Why is the ocean salty?

Oceans cover about 70 percent of the Earth's surface and about 97 percent of all water on and in the Earth is saline—there's a lot of salty water on our planet. By some estimates, if the salt in the ocean could be removed and spread evenly over the Earth’s land surface it would form a layer more than 500 feet (166 meters) thick, about the height of a 40-story office building (NOAA). But, where did all this salt come from? Salt in the ocean comes from rocks on land. Here's how it works:

From precipitation to the land to the rivers to the sea:

The rain that falls on the land contains some dissolved carbon dioxide from the surrounding air. This causes the rainwater to be slightly acidic due to carbonic acid. The rain physically erodes the rock and the acids chemically break down the rocks and carries salts and minerals along in a dissolved state as ions. The ions in the runoff are carried to the streams and rivers and then to the ocean. Many of the dissolved ions are used by organisms in the ocean and are removed from the water. Others are not used up and are left for long periods of time where their concentrations increase over time.

The two ions that are present most often in seawater are chloride and sodium. These two make up over 90% of all dissolved ions in seawater. By the way, the concentration of salt in seawater (salinity) is about 35 parts per thousand. In other words, about 3.5% of the weight of seawater comes from the dissolved salts; in a cubic mile of seawater the weight of the salt, as sodium chloride, would be about 120 million tons. And, just so you don't think seawater is worthless, a cubic mile of it also can contain up to 25 pounds of gold and up to 45 pounds of silver! Before you go out and try alchemy on seawater, though, just think about how big a cubic mile is (1 cubic mile contains 1,101,117,147,000 gallons!).

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All water on Earth in a sphere, placed over a "dry" globe
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This image shows blue spheres representing relative amounts of Earth's water in comparison to the size of the Earth. Are you surprised that these water spheres look so small? They are only small in relation to the size of the Earth. These images attempt to show three dimensions, so each sphere represents "volume." They show that in comparison to the volume of the globe, the amount of water on the planet is very small. Oceans account for only a "thin film" of water on the surface.

Spheres representing all of Earth's water, Earth's liquid fresh water, and water in lakes and rivers

The largest sphere represents all of Earth's water. Its diameter is about 860 miles (the distance from Salt Lake City, Utah, to Topeka, Kansas) and has a volume of about 332,500,000 cubic miles (mi3) (1,386,000,000 cubic kilometers (km3)). This sphere includes all of the water in the oceans, ice caps, lakes, rivers, groundwater, atmospheric water, and even the water in you, your dog, and your tomato plant.

Liquid fresh water

How much of the total water is fresh water, which people and many other life forms need to survive? The blue sphere over Kentucky represents the world's liquid fresh water (groundwater, lakes, swamp water, and rivers). The volume comes to about 2,551,100 mi3 (10,633,450 km3), of which 99 percent is groundwater, much of which is not accessible to humans. The diameter of this sphere is about 169.5 miles (272.8 kilometers).

Water in lakes and rivers

Do you notice the "tiny" bubble over Atlanta, Georgia? That one represents fresh water in all the lakes and rivers on the planet. Most of the water people and life of earth need every day comes from these surface-water sources. The volume of this sphere is about 22,339 mi3 (93,113 km3). The diameter of this sphere is about 34.9 miles (56.2 kilometers). Yes, Lake Michigan looks way bigger than this sphere, but you have to try to imagine a bubble almost 35 miles high—whereas the average depth of Lake Michigan is less than 300 feet (91 meters).

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Mineral: Halite (NaCl)
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This photo shows a road salt storage stockpile that is awaiting use for pavement deicing, Port of Milwaukee, WI. The use of salt to deice pavement can be harmful to aquatic life in urban streams. The USGS is involved in studies that focus on the influence of winter runoff on aquatic ecosystems.