Weathering

Weathering is the in situ breakdown of rocks, minerals, soils and artificial materials through prolonged exposure to water, atmospheric gases, sunlight and biological activity. It differs from erosion, which involves the physical removal and transport of material by agents such as water, wind, ice or gravity. Weathering contributes significantly to shaping Earth’s landscapes and plays a central role in the rock cycle, with sedimentary rocks—formed from weathered material—covering around two-thirds of the continents and extensive areas of the ocean floor.

Nature and Significance of Weathering

Weathering encompasses both physical and chemical processes, with biological activity often contributing to both. The immediate products of weathering combine with organic matter to form soil, making weathering essential to ecosystem development, nutrient cycling and agricultural productivity. Many distinctive landforms arise from the interplay of weathering, erosion and redeposition; for example, tafoni structures, inselbergs, and cavernous surfaces in arid or coastal regions owe their origins to differential weathering patterns.
Water acts as the principal agent in both major categories of weathering, although atmospheric gases such as oxygen and carbon dioxide, together with the biochemical activities of organisms, play vital roles in altering mineral composition and accelerating rock decay.

Physical Weathering

Physical, or mechanical, weathering refers to the disintegration of rock without alteration of its chemical composition. It arises from mechanical stresses generated by temperature changes, pressure changes, or the intrusion of expanding materials into rock voids. These processes fragment rocks into progressively smaller particles, increasing the surface area available for subsequent chemical weathering.
Major forms of physical weathering include:
• Freeze–thaw and frost-related processes: Frost weathering results from the formation of ice within rock pores and fractures. Traditional explanations emphasised frost wedging, in which porewater freezes, expands and exerts sufficient pressure to widen cracks. Although water expands by approximately 9% on freezing, ice in open, straight fractures cannot build high pressure because it simply expands outward. Frost wedging therefore requires complex, narrow fractures and near-saturation by water—conditions that, while possible, are not universally common.
A more influential mechanism is ice segregation. Rock temperatures just below freezing allow thin premelted water films to persist on ice grain surfaces. These draw additional water through capillary action toward forming ice lenses, exerting pressures that may exceed those from frost wedging and are capable of prising rocks apart. Ice segregation effectively operates in climates with frequent freeze–thaw cycling and temperatures hovering around 0°C.
• Thermal stress weathering: Temperature fluctuations cause expansion and contraction within rock bodies. When differential heating occurs—typically where one rock surface is exposed and the remainder is constrained—stress accumulates. Thermal shock may cause immediate fracturing, although more commonly repeated cycles lead to thermal fatigue, which gradually weakens the material. In deserts, large diurnal temperature variations promote this form of weathering, but it is also significant in cold regions and in areas exposed to wildfires. Modern research has renewed interest in thermal weathering after earlier laboratory experiments underestimated its importance due to unrealistic experimental conditions.
• Pressure release (unloading): Intrusive igneous rocks formed deep within the Earth are subjected to high overburden pressures. When erosion removes the overlying material, the reduction in pressure allows the rock to expand, producing stress parallel to the exposed surface. This leads to exfoliation, in which sheets of rock detach over time. Pressure release contributes to joint formation, spalling in mines and quarries, and exfoliation domes in granitic terrains. The retreat of glaciers can enhance this process by removing substantial overburden.
• Salt crystallisation: Salt weathering, or haloclasty, occurs when saline solutions infiltrate fractures and evaporate, leaving behind salt crystals. These can grow through capillary action and form salt lenses that exert high pressures, particularly when composed of sodium or magnesium salts. Salts generated by oxidation of pyrite may also crystallise as gypsum or iron(II) sulphate, contributing further to disintegration. Salt weathering is common in arid environments and along coastal zones and contributes to features such as tafoni.
• Biological mechanical effects: Organisms may facilitate mechanical weathering. Plant roots penetrating fractures exert prying forces that widen cracks. Burrowing animals disturb soil–rock interfaces, and lichens may pluck mineral grains from exposed surfaces. Although often secondary to abiotic mechanisms, these processes can accelerate rock fragmentation substantially, especially in environments with abundant vegetation.

Chemical and Biological Weathering

Chemical weathering involves reactions that alter the mineralogical composition of rocks, often in the presence of water and atmospheric gases. Processes include oxidation, hydrolysis, carbonation and solution. Chemical alteration weakens rocks, making them more susceptible to physical breakdown. Biological weathering is sometimes regarded as a subcategory of chemical weathering, as organisms produce acids and chelating agents that dissolve and transform minerals. Microorganisms, plants and lichens contribute to mineral decay through biochemical pathways, playing a major role in soil formation.

Interdependence of Weathering Processes

Although physical and chemical weathering are classified separately, they often act synergistically. Physical weathering increases the rock’s surface area, enhancing chemical reactions. Chemical weathering alters minerals, making them more friable and prone to mechanical breakdown. This interplay underlies the gradual sculpting of landscapes and the generation of sediments transported by erosion.

Weathering and Landscape Evolution

Weathering exerts a controlling influence on geomorphological development. Differential weathering of rock types results in varied landforms, as harder rocks resist breakdown while softer rocks decay more rapidly. Features such as cliffs, tors, arches and pedestal rocks often owe their form to contrasts in weathering resistance. Jebel Kharaz in Jordan, for example, exhibits striking shapes formed through erosion of differentially weathered sandstone.
In cold regions, frost-related processes dominate; in arid regions, salt and thermal stress weathering are more prominent; and in temperate climates, chemical and biological weathering take precedence. Over geological timescales, weathering contributes to nutrient cycles, climate regulation through carbon sequestration in minerals, and the continual reshaping of Earth’s surface.