The history of planet Earth is already about 7 billion years old. During this time, our common home has undergone significant changes, which was a consequence of the change in periods. in chronological order, they reveal the entire history of the planet from its very appearance to the present day.

Geological chronology

The history of the Earth, presented in the form of aeons, groups, periods and eras, is a certain grouped chronology. At the first international congresses of geology, a special chronological scale was developed, which represented the periodization of the Earth. Subsequently, this scale was replenished with new information and changed, as a result, now it reflects all geological periods in chronological order.

The largest subdivisions in this scale are eonothems, eras and periods.

Formation of the Earth

The geological periods of the Earth in chronological order begin their history precisely with the formation of the planet. Scientists have come to the conclusion that the Earth formed about 4.5 billion years ago. The very process of its formation was very long and, possibly, began 7 billion years ago from small cosmic particles. Over time, the force of gravity grew, along with it the speed of bodies falling on the forming planet increased. Kinetic energy was transformed into heat, resulting in a gradual heating of the Earth.

The core of the Earth, according to scientists, was formed over several hundred million years, after which the planet began to gradually cool down. Currently, the molten core contains 30% of the Earth's mass. The development of other shells of the planet, according to scientists, is not yet complete.

Precambrian aeon

In the geochronology of the Earth, the first eon is called the Precambrian. It spans 4.5 billion to 600 million years ago. That is, the lion's share of the planet's history is covered first. However, this eon is divided into three more - katarchean, archean, proterozoic. Moreover, often the first of them stands out as an independent eon.

At this time, the formation of land and water took place. All this happened during active volcanic activity throughout almost the entire eon. Shields of all continents were formed in the Precambrian, but traces of life are very rare.

Catarchean eon

The beginning of the history of the Earth - half a billion years of its existence in science is called katarchean. The upper boundary of this eon is at around 4 billion years ago.

Popular literature portrays the catarchy for us as a time of active volcanic and geothermal changes on the Earth's surface. However, in reality, this is not true.

The Catarchian Eon is a time when volcanic activity did not appear, and the surface of the Earth was a cold inhospitable desert. Although quite often there were earthquakes that smoothed the landscape. The surface looked like a dark gray primary material covered with a layer of regolith. A day at that time was only 6 hours.

Archean eon

The second main eon of the four in the history of the Earth lasted about 1.5 billion years - 4-2.5 billion years ago. Then the Earth did not yet have an atmosphere, therefore there was no life yet, however, bacteria appeared in this eon, due to the lack of oxygen, they were anaerobic. As a result of their activities, today we have deposits of natural resources such as iron, graphite, sulfur and nickel. The history of the term "archaea" dates back to 1872, when it was proposed by the famous American scientist J. Dan. The Archean Eon, unlike the previous one, is characterized by high volcanic activity and erosion.

Proterozoic eon

If we consider the geological periods in chronological order, the next billion years were occupied by the Proterozoic. This period is also characterized by high volcanic activity and sedimentation, and erosion also continues over huge areas.

The formation of the so-called. mountains Currently, they are small hills on the plains. The rocks of this eon are very rich in mica, non-ferrous metal ores and iron.

It should be noted that in the Proterozoic period the first living things appeared - the simplest microorganisms, algae and fungi. And by the end of the eon, worms, marine invertebrates, and mollusks appear.

Phanerozoic eon

All geological periods in chronological order can be divided into two types - explicit and hidden. Phanerozoic refers to the explicit. At this time, a large number of living organisms with mineral skeletons appear. The epoch preceding the Phanerozoic was called hidden because practically no traces of it were found due to the absence of mineral skeletons.

The last about 600 million years of our planet's history are called the Phanerozoic eon. The most significant events of this eon are the Cambrian explosion, which occurred about 540 million years ago, and the five largest extinctions in the history of the planet.

Eras of the Precambrian Aeon

There were no universally recognized eras and periods during the Catarchean and Archean times, so we will skip their consideration.

The Proterozoic consists of three great eras:

Paleoproterozoic- that is, the ancient one, which includes the Siderius, the Riasian period, the Orosirian and Staterias. By the end of this era, the concentration of oxygen in the atmosphere had reached its present level.

Mesoproterozoic- average. Consists of three periods - potassium, ectasia and stheny. During this era, algae and bacteria reached their greatest flourishing.

Neoproterozoic- new, consisting of tonium, cryogeny and ediacaria. At this time, the formation of the first supercontinent, Rodinia, takes place, but then the plates diverged again. The coldest ice age took place in an era called the Mesoproterozoic, during which most of the planet froze over.

Eras of the Phanerozoic aeon

This eon consists of three great eras, sharply different from each other:

Paleozoic, or the era of ancient life. It began about 600 million years ago and ended 230 million years ago. The Paleozoic consists of 7 periods:

1. Cambrian (a temperate climate is formed on Earth, the landscape is low, during this period all modern types of animals are born).
2. Ordovician (the climate on the entire planet is warm enough, even in Antarctica, while the land is sinking significantly. The first fish appear).
3. The Silurian period (the formation of large inland seas takes place, while the lowlands become drier due to the uplift of the land. The development of fish continues. The Silurian period is marked by the appearance of the first insects).
4. Devon (emergence of the first amphibians and forests).
5. Lower Carboniferous (dominance of ferns, distribution of sharks).
6. Upper and Middle Carboniferous (appearance of the first reptiles).
7. Perm (most of the ancient animals are dying out).

Mesozoic, or the time of reptiles. Geological history consists of three periods:

1. Triassic (seed ferns die out, gymnosperms dominate, the first dinosaurs and mammals appear).
2. Jura (part of Europe and the western part of America is covered with shallow seas, the appearance of the first toothed birds).
3. Chalk (the emergence of maple and oak forests, the highest development and extinction of dinosaurs and toothed birds).

Cenozoic, or the time of mammals. Consists of two periods:

1. Tertiary. At the beginning of the period, predators and ungulates reach their dawn, the climate is warm. There is a maximum spread of forests, the oldest mammals are dying out. Approximately 25 million years ago, humans appear and in the Pliocene epoch.
2. Quaternary. Pleistocene - large mammals are dying out, human society is emerging, 4 ice ages occur, many plant species are dying out. The modern era - the last ice age is coming to an end, gradually the climate takes on its present form. Human supremacy over the entire planet.

The geological history of our planet has a long and contradictory development. In this process, several extinctions of living organisms took place, ice ages were repeated, periods of high volcanic activity were observed, there were eras of the dominance of different organisms: from bacteria to humans. The history of the Earth began about 7 billion years ago, it was formed about 4.5 billion years ago, and only less than a million years ago, man ceased to have competitors in all living nature.

The history of our planet still holds many mysteries. Scientists from various fields of natural science have contributed to the study of the development of life on Earth.

Our planet is believed to be about 4.54 billion years old. This entire time period is usually divided into two main stages: Phanerozoic and Precambrian. These stages are called eons or eonothems. Eons, in turn, are divided into several periods, each of which is distinguished by a set of changes that took place in the geological, biological, atmospheric state of the planet.

1. Precambrian, or cryptose- this is an eon (the time period of the Earth's development), covering about 3.8 billion years. That is, the Precambrian is the development of the planet from the moment of formation, formation of the earth's crust, protoocean and the emergence of life on Earth. By the end of the Precambrian, highly organized organisms with a developed skeleton were already widespread on the planet.

The eon includes two more eonothems - katarchean and archaea. The latter, in turn, includes 4 eras.

1. Catarchaeus- this is the time of the formation of the Earth, but there was still neither a core nor an earth's crust. The planet was still a cold space body. Scientists assume that during this period there was already water on the Earth. Catarchean lasted for about 600 million years.

2. Archaea covers a period of 1.5 billion years. During this period, there was still no oxygen on the Earth, the formation of deposits of sulfur, iron, graphite, nickel took place. The hydrosphere and atmosphere were a single vapor-gas envelope that enveloped the globe in a dense cloud. The sun's rays practically did not penetrate through this veil, so darkness reigned on the planet. 2.1 2.1. Eoarcheus- this is the first geological era, which lasted about 400 million years. The most important event in the Eoarchean is the formation of the hydrosphere. But there was still little water, the reservoirs existed separately from each other and until they merged into the world ocean. At the same time, the earth's crust becomes solid, although asteroids are still bombing the Earth. At the end of the Eoarchean, the first supercontinent in the history of the planet, Vaalbara, is formed.

2.2 Paleoarchean- the next era, which also lasted for about 400 million years. During this period, the core of the Earth is formed, the intensity of the magnetic field increases. A day on the planet lasted only 15 hours. But the oxygen content in the atmosphere increases due to the activity of the bacteria that have appeared. Remains of these first forms of the Paleoarchean era of life have been found in Western Australia.

2.3 Mesoarchean also lasted about 400 million years. In the Mesoarchean era, our planet was covered by a shallow ocean. The land areas were small volcanic islands. But already during this period, the formation of the lithosphere begins and the mechanism of plate tectonics is triggered. At the end of the Mesoarchean, the first ice age is observed, during which snow and ice form for the first time on Earth. Biological species are still represented by bacteria and microbial life forms.

2.4 Neoarchean- the final era of the Archean eon, the duration of which is about 300 million years. Colonies of bacteria at this time form the first stromatolites on Earth (limestone deposits). The most important event in Neoarchean is the formation of oxygenic photosynthesis.

II. Proterozoic- one of the longest time periods in the history of the Earth, which is usually divided into three eras. During the Proterozoic, the ozone layer appears for the first time, the world's oceans reaches almost modern volume. And after the longest Huronian glaciation, the first multicellular life forms appear on Earth - mushrooms and sponges. It is customary to divide the Proterozoic into three eras, each of which contained several periods.

3.1 Paleo-Proterozoic- the first era of the Proterozoic, which began 2.5 billion years ago. At this time, the lithosphere is fully formed. But the old forms of life due to the increase in oxygen content have practically died out. This period was called the oxygen catastrophe. By the end of the era, the first eukaryotes appear on Earth.

3.2 Mesoproterozoic lasted approximately 600 million years. The most important events of this era: the formation of continental masses, the formation of the supercontinent Rodinia and the evolution of sexual reproduction.

3.3 Neo-Proterozoic... During this era, Rodinia disintegrates into about 8 parts, the Mirovia superocean ceases to exist, and at the end of the era, the Earth is covered with ice almost to the equator. In the Neoproterozoic era, living organisms for the first time begin to acquire a hard shell, which will later serve as the basis of the skeleton.

III. Paleozoic- the first era of the Phanerozoic eon, which began about 541 million years ago and lasted about 289 million years. This is the era of the emergence of ancient life. The supercontinent Gondwana unites the southern continents, a little later the rest of the land is joined to it and Pangea appears. Climatic zones begin to form, and flora and fauna are represented mainly by marine species. Only by the end of the Paleozoic, land development begins, and the first vertebrates appear.

The Paleozoic era is conventionally divided into 6 periods.

1. Cambrian period lasted 56 million years. During this period, the main rocks are formed, and a mineral skeleton appears in living organisms. And the most important event in the Cambrian is the emergence of the first arthropods.

2. Ordovician period- the second period of the Paleozoic, which lasted 42 million years. This is the era of the formation of sedimentary rocks, phosphorites and oil shale. The organic world of the Ordovician is represented by marine invertebrates and blue-green algae.

3. Silurian period covers the next 24 million years. At this time, almost 60% of living organisms that existed before die out. But the first cartilaginous and bony fish in the history of the planet appear. On land, the Silurian is marked by the emergence of vascular plants. Supercontinent approaches and form Laurasia. By the end of the period, ice melting was noted, sea level rose, and the climate became milder.

4. Devonian period is distinguished by the rapid development of various forms of life and the development of new ecological niches. Devon covers a time span of 60 million years. The first terrestrial vertebrates, spiders, insects appear. Lungs are formed in sushi animals. Although, as before, fish predominate. The kingdom of the flora of this period is represented by prop-ferns, horsetails, lyre and goose seeds.

5. Carboniferous period often referred to as carbon. At this time, Laurasia collides with Gondwana and the new supercontinent Pangea appears. A new ocean is also being formed - Tethys. This is the time of the appearance of the first amphibians and reptiles.

6. Permian period- the last period of the Paleozoic, which ended 252 million years ago. It is believed that at this time a large asteroid fell to Earth, which led to significant climate change and the extinction of almost 90% of all living organisms. Most of the land is covered with sands, the most extensive deserts that have existed in the entire history of the development of the Earth appear.

IV. Mesozoic- the second era of the Phanerozoic eon, which lasted almost 186 million years. At this time, the continents take on an almost modern shape. And the warm climate contributes to the rapid development of life on Earth. Giant ferns disappear, and angiosperms appear to replace them. The Mesozoic is the era of the dinosaurs and the appearance of the first mammals.

In the Mesozoic era, there are three periods: Triassic, Jurassic and Cretaceous.

1. Triassic period lasted just over 50 million years. At this time, Pangea begins to split, and the inland seas gradually become shallow and dry. The climate is mild, the zones are not pronounced. Almost half of the land plants are disappearing as deserts spread. And in the kingdom of fauna, the first warm-blooded and land reptiles appear, which became the ancestors of dinosaurs and birds.

2. Jurassic period covers an interval of 56 million years. A humid and warm climate reigned on Earth. The land is covered with thickets of ferns, pines, palms, cypresses. Dinosaurs reign on the planet, and numerous mammals were notable for their small stature and thick hair.

3. Cretaceous period- the longest period of the Mesozoic, lasting almost 79 million years. The split of the continents is practically coming to an end, the Atlantic Ocean is significantly increasing in volume, ice sheets are forming at the poles. The increase in the water mass of the oceans leads to the formation of the greenhouse effect. At the end of the Cretaceous period, a catastrophe occurs, the reasons for which are still not clear. As a result, all dinosaurs and most species of reptiles and gymnosperms became extinct.

V. Cenozoic- This is the era of animals and Homo sapiens, which began 66 million years ago. The continents at this time acquired their modern shape, Antarctica occupied the south pole of the Earth, and the oceans continued to grow. The plants and animals that survived the Cretaceous catastrophe found themselves in a completely new world. Unique communities of life forms began to form on each continent.

The Cenozoic era is divided into three periods: Paleogene, Neogene and Quaternary.

1. Paleogene period ended about 23 million years ago. At this time, a tropical climate reigned on Earth, Europe was hiding under evergreen tropical forests, only deciduous trees grew in the north of the continents. It was during the Paleogene period that mammals developed rapidly.

2. Neogene period covers the next 20 million years of the planet's development. Whales and bats appear. And, although saber-toothed tigers and mastodons still roam the earth, the fauna is increasingly acquiring modern features.

3. Quaternary period began more than 2.5 million years ago and continues to this day. Two major events characterize this time period: the ice age and the emergence of man. The Ice Age completely completed the formation of the climate, flora and fauna of the continents. And the appearance of man marked the beginning of civilization.

The term "Anthropocene" was coined by Eugene Stormer in the mid-1980s, but it gained popularity thanks to the specialist in atmospheric chemistry, Nobel laureate Paul Krutzen. At the beginning of the 21st century, both the very idea of ​​the Anthropocene and its rationale were enthusiastically accepted by the ecological community in the United States, the European Union and a number of other countries, which made the issue not only scientific, but also political.

Science and politics

Indeed, if we define the boundary of the Anthropocene on geological maps, the changes will be small. But its recognition by the scientific community will strengthen the position of conservationists, will be a new argument in favor of the Paris Climate Agreement 2015 and will confirm that human influence on the planet is an indisputable fact.

What characterizes the onset of a new geological era? Several features distinguish the Anthropocene from the previous history of the Earth. Traces of nuclear and thermonuclear explosions have been present in the deposits since the middle of the 20th century. Radionuclides (plutonium-239, cesium-137, strontium-90) are recorded all over the world. The longest-lived of these, plutonium-239, has a half-life of about 24,000 years.

The most dramatic changes began after 1950: a leap is observed both in economic development and in the impact on the biosphere of the planet as a whole.

Other characteristic traces of the Anthropocene may also persist for a long time. The carbon dioxide content in the atmosphere is increasing due to the burning of fossil fuels. Materials that did not exist before appear and accumulate - reinforced concrete, plastics, metallic aluminum, many types of glass. Soils are polluted with phosphates and other mineral fertilizers, people cut down forests and plow up the steppes. Mass extinction is occurring.

At the edge of extinction

There have already been several very large extinctions in Earth's history. Their traces in the rocks are easy to identify: the lower layer, extremely rich in fossils, is overlapped by the next, in which they are practically absent, and if there are, then they are completely different. Naturally, such explicit transitions serve as convenient boundaries for dividing eras.

The first recorded mass extinction, the Ordovician-Silurian, occurred about 440 million years ago. The Devonian extinction marked the beginning of the Carboniferous period. The most famous extinction, the Cretaceous-Paleogene, cost the lives of dinosaurs about 65 million years ago. But can what is happening now be called extinction?

It is believed that man has already contributed to the desertification of the Sahara, the disappearance of mammoths and other representatives of the glacial fauna. But this process is far from over: according to WWF, the World Wildlife Fund, over the past 40 years, the total number of animals has decreased by 60%. For a couple of decades, the diversity of island butterflies has dropped by 71%, birds - by 56%, plants - by almost a third.

At the same time, man turns out to be a powerful evolutionary factor for species that manage to survive in the new world. For example, a wide collection and shooting of animals is aimed primarily at the largest individuals. Small animals, small fish are more likely to survive, which triggers previously non-existent adaptation mechanisms. Indeed, in regions where various species are objects of hunting or fishing, there is a threefold acceleration of changes in their marker features - and a general decrease in size.

Paleontologists of the future will surely be able to find traces of these processes in the sediments. In the stratigraphy, evidence will be preserved of both the rather rapid, in geological terms, the disappearance of many species, and their unexpected crushing before death - the boundary of the Anthropocene can be set by this “extinction line”. But minerals are also ready to give their reference to the new era.

Garbage and diamonds

Carnegie Institute geologist Robert Hazen has calculated that of the more than 5,200 minerals described by modern science, almost 4% owe their appearance to humans. Some of them were formed after people took them out of the ground and for the first time they began to be affected by erosion factors. Others are artificial by their origin: they are formed during the decomposition of garbage or they are simply produced in factories. Many of these compounds are extremely inert and can persist for a long time, even by geological standards. Their appearance in geological deposits can also be considered one of the markers of the onset of a new era.

But the nuclear tests, which began in the 1940s, left not only scattered radionuclides. Geologists of the future will recognize them by their characteristic glassy cavities in the earth's crust - traces of deep underground nuclear explosions. The multi-colored "glass" formed under the influence of enormous temperatures can last for hundreds of millions of years. However, ordinary bottle glass, resistant to the action of both acidic and alkaline environments, is quite capable of persisting for more than one million years.

Steel and reinforced concrete structures will give less hope of detecting the onset of the Anthropocene. They are less resistant to temperature fluctuations, and most importantly, to water. The iron, which hardens the concrete, will help it break down over time, rusting and eroding the structure from the inside. But for a very long time, products and structures made of metals such as titanium, alloy steel and others that could not arise naturally on our planet will survive.

The “time of people” also has one more marker - this is the concentration of gold, platinum and cut precious stones. A single whimsically cut stone can tell a lot about our technology and culture. Garbage dumps, industrial waste and jewelery will be key indications of the beginning of the Anthropocene from a mineralogical point of view. If only the scientists themselves agree to consider it a full-fledged period of geochronology.

Between age and age

The recognition of the Anthropocene in science is still far from complete. At the moment, there is only a positive recommendation from the Working Group on the Anthropocene, which will be analyzed by the stratigraphic commission at the International Union of Geological Sciences (IUGS). Only this body can make decisions concerning the International Geochronological Scale, and it is vigilant about the clarity and unambiguity of its wording.

Stratigraphers develop criteria and make decisions to define the boundaries of geological ages, eras, periods, eras and eons. In order for them to consider the "candidacy" of the Anthropocene more favorably, the working group voiced a number of compromise options. For example, considering the Anthropocene as a new geological epoch, we must state the end of the previous one, the Holocene. If there are not enough reasons to take such a step, the Anthropocene can be designated as an event of a lower level - the geological age.

According to the chairman of the stratigraphic commission, professor at the University of California at Long Beach Stanley Finney, it is too early to talk about the recognition of the Anthropocene as a stratigraphic unit. Even supporters of the adoption of this term are not entirely sure from what point to count the beginning of an era. Accumulation of aluminum? Extinction of species?

If the extinction of the Pleistocene fauna, including mammoths and cave bears, we associate with human activity, is it not easier to rename the Holocene to the Anthropocene? And if you count from the accumulation of industrial products, isn't it worth starting from the 18th century? Or draw the border by the date of the first nuclear test? In this case, the Anthropocene began on July 16, 1945 at 14:29 Moscow time.

The emergence of the Earth and the early stages of 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 gas and dust scattered in the protosolar system. One of the most probable variants of the Earth's origin is as follows. First, the Sun and a flattened rotating circumsolar nebula from an interstellar gas and dust cloud formed under the influence of, for example, an explosion of a nearby supernova. Further, the evolution of the Sun and the circumsolar nebula took place with the transfer of the angular momentum from the Sun to the planets in an electromagnetic or turbulent-convective way. Subsequently, the "dusty plasma" condensed into rings around the Sun, and the material of the rings formed the so-called planetesimals, which condensed to the planets. After that, a similar process was repeated around the planets, which led to the formation of satellites. It is believed that this process took about 100 million years.

It is assumed that further, as a result of the differentiation of the Earth's matter under the influence of its gravitational field and radioactive heating, a shell, the Earth's geosphere, different in chemical composition, state of aggregation and physical properties, arose and developed. The heavier material formed a core, probably composed of iron with an admixture of nickel and sulfur. Some 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. The composition of the earth's crust has already been 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 falling out of a gas and dust nebula and sticking 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 AI, Be that have become extinct since then. Cl, etc. As a result, complete (in the core) or partial (in the mantle) melting of matter could occur. In the initial period of its existence, up to about 3.8 billion years, the Earth and other terrestrial planets, as well as the Moon, were subjected to intense bombardment by small and large meteorites. A consequence of this bombardment and an 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 in space. A little later, the hydrosphere began to form. The atmosphere and hydrosphere thus formed were replenished during the 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. Crater formation could provoke outpouring of magma with the formation of basalt fields, similar to those covering the lunar "seas". This is probably how the primary crust of the Earth was formed, which, however, was not preserved on its present surface, with the exception of relatively small fragments in the "younger" continental type crust.

This crust, which already contains granites and gneisses, although 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 mode of formation of the oldest continental crust is still largely unclear. In the composition of this crust, which is ubiquitously metamorphosed under conditions of high temperatures and pressures, rocks are found, the textural features of which indicate their accumulation in the aquatic environment, i.e. in this distant epoch the hydrosphere already existed. The emergence of the first crust, similar to the modern one, required the supply of large amounts of silica, aluminum, 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, the gray-gneiss crust was widespread on the area of ​​modern continents, so named after the predominant type of its constituent rocks. In our country, for example, it is known on the Kola Peninsula and in Siberia, in particular in the basin of the river. Aldan.

The principles of periodization of the geological history of the Earth

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

Basic in relative geochronology is biostratigraphic, or paleontological, method of establishing the relative age and sequence of rocks. This method was proposed by W. Smith at the beginning of the 19th century, and then developed by J. Cuvier and A. Bronyard. The fact is that in most sedimentary rocks you can find the remains of animals or plant organisms. J. B. Lamarck and Charles Darwin established that animals and plant organisms during geological history have gradually improved in the struggle for existence, adapting to changing living conditions. Some animals and plant organisms died out at certain stages of the Earth's development, and others, more advanced ones, came to replace them. Thus, according to the remains of more primitive ancestors who lived earlier, found in any layer, one can judge about the relatively older age of this layer.

Another method of geochronological dissection of rocks, which is especially important for the dissection of magmatic formations of the ocean floor, is based on the property of magnetic susceptibility of rocks and minerals formed in the Earth's magnetic field. With a change in the orientation of the rock relative to the magnetic field or the field itself, part of the "innate" magnetization is retained, and the change in polarity is imprinted in the change in the orientation of the remanent magnetization of the rocks. At present, a scale has been established for the change of such eras.

Absolute geochronology - the doctrine of measuring geological time, expressed in conventional 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 to analyze the ratio of radioactive substances and their decay products in rocks formed in different epochs.

The oldest rocks are currently found in West Greenland (3.8 billion years). The greatest age (4.1 - 4.2 billion years) was obtained from zircons from Western Australia, but here zircon occurs in a redeposited state in Mesozoic sandstones. Taking into account the concept of the simultaneous formation of all 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 taken to be 4.6 billion years.

In 1881, at the II International Geological Congress in Bologna (Italy), the main subdivisions of the combined stratigraphic (for separating layered sedimentary rocks) and geochronological scales were approved. On 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 the most ancient life; 2) Paleozoic - the era of ancient life; 3) Mesozoic - the era of middle life; 4) Cenozoic - the era of new life. In 1887, the Proterozoic, the era of primary life, was separated from the Archean era. Later, the scale was improved. One of the options for the modern geochronological scale is presented in table. 8.1. The Archean era is divided into two parts: the early (older than 3500 million years) and the late Archean; Proterozoic - also into two: early and late Proterozoic; in the latter, the Riphean (the name comes from the ancient name of the Ural Mountains) and the Vendian periods are distinguished. The Phanerozoic zones are subdivided into the Paleozoic, Mesozoic and Cenozoic eras and consists of 12 periods.

Table 8.1. Geochronological scale

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

Let us briefly consider the main stages of 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 stretching, 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 green-stone belts (by this name they owe the predominant greenschist low-temperature metamorphism of basaltic rocks). Along with basalts, among the lavas of the lower, the main in terms of the thickness of the section of these belts, there are high-magnesian lavas, which indicate a very large degree of partial melting of mantle material, which indicates a high heat flux, much higher than the modern one. The development of greenstone belts consisted in a change in the type of volcanism in the direction of an increase in the content of silicon dioxide (SiO 2), in compression deformations and metamorphism of sedimentary-volcanogenic filling, 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 in the evolution of the earth's crust ended 3.0-2.5 billion years ago with the mass 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 a higher stage, led to the formation of a mature continental crust over most of the area of ​​modern continents. However, this crust also 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 in the end early Proterozoic(2.0-1.7 billion years ago) folded systems that re-soldered 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 boundary, the entire continental crust constituted a single massif - the supercontinent Megageu (mainland), which on the other side of the globe was opposed by the ocean - the predecessor of the modern Pacific Ocean - Megatalassa (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, albeit more slowly. It is possible that the prototype of Megatalassa appeared even earlier, at the end of the Archean.

In the Katarchean and the beginning of the Archean, the first traces of life appeared - bacteria and algae, and in the Late Archean algal lime 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: under the influence of the vital activity of plants, free oxygen appeared in it, while the Catarchean 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 split gradually, 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, Mediterranean, Ural-Okhotsk, which divided the continents of 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. Simultaneously, the mobile belts developed along the periphery of the Pacific Ocean, the depth of which also increased. The 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 continental margins (the latter on the periphery of the Pacific Ocean). These belts were dissected into marginal seas and island arcs, their sedimentary-volcanic strata underwent complex fold-thrust and then fault-strike-slip deformations, granites were introduced into them, and on this basis folded mountain systems were formed. This process was uneven. A number of intense tectonic epochs and granite magmatism are distinguished in it: the Baikal - at the very end of the Proterozoic, the Salair (from the Salair ridge in Central Siberia) - at the end of the Cambrian, the Takovo (from the Takov Mountains in the eastern United States) - at the end of the Ordovician, the Caledonian ( from the ancient Roman name of Scotland) - at the end of the Silurian, Acadian (Acadia - the old name of the northeastern states of the United States) - in the middle Devonian, Sudeten - at the end of the Early Carboniferous, 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 in the Caledonian era of tectogenesis, the last three in the Hercynian, or Varissian. In each of the listed tectonic epochs, certain parts of the mobile belts turned into folded mountain structures, and after destruction (denudation) they became part of the basement of young platforms. But some of them experienced some revitalization in subsequent orogeny eras.

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 Sino-Korean. As a result, the supercontinent Laurasia was formed, and the withering away of the western Mediterranean belt led to its unification with the southern supercontinent - Gondwana - into one continental block - Pangea. In the late Paleozoic - early Mesozoic, the eastern part of the Mediterranean belt 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 in the late Proterozoic, and at the very dawn of the Phanerozoic, almost all types of invertebrates existed, but they were still devoid of shells or shells, which are known from the Cambrian. In the Silurian (or already in the Ordovician), vegetation began to emerge on land, and at the end of the Devonian there were forests that were 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 separation of modern continents. At the beginning of the stage, in the Triassic, Pangea still existed, but already in the early Jurassic period it again split into Laurasia and Gondwana due to the emergence of the latitudinal ocean Tethys, 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 included the Central Atlantic. From here, at the end of the Jurassic, the process of continental spreading spread to the north, creating the North Atlantic during the Cretaceous and 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 from the south. This marked the beginning of the collapse of Gondwana, which existed as a whole throughout the Paleozoic. At the end of the Cretaceous, the North Atlantic merged with 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 were formed, all modern continents were isolated, and the appearance of the Earth acquired a form that was mostly close to the present. However, there were no modern mountain systems yet.

Intensive mountain building began in the Late Paleogene (40 million years ago), culminating in the last 5 million years. This stage of the formation of young folded-cover mountain structures, the formation of revived arched-block mountains is distinguished as neotectonic. In fact, the neotectonic stage is a substage of the Mesozoic-Cenozoic stage of the Earth's development, since it was at this stage that the main features of the Earth's modern 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 in the late Pliocene, humans appeared. At the end of the Early Cretaceous, angiosperms appeared and the land acquired a grass cover. At the end of the Neogene and Anthropogene, the high latitudes of both hemispheres were covered by 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 in the late Carboniferous - early Permian; both were common within Gondwana.

QUESTIONS FOR SELF-CONTROL

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

What is the internal structure of the Earth? What is the basis for making a conclusion about its structure?

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? What is the basis for the conclusion about the chemical composition of the entire Earth and the earth's crust?

What are the main types of the earth's crust 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 are taking place within it? What is weather and climate?

Give a definition of endogenous processes. What endogenous processes do you know? Describe them briefly.

What is the essence of plate tectonics? What are its main provisions?

10. Give the definition of exogenous processes. What is the main essence of these processes? What endogenous processes do you know? Describe them briefly.

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

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

What is the basis for 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?

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