There are various scientific areas within which the structure of the Earth is studied. One such discipline is petrography. The importance of this science is difficult to overestimate. She works closely with many disciplines. Let us consider in more detail what about petrography.
Definition
The name of the discipline uses the Greek word pétros (stone). To the question of what petrography studies, a brief answer can be given this way - it studies stone. However, such a definition does not provide a complete understanding of the objectives and goals of the discipline.
More fully, petrography is a science that studies rocks, their chemical and mineralogical composition, textures, structures, bedding conditions, patterns of origin, distribution, and change.
The general science of rocks is usually divided into two parts. The first is based on the basics of petrography , suggesting mainly the use of descriptive methods. The second part is petrology. In the framework of this direction, a study of genetic relationships is being carried out. In the literature, however, these terms are often regarded as synonyms.
Specificity
Petrography is a discipline of the geological cycle. As mentioned above, it is closely connected with a variety of scientific fields.
For example, in practice, the use of mineralogy and petrography methods is common. In addition, the discipline is associated with geochemistry, tectonics, volcanology, stratigraphy, etc.
Classification
It is carried out depending on the type of test breed. So, there is petrography:
- Igneous rocks. In this area, crystalline rocks are studied, which arose mainly during the solidification and crystallization of magma. These processes are accompanied by the splitting and dissolution of the host structures. This led to the appearance of rock types of different composition and fossils associated with them. Magmatic petrography is aimed at obtaining information about their material composition, physicochemical conditions under which magma solidified, the peculiarities of their interaction with other rocks, etc.
- Metamorphic rocks. This direction studies structures that have changed their chemical and mineral composition under the influence of new conditions. Depending on the changes, rocks of various metamorphic facies are distinguished. Their mineral composition is determined mainly by the temperature and pressure of the environment.
Some breeds are intermediate. Some structures during their formation partially melt, while others are formed under the influence of metamorphism processes.
Research methods
Everything that petrography studies is connected with the bowels of the earth. Special methods are used to study the structure and composition of rocks.
First of all, it is necessary to say about crystal-optical methods. They allow you to examine fine-grained aggregates using a polarizing microscope and other devices.
Spectral analysis and the fluoroscopic method are also quite widely used in petrography. These techniques allow the determination of impurities present in structures in small quantities. The chemical composition is determined using microanalyzers directly in the rock.
Another common method of petrography is the study of physical constants. These include indicators of hardness, density, thermal expansion, magnetic properties, compressibility, viscosity, etc.
Since the middle of the last century, mathematical methods have been actively used. They make it possible to evaluate the reliability of the results of spectral or chemical analyzes, the construction of rock classifications, the determination of attributes for different types of minerals.
It should be said that petrography studies not only the rocks themselves, but also the conditions in which they are found. Thus, the study of minerals involves a complex set of measures. However, any research always begins in the field.
A generalization of the obtained materials allows us to determine the role of different rocks in the formation of the Earth's crust.
Sections of science
They stand out depending on the methods used. The main sections of petrography are :
- petrophysics;
- petrochemistry;
- petrotectonics.
In addition, there are areas such as technical, experimental, physico-chemical, space petrography.
The determination of the complex of chemical interactions in certain rocks in their natural combinations is carried out in the framework of petrochemistry. The development of geophysical, engineering and geological research has led to the emergence of a new section - petrophysics. In the framework of this direction, a relationship is established between the physical characteristics of rocks with composition, history of formation and structure.
Petrotectonics is the section of petrography that studies the relationship between geometric patterns in rock microstructures and movements / deformations in them. Using special methods, the acting forces and stresses are determined. Petrotectonics is based on microstructural analysis, which allows one to establish the predominant spatial orientation of the linear and planar components of the rock structure.
Physico-chemical petrography is the science of the relationship between the mineral and chemical composition of minerals and the general conditions for their formation. The studies use the general laws of thermodynamics.
Experimental petrography is a direction in the framework of which modeling of natural processes of rock formation and their mineral associations is carried out.
Separately, it should be said about technical petrography. It began to develop thanks to the activities of the Soviet geologist Belyankin. In this area, the mineral composition of porcelain, cement, slag, glass, stone casting, ceramics and other technical products is determined. Research results are of great importance for metallurgical and silicate production. At the same time, technical petrography methods are used to decode many rock formation processes.
Cosmic petrography, which arose in the 70s of the last century, studies the rocks of the moon and other cosmic bodies.
History of Petrography
Until the middle of the XIX century, the problems of studying rocks were partially solved by the methods of general geology. During this period, the foundations of mineralogy and petrography formed.
In the framework of the research, all rocks were divided into sedimentary, metamorphic, and igneous according to their genesis. Mineralogy and petrography began to develop actively after Sorby demonstrated the possibility of analyzing the mineral composition of minerals in thin sections under a microscope.
Subsequently, a polarizing microscope began to be used in practice, and further crystal-optical research methods were improved. After some time, a theodolite method for studying constants using a universal table was developed.
Subsequently, methods for studying minerals by crystal-optical properties were introduced into practice. Thanks to the use of theodolite method for studying rocks, microstructural analysis began to develop. At the same time, chemical methods began to improve.
All this made it possible to create a quantitative-mineralogical and chemical classification of igneous rocks. The separation was based on different methods for recalculating chemical analyzes of minerals.
Science at the turn of the XIX-XX centuries.
During this period, scientists began to actively study the problem of genesis and causes of breed diversity. Assumptions were made about the division of primary magma into private ones, about the process of assimilation of host rocks by it.
At the end of the 19th century, Levinson-Lessing was able to show that the initial source of the formation of igneous rocks distributed on the earth's surface are 2 fundamentally different magmas - primary and acidic. In the 1920s, this idea was supported by Reilly. After a decade, Bowen hypothesized that basaltic magma exists in the bowels, from which almost all igneous rocks could form during crystallization differentiation (separation from residual magma when floating out or immersed crystals). This assumption has gained great popularity in the scientific community. In the course of further research in nature, real cases of such differentiation were revealed.
Granite research
Researchers paid special attention to these breeds. Granites occurring in deeply metamorphosed migmatite and gneiss layers were studied.
Even at the beginning of the twentieth century, Södergolm discovered some features in these rocks, which were rather difficult to explain. He suggested that such granites cannot be considered igneous. Rather, in his opinion, they were formed as a result of metasomatic granitization or ultrametamorphism under the influence of deep-seated processes.
Introducing New Concepts
In the writings of Korzhinsky, the foundations of a physicochemical study of the paragenesis of minerals were formulated. The elements that make up the rocks were classified into groups depending on the function that they perform in mineral formation.
A concept appeared about systems with moving components, in which the equilibrium conditions of chemical indicators are determined by specific thermodynamic potentials. The concept of differential mobility arose. All this has significantly expanded the scope of application of methods for studying the paragenesis of minerals.
Korzhinsky was able to show that magmatism in the crust of the planet develops in interaction with transmagmatic solutions (fluids). He substantiated the role of magmatic substitution in the formation of eruptive structures in deep conditions. Korzhinsky developed the concept of metasomatic zoning.
Science at the present stage
In the 60s-70s of the last century, based on the results of experimental, petrological, and geophysical studies, scientists again began to discuss the possibility of granite formation due to melting from deep layers.
Many scientists recognize the existence of 2 types of granites. The first was formed from paligenic magma, which has a relatively low temperature. It arose as a result of partial melting of crustal rocks under conditions of water saturation. During crystallization at the site of magma, weakly displaced or autochthonous (non-displaced) granites are formed.
The second type is formed from acidic melts. They are formed during the transformations of basaltic magma, which, in turn, comes from the lower parts or the upper mantle of the crust. High-temperature acidic melts can reach the surface, forming both intrusive granites and their effusive analogues.
Lithology
As part of this research area , sedimentary rocks are being studied. This direction was isolated at the turn of the XIX-XX centuries. due to paleogeographic, stratigraphic and other studies. The key task of science is the analysis of the material composition of rocks and minerals associated with them.
Of particular importance for the separation of lithology in an independent direction were the materials that were obtained on the Challenger vessel, the research of Walter (a German geologist), devoted to the problems of sedimentary rock formation.
Significant contribution to the development of the direction was made by such scientists as Zavaritsky, Arkhangelsk, Shvetsov, Baturin, Strakhov, Rukhin and a number of foreign scientists.
Petrology
In the framework of this science, adjacent to petrography, studies of the structural and texture specificity of metamorphic and igneous rocks are carried out. Within the framework of the discipline, their classification was created, including according to the mineral composition. However, petrography covers a wider range of objects; it explores not only metamorphic and magmatic structures. In relation to the former, these two sciences are often regarded as identical. However, it is within the framework of petrology that the genetic relationships between the rocks are studied.
Petrophysics
This is another related discipline with petrography. It is considered as a science about the physicochemical properties of rocks. Petrophysics arose on the basis of petrography, laboratory methods of analysis, and geophysics. A key area is the consideration of the characteristics of structures as geological bodies, taking into account their specific nature.
The concept of "petrophysics" was first proposed in 1953 by the German physicist Frelich. The first methodological guide to the discipline was the work of Kobranova, published in 1962.
The key task of petrophysics is the study of the physical characteristics of rocks and the classification of types of structures, facies, strata according to a combination of properties. Rock parameters are studied in the massif using cosmophysical, geophysical and geological methods, in laboratory conditions by determining parameters under high temperatures and pressure.
There is a correlation between the physical characteristics of the rocks (porosity, density, etc.) and the petrographic-structural indicators. It is detected by petrophysical analysis.
Research is carried out in conjunction with other traditional methods. The received data is displayed on special cards. According to this information, researchers reconstruct the geodynamic conditions for the formation of geological bodies, establish the type of deformation, stress and identify patterns of distribution of ore deposits, oil and gas deposits. All this provides an effective solution to engineering problems.
For example, thanks to petrophysical studies, the nature of anisotropy of the elastic characteristics of rocks was revealed, its dependence on petrostructural factors, and the relationship with deformation changes were established. The data obtained made it possible to reconstruct the geodynamic stresses that occurred during the formation of intrusive bodies, the formation of large-scale tectonic elements, especially fault zones, as well as ore-bearing structures.
In the framework of petrology, the laws of changes in the physical characteristics of magmatic complexes under different conditions of their formation have been studied. In the course of the research, the tasks that performed the pre-hydrothermal-metasomatic processes were established. They contributed to the formation of porous, brittle and permeable rocks, which are easily replaced by ore substance.
Developments in petrophysics were introduced into other geological and physical research areas. For example, they had special significance for seismology. Scientific and methodological developments were used to interpret the profiles of seismic deep zoning. Of no small importance were the developments for tectonics. In particular, they helped to establish the nature of violations and the discovery of hidden faults.
Conclusion
As you can see, the study of rocks involves the application of a variety of methods and techniques. Recently, computer programs have been used in research. They significantly speed up the process of collecting and processing data. Given the number of unresolved issues, it seems that the science of rocks will continue to improve.