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Weathering

Weathering

Coasts change a lot over time. For all the wonderfully unique landforms to come about, many processes have been operating. One of the processes that are often at work in coastal environments is weathering. Not (always) to do with the weather and certainly not the same as erosion, this process has helped shape the coastlines we love worldwide. So, what is it? How does it work? Is it always the same in every place? These are the many questions we must look at to fully understand weathering at the coast. Let's delve in.

Weathering definition

Weathering is a coastal process that is often confused with erosion. It is important to remember that these are not the same, although they sometimes happen together. The definition for weathering is:

Weathering means rocks that are decomposing and disintegrating in situ, or in place.

So, how is this different from erosion? Whereas weathering happens in situ with no movement of rocks and minerals, erosion involves the transport of rocks and minerals by factors such as water, ice, and waves.

Weathering and erosion both involve material (rock) being broken down. It is crucial to make the difference between the two extremely clear in your exam answers.

Types of weathering

There are 2 main types of weathering:

  1. Mechanical weathering
  2. Chemical weathering

That said, there is a third type of weathering, namely biological weathering. However, this is not a separate type of weathering but a combination of mechanical or chemical weathering alongside a biological factor.

When discussing weathering in exam answers, try to be specific about what type of weathering you are talking about. This shows a greater depth of understanding and can help you to unlock those higher marks.

Mechanical weathering

Mechanical weathering, also known as physical weathering, is achieved through physical processes, such as temperature fluctuation, frost action and abrasion. Rock is broken up or disintegrated because of a variety of physical factors.

Mechanical weathering involves disintegration or rocks breaking up into smaller pieces with no chemical composition changes.

While there are different types of weathering and weathering processes, considering our primary focus is on coasts, we will be looking at the weathering types that are relevant to coastal landscapes, namely:

  • Saltwater crystal growth
  • Frost weathering
  • Wetting and drying

Let's look at the mechanical weathering processes applicable to coasts in a bit more detail.

Saltwater crystal growth

When salty seawater collects in cracks in rocks, crystals form when that salt water evaporates, leaving only the salt particles behind. When the temperatures rise, the crystals heat up and expand. The crystals put immense pressure on the rock, causing it to break. Another name for saltwater crystal growth is haloclasty.

Arid climates where warm temperatures cause strong evaporation and areas along coasts are the most common climates where you will see salt crystal growth processes.

An example of salt weathering is a so-called tafone (plural: tafoni), a small to large cavity feature in rocks. They often look like honeycombs or Swiss cheese. One place to find tafoni is at Elgon on the Isle of Skye.

Weathering Tafone at Elgol, Isle of Skye StudySmarterFig. 1 - Tafone at Elgol, Isle of Skye

Frost weathering

Frost weathering, also known as ice wedging, is actually a collective term for the various processes involving ice, such as freeze-thaw, frost wedging and frost shattering.

Freeze-thaw, also known as ice crystal growth, happens when water seeps into rock cracks, freezes, and thaws. When water freezes, it expands, causing the surrounding rock to crack even further; we call this process ice wedging. Then, when temperatures rise, the ice in the cracks melts, leaving behind a bigger crack. When this cycle happens repeatedly, the cracks will continue to get bigger, weaken the rock, and eventually, the rock will break apart, a process called frost shattering.

Tors, large, free-standing rocky outcrops with an abrupt rise from the smooth and gentle slopes surrounding them, are an example of how erosion and weathering will affect rock. You can see an example in Dartmoor National Park.

Weathering A tor, a freeze-weathering example in Dartmoor National Park StudySmarterFig. 3 - A tor, a freeze-weathering example in Dartmoor National Park

Wetting and drying

Wetting and drying are common along coastlines. Coasts are often rich in clay; these clay-rich soils will expand when they are wet and contract when they are dry. This process of expanding and contracting cracks the rocks, making them vulnerable to freeze-thaw and salt crystallisation.

An example is the Wash, an estuary on the East coast in East Anglia, UK. This estuary is located on the coast and has a lot of clay. This clay often gets wet and then dries, cracking the clay.

Weathering The Wash estuary in East Anglia, an example of wetting and drying StudySmarterFig. 4 - The Wash estuary in East Anglia, an example of wetting and drying

Mechanical weathering effects on coasts

The table below summarises the information mentioned above and the effects on coasts in particular.

Weathering processEffect on coasts
Saltwater crystal growthSaltwater collects in cracks in rocks and cliff faces; warm (er) temperatures evaporate the water and leave the salt crystals behind. The heat expands the crystals, exerting enormous pressure on the rock, causing it to disintegrate. This is the primary weathering that occurs in coastal areas.
Freeze-thawWater collects in cracks in rocks. This water freezes, making the water expand and creating a bigger crack. When it thaws, the water is released from the crack, leaving behind a bigger crack than before. Repeated freezing and thawing will eventually cause the rock to split/disintegrate. This type of weathering is most effective on high latitude coasts that have significant rainfall.
Wetting and drying (water-layer weathering)The clay-rich soil you often find on the coast expands when wet and contracts when dry. The constant expansion and contraction will lead the clay-rich rock to crack. This expansion and contraction of minerals are most effective on clay and in micro-tidal environments.

Table 1

Chemical weathering

Chemical weathering involves the decomposition of rocks because of a chemical reaction between the rock and water. This type of weathering changes the chemical composition of a rock.

So, how does chemical weathering work? Both rain- and seawater can be weak acids. With rocks already being relatively chemically unstable, the water, mostly in combination with other chemical substances, such as oxygen and carbon dioxide, will change the composition of the rock, gradually leading to the disintegration of the rock.

Several types of chemical weathering are found along coasts:

Chemical weatheringEffect
HydrolysisThis happens in certain rocks that are (partially) composed of minerals that react to water or chemicals in the water. These minerals will break down after contact with the water leading to a lesser resistance to weathering. Eventually, the rock will disintegrate.
SolutionWhether rock minerals are soluble in water depends on the temperature and acidity of the water. Limestone is particularly susceptible to carbonation, even though it may be less soluble in seawater. When water, rich in carbonic acid, sprays the rock, it leads to honeycomb weathering (see the tafone example).
CarbonationCarbonation will attack the calcium carbonate in limestones and other carbonate rocks. This will eventually weaken the rock, causing it to break down.
HydrationWhen minerals absorb water, it weakens their crystal structure, making the rock more susceptible to other weathering processes.
ChelationPlant roots and decaying organic matter will produce organic acids. These acids will bind to metal ions, causing weathering.
OxidationThis happens when there are iron elements present within the rock. When it comes into contact with water, it will essentially rust, causing the rocks to disintegrate.

Table 2

As mentioned, chemical weathering occurs when rocks and water have a chemical reaction. An example of that is limestone. When rainwater is mixed with carbon dioxide in the atmosphere, it forms carbonic acid. When carbonic acid reacts with the calcium carbonate in limestone, it forms calcium bicarbonate. Bicarbonate, in turn, is soluble in water. So, when carbonation occurs, limestone gets weathered, weakening a limestone cliff.

Weathering Weathered (right) vs unweathered (left) limestone in State College, Pennsylvania, US StudySmarterFig. 5 - Weathered (right) vs unweathered (left) limestone in State College, Pennsylvania, US

Biological weathering

As mentioned earlier, biological weathering can also have an effect, although it is often a combination of mechanical or chemical weathering alongside a biological agent.

Biological weathering is when plants, animals and microbes weaken rock that will eventually disintegrate.

There are several biological agents responsible for biological weathering. Let's look at them in more detail.

Plants

As you know, plants require roots to get water and nutrients to grow. These roots, however, can grow into cracks in rocks. When roots continue to grow in these cracks, acting as a wedge, they put pressure on the rock until it cracks even further, eventually disintegrating. The same process can happen when seeds fall into the cracks and start to grow.

Remember: cracks in rocks are part of mechanical weathering. The plant roots growing into the cracks are biological weathering.

Animals

Burrowing animals, such as rabbits and moles, can move rock fragments to the surface. Not only will the exposed rocks themselves be susceptible to mechanical or chemical weathering, but also the cracks and crevices left behind (in place of the displaced rocks) are now open to mechanical and chemical weathering.

Apart from burrowing animals, there are also rock-boring species, such as piddocks (Pholadidae), a mollusc species. Boring themselves into the rock creates openings that are then susceptible to weathering. They also secrete chemicals that can dissolve rocks.

Weathering Piddocks boring themselves into rock, an example of biological weathering by animals StudySmarterFig. 7 - Piddocks boring themselves into rock, an example of biological weathering by animals

Lichens

Lichens are biological agents that cause mechanical weathering. Lichen is often found on the coast, grows on a rock's surface and produces organic chemicals that can break down the rock's outer layer by altering its chemical composition.

Lichens, one of the first living organisms to settle on a barren rock, are made up of algae and fungi, two organisms that depend on one another through their interaction. One (algae) produces food, and the other (fungi) provides water and protection. Lichens produce a diluted, acidic solution that can slowly cause some minerals in the rock to decompose.

Weathering Lichen on rocks, an example of biological weathering by a microorganism StudySmarterFig. 8 - Lichen on rocks, an example of biological weathering by a combination of algae and fungi

Remember: while the compound itself is produced through a biological process, they result from a biochemical reaction. These reactions will accelerate chemical and mechanical weathering. However, because the process starts with a biological agent, the lichen, it is classified as biological weathering.

Examples of weathering

As you noticed, several examples of weathering were mentioned throughout this explanation. Let's summarise those examples:

ExampleEffect
Saltwater crystal growthA tafone, for example at Elgon on the Isle of Skye.
Frost weatheringA tor, which you can find in Dartmoor National Park.
Wetting and dryingThe Wash estuary on the East Anglia coast, UK.
Chemical weatheringLimestone can carbonise, making it susceptible to weathering, and weakening a limestone cliff.
PlantsRoots or seeds grow in/through cracks in the rock, eventually causing it to disintegrate.
AnimalsBurrowing or rock-boring animals can move rocks to the surface where they are open to weathering; these animals also secrete chemicals causing rocks to dissolve.
LichensLichens produce acids that decompose the rock.

Table 3

Weathering - Key takeaways

  • Weathering means that rocks decompose and/or disintegrate in situ. This means the process happens in place, compared to erosion, which means the transportation of minerals and rocks.
  • There are 3 types of weathering:
    1. Mechanical
    2. Chemical
    3. Biological
  • Mechanical weathering is achieved through physical processes such as freeze-thaw, and it involves rocks disintegrating into small(er) pieces.
  • Chemical weathering involves the decomposition of rocks by chemical reactions, changing the chemical composition of a rock.
  • Biological weathering can be caused by plants, animals, and lichen. This process happens in conjunction with mechanical or chemical weathering.

References

  1. Fig. 1 - Tafone at Elgol, Isle of Skye (https://en.wikipedia.org/wiki/File:Tafoni_at_Elgol,_Isle_of_Skye.jpg) by Kalense (https://en.wikipedia.org/wiki/User:Kalense) Licensed by CC BY 3.0 (https://creativecommons.org/licenses/by/3.0/deed.en)
  2. Fig. 3 - Frost weathering example in Dartmoor National Park (https://commons.wikimedia.org/wiki/File:Haytor_main_buttress2.jpg) by Herby (https://commons.wikimedia.org/wiki/User:Herbythyme) Licensed by CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0/deed.en)
  3. Fig. 4 - The Wash estuary, an example of wetting and drying (https://en.wikipedia.org/wiki/File:Dried_mud_creeks_on_the_shores_of_the_Wash_-_geograph.org.uk_-_10669.jpg) by Alan Parkinson (https://www.geograph.org.uk/profile/117) Licensed by CC BY-SA 2.0 (https://creativecommons.org/licenses/by-sa/2.0/deed.en)
  4. Fig. 7 - Biological weathering by rock boring animal (https://commons.wikimedia.org/wiki/File:Pholades_niches.jpg) by lamiot (https://commons.wikimedia.org/wiki/User:Lamiot) Licensed by CC BY-SA 3.0 (https://creativecommons.org/licenses/by-sa/3.0/deed.en)
  5. Fig. 8 - Biological weathering by microoranism: Lichen (https://en.wikipedia.org/wiki/File:Lava_z14.jpg) by Zayance (no profile available) Licensed by CC BY-SA 2.5 (https://creativecommons.org/licenses/by-sa/2.5/deed.en)

Frequently Asked Questions about Weathering

Weathering means rocks decomposing and disintegrating in situ, or in place

The three types of weathering are:

  1. Mechanical weathering
  2. Chemical weathering
  3. Biological weathering

Biological weathering can occur in various ways, depending on the biological factor:

  • Plants/seeds - roots and seeds can grow in the cracks of rocks eventually making the rock disintegrate into small(er) pieces.
  • Animals - can burrow into the soil or rock. Not only will the cavity be vulnerable to weathering, the rocks removed from the burrow are now open to weathering.
  • Lichens - these are biological agents breaking down the rock's outer layer by altering the chemical composition of the rock. This will cause it to decompose in due time.

Biological weathering has many effects. It causes rocks to become weaker and eventually results in them disintegrating or decomposing.

An example of weathering is salt water crystal growth. This is when salt water collects in cracks in rocks and cliff faces. When the water evaporates, salt crystals are left behind. When the temperature rises, the salt crystals expand, putting pressure on the rock, eventually causing it to break apart.

Final Weathering Quiz

Question

TRUE or FALSE: Weathering means rocks decomposing and disintegrating in situ, or in place 

Show answer

Answer

True.

Show question

Question

TRUE or FALSE: weathering and erosion are the same thing

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Answer

False.

Show question

Question

Which statements are FALSE?

Show answer

Answer

Biological weathering is a stand-alone process that does not happen in combination with either mechanical or chemical weathering.

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Question

TRUE or FALSE: Mechanical weathering is when plants, animals, and lichens weaken rock which will then eventually disintegrate. 

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Answer

False.

Show question

Question

Which statement is FALSE?

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Answer

Frost weathering is also known as ice shattering.

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Question

TRUE or FALSE: physical weathering and mechanical weathering are not the same.

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Answer

False.

Show question

Question

Which statements are TRUE?

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Answer

Mechanical weathering is also known as physical weathering.

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Question

Which statements are TRUE?

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Answer

Chemical weathering involves decomposition of rocks because of a chemical reaction between the rock and water.

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Question

TRUE or FALSE: erosion and weathering happen at the same time

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Answer

True - sometimes

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Question

TRUE or FALSE: another term for saltwater crystal growth is haloclasty

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Answer

True.

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Question

Give an example of salt weathering, and explain the example. Where can you find such an example?

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Answer

An example of salt weathering is a tafone, which is a small to large cavity feature in rocks. It often looks likes honeycombs or Swiss cheese. You can find an example of a tafone at Elgon on the Isle of Skye in Scotland.

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Question

TRUE or FALSE: wetting and drying is not common occurrence along coastlines.

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Answer

False.

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Question

Explain the process of salt water crystal growth.

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Answer

Salty sea water collects in cracks in the rocks where salt crystals will form when the water evaporates, leaving only salt particles behind. When temperatures rise, the crystals heat up and expand, putting immense pressure on the rock, and causing it to break.

Show question

Question

What are the 3 types of weathering?

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Answer

Chemical

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Question

Which statement is FALSE?

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Answer

Weathering is a coastal process often confused with erosion.

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