How old is the Earth?

The earth has a total history of 4.6 billion years, but humans only came into being about 3 million years ago, and the history of human civilization is only about 6,000 years old, which is just a brief moment in the long history. Humanity's understanding of the long early history cannot be directly observed. However, the history of the earth has its own development laws and cyclical systems. Therefore, the history of the earth presents obvious stages, based on the deformation of various types of rocks, fossils, and rock formations. Geological records such as signs, relationships between rock layers or rock masses, and detection and research using modern scientific and technological means such as radioisotope decay measurement, amino acid racemization measurement, paleomagnetic method, etc., can divide the evolution and development history of the earth into the following five stages:

1. The birth of the earth and its childhood

The earth is a member of the solar system, and it is closely related to the origin of the solar system. In this way, understanding the formation and early evolution history of the earth is of course inseparable from exploring the origin of the entire solar system, and the solar system is one of many stars. Therefore, we can speculate on the origin of the solar system and even the earth based on the general laws of stellar evolution.

The evolution of a star can be roughly divided into three stages. The first stage is the gravitational contraction stage, that is, the mutual gravity between diffuse nebulae concentrates them into clusters of nebulae; the second stage is the nuclear reaction stage, where the original nebula They collide with each other and generate heat, and violent nuclear reactions take place inside; the third stage is the aging stage, that is, the hydrogen and nitrogen used as nuclear fusion fuel are gradually exhausted.

According to the general laws of stellar evolution, it can be inferred that about 5 to 6 billion years ago, a nebula began to gather. During the process of gravitational contraction, most of the material in the nebula entered the center and formed The original sun began to take form and began to shine. After that, the huge energy generated by the internal nuclear reaction caused it to radiate light and heat all the time.

The Earth may have first been formed by the aggregation of large and small nebulae. It is generally believed that it had grown to a mass similar to that of the modern Earth 4.7 billion years ago. At this time, the earth was just a collection of many planetesimals, called the proto-Earth. The proto-Earth was constantly heated under the action of gravitational contraction and the heat generated by the decay of internal radioactive elements. When the internal temperature of the proto-Earth reached enough to melt iron, nickel and other elements , iron, nickel and other elements quickly concentrated toward the center of the earth, forming the earth's core and mantle around 4.6 billion years ago, and the earth's crust initially differentiated. The original crust was relatively weak, and the internal temperature of the earth was very high. Therefore, volcanoes were frequently active, and many gases ejected from the volcanoes formed the original atmosphere, such as CH4, NH3, H2, H2O (water vapor), H2S, HCH, etc., but No free oxygen (the oxygen in the atmosphere now is accumulated over a long period of time after the emergence of photosynthetic organisms cyanobacteria and green plants). This reducing atmosphere forms a series of organic small molecule compounds, including amino acids, nucleotides, etc., under the energy of lightning, ultraviolet, shock waves, rays, etc. (This has been used by the spark discharge device designed by American scientist Miller to simulate thunder and lightning. The experiment of synthesizing inorganic substances into organic substances has been confirmed). These organic small molecule compounds either fell directly into the primitive ocean, or were collected into the primitive ocean through lakes and rivers. They accumulated and interacted with each other for a long time in the middle layer of the ocean. Under appropriate conditions, they were further condensed into proteins and nucleic acids with primitive structures and non-specific functions. Such as biological macromolecules, these biological macromolecules accumulated in the primitive ocean, their concentration continued to increase, and they condensed into droplets to form a multi-molecular system. Under certain evolutionary probability and suitable environmental conditions, and after a long period of continuous evolution, primitive life forms with metabolism and self-reproduction capabilities were finally formed about 3.5 billion years ago. This is the first stage of life evolution, that is, the non-cellular life stage, realizing the process of transformation from non-life to life.

The childhood of the earth started from the formation period 4.6 billion years ago and lasted approximately 3 billion years ago, which is 1.516 billion years ago. Of course, not much is known about Earth’s childhood, and it is still a subject that needs further exploration.

2. The Earth’s Youth Period

From about 3 billion years ago to 570 million years ago, the Earth entered its youth period, which is the pre-Paleozoic Era. Although this period lasted for a long time, and the atmosphere, water, and biosphere also developed greatly, the evolution of the biological world was very slow. Until the end of the Pre-Paleozoic Era, there were only fungi, algae, and some lower protozoa on the earth. , brachiopods, etc. This is in sharp contrast to the rapid development of the biological world after the Cambrian.

The earth's entry into its teenage years was marked by the earliest appearance of a small continental core. Later, continents were formed by the gradual expansion of the continental core. The small stable continental core found on the earth with conclusive evidence was formed in 2.8 billion years ago, located in southern Africa. Until 2.5 billion years ago, several small stable land masses were formed in each continent. Later, around 1.7 billion years ago, the Earth experienced the most significant stable continent formation event. The area of ??the stable continent increased greatly in a relatively short historical period, and the continent was almost close to its current size. However, the formed continental lithosphere (also called the original platform) is still relatively weak, retains considerable mobility, and has not reached true stability.

The transition period from the original platform to the platform is from about 1.7 billion years ago to about 1.4 billion years ago. According to scientists’ research and analysis of the data, the original platform has been repeatedly moved from It was shattered by the force inside the Earth, and was constantly cemented by the magma material coming up from below. It became thicker and more stable. Therefore, about 1.4 billion years ago was the period when stable continents were finally formed. The Earth's lithosphere The evolution has entered a new stage.

During this period, the development of the biological world entered the second stage, that is, the prokaryotic cell stage. At this stage, life already had a cell form and a real cell membrane, but there was no real cell nucleus and it was indistinguishable. True nuclear membrane and nucleolus. Mainly represented by cyanobacteria that were most prevalent 2.8 to 2 billion years ago, they can carry out true photosynthesis, absorb carbon dioxide, and release oxygen, so that the reducing atmosphere of the early earth was gradually replaced by an oxidizing atmosphere, and then entered the In the third stage of evolution, eukaryotic cells appeared. The development from prokaryotic cells to eukaryotic cells is the most important evolution completed in the biological world.

3. The Paleozoic Era of the Earth

The stratigraphy of the Paleozoic Era can be divided into early and late periods. The Early Paleozoic Era is divided into three periods: Cambrian, Ordovician and Silurian. It is about 570 million to 400 million years ago; the Late Paleozoic Era includes the Devonian, Carboniferous, and Triassic periods, which are 400 million to 230 million years ago. These 340 million years are the age of the oldest life. The earth has experienced billions of years of evolution during this period. Is the material composition and structure of the atmosphere, hydrosphere, and lithosphere different from the conditions on the earth today? 绻婧屯综合阏?寮?Mechanics Nanjiu?啫師?芟Du?ShuI?Technetium?oxime upamine pen⑹forcing the tomb?Jun⒅rank howling up dark hexagram 龀ぃ?Struggling to force the posthumous title Xiedu Ye? faction Since the Cambrian period, the platform has experienced long-term weathering, erosion, transportation and other external geological processes. The height difference on the earth's surface has been reduced (i.e., flattening), and low-lying areas have been repeatedly damaged. The sea water is flooding and the shallow sea area is expanding. This period was the earliest period when usable coal appeared on the earth. For example, stone coal, a kind of coal in southern my country, was formed by the accumulation and petrification of a large number of remains of marine plants living in coastal and shallow seas. By the end of the Silurian Period, major changes in the geosyncline area around the platform and between the platforms occurred, which were caused by the Caledonian movement, which lasted for several million years. The original low-level areas were raised again, and the simple landform became complex. After this change, tilting and folding occurred in some places, faults occurred in other places, and the total area of ??the continent expanded. As the flattening process continued again, the earth's terrain gradually became flat again, and sea subsidence occurred again in several areas of the Pacific Ocean. In the middle of the Carboniferous, the scale of sea subsidence reached its maximum. Beginning in the late Carboniferous, strong tectonic movements caused the sedimentary and volcanic rocks in the geosynclines to fold violently, transforming them into fold mountain systems. Tectonic movements came and went, and continued until the end of the Late Paleozoic before being completed. This movement is called the Variscan (Alpine) Variscan) movement in the mountains.

The Variscan movement caused the geosyncline between Europe and Africa, the Ural geosyncline between the Eastern European platform and the Siberian platform, and the vast geosyncline between the Siberian, Central Asian and Chinese platforms. The Appalachian geosyncline in the East, North America, and the eastern edge of the continent was transformed into a fold mountain system, and the seawater retreated, connecting the Eurasian continent into one. The global continental blocks have reached the greatest degree of proximity to each other, forming a globally unified continent - Pangaya. The total continental area is almost the same as the total continental area on the earth today.

At the end of the Pre-Paleozoic Era, plants and animals had diverged.

In the plant kingdom, cyanobacteria and fungi flourish; in the animal kingdom, lower invertebrates have appeared. Entering the Cambrian period, red algae and green algae in the plant kingdom began to flourish; in the animal kingdom, several categories of invertebrates Animals, especially trilobites, suddenly began to flourish. In the oceans of the Ordovician, algae developed widely in the plant kingdom, and cephalopods were the majority of marine invertebrates. In the late Ordovician, primitive jawless fish-shaped vertebrates - jawless - appeared. kind. Real fish appeared in the late Silurian period. By the time of the Devonian period, fishes were already very prosperous and were the most advanced animals at that time. Among them was a kind of lobe-finned fish, which later developed into amphibians.

Due to the Caledonian Movement, the continental area expanded, some oceans disappeared, and the environment changed drastically, allowing those biological species with strong adaptability to survive. In the middle of the Devonian period, terrestrial plants developed greatly, and many species had grown into large trees, and insects and amphibians appeared. By the middle of the Carboniferous period, forests appeared, and insects further developed into the air. At the same time, reptiles evolved from amphibians also appeared. Later, the Wariscian movement caused the sea water to recede, and the continental area expanded even more, making the biological world move towards The process of marching into the continent has greatly advanced. In general, during the Paleozoic era, the plant kingdom evolved from lower aquatic algae to higher terrestrial plants, and the animal kingdom evolved from lower marine invertebrates to fish. species and terrestrial reptiles, completed their march to the mainland.

4. The Mesozoic Era of the Earth

The Mesozoic Era is divided into three periods: Triassic, Jurassic, and Cretaceous. It started from 230 million years ago and ended 67 million years ago. The time is about 160 million years.

After the beginning of the Mesozoic Era, the development of earth history took a new turn. Pangaya gradually disintegrated, and each land mass gradually tended to drift to its modern position. The lithosphere also experienced a series of important changes. change. Twenty to thirty million years have passed since the beginning of the Mesozoic Era. By the end of the Triassic Period, North and South America and Eurasia and Africa were splitting apart. Several land masses in the south were also splitting apart and starting to move away from each other. In the late Jurassic, the land masses were further divided, and a huge rift in the general north-south direction was created between North America and Eurasia, and between South America and Africa. The land masses moved to both sides, and the seawater infiltrated. It is the future Atlantic Ocean; after another 70 million years, in the late Cretaceous period, the situation changed further, and the continents continued to move away from each other. The most significant thing is that the distance between South America and Africa increased, which means that the South Atlantic Ocean has obvious expansion.

What is the historical basis for the above-mentioned division of the continent during the Mesozoic Era? What was the reason for the split? This starts with the following hypothesis.

The first is the continental drift hypothesis proposed by the Austrian geophysicist A.L. Wegenge (1880-1930) in 1912. He believed that the earth is a celestial body that changes from hot to cold, and its surface first It cools and condenses into a solid crust. The upper layer of the crust is a lighter silicon-aluminum layer, and its lower layer is a heavier silicon-magnesium layer, which is in a molten state. Just as ice floats on water, continents float on its base, a layer of silicon and magnesium. Due to the earth's eastward rotation and tidal forces, the original continent of Panga slowly moved westward, and later cracks appeared and disintegrated. He also believed that the Pacific Ocean was an ancient ocean that existed together with the original continent. Later, as the American continent drifted westward, its scope gradually shrank, and its reduced area was equal to the expanded area of ??the Atlantic Ocean. The Indian Ocean only appeared after the separation of Australia and Antarctica. As for the Arctic Ocean, it was originally part of the Pacific Ocean. Evidence for continental drift has been found in geological and paleontological literature: the Sierra Mountains on the east coast of South America and the Cape Mountains on the west coast of Africa not only have the same geological structure, Moreover, the composition and age of their mineral deposits are the same; secondly, there is paleontological data. Paleontological research from that era proved that on several continents in the southern hemisphere, 64% of the species of reptiles in the Carboniferous period were identical. During the Triassic, that is, after several continents in the southern hemisphere were speculated to have split for a period of time, the number of homologous reptiles in several continents had dropped to 34%; again, based on paleoclimate data, using the special paleoclimate conditions Analysis of sediments, such as coal seams formed by tropical plants that reflect the ancient equatorial climate, and salt deposits that reflect hot and dry climate conditions, found that they have reached today's high latitudes, while the glaciers that reflect the ancient polar regions have reached today's high latitudes. Today's equatorial region is also known as polar shift. However, this hypothesis fell into disfavor after its popularity.

Until the early 1950s, with the rise of paleomagnetism, research proved that the trajectory of continental drift was consistent with paleomagnetism. The earth's magnetic field is divided into north and south poles. The magnetic records of that time are preserved in rock layers formed hundreds of millions of years ago. Using precision instruments to measure the residual magnetism of rocks, we can know the position and movement of the magnetic north pole of different continents in different geological eras. , research shows that the measured magnetic north pole of each continent has moved differently in the corresponding geological era, and finally converges at the magnetic north pole today.

Secondly, in the early 1960s, American scholars H.H. Hess (1906~?) and R.S. Dietz (1914~?) proposed the seafloor spreading hypothesis. The basic idea of ??this hypothesis is: Hot, plastic material rises up from the asthenosphere below, passes through cracks in the lithosphere, and invades the future ridge axis. The outflowing magma condenses into a new ocean floor and pushes the original ocean floor. Expanding to both sides, the continent drifted with it. Over time, the new ocean floor continues to widen, and the cracked continental crust is carried farther away from the ocean rift.

Since new oceanic lithosphere is constantly being produced from each ocean, the old oceanic lithosphere is moving outward, and the oceans are expanding. If this continues, isn't the size of the earth getting larger and larger? It was not until later that the seafloor spreading hypothesis and the continental drift hypothesis were combined to explain this problem. That is, the ever-growing oceanic lithosphere returned to the asthenosphere and died in other parts of the earth. This has nothing to do with the global trend. The study of seismic activity zones is closely related. This resulted in the formation of a complete and systematic theory in earth science that can explain the various movements occurring in the upper layers of the earth from a macro perspective - the theory of plate tectonics. This theory divides the earth's crust into the Pacific Plate, the Indian Ocean Plate, the Eurasian Plate, the African Plate, the Antarctic Plate and the American Plate, and each plate is divided into several small pieces. All these plates form a layer of lithosphere. The junction of each plate is the active zone of the earth's crust, and the plates move as the ocean floor spreads. Near the ocean ridges are plate growth zones, including the Mid-Atlantic Ridge, the Mid-Indian Ridge, and the East Pacific Rise. Near the trench is the plate subduction zone, which is the trench portion of the eastern and western edges of the Pacific Ocean. When a denser plate subducts toward a less dense plate, it causes strong earthquakes and volcanoes; the subduction forms islands or tall mountains.

The climatic conditions of the Mesozoic Era were generally conducive to the development of animals and plants. The plants in the early Mesozoic Era were dominated by gymnosperm conifers, cycads, ginkgo and some true ferns. By the late Mesozoic Era, plants that could actually bloom and bear fruit appeared - angiosperms. Angiosperms are the highest category in the plant kingdom, and they have significant advantages in spreading and reproducing offspring. In the animal kingdom, the Mesozoic Era is often called the Age of Reptiles. Among them, dinosaurs were the most prosperous and became the overlord of the earth during the Jurassic Period. However, they suddenly became extinct during the Cretaceous Period. The reason for this is still unclear. The scientific mystery of explanation. Birds and mammals, two more advanced vertebrates that developed from reptiles, also appeared during the Mesozoic.

5. The Cenozoic Era of the Earth

The Cenozoic Era is the latest era in geological history. The entire Cenozoic Era, including modern times, is about 67 million years old. It started from the third Period and Quaternary period.

Although the Cenozoic Era lasted for a relatively short period, during this period, the distribution of sea and land on the earth's surface, climate conditions, and the biological interface gradually evolved to what they are today.

The most prominent events in the Cenozoic Era were the proximity of Africa and Europe and the collision of the Indo-Pak subcontinent and Asia. As a result, part of the upper material in the lithosphere pushed each other, forming a tectonic plate spanning the northern and southern hemispheres. , the most majestic mountain system and plateau that stretches almost half of the earth. It starts from the Atlas Mountains in northern Africa in the west, passes through the Alps in southern Europe, extends to the Carpathian Mountains in the east, and connects to the Caucasus Mountains, the plateaus of Turkey and Iran, and Mountains, Pamir Plateau and mountains, to the east are the Himalayas and the Tibetan Plateau, the roof of the world, and further to the southeast, the mountains of Indochina and the Indonesian islands are also connected to it. This is the product of the Alpine orogeny and the Himalayan orogeny.

The mutual compression between the Pacific Ocean and the surrounding continents has also caused the structural belts on the continental margins to continue to undergo strong deformation and magmatism, accompanied by strong seismic activities. These effects are still going on until modern times.

And the faults formed by the movements of various geological historical periods were cut into large and small fault blocks. Under the influence of various effects on the continental margin and the movement of lithospheric materials, they pushed each other, pulled apart or rose and fell relative to each other, forming Mountains, plateaus, basins and plains.

There are two main categories of animals in the early Cenozoic Era: ancient ungulates and ancient carnivores. With their evolution, in the middle and late Tertiary Period, ancient ungulates and then Perissodactyls , such as horses, rhinoceros, etc., followed by artiodactyls, such as sheep, cattle, etc.; ancient meat-eating animals also gradually evolved into various beasts, such as lions, leopards, tigers, etc. After billions of years of evolution, living things have gone through many stages of development from scratch, from low-level to high-level, and finally gave birth to the flower of life - human beings in the latest geological historical period. Human evolution is the result of long-term evolution of the biological world.

In general, the factors that contribute to the evolution of the earth are nothing more than internal and external aspects. External factors are the forces acting in the atmosphere, hydrosphere, and biosphere outside the earth. The geological effects they cause are weathering, erosion, sedimentation, etc. Its main energy source is solar energy and the earth's gravity. In addition, there are the tidal forces of the sun and the moon on the earth, and the impact of meteorites in the history of the earth. There are two main aspects of internal factors: one is the heat generated by the decay of radioactive elements contained in the earth's interior; the other is the energy converted from gravitational energy. Internal and external factors are interdependent and contradictory, and jointly determine the material movement on the surface and inside of the earth.

If we count from the publication of Ryle's famous book "Principles of Geology" in the mid-19th century, it has been more than a hundred years now. Through the efforts of many earth scientists, coupled with the development and technological progress of basic disciplines such as astronomy, physics, chemistry, biology, mathematics, etc., great progress has been made in the study of earth evolution. However, due to the complexity of the problem, scientists have been unable to solve some problems involving the evolution of the earth.

Throughout the development of the scientific earth history, It can be said that earth scientists are on the eve of making a new leap in understanding. Future earth scientists will certainly be able to push the important basic discipline of scientific earth history into a new period of development.

Reference materials: http://202.101.108.27/viewstaticres/SysContent6/d1/dd2/ddd94/792857862094/792857862094.htm