Science Puzzle
Where Does White Sand Come From?
Many beaches in Britain are golden or brown, made largely of tiny grains of the mineral quartz, worn from rocks over thousands of years. The beaches of tropical islands such as Hawaii and the Maldives are often a dazzling white instead, and their sand is made of something quite different.
A lot of that white sand comes from a surprising source. What is it?
The Answer
Fish poo. Much of the white sand on tropical beaches has passed through the guts of parrotfish.
Parrotfish have teeth fused into a hard, beak-like plate, which they use to scrape and bite at coral and rock to get at the algae growing on it. They swallow chunks of coral skeleton along with their food. A second set of teeth in their throats grinds it down, and what comes out the other end is fine white sand. Divers near a reef can often hear them crunching.
Coral skeletons are made of calcium carbonate, the same white mineral as chalk and sea shells, which is why the sand is so pale. A single large parrotfish can produce hundreds of kilograms of sand in a year. A study of one small reef island in the Maldives found that parrotfish had made most of the island's new sand.
Not all white sand comes from parrotfish. Broken shells, other reef creatures and the slow wearing of coral by waves all add to it. But the next time you lie on a white beach, there's a good chance you're lying on fish poo.
The principle: Living things shape the land. Animals and plants don't just live on Earth's surface; by breaking down, building up and moving material, they help make it.