Assess the extent to which rates of coastal recession and stability depend on lithology PDF

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Imogen Goode

‘Assess the extent to which rates of coastal recession and stability depend on lithology.’ (12) There are numerous ways in which rates of coastal recession and stability depends on lithology, however other factors can affect the rates too. Firstly, rock type affects the rate of coastal recession and stability, there are three main rock types which are Igneous, Metamorphic and Sedimentary. Igneous rocks such as Granite and Basalt will erode very slowly due to the crystalline structure, the interlocking crystals create a hard and strong rock, they can also have few joints, so there are limited weaknesses that erosion can exploit. Metamorphic rocks such as Slate and Marble have a slow erosion rate as they are also crystalline, they also exhibit a feature called foliation, where crystals are all orientated in one direction and he rock is usually folded and heavily fractured which allows the rock to be exploited to coastal recession. However, Sedimentary rocks such as Limestone and Sandstone erode much faster due to the rock being clastic, rocks with many bedding places and fractures (e.g. Shale) are easily exploited to erosion. So, the type of rock has an impact on the rate at which it will cause coastal recession and its stability. Furthermore, the weakness of the rock can influence the rates of coastal recession and stability. Permeable rocks allow water to flow through them, and include many sandstones and limestones, however impermeable rocks do not allow groundwater flow and include clays, mudstones and most igneous and metamorphic rocks. Permeable rocks will erode much quicker as groundwater flow through rock layers can weaken rocks by removing the cement that binds sediment in the rock. It can also create high pore water pressure within cliffs, this affects their stability. Water emerging from below the cliff face at a spring can cause surface runoff erosion. This will allow the rate of cliff recession to be faster and weaken their stability. In addition, weathering can also affect the rate of coastal recession and stability. Mechanical weathering breaks down rocks due to the exertion of a physical force, it can proceed with freeze-thaw and salt crystallisation. Any rocks with cracks and fissures or high up cliffs away from salt spray are vulnerable. Chemical weathering involves a chemical reaction and the generation of new chemical compounds such as carbonation, hydrolysis and oxidation, Limestone and other carbonate rocks are vulnerable, so are Igneous rocks and Metamorphic rocks that contain feldspar and other silicate minerals. Sandstone, siltstone and shales also often contain compounds that can be oxidised. Finally, there is biological weathering, this often speeds up mechanical or chemical weathering through the action of plants, bacteria or animals by plant roots or rock boring. This can produce rock falls off cliff tops where there is vegetation and on sedimentary rocks. So, weathering can affect different rock types and allow rates of erosion to be affected. However, wave processes can affect the rates of coastal recession and stability too. Most erosion will occur when waves are at their largest (influenced by wind, speed and fetch meaning it has enough energy to

Imogen Goode

pick up sediment and it can hit the cliff face), waves approaching at a 90degree angle and when tide is high (propelling waves higher up the cliff face), heavy rainfall and if debris from previous erosion has had time to be removed from the cliff foot. There are four processes of erosion: hydraulic action (affects heavily jointed/fissured sedimentary rocks), abrasion (affects softer sedimentary rocks), attrition (affects softer rocks) and solution (affects mainly limestone). So, depending on the wave, this can affect the rates of coastal recession and stability along with lithology. In conclusion, lithology plays a large part in the rate at which coastal recession happens and the stability, however other factors such as characteristics of the waves at the coast can also be responsible for the recession and stability....