Weathering and erosion are natural processes which reshape Earth’s rock into ever-shifting terrain, shaping rivers, streams and oceans in their path.
Once weathering breaks rocks into small pieces, erosion carries them to different locations where gravity deposits them into layers of sedimentary deposit.
Physical
Physical weathering breaks rocks and minerals down into smaller pieces without altering their chemical makeup, with pieces then carried away by erosion to be deposited elsewhere. Physical weathering is most evident at beaches or when looking at Apple Rock which has split apart due to expanding and contracting ice sheets expanding onto it and splitting its rocks apart.
Water is one of the primary agents of physical weathering. It may seep into cracks and gaps in rocks, freeze if temperatures dip, then thaw during winter – this process known as cry fracturing can cause small cracks to widen into larger ones, eventually breaking apart rock surfaces.
Plants and burrowing animals can also contribute to physical weathering. As plants grow, their roots trample and erode rock surfaces while root action enlarges cracks via wet-dry cycles and root action. Furthermore, stress along natural zones of weakness in rocks may cause physical weathering; examples include bedding planes, fractures joints or bedding striations. This form of weathering most commonly occurs with sedimentary rocks which contain less stable silicate minerals (quartz, K-, Na rich feldspars) than amphiboles and pyroxenes found in igneous rocks.
Chemical
Chemical weathering involves breaking down rock through chemical reactions. It typically occurs in warm, wet conditions; for instance, when slightly acidic rain or sea water interacts with limestone it erodes through carbonation, as can other rocks containing calcium such as chalk. As such it results in the topography known as Karst which features sinkholes, caves and distinct rock formations.
Most minerals form at extreme pressures and temperatures deep within Earth’s crust, but must eventually change in order to become stable at lower temperatures and pressures nearer its surface. This chemical weathering alters hillslope form while also impacting sediment transport and landscape morphology.
Chemical weathering includes various processes like abrasion, dissolution and oxidation that break down plants and bacteria that contribute to ecosystem nutrition cycles. Chemical weathering also results in the breakdown of plants that help create ecosystem diversity through cycling nutrients through ecosystems.
Biological
Biological weathering refers to rock erosion caused by living organisms, using both biochemical and physical processes for its breakdown.
Mosses and lichen, as they grow on rocks, cause them to break down through biological weathering processes that alter their chemical makeup and increase fragility, leaving the rock vulnerable to other weathering processes as well as increasing chances for cracking in its structure.
Lichens and cyanobacteria produce organic acids which aid in further erosion of rocks.
Biological weathering is particularly crucial in arid regions, where rock can often be more fragile. Weeds growing into gaps in concrete can expand these holes and break it, then fall away leaving an opportunity for further biological weathering to continue until all rock has been decomposed.
Environmental
Weathering affects all aspects of a landscape and plays a vital role in the geologic cycle. It changes rock materials while releasing chemical compounds which affect soil development, atmospheric chemistry and temperature.
Physical weathering erodes rocks, often breaking them apart in the process. Frost cracking occurs when water in cracks freezes and expands, widening and eventually breaking apart a rock’s structure. Wind is another physical weathering agent; it carries dust and sand between places as it erodes smooth, curved layers found at Arches National Park and Goblin Valley State Park in Utah that give these places their distinct look.
Erosion is a mechanical process driven by water, gravity (see Chapter 10), wind or ice (see Chapter 14) that transports rock and sediment away from their original sites of weathering to new ones. Liquid water is the principal agent behind erosion; other natural forces contributing include avalanches, landslides and mass wasting (see Chapter 16). Human activities may hasten this process too – for instance air pollution causes rocks to erode faster by emitting chemicals into the atmosphere and thus speeding up weathering rates even further.