The Science Behind The Black Sands At Ocean Beach

The Science Behind The Black Sands At Ocean Beach
A woman walks on the black sands on Ocean Beach, with regular sand peeking through. (Photo: sanguinetangox/Flickr
By Fiona Lee - Published on April 11, 2016.

Over at SFGate, Amy Graff asks an intriguing question: "What's with the black sands of Ocean Beach?" 

Black sand is commonly found on Ocean Beach and other beaches in San Francisco, including Fort Funston and Baker Beach. But because it only shows up during certain times of the year, theories have abounded, with locals arguing that the sands are volcanic, that they come from oil slicks from tankers passing through, or that the beach is simply just dirty. 

As it turns out, the sand is largely composed of an iron ore called magnetite, which, as its name suggests, is highly magnetic. It's derived from the erosion of igneous rocks in the Sierra Nevadas. 

"Most of the sand at Ocean Beach is from this source," the Exploratorium's Ken Finn told SFGate. "[It's] carried here by the tributaries of the San Joaquin and Sacramento rivers, which meet in the delta and flow through San Francisco Bay and out the gate. The prevailing ocean current helps push this existing sediment flow south, and wave action pushes it onto Ocean Beach."

"Black sand can be seen as a layer on top of silica sand in regions with high wave energy," notes The San Francisco Dog Walker "This weight enables it to remain when high-energy waves wash the lighter sand grains out into the surf zone."

For amateur geologists, the black sands can also provide a unique opportunity to study the minerals that comprise them. "Doris Sloan, considered by some the dean of Bay Area geology, suggests bringing along a hand lens or magnifying glass the next time you go to Ocean Beach," writes Paul Judge in the forums of the Western Neighborhood Project. "You’ll see tiny polished grains of clear quartz, opaque milky feldspar, and fragments of red chert, green serpentine, black magnetite and an occasional pink garnet."

But even if you think you see some tar on the beach, it's probably just sand, Judge notes. "Since magnetite is denser than the other material, it sometimes concentrates into delicate ribbons of black sand that might be mistaken for evidence of oil or tar."