Summary: A One-Way Street | AI Learning Guide

So, to answer our original question: Why don't oceans drain? Because the only significant water movement between land and sea is a one-way street. Rain falls on land, becomes groundwater, and flows out to the ocean. Once in the ocean, the saltwater is too dense to sink into the crust, and the freshwater is too light to mix deeply. The ocean is held in place by the shape of the Earth and the physics of density. In the next chapter, we'll look at an even slower process: how some water gets trapped inside rocks during subduction.

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The Stability of the Ocean Basin

To summarize: Water moves from land to ocean via SGD. Because freshwater is less dense than saltwater, it floats on the surface and exits the basin. The denser saltwater remains in the deep ocean basin, held by gravity and geological barriers. There is no mechanism for the entire ocean to drain into the Earth's crust.