英语听力：自然百科 为什么海水是咸的？

This week on Why Tell Me Why, we are finding out why the oceans are so salty.

So the oceans are salty because the oceans have been around, let’s say, for over five hundred million years. And over the course of time what we found out is that we’ve always had precipitation--the hydrological(水文的) cycle on the planet. Rain water, fresh water on the planet is slightly acidic(酸性的). It has dissolved(溶解) carbon dioxide.

When that slightly acidic rain water beats down on the continents, the rocks, just tend to dissolve the minerals in the rocks. Those minerals, basically contain what we call ions(离子) or salts,calcium chloride(氯化钙), magnesium chloride(氯化镁). They run off in the rivers on the planet.

Those rivers, basically take that water, deposited with those salts in the oceans; and over hundreds of millions of years, these salts continue to build up in the world oceans.

When we look at the total oceans, there's some quadrillion tons, a million billion tons of salt in the world’s oceans as a result of this weathering(风化) of these ions being dissolved from their rocks on land, going into rivers, the rivers emptying into the oceans, and the salt is basically trapped in the oceans, never to leave again.

If for example we were to evaporate(蒸发) all the water in the oceans, it’s estimated that the salt left behind would cover the planet five-hundred-feet thick across the entire planet, something like a forty-story building.