7/25/2019 Why is the Sea Salty

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Why is the sea salty?

Explanation text by Diondra Rizkiawan

Everyone who has been to the beach knows that seawater is salty.

Everyone also knows that fresh water in rain, rivers, and even ice is notsalty. Why are some of Earths waters salty and others not? There aretwo clues that give us the answer. First, fresh water is not entirelyfree of dissolved salt. Even rainwater has traces of substancesdissolved in it that were !icked u! during !assage through theatmos!here. "uch of this material that washes out of theatmos!here today is !ollution, but there are also natural substances!resent.#s rainwater !asses through soil and !ercolates through rocks, itdissolves some of the minerals, a !rocess called weathering. This is thewater we drink, and of course, we cannot taste the salt because its

concentration is too low. Eventually, this water with its small load ofdissolved minerals or salts reaches a stream and $ows into lakes andthe ocean. The annual addition of dissolved salts by rivers is only a tinyfraction of the total salt in the ocean. The dissolved salts carried by allthe worlds rivers would e%ual the salt in the ocean in about &'' to (''million years.# second clue to how the sea became salty is the !resence of salt lakessuch as the )reat *alt +ake and the ead *ea. -oth are about ' timessaltier than seawater. Why are these lakes salty while most of theworlds lakes are not? +akes are tem!orary storage areas for water./ivers and streams bring water to the lakes, and other rivers carry

water out of lakes. Thus, lakes are really only wide de!ressions in ariver channel that have 0lled with water. Water $ows in one end andout the other.The )reat *alt +ake, ead *ea, and other salt lakes have no outlets. #llthe water that $ows into these lakes esca!es only by eva!oration.When water eva!orates, the dissolved salts are left behind. *o a fewlakes are salty because rivers carried salts to the lakes, the water inthe lakes eva!orated and the salts were left behind. #fter years andyears of river in$ow and eva!oration, the salt content of the lake waterbuilt u! to the !resent levels. The same !rocess made the seas salty./ivers carry dissolved salts to the ocean. Water eva!orates from the

oceans to fall again as rain and to feed the rivers, but the salts remainin the ocean. -ecause of the huge volume of the oceans, hundreds ofmillions of years of river in!ut were re%uired for the salt content tobuild to its !resent level./ivers are not the only source of dissolved salts. #bout twenty yearsago, features on the crest of oceanic ridges were discovered thatmodi0ed our view on how the sea became salty. These features, knownas hydrothermal vents, re!resent !laces on the ocean $oor where sea

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water that has see!ed into the rocks of the oceanic crust, has becomehotter, and has dissolved some of the minerals from the crust, now$ows back into the ocean. With the hot water comes a largecom!lement of dissolved minerals. Estimates of the amount ofhydrothermal $uids now $owing from these vents indicate that the

entire volume of the oceans could see! through the oceanic crust inabout ' million years. Thus, this !rocess has a very im!ortant e1ecton salinity. The reactions between seawater and oceanic basalt, therock of ocean crust, are not one2way, however3 some of the dissolvedsalts react with the rock and are removed from the water.# 0nal !rocess that !rovides salts to the oceans is submarinevolcanism, the eru!tion of volcanoes under water. This is similar to the!revious !rocess in that seawater is reacting with hot rock anddissolving some of the mineral constituents.Will the oceans continue to become saltier? 4ot likely. 5n fact the seahas had about the same salt content for many hundred of millions if

not billions of years. The salt content has reached a steady state.issolved salts are being removed from seawater to form new mineralsat the bottom of the ocean as fast as rivers and hydrothermal!rocesses are !roviding new salts.

We cansummari6e thisdiscussion.Wherever water

comes into contact with the rocks of Earths crust, either on land or inthe ocean or within the oceanic crust, some of the minerals in the rockdissolve and are carried by the water to the ocean. The salt content ofseawater does not change because new minerals are forming on thesea $oor at the same rate as salt is added. Thus, the salt content of thesea is at steady state.