The sands of time.

Summer's nearly here and you can't wait to be out of the classroom and on the beach! The wind blowing through your hair, the waves lapping at your towel, your brain on vacation.

But not for long. Something about that crunchy sand between your toes sets your wheels turning. Where does sand come from? How old is it? How did it get those warm colors and odd shapes? What is sand anyway?

As the naturalist Rachael Carson once said, "Sand is a substance that is beautiful, mysterious, and infinitely variable. Each grain on the beach is the result of processes that go back into the shadowy beginnings of life, or Earth itself."

To find out what she means, check out the photos on these pages. Then pick up your magnifying glass and head for the beach to start observig.

On a single beach, you might find sand grains in a rainbow of colors--black green, red, pink, white, beige, and more. Basically, any mineral particle measuring between 0.6 mm (about the size of a pinhead) and 2.0 mm in diameter is considered a sand grain. The vast majority of these billions of tiny particles come from the erosion, or wearing away, of rocks over thousands or millions of years. To see how, trace the "life" cycle of a single grain of quartz sand, the most abundant kind in the world.

Quartz sand grains are "born" deep beneath Earth's crust, where hot, thick, liquid rock called magma churns and rises toward the surface. As it rises, the magma cools, and individual chemical compounds solidify. They form distinct 3-D shapes called mineral crystals. Heat and pressure squeeze these tiny mineral crystals together to form solid igneous rock. A mixture of quartz, feldspar, and other crystals, for example, forms solid granite.

Solid, but . . . the minute that granite is exposed on Earth's surface, it begins to wear away. Tiny particles of dust and sand carried by the wind or water grind away at the bonds that hold the individual crystals together. This process, called mechanical weathering, physically breaks the rock into smaller and smaller pieces. But that's not all.

Water (and other chemicals dissolved in it) can also weather the rock chemically. That is, the chemicals dissolve some crystals (feldspar, for example) while leaving others (quartz) behind. The result of these weathering processes: individual grains of pure quartz sand.

Over millions of years, weathering can level a rocky mountain, grain by grain. Rivers and rains carry the sand grains to the sea. Ocean currents transport them thousands of miles, spreading the grains across the seafloor and on beaches all over the world.

So grab a handful of the grains beneath your beach blanket and think about where they've been--and where they might go. The pictures and clues on these pages may help you find some answers--and maybe even more questions. One thing's for sure: You'll never look at sand the same way again.