Weathering is the geological process that breaks down rock material on the Earth’s surface. This slow-motion process transforms solid rock into smaller fragments and chemically altered materials, which form the foundational components of soil. Weathering differs from erosion, which is the physical removal and transport of those weathered particles by agents such as wind, water, ice, or gravity.
Physical Processes of Breakdown
Physical weathering breaks rocks into smaller pieces without changing their chemical makeup, relying on mechanical stress to overcome the rock’s internal strength. Frost wedging exploits existing cracks in cold climates when water seeps into fractures and freezes. Since water expands by approximately nine percent upon turning into ice, it exerts immense pressure, forcing the crack to widen.
Exfoliation results from the release of pressure on deeply buried rock masses. When overlying material is removed, the rock expands upward, causing outer layers to peel away in sheets parallel to the surface. Abrasion involves the physical grinding action of rock against rock, similar to sandpaper, as particles are carried by wind, water, or glaciers. The constant friction rounds and smooths rock fragments, such as the polished pebbles found in riverbeds.
Chemical Alteration of Rock Material
Chemical weathering involves reactions between rock minerals and external agents like water, oxygen, and atmospheric gases, which alter the rock’s chemical composition and internal structure. The most widespread type of alteration is hydrolysis, where hydrogen ions from water react with minerals, most commonly silicates like feldspar. This reaction breaks down the original mineral structure and forms new, softer minerals such as clay.
Oxidation occurs when oxygen reacts with iron-bearing minerals, causing the iron atoms to lose electrons. This reaction forms iron oxides, the red or brown compounds commonly known as rust. Dissolution, or carbonation, involves minerals dissolving in water, often aided by natural acids. Atmospheric carbon dioxide dissolves in rainwater to form a weak carbonic acid, which is highly effective at dissolving soluble rocks like limestone and marble.
Biological Contributions to Weathering
Living organisms accelerate both physical and chemical weathering. A physical form of biological weathering is root wedging, where the growth of plant roots pushes into rock cracks and joints. As roots thicken and expand, they exert considerable mechanical pressure, prying the rock apart in a manner analogous to frost wedging.
Organisms also contribute to chemical breakdown by secreting organic acids. Lichens and mosses, for example, colonize bare rock and release chelating agents like oxalic acid, which dissolve mineral matter and create microscopic pits. Burrowing animals like earthworms and rodents physically move rock fragments and expose fresh, unweathered material. This action increases the rock’s surface area, making it more susceptible to chemical attack and further physical disintegration.
Environmental Factors That Control the Rate of Breakdown
The rate at which rocks break down is controlled by the environmental conditions of a region, primarily its climate and the properties of the rock itself. Climate determines which weathering process is dominant; warm, humid environments favor chemical weathering because water is required for most decomposition reactions. For every \(10^{\circ}\text{C}\) increase in average temperature, the rate of these chemical reactions roughly doubles.
Conversely, in cold, moist climates with frequent temperature fluctuations around the freezing point, physical weathering by frost wedging is the predominant force. Rocks containing minerals that dissolve easily, such as calcite in limestone, weather faster than those made of chemically stable minerals like quartz. The presence of pre-existing fractures, joints, or bedding planes provides pathways for water and ice penetration, dramatically increasing the rate of mechanical breakdown.