# weathering

In geosciences, weathering refers to the natural decomposition of rock as a result of its exposed location on or near the earth's surface. Several processes play together that cause physical destruction and / or chemical change in the rock - abiotic or biotic. Depending on the type of weathering, the rock-forming minerals are retained (physical weathering) or are dissolved or transformed (chemical weathering).

## introduction

### General

The shape of the earth's surface is shaped by processes within and below the earth's crust (endogenous factors) as well as by processes that act on or near the surface and largely depend on the prevailing climatic conditions (exogenous factors). The most important endogenous factors are volcanism and tectonics . Weathering, together with erosion and sediment transport and deposition, is one of the exogenous factors (see also →  Rock cycle ).

The weathering does not act on its own, but is often the first link in a chain of exogenous processes, especially in high-lying terrain with steep slopes. A high relief energy ensures that weathering products are quickly eroded and deposited again as sediment at a point with less relief energy . Even terrain can be affected by erosion (see → hull area), but it is much less effective there. Therefore, the products of rock weathering can form loose surface layers there, which are known as regolith . The regolith is the depth into the unaltered rock, generally considered solid rock (just the Pending ) is called. The Soil Science speaks here from the C horizon .

When it comes to weathering processes, a rough distinction is usually made between:

• Physical processes - mostly the mechanical weakening or destruction of the rock structure as a result of an increase in volume of individual components of the same, which can have various causes.
• Chemical processes - decomposition of individual or all components of the rock structure.
• Biogenic processes - rock-weakening effects of the activity of living things.

A sharp distinction between these three forms of weathering, each of which can be further subdivided, is not always possible. Biogenic weathering by plants is partly of a physical nature ( turgor pressure ) and partly of a chemical nature (caustic effect). In addition, the effectiveness of a form of weathering often requires other forms of weathering that have previously attacked: Chemical weathering is more effective in a rock body that has already been severely disrupted by physical processes (which, however, can also be endogenous). On the other hand, even after thousands of years, rock surfaces polished smooth by glacier ice often show no significant signs of chemical weathering.

### Synonyms and definition of terms

Not only naturally occurring rocks are subject to weathering processes, but also buildings and works of art made of natural stone . In the latter case, stone damage is also popularly spoken of.

In general terms , “weathering” is understood to mean the natural decomposition of materials that are exposed to the direct influence of weathering . In addition to rock, this also applies to organic materials such as wood and metallic materials , glass , ceramics and plastics . In the case of organic materials, this form of "weathering" falls under the generic term rotting , in metals, glass, ceramics and plastics under the generic term corrosion .

Rotting and rock weathering are the most important processes in soil formation .

## Physical weathering

A stone that has become brittle due to physical weathering: 1 as found, 2  after lightly pressing

Physical weathering (also physical or mechanical weathering ) is a broad term that includes several quite different physical processes. What they have in common is that they all break up the hard, massive rock in the vicinity into fragments, the size of which can range from large blocks to fine sand and silt . Since this also happens through the rubbing and crushing effect of the work of rivers, waves and currents, wind and glacier ice, these processes are also sometimes assigned to physical weathering. However, because this is a matter of external mechanical effects, it should be referred to as erosion rather than weathering.

### Frost weathering

A stone fragmented by frost blasting in southern Iceland

Frost weathering (also frost cracking ) is caused by the volume expansion of freezing water in the pores and crevices and is one of the most important processes of physical weathering. Accordingly, their occurrence is restricted to areas with cold winters, i. i.e., to higher geographical latitudes ( polar regions and cold-temperate climate ) as well as the nival altitude level in mountain regions.

A pressure of over 200 MPa can occur during frost bursting. At −5 ° C the pressure is 50 MPa. At −22 ° C, the maximum pressure is reached at 211.5 MPa. This leads to an increase in volume of up to 9%. At even higher pressure, the ice changes into a different, less space-consuming shape.

Almost everywhere the rock is criss-crossed by crevices, the so-called fissures . Solidified rock is rarely free of crevices through which the water can get into the interior of the rock (crevice frost). In sedimentary rocks , the strata form a natural series of levels of relatively low resistance in the rock; the layer surfaces and the fissures cross at right angles to each other. Comparatively low forces are sufficient to separate blocks from the existing rock formation, bounded by fissures and strata surfaces, while much more force is required to create new, fresh crevices in the solid rock. The process of separating blocks from the upcoming is called block disintegration .

When coarse-grained solidification rock is weakened by chemical decomposition, water can penetrate the rock along the interfaces between the mineral grains; here the water can freeze and separate the mineral grains from each other due to the strong pressure of the resulting increase in volume. This process is called granular decay . The resulting product is fine gravel or coarse sand in which each grain consists of a single mineral particle that has been separated from its neighbors along the original crystal or grain boundary.

Frost cracking can also occur in building materials that have been wetted , for example, by diffused moisture with subsequent condensation by cooling below the dew point .

The effect of frost weathering can be observed in all climates that have a winter season with many changes in frost. Where the surrounding rock is exposed on rocks and mountain peaks, blocks are separated from rock by water that freezes in the fissures. Under particularly favorable conditions, such as those found on high mountain peaks and in the arctic tundra , large, angular rocks collect in a layer of rubble that completely covers the underlying rock. The name Felsenmeer refers to such extensive ceilings made of coarse blocks of stone.

Frost weathering separates rock fragments from rock faces in high mountains, which fall down to the base of the wall. Where the production of this rubble occurs at a high rate, the fragments at the foot of the rock walls accumulate to form rubble heaps . Frost weathering is a predominant process in the arctic tundra and is a factor in the development of a wide variety of different soil structures and landforms.

### Salt weathering

Rock recess in Mesa Verde National Park , Colorado , USA
Salt fray at the Theatinerkirche in Munich

The effect of frost weathering through growing ice crystals is very similar to the effect of the growth of salt crystals in crevices and pores of the rock. This process, called salt blasting , is common in arid climates. During long periods of drought, water is drawn from inside the rock to the surface by capillary forces . This water contains dissolved mineral salts. When it evaporates , tiny salt crystals remain.

The growth or crystallization pressure of these crystals is able to cause the granular disintegration of the outer rock shell. Crystallization from supersaturated solutions produces a pressure of 13 MPa, and when growing salt crystals, 4 MPa. The same process can also be observed on building blocks and concrete in cities. Road salt that is scattered on roads in winter leads to considerable disintegration of the area near the ground of stone and concrete structures.

Sandstone rock walls are particularly prone to rock disintegration from salt blasting. If seepage water escapes at the foot of a sandstone wall, as it cannot penetrate a denser, impermeable layer of rock ( clay slate, for example), the ongoing evaporation of this water leaves the salts carried along in the pores of the sandstone close to the surface. The pressure of the growing salt crystals tears off small scales and splinters from the sandstone. Separated grains of sand are carried away by gusts of wind or washed away by rainwater that runs off the rock face.

As the foot of the wall recedes, a niche or shallow cave gradually emerges there. In the southwestern United States (for example in Mesa Verde National Park ) many of these niches were inhabited by Indians; they enclosed the natural hollow forms with stone walls. These rock niche settlements (English: cliff dwellings ) were not only protected from bad weather, but also from enemy attacks.

Salt weathering is generally typical for regions with an arid climate , as the high evaporation rates and the low amounts of precipitation favor the precipitation of salts in the pore space of the rock. In humid climates, this type of weathering occurs primarily on the coast of the sea, especially on walls or rocks that protrude directly into the sea.

### Hydration weathering

Hydration weathering is the disintegration of the original rock structure as a result of the increase in volume of mineral grains due to the storage of water molecules in the crystal lattice of the corresponding minerals ( hydration or hydration). The hydration weathering must not be confused with the hydrolysis , in which the minerals are converted by chemical reactions with water ions (chemical weathering).

### Rust weathering

Rust weathering (also rust detonation ) only occurs in rocks that contain (non-oxic) iron ore minerals. Corresponding mineral grains experience an increase in volume on contact with meteoric water due to oxidation and thus the formation of iron oxides, hydroxides, oxide hydroxides and oxide hydrates. The increase in volume bursts the original rock structure, whereby the explosive effect can affect very extensive areas of a rock body. In mountainous areas, rust blasts can cause severe rock falls and avalanches . Rust blasting also often destroys stone cultural assets, since in earlier times iron dowels and iron anchors were often used for installation in buildings.

### Swelling pressure weathering

Due to swellable clay minerals , when changing between moisture penetration and drying, volume fluctuations occur, which can destroy the rock structure.

### Pressure relief weathering

Exfoliation of granite

A peculiar, widespread process, which is related to physical weathering, occurs through pressure relief: the reaction of the rock to the reduction of previously existing compressive forces that constrict the rock body when overlying rock masses are removed.

Rocks formed at great depths below the surface of the earth (especially solidification and metamorphic rocks ) are in a compressed state because of the load of the overlying rocks. When these rocks come to the surface, they expand a little; in the process, thick rock shells break loose from the rock mass below. This process is also called exfoliation . The dividing surfaces between the shells form a system of gaps called pressure relief joints .

This fracture structure is best formed in massive, previously poorly fractured rocks, such as granite; because in an already closely fractured rock the expansion would only lead to an expansion of these existing fractures.

The rock shells that are created by the depressurization are generally parallel to the land surface and are therefore inclined towards the valley floors. On granite coasts the bowls are inclined at all points towards the sea. The pressure relief vent can be seen very well in quarries, where it greatly facilitates the extraction of large blocks of rock.

Half Dome in Yosemite National Park with rock shells

Where the pressure relief fissures have developed over the summit area of a single large, solid rock body, a created Exfoliationskuppe (English: exfoliation dome ). These knolls are among the largest landforms that have been created mainly by weathering. In the region of the Yosemite Valley in California, where such peaks impressively shape the landscape, individual rock shells are six to 15 meters thick.

Other types of large, smooth rock domes without such a shell structure are not real exfoliation domes, but are created by the granular disintegration of the surface of a uniform mass of hard, coarse-grained intrusive solidification rock that lacks crevices. Examples are the Sugar Loaf in Rio de Janeiro and Stone Mountain in Georgia (USA). These smooth mountain tops tower conspicuously above their surroundings made of less resistant rock.

### Thermal weathering

Thermal weathering (insulation weathering) is one of the physical types of weathering, but is usually listed as a special category. It is caused in solid materials by spatial and temporal temperature differences and the resulting volume changes. these can

## Chemical weathering

Under the chemical weathering of the whole is understood all those processes that lead to chemical modification or complete solution of rocks under the influence of precipitation and groundwater near the surface or ground water lead. The physical properties of the rock usually change with the mineral stock. The water dissolves elements or compounds from the minerals (up to complete dissolution) or elements or compounds that have already been dissolved in the water are newly incorporated into the minerals. Because chemical weathering is linked to water, it only plays an important role in regions with a humid climate . In regions with a large excess of water, the substances released from the rock are often carried away into rivers and ultimately end up in the sea.

### Solution weathering

Solution weathering is the solution of rocks consisting predominantly of minerals that are soluble in pure water, e.g. B. gypsum (CaSO 4  · 2H 2 O), halite (NaCl) or sylvine (KCl). These rocks are therefore humidem climate rarely naturally open-minded because they are usually already dissolved below the ground surface. Special weathering phenomena of the solution weathering are the salt level and the gypsum cap in the roof area of salt domes .

Since solution is traditionally counted as chemistry, solution weathering is assigned to chemical weathering. Since it is in principle reversible and the chemical composition of the rock is not changed, but only the crystal structure is destroyed, it can also be understood as a physical type of weathering.

### Carbonic acid weathering

Surface of a chemically weathered limestone

Calcium carbonate (CaCO 3 , calcite , aragonite ) is only very poorly soluble in pure water. However, if the water combines with carbon dioxide (CO 2 ) from the air,

${\ displaystyle \ mathrm {H_ {2} O + CO_ {2} \ rightleftharpoons H_ {2} CO_ {3}}}$,

to form carbonic acid . It converts the carbonate according to the reaction equation

${\ displaystyle \ mathrm {CaCO_ {3} + H_ {2} CO_ {3} \ rightleftharpoons Ca (HCO_ {3}) _ {2}}}$

in calcium hydrogen carbonate , which is always completely dissolved in water. This process is called carbonation because a salt of carbonic acid reacts again with carbonic acid. For the same reason, hydrogen carbonate is also known as bi- or double carbonate . In higher concentrations, CO 2 can also come from soil organisms or from the decomposition of organic substances (see also chemical-biotic weathering ).

The reaction of carbonic acid with carbonate rocks ( limestone , dolomite , carbonatite , marble ) creates many interesting surface shapes on a small scale. The surface of exposed limestone is typically coated with a complex pattern of pans, grooves, furrows and other depressions. In some places they reach the extent of deep furrows and high, wall-like rock ribs, which humans and animals can no longer cross in the normal way. This creates bizarre karst landscapes in areas whose surface geology is dominated by limestone . However, the dissolution of carbonate rock is not limited to the surface of the terrain, but also takes place underground through seepage (carbonate) surface water. This leads to the formation of extensive caves and cave systems and subsequently to sinkholes and poljes . The chemical stability of calcium hydrogen carbonate, however, depends on pressure and temperature. If the solution warms up or if the pressure is released, the chemical reaction equilibrium shifts to the disadvantage of carbonic acid and calcium hydrogen carbonate. In the course of this, the hydrogen carbonate breaks down, releasing CO 2 , and calcium carbonate precipitates . In this way, u. a. Spring limestone and stalactites in limestone caves.

The effect of carbonic acid is a dominant factor for denudation in limestone areas with a humid climate, not least because of the intensive biotic CO 2 -generating processes there. In humid climates, limestones are therefore relatively susceptible to weathering and can form large valley zones and other areas of low terrain, while neighboring ridges and plateaus are made of rock that is more resistant to weathering under the prevailing conditions. The investigation of a valley cut into limestone in Pennsylvania showed that the surface of the land was deepened by an average of 30 cm in 10,000 years through the effects of carbonic acid alone.

The reverse is true for dry climates . There, the influence of carbonic acid weathering is much less due to the absence of liquid water and the associated lower biotic activity, and limestone and dolomite form high ridges and plateaus. For example, the edges of the Grand Canyon and the adjacent plateaus are underlain by layers of dolomite. Sandstone layers made from quartz grains cemented together with calcium carbonate (so-called carbonate- cemented sandstones ) also weather relatively slowly in a dry climate.

Another calcium compound susceptible to carbonic acid weathering is calcium hydroxide (Ca (OH) 2 , portlandite ), which is rather rare in nature . It weathers according to the reaction equation

${\ displaystyle \ mathrm {Ca (OH) _ {2} + H_ {2} CO_ {3} \ \ rightarrow \ CaCO_ {3} +2 \; H_ {2} O}}$

to calcium carbonate, which subsequently continues to weather. Calcium hydroxide, as slaked lime, is an important component of concrete. In reinforced concrete , the reaction of carbonic acid with calcium hydroxide, which is also known as carbonation , but in which calcium carbonate is generated instead of decomposed, promotes corrosion of the reinforcement , which can result in serious structural damage.

In addition to calcium carbonate and calcium hydroxide, the silicate mineral olivine , which is a component of many volcanic rocks, can also be used according to the reaction equation

${\ displaystyle \ mathrm {Mg_ {2} SiO_ {4} +2 \; H_ {2} O + 4 \; CO_ {2} \ longrightarrow 2 \; Mg (HCO_ {3}) _ {2} + SiO_ { 2}}}$

can be almost completely resolved, the equation above summarizing a multiphase process with several individual reactions.

In the humid climates of the lower latitudes, mafic rock, especially basalt , is intensely attacked by mostly biogenic soil acids. In the interplay with chemical weathering through hydrolysis ( see below ), land forms are created which, as so-called silicate karst, are very similar to carbonate karst. The effects of chemical weathering of basalt can be seen, for example, in the impressive furrows, rock ridges and towers on the slopes of deep mountain niches in parts of the Hawaiian Islands .

### Sulfuric acid weathering

Acid rain attacks limestone and turns it into gypsum. As a result, sculptures lose their definition.

This form of weathering also mainly affects rocks with larger proportions of calcium carbonate ( limestone , sand-lime brick , marble ). Acid rain contains small amounts of sulfuric acid (H 2 SO 3 ) or sulfuric acid (H 2 SO 4 ) due to the uptake of sulfur dioxide (SO 2 ) and sulfur trioxide (SO 3 ) from higher air layers . Both sulfur oxides originate mainly from man-made and volcanic emissions. Certain bacteria can increase the proportion of sulfuric acid in rainwater after it hits the ground or other surfaces by oxidizing the sulfuric acid it contains .

When carbonate rocks come into contact with acid rain, the sulfuric acid displaces the weaker carbonic acid from its calcium salt. Calcium carbonate (calcite) produces calcium sulfate (gypsum) and carbon dioxide (CO 2 ):

${\ displaystyle \ mathrm {CaCO_ {3} + H_ {2} SO_ {4} + H_ {2} O \ longrightarrow CaSO_ {4} \ cdot 2 \; H_ {2} O + CO_ {2}}}$ .

The water solubility of gypsum is much better than that of calcite, and the rock therefore weathers off more quickly after plastering.

Since it generates CO 2 instead of binding atmospheric CO 2 , as is the case with carbonic acid weathering and the subsequent biogenic precipitation of calcium carbonate in the oceans, sulfuric acid weathering can influence the carbon cycle . The reduction of man-made sulfur oxide emissions thus has a certain relevance in the debate about effective measures against global warming , because at least regionally, sulfuric acid weathering now makes a significant contribution to natural carbonate weathering.

In urban areas, sulfuric acid weathering accelerates the aging and destruction of historical building facades, monuments and the like. As the first visible sign , marble sculptures lose the typical sheen of their polished surface. Subsequently, they lose their contour sharpness and in extreme cases can lose the entire sculptured surface. Since gypsum is hygroscopic, soot particles contained in the rain can be bound into the plastered surface - so-called black crusts are formed. These are denser than the marble and reduce the water vapor diffusion capacity of the rock. Damage zones then run parallel to the surface and at some point the black crust flakes off over a large area - the sculpted surface is also lost in this process. Because of the acid rain, most of the marble sculptures have now been moved to museums and replaced by casts made of material that is insensitive to acid rain.

### hydrolysis

During hydrolysis ( hydrolytic weathering ), the ions in the crystal lattice of certain minerals are bound to H + and OH - ions, which are permanently formed in water through autoprotolysis , whereby the ion lattice disintegrates. Hydrolysis is an important process of soil formation because it forms the initial reaction of the conversion of common silicate minerals (e.g. feldspars and mica ) into clay minerals (e.g. illite , kaolinite , montmorillonite , smectite ). For example, potassium feldspar decays according to the reaction equation

${\ displaystyle \ mathrm {KAlSi_ {3} O_ {8} + H ^ {+} \ + OH ^ {-} \ longrightarrow HAlSi_ {3} O_ {8} + KOH}}$

in aluminosilicic acid and potassium hydroxide. The latter is converted into potassium carbonate ("potash", K 2 CO 3 ) by reaction with carbonic acid and, since it is easily soluble in water, it is carried away from the rock with the fissure, pore or surface water. The aluminosilicic acid reacts with water according to the reaction equation

${\ displaystyle \ mathrm {2 \; HAlSi_ {3} O_ {8} +9 \; H_ {2} O \ longrightarrow Al_ {2} Si_ {2} O_ {5} (OH) _ {4} +4 \ ; H_ {4} SiO_ {4}}}$

to kaolinite and orthosilicic acid. The latter is again soluble and is removed. However, changing the chemical environment on the road can be obtained from this weathering solution SiO 2 fail and then forms chalcedony crusts ( Silcretes ).

In general, the more humid the climate, the higher the temperature and the lower the pH value, the more intensive the hydrolysis. In the warm and humid climates of the tropical and subtropical zones, igneous rocks and metamorphic rocks are often weathered to depths of 100 meters through hydrolysis and oxidation. Geologists who first investigated such deep weathering of the rock in the southern Appalachians called this weathering layer saprolith (literally "rotten rock"). For the civil engineer, deeply weathered rock means a risk when building highways, dams or other heavy-duty structures. Although saprolite is soft and can be moved by excavators without a lot of blasting work, there is a risk that the material will yield under heavy loads, as it has undesirable plastic properties due to its high content of swellable clay minerals .

## Biotic weathering

Road asphalt broken up by tree roots

Biotic weathering (also called biological or biogenic weathering) is understood to be weathering caused by the influence of living organisms and their excretion or decomposition products. These effects can be of a physical nature (example: root blasting) or they can consist of a chemical effect. In some cases it is difficult to differentiate between biotic and abiotic weathering. The biotic weathering processes are sometimes also classified in the literature in the categories of physical or chemical weathering.

### Mechanical-biotic weathering

Mechanical-biotic weathering is mainly root blasting . In crevices of the rock and into tiny gaps between mineral grains into growing plant roots exert through their thickness growth of a force whose tendency is to expand these openings. You can occasionally see trees with their lower trunk and roots firmly wedged in a chasm in the massive rock. In individual cases, it remains to be seen whether the tree actually managed to drive the rock blocks further apart on both sides of the gap, or whether it only filled the space in the gap that was already there. In any case , what is certain is that the pressure exerted by the growth of tiny roots in hairline cracks in the rock loosens countless small scales and grains. The lifting and breaking of concrete pavement slabs from the growth of roots from nearby trees is well known evidence of the effective contribution of plants to mechanical weathering.

### Chemical-biotic weathering

Chemical-biotic weathering is caused by microorganisms , plants and animals , and is one of those phenomena that are summarized under the term biocorrosion . For example, the organic acids secreted by plant roots attack minerals and break the rock down into individual components. The humus , which consists of microbially degraded remains of dead plants and animals, contains a large proportion of humic acids , which have a rock-destroying effect. The formation of microbial acids, oxidation and reduction can lead to the dissolution of minerals.

The effect of carbonic acid is in many cases intensified by the effect of simple organic acids. They arise from the microbial decomposition of dead organic matter or are given off by the roots of living plants. They go with metals, especially iron (Fe), aluminum (Al) and magnesium (Mg), very stable, water-soluble part water-insoluble compounds in part a so-called organometallic complexes ( chelates , chelates ). This chelation is an important weathering reaction. The word "chelate" means "like crab claws" and refers to the very close bond that organic molecules form with metal cations.

In the case of soluble complexes, these are shifted in the soil profile with the movement of seepage water and removed from the weathering mechanism. Chelating substances that are mainly released during microbial degradation processes include citric acid , tartaric acid and salicylic acid .

Furthermore, microorganisms and the respiration of the plant roots can increase the carbon dioxide content in the soil through the formation of carbon dioxide and thus accelerate dissolution processes. Anaerobic bacteria partially cause reduction processes by using certain substances as electron acceptors for their energy metabolism and thereby making them water-soluble, for example by reducing iron from the trivalent to the divalent form. Compounds of divalent iron are much more soluble in water than those of trivalent iron, which is why iron can be mobilized and relocated relatively easily through microbial reduction.

## Special signs of weathering

### Wool sack weathering

Paleozoic sandstones formed by wool sack weathering in Świętokrzyski National Park , Świętokrzyski Mountains, Poland

The formation of typical shapes in the rock that occurs through various weathering processes is referred to as wool sack weathering. Initially, an approximately right-angled network of fissures forms in the rock, which can be traced back to physical weathering, but can also develop in igneous rock due to a decrease in volume when it cools. Water penetrates into the rock in the fissures and sets chemical weathering processes (e.g. the hydrolysis of feldspars) in motion. From the fissures, the decomposition advances into the rock, which happens particularly quickly at corners and edges, since this is where the ratio of attack surface to rock volume is greatest. When exposed on the surface, the rock attacked by weathering is preferentially eroded, which gives the previously unweathered, exposed cores of the blocks a rounded, wool-sack-like shape.

### Grudge

The structure of granitic rocks ( granite , granodiorite ) breaks down into individual mineral grains through hydrolysis of the feldspars and mica or through temperature weathering . This by the grain size forth sandy to fine-pebble material is crus called and the corresponding process is called Vergrusung or Abgrusung . Gravel formation is often associated with wool sack weathering.

### Alveolar weathering

Tafoni on a wall in Gozo , Malta

The mechanisms behind alveolar weathering are not exactly understood. Presumably, depending on the local conditions, it arises from various types of weathering (salt weathering, carbonic acid weathering) in conjunction with erosion from wind and water. Sandstones are primarily affected. The resulting honeycomb-like structures are called tafoni .

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