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Subaerial Processes

Subaerial Processes

Subaerial processes, which are land-based, are often described as unseen processes. They work together to influence the rates of coastal recession, and they change the shape of coastlines. Subaerial processes include weathering and mass movement. These processes form part of the coastal erosion process.

  • Weathering is the gradual breakdown of rocks in situ, and it weakens rocks above the high tide mark.
  • Mass movement is the movement of materials downslope at a range of speeds due to gravity.

Examples of subaerial processes

Let’s look at weathering and mass movement in more detail. Note that weathering can be broken down into mechanical, chemical, and biological weathering, and mass movement can be broken down into flows and slides.

Weathering

Weathering is the breaking down of rock in situ and is affected by climatic conditions. Temperature and precipitation have a massive effect on the type of weathering experience. The lithology of the rock also plays a major role because some rocks are more prone to weathering.

Lithology refers to the general physical characteristics of rocks in a certain area.

There are three different types of weathering: mechanical, chemical, and biological.

Mechanical weathering

Mechanical weathering, also known as physical weathering because it is caused by physical forces, is the breaking down of rocks above the waterline. During this process, the chemical composition of the rocks is unchanged. Water also plays a prominent role in the process. Here are three types of mechanical weathering:

  • Freeze-thaw (frost shattering) occurs in rocks that are porous or permeable. Because of this, water can enter the rock, where it freezes. When water freezes, it expands by approximately 10%, and this expansion exerts pressure on the rocks, forcing cracks in the rocks to widen. With repeated freezing and thawing, rock fragments break away, fall to the cliff face, and collect at the bottom of the cliff as scree. In the United Kingdom, freezing is relatively uncommon at the coasts, especially in the south.
  • Salt crystallisation (salt weathering) happens when sea spray gets through cracks in the rocks. When salt from the sea spray evaporates, it leaves behind salt crystals in the rocks. These crystals grow over time and eventually (through pressure) cause the rocks to break up. The salt can also corrode rocks, especially if they contain traces of iron. Porous rocks, such as sandstone, are particularly vulnerable. The effect is more significant in hotter countries with dryer climates.
  • Wetting and drying. Frequent wetting and drying cycles are standard on the coast. Rocks rich in clay expand when they get wet and then contract when they dry. This repeated process causes them to crack and break up over time.

Chemical Weathering

Chemical weathering is the erosion of rock through a chemical reaction where water reacts with the chemical components in the rocks. This happens through carbonation, hydrolysis, and oxidation.

  • Carbonation is the slow dissolution of rocks due to rainfall. Limestone and other carbonate rocks are particularly vulnerable to this type of weathering.
  • Hydrolysis is the breakdown of minerals to form new clay minerals due to the effects of water and dissolved carbon dioxide. Hydrolysis is when acidic rainwater breaks down the rock, causing it to rot. Rocks that are particularly vulnerable to this type of weathering are igneous rocks and metamorphic rocks that contain silicate minerals.
  • Oxidation adds oxygen to minerals, especially iron compounds. Oxidation is when rocks are broken down by oxygen and water. Sandstone and shale often contain iron compounds that can be oxidized.

Biological weathering

Biological weathering is the breaking down of rocks by living organisms, such as plants and animals. This occurs in several ways:

  • Plant roots start to grow into small cracks in the cliff face. As the roots grow and thicken, the crack widens and the rocks begin to break up.
  • Water in decaying vegetation becomes acidic. This leads to increased chemical weathering.
  • Animals dig burrows into the cliff. This breaks up the rocks.
  • Marine organisms are capable of burrowing into cliffs or secreting acid, which breaks up the cliffs.

Piddocks are clam-like shellfish that make their homes in rocks like clay and sandstone. They are an example of biological weathering.

Mass movement

Mass movement is the movement of materials downslope at a range of speed because of gravity. Water acts as the common lubricant involved in mass movement. Examples of mass movement are flows and slides.

Flows

  • Soil creep. This process takes place slowly and continuously, not exceeding 1 centimetre a year. It generally happens on a slope that is above 5 degrees. Soil creep is caused by the expansion and contraction of the material. It can also occur as a result of freeze-thaw: as the sediment is frozen, it is lifted onto the surface, and then as the ice melts, the particles move further down the slope.
  • Earth and mudflows. An increase in the amount of water (heavy rains) can reduce friction. This causes the soil and mud to flow over the underlying bedrock. The material is wet enough to flow rapidly and contains at least 50 percent of sand, silt, and clay-sized particles. Mudflows are also known as mudslides.

Slides

  • Rockfalls are most likely to occur when rock faces/cliffs are exposed to mechanical weathering (such as freeze-thaw). Rockfall occurs on slopes over 40 degrees. The material that breaks away falls down to form scree (talus) at the foot of the slope/cliff. The undercutting of cliffs by the creation of wave-cut notches can lead to large falls. Block falls involve larger pieces of rock.
  • Rock/debris slides. Rocks that are jointed or have bedding planes roughly parallel to the slope/cliff face are vulnerable to landslides. An increase in the amount of water can reduce friction, causing sliding. The rocks slide over the underlying rock.
  • Slumps occur in saturated conditions where softer rock, such as boulder clay, retains rainwater and runoff on moderate to steep slopes. They are also common where softer clay or sand overlies more resistant or impermeable rocks, such as limestone or granite. As the rock becomes saturated, a slip plane forms, and the weight causes the cliff to rotate down the slope slowly.
  • Landslide. In areas of more resistant cliff material, erosion is more significant at the base where the continuous breaking of waves produces a wave-cut notch. The bigger the notch becomes, the heavier the cliff above the base becomes. Eventually, the base can no longer support the material above it and a landslide occurs.

Subaerial processes versus marine processes

Cliffs are eroded at the coast through marine processes and subaerial processes. The difference between the two types of erosion is that marine processes occur at the waterline and the subaerial processes occur above it.

Types of marine processes include hydraulic action (expanding trapped air causes the cracks in the cliff to expand), corrosion, and abrasions.

Subaerial processes Cliffs at the coastline StudySmarterCliffs at the coastline. Saltburn-by-the-Sea, UK, unsplash

The role of subaerial processes on the coast

As we mentioned at the beginning of this explanation, subaerial processes work together to influence the rates of coastal recession. These processes form part of the coastal erosion process.

The Holderness Coast in the UK is the fastest eroding coastline in Europe.

Subaerial Processes - Key takeaways

  • Subaerial processes work together to influence the rates of coastal recession.
  • Subaerial processes include weathering and mass movement.
  • Weathering is the gradual breakdown of rocks in situ, and it weakens rocks above the high tide mark.
  • Mass movement is the movement of materials downslope at a range of speeds due to gravity.
  • Mechanical, chemical, and biological weathering are types of weathering processes.
  • Mass movement can be divided into flows and slides.

Frequently Asked Questions about Subaerial Processes

Subaerial processes are land-based processes and occur above the waterline. They include weathering and mass movement, which happen on the cliff rocks. These processes operate on the cliff lithology to weaken them and provide material for erosion.

Subaerial processes, which are land-based, are often described as unseen processes. They work together to influence the rates of coastal recession, and they change the shape of coastlines. Subaerial processes include weathering and mass movement. These processes form part of the coastal erosion process.


Subaerial processes work together to influence the rates of coastal recession. They include weathering and mass movement.

Longshore drift is not a direct subaerial process. However, it does indirectly remove the scree (broken rock fragments) collected at the base of the cliff face. Scree forms from weathering processes.

Cliffs are eroded at the coast through marine processes and subaerial processes. The difference between the two types of erosion is that marine processes occur at the waterline and the subaerial processes occur above it.

Final Subaerial Processes Quiz

Question

What are the processes of weathering at the coast?

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Answer

Mechanical, chemical, and biological.

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Question

What is mechanical weathering?

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Answer

Mechanical weathering is the breaking down of rocks above the waterline. During mechanical weathering, the chemical composition of the rocks is unchanged. 

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Question

What is chemical weathering?


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Answer

Chemical weathering is the erosion of rock through a chemical reaction. Water reacts with the chemical components in the rocks, and this is done through either carbonation, hydrolysis, or oxidation.

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Question

What is biological weathering?

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Answer

Biological weathering is the breaking down of rocks by living organisms, such as plants and animals.

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Question

What is mass movement?


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Answer

Mass movement is the movement of soil and rocks in large quantities downslope as a result of gravity.

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Question

Explain slumping.


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Answer

Slumping occurs in saturated conditions where softer rock retains rainwater and runoff on moderate to steep slopes. It is also common where softer clay or sand overlies more resistant or impermeable rocks. As the rock becomes saturated, a slip plane forms and the weight causes the cliff to slump.

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Question

Explain a landslide.


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Answer

In areas of more resistant cliff material, erosion is more significant at the base where the continuous breaking of waves produces a wave-cut notch. The bigger the notch becomes, the heavier the cliff above the base becomes. Eventually, the base can no longer support the material above it and a landslide occurs. 


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Question

 Explain freeze-thawing.


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Answer

Freeze-thaw occurs in rocks that are porous or permeable. This allows the water to enter the rock and freeze. When water freezes, it expands by approximately 10%. This expansion exerts pressure on the rocks, forcing cracks in the rocks to widen. With repeated freezing and thawing, rock fragments break away, fall to the cliff face, and collect at the bottom of the cliff as scree. 

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Question

Explain salt weathering.


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Answer

Salt crystallisation happens when sea spray penetrates cracks in the rocks. When salt evaporates, it leaves behind salt crystals. These grow over time and exert stresses on the rock, causing it to break up. The salt can corrode rocks, especially if they contain a trace of iron. 

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Question

In which conditions does carbonation occur?


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Answer

In warm, wet conditions.

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Question

Explain hydrolysis weathering.


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Answer

Hydrolysis is when acidic rainwater breaks down the rock, causing it to rot.

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Question

What is oxidation weathering?


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Answer

Oxidation is when rocks are broken down by oxygen and water.


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Question

Name a coastline in the UK that is a good example of mass movement?


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Answer

The Holderness Coast is the fastest eroding coastline in Europe.

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Question

What's the name usually given to the rockfall at the bottom of a cliff face?


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Answer

Scree.

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Question

Explain what is meant by the term subaerial processes.


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Answer

Subaerial processes include weathering and mass movement. Weathering weakens rocks above the high tide mark. It is the gradual breakdown of rocks in situ. It does not involve any movement. Mass movement is the movement of materials downslope at a range of speeds.

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