black sand country

Talk among prospectors in the most lively days of the gold rush in the American Southwest often turned to the topic of black sand, for miners believed that the dark mineral on a riverbank or shore was a telltale sign of nearby gold. But they were a bit off in that assumption: iron-rich and heavy black sand does remain in streambeds and beaches while other minerals are swept away by water, and gold—also heavy—stays put while lighter materials are washed off; but the presence of one doesn’t necessarily signal the presence of the other. Though not as valuable as gold strikes, black sand regions hold their own kind of fascination. For instance, the beach at Fort Funston in the Golden Gate National Recreation Area is the ideal place to try a magnet experiment: the dark-colored minerals there (magnetite) will cling to the ends of a magnet that is dipped into the sand. And many visitors flock to Hawai‘i to watch black sand forming along the coastal fringes of the island’s active volcanoes. When lava from Hawai‘i’s Mauna Loa, for example, comes down headlong, like a curtain, into the cold ocean water, some of it instantly “freezes,” reforming itself into shards of black glass. Erosion of the underlying mafic (dark) basalts by wave action produces the sand, rich in iron and magnesium, that is characteristic of Hawai‘i’s black beaches.

Elizabeth Cox