Two Primary Sources of Salt in Seawater

Where Ocean Salinity Comes From

Seawater's dissolved salts originate from two main geological processes:

1. Hydrothermal Vents

At mid-ocean ridges, seawater percolates down through fractured crust, is heated by magma, and reacts with basalt. This leaches metals and minerals (including sodium, chloride, magnesium, sulfate) from the rock. The superheated, mineral-rich fluid jets back into the ocean, contributing dissolved salts.

2. Riverine Input (Weathering of Continental Rocks)

Rainwater (slightly acidic from CO₂) weathers silicate and carbonate rocks on land. Rivers carry the dissolved ions—sodium, calcium, magnesium, potassium, chloride, sulfate, bicarbonate—to the sea. Over geological time, this continuous flux builds ocean salinity.

Additional minor sources include volcanic emissions, atmospheric deposition, and seabed sediment dissolution. The balance of input and removal (e.g., evaporite formation, biological uptake) maintains relatively stable salinity (~35 g/kg).