BioEd Online

A year after firefighters extinguished a small wildfire in the southern California mountains, geologists are still struggling to understand how a landslide apparently sparked the blaze.

One possibility is that the landslide exposed a mix of minerals to the air, which promptly began oxidizing. This chemical reaction may have given off enough heat to trigger the fire, says Allen King, a geologist with the USDA Forest Service in Goleta, California.

But that explanation remains his top choice simply because the other alternatives - including geothermal heat, radioactivity, and an alien conspiracy - have all been ruled out. Hoping to elicit other insights, King described the mystery to geologists on Monday during a Geological Society of America meeting in Salt Lake City, Utah.

"It's a fascinating story," says forest service geologist Joseph Gurrieri.

The tale began sometime in the past several years, when a three-acre landslide occurred in the Dick Smith Wilderness north of Santa Barbara. Nobody noticed it at the time. But in August 2004, someone spotted a small fire burning in the area and reported it to authorities. Firefighters put it out. Then they tried to extinguish the heat coming from the ground. They kept trying.

Several days later, they called King, the geologist responsible for that region of forest. They told him they couldn't get the dirt to cool down. "I really didn't know if they were just pulling my leg," he says.

Heated debate

King and other geologists visited the site. Someone stuck a thermometer into the ground, and it rocketed to over 200 °C, the highest it could register. Later, more sophisticated measurements revealed temperatures reaching a maximum of just over 300 °C at a depth of 3.5 metres. The ground was slightly cooler at deeper depths, suggesting the heat source was concentrated near the surface.

Firefighters shot an infrared movie of the area, revealing blotches of heat glowing on the landslide. Gas sampling detected carbon dioxide, carbon monoxide, and various sulphur-containing gases. There was a little helium, suggesting that the source probably didn't originate in a deep volcanic chamber. King's originally favored theory - that the heat was coming from geothermal sources - was soon ruled out.

And so King is left with the chemical-reaction idea. The rocks are mostly shale, with bits of pyrite crystals and carbonaceous material. "Our hypothesis is when that comes in contact with oxygen, it generates heat," says King.

Similar reactions can sometimes trigger spontaneous fires inside mines that are being worked. But King says he could not find any examples of such a thing happening in a natural setting.

A meeting of forest service geologists two weeks ago wasn't able to come up with a better idea. So at the Salt Lake convention, King stood hopefully by his poster, waiting for someone to help explain the landslide that started a forest fire.

If you have an answer to King's dilemna, write to us at news@nature.com.