Denudation

Denudation incorporates the mechanical, biological, and chemical processes of erosion, weathering, and mass wasting. Denudation can involve the removal of both solid particles and dissolved material. These include sub-processes of cryofracture, insolation weathering, slaking, salt weathering, bioturbation, and anthropogenic impacts. Factors affecting denudation include: • Anthropogenic (human) activity, including agriculture, damming, mining, and deforestation; • Biosphere, via animals, plants, and microorganisms contributing to chemical and physical weathering; • Climate, most directly through chemical weathering from rain, but also because climate dictates what kind of weathering occurs; • Lithology or the type of rock; • Surface topography and changes to surface topography, such as mass wasting and erosion; and orogeny, the process that forms mountains. ==Historical theories==

The effects of denudation have been written about since antiquity, although the terms "denudation" and "erosion" have been used interchangeably throughout most of history. In the Age of Enlightenment, scholars began trying to understand how denudation and erosion occurred without mythical or biblical explanations. Throughout the 18th century, scientists theorized valleys are formed by streams running through them, not from floods or other cataclysms. In 1785, Scottish physician James Hutton proposed an Earth history based on observable processes over an unlimited amount of time, which marked a shift from assumptions based on faith to reasoning based on logic and observation. In 1802, John Playfair, a friend of Hutton, published a paper clarifying Hutton's ideas, explaining the basic process of water wearing down the Earth's surface, and describing erosion and chemical weathering. Between 1830 and 1833, Charles Lyell published three volumes of Principles of Geology, which describes the shaping of the surface of Earth by ongoing processes, and which endorsed and established gradual denudation in the wider scientific community. As denudation came into the wider conscience, questions of how denudation occurs and what the result is began arising. Hutton and Playfair suggested over a period of time, a landscape would eventually be worn down to erosional planes at or near sea level, which gave the theory the name "planation". Publication of the Davisian cycle of erosion caused many geologists to begin looking for evidence of planation around the world. Unsatisfied with Davis's cycle due to evidence from the Western United States, Grove Karl Gilbert suggested backwearing of slopes would shape landscapes into pediplains, and W.J. McGee named these landscapes pediments. This later gave the concept the name pediplanation when L.C. King applied it on a global scale. The dominance of the Davisian cycle gave rise to several theories to explain planation, such as eolation and glacial planation, although only etchplanation survived time and scrutiny because it was based on observations and measurements done in different climates around the world and it also explained irregularities in landscapes. The majority of these concepts failed, partly because Joseph Jukes, a popular geologist and professor, separated denudation and uplift in an 1862 publication that had a lasting impact on geomorphology. These concepts also failed because the cycles, Davis's in particular, were generalizations and based on broad observations of the landscape rather than detailed measurements; many of the concepts were developed based on local or specific processes, not regional processes, and they assumed long periods of continental stability. Penck's theory, while ultimately being ignored, returned to denudation and uplift occurring simultaneously and relying on continental mobility, even though Penck rejected continental drift. The Davisian and Penckian models were heavily debated for a few decades until Penck's was ignored and support for Davis's waned after his death as more critiques were made. One critic was John Leighly, who stated geologists did not know how landforms were developed, so Davis's theory was built upon a shaky foundation. From 1945 to 1965, a change in geomorphology research saw a shift from mostly deductive work to detailed experimental designs that used improved technologies and techniques, although this led to research over details of established theories, rather than researching new theories. Through the 1950s and 1960s, as improvements were made in ocean geology and geophysics, it became clearer Wegener's theory on continental drift was correct and that there is constant movement of parts (the plates) of Earth's surface. Improvements were also made in geomorphology to quantify slope forms and drainage networks, and to find relationships between the form and process, and the magnitude and frequency of geomorphic processes. Through these findings and improvements in geophysics, the study of denudation shifted from planation to studying which relationships affect denudation–including uplift, isostasy, lithology, and vegetation–and measuring denudation rates around the world. ==Measurement==

Denudation is measured in the wearing down of Earth's surface in inches or centimeters per 1000 years. This rate is intended as an estimate and often assumes uniform erosion, among other things, to simplify calculations. Assumptions made are often only valid for the landscapes being studied. Measurements of denudation over large areas are performed by averaging the rates of subdivisions. Often, no adjustments are made for human impact, which causes the measurements to be inflated. Calculations have suggested soil loss of up to caused by human activity will change previously calculated denudation rates by less than 30%. The only areas at which there could be equal rates of denudation and uplift are active plate margins with an extended period of continuous deformation. Denudation is measured in catchment-scale measurements and can use other erosion measurements, which are generally split into dating and survey methods. Techniques for measuring erosion and denudation include stream load measurement, cosmogenic exposure and burial dating, erosion tracking, topographic measurements, surveying the deposition in reservoirs, landslide mapping, chemical fingerprinting, thermochronology, and analysis of sedimentary records in deposition areas. The most common way of measuring denudation is from stream load measurements taken at gauging stations. An important equation for denudation is the stream power law: E=KA^mS^n, where E is erosion rate, K is the erodibility constant, A is drainage area, S is channel gradient, and m and n are functions that are usually given beforehand or assumed based on the location. Preliminary research into using cosmogenic isotopes to measure weathering was done by studying the weathering of feldspar and volcanic glass, which contain most of the material found in the Earth's upper crust. The most common isotopes used are 26Al and 10Be; however, 10Be is used more often in these analyses. 10Be is used due to its abundance and, while it is not stable, its half-life of 1.39 million years is relatively stable compared to the thousand or million-year scale in which denudation is measured. 26Al is used because of the low presence of Al in quartz, making it easy to separate, and because there is no risk of contamination of atmospheric 10Be. This technique was developed because previous denudation-rate studies assumed steady rates of erosion even though such uniformity is difficult to verify in the field and may be invalid for many landscapes; its use to help measure denudation and geologically date events was important. On average, the concentration of undisturbed cosmogenic isotopes in sediment leaving a particular basin is inversely related to the rate at which that basin is eroding. In a rapidly-eroding basin, most rock will be exposed to only a small number of cosmic rays before erosion and transport out of the basin; as a result, isotope concentration will be low. In a slowly-eroding basin, integrated cosmic ray exposure is much greater and isotope concentration will be much higher. Problems in measuring denudation include both the technology used and the environment. The two main problems with dating methods are uncertainties in the measurements, both with equipment used and with assumptions made during measurement; and the relationship between the measured ages and histories of the markers. In a study by James Gilully, the presented data suggested the denudation rate has stayed roughly the same throughout the Cenozoic era based on geological evidence; however, given estimates of denudation rates at the time of Gilully's study and the United States' elevation, it would take 11-12 million years to erode North America; The research on denudation is primarily done in river basins and in mountainous regions like the Himalayas because these are very geologically active regions, which allows for research between uplift and denudation. There is also research on the effects of denudation on karst because only about 30% of chemical weathering from water occurs on the surface. Denudation has a large impact on karst and landscape evolution because the most-rapid changes to landscapes occur when there are changes to subterranean structures. and vegetation impact denudation. Research is also being done to find the relationship between denudation and isostasy; the more denudation occurs, the lighter the crust becomes in an area, which allows for uplift. The work is primarily trying to determine a ratio between denudation and uplift so better estimates can be made on changes in the landscape. In 2016 and 2019, research that attempted to apply denudation rates to improve the stream power law so it can be used more effectively was conducted. == Examples==

, Chile, a volcano without effects of erosion and denudationB) Chachahén Volcano, Mendoza Province, Argentina, a volcano with strong effects of erosion but no denudationC) Cardiel Lake, Santa Cruz Province, Argentina, a volcanic area with strong effects of denudation, exposing subvolcanic rock body. Denudation exposes deep subvolcanic structures on the present surface of the area where volcanic activity once occurred. Subvolcanic structures such as volcanic plugs and dikes are exposed by denudation. File: Ladahk Mountains.jpg|A mountain road in Ladakh that shows signs of mass wasting and erosion that result in bedrock exposure. File:Betsiboka Estuary Erosion.jpg|Satellite images that show the extreme erosion in the Betsiboka Estuary in Madagascar due to deforestation, which results in rapid denudation and one of the fastest changing coastlines. File:Weathering (in Portugal).jpg|Cliffs of a coastline in Portugal that have denuded due to erosion and weathering primarily from water and salt. Other examples include: • Earthquakes causing landslides; • Haloclasty, the build-up of salt in cracks in rocks leading to erosion and weathering; • Ice accumulating in the cracks of rocks; and • Microorganisms contributing to weathering through cellular respiration. == References ==