Weathering

Weathering is the physical disintegration and chemical decomposition of older rock that produces solid particulate residues - resistant minerals and rock fragments - and dissolved chemical substances. ^[1] It is the first step in the chain of processes that produce sedimentary rocks. ^[1] That position at the very beginning of the sedimentary cycle means that weathering sets limits on everything that follows - the grain types available for transport, the dissolved ions available for chemical precipitation, and ultimately the composition of the sedimentary rock record.

Weathering operates through chemical, physical, and biological processes, though chemical processes are by far the most important. ^[1] The dominance of chemical weathering comes down to water. Because some water is present in almost every environment, chemical weathering processes are commonly far more important than physical weathering, even in arid climates. ^[1] That said, the low temperatures of the weathering environment - generally below 30°C - mean that even the dominant chemical processes proceed very slowly. ^[1] Speed and dominance are different things: chemical weathering governs the outcome even when it moves slowly.

The composition of terrigenous sedimentary rocks such as sandstones is controlled not only by the composition of the source rock but also by the nature, intensity, and duration of weathering and soil-forming processes. ^[1] This is why two sandstones derived from the same granite - but subjected to different weathering intensities - can have very different mineralogy. A briefly weathered granite supplies fragments of feldspar alongside quartz; a granite subjected to prolonged intense weathering supplies almost nothing but quartz, because all the less stable minerals have been destroyed.

Types of Weathering

Physical weathering is the process by which rocks are broken into smaller fragments through a variety of causes, but without significant change in chemical or mineralogical composition. ^[1] Its role is to increase the surface area of rock exposed to chemical attack, which is why physical and chemical weathering are not truly independent. In most environments, except extremely cold or very dry climates, the two act together and it is difficult to separate their effects. ^[1]

Chemical weathering involves changes that can alter both the chemical and mineralogical composition of rocks, as minerals are attacked by water and dissolved atmospheric gases such as oxygen and carbon dioxide. ^[1] Some components of the minerals dissolve and are removed in solution; others recombine in place and crystallize to form new mineral phases. ^[1] The net result is that the original rock fabric is disrupted, producing both a residual accumulation of resistant grains and a new suite of secondary minerals that were not present in the original rock.

Three Products of Weathering

Subaerial weathering generates three types of products that are important to the formation of sedimentary rocks. ^[1] Each product type takes a different path and ends up in a different kind of sedimentary rock, which is why understanding these three categories is fundamental to understanding the diversity of the sedimentary record.

The first product type is source-rock residues: chemically resistant minerals and rock fragments that survive weathering largely intact, derived particularly from siliceous rocks such as granite, rhyolite, gneiss, and schist. ^[1] Quartz is the archetypal survivor here - it resists chemical attack so effectively that it accumulates as the dominant residual mineral in mature sediments. These residues, after erosion and transport, are the building blocks of sandstones, conglomerates, and mudrocks.

The second product type is secondary minerals formed in place by chemical recombination and crystallization, largely as a result of hydrolysis and oxidation. ^[1] Clay minerals are the dominant secondary minerals produced this way. Clay minerals do not exist in unweathered granite or basalt; they are created during weathering when silicate minerals break down under the attack of acidic water. These clay minerals become the principal component of shales and mudrocks.

The third product type is soluble constituents released from parent rocks mainly by hydrolysis and simple solution. ^[1] These dissolved ions are the invisible but geochemically critical output of weathering. They enter groundwater and rivers, travel to the ocean, and are eventually extracted by biological and chemical processes to form limestone, chert, and evaporite deposits.

Soluble Products and Their Fate

The soluble materials produced by chemical weathering are removed from the weathering site in surface water or soil groundwater more or less continuously throughout the weathering process. ^[1] They ultimately make their way into rivers and are carried to the ocean. ^[1] The most abundant inorganic constituents of rivers, representing the principal soluble products of weathering, are - in order of decreasing abundance - HCO₃^- (bicarbonate), Ca²⁺, H₄SiO₄ (silicic acid), SO₄^2- (sulfate), Cl^-, Na⁺, Mg²⁺, and K⁺. ^[1] These dissolved constituents are the raw materials from which chemically and biochemically deposited rocks such as limestones and cherts are formed in the oceans. ^[1]

The dominance of bicarbonate in this list reflects the importance of carbon dioxide in driving chemical weathering. When rainwater dissolves CO₂ from the atmosphere or soil, it forms carbonic acid, which dissociates to release H⁺ ions - the primary agent of hydrolysis. Those H⁺ ions attack silicate and carbonate minerals, liberating calcium and other cations that become part of the river load. Bicarbonate is the counter-ion left in solution when carbonates dissolve in this way.

Controls on Weathering Rate

Weathering processes proceed at different rates depending on the climate and the mineral composition and grain size of the rocks undergoing weathering. ^[1] Climate sets the most powerful control: physical weathering is most effective in moderately cold climates (through freeze-thaw cycling) and arid climates (through salt crystallization), whereas chemical weathering is accelerated in hot, humid climates. ^[1]

Rock type creates dramatic contrasts in weathering rate. Limestones weather rapidly by solution in wet climates and much more slowly in very arid or very cold climates. ^[1] Quartz-rich sandstones cemented with silica cement, by contrast, weather very slowly under most climatic conditions. ^[1] The contrast exists because limestone is dominated by calcite, which is soluble in even mildly acidic water; quartz sandstone is dominated by covalently bonded quartz, which resists nearly all chemical attack under surface conditions.

Slope also matters. Weathering tends to be more effective on low to moderate slopes than on steep slopes because water is more likely to be retained on gentle terrain, allowing material to stay in contact with weathering agents for longer before erosion removes it. ^[1] On very steep slopes, the weathered mantle is stripped away so rapidly by erosion that soil barely accumulates, and the fresh rock surface is perpetually re-exposed rather than being progressively chemically altered.

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