Serpentinite Presidio of San Francisco National Park Service
Serpentinite is a rare rock type that is most often developed in subduction zones where one tectonic plate is subducted under another The serpentinite at the Presidio is probably made from rock scraped off the bottom of ocean crust that is found east of the subduction zone These deep ocean crust and upper mantle rocks were then altered by pressure and hot fluids that circulated through them
