Mountains, eventually
Where Does a Beach Come From?
Most sand is the hardest leftover of broken-down rock, carried to the coast by rivers over ages.
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Step 1 of 5
Sand is a size, not a substance.
Anything ground down to between about a sixteenth of a millimetre and two millimetres counts as sand, whatever it is made of.
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Step 2 of 5
Rock is broken down slowly and relentlessly.
Water freezing in cracks, roots wedging in, and grains knocking together in rivers reduce solid rock to fragments over long ages.
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Step 3 of 5
Only the toughest mineral survives the trip.
Softer minerals are ground to dust and washed away, while quartz resists both, which is why most beaches are pale quartz sand.
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Step 4 of 5
Rivers do the carrying.
Grains are bounced downstream over enormous distances and timescales, so beach sand may have started as a mountain hundreds of miles inland.
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Step 5 of 5
Not all sand comes from rock.
Tropical white sand is often ground-up coral and shell, and black beaches are volcanic glass, so a beach's colour reveals its origin.
Much white Caribbean sand has passed through parrotfish, which grind coral to eat the algae and excrete the rest as sand.
The short version
Sand is the hardest leftover of rock broken down over ages and carried to the coast, though tropical sand is often ground coral instead.
Try it yourself
Look at beach sand through a magnifying glass. You can pick out clear quartz, dark specks and often broken shell fragments.
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