Definition and formation: Coal balls are calcareous masses of fossil peat found in coal beds. They are formed in the original peat before it undergoes coalification (DeMaris and others, 1983; Scott and others, 1996). Individual coal balls can be inches to many feet in diameter, and coal-ball clusters may occupy a small part (concentrated along the top or bottom) or the entire height of a seam. In one eastern Kentucky mine, an operator reported finding “real wood” in a seam, which turned out to be well-preserved fossil wood in a coal ball. In the Illinois basin, coal balls are commonly pyritized, and sometimes stained red from iron, so are sometimes termed “red rock.” Silica-rich coal balls have also been reported in a southern Illinois coal mine and may occur in western Kentucky (Nelson, 1983).

Typical appearance of coal balls (brown masses) in an underground coal mine in the Springfield (Western Kentucky No 9) coal bed in Kentucky.
Small coal balls concentrated in the Herrin coal bed in an underground mine. The coal balls have a brown to reddish hue, and are locally termed “red rock.” The blue band is a regional parting in the coal. Photograph courtesy of John Popp.


Discontinuities and obstacles: Individual small coal balls can be mined through without significant issue, but larger coal balls or concentrations of coal balls can slow mining, because they are hard and wear down bits on continuous-miner machines. In surface mines, large coal-ball masses or concentrations may need to be blasted (see, for example, Nelson, 1983). Encountering large coal-ball masses during mining is similar to mining through limestone.

Potential roof-fall hazards: Coal balls occur in coal seams and not roof rocks, so are not roof-fall hazards. Isolated rock masses can also occur in mine roofs, but are discussed under Concretions. Coal balls can also be associated with cutouts and roof rolls (paleochannels), which are discussed separately.

Trends: Most coal-ball occurrences do not follow predictable trends; however, concentrations of coal balls generally only occupy small areas, so if they do pose a local obstacle, they can usually be easily bypassed. In at least one case, coal balls were encountered near a sandstone paleochannel (cutout) margin, so might have followed the channel trend (Greb and others, 1999).

Known Kentucky occurrences: Coal balls are very rare in eastern Kentucky mines, but many have been reported in western Kentucky (Greb and others, 1999). They are most common in the Springfield (W. Ky. No. 9) and Herrin (W. Ky. No. 11) coal beds (Krausse and others, 1979; Nelson, 1983).

Coal balls (brown masses) in an underground coal mine in the Amburgy coal bed, eastern Kentucky, and details of samples showing permineralized woody tissue (brown) in coal (black).

Planning and mitigation: When mining the Springfield and Herrin coals, miners can be made aware of what coal balls are and what they look like. Because they normally don’t follow a trend, they are hard to plan for.

Last Modified on 2018-06-22
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