Weathering

Under weathering is defined as the natural decomposition of rocks. Several processes play together, the physical disintegration and chemical alteration of the rock because of its prominent location at or near the earth's surface - caused abiotic or biotic - Making. Examples of physical forces are the impacts of soil and rain water, ice, temperature fluctuations and root pressure. The result of weathering of rock destruction with get the rock-forming minerals depending on the weathering remain ( physical weathering ) or to or newly formed ( chemical weathering ).

The products of the weathering of rocks usually accumulate as a loose surface layers, which are called regolith. The regolith goes down into the solid, unaltered rock, which is generally regarded as the bedrock ( the short Pending ) or designated as bedrock. The soil science, however, speaks of the C horizon.

The weathering processes on structures made of natural stone are popular also called stone feeding.

3.1 Wollsackverwitterung
3.2 carbonic acid weathering
3.3 weathering solution with pure water
3.4 hydrolysis

4.1 Mechanical- biotic weathering
4.2 Chemical and biotic weathering

Classification and Structure

The shape of the surface is both processes within and below the earth's crust formed ( endogenous ) and of processes which act on or near the surface and to a large extent climatic causes ( exogenous factor ). The most important endogenous factors are volcanism and tectonics. The weathering belongs together with erosion and sediment transport and deposition to the exogenous factors.

The weathering effects are usually broken down 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 Wollsackverwitterung.
Biogenic Influences: physical and biotic weathering ( for example, root blasting) and chemical- biotic weathering (eg, carbonic acid weathering, weathering by formation of other acids and oxidizing weathering ).

A sharp separation between these three weathering forms is not always possible. Thus, the biogenic weathering by plants partly physical ( turgor pressure ) and partly chemical in nature ( corrosivity). In addition, the efficacy of weathering form frequently uses other before attacking weathering forms ahead: One active chemical weathering is usually preceded by a relaxation of the rock formation. When polished by glacial ice surfaces show, for example, even after thousands of years often no appreciable signs of chemical weathering.

Physical weathering

Physical weathering (including physical or mechanical weathering ) is a broad term that includes several quite different physical processes; they have in common is that they break down all the hard, solid bedrock into fragments ranging in size from large blocks can range up to fine sand and silt.

Through the rock disintegration due to physical weathering mineral particles are formed of different grain size classes, but also by the grinding and crushing effect of the work of rivers, waves and currents, wind, and glacial ice. In nature usually work two or more of these processes simultaneously on the rock one, along with the chemical processes of rock disintegration.

Frost weathering

The frost weathering (including frost damage ) is caused by the volume expansion of freezing, located in the pores and crevices could water and is one of the most important processes of physical weathering. Accordingly, their occurrence is limited to areas with cold winters, that is, to higher latitudes and higher altitudes in mountainous regions.

When frost shattering a pressure of about 200 MPa can occur. At -5 ° C, the pressure is 50 MPa. At -22 ° C the pressure maximum is reached 211.5 MPa. This leads to a volume increase of up to 9%. At still higher pressure the ice is transferred to a different, less space beanspruchende form.

Almost everywhere the bedrock of columns running through it, the so-called clefts. Solidification rocks are rarely free of fissures through which the water can pass into the interior of the rock (columns of frost ). In sedimentary rocks the layer surfaces form a natural series of relatively low levels resistiveness in the rock; the bedding planes and fractures intersect each other at right angles. Comparatively satisfy small forces to separate bounded by fractures and bedding planes blocks from the bedrock bandage, while much more force is needed to produce fresh new columns in the solid bedrock. The process of separation of blocks from the forthcoming called block disintegration.

If coarse-grained igneous rock is weakened by chemical decomposition, water can penetrate along the interfaces between the mineral grains in the rock; here, the water may freeze and the mineral grains separated from each other by the strong pressure from the increase in volume occurring. This process is called granular disintegration. The resulting product is a fine gravel or coarse sand, in which each particle consists of a single mineral particles, which 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 freeze-thaw cycles. Where the bedrock is exposed on rocks and mountain peaks, blocks by water that freezes in the caves, separated from the rock. Under particularly favorable conditions, as they occur in high mountain summits and in the arctic tundra, to large, angular rock fragments accumulate in a layer of debris, which completely conceals the underlying bedrock. The name Felsenmeer refers to those vast ceiling of rough stone blocks.

From rock walls in the high mountains, the frost action separates from rock fragments fall down to the foot of the wall. Where the production of this debris occurs at a high rate, the fragments accumulate to scree at the foot of the cliffs. Frost weathering is a predominant process in the Arctic tundra and a factor in the development of a wide variety of there occurring soil structures and landforms.

Salt weathering

The effect of frost action by growing ice crystals is very similar to the effect of the growth of salt crystals in the column and pores of the rock. This salt fretting called process is widely used in dry climates. During long periods of drought, water is drawn from the interior of the stone by capillary action to the surface. This water contains dissolved mineral salts. In its evaporation 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 creates a pressure effect of 13 MPa, and the growth of salt crystals 4 MPa. The same process can also be observed in the blocks and concrete in the cities. Road salt, which is spread on roads in the winter, leading to considerable decay of the near-surface area of stone and concrete structures.

Sandstone cliffs are particularly vulnerable to decay by salt rock blasting. Kick out at the foot of a sandstone wall seepage, because it can not penetrate into a denser, impermeable rock layer ( shale, for example) that leaves occurring there continued evaporation of this water entrained salts in the near-surface pores of the sandstone. The pressure of the growing salt crystals tears of sandstone from small scales and splinters. Severed sand grains are carried away by gusts of wind or washed away by rain water that runs over the rock wall.

With the receding of the wall toe, there arises gradually a niche or shallow cave. In the southwestern United States (for example, in the Mesa Verde National Park ) many such niches of Indians were inhabited; with stone walls, they joined a natural hollow forms. This rock niche settlements (English: cliff dwellings ) were not only protected from bad weather, but also from enemy attacks.

The salt weathering is usually associated with arid climates, in conjunction, since there the intense heat leads to very strong evaporations. It is this form of weathering but also on coasts recognize very well, particularly in walls or rock projecting 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 Tafoni called weathering form is caused by simultaneous chemical and physical weathering by the salt water.

Pressure relief weathering

A peculiar, ubiquitous process, which is related to the physical weathering, caused by pressure relief: the response of the rock to the reduction in pre-existing, the rock body constricting pressure forces when overlying rock masses are removed.

Rocks, which were formed at great depth below the surface (especially setting and metamorphic rocks ), are in a compressed state due to the load of the overlying rock it. If these rocks reach the surface, they expand slightly; thereby breaking thick rock shells from the rock mass beneath it going. This process is also called exfoliation. The separation surfaces between the shells form a system of columns that will be referred to as pressure relief gaps.

This gap structure is best developed in massive, previously cleft- poor rocks such as granite; because in an already fractured rock engständig the expansion would only lead to an extension of these existing gaps.

The rock shells, which are generated by the pressure relief, are generally parallel to the land surface and are therefore inclined to the valley bottoms out. At Granite coasts, the shells are inclined at all points seaward. The Druckentlastungsklüftung is very good to see in quarries, where they greatly facilitated the removal of large stone blocks.

Where is the pressure relief fissures have developed over the summit area of a single large, solid rock body, a Exfoliationskuppe arises (English: exfoliation dome). These peaks are among the largest landforms that have been generated mainly by weathering. In the region of the Yosemite Valley in California, where such impressive peaks dominate the landscape, possess distinct rock shells thicknesses of between six and 15 meters.

Other types of large, smooth rock domes without such a shell structure are not real Exfoliationskuppen, but caused by the granular disintegration of the surface of a unit mass of a hard, coarse-grained intrusive rock solidification, the gaps are missing. Examples are the Sugar Loaf in Rio de Janeiro and Stone Mountain in Georgia ( USA). These smooth hilltops protrude in a conspicuous manner on its environment of less resist permanent rock.

Swelling pressure weathering

By swellable clay minerals occur when switching between moistening and drying to volume fluctuations, which can destroy the rock mass.

Hydration weathering

Under hydration weathering is defined as the storage of water molecules in the crystal lattice of minerals. Due to the volume increase caused thereby it comes to Vergrusungserscheinungen. The hydration is a form of physical decomposition and should not be confused with the hydrolysis, in which the minerals with the charged ions in the water react ( chemical weathering ). Occasionally, the synonym hydration in the literature is used in place of hydration, which is internationally but uncommon. The hydration weathering is generally counted for chemical weathering.

Thermal weathering

The thermal decomposition ( Insolationsverwitterung ) is one of the physical weathering types, but is usually performed as a special category. It is caused in solid materials due to temperature differences. This can

Natural causes ( solar radiation, wind, freezing, radiation, weather, temperature increase in the interior of the earth and the like) or
On technical measures to decline (friction, aging / corrosion, radioactivity, heating and others)

Chemical weathering

Under the chemical weathering is defined as the totality of all of the processes that lead to chemical change or complete dissolution of minerals. Through them, rocks are altered in their properties and substances released into the environment or from the local community in the mineral constituents involved ( for example, in damp buildings or the formation of conglomerates ). With the numerous chemical weathering processes of the micro-technical etching up to a large-scale effect of acid rain are connected.

Wollsackverwitterung

Main article: Wollsackverwitterung

If the chemical weathering penetrates into the bedrock, the cleft- limited blocks are attacked in the rock association from all sides. From the clefts forth the decomposition proceeds in the interior of the block, creating concentric shells soft rock, which secrete onion skins with further weathering and give the still unweathered core of the angular limited block a rounded, wool bag-like shape - hence the name Wollsackverwitterung.

An externally similar but physically different kind is the onion skin weathering.

Carbonic acid weathering

Weathering solution with pure water

The solution weathering is the solution of alkali and alkaline earth metal salts (eg, sodium chloride (NaCl ), potassium salt (KCl ), chlorides, carbonates, sulfates such as anhydrite (CaSO4 ) ), go with minerals in pure water in solution or crystal absorb water (for example, converting anhydrite to gypsum ). In the simplest case, salts are dissolved in the soil by seeping water, which can cause cavities. In the solution weathering usually only small lattice structures remain. In some cases arise honeycomb-like cavities. In this case one speaks of alveolischen weathering forms.

Since solution is traditionally counted on the chemistry, assigns it to the solution weathering chemical weathering. However, because it is reversible, and the chemical composition of the rock will not be changed, only the crystal structure is destroyed, it can be referred to as physical Verwitterungsart.

The solution weathering, the carbonate weathering as a special form of solution weathering, is a visually recognizable mostly well weathering process.

Hydrolysis

In the hydrolysis ( hydrolytic decomposition ) cations which are bound to the basic structure of the rock (usually K, Mg, Ca, Mn, Fe ) is dissolved H ions. Thus, the backbone is unstable and decomposes. The hydrolysis responsible for the soil formation. Hydrolysis of silicate rocks are clay minerals ( mica), which take various forms depending on the climate ( in temperate climates: illite, in a tropical climate: kaolinite ). Generally, the moist climate, the higher the temperature and the lower the pH, the more intense is hydrolysis. In warm and humid climates of the equatorial, tropical and subtropical zone solidification rocks and metamorphic rocks by hydrolysis and oxidation are often weathered to depths of 100 meters. Geologists who examined such deep weathering of the rock first in the southern Appalachians, this weathered saprolite layer called (literally " rotten rock "). For the Civil deeply weathered rock is a risk in the construction of highways, dams or other hard-to -heavy structures.

One hand, the saprolite is soft and can be moved without much blasting of excavators, but on the other hand there is the danger that the material yields under a heavy load. This type of regolith may have due to its high content of certain water -absorbing and thereby crescendo clay minerals such as montmorillonite, unwanted plastic properties.

The hydrolysis of the granite is accompanied by granular disintegration, the physical separation of the rock in its mineral grains. The associated Zurundung rock edge, this process generates diverse forms of blocks and rocks. In arid regions such forms are particularly striking. At most, there is little deserts moisture for the hydrolysis, it proceeds only slowly progressing, compared with humid climates.

The product of the granular disintegration of felsitischem igneous rock is a fine gravel, Grus called, is primarily composed of mineral grains of quartz and feldspar teilverwittertem.

Biotic weathering

Under biotic weathering ( biological or biogenic weathering called ) refers to weathering due to the influence of living organisms and their excretion and decomposition products. These effects may be physical in nature (example: root blasting) or in a chemical action exist. Biotic and abiotic weathering is sometimes difficult to distinguish. The biotic weathering processes are sometimes classified in the literature in the categories of physical and chemical weathering.

Mechanical- biotic weathering

Mechanical- biotic weathering is mainly the root blast. In clefts of the rock and in tiny spaces between mineral grains in growing plant roots by practicing their growth in thickness from a force whose tendency is to extend these openings. One sees occasionally trees whose lower trunk and its roots are firmly wedged in a gap of the solid rock. It will remain open in each individual case whether the tree has actually managed to both sides of the divide disperse the rock blocks away, or whether he merely filled in the existing space of the column. In any case, make sure, however, that the pressure that the growth of tiny roots exerts hairline cracks in the rock, countless small rock flakes and grains loosens. Lifting and cracking of concrete paving slabs by the growth of roots near trees is a well-known proof of the effective contribution of plants to mechanical weathering.

Chemical- biotic weathering

Chemical- biotic weathering is caused by micro-organisms, plants and animals, and is one of those phenomena which are grouped under the term bio-corrosion. For example, grab the secreted by plant roots organic acids to break down the rock minerals and thereby into individual components. Composed of microbially partially degraded remains of dead plants and animals humus contains a large proportion of humic acids, which act rock- destructive. Microbial acid formation, oxidations and reductions may occur to the dissolution of minerals.

The effect of carbon dioxide is increased in many cases by the action of simple organic acids. They result from the microbial decomposition of dead organic matter or living plants by the roots be delivered. Proceed with metals, especially iron (Fe ), aluminum ( Al ) and magnesium (Mg ), very stable, partially water-soluble, partially water-insoluble compounds, so-called metal- organic complexes ( chelates, chelates ). This chelation is an important weathering reaction. The word " chelate " means "looks like a crab claw " and refers to the very close ties that enter into organic molecules with metal cations.

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

Furthermore, microorganisms and increase the respiration of plant roots by carbon dioxide formation the carbon dioxide content in the soil, accelerating dissolution processes. Anaerobic bacteria cause partial reduction processes by using certain substances as electron acceptors for their energy metabolism and thereby render them water soluble, for example, by the reduction of iron from the trivalent to the divalent form. Compounds of divalent iron are much more soluble than the trivalent why iron can be mobilized and displaced by microbial reduction relatively easily in water.

Pictures of Weathering

Dimension stone Desert climate Aging (disambiguation) Spheroidal weathering Limestone Alps Dolostone Halite Manganese Aluminium Frank Press Hans Georg Wunderlich

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