2 December 2008

Burning landslides revisited…

Posted by Dave Petley

Back in August I posted on the strange phenomenon of burning landslides. This is an interesting and surprisingly common phenomenon in which a slope failure exposes materials in the soil or rock that oxidises to generate large amounts of heat. At times this effect can be strong enough to actually ignite – in Dorset in England for example organic matter in the Kimmeridge Clay (which is the main source rock for North Sea Oil) has often ignited after landslides.

In the latest edition of the journal Geology, a paper by Robert Mariner and his colleagues at the USGS at Menlo Park in California report upon a forest fire that occurred about 40 km northeast of Santa Barbara. The fire was quickly extinguished, but subsequent investigations showed the seat of the blaze was a 7 hectare landslide, from which hot gas was being emitted via a series of fumaroles. The team measured the temperatures in some of the fumaroles and also in a borehole drilled near to one of them. The measured temperatures were as high as 262 C in a fumarole and 307 C in the borehole. The ignition temperature for dry grass is 150-200 C, so it is not hard to work out where the fire started!

The team report suggest that pyrite in the shale bedrock was exposed by the slide and oxidised rapidly, generating heat. The temperatures were high enough to ignite carbonaceous matter in the shale, which then burnt to create the fumaroles, which in turn started the forest fire. This view is supported by the isotopic composition of the gases emitted by the fumaroles.

Interestingly, the paper notes that the team are “surprised that the phenomenon is rare because siltstone-shale sequences commonly have pyrite and organic matter and are involved in landslides”, suggesting that maybe this happens more often than we realise.

Overall this is an excellent paper on a slightly esoteric but very interesting topic.

Robert H. Mariner, Scott A. Minor, Allen P. King, James R. Boles, Karl S. Kellogg, William C. Evans, Gary A. Landis, Andrew G. Hunt, and Christy B. Till, 2008. A landslide in Tertiary marine shale with superheated fumaroles, Coast Ranges, California. Geology, 36 (12) 959-62.