"Serpentinite is a rare rock type that is developed only in subduction zones, where one tectonic
plate is subducted under another. This 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 in the subduction zone and on their way to the surface. They are now at the Earth's surface
because the region has experienced uplift after subduction ceased, and because these altered rocks are now lighter than surrounding rocks and are therefore rising to the surface."