Salt is the only mineral that people eat—the only dietary mineral that's a real mineral. It's a commonplace substance sought after by animals and humans since the beginning. Salt comes from the sea and from solid layers underground, and that's all most of us need to know. But if you're curious, let's go a bit deeper into the subject.
The Truth About Sea Salt
The sea collects salt, as we all know, but in fact that isn't really true. In truth, the sea only collects the ingredients of salt. Here's how that works.
The sea takes in dissolved matter from two sources, rivers that enter it and volcanic activity on the seafloor. The rivers provide mainly ions—unpaired atoms with a lack or an excess of electrons—from the weathering of rocks. The major ions are various silicates, various carbonates, plus the alkali metals sodium, calcium and potassium. The seafloor volcanoes provide mainly hydrogen and chloride ions. All of these mix and match: sea organisms build shells from calcium carbonate and silica, clay minerals take up potassium, the hydrogen is snapped up in lots of different places. After all the electron swapping is done, sodium ion (from rivers) and chloride ion (from volcanoes) are the two survivors.
Water loves these two ions and can hold large amounts of them in solution. But when sodium and chloride become concentrated enough they form an association and drop out of the water—precipitate—as solid salt, sodium chloride, the mineral halite. When we taste salt, our tongues are instantly dissolving it into sodium and chloride ions again.
Halite is a very delicate mineral that way. It doesn't last long on the Earth's surface, unless water never touches it. Salt is also physically weak. Under quite moderate pressure, rock salt—the stone composed of halite—flows much like ice. The dry Zagros Mountains, in the Iranian desert, feature some notable salt glaciers. So does the continental slope of the Gulf of Mexico, where there's so much buried salt that it can emerge faster than the sea dissolves it.
Besides flowing downward as glaciers, salt can rise upward into overlying rock beds as buoyant, balloon-shaped bodies. These salt domes are widespread in the south-central United States, for example. They're noteworthy because often petroleum rises along with them, making them attractive drilling targets. They are also handy for mining salt.
Salt beds are forming today in playas and larger isolated mountain basins like the Great Salt Lake of Utah or the Salar de Uyuni of Bolivia. (In those places, the chloride comes from land volcanism.) But the large underground salt beds that are mined in many countries formed at sea level, in a very different setting from today's world.
Why Salt Exists Above Sea Level
Most of the land we live on today is only temporarily above sea level because the ice of Antarctica is holding so much water out of the ocean. Over all of geologic history, on average, the sea sits as much as 200 meters higher than today. And in the shallow, flat-bottomed seas that normally cover much of the continents, subtle vertical crustal motions can isolate large areas of water that dry up and precipitate their salt. Once formed, these salt beds can be easily covered by limestone or shale and preserved. In a few million years, maybe less, this natural salt harvest might start happening again as the ice caps melt and the sea rises.
PS: The thick salt beds under southern Poland have been mined for many centuries. Today, the great Wieliczka mine, with its chandeliered salt ballrooms and carved salt chapels, is a world-class tourist attraction. Other salt mines are doing the same—changing their image from the worst kind of workplaces to magical subterranean playgrounds. Visit the Salt links list for more.