【英语时差8,16】沙子的地质情况

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 battered¹ 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 wavy² lines of black mixed in with the white, and certain beaches in Hawaii are almost entirely³ black sand. This black sand comes from volcanos. When lava⁴ 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, volcanic⁵ 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.  

1 battered   

adj.磨损的；v.连续猛击；磨损

参考例句：

• He drove up in a battered old car.他开着一辆又老又破的旧车。
• The world was brutally battered but it survived.这个世界遭受了惨重的创伤,但它还是生存下来了。

2 wavy   

adj.有波浪的，多浪的，波浪状的，波动的，不稳定的

参考例句：

• She drew a wavy line under the word.她在这个词的下面画了一条波纹线。
• His wavy hair was too long and flopped just beneath his brow.他的波浪式头发太长了，正好垂在他的眉毛下。

3 entirely   

ad.全部地，完整地；完全地，彻底地

参考例句：

• The fire was entirely caused by their neglect of duty. 那场火灾完全是由于他们失职而引起的。
• His life was entirely given up to the educational work. 他的一生统统献给了教育工作。

4 lava   

n.熔岩，火山岩

参考例句：

• The lava flowed down the sides of the volcano.熔岩沿火山坡面涌流而下。
• His anger spilled out like lava.他的愤怒像火山爆发似的迸发出来。

5 volcanic   

adj.火山的；象火山的；由火山引起的

参考例句：

• There have been several volcanic eruptions this year.今年火山爆发了好几次。
• Volcanic activity has created thermal springs and boiling mud pools.火山活动产生了温泉和沸腾的泥浆池。