Why is the ocean salty?

Some areas of the ocean are saltier than others. This image shows methane mussels living at the edge of a underwater brine pool in a cavern at a depth of 650 feet in the Gulf of Mexico. The pool of brine in the foreground is nearly four times as salty as seawater and is so dense that a submarine can float on the pool (in fact, this photo was shot from a submarine).

Ocean salt primarily comes from rocks on land

You may know that the oceans cover about 70 percent of the Earth's surface, and that 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. But, where did all this salt come from? If you get into folk stories and mythology you will see that almost every culture has a story explaining how the oceans became salty.

 The answer is really very simple. 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 35 of 1,000 (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 (at a concentration of 0.000005 parts per million). 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!).

Salt in the ocean comes from rocks on land.
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 (which forms from carbon dioxide and water).
As the rain erodes the rock, acids in the rainwater break down the rock. This process creates ions, or electrically charged atomic particles. These ions are carried away in runoff to streams and rivers and, ultimately, 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.
Two of the most prevalant ions in seawater are chloride and sodium. Together, they make up over 90 percent of all dissolved ions in the ocean. Sodium and Chloride are 'salty.'
The concentration of salt in seawater (salinity) is about 35 parts per thousand, on average. Stated in another way, about 3.5 percent of the weight of seawater comes from the dissolved salts.

Why is the ocean salty? Why is the ocean salty? Reviewed by Suman Ranganath on January 13, 2018 Rating: 5

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