In the rough waters beneath the iconic Point Bonita Lighthouse sits a 180-million-year-old rock that geologists from all over the world travel to marvel at. “It is a prime example of pillow basalt,” said Marin Headlands Park Ranger James Manzolillo. 

Pillow basalt is a volcanic rock that forms when lava erupts on the ocean floor, then the rapid cooling of that lava from the cold seawater forms its unique, trademark pillowy structures.  

“Sometimes gas gets trapped inside of the rock—as it’s forming so quickly—and within those gas bubbles other minerals can form,” added Manzolillo, explaining that carnelian is a red-orange mineral commonly found within pillow basalt.  

The pillow basalt at the Point Bonita Lighthouse has another feature that excites visitors: it resembles the head of an elephant! “When the waves crash over it, it looks like water is dripping out of an elephant's mouth,” said Manzolillo. "It’s a really picturesque and stunning example of pillow basalt." 

Tip: To see the pillow basalt, walk across the Point Bonita Lighthouse bridge and look down and to the left. It’s in the water on the side of the lighthouse facing San Francisco.  

Plan your own visit to the Point Bonita Lighthouse to see the pillow basalt by checking out our Point Bonita Lighthouse page. Reminder: be mindful of seasonal open hours!