What is biotic weathering

weathering

Under weathering one understands the decomposition of rock. Several processes play together that cause the physical decay and the chemical and biogenic decomposition of the rock due to its exposed location on or near the surface of the earth. Examples of such physical forces are the effects of water, ice, wind and temperature changes. The result of weathering is rock destruction, in which, depending on the type of weathering, the rock-forming minerals are retained (physical weathering), or are reformed or newly formed (chemical weathering). In contrast to endogenous volcanic and tectonic factors, climate-related weathering, together with erosion, transport and sedimentation, is one of the climate-related exogenous factors that shape the earth.

The products of rock weathering usually collect as loose surface layers, the regolith. The regolith merges downward into the solid, unchanged rock, commonly known as the pending rock (short "the upcoming") or as grown rock referred to as. Soil science, on the other hand, speaks of the C horizon.

The weathering processes on structures made of natural stone are also popularly referred to as stone corrosion.

structure

The effects of weathering are usually divided into:

  • Physical influences: Frost weathering, salt weathering, pressure relief weathering, swelling pressure weathering, hydration weathering and thermal weathering.
  • Chemical influences: Carbonic acid weathering, solution weathering, hydrolysis and wool sack weathering.
  • Biogenic influences: Physical-biotic weathering (e.g. root blasting) and chemical-biotic weathering (e.g. carbonic acid weathering, weathering through the formation of other acids and oxidation weathering).

A sharp distinction between these three forms of weathering 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 one form of weathering often requires other forms of weathering that have previously attacked: An effective chemical weathering is usually preceded by a loosening of the rock structure. For example, surfaces polished by glacier ice often show no noteworthy signs of chemical weathering even after thousands of years.[1]

Physical weathering

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.

The rock disintegration as a result of physical weathering creates mineral particles of various grain size classes, but also through the rubbing and crushing effect of the work of rivers, waves and currents, wind and glacier ice. In nature, two or more of these processes usually act simultaneously on the rock, along with chemical processes of rock decomposition.

Frost weathering

A stone fragmented by frost blasting in southern Iceland

The frost weathering is through Change of frost caused, i.e. by the repeated growth and melting of ice crystals in the pores or natural crevices of the rock, and is one of the most important processes of physical weathering. Of course, their occurrence is limited to those climates of the middle and higher latitudes, whose winters are cold, as well as to cold climatic altitudes in high mountains.

Pressures of over 200 MPa can occur here. 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.[2]

Almost everywhere the rock is criss-crossed by fissures, the so-called fissures. Solidified rocks are 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 intersect at right angles to each other. Comparatively low forces are sufficient to separate blocks from the existing rock formation, which are delimited by fissures and strata surfaces, while much more force is required to create new, fresh crevices in the solid existing rock. The process of separating blocks from the pending 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 will granular decay called. 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.

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 rock below. The name Felsenmeer refers to such extensive blankets made of coarse stone blocks.

Frost weathering separates rock fragments from rock faces in the 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 in heaps of rubble. Frost weathering is a predominant process in the arctic tundra and a factor in the development of a wide variety of different soil structures and landforms found there.

Salt weathering

Rock niche in Mesa Verde National Park
Salt weathering on a wall in Gozo, Malta

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 Salt blast called process is widespread in arid climates. During long dry periods, 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 are left behind.

The growth or also Crystallization pressure this crystal is able to cause the granular disintegration of the outer rock shell. Crystallization from supersaturated solutions produces a pressure of 13 MPa, and when salt crystals grow 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 susceptible 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.

The weathering of salt is usually associated with arid climates, as the great heat there leads to very strong evaporation. This form of weathering can also be seen very well on coasts, especially on walls or rocks that protrude into the sea. They often have spherical to kidney-shaped cavities with a diameter of a few centimeters to half a meter, which resemble the shape of a honeycomb pattern. This form of weathering, called Tafoni, is caused by simultaneous chemical and physical weathering by the salt water.

Pressure relief weathering

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 through the removal of overlying rock masses. Rocks formed at great depths below the surface of the earth (particularly 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 will too Exfoliation called. The dividing surfaces between the shells form a system of columns known as Pressure relief joints are designated.

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

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

Where the pressure relief fissures have developed over the summit area of ​​a single large, massive rock body, one arises Exfoliation dome (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.

Swelling pressure weathering

A stone that has become crumbly due to changes in temperature (day / night differences) (1. Immediately after the exposure; 2. After pressing lightly)

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

Hydration weathering

Hydration weathering means the storage of water molecules in the crystal lattice of minerals. The increase in volume caused by this leads to scaling phenomena. Hydration is a form of physical weathering and must not be confused with hydrolysis, in which the minerals react with the charged ions of the water (chemical weathering). Occasionally, the synonym hydration is used in the literature instead of hydration, but this is unusual internationally. Hydration weathering is generally also counted as chemical weathering.

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 temperature differences. these can

  • have natural causes (solar radiation, wind, frost, radiant weather, temperature increase in the interior of the earth and the like) or
  • go back to technical measures (friction, aging / corrosion, radioactivity, heating and others)

Chemical weathering

Chemical weathering is understood to mean the entirety of all those processes that lead to the chemical change or complete dissolution of minerals. They change the properties of rocks and release substances in their surroundings or integrate them from the surroundings into the mineral stock (for example in damp structures or the formation of conglomerates). Numerous processes from micro-technical etching to the large-scale effect of acid rain are associated with chemical weathering.

Weathering of wool sacks

Rocks fissured by wool sack weathering in Świętokrzyski National Park, Poland

Main article: Wool sack weathering

When the chemical weathering penetrates the rock, the fractured blocks in the rock are attacked from all sides. From the fissures, the decomposition advances into the interior of the block, creating concentric shells of soft rock which, as the weathering continues, separate in the form of an onion skin and give the still unweathered core of the angularly delimited block a rounded, wool-sack-like shape - hence the name Weathering of wool sacks.

Onion skin weathering is an outwardly similar but physically different type.

Carbonic acid weathering

Weathering of limestone through chemical processes

Carbonate rocks (limestone, marble) are particularly susceptible to the effects of the carbonic acid contained in rainwater and especially in soil water. Mineral calcium carbonate (calcite) is dissolved and breaks down into calcium and hydrogen carbonate ions. In regions with a large excess of water, these ions are carried away in rivers.

The reaction of carbonic acid with limestone creates many interesting, mostly relatively small, surface shapes. The surface of exposed limestone is typically covered 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. They create the bizarre cart fields in the Karst and Limestone Alps.

In the humid climates of the lower latitudes, mafic rock, especially basalt lava, is intensively attacked by mostly biogenic soil acids. This creates landforms that are very similar to the forms produced by carbonation of massive limestone in humid climates at higher latitudes. The effects of the removal of basaltic lava in solution can be seen in the impressive furrows, rock ridges and towers on the slopes of deep mountain niches in parts of the Hawaiian Islands.

The effect of carbonic acid is a dominant factor for denudation in limestone areas with a humid climate because of the intense biotic CO there2- forming processes. The investigation of a valley cut into limestone in Pennsylvania showed that the surface of the land was lowered by an average of 30 cm in 10,000 years through the effects of carbonic acid alone. As expected, in humid climates beneath the large valley zones and other areas of low terrain there are often carbotnate rocks in contrast to other, less vulnerable rocks that make up neighboring ridges and plateaus.

The opposite applies to dry climates; there the weathering effects are much less due to the lower biotic activity and limestone and dolomite form the high ridges and plateaus. The edges of the Grand Canyon and the adjacent plateaus, for example, are underlain by layers of dolomite. Sandstone layers made of quartz grains cemented together with calcium carbonate also only weather slightly in a dry climate.

The effect of carbonic acid also leads to the removal of carbonate rock deep below the land surface and thus to the formation of caves and cave systems in the limestone. The movement of the groundwater plays a key role in this process.

Solution weathering

Alveolic weathering in the Marès calcarenite in an old quarry on the Punta de n’Amer peninsula

Solution weathering mainly attacks salt and carbonate rocks, the minerals of which dissolve or absorb water of crystallization (for example, conversion of anhydrite to gypsum). In the simplest case, salts are dissolved in the soil by seeping in water, which can create cavities. In carbonate rocks, this process can lead to the formation of caves and other karst phenomena.When the solution is weathered, mostly only the smallest lattice structures remain. In some cases honeycomb-like cavities are created. In this case one speaks of alveolic weathering forms.
Since solution is traditionally counted as chemistry, solution weathering is assigned to chemical weathering. However, since it is reversible and the chemical composition of the rock is not changed, but only the crystal structure is destroyed, solution weathering is actually a physical type of weathering.

Solution weathering is a weathering process that can usually be easily recognized optically, in which some rocks are dissolved. Limestone, which mainly consists of calcium carbonate, is particularly affected by this form of weathering, as it can be completely dissolved by carbonic acid.

This acid is created by the reaction of water with carbon dioxide. The latter comes either from the atmosphere or, in a higher concentration, biotically formed, from the soil:

$ \ mathrm {H_2 O + CO_2 \ longrightarrow H_2CO_3} $

From this follows a reaction equation in which calcium carbonate dissolves in the presence of water and carbon dioxide:

$ \ mathrm {CaCO_3 + CO_2 + H_2O \ longrightarrow Ca ^ {2+} + 2 \; HCO_3 ^ -} $

Water in connection with carbon dioxide attacks many minerals. For example, olivine, which is found in many volcanic rocks, can be almost completely dissolved after a series of reactions. Here is a simplified reaction equation:

$ \ mathrm {Mg_2SiO_4 + 2 \; H_2O + 4 \; CO_2 \ longrightarrow 2 \; Mg (HCO_3) _2 + SiO_2} $

hydrolysis

During hydrolysis (hydrolytic weathering) are cations that are bound to the basic structure of the rock (mostly K, Mg, Ca, Mn, Fe) through H + -Ions dissolved. This makes the basic structure unstable and crumbles. Hydrolysis is responsible for soil formation. Through hydrolysis, silicate rocks (mica) are transformed into clay minerals, which take on different forms depending on the climate (in the temperate climate: illite, in the tropical climate: kaolinite). In general, the more humid the climate, the higher the temperature and the lower the pH, the more intensive the hydrolysis. In the warm and humid climates of the equatorial, tropical and subtropical zones, solidification 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 Saprolite (literal: "rotten rock"). For the civil engineer, deeply weathered rock means a risk when building highways, dams or other heavy-duty structures.

On the one hand, the saprolite is soft and can be moved by excavators without a lot of blasting work, but on the other hand there is a risk that the material will yield under heavy loads. This type of regolith can have undesirable plastic properties because of its high content of certain water-absorbing and thereby swelling clay minerals, such as montmorillonite.

The hydrolysis of the granite is accompanied by granular decay, the physical breakdown of the rock into its mineral grains. By rounding the edges of the rock, this process creates a variety of shapes of blocks and rocks. Such forms are particularly noticeable in arid areas. Most deserts have sufficient moisture for hydrolysis; it only progresses more slowly than in more humid climates.

The product of the granular decay of Felsitic solidification rock is a fine gravel, Grus called, which consists mainly of mineral grains of quartz and partially weathered feldspar.

Biotic weathering

Biotic weathering (also called biological or biogenic weathering) means weathering through the influence of living organisms and their excretion or decomposition products.[3][4] These effects can be of a physical nature (example: root blasting) or consist of a chemical effect. Biotic and abiotic weathering is difficult to differentiate in some cases.[5] 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 that Root blast. Plant roots growing into crevices in the rock and into tiny gaps between mineral grains exert a force through their growth, the tendency of which is to widen these openings. You can occasionally see trees with their lower trunk and roots firmly wedged in a crevice in the massive rock. In individual cases, it remains open 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, it is certain 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 of nearby trees is well known evidence of the effective contribution of plants to mechanical weathering.

Chemical-biotic weathering

Weathering through chemical-biotic processes is caused by microorganisms, plants and animals. For example, the organic acids secreted by plant roots attack minerals and thereby break down the rock 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.

In many cases, the effect of carbonic acid is 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. With metals, especially iron (Fe), aluminum (Al) and magnesium (Mg), they form very stable, partly water-soluble, partly water-insoluble compounds, so-called organometallic complexes (Chelate complexes, 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 the soluble complexes, these are shifted in the soil profile with the movement of seepage water and withdrawn from the weathering mechanism. Chelating substances, which 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 carbonic acid content in the soil through the formation of carbon dioxide and thus accelerate dissolution processes. Anaerobic bacteria sometimes 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 readily soluble in water than those of trivalent iron, which is why iron can be mobilized and relocated relatively easily through microbial reduction.

literature

  • F. Press, R. Siever: General Geology - Introduction to the Earth System. 3rd edition, Spektrum Akademischer Verlag, Heidelberg 2003, ISBN 3-8274-0307-3.
  • H. Gebhardt, R. Glaser, U. Radtke, P. Reuber (Eds.): geography. 1st edition, Spektrum Akademischer Verlag, Heidelberg 2007, ISBN 3-8274-1543-8.
  • A. H. Strahler, A. N. Strahler: physical geography. 3rd edition, UTB, Stuttgart 2005, ISBN 3-8252-8159-0.
  • H. J. de Blij, P. O. Muller, R. S. Williams Jr .: Physical Geography - The global environment. 3. Edition. Oxford University Press, Oxford 2004, ISBN 0-19-516022-3.
  • Kurt Konhauser: Introduction to Geomicrobiology. Blackwell Publishing, Malden, Oxford, Carlton 2007, ISBN 978-0-632-05454-1.
  • Henry Lutz Ehrlich, Dianne K. Newman: Geomicrobiology. 5th edition, CRC Press, Boca Raton 2009, ISBN 978-0-8493-7906-2.

Web links

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Individual evidence

  1. ↑ Hans-Georg Wunderlich: Introduction to geology. Volume I. Exogenous Dynamics, B. I.-Wissenschaftsverlag, Mannheim, Vienna and Zurich, 1968, p. 39
  2. ↑ Herbert Louis, Klaus Fischer: General geomorphology, de Gruyter, 1979, p. 113 ff.
  3. ↑ F. J. Stevenson: Humus chemistry: Genesis, Composition, Reactions, Crystal Dreams Publishing, 1994, p. 474
  4. ^ Francis George Henry Blyth, M. H. De Freitas: A geology for engineers, 7th ed., Elsevier, 1984, p. 31
  5. ↑ Greg John Retallack: Soils of the past. An introduction to paleopedology, 2nd ed., Blackwell Science, Oxford, 2001, p. 75