Rocks that come to the surface of the earth are constantly in contact with the atmosphere, biosphere, hydrosphere. Under the influence of negative environmental factors, the rocks begin to transform and collapse. This process can take hundreds or thousands of years. As a result, a weathering crust forms on the surface of the earth.
Definition and main types
The weathering crust is thus a layer of secondary, in most cases loose sedimentary rocks, located in the upper layers of the lithosphere and formed as a result of the destruction of mountain ranges under the influence of external factors. There are only three main types of eluvium, formed as a result of processes:
- physical;
- chemical;
- biological.
Of course, such a division is somewhat arbitrary. In the overwhelming majority of cases, the weathering crust is formed under the influence of all three of these factors in combination. In this case, we can only speak about the predominance of the conditions for the formation of the sedimentary layer.
A bit of history
For the first time, the term "weathering crust" was introduced into use by the Swiss scientist A. Game in 1879. A systematic study of such geological layers subsequently began in Russia. A great contribution to such research at the end of the 19th century, for example, was made by outstanding Russian scientists N. A. Bogoslovsky, K. D. Glinka, P. A. Zemyatchensky. Initially, geologists did not distinguish weathering crust from soil. The domestic scientist V. V. Dokuchaev clearly divided these concepts.
As an independent branch of geology, the science of weathering crusts was formed only at the beginning of the 20th century. The founders of the new direction at the same time were also Russian scientists - I. I. Ginzburg, B. B. Polynov. Of course, some foreign researchers and enthusiasts also made a significant contribution to the development of this section of geology - the Swede O. Tamm, the American W. Keller, the German G. Garrassovets and many others.
Physical forces of weathering
In this case, the weathering crust is a layer formed from the parent rock, crushed and disintegrated without significant change in the mineral composition. Such crusts are very common in the Arctic and Antarctic, in mountains, deserts and semi-deserts. Physical weathering occurs mainly as a result of:
- numerous cycles of thawing and freezing water;
- temperature changes;
- action of the root system of plants;
- digging holes for animals;
- crystallization of s alts contained in capillary water.
Large fragments in the weathering crusts of this species are usually located nearfoothills or in depressions. At the same time, small ones are carried away by water and wind, sometimes for hundreds of kilometers.
Scientists distinguish five main types of physical weathering:
- snowy;
- frosty;
- insolation (in deserts);
- ice;
- biological.
Chemical processes destruction
Rocks emerging on the surface of the earth, of course, can be transformed not only under the influence of physical factors. It happens that weathering also occurs due to complex chemical processes occurring in the parent massif. Thus, the rocks are also destroyed quite often. The main factors in the chemical formation of the weathering crust are:
- strong organic acids;
- water;
- hydrogen sulfide;
- carbonic acid;
- oxygen;
- ammonia;
- biological activity of microorganisms.
In the thickness of the parent rock, processes of leaching, oxidation, dissolution, hydrolysis, etc., can occur, leading to a violation of its structure.
Biological weathering
This type of destruction is a combination of physical and chemical processes. For example, the roots of trees and shrubs can grow into the parent rock to get water and nutrients. As they develop, they split the array more and more. Animals do the same when they burrow. Of course, one gopher or, for example, an oak tree cannot destroy a whole rock. But in the resultingfor their vital activity, the cavity will subsequently get water. As a result, the weathering crust is formed. The destruction of the parent rock in this case can occur both under the influence of physical factors and chemical reactions.
Building
Weathering bark is an array located directly under the soil. It differs from the latter primarily in that it does not contain humus formation processes. The structure of the weathering crust in most cases is not too complicated. With sufficiently long transformation processes, clearly defined horizons are distinguished in it. For example, layers in the eluvium from bottom to top can be arranged as follows:
- crushed stone or clastic - slightly altered, slightly cracked, granite;
- hydromicaceous - usually gray in color, easy to break with hands;
- kaolin - mineral clay mass with separate areas of loose gravel material.
This structure of the weathering crust is usually observed in granite areas.
Development stages
The most favorable conditions for the formation of eluvium are a leveled relief and a hot climate. There are four stages in the development of the weathering crust:
- with predominance of physical weathering;
- removal of easily soluble elements - sulfur, chlorine, lime;
- formation of kaolins with removal of calcium, potassium and magnesium;
- formation of laterites.
Laterite weathering cruston rocks enriched with titanium, iron and aluminum, it develops well in tropical conditions.
Types by place and conditions of education
Weathering crusts can, of course, differ not only in the way they are formed. Also, such arrays are classified by composition. In this regard, the following types of weathering crust are distinguished:
- rocky - formed mainly in the mountains;
- clastic - also most often formed in mountainous areas, represented by unrounded debris;
- small-earth carbonate - formed on igneous rocks, or loess-like loams (Armenia, Crimea, Mongolia);
- fine-grained siallitic - crusts with a complex of siallitic materials (northern Russian Plain);
- clayey - formed mainly in dry climates;
- clayey ferruginous - formed in tropical and subtropical zones;
- ferritic;
- bauxite - containing a large amount of aluminum hydroxide.
Morphogenetic Species
In this regard, the following types of weathering crust are distinguished:
- areal;
- linear.
The first type of formations covers very large areas of several hundreds and thousands of square kilometers. In this case, linear weathering crusts develop along tectonically weakened zones. Therefore, they form only small local zones in accordance with the strike of areas of different activity.
The dissection of the relief can greatly impede the formation of the crustweathering. The uplift of sites often exceeds the rate of eluvium formation. As a result, the weathering crust undergoes denudation until it is completely formed. In this case, huge masses of coarsely dispersed material are carried out into the final runoff reservoirs. For example, r. The Ob annually replenishes the ocean by 394 km3 different kinds of rocks.
What can be the power
The formation of the weathering crust on Earth has been going on for many thousands of years. Of course, in different places on the planet, such processes did not take the same time intervals. The rocks that arose at the stage of the formation of the planet were destroyed longer, those formed in later periods - a shorter time. Therefore, all weathering crusts on earth can be conditionally divided into modern and ancient.
The first type of eluvium usually does not have too much power. Such weathering crusts have not yet fully formed and often do not even have clear horizons. Ancient eluvium usually form very thick massifs with pronounced layering.
In different places on the planet, depending on the duration of formation, the weathering crust can have a thickness of several meters to several hundred meters. In most cases, the thickness of the eluvial subsoil layer is 30–40 m. The weathering crust is thickest in tropical and subtropical regions. The thinnest eluviums are usually observed in deserts and steppes.
Ancient weathering crusts, in turn, are subdivided into:
- Precambrian;
- Upper Paliozoic;
- Triassic-Jurassic;
- Cretaceous-Paleogene;
- Pleothin-Quaternary.
Such crusts, already after formation, were often subjected to repeated whitening processes: chamotization, kaolinization, pyritization, gleyization, carbonatization, salinization, etc. At present, such eluviums on the earth are very well preserved, mainly where younger ones lie above them rocks that keep them from destruction.
Underwater weathering
The products of destruction of rocks, of course, can accumulate and form whole geological masses not only on the land surface. The weathering crust is also present on the bottom of the seas and oceans. In this case, the destruction of the rock (halmyrolysis) occurs mainly under the action of:
- mineralized sea water;
- fluctuations in water temperature;
- pressure;
- changes in the gas regime, etc.
Precipitation accumulates at the bottom of the seas and reservoirs usually faster than on land. Sometimes, during halmyrolysis, underwater hard shells of different composition are formed: calcareous, iron-manganese, dolomite, etc. The thickness of such layers usually does not exceed 1 m.
What minerals can occur
Study of the weathering crust has not only theoretical (restoration of the paleogeographic setting of the time of formation), but also practical significance. The fact is that such geological formations are often rich in various valuable minerals:
- ironore;
- bauxites;
- manganese;
- nickel ores;
- cob alts, etc.
In ancient weathering crusts, in some cases, various kinds of metals can accumulate in separate areas in quantities greater than even in the parent rock. For example, this is how many deposits that are now industrially developed in the Urals were formed.
Also quite valuable from the point of view of human economic use can be various kinds of clay formations of weathering crusts. Such material is used as a ceramic or refractory raw material, it is distinguished by bleaching and other valuable properties. Of course, the most rich in various kinds of minerals are ancient crusts.
Alluvial deposits
Weathering crusts are thus formations that are of great economic importance in our time in terms of mining metals and clay. In addition, in such strata there are often scattered deposits of gold, platinum, silver, diamonds, etc. of a large area. In such areas, the extraction of precious stones and precious metals is carried out, including in an industrial way. Such deposits can be found in both ancient and modern weathering crusts. Gold, diamonds or platinum in this case are simply carried out by water flows from the thickness of the collapsing parent rock and accumulate, for example, in shallows or river bends.
What is illuvium
Usually barkweathering geologists call eluvium. But there is another type of massifs, formed by fragments not of the parent rock in this particular area, but brought from outside. Such weathering crusts are called infiltration. Their composition may vary. For example, carbonate, sulfate, s alt, and siliceous illuviums are distinguished. Of course, various kinds of deposits in weathering crusts of this type also form quite often.
