Posted by The San Francisco Dog Walker
So, what is that black stuff on the beach in Fort Funston? Is it oil left over from the Cosco Busan spill? No, in fact it is the chalky substance-veins of volcanic ash thrown from Mount Lassen thousands of years ago. After all of our heavy storms this winter, most of the top layer of silica sand had been blown or washed away to leave dark colored streaks on the beaches. This black sand is a kind of iron ore called magnetite. The magnetite is a part of the cliff walls, and as the sandstone erodes, it leaves the heavier iron-based magnetite on the beach.
Black sand can be seen as a layer on top of silica sand in regions with high wave energy. This weight enables it to remain when high-energy waves wash the lighter sand grains out into the surf zone.
Take a magnet with you to the beach and run it through the sand, magnetite will stick to the ends. The dark minerals in beach sand at right, from Fort Funston are primarily magnetite and amphiboles, which are non-magnetic black minerals. Both of these mineral types tend to fracture into very small grains that collect on the surface of the sand, by virtue of being smaller and, therefore, lighter.