Why are oceans salty but not lakes

Why is the sea salty and rivers and lakes aren’t? Also, what’s the difference between sea salt and regular table salt?

The earth’s water goes through a continuous cycle of being evaporated and rained back down. Whenever evaporation happens, water goes up as vapor with essentially no salt in it.

Water falls all over the planet, but when it passes through soil and rocks it slowly dissolves bits of minerals, including sodium chloride (salt). This means rivers and lakes have tiny fragments of salt in them, which — little by little — are carried into the sea.

Rivers and lakes are replenished with fresh rainwater, but oceans are a sort of dumping ground where water with accumulated salt keeps adding to the salinity. There are also vents and volcanoes under the ocean that increase the amount of minerals, especially salt.

A detailed account of why some elements and compounds are more abundant is a long story, but take it as fact that of all minerals, sodium chloride is the most common highly soluble one.

This might lead you to worry that the oceans will keep getting saltier with time, but other processes, such as the formation of minerals at the bottom of the ocean, take some salt out.

Sea salt is obtained by evaporating sea water, so it tends to include tiny pieces of whatever else is in the sea, affecting taste and color.

Table salt is mined from underground deposits, but is usually processed to remove impurities and is nearly 98 percent sodium chloride. Sea salt is about 85 percent pure (with 15 percent mainly other minerals).

Iodine is usually added to table salt to help avoid dietary deficiencies, and chemicals like calcium silicate are mixed in to absorb moisture to stop the salt from clumping.

Ask Dr. Knowledge is written by Northeastern University physicist John Swain. E-mail questions to or write to Dr. Knowledge, c/o The Boston Globe, PO Box 55819, Boston, MA 02205-5819.

It all comes down to a thing called the water- or hydrological cycle.

Rain, which is fresh water, falls from clouds onto the land and finds its way into lakes and rivers, and also through the ground, back to the sea, picking up salts and minerals as it goes.

Once it reaches the sea, the water can be evaporated again to form new clouds containing fresh water, and the salt is left behind; so, over millions of years, the oceans have slowly been accumulating salt washed off the land by fresh water.

So is the sea becoming more salty now?

Probably not, because if the level of salt rises any further the extra is removed by various processes, including chemical reactions, so the sea is now about as salty as it is going to get.

That's not to say you can't get saltier seas – like the Dead Sea: these are just bodies of water cut off from the main ocean and in which more water is evaporating than being returned by rivers, so the water becomes more concentrated, or saltier...

Why is the sea water salty, and not the water of the big lakes? Is the salt concentration changing over time?

It is thought that the salt in the oceans stems from erosion of bedrock on continents, where the minerals from these rocks are eventually carried out by rivers to the oceans. Over time, the oceans, which act as the final sink for almost all rivers, become more salty. Rivers that don't make it to the ocean are trapped in basins known as "endoheric" basins, instead ending in a lake (like in the case of the Dead Sea) that also gets saltier over time. Most big lakes with outlets are fed by fresh water, either from the atmosphere as rain, from the melting of glaciers, or from underground aquifers, leaving them less salty.

People don't really know what happens to the salt concentration (or salinity) of the oceans over time; Accurate salinity maps of oceans are hard to produce since oceans are so big. As the Earth's temperature rises in the future (both from natural and human effects) - two things will happen to the salinity. First, increased evaporation over oceans will tend to make the salinity rise. Second, increased melting at the poles will bring more freshwater into the oceans, which will decrease the salinity. Which of these two effects dominates the water cycle in the future will determine the change in salt concentration.

For more information about the salinity data that is available (in North America), check out the National Oceanographic Data Center's database.

This page was last updated on June 27, 2015.

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