Geology You Use Every Day

Where Does Salt Come From?



What is salt?

There are many ionic salts. What we think of as table or road salt is an ionic salt called sodium chloride (NaCl).

It dissolves in water much better than any other mineral. As science writers Penny Le Couteur and Jay Burreson put it:

As life is thought to have developed in the oceans and as salt is essential for life, without this solubility of salt life as we know it would not exist…It is this solubility of sodium chloride that [also] makes salt…such a good preservative.

A salt crystal is made out of a regular lattice of sodium (positive charge) and chloride (negative charge) ions that are held together by the strong attraction of their opposite charges.

A single grain of table salt under an electronic microscope.  Image by Chhe
A single grain of table salt under an electronic microscope. Image by Chhe

Water molecules are partly charged, with the H2 side mildly positive and the O side mildly negative.

Let’s go back to Le Couteur and Burreson, who have a better grip of chemistry than I do:

Although the attraction between a positive sodium ion and the negative end of water molecules (and the attraction between negative chloride ions and the positive end of water molecules) are similar to the attractive force between Na+ ions and Cl ions, what ultimately accounts for the solubility of salt is the tendency for these ions to disperse randomly.

How does salt form?

If you took all the NaCl in the sea and spread it out evenly over land, the layer would be more than 500 feet thick.

That’s a lot of salt. Since only fresh water empties into the ocean, where does it all come from?

Weathering and erosion load fresh water with a lot of calcium, silica, sodium, chloride, and other elements and minerals. Fresh-water sources pour an estimated 4 billion tons of this material into the ocean every year.

Sediment plumes from the Mississippi and Atchafalaya rivers.  NASA
Sediment plumes from the Mississippi and Atchafalaya rivers. NASA

Dissolved materials from the atmosphere and from underwater volcanic activity also end up in the mix.

Ocean water then gets concentrated due to evaporation, especially around the tropics.

Sodium chloride builds up in it because sea life, chemical reactions and physical-chemical reactions remove a lot of the calcium, magnesium, bicarbonate, and silica.

Eventually a little over 85% of the dissolved elements in sea water is NaCl.

Collecting salt

If a sea’s water supply changes, it can evaporate away, leaving behind all its salt. Over time, the salt bed may end up buried underground, forming a halite deposit. Halite on or near the Earth’s surface has been mined for thousands of years, and it still is mined today.

If underground water circulates through the salt bed, a brine spring may form. Its waters, up to ten times as salty as the sea, then can be boiled off to retrieve the NaCl.

Evaporating sea water under the sun is still the most common method of producing salt today, at least in tropical lands near the ocean.

Where is salt found?

Human history and the presence of salt are intertwined because the human body, on average, contains about 4 ounces of NaCl which gets used up and needs to be replaced in order for life to go on. People who don’t live near a salt source must trade for it or die.

Salt also was the best way to preserve food until canning and refrigeration were developed during the last hundred and fifty years or so.

It used to be called white gold and sometimes was traded for gold, ounce for ounce. Only in recent times, when industrial demand for NaCl brought the price down, did salt become commonplace and cheap.

You can find NaCl sodium chloride, i.e., halite, on every continent – wherever an ancient sea evaporated.

Salt being harvested in central New York State circa 1908.  Source
Salt being harvested in central New York State circa 1908. Source

Remains of salt works, where Neolithic people evaporated salt from brine springs, have been found in Romania and China that date back over 8,000 years.

The Romans paid soldiers a salarium, money with which to buy salt – today we buy much more than that with our salaries. They also salted leafy vegetables, a habit we have memorialized with the word “salad.”

Venice started out in a sea marsh precisely because salt could be harvested from the water there. In Austria, the city of Salzburg (“salt castle”) sits on the River Salzach (“salt river”) atop an extensive deposit of – wait for it – halite.

Today, in Africa, traditional Tuareg salt caravans still travel the Sahara, though mostly by truck now instead of camels.

The biggest salt bed in the world is at Salar de Uyuni in Bolivia. Tourists there can stay at a hotel that has been constructed out of halite.

They can take some awesome photos, too, if the salt pan happens to be wet.  Ezequiel Cabrera
They can take some awesome photos, too, if the salt pan happens to be wet. Ezequiel Cabrera

Sodium chloride dissolves in water very easily. It’s essential to life as well as useful. For a long time it was worth its weight in gold, but today we think nothing of pouring salt on roads by the ton in order to drive safely during winter. We still harvest today just as our ancient ancestors did – by evaporating sea water, boiling down brine springs, or mining it out of the ground.


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Front page image: W. J. Pilsak


About BJ Deming

After getting an associate's degree in forestry, I studied geology as an undergraduate back in the 1980s but went into medical transcription instead. It just worked out better for me. The Internet renewed my interest in geoscience as a hobby, and when I retired in 2014, I decided to write a book about cat evolution. That started a new career for me (enormous fun but not self-supporting yet). Right now, besides blogging I am finishing up the first two books in a self-published ebook series about the cat family and its history. Thanks for your interest!

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