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CIE A-Level Geography Notes

8.1.2 Marine Erosion

Hydraulic Action and Cavitation

Mechanisms

  • Hydraulic Action: This phenomenon occurs when waves crash against the coastline, exerting immense pressure. The force of the water compresses air in rock cracks, causing stress on the rock structure. Repeated compression and release of pressure eventually lead to the fragmentation of the rock.
  • Cavitation: As waves pull back, the compressed air in rock fissures rapidly expands, creating an explosive effect. This process can violently eject rock fragments, further contributing to coastal erosion.

Effects on Coastal Landscapes

  • Formation of Coastal Features: Hydraulic action and cavitation are instrumental in forming distinctive coastal features like sea caves, which may evolve into arches and stacks over time.
  • Cliff Retreat: These processes are central to the gradual but relentless retreat of cliff faces, reshaping coastal margins.
  • Altering Shoreline Profiles: The continuous impact of hydraulic action modifies shoreline profiles, resulting in diverse coastal topographies.

Corrasion/Abrasion

Mechanism

  • Sediment as Erosive Tools: When waves laden with sediment (ranging from fine sand to larger pebbles) strike the coast, they abrade the rock surface. This process, akin to sandpapering, is known as corrasion or abrasion.
  • Intensity Variations: The intensity of corrasion depends on factors such as wave energy, the hardness of the rock, and the size and composition of the sediment.

Impact on Coastal Landforms

  • Smoothing of Surfaces: Abrasion smoothens and levels rock surfaces, contributing significantly to the formation of features like wave-cut platforms.
  • Sculpting of Features: The abrasive action of sediment shapes various coastal features, including the undercutting of cliffs and the sculpting of unique rock formations.

Solution

Chemical Processes

  • Dissolution of Rocks: This process involves the chemical interaction between water and rock. Rainwater, often acidic due to dissolved carbon dioxide, reacts with minerals in the rock, leading to their dissolution.
  • Carbonation: A specific chemical reaction where carbon dioxide in seawater forms carbonic acid, which dissolves calcium carbonate in rocks like limestone and chalk.

Effects on Coastal Rock

  • Weakening of Structures: Chemical processes like solution lead to the gradual weakening of rock structures, making them more susceptible to other forms of erosion.
  • Creation of Unique Landforms: Features such as limestone pavements, karst landscapes, and sea caves often result from solution processes.

Attrition

Breakdown of Sediment

  • Collision of Particles: Attrition occurs as sediment particles collide with each other and the coastline. These collisions cause the particles to break down into smaller, smoother fragments.
  • Gradual Process: The process of attrition is gradual and continuous, contributing to the long-term evolution of coastal sediment composition.

Role in Coastal Environments

  • Reduction in Sediment Size: Attrition results in finer, more uniform sediment along coastlines, influencing the texture of beaches.
  • Beach Formation: The breakdown of larger rocks into smaller grains through attrition is a key factor in the formation of sandy beaches.
A diagram showing types of erosion.

Image courtesy of teleskola.mt

FAQ

Human activities can significantly influence the rate of attrition on coastal beaches. Coastal engineering structures like groynes, breakwaters, and sea walls alter wave patterns and sediment transport, which can affect the natural process of attrition. For example, groynes may trap sediment on one side, reducing the amount of material available for attrition on the other side. This can lead to beaches with coarser, less eroded sediment. Additionally, activities such as dredging and aggregate extraction remove sediment from the coastal system, potentially reducing the material available for natural attrition processes. Pollution, particularly from oil spills and industrial waste, can also affect the physical properties of sediment, influencing the rate and nature of attrition. Furthermore, climate change-induced factors like rising sea levels and increased storm frequency can alter wave dynamics, potentially intensifying attrition processes. Therefore, human interventions and environmental changes play a significant role in shaping the attrition dynamics on coastal beaches.

The process of solution varies significantly with different rock types due to their varying mineral compositions and solubility. Rocks like limestone and chalk, which are primarily composed of calcium carbonate, are highly susceptible to solution. The carbonic acid in seawater reacts with calcium carbonate, leading to the dissolution of these rocks. This reaction is more pronounced in areas with acidic water, often influenced by environmental factors such as pollution or organic decay. Conversely, rocks like granite and basalt show much less susceptibility to solution due to their composition of less soluble minerals. These variations in rock solubility play a crucial role in shaping different coastal landscapes. For instance, limestone coastlines often feature distinctive landforms like sea caves and limestone pavements, whereas granite coastlines tend to be more rugged and less affected by chemical erosion.

Seasonal changes can have a profound impact on marine erosion processes such as hydraulic action and corrasion. During stormier seasons, typically autumn and winter in many regions, increased wind speed and storm frequency result in more energetic waves with greater erosive power. This heightened wave activity enhances the hydraulic action, leading to more significant erosion of coastal features like cliffs and rock formations. The increased wave energy also means more sediment is suspended and transported by the waves, intensifying the corrasion process. In contrast, during calmer seasons like spring and summer, the reduced wave energy results in less intense hydraulic action and corrasion. However, these seasonal variations can be influenced by geographical location and climate patterns. For instance, in tropical regions, monsoon seasons bring about drastic changes in wave conditions, significantly affecting the rate of marine erosion. Understanding these seasonal dynamics is essential for coastal management and planning, especially in areas prone to severe weather events and erosion.

Wave frequency, or the number of waves breaking on the coast over a given time, is a crucial factor in the process of corrasion/abrasion. Higher wave frequency results in more frequent impacts of sediment-laden water on the coast, enhancing the abrasive effect. This continuous bombardment by sediment-rich waves accelerates the erosion of coastal rocks and cliffs. The intensity of abrasion is also influenced by the energy and height of the waves; higher energy waves carry more sediment and have greater erosive power. Furthermore, the type and size of the sediment in the waves play a role. Larger and harder particles like pebbles and gravel can cause more significant abrasion compared to finer sediments like sand. Therefore, coastlines exposed to high wave frequencies and energy levels, particularly those with larger sediment particles, are more prone to rapid erosion through corrasion/abrasion.

The rate of marine erosion greatly depends on the type of coastline, primarily influenced by the geological composition and structure of the coast. Rocky coastlines composed of harder, more resistant rocks, such as granite or basalt, tend to erode slower due to their durability against erosive forces like wave action and abrasion. In contrast, softer rock types, such as chalk or sandstone, erode more rapidly under the same conditions. The structural features of the coast, including the presence of faults and joints, also play a significant role. Coasts with more cracks and joints are more susceptible to hydraulic action, leading to faster erosion. Moreover, the angle of wave approach and the frequency of storm events can accelerate erosion in certain areas, while sheltered areas may experience reduced erosion rates. Understanding these variations is crucial for predicting and managing coastal changes and for implementing appropriate coastal defense strategies.

Practice Questions

Explain how hydraulic action contributes to the formation of coastal features such as caves, arches, and stacks.

Hydraulic action plays a crucial role in sculpting coastal landscapes. This process involves the force of waves compressing air in cracks within coastal rocks, creating pressure that weakens the rock structure over time. In areas of softer rock, hydraulic action can carve out large cavities, forming sea caves. As these caves enlarge and extend towards the top of the cliff, they can create natural arches. Continued erosion and the eventual collapse of the arch's roof result in the formation of isolated stacks. Hydraulic action, therefore, is fundamental in shaping these distinctive coastal features, showcasing the power of wave action on coastal topography.

Describe the process of attrition and its impact on the formation of sandy beaches.

Attrition is a key process in the formation of sandy beaches. It involves the gradual breakdown of rock and sediment particles as they collide with each other in the coastal environment. These collisions, driven by wave action, reduce larger, rougher particles into smaller, smoother grains. Over time, this results in the accumulation of fine sand along the shoreline. Attrition's contribution to beach formation is significant as it not only influences the texture and composition of the beach but also affects the colour and appearance of the sand. The process exemplifies the gradual yet transformative power of coastal processes in shaping beach landscapes.

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