Where Does Salinity Come From? Ocean Salt Explained

97% of all water on Earth is salty, and the ocean covers about 70% of the entire Earth’s surface. But where does salinity come from, and why are some bodies of water saltier than others? The answer lies in millions of years of geological processes — rock weathering, river runoff, volcanic activity and evaporation — that have gradually accumulated dissolved minerals in the world’s oceans.

What Is Salinity?

Salinity is defined as the concentration of dissolved inorganic salts in seawater, typically measured in grams of salt per kilogram of seawater or parts per thousand (‰), with values generally ranging from near zero near river mouths to over 40‰ in areas like the Red Sea. ScienceDirect

According to NASA, in a kilogram of seawater there are approximately 34.482 grams of dissolved salt. The dissolved matter is mostly sodium chloride (NaCl) — sodium and chloride together make up about 85% by weight of all dissolved matter in seawater. NASA Salinity

Where Does Salinity Come From? The Main Sources

1. Rock Weathering and River Runoff

The primary source of ocean salinity is the natural weathering of rocks on land. Rain that falls on land is slightly acidic, and this acid erodes rocks. The rocks are made up of salts and minerals that then run into streams, rivers and oceans. Nessf

The accumulation of salts in oceans and seas occurred over several thousands of years, largely as the result of the natural weathering of rocks and of rain runoff carrying minerals into bodies of water. Encyclopedia Britannica

This is also why river water is fresh — when water evaporates from the ocean surface, the salts don’t evaporate with it. It’s this freshwater that eventually ends up in rivers. FutureLearn

2. Hydrothermal Vents

Salt also comes from the bottom of the sea. Hydrothermal vents are found on the crest of oceanic ridges and release dissolved minerals to the ocean. These are locations where seawater that has sunk into the Earth’s crust flows back into the ocean, bringing with it salts from the crust. Nessf

Estimates indicate that the entire volume of the world’s oceans could circulate through hydrothermal vent systems every 10–20 million years, giving this process a very significant effect on salinity. usgs

Source: USGS — Why Is the Ocean Salty?

3. Submarine Volcanism

Underwater volcanic eruptions also contribute to ocean salinity by releasing dissolved minerals directly into seawater. When seawater reacts with hot volcanic rock, it dissolves mineral constituents that remain in solution, gradually increasing the salt content of the oceans over geological timescales.

4. Evaporation

Evaporation concentrates salinity in enclosed or semi-enclosed bodies of water. Regions of high evaporation have elevated surface salinities, while regions of high precipitation have depressed surface salinities. This explains why the Red Sea, with limited freshwater input and high evaporation, has surface salinity rising to 41 psu, while the Baltic Sea can drop to 10 psu or less. Encyclopedia BritannicaEncyclopedia Britannica

Where Does Saline Come From?

Saline — a solution of salt dissolved in water — comes from the same source as ocean salinity: sodium chloride (NaCl) dissolved in water. In industrial and medical contexts, saline is produced by dissolving food-grade or pharmaceutical-grade sodium chloride in purified water at specific concentrations. The raw material — sodium chloride — is extracted from underground rock salt deposits, solar evaporation ponds or vacuum evaporation plants.

Why Is the Ocean Saltier Than Rivers?

Salt that is found on land can come from evaporated ancient oceans. For instance, in the south of Spain today, there are large deposits of gypsum and salt that formed during the Messinian Salinity Crisis. FutureLearn

Rivers carry dissolved minerals from rocks to the ocean, but river water remains fresh because the salt accumulates in the ocean while freshwater evaporates back into the atmosphere. Over millions of years, this cycle has progressively increased ocean salinity to its current average of approximately 3.5%.

The concentration of salt in seawater varies with temperature, evaporation and precipitation. Salinity is generally low at the equator and at the poles, and high at mid-latitudes. The average salinity is about 35 parts per thousand. NOAA National Ocean Service

Source: NOAA Ocean Service — Why Is the Ocean Salty?

Salinity Around the World: Key Variations

The range of salinity observed in the open ocean is from 33 to 37 grams of salt per kilogram of seawater. Salt concentrations as high as 256 psu have been found in hot but dense pools of brine trapped in depressions at the bottom of the Red Sea. Encyclopedia Britannica

Notable examples of salinity variation:

• Open ocean average — 35 psu
• Red Sea — up to 41 psu due to high evaporation and limited freshwater input
• Baltic Sea — as low as 10 psu due to high freshwater input from rivers
• Dead Sea — approximately 315 psu, nearly nine times the salinity of the ocean, with no outlet to dilute its mineral concentration

Source: Britannica — Salinity

The Connection Between Salinity and Salt Production

Understanding where salinity comes from explains why certain regions of the world are ideal for salt production. Areas with high evaporation rates, shallow coastal waters and low rainfall — such as the Mediterranean coast, the Indian subcontinent and the Chilean Atacama — have historically been centres of solar salt production. The same geological processes that created ocean salinity over millions of years also formed the underground rock salt deposits mined today in the USA, Germany, Canada and Poland.

Frequently Asked Questions

• Where does salinity come from in the ocean?
  Ocean salinity comes primarily from rock weathering and river runoff, hydrothermal vents on the ocean floor, submarine volcanism and the accumulation of minerals over millions of years of evaporation cycles.
• Where does saline come from?
  Saline solution is produced by dissolving sodium chloride in purified water. The sodium chloride itself comes from underground rock salt deposits, solar evaporation or vacuum evaporation processes.
• Why is the ocean salty but rivers are not?
  Because salt accumulates in the ocean while freshwater evaporates back into the atmosphere. Rivers carry minerals to the sea but the water — not the salt — evaporates, progressively concentrating salinity over geological timescales.
• What is the average salinity of the ocean?
  The average salinity of the ocean is approximately 35 parts per thousand — meaning 35 grams of dissolved salt per kilogram of seawater.