Salt is the only mineral that people eat-it's the only dietary mineral that's really a mineral. It's a common substance that's been sought by animals and humans alike since the beginning of time. 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.
The Truth About Sea Salt
We all know that the sea collects salt, but that isn't really true. The sea only collects the ingredients of salt. Here's how it works.
The sea takes in dissolved matter from two sources: rivers that enter it and volcanic activity on the seafloor. The rivers mainly provide mainly ions from the weathering of rocks-unpaired atoms with a lack or excess of electrons. The major ions are various silicates, various carbonates, and the alkali metals sodium, calcium, and potassium.
Seafloor volcanoes mainly provide hydrogen and chloride ions. All these mix and match: sea organisms build shells from calcium carbonate and silica, clay minerals take up potassium, and 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 sodium and chloride form an association and drop out of the water when they become concentrated enough. They precipitate as solid salt, sodium chloride, the mineral halite.
When we taste salt, our tongues instantly dissolve it into sodium and chloride ions again.
Halite is a very delicate mineral. It doesn't last long on the earth's surface unless water never touches it. Salt is also physically weak. Rock salt-the stone composed of halite-flows much like ice under quite moderate pressure. 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 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 U.S. They're noteworthy because petroleum often rises along with them, making them attractive drilling targets. They're also handy for mining salt.
Salt beds form in playas and larger isolated mountain basins like the Great Salt Lake of Utah and the Salar de Uyuni of Bolivia. The chloride comes from land volcanism in these places. 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 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, the sea sat as much as 200 meters higher than it does today. Subtle vertical crustal motions can isolate large areas of water in the shallow, flat-bottomed seas that normally cover much of the continents and 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.
The thick salt beds under southern Poland have been mined for many centuries. The great Wieliczka mine, with its chandeliered salt ballrooms and carved salt chapels, is a world-class tourist attraction. Other salt mines are also changing their image from the worst kind of workplaces to magical subterranean playgrounds.