Rocks of the Monterey Formation rich in organic matter have undergone spontaneous subsurface combustion (combustion metamorphism) at many localities, particularly during the Pleistocene. At the best studied locality, the Grimes Canyon area, the burnt-out rock body is 20 km long, 1 to 3 km wide, and at least 400 m thick. The original rocks were mainly mudstones with lesser amounts of diatomites, phosphorites, shales, some cherts, and carbonate rocks. The trace element composition of the parent rocks is that of oil shales; Ba, Cr, Ni, Sb, and Se contents are higher. Rock melting started below 1,000°C. The initial melt (5% of rock volume) is highly differentiated and has a major, minor, and trace element composition indistinguishable from high-calcium granites. These me ts intruded to form sills and dikes, and cooled to obsidian-like glasses. As the temperature rose, the melt changed composition and approached the composition of the parent rocks. These melts formed intrusive stocks and cooled to crystalline rocks resembling, in the field, scoriaceous basalt. At the highest temperatures, in excess of 1,650°C, phosphorites were molten; the resulting melt had the composition of apatite and was immiscible with the coexisting silicate melt, with which it formed an emulsion.

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