They burn for decades

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.

Maureen Sullivan, “How Does a Fire Burn for 92 Years?”, Slate, June 12, 2002

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.

Underground coal fires can burn for decades
The mining town of Centralia, Pennsylvania, was abandoned some 60 years ago because of a fire burning beneath it. The fire still burns today. (Don Emmert/AFP/Getty Images)

Featured image: Coal mine fire near Wise Hill in Craig, Colorado, started soon after the mine closed in 1970. (Photo courtesy of Tetra Tech)

15 thoughts on “They burn for decades

      1. Yes, we have lots of old mine workings here and the problems that the old coal tips have caused over the past 80 or 90 years are awful. They’re not burning, but they move. 🙁

      2. There was a lot of mining done in Colorado back in frontier days, and the old shafts are a constant hazard. Cave-ins, polluted water, people wandering in and getting lost or trapped, etc. And that’s just the old gold and silver mines that don’t burn.

  1. Very interesting. When I came to Boulder more than 50 years ago, I saw smoke coming from that location and always wondered about it. I knew it was from an old coal mine but, like you, I couldn’t figure out how it kept burning.

  2. You have an enquiring mind, Colorado.
    I am happy to note that the photo of the road emitting smoke comes from an abandoned town: but we have some country roads that the image reminds me of ..

    1. I thought about you, Aussie, and figured you must surely know all about your Burning Mountain fire. 5,500 years! Wow!

      It’s sad that a whole town in Pennsylvania had to be abandoned because of one of these fires. But look what it’s done to that road. Sure wouldn’t want that under my house.

  3. This is just so weird – (like molten core, volcanoes, and paint pots are enough naturally, mankind has to add its’ 2 cents…)
    Thanks for the research – has intrigued me for a long time.
    (We’re having brush fires, even massive mulch piles spontaneously catching fire…like trash fires they also burn for long times – not as weird as underground fires though)
    Too hot to think of roasting marshmallows and fireplaces right now…although ski slope openings has been released?
    Whew, at least the thought of that is cooling

    1. I don’t follow ski information, so can’t tell you about that. But the fires everywhere, all kinds of fires, seem to be sending a message this year. And it’s not a good one.

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