Chemical Weathering Easy Definition Sedimentary Rock Easy Definition

Chemical Weathering

Chemical weathering is an important mechanism driving elemental fractionation away from parental bedrock signatures.

From: Chemostratigraphy , 2015

Mountain and Hillslope Geomorphology

S.M. Mudd , ... E.J. Gabet , in Treatise on Geomorphology, 2013

7.5.4 Conclusions

Chemical weathering affects hillslope form by directly removing mass from hillslopes. The transformation of primary minerals to secondary minerals can also affect the hydrology, biota, and stability of hillslopes, which in turn influence sediment transport and landscape morphology. Erosion in evolving landscapes can modulate and be modulated by chemical weathering; for example, pulses of accelerated erosion can lower the residence time of hillslope materials, thereby increasing their chemical weathering rates, which in turn accelerate the incision signal. We have presented statements of mass conservation that allow quantification of the important feedbacks between chemical weathering and hillslope form; however, many of the constitutive relationships required to solve these equations are still lacking. For example, we do not know if chemical weathering influences the efficiency of sediment transport or how it influences the rate at which soil is produced at a given soil thickness. Studies quantifying these relationships are required before we can fully understand how chemical weathering influences hillslope form and the nature of the feedbacks between chemical weathering and landscape evolution. These studies are necessary if we are to develop an integrative understanding of dynamic feedbacks between climate, biogeochemical cycles, and landscape evolution.

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Elements of exploration geochemistry

Athanas Simon Macheyeki , ... Feng Yuan , in Applied Geochemistry, 2020

1.1.2.1.2 Chemical weathering

Chemical weathering involves the interaction of rock with mineral solutions (chemicals) to change the composition of rocks. In this process, water interacts with minerals to create various chemical reactions and transform the rocks. Chemical weathering is a gradual and ongoing process as the mineralogy of the rock adjusts to the near-surface environment. Secondary minerals develop from the original primary minerals of the rock. In this the processes oxidation and hydrolysis are the most frequent chemical processes that take place. Chemical weathering is enhanced by such geological agents as the presence of water and oxygen, as well as biological agents as the acids produced by microbial and plant root metabolism.

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Principles of Geology

Nicholas P. Cheremisinoff Ph.D. , in Groundwater Remediation and Treatment Technologies, 1997

Chemical Weathering

Chemical weathering, on the other hand, is an actual change in composition as minerals are modified from one type to another. Many, if not most of the changes are accompanied by a volumetric increase or decrease, which in itself further promotes additional chemical weathering. The rate depends on temperature, surface area, and available water.

The major reactions involved in chemical weathering are oxidation, hydrolysis, and carbonation. Oxidation is a reaction with oxygen to form an oxide, hydrolysis is reaction with water, and carbonation is a reaction with CO₂ to form a carbonate. In these reactions the total volume increases and, since chemical weathering is most effective on grain surfaces, disintegration of a rock occurs.

Quartz, whether vein deposits or individual grains, undergoes practically no chemical weathering; the end product is quartz sand. Some of the feldspars weather to clay and release calcium, sodium, silica, and many other elements that are transported in water. The iron-bearing minerals provide, in addition to iron and magnesium, weathering products that are similar to the feldspars.

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Surface and Ground Water, Weathering, and Soils

J. Viers , ... J. Gaillardet , in Treatise on Geochemistry, 2007

5.20.1 Introduction

Chemical weathering of rocks is a spontaneous (i.e., irreversible) thermodynamic process leading to a more stable state for natural materials under a given set of conditions (e.g., temperature and pressure). It results from the reaction of aqueous, acidic, and oxidizing solutions with the minerals in rocks and soils. Chemical weathering studies are of fundamental importance for several reasons:

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Chemical weathering of continental rocks is the major chemical process by which soils are generated. Soils constitute a fundamental reservoir of (macro- and micro-) nutrients essential for the normal healthy growth of living organisms (plants, animals, and humans at the end of the food chain). Chemical weathering controls the formation and evolution of soil, particularly (1) the concentration and distribution of chemical elements in the soil including the elements that control soil fertility and (2) the physical properties of the soil (Nahon, 1991). This is particularly important for nutrient deficient soils of tropical countries, which represent one-third of the continental surface, constitute huge fresh water reservoirs, and where 50% of the world population is living (Tardy, 1993). This is illustrated in Figure 1, which is a comparison of the chemical composition (sodium (Na), magnesium (Mg), potassium (K), calcium (Ca), phosphorus (P), silicon (Si), aluminium (Al), iron (Fe), manganese (Mn), and titanium (Ti)) of some nutrient-depleted Amazonian soils with that of an agricultural soil from California (San Joaquin soil, NIST, USA). Chemical weathering is a key process in the cycle of the elements at the Earth's surface. Within the different reservoirs (continent, ocean, and atmosphere) chemical weathering is the major source of elements delivered by rivers to the oceans (Martin and Whitfield, 1983). A consequence of the processes considered above is that chemical weathering contributes to denudation, the lowering of land surface by removal of material, and controls landscape development (Milliman and Meade, 1983; Walling and Fang, 2003). Understanding the rates of chemical weathering for watersheds located all around the world is fundamental for soil resource management.

Figure 1. Chemical composition of different Amazonian soils (Kronberg and Fyfe, 1983). Concentrations have been normalized to an estimate of the chemical composition of the upper continental crust (Taylor and McLennan, 1985). For comparison the chemical composition of an agricultural soil from the United States of America (San Joaquin soil, Standard Reference Material, NIST, USA) is also given.

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Chemical weathering is also strongly linked to climate because chemical weathering of silicate rock consumes CO₂ and over geological time has played a key role in climate regulation. Chemical weathering rates (CWR) of silicates (1) and carbonates (2) on the continents and carbonate precipitation (3) in the oceans can be represented by the following reactions:

(1) ${\text{CaAl}}_2{\text{Si}}_2{\text{O}}_8\left(\text{s}\right)+2{\text{CO}}_2\left(\text{g}\right)+3{\text{H}}_2\text{O}\left(\text{aq}\right)={\text{Ca}}^{2+}\left(\text{aq}\right)+2{\text{HCO}}_{\text{3}}^{-}\left(\text{aq}\right)+{\text{Al}}_2{\text{Si}}_2{\text{O}}_5{\left(\text{OH}\right)}_4\left(\text{s}\right)$

(2) ${\text{CaCO}}_3\left(\text{s}\right)+{\text{H}}_2\text{O}\left(\text{aq}\right)+{\text{CO}}_2\left(\text{g}\right)={\text{Ca}}^{2+}\left(\text{aq}\right)+2{\text{HCO}}_{\text{3}}^{-}\left(\text{aq}\right)$

(3) ${\text{Ca}}^{2+}\left(\text{aq}\right)+2{\text{HCO}}_{\text{3}}^{-}\left(\text{aq}\right)={\text{CaCO}}_3\left(\text{s}\right)+{\text{H}}_2\text{O}\left(\text{aq}\right)+{\text{CO}}_2\left(\text{g}\right)$

Examination of these three reactions reveals that dissolution of carbonates has no net effect on the CO₂ balance of the atmosphere because 1   mol of CO₂ is consumed for the dissolution of one mole of CaCO₃, while 1   mol is released during the precipitation of CaCO₃ in the oceans. More precisely, carbonate dissolution will have an effect on the CO₂ balance of the atmosphere only on a timescale that is similar to or shorter than the residence time of HCO₃ ⁻ in the oceans (100,000 years). On longer timescales, limestone weathering and limestone deposition will cancel each other out. By contrast, the dissolution of 1   mol of CaAl₂Si₂O₈ will consume 2   mol of CO₂. If Ca is then deposited as limestone in the ocean, only 1   mol of CO₂ is returned to the atmosphere. Thus, silicate weathering results in a net consumption of CO₂. Other processes may consume CO₂ (exchange between Na, K, and Ca from the particle surface in seawater) or release CO₂ (precipitation of secondary silicates in seawater) but are still poorly understood and badly quantified (see Chapter 6.11).

Even though there is still some debate in the scientific community, authors generally consider that increased rock weathering 300–400   Ma caused a decrease in the atmospheric concentration of CO₂ (Knoll and James, 1987; Berner, 1992, 1994). All existing numerical models describing the evolution of global biogeochemical cycles at the geological timescale are based on the work by Walker et al. (1981). These authors argue for a direct dependency of continental silicate weathering on climate (silicate weathering increases with temperature and runoff, both enhanced under higher $P_{{\text{CO}}_{\text{2}}}$ values). At the geological scale (10⁶–10⁹ years), the main factors driving the climate, in close connection with silicate weathering, are the solar constant, CO₂ degassing rate, and continental configuration. Walker et al. (1981) proposed a law (model) to link quantitatively global mean temperature, CO₂ degassing rate, and CO₂ consumption by continental weathering. In this model, the rate of continental weathering was controlled by temperature and runoff. Another model was proposed by Raymo (1991) in which chemical weathering was not controlled by temperature but by tectonic effects. Orogenesis induces a lowering of atmospheric CO₂ pressure through chemical weathering processes. This leads to lower temperatures that create glaciated environments, again increasing erosional processes. However, this hypothesis leads to the existence of long-term disequilibria between the CO₂ outgassing and the consumption by continental silicate weathering. However, several questions arise from these models. Is continental weathering controlled only by temperature and runoff at the global scale? Is it possible to reconstruct the relationship between temperature, runoff, and the partial pressure of CO₂ in the atmosphere? What is the role of physical erosion? Is it possible to establish general laws from field observations?

There is no doubt that the increasing number of studies dealing with chemical weathering during recent decades is related to increasing concern about global climate change. This chapter will consider these questions. The objective is to estimate CWRs of silicates, and to define which parameters control these rates at a global scale on the basis of the chemical composition of rivers draining both small and large watersheds. The importance of parameters controlling CWRs should be evaluated and included in climate models (Dupré et al., 2003).

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Weathering and Soils Geomorphology

J.C. Dixon , in Treatise on Geomorphology, 2013

4.14.1 Introduction

Chemical weathering has long been believed to be a relatively insignificant geomorphic process in cold climates due to the combination of low temperatures, limited liquid moisture, and associated lack of vegetation. Chemical reaction rates have largely been viewed to be temperature driven. This view has persisted despite the fact that Tamm (1924) clearly established that chemical reactions at prevailing Earth surface temperatures are not inhibited in the coldest climates. This view however has persisted in geomorphology as a consequence of the emphasis placed on freeze-thaw weathering by łoziński (1909, 1912) in his definition of the periglacial realm and is emphasized in the classic work of Peltier (1950) who deductively reasoned that as temperature decreases chemical weathering in cold climates becomes progressively less significant.

Beginning in the 1960s geomorphologists and geochemists began to realize that there was considerable evidence for the operation of chemical weathering in arctic and alpine environments. This change of perspective was based on empirical measurements and the realization that in fact temperature is only one, and a subordinate one at that, control on rates of chemical weathering. In fact more significant is the availability of moisture (specifically H⁺ ion availability) in driving chemical reactions. Second, there is an abundance of organic acids available in association with arctic and alpine plants that facilitate the migration of even the most resistant cations as chelates. Amongst the earliest geomorphologal studies to demonstrate the important role of chemical weathering in landscape denudation in alpine environments was Jackli (1956) who demonstrated that solute transport accounted for greater 'sediment' transport than all other hillslope processes and glacial transport. Similar results were obtained by Rapp (1960) from his detailed study of hillslope processes in the arctic alpine environment of northern Sweden. This study was seminal in that it showed that with respect to hillslope denudation, solute transport was not only substantial but dominant. Since the work of Rapp there has been an ever increasing body of literature that has shown the dominance of chemical weathering over physical weathering in cold climate landscape denudation.

Following the work of Jäckli and Rapp empirical studies by Reynolds (1971) and Reynolds and Johnson (1972) clearly established that primary minerals weather into secondary clay products in alpine environments and that meltwater streams draining different lithologic environments had distinctive compositions controlled by bedrock composition rather than by precipitation, aeolian infall, or evaporation. Importantly, stream waters draining different watersheds had total ionic concentrations that were significantly greater than that of incoming precipitation.

These four seminal papers established that chemical weathering in cold climates was a reality and that this set of processes collectively could be responsible for substantial and significant landscape denudation. The most important aspect of cold climate chemical weathering is that associated solutes are transported in a highly efficient manner from the drainage basin and runoff waters from snowmelt and glacial ablation transport exceptionally large quantities of dissolved mass.

This chapter considers the nature of chemical weathering processes operating in cold climates, controls on chemical weathering, bedrock weathering in cold climates, glacial foreland weathering, arctic/alpine soil formation, and geochemical landscape denudation.

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Cliffs and Rock Coasts☆

A.S. Trenhaile , in Reference Module in Earth Systems and Environmental Sciences, 2016

Chemical weathering

Chemical weathering is promoted on rock coasts by alternate immersion and exposure in the intertidal zone and by spray and splash in the supratidal zone ( Fig. 3c ). Coastal zones therefore provide the water that is needed for chemical reactions and the runoff to remove the soluble products. Chemical weathering reduces rock hardness and it is particularly important along discontinuities, which facilitates wave quarrying, and in hot, wet climates where cliff retreat may largely result from the removal of fine-grained, weathered material by fairly weak waves. Chemical and salt weathering can produce tafoni ( Fig. 3d ) and honeycombs, and they make important contributions to water-layer leveling, a suite of processes operating around the edges of pools of standing water that lowers, smoothes, and levels shore platform surfaces (Bartrum and Turner, 1928). Chemical weathering can result in case hardening, or strengthening, of rock surfaces with silica, iron, and other cementing agents from evaporating saline solutions, and chemically weathered softening or rotting of the rock interior ( Fig. 3d ). Frame weathering, which often occurs in conjunction with water-layer leveling, results from the movement of dissolved ions along joint planes, which can deplete or impregnate them with precipitates. If impregnated joints become more resistant than the joint blocks, then weathering pits with raised rims develop, whereas if the joints are weaker, small plateaus can be produced, sometimes in a form reminiscent of miniature volcanoes.

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Estuarine and Coastal Geology and Geomorphology

A.S. Trenhaile , in Treatise on Estuarine and Coastal Science, 2011

3.07.2.2.2 Chemical weathering

Chemical weathering is promoted on rock coasts by alternate immersion and exposure in the intertidal zone and by spray and splash in the supratidal zone ( Figure 3) (c). Coastal zones therefore provide the water that is needed for chemical reactions and the runoff to remove the soluble products. Chemical weathering reduces rock hardness and it is particularly important along discontinuities, which facilitates wave quarrying, and in hot, wet climates where cliff retreat may largely result from the removal of fine-grained, weathered material by fairly weak waves. Chemical and salt weathering can produce tafoni ( Figure 3 (d)) and honeycombs, and they make important contributions to water-layer leveling, a suite of processes operating around the edges of pools of standing water that lowers, smoothes, and levels shore platform surfaces (Bartrum and Turner, 1928). Chemical weathering can result in case hardening, or strengthening, of rock surfaces with silica, iron, and other cementing agents from evaporating saline solutions, and chemically weathered softening or rotting of the rock interior ( Figure 3 (d)). Frame weathering, which often occurs in conjunction with water-layer leveling, results from the movement of dissolved ions along joint planes, which can deplete or impregnate them with precipitates. If impregnated joints become more resistant than the joint blocks, then weathering pits with raised rims develop, whereas if the joints are weaker, small plateaus can be produced, sometimes in a form reminiscent of miniature volcanoes.

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Surface and Ground Water, Weathering, and Soils

D.E. Granger , C.S. Riebe , in Treatise on Geochemistry, 2007

5.19.4.3.2.2 Chemical weathering as a function of altitude

Chemical weathering depends on a wide variety of interrelated factors, including temperature, precipitation, and vegetative cover. All of these factors can vary over short spatial scales on steep mountains. To illustrate how chemical weathering rates vary in a mountain setting, we present an example from a 2-km long ridge spanning 2,090–2,750  m in altitude in the Santa Rosa Mountains, Nevada (Riebe et al., 2004b). The ridge encompasses marked contrasts in vegetative cover, snow depth, and mean annual air temperature (from 3.6 to 0.4   °C), and is underlain by roughly uniform bedrock.

Six sites were selected on the ridgeline itself, and in small steep catchments along it. Each of the sampling areas was small (<1.0   ha). The aim in sampling was to capture local variability and avoid sampling from single, potentially anomalous points, while staying within a narrow elevation range, so that each location would represent a distinct set of climatic conditions. In all, 49 samples of soil, 6 samples of saprolite (from the bases of several soil pits), and 28 samples of parent rock were obtained and analyzed. To quantify denudation rates along the transect, sediments from four sites were analyzed for concentrations of cosmogenic nuclides in sediment and saprolite.

CDFs decrease rapidly with increasing altitude (Figure 17a), dropping from 0.20 at the lowest sample to essentially 0.0 at the highest. In contrast, denudation rates vary by only a factor of 1.4 and show no clear trends with elevation (Figure 17b). Chemical weathering must therefore be slower at higher altitudes, because weathering is less intense for the same rate of sediment supply. Thus chemical weathering rates decline rapidly with increasing altitude to essentially zero above the upper limits of aspen and woody brush (Figure 17c). The altitudinal decline in weathering rates of individual elements is similarly sharp (Riebe et al., 2004b).

Figure 17. CDFs, denudation rates, and weathering rates plotted against altitude (lower axes) and temperature (upper axes). Vertical bar marks upper limits of trees and brush. CDFs decrease from 0.2 to 0.0 (a), whereas denudation rates vary by a factor of &lt;1.5 across the site (b), consistent with a decrease in chemical weathering rates (c) with increasing altitude. Lines through data are linear regressions. Weathering rates at the highest and lowest locations were calculated using a site-wide average denudation rate. Because the CDF is 0 within error at the highest site, the weathering rate there must be 0 within error as well, irrespective of denudation rate. Reproduced by permission of Elsevier from Riebe et al. (2004b).

The decrease in elemental chemical weathering rates of Si and Na with increasing altitude is sharper than predicted based on the temperature sensitivity of silicate weathering in granitic watersheds, but is consistent with an earlier report based on solute flux data from an altitude transect in the Swiss Alps (Drever and Zobrist, 1992). Weathering rates at Santa Rosa Mountain appear to be affected by factors other than temperature. For example, the progressive decline in vegetative cover and the increase in snow cover and duration of freezing with altitude are potentially important factors (Riebe et al., 2004b). If that is the case, then weathering rates may be particularly sensitive to differences in elevation at higher sites, where variations in vegetative cover and snow depth may be particularly sharp.

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The Precambrian Earth

P.G. Eriksson , ... O. Catuneanu , in Developments in Precambrian Geology, 2004

Relative Rates of Chemical Weathering and Mechanical Erosion

Where chemical weathering rates are much more rapid than rates of erosion, profiles are well developed, with the exposed surface being composed primarily of aluminous, secondary clay minerals and quartz ( Figs. 5.10-3a, 5.10-4a). If mechanical erosion is commensurate with the rate of chemical weathering (Fig. 5.10-4b), the weathered mantle capping source rock will be less well developed and the exposed surface will be composed of both clay minerals (with complex clays, smectites, illites, vermiculites dominating kaolinite and gibbsite) and primary minerals. Where the rate of mechanical erosion greatly dominates over the rate of chemical weathering (Fig. 5.10-4c) a disaggregated weathering profile may be very thin or absent and primary bedrock may constitute much of the exposed surface. Any detritus derived from the exposed surfaces is composed mostly of primary minerals (with minor complex clays). Although derived from the same bedrock source, the composition of detritus derived from each of these hypothetical profiles would differ greatly, the differences being due solely to the relative rates of chemical weathering and mechanical erosion.

Fig. 5.10-4. Hypothetical, idealised profiles with illustration of the relative intensities of chemical weathering and mechanical erosion required to produce a profile with the properties shown. The sequence (a)–(c) illustrates the effect of increasing intensity of mechanical erosion relative to chemical weathering.

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Component

Joseph A. DiPietro , in Landscape Evolution in the United States, 2013

Weathering, Erosion, and Deposition

The weathering process includes all changes that result from exposure of rock material (or any material) to the atmosphere. If you leave your bicycle outside for a year or two, it will begin to rust, which is a form of weathering. The weathering process includes physical changes that break the rock into smaller pieces, and chemical changes by which the rock reacts with water, air, and organic acids and partly or wholly dissolves. Physical weathering is equivalent to hitting a rock with a hammer. Chemical weathering is equivalent to pouring acid on a rock. The residual product of weathering is unconsolidated sediment.

An important form of physical weathering is frost cracking (also known as ice wedging). Upon freezing, water expands by about 9%. Frost cracking occurs where water collects in a small crack in a rock and then freezes. The expansion causes the crack to propagate (become wider and longer) such that the rock eventually breaks in half. Frost cracking is particularly active along mountaintops in the west where temperature is above freezing during the day and below freezing at night for much of the year. Piles of broken rock litter summit areas above tree line in areas such as the Rocky Mountains as shown in Figure 2.1.

FIGURE 2.1. Looking along the ridge of Mount Audubon, Front Range, Colorado. The large rock pile that litters the ridge formed via frost cracking.

Chemical weathering is the partial or complete dissolution of rock. Water is naturally slightly acidic and becomes more acidic when in contact with dead and decaying plant matter. Many minerals only partially dissolve and, in doing so, leave behind a clay residue. An example is the partial dissolution of potassium feldspar, which is a major mineral in granitic rock and which is also common in sandstone and some types of metamorphic rock. The following chemical equation describes the process:

$\begin{array}{l}2{\text{KAlSi}}_3{\text{O}}_8+\left(2{\text{H}}^{+}+9{\text{H}}_2\text{O}\right)={\text{Al}}_2{\text{Si}}_2{\text{O}}_5{\left(\text{OH}\right)}_4\\ +(2{\text{K}}^{+}+4{\text{H}}_4{\text{SiO}}_4)\end{array}$

$\begin{array}{l}\text{Potassium}\text{feldspar}+\text{Acidic}\text{water}=\text{Clay}\\ +\text{Components}\text{in}\text{solution}\text{in}\text{water}\end{array}$

The reaction removes potassium (K) and some of the silica (SiO₂) from feldspar. Both go into solution in water. The potassium then becomes available for plants to absorb. The dissolved silica (silicic acid; H₄SiO₄) may enter the groundwater system and precipitate around sand grains to form sandstone.

Quartz, which is crystalline silica, is the only common mineral that is not strongly affected by chemical weathering. It does not dissolve in water. All other common minerals are either dissolved completely or are partially dissolved and reduced to clay. For this reason quartz and clay are the two most abundant minerals in sedimentary rock. Chemical weathering is most effective when water is present. Therefore, the amount of chemical weathering in an area is controlled largely by the amount of available water.

The weathering process disaggregates rock, but it does not remove rock material from its original location. Erosion is the physical removal of rock and sediment from its original location by an agent such as water, ice, air, gravity, or animal/human interference. If erosion does not occur, unconsolidated sediment will remain in place, mix with organic material, and become soil. The breakdown of rock through the weathering process facilitates erosion.

Deposition is the accumulation of weathered and eroded sediment such as in a lake or subsiding basin. Deposition can occur in one of three environments: marine, nonmarine, or transitional. The term marine refers to something found in or produced by the sea. Marine environments include shallow marine, deep marine, and reef. Nonmarine environments are those that form on land, such as a desert, lake, river, soil, or glacier. Transitional environments form along coastlines where there is a component of both land and ocean. Such environments include beach, delta, and estuary.

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