Weathering

Weathering Definition

Weathering is the process by which rocks and minerals are broken down into smaller particles over time due to various natural forces and environmental factors.

Physical Weathering

Physical weathering is the process of breaking down rocks into smaller fragments through mechanical forces, like temperature changes, frost, and abrasion, without altering their chemical composition.

Freeze-thaw

Freeze–thaw occurs when water gets into joints and cracks in the rock. This water then freezes. As it freezes it expands. To give you a rough idea it expands by about 10% and exerts pressure up to a maximum of 2100 kg/cm2 at –22°C.

Freeze–thaw is most effective in environments where moisture is plentiful and there are frequent fluctuations above and below freezing point.

Thermal Stress

Thermal stress is caused by the Heating and cooling of rocks. Different minerals expand and contract at different temperatures. This can cause granular disintegration in rocks composed of different minerals, for example, granite contains quartz, feldspar, and mica. And all of these minerals expand and contract differently as they get heated and cooled.

Disintegration is found in hot desert areas where there is a large diurnal temperature range. In many desert areas, daytime temperatures exceed 40°C whereas night-time ones are a little above freezing. Rocks heat up by day and contract by night. As rock is a poor conductor of heat, stresses occur only in the outer layers. This causes peeling or exfoliation to occur. Moisture is essential for this to happen.

The different minerals in the granite expand differently when heated and cooled.

Salt Crystallisation

Salt crystallization causes the decomposition of rock by solutions of salt.

When water evaporates, salt crystals may be left behind. As the temperature rises, the salts expand and exert pressure on the rock. Sodium sulfate (Na 2 SO 4) and sodium carbonate (Na 2CO3) expand by about 300%. This creates pressure on joints, forcing them to crack. This mechanism is frequent in hot desert regions where low rainfall and high temperatures cause salts to accumulate just below the surface.

Salt crystals forming on rocks in the desert

Pressure Release

Pressure release is the process whereby overlying rocks are removed by erosion. This causes underlying rocks to expand and fracture parallel to the surface. The removal of a great weight, such as a glacier, has the same effect.

The rock is under great pressure due to the top layer

When the top layer is eroded the pressure is eased and the rock can expand. As it expands it can fracture.

Chemical Weathering

Water is the key medium for chemical weathering. Unlike mechanical weathering, chemical weathering is the most effective sub-surface because percolating water has gained organic acids from the soil and vegetation.

Hydrolysis

Hydrolysis is a form of chemical weathering in which only part of a mineral is taken into solution. The rest of the mineral is transformed into a new solid material.

Water Infiltration:

• The process begins with the infiltration of water into rocks or minerals. Water is a universal solvent and plays a key role in the chemical weathering of minerals.

1. Presence of Minerals:
   • The rocks or minerals that undergo hydrolysis typically contain primary minerals, such as feldspar, which is common in many types of rocks.
2. Initiation of Hydrolysis:
   • Water molecules come into contact with the minerals, leading to the initiation of hydrolysis. The water molecules contain ions (H+ and OH-) that can react with the mineral’s crystal structure.
3. Chemical Reaction:
   • The hydrolysis reaction involves the breaking of chemical bonds within the mineral. For example, the hydrolysis of feldspar can be represented as: Feldspar+Water→Clay Minerals+Dissolved Ions
   • In this reaction, the feldspar mineral reacts with water to produce clay minerals and dissolved ions.
4. Formation of Clay Minerals:
   • The primary outcome of hydrolysis is the formation of clay minerals. These are secondary minerals with a layered structure. The original mineral is transformed into new minerals through the hydrolysis process.
5. Release of Dissolved Ions:
   • As the mineral undergoes hydrolysis, various ions are released into the surrounding water. Common ions include potassium (K+), sodium (Na+), calcium (Ca2+), and silica (SiO2).
6. Accumulation of Clay:
   • Over time, the clay minerals accumulate as a product of hydrolysis. These clay minerals often have different physical and chemical properties compared to the original minerals.
7. Soil Formation:
   • The accumulation of clay minerals contributes to soil formation. Clay-rich soils are common in areas where hydrolysis weathering has been active.
8. Nutrient Cycling:
   • The dissolved ions released during hydrolysis, such as potassium and calcium, contribute to nutrient cycling in soils. These ions can be taken up by plants, influencing the overall ecosystem.

Hydration

Hydration is the process whereby certain minerals absorb water, expand, and change. For example, anhydrite is changed to gypsum. Although it is often classified as a type of chemical weathering, mechanical stresses occur as well. When anhydrite absorbs water to become gypsum it expands by about 0.5%. Shales and mudstones increase in volume by around 100% when clay minerals absorb water.

In the diagram below you can see that the rock is made of several different minerals. These minerals expand differently when water is absorbed. The expansion puts pressure on the rock and can cause it to crack.

Oxidation

Oxidation weathering involves the reaction of iron-containing minerals with oxygen, leading to the formation of iron oxides and altering the color and composition of rocks. This process is important in the context of soil development, as iron oxides contribute to the color and properties of soils. The presence of rust-colored rocks is often indicative of the occurrence of oxidation weathering in a particular geological setting.

The outer edge of the rock is a reddish color because of oxidation. The core has not had contact with oxygen and is the original color of the rock.

1. Exposure to Oxygen:
   • The process of oxidation begins when these iron-containing minerals are exposed to oxygen (O2) in the atmosphere or in water.
2. Initiation of Oxidation Reaction:
   • Oxygen reacts with iron in the mineral, leading to the initiation of oxidation. The reaction involves the transfer of electrons from iron to oxygen.
3. Chemical Reaction:
   • The general oxidation reaction can be represented as follows: Iron-containing mineral+Oxygen→Iron Oxide (Rust)
4. Formation of Iron Oxides:
   • The primary outcome of oxidation weathering is the formation of iron oxides, commonly known as rust. The iron in the mineral combines with oxygen to form various iron oxide minerals.
5. Color Change in Rocks:
   • As iron oxides form, they often impart a characteristic reddish-brown color to the affected rocks. This coloration is a visual indicator of oxidation weathering.
6. Weakening of Minerals:
   • The process of oxidation can weaken the structure of minerals, making them more susceptible to physical weathering processes.
7. Particle Disintegration:
   • Over time, the affected minerals may disintegrate into smaller particles, contributing to soil formation. The iron oxides can become a component of the resulting soil.
8. Environmental Factors:
   • The rate of oxidation is influenced by environmental factors such as temperature, humidity, and the presence of organic matter.

Solution

Solution is the process in which a mineral dissolves completely without producing any new solid substance. Rainwater easily dissolves soluble minerals, such as halite or gypsum, but can also dissolve highly resistant minerals such as quartz, given sufficient time. This process is particularly important in the development of karst landscapes and can contribute to the enrichment of soils with dissolved minerals.

1. Water Infiltration:
   • The process begins with the infiltration of water into rocks or minerals. Water, as a solvent, plays a crucial role in solution weathering. In the picture, this is in the form of rain.
2. Contact with Soluble Minerals:
   • Water comes into contact with the soluble minerals, leading to the dissolution process. In the picture, you can see the water runs off the rock and pools in the low points. This allows the water more time to dissolve these areas so they dissolve faster leading to cracks. this can also lead to the creation of caves, limestone pavements, and many other features.
3. Formation of Solution:
   • A solution is formed as the mineral dissolves in water. The dissolved ions, such as sodium (Na+), chloride (Cl-), or calcium (Ca2+), become part of the solution.
4. Dissolution Reaction:
   • The general dissolution reaction can be represented as follows:
   Soluble mineral+Water→Dissolved ions in solution and here is a real example Quartz + Water → silicic acid Acid rain falls on the rock. The acidic water dissolves channels in the rock.

Biological Weathering:

The next type of weathering is biological and it has elements of chemical weathering and physical weathering. Biological weathering as the name suggests is carried out by living organisms both big and small.

Roots and Plants

There are a few different ways in which plants can help in the weathering process. The first way is the most visible form as plants grow between cracks in the rock. Over time the plants and the roots grow and push outwards this pushes on the crack and widens it.

The most common forms of biological weathering result from chelation.

1. Root Growth and Penetration:
   • The process begins with the growth and penetration of plant roots into the soil. As roots extend, they come into contact with minerals present in the soil.
2. Release of Organic Compounds:
   • Specialized cells in the roots release organic compounds, often in the form of organic acids. Common examples include citric acid, oxalic acid, and malic acid.
3. Chelation Process:
   • Organic acids released by the roots contain functional groups that can form complexes with metal ions present in minerals. This process is known as chelation. The organic acid wraps around the metal ions, creating stable, water-soluble complexes.
4. Breaking Mineral Bonds:
   • The chelating compounds weaken the bonds between mineral particles. In particular, they target metal cations, such as calcium, magnesium, iron, and aluminum, that are essential components of minerals.
5. Mineral Dissolution:
   • As the chelation process continues, mineral particles start to dissolve. The chelating agents enhance the solubility of minerals, leading to the release of metal ions into the soil solution.
6. Nutrient Uptake:
   • The dissolved metal ions, now in a more available form, can be absorbed by the plant roots. This is especially important for essential nutrients like calcium, magnesium, and iron.
7. Acidification of the Rhizosphere:
   • The release of organic acids during chelation weathering contributes to the acidification of the rhizosphere, the soil region influenced by root activity. This localized decrease in pH further aids in mineral weathering.

The roots give off hydrogen and acids which help break down rocks and allow the plant to absorb the minerals and metals it needs for growth.

In addition to chelation carbon dioxide can also play a big role, bacterial activity and the respiration of plant roots raise CO2 levels in the soil, thereby aiding solution.

Last but not least decaying remains of dead plants in soil may form organic acids which, when dissolved in water, cause chemical weathering.

Animal Impact

One of the easiest to see is the weathering from footsteps from people or animals. We can see this on the doorstep here. over hundreds of years of people coming in and out of the house has worn down the steps. We can also see this on animal paths, overtime many animals walk the same tracks repeatedly which compresses the soil and also weathers rocks in the same way as the footsteps on the doorstep.

Weathered door step.

Lichens

Another important and often overlooked organism is lichen, Lichens on rocks are among the most effective biological agents of chemical weathering. For example, an experimental study on hornblende granite in New Jersey, US, demonstrated a 3x – 4x increase in weathering rate under lichen-covered surfaces compared to recently exposed bare rock surfaces. Lichens have also been observed to pry mineral grains loose from bare shale with their hyphae (root-like attachment structures), a process described as plucking, and to pull the fragments into their body, where the fragments then undergo a process of chemical weathering not unlike digestion

Lichens weathering the rock.

Misconceptions

1. Misconception: Weathering Only Affects Rocks:
   • Clarification: Weathering can affect not only rocks but also various materials, including soil, minerals, and even human-made structures. It’s a broader process than just rock breakdown.
2. Misconception: Weathering and Erosion Are Interchangeable:
   • Clarification: Weathering is the breakdown of rocks, while erosion involves the movement and transport of the weathered particles. They are related but distinct processes.
3. Misconception: Weathering Only Results in Breakdown, Not Formation:
   • Clarification: While weathering can break down rocks, it also contributes to the formation of new minerals and soil. It’s a dynamic process involving both degradation and creation.