Exploring coasts as dynamic and changing systems
This lesson focuses on two coastal processes: longshore drift, as an example of transportation, and erosion. Students will need to know about the process of longshore drift in order to apply their knowledge to a case study on Ghana in lesson four.
What is longshore drift?
Longshore drift is the process of sediment transportation across a beach. It is dependent on the prevailing wind which affects the waves and as a result the swash (waves up the beach). Sediment is transported with swash at a 45 degree angle up the beach and 90 degree angle back to the sea with backwash. As the waves move across the beach, sand and sediment is transported in the direction of the wave movement. See this website for a diagram.
How are arches, caves, stacks and stumps formed?
Weaknesses in rock are exposed to erosion through processes of abrasion, solution and hydraulic action. These processes widen these weaknesses into cracks, which slowly become larger and develop into small caves. As the cave continues to erode, the back wall becomes thinner until eventually it is completely eroded away leaving an arch. Subsequent erosion under the arch eventually leads to a weakening of the structure until the roof of the arch collapses and leaves a feature called a stack. Over time the stack will continue to erode and finally collapse into a stump. Go to the BBC Learning Zone website to see this process in action.
What is solution?
Acids in water create chemical reactions which dissolve and erode rock such as limestone and chalk.
What is abrasion?
Pebbles and sediment being transported by waves scrape like sandpaper against rocks causing the rock to breakdown
What is attrition?
Rocks and pebbles being transported in the water collide and break into smaller fragments. This process causes transported rocks to become smaller and rounder.
What is hydraulic action?
Water is forced into cracks in rock. Air becomes compressed and the increased pressure causes cracks to widen and deepen.
You will be shown a YouTube clip of longshore drift. (It may be necessary to play the clip twice). What happens to the sand particle? Does it move in a particular direction? Why do you think this might be the case? Movement of sand and sediment across a beach is called longshore drift and is a form of transportation. A diagram to show this process can be seen on the longshore drift reference sheet.
You will be shown a YouTube clip introducing you to different types of erosion (again you might want to watch the clip twice). Using your reference sheet to help you with the definitions, look back to the image of the cave, arch and stack in Pembrokeshire from your first lesson on coasts. Using the new vocabulary you have just been introduced to, try and explain how you think a cave might form.
Next you will be given an A3 piece of paper and some Plasticine. Working in pairs your task is to model a cave, arch and stack to show how the feature was formed. Place your 3D model in the centre of your A3 paper and annotate the diagram to explain how it was formed. You must use the new subject specific vocabulary you have learnt. When you have completed your model and annotations take a photograph of your work, print it out and put all the images up on the classroom wall.
There is one final feature that is missing from the cave, arch, stack series; a stump. What do you think a stump is and how might it form? Modify your model to show the addition of a stump and explain how it occurred.
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