The strange phenomenon of burning landslides

The Los Angeles Times today carries an interesting story about a landslide in Ventura County, to the north of Los Angeles. Ventura County is usually brought into focus by the La Conchita landslide, currently the subject of a court hearing, but in this case the issue is rather different. In the words of the paper:

"A patch of land in Ventura County's section of Los Padres National Forest where the ground recently heated up to 812 degrees continues to puzzle firefighters and geologists after weeks of monitoring. "It's a thermal anomaly," said Ron Oatman, spokesman for the Ventura County Fire Department. Firefighters responded to reports of a blaze there a month and a half ago, when observers noticed smoke rising from the parched scrub. But when they arrived, they found no flames...The hot spot is in an area considered to be an active landslide that has shifted for more than 60 years. Several hundred feet below its cracked surface lie pockets of gas, tar and oil."

The suggestion is that the landslide has opened cracks and fissures in the ground that have penetrated down to a pocket of hydrocarbon, which has now spontaneously started to combust. This probably sounds a little weird and surprising, but interestingly there are quite a few reports of such events from around the world. The area in which this occurs with which I am most familiar is Dorset on the south coast of England. Indeed, near to the town of Ringstead in Dorset there is a coastal landslide known as "Burning Cliff" (Fig. 1).

Fig. 1: Burning Cliff in Dorset as shown on Google Earth.

Perhaps the most famous example however is that of the "Lyme Volcano", which occurred on the Black Ven landslide (see this previous post). This well-recorded event occurred in 1908 when a mound of material on the landslide, probably representing a rock fall deposit, smoldered over several months. It became a sufficiently important tourist attraction that when the heat and smoke started to diminish the local sought to maintain it by throwing paraffin onto it.

So what is the cause of this rather strange phenomenon? In general these events always occur in deposits containing reasonable amounts of hydrocarbon - in the case of the Ventura County event above the landslide is located in a production onshore oil field. In the case of Black Ven Dorset, the Liassic rocks of Black Ven contain about 7% organic materials and represents the source rock for the Wessex Basin. In Ringstead, the Kimmeridge Clay is the cause - this is the main source rock for North Sea oil, containing as much as 20% organics. Whilst I am not sure what the source would be of the ignition for the Ventura County event, in Dorset this is provided by oxidation of the abundant iron pyrites in the shale. Thus, when the landslide occurs the pyrite is exposed. This oxidises, producing heat that then ignites the hydrocarbon in the cliff.

If you are interested in this strange phenomenon then can I recommend the following web page by Ian West of Southampton University, which provides a very details account of such events in Dorset:

West, I. undated. Burning Cliffs of Dorset. http://www.soton.ac.uk/~imw/kimfire.htm