From what I read they've been at 3.5% salt equilibrium for millions of years. How is this possible when rivers are constantly flowing dissolving salt into them?
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$\begingroup$ Related if not duplicate: earthscience.stackexchange.com/q/20432/18081 $\endgroup$– Jean-Marie PrivalJul 6 at 20:05
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Because salt gets buried over time. Most of the salt we mine is old seas / oceans that dried up and then got buried. Subduction zones carry salt-laden and water-laden sediments very deep underground. The water tends to escape by volcanism. The salt does not.
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$\begingroup$ So the ocean gets less deep over time? Because the ocean floor is getting higher with the salt buildup and formation of new surface $\endgroup$– GroveishJul 13 at 22:41
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$\begingroup$ @Groveish Google "subduction zone". The continents are in general much older than are the oceans. The oldest continental crust is over 3 billion years old. The oldest ocean crust is less than 200 million years old. $\endgroup$ Jul 14 at 4:51