glass - display specimen of natural glass formed by combustion metamorphism of shale

The most commonly encountered geologic glass is obsidian. Occasionally a lightning strike on a sand dune fuses the sand into a glassy tube called a fulgurite. Vitrophyre, another geologic glass, forms in a volcanic ash bed when the ash layers are thick enough to form an insulating blanket, and the temperature of the extruded ash is high enough to melt it. This glass, formed here in phosphatic layers in shale of the Monterey Formation, is uncommon, though deposits are also known from Israel, Jordan, Russia, Iran, India, Australia and Canada.

Combustion metamorphosed shales in the Monterey Formation form a belt roughly 3 km wide and 20 km long in Ventura County, California. Spontaneous subsurface combustion, probably occurring in the late 19th century, occurred when naturally occurring organic matter, likely petroleum, served as a fuel. Temperatures reached as high as 1600˚C and melted phosphatic layers in the shale that alternate with layers poor in P₂O₅.

The phosphate-poor layers became sintered, but did not melt. The melted phosphatic layers formed pseudomagmas that formed small-scale intrusions such as dikes, sills and laccoliths. The glass is either slaggy and vesicular or more dense and resembling obsidian. This specimen was collected from Grimes Canyon, just south of Fillmore, California and is a wonderfully colorful example of black glass and sintered shale.

The first two photos are of the same specimen.

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