Geochronology- the sequence of geological events in time, their duration and subordination:

– relative geochronology reflects the natural stages in the history of the Earth's development, based on the principle of stratification sequence and using the method of biostratigraphic constructions;

- absolute geochronology determines the age and duration of subdivisions of the geochronological scale in time intervals equal to the modern astronomical year (in astronomical units). It is based on the study of radioactive decay products in minerals.

Geochronological(geohistorical) scale – a hierarchical system of geochronological subdivisions equivalent to units of the common stratigraphic scale.

Stratigraphic division(unit) - a set of rocks that make up a certain unity in terms of a set of features (features of the material composition, organic residues), which allows you to distinguish it in a section and trace it across the area.

The patterns of development and formation of the earth's crust are studying historical geology. The age of rocks is absolute and relative.

Absolute age - the duration of the existence (life) of the breed, expressed in years. To determine it, methods are used based on the use of radioactive transformation processes that take place in some chemical elements (uranium, potassium, rubidium) that make up rocks. The age of igneous rocks, as well as chemical sediments, is equal to the age of their constituent minerals. Other rocks are younger than their constituent minerals.

The ratio of the amounts of the radioactive initial isotope and the stable element formed from it gives an idea of ​​the age of the host rocks. Methods for determining absolute age get their name from the products of radioactive decay: uranium-lead (lead), helium, potassium-argon (argon), potassium-calcium, rubidium-strontium and others. So, knowing how much lead is formed from 1 g of uranium per year, determining their combined content in a given mineral, one can find the absolute age of the mineral and the rock in which it is located. Carbon 14 C, whose half-life is 5568 years, can be used to determine the age of formations that appeared later. The absolute age of rocks can be determined using the geochronological scale of the earth's crust (table). Determining the absolute age of rocks is a very difficult task, the solution of which became possible only in the 1950s.

Geochronological scale of the earth's crust

(eonotemes)		Period (system)		Typical organisms	Abs. age, million years
Neochron (Phanerozoic)	Cenozoic Kz ("era of new life")	Quaternary (anthropogenic) Q
		Tertiary Tr		Mammals, flowering plants
			Paleogene P
	Mesozoic Mz ("era of middle life")	Cretaceous K		Cephalopods, mollusks and reptiles
		Triassic T
	Paleozoic Pz ("era of ancient life")	Perm P		Amphibians and spores
		Carboniferous C
		Devonian D		Fish, brachiopods
		Silurian S		invertebrates
		Ordovician O
		Cambrian Cm
Paleochron (cryptozoic)	Proterozoic PR			Rare remains of primitive forms
	Archean (Archaeozoic) AR
Planetary stage of the Earth					Over 4500

The younger the determined age of the mineral, the more it is required for analysis, since the decomposition products do not have time to accumulate.

Geologists have to deal with rock masses that have accumulated over the long geological history of the planet. It is necessary to know which of the rocks that make up 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 distant from each other.

The doctrine of the sequence of formation and age of rocks is called geochronology. Methods of relative and methods of absolute geochronology differ.

Relative geochronology

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

These methods are based on a few simple principles. In 1669, Nicolò Steno formulated the principle of superposition, which states, that in undisturbed occurrence each overlying layer is younger than the underlying one. Note that the definition emphasizes the applicability of the principle only in conditions of undisturbed occurrence.

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 studies the sequence of formation and division of sedimentary, volcanic-sedimentary and metamorphic rocks that make up the earth's crust.

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

Another important principle to be noted is that the time of transformation or deformation of rocks is younger than the age of formation of these rocks.

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

The sequence of events is as follows. Initially, the accumulation of sedimentary strata of the lower layer (1) occurred, then, sequentially, 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 overwhelming majority of cases occurs in the form of horizontally lying layers, which is how the formed layers (1-5) originally occurred. Later, these sequences were deformed (6), and a body of igneous rocks 7 was intruded into them. Then, again horizontally, the accumulation of the overlying layer began, overlying the intruded magmatic body. At the same time, taking into account that the formed 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 magmatic body 10.
We emphasize that, considering the history of the geological development of the territory, the section of which is shown in the figure, we used only 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 enclosed 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 epoch. 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 supplemented by another important provision, stating that fossil flora and fauna replace each other in a certain order. Thus, the basis of all biostratigraphic methods is the provision on the 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 is reduced to comparing the fossils found in them with data on the time of existence of these organisms in geological history. As a rough analogy of the essence of the method, we can cite the well-known methods for determining age in archeology: if only stone tools were found during excavations, then the culture belongs to the Stone Age, the presence of bronze tools gives grounds for attributing it to the Bronze Age, etc.

Among biostratigraphic methods, the method of guiding forms has long remained the most important. Ruling 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 wide 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 that describe which time interval certain forms are characteristic of. The first of these atlases was created in the middle of the 19th century by the paleontologist G. Bronn.

To date, the main biostratigraphy is method for the analysis of organic complexes. With this method, inference of relative age is based on information about the entire fossil assemblage, rather than on finds of single guide forms, which greatly improves accuracy.

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

The principal model for using fossils to correlate remote sections is shown in the figure.

Layers containing the same complex of fossils are of the same age.

Absolute geochronology

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

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

Many radioactive isotopes are used to determine the age: 238 U, 235 U, 40 K, 87 Rb, 147 Sm, etc. etc. The results of determining the age of geological objects are expressed in 106 and 109 years, or in the values ​​of the International System of Units (SI): Ma and Ga. This abbreviation means, respectively, "million. years” and “billion years” ( from lat. Mega anna - million years, Giga anna - billion years).

Consider age determination by rubidium-strontium isochron 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 application 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 isotopic composition of strontium (used to determine the formation conditions 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 strontium initially contained in the mineral (87 Sr) 0 and strontium that arose during the radioactive decay of 87 Rb during the period of the mineral's existence:

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

The resulting equation has two unknowns: the 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 the coordinates 87 Sr/ 86 Sr – 87 Rb/ 86 Sr. In the case of correctly selected samples, all points lie along one straight line - isochrones (hence, they have the same age). The age of the analyzed samples is calculated from the isochron slope, 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 lie on one line, we can talk about incorrect sampling. To avoid this, the following main conditions must be observed:

• samples must be taken from the same geological object (i.e. must be known to be of the same age);
• in ai the rocks to be followed should show no evidence of superimposed transformations that could lead to redistribution of isotopes;
• samples must have the same isotopic composition of strontium at the time of occurrence (it is unacceptable to use different rocks when constructing one isochrone).

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

Currently, the most accurate is samarium - neodymium method, accepted as a standard against which the data of other methods are compared. It's connected about the fact that, due to geochemical features, these elements are the least affected by 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 isotope 14 C, which is formed in the upper atmosphere as a result of the impact of cosmic radiation on atmospheric gases (nitrogen, argon, oxygen). Subsequently, 14 C, like a non-radioactive carbon isotope, forms carbon dioxide CO 2, and in its composition is involved in photosynthesis, thus being in the composition of plants and, further, the food chain is transferred to animals. 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 life. Intensive mixing of air masses in the atmosphere and the active participation of carbon in the global cycle of chemical elements leads to equalization of 14 C concentrations in the atmosphere, hydrosphere and biosphere. For living organisms, the equilibrium state is reached at the specific activity of 14 C, which is 13.56 ± 0.07 decays per minute per 1 gram of carbon. If the organism dies, then the supply of 14C stops; as a result of radioactive decay (transition to non-radioactive 14 N), the specific activity of 14 C decreases. By measuring the activity value in the sample and comparing it with the specific activity value in living tissue, it is easy to calculate the time of termination of the organism's vital activity using the formula

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Radiocarbon dating makes it possible 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 archeology.

In conclusion of the consideration of the methods of isotope geology, it should be noted that, despite obtaining “absolute” dates expressed in years, we are dealing with model age- the results obtained inevitably contain some error and, moreover, the duration of the astronomical year has changed in the course of a long geological history.

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

Studying the rhythm of banded clays makes it possible not only to determine the absolute age, but also to correlate the sections located not far from each other, comparing the thicknesses 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 non-universality.

Periodization of geological history. Stratigraphic and geochronological scales

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

The main information about the geological history of the Earth is carried by layers of rocks, in which, as on the pages of a stone chronicle, the changes that took place on the planet and the evolution of the organic world are imprinted (the latter is “imprinted” in fossil complexes 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 the stratigraphic units that make up the earth's crust and reflect the stages of historical development passed by the earth. 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 the geochronological scale.

Geological 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 geologic time scale (or geochronometric scale) is a sequential series of datings of the lower boundaries of common stratigraphic units, expressed in units of time (more often in millions of years) and calculated using absolute dating methods.

The object of the geochronological scale is the geochronological subdivisions - the intervals of geological time during which the rocks that are part of this stratigraphic subdivision were formed.

All stratigraphic units correspond to units of the geochronological scale.

At the same time, almost all stratigraphic units of the eonoteme-system rank have common, generally accepted international names.

The largest stratigraphic units are acrothemes and eonotemes. The Archean and Proterozoic acrothemes are combined under the name "Precambrian" (i.e., rock strata accumulated before the Cambrian period - the first period of the Phanerozoic) or "cryptozoic". The boundary of the Precambrian and Phanerozoic is the appearance in the layers of rocks of the remains of skeletal organisms. In the Precambrian, organic remains are rare, since soft tissues are quickly destroyed before they can be buried. The term "cryptozoic" itself was formed by merging the roots of words "cryptos" - hidden and "zoe" - life. When dividing the Precambrian strata into fractional stratigraphic units, the methods of isotopic geochronology play an important role, since organic remains are rare or absent at all, are difficult to determine and, most importantly, are not subject to rapid evolution (similar microfauna complexes remain unchanged over huge time intervals, which does not allow dismembering thickness on this basis).

Eonotems include eratems. 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 eratems 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 by 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 (Anthropogenic). The names of most systems come from the geographical names of the localities where they were first established. For each system on geological maps, a certain color is accepted, which is international, and an index formed by the initial letter of the Latin name of the system.

The 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 divisions are manifested in the difference between fauna and flora at the level of genera or groups. The names of 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 division is divided into tiers. Tier- deposits formed during century; centuries are several million years long.

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

To regional stratigraphic units include horizon and lona.

Horizon- the main regional subdivision of the stratigraphic scale, uniting deposits of the same age, characterized by a certain complex of lithological and paleontological features. The horizons are given geographic 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 of the Sea of ​​Azov, corresponds to the thickness of river sands that 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 near st. Khapry. Let us add that the term “horizon”, used without a geographical name, is understood as a layer or a pack of layers identified on the basis of some 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 the given region, and reflects a certain phase in the development of the organic world of the given region. The name of the womb is given according to the type-index. The geochronological equivalent of the womb is time.

Local stratigraphic units are rock strata distinguished by a number of features, 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 given a geographical name according to the characteristic place of its development. Most often, the complexes are distinguished during the dismemberment of metamorphic strata.

Series covers a fairly thick and complex rock mass for which there are some common features: similar formation conditions, the predominance of certain types of rocks, a close degree of deformation and metamorphism, etc. Series usually correspond to a single major cycle of development of the territory.

Basic unit of local stratigraphic units is a retinue. Retinue is a stratum of rocks formed in a certain physical and geographical setting and occupying a certain stratigraphic position in the section. The main features of the suite are the presence of stable lithological features throughout the entire area of ​​its distribution and a clear expression of 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.

In the course of work, a geologist often has to use also auxiliary stratigraphic units- thickness, pack, layer, deposit, etc., usually named according to characteristic rocks, color, lithological features or characteristic organic remains (limestone sequences, layers with Matra fabriana, etc.).

Hey! In this article I want to tell you about the geochronological column. This is a column of periods of the Earth's evolution. And also more about each era, thanks to which you can draw a picture of the formation of the Earth throughout its history. What types of life first appeared, how did they change, and how much did it take.

The geological history of the Earth is divided into large intervals - eras, eras are divided into periods, periods are divided into epochs. Such a division was associated with events that took place on. The change in the abiotic environment influenced the evolution of the organic world on Earth.

Geological eras of the Earth, or geochronological scale:

And now about everything in more detail:

Designations:
eras;
periods;
Epochs.

1. Catharchean era (from the creation of the Earth, about 5 billion years ago, to the origin of life);

2. Archean era , the most ancient era (3.5 billion - 1.9 billion years ago);

3. Proterozoic era (1.9 billion - 570 million years ago);

Archean and Proterozoic are still combined into Precambrian. The Precambrian covers the largest part of geological time. Formed, areas of land and sea, active volcanic activity took place. Shields of all continents were formed from Precambrian rocks. Traces of life are usually rare.

4. Palaeozoic (570 million - 225 million years ago) with such periods :

Cambrian period(from the Latin name for Wales)(570 million - 480 million years ago);

The transition to the Cambrian is marked by the unexpected appearance of a huge number of fossils. This is a sign of the beginning of the Paleozoic era. Marine life flourished in numerous shallow seas. Trilobites were especially widespread.

Ordovician period(from the British Ordovician tribe)(480 million - 420 million years ago);

On a significant part of the Earth it was soft, most of the surface was still covered by the sea. The accumulation of sedimentary rocks continued, mountain building took place. There were reef builders. An abundance of corals, sponges and molluscs has been noted.

Silurian (from the British Silur tribe)(420 million - 400 million years ago);

Dramatic events in the history of the Earth began with the development of jawless fish (the first vertebrates), which appeared in the Ordovician. Another significant event was the appearance in the late Silurian of the first terrestrial.

Devonian (from Devonshire in England)(400 million - 320 million years ago);

In the early Devonian, mountain building movements reached their peak, but basically it was a period of spasmodic development. The first seed plants settled on land. A great variety and number of fish-like species was noted, the first terrestrial animals- amphibians.

Carboniferous or Carboniferous period (from the abundance of coal in the seams) (320 million - 270 million years ago);

Mountain building, folding, and erosion continued. In North America, swampy forests and river deltas were flooded, and large carbonaceous deposits formed. The southern continents were covered by glaciation. Insects spread rapidly, the first reptiles appeared.

Permian period (from the Russian city of Perm)(270 million - 225 million years ago);

A large part of Pangea - the supercontinent that united everything - was dominated by conditions. Reptiles spread widely, modern insects evolved. A new terrestrial flora developed, including conifers. Several marine species have disappeared.

5. Mesozoic era (225 million - 70 million years ago) with such periods:

Triassic (from the tripartite division of the period proposed in Germany)(225 million - 185 million years ago);

With the advent of the Mesozoic era, Pangea began to disintegrate. On land, the dominance of conifers was established. Diversity among reptiles is noted, the first dinosaurs and giant marine reptiles appeared. Primitive mammals evolved.

Jurassic period(from mountains in Europe)(185 million - 140 million years ago);

Significant volcanic activity was associated with the formation of the Atlantic Ocean. Dinosaurs dominated the land, flying reptiles and primitive birds conquered the air ocean. There are traces of the first flowering plants.

Cretaceous period (from the word "chalk")(140 million - 70 million years ago);

During the maximum expansion of the seas, chalk deposits occurred, especially in Britain. The dominance of dinosaurs continued until the extinction of them and other species at the end of the period.

6. Cenozoic era (70 million years ago - up to our time) with such periods and epochs:

Paleogene period (70 million - 25 million years ago);

— Paleocene epoch ("the oldest part of the new epoch")(70 million - 54 million years ago);
— Eocene epoch ("dawn of a new era")(54 million - 38 million years ago);
— Oligocene era ("not very new")(38 million - 25 million years ago);

Neogene period (25 million - 1 million years ago);

— Miocene epoch ("comparatively new")(25 million - 8 million years ago);
— Pliocene epoch ("very new")(8 million - 1 million years ago);

The Paleocene and Neocene periods are still combined into the Tertiary period. With the advent of the Cenozoic era (new life), there is an abrupt spread of mammals. Many large species have evolved, although many have become extinct. There has been a sharp increase in the number of flowering plants. With the cooling of the climate, herbaceous plants appeared. There has been a significant uplift.

Quaternary period (1 million - our time);

— Pleistocene era ("newest")(1 million - 20 thousand years ago);

— Holocene epoch(“a completely new era”) (20 thousand years ago - our time).

This is the last geological period that includes the present. Four major glaciations alternated with warming periods. The number of mammals has increased; they have adapted to. There was a formation of man - the future ruler of the Earth.

There are also other ways of dividing eras, epochs, periods, eons are added to them, and some epochs are still divided, like in this table, for example.

But this table is more complicated, the confusing dating of some eras is purely chronological, not based on stratigraphy. Stratigraphy is the science of determining the relative geologic age of sedimentary rocks, subdividing rock strata, and correlating different geological formations.

Such a division, of course, is relative, since there was no sharp distinction between today and tomorrow in these divisions.

But still, at the turn of neighboring eras and periods, significant geological transformations mainly took place: the processes of formation of mountains, the redistribution of seas, changing of the climate etc.

Each subsection was characterized, of course, by the originality of flora and fauna.

, and can be found in the same section.

Thus, these are the main eras of the Earth, on which all scientists rely 🙂

Geological table- this is one of the ways to represent the stages of development of the planet Earth, in particular life on it. The table records eras, which are subdivided into periods, their age, duration are indicated, the main aromorphoses of flora and fauna are described.

Often in geochronological tables, earlier, i.e. older, eras are written at the bottom, and later, i.e., younger ones, at the top. Below are data on the development of life on Earth in natural chronological order: from oldest to newest. Tabular form omitted for convenience.

Archean era

It began about 3500 million (3.5 billion) years ago. Lasted about 1000 million years (1 billion).

In the Archean era, the first signs of life on Earth appear - single-celled organisms.

According to modern estimates, the age of the Earth is more than 4 billion years. Before the Archean, there was the Catharchean era, when there was no life yet.

Proterozoic era

It began about 2700 million (2.7 billion) years ago. It lasted more than 2 billion years.

Proterozoic - the era of early life. In the layers belonging to this era, rare and few organic remains are found. However, they belong to all types of invertebrates. It is also likely that the first chordates appear - non-cranial.

Palaeozoic

It began about 570 million years ago and lasted more than 300 million years.

Paleozoic - ancient life. Starting from it, the process of evolution is better studied, since the remains of organisms from the upper geological layers are more accessible. Hence, it is customary to consider each era in detail, noting the changes in the organic world for each period (although their periods are distinguished both in the Archean and in the Proterozoic).

Cambrian Period (Cambrian)

Lasted about 70 million years. Marine invertebrates and algae thrive. Many new groups of organisms appear - the so-called Cambrian explosion occurs.

Ordovician period (Ordovician)

Lasted 60 million years. The heyday of trilobites, racoscorpions. The first vascular plants appear.

Silurian (30 Ma)

• Bloom of corals.
• The appearance of scutellum - jawless vertebrates.
• The appearance of psilophyte plants that have come to land.

Devonian (60 Ma)

• The flowering of corymbs.
• The appearance of lobe-finned fish and stegocephalians.
• Distribution on land of higher spores.

Carboniferous period

Lasted about 70 million years.

• The rise of amphibians.
• Appearance of the first reptiles.
• The emergence of flying forms of arthropods.
• Decline in the number of trilobites.
• Blossoming ferns.
• The emergence of seed ferns.

Perm (55 million)

• The spread of reptiles, the emergence of animal-toothed lizards.
• Trilobite extinction.
• Disappearance of coal forests.
• Distribution of gymnosperms.

Mesozoic era

The era of middle life. It began 230 million years ago and lasted about 160 million years.

Triassic

Duration - 35 million years. The flowering of reptiles, the appearance of the first mammals and true bony fish.

Jurassic period

Lasted about 60 million years.

• Dominance of reptiles and gymnosperms.
• Appearance of Archeopteryx.
• There are many cephalopods in the seas.

Cretaceous period (70 million years)

• The emergence of higher mammals and true birds.
• Widespread distribution of bony fish.
• Reduction of ferns and gymnosperms.
• The emergence of angiosperms.

Cenozoic era

The era of new life. It began 67 million years ago, lasts, respectively, the same amount.

Paleogene

Lasted about 40 million years.

• Appearance of tailed lemurs, tarsiers, parapithecus and dryopithecus.
• An explosion of insects.
• The extinction of large reptiles continues.
• Entire groups of cephalopods are disappearing.
• dominance of angiosperms.

Neogene (about 23.5 Ma)

dominance of mammals and birds. The first representatives of the genus Homo appeared.

Anthropogene (1.5 Ma)

Appearance of Homo sapiens species. The animal and plant world takes on a modern look.

Stages of development of the planet. Of great importance for geographical science is 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 the planet Earth is divided into two stages: planetary and geological.

planetary stagecovers 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 make up the solar system. You know that the Earth is one of the 8 planets of the solar system from the 6th grade course. Planet Earth was formed 3.5-5 billion years ago. This stage ended with the appearance of the primary lithosphere, atmosphere and hydrosphere (3.7-3.8 billion years ago).

Geological stagebegan with the appearance of the first rudiments of the earth's crust, which continues to the present. During this period, various rocks were formed. The earth's crust has repeatedly been subjected to slow ups and downs under the influence of internal forces. During periods of subsidence, the territories were flooded with water and sedimentary rocks (sands, clays, etc.) were deposited at the bottom, and during periods of uplift of the sea bottom, plains formed here, composed of these sedimentary rocks.

Thus, the original structure of the earth's crust began to change. This process continued uninterrupted. At the bottom of the seas and depressions of the continents, a sedimentary layer of rocks accumulated, among which were the remains of plants and animals. Each geological period corresponds to their specific forks, because the organic world is in constant development.

Determination of 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 relative age of rocks, it is necessary to know the patterns of successive occurrence of layers of sedimentary rocks of different composition. Their essence is as follows: if the layer of sedimentary rocks lies in an undisturbed state, as they were deposited one after another on the bottom of the moraines, then this means that the layer lying below was deposited earlier, and the layer lying above was formed later, therefore, he is younger.

Indeed, if there is no lower layer, then it is clear that the upper layer covering it cannot be formed, therefore, the lower the sedimentary layer is located, the greater its age. The topmost layer is considered the youngest.

In determining the relative age of rocks, the study of the successive occurrence of sedimentary rocks of different compositions and the fossilized remains of animal and plant organisms contained in them is of great importance. 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 table-scale is constantly being improved. The current state of the table is given on p. 45.

The scale units are era. They are divided into periods, which are subdivided into era. The five largest of these divisions (eras) bear names associated with the nature of the life that existed then. For example, ar-hey- early life time p[utherozoic- the era of primary life, Paleozoic- the era of ancient life, mesozoic- the era of middle life, Cenozoic - era of new life.

Eras are subdivided into shorter periods of time - periods(sometimes called systems). Their names are different. Some of them come from the names of rocks that are most characteristic of this time (for example carbonic period in the Paleozoic and Cretaceous in the Mesozoic). Most of the periods are named after those localities in which the deposits of one or another period are most fully represented and where these deposits were first characterized. The earliest period of the Paleozoic Cambrian got its name from the Cambrian - an ancient state in the west of England. Names of the next periods leozoic - Ordovician and Silurian- come from the names of the ancient tribes of the Ordovicians and Silures, who inhabited the territory of present-day Wales.

To distinguish between the systems of the geochronological table, conventional signs are adopted. Geological eras are indicated by indices (signs) - the initial letters of their Latin names (for example, archaean - AR ), and period indices - by the first letter of their Latin names (for example, Permian P).

Definition absolute age of rocks began at the beginning of the 20th century, after the law of decay of radioactive elements was discovered. Its essence is as follows. In the bowels of the Earth are radioactive elements, such as uranium. Over time, it slowly, at a constant rate, decays into helium and lead. The helium dissipates, while the lead remains in the rock. Knowing the decay rate of uranium (out of 100 g of uranium, 1 g of lead is released over 74 million years), it is possible to calculate how many years ago it was formed by the amount of lead contained in the rock.

The use of radiometric methods 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 the relative and absolute methods of reckoning, a geochronological table was compiled.

1. What stages is the geological history of the Earth's development divided into?

2. What stage of the development of the Earth is geological?

3*. How is the relative and absolute age of rocks determined?

1. Compare the duration of geological eras and periods according to the geochronological table.