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The Biology of Sand

On our last program, we learned how sand is formed by the erosion of rock.  Large rocks are battered by wind and water, and eventually ground up into sand.  This type of erosion accounts for most of the sand on our planet, but it's not the whole story.

Our oceans aren't just salty water:  They're teeming with life.  Mixed in with the sand on an ocean beach, you're likely to find a number of seashells.  Many ocean-going life forms draw calcium from the water around them, and use it to secrete a hard exoskeleton, or shell.  The big shells you see on the beach are an example of this, but there are many microscopic species of algae that do the same thing.  The hard, white material of all these shells is ground by the ocean waves into the brilliant, white sand you see on some tropical beaches.

Certain tropical beaches are known for having pink sand.  This pink comes from the colorful shell of a single-celled marine organism called rubrum-red.

Some single-celled organisms don't make a shell from calcium material.  They extract silicates from the ocean instead, and secrete a shell of solid glass.  The giant loggerhead sponge secretes glass too, producing crystals that resemble fiberglass.  About half a loggerhead sponge's weight comes from tiny, internal spikes of glass.

Just like the calcium-based shell material, this natural marine glass is pulverized by the waves into a delicate white sand.  Because of this, even the softest, cleanest beach in the world might be literally covered with broken glass!

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