Basalt

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.