Quantifying the amount of salt on Earth requires looking beyond common table salt (primarily sodium chloride, or NaCl) to encompass the vast reservoir of dissolved solids across the planet. Quantification involves estimating the mass of NaCl and other mineral salts, such as chlorides, sulfates, and carbonates, dissolved in water or trapped in the Earth’s crust. Scientific measurements show that the total mass of salt is overwhelmingly concentrated in a single global location. This inventory provides a powerful perspective on the scale of geological and oceanic processes.
Quantifying the Earth’s Salt: The Oceanic Majority
The vast majority of the planet’s salt resides within the oceans, representing the primary reservoir of dissolved minerals. Estimates place the total mass of these dissolved solids at approximately 50 quadrillion metric tons. This immense quantity gives seawater its characteristic brininess, maintaining an average salinity of about 35 parts per thousand (ppt). This means 35 grams of salt are present in every one thousand grams of seawater.
The concentration of salt is not uniform across the globe but varies based on localized environmental conditions. In regions with high evaporation and limited freshwater inflow, such as the Red Sea, salinity can rise above 40 ppt. Conversely, areas near large river mouths or at the poles see a decrease due to freshwater dilution from runoff or melting ice. If all the oceanic salt were dried and spread evenly across the Earth’s landmasses, it would form a solid layer more than 500 feet (166 meters) thick.
Terrestrial Salt Reserves and Crustal Content
Salt is also stored in massive quantities beneath the continents, locked away in solid form within the Earth’s crust. These terrestrial reserves are chiefly found in evaporite deposits, which are remnants of ancient seas that dried up millions of years ago. For instance, the Messinian Salinity Crisis created a giant deposit beneath the Mediterranean Sea basin, containing an estimated 821,000 to 927,000 cubic kilometers of salt and associated sediments.
These deposits are primarily composed of halite (rock salt), alongside other less soluble mineral salts. While these massive deposits are geographically concentrated, the elements that form salt are also distributed throughout the Earth’s rocks. Elemental analysis shows that sodium, a key component of common salt, constitutes approximately 2.83% of the Earth’s crust by mass. Chlorine, the other main element in sodium chloride, is also a constituent of the crust, though its abundance is much lower.
The Global Salt Cycle: How Salt Moves
The salt on Earth is not static but constantly mobilized by a slow, continuous geological process known as the global salt cycle. This process begins with the weathering of continental rocks, where rainwater, made slightly acidic by dissolved carbon dioxide, breaks down minerals. Dissolved ions, including sodium and chloride, are then transported by rivers and streams as runoff into the oceans.
While river discharge is the most visible mechanism, other geological processes also contribute to the cycle. Submarine volcanism and hydrothermal vents at the seafloor release additional dissolved minerals into the water column. Over immense timescales, tectonic activity and the formation of sedimentary basins can trap and bury seawater, leading to the formation of massive evaporite deposits that constitute the terrestrial reserves.