The common belief that all salt originates from the sea is understandable, given the ocean’s vast salt content, but it is an incomplete picture. Salt, or sodium chloride (NaCl), is a ubiquitous compound found in many different geological environments. While oceans hold an immense quantity of dissolved salt, a significant portion of the world’s supply is extracted from terrestrial sources. Global salt production relies on three distinct resource types: surface water evaporation, underground rock deposits, and the extraction of artificial brine.
Salt Production from Seawater Evaporation
The traditional method of creating “sea salt” relies on the natural processes of sun and wind to concentrate saline water. This process, known as solar evaporation, is the oldest and most energy-efficient technique for salt production. It is used extensively in regions with high solar intensity and low rainfall, such as the Mediterranean and parts of Australia.
The process begins by directing seawater or highly saline lake water, like from the Dead Sea, into a series of shallow, interconnected ponds. In the first set of ponds, called concentrating ponds, the water slowly evaporates, increasing the brine’s salt concentration. Once the brine reaches a specific saturation point, it is moved to crystallization ponds.
In the final crystallization ponds, the sodium chloride precipitates out as solid crystals, which are then harvested using specialized mechanical equipment. This method yields a salt that often contains trace minerals, which can contribute to its unique flavor profile. Industrial solar salt is generally close to 100% purity, but this method is only one component of the world’s total salt supply.
Extraction of Underground Rock Salt Deposits
A major alternative to sea salt is rock salt, or halite, which is mined from immense underground deposits. These deposits are the solidified remnants of ancient oceans and saline lakes that evaporated millions of years ago. The dried-up salt layers were subsequently buried and compressed under layers of sediment and rock, preserving them far beneath the surface.
Extracting rock salt involves conventional mining techniques, often called dry mining or shaft mining. Deep shafts are sunk to access the salt beds, which can be hundreds of feet thick. Miners use the “cut and blast” technique, where explosives break the salt into large chunks.
Alternatively, continuous mining machines bore directly into the salt face, grinding it into smaller pieces. A system of pillars, which are columns of untouched salt, is left in place to provide structural support for the mine roof, ensuring safety and stability. The resulting rock salt is then crushed and screened into various sizes, commonly used for de-icing roads due to its less refined nature.
Harvesting Salt Through Brine Pumping
A third method for obtaining salt from subterranean sources is solution mining, also known as brine pumping. This technique is distinct because it uses water to dissolve the underground salt before bringing it to the surface, bypassing the need for physical mining shafts. The process begins by drilling wells deep into the salt formation.
Freshwater is injected down one well, dissolving the salt to create a saturated solution called brine. This concentrated brine is forced back up a separate well to the surface. The resulting liquid is piped to a facility for purification, where impurities are removed.
For the final step, the purified brine is evaporated to crystallize the salt. This is often achieved using large, closed vessels called vacuum evaporators. The vacuum reduces the water’s boiling point, allowing the brine to crystallize at lower temperatures and yielding a very high-purity, fine-textured salt suitable for food processing and chemical manufacturing.
Geological History of Salt Formation
The existence of salt in oceans and massive terrestrial deposits is explained by large-scale geological and hydrological cycles spanning billions of years. The initial source of ocean salt was the chemical weathering of rocks on land, where slightly acidic rainwater dissolved minerals. These dissolved ions, including sodium and chloride, were carried by rivers into the oceans, gradually increasing the water’s salinity.
Salt is also introduced through hydrothermal vents in the seafloor, where seawater reacts with hot rocks beneath the crust. Over geological time, the concentration of salt in the ocean has been relatively stable because mechanisms like the burial of sediments and the formation of evaporite deposits remove salt from the water. The average salinity of the modern ocean is about 35 parts per thousand.
The massive rock salt deposits found beneath continents are the result of ancient evaporation cycles. During periods of continental drift, parts of the ocean were cut off, forming shallow, restricted bodies of water known as epeiric seas. In arid climates, the water in these isolated basins evaporated completely, leaving behind thick layers of salt called evaporites.
Through tectonic forces and sedimentation, these evaporite layers were buried deep underground and compressed into the hard rock salt we mine today. The size of these deposits, found in places like the Gulf of Mexico and parts of the Middle East, demonstrates that salt’s origin is deeply integrated into Earth’s crustal history.