Weathering is the set of processes that physically disintegrate and chemically decompose rocks and minerals found at or near the Earth’s surface. This constant exposure to the atmosphere, water, and biological activity causes the parent rock material to become unstable. The products of this breakdown are varied, forming the loose surface material that covers the solid bedrock. These resulting materials are important to geology and the formation of life-supporting systems.
Clastic Sediments: Products of Mechanical Breakdown
Mechanical weathering involves the physical reduction of rock and mineral size without altering their chemical composition. This disintegration generates detrital or clastic particles, which are fragmented debris broken off from the parent rock. The resulting loose material, known as sediment, is classified based on the size of these fragments, ranging from large boulders down to clay-sized particles.
Particle size categories include coarse-grained gravel, medium-grained sand, and the finer materials of silt and clay. Quartz is the most commonly found mineral grain in these sediments due to its exceptional hardness and chemical resistance to surface weathering processes. Other minerals that are more susceptible to chemical breakdown, such as feldspar and iron-rich silicates, are much less abundant in mature sediments.
The energy of the transporting medium, such as a river current or wind, dictates how well sorted the sediment is, affecting the range of particle sizes present in a deposit. High-energy currents can carry larger fragments, but as the energy decreases, heavier particles settle out, leading to size sorting.
Initially, fragments freshly broken from bedrock tend to have angular shapes, reflecting the fractures in the source rock. These particles become progressively smoother and more rounded as they are transported over distance by air or water, a process that gives geologists clues to their travel history. This accumulation of unconsolidated sediment forms the raw material that can eventually be lithified into clastic sedimentary rocks such as conglomerate, sandstone, and shale.
Dissolved Matter and Solutes
Chemical weathering processes, such as dissolution, produce products that are not solid fragments. Dissolution occurs when minerals break down and their constituent elements are released as ions, or solutes, into the surrounding water. Minerals like halite (table salt) or gypsum dissolve readily in water, separating into their respective ions, such as sodium and chloride.
Other minerals, particularly calcite found in limestone, dissolve when exposed to weak acids, such as carbonic acid. Carbonic acid forms when carbon dioxide from the atmosphere or soil reacts with water. This reaction releases calcium and bicarbonate ions into the solution, which can eventually lead to the formation of extensive underground cave systems.
The dissolution of common silicate minerals releases various ions, including sodium, potassium, calcium, iron, and magnesium. These dissolved ions are carried away from the site of weathering by groundwater and river systems. They are eventually transported to the oceans, where they contribute significantly to the overall salinity of the seawater. Alternatively, these solutes may precipitate elsewhere to form chemical sedimentary rocks, such as evaporites like rock salt or gypsum, when the water evaporates.
Secondary Minerals: Newly Formed Solids
Some chemical weathering reactions do not simply dissolve the parent minerals but instead transform them into entirely new solid compounds known as secondary minerals. This process, often involving hydrolysis, occurs when primary minerals that formed under high pressure and temperature become unstable at the Earth’s surface. The most significant group of these newly formed solids are the clay minerals, which are hydrous sheet silicates.
Feldspars, which are abundant in many rocks, commonly undergo hydrolysis when exposed to slightly acidic water, altering them into clay minerals like kaolinite. The chemical structure of these clays involves the combination of tetrahedral silica sheets and octahedral alumina sheets. This layered structure gives clay minerals a high surface area relative to their volume, which is important for their ability to adsorb water and nutrients.
Different environmental conditions produce various types of clays. Kaolinite forms in well-drained, acidic environments, while smectite forms in poorly drained, alkaline conditions and exhibits a high capacity for swelling.
Oxidation is another process that creates secondary minerals when iron-bearing silicates are exposed to oxygen and water. This reaction produces iron oxides and hydroxides, such as hematite and goethite. These compounds are highly stable in surface conditions and are responsible for the distinctive red, yellow, and brown coloration seen in many soils and weathered rock profiles.
The Ultimate Accumulation: Regolith and Soil
The ultimate fate of all the weathering products is their accumulation as a layer of unconsolidated material covering the solid, unweathered bedrock. This blanket of loose, heterogeneous material is defined as the regolith. The regolith is composed primarily of the clastic fragments, secondary minerals, and residual primary minerals that resist breakdown.
A major component of the regolith is saprolith, which represents partially weathered rock that still retains the structure of the original parent material. While the regolith covers the entire Earth’s surface, the term soil refers specifically to the uppermost layer of the regolith. Soil is a much more complex mixture, distinguished by its incorporation of organic matter, along with water and air, making it capable of supporting plant life.
The regolith is generally inorganic, but decaying plant and animal material transforms the upper portion into fertile soil. The properties of the soil layer are directly determined by the type and proportion of the weathering products it contains. For example, the mix of sand, silt, and clay-sized clastic particles dictates the soil’s texture and drainage. Furthermore, the presence of secondary clay minerals and trapped solutes influences the soil’s nutrient-holding capacity and overall fertility.