The earth has been the birthday star for 4.6 billion years, which originated from the primitive solar nebula. About 3-4 billion years ago, the most primitive single-celled life began to appear on the earth, and then gradually evolved, and various different creatures appeared. The average equatorial radius of the earth is 6378. 14 km, which is 2 1 km longer than the polar radius.
The internal structure of the earth can be divided into three layers: crust, mantle and core. Under the action of the earth's gravity, a large number of gases gather around the earth to form a blanket, which is the earth's atmosphere.
The earth is like a top, rotating from west to east along its axis of rotation. Her rotation period is 23 hours, 56 minutes and 4 seconds, which is about 24 hours. At the same time, the earth revolves around the sun, and its orbit is elliptical, with a half-length of 149597870 km. One revolution takes 365.25 days, which is one year.
After the birth of the solar system about 5 billion years ago, the earth began to form about 500 million years later. The earth was formed by fractionation, collapse and condensation of the primitive solar nebula. First, planetesimals gather into planetary tires, and then proliferate to form the primitive earth.
The planetesimals obtained from the primitive earth are relatively cold, but each planetesimal falling on the primitive earth has high motion energy, which is converted into heat energy through impact; In addition, due to the accumulation of planetesimals, the external weight of the earth's planets increases, and the internal parts are compressed, and the energy consumed by the compressed parts is converted into heat and preserved; Coupled with the accumulation of heat generated by the decay of radioactive elements such as uranium, thorium and potassium, the earth began to get hot, and eventually the temperature in most areas exceeded the melting point of iron. Metal iron, nickel and iron sulfide in the primitive earth flowed to the center of the earth after melting due to their high density, forming a liquid iron core.
Subsequently, the average temperature of the earth rose further, causing most of the materials on the earth to melt, and the melted materials lighter than the parent material floated up, bringing the tropics to the surface, and then sinking down after cooling. This material movement under the control of convection makes the primitive earth undergo global differentiation and evolve into a layered earth, that is, the most primitive core, with an iron core at the center and a light material with low melting point on the surface, and the core further proliferates and expands to form the crust. Between the core and the crust is the mantle. Differentiation is the most important function of the earth, which leads to the formation of crust and continent, as well as the formation of atmosphere and ocean.
Most experts believe that the water on the earth appeared about 4-3.6 billion years ago, when the earth was still in its formative stage and was constantly hit by thousands of comets and meteorites. Scientists believe that many of these asteroids that hit the earth are rich in water resources-their forms include steam, liquid water and ice.
The water formed by the combination of hydrogen and oxygen was initially hidden in some minerals. When the primitive earth became hot and partially melted, water was released and moved to the surface with lava, most of which escaped as steam, and the rest gradually filled the ocean in the long geological history. In the process of differentiation caused by the warming of the primitive earth, the gas released from the interior of the earth formed the atmosphere. The atmospheric composition of the early earth was different from that of the modern world. It is precisely because of the energy of ultraviolet radiation that the original atmospheric components react, from inorganic substances to organic small molecules, and then develop into a multi-molecular system composed of organic polymer substances, and then evolve into cells, so that life can begin and evolve.
After the early stage of differentiation, the mantle consolidated, the original crust and continent developed, and the ocean and atmosphere formed.
The changes of the core and mantle play a leading role in the changes of the earth's magnetic field. The evolution of geological structures, the formation and movement of plates, earthquakes, volcanoes and other natural phenomena show that the interior of the earth is in a state of thermal and mechanical imbalance, and there is a huge source of force, which makes the movement continue.
Two measurable physical properties of the core are magnetic field and heat. The earth's core influences the mantle in two important direct ways. One is to provide heat to the bottom of the mantle and stimulate thermal convection in the depth of the mantle, that is, the output of heat is through conduction and convection; The second is to exert mechanical torque on the mantle. This interaction and other earth processes, including atmospheric movement, determine the length of a day and the direction of the earth's axis in space.
Mantle convection is a thermal mode in the mantle and a movement process of mantle materials. This thermal convection in the mantle is an effective way to transport energy, momentum and mass from the earth's interior to the earth's surface, and it is likely to be the driving force of the earth's evolution.
The top layer of the earth is a hard rock layer with a thickness of about 100 km, called lithosphere, which includes the top of the crust and the upper mantle. Below the lithosphere is the low-velocity layer of the upper mantle, with a little melting, but the solid medium will have rheological characteristics under high temperature and high pressure for a long time, and the whole low-velocity layer can flow and deform, so it is called asthenosphere, and its lower boundary is about 220 kilometers deep. The lithosphere is not a whole, but several rigid plates separated by tectonic active zones and constantly moving relative to each other. The global lithosphere was first divided into six plates: Eurasia plate, America plate, Africa plate, Pacific plate, India-Australia plate and Antarctic plate. The boundaries of these plates are not continental margins, but ocean ridges, island arc structures and horizontal faults. Except that the Pacific plate is completely water, the rest are land and sea. Most earthquakes and volcanoes occur at plate boundaries. Plate tectonics is of great significance to the connection and separation of continental plates and the migration and evolution of biological species.
According to the theory of plate tectonics, the tectonic movement and seismic activity in the upper earth are mainly the result of the interaction of these plates. Plate deformation mainly occurs at its boundary, and intra-plate deformation is mainly large-scale orogeny. There are Pacific Rim seismic belt, Eurasian seismic belt and a long weak seismic belt in the Atlantic Ocean on the surface of the earth, which is the boundary of plates.
America, Africa, Europe and Greenland were all connected for a long time 200 million years ago. They began to split about 200 million years ago and then expanded to form the Atlantic Ocean. This process is called "dispersion"; However, the Indian plate only drifted near Asia 0.7-0.6 billion years ago, and then collided with the Eurasian plate. This process is called "convergence". The plates will separate, collide and slide along the transform fault, which is the core of plate tectonic theory.
In the process of plate collision, the heavier oceanic lithosphere is inserted into the mantle under the lighter continental lithosphere, which is called "subduction". It is precisely because of the subduction of the Indian plate that the Qinghai-Tibet Plateau in China has become the region with the largest upper crust thickness and the highest altitude in Cenozoic, which has a great impact on the global environment.
Due to the accumulation and continuous movement of plates, many favorable conditions have been created for the formation of minerals. In the convergence area, the lithosphere subducted under the continental or island arc and remelted, and the ore-bearing solution upwelled. Many sulfide deposits in the world are related to plate convergence. In the marginal sea area between the island arc and the mainland, sediments contain a lot of organic matter, which creates oil-generating conditions. The East China Sea, the Yellow Sea and the South China Sea are such areas. The discrete boundary of the plate is where the new seabed is formed. Seawater invades rock cracks, dissolves upwelling materials in the mantle and produces hot water deposits.
According to British media reports, the latest research shows that there may be about 60 billion "livable" planets in the Milky Way where the earth is located, all of which have the conditions to support the existence of life. This discovery means that there may be extraterrestrial life not far from the earth.