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Prentice K. Stout

For the casual beachcomber sand is something to walk on; for the sun worshiper it is something to lie on; and for children it is castle-building material. Sand particles range in diameter from 0.0625 mm to 2 mm. (Note: a millimeter (mm) is approximately as long as this dash:-.) Not all beaches are composed of fine sand grains. Some, called cobble beaches, are made up of rather large rocks.

Because beaches are composed of the same rocky material as the adjacent land, the color of the beach is determined by the rocks of that area. In Rhode Island, 90 percent of beach sand is composed of quartz, a compound of silicon and oxygen (SiO2). This compound can be colorless, milky white, brown, or light pink. A combination of these colors gives our beaches their characteristic color of light tan. In some parts of the world, such as Hawaii, there are black sand beaches composed of pulverized lava. In Bermuda, coral remains color the sand pink. Sand is rare on the deep ocean floor, which is mainly mud.

Most sand is a product of the weathering of rocks on land areas inland from the beaches. Weathering processes such as rain, freezing and thawing, and chemical reactions break down the rocks into fragments, and streams and rivers transport these particles to the oceans. Once at the seashore, they are moved by offshorecurrents and waves from one area to another. By a process of natural sorting, the heavier rocks will tend to sink to the bottom layer of the beach, while the lighter particles will be found on the top layer.

After each wave strikes the shore, its turbulence lifts the sand into suspension and carries the particles farther along the beach Hence a beach profile is never the same from one minute to the next. Offshore currents can carry sand for miles along a beach and redeposit it. During the relatively calm summer months, gentle wave action deposits more sand on the beach, which is then torn away by winter storms. If there is a layer of stones and rocks below the sand, these will be exposed after the storm. The sand will return and cover the rocks again in periods of calmer weather.

Not only water transports sand-the wind also does its share. Whether carried by wind or water, sand particles are subject to abrasion. You can see the effects of abrasion by looking at sand particles through a hand lens or a microscope; the surface appears frosted where the grains have been worn down. Wind transported sand tends to be more rounded-that is, more worn down by abrasion-than its waterborne counterparts. Why should this be? If you pick up some damp sand, you will notice that it tends to cling together, whereas dry sand does not. The clumping of wet sand is due to a film of water that surrounds each particle. Since water molecules attract each other by a physical force called capillary attraction, the water "envelopes" hold their imprisoned sand grains together with remarkable strength. It is this watery cloak that prevents the sand grains from rubbing together, thus reducing further wearing. Windblown sand, which lacks this watery buffer as well as the weight of the water envelope, tends to abrade much faster and travel farther.

The crevices and angles of certain types of sand grains form a habitat for a diversity of life forms. The watery cushion that surrounds wet sand permits bacteria, algae, and minute animals to inhabit those areas. More life is found on beaches whose sands are derived from weathered crystalline rocks than on those whose sands are composed of calcium carbonate (the chemical substance of shells). This is because calcium carbonate releases alkaline substances that upset the internal bodily balances of some life forms.

Sand is classified according to the diameter of the particles, as shown below:

very fine 0.05 - 0.1 mm
fine 0.1 - 0.25 mm
medium 0.25 - 0.5 mm
coarse 0.5 - l mm
very coarse 1 - 2 mm

The size of sand grains directly affects the amount of water retention: the coarser the particles, the less retention. Conversely, sand composed of fine particles retains more water. This factor will have a profound effect on animal life between the sand grains. Sand of very fine particles, with its potential to retain more water, makes moving about more difficult. In addition, there is less downward mixing of oxygen. In areas such as this, most life is found a few millimeters below the surface.

Commercial use of sand is enormous. About 994 million tons of sand and gravel are processed each year, and 90 percent of this amount goes to building structures and roads. In the late 1940s, Jamaica Bay near New York City yielded 53 million cubic yards of sand for the construction of Kennedy Airport. The removal of such large quantities of sand can have a detrimental effect on the total ocean beach sand transport system.

Revised October 1988


Related reading: "Beach Processes in Southern Rhode Island." Rhode Island Sea Grant Fact Sheet. Order P923.

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