• East view from Crissy Field overlook with old Coast Guard station on left and city on right

    Presidio of San Francisco

    California

Basalt

Basalt exposure

Named for the pillow shapes formed by rapid cooling in underwater lava flows, these pillow basalts found on north Baker Beach at the Presidio, formed in the deep ocean at a mid-ocean ridge in the central Pacific
basin.

Will Elder, NPS

Basalts are a type of volcanic rock and are the most common crust rock. Volcanic rocks are extruded above land or the seafloor, resulting in very fine-grained textures. Basalts have less silica and more iron than other volcanic rocks and tend to be black in color. The Franciscan Complex contains pieces of oceanic crust that include basalts erupted at mid-ocean ridge and island arc settings in the Pacific.

Many times these Franciscan basalts have been metamorphosed by hydrothermal activity into a greenish rock known as greenstone.

Learn more about pillow basalt.

Did You Know?

Crissy Field around 1940

During 1941 and 1942, Japanese-American language specialists were trained at the Military Intelligence Service Language School at Crissy Field.