WHY is water in the oceans and seas salty while fresh in rivers?

Most of the salts in seawater were formed at the beginning of geologic history, at the origin of the oceans. Volcanic gases present in the atmosphere reacted with water to form acids. These reacted with the metal silicates in the rocks of the ocean floor, producing salts. Some more salts were brought to the seas by the rivers, which washed them out of the minerals of the land. The river water that enters the sea evaporates and the salts accumulate.

The water in rivers is normally fresh since they are fed by precipitation, which does not contain salt, and they flow in channels that have long since been washed away. Yet in some arid mountainous areas (e.g., the Kopetdag), saline rivers do exist.

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