How can igneous intrusions change bituminous coal into anthracite?

Igneous intrusions transform bituminous coal into anthracite through the process of thermal metamorphism. When molten igneous rock forces its way into coal-bearing strata—often forming structures like sills—it subjects the surrounding coal to intense, localized heat that exceeds the temperatures typically found in sedimentary basins.1

For this conversion to occur, the coal must be exposed to temperatures ranging from approximately 170 to 250 °C (340 to 480 °F). This extreme heating drives off volatile matter and increases the relative concentration of fixed carbon, effectively upgrading the coal to a higher rank. This process is a primary example of how geological heat sources can drive coalification beyond the levels achieved by standard burial and geothermal gradients alone.1