What geological period are we living in? History of the geological development of the earth.

The origin of the earth and early stages its formation

One of the important tasks of modern natural science in the field of Earth sciences is the restoration of the history of its development. According to modern cosmogonic concepts, the Earth was formed from the gas and dust matter scattered in the protosolar system. One of the most probable variants of the origin of the Earth is as follows. Initially, the Sun and a flattened rotating circumsolar nebula were formed from an interstellar gas and dust cloud under the influence of, for example, the explosion of a nearby supernova. Next, the evolution of the Sun and the circumsolar nebula took place with the transmission of the moment of momentum from the Sun to the planets by electromagnetic or turbulent-convective methods. Subsequently, the "dusty plasma" condensed into rings around the Sun, and the material of the rings formed the so-called planetesimals, which condensed to planets. After that, a similar process was repeated around the planets, which led to the formation of satellites. This process is believed to have taken about 100 million years.

It is assumed that further, as a result of the differentiation of the Earth's substance under the influence of its gravitational field and radioactive heating, different in chemical composition, state of aggregation and physical properties of the shell - the Earth's geosphere - arose and developed. The heavier material formed a core, probably composed of iron mixed with nickel and sulfur. Somewhat lighter elements remained in the mantle. According to one of the hypotheses, the mantle is composed of simple oxides of aluminum, iron, titanium, silicon, etc. About the composition earth's crust already discussed in sufficient detail in § 8.2. It is composed of lighter silicates. Even lighter gases and moisture formed the primary atmosphere.

As already mentioned, it is assumed that the Earth was born from a cluster of cold solid particles that fell out of a gas and dust nebula and stuck together under the influence of mutual attraction. As the planet grew, it warmed up due to the collision of these particles, which reached several hundred kilometers, like modern asteroids, and the release of heat not only by naturally radioactive elements now known to us in the crust, but also by more than 10 radioactive isotopes Al, Be, which have since died out. Cl, etc. As a result, complete (in the core) or partial (in the mantle) melting of the substance could occur. In the initial period of its existence, up to about 3.8 billion years, the Earth and other planets of the terrestrial group, as well as the Moon, were subjected to increased bombardment by small and large meteorites. The consequence of this bombardment and the earlier collision of planetesimals could be the release of volatiles and the beginning of the formation of a secondary atmosphere, since the primary, consisting of gases captured during the formation of the Earth, most likely quickly dissipated into outer space. A little later, the hydrosphere began to form. The atmosphere and hydrosphere formed in this way were replenished in the process of degassing of the mantle during volcanic activity.

The fall of large meteorites created vast and deep craters, similar to those currently observed on the Moon, Mars, Mercury, where their traces have not been erased by subsequent changes. Cratering could provoke magma outpourings with the formation of basalt fields similar to those covering the lunar "seas". Thus, the primary crust of the Earth was probably formed, which, however, has not been preserved on its modern surface, with the exception of relatively small fragments in the “younger” crust of the continental type.

This crust, containing in its composition already granites and gneisses, however, with a lower content of silica and potassium than in “normal” granites, appeared at the turn of about 3.8 billion years and is known to us from outcrops within the crystalline shields of almost all continents. The method of formation of the oldest continental crust is still largely unclear. This crust, metamorphosed everywhere under conditions of high temperatures and pressures, contains rocks whose textural features indicate accumulation in the aquatic environment, i.e. in this distant epoch the hydrosphere already existed. The appearance of the first crust, similar to the modern one, required the supply of large amounts of silica, aluminum, and alkalis from the mantle, while now mantle magmatism creates a very limited volume of rocks enriched in these elements. It is believed that 3.5 billion years ago, gray-gneiss crust, named after the predominant type of its constituent rocks, was widespread on the area of ​​modern continents. In our country, for example, it is known on the Kola Peninsula and in Siberia, in particular in the basin of the river. Aldan.

Principles of periodization of the geological history of the Earth

Further events in geologic time are often determined according to relative geochronology, categories "old", "younger". For example, some era is older than some other. Separate segments of geological history are called (in decreasing order of their duration) zones, eras, periods, epochs, centuries. Their identification is based on the fact that geological events are imprinted in rocks ah, and sedimentary and volcanogenic rocks are located in the earth's crust in layers. In 1669, N. Stenoy established the law of stratification sequence, according to which the underlying layers sedimentary rocks older than the overlying ones, i.e. formed before them. Thanks to this, it became possible to determine the relative sequence of the formation of layers, and hence the geological events associated with them.

The main method in relative geochronology is the biostratigraphic, or paleontological, method of establishing the relative age and sequence of the occurrence of rocks. This method was proposed by W. Smith at the beginning of the 19th century, and then developed by J. Cuvier and A. Brongniard. The fact is that in most sedimentary rocks one can find the remains of animal or plant organisms. J.B. Lamarck and C. Darwin established that animals and plant organisms in the course of geological history gradually improved in the struggle for existence, adapting to changing living conditions. Some animal and plant organisms died out at certain stages of the development of the Earth, they were replaced by others, more perfect ones. Thus, according to the remains of earlier living more primitive ancestors found in some layer, one can judge the relatively older age of this layer.

Another method of geochronological separation of rocks, especially important for the separation of igneous formations of the ocean floor, is based on the property of the magnetic susceptibility of rocks and minerals formed in the Earth's magnetic field. With a change in the orientation of the rock relative to magnetic field or the field itself, part of the "inherent" magnetization is preserved, and the change in polarity is imprinted in a change in the orientation of the remanent magnetization of rocks. Currently, a scale for the change of such epochs has been established.

Absolute geochronology - the doctrine of the measurement of geological time, expressed in ordinary absolute astronomical units(years), - determines the time of occurrence, completion and duration of all geological events, primarily the time of formation or transformation (metamorphism) of rocks and minerals, since the age of geological events is determined by their age. The main method here is the analysis of the ratio of radioactive substances and their decay products in rocks formed in different eras.

The oldest rocks are currently established in West Greenland (3.8 billion years). The oldest age (4.1 - 4.2 Ga) was obtained from zircons from Western Australia, but the zircon here occurs in a redeposited state in Mesozoic sandstones. Taking into account the concept of the simultaneity of the formation of all the planets of the solar system and the moon and the age of the most ancient meteorites (4.5-4.6 billion years) and ancient lunar rocks (4.0-4.5 billion years), the age of the Earth is assumed to be 4.6 billion years.

In 1881, at the II International Geological Congress in Bologna (Italy), the main divisions of the combined stratigraphic (for separating layered sedimentary rocks) and geochronological scales were approved. According to this scale, the history of the Earth was divided into four eras in accordance with the stages of development of the organic world: 1) Archean, or Archeozoic - the era of ancient life; 2) Paleozoic - era ancient life; 3) Mesozoic - era average life; 4) Cenozoic - the era of new life. In 1887, the Proterozoic, the era of primary life, was singled out from the Archean era. Later the scale was improved. One of the variants of the modern geochronological scale is presented in Table. 8.1. The Archean era is divided into two parts: early (older than 3500 Ma) and late Archean; Proterozoic - also into two: early and late Proterozoic; in the latter, Riphean is distinguished (the name comes from the ancient name Ural mountains) and Vendian periods. The Phanerozoic zone is subdivided into the Paleozoic, Mesozoic and Cenozoic eras and consists of 12 periods.

Table 8.1. Geological scale

The main stages of the evolution of the earth's crust

Let us briefly consider the main stages in the evolution of the earth's crust as an inert substrate, on which the diversity of the surrounding nature has developed.

INapxee The still rather thin and plastic crust, under the influence of extension, experienced numerous discontinuities, through which basaltic magma again rushed to the surface, filling troughs hundreds of kilometers long and many tens of kilometers wide, known as greenstone belts (they owe this name to the predominant greenschist low-temperature metamorphism of basalt breeds). Along with basalts, among the lavas of the lower, most thick part of the section of these belts, there are high-magnesian lavas, indicating a very high degree of partial melting of the mantle substance, which indicates a high heat flow, much higher than the modern one. The development of greenstone belts consisted in a change in the type of volcanism towards an increase in the content of silicon dioxide (SiO 2 ) in it, in compressional deformations and metamorphism of sedimentary-volcanogenic fulfillment, and, finally, in the accumulation of clastic sediments, indicating the formation of a mountainous relief.

After the change of several generations of greenstone belts, the Archean stage of the evolution of the earth's crust ended 3.0 -2.5 billion years ago with the massive formation of normal granites with a predominance of K 2 O over Na 2 O. Granitization, as well as regional metamorphism, which in some places reached the highest stage, led to the formation of a mature continental crust over most of the area of ​​modern continents. However, this crust turned out to be insufficiently stable: at the beginning of the Proterozoic era, it experienced crushing. At this time, a planetary network of faults and cracks arose, filled with dikes (plate-like geological bodies). One of them, the Great Dike in Zimbabwe, is over 500 km long and up to 10 km wide. In addition, rifting appeared for the first time, giving rise to zones of subsidence, powerful sedimentation and volcanism. Their evolution led to the creation at the end early Proterozoic(2.0-1.7 billion years ago) of folded systems that re-soldered the fragments of the Archean continental crust, which was facilitated by a new era of powerful granite formation.

As a result, by the end of the Early Proterozoic (by the turn of 1.7 billion years ago), a mature continental crust already existed on 60–80% of the area of ​​its modern distribution. Moreover, some scientists believe that at this turn, the entire continental crust formed a single massif - the Megagea supercontinent ( big land), which on the other side of the globe was opposed by the ocean - the predecessor of the modern Pacific Ocean- Megathalassa (big sea). This ocean was less deep than modern oceans, because the growth of the volume of the hydrosphere due to degassing of the mantle in the process of volcanic activity continues throughout the subsequent history of the Earth, although more slowly. It is possible that the prototype of Megathalassa appeared even earlier, at the end of the Archean.

In the Catarchean and the beginning of the Archean, the first traces of life appeared - bacteria and algae, and in the late Archean, algal calcareous structures - stromatolites - spread. In the Late Archean, a radical change in the composition of the atmosphere began, and in the Early Proterozoic, a radical change in the composition of the atmosphere began: under the influence of plant life, free oxygen appeared in it, while the Catharchean and Early Archean atmosphere consisted of water vapor, CO 2 , CO, CH 4 , N, NH 3 and H 2 S with an admixture of HC1, HF and inert gases.

In the Late Proterozoic(1.7-0.6 billion years ago) Megagea began to gradually split, and this process sharply intensified at the end of the Proterozoic. Its traces are extended continental rift systems buried at the base of the sedimentary cover of ancient platforms. Its most important result was the formation of vast intercontinental mobile belts - the North Atlantic, the Mediterranean, the Ural-Okhotsk, which divided the continents North America, Eastern Europe, East Asia and the largest fragment of Megagea - the southern supercontinent Gondwana. The central parts of these belts developed on the oceanic crust newly formed during rifting, i.e. the belts were ocean basins. Their depth gradually increased as the hydrosphere grew. At the same time, mobile belts developed along the periphery of the Pacific Ocean, the depth of which also increased. Climatic conditions became more contrasting, as evidenced by the appearance, especially at the end of the Proterozoic, of glacial deposits (tillites, ancient moraines, and water-glacial sediments).

Paleozoic stage The evolution of the earth's crust was characterized by the intensive development of mobile belts - intercontinental and marginal continental (the latter on the periphery of the Pacific Ocean). These belts were divided into marginal seas and island arcs, their sedimentary-volcanic strata experienced complex fold-thrust, and then normal-shear deformations, granites were introduced into them and on this basis folded mountain systems were formed. This process proceeded unevenly. It distinguishes a number of intense tectonic epochs and granitic magmatism: Baikal - at the very end of the Proterozoic, Salair (from the Salair ridge in Central Siberia) - at the end of the Cambrian, Takov (from the Takov Mountains in the east of the USA) - at the end of the Ordovician, Caledonian ( from the ancient Roman name of Scotland) - at the end of the Silurian, Acadian (Acadia - old name the northeastern states of the USA) - in the middle of the Devonian, the Sudetenian - at the end of the Early Carboniferous, the Saal (from the Saale River in Germany) - in the middle of the Early Permian. The first three tectonic epochs of the Paleozoic are often combined into the Caledonian era of tectogenesis, the last three into the Hercynian or Varisian. In each of the listed tectonic epochs, certain parts of the mobile belts turned into folded mountain structures, and after destruction (denudation) they were part of the foundation of young platforms. But some of them partially experienced activation in subsequent epochs of mountain building.

By the end of the Paleozoic, the intercontinental mobile belts were completely closed and filled with folded systems. As a result of the withering away of the North Atlantic belt, the North American continent closed with the East European, and the latter (after the completion of the development of the Ural-Okhotsk belt) - with the Siberian, Siberian - with the Chinese-Korean. As a result, the supercontinent Laurasia was formed, and the dying off of the western part of the Mediterranean belt led to its unification with the southern supercontinent - Gondwana - into one continental block - Pangea. The eastern part of the Mediterranean belt at the end of the Paleozoic - the beginning of the Mesozoic turned into a huge bay of the Pacific Ocean, along the periphery of which folded mountain structures also rose.

Against the background of these changes in the structure and relief of the Earth, the development of life continued. The first animals appeared as early as the late Proterozoic, and at the very dawn of the Phanerozoic, almost all types of invertebrates existed, but they still lacked the shells or shells that have been known since the Cambrian. In the Silurian (or already in the Ordovician), vegetation began to land on land, and at the end of the Devonian there were forests that became most widespread in the Carboniferous period. Fish appeared in the Silurian, amphibians in the Carboniferous.

Mesozoic and Cenozoic eras - the last major stage in the development of the structure of the earth's crust, which is marked by the formation of modern oceans and the isolation of modern continents. At the beginning of the stage, in the Triassic, Pangea still existed, but already in the early Jurassic, it again split into Laurasia and Gondwana due to the emergence of the latitudinal Tethys ocean, stretching from Central America to Indochina and Indonesia, and in the west and east it merged with the Pacific Ocean (Fig. 8.6); this ocean also included the Central Atlantic. From here, at the end of the Jurassic, the process of moving apart the continents spread to the north, creating the North Atlantic during the Cretaceous period and the early Paleogene, and starting from the Paleogene, the Eurasian basin of the Arctic Ocean (the Amerasian basin arose earlier as part of the Pacific Ocean). As a result, North America separated from Eurasia. In the Late Jurassic, the formation of the Indian Ocean began, and from the beginning of the Cretaceous, the South Atlantic began to open up from the south. This meant the beginning of the disintegration of Gondwana, which existed as a whole throughout the Paleozoic. At the end of the Cretaceous, the North Atlantic joined the South, separating Africa from South America. At the same time, Australia separated from Antarctica, and at the end of the Paleogene, the latter separated from South America.

Thus, by the end of the Paleogene, all modern oceans took shape, all modern continents became isolated, and the appearance of the Earth acquired a form that was basically close to the present. However, there were no modern mountain systems yet.

From the Late Paleogene (40 million years ago), intensive mountain building began, culminating in the last 5 million years. This stage of the formation of young fold-cover mountain structures, the formation of revived arch-block mountains is distinguished as neotectonic. In fact, the neotectonic stage is a sub-stage of the Mesozoic-Cenozoic stage of the Earth's development, since it was at this stage that the main features of the modern Earth relief took shape, starting with the distribution of oceans and continents.

At this stage, the formation of the main features of modern fauna and flora was completed. The Mesozoic era was the era of reptiles, mammals began to predominate in the Cenozoic, and man appeared in the late Pliocene. At the end of the Early Cretaceous, angiosperms appeared and the land acquired grass cover. At the end of the Neogene and Anthropogene, the high latitudes of both hemispheres were covered by a powerful continental glaciation, the relics of which are the ice caps of Antarctica and Greenland. This was the third major glaciation in the Phanerozoic: the first took place in the late Ordovician, the second - at the end of the Carboniferous - the beginning of the Permian; both were common within Gondwana.

QUESTIONS FOR SELF-CHECKING

What are spheroid, ellipsoid and geoid? What are the parameters of the ellipsoid adopted in our country? Why is it needed?

What is internal structure Earth? On the basis of what is the conclusion about its structure made?

What are the main physical parameters of the Earth and how do they change with depth?

What is the chemical and mineralogical composition of the Earth? On the basis of which the conclusion is made about chemical composition the whole earth and the earth's crust?

What are the main types of the earth's crust are currently distinguished?

What is the hydrosphere? What is the water cycle in nature? What are the main processes occurring in the hydrosphere and its elements?

What is atmosphere? What is its structure? What processes take place within it? What is weather and climate?

Define endogenous processes. What endogenous processes do you know? Briefly describe them.

What is the essence of tectonics lithospheric plates? What are its main provisions?

10. Define exogenous processes. What is the main essence of these processes? What endogenous processes do you know? Briefly describe them.

11. How do endogenous and exogenous processes interact? What are the results of the interaction of these processes? What is the essence of the theories of V. Davis and V. Penk?

What are the current ideas about the origin of the Earth? How was its early formation as a planet?

On the basis of what is the periodization of the geological history of the Earth?

14. How did the earth's crust develop in the geological past of the Earth? What are the main stages in the development of the earth's crust?

LITERATURE

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Geological dictionary. T. 1, 2. M., 1978.

GorodnitskyA. M., Zonenshain L.P., Mirlin E.G. Reconstruction of the position of the continents in the Phanerozoic. M., 1978.

7. Davydov L.K., Dmitrieva A.A., Konkina N.G. General hydrology. L., 1973.

Dynamic Geomorphology / Ed. G.S. Anan'eva, Yu.G. Simonova, A.I. Spiridonov. M., 1992.

Davis W.M. Geomorphological essays. M., 1962.

10. Earth. Introduction to general geology. M., 1974.

11. Climatology / Ed. O.A. Drozdova, N.V. Kobysheva. L., 1989.

Koronovsky N.V., Yakusheva A.F. Fundamentals of Geology. M., 1991.

Leontiev O.K., Rychagov G.I. General geomorphology. M., 1988.

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Monin A.S. History of the Earth. M., 1977.

Neklyukova N.P., Dushina I.V., Rakovskaya E.M. and etc. Geography. M., 2001.

Nemkov G.I. and etc. Historical geology. M., 1974.

Restless landscape. M., 1981.

General and field geology / Ed. A.N. Pavlova. L., 1991.

Penk V. Morphological analysis. M., 1961.

Perelman A.I. Geochemistry. M., 1989.

Poltaraus B.V., Kisloe A.V. Climatology. M., 1986.

26. Problems of Theoretical Geomorphology / Ed. L.G. Nikiforova, Yu.G. Simonov. M., 1999.

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Sorokhtin O.G., Ushakov S.A. Global evolution of the Earth. M., 1991.

Ushakov S.A., Yasamanov H.A. Continental drift and the Earth's climate. M., 1984.

Khain V.E., Lomte M.G. Geotectonics with the basics of geodynamics. M., 1995.

Khain V.E., Ryabukhin A.G. History and methodology of geological sciences. M., 1997.

Khromov S.P., Petrosyants M.A. Meteorology and climatology. M., 1994.

Schukin I.S. General geomorphology. T.I. M., 1960.

Ecological functions of the lithosphere / Ed. V.T. Trofimov. M., 2000.

Yakusheva A.F., Khain V.E., Slavin V.I. General geology. M., 1988.

Billions of years ago, our Earth was a bare, lifeless planet. And now life appeared on its surface - those first, most primitive forms of living beings, the development of which led to the endless diversity of the nature around us. How did this development take place? How did animals and plants appear on Earth, how did they change? This book will answer some of these questions. Its author, the outstanding Soviet scientist Academician V. L. Komarov, described in it the history of the flora of the Earth - from the simplest unicellular bacteria to the highly developed flowering plants of our time. The author draws this long path of development in close connection with common history Earth, with its changes natural conditions, topography, climate. The book is written in a popular way, is easy to read and will be of great benefit to the widest range of readers who have elementary information from the field of biology in the volume of a school course.

So far, only geological and climatic forces have acted in nature. As we have seen, they have always strongly influenced the vegetation and contributed to its greater and greater diversity. Now a completely new factor has appeared: man.

Born in the Tertiary period, according to various estimates, 600,000 - 1,000,000 years before our time, in ape-like forms, he met the ice age still unarmed. But in many places it was impossible to escape from the glacier; cold drove man into the caves, which became his first dwelling, and forced him to invent devices for maintaining fire. From this moment on, man becomes an industrial being and, increasing his activity, begins to influence nature more strongly than any living being. He cuts down forests, raises virgin soil, breaks through canals, blows up and digs up entire mountains, and generally changes the face of the Earth at his own discretion.

In relation to vegetation, man destroys the forest flora, destroys steppe plants and many others, and creates in their place his own special world, the world of cultivated plants, which would never exist if not for man. The modern period of the development of terrestrial vegetation is precisely characterized by the replacement by man of the flora inherited from former times by cultivated vegetation.

We have seen that the conditions plant life on Earth, they first put forward, as the pioneers of the primary settlement of the earth's crust, a group of bacteria known under the general name of chemotrophic, i.e., those whose nutrition is reduced to a small number of clearly expressed chemical reactions and does not need previously formed organic matter.

The age of bacteria was later replaced by the age of algae, which in the waters of the ancient oceans reached a significant variety of shapes and colors.

The age of algae was replaced on the primary continents by the age of psilophytes, which gave vegetation resembling in its own way general view and the size of modern thickets of large mosses.

The age of psilophytes gave way to the age of fern-like plants, which already formed extensive forests on swampy soils. This vegetation contributed a lot to the fact that both the composition of the air and the accumulation of a mass of nutrients made possible the emergence of the first land vertebrates. At the same time, the main masses of coal were accumulated.

The age of ferns gave way to the age of cone-bearing plants. For the first time, the surface of the continents took on a modern appearance in some places, and the possibility of the existence of higher animals was even closer.

The age of the cone-bearing plants was gradually replaced by the age of flowering plants, when all the plants that exist today were formed one after the other.

It must be said that the advent of a new century or period never completely destroyed the previous one. vegetable world. Always a part of the past population of the Earth was preserved and continued to exist along with the new world. So, bacteria when they appear higher vegetation not only did not disappear, but also in the soil and in the organic matter, so generously created higher plants found new sources of livelihood. Algae, once developed, continue to grow and improve along with higher plants. Moreover, they are not competitors to them, since some inhabit the coastal sea areas, while others mainly land.

Finally, the coniferous forests of our time continue to exist along with the deciduous ones, and their shade gives shelter to fern-like plants, since this heritage of the foggy and humid Carboniferous period is afraid of open habitats where it is harmed by the sun's rays, and seeks shade.

Thus the history of the earth's crust led to the creation of a rich and varied plant world, starting from the materials provided by the inorganic world and ending with the creation of what surrounds us and provides us with everything we need for life.

“Zoology and botany are still fact-gathering sciences until paleontology - Cuvier - joins here, and soon after the discovery of the cell and the development of organic chemistry. Thanks to this, comparative morphology and comparative physiology became possible, and since then both have become genuine sciences.

F. Engels

Geological time and methods for its determination

In the study of the Earth as a unique cosmic object, the idea of ​​its evolution occupies a central place, therefore an important quantitative evolutionary parameter is geological time. The study of this time is engaged in a special science called Geochronology- geological reckoning. Geochronology may be absolute and relative.

Remark 1

Absolute geochronology deals with the determination of the absolute age of rocks, which is expressed in units of time and, as a rule, in millions of years.

The determination of this age is based on the rate of decay of isotopes of radioactive elements. This speed is a constant value and does not depend on the intensity of physical and chemical processes. Age determination is based on nuclear physics methods. Minerals containing radioactive elements, during the formation of crystal lattices, form closed system. In this system, the accumulation of radioactive decay products occurs. As a result, the age of the mineral can be determined if the rate of this process is known. The half-life of radium, for example, is $1590$ years, and the complete decay of the element will occur in $10$ times the half-life. Nuclear geochronology has its leading methods − lead, potassium-argon, rubidium-strontium and radiocarbon.

Methods of nuclear geochronology made it possible to determine the age of the planet, as well as the duration of eras and periods. Radiological time measurement proposed P. Curie and E. Rutherford at the beginning of the $XX$ century.

Relative geochronology operates with such concepts as " early age, middle, late. There are several developed methods for determining the relative age of rocks. They fall into two groups - paleontological and non-paleontological.

First play a major role due to their versatility and ubiquity. The exception is the absence of organic remains in the rocks. With the help of paleontological methods, the remains of ancient extinct organisms are studied. Each rock layer has its own complex of organic remains. In each young layer there will be more remains of highly organized plants and animals. The higher the layer lies, the younger it is. A similar pattern was established by the Englishman W. Smith. He owns the first geological map of England, on which the rocks were divided by age.

Non-paleontological methods determinations of the relative age of rocks are used in cases where there are no organic remains in them. More efficient then will be stratigraphic, lithological, tectonic, geophysical methods. Using the stratigraphic method, it is possible to determine the sequence of stratification of layers in their normal occurrence, i.e. the underlying layers will be older.

Remark 3

The sequence of formation of rocks determines relative geochronology, and their age in units of time determines already absolute geochronology. A task geological time is to define chronological order geological events.

Geological table

To determine the age of rocks and their study, scientists use various methods, and for this purpose a special scale was drawn up. Geological time on this scale is divided into time periods, each of which corresponds to a certain stage in the formation of the earth's crust and the development of living organisms. The scale is called geochronological table, which includes the following divisions: eon, era, period, epoch, century, time. Each geochronological unit is characterized by its own set of deposits, which is called stratigraphic: eonoteme, group, system, department, tier, zone. A group, for example, is a stratigraphic unit, and the corresponding temporal geochronological unit is era. Based on this, there are two scales - stratigraphic and geochronological. The first scale is used when it comes to deposits, because in any period of time some geological events took place on the Earth. The second scale is needed to determine relative time. Since the adoption of the scale, the content of the scale has been changed and refined.

The largest stratigraphic units at present are eonotemes - Archean, Proterozoic, Phanerozoic. In the geochronological scale, they correspond to zones of different duration. According to the time of existence on Earth, they are distinguished Archean and Proterozoic eonotemes covering nearly $80$% of the time. Phanerozoic eon in time is much less than the previous eon and covers only $ 570 $ million years. This ionoteme is divided into three main groups - Paleozoic, Mesozoic, Cenozoic.

The names of eonotems and groups are of Greek origin:

• Archeos means ancient;
• Proteros - primary;
• Paleos - ancient;
• Mezos - medium;
• Cainos is new.

From the word " zoiko s”, which means vital, the word “ zoi". Based on this, the eras of life on the planet are distinguished, for example, the Mesozoic era means the era of average life.

Eras and periods

According to the geochronological table, the history of the Earth is divided into five geological eras: Archean, Proterozoic, Paleozoic, Mesozoic, Cenozoic. The eras are further subdivided into periods. There are much more of them - $12$. The duration of the periods varies from $20$-$100$ million years. The last one points to its incompleteness. Quaternary period of the Cenozoic era, its duration is only $1.8 million years.

Archean era. This time began after the formation of the earth's crust on the planet. By this time there were mountains on the Earth and the processes of erosion and sedimentation had come into play. The Archean lasted for approximately $2 billion years. This era is the longest in duration, during which volcanic activity was widespread on Earth, there were deep uplifts, which resulted in the formation of mountains. Most of the fossils under the influence high temperature, pressure, displacement of masses, was destroyed, but little data about that time was preserved. In the rocks of the Archean era, pure carbon is found in dispersed form. Scientists believe that these are altered remains of animals and plants. If the amount of graphite reflects the amount of living matter, then there was a lot of it in the Archaean.

Proterozoic era. In terms of duration, this is the second era, spanning $1 billion years. During the era there was a deposition a large number rainfall and one significant glaciation. Ice sheets extended from the equator to $20$ degrees of latitude. Fossils found in the rocks of this time are evidence of the existence of life and its evolutionary development. Spicules of sponges, remains of jellyfish, fungi, algae, arthropods, etc. have been found in the Proterozoic deposits.

Palaeozoic. This era stands out six periods:

• Cambrian;
• Ordovician,
• Silur;
• Devonian;
• Carbon or coal;
• Perm or Perm.

The duration of the Paleozoic is $370$ million years. During this time, representatives of all types and classes of animals appeared. Only birds and mammals were missing.

Mesozoic era. The era is divided into three period:

• Triassic;

The era started about $230 million years ago and lasted $167 million years. During the first two periods Triassic and Jurassic – most of mainland areas rose above sea level. The climate of the Triassic is dry and warm, and in the Jurassic it became even warmer, but was already humid. In the state Arizona there is a famous stone forest, existing since Triassic period. True, only trunks, logs and stumps remained from the once mighty trees. At the end of the Mesozoic era, or rather in the Cretaceous period, a gradual advance of the sea takes place on the continents. The North American continent experienced a subsidence at the end of the Cretaceous, and as a result, the waters of the Gulf of Mexico joined with the waters of the Arctic basin. The mainland was divided into two parts. The end of the Cretaceous period is characterized by a large uplift, called Alpine orogeny. At this time, the Rocky Mountains, the Alps, the Himalayas, the Andes appeared. In the west of North America, intense volcanic activity began.

Cenozoic era. This is a new era that has not yet ended and continues at the present time.

The era was divided into three periods:

• Paleogene;
• Neogene;
• Quaternary.

Quaternary period has whole line unique features. This is the time of the final formation of the modern face of the Earth and ice ages. New Guinea and Australia became independent, moving closer to Asia. Antarctica has remained in its place. Two Americas united. Of the three periods of the era, the most interesting is quaternary period or anthropogenic. It continues today, and was allocated in $1829$ by a Belgian geologist J. Denoyer. Coolings are replaced by warmings, but its most important feature is appearance of man.

Modern man lives in the Quaternary period of the Cenozoic era.

Reasons for the allocation of the Quaternary period

Starting from the Oligocene, the climate on Earth began to steadily cool, which was accompanied by an equally steady decrease in sea level. Both of these processes were not strictly unidirectional - they were oscillatory, but the general trend persisted. Simultaneously, land contours became more and more modern, zonal-sectoral landscape-climatic zones close to modern ones were established. The cooling was accompanied by an increase in the amplitude of climate fluctuations, and these fluctuations began to noticeably affect the entire natural environment - during periods of cold snaps, a massive offensive of cold-loving tundra-steppe vegetation occurred, the corresponding fauna spread, configuration natural areas changed sharply in the direction of reduction of low-latitude transition zones and growth of high-latitude ones. During periods of warming, cold-loving flora and fauna almost disappeared, and low-latitude transition zones became more widespread. At the same time, with each new warming of relic tropical plants in temperate zones became less and less.

All this led to the fact that over several million years the physical and geographical situation on Earth has changed dramatically and turned out to be incomparable with any of the previously existing ones. It took allocation last stage development of the geographical shell in a special geological period. This happened in 1825, when approximately the last million years of the history of the Earth were combined into a special - Quaternary period. It is sometimes called the Anthropogenic period or the Pleistocene.

A special period in the history of the Earth has its own unique features, distinguishing it from all other geological periods:

1. It is unusually short. Its duration is only 1.8 million years (in Russia - 1.65 million years).

2. Deposits of the Quaternary age are extremely young and therefore: a) are everywhere preserved and cover the Earth with an almost continuous cover; b) are characterized by extreme genetic diversity, diversity and facies variability of lithological composition; c) have an almost exclusively continental genesis (except, of course, Quaternary and modern deposits accumulating in the seas and oceans); d) have low power due to the short duration of their formation.

3. Well-preserved natural and biotic complexes (and not just individual leading fossils).

The main events of the Quaternary period are the following:

1. Sharp and repeated climate fluctuations, which led to glaciations in high latitudes in the second half of the period (which has not happened, at least since the Carboniferous). Warm epochs are called thermochrons, cold epochs are called cryochrons. These fluctuations and glaciations have been recorded in thousands of outcrops, where special glacial deposits are exposed - boulder loams (moraines) and others, as well as, in analyzes of the fauna and flora of those times, in the composition of oxygen isotopes and various other traces of past eras.

2. The appearance of man. If glaciations have already occurred on Earth's continents in the past, then this event is unique, having no analogues either in the history of the Earth or in the history of other celestial bodies accessible to study. The emergence and development of man led to the emergence on Earth of a fundamentally new suprabiotic community - humanity. It was humanity that first touched the noosphere - the sphere of the mind, which some consider the highest state of the earth's biosphere (according to V.I. Vernadsky), and others - an intangible substance that is not included in geographical envelope, but perceived by man and contributing to his geo-forming role (according to E. Leroy and P. Teilhard de Chardin).

A few words must be said about the lower boundary of the Quaternary period and its periodization. Although the first signs of continental glaciation appeared only 780 thousand years ago. n., the lower limit of the Quaternary is carried out in the countries of Western Europe at the turn of 1.8 million years. It was approved in 1932 according to the established signs of cooling. sea ​​water in marine sections of southern Italy, at the base of the Calabrian stage. In 1948, this border was legalized everywhere except the USSR. But in 1990, in our country, the boundary of the Quaternary period was lowered to the boundary of 1.65 million years. n. and began to be traced along the lower boundary of the Apsheroian stage (analogous to the Calabrian). The time interval between the new and old boundaries of the Quaternary period, i.e. between 1.65 and 0.78 million liters. n. was called the Eopleistocene and the former Quaternary the Neopleistocene (although it is often referred to simply as the Pleistocene) (see 7.1).

And the universe. For example, the hypotheses of Kant - Laplace, O.Yu. Schmidt, Georges Buffon, Fred Hoyle and others. But most scientists tend to believe that the Earth is about 5 billion years old.

The unified international geochronological scale gives an idea of ​​the events of the geological past in their chronological sequence. Its main divisions are the eras: Archean, Proterozoic, Paleozoic, Mesozoic. Cenozoic. The oldest interval of geological time (Archaean and Proterozoic) is also called the Precambrian. It covers a large period - almost 90% of the whole (the absolute age of the planet, according to modern concepts, is taken to be 4.7 billion years).

Within the eras, smaller time intervals are distinguished - periods (for example, the Paleogene, Neogene and Quaternary in the Cenozoic era).

In the Archean era (from the Greek - original, ancient), crystalline rocks (granites, gneisses, schists) were formed. In this era, powerful mountain-building processes took place. The study of this era allowed geologists to assume the presence of the seas and living organisms in them.

The Proterozoic era (the era of early life) is characterized by rock deposits in which the remains of living organisms are found. During this era, the most stable areas, platforms, formed on the surface of the Earth. Platforms - these ancient cores - became the centers of formation.

The Paleozoic era (the era of ancient life) is distinguished by several stages of powerful mountain building,. In this era, the Scandinavian mountains, the Urals, Tien Shan, Altai, Appalachians arose. At this time, animal organisms with a solid skeleton appeared. Vertebrates first appeared: fish, amphibians, reptiles. Ground vegetation appeared in the Middle Paleozoic. Tree ferns, club mosses, and others served as material for the formation of coal deposits.

The Mesozoic era (the era of middle life) is also characterized by intense folding. Mountains formed in areas adjacent to. Reptiles dominated among animals (dinosaurs, proterosaurs, etc.), birds and mammals first appeared. The vegetation consisted of ferns, conifers, angiosperms appeared at the end of the era.

In the Cenozoic era (the era of new life), the modern distribution of continents and oceans takes shape, and intense mountain-building movements take place. Mountain ranges are formed on the shores of the Pacific Ocean, in the south of Europe and Asia (, the Himalayas, the Cordillera Coast Ranges, etc.). At the beginning of the Cenozoic era, the climate was much warmer than today. However, the increase in land area due to the rise of the continents led to a cooling. Extensive ice sheets appeared in the north and. This led to significant changes in the flora and fauna. Many animals have died out. Plants and animals appeared close to modern ones. At the end of this era, man appeared and began to intensively populate the land.

The first three billion years of the development of the Earth led to the formation of land. According to scientists, at first there was one continent on Earth, which subsequently split into two, and then there was another division and, as a result, to today formed five continents.

The last billion years of the Earth's history is associated with the formation of folded regions. At the same time, several tectonic cycles (epochs) are distinguished in the geological history of the last billion years: Baikal (end of the Proterozoic), Caledonian (early Paleozoic), Hercynian (late Paleozoic), Mesozoic (Mesozoic), Cenozoic or Alpine cycle (from 100 million years to present tense).
As a result of all the above processes, the Earth acquired a modern structure.