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Why Is the Sea Salty?

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Pacific Ocean / Jennifer Kennedy, Licensed to About.com

Pacific Ocean

© Jennifer Kennedy, Licensed to About.com
Question: Why Is the Sea Salty?

Ocean water is commonly called salt water. This is because the ocean is made up of freshwater plus minerals commonly called "salts." The minerals present in largest amounts are sodium and chloride, which combine to form what we use as table salt. But ocean water also includes other minerals such as calcium, magnesium, potassium, sulfate, strontium, bromine and carbon, plus are other minerals - in fact, it is thought that all the naturally-occurring elements on Earth are present in ocean water in some amount.

The amount of minerals in the ocean has stayed about the same for millions of years, across all the oceans. How did these "salts" get into the ocean, and remain in a fairly constant ratio?

Answer:

Nobody knows for sure how the ocean formed, but the process is thought to have started about 4 billion years ago. The ocean is thought to have originated from water vapor present in the Earth. The Earth's surface, after The Earth's surface and atmosphere were hot, but eventually cooled, and water vapor eventually evaporated, formed clouds and caused rain. Over a long time, the rain poured into depressions on the Earth's surface, creating the first oceans.

The "salts" in ocean water come from several sources. These include:

  • Rocks on land - As rain water, which is slightly acidic, flows across rocks, it erodes them, causing minerals to be added to the fresh water. This water is then carried from land to the ocean by rivers and streams, and through underground passages.
  • Volcanic eruptions - These eruptions cause out-gassing, or the emission of large amounts of water vapor. The minerals from this out-gassing can end up in the ocean.
  • Hydrothermal vents - The sea water seeps into cracks in the ocean floor, is heated up by magma, and then the mineral-rich water is released through hydrothermal vents and into the ocean.
  • Wind - Small amounts of minerals may also be picked-up by wind as it blows across the land toward the ocean.

How Does the Ocean's Salinity Stay the Same?

The processes are not completely understood, but the salinity of the ocean stays about the same over time. It hovers around about 35 parts per thousand, which means there are 35 grams of salt per liter, or that about 3.5% of the weight of sea water comes from salts. When water evaporates from the ocean, the salts stay behind. Some of the minerals in the ocean sink to the ocean floor and become part of the ocean bottom, or are used by marine animals to build skeletons or shells. Water is continually added to the ocean through rainfall and runoff. Through these and other complex processes, the salinity in the ocean remains fairly constant over time.

Resources and Further Information

  • Helmenstine, A.M. Why Is the Ocean Salty?. About.com. Accessed March 18, 2013.
  • NOAA. Salinity Data. NOAA National Oceanographic Data Center. Accessed March 18, 2013.
  • Rice, T. 2009. "Why Is the Sea Salty." In Do Whales Get the Bends?. Sheridan House: New York.
  • Swenson, H. Why Is the Ocean Salty? U.S.G.S. Accessed March 18, 2013.
  • Tarbuck, E.J., F. K. Lutgens and D. Tasa. 2009. Earth Science. Pearson Prentice-Hall: New Jersey.
  • U.S.G.S. Why Is the Ocean Salty? Accessed March 18, 2013.
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