Igneous intrusions and extrusions occur in many coal-bearing deposits throughout the world. Dikes, sills, volcanic flows, and tuffs are the most common igneous units associated with coal deposits exploited for coal and CBM.
Igneous intrusions, such as dikes and sills, interrupt the continuity of coal seams and, occasionally, can limit mine development. They also can create barriers to the migration of methane and, therefore, restrict the size of coal reservoirs. Dikes are steeply-dipping, tabular, discordant igneous units, whereas sills are concordant with bedding (Pictures 1, 2, 3, 4, and 5).