2.2.3 Marine Processes

syllabus_link_buttonDemonstrate an understanding of wave processes in eroding a coastline and re-sorting and depositing materials removed through erosion.  Candidates should understand the types of waves and the components of waves, swash and backwash. 

wave costa rica
Waves breaking, Costa Rica

Waves

Waves are the result of the wind blowing on the oceans or seas.  The  bigger the fetch the larger and more powerful the waves are likely to be.  There are 2 main types of wave:

Constructive waves: these have a strong swash and a weaker backwash, they deposit more material on a beach than they remove and so build up beaches.

Destructive waves: these have a stronger backwash and remove more material than they deposit.  They gradually destroy beaches.

 

syllabus_link_buttonThe erosional processes of wave action should include an understanding of corrasion, hydraulic action, corrosion and attrition. 

Coastal Erosion Processes

The same erosional forces apply in marine environments as in river ones.  Hydraulic action from the force of the waves.  Abrasion occurs with sand and pebbles scraping against other material.  Attrition happens when pebbles, shells and rocks collide as the are moved by waves.  Corrosion is present as the sea is slightly acid and dissolves limestone rocks and cliffs.

 

syllabus_link_buttonTransport of material along a coastline should be appreciated; onshore and offshore movements together with an understanding of movement along a coastline (longshore drift).

Longshore Drift 

Longshore Drift Process
Longshore Drift Process

Material is transported along coastlines.  The direction of the wind influences the direction that they waves break upon the coastline.  The swash usually comes up the beach at an angle bringing material with it.  The backwash always retreats perpendicularly as gravity exerts its force.  This results in material slowly being moved along the beach.





syllabus_link_buttonUnderstand the action of wind in shaping coastal sand dunes.

sand dunes
Coastal Sand Dunes

Sand Dunes

Sand dunes form on beaches where constructive waves desposit sand which is then blown further back by the wind.  The sand builds up behind the beach forming mounds or dunes.  Grasses then slowly colonise the dunes and hold the sand in place reducing the amount that is blown further inland.  If vegetation does not stabilise the dunes the wind will continue to change their shape and size.

 

syllabus_link_buttonDescribe the conditions required for the development of coral reefs.

Coral Reefs

Coral reefs establish in areas of warm water, generally at 20OC or above. They typically form around tropical sea islands.  They tend to form in shallow water where there is sufficient sunlight reaching them.  They appear at river mouth locations where freshwater brings sediments that cloud the water and reduce the ability of sunlight to pass through the water.

 

syllabus_link_buttonDescribe fringing and barrier reefs and atolls.

Fringing reefs are reefs that attach directly to an island, they are close to shore, sometimes with a shallow lagoon between them and the shoreline.

Barrier reefs are reefs that are further offshore, often with a stretch of deep water between them and the main land.

Atoll reefs are generally round creating a circular lagoon that doesn’t have an island in.

 

syllabus_link_buttonA study should be made of the following coastal landforms:

Cliffs, wave-cut platforms, caves, arches, stacks, bay and headland coastlines, beaches, spits and bars, coastal sand dunes and marsh.

Bays and Headlands

Coastlines are composed of different types of rock which erode and weather at different rates.

Softer rocks erode more easily and quickly, forming bays whereas the harder rocks take much longer and stick out as headlands.

The harder rock of headlands is associated with some distinct features.

 

Headland Features

Weaknesses in the rock are widened through hydraulic action as the waves force water and air into small cracks.  The cracks are widened gradually into caves.  Erosion continues until the cave is eroded all the way through the headland creating an arch.  Eventually the roof of the arch collapses and a stack is left standing.  Over time the base of the stack erodes causing it to collapse.  A stump is left behind.

Headland features caused by erosion
Headland features caused by erosion
erosion
Southern Australian Coastline, stacks and stumps










Wave-cut Platforms

Wave Cut Platform
Wave Cut Platform

These are found between the high & low tide levels where the waves have eroded the cliffs, undercutting them and causing them to collapse. The cliffs retreat backwards.

The platform usually slopes gently downwards and may have rocks from the collapsed cliffs. The platform is usually exposed at low tide but underwater during high tide.

 

Deposition Features

Sand is moved along the coastline due to longshore drift.  In areas of the coastline sheltered by headland that stick out the waves and currents will have less energy.  In these areas sand will be deposited and may start to build up  forming spits and possibly bars.  The area behind spits and bars is protected from the waves and usually becomes saltmarsh

Test Yourself

Past Paper Style Questions

Grab a Pen and start practising for the exam

1) Describe the difference between constructive and destructive waves. [3 marks]

2) Explain how headland is formed along some stretches of coastline. [3 marks]

3) Explain why coral reefs are only found in some locations. [5 marks]

4) With reference to an example that you have studied, describe the impacts of a named tropical storm or hurricane. [7 marks]

5) For a named area that you have studied describe the impacts of coastal erosion and the methods that have been used to protect the coast. [7 marks]