Weathering is the in place (in situ) breakdown of rock materials at or near Earth’s surface. Weathering processes are fundamentally important in the generation of the soils, sediments and sedimentary rocks that cover more than 80% of Earth’s surface. Most sediment originates as solid detrital particles and dissolved solids produced during weathering. These materials are subsequently eroded and dispersed by water, wind, glaciers and mass flows across Earth’s surface to be deposited as detrital and biochemical sediments.
Weathering is also the dominant process in the production of soils upon which so many essential human activities depend. Soils and sediments provide critical wildlife habitats in both terrestrial and aquatic environments, serve as aquifers and aquitards critical for the storage and transmission of water, contain critical supplies of coal, petroleum, natural gas and ore deposits, and are widely used as raw materials in the construction of roads, dams, buildings and other structures. Soils are essential to agriculture and the production of forest products. Weathering processes determine soil texture, soil nutrient and water retention properties and therefore the types of crops that can be successfully grown in a given area. In short, we could not survive on Earth without the sediment and soil- producing processes involved in weathering.
Weathering involves an interactive set of physical, chemical and biological processes that result in the in situ breakdown of rock material at or near Earth’s surface. Weathering may occur in the original source area where bedrock is exposed, or in rock materials that have been eroded, transported and deposited thousands of kilometers away from their original source area.
Weathering and erosion are two important, but different, sets of processes. The distinguishing factor between weathering and erosion is that weathering processes involve the breakdown of rock material in a particular location, whereas erosion processes involve the removal of rock material from a geographic location which initiates its transportation to another location. Weathering and erosion are intimately related because weathering generally breaks rock materials down into smaller detrital, organic or dissolved constituents whose small size (unless they are cohesive) makes them more easily removed by erosion and dispersed by transportation. As will be seen in the discussions that follow, different rock materials weather at different rates, a process called differential weathering. Less resistant rocks that break down more rapidly tend to be eroded more rapidly. More resistant rocks that weather more slowly tend to erode more slowly.
Differential weathering, combined with differences in rock durability, tends to produce differential erosion as the products of weathering are removed at different rates. Figure illustrates differential erosion between durable, cliff-forming sandstones and more easily eroded mudrocks in southern Utah.Major weathering processes may be subdivided into disintegration and decomposition processes. Disintegration is the breakdown of larger, more coherent rock bodies into smaller fragments of the same composition. As discussed below, disintegration may involve both physical (mechanical) and biological processes. Disintegration generates an increased number of smaller rock or mineral fragments of the original material that is being disintegrated. When such fragments are eventually transported and deposited as sediment they may allow us to recognize the kinds of rock material originally exposed in the source area and to infer their dispersal pathways. This may allow us to pinpoint the source or provenance of a particular sediment as being from a particular place where such rocks are (or were) exposed.
Decomposition is any breakdown of rock materials that involves changes in chemical composition. Decomposition generally alters a rock’s mineralogy so that minerals stable at higher temperatures or pressures are altered to minerals stable at the temperatures and pressures near Earth’s surface. Decomposition involves both inorganic and organic chemical processes and is strongly dependent on the availability of water, which plays a significant role in decomposition processes. In the sections that follow, we will examine how the types of weathering processes and the degree of weathering that occur at a particular place vary greatly depending upon factors such as climate, rock type, slope and time. Disintegration is more prevalent in cold and dry climates; decomposition processes dominate in warmer and wetter climates (Figure). Steep slopes favor short-term, incomplete decomposition as rock materials are removed rapidly by erosion before decomposition is complete, whereas gentle slopes favor longer term, more thorough weathering.
