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Coasts

  • Rock Cycle
  • Erosion and Weathering
  • Headlands and Bays
  • Cliffs and Wave-cut Platforms
  • Beaches and Sand Dunes
  • Spits and Bars
  • Coastal Protection

What is the rock cycle?

Rock is a made from a mixture of different minerals.

 

Rocks are one of earth's natural resources to make up the lithosphere. 

 

Humans forever interact with rocks. For example...

  • Mountains are made of rock and they are used for mining, quarrying and hiking. 
  • Roofs are made of slate, a rock.
  • Buildings are made of blocks of rock.
  • Roads are built from crushed rock.
  • Rock is hidden under the soil.
  • Rock is used to build walls and bridges.
  • Rocks are used to make kitchen surfaces and tiles (granite and limestone). 
  • Diamonds and gold used for jewellery are obtained from rocks.

There are many different types of rock but all rocks can be categorised into three groups; sedimentary, igneous and metamorphic. The rocks in each group forms in different ways. 

  • Sedimentary rock forms when rocks are eroded and the minerals are transported (perhaps by a river) to another place and deposited. Over time, the layers of deposited sediment build up and the weight compresses the minerals together to become stuck and form a new rock. Sandstone and limestone are examples of sedimentary rock. 
  • Igneous rock forms when rock is heated, melts, cools and solidifies again. These types of rocks tend be found near volcanoes. This is because magma is melted rock, when it erupts through the earth's surface it cools and hardens as a result of much lower temperatures. Granite and basalt are examples of igneous rocks.
  • Metamorphic rock forms when rock changes underground without melting. Heat and pressure of rocks above can change rock without melting it. Slate is an example of a metamorphic rock.

About 65% of earth's crust is igneous rock whilst only around 8% is sedimentary rock. Rock changes all of the time experiencing a cycle.

Why does the coastline change? 

 

The coastline is where the land meets the sea. For millions of years, the coastline has been broken down, changed and built up. Geomorphology is the term used for studying the formation of land and the processes that shape the land, such as weathering, erosion, transportation and deposition. 

 

Weathering is the breakdown, but not removal, of rocks in their original place. Weathering weakens and decays rocks which changes their structure or appearance. Weathering is mostly caused by precipitation (rain, snow, sleet and hail) and changes in temperature, but it can also be caused by atmospheric gases. 

  • Chemical weathering is the weakening and decomposition of rocks through chemical changes, that usually involve water. Carbonation is an example of chemical weathering. 
  • Mechanical weathering is the weakening and disintegration (break up) of rocks. Piles of rocks, known as scree can be found at the foot of cliffs where this occurs. It usually caused by extreme changes in temperature. 

Carbonation is an example of chemical weathering. Rainwater reacts with carbon dioxide (CO2) in the atmosphere to form a weak solution of carbonic acid (H2CO3). When this acidified water comes into contact with rocks, it causes a chemical reaction that slowly dissolves the rock away.

 

Freeze-thaw weathering is an example of mechanical weathering. Water collects in cracks or holes in rocks. The water freezes at night and expands causing the cracks or holes in rocks to become bigger. When the temperature increase and the ice thaws (melts) the water seeps deeper into the rock. The process continues (repeated freezing and thawing) until fragments of the rock disintegrate (break up) and fall to the foot of the cliff. 

 

Erosion is where waves wear away the coast and involves the removal of rock fragments. Waves erode by hydraulic action, abrasion and attrition.

  • Hydraulic action is the force of waves as they smash onto cliffs. The forward surge of water compresses water and air into cracks and holes in the rock. As the wave retreats the air is released (causing an explosive effect) and eventually the rock breaks apart.
  • Abrasion happens when waves have enough energy to hurl sand and shingle at a cliff. The rocks scratch and scrape against each causing smaller particles to break off (sandpaper effect) and it often leaves a smooth surface.
  • Attrition happens when rock fragments carried by the sea bump into each other, wearing away jagged edges to make smooth, rounded pebbles which often break up into smaller pieces. 

How are headlands and bays formed? 

 

The change in coastlines doesn't just depend on geomorphic processes, but also on geology. Geology is the study of the earth's crust, the rocks it is made up of and their characteristics. Some rocks are more resistant to weathering and erosion than others, which affects the rate at which they change. Hard rocks including chalk, limestone and sandstone erode slower than softer rocks such as clays. This is known as differential erosion and it occurs at a discordant coastline.

A discordant coastline is a coastline that is made up of different layers of hard and soft rock at right angles to the shore, whereas a concordant coastline is typically made up of the same type of rock, meaning it is eroded at the same rate (see the image below). This differential erosion forms headlands and bays along a discordant coastline.

  1. Destructive waves erode the coastline by hydraulic action and abrasion.
  2. The harder rock such as chalk, limestone or sandstone is more resistant to erosion and so the waves energy is focussed on the layers of softer rock such as clays either side of the hard rock.
  3. The softer rock erodes much faster, forming a crescent-shaped indentation in the coastline, known as a bay.
  4. Over time, a beach may form in the bay as sand and shingle is deposited by constructive waves.
  5. The hard rock then juts (points) out to sea. This becomes known as a headland and is surrounded by water on all three sides. It erodes much slower than the bays either side of it. 

What is a wave-cut platform? 

 

Wave-cut platforms are areas of visible bedrock found at the base of cliffs. They are usually exposed at low tide and have a gentle slope of about 3-4 degrees. They may be pitted with rock pools and be smooth in some areas as a result of attrition.

How are stacks formed? 

 

Despite the hardness of the rocks, headlands are constantly being eroded by the waves as a result of wave refraction. Wave refraction is explained below. 

The image below shows what happens as a result of wave refraction. This is how caves, arches and stacks form along the coastline. 

What are the characteristics of beaches? 

 

The waves do not only erode the coastline. They also transport material away from cliffs and beaches. Longshore drift is the process of transporting beach material (such as sand and shingle) and is explained in the image below. 

When the waves lose their energy and they can no longer transport the eroded material, it is deposited. The deposits of sand and shingle build up and over time, a beach is formed.

 

Sandy beaches often form in sheltered bays by constructive waves (low energy) whereas pebble beaches are found in exposed coastal areas where there are no or few headlands, and are formed by destructive waves (high energy). 

 

A beach profile shows the gradient from the back of the beach to the sea and the characteristics in between. A sandy beach generally has a gentle, fairly flat profile, whereas a pebble beach has a steep, stepped profile.

berm is a terrace on a beach that has formed in the backshore (back of the beach). On broad beaches there may be three or more berms formed under different wave conditions. Berms are formed in calm weather when constructive waves transport material onto the beach, whilst existing berms are moved up the beach by storms and then new berms are formed changing the beach profile. Large boulders are found at the back of pebble beaches whereas sand dunes are found at the back of sandy beaches. Can you see the berms on the beach profiles to the right?

How are sand dunes formed? 

 

Sand dunes are heaps of sand found in the backshore of sandy beaches. For a sand dune to form, there needs to a wide flat sandy beach, a large supply of sand, on-shore wind and an obstacle at the back of the beach.

How are spits and bars formed? 

 

Transportation and deposition continues along beaches and the coastline continues to change. This can lead to other landforms being created such as spits and bars. A spit is a sand or shingle beach that is joined to the land but juts (points) out to sea. Spits form on coasts where there is significant longshore drift. If the coastline changes direction and bends sharply, sediment is then deposited out to sea

Can we protect the coast? 

 

Unless they are protected, all landforms are vulnerable to erosion and change. We live on an island, so there is coast all around us. Our coastline is used for various industries such as tourism, sports, fishing, trade (ports), agriculture, oil extraction and construction (gravel taken from the sea bed to construct roads). 

 

Every so often, the sea reminds us of the risks of living by the sea. Look at the images below. 

The flooding in the image above has not been caused by rainwater, instead it has been caused by gale force winds (strong winds) pushing the waves towards the coast. In some places the seawater surges forward and is swept along streets, and into shops and homes. 

 

The image of the house on the beach has been caused by rapid erosion of the cliffs, causing them to collapse and retreat towards residential areas. The UK has some of the fastest eroding coastlines in Europe. 

 

It is important to protect the coastline from erosion and storm surges, and the UK has many successful coastal defences. 

 

We can protect the coast in similar ways we protect areas from flooding; using hard and soft engineering strategies. Hard engineering is invasive and more expensive and usually makes the landscape look unnatural. Soft engineering is less invasive, cheaper and often blends in with the natural landscape. 

Hard engineering strategies

  • A sea wall gives people a sense of security. It often has a promenade on top of it, which doubles up as cycle route outside peak walking periods. Steps at the base of a wall act as seating areas for beach users.
  • If well maintained, sea walls can last for many years.
  • They restrict access to the beach and if waves break over the sea wall (overtopping), coastal flooding may occur.
  • At about £5,000 per linear metre, sea walls are expensive to build. Repairs are also expensive.
  • Groynes create a wider beach, which can be popular with tourists and provide useful structures for people interested in fishing.
  • At £5,000 each, groynes are relatively cheap and, if well maintained, can last up to 40 years. A larger beach, with more space for activities, attracts more tourists, which boosts the local economy.
  • Groynes are barriers, which impede walking along a beach. They are also dangerous, as they have deep water on one side and shallow water on the other. This is a particular hazard to children who find it hard to resist climbing on them.
  • Groynes are ineffective in stormy conditions and need regular maintaining so they do not rot.

Soft engineering strategies

Beach nourishment involves adding new sediment to beaches. 

  • A wider beach means more room for beach users. People living along the seafront are more protected from coastal flooding.
  • Relatively cheap and easy to maintain.
  • Blends in with existing beach.
  • Although cheaper than hard engineering options, this has high overheads as it costs around £300,000 to hire a dredger.
  • Needs constant maintenance unless structures are built to retain the beach, as sediment may be taken away by longshore drift.

Managed retreat allows the sea to flood an area with low-value.

  • It is cheaper than paying for other engineering strategies. 
  • It creates a saltmarsh which can provide habitats for wildlife and attract bird watchers to the area.
  • Land is lost to the sea.
  • Often floods farmland and requires compensation to be paid to the land owner. This can be around £5,000 - £10,000 per hectare. 

Questions

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