The evolution of living beings can only be understood in the context of geological time.
Geochronological (stratigraphic) timeline - this is a scale of relative geological time, built on the basis of the stages of formation determined by paleontology and historical geology earth's crust and life on the planet. It is a sequence of stratigraphic elements in the order of their formation, in the form of a complete composite ideal section of all terrestrial deposits without gaps and overlaps, and is a standard for the correlation of any stratigraphic units. The boundaries between stratigraphic elements are drawn by events of marked evolutionary or geological change. The doctrine of the chronological sequence of formation and age of the rocks that make up the earth's crust is called geochronology .
Distinguish between relative and absolute geochronology.
task relative geochronology is the determination of the relative age of rocks: determining which deposits found in the earth's crust are older and which are younger. There are several methods for determining the relative age of rocks.
First method - stratigraphic. He proceeds from a completely unclear and logical notion that each layer of sedimentary rocks was formed before the layer that overlies it.
Second method - paleontological. It allows you to establish the relative age of rocks and compare them in geological sections belonging to different areas or regions. Establishment is made according to the nature of various organic remains found in the layers (petrified sea shells, animal bones, leaf prints, etc.).
task absolute geochronology is to determine the true duration of individual periods and epochs in the life of the Earth, as well as its geological age as a whole.
The geochronological age of rocks is determined by units such as era, period, epoch, and century.
Era - the largest stage in the history of the development of the Earth, in which a group of deposits was formed. There are five eras (starting from the more ancient ones): Archean, Proterozoic, Paleozoic, Mesozoic and Cenozoic.
Each era covers several periods. The period corresponds to the time of formation of the rock system. The periods are subdivided into several epochs, which correspond to rock divisions. Epochs are subdivided into centuries, which correspond to tiers as a set of rocks formed in a particular century.
Archean(era of primary life) and Proterozoic(era of ancient life) era farthest from us in time (about 1.5 billion years). At this time, the most ancient rocks were formed that make up the rigid foundation of the earth's crust. Rocks archean era bear only traces of primitive organic forms, testifying to the origin of life on Earth at this time. The Proterozoic era coincides in time with the beginning of the development of various algae, bacteria and invertebrates on Earth.
Palaeozoic(era of ancient life) - a period of time removed from us by about 600 million years and lasting about 350 million years. This era and the breeds related to it have been studied in more detail. The Paleozoic era is characterized by the flourishing of organic life in the seas and oceans and its emergence on land. On land, large amphibians become dominant, and at the end of the era, the first reptiles. In the Carboniferous period of the era, tree-like ferns, horsetails, etc.
The Paleozoic era is divided into six periods (starting from the more ancient ones): Cambrian (Cm), Ordovician (O), Silurian (S), Devonian (D), Carboniferous (C) and Permian (P).
Mesozoic era(the era of average life) lasting 185 million years is the heyday of giant reptiles on land (giant lizards - dinosaurs, flying pterodactyls, etc.). Vegetable world and the world of insects in the Mesozoic have some features in common with our time. At this time, the first representatives of mammals and birds appear on Earth, which developed in the next, Cenozoic era.
The Mesozoic era is divided into three periods: Triassic (T), Jurassic (J) and Cretaceous (Cr).
Cenozoic era(era of new life) - the youngest (about 40 ... 50 million years BC), replacing mesozoic era. Life at this time takes on forms that are closer and closer to our time.
The Cenozoic era is divided into three periods: Paleogene (Pg), Neogene (N) and Anthropogenic (Ap), or Quaternary (Q). The Quaternary period is the last period in the development of the organic world, during which man appeared.
Rocks up to the Quaternary age are called indigenous, and the continental Quaternary age - coverslips. Within bedrocks, in general, older rocks are more durable than younger ones, while Quaternary cover formations are less durable than bedrocks. But there is no direct connection between the age of rocks and their strength, and sometimes young rocks are more durable than ancient ones.
As a result of studying the age, composition, conditions of occurrence and distribution of rocks, geological maps are compiled that show the outcrops of bedrocks on the surface of the earth. Deposits of the Quaternary time on geological maps, as a rule, do not show; for them, special maps of Quaternary (cover) deposits are compiled. They do this for the reason that the rocks until the Quaternary time in the vast majority of cases are of marine origin and are distinguished by a well-identified regularity in the structure of the layers, both in plan and in depth. The rocks of the Quaternary age, on the contrary, in most cases are of continental origin (formed within the land). These rocks are characterized by an extremely variable composition, and the boundaries of their distribution are usually determined by the existing terrain.
The history of the planet Earth already has about 7 billion years. During this time, our common home has undergone significant changes, which was the result of changing periods. in 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 epochs, is a certain grouped chronology. At the first international congresses of geology, a special chronological scale was developed, which represented the periodization of the Earth. Subsequently, this scale was replenished with new information and changed, as a result, now it reflects all geological periods in chronological order.
The largest subdivisions in this scale are eonotemes, eras and periods.
Formation of the Earth
The geological periods of the Earth in chronological order begin their history precisely with the formation of the planet. Scientists have come to the conclusion that the Earth was formed about 4.5 billion years ago. The very process of its formation was very long and, possibly, began as early as 7 billion years ago from small cosmic particles. Over time, the gravitational force grew, along with it, the speed of bodies falling on the forming planet increased. Kinetic energy was transformed into heat, resulting in a gradual heating of the Earth.
The core of the Earth, according to scientists, was formed over several hundred million years, after which the gradual cooling of the planet began. Currently, the molten core contains 30% of the mass of the Earth. 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 history of the planet is covered by the first. However, this eon is divided into three more - Katarchean, Archean, Proterozoic. And often the first of them stands out in 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. Shields of all continents were formed in the Precambrian, but traces of life are very rare.
Catharhean eon
The beginning of the history of the Earth - half a billion years of its existence in science is called katarchey. The upper limit of this eon is at around 4 billion years ago.
Popular literature portrays the Catarchean as a time of active volcanic and geothermal changes on the Earth's surface. However, this is not actually true.
Catharhean eon - the time when volcanic activity did not appear, and the surface of the Earth was a cold, inhospitable desert. Although quite often there were earthquakes that smoothed the landscape. The surface looked like a dark gray primary substance covered with a layer of regolith. The day at that time was only 6 hours.
archean eon
The second main eon out of four in the history of the Earth lasted about 1.5 billion years - 4-2.5 billion years ago. Then the Earth did not yet have an atmosphere, and therefore there was no life yet, but in this eon bacteria appear, due to the lack of oxygen they were anaerobic. As a result of their activities, today we have deposits of natural resources such as iron, graphite, sulfur and nickel. The history of the term "archaea" dates back to 1872, when it was proposed by the famous American scientist J. Dan. The Archean eon, unlike the previous one, is characterized by high volcanic activity and erosion.
Proterozoic eon
If we consider the geological periods in chronological order, the next billion years took 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. 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 the first living creatures appeared in the Proterozoic period - the simplest microorganisms, algae and fungi. And by the end of the eon, worms, marine invertebrates, and mollusks appear.
Phanerozoic eon
All geological periods in chronological order can be divided into two types - explicit and hidden. Phanerozoic refers to explicit. At this time, a large number of living organisms with mineral skeletons appear. The era preceding the Phanerozoic was called hidden because its traces were practically not found due to the absence 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 major eras:
Paleoproterozoic- i.e. ancient, including siderium, riasian period, orosirium and staterium. By the end of this era, the concentration of oxygen in the atmosphere reached its present level.
Mesoproterozoic- average. It consists of three periods - potassium, ectasia and stenia. In this era, algae and bacteria reached their greatest prosperity.
Neoproterozoic- new, consisting of tonium, cryogenium and ediacarium. At this time, the formation of the first supercontinent, Rodinia, takes place, but then the plates parted again. The coldest ice age took place during an era called the Mesoproterozoic, during which most of the planet froze over.
Eras of the Phanerozoic eon
This eon consists of three large epochs, which differ sharply from each other:
Paleozoic, or an era of ancient life. It began about 600 million years ago and ended 230 million years ago. The Paleozoic consists of 7 periods:
- Cambrian (a temperate climate is formed on Earth, the landscape is low-lying, during this period all modern types of animals originate).
- Ordovician (the climate on the entire planet is quite warm, even in Antarctica, while the land sinks significantly. The first fish appear).
- Silurian period (the formation of large inland seas takes place, while the lowlands become increasingly arid due to land uplift. The development of fish continues. The Silurian period is marked by the appearance of the first insects).
- Devon (appearance of the first amphibians and forests).
- Lower Carboniferous (dominance of ferns, distribution of sharks).
- Upper and Middle Carboniferous (appearance of the first reptiles).
- Perm (most of the ancient animals are dying out).
mesozoic, or the time of the reptiles. Geological history consists of three periods:
- Triassic (seed ferns die out, gymnosperms dominate, the first dinosaurs and mammals appear).
- Jura (part of Europe and the western part of America is covered with shallow seas, the appearance of the first toothed birds).
- Chalk (appearance of maple and oak forests, the highest development and extinction of dinosaurs and toothed birds).
cenozoic, or the time of mammals. Consists of two periods:
- Tertiary. At the beginning of the period, predators and ungulates reach their dawn, the climate is warm. There is a maximum spread of forests, the oldest mammals are dying out. Approximately 25 million years ago, a person appears and in the Pliocene era, a person arises.
- Quaternary. Pleistocene - large mammals die out, originate human society 4 ice ages occur, many plant species die out. The modern era - the last ice age ends, gradually the climate takes on its present form. The supremacy of man on the whole planet.
The geological history of our planet has a long and contradictory development. In this process, there was a place for several extinctions of living organisms, ice ages repeated, periods of high volcanic activity were observed, there were eras of the dominance of various organisms: from bacteria to humans. The history of the Earth began about 7 billion years ago, it was formed about 4.5 billion years ago, and less than a million years ago, man ceased to have competitors in all living nature.
| Akron (acrotheme) | Aeon (eonoteme) | Era (erathema) | Period (system) | Epoch (Department) | completion, years ago | Tectonic cycles | Main developments |
|---|---|---|---|---|---|---|---|
| fz Phanerozoic | kz Cenozoic | Quaternary | Holocene | Continues these days | Alpine cycle There are only 2 belts on Earth. The Tethys Ocean disappears. From the end of the Neogene, the glaciation begins in Antarctica. Tt.o. Neogene - the largest geocratic period of the Earth. The area of the continents was larger than modern. All shelf zones were part of the continents. | Extinction of many large mammals. | |
| Pleistocene | 11 400 | The emergence of modern man. | |||||
| Neogene | Pliocene | 1.81 million | |||||
| Miocene | 5.33 million | ||||||
| Paleogene | Oligocene | 23.0 million | The appearance of the first great apes. | ||||
| Eocene | 37.2 million | The emergence of the first "modern" mammals. | |||||
| Paleocene | 55.8 million | ||||||
| mz Mesozoic | Chalky | 66.5 million | Pacific cycle On Earth there is 1 continent, 2 oceans and 3 belts. The dominance of land on Earth, the climate is hot dry. The split of Gondwana completely. | The first placental mammals. Dinosaur extinction. | |||
| Jurassic | 146 million | The appearance of marsupials and the first birds. The rise of the dinosaurs. | |||||
| Triassic | 200 million | The first dinosaurs and egg-laying mammals. | |||||
| pz Paleozoic | Permian | 251 million | Herzing cycle In the Carboniferous, a new supercontinent, Angarida, at that time Eria and Gondwana already existed. Eria + Angarida = Laurasia Laurasia + Gondwana = Pangea But immediately a split begins (at the end of Perm). At the end of Perm, the first great extinction of organisms. | About 95% of all existing species died out. | |||
| Coal | 299 million | The appearance of trees and reptiles. | |||||
| Devonian | 359 million | The appearance of amphibians and spore plants. | |||||
| S Silurian | 416 million | Caledonian cycle At this stage, there were 6 ancient platforms on Earth. The largest transgression with max in the Ordovician, Gondwana remains dry land. At the beginning of the Silurian - glaciation. At the end of the Caledonian phase, the supercontinent Eria formed. | Exit of life to land: scorpions and later the first plants. The appearance of fish. | ||||
| O Ordovician | 443 million | The pelagial is inhabited by cephalopods | |||||
| E Cambrian | 488 million | Appearance a large number new groups of organisms. | |||||
| PR Proterozoic | Riphean (Neoproterozoic) | Ediacarus (obsolete Vendian) | 542 million | Baikal cycle 5 geosynclinal belts are laid. Formed Pacific Ocean(800 million years ago) At the end of the Riphean, all continents are connected southern hemisphere- Gondwana. The climate is warm everywhere, at the end of the Riphean glaciation. The atmosphere is saturated with oxygen (1% of the current level) | The first multicellular animals. | ||
| cryogeny | 600 million | ||||||
| Tony | 850 million | ||||||
| Late (Mesoproterozoic) | Stenius | 1.0 billion | |||||
| Ectasia | 1.2 billion | ||||||
| potassium | 1.4 billion | ||||||
| Early (Paleoproterozoic) | Statery | 1.6 billion | Karelian cycle Revolutionary stage. At the end of it, huge sections of the ZK become rigid and stable. Real platforms are being formed. | ||||
| Orosirium | 1.8 billion | ||||||
| Riasius | 2.05 billion | ||||||
| siderius | 2.3 billion | ||||||
| AR archaeus | Late | neoarchean | 2.5 billion | White Sea cycle Formation of a real continental ZK. | |||
| Mesoarchean | 2.8 billion | ||||||
| Early | paleoarchaean | 3.2 billion | Soami cycle On Earth, a hydrosphere is formed, which is represented by shallow oceans, in the form of islands there are cores of the protocontinental crust. | ||||
| Eoarchean | 3.6 billion | The emergence of primitive unicellular organisms. | |||||
| 3.8 billion | Early geological stage The Earth is formed as a result of rotation. Substance differentiation begins. A basaltic crust is formed, but it is phantom. | Formation of the Earth 4.57 billion years ago | |||||
Geological table
This is a list of time divisions or intervals, in the order of their hierarchy.
Chronometric scale
This is an isotopic age scale based on the radioactive decay of elements, from their formation to the present day.
Akron is a time period lasting 2 billion years.
Eon - a gap of 1 billion years.
An era is hundreds of millions of years.
Period - tens of million years
Epoch - tens of millions of years.
Stratigraphic scale
This is the rock scale. Represents a complete perfect section of the Earth's crust
See also: Evolution of the geographic shell of the earth, Geochronological scale (original article).
Geological chronology, or geochronology, based on finding geological history the most well-studied regions, for example, in Central and Eastern Europe. Based on broad generalizations, comparison of the geological history of various regions of the Earth, patterns of evolution of the organic world at the end of the last century, at the first International Geological Congresses, the International Geochronological Scale was developed and adopted, reflecting the sequence of time divisions during which certain sediment complexes were formed, and the evolution of the organic world . Thus, the international geochronological scale is a natural periodization of the history of the Earth.
Among the geochronological divisions are distinguished: eon, era, period, epoch, century, time. Each geochronological subdivision corresponds to a set of deposits, identified in accordance with the change in the organic world and called stratigraphic: eonoteme, group, system, department, stage, zone. Therefore, the group is a stratigraphic unit, and the corresponding temporal geochronological unit is represented by an era. Therefore, there are two scales: geochronological and stratigraphic. The first is used when talking about relative time in the history of the Earth, and the second when dealing with deposits, since in every place the globe in any period of time there were some geological events. Another thing is that the accumulation of precipitation was not ubiquitous.
- The Archean and Proterozoic eonotemes, covering almost 80% of the time of the Earth's existence, are distinguished in the Cryptozoic, since the skeletal fauna is completely absent in the Precambrian formations and the paleontological method is not applicable to their division. Therefore, the division of Precambrian formations is based primarily on general geological and radiometric data.
- The Phanerozoic eon covers only 570 million years, and the division of the corresponding eonoteme of deposits is based on a wide variety of numerous skeletal fauna. The Phanerozoic eonoteme is subdivided into three groups: Paleozoic, Mesozoic and Cenozoic, corresponding to major stages in the natural geological history of the Earth, the boundaries of which are marked by rather abrupt changes in the organic world.
The names of eonotems and groups come from Greek words:
- "archeos" - the most ancient, most ancient;
- "proteros" - primary;
- "paleos" - ancient;
- "mesos" - medium;
- "kainos" - new.
The word "cryptos" means hidden, and "phanerozoic" means explicit, transparent, since the skeletal fauna appeared.
The word "zoi" comes from "zoikos" - life. Therefore, "Cenozoic era" means the era of new life, and so on.
Groups are subdivided into systems, the deposits of which were formed during one period and are characterized only by families or genera of organisms characteristic of them, and if these are plants, then by genera and species. Systems have been identified in different regions and at different times since 1822. At present, 12 systems are distinguished, the names of most of which come from the places where they were first described. For example, the Jurassic system - from the Jura Mountains in Switzerland, the Permian - from the Perm province in Russia, the Cretaceous - according to the most characteristic rocks - white writing chalk, etc. The Quaternary system is often called Anthropogenic, since it is in this age interval that a person appears.
The systems are subdivided into two or three divisions, which correspond to the early, middle, and late eras. The departments, in turn, are divided into tiers, which are characterized by the presence of certain genera and species of fossil fauna. And, finally, the stages are subdivided into zones, which are the most fractional part of the international stratigraphic scale, which corresponds to time in the geochronological scale. The names of the tiers are usually given according to geographical names areas where this tier was identified; for example, the Aldanian, Bashkirian, Maastrichtian stages, etc. At the same time, the zone is designated by the most characteristic type of fossil fauna. The zone covers, as a rule, only a certain part of the region and is developed over a smaller area than the deposits of the stage.
All subdivisions of the stratigraphic scale correspond to the geological sections in which these subdivisions were first identified. Therefore, such sections are reference, typical, and are called stratotypes, which contain only their own complex of organic remains, which determines the stratigraphic volume of a given stratotype. The determination of the relative age of any layers consists in comparing the discovered complex of organic remains in the studied layers with the complex of fossils in the stratotype of the corresponding division of the international geochronological scale, i.e. the age of the deposits is determined relative to the stratotype. That is why the paleontological method, despite its inherent shortcomings, remains the most important method for determining the geological age of rocks. Determining the relative age of, for example, the Devonian deposits only indicates that these deposits are younger than the Silurian, but older than the Carboniferous. However, it is impossible to establish the duration of the formation of Devonian deposits and give a conclusion about when (in absolute chronology) the accumulation of these deposits occurred. Only methods of absolute geochronology are able to answer this question.
Tab. 1. Geological table
| Era | Period | Epoch | Duration, Ma | Time from the beginning of the period to the present day, million years | Geological conditions | Vegetable world | Animal world |
| Cenozoic (time of mammals) | Quaternary | Modern | 0,011 | 0,011 | End of the last ice age. The climate is warm | The decline of woody forms, the flowering of herbaceous | Age of Man |
| Pleistocene | 1 | 1 | repeated glaciations. four ice ages | Extinction of many plant species | Extinction of large mammals. The origin of human society | ||
| Tertiary | Pliocene | 12 | 13 | Mountains continue to rise in the west North America. Volcanic activity | Decay of forests. Spread of meadows. flowering plants; development of monocots | The emergence of man from the great apes. Types of elephants, horses, camels, similar to modern | |
| Miocene | 13 | 25 | The Sierras and the Cascade Mountains formed. Volcanic activity in the northwestern United States. The climate is cool | The culminating period in the evolution of mammals. The first great apes | |||
| Oligocene | 11 | 30 | The continents are low. The climate is warm | Maximum distribution of forests. Strengthening the development of monocotyledonous flowering plants | Archaic mammals are dying out. The beginning of the development of anthropoids; ancestors of most extant genera of mammals | ||
| Eocene | 22 | 58 | The mountains are blurred. There are no inland seas. The climate is warm | Diverse and specialized placental mammals. Ungulates and carnivores flourish | |||
| Paleocene | 5 | 63 | Distribution of archaic mammals | ||||
| Alpine orogeny (minor destruction of fossils) | |||||||
| Mesozoic (time of reptiles) | Chalk | 72 | 135 | At the end of the period, the Andes, the Alps, the Himalayas, Rocky Mountains. Prior to this, inland seas and swamps. Deposition of writing chalk, shale | The first monocots. The first oak and maple forests. Decline of gymnosperms | Dinosaurs reach the highest development and die out. Toothed birds are dying out. Appearance of the first modern birds. Archaic mammals are common | |
| Yura | 46 | 181 | The continents are quite elevated. Shallow seas cover parts of Europe and the western United States | The value of dicots increases. Cycadophytes and conifers are common | The first toothed birds. Dinosaurs are large and specialized. Insectivorous marsupials | ||
| Triassic | 49 | 230 | Continents are elevated above sea level. Intensive development of arid climate conditions. Widespread continental deposits | The dominance of the gymnosperms, already beginning to decline. Extinction of seed ferns | The first dinosaurs, pterosaurs and egg-laying mammals. Extinction of primitive amphibians | ||
| Hercynian orogeny (some destruction of fossils) | |||||||
| Paleozoic (era of ancient life) | Permian | 50 | 280 | Continents are raised. Appalachian mountains formed. Dryness is getting worse. Glaciation in the southern hemisphere | Decline of club mosses and ferns | Many ancient animals are dying out. Animal reptiles and insects develop | |
| Upper and Middle Carboniferous | 40 | 320 | The continents are initially low-lying. Vast swamps in which coal was formed | Large forests of seed ferns and gymnosperms | The first reptiles. Insects are common. Distribution of ancient amphibians | ||
| Lower Carboniferous | 25 | 345 | The climate is initially warm and humid, later, due to the rise of the land, it becomes cooler. | Club mosses and fern-like plants dominate. Gymnosperms are spreading more and more | Sea lilies reach their highest development. Distribution of ancient sharks | ||
| Devonian | 60 | 405 | Inland seas are small. Land elevation; development of an arid climate. Glaciation | First forests. Land plants are well developed. First gymnosperms | The first amphibians. Abundance of lungfish and sharks | ||
| Silurus | 20 | 425 | Vast inland seas. Low-lying areas are getting drier as the land rises | The first reliable traces of land plants. Algae dominate | Marine arachnids dominate. The first (wingless) insects. Increased development of fish | ||
| Ordovician | 75 | 500 | Significant land sink. The climate is warm, even in the Arctic | Probably the first land plants appear. Abundance of seaweed | The first fish are probably freshwater. Abundance of corals and trilobites. Various clams | ||
| Cambrian | 100 | 600 | The continents are low, the climate is temperate. The most ancient rocks with abundant fossils | Seaweed | Trilobites and lechenopods dominate. The origin of most modern animal phyla | ||
| Second great orogeny (significant destruction of fossils) | |||||||
| Proterozoic | 1000 | 1600 | Intensive process of sedimentation. Later - volcanic activity. Erosion over large areas. Multiple glaciations | Primitive aquatic plants - algae, fungi | Various marine protozoa. By the end of the era - molluscs, worms and other marine invertebrates | ||
| First great mountain building (significant destruction of fossils) | |||||||
| archaeus | 2000 | 3600 | Significant volcanic activity. Weak sedimentation process. Erosion on large areas | Fossils are absent. Indirect evidence of the existence of living organisms in the form of deposits of organic matter in rocks | |||
The problem of determining the absolute age of rocks, the duration of the existence of the Earth has long occupied the minds of geologists, and attempts to solve it have been made many times, for which various phenomena and processes have been used. Early ideas about the absolute age of the Earth were curious. A contemporary of M. V. Lomonosov, the French naturalist Buffon determined the age of our planet at only 74,800 years. Other scientists gave different figures, not exceeding 400-500 million years. It should be noted here that all these attempts were doomed to failure in advance, since they proceeded from the constancy of the rates of processes, which, as is known, changed in the geological history of the Earth. And only in the first half of the XX century. there was a real opportunity to measure the truly absolute age of rocks, geological processes and the Earth as a planet.
| Tab.2. Isotopes used to determine absolute ages | ||
| parent isotope | Final product | Half-life, billion years |
| 147cm | 143 Nd+He | 106 |
| 238 U | 206 Pb+ 8 He | 4,46 |
| 235 U | 208 Pb+ 7 He | 0,70 |
| 232Th | 208 Pb+ 6 He | 14,00 |
| 87Rb | 87 Sr+β | 48,80 |
| 40K | 40 Ar+ 40 Ca | 1,30 |
| 14C | 14 N | 5730 years |
A very important characteristic of rocks is their age. As shown above, many properties of rocks, including engineering-geological ones, depend on it. In addition, on the basis of studying, first of all, the age of rocks, historical geology recreates the patterns of development and formation of the earth's crust. An important section of historical geology is geochronology - the science of the sequence of geological events in time, their duration and subordination, which it establishes by determining the age of rocks based on the use of various methods and geological disciplines. The relative and absolute age of rocks is distinguished.
In assessing relative age, older and younger rocks are distinguished by highlighting the time of an event in the history of the Earth in relation to the time of another geological event. Relative age is easier to determine for sedimentary rocks in their undisturbed (close to horizontal occurrence) occurrence, as well as for volcanic and less often metamorphic rocks interbedded with them.
The stratigraphic (stratum - layer) method is based on the study of the sequence of occurrence and the relationship of layers of sedimentary deposits, based on the principle of superposition: each overlying layer is younger than the lower one. It is used for strata with undisturbed horizontal occurrence of layers (Fig. 22). This method should be carefully applied when the layers are folded; first, their tops and bottoms must be determined. Young is the layer 3 , and the layers 1 and 2 - more ancient.
Lithologo - petrographic method is based on the study of the composition and structure of rocks in adjacent sections of wells and the identification of rocks of the same age - correlation of sections . Sedimentary, volcanic and metamorphic rocks of the same facies and age, such as clays or limestones, basalts or marbles, will have similar textural and structural features and composition. Older rocks tend to be more altered and compacted, while younger ones are slightly altered and porous. It is more difficult to use this method for thin continental deposits, the lithological composition of which is rapidly changing along the strike.
The most important method for determining relative age is paleontological ( biostratigraphic ) method , based on the allocation of layers containing various complexes of fossil remains of extinct organisms. The method is based on the principle of evolution : life on Earth develops from simple to complex and does not repeat itself in its development. The science that establishes the pattern of development of life on Earth by studying the remains of fossil animals and plant organisms - fossils ( fossils) contained in the strata of sedimentary rocks is called paleontology. The time of formation of one or another rock corresponds to the time of death of organisms, the remains of which were buried under layers above the accumulated sediments. The paleontological method makes it possible to determine the age of sedimentary rocks in relation to each other, regardless of the nature of the occurrence of layers, and to compare the age of rocks occurring in distant parts of the earth's crust. Each segment of geological time corresponds to a certain composition of life forms or guiding organisms (Fig. 23-29). Leading fossils ( forms ) lived for a short period of geological time on vast areas, as a rule, in reservoirs, seas and oceans. Starting from the second half of the twentieth century. actively began to apply the micropaleontological method, including spore - pollen, to study organisms invisible to the eye. On the basis of the paleontological method, schemes of the evolutionary development of the organic world were drawn up.
Thus, based on the above methods for determining the relative age of rocks to late XIX in. a geochronological table was compiled, which includes subdivisions of two scales: stratigraphic and corresponding geochronological.
Stratigraphic subdivision (unit) - a set of rocks that make up a certain unity in terms of a set of features (features of the material composition, organic remains, etc.), which allows you to distinguish it in the section and trace about the area. Each stratigraphic unit reflects the peculiarity of the natural geological stage of the development of the Earth (or a separate area), expresses a certain geological age and is comparable to a geochronological unit.
Geochronological (geohistorical) scale - a hierarchical system of geochronological (temporal) divisions, equivalent to units of the general stratigraphic scale. Their ratio and subdivision is shown in Table. fifteen.
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isolated in the UK, Perm - in Russia, etc. (Table 16).
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Absolute age - the duration of the existence (life) of the breed, expressed in years - in time intervals equal to the modern astronomical year (in astronomical units). It is based on measuring the content of radioactive isotopes in minerals: 238U, 232Th, 40K, 87Rb, 14C, etc., their decay products and knowledge of the experimentally revealed decay rate. The latter has a half-life – the time it takes for half of the atoms of a given unstable isotope to decay. The half-life varies greatly for different isotopes (Table 17) and determines the possibilities of its application.
Methods for determining the absolute age got their name from the products of radioactive decay, namely: lead (uranium-lead), argon (potassium-argon), strontium (rubidium-strontium), etc. The most commonly used potassium-argon method, since the 40K isotope contained in many minerals (mica, amphiboles, feldspars, clay minerals), decomposes with the formation of 40Ar and has a half-life of 1.25 billion years. Calculations made using this method are often verified by the strontium method. In these minerals, potassium is isomorphically replaced by 87Rb, which, upon decay, transforms into the 87Sr isotope. With the help of 14C, the age of the youngest Quaternary breeds is determined. 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.
The use of these methods is complicated by the fact that rocks during their "life" experience various events: magmatism, metamorphism, and weathering, during which the minerals "open", change and lose the isotopes and decay products partially contained in them. Therefore, the term "absolute" age used is convenient to use, but is not absolutely accurate for the age of rocks. It is more correct to use the term "isotopic" age. A systematic correlation is made between the divisions of the relative geochronological table and the absolute age of the rocks, which is still specified and given in tables.
Geologists, builders and other professionals can obtain information about the age of rocks by studying geological maps or related geological reports. On the maps, the age of rocks is shown by the letter and color that are accepted for the corresponding subdivision of the geochronological table. Comparing the relative age of specific rocks shown by letter and color with the absolute age of the unified geochronological table, we can assume the absolute age of the studied rocks. Civil engineers must have an understanding of the age of rocks and its designation, and also use them when reading geological documentation (maps and sections) compiled when designing buildings and structures.
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Of particular interest is the Quaternary period (Table 18). The deposits of the Quaternary system cover the entire earth's surface, their sequences contain remains ancient man and his household items. In these sequences, various deposits (facies) alternate and replace each other in area: eluvial, alluvial , moraine and fluvioglacial, lacustrine - marsh. Deposits of alluvial gold and other valuable metals are confined to alluvium. Many breeds of the Quaternary system are raw materials for the production of building materials. A large place is occupied by deposits of the cultural layer , resulting from human activity. They are distinguished by considerable friability and great heterogeneity. Its presence can complicate the construction of buildings and structures.