Rock Talk By Larry Larason
Red Dog Gravel's Fiery Origin
Drive almost any street in Gallup, New Mexico and you see houses with gravel in their yards. Homeowners usually plant stones after they give up on growing grass. Everyone knows that a ton of gravel and a couple of yuccas make a fine New Mexican "lawn." You see pink granite, gray limestone, red cinders, or river pebbles and cobbles. And, around some of the older homes, red dog gravel.
These colorful stones, also called "clinker", are not as popular around Gallup as they once were. I suspect this has to do with lowered supply due to the decline of coal mining, but even gravel seems to go through fashion cycles.
Red dog gravel occurs in irregular lumps that mostly have a red to orange color, although it may sometimes be purple to black. It resembles unglazed ceramics, and, in one sense, that's what it is. Coal forms from plant material that grew in swampy places, so it's normal that shale containing clay may appear in beds above or below the coal. If the coal catches fire, the shale will be baked into a brick-like rock, which has been fused but not melted. However, coal bed fires can generate temperature of up to 1700 degrees C, so in some places the shale actually has melted; the result resembles basalt and may be referred to as "para-lava."
Because it is so colorful, clinker exerts a certain fascination. The ancient Chacoans found it interesting enough to make pendants, gaming pieces, and medicine stones from it. An excavation in Heaton Canyon near Gallup in the 1980s turned up several examples of these objects. One of the pendants had an inlaid piece of turquoise. The red dog they used was soft enough to shape by rubbing it on sandstone.
Natural sub-surface fires in coal beds start where erosion has exposed the coal to air at the surface. Because it often contains sulfur, coal may spontaneously combust under certain conditions. Brush fires or lightning strikes also may ignite it. Presently there are at five such fires on the Southern Ute Reservation in Colorado, one of which has been burning since at least the 1930s. In 1999 a Park Service firefighter working in that area was overcome by hydrogen sulfide gas coming from a vent over the subterranean fire. The tribe spent a million dollars in 2000-2001 to have "grout" poured into vents in an effort to extinguish the fire, but it had no effect.
Wherever there is coal it is likely that there will be evidence of ancient subsurface fires. When Lewis and Clark passed through Montana-Wyoming in the early 1800s, they encountered a river bounded by boulders of clinker. They named it the Redstone River. Later explorers, smelling the smoke from coal fires in the area, were reminded of burning gun powder, and gave the stream its modern name: Powder River. Geologists estimate that between 30 and 50 billion tons of coal have burned in the Powder River Basin to produce the quantity of clinker found there, some of which occurs in beds nearly 200 feet thick.
Coal bed fires are long-lived and mostly impossible to control. Subterranean coal fires are burning all around the world even now, especially in India and China. China's coal bed fires, alone, burn twenty to thirty million tons each year and produce as much carbon dioxide as all the gasoline burning motor vehicles in the United States. The coal fires in India have reduced part of that country to a wasteland. In Australia a coal bed fire in New South Wales, known as Burning Mountain, is estimated to have been afire for 2000 to 6000 years; the first explorers who saw it thought it was a volcano.
The best known sub-surface fire in the United States is burning beneath Centralia, Pennsylvania. The town had to be mostly abandoned after coal in old mine tunnels beneath the town caught fire in 1962. Millions were spent in trying to control the fire, but it is still active. Although a handful of stubborn people still live in Centralia, they face the constant threats of subsidence and poisonous gases, including hydrogen sulfide, which combines with moisture to produce sulfuric acid.
Clinker can be observed at several places near Gallup, but my favorite place to see red dog in its "natural habitat" is along NM 371 north of Crownpoint. You can see the red clinkers decorating hills west of the Tsaya Trading Post, then again in several small badlands ahead. The best place to see it is in the Bisti Badlands, but if you don't turn off for the Bisti Wilderness Area, you can see spots where Red Dog has been quarried out of road cuts around Milepost 71 before the highway dips into Hunter Wash. The coal fires in the Bisti area were extensive; however enough coal remained to sustain several small mines. The entire badlands was slated for mining before the wilderness area was created.
The coal fires in the San Juan Basin and the Gallup region were all prehistoric. If they were burning now the region might be suffering a counterpart of the Asian Brown Haze – a three-kilometer thick cloud of aerosols and particulates that forms in December through May and covers most all of southeastern Asia. This haze is produced by copious use of coal and other organic fuels, as well as smoke from the extensive coal bed fires in India. It was blamed, at least in part, for as many as 60,000 deaths from respiratory failure in just India during the early 1990s. Be thankful that we are not suffering such a miserable situation!