How does sand form?

Sand is the end product of many things, including decomposed rocks, organic by-products, and even parrotfish poop.

giant bumphead parrotfish

The giant bumphead parrotfish is an amazing fish that can live to be 40 years old, growing up to four feet long and 100 pounds. They use their large head bumps to literally bump heads during competitive displays, when large numbers of fish aggregate to spawn on a lunar cycle. The bumphead parrotfish excretes white sand, which it may produce at the rate of several hundred pounds a year!

The environmentalist Rachel Carson wrote, "In every curving beach, in every grain of sand, there is a story of the Earth."

Sand comes from many locations, sources, and environments. Sand forms when rocks break down from weathering and eroding over thousands and even millions of years. Rocks take time to decompose, especially quartz (silica) and feldspar.

Often starting thousands of miles from the ocean, rocks slowly travel down rivers and streams, constantly breaking down along the way. Once they make it to the ocean, they further erode from the constant action of waves and tides.

The tan color of most sand beaches is the result of iron oxide, which tints quartz a light brown, and feldspar, which is brown to tan in its original form. Black sand comes from eroded volcanic material such as lava, basalt rocks, and other dark-colored rocks and minerals, and is typically found on beaches near volcanic activity. Black-sand beaches are common in Hawaii, the Canary Islands, and the Aleutians.

The by-products of living things also play an important part in creating sandy beaches. Bermuda's preponderance of pleasantly pink beaches results from the perpetual decay of single-celled, shelled organisms called foraminifera.

Less common but no less inviting beaches, devoid of quartz as a source of sand, rely on an entirely different ecologic process. The famous white-sand beaches of Hawaii, for example, actually come from the poop of Bumphead parrotfish. The fish bite and scrape algae off of rocks and dead corals with their parrot-like beaks, grind up the inedible calcium-carbonate reef material (made mostly of coral skeletons) in their guts, and then excrete it as sand. At the same time that it helps to maintain a diverse coral-reef ecosystem, a large Bumphead (which can grow over four feet long and weigh up to 100 pounds) can produce hundreds of pounds of white sand each year!

So the next time you unfurl your beach towel down by the shore, ponder the sand beneath you, which, as Rachel Carson said, is telling you a story about the Earth. You may be about to comfortably nestle down in the remains of million-year-old rocks. Then again, you may soon come to rest upon an endless heap of parrotfish poop.

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Sand is measured according to specific parameters. If more than 50 percent of the material is larger in diameter than 75 microns (.03 inches) but smaller than .18 inches, it is said to be sand. If the average particle size is smaller, it is considered to be silt or clay, and if the average particle size is larger, it is garden-variety gravel.


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