Weathering

What Weathering Really Is

Weathering is the action of the elements of weather and climate on earth materials. It is not a single process but a family of processes that act either individually or together, all aimed at reducing rocks to a fragmental state. The formal definition captures both sides of the job: weathering is the mechanical disintegration and chemical decomposition of rocks through the actions of various elements of weather and climate.

The key feature that separates weathering from erosion is motion. In weathering, very little or no movement of materials takes place — it is an in-situ or on-site process. The small amount of motion that occasionally occurs during weathering is not the same as transportation, because transportation implies the actual carrying away of material by agents like rivers, wind or glaciers. Weathering only prepares the material; it does not move it.

Weathering processes are conditioned by complex geological, climatic, topographic and vegetative factors. Among these, climate is of particular importance. Not only do weathering processes differ from one climate to another, but the depth of the weathering mantle also varies with climatic regimes. In humid tropical climates the mantle is deep; in arid or cold climates it is shallow.

The Three Major Groups

Weathering processes fall into three broad groups:

1. Chemical weathering
2. Physical or mechanical weathering
3. Biological weathering

Very rarely does any one of these operate completely by itself. In practice, one process usually dominates, but others are almost always at work alongside it.

Chemical Weathering Processes

Chemical weathering is a group of processes — solution, carbonation, hydration, oxidation and reduction — that act on rocks to decompose, dissolve or reduce them to a fine clastic state. The agents are chemical reactions involving oxygen, surface water and/or soil water, and other acids.

For these reactions to speed up, three things must be present: water, air (oxygen and carbon dioxide), and heat. Beyond the carbon dioxide already present in the atmosphere, the decomposition of plants and animals adds to the quantity of carbon dioxide underground. This is why soil air is often richer in carbon dioxide than the atmosphere above.

The chemical reactions acting on minerals in rocks are very much like the reactions you would perform in a laboratory — the same principles of solution, oxidation and acid action apply.

Physical Weathering Processes

Physical or mechanical weathering depends on applied forces. These forces can be grouped into three types:

• Gravitational forces — overburden pressure, load and shearing stress
• Expansion forces — caused by temperature changes, crystal growth or animal activity
• Water pressures — controlled by wetting and drying cycles

Many of these forces act both at the surface and within the earth materials, leading to rock fracture. Most physical weathering processes are caused by thermal expansion and pressure release.

These processes are small and slow, but they can cause great damage to rocks. The reason is fatigue: rocks suffer continued repetition of contraction and expansion, and over long periods this repeated stress weakens and eventually fractures them.

Biological Activity and Weathering

Biological weathering is the contribution to, or removal of, minerals and ions from the weathering environment, along with physical changes caused by the growth or movement of organisms.

Burrowing and wedging by organisms like earthworms, termites and rodents help in two ways: they expose new surfaces to chemical attack, and they assist in the penetration of moisture and air into the ground.

Human beings also contribute, often unintentionally. By disturbing vegetation, ploughing and cultivating soils, humans help in mixing earth materials and creating new contacts between air, water and minerals.

Decaying plant and animal matter produces humic, carbonic and other acids, which enhance the decay and solubility of some elements.

Plant roots exert tremendous pressure on earth materials, mechanically breaking them apart as they grow.

Special Effects of Weathering: Exfoliation

Exfoliation is a result, not a process in itself. It has already been explained under the physical weathering processes of unloading, thermal contraction and expansion, and salt weathering.

Exfoliation is the flaking off of more or less curved sheets or shells from over rocks or bedrock, resulting in smooth and rounded surfaces. It can occur due to expansion and contraction induced by temperature changes.

Two distinct landforms arise from exfoliation:

• Exfoliation domes — result from unloading
• Tors — result from thermal expansion

Significance of Weathering

Weathering is far more than just rock breakage. It prepares the way for the formation of regolith and soils, and it sets the stage for erosion and mass movements.

The link to life on Earth is direct: biomes and biodiversity are basically a result of forests, and forests depend upon the depth of weathering mantles. Where weathering mantles are deep, vegetation thrives; where they are thin, life is sparse. …