Coastal Zones

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  • Coastal Zones
    • Coastal Weathering and Erosion
      • Mechanical Weathering - breakdown of rock without changing its chemical composition
        • Freeze-thaw weathering is the main type of mechanical weathering that affects coasts
          • Happens when temperature alternates above and below 0*C (freezing point of water)
            • Water gets into rock that has cracks has cracks (e.g. Granite)
              • When the water freezes it expands, which puts pressure on the rock. When the water thaws it contracts, which releases the pressure on the rock.
                • Repeated freezing + thawing widens the cracks and cause the rock to break up.
      • Chemical Weathering - breakdown of rock by changing its chemical composition
        • Carbonation weathering is a type of chemical weathering that happens in warm + wet conditions
          • Rainwater has carbon dioxide dissolved in it, which makes it a weak carbonic aicd
            • Carbonic acid reacts with rock that contains calcium carbonate, e.g. carboniferous limestone, so the rocks are dissolved by the rainwater
      • Mass Movement - shifting of rocks and loose material down a slope e.g. a cliff
        • Happens when the force of gravity acting on a slope is greater than the force supporting it
          • Causes coasts to retreat rapidly
            • More likely to happen when the material is full of water (acts as lubricant)
              • Slides - Material shifts in a straight line
              • Slumps - Material shifts with a rotation
      • Erosion - wears away the coast
        • Four processes
          • Hydraulic Power - waves crash against rock + compress the air in the cracks. This puts pressure on the rock. Repeated compression widens the cracks and makes bits of rock break off
          • Abrasion - eroded particles in water scrape and rub against rock, removing small pieces
          • Attrition - eroded particles in water smash into each other + break into smaller fragments. Their edges also get rounded off as they rub together
          • Solution - weak carbonic acid in seawater dissolves rock like chalk + limestone
        • Waves that carry out erosional processes are called destructive waves
          • They have a high frequency (10-14 waves per minute), they're high + steep, their backwash is more powerful than their swash so material is removed from the coast
    • Coastal Landforms cause by Erosion
      • Wave-cut Platforms
        • Waves cause most erosion at the foot of a cliff, this forms a wave-cut notch, which is enlarged as erosion continues
          • The rock above the notch becomes unstable + eventually collapses. The collapsed material is washed away + a new wave-cut notch starts to form.
            • Repeated collapsing results in the cliff retreating
              • A wave-cut platform is the platform that's left behind as the cliff retreats
      • Headlands + Bays form where erosion resistance is different
        • Some types of rock are more resistant to erosion than others
          • Headlands + bays form where there are alternating bands of resistant + less resistant rock along a coast
            • Less resistant rock (e.g. clay) is eroded quickly + this forms a bay (which have a gentle slope)
              • Resistant rock (e.g. chalk) is eroded more slowly + it's left jutting out, forming a headland (which have steep sides)
      • Headlands are eroded to form Caves, Arches + Stacks
        • Headlands are usually made of resistant rocks that have weaknesses like cracks
          • Waves crash into the headlands + enlarge the cracks - mainly by hydraulic power + abrasion
            • Repeated erosion + enlargement of the cracks causes a cave to form
              • Continued erosion deepens the cave until it breaks through the headland - forming an arch e.g. Durdle Door in Dorset
                • Erosion continues to wear away the rock supporting the arch, until it eventually collapses
                  • This forms a stack - an isolated rock that's separate from the headland e.g. Old Harry in Dorset
    • Coastal Transportation and Deposition
      • Transportation is the movement of material (along coasts this is called longshore drift)
        • Waves follow the direction of prevailing wind
          • The usually hit the coast at an oblique angle
            • Swash carries material up the beach in the same direction as the waves
              • Backwash carries material down the beach at right angles, back towards the sea
                • Overtime, material zigzags along the coast
        • Four Processes
          • Traction - large particles like boulders are pushed along the sea bed by the force of water
          • Saltation - pebble-sized particles are bounced along the sea bed by the force of water
          • Suspension - small particles like silt and clay are carried along in the water
          • Solution - soluble materials dissolve in the water and are carried along
      • Deposition is carrying material in the sea water and being dropped on the coast
        • Coasts build up when the amount of deposition is greater than the amount of erosion
          • Amount of material deposited on an area of coast is increased when
            • There's lots of erosion elsewhere on the coast (lots of material available)
            • Lots of transportation of material into the area
            • Low energy (slow) waves carry material to the coast but they're not strong enough to take a lot of material away. This means there's lots of deposition + very little erosion
        • Wave that deposit more material than they erode are called constructive waves
          • They have a low frequency (6-8 waves per min), they're low + long, the swash is powerful + it carries material up the  coast and the backwash is weak + doesn't take a lot of material back down the coast
    • Coastal Landforms Caused by Deposition
      • Beaches are formed by Deposition
        • Beaches are found on coasts between the high water mark + the low water mark
          • They're formed by constructive waves depositing material like sand + shingle
            • Sand Beaches - Flat + wide - sand particles are small + the weak backwash can move them back down the beach, creating a long, gentle slope
            • Shingle Beaches - Steep + narrow - shingle particles are large + the weak backwash CAN'T move them back down the beach, creating a steep slope
      • Spits + Bars are formed by Longshore Drift
        • Spits form at sharp bends in the coastline e.g. at a river mouth
          • Longshore drift transports sand + shingle past the bend + deposits it in the sea
            • Strong winds + waves can curve the end of the spit (forming a recurved end)
              • Sheltered area behind the spit is protected from waves - lots of material accumulates in this area, which means plants can grow there/
                • Overtime, the sheltered area can become a mud flat or a salt marsh
        • Bars form when a spit joins two headlands together, e.g. there's a bar at Slapton in Devon
          • The bar cuts off the bay between the headlands from the sea
            • This means a lagoon can form behind the bar

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