Weathering is the breakdown and alteration of rocks, soils, and minerals. It occurs when these materials contact water, atmospheric gases, sunlight, and living organisms. Weathering works directly on site, without involving the movement or transport of broken material. It shapes landscapes and contributes to soil formation.
Physical Breakdown of Rocks
Physical weathering, also known as mechanical weathering, breaks rocks into smaller pieces without changing their chemical composition. It occurs through forces that fracture or fragment rocks. The smaller pieces retain the original rock’s mineral composition and proportions, just in a reduced size.
Frost wedging is a common physical process where water seeps into rock cracks and then freezes. As water turns into ice, its volume expands by approximately 9%, exerting pressure that widens the cracks. Repeated freeze-thaw cycles split the rock apart, often observed in colder climates or at higher elevations. Abrasion is another mechanism, occurring when one rock bumps against another. This occurs as rocks tumble down slopes or when particles carried by wind or water collide with rock surfaces.
Exfoliation, or pressure release, occurs when deeply buried rocks are brought to the surface. Intrusive igneous rocks, like granite, form under immense pressure from overlying material. When erosion removes this overburden, the pressure is released, causing them to expand and crack in layers parallel to the surface, similar to an onion peeling. Thermal expansion and contraction also contribute. As rocks are alternately heated and cooled, they expand and contract. Differing expansion rates of various minerals can create internal stresses, leading to fracturing.
Chemical Transformation of Rocks
Chemical weathering involves reactions that change the mineral composition of rocks. This process converts original minerals into secondary minerals or removes substances as dissolved solutes. Water is a primary agent in these chemical reactions, often aided by atmospheric gases and biological chemicals.
Dissolution is where minerals dissolve completely in water without forming new solid substances. Limestone, for instance, readily dissolves in slightly acidic water, formed when carbon dioxide from the atmosphere or soil dissolves in rainwater, creating carbonic acid. It forms caves and karst landscapes. Oxidation is another significant reaction, observed when oxygen reacts with iron in rocks. This reaction converts iron (Fe2+) into iron oxides and hydroxides (Fe3+), such as rust, which weakens the rock and often gives it a reddish-brown color.
Hydrolysis occurs when water reacts with minerals, changing their composition. For example, feldspar minerals in granite can react with water to form new clay minerals. This transformation makes the rock more susceptible to further breakdown. Carbonation, a specific dissolution type, involves carbonic acid reacting with calcium or magnesium carbonates, dissolving and carrying them away. These chemical changes fundamentally alter the rock’s structure and stability.
Biological Contributions to Weathering
Living organisms contribute to both the physical and chemical breakdown of rocks. This biological weathering results from direct mechanical force or chemical reactions produced by organisms. Many forms of life, including plants, bacteria, fungi, and animals, participate in these processes.
Plant roots cause physical weathering as they grow into cracks and fractures in rocks. As these roots expand, they exert pressure, wedging the rock apart. This root wedging widens fissures, making the rock more vulnerable to other weathering agents. Burrowing animals, such as rabbits, moles, or earthworms, also contribute to physical weathering by digging into the ground. Their activities disturb soil and rock, bringing new material to the surface for further weathering.
Microorganisms like lichens and bacteria significantly contribute to chemical weathering. Lichens, a symbiotic relationship of fungi and algae, attach to rock surfaces and produce organic acids. These acids react with rock minerals, helping to dissolve them. Certain bacteria also secrete chemicals that break down minerals, making soil more acidic and increasing rock degradation. Human activities, including mining, quarrying, and construction, also act as agents of biological weathering by disrupting rock formations and exposing them to environmental factors.
What Influences Weathering
Several factors collectively determine the rate and types of weathering that occur in a given environment. The interplay of these influences explains why weathering varies across geographical regions. Climate, encompassing both temperature and precipitation, is a primary control.
Warm, wet climates accelerate chemical weathering because reactions proceed more rapidly at higher temperatures, and abundant water is necessary. In contrast, cold climates with frequent freeze-thaw cycles favor physical weathering, as ice expansion repeatedly stresses rocks. Rock type and mineral composition also play a role. Rocks composed of less stable minerals, such as olivine and pyroxene, weather more quickly than those with stable minerals like quartz. Fractures, high porosity, and permeability allow water and air to penetrate more easily, increasing susceptibility to weathering.
Topography, including slope and land exposure, influences water accumulation and rock exposure to weathering agents like wind and sun. Steeper slopes might shed water quickly, while flatter areas might retain it, affecting dominant weathering processes. Exposure time also directly impacts weathering extent. As physical weathering breaks rocks into smaller pieces, it increases exposed surface area, accelerating chemical reactions.