Briefly talk about how coal is formed.

Formation of Coal Coal is a kind of combustible sedimentary rock accumulated by ancient plant remains in lakes, bays, shallow seas and other places through complex biochemical and physical-chemical actions. The chemical composition of coal is mainly carbon, hydrogen, oxygen, nitrogen, sulfur and other elements. Under the microscope, spores and pollen composed of plant cells can be found in coal, and plant fossils can also be found in coal seams, which can prove that coal is made up of plant remains. Scientists found in geological survey that there were periods of humid climate and flourishing plants on the earth, such as Carboniferous, Permian (about 300 million years ago) and Jurassic (about 65.438+300 million years ago ~ 65.438+800 million years ago). At that time, a large number of prolific plants piled up in closed lakes, swamps or bays, and were quickly covered by sediment. Hundreds of millions of years later, plants turned into coal, and soil and sand turned into sandstone or shale. Due to the rhythm of crustal movement and repeated accumulation, there are often many coal seams in the same area, and each layer of coal is separated by rocks. The process from plant to coal can be divided into three stages: (1) bacterial decomposition stage, that is, peatization stage. When plants accumulate underwater and are covered with mud and sand, oxygen is gradually cut off, and anaerobic bacteria participate in the action, which promotes the decomposition of organic matter and generates peat. Through this effect, the components of hydrogen and oxygen in plant debris gradually decrease, while the components of carbon gradually increase. Peat is loose, brown and dull, and its specific gravity is very small. You can see the residue of organic matter, which can be ignited by burning with matches. The smoke is thick and the dust is flying. (2) coalification stage, namely lignite stage. When peat is covered by sediment to form a roof, it becomes a completely closed environment, the bacterial action gradually stops, peat begins to compress, dehydrate and cement, and the carbon content further increases, which is called coalification. Brown or nearly black lignite, dull luster, basically no organic residue, denser than peat, can be ignited with matches and has smoke. (3) Metamorphic stage, that is, bituminous coal and anthracite stage. Lignite is formed at low temperature and low pressure. If lignite is buried deep underground, it will be subjected to high temperature and high pressure, which will change the chemical composition of lignite, mainly because of the decrease of moisture and volatile matter and the relative increase of carbon content. Physical properties have also changed, mainly due to the increase of density, specific gravity, luster and hardness, becoming bituminous coal. This effect is the metamorphism of coal. Bituminous coal is black, shiny and dense, and can be lit by candles, with bright flame and smoke. Bituminous coal further deteriorated into anthracite. Anthracite is black, hard and shiny, and can't be lit with candles, so it burns smokeless. Coal is called black gold and industrial food. Since18th century, it has been one of the main energy sources used in the human world. Although its important position has been replaced by oil, it will inevitably decline in the future for a long time, and coal will become one of the irreplaceable energy sources in human production and life because of its huge reserves and rapid development of science and technology. Coal is a very thick layer of black humus, which has been accumulated on the ground by the branches and roots of plants for millions of years. Due to the change of the earth's crust, it is buried underground for a long time and isolated from the air. After a series of complicated physical and chemical changes at high temperature and high pressure, black fossils are formed, which is the formation process of coal. The thickness of coal seam in coal mine is related to the speed of crustal decline and the accumulation of plant residues in this area. The earth's crust is falling rapidly and the plant remains are piled thick, so the coal seam in this coal mine is thick. On the contrary, the earth's crust is slowly falling, and the plant remains are piled very thin, so the coal seam of this coal mine is very thin. Due to the tectonic movement of the earth's crust, the original horizontal coal seam was folded and fractured. Some coal seams are buried deep underground, while others are pushed out of the surface or even exposed to the ground, which is easy to be found. There are also some coal seams that are relatively thin and small in area and have no mining value. So far, there is no updated statement about the formation of coal. Is coal formed in this way? Whether some expositions should be further studied and discussed. Large coal mines have thick coal seams and excellent coal quality, but the area is generally not very large. If it is formed by the natural growth of leaves and roots of plants for millions of years, its area should be very large. Because the earth was full of forests and grasslands in ancient times, there should be traces of coal storage everywhere underground; The coal seam is not necessarily very thick, because the leaves and roots of plants rot into humus, which will be absorbed by plants. If it is repeated, it will not be so concentrated when buried underground, and the boundary between soil layer and coal seam will not be so clear. However, the undeniable fact and basis is that coal is indeed formed by the systematic evolution of plant residues, which is an unbreakable truth. As long as you look at the coal carefully, you can see the traces of leaves and roots of plants; If you slice coal and observe it under a microscope, you can find a very clear plant tissue structure. Sometimes things like tree trunks are still preserved in coal seams, and some coal seams are still wrapped with complete insect fossils. Why it is so concentrated and of such high quality is worth discussing. I remember when I was in primary school, my family lived in a village not far from the city. Whenever the midsummer rainy season comes, after a rainstorm, there will be a swift stream in the middle of the village, and many of our children will go to play in it. Due to the extension of heavy rain, rivers will become smaller and smaller and eventually dry up. But before the water is cut off, you will find that many water flows are blocked by floating objects such as wooden sticks and weeds, forming small water barriers. In order to make the water flow smoothly, we tear open those small water barriers from time to time and sometimes build a dam on the stream. Even living in the city now, after a heavy rain, there will be all kinds of floating objects blocking the water flow and the sewer mouth in many places on the street, and many floating objects are concentrated in one place. It can be inferred that the formation of coal may be directly related to the flood. Without such a powerful force and carrying function as flood, the formation of coal would never be so concentrated and of such high quality. We can imagine that in the geological and historical period millions of years ago, because of the very suitable climatic conditions, lush and tall plants grew on the ground, and a large number of plants grew in coastal and inland swamp areas. At that time, the rainfall was quite abundant. When natural disasters such as the once-in-a-century flood or tsunami come, grasslands and large forests will be flooded, and plants, large and small, there will be uprooted and floating on the water. The soil at the roots of plants will also be washed away. These trees and grass with roots and branches will also climb and entangle each other and go down the river. Once they are washed to shallows and bays, they will run aground, where they will settle down and screen out all the floating objects like a sieve. Soon, a barrier will be formed here, and this place will also be the accumulation place of plant remains (and many animal remains) in the next flood. When the flood recedes, a winding pile of plant debris will form here. After long-term geological changes, this pile of plant debris will gradually be buried underground and eventually evolve into today's coal mine. Then some people may ask, why didn't this happen when China was hit by a rare flood in 1998? I think that's because the current forest coverage rate in China is very low. Most places with forests are at high altitudes, and the plains are full of grain fields, almost to the point where there is no forest to flood, but some farmland shelterbelts are flooded, and the trees in farmland shelterbelts are rare, and the roots of trees are well developed, so they can firmly grasp the land, and short-term soaking and impact will not cause much damage. The trees in the forest are different. Many trees live together. In order to absorb the energy of the sun, they grow up desperately, and their roots are underdeveloped. Once a tree is uprooted by the flood, it will be destroyed by the flood, just like putting a raft down the river, unstoppable, and finally all piled up in one place. In addition, due to the enhancement of human understanding of nature, the ability to resist sudden natural disasters has been continuously improved. Building water conservancy projects, building strong dams and strengthening river embankments have greatly slowed down the impact of fierce floods, and the phenomenon of flooding has become less, even obediently obeying the call of human beings, turning fierce floods into electric energy, kinetic energy and heat energy, benefiting mankind and serving human society. Not only floods have the ability to carry animals and plants, but tides, typhoons and tsunamis also have this ability. Tsunamis caused by earthquakes, volcanic eruptions and other factors can make the waves rise by more than 30 to 40 meters, and instantly wipe out the animals and plants on an island; Plunder all living things near the coastline. Moreover, the material on the surface cannot wait for the earth to sink forever, and the material on the surface is constantly circulating. Therefore, the flood theory is reasonable, convincing and convincing. The material on the earth's surface can't wait for the earth to sink forever. The material on the earth's surface is constantly circulating. Therefore, the flood theory is reasonable, convincing and convincing. It is an irrefutable fact that coal is indeed formed by the systematic evolution of plant residues. As long as you look at the coal carefully, you can see the traces of leaves and roots of plants. If you slice coal and observe it under a microscope, you can find a very clear plant tissue structure. Sometimes things like tree trunks are still preserved in coal seams, and some coal seams are still wrapped with complete insect fossils. Why it is so concentrated and of such high quality is worth discussing. Because a large number of ancient plants have been deposited, buried deep underground and subjected to high pressure and high temperature, after hundreds of millions of years, coal mines, like other mines, are layered and not everywhere. If they were accumulated by surface plants, they would not be so concentrated and should be everywhere, so I think what the book says is wrong. Carbon exists in the earth. Most of the carbon on the surface exists in the form of compounds, while the carbon in the center of the earth exists in the form of simple substances. When the carbon in the center of the earth is sprayed to the surface, it is partly diamond, partly graphite and mostly coal (different substances are formed under different conditions), which is the same as the origin of most other mines. Plants are pressed underground and will form coal under the condition of long-term hypoxia and high pressure. In Carboniferous, the plants on the earth flourished, which provided a strong material basis for the formation of coal, and the subsequent orogeny provided external conditions for the formation of coal. Many years later, there was coal.