Biological Weathering: Definition, Agents, Types, and Impact

Biological Weathering is a fundamental geological process where the disintegration and decay of rocks and minerals occur due to the activity of living organisms. 

This process is crucial for breaking down the Earth's surface materials, playing a vital role in soil formation and the nutrient cycle. Students studying Earth sciences must understand the intricate mechanisms of Biological Weathering, which involves both physical and chemical actions by plants, animals, and microorganisms.

Biological Weathering

Biological Weathering refers to the breakdown of rocks and the removal of minerals and ions from the Earth's crust as a direct result of the growth, movement, or chemical secretions of living organisms. It is sometimes also referred to as Organic Weathering.

Unlike physical (mechanical) weathering, which uses force, or chemical weathering, which uses chemical reactions from water and air, biological weathering is specifically induced by biotic factors (living things). However, this process often combines both physical and chemical means to achieve the disintegration of rock material.

Four Major Agents of Biological Weathering

The primary agents responsible for causing this type of rock breakdown are categorised into four main groups:

1. Plants (Flora): The most recognisable agents, where the growth of roots is a major factor.

2. Animals (Fauna): Burrowing creatures that disturb the soil and rock fragments.

3. Microorganisms: Tiny life forms like bacteria, fungi, algae, and lichens that secrete chemical compounds.

4. Humans (Anthropogenic Activities): Human actions like mining, construction, and agriculture significantly speed up the weathering process.

Types of Biological Weathering

Biological weathering can be broadly classified into two categories based on the mechanism of rock disintegration:

Biological Weathering by Physical Means

This involves a mechanical force or pressure exerted by the organism that physically breaks the rock apart without changing its chemical composition.

• Action of Plant Roots (Root Wedging): As trees and plants grow, their roots penetrate the soil to search for nutrients and water. The roots enter pre-existing cracks, joints, and crevices in rocks. As the roots widen and increase in mass, they exert immense pressure on the rock walls, causing the cracks to widen further and eventually leading to the splitting and breakage of the rock into smaller fragments.

• Burrowing and Tunnelling Animals: Animals like earthworms, rodents (shrews, moles, rabbits), and ants dig holes and passages in the ground for shelter or nesting. This action loosens the soil and rock particles, bringing fresh rock material to the surface, where it is more vulnerable to other forms of weathering (chemical and physical). Marine organisms like the Piddock shell also bore into rocks for protection.

• Human Activities: Activities like ploughing, cultivation, construction, and quarrying forcefully expose rock surfaces and crush soil particles, making human activity one of the most effective and rapid agents of physical and biological weathering.

Biological Weathering by Chemical Means

This involves the chemical alteration or corrosion of rock minerals due to organic compounds secreted by living organisms. This mechanism is also why biological weathering is often called Organic Weathering.

• Organic Acids from Plants and Roots: Some plant roots, in their quest to extract minerals from the rock, secrete organic acids. These acids react with the rock's minerals, dissolving them and weakening the rock structure.

• Role of Lichens, Algae, and Bacteria: Microscopic organisms like algae, moss, lichens, and bacteria thrive on bare rock surfaces. Lichens and certain fungi and bacteria produce humic acids and other organic acids (like oxalic acid) as metabolic byproducts. These powerful acids chemically dissolve the rock's minerals, releasing nutrients that the organism then consumes in a process called the ion-exchange mechanism. This change in the rock’s chemical composition gradually erodes the outer layer.

• Decomposition of Organic Matter: When plants and animals die, their organic matter decomposes. This decomposition process releases carbon dioxide (CO₂) into the soil. This CO₂ reacts with water (H₂O) present in the soil to form a weak carbonic acid (H₂CO₃). This acid is capable of dissolving minerals and degrading the rock surfaces, contributing to chemical weathering.

Significance of Biological Weathering

Biological weathering is a vital process that has far-reaching consequences for the Earth’s surface and ecosystem:

• Soil Formation: It is fundamental to the formation of soil. Breaking down large rocks into smaller particles like silt, clay, and sand it provides the essential mineral components of fertile soil.

• Nutrient Cycling: The process of dissolving rock minerals liberates essential nutrients (like phosphorus, potassium, and calcium) from the rock material, making them available in the soil for plants to absorb. Without this process, plant life would not be able to obtain these minerals from the soil.

• Landscape Modification: Over long periods, the cumulative effect of biological weathering, often working alongside physical and chemical weathering, significantly weakens the Earth's surface materials and contributes to the overall shaping and degradation of landscapes.