Ramona L. Harding, age 17, of Sarasota, Florida, for her question:
How many metals and minerals are in seawater?
Suppose we could extract all the minerals and metals from the sea. This material would cover all the continents with a layer 500 feet thick. Florida and all the other areas of dry land would be 500 feet higher than they are now. More than 99 per cent of the recovered material would be composed of five salty chemicals. However, it also would contain gold, silver and most of all the metals and minerals found in the earth's crust.
The contents of the ocean tell a story of erosion that has been going on for several billion years. Every day, rains and running water dissolve chemicals in the earth's rocks, wash them away and finally dump them into the ocean. Every day, the sun evaporates water vapor from the surface of the sea but almost all the dissolved minerals stay behind. Hence the salty sea is getting saltier all the time.
It is estimated that the world ocean now contains about 50 quadrillion tons of dissolved minerals, some of which are metals. Since this figure is 50 plus 15 zeros, it is easier to imagine the situation if we concentrate on a smallish sample. So let's consider the mineral and metal contents in a cubic mile of average seawater. The tank to hold this amount would be a mile long, a mile wide and mile high.
The dissolved chemicals make up about 3.5 per cent of our liquid sample. If we evaporate 96.5 per cent water, we would have about 166 million tons of dry deposits. The major portion of the sediment, about 77.$ per cent, would be sodium chloride, or ordinary table salt. About 10.9 per cent would be magnesium chloride, 4.7 per cent magnesium sulphate, 3.6 per cent calcium sulphate and 2.5 per cent potassium sulphate.
These five salty chemicals make up more than 99 per cent of the minerals and metals dissolved in the sea. This still leaves 1,175,000 tons of other materials in our sample deposit. And somewhere among this mountain of sediments we could find traces of more than 60 of the chemical elements that are present in the earth's crust.
However, the proportion of different chemicals found in the sea differ from the contents of the earth's crust. For example, our cubic mile of seawater contains 6,125,000 tons of magnesium and 47 tons of iron. In the earth's crust, there is two times more iron than magnesium. There are only 47 tons of aluminum in our seawater sample, yet aluminum is the most plentiful metal of the dry land.
The sea's most abundant elements are in the form of salty chemical compounds. Our cubic mile sample also contains 14 tons of dissolved copper, plus the same percentage of lead and tin. It also contains 1.4 tons of dissolved silver. And it is estimated that about 38 pounds of precious gold is dissolved in every cubic mile of seawater.