What is the Difference Between Chemical and Mechanical Weathering

Weathering is a natural process that breaks down rocks into smaller pieces. There are three types of weathering: biological weathering, chemical weathering, and physical weathering. This article discusses the difference between chemical and mechanical weathering in detail.

What is the difference between chemical and mechanical weathering? Chemical weathering involves the breakdown of rocks through chemical reactions, whereas mechanical weathering involves physically breaking down rocks into smaller pieces without changing their chemical composition.

Key Areas Covered

1. What is Chemical Weathering
      – Definition, Features 
2. What is Mechanical Weathering
      – Definition, Features 
3. Similarities Between Chemical and Mechanical Weathering
      – Outline of Common Features
4. Difference Between Chemical and Mechanical Weathering
      – Comparison of Key Differences
5. FAQ: Chemical and Mechanical Weathering
      – Answers to Frequently Asked Questions

Key Terms

Chemical Weathering, Mechanical Weathering, Physical Weathering

What is Chemical Weathering

Chemical weathering is a geological process that alters the composition of rocks and minerals through chemical reactions. Unlike physical weathering, chemical weathering alters the composition of the rocks and minerals and forms new materials with different compositions. This process can be observed in Earth’s crust. Temperature, moisture, and the presence of certain substances influence this process.

Several types of reactions are involved in chemical weathering. One is hydration. Here, minerals absorb water molecules and undergo changes in structure. For example, when anhydrite, a mineral containing calcium sulfate, reacts with water, it forms gypsum, a softer and more soluble mineral.  These kinds of reactions are commonly observed in desert regions where there are fluctuating moisture levels.

Figure 1: Comparison of Unweathered (left) and Weathered Ordovician Limestone

One other common process is oxidation, where minerals react with atmospheric oxygen to form new substances. These types of reactions are mostly seen in iron-bearing minerals such as pyrite and hematite. Ultimately, the formation of iron oxide, commonly known as rust, can be observed. This process is evident in the reddish-brown coloration often seen in weathered rocks and soils.

Acid rains also contribute to chemical weathering. Rain that carry dissolved carbon dioxide reacts with minerals like limestone and marble to form soluble compounds like calcium bicarbonate. Over time, this can lead to the formation of features such as caves and sinkholes.

Another cause of chemical weathering is the secretions of organic acids by plants and microbes. This process is vital in the formation of new soil. Here, there is an interaction of organic matter with minerals, resulting in fertile soil.

What is Mechanical Weathering

Mechanical weathering is a process by which minerals and rocks are broken down into smaller pieces physically. Hence, this process is also referred to as physical weathering. The chemical composition of rocks is not altered in this process. Environmental factors such as temperature changes, pressure, frost action, and abrasion cause the physical weathering process.

When rocks are exposed to alternating periods of heating and cooling, they expand and contract. With time, this repeating process weakens the structure of the rock; thus, the rock cracks and breaks down into smaller pieces. This process is especially common in arid regions where temperature differences between day and night are pronounced.

Figure 2: Weathered Rock

Pressure is another force that is responsible for the weathering of rocks. As overlying rock layers erode, the pressure on underlying rocks decreases, leading to the expansion of those rocks. This expansion can cause fractures and disintegration, especially in rocks with existing weaknesses.

Frost action occurs in environments where water rapidly freezes and thaws within rock fractures. Here, pressure is exerted on surrounding rock walls due to the expansion of water, which exerts pressure on the surrounding rock walls. With each successive freeze-thaw cycle, the cracks widen, eventually causing the rock to fragment.

Abrasion, due to friction and impact, also causes the breakdown of rocks into smaller pieces. It occurs through processes such as wind erosion, where particles carried by the wind collide with rock surfaces, gradually wearing them away. Water erosion in rivers and lakes also smoothens and rounds the rocks, changing their shape and breaking them into smaller pieces.

Similarities Between Chemical and Mechanical Weathering

1. Chemical weathering and mechanical weathering both contribute to the breakdown of rocks.
2. They play roles in the erosion process.

Difference Between Chemical and Mechanical Weathering

Definition

• Chemical weathering involves the breaking down of rocks through chemical reactions while mechanical weathering involves the breaking down of rocks as a result of physical processes without altering the chemical composition of the rocks.

Cause

• Chemical weathering is often caused by water, acids, oxygen, and other chemical agents, whereas mechanical weathering is primarily caused by physical forces such as frost wedging, abrasion, and pressure.

Results

• Chemical weathering typically results in the formation of new minerals or mineral structures that are more stable under surface conditions. On the other hand, mechanical weathering results in the fragmentation of rocks into smaller particles without altering their chemical composition.

Examples

• Examples of chemical weathering include dissolution, hydrolysis, and oxidation, whereas examples of mechanical weathering include frost wedging, exfoliation, and root pry.

Conclusion

Chemical weathering involves the alteration of rocks through chemical reactions, leading to the formation of new minerals and structures. On the other hand, mechanical weathering breaks down rocks physically into smaller pieces without changing their chemical composition. Despite the difference between chemical and mechanical weathering, both processes are driven by various environmental factors and contribute significantly to the shaping of Earth’s surface over time.

FAQ: Chemical and Mechanical Weathering

1. What are the three types of weathering of rocks?

Biological weathering, mechanical weathering, and chemical weathering are the three types of weathering of rocks. Biological weathering is the breakdown of rocks and minerals by the action of living organisms, such as plants, animals, and microbes. Chemical weathering involves the breakdown of rocks through chemical reactions, whereas mechanical weathering involves physically breaking down rocks into smaller pieces without changing their chemical composition.

2. What are the five main types of mechanical weathering?

The five main types of mechanical weathering are thermal expansion, frost weathering/wedging, exfoliation, abrasion, and salt weathering.

1. Thermal expansion happens when temperature changes cause rocks to expand and contract, which leads to cracking.
2. Frost weathering occurs when water in rock cracks freezes and expands, breaking the rock apart.
3. Exfoliation, on the other hand, is the peeling away of outer rock layers due to pressure release or temperature shifts.
4. Abrasion is the scraping of rock surfaces by other rocks or particles carried by wind, water, or ice, causing gradual wear.
5. Salt weathering involves the growth of salt crystals in rock cracks, which exerts pressure and leads to rock disintegration.

3. What are examples of chemical weathering?

Some examples of chemical weathering are rust, which happens through oxidation, acid rain, which is caused by carbonic acid dissolving rocks, and secretions of organic acids by plants and microbes.

4. What are two types of chemical weathering?

Hydrolysis and oxidation are two types of chemical weathering. Hydrolysis involves the breakdown of rock by acidic water to produce clay and soluble salts, whereas oxidation involves the breakdown of rock by oxygen and water.

5. What are the main causes of chemical weathering?

The causes of chemical weathering are chemical reactions such as oxidation, carbonation, hydrolysis, and acid-base reactions. These processes often involve interactions with water, gases, and organic materials, and break down rocks by altering their chemical composition.

Reference:

1. “Chemical Weathering – An Overview.” Science Direct.
2. “Physical Weathering.” The Rock Cycle. The Geological Society.

Image Courtesy:

1. “Weathering Limestone State College PA” By Jstuby at English Wikipedia – Own work (Public Domain) via Commons Wikimedia
2. “KharazaArch” By Etan J. Tal – Own work (CC BY 3.0) via Commons Wikimedia