Published: 20 January, 2005

Earth's rinsing process creates the salt of the ocean

Where does the salt in the sea come from?

This is a very good question - one that forms an important part of the field of chemical oceanography. While modern oceanographers are still working out the details, Antoine Lavoisier, sometimes called "the father of modern chemistry", gave a simple answer over 200 years ago. Lavoisier said that the oceans are the "rinsings of the Earth". What he meant was that salts are washed into the oceans from the land. These "salts" - including calcium carbonate (limestone), magnesium sulfate (epsom salt) as well as table salt (sodium chloride) - are produced when rocks react with water in a process that geologists call "weathering". Weathering breaks down the minerals in rocks to clays and salts that dissolve in the water. The salts are carried by the water as it makes its way back to sea through streams and rivers.

Weathering is such a slow process we hardly notice it. But if you go to one of the local cemeteries you can see its effects. Look at some of the oldest headstones and you'll see that the inscriptions are very hard to read - that's the effect of weathering in just one or two hundred years. Imagine this process over eons - that's a lot of salt!

However, scientists today know that this is only part of the answer - for example there is hardly any chlorine or sulfur in most rocks so those elements couldn't come from weathering. They come from volcanoes - which spew out gases containing sulfur and chlorine as well as lava.

You might think that with salts being constantly added, that the sea is becoming steadily saltier - but that's not so. Salts are also being constantly removed from the sea. For example, clams remove calcium carbonate to build their shells. When part of the sea dries up, it can leave a deposit of salt called an "evaporite". During Silurian time, 420 million years ago, the sea that covered the Ithaca area dried up and left a vast deposit of salt. That salt now lies more than 2000 ft. beneath the surface. Cargill Salt Company in Lansing mines this salt from beneath Cayuga Lake. It's this salt that is spread on the roads around here, making it possible to drive in the winter.