Coastal Geomorphology: Coasts

TL;DR

Coasts are dynamic environments shaped by powerful natural forces like waves, tides, and weather, creating distinct landforms. These forces can either erode (wear away) or deposit (build up) material, leading to unique coastal features. Understanding these processes helps us predict how coasts change and why certain landforms appear where they do.

1. The Mental Model

Think of coasts as a continuous battleground where the land meets the sea. The sea is constantly trying to reshape the land using its energy, either by smashing pieces off (erosion) or by dropping off bits it's carried (deposition).

2. The Core Material

Coasts are constantly changing due to a combination of marine (sea-related) and terrestrial (land-related) processes. For your Y9 CHMS exam, you'll need to know about the main processes and the landforms they create.

Coastal Processes

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There are two main categories of coastal processes: erosion (wearing away) and deposition (building up).

2.1 Erosion Processes

These are the ways the sea wears away the land.

• Hydraulic Action: This is the sheer force of the water itself. Waves crash against the rock, trapping air in cracks. The pressure compresses this air, then as the wave retreats, the pressure is released, causing the air to expand explosively. This repeated compression and expansion weakens and eventually breaks off pieces of rock.
• Abrasion (Corrasion): This is like sandpaper. Waves pick up rocks, pebbles, and sand, and hurl them against the cliff face, grinding away at it. The more sediment the waves carry, the more effective abrasion is.
• Attrition: Rocks and pebbles carried by waves crash into each other, becoming smaller, smoother, and more rounded over time. This doesn't directly erode the cliff but reduces the size of the material available for abrasion.
• Corrosion (Solution): This is chemical erosion. Seawater, which is slightly acidic, can dissolve certain types of rock, especially limestone or chalk.

2.2 Transportation Processes

Once material is eroded, it needs to be moved.

• Longshore Drift: This is the most important process for moving sediment along a coast. Waves usually approach the shore at an angle (due to prevailing winds). They push sediment up the beach at this angle. Then, as the wave breaks and water retreats (backwash), it pulls the sediment straight back down the beach due to gravity. This zig-zag movement slowly shifts material along the coast.

graph LR
    A["Wave approaches at an angle"] --> B["Swash pushes sediment up beach (diagonal)"]
    B --> C["Backwash pulls sediment down beach (straight out)"]
    C --> D["Net movement of sediment along the coast"]
    D --> A

2.3 Deposition Processes

When the sea loses energy, it drops the material it's been carrying. This typically happens in sheltered bays, estuaries, or when waves have travelled a long distance and lost their power. Larger, heavier sediments are dropped first, followed by smaller, lighter ones.

Coastal Landforms

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These are the features created by erosion and deposition.

2.4 Erosional Landforms

These are usually found on high-energy coasts where powerful waves are common.

• Cliffs: Steep rock faces formed by erosion.
• Wave-Cut Notch: An indentation at the base of a cliff, formed by hydraulic action and abrasion, where waves concentrate their energy.
• Wave-Cut Platform: A wide, flat rock surface left at the base of a retreating cliff. As the wave-cut notch enlarges, the rock above it becomes unstable and collapses, and the process repeats, leaving a platform behind at low tide.
• Headlands and Bays: Headlands are resistant rock that sticks out into the sea, while bays are curved inlets formed where softer rock has been eroded more quickly.
• Caves, Arches, Stacks, and Stumps: These form in headlands.
  • Cave: A crack in a headland (often a weakness like a fault) is widened by hydraulic action and abrasion.
  • Arch: If two caves on opposite sides of a headland join, or a single cave erodes all the way through, it forms an arch.
  • Stack: When the top of an arch collapses due to gravity and weathering, a pillar of rock is left standing isolated from the headland.
  • Stump: Over time, the stack is eroded by waves and collapses, leaving a small, low-lying piece of rock.

2.5 Depositional Landforms

These are usually found on low-energy coasts where waves are less powerful, allowing sediment to settle.

• Beaches: Accumulations of sand, shingle, or pebbles. They are formed by deposition and can be shaped by longshore drift.
• Spits: Long, narrow ridges of sand or shingle attached to the land at one end and extending out into the sea (or across an estuary) at the other. They form when longshore drift carries sediment along a coast, then the coastline changes direction (e.g., at a river mouth or a bend), causing the sediment to be deposited outwards. The end of a spit often curves inwards due to prevailing winds or currents, forming a hooked end.
• Bars: A bar forms when a spit grows all the way across a bay, connecting two headlands and trapping a lagoon behind it.
• Tombolos: Similar to spits, but they connect an offshore island to the mainland.

3. Worked Example

Let's imagine a coastline with alternating bands of hard and soft rock, exposed to prevailing winds from the west, creating waves that approach the coast from the west.

1. Initial stage: The coastline is relatively straight.
2. Differential Erosion: Waves will erode the softer rock much faster than the harder rock through hydraulic action and abrasion.
3. Headland and Bay Formation: The softer rock areas will begin to recede inwards, forming bays. The harder rock areas will resist erosion and stand out as headlands.
4. Erosional Features on Headlands: On the headlands, weaknesses like cracks will be exploited by hydraulic action and abrasion, forming caves. If these caves deepen and meet, or erode all the way through, they'll become arches. Eventually, the roof of an arch will collapse, leaving a stack, which will eventually erode into a stump.
5. Depositional Features in Bays: In the sheltered bays, the wave energy is much lower. Sediment eroded from the headlands and transported by longshore drift will accumulate here, forming beaches. If a river enters the bay, it will also contribute sediment.
6. Spit Formation: If the coastline suddenly changes direction (e.g., at a river mouth) while longshore drift is active, sediment will continue to be deposited in the original direction of drift, building up a narrow ridge of sand or shingle called a spit. This spit might develop a hooked end due to secondary wind/wave directions.

This example shows how both erosional and depositional processes work together to shape a complex coastline.

4. Key Takeaways

• Coasts are shaped by the opposing forces of erosion (wearing away) and deposition (building up).
• Hydraulic action, abrasion, attrition, and corrosion are the main ways the sea erodes land.
• Longshore drift is key to moving sediment along the coast, often at an angle.
• High-energy coasts usually have erosional landforms like cliffs, wave-cut platforms, headlands, caves, arches, stacks, and stumps.
• Low-energy, sheltered coasts usually have depositional landforms such as beaches, spits, bars, and tombolos.
• The type of rock (hard vs. soft) significantly influences whether an area forms a headland or a bay.

Common Mistakes to Avoid:
- Don't confuse abrasion (waves hitting rocks against the cliff) with attrition (rocks hitting each other).
- Remember corrosion is chemical (dissolving), not physical.
- Don't forget that wave direction is crucial for understanding longshore drift and spit formation.
- Incorrectly labelling features: make sure you know the order of cave → arch → stack → stump.

5. Now Try It

Imagine you're visiting a coastal area. For 15 minutes, sketch a simple diagram of a coastline that includes at least one erosional landform (like a headland with a stack) and at least one depositional landform (like a spit or a beach). Label each feature and add arrows to show the direction of longshore drift. Success looks like clearly labelled features that make sense in their location based on the processes we've discussed.