Almost everyone has heard of the 2021 Marshall Fire that burned in Boulder County, Colorado. It killed two people, destroyed 1,084 homes and 7 businesses, and covered 6,026 acres. Originally it was suspected that the old Lewis Coal Mine fire, which has been burning for more than 50 years, was the ignition point. (Ultimately it was determined that a neglected trash fire and a downed power line caused the Marshall fire.)
I wondered then and now how a coal fire can continue to burn underground for so many years. After all, fires need oxygen to burn, don’t they?
Searching for an explanation, I found this from 2002:
One of the wildfires burning in Colorado was started by flames from an underground coal fire that “may have been burning since 1910.” How can a fire burn underground for 92 years, and why hasn’t anyone put it out before now? Underground fires usually begin when a coal seam juts up through the ground’s surface. The coal can be ignited in three ways: by human accident, by lightning, or by spontaneous combustion—the process by which the explosive gases contained in coal combine with oxygen and heat up to the point where they burst into flame. (This process typically starts underground where the heat can’t be dissipated into air.) When the seam ignites, the flames spread to burn the adjacent, underground coal.
But fire needs oxygen to burn. So what keeps underground fires burning for decades? Once a portion of the coal has burned, it turns to ash. Since the ash can’t support the weight of rock layers above, the layers buckle, creating cracks and crevices where oxygen can get through and rejuvenate the fire. Underground fires are also sustained by mineshafts, which provide a steady stream of oxygen to the inferno.
Why don’t forest rangers just put out underground fires? First, they have to find them. Underground fires are hard to pinpoint from the surface, and rangers often need satellite equipment (not yet widely available) to properly do the job. If they manage to find the fire, a special foam called cellular grout can be pumped into the ground to fill up holes and keep oxygen out. Unfortunately, cellular grout is prohibitively expensive, and it’s difficult to plug all the leaks. Even good old-fashioned water is useless. Coal fires can reach temperatures of 1200 degrees Fahrenheit, so water dumped on them evaporates instead of putting them out.
Although it was determined that the Lewis Mine fire was not to blame, officials have decided recently to take action because it has begun burning closer to the surface and threatens the integrity of a water supply conduit. Mitigation efforts will include excavating to a depth below the burning coal seam, some 30 feet, over an area as large as 1.5 acres, then turning and mixing the smoldering coal with soil and rock and, if necessary, dousing it with water. Finally, the excavated material will be graded and restored to match the surrounding area.
Coal seam fires are relatively common in mineral-rich western states. In 2019 it was estimated that Colorado had more than three dozen coal seam fires burning underground in abandoned mines.
The world’s longest burning fire is thought to be the Burning Mountain fire in Australia, estimated to have begun some 5,500 years ago.
Featured image: Coal mine fire near Wise Hill in Craig, Colorado, started soon after the mine closed in 1970. (Photo courtesy of Tetra Tech)