Coastal Erosion and Deposition: Formation of Landforms and Coral Reefs
Coastal Erosion and Deposition
Headlands and Bays (Port Campbell National Park, Australia)
Formed along coasts with alternating resistant (harder) and less resistant (softer) rock. Resistant rock erodes slower, forming headlands that jut into the sea. Softer rock erodes faster, creating bays. Exposed headlands are more vulnerable to erosion than sheltered bays.
Cliffs and Wave-Cut Platforms (12 Apostles, Limestone Port Campbell National Park, Australia)
Large waves breaking against cliffs cause significant erosion. The cliff foot is undercut, forming a wave-cut notch. As the notch grows, the cliff above collapses, retreating and often increasing in height. The gently sloping land left behind is the wave-cut platform.
Caves, Arches, and Stacks
Weaknesses in cliffs erode first. The sequence progresses from a cave to an arch that cuts through the headland. The arch widens, and the unsupported rock above collapses, forming a stack. Further erosion leaves only a stump.
Deposition by the Sea
Deposition occurs in sheltered areas where sand and shingle accumulate. Beaches are the most common coastal deposition feature. While rocky beaches form through erosion, sand and shingle beaches result from deposition.
Shingle Beaches
Steeper slopes with larger shingle and pebbles at the top due to stronger swash than backwash.
Sandy Beaches
Gentler slopes with shingle at the top and fine sand at the bottom.
Waves
Most waves are caused by wind friction on the sea surface. Wave size depends on wind speed, duration, and fetch (distance over which the wind blows).
Destructive Waves
Weak swash, strong backwash. Remove material from the beach.
Constructive Waves
Strong swash, weak backwash. Build up the beach. Often low waves with long wavelengths.
Marine Transportation
Similar to rivers, the sea transports eroded material through suspension, solution, traction, and saltation.
Longshore Drift
Sediments move along the beach due to waves approaching at an angle. Obstructions can disrupt this process, leading to beach depletion. Groynes (barriers) can help trap sediments.
Marine Deposition
Constructive waves carry sand and shingle up the beach. Larger materials are deposited higher up. Backwash carries smaller materials back down, sorting the beach material by size.
Coral Reefs
Formed by the continuous growth of coral on a rock base.
Fringing Reef
Coral platforms attached to the mainland, with a shallow lagoon above.
Barrier Reef
Coral grows in shallower areas away from the mainland, forming a lagoon between the reef and the shore.
Atoll
A ring of coral reefs surrounding a lagoon, formed as the island it initially surrounded subsides or sea level rises.
Coral Growth Requirements
- Warm water (23-25°C)
- Clear and shallow salt water (no deeper than 50 meters)
- Plenty of sunlight
The Great Barrier Reef, Australia
A World Heritage Area, crucial to Australia’s economy. It faces threats from pollution, overfishing, dredging, coastal development, and tourism.
Threats to the Great Barrier Reef
- Industry and sewage pollution
- Overfishing and destructive fishing practices
- Dredging and sediment runoff
- Coastal development and habitat destruction
- Tourism impacts