沙子是怎么形成的？

Sand comes from many places. It can be made of broken-down rock, the remains of living things and even fish poop. Most sand forms when rocks are broken down by weathering and erosion over thousands or even millions of years. Rocks take a long time to break down, especially hard minerals like quartz and feldspar. Often starting thousands of kilometers from the sea, pieces of rock slowly travel down rivers and streams, breaking into smaller and smaller pieces on the way. Once they reach the ocean, waves and tides wear them down even more. The light brown color of most sandy beaches comes from iron oxide, which tints quartz a light brown, and from feldspar, which is naturally brown or tan. Black sand comes from volcanic rock such as lava and basalt, so it is usually found on beaches near volcanoes. Black-sand beaches are common in Hawaii and the Canary Islands, and on the west coast of New Zealand's North Island. Living things play a part too. The pink beaches of Bermuda get their color from the remains of tiny single-celled creatures with shells, called foraminifera. And some of Hawaii's famous white-sand beaches come from parrotfish. These fish use their beak-like mouths to scrape algae off rocks and dead coral. They grind up the hard coral in their guts and then pass it out as fine white sand. A single large parrotfish can produce more than a hundred kilograms of sand a year! So next time you lie down on a beach, think about the sand beneath you. You may be resting on the remains of million-year-old rocks – or on a huge pile of parrotfish poop.