A landslide in Tertiary marine shale with superheated fumaroles, Coast Ranges, California

In August 2004, a National Forest fire crew extinguished a 1.2 ha fire in a wilderness area ~40 km northeast of Santa Barbara, California. Examination revealed that the fire originated on a landslide dotted with superheated fumaroles. A 4 m borehole punched near the hottest (262 °C) fumarole had a maximum temperature of 307 °C. Temperatures in this borehole have been decreasing by ~0.1 °C/d, although the cooling rate is higher when the slide is dry. Gas from the fumaroles and boreholes is mostly air with 3–8 vol% carbon dioxide and trace amounts of carbon monoxide, methane, ethane, and propane. The carbon dioxide is ¹⁴C-dead. The ratios of methane to ethane plus propane [C₁/(C₂ + C₃)] range from 3.6 to 14. Carbon isotope values for the CO₂ range from −14‰ to −23‰ δ¹³C. ³He/⁴He values range from 0.96 to 0.97 times that of air. The anomalous heat is interpreted to be due to rapid oxidation of iron sulfide augmented by combustion of carbonaceous matter within the formation.