Basalt at Point Bonita

Pillow basalt arch with waves washing around it at Point Bonita seen from above.

Quick Facts

Amenities

1 listed

Historical/Interpretive Information/Exhibits

Below the suspension bridge to the left is a spectacular stack of pillow basalt rock formations that attracts geologists from around the world. The tunnel and trail you just walked is another example of pillow basalt, an igneous rock that only forms when lava erupts underwater.


The rapid cooling of lava by cold water forms the pillow-shaped bodies, which often break open and release more lava from inside. The volcano that erupted these rocks did so far out in the Pacific Ocean over 190 million years ago. Continental drift brought them to this place millions of years ago.