December 2008 Geology and GSA Today media ...(ll impact craters. A landslide in Ter...)

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

Robert H. Mariner et al., U.S. Geological Survey, MS 434, 345 Middlefield Road, Menlo Park, California, 94025, USA. Pages 959-962.

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 (gas vents). A temperature of 262 C (504 F) was measured in one of the superheated fumaroles. A temperature of 307 C (645 F) has been measured at a depth of about 3 meters (~10 feet) in a borehole near this fumarole. Temperatures in this borehole are decreasing by ~0.1 C/day. Gas collected from the fumaroles and boreholes is mostly air with 3% to 8% carbon dioxide and trace amounts of carbon monoxide, methane, ethane, and propane. The chemical and isotopic composition of the gas show that it is not associated with volcanic activity, a geothermal system, an oil field fire, or spontaneous combustion of modern organic material incorporated in the slide. Pyrite (iron sulfide) and fine-grained carbonaceous matter (essentially small fragments of low-grade coal) typically occur disseminated in this early Tertiary sandstone-shale sequence. However, local concentrations of secondary minerals normally produced when pyrite oxidizes show that high concentrations of pyrite do occur in this formation. Mariner et al. speculate that pyrite in the jumbled blocks of the Juncal shale, involved in the landslide, oxidized rapidly when air was introduced during the initial landslide movement, and this heated the rock enough to ignite dispersed solid carbonaceous matter present in the shale.

Diachronous dawn of Africa's Middle Stone Age: New 40Ar/39Ar ages from the Ethiopian Rift

Leah E. Morgan and Paul R. Renne, Department of Earth and Planetary Science,
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