Why Is the Ocean Salty?
Rivers and lakes are mostly fresh, yet the ocean they flow into is salty enough that you cannot drink it. That raises a fair question: if rivers carry fresh water to the sea, where does all the salt come from? Two US science agencies, NOAA and the US Geological Survey (USGS), give the same core answer. Most of it comes from rocks.
1. How much salt are we talking about?
According to both agencies, the average salinity of seawater is about 35 parts per thousand. Put another way, about 3.5 percent of the weight of seawater is dissolved salts. A kilogram of seawater holds roughly 35 grams of dissolved salts.
The USGS adds some numbers for scale. A cubic mile of seawater would contain about 120 million tons of salt, measured as sodium chloride. And by some estimates, if all the salt in the ocean could be removed and spread evenly over Earth's land, it would form a layer more than 500 feet thick, about the height of a 40-story office building.
2. Source 1: rocks on land
NOAA calls rocks on land the major source of salts dissolved in seawater. The process works like this:
- Rain is slightly acidic. The USGS explains that rain picks up some carbon dioxide from the air, which forms a weak acid called carbonic acid.
- Rain breaks down rock. Rain wears rocks down physically, and the weak acid breaks them down chemically, releasing salts and minerals as ions, tiny electrically charged particles.
- Rivers carry the ions to the sea. The dissolved ions travel in runoff to streams and rivers, and then to the ocean.
3. Why rivers do not taste salty
If rivers carry salt, why don't they taste salty? The amount of dissolved minerals in a river at any moment is small. The difference is what happens next. Rivers keep flowing, so their water is constantly replaced. The ocean is the end of the line.
NOAA and the USGS both describe what happens to the ions once they arrive. Many are used by ocean organisms and removed from the water. Others are not used up, so they stay for long periods and their concentrations increase over time. Meanwhile, water leaves the ocean mostly by evaporation, which takes the water but leaves the salt behind. Over enormous stretches of time, that slow buildup made the sea salty.
4. Source 2: openings in the seafloor
NOAA describes a second source: hydrothermal fluids from vents in the seafloor. Ocean water seeps into cracks in the seafloor and is heated by magma. The heat drives chemical reactions. The water tends to lose oxygen, magnesium, and sulfates, and picks up metals such as iron, zinc, and copper from surrounding rocks. The heated water then flows back out through the vents, carrying those metals with it.
Underwater volcanic eruptions also release minerals directly into the ocean.
5. Source 3: salt domes
NOAA also mentions salt domes, vast deposits of salt that form over geological time and are found underground and under the sea around the world. They are common along the continental shelf of the northwestern Gulf of America (formerly the Gulf of Mexico), where they add to the ocean's saltiness.
6. What the salt is made of
Ocean "salt" is a mix of ions, but two dominate: chloride and sodium, the same pair that makes up table salt. NOAA says these two make up around 85 percent of all dissolved ions in the ocean, while the USGS puts their share at over 90 percent. Either way, they are the large majority. NOAA adds that magnesium and sulfate make up about another 10 percent, and other ions are found in very small amounts.
7. Is the ocean equally salty everywhere?
No. NOAA explains that salinity varies with temperature, evaporation, and precipitation. In general, salinity is low at the equator and at the poles, and higher at mid-latitudes. Rain adds fresh water and dilutes the salt, while evaporation removes water and concentrates it.
8. A surprising extra: gold
The USGS notes that a cubic mile of seawater can contain up to 25 pounds of gold and up to 45 pounds of silver. Before anyone gets ideas, it also points out how big a cubic mile is: about 1.1 trillion gallons of water. The metals are spread far too thin to collect easily.
9. Key takeaways
- Seawater averages about 35 parts per thousand salt, or about 3.5 percent of its weight.
- Most ocean salt comes from rocks on land, broken down by slightly acidic rain and carried to the sea by rivers.
- The ocean stays salty because evaporation removes water but leaves salt behind, so salt builds up over time.
- Hydrothermal vents, underwater volcanoes, and salt domes add more salts and minerals.
- Chloride and sodium make up the large majority of dissolved ions, and salinity varies from place to place.