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

This orbicular diorite 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 two and one-half ton block of orbicular diorite, from the southern Sierra Nevada, crystallized 100 million years ago. The orbicular structures formed within a vertical pipe which conducted hot magmatic fluids, with suspended rock fragments, along the margins of a cooling granitic intrusion. The rock fragments served as nuclei which became coated with minerals which precipitated out of the magmatic fluid. Eventually, multiple mineral layering created spherical shapes. Note that the irregular shapes are usually closely-packed and touch one another; areas where there are apparently no orbicules, are generally occupied by inconspicuous unrimmed fragments.”