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About Sand

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black sand

Hawaiian black sand is a lithic sediment, made of tiny rock bits

Photo (c) Andrew Alden, licensed to About.com (fair use policy)

Sand is everywhere; in fact sand is the very symbol of ubiquity. Let's learn a little more about sand.

Sand Terminology

Technically, sand is merely a size category. Sand is particulate matter that's larger than silt and smaller than gravel. Different specialists set different limits for sand:

  • Engineers call sand anything between 0.074 and 2 millimeter, or between a U.S. standard #200 sieve and a #10 sieve.
  • Soil scientists classify grains between 0.05 and 2 mm as sand, or between sieves #270 and #10.
  • Sedimentologists put sand between 0.062 mm (1/16 mm) and 2 mm on the Wentworth scale, or 4 to –1 units on the phi scale, or between seives #230 and #10. In some other nations a metric definition is used instead, between 0.1 and 1 mm.

In the field, unless you carry a comparator with you to check against a printed grid, sand is anything big enough to feel between the fingers and smaller than a matchhead.

From a geological viewpoint, sand is anything small enough to be carried by the wind but big enough that it doesn't stay in the air, roughly 0.06 to 1.5 millimeters. It indicates a vigorous environment.

Sand Composition and Shape

Most sand is made of quartz or its microcrystalline cousin chalcedony, because that common mineral is resistant to weathering. The farther from its source rock a sand is, the closer it is to pure quartz. But many "dirty" sands contain feldspar grains, tiny bits of rock (lithics), or dark minerals like ilmenite and magnetite.

In a few places, black basalt lava breaks down into black sand, which is almost pure lithics. In even fewer places, green olivine is concentrated to form green sand beaches.

The famous White Sands of New Mexico are made of gypsum, eroded from large deposits in the area. And the white sands of many tropical islands are a calcite sand formed from coral fragments or from tiny skeletons of planktonic sea life.

The look of a sand grain under the magnifier can tell you something about it. Sharp, clear sand grains are freshly broken and have not been carried far from their rock source. Rounded, frosted grains have been scrubbed long and gently, or perhaps recycled from older sandstones.

All of these attributes are the delight of sand collectors around the world. Easy to collect and display (a little glass vial is all you need) and easy to trade with others, sand makes a great hobby.

Sand Landforms

Another thing that matters to geologists is what the sand makes—dunes, sandbars, beaches.

Dunes are found on Mars and Venus as well as Earth. Wind builds them and sweeps them across the landscape, moving a meter or two per year. They are eolian landforms, formed by air movement. Have a look at a desert dune field.

Beaches and riverbeds are not always sandy, but those that are have a variety of different landforms built of sand: bars and spits and ripples. My favorite of these is the tombolo.

Sand Sounds

Sand also makes music. I don't mean the squeaking that beach sand sometimes does when you walk on it, but the humming, booming or roaring sounds that large desert dunes produce when sand tumbles down their sides. Sounding sand, as the geologist calls it, accounts for some eerie legends of the deep desert. The loudest singing dunes are in western China at Mingshashan, although there are American sites like the Kelso Dunes in the Mojave Desert, where I have made a dune sing.

You can hear sound files of singing sand at Caltech's Booming Sand Dunes research group site. Scientists from this group claim to have solved the mystery in an August 2007 paper in Geophysical Review Letters. But surely they have not explained away its wonder.

The Beauty and Sport of Sand

That's enough about the geology of sand, because the more I poke around the Web the more I feel like getting out to the desert, or the river, or the beach.

Geo-photographers love dunes. But there are other ways to love dunes besides looking at them. Sandboarders are a hardy bunch of people who treat dunes like big waves. I can't imagine this sport growing into a big-money thing like skiing—for one thing, the lift lines would have to be moved every year—but it does have its own journal, Sandboard Magazine. And when you've perused a few articles, you may come to give sandboarders more respect than the sand miners, offroaders and 4WD drivers who threaten their beloved dunes.

And how could I ignore the simple, universal joy of just playing with sand? Kids do it by nature, and a few continue to be sand sculptors after they grow up, like the "Earth artist" Jim Denevan. Another group of pros on the world circuit of sand-castle contests build the palaces shown at Sand World.

The village of Nima, Japan, may be the place that takes sand the most seriously. It hosts a Sand Museum. Among other things there is, not an hourglass, but a yearglass . . . The townspeople gather on New Year's Eve and turn it over.

PS: The next grade of sediment, in terms of fineness, is silt. Deposits of silt have their own special name: loess. See the Sediment and Soil list for more links about the subject.

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