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How big is a Grain of Sand?

500 micrometres!

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Grain of Sand

A grain of sand is a small particle of rock, mineral, shell or coral. Geologists define sand by size alone: any grain from 0.0625 to 2 millimeters across. The explorer shows a grain 0.5 millimeters, or 500 micrometers, across, the size that separates medium sand from coarse sand. A micrometer is a millionth of a meter.

Size

The size limits come from a grade scale that C. K. Wentworth published in 1922, and geologists, including the U.S. Geological Survey, have long used it as the standard. Grains from 0.0625 to 2 millimeters across are sand. Smaller grains are silt, down to about 0.004 millimeters, and anything smaller is clay. Larger grains are gravel.

The scale splits sand into five grades, each twice as wide as the one below it: very fine, fine, medium, coarse and very coarse. Medium sand is 0.25–0.5 millimeters across, and coarse sand is 0.5–1 millimeter. To measure a sample, geologists shake it through a stack of sieves, with the widest mesh at the top. Each grain stops at the first mesh too fine for it to pass.

Size comparisons

  • A grain 0.5 millimeters across is about five times as wide as a human hair.
  • About 130 grains lined up in a row would stretch across a tennis ball, which is about 6.7 centimeters (2.6 inches) wide.
  • A grain of table salt is about the same size, and a 0.7-millimeter mechanical pencil lead is 1.4 times as wide.
  • If a 0.5-millimeter grain were enlarged to the size of a basketball, 24 centimeters (9.4 inches) across, the largest grains of silt would be 3 centimeters (1.2 inches) across. The largest grains of sand would be about 1 meter (3 feet) across.

What sand is made of

Mineral sand forms as rock breaks down through weathering, and water and wind carry the grains away. Granite, the rock that makes up most mountains on the continents, breaks down into grains of quartz, feldspar, mica and magnetite. Quartz is hard-wearing and abundant, so the sand on most beaches along the coasts of the continental United States is quartz sand.

Basalt, the dark rock of lava flows and volcanic islands, contains no quartz. It wears down into black, gray or brownish red grains. Papakōlea Beach on Hawaii's Big Island has sand made of olivine, a shiny mineral found in basalt that ranges from olive green to almost brown.

Other sand comes from living things. Coral skeletons, shells and sea urchin spines are made mostly of calcium carbonate. When the animals die, these hard parts are worn into sand by tumbling waves and by the grinding of animals such as parrotfish.

Why grain size varies

The size of the grains on a beach reflects the waves that shaped it. Beaches exposed to strong waves usually have larger grains than calmer ones. Steep beaches also tend to have larger grains, because waves can throw large particles high up the slope. On flatter beaches, grains are rolled back and forth and broken into smaller pieces.

Sources

Scale of the Universe > Grain of Sand
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