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USGS Geology in the Parks

Black Sand

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Black sand beach

Black sand can be seen as a layer on top of silica sand in regions with high wave energy, on the flanks of volcanoes, and in areas where most of the source rock is mafic, or dark-colored and poor in silica. It can be composed of a number of different dark minerals - most are iron-rich and heavier than silica sand. This weight enables it to remain when high-energy waves wash the lighter sand grains out into the surf zone.

Lava meets the oceanVolcanoes such as Hawai'i's Mauna Loa produce lava flows that come into contact with the cold ocean water, as shown at right. The extreme difference between the ocean temperature and the temperature of the flowing lava cause the lava to fracture into tiny shards of black glass. This glass is collected by waves into beaches. Lava flows also harden and form layers like the cliffs seen in the image above. These are eroded by wave action and are broken into grains and clasts which are included in the black sand.

An example of Gabbro Mafic rocks such as basalts (like those produced on Mauna Loa), gabbro (an intrusive igneous or plutonic rock), and other dark-colored low-silica rocks will erode down to components that are also dark. The serpentinized basalt at left is a metamorphic rock - basalt that was pressurized and subjected to high heat. The west coast has a great deal of this type of rock, and much of the black sand found on beaches in that region come from rocks like this one.

Magnetite sand

Some dark-colored minerals, like magnetite, are magnetic - take a magnet with you to the beach and run it through the sand, magnetite will stick to the ends. You might want to cover it with plastic first, the magnet will still work, and it's easier to get clean. The dark minerals in beach sand at right, from Fort Funston, (Golden Gate National Recreation Area, California) 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. This is an ideal locale type to try out a magnet. The other grain types in this particular image are primarily multi-colored chert, a type of silica.

Waves crashing against bluffs

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