Large boulder of serpentinite embedded in sheared serpentinite matrix at North Baker Beach.

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.

Since it is derived from mantle material, the chemistry of serpentinite is unlike that of most rocks in the earth's crust. Serpentinite rock is mostly composed of a mineral called serpentine. This mineral is low in potasium and calcium, both of which are plant nutrients, and also contains high levels of potentially toxic elements such as magnesium, chromium, and nickel. Plants that live on serpentinite are adapted to survive in these unusual chemical conditions.

Because serpentinite outcrops form small isolated areas of unusual soil chemistry, endemic plants adapted to these chemical conditions, and the microclimates of a particular outcrop, often evolve there. Several of the Presidio's endangered plant species (Presidio clarkia and Raven's manzanita) are endemic species adapted to the conditions developed on the serpentinite outcrops here.

Serpentinite boulder on beach showing asbestos vein network. Fibrous asbestos is one of the forms of the mineral serpentine.