英語時差：沙子的地質(zhì)情況

On the Earth's surface, sand is everywhere. In fact, it's even more common than rock. On today's Moment of Science, we'll learn where all this sand comes from.

Most grains of sand originally started as part of a larger rock. A giant rock might seem solid and permanent, but it's actually quite temporary over geological periods of time. Rocks are constantly battered by wind and rain. They're cracked apart by the roots of plants, and ground together by ocean waves. Over millions of years, even the sturdiest of rocks will be broken apart by this type of erosion.

Most sand is made of quartz--the most common mineral on Earth--but there are lots of other minerals that break up into sand. Next time you're at an ocean beach, examine a handful of sand carefully. You'll probably see grains in a great variety of colors. Each color came from a different type of rock or mineral, somewhere near the beach.

Many beaches have wavy lines of black mixed in with the white, and certain beaches in Hawaii are almost entirely black sand. This black sand comes from volcanos. When lava from a volcano cools, it often turns into black rock. At the ocean, the black rock is broken up by waves, and eventually turned into sand.

Black, volcanic sand has a lot of iron in it. You can test this yourself by bringing a magnet along on your next trip to the beach. If you find some black sand, chances are it will stick to the magnet.

While weathered rocks account for most of the sand on Earth, they're not the whole story. On our next program, we'll learn how living organisms can contribute to a beach's sand.