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Geo-Joint: Underground Fires

Posted on October 24 2017

A forest fire is a huge spectacle, as towering trees explode in flame, and thick smoke billows away from the roaring inferno. Though they sometimes burn for weeks or months, the hard work of fire crews, retardant-dropping airplanes, or the change of seasons that brings rain and snow eventually wins out over the flames. Not so with peat fires. Peat is a deposit of preserved organic material that collects in bogs or on forest floors to a depth of a few feet to as much as 33 feet. Formed in poorly drained depressions in wet climates, this accumulation of peat moss over decades or longer is a rich store of carbon. People have long cut material from bogs and dried it for fuel. A brick-sized chunk can burn for an hour. As long as these features remain sufficiently wet, they are very stable and have been known to preserve the bodies of people who have died in them and been buried under many years of accumulated mosses and other debris.

Peat also forms on rainforest floors after long years of accumulation of fallen tree leaves and branches, whole trees, vines and other plant materials. Decomposition occurs much faster in the warm environment of a tropical forest, but there is a rich and continuous supply of organic detritus. Mosses and ferns, which can thrive in the low-light conditions, are the major component of the living forest floor community, similar to that in a bog. In either case, there is abundant water and if undisturbed, the chemistry of the environment perpetuates itself, building up the peat layer.

Trouble comes when the moisture content of a peat bog drops to a lower level as a result of some outside factor. A change of climate or shifting drainage patterns can desiccate the peat. If in that drier state a fire is started by accident or design, the effects can be long-lasting. Peat fires are not scenes of leaping flame but rather, slow smoldering. They can burn very deep, making extinguishing them quite difficult. Sometimes a large area must be flooded with the water from a diverted stream in order to smother the burning material. Bog fires can burn for months, or years. In the rainforest, the peat below the trees is not in danger of fire until the forest is cut down. As the cover is removed, the land may dry out. Or, as is common, the peat areas are trenched and drained by agriculture interests hoping to start a plantation for palm oil on clearcut land. But fire here is often at hand. One of the frequently used methods of clearing the unused biomass left over from a massive logging operation is to burn it off. If the peat underlying the burning cut vegetation catches fire, a peat fire of enormous dimensions may result. These fires burn slow and hot and release not only huge amounts of CO2, but also methane, a potent greenhouse gas. Some peat fires can only be defeated by a strategy similar to cutting lines around a forest fire and letting the interior burn. Trenches are cut down to bedrock all around the burning peat, and once the encircled material is burned, the fire is over. However, this is an expensive and difficult method.

Peat bogs only go so deep, but there is another kind of underground fire that can burn very far below the surface. These are coal seam fires. They can start by natural means, such as a lightning strike where a coal deposit reaches the surface, but much more likely points of ignition are in mines. Mine shafts also provide access to air, which facilitates burning. Coal fires are known to burn for decades, such as the one in Centralia, Pennsylvania, started in 1962. A fire at a waste dump spread to a surface outcrop of a coal seam, and moved to an abandoned coal mine. It was soon out of control, and continued to burn under the town of Centralia, causing the expensive relocation of almost all the town’s residents. It is still burning today, and is predicted to continue for at least 250 years.

But that quarter-millennium is nothing compared to Burning Mountain, or Mt. Wingen, in New South Wales, Australia. The coal fire there has been smoldering for an estimated 6,000 years. Early settlers thought the smoke and heat were coming from a volcano, but it was merely a smoking hill. Now burning at a depth of around 100 feet, it moves at a speed of about 3 feet per year, changing the appearance of the surface as it goes. Vegetation dies off as the heated ground and noxious gasses approach, leaving chemical deposits on the rock and creating cracks and areas of subsidence. How it began is of course unknown, but possible causes are lightning, internal combustion, or grass fires either natural or started by Aboriginal Australians.

Some people are actively trying to set fires in coal seams. However, their aim is to produce energy. Underground coal gasification (UCG) is a method of accessing the energy bound up in coal deposits that are too deep to mine. These deposits are below drinkable groundwater stores, usually deeper than 1,200 feet. Some UCG operations inject chemicals into coal deposits to free up gases, and others utilize combustion. In these, two wells are drilled into a deep coal seam, one to deliver air or pure oxygen and an igniter, and the other to draw off the emerging gas and impurities as the coal burns. Sounds simple, but the process has its problems. Some of that stuff that comes up with the usable gas is pretty noxious material, and there can be land subsidence as the coal seam is reduced to ash. Uncontrollable, long-term burning does not appear to be an issue, however, as sealing the air intake well and flooding the whole operation with saline groundwater snuffs out the deep fire. The process has enough drawbacks, including excessive greenhouse gas production, that it hasn’t taken off in a big way yet.

So the fires down below burn in a number of different forms, some more hellish than others, but all distinctively different from most of the conflagrations we see at the surface.

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