How does biological weathering occur?

Biological weathering happens when living things (plants, animals, and microbes) break rocks by chemical reactions with moisture and by physical stress like root growth and burrowing.

What is biological weathering?

Biological weathering is the breakdown of rocks due to living organisms. It is different from pure physical weathering (like cracking due to heat and cold) and from pure chemical weathering (like dissolution by rainwater alone). In biological weathering, life drives the damage. Roots push into cracks, animals dig tunnels, and tiny organisms release weak acids. 

Over time, these actions widen joints, change minerals, and turn hard rock into smaller pieces and soil. This process is slow, but it shapes slopes, creates fertile ground, and prepares land for plants and humans.

Main ways biological weathering works

• Root wedging (physical stress): Plant roots search for water and nutrients. They enter tiny cracks in rocks. As roots grow thicker, they push the crack walls apart. This mechanical pressure widens fractures, causes flakes to detach, and finally breaks blocks into smaller fragments.
• Organic acids (chemical reaction): Lichens, mosses, fungi, and bacteria produce weak acids (like oxalic acid). These acids react with rock minerals in the presence of moisture. The reaction dissolves or loosens minerals such as feldspar and calcite, making the rock softer and easier to crumble.
• Humic substances from decaying matter: When leaves and roots rot, the soil makes humic and fulvic acids. Rainwater carries these acids into cracks. The acids chelate (bind) metal ions like iron and aluminum, changing the mineral structure and speeding breakdown.
• Carbon dioxide from respiration: Roots and microbes release CO₂. Dissolved in water, CO₂ forms carbonic acid. This weak acid reacts with minerals (especially in limestone and marble), leading to gradual dissolution.
• Burrowing and boring (physical disturbance): Ants, termites, earthworms, rodents, and even shell-boring marine organisms move grains, open voids, and expose fresh rock surfaces to air and water. More exposure means faster chemical reactions.
• Salt and moisture trapping by vegetation: Plants can trap moisture and salts at rock surfaces. Repeated wetting and drying, along with salt growth, stresses the rock and encourages granular disintegration.

Role of moisture and mineral reactions?

Water is the partner of biology in this process. Roots carry water; microbes live in thin water films; decayed matter holds moisture like a sponge. With moisture present, acids can dissolve minerals, and ions can move. For example:

Where you can observe it?

• Tree roots lifting paving stones and splitting garden walls.
• Lichens coloring bare rock faces; under the lichen, the surface turns powdery.
• Termite mounds and burrows loosening soil on slopes.
• Cracked rocks along forest edges where roots are dense and soils stay moist.

Why biological weathering matters?

• Soil formation: It creates fine particles and releases nutrients, building fertile soil.
• Landscape change: It prepares rock for faster physical and chemical weathering.
• Engineering impact: It can weaken foundations, retaining walls, and road cuts.
• Ecological benefits: It cycles elements like carbon, phosphorus, and iron.