Significant volumes of in-situ coal have burned naturally in many coal deposits throughout the world. For example, there is approximately 2,580 km (1,600 mi) of burnt rock derived from the in-situ burning of coal seams in the Powder River Basin of Wyoming and Montana. That represents some 27 to 36 billion tonnes (30 to 40 billion tons) of coal that burned naturally since Pliocene time, and excludes burnt rock that was eroded (Heffern and Coates, 1997). In some deposits, natural combustion is still underway (Heffern et al., 1993).
During the burning process, overburden rock is baked. This produces a thermally-altered rock referred to as red dog, scoria, burn, and, most commonly, clinker (Pictures 1 and 2). An understanding of the origin of clinker provides insight on the extent of mineable coal and clinker resources and helps in resource estimation and design of efficient mine development plans.