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Roughly 71 % of Earth’s surface is water, yet only about 3.5 % is freshwater. The rest—our oceans—is briny, and the difference stems from the planet’s volcanic past. Early eruptions released vast amounts of minerals, including salt ions, into a hot atmosphere rich in water vapor. As the planet cooled, the vapor condensed, producing continuous rainfall that filled basins and created the oceans. That rain also carried the previously dispersed salt ions back to the sea, establishing its salinity.
Once formed, oceans have gradually become saltier through processes such as weathering and soil erosion. Acidic rain dissolves minerals from rocks; rivers then transport roughly four billion tons of dissolved salts to the sea each year. Hydrothermal vents further contribute minerals from the Earth’s interior. Although salt keeps entering the ocean, marine organisms absorb it, maintaining a dynamic equilibrium in salt‑water ecosystems.
Lake waters remain fresh largely because of the water cycle. While rainwater also leaches salts from surrounding geology, rivers and streams remove these ions, preventing accumulation in lakes. Freshwater that falls as precipitation originates from ocean evaporation, which leaves salt behind. Thus, the water that fills lakes is fresh by default. Lakes that do not drain—such as the Dead Sea and the Great Salt Lake—accumulate salt and become hypersaline. The Caspian Sea, the world’s largest inland body of water, illustrates this balance: its northern basin receives the Volga River’s fresh inflow, whereas its southern basin, lacking an outlet, is salty like an ocean.
