When smoke issues from the rocky cliffs at Hope Ranch, as it does every few years, the immediate thought is "Volcano!" or, for the cognoscenti, "Solfatara!" Well, both are incorrect, said Jim Boles, a professor emeritus at UC Santa Barbara who specializes in geology. It's been about 30 million years since Santa Barbara saw any volcano action.
The correct answer lies in the folds of uplifted earth visible at the cliffs from the collision of tectonic plates over the eons. Compressed in those layers are the ingredients for spontaneous combustion — iron sulfides and hydrocarbons — when oxygen is added.
Tiny flecks of minerals like pyrite — fool's gold — and marcasite are among the iron sulfides trapped in the layers of rock underlying coastal Southern California. As the Hope Ranch cliff face slides from time to time, fissures open that expose the iron sulfides to oxygen. In reacting to the oxygen, they give off heat. And if hydrocarbons are present — generally the offshore tars among the layers of uplift that created the cliffs — it can all go poof!