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Why is increasing temperature considered a major cause of coalification?

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coal utilization Cross-regenerative coke oven. (A) Cross section, showing the alternating arrangement of flue walls and ovens; (B) longitudinal section, showing (left) a series of combustion flues in a single flue wall and (right) part of a long, slotlike oven.

Increasing temperature is considered the primary driver of coalification because it facilitates the chemical and physical transformation of organic matter into higher-rank coals, such as anthracite.1

While factors like duration and pressure are also present during burial, evidence suggests that temperature is the most significant influence. As organic material is buried, it is exposed to the Earth's geothermal gradient, which typically increases with depth. Research indicates that temperatures below 200 °C (about 390 °F) are sufficient to advance coal through its various ranks.1

Conversely, increasing pressure is not considered a primary cause of coalification. In some cases, high overburden pressure may even hinder the process by trapping volatile compounds, such as methane, that must be released for the coal to mature. Therefore, the progressive heating of buried sedimentary layers is the widely accepted mechanism for the conversion of vegetable matter into coal.1