Short answer: the ocean is salty because rain slowly dissolves rock on land, rivers carry those dissolved minerals to the sea, and some of them never leave. Add hot water squirting out of cracks in the seafloor and the odd underwater volcano, and you get a planet-sized bowl of brine. Here is how that actually works, with the receipts.
How salty is it, exactly?
Seawater averages about 35 parts per thousand of salt. Put another way, about 3.5 percent of the weight of seawater is dissolved salts (NOAA Ocean Service; USGS Water Science School).
That sounds small until you scale it. The USGS works out that one cubic mile of seawater holds about 120 million tons of salt, counted as sodium chloride. And if you pulled all the salt out of the ocean and spread it over the land, NOAA estimates it would make a layer more than 500 feet thick, roughly the height of a 40-story office building (USGS).
Bonus weirdness from the same USGS page: seawater also carries gold. Up to about 25 pounds per cubic mile. It is spread so thin, at 0.000005 parts per million, that nobody is getting rich off it.
Step one: rain eats rocks
Rain is not perfectly pure water. As it falls, it picks up some carbon dioxide from the air, which makes it slightly acidic thanks to carbonic acid (USGS).
That weak acid works on rock. Over and over, for a very long time, it breaks rock down and frees up charged particles called ions. The runoff carries those ions into streams and rivers, and the rivers carry them to the ocean. NOAA puts it simply: ocean salt comes primarily from rocks on land and openings in the seafloor (NOAA).
Step two: the ocean keeps some of it
So why don't rivers taste like the sea? Because they are the delivery truck, not the warehouse. Rivers carry small amounts of dissolved minerals and keep moving. The ocean is where they pile up.
Once ions reach the sea, many of them get used by ocean life and removed from the water. But others are not used up. Those stay for long periods, and their concentrations increase over time (USGS; NOAA). The leftovers are what make seawater taste like seawater.
Step three: the seafloor leaks
Land is not the only source. Ocean water seeps into cracks in the seafloor and gets heated by magma. That hot water loses oxygen, magnesium and sulfates, and picks up metals like iron, zinc and copper from the surrounding rock. Then it comes back out through hydrothermal vents (NOAA).
This is not a side process. The USGS estimates the entire volume of the world's oceans could circulate through hydrothermal vent systems every 10 to 20 million years. The whole ocean, through the plumbing of the planet, on repeat.
Underwater volcanic eruptions add more. They release minerals straight into the sea (NOAA).
What is actually in the salt?
Mostly the stuff in your salt shaker. Chloride and sodium make up around 85 percent of all dissolved ions in the ocean. Magnesium and sulfate make up about another 10 percent. Everything else is there in very small amounts (NOAA).
Not all seawater is equally salty
Salinity shifts with temperature, evaporation and precipitation. NOAA says it is generally low at the equator and at the poles, and high at the mid-latitudes (NOAA). Rain adds fresh water. Evaporation takes water away and leaves the salt behind.
Scientists now track this from orbit. NASA measures sea surface salinity with satellites (NASA), and NOAA's Climate.gov has mapped how surface saltiness shifted between 2004 and 2013 (Climate.gov).
The truly weird part: salt lakes on the seafloor
Salt also sits in giant buried deposits called salt domes. They form over geological timescales and are found underground and undersea around the world. They are common on the continental shelf of the northwestern Gulf of America (NOAA).
Where that salt meets seawater, you get brine: water far saltier than the ocean around it. NOAA's Flower Garden Banks National Marine Sanctuary has a brine seep at East Flower Garden Bank, about 240 feet down (NOAA; Flower Garden Banks NMS). Extra-salty water, leaking out of the seafloor, inside the salty sea.
The one-line version
- Slightly acidic rain breaks down rock on land.
- Rivers carry the dissolved ions to the sea.
- Marine life uses some; the rest build up.
- Hydrothermal vents and underwater volcanoes add more from below.
- Result: about 3.5 percent salt by weight, mostly sodium and chloride.
Next time you get a mouthful of ocean, that is millions of years of rock, rain and volcano in one gulp. Deeply strange.
Sources: NOAA Ocean Service, "Why is the ocean salty?" · USGS Water Science School, "Why is the ocean salty?" · NASA PO.DAAC, sea surface salinity · Flower Garden Banks NMS, brine seep