2.2.3. Marine processes

Types of waves:

• Constructive waves: have a low wave height & usually the beach gradient is gentle.

• The waves spill forward gently creating a strong swash. The water drains away through the beach material so the backwash is weak.

• These waves deposit material & build up beaches.

• Destructive waves: have a high wave height & the beach tends to be steep.

• The wave plunges forward onto the beach so the swash is weak, but the rotation of the water causes a strong backwash.

• These waves tend to erode beaches.

Components of Waves:

Swash: when a wave breaks & washes up the beach.

Backwash: when the water drains away back into the oncoming wave.

The size of waves depends upon three factors:

1. The strength of the wind.
2. The length of time the wind has been blowing.
3. The fetch or distance over which the wind can blow.

Methods of erosion:

Corrasion – large waves hurl beach material at the cliff

Corrosion – salts and acids slowly dissolve a cliff

Attrition – waves cause stones to collide and disintegrate

Hydraulic action – force of waves compresses air in the cliffs

Longshore Drift:

• When waves approach the coast at an angle the swash moves up the beach at an angle.

• The backwash, however, drains straight back down the beach.

• This will gradually move sand/stones along the beach in a zig-zag motion.

Cliff and wave-cut platform formation:

• Wave erosion is concentrated at the foot of the cliff so a wave-cut notch is formed.

• The cliff is undercut & collapses.

• Repeated collapse causes retreat of the cliff producing a platform of flat rock at the cliff foot extending out to sea.

Caves, Arches and Stacks:

•A band of weaker rock extends through a headland.

• More erosion occurs producing caves on both sides of the headland.

• Continued erosion produces an arch through the headland.

• Eventually the roof is weakened & collapses forming a stack

Beaches: In bays the waves diverge outwards. The wave energy is dissipated creating a low energy environment hence deposition to form beaches.

Bays and Headlands:

• Bays are formed due to softer rock getting eroded easily.

• Headlands are usually formed since they are made of resistant rock and so is eroded more difficultly.

Spits:

• Spits form when the coastline changes direction. Longshore drift continues to carry material in the same direction.

• Sand & shingle is built up to form the spit.

• The end of the spit curves round due to wave refraction or secondary winds.

Sand Dunes:

• Sand dunes form behind wide sandy beaches.

• Onshore winds pick up the dry sand from above the high-water mark & carry it landward by saltation.

• If they encounter an obstacle the wind loses energy & deposits sand in the lee of the obstacle.

• Eventually a dune is formed. Plants then grow on it which stabilize it & trap more sand.

Salt Marsh:

• Mud is deposited by the tides. The beach builds up above sea level forming mudflats.

• Plants start to grow in the mud & trap more sediment. This forms into a salt marsh.

Coral Reefs:

-Conditions required for the growth of coral reef:

   Warm water/seas; temperatures above 20 degrees C (dev)

   Shallow water; not more than 60 metres deep (dev)

   Water free from sediment/clear/availability of light;

   Plentiful supply of oxygen in water/unpolluted;

   Plentiful supply of plankton;

   Lack of strong currents etc. 

- Fringing reef: Coral reefs grow in the shallow water of the coast in tropical areas (4A)

- Barrier reef: Due to plate tectonics the island starts to sink the reef grows to keep up with the sinking, but a lagoon develops between the reef & the land (4B)

- Coral atoll: These form around islands that are sinking. The coral growth keeps up with this & the island keeps sinking (4C)

- Eventually the island sinks below sea level forming a ring of coral with a lagoon in the centre.