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Why is seawater saltier than freshwater?

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Diagram illustrating Earth's environmental spheres, with labeled sections showing the geosphere (land and crust), biosphere (living things), atmosphere (air), and hydrosphere (water), each extending vertically from the Earth's surface. — hydrosphere Earth's environment includes the atmosphere, the hydrosphere, the lithosphere, and the biosphere.

Seawater is significantly saltier than freshwater primarily because the ocean acts as a long-term collector of minerals. As rain and other precipitation wash over the landscape, they physically and chemically erode rocks, carrying dissolved salts into rivers and streams that eventually deposit these minerals into the sea.12

While inland bodies of water typically remain fresh because they are continually replenished by precipitation and have outlets that keep water moving, the ocean collects salts from all incoming streams. When water evaporates from the ocean surface as part of the water cycle, the salty chemicals are left behind. Over millennia, this process of mineral deposition and accumulation has concentrated salts in the ocean. Additionally, hydrothermal vents and undersea volcanic activity contribute further salts directly to seawater.13

It is important to note that the ocean is not a simple accumulator; it functions as a dynamic system where salts are also removed through the formation of marine sediments.4