The ocean is salty primarily because of dissolved salts that originate from rocks on land and from openings in the seafloor.
Where Does Ocean Salt Come From?
The salt in seawater comes from two main sources, as highlighted by the National Ocean Service:
1. Runoff from Land
This is the major source of salts dissolved in the ocean. Here's how it works:
- Acidic Rainwater: Rainwater isn't pure H₂O. It's slightly acidic because it picks up carbon dioxide from the atmosphere as it falls.
- Rock Erosion: This slightly acidic rainwater falls on land and erodes rocks. As it flows over and through rocks and soil, it dissolves minerals, including salts.
- River Transport: These dissolved salts and minerals are then carried by streams and rivers into the oceans.
Over millions of years, this continuous process of erosion and transport has deposited vast amounts of salt into the ocean basins.
2. Openings in the Seafloor
Another significant contributor to ocean salinity comes from openings in the seafloor. These often involve hydrothermal vents located along mid-ocean ridges.
- Water Circulation: Seawater seeps into cracks in the ocean floor.
- Heating and Mineral Exchange: This water is heated by molten rock below the crust. As it heats up, it reacts with the surrounding rocks, dissolving some minerals from the crust while depositing others.
- Vent Expulsion: This hot, mineral-rich fluid then gushes back out into the ocean through hydrothermal vents. These fluids carry dissolved minerals and salts, adding to the ocean's overall salinity.
| Source of Salt | How it Works | Primary Contribution |
|---|---|---|
| Runoff from Land | Acidic rain erodes rocks, rivers carry dissolved salts | Major source |
| Seafloor Openings | Seawater reacts with hot rocks, minerals expelled | Significant contributor |
While rivers constantly bring new salt to the ocean, evaporation removes pure water, leaving the salts behind. This balance, over geological timescales, maintains the ocean's salinity.
