Ocean salt is just rock that took a very long bath 🛁

Seawater tastes salty because it is, in a very real sense, dissolved rock. Rain falling through the atmosphere picks up a small amount of carbon dioxide, forming weak carbonic acid. That mildly acidic water slowly attacks the minerals in rocks on land, releasing ions like sodium, calcium, potassium, magnesium, chloride and sulfate. Rivers then carry those dissolved ions to the sea. ^[1][2]

Once in the ocean, the water leaves but most of the dissolved ions stay. Evaporation lifts pure water vapor into the atmosphere, and when that vapor condenses and falls as rain, it starts the cycle again. Over geological time, this one-way traffic of ions from land to sea has concentrated the ocean's dissolved load. ^[1][3]

Why the sea isn't getting saltier forever

If rivers kept adding salt and nothing removed it, the ocean would be far saltier than it is. Instead, the system is roughly in balance. Ions are pulled out of seawater by processes including the formation of evaporite minerals in shallow seas, adsorption onto clay particles, incorporation into hydrothermal fluids at mid-ocean ridges, and the formation of carbonate shells and sediments by marine organisms. ^[2][4]

That average hides real variation. The Atlantic tends to be saltier than the Pacific because of differences in evaporation, rainfall and river input. Coastal areas near large river mouths are fresher, while subtropical seas where evaporation is intense can be saltier. Salinity is usually reported in practical salinity units (PSU), which are numerically close to grams of dissolved salts per kilogram of seawater. ^[3]

Four billion years in the making

1. ~4.4 billion years ago

   The first oceans form as Earth cools enough for liquid water to condense and accumulate.

2. ~3.5 billion years ago

   Life appears in the oceans; biological processes begin to influence the chemistry of seawater.

3. ~600 million years ago

   Large-scale formation of evaporite deposits and carbonate reefs removes dissolved ions from the sea.

4. Today

   Ocean salinity sits near 35 g/L, held roughly steady by a balance of inputs and outputs.

The ions themselves have different stories. Sodium and chloride are the most abundant dissolved constituents in seawater, which is why table salt (sodium chloride) dominates the taste. But other ions matter too: calcium is used by corals and shellfish to build skeletons, and magnesium is incorporated into many minerals. The ocean's chemistry is not just a passive record of erosion; it is actively shaped by life and by the seafloor. ^[2][4]

So the next time you taste the sea, you are tasting a four-billion-year chemical experiment: rock weathered by rain, delivered by rivers, concentrated by evaporation, and recycled through the crust and the biosphere. ^[1][2][3]