This natural oil seep, at the base of a sea cliff near Santa Barbara, covers the beach sand with an impressive build-up of black asphaltum. Naturally occurring gas seeps, such as this one in Humboldt County, contain methane, a greenhouse gas that contributes to global warming. Natural oil seeps like this one, at Tar Water Creek in the Santa Cruz Mountains, can degrade water quality and endanger wildlife. Oil is a common companion of mercury, the most toxic of natural metals. This chunk of mercury ore, from the abandoned New Almaden mercury mine near San Jose, California, has oil hiding inside tiny vugs and in the area stained brown on the righthand side of the specimen.
Geologists think hot fluids carried the mercury and oil from deep underground.