The geological code of "Flame Mountain"

The "Flame Mountain" in the Turpan Basin of Xinjiang has the well-known classic scene of "Wukong borrowing the banana fan three times" in "Journey to the West". It is said that "there is no spring or autumn, and it is hot all year round. That mountain is the only way to the west, but there is a flame of 800 miles, and there is no grass growing around it"; it is called "Flame Mountain" in "Classic of Mountains and Seas", and "Kiziltak" in Uyghur, which means Red Mountain; it was also called "Red Stone Mountain" during the Sui and Tang Dynasties. The body of the Flame Mountain is full of gullies, twists and turns, and no grass grows. In the hot summer, the exposed surface can reach a temperature of 75℃ under the baking sun, and the heat waves roll, making people breathless.

So how was the "Flaming Mountain" formed?

This is due to the spontaneous combustion of coal. Coal is the black humus formed by the branches, leaves and rhizomes of plants accumulated on the ground. Due to the movement of the earth's crust, it is constantly buried underground and isolated from the air for a long time. Under high temperature and high pressure, it undergoes a series of complex physical and chemical reactions to form black combustible sedimentary rocks. In arid and water-scarce areas or areas where coal seams are exposed to the surface, coal catches fire without being ignited, which is called "spontaneous combustion of coal." This is because when the self-igniting coal encounters oxygen in the air, the heat generated by oxidation is greater than the heat dissipated to the surrounding environment, resulting in heat accumulation, which causes the coal temperature to rise to the ignition point and catch fire. Now everyone may understand "why coal trucks are often watered and why coal storage stations intermittently sprinkle water on coal piles."

Spontaneous combustion of coal seams is not uncommon in my country. It can be seen in Xinjiang, Inner Mongolia, Ningxia, Shanxi and other places. For example, in the Liuhuanggou coalfield 42 kilometers away from Urumqi, since the Guangxu period of the Qing Dynasty, there have been "cracks crisscrossing, thick smoke filled the air, and flames roaring in the cracks of the rocks for years." After four years of cleaning work, the fire area of ​​this coalfield was extinguished in 2003.

The product of the spontaneous combustion of coal seams is "burned rock".

The color of burnt rock is generally yellow, orange, red and purple-black, and the color varies with the degree of baking. Its common structural features are pore-like structure and brecciated structure. Slag-like structure can be seen in those with a high degree of baking. The pores are relatively developed, and the pores are generally round or oval. It has a slightly shiny ablation flow surface and shows faint wrinkles in some parts. In 2017, the Hebei Provincial Regional Geological Survey Institute discovered the output of burnt rock for the first time in the northern part of Wanquan County, Hebei Province.

The various characteristics of burnt rocks are closely related to the distance from the fire zone. The burnt rocks closest to the fire zone are subjected to the highest degree of baking. After formation, they are partially or completely melted, and then quickly cooled to be molten, slag-like or honeycomb-like. The degree of baking of burnt rocks gradually decreases as they move away from the combustion zone until they transition to normal rocks. This is because the original rock structure, structure and composition will change to varying degrees due to the different effects of the high temperature generated by the combustion of coal seams. According to the degree of ablation, they can be divided into ablated rocks, baked rocks and weakly baked rocks from heavy to light. Therefore, burnt rocks are often used as effective prospecting signs for coal mines.

Spontaneous combustion of coal is a serious waste of coal resources, and is accompanied by various geological disasters such as surface cracks, collapse, and landslides. Combustion also produces a large amount of toxic and harmful gases such as carbon dioxide, carbon monoxide and sulfur dioxide, which endanger people's lives and safety and cause serious air pollution.

How to prevent spontaneous combustion of coal? We should strengthen the research on the geological background, storage conditions, coal quality, and burnt rocks of coal seams. Burnt rocks, as another code for studying "Flame Mountain", can further reveal the process and law of spontaneous combustion of coal fields, so as to help us better protect, mine and utilize coal resources.