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The Most Intimate Mineral |
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Salt pervades the Earth and seas and, not least, our lives.
Salt is the only mineral that people eat; 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 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 ionsunpaired atoms with a lack or an excess of electronsfrom the weathering of rocks: various silicates, various carbonates, plus ions of 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 calcium carbonate and silica shells, clay minerals take up potassium, the hydrogen is snapped up in lots of different places. After all the electron swapping is done, sodium ion and chloride ion 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 waterprecipitateas 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 flimsy mineral that way. It doesn't last long on the Earth's surface, unless water never touches it. In fact, it's physically weak too. Under quite moderate pressure, it flows like ice does. 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.
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. 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.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.