Chronology of eras and periods. Geological periods in chronological order. Geological history of the Earth. Major geological events

The history of planet Earth already goes back approximately 7 billion years. During this time our common Home has undergone significant changes, which was a consequence of changing periods. V chronological order 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 eons, 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 divisions on 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 concluded that the Earth was formed approximately 4.5 billion years ago. The process of its formation itself was very long and may have begun 7 billion years ago from small cosmic particles. Over time, the gravitational force grew, and along with it, the speed of the bodies falling onto the forming planet increased. Kinetic energy transformed into heat, resulting in a gradual warming of the Earth.

The Earth's core, according to scientists, was formed over several hundred million years, after which the gradual cooling of the planet began. Currently, the molten core contains 30% of the Earth's mass. The development of other shells of the planet, according to scientists, has not yet been completed.

Precambrian eon

In the geochronology of the Earth, the first eon is called the Precambrian. It covers the time 4.5 billion - 600 million years ago. That is, the lion's share of the planet's history is covered by the former. 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 occurred. All this happened during active volcanic activity for almost the entire eon. The shields of all continents were formed in the Precambrian, but traces of life are very rare.

Catarchaean Eon

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

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

The Catarchaean Eon is a time when volcanic activity did not manifest itself, and the surface of the Earth was a cold, inhospitable desert. Although earthquakes occurred quite often, which smoothed the landscape. The surface looked like dark gray primordial material covered with a layer of regolith. A day at that time was only 6 hours long.

Archean eon

The second main eon of four in the history of the Earth lasted about 1.5 billion years - 4-2.5 billion years ago. At that time, the Earth did not yet have an atmosphere, therefore there was no life yet, however, during this eon, bacteria appeared; 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 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 continues over vast areas.

The formation of the so-called occurs. 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 beings appeared - simple 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 - obvious and hidden. Phanerozoic belongs to the obvious ones. At this time, a large number of living organisms with mineral skeletons appear. The era preceding the Phanerozoic was called hidden because practically no traces of it were found due to the lack of mineral skeletons.

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

Eras of the Precambrian Eon

During the Katarchean and Archean there were no generally recognized eras and periods, so we will skip their consideration.

The Proterozoic consists of three large eras:

Paleoproterozoic- i.e. ancient, including the Siderian, Rhiasian period, Orosirium and Staterium. By the end of this era, oxygen concentrations in the atmosphere had reached modern levels.

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

Neoproterozoic- new, consisting of Thonium, Cryogenium and Ediacaran. At this time, the formation of the first supercontinent, Rodinia, occurred, but then the plates diverged again. The coldest ice age occurred during an era called the Mesoproterozoic, during which much of the planet froze.

Eras of the Phanerozoic eon

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

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

1. Cambrian (a temperate climate was formed on Earth, the landscape was lowland, during this period the birth of all modern types animals).
2. Ordovician (the climate throughout the planet is quite warm, even in Antarctica, while the land subsides significantly. The first fish appear).
3. Silurian period (large inland seas are formed, while the lowlands become drier due to the rise of land. The development of fish continues. The Silurian period is marked by the appearance of the first insects).
4. Devonian (appearance of the first amphibians and forests).
5. Lower Carboniferous (dominance of pteridophytes, distribution of sharks).
6. Upper and Middle Carboniferous (appearance of the first reptiles).
7. Perm (most ancient animals die 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 West Side America is covered with shallow seas, the appearance of the first toothed birds).
3. Chalk (appearance of maple and oak forests, highest development and the 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 expansion of forests, oldest mammals are dying out. Approximately 25 million years ago, humans appeared and in the Pliocene era.
2. Quaternary. Pleistocene - large mammals die out, human society emerges, 4 ice ages occur, many plant species become extinct. Modern era - the last ice age ends, the climate gradually takes on its current form. The primacy of man on the entire planet.

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

Life on Earth began over 3.5 billion years ago, immediately after the completion of the formation of the earth's crust. Throughout time, the emergence and development of living organisms influenced the formation of relief and climate. Also, tectonic and climatic changes that occurred over many years influenced the development of life on Earth.

A table of the development of life on Earth can be compiled based on the chronology of events. The entire history of the Earth can be divided into certain stages. The largest of them are eras of life. They are divided into eras, eras into epochs, epochs into centuries.

Eras of life on Earth

The entire period of the existence of life on Earth can be divided into 2 periods: the Precambrian, or cryptozoic (primary period, 3.6 to 0.6 billion years), and the Phanerozoic.

The Cryptozoic includes the Archean (ancient life) and Proterozoic (primary life) eras.

The Phanerozoic includes the Paleozoic (ancient life), Mesozoic (middle life) and Cenozoic ( new life) era.

These 2 periods of life development are usually divided into smaller ones - eras. The boundaries between eras are global evolutionary events, extinctions. In turn, eras are divided into periods, and periods into epochs. The history of the development of life on Earth is directly related to changes in the earth’s crust and the planet’s climate.

Eras of development, countdown

The most significant events are usually identified in special time intervals - eras. Time is counted down in reverse order, from ancient life to modern life. There are 5 eras:

1. Archean.
2. Proterozoic.
3. Paleozoic.
4. Mesozoic.
5. Cenozoic.

Periods of development of life on Earth

The Paleozoic, Mesozoic and Cenozoic eras include periods of development. These are smaller periods of time compared to eras.

Palaeozoic:

• Cambrian (Cambrian).
• Ordovician.
• Silurian (Silurian).
• Devonian (Devonian).
• Carboniferous (carbon).
• Perm (Perm).

Mesozoic era:

• Triassic (Triassic).
• Jurassic (Jurassic).
• Cretaceous (chalk).

Cenozoic era:

• Lower Tertiary (Paleogene).
• Upper Tertiary (Neogene).
• Quaternary, or Anthropocene (human development).

The first 2 periods are included in the Tertiary period lasting 59 million years.

Table of the development of life on Earth

Era, period	Duration	Live nature	Inanimate nature, climate
Archean era (ancient life)	3.5 billion years	The appearance of blue-green algae, photosynthesis. Heterotrophs	The predominance of land over the ocean, the minimum amount of oxygen in the atmosphere.
Proterozoic era (early life)	2.7 billion years	The appearance of worms, mollusks, the first chordates, soil formation.	The land is a rocky desert. Accumulation of oxygen in the atmosphere.
The Paleozoic era includes 6 periods:
1. Cambrian (Cambrian)	535-490 Ma	Development of living organisms.	Hot climate. The land is deserted.
2. Ordovician	490-443 Ma	The appearance of vertebrates.	Almost all platforms are flooded with water.
3. Silurian (Silurian)	443-418 Ma	Exit of plants to land. Development of corals, trilobites.	with the formation of mountains. The seas dominate the land. The climate is varied.
4. Devonian (Devonian)	418-360 Ma	The appearance of mushrooms and lobe-finned fish.	Formation of intermountain depressions. Prevalence of dry climate.
5. Coal (carbon)	360-295 Ma	The appearance of the first amphibians.	Subsidence of continents with flooding of territories and the emergence of swamps. There is a lot of oxygen and carbon dioxide in the atmosphere.
6. Perm (Perm)	295-251 Ma	Extinction of trilobites and most amphibians. The beginning of the development of reptiles and insects.	Volcanic activity. Hot climate.
The Mesozoic era includes 3 periods:
1. Triassic (Triassic)	251-200 million years	Development of gymnosperms. The first mammals and bony fish.	Volcanic activity. Warm and sharply continental climate.
2. Jurassic (Jurassic)	200-145 million years	The emergence of angiosperms. Distribution of reptiles, appearance of the first bird.	Mild and warm climate.
3. Cretaceous (chalk)	145-60 million years	The appearance of birds and higher mammals.	Warm climate followed by cooling.
The Cenozoic era includes 3 periods:
1. Lower Tertiary (Paleogene)	65-23 million years	The rise of angiosperms. Development of insects, appearance of lemurs and primates.	Mild climate with distinct climatic zones.
2. Upper Tertiary (Neogene)	23-1.8 million years	The appearance of ancient people.	Dry climate.
3. Quaternary or Anthropocene (human development)	1.8-0 Ma	The appearance of man.	Cold weather.

Development of living organisms

The table of the development of life on Earth involves division not only into time periods, but also into certain stages of the formation of living organisms, possible climatic changes (ice age, global warming).

• Archean era. The most significant changes in the evolution of living organisms are the appearance of blue-green algae - prokaryotes capable of reproduction and photosynthesis, and the emergence of multicellular organisms. The appearance of living protein substances (heterotrophs) capable of absorbing dissolved in water organic matter. Subsequently, the appearance of these living organisms made it possible to divide the world into plant and animal.

• Mesozoic era.

• Triassic. Distribution of plants (gymnosperms). Increase in the number of reptiles. The first mammals, bony fish.
• Jurassic period. The predominance of gymnosperms, the emergence of angiosperms. The appearance of the first bird, the flourishing of cephalopods.
• Cretaceous period. Distribution of angiosperms, decline of other plant species. Development of bony fishes, mammals and birds.

• Cenozoic era.
  • Lower Tertiary period (Paleogene). The rise of angiosperms. Development of insects and mammals, appearance of lemurs, later primates.
  • Upper Tertiary period (Neogene). The formation of modern plants. The appearance of human ancestors.
  • Quaternary period (Anthropocene). Formation of modern plants and animals. The appearance of man.

Development of inanimate conditions, climate change

The table of the development of life on Earth cannot be presented without data on changes in inanimate nature. The emergence and development of life on Earth, new species of plants and animals, all this is accompanied by changes in inanimate nature and climate.

Climate Change: Archean Era

The history of the development of life on Earth began through the stage of dominance of land over water resources. The relief was poorly outlined. Prevails in the atmosphere carbon dioxide, the amount of oxygen is minimal. Shallow waters have low salinity.

The Archean era is characterized by volcanic eruptions, lightning, and black clouds. The rocks are rich in graphite.

Climatic changes in the Proterozoic era

The land is a rocky desert; all living organisms live in water. Oxygen accumulates in the atmosphere.

Climate Change: Paleozoic Era

During various periods of the Paleozoic era the following occurred:

• Cambrian period. The land is still deserted. The climate is hot.
• Ordovician period. The most significant changes are the flooding of almost all northern platforms.
• Silurian. Tectonic changes and conditions of inanimate nature are varied. Mountain formation occurs and the seas dominate the land. Areas of different climates, including areas of cooling, have been identified.
• Devonian. The prevailing climate is dry, continental. Formation of intermountain depressions.
• Carboniferous period. Subsidence of continents, wetlands. The climate is warm and humid, with a lot of oxygen and carbon dioxide in the atmosphere.
• Permian period. Hot climate, volcanic activity, mountain building, drying out of swamps.

During the Paleozoic era, mountains were formed. Such changes in relief affected the world's oceans - sea basins were reduced, and a significant land area was formed.

The Paleozoic era marked the beginning of almost all major oil and coal deposits.

Climatic changes in the Mesozoic

The climate of different periods of the Mesozoic is characterized by the following features:

• Triassic. Volcanic activity, the climate is sharply continental, warm.
• Jurassic period. Mild and warm climate. The seas dominate the land.
• Cretaceous period. Retreat of the seas from the land. The climate is warm, but at the end of the period global warming gives way to cooling.

In the Mesozoic era, previously formed mountain systems are destroyed, the plains go under water ( Western Siberia). In the second half of the era, the Cordillera, the mountains of Eastern Siberia, Indochina, and partly Tibet were formed, and the mountains of Mesozoic folding were formed. The prevailing climate is hot and humid, promoting the formation of swamps and peat bogs.

Climate Change - Cenozoic Era

During the Cenozoic era, a general rise of the Earth's surface occurred. The climate has changed. Numerous glaciations of the earth's surfaces advancing from the north changed the appearance of the continents of the Northern Hemisphere. Thanks to such changes, the hilly plains were formed.

• Lower Tertiary period. Mild climate. Division into 3 climatic zones. Formation of continents.
• Upper Tertiary period. Dry climate. The emergence of steppes and savannas.
• Quaternary period. Multiple glaciations of the northern hemisphere. Cooling climate.

All changes during the development of life on Earth can be written down in the form of a table that will reflect the most significant stages in the formation and development modern world. Despite the already known research methods, even now scientists continue to study history, making new discoveries that allow modern society find out how life developed on Earth before the advent of man.

Geologists have to deal with rock strata that have accumulated over the long geological history of the planet. It is necessary to know which of the rocks composing the study area are younger and which are older, in what sequence they were formed, to which intervals of geological history the time of their formation belongs, and also to be able to compare the age of rock strata that are distant from each other.

The study of the sequence of formation and age of rocks is called geochronology. There are differences between relative and absolute geochronology methods.

Relative geochronology

Methods of relative geochronology are methods for determining the relative age of rocks, which only record the sequence of formation of rocks relative to each other.

These methods are based on several simple principles. In 1669, Nicolo Steno formulated the principle of superposition, which states that that in an undisturbed bedding, each overlying layer is younger than the underlying one. Please note that the definition emphasizes the applicability of the principle only in undisturbed conditions.

The method of determining the sequence of formation of layers, based on the Steno principle, is often called stratigraphic. Stratigraphy is a branch of geology that deals with the study of the sequence of formation and division of the strata of sedimentary, volcanogenic-sedimentary and metamorphic rocks that make up earth's crust.

The next most important principle, known as intersection principle, formulated by James Hutton. This principle states that any body crossing the thickness of the layers is younger than these layers.

One more thing should be noted important principle, saying that time of transformation or deformation of rocks younger than the age of formation of these rocks.

Let us consider the use of these principles using the example of sedimentary rock strata intruded by several cutting igneous bodies.

The sequence of events is as follows. Initially, the accumulation of sedimentary strata of the lower layer (1) occurred, then, successively, the accumulation of overlying layers (2, 3, 4, 5), each of which is younger than the underlying one. The accumulation of sedimentary rocks in the vast majority of cases occurs in the form of horizontally lying layers, which is how the formed layers originally lay (1-5). Later, these strata were deformed (6), and a body of igneous rocks 7 was intruded into them. Then, again horizontally, the accumulation of the overlying layer overlying the intruded magmatic body began. Moreover, taking into account that the resulting layer lies on a leveled horizontal surface, it is obvious that its accumulation was preceded by the leveling of the territory - its erosion (8). Following the erosion of the territory, the next layer (9) accumulated. The youngest formation is magma body 10.
We emphasize that, when considering the history of the geological development of the territory, the section of which is shown in the figure, we used exclusively relative time, determining only the sequence of formation of bodies.

Another large group of relative geochronology methods isbiostratigraphic methods . These methods are based on the study fossils - fossil remains of organisms contained in layers of rocks: in layers of rocks of different ages there are different complexes of remains of organisms that characterize the development of flora and fauna in a particular geological era. The methods are based on the principle formulated by William Smith: sediments of the same age contain the same or similar remains of fossil organisms. This principle is complemented by another important provision, which states that fossil flora and fauna replace each other in a certain order. Thus, the basis of all biostratigraphic methods is the assumption of continuity and irreversibility of changes in the organic world - the law of evolution of Charles Darwin. Each segment of geological time is characterized by certain representatives of flora and fauna. Determining the age of rock strata comes down to comparing the fossils found in them with data on the time of existence of these organisms in geological history. As a rough analogy to the essence of the method, we can cite the well-known methods for determining age in archeology: if only stone tools are found during excavations, then the culture belongs to the Stone Age, the presence of bronze tools gives grounds for its attribution to the Bronze Age, etc.

Among biostratigraphic methods, the method of guiding forms has long remained the most important. Guiding forms are the remains of extinct organisms that meet the following criteria:

• these organisms existed for a short period of time,
• were distributed over a large area,
• their fossil parts are found and easily identified.

When determining the age, among the fossils found in the studied layer, the most characteristic ones are selected, then they are compared with atlases of guiding forms, which describe which time intervals are characteristic of certain forms. The first of these atlases was created in the middle of the 19th century by paleontologist G. Bronn.

Today, the main thing in biostratigraphy is method of analysis of organic complexes. When using this method, the conclusion about the relative age is based on information about the entire complex of fossils, and not on the finds of single leading forms, which significantly increases accuracy.

In the course of geological research, the task is not only to subdivide strata by age and assign them to any interval of geological history, but also to compare - correlations– sequences of the same age that are distant from each other. The simplest method of identifying coeval strata is to trace layers in the area from one outcrop to another. Obviously, this method is only effective in conditions of good exposure. More universal is the biostratigraphic method of comparing the nature of organic remains in distant sections - layers of the same age have the same complex of fossils. This method allows for regional and global correlation of sections.

The principle model for using fossils to correlate distant sections is shown in the figure.

Layers containing the same complex of fossils are coeval

Absolute geochronology

Absolute geochronology methods make it possible to determine the age of geological objects and events in time units. Among these methods, the most common are those of isotope geochronology, based on calculating the decay time of radioactive isotopes contained in minerals (or, for example, in wood remains or in fossilized animal bones).

The essence of the method is as follows. Some minerals contain radioactive isotopes. From the moment such a mineral is formed, the process of radioactive decay of isotopes occurs in it, accompanied by the accumulation of decay products. The decay of radioactive isotopes occurs spontaneously, with constant speed, independent of external factors; the number of radioactive isotopes decreases in accordance with the exponential law. Taking into account the constancy of the rate of decay, to determine the age it is sufficient to establish the amount remaining in the mineral radioactive isotope and the amount of stable isotope formed during its decay. This dependence is described the main equation of geochronology:

To determine age, many radioactive isotopes are used: 238 U, 235 U, 40 K, 87 Rb, 147 Sm, etc. The names of isotope-geochronological methods are usually formed from the names of radioactive isotopes and the final products of their decay: uranium-lead, potassium-argon and etc. The results of determining the age of geological objects are expressed in 106 and 109 years, or in the values International system units (SI): Ma and Ga. This abbreviation means, respectively, “million. years" and "billion years" ( from lat. Mega anna – million years, Giga anna – billion years).

Let's consider determination of age by rubidium-strontium isochronous method. As a result of the decay of the radioactive isotope 87 Rb, a non-radioactive decay product is formed - 87 Sr, the decay constant is 1.42 * 10 -11 years -1. The use of the isochron method involves the analysis of several samples taken from the same geological object, which increases the accuracy of age determination and allows the calculation of the initial strontium isotopic composition (used to determine the conditions of formation of the rock).

In the course of laboratory studies, the contents of 87 Rb and 87 Sr are determined, while the content of the latter is the sum of the strontium initially contained in the mineral (87 Sr) 0 and the strontium generated during the radioactive decay of 87 Rb during the period of existence of the mineral:

In practice, it is not the contents of these isotopes that are measured, but their ratios to stable isotope 86Sr, which gives more accurate results. As a result, the equation takes the form

The resulting equation has two unknowns: time t and the initial ratio of strontium isotopes. To solve the problem, several samples are analyzed, the results are plotted as points on a graph in coordinates 87 Sr/ 86 Sr – 87 Rb/ 86 Sr. In the case of correctly selected samples, all points lie along the same straight line - isochrones (hence, they have the same age). The age of the analyzed samples is calculated from the isochron slope angle, and the initial strontium ratio is determined from the intersection of the 87 Sr/ 86 Sr isochron axis.

If the points on the graph do not fall on the same line, we can talk about incorrect sample selection. To avoid this, the following main conditions must be observed:

• samples must be taken from one geological object (i.e., be obviously of the same age);
• in II the following rocks should not show signs of superimposed transformations that could lead to redistribution of isotopes;
• samples must have the same strontium isotopic composition at the time of occurrence (the use of different rocks when constructing one isochron is unacceptable).

Without dwelling on methods for determining age by other methods, we will only note the features of some of them.

Currently considered the most accurate samarium – neodymium method, adopted as the standard against which data from other methods are compared. It's related with the fact that, due to geochemical characteristics, these elements are least susceptible to the influence of superimposed processes, often significantly about distorting or nullifying the results of age determinations. The method is based on the decay of the 147 Sm isotope with the formation of 144 Nd as the final decay product.

The potassium-argon method is based on the decay of the radioactive isotope 40 K. This method has long been widely used to determine the age of all genetic types of rocks. It is most effective in determining the time of formation of sedimentary rocks and minerals, such as glauconite. When applied to igneous and especially metamorphic rocks affected by superimposed alterations, this method often gives “rejuvenated” dates due to the loss of mobile argon.

Radiocarbon method is based on the decay of the 14 C isotope formed in the upper layers of the atmosphere as a result of the impact of cosmic radiation on atmospheric gases (nitrogen, argon, oxygen). Subsequently, 14 C, like the non-radioactive carbon isotope, forms carbon dioxide CO 2, and in its composition is involved in photosynthesis, thus ending up in plants and, further, transmitted to animals in the food chain. 14 C enters the hydrosphere as a result of the exchange of CO 2 between the atmosphere and the World Ocean, then it ends up in the bones and carbonate shells of aquatic inhabitants. Intensive mixing of air masses in the atmosphere and Active participation carbon in the global cycle chemical elements leads to equalization of 14 C concentrations in the atmosphere, hydrosphere and biosphere. For living organisms, the equilibrium state is achieved at a specific activity of 14 C of 13.56 ± 0.07 disintegrations per minute per 1 gram of carbon. If the body dies, the supply of 14C stops; as a result of radioactive decay (transition into non-radioactive 14 N), the specific activity of 14 C decreases. By measuring the value of activity in the sample and comparing it with the value of specific activity in living tissue, it is easy to calculate the time of cessation of vital activity of the organism using the formula

///////////////

Radiocarbon dating allows us to determine the age of samples containing carbon (bones, teeth, shells, wood, coal, etc.) up to 70 thousand years old. This determines its use in Quaternary geology and, especially, in archaeology.

To conclude the consideration of the methods of isotope geology, it should be noted that, despite obtaining “absolute” dating, expressed in years, we are dealing with model age– the results obtained inevitably contain some error and, moreover, the length of the astronomical year has changed over the course of long geological history.

Another group of absolute geochronology methods is presented seasonal and climatic methods. An example of such a method is warvochronology– a method of absolute geochronology, based on the calculation of annual layers in the “ribbon” deposits of periglacial lakes. For periglacial lakes, the characteristic deposits are the so-called “band clays” - clearly layered sediments consisting of large number parallel tapes. Each ribbon is the result of an annual sedimentation cycle in lakes that are frozen most of the year. It always consists of two layers. The upper - winter - layer is represented by dark-colored clays (due to enrichment with organic matter), formed under the ice cover; the lower one - summer - is composed of coarser-grained, light-colored sediments (mainly fine sands or silty-clayey sediments), formed due to material brought into the lake by melted glacial waters. Each pair of such puffs corresponds to 1 year.

The study of the rhythmicity of ribbon clays allows not only to determine the absolute age, but also to correlate sections located close to each other, comparing the thickness of the layers.

The calculation of annual layers in the sediments of salt lakes is based on a similar principle, where in summer, due to increased evaporation, active precipitation of salts occurs.

The disadvantages of seasonal-climatic methods include their lack of universality.

Periodization of geological history. Stratigraphic and geochronological scales

Operating with the category of relative time, it is necessary to have a universal scale of periodization of history. Thus, in relation to the history of mankind, we use the expressions “before our era”, “in the Renaissance”, “in the 20th century”, etc., relating any event or object of material culture to a certain time interval. A similar approach has been adopted in geology; the International Geochronological Scale and the International Stratigraphic Scale have been developed for these purposes.

The main information about the geological history of the Earth is carried by layers of rocks, in which, as on the pages of the stone chronicle, the changes that took place on the planet and the evolution of the organic world are imprinted (the latter is “impressed” in complexes of fossils contained in layers of different ages). Layers of rocks that occupy a certain position in the general sequence of strata and are distinguished on the basis of their inherent features (more often - a complex of fossils) are stratigraphic units. The rocks that make up the stratigraphic units were formed over a certain interval of geological time, and, therefore, reflect the evolution of the earth's crust and the organic world over this period of time.

- a scale showing the sequence and subordination of stratigraphic units that make up the earth’s crust and reflect the stages the earth has passed through historical development. The object of the stratigraphic scale is the layers of rocks. The basis of the modern stratigraphic scale was developed in the first half of the 19th century and was adopted in 1881 at the II session of the International Geological Congress in Bologna. Later, the stratigraphic scale was supplemented by a geochronological scale.

Geochronological scale– a scale of relative geological time, showing the sequence and subordination of the main stages of the geological history of the Earth and the development of life on it. The object of the geochronological scale is geological time.

The geological time scale (or geochronometric scale) is a sequential series of dates lower limits general stratigraphic units expressed in units of time (usually millions of years) and calculated using absolute dating methods.

The object of a geochronological chart is geochronological divisions - intervals of geological time during which the rocks that make up a given stratigraphic division were formed.

All stratigraphic units correspond to units of the geochronological scale.

Moreover, almost all stratigraphic units of the eonotema-system rank have uniform generally accepted international names.

The largest stratigraphic units are acrothemes and eonothems. The Archean and Proterozoic acrotems are combined under the name “Precambrian” (i.e., rock strata that accumulated before the Cambrian period - the first period of the Phanerozoic) or “cryptozoic”. The boundary between the Precambrian and Phanerozoic periods is the appearance of remains of skeletal organisms in rock layers. In the Precambrian, organic remains are rare because soft tissues are quickly destroyed before they can be buried. The term “cryptozoic” itself is formed by merging the roots of the words "cryptos" - hidden And "zoe" - life. When dividing Precambrian strata into fractional stratigraphic units, the most important role is played by the methods of isotope geochronology, since organic remains are rare or absent, are difficult to determine and, most importantly, are not subject to rapid evolution (same-type microfaunal complexes remain unchanged over vast periods of time, which does not allow dissection thickness according to this feature).

Eonothems include erathemes. Eratema, or group- deposits formed during era; The duration of eras in the Phanerozoic is the first hundreds of millions of years. Eratems reflect major stages in the development of the Earth and the organic world. The boundaries between erathems correspond to turning points in the history of the development of the organic world. In the Phanerozoic, three erathems are distinguished: Paleozoic, Mesozoic and Cenozoic.

Eratems, in turn, include systems in their composition. System are deposits formed during period; The duration of the periods is tens of millions of years. One system differs from another in the complexes of fauna and flora at the level of superfamilies, families and genera. In the Phanerozoic, 12 systems are distinguished: Cambrian, Ordovician, Silurian, Devonian, Carboniferous (Carboniferous), Permian, Triassic, Jurassic, Cretaceous, Paleogene, Neogene and Quaternary (Anthropogene). The names of most systems come from the geographical names of the areas where they were first installed. For each system on geological maps, a certain color is adopted, which is international, and an index formed by the initial letter of the Latin name of the system.

Department- part of the system corresponding to deposits formed during one era; The duration of epochs is usually the first tens of millions of years. Differences between departments are manifested in differences in fauna and flora at the genus or group level. The names of the departments are given according to their position in the system: lower, middle, upper, or only lower and upper; eras are respectively called early, middle, late.

The department is divided into tiers. Tier- deposits formed during century; The duration of centuries is several million years.

Along with the main divisions of the stratigraphic and geochronological scales, regional and local divisions are used.

To regional stratigraphic units include the horizon and the bosom.

Horizon- the main regional division of the stratigraphic scale, uniting sediments of the same age, characterized by a certain set of lithological and paleontological features. Horizons are given geographical names corresponding to the places where they are best represented and studied. The geochronological equivalent is time. For example, the Khaprovsky horizon, common on the coast of the Taganrog Bay Sea of ​​Azov, corresponds to the thickness of river sands formed at the end of the Neogene period. The stratotype (the most representative section of the stratigraphic horizon, which is its standard) of this horizon is located at station. Khapra. Let us add that the term “horizon”, used without a geographical name, is understood as a layer or pack of layers distinguished on the basis of any features (paleontological or lithological), that is, it is a designation for free use.

Lona is part of the horizon distinguished by the complex of fauna and flora characteristic of a given region, and reflects a certain phase of development of the organic world of a given region. The name of the womb is given by the index species. The geochronological equivalent of the womb is time.

Local stratigraphic units are rock strata distinguished by a number of characteristics, mainly by lithological or petrographic composition.

Complex- the largest local stratigraphic unit. The complex has a very large thickness, a complex composition of rocks formed during some major stage in the development of the territory. The complex is assigned geographical name according to the characteristic place of its development. Most often, complexes are identified during the dissection of metamorphic strata.

Series covers a fairly thick and complex composition of rocks for which there are some general signs: similar conditions of formation, the predominance of certain types of rocks, similar degrees of deformation and metamorphism, etc. The series usually correspond to a single large cycle of development of the territory.

The main unit of local stratigraphic units represents is retinue. Retinue represents a sequence of rocks formed in a certain physical-geographical setting and occupying a specified stratigraphic position in the section. The main features of the formation are the presence of stable lithological features over the entire area of ​​its distribution and clearly defined boundaries. The formation gets its name from the geographical location of the stratotype.

The boundaries of local stratigraphic units often do not coincide with the boundaries of units of a single stratigraphic scale.

When working as a geologist, you often have to use auxiliary stratigraphic units- thickness, unit, layer, deposit, etc., usually named by characteristic rocks, color, lithological features or characteristic organic remains (stratum of limestone, layers with Matra fabriana, etc.).

Geological chronology and geochronological table
Great value for geographical science has the ability to determine the age of the Earth and the earth's crust, as well as the time of significant events that occurred in the history of their development.
The history of the development of planet Earth is divided into two stages: planetary and geological.
The planetary stage covers the period of time from the birth of the Earth as a planet to the formation of the earth's crust. The scientific hypothesis about the formation of the Earth (as a cosmic body) appeared on the basis of general views on the origin of other planets that are part of solar system. You know from the 6th grade course that the Earth is one of the 9 planets in the solar system. Planet Earth was formed 4.5-4.6 billion years ago. This stage ended with the appearance of the primary lithosphere, atmosphere and hydrosphere (3.7-3.8 billion years ago).
From the moment the first rudiments of the earth's crust appeared, a geological stage began, which continues to the present day. During this period, various rocks were formed. The earth's crust has repeatedly been subjected to slow uplifts and subsidences under the influence of internal forces. During the period of subsidence, the territory was flooded with water and sedimentary rocks (sands, clays, etc.) were deposited at the bottom, and during periods of rising the seas retreated and in their place a plain composed of these sedimentary rocks arose.
Thus, the original structure of the earth's crust began to change. This process continued continuously. At the bottom of the seas and continental depressions, a sedimentary layer of rocks accumulated, among which the remains of plants and animals could be found. Each geological period corresponds to their individual species, because the organic world is in constant development.
Determining the age of rocks. In order to determine the age of the Earth and present the history of its geological development, methods of relative and absolute chronology (geochronology) are used.
To determine the relative age of rocks, it is necessary to know the patterns of sequential occurrence of layers of sedimentary rocks different composition. Their essence is as follows: if layers of sedimentary rocks lie in an undisturbed state in the same way as they were deposited one after another on the bottom of the seas, then this means that the layer lying below was deposited earlier, and the layer lying above was formed later, therefore , he's younger.
Indeed, if there is no lower layer, then it is clear that the one covering it upper layer cannot form, therefore the lower the sedimentary layer is located, the older its age. The topmost layer is considered the youngest.
In determining the relative age of rocks great importance has a study of the successive occurrence of sedimentary rocks of different compositions and the fossilized remains of animal and plant organisms contained in them. As a result of the painstaking work of scientists to determine the geological age of rocks and the time of development of plant and animal organisms, a geochronological table was compiled. It was approved at the II International Geological Congress in 1881 in Bologna. It is based on the stages of life development identified by paleontology. This scale table is constantly being improved. Current state tables are given on p. 43.
The units of the scale are eras, divided into periods which are subdivided into epochs. The five largest of these divisions - eras - bear names associated with the nature of life that existed then. For example, Archean is the time of earlier life, Proterozoic is the era of primary life, Paleozoic is the era of ancient life, Mesozoic is the era of middle life, Cenozoic is the era of new life.
Eras are divided into shorter periods of time - periods. Their names are different. Some of them come from the names of rocks that are most characteristic of this time (for example, the Carboniferous period in the Paleozoic and the Mothic period in the Mesozoic). Most periods are named after the localities in which the deposits of a particular period are most fully developed and where these deposits were first characterized. The oldest period of the Paleozoic - the Cambrian - received its name from Cambria, an ancient state in the west of England. The names of the following periods of the Paleozoic - Ordovician and Silurian - come from the names of the ancient tribes of the Ordovicians and Silurians who inhabited the territory of what is now Wales.
To distinguish the systems of the geochronological table, conventional signs are adopted. Geological eras are designated by indices (signs) - initial letters their Latin names (for example, Archaean - AR), and period indices - the first letter of their Latin names (for example, Permian - P).
Determination of the absolute age of rocks began at the beginning of the 20th century, after scientists discovered the law of decay of radioactive elements. In the bowels of the Earth are radioactive elements, for example, uranium. Over time, it slowly, at a constant rate, decays into helium and lead. The helium dissipates, but the lead remains in the rock. Knowing the rate of decay of uranium (from 100 g of uranium, 1 g of lead is released over 74 million years), from the amount of lead contained in the rock, we can calculate how many years ago it was formed.
The use of radiometric methods has made it possible to determine the age of many rocks that make up the earth's crust. Thanks to these studies, it was possible to establish the geological and planetary age of the Earth. Based on relative and absolute methods chronology and a geochronological table was compiled.
1. What stages is the geological history of the Earth's development divided into?
2. Which stage of the Earth’s development is geological? 3.* How is the age of rocks determined?
4. Compare by geochronological table duration of geological eras and periods.

Stratigraphic (g eochronological) scale– a geological time scale, the stages of which are highlighted by paleontology according to the development of life on Earth.

The two names of this scale have different meanings: the stratigraphic scale serves to describe the sequence and relationships of rocks that make up the earth’s crust, and the geochronological scale to describe geological time.

These scales differ in terminology; you can see the differences in the table below: General stratigraphic	subdivisions (stratons) Divisions
geochronological scale	Akrotema
Akron	Eonothema
Eon	Eratema
Era	System
Period	Department
era	Tier

Century Thus, we can say that, for example, the limestone sequence belongs to the Cretaceous system , but limestones formed in the Cretaceous.

period Systems, departments, tiers can be upper or lower, and periods, eras and centuries -

early or late.

These terms should not be confused.

Phanerozoic Phanerozoic The eon includes three eras, the names of which should be known to many: Paleozoic (era of ancient life), Mesozoic (middle life era) and Cenozoic

(era of new life). Eras are in turn divided into periods. Paleozoic: Cambrian, Ordovician, Silurian, Devonian, Carboniferous, Permian; Mesozoic: Triassic, Jurassic, Cretaceous; Cenozoic: Paleogene, Neogene and Quaternary. Each period has its own letter designation and its own color for designation on geological maps.

Remembering the order of periods is quite simple using a mnemonic device. The first letter of each word in the two sentences below corresponds to the first letter of the period: TO every ABOUT educated WITH student D Remembering the order of periods is quite simple using a mnemonic device. The first letter of each word in the two sentences below corresponds to the first letter of the period: olzhen urit P apiros. T s, YU rchik, M urit al, go away N ID H

	inarik.	Symbol
Color	€	Cambrian
Bluish green	Ordovician	O
Olive	Silur	S
Gray-green	Devonian	D
Brown	Carbon	C
Grey	Permian	P
Yellow-brown	Triassic	T
Violet	Yura	J
Blue	Chalk	K
Light green	Paleogene	P*
Orange	Neogene	N
Yellow	Quaternary	Q

Yellowish gray

*Paleogene symbol may not be displayed, because not found in all fonts: this is the ruble symbol (P with a horizontal bar)

Precambrian Archaean And Proterozoic Akrons are the more ancient divisions, in addition, they account for most of the existence of our planet. If the Phanerozoic lasted about 530 million years, then the Proterozoic alone