Why are the oceans salty?

Answer

Salt disolves well in water streams/rivers pick it up and drain it to the ocean. There the water can evaporate but the salt stays.

Answer

Ocean water is a complex solution of mineral salts and of decaying/decayed biological matter that results from the abundance of life in the seas. Most of the ocean's salts were derived from gradual processes such the breaking up of the cooled igneous rocks of the Earth's crust by weathering and erosion, the wearing down of mountains, and the dissolving action of rains and streams which transported their mineral washings to the sea. Some of the ocean's salts have been dissolved from rocks and sediments below its floor. Other sources of salts include the solid and gaseous materials that escaped from the Earth's crust through volcanic vents or that originated in the atmosphere. This process has been occurring for hundreds of millions of years, and therefore early oceans wouldn't have been as salty as they are today.

The ocean is not 'diluted' by the addition of fresh water through rain and rivers because the saltiness of the ocean is the result of several natural influences and processes, the salt load of the streams entering the ocean is just one of these factors. In addition, salts become concentrated in the sea because the sun's heat distills or vaporizes almost pure water from the surface of the sea and leaves the salts behind (this process is part of the continual exchange of water between the Earth and the atmosphere that is called the hydrologic cycle).

Why the Ocean is Salty

Most of the salt in the oceans came from land. Over millions of years, rain, rivers, and streams have washed over rocks containing the compound sodium chloride (NaCl), and carried it into the sea. You may know sodium chloride by its common name: table salt! Some of the salt in the oceans comes from undersea volcanoes and hydrothermal vents. When water evaporates from the surface of the ocean, the salt is left behind. After millions of years, the oceans have developed a noticeably salty taste.

Salty Ocean

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:

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). The rain erodes the rock and the acid breaks down the rocks and carries it along in a dissolved state as ions. The ions in the runoff are carried to the streams and rivers 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 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 tons of gold and up to 45 tons of silver! Before you go out and try alchemy on seawater, though, just think about how big a cubic mile is.

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 m) thick, about the height of a 40-story office building.

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