Salt is created by a chain of geologic processes that begins deep inside the Earth and stretches across billions of years. The sodium and chlorine that make up table salt were originally released from volcanic eruptions and the weathering of rocks on the young Earth’s surface, eventually accumulating in the ocean. When ancient seas evaporated in the right climatic and tectonic conditions, thick deposits of rock salt formed underground, and those deposits are still the primary source of the salt we mine and consume today.
How the Ocean Got Its Salt
Earth’s oceans were not always salty. In the planet’s earliest history, intense volcanic activity vented gases from the mantle, including hydrogen chloride, which dissolved in rainwater and washed into the nascent seas. At the same time, freshwater runoff eroded minerals from newly forming continental crust, carrying dissolved sodium, calcium, magnesium, and other ions into the ocean basins. Over hundreds of millions of years, these two processes built up the ocean’s dissolved salt content to roughly the concentration we measure today.
The balance between what flows in and what gets removed keeps ocean salinity relatively stable. Rivers continuously deliver dissolved minerals, while processes like the formation of new seafloor minerals at mid-ocean ridges and the precipitation of evaporite deposits pull ions back out. Researchers studying the residence times of various elements in seawater have traced the ocean’s salinity to both early mantle degassing and long-term continental growth, confirming that no single event salted the seas.1Bulletin de la Société Géologique de France. Seawater residence times of some elements of geochemical interest and the salinity of the oceans Stable and radiogenic isotope systems preserved in ancient carbonate shells serve as chemical time capsules, allowing scientists to reconstruct how ocean composition has shifted across geologic eras.2PubMed. Isotopic history of seawater: the stable isotope character of the global ocean at present and in the geological past
From Seawater to Stone
The vast underground salt deposits found on every continent started as shallow seas that dried up. When a body of seawater becomes partially or fully cut off from the open ocean, evaporation can outpace any fresh inflow. As the water shrinks, the dissolved minerals concentrate and begin to crystallize out in a predictable order. Calcium carbonate (limestone) precipitates first, followed by calcium sulfate (gypsum), and then sodium chloride (halite, or common rock salt). This sequence was worked out through thermodynamic modeling of seawater chemistry: after gypsum converts to anhydrite, halite is the next major phase to drop out of solution.3Geochimica et Cosmochimica Acta. Mineral equilibria in a six-component seawater system, Na-K-Mg-Ca-SO4-Cl-H2O, at 25°C
If evaporation continues past the halite stage, more exotic minerals appear. Magnesium sulfates like epsomite crystallize, followed by potassium-bearing minerals such as carnallite. These later-stage “bittern salts” are commercially important as sources of potash fertilizer and industrial magnesium, but they make up a much smaller fraction of most evaporite deposits than halite does.
Not every shallow sea produced a giant salt deposit. The formation of what geologists call mega-evaporites required a specific combination of arid climate, tectonic positioning, and sea-level conditions. These settings tended to cluster near the latitudinal equivalents of today’s horse latitudes, the subtropical belts where descending dry air creates deserts. When all those factors aligned, enormous volumes of seawater were drawn into subsea-level depressions and evaporated, leaving salt layers that can be hundreds of meters thick.4Earth-Science Reviews. Evaporites through time: Tectonic, climatic and eustatic controls in marine and nonmarine deposits Some of the most famous examples include the Permian Basin deposits beneath parts of Texas and New Mexico, the Zechstein Basin under the North Sea, and the massive Messinian salts beneath the Mediterranean.
When Buried Salt Starts to Move
Once a salt layer gets buried under enough sediment, it behaves in ways that no other rock type does. Rock salt is essentially a crystalline solid that flows under sustained pressure, almost like an extremely slow-moving fluid. Because slightly impure rock salt has a density of roughly 2,200 kilograms per cubic meter, it is lighter than most compacted sedimentary rocks stacked above it. That density contrast creates buoyancy: the salt wants to rise, and the heavier overburden wants to sink.
This gravitational instability eventually produces salt structures called diapirs, columns or mushroom-shaped blobs of salt that push upward through overlying rock. Reaching the surface by buoyancy alone typically requires burial beneath at least about 3,000 meters of sediment before the average density of the entire overburden exceeds that of salt.5Earth-Science Reviews. Terra infirma: Understanding salt tectonics Where diapirs reach or approach the surface, you get features like salt domes, which are prized by the petroleum industry because oil and gas often accumulate in the warped rock layers around them. Salt domes also serve as sites for strategic petroleum reserves and, increasingly, for underground energy storage.
Salt Flats and Landlocked Basins
Not all salt deposits come from ancient marine evaporation. In arid inland basins with no outlet to the sea, rivers and groundwater carry dissolved minerals to a low point where the water evaporates, leaving salt behind. These endorheic (closed-drainage) systems produce the dazzling white salt flats found in places like the Bonneville Salt Flats in Utah, the Salar de Uyuni in Bolivia, and numerous playas across central Asia and the Middle East.
The chemistry in these settings differs from a drying ocean because the dissolved minerals depend on local geology. A study of Pilot Valley in the Great Basin showed that groundwater dissolving sulfate and chloride minerals along its flow path is the main driver of the basin’s brine chemistry. During wet periods, episodic flooding redissolves precipitated salts and carries them to lower ground, where they recrystallize into a fresh crust during the dry season.6Hydrogeology Journal. Shallow groundwater chemical evolution, isotopic hyperfiltration, and salt pan formation in a hypersaline endorheic basin: Pilot Valley, Great Basin, USA The result is a salt pan that grows incrementally, layer by layer, over thousands of years. Some of these continental salt flats are major sources of lithium, boron, and other minerals alongside sodium chloride.
Brine Pools on the Ocean Floor
One of the stranger places salt reveals itself is at the bottom of the sea. In certain deep-sea environments, seawater dissolves buried evaporite layers beneath the seafloor, producing ultra-salty brines that seep upward and collect in depressions on the ocean floor. Because these brines are far denser than ordinary seawater, they pool like underwater lakes, complete with a visible shoreline where the brine meets the surrounding water column.
In the Red Sea, multiple brine pools have been discovered whose chemistry traces back to Miocene-age evaporite deposits buried under the seabed. The NEOM Brine Pools in the Gulf of Aqaba, for instance, are thought to originate from dissolution of sub-seabed halite.7Communications Earth & Environment. Discovery of the deep-sea NEOM Brine Pools in the Gulf of Aqaba, Red Sea The Afifi brine pool, the shallowest and southernmost reported in the Red Sea, likewise appears to draw its salinity from dissolution of underlying evaporitic sediments after diagenetic reactions concentrated the pore waters into brines.8Scientific Reports. Discovery of Afifi, the shallowest and southernmost brine pool reported in the Red Sea In the Mediterranean, the connection is even more direct: Messinian-age evaporites exposed on the seafloor dissolve into dense brines that settle into collapsed basins, sometimes creating deep anoxic environments hostile to most marine life.9Sedimentary Geology. Exhumation of Messinian evaporites in the deep-sea and creation of deep anoxic brine-filled collapsed basins
These brine pools support specialized microbial communities that thrive in extreme salinity and low oxygen, making them a focus of astrobiology research. They also serve as natural laboratories for understanding how salt cycles between solid deposits, dissolved brines, and back again on geologic timescales.
Mining Salt Underground
Humans have been extracting salt from the Earth for thousands of years, and the basic methods fall into two broad categories: mining it as a solid rock and dissolving it to pump out the brine.
Underground rock salt mining typically uses a room-and-pillar method. Miners carve out large chambers (the “rooms”) in the salt bed while leaving thick columns of salt (the “pillars”) in place to support the roof.10Journal of the Southern African Institute of Mining and Metallurgy. Stability evaluation of room-and-pillar rock salt mines by using a flat jack technique – A case study Getting the pillar dimensions right is a careful balancing act: too narrow and the roof collapses, too wide and you leave valuable salt in the ground.11Gornyi Zhurnal. Geomechanical justification of room-and-pillar dimensions for rock salt and polymineral salt mining The salt pulled from these mines is crushed and screened but often not heavily refined, which is why road de-icing salt sometimes has a grayish or pinkish tint from trace minerals. Mines like the one beneath the city of Detroit and the Wieliczka mine in Poland illustrate how immense these underground operations can become, with tunnels extending for hundreds of kilometers at depths of several hundred meters.
Solution Mining and Brine Wells
When a salt deposit is too deep or too irregularly shaped for conventional mining, solution mining offers an alternative. A well is drilled into the salt formation, and fresh water or unsaturated brine is pumped down. The water dissolves the salt underground, and the resulting saturated brine is pumped back to the surface through a separate pipe or an annular space in the same borehole. The brine is then sent to an evaporation plant to crystallize out pure salt.
This technique also creates large underground cavities, which have become valuable in their own right. Salt caverns formed by solution mining are increasingly used for energy storage, whether for natural gas reserves or compressed air. Researchers have developed multi-well combined solution mining methods to optimize how quickly and efficiently these caverns are created, finding that adjusted configurations can reduce the energy consumed during brine injection by close to 19 percent while maintaining the same dissolution rate.12Energy. Multi-well combined solution mining for salt cavern energy storages and its displacement optimization
Solar Evaporation Ponds
The oldest and simplest salt production method is also still one of the most widely used, particularly in warm, dry climates. Seawater or naturally occurring brine is channeled into broad, shallow ponds and left to evaporate under the sun. As the water level drops, dissolved minerals crystallize on the pond floor. Operators often use a series of interconnected ponds, moving the increasingly concentrated brine from one to the next so that less-desirable minerals like gypsum precipitate early and can be separated before the halite stage.
Solar salt production dominates in countries around the Mediterranean, along the coasts of India and Australia, and in parts of Latin America. The process requires virtually no energy input beyond pumping the initial water, but it demands vast areas of flat, low-lying land and a reliably arid climate. A single large solar salt works can cover thousands of hectares. The tradeoff is time: it can take two years or more from the moment seawater enters the first pond to the harvest of crystallized salt from the final one.
Turning Raw Salt into a Finished Product
Raw salt, whether mined as rock or harvested from brine, contains impurities. Calcium, magnesium, and sulfate ions are the usual culprits, along with traces of clay, iron oxides, or organic matter. For food-grade and many industrial uses, these need to be removed.
The most common refining method for brine-derived salt involves chemical purification followed by vacuum evaporation. Sodium carbonate and sodium hydroxide are added to the brine to precipitate calcium and magnesium as insoluble compounds that can be filtered out. Industrial research has shown that optimizing the dosages of these reagents can push calcium and magnesium removal to nearly 100 percent.13Chemical Engineering Journal. Integrated brine purification and scale mitigation strategy for vacuum pan salt production: Mechanistic insights and industrial solutions The purified brine is then fed into vacuum pans, sealed vessels where reduced pressure lowers the boiling point and drives off the water, leaving fine, uniform salt crystals. This vacuum-pan salt is what you typically find in a grocery store salt canister: small, white, free-flowing crystals with an anti-caking agent added.
Not all salt goes through this process. Specialty salts like fleur de sel, Maldon flakes, and various “finishing salts” are prized precisely for their retained trace minerals and distinctive crystal shapes. These products skip heavy purification and are harvested by hand or with minimal processing, which is part of why they cost many times more per kilogram than ordinary table salt.
Environmental Costs of Salt Extraction
Salt extraction can leave a significant footprint. Underground mining removes material that once supported overlying rock and soil, and if pillars fail or cavities are poorly managed, the ground above can subside. In Maceió, Brazil, solution mining of rock salt beneath the city contributed to cumulative ground subsidence of up to 3.83 meters in the most affected neighborhoods. The Bebedouro neighborhood alone saw subsidence reaching about a third of a meter across more than 40 hectares, correlating with structural damage to buildings and increased vulnerability to flooding.14Environmental Challenges. An assessment of ground subsidence from rock salt mining in Maceió (Northeast Brazil) from 2019 to 2023 using remotely sensed data Thousands of residents were eventually displaced.
Solar evaporation operations alter coastal ecosystems by converting wetlands and tidal flats into industrial ponds. The hypersaline discharge from these ponds, if returned to the sea, can harm marine organisms in the immediate outfall area. Inland brine extraction and disposal raise similar concerns about salinization of freshwater aquifers and soil. These issues are manageable with proper engineering and regulation, but they underscore that even something as seemingly benign as salt production carries real environmental trade-offs.
Salt Beyond Earth
Salt formation is not unique to our planet. Wherever liquid water interacts with rock, dissolved minerals will concentrate and eventually precipitate if conditions allow. One of the most intriguing cases is Europa, the ice-covered moon of Jupiter, where theoretical models predict that the nonicy material on the surface is dominated by magnesium and sodium sulfate salts. These models, built independently of direct observation, match spectroscopic data from spacecraft and suggest the salts originate from a subsurface ocean in contact with a rocky mantle.15Journal of Geophysical Research: Planets. Composition and stability of salts on the surface of Europa and their oceanic origin
Mars shows evidence of ancient evaporite deposits and recurring brine seeps. Saturn’s moon Enceladus vents salty water ice from its south pole. Even some meteorites contain tiny halite crystals with fluid inclusions that preserve samples of water from the early solar system. The message from planetary science is clear: given water, rock, and enough time, salt formation is practically inevitable. The specific salts that form depend on the local chemistry, but the general process of dissolution, transport, and precipitation is universal.
Why Animals Go to Extreme Lengths for Salt
Salt matters to living things because sodium and chloride are essential for nerve signaling, muscle contraction, and maintaining fluid balance. Plants tend to be low in sodium, which creates a chronic shortage for herbivores, especially those living far from the coast. In the western Amazon, the phenomenon is dramatic: parrots, tapirs, monkeys, and other animals congregate at natural mineral licks to ingest sodium-rich clay and soil. Research has shown that lick visitation increases with distance from the ocean, consistent with the idea that atmospheric salt deposition declines inland and the animals are compensating for what their plant-based diet cannot provide.16Biotropica. Lust for Salt in the Western Amazon
This biological demand is what made salt one of the earliest traded commodities in human history. Inland communities that could not harvest it from the sea relied on brine springs, salt-encrusted soils, or trade networks stretching hundreds of kilometers. The economic and strategic importance of salt deposits shaped settlement patterns, taxation systems, and even military campaigns for millennia. Modern refrigeration and global shipping have made salt cheap and ubiquitous, but the underlying geologic and biological forces that put it in the ground and made it indispensable have not changed at all.