2.2.1 Weathering
Candidates should be able to:
• Recognise that weathering involves the breakdown of rock in-situ and, as such, should be distinguished from erosion.
• Describe what is meant by different types of weathering – physical/mechanical (freeze-thaw action, exfoliation), chemical (carbonation, oxidation) and biological.
• Explain the main factors influencing the type and rate of weathering – climate and rock features (mineral composition, grain size of the rock, presence of lines of weakness). The influence of climate on the rate of weathering could be illustrated with reference to simple explanation as to why weathering is more rapid in humid tropical regions of the world than in temperate regions.
Weathering: Is the break-up and decomposition of rocks in-situ (in their place of origin). Weathering does not involve the movement of material and this makes it different to erosion.
Weathering can be broken into either two or three categories:
Chemical Weathering: The break down of rocks caused by a change in their chemical make-up.
Physical or Mechanical Weathering: The break down of rocks caused by physical processes with no change in the rocks chemical make up.
Biological Weathering: Biological is sometimes included within physical weathering. Biological weathering is when flora and fauna break down the rock e.g. growing roots systems or burrowing animals.
Types of Chemical Weathring
Carbonation or solution
This is caused by carbonic acid which occurs naturally in rainwater. Although only a very weak acid, it chemically reacts with rocks such as limestone and slowly dissolves them. Areas that have standing rainwater are going to experience higher rates of carbonation, therefore its is less likely to happen in dry countries and on steep slopes. The dissolved rock then gets washed away.
Hydrolysis
Hydrogen in water reacts with minerals in the rocks. Instead of dissolving the rock, the water actually combines with the rock. One example of hydrolysis is feldspar found in igneous rocks such as granites can be turned into a form of clay.
Hydration
Certain rocks are able to absorb water into their structure, causing them to swell (get bigger). This not only exerts pressure because of the growing size but causes the chemical structure to change.
Oxidation
This occurs when iron compounds within rock, reacts with oxygen to produce a reddish-brown coating.
Types of Physical (mechanical) Weathering
Freeze-thaw or Frost Shattering
This occurs in areas with moisture and a high diurnal (daily) temperature range that falls below zero. As water freezes it expands placing pressure on the rock around it. When it thaws more water is able to enter the crack and then it is able to freeze again. This process continues, slowly breaking down the rock.
Exfoliation or Onion Weathering
Again this tends to happen is areas with a high diurnal temperature range (very hot in the day and cold at night). During the day rock heats up and expands and during the night it cools down and contracts. These changes in temperature and shape cause the rock to weaken and layers beginning to peel off, just like an onion.
Pressure Release
When rocks above are weathered, or eroded and then removed, rock underneath experiences and reduction in pressure placed upon it. The removal of pressure allows the rocks to expand causing them to fracture. When a glacier melts a similar process happens.
Salt Crystal Growth
Common in hot areas with high rates of evaporation. Water is evaporated leaving behind salt crystals. The salt can attack the rock chemically, but they also expand placing pressure on the rocks structure.
Factors Affecting the Rate of Weathering
Geology: The type of rock, is extremely important in affecting rates of weathering. Rocks with cracks are more likely to experience increased rates of physical and chemical weathering. Cracks allow water to get. Soft rocks are much more vulnerable to weathering than strong rocks. Also the chemical composition of rocks is also important. For example limestone's that have large amounts of calcium carbonate are more vulnerable to carbonation.
Vegetation: Areas of land that have vegetation are more likely to experience rapid biological weathering. However, they are also likely to insulate the rock from large temperature ranges, reducing some physical weathering. Vegetation will intercept rainwater, reducing rates of some chemical weathering. Vegetation can hold rain water in-situ though increasing chemical weathering and some mosses contain chemicals that can increase chemical weathering.
Climate: Climate is very important because hot temperatures increase the rates of chemical reaction, therefore increasing chemical weathering. Areas with high diurnal temperature ranges will see an increase in some physical weathering. Wet areas are going to see an increase in chemical weathering. Wet and warm areas are also likely to see an increase in the amount of vegetation, increasing biological weathering.
Relief: A steep relief can increase some forms of physical weathering, but can slow chemical weathering. Most forms of chemical weathering need rain to be stationary, but steep slopes encourage fast surface run-off. However, a steep slope will cause weathered rocks to fall away quicker exposing fresh rock beneath to be weathered.
Aspect: This is the direction a slope is facing. The direction it faces can affect the amount of sunshine it receives. If a slope is facing the sun it might have more vegetation growing on it, increasing biological weathering. If it is not facing the sun it might have less vegetation increasing the rates of chemical weathering and physical weathering.
Humans: Humans can influence rates of weathering in many ways, they can add chemicals to water courses, they can deforest or forest areas, they can introduce animals or remove animals.
Why is weathering more rapid in tropical areas than temperate areas?
Briefly tropical areas tend to experience more weathering because of :
Large amounts of rainfall increasing chemical weathering
Large amounts of vegetation increasing biological weathering
They are nearer to the equator so there are high temperatures and faster rates of chemical reaction
Some areas (high areas and desert areas) have higher diurnal temperature range.
Limestone Weathering Features
Weathering can create many features, especially in limestone's. Three of the most impressive are briefly mentioned below:
Limestone Pavement
As the name suggests, these features occur in limestone rock. Limestone that is exposed i.e. has no vegetation cover is attacked by the weather. Rainwater can causing carbonation and also freeze-thaw weathering. The high exposed bits of the pavement are called clints and the cracks in between are called grykes.
Karsts
Karsts are some of the most impressive features found on our planet. Below is the UNESCO world heritage site, Halong Bay in Vietnam. Karsts are mainly formed by carbonation over millions of years. Some weaker areas are completely weathered, leaving stronger sections remaining. The remaining sections are called karsts.
Stalactites and Stalagmites
These are strictly depositional features and are made because limestones are being weathered elsewhere. They are created by water with large amounts of dissolved calcium in them. The dissolved water leaves behind the calcium which over time build up to create stalactites and stalagmites.
IGCSE and GCSE Weathering
Specification:
2.2.1 Weathering
Candidates should be able to:
• Recognise that weathering involves the breakdown of rock in-situ and, as such, should be distinguished from erosion.
• Describe what is meant by different types of weathering – physical/mechanical (freeze-thaw action, exfoliation), chemical (carbonation, oxidation) and biological.
• Explain the main factors influencing the type and rate of weathering – climate and rock features (mineral composition, grain size of the rock, presence of lines of weakness). The influence of climate on the rate of weathering could be illustrated with reference to simple explanation as to why weathering is more rapid in humid tropical regions of the world than in temperate regions.
Weathering: Is the break-up and decomposition of rocks in-situ (in their place of origin). Weathering does not involve the movement of material and this makes it different to erosion.
Weathering can be broken into either two or three categories:
Chemical Weathering: The break down of rocks caused by a change in their chemical make-up.
Physical or Mechanical Weathering: The break down of rocks caused by physical processes with no change in the rocks chemical make up.
Biological Weathering: Biological is sometimes included within physical weathering. Biological weathering is when flora and fauna break down the rock e.g. growing roots systems or burrowing animals.
Types of Chemical Weathring
This is caused by carbonic acid which occurs naturally in rainwater. Although only a very weak acid, it chemically reacts with rocks such as limestone and slowly dissolves them. Areas that have standing rainwater are going to experience higher rates of carbonation, therefore its is less likely to happen in dry countries and on steep slopes. The dissolved rock then gets washed away.
Hydrogen in water reacts with minerals in the rocks. Instead of dissolving the rock, the water actually combines with the rock. One example of hydrolysis is feldspar found in igneous rocks such as granites can be turned into a form of clay.
Certain rocks are able to absorb water into their structure, causing them to swell (get bigger). This not only exerts pressure because of the growing size but causes the chemical structure to change.
This occurs when iron compounds within rock, reacts with oxygen to produce a reddish-brown coating.
Types of Physical (mechanical) Weathering
This occurs in areas with moisture and a high diurnal (daily) temperature range that falls below zero. As water freezes it expands placing pressure on the rock around it. When it thaws more water is able to enter the crack and then it is able to freeze again. This process continues, slowly breaking down the rock.
Again this tends to happen is areas with a high diurnal temperature range (very hot in the day and cold at night). During the day rock heats up and expands and during the night it cools down and contracts. These changes in temperature and shape cause the rock to weaken and layers beginning to peel off, just like an onion.
When rocks above are weathered, or eroded and then removed, rock underneath experiences and reduction in pressure placed upon it. The removal of pressure allows the rocks to expand causing them to fracture. When a glacier melts a similar process happens.
Common in hot areas with high rates of evaporation. Water is evaporated leaving behind salt crystals. The salt can attack the rock chemically, but they also expand placing pressure on the rocks structure.
Factors Affecting the Rate of Weathering
Geology: The type of rock, is extremely important in affecting rates of weathering. Rocks with cracks are more likely to experience increased rates of physical and chemical weathering. Cracks allow water to get. Soft rocks are much more vulnerable to weathering than strong rocks. Also the chemical composition of rocks is also important. For example limestone's that have large amounts of calcium carbonate are more vulnerable to carbonation.
Vegetation: Areas of land that have vegetation are more likely to experience rapid biological weathering. However, they are also likely to insulate the rock from large temperature ranges, reducing some physical weathering. Vegetation will intercept rainwater, reducing rates of some chemical weathering. Vegetation can hold rain water in-situ though increasing chemical weathering and some mosses contain chemicals that can increase chemical weathering.
Climate: Climate is very important because hot temperatures increase the rates of chemical reaction, therefore increasing chemical weathering. Areas with high diurnal temperature ranges will see an increase in some physical weathering. Wet areas are going to see an increase in chemical weathering. Wet and warm areas are also likely to see an increase in the amount of vegetation, increasing biological weathering.
Relief: A steep relief can increase some forms of physical weathering, but can slow chemical weathering. Most forms of chemical weathering need rain to be stationary, but steep slopes encourage fast surface run-off. However, a steep slope will cause weathered rocks to fall away quicker exposing fresh rock beneath to be weathered.
Aspect: This is the direction a slope is facing. The direction it faces can affect the amount of sunshine it receives. If a slope is facing the sun it might have more vegetation growing on it, increasing biological weathering. If it is not facing the sun it might have less vegetation increasing the rates of chemical weathering and physical weathering.
Humans: Humans can influence rates of weathering in many ways, they can add chemicals to water courses, they can deforest or forest areas, they can introduce animals or remove animals.
Why is weathering more rapid in tropical areas than temperate areas?
Briefly tropical areas tend to experience more weathering because of :
Limestone Weathering Features
Weathering can create many features, especially in limestone's. Three of the most impressive are briefly mentioned below:
As the name suggests, these features occur in limestone rock. Limestone that is exposed i.e. has no vegetation cover is attacked by the weather. Rainwater can causing carbonation and also freeze-thaw weathering. The high exposed bits of the pavement are called clints and the cracks in between are called grykes.
Karsts are some of the most impressive features found on our planet. Below is the UNESCO world heritage site, Halong Bay in Vietnam. Karsts are mainly formed by carbonation over millions of years. Some weaker areas are completely weathered, leaving stronger sections remaining. The remaining sections are called karsts.
These are strictly depositional features and are made because limestones are being weathered elsewhere. They are created by water with large amounts of dissolved calcium in them. The dissolved water leaves behind the calcium which over time build up to create stalactites and stalagmites.