The sea gets its salt from ordinary rain and river rock
The ocean was not filled with saltwater all at once. When rain falls through the air, it absorbs carbon dioxide and becomes slightly acidic. As this rain washes over rocks on land, it slowly breaks down minerals, freeing charged ions like sodium and chloride. Rivers quietly wash billions of tons of these dissolved minerals into the oceans. Once there, evaporating water leaves the salt behind, steadily seasoning the sea over billions of years.
The Chemistry of Rain and Weathering
The journey that makes the ocean salty begins in the atmosphere. Water vapor condenses into clouds and falls through the air as precipitation, absorbing carbon dioxide along the way. When carbon dioxide dissolves into water, it reacts to form weak carbonic acid. Although this acid is mild—far too weak to harm living organisms directly—it gives natural rainwater a slightly acidic quality that turns it into an effective chemical solvent over vast periods of time.
When this mildly acidic rain strikes land, it washes over exposed rocks, soil, and boulders. The acid slowly breaks down the rock through a process known as chemical weathering. As the rock structure decomposes, its constituent minerals separate into electrically charged particles called ions. Among the most common ions freed by this constant chemical erosion are sodium, calcium, magnesium, and chloride. These dissolved ions are picked up by surface moisture and trickling runoff, starting their slow migration toward the sea.
Why River Water Tastes Fresh
Runoff gathers into small brooks, which flow into streams and eventually merge into major river systems. Rivers are the primary conduits carrying weathered land minerals directly into the world's oceans, washing billions of tons of dissolved salts out to sea every year. Yet despite this enormous and continuous transport of dissolved rock, river water does not taste salty to human senses.
The reason river water remains fresh lies in the balance between volume and transit time. In flowing freshwater, the concentration of dissolved minerals is exceptionally low compared to the sheer volume of water moving downstream. Furthermore, rivers are constantly replenished with newly fallen rain and melting snow, flushing minerals through the channel rather than allowing them to accumulate. The dissolved substances only begin to concentrate once they reach the ocean, which acts as a permanent terminus where water leaves only through evaporation.