Salt reaches your table from three broad sources: the ocean, underground rock deposits left by ancient seas, and surface accumulations in landlocked basins and desert flats. Each of these traces back, ultimately, to the same geochemistry that has been cycling minerals between rock, water, and atmosphere for billions of years. The story is more layered than most people realize, involving everything from volcanic vents on the ocean floor to catastrophic episodes where entire seas evaporated.
How Salt Gets Into the Ocean
Seawater is salty because dissolved minerals have been washing into it for eons. Rain is slightly acidic thanks to dissolved carbon dioxide, and when it falls on land, it slowly breaks down rock. Rivers carry the dissolved ions, especially sodium and chloride, downstream to the sea. Studies of major river systems show that the chemistry of highland rivers is dominated by the weathering of carbonate rocks, with calcium, magnesium, and bicarbonate making up the bulk of dissolved ions, while lowland rivers pick up additional sodium and potassium from silicate weathering and contact with saline soils and groundwater.1ScienceDirect. Major ion chemistry of the Ganga-Brahmaputra river system: Weathering processes and fluxes to the Bay of Bengal All of that dissolved material eventually reaches the ocean, where water evaporates but the salts stay behind, concentrating over geologic time.
Rivers are not the only contributor. Along mid-ocean ridges, seawater seeps into cracks in the seafloor, gets superheated by magma, and returns through hydrothermal vents loaded with dissolved minerals. Chloride is the dominant anion in these fluids, and its concentration can range widely, from roughly a third of normal seawater levels at some vents to about double at others.2Nature. Halide systematics of submarine hydrothermal vents These vents have been reshuffling ocean chemistry for as long as plate tectonics has been active. The balance between what rivers deliver, what vents add and remove, and what gets locked away in sediments determines how salty the ocean is at any given point in Earth’s history.3Bulletin de la Société Géologique de France. Seawater residence times of some elements of geochemical interest and the salinity of the oceans
When Seas Dry Up and Leave Salt Behind
The massive underground salt deposits mined around the world are remnants of ancient bodies of water that evaporated. When a shallow sea or a restricted marine basin loses water faster than it is replenished, the dissolved minerals precipitate out in a predictable sequence. Calcium carbonate settles first. Then comes gypsum (calcium sulfate). Only after a great deal more water has evaporated does halite, the mineral name for common table salt (sodium chloride), crystallize out. If evaporation continues further still, rarer minerals like epsomite and carnallite form from the remaining magnesium-rich brine.4Geochimica et Cosmochimica Acta. Mineral equilibria in a six-component seawater system, Na-K-Mg-Ca-SO4-Cl-H2O, at 25°C
These evaporite sequences can be staggeringly thick. In some cases, repeated cycles of flooding and evaporation stacked salt beds hundreds of meters deep. The Permian Basin beneath western Texas, Oklahoma, and eastern New Mexico holds brines with salinities ranging from 45 to over 300 grams per liter, reflecting both ancient evaporated seawater and more recent dissolution events where meteoric water re-dissolved older deposits of halite and anhydrite.5GSA Bulletin. Origin, distribution, and movement of brine in the Permian Basin (U.S.A.): A model for displacement of connate brine Similarly, the massive evaporite formations underlying the Canadian prairies, which supply much of the world’s potash, are classified as “salt giants” recording long periods of arid conditions over restricted seas.6FACETS. The geology of Canadian potash: a critical mineral for feeding the world Deep brine systems in Alberta’s Devonian aquifers tell a complex story involving original seawater, partial evaporation, dolomitization, and the later influx of freshwater that dissolved and redistributed the salts over hundreds of millions of years.7Journal of Geochemical Exploration. New insights into the origin and migration of brines in deep Devonian aquifers, Alberta, Canada
The Messinian Salinity Crisis
Perhaps the most dramatic single episode of salt deposition in Earth’s recent geological past happened between about 5.97 and 5.33 million years ago, when the Mediterranean Sea was largely cut off from the Atlantic Ocean. Tectonic shifts narrowed and eventually restricted the gateway at what is now the Strait of Gibraltar, and the isolated Mediterranean began to evaporate. The result was the precipitation of roughly a million cubic kilometers of salt.8PubMed Central. Kilometric sea level changes during the Messinian salinity crisis caused by river erosion and climate
This was not a simple, one-shot evaporation. The tectonic setting of the basin determined the timing, while orbital-scale climate cycles and sea-level fluctuations drove repeated rounds of carbonate, gypsum, and halite precipitation.9Nature Reviews Earth & Environment. Causes and consequences of the Messinian salinity crisis During the most intense phase, halite accumulated across the entire Mediterranean floor following a rapid drawdown event in which sea level dropped an estimated 1.7 to 2.1 kilometers in the eastern basin and around 850 meters in the western basin.10Nature Communications. Chlorine isotopes constrain a major drawdown of the Mediterranean Sea during the Messinian Salinity Crisis Imagine most of the Mediterranean reduced to a chain of hyper-salty lakes surrounded by vast salt flats. The deposits left behind sit beneath the modern seafloor and, in some places, are kilometers thick. It is one of the clearest examples of how geological processes can concentrate salt on an almost unimaginable scale.
Salt Domes and What Happens After Burial
Once buried under layers of sediment, salt does not simply sit still. Rock salt is less dense than the sedimentary rock piled on top of it, and under enough pressure it becomes plastic, flowing like an extremely slow liquid. Over millions of years, the salt can push upward through overlying layers, forming mushroom-shaped structures called salt domes. Two sources of pressure drive this movement: the static weight of the sediments above and, in some settings, lateral tectonic compression. Growth can proceed by active upward thrusting or by “downbuilding,” where surrounding sediments sink as the salt body holds its position.11AAPG Bulletin. Mechanics of Formation of Salt Domes with Special Reference to Gulf Coast Salt Domes of Texas and Louisiana
Salt domes matter for more than just the salt industry. They create traps for oil and natural gas, which is why petroleum geologists have studied them intensively since the early twentieth century. The salt itself is mined in some domes, and the cavities left behind are increasingly repurposed for energy storage, including compressed air and hydrogen. Many of the Gulf Coast domes in Texas and Louisiana are so well mapped that engineers can model their internal geometry in detail.
Salt Flats and Landlocked Basins
You do not need an ancient ocean to produce a salt deposit. In arid regions with no outlet to the sea, water collects in low-lying basins, evaporates, and leaves its dissolved minerals behind. The Great Salt Lake in Utah, the salt pans of the Bonneville Flats, and the playas of Iran and central Asia all form this way. In these endorheic (closed) basins, all groundwater is ultimately lost to evapotranspiration at the topographic low point. As fresh water flows toward the basin center, it transitions from fresh to hypersaline near the boundary between the alluvial fan and the playa, forming an inverted salinity gradient where extremely salty water sits near the surface above fresher water below.12Hydrogeology Journal. Shallow groundwater flow and inverted fresh/saline-water interface in a hypersaline endorheic basin (Great Basin, USA)
These surface salt flats are often harvested by simply scraping the crust or pumping brine into shallow pools. In some commercial operations, particularly for lithium and potash alongside sodium chloride, solar evaporation ponds do the concentrating work. The process is slow but cheap, relying entirely on sunlight and dry air. Salt flats also form in places like the Salar de Uyuni in Bolivia, where seasonal flooding dissolves the surface, mixes the minerals, and leaves a fresh, flat crust as the water evaporates again. Each of these settings produces salt with a slightly different mineral profile, which is why salts from different geographic sources can taste subtly different.
How Salt Is Actually Harvested
Three main methods move salt from its natural source to the human supply chain: solar evaporation, conventional mining, and solution mining.
Solar evaporation is the oldest technique, practiced for thousands of years along coastlines and at inland salt lakes. Seawater or brine is channeled into a series of shallow ponds. As water evaporates, minerals precipitate in stages. Calcium carbonate drops out first, when salinity reaches about double that of normal seawater. Gypsum follows at roughly three times seawater salinity. Finally, sodium chloride crystallizes when salinity climbs to about ten times that of seawater.13Elsevier / Estuarine, Coastal and Shelf Science. Permanent salt evaporation ponds in a semi-arid Mediterranean region as model systems to study primary production processes under hypersaline conditions By moving brine progressively through different ponds, producers can separate impurities at each stage and collect relatively pure halite from the final crystallizer ponds. This method works best in warm, dry climates with low rainfall and is still the dominant technique across much of Asia, Africa, and parts of South America.
Conventional mining targets underground rock-salt deposits. Miners extract the salt in solid form, typically using a room-and-pillar method where large chambers are carved out while pillars of salt are left standing to support the ceiling. The Khewra Salt Mine in Pakistan, one of the oldest and largest, has been in operation for centuries using variations of this approach. Rock salt mined this way often retains trace minerals that give it color, which is why Himalayan pink salt looks the way it does.
Solution mining takes a different approach. Instead of sending miners underground, water is injected into a salt deposit through boreholes. The water dissolves the salt, and the resulting brine is pumped back to the surface. This is the method of choice for deep or irregularly shaped deposits where conventional tunneling would be impractical. Researchers continue to refine the technique, with recent work showing that optimizing the displacement of injected water can cut energy consumption by close to a fifth compared to standard methods.14Energy. Multi-well combined solution mining for salt cavern energy storages and its displacement optimization Studies of low-grade salt deposits have also compared different mining geometries, finding that continuous solution mining through multiple layers can produce caverns over twice as large as layered approaches while cutting per-unit costs to about 40 percent.15International Journal of Mining Science and Technology. Schemes comparation of layered and continuous solution mining in bedded salt formations by horizontal interconnected wells The caverns left behind by solution mining are increasingly valuable in their own right, finding use as storage sites for natural gas, compressed air, and hydrogen.16Frontiers in Energy Research. Feasibility assessment of solution mining and gas storage in salt caverns: a case study of the Sanshui salt mine
Why Humans Crave Salt in the First Place
Humans find moderate salt concentrations in food genuinely pleasurable. This preference is shared with other omnivores, but people tend to eat more salt than you would expect from diet alone. One reason is sweating: because humans lose sodium through the skin far more than most animals do, we may have evolved a higher baseline appetite for salt. Cross-cultural studies find that salt intake is remarkably consistent around the world, suggesting the preference is biological rather than purely cultural.17Current Biology. Human Taste: From Evolutionary Origins to Modern Health Carnivores get salt with every bite of meat, while herbivores can become sodium-depleted and will actively seek out salt licks. Omnivores like us land somewhere in between, but our sweating habit tips the balance toward a stronger preference.
High salt concentrations, on the other hand, are aversive. The body’s taste system is finely tuned: concentrations near the body’s own isotonic level taste good, while concentrations far above it trigger rejection, since too much salt disrupts the osmotic balance of body fluids. That push-and-pull shaped our ancestors’ foraging behavior and, in the modern food environment where salt is abundant, has contributed to overconsumption. Easy access to salty, energy-dense foods has helped drive diet-related health problems in populations that no longer face the scarcity our taste preferences evolved to solve.18PubMed Central. An evolutionary perspective on food and human taste
Road Salt and Its Toll on Freshwater
One of the largest modern uses of mined salt has nothing to do with food. In cold climates, enormous quantities of sodium chloride are spread on roads every winter to keep them ice-free. What happens afterward is a growing environmental concern. The chloride in road salt does not break down. It washes into soil, seeps into groundwater, and eventually raises the year-round chloride concentration in rivers and lakes.19Science of The Total Environment. The effects of road salt on freshwater ecosystems and solutions for mitigating chloride pollution – A review
The ecological effects ripple through entire food webs. Road salt negatively affects organisms at every trophic level, from biofilms coating stream rocks to fish at the top. Species-level effects are often sub-lethal, meaning the salt does not kill outright but reduces growth and reproduction, effects that predation and other natural stressors can amplify. At the community level, biodiversity declines as salt-sensitive species drop out and salt-tolerant ones take over. This shift can have unexpected consequences, including potentially favoring mosquito host species and altering disease transmission dynamics.20Freshwater Biology. A review of the species, community, and ecosystem impacts of road salt salinisation in fresh waters Contaminated wetlands may export more greenhouse gases, salinized streams tend to export more nitrogen and carbon, and lakes can develop disrupted oxygen dynamics that release phosphorus from sediments.
Even relatively low chloride levels can cause damage. In laboratory experiments with the common water flea, survival dropped by over 60 percent at a chloride concentration of 120 milligrams per liter compared to controls, and the number of offspring produced declined steadily with increasing chloride, suggesting that current regulatory thresholds may not be protective enough for some freshwater organisms.21PubMed Central. Impacts of water hardness and road deicing salt on zooplankton survival and reproduction This is prompting researchers and municipalities to look harder at alternatives like beet juice-based brines and more targeted application methods, though no replacement has yet matched the cost-effectiveness of plain sodium chloride on a large scale.
Salt Beyond Earth
The same geochemistry that produces salt on Earth operates elsewhere in the solar system. Saturn’s moon Enceladus, which shoots plumes of water vapor and ice from its south pole, has been a particular focus. Early analysis of those plumes found no firm detection of sodium and chloride in the gas phase, which researchers interpreted as consistent with salts accumulating at the boundary between the subsurface ocean and the ice shell rather than being carried aloft in the vapor.22Geophysical Research Letters. An oceanic composition on early and today’s Enceladus Later observations by the Cassini spacecraft did detect sodium salts in the ice grains of the plume, supporting the existence of a salty ocean beneath the ice. On Mars, chloride-bearing deposits have been mapped from orbit across large swaths of the southern highlands, interpreted as the evaporative residue of ancient surface water. Wherever liquid water interacts with rock and then evaporates or freezes, salt formation follows. It is one of the most universal geochemical processes we know of, which is part of why the detection of specific salts on other worlds gets planetary scientists so excited: salt deposits are indirect evidence of sustained liquid water, and sustained liquid water is the prerequisite most commonly invoked for the possibility of life.