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Readers Want to Know: Why is This Sand Black?

A curious beachgoer brought a small bag of sand into Minnesota Sea Grant this fall. It was black, fine and from Lake Superior's South Shore where eroded sandy-colored sandstone and glacial till typically dominate the scene. The question that came along with the sand was, "Why is it black?"

Bands of jet-black sand commonly get washed to near the high-water line along many beaches in the Great Lakes region. Howard Mooers, who holds the prestigious Robert L. Heller Professorship of Geology at the University of Minnesota Duluth, said that most of this black sand is magnetite that was ground out of the region's bedrock by glaciers during the last ice age and by tens of thousands of years of weathering and water-mediated erosion.

Magnetite is heavy and ferromagnetic, meaning it is attracted to a magnet. In fact, magnetite is the most magnetic of all the naturally occurring minerals on Earth. Next time you go to a Great Lakes beach, take a magnet and see for yourself!

There are other black sands around Lake Superior, including the sand on Black Beach in Silver Bay, Minnesota. This beach, which opened to the public in 2015, is a testament to the industry that created Silver Bay itself. It's made of unusable taconite tailings that were once added to the lake. Non-magnetic black sand can also be found along Lake Superior's Keweenaw Peninsula. In this region of northern Michigan, stamp mills crushed basalt rock to extract copper in the 19th and early 20th centuries. The resulting black sand deposits may contain trace amounts of harmful heavy metals and may be hazard to humans and other species.

By Sea Grant Staff
December 2016

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