Anyone who has swallowed a mouthful of seawater at the beach already knows the ocean is salty, but why is the ocean salty in the first place is a question that traces back to rocks, rivers, and geological processes happening over enormous stretches of time. The short version is that rainwater slowly dissolves minerals out of rocks on land, rivers carry those dissolved minerals to the sea, and the water molecules eventually evaporate back into the atmosphere while the salt stays behind. This article breaks down exactly where that salt originates, why the ocean has not simply kept getting saltier forever, and why some seas are noticeably saltier than others.
Quick Answer
The ocean is salty primarily because rainwater dissolves minerals out of rocks on land, and rivers carry those dissolved salts into the sea. Underwater hydrothermal vents and volcanic activity add more dissolved minerals from the ocean floor. Because water evaporates from the ocean surface while the dissolved salts stay behind, salt has gradually accumulated in seawater over geological time.
Quick Facts
- Average ocean salinity is about 35 parts per thousand, meaning roughly 35 grams of dissolved salt per kilogram of seawater.
- Sodium and chloride together make up about 85 percent of the dissolved ions in seawater.
- Rivers deliver mineral-rich runoff to the ocean, while hydrothermal vents on the seafloor add dissolved minerals from inside the Earth’s crust.
- The entire volume of the world’s oceans is estimated to circulate through hydrothermal vent systems roughly every 10 to 20 million years.
- Ocean salinity varies by region, from about 40 parts per thousand in the Red Sea to as low as 7 to 8 parts per thousand in parts of the Baltic Sea.
Where Does the Salt in the Ocean Come From?
Most of the salt in the ocean originates from the weathering of rocks on land. Rainwater absorbs small amounts of carbon dioxide from the atmosphere as it falls, which makes it slightly acidic. That mildly acidic rainwater slowly dissolves minerals out of rocks and soil as it flows across the landscape, a process called chemical weathering. This releases ions such as sodium, chloride, sulfate, and magnesium into the water.
Salt does not come only from the land, however. Hydrothermal vents located along mid-ocean ridges are another major source. These vents form where seawater seeps down into cracks in the ocean floor, gets heated by magma beneath the crust, dissolves minerals out of the surrounding rock, and then flows back out into the ocean carrying those dissolved minerals with it. According to the U.S. Geological Survey, the entire volume of the world’s oceans could cycle through these hydrothermal vent systems roughly every 10 to 20 million years, making this an important ongoing contributor to ocean salinity. Underwater volcanic eruptions add further dissolved minerals directly into the water as well.
How Do Rivers Add Salt to the Ocean?
Rivers act as the delivery system that carries dissolved minerals from weathered rock on land all the way to the sea. As rainwater and runoff move across the landscape, they pick up dissolved ions from rocks and soil, and rivers then transport that mineral-rich water downstream. Every river on Earth carries at least some dissolved salt, even rivers that taste completely fresh to drink, because the concentration in any single river is extremely low compared to the ocean.
Over the course of geological time, however, the constant, ongoing discharge of countless rivers has added an enormous cumulative amount of dissolved salt to the world’s oceans. Rivers alone do not make the ocean feel salty at any given moment, but their steady, long-term contribution is one of the two largest sources of ocean salt, alongside hydrothermal activity on the seafloor.
Read This How Deep Is the Ocean? Exploring Earth’s Deepest Waters
Why Doesn’t the Ocean Become Saltier Forever?
Although rivers and hydrothermal vents continuously add dissolved salt to the ocean, salinity is not simply increasing without limit. Several natural processes remove salt from seawater at roughly the same pace that new salt is being added, keeping the ocean’s overall salinity in what scientists describe as a dynamic, quasi-steady state rather than a constantly rising figure.
Some of the dissolved minerals brought in by hydrothermal vent systems actually react with the surrounding basalt rock and are pulled back out of the seawater during that same circulation process. Other salt is removed through the formation of evaporite mineral deposits, the burial of salt-laden sediments and pore water on the seafloor, the formation of certain clay minerals, and the uptake of dissolved minerals by marine organisms building shells and skeletons. Because addition and removal are roughly balanced over long timescales, the ocean’s overall salt content has remained relatively stable rather than climbing indefinitely.
How Does Salt Leave the Ocean?
Salt leaves the ocean through several different pathways, most of which happen slowly and are easy to overlook compared to the more visible process of rivers adding salt in. Evaporite deposits form when seawater in shallow, enclosed basins evaporates enough that dissolved salts crystallize out and settle as solid mineral layers, sometimes creating large underground or undersea salt deposits over time.
Chemical reactions between seawater and oceanic crust during hydrothermal circulation also remove certain elements, such as magnesium and sulfate, even as the same process adds other minerals back into the water. Marine organisms play a role too, since many sea creatures extract dissolved calcium and other minerals from seawater to build shells and skeletons, which eventually settle to the seafloor as sediment when the organisms die. Some ocean sediment, including buried salts, is even carried down into the Earth’s mantle at subduction zones, permanently removing that material from active circulation in the ocean.
Read This Why Do Dolphins Jump Out of Water? 7 Scientific Reasons Explained
Why Is Seawater Saltier Than Freshwater?
Seawater is saltier than river water or rainwater because of what happens after water evaporates from the ocean’s surface. When the sun heats the ocean, water molecules evaporate into the atmosphere, but the dissolved salts they were carrying do not evaporate along with them. That evaporated water eventually falls back to Earth as rain, which is why rain and the rivers it feeds are relatively fresh, even though that same rainwater will go on to dissolve more minerals as it flows toward the sea.
The ocean, by contrast, acts as the final collection point for that mineral-laden runoff. Since only the water evaporates while the salt stays behind, and rivers keep delivering fresh mineral loads, the ocean has slowly accumulated far more dissolved salt than any single river or lake typically holds. This is also why closed inland bodies of water with no outlet, such as the Dead Sea, can become dramatically saltier than the ocean itself over time, since they lose water to evaporation without ever draining any of that accumulated salt away.
Are All Oceans and Seas Equally Salty?
No, salinity varies noticeably across different oceans and seas, mainly because of differences in evaporation rates, freshwater input, and how enclosed or open a body of water is. The Red Sea is one of the saltiest bodies of water connected to the open ocean, with salinity often exceeding 40 parts per thousand, largely because it sits in a hot, dry region with high evaporation and very little freshwater inflow from rivers or rainfall.
The Baltic Sea sits at the opposite extreme, with salinity as low as roughly 7 to 8 parts per thousand in some areas, since it receives large amounts of freshwater from surrounding rivers and has only a narrow connection to the North Sea and Atlantic Ocean. The Mediterranean Sea also tends to run saltier than the open ocean average due to high evaporation, while regions like the Bay of Bengal run lower than average because of substantial freshwater discharge from major rivers. Among the major ocean basins, the Atlantic Ocean is generally considered the saltiest, while the Arctic Ocean tends to be the least salty due to lower evaporation rates and significant freshwater input from melting ice and river discharge.
Read This How Deep Is the Ocean? Exploring Earth’s Deepest Waters
How Salty Is the Ocean?
The average salinity of the world’s open oceans is about 35 parts per thousand, meaning that a kilogram of average seawater contains roughly 35 grams of dissolved salt, or about 3.5 percent salt by weight. Sodium and chloride ions, the two components of ordinary table salt, together make up about 85 percent of all the dissolved ions in seawater, with magnesium and sulfate contributing most of the remainder, along with smaller amounts of many other dissolved minerals.
This average masks meaningful regional variation, since typical open-ocean salinity generally ranges from about 33 to 37 parts per thousand depending on location, while enclosed or semi-enclosed seas can fall well outside that range in either direction, as seen in the unusually salty Red Sea or the comparatively fresh Baltic Sea.
Frequently Asked Questions
What is the main reason the ocean is salty?
The ocean is salty mainly because rivers carry dissolved minerals from weathered rock on land into the sea, while hydrothermal vents add further dissolved minerals from the ocean floor.
Why doesn’t rain make the ocean less salty?
Rain adds freshwater to the ocean, but that water eventually evaporates again while the dissolved salt stays behind, so rainfall does not reduce the ocean’s overall salt content.
Is the ocean getting saltier over time?
Overall ocean salinity has remained relatively stable over long timescales, since natural processes remove salt from seawater at roughly the same rate that rivers and hydrothermal vents add it.
Why is the Dead Sea so much saltier than the ocean?
The Dead Sea has no outlet, so water is lost only through evaporation while the dissolved salt it carries stays behind and accumulates, making it far saltier than the open ocean.
What is the average salinity of ocean water?
The average salinity of the open ocean is about 35 parts per thousand, or roughly 35 grams of dissolved salt per kilogram of seawater.
Conclusion
The ocean’s saltiness is not a mystery so much as the long-term outcome of ordinary geological processes playing out over millions of years, from rainwater dissolving minerals on land to hydrothermal vents reshaping seawater chemistry deep on the ocean floor. Because the sources and sinks of salt have roughly balanced out over time, the sea has settled into the salty, life-sustaining chemistry familiar to anyone who has ever taken a swim in it. The next splash of seawater is really a small sample of billions of years of rock, rain, and ocean chemistry working together.
Ethan Carter is a nature enthusiast and educational writer at Wonderexplained. He creates well-researched articles about animals, oceans, weather, and the natural world, making complex topics easy to understand. His goal is to provide accurate, engaging, and reader-friendly content that inspires curiosity and lifelong learning.
