Most people are aware that the oceans cover 70 percent of the earth’s surface. In addition to this, 97 percent of water on and in Earth is saline. In fact, the amount of salt in the ocean is so large that it would form a layer over 500 feet thick on land, the equivalent of a 40-story office building. While no one knows for sure what causes the oceans to be so salty, there are a few theories, including a folklore explanation.

Sources of ocean salt

The majority of ocean salt comes from hydrothermal vents. These vents are underground formations where low-salinity ocean water seeps in and becomes superheated by magma. As the water flows back up to the surface, it is mineralized. Volcanic outgassing also contributes to the ocean’s saltiness. This type of saltiness has important effects on the environment, both on land and on the sea.

Ocean salt comes from two main sources: runoff from land and hydrothermal vents from deep-sea volcanoes. Though all oceans are connected to each other, seawater salinity is not uniformly high around the globe. In areas with higher evaporation, sea water becomes saltier and less pristine, while in dry regions, the amount of precipitation decreases. Likewise, Mediterranean Sea water is saltier than the Black Sea and the Atlantic. Both come from the breakdown of minerals in the land, which are carried by rivers to the sea.

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Although the total amount of salt and salinity of ocean water vary from region to region, the ratio of major constituents and dissolved gases is roughly constant. In some areas, sea water is up to four times saltier than seawater, while in others it is less than half that amount. In general, seawater is about 35 parts per thousand (ppt) salinity. As a result, only 3.5 percent of the water is comprised of dissolved salts.

Erosion

Did you know that salt is an important part of the Earth’s ecosystem? The world’s oceans contain a great deal of salt, even though they are not all the same? Erosion has two main effects on salt concentration: one is the accumulation of salt atop mountains and a decrease in its concentration at sea level. Both processes occur over time, and the two can balance each other out.

The seas did not always appear to be so salty, but rainfall has contributed to the problem. Over millions of years, rainfall has eroded the surface of rocks, displacing mineral salts with it. As this salt solution is washed off the surface, it flows into rivers and streams, where it dissolves rock-based minerals. The salt solution ultimately ends up in seas, which act as a reservoir for this material.

Volcanic eruptions

Submarine volcanoes are another source of salt in the oceans. Like hydrothermal vents, these volcanoes erupt underwater and displace mineral components in the ocean basin. Because of their high salt content, ocean water is heavier than less brackish water and forms bowls beneath the brackish layer. In addition to altering the balance of ocean currents, high salinity also impairs marine life.

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Volcanic eruptions release water vapor, carbon dioxide, and other chemicals. This water is heated by magma at the Earth’s core. As the water is heated, it reacts with the mineral constituents of the rocks to produce sodium chloride. This salt is found in lakes, rivers, and oceans, so it’s no surprise that they are salty. Volcanic eruptions, however, also contribute to the salty composition of the oceans.

Carbonic acid

Oceans are salty because the carbon dioxide from the atmosphere dissolves in rainwater. Water also contains carbon dioxide, which is slightly acidic. The rain then flows through the land and carries mineral salts with it. The salts are carried by the runoff water to nearby rivers and lakes, where they eventually reach the sea. The amount of dissolved salts in seawater is around 35 parts per thousand by weight.

Despite the corrosive effects of carbon dioxide, the ocean has a large capacity to resist changes in pH. The ocean has around 38 billion tons of carbonate and bicarbonate ions, and this allows it to absorb CO2 without becoming acidic. In fact, the ocean has the greatest buffering capacity in the world! It can absorb more CO2 and still remain pH 7.

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