such as caves and wave-cut notches, are often controlled by the location of faults as is the case at the Northumberland heritage coast which had a large concentration of faults at the bottom of the cliff face which has been removed to expose hard layer of limestone. This also affects the cliff profile as soft rock at the bottom of a cliff face which can easily be eroded creates a horizontal dip.
Using examples, discuss the importance of mass movements and weathering in influencing the stability of cliffs.
- Weathering is the in situ breakdown of rocks by chemical, mechanical or biological agents. No movement involved. Affects all coastlines. Weathering weakens cliffs by opening up cracks and fissures and causing erosion
- e.g mechanical weathering breaks down rocks by the exertion of a physical force and doesn’t involve any chemical change. Freeze-thaw, water expands by 9% in volume, which exerts a force in cracks and fissures. Repeated cycles force cracks to open and loosen rock
- Mass movement is the downslope movement of rock and soil. There are 5 types, a rock fall, toople, translational slide, rotational slide, and flow. Mass movement makes the cliff unstable.
- e.g. a topple is caused when the cliff has seaward dip and is undercut by erosion. Eventually the undercut will be too great and the rock will fall causing the cliff to recede.
Outline the difference in the meaning of the terms ‘isostatic’ and ‘eustatic’.
- Eustatic is a global sea level change
- Isostatic is a local sea level change
- Eustatic changes due to change in amount of volume of water in sea
- Isostatic changes due to increase/decrease in height of land
Use examples of named landforms to explain the meaning of the term ‘emergent coast’.
- Emergent coastlines are ones that have risen or emerged from the sea. This, as we have seen, may be due to eustatic change – a fall in sea level, or isostatic change - a rise in the level of the land.
- Example is Gruinard Bay, West Scotland
- A clear ledge in the way, called a raised beach
- Towards the back of the raised beach there is a sharp rise in the level of the land that marks the position of cliffs that were once on the coast, now known as relic cliffs
- Sometimes caves, arches, stacks and stumps are found alongside the relic cliffs.
Suggest why coastlines in the north of the UK are usually emergent, whereas in the south they are submergent.
At the end of the Ice Age the loss of weight led to the British Isles tilting the other way – upwards in the north and west, but downwards in the south and east. Coastlines in Scotland rose above sea level, while coastlines in the south of England became submerged. The process of changes in the level of the land is called isostasy. These movements take place very slowly and the south of England is still sinking today as a result of the loss of ice and its weight at the end of the Ice Age.
