Plumes of smoke rising from the cliffs near Hope Ranch on Thursday prompted a response from local fire crews — and renewed confusion over what many residents refer to as the “Hope Ranch volcano.”

For the record: There are no volcanoes in Santa Barbara County.

What does exist is a recurring geological phenomenon along the bluffs, one that firefighters say typically requires little intervention.

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According to research from Jim Boles, the cliffs contain layers of iron sulfides and hydrocarbons. When landslides or erosion expose those materials to oxygen, a chemical reaction generates heat. Under the right conditions, that heat can build to the point of ignition, producing smoke — and occasionally small brush fires.