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USGS Geology in the Parks

Green Sand

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Steeply eroding slopeGreen sand beaches are very rare, but two popular ones lie within the US: one on Hawai'i, one in the territory of Guam. Green sand is composed primarily of olivine crystals which erode out of basalt (lava) flows. The crystals are heavier than most sand types on the beach and remain behind when lighter sand grains are washed away by strong wave activity.

Mineral specimen of dunite

Olivine is a silicate mineral that contains iron and magnesium. These density the of the mineral, a two elements, make it heavy. Olivines form at high temperatures and are green and glassy. They are common in basalts, especially those found of the ocean-floor, and in ultramafic rocks. Rocks made up entirely of olivine, such as the example at right, are called dunite, and gem-quality olivines are called peridot.

Take a short field trip to Hawai'i's green sand beach

Home > Barrier Island Intro > Flat Coast > Wetlands > Lagoon/Sound > Dune > Beach


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