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, rocks begin to transform and collapse. This process can take place 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, therefore, 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. The main types of eluvium, there are only three, formed as a result of processes:
- physical;
- chemical;
- biological.
Of course, such a unit is somewhat arbitrary. In the vast majority of cases, weathering crust is formed under the influence of all three of these factors in the complex. In this case, we can only talk about the predominance of the conditions of formation of the sedimentary layer.
A bit of history
For the first time, the term “weathering crust” was introduced by the Swiss scientist A. Geim 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 domestic scientists N. A. Bogoslovsky, K. D. Glinka, P. A. Zemyatchensky. Initially, geologists did not distinguish weathering crust from soil. The Russian scientist V.V. Dokuchaev clearly separated these concepts.
As an independent section of geology, the science of weathering crusts was formed only at the beginning of the 20th century. At the same time, the Russian scientists, I. I. Ginzburg, B. B. Polynov, also became the founders of the new direction. Of course, some foreign researchers and enthusiasts - the Swede O. Tamm, the American W. Keller, the German G. Garrassovets and many others made a significant contribution to the development of this branch of geology.
Physical weathering forces
In this case, the weathering crust is a layer formed from the parent rock, fragmented and disintegrated without a significant change in mineral composition. Such crusts are very common in the Arctic and Antarctic, in the mountains, deserts and semi-deserts. Physical weathering occurs mainly as a result of:
- numerous cycles of thawing and freezing water;
- temperature changes;
- actions of the root system of plants;
- digging holes with animals;
- crystallization of salts contained in capillary water.
Large debris in the weathering crusts of this species is usually located at the foot of the mountains or in depressions. 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 that come to the surface of the earth, of course, can be transformed not only under the influence of physical factors. Weathering also happens due to complex chemical processes taking place in the maternal 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;
- carbon dioxide;
- oxygen;
- ammonia;
- biological activity of microorganisms.
In the thickness of the mother rock, 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 sprout into the mother breed to get water and nutrients. As they develop, they more and more split the array. Animals do the same, digging holes. Of course, one gopher or, for example, an oak tree cannot destroy a whole rock. But in the cavity formed due to their vital activity in the future, water will surely fall. As a result of this, the weathering crust is formed. The destruction of the parent breed in this case can occur both under the influence of physical factors and chemical reactions.
Structure
Weathering crust is an array located directly beneath the soil. It differs from the latter primarily in that no processes of humus formation occur in it. The structure of the weathering crust in most cases is not too complicated. With sufficiently long conversion processes, distinct horizons are distinguished in it. For example, layers in eluvium from the bottom up can be located as follows:
- crushed stone or clastic - slightly changed, slightly cracked granite;
- hydromica - usually has a gray color, is easily broken by hands;
- kaolin - a mineral clay mass with separate sections of loose crushed stone material.
Such a structure of the weathering crust is usually observed in granite areas.
Developmental stages
The most favorable conditions for the formation of eluvium are a leveled topography and a hot climate. Four stages of development of the weathering crust are distinguished:
- with a predominance of physical weathering;
- removal of readily soluble elements - sulfur, chlorine, lime;
- the formation of kaolins with the removal of calcium, potassium and magnesium;
- laterite formation.
The lateritic weathering crust on rocks enriched in titanium, iron and aluminum is well developed in the tropics.
Types by place and conditions of education
Weathering crusts can vary, of course, not only by the method of formation. Also, such arrays are classified by composition. In this regard, the following types of weathering crust are distinguished:
- rocky - formed mainly in the mountains;
- debris - also most often formed in mountainous areas, represented by neokatannye debris;
- fine-earth carbonate - formed on igneous rocks, or loess-like loams (Armenia, Crimea, Mongolia);
- fine-grained siallitic - crusts with a complex of siallitic materials (north of the Russian Plain);
- oblique - formed mainly in a dry climate;
- glandular glandular - formed in the 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:
The first type of formations covers very large areas of several hundred and thousand 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 complicate the formation of the weathering crust. Puffing up sites often exceeds the rate of eluvium formation. As a result, the weathering crust undergoes denudation until complete formation. In this case, huge masses of coarsely dispersed material are carried into the final water bodies of the runoff. For example, p. The Ob annually replenishes the ocean with 394 km 3 of various kinds of rocks.
What might be the power
The formation of the weathering crust on Earth took place over many thousands of years. Of course, in different places on the planet such processes did not take the same intervals. Rocks that arose at the stage of planet formation were destroyed longer, formed in later periods - a shorter time. Therefore, all weathering crusts on the earth can be divided into modern and ancient.
The first type of eluvium usually has not too much power. Such weathering crusts have not yet formed completely and often do not even have clear horizons. Ancient eluviums usually form very thick massifs with pronounced stratification.
In different places of 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 has the greatest thickness in tropical and subtropical regions. The thinnest eluvia are usually observed in deserts and steppes.
Ancient weathering crusts, in turn, are divided into:
- Precambrian;
- Upper Paliozoic;
- Triassic Jurassic;
- Cretaceous-Paleogene;
- pleotin-quaternary.
After formation, such barks often underwent repeated whitening processes: chamotization, kaolinization, pyritization, gleying, carbonation, salinization, etc. Nowadays, such eluviums on the earth are very well preserved mainly where younger rocks lie above them, preserving them from destruction.
Underwater weathering
Of course, the products of rock destruction can accumulate and form entire geological massifs not only on the land surface. Weathering crust is also present at the bottom of the seas and oceans. In this case, the destruction of the rock (halmirolysis) occurs mainly under the influence of:
- mineralized sea water;
- fluctuations in water temperature;
- pressure
- changes in gas regime, etc.
Precipitation accumulates at the bottom of the seas and ponds, usually faster than on land. Sometimes during halmirolysis, underwater hard shells of various compositions are formed: calcareous, iron-manganese, dolomite, etc. The thickness of such layers usually does not exceed 1 m.
What minerals can occur
The study of the weathering crust is not only theoretical (restoration of the paleogeographic setting at the time of formation), but also of practical importance. The fact is that such geological formations are often rich in all kinds of valuable minerals:
- iron ores;
- bauxite;
- manganese;
- nickel ores;
- cobalts, etc.
In ancient weathering crusts, in some cases, various metals can accumulate in separate areas in quantities greater than even in the parent rock. For example, it was in this way that many deposits were formed that are currently being developed industrially in the Urals.
Clay formations of weathering crust of various kinds can also be quite valuable from the point of view of economic use by humans. Such material is used as a ceramic or refractory raw material; it differs in bleaching and other valuable properties. Of course, the richest in various kinds of minerals are ancient bark.
Alluvial deposits
Weathering crusts are, therefore, formations that are of great national importance in our time in terms of the extraction of metals and clay. In addition, loose deposits of gold, platinum, silver, diamonds, etc. of a large area are often found in such layers. In such areas, the extraction of precious stones and precious metals is also carried out industrially. Such deposits can be found in both ancient and modern weathering crusts. Gold, diamonds or platinum in this case are simply carried away by water flows from the thickness of the collapsing parent rock and accumulate, for example, on the shallows or river bends.
What is illusion?
Usually geologists call the weathering crust eluvium. But there is another type of massif, formed by fragments not of the mother rock in this particular area, but brought from outside. Such weathering crusts are called infiltration. The composition they can have is very different. For example, there are carbonate, sulfate, salt, and siliceous illusions. Of course, various deposits in the weathering crusts of this type are also formed quite often.