Blueschist

Blueschist is a distinctively Californian rock that forms by the metamorphism of sodium-rich basic rocks at high pressures and low temperatures. Conditions are good for that in northern California, where marine crust and sediments were subducted beneath the North American plate and kneaded by changing tectonic motions while sodium-rich fluids marinated the rocks. It's a schist because all traces of original structure in the rock have been wiped out along with the original minerals, and strongly layered fabric has been imposed.

Petrologists prefer to talk about the glaucophane-schist facies, because not all blueschist is all that blue. Consider this hand specimen, which displays many different shades. Glaucophane and lawsonite are the major blue species in this rock type, but jadeite, epidote, and quartz are also common. It's hard to tell what's what on this specimen without a microscope.

As a landscaping stone, blueschist is responsible for some striking, even garish effects as in the low wall shown below.

_{(c) 2000 Andrew Alden, licensed to About.com, Inc.}

For more photos see the Metamorphic Rocks Gallery.

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