Blueschist - Menlo Park, California - Rock and Mineral Displays on Waymarking.com

This blueschist block is one example of a series of large blocks of rocks from various locations, which are on display around the USGS Campus in Menlo Park. There is a self-guided tour of the campus available at the visitor center.
Text of Sign: “This seven ton monolith of contorted and lineated blueschist from the Franciscan Complex was collected from the Tiburan Peninsula five miles north of San Francisco. Similar blueschist blocks from that locality have been dated at about 160 million years. The rock contains galaucophane, which imparts the blue color, white mica, epidote, sphene, and 2 mm-diameter crystals of red garnet. The minerals comprising blueschist crystallize under rather unique conditions of metamorphism which are: high pressure, from burial at depths greater than about 18 km, and relatively low temperatures near 400° C.”