How is coal formed?

Some people say that coal is made of stone. But as long as you look carefully, you can see traces of leaves and roots of plants in some coal blocks. If coal is sliced and observed under a microscope, we can find very clear plant cell tissues, spores, pollen and so on. Fossils of plants can also be found in coal seams. In the famous Fushun coal mine in China, there are a lot of amber in the coal seam, some of which contain complete insect fossils, and amber is made of resin secreted by trees. Through research, it is found that coal is formed by complex changes of ancient terrestrial plant remains.

So, how did ancient terrestrial plants become coal? It turns out that in the geological and historical period, the environment in some periods is very conducive to the formation of coal. During these periods, due to the suitable climatic conditions, dense and tall plants are growing everywhere on the ground, and a large number of higher or lower plants are also growing in lakes or swamps. Due to the subsequent crustal changes, these plants were quickly covered by sediments. After a long period of pressure, geocentric heat and the action of bacteria, the original volatile substances such as oxygen and nitrogen all escaped, and most of the rest were "charcoal". This process is called carbonization. The plants buried by sediments are carbonized to form carbon mud, and then the carbon mud is buried deeper and deeper, and gradually becomes coal under the action of pressure and temperature, and the sediments become sand shale.