Can a landslide spontaneously combust? It can, if it contains the right kind of rock.

In August 2004, fire crews attending a wildfire near Santa Barbara, California, traced the source of the blaze to a recent landslide, but they had no idea how the fire got started.

A few weeks later, Robert Mariner of the US Geological Survey in Menlo Park, California, and his colleagues visited the site. They found that the temperature of the rocks in the landslide must have reached just over 300 °C - hot enough to start a fire.

By measuring the temperature and composition of the air from boreholes, they were able to rule out the possibility of a geological ignition from volcanic activity or seepage of flammable natural gas, so it appears that a chemical reaction in the rocks must have been responsible. They believe that the landslide exposed a mineral called pyrite to the air, causing an oxidation reaction that heated a nearby seam of low-grade coal to the temperatures they inferred.

Ian West at the University of Southampton, UK, thinks that landslide fires may be more common than we realise. "There have been a few along the UK's Dorset coast in the last few hundred years, and there are records of a huge fire in the Dead Sea area, dating from King Solomon's time, which may have started this way," he says.

Journal reference: Geology, DOI: 10.1130/G25285A.1