Grades 3–5 · Earth & Space
Why is the ocean salty?
Ocean water contains dissolved salts. A dissolved salt has separated into tiny charged pieces called ions, which spread through the water. Many of these ions begin in rocks on land. Rainwater slowly reacts with minerals in the rocks, releasing ions that can travel with flowing water.
In the diagramRock weathering supplies dissolved salts to the oceanWater evaporates; dissolved salts largely stay behindLand rocksRivers carrydissolved ionsOcean water + saltsWater vaporleavesSome ions enter sedimentsSeafloor reactionsSaltiness reflects both ion transfers and water movement.
Streams and rivers carry these dissolved materials toward the sea. River water contains salts too, although usually in much smaller concentrations than seawater. Reactions between seawater and hot rocks beneath the seafloor also exchange dissolved materials with the ocean.
Sunlight supplies energy for ocean water to evaporate. Water molecules enter the air as vapor, while most dissolved salts stay in the water. Removing water can therefore make the remaining water saltier. Rain and incoming river water can dilute it again.
Salts do not simply build up forever without leaving. Living things take up some ions, and chemical reactions can place others into seafloor sediments. The ocean’s saltiness reflects these inputs, removals and movements of water over long periods.
About this explanation
Starting knowledge: Know that rivers reach the ocean, water can evaporate and salt can dissolve.
Sources
- NOAA National Ocean Service — Why is the ocean salty?
- NOAA National Ocean Service — Why is the ocean salty, but rivers flowing into it are not?