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Hess (1962), and those who followed, realized that sea floor spreading causes the outer layer of Earth to grow substantially over time. If Earth's circumference is relatively constant and Earth's lithosphere is growing and being extended horizontally at divergent plate boundaries over long periods of time, then there must be places where it is undergoing long‐term horizontal shortening of similar magnitude. As ocean lithosphere ages and continues to move away from oceanic spreading centers, it cools, subsides, and becomes denser over time. The increased density eventually causes the strong ocean lithosphere to become denser than the underlying, weak asthenosphere. As a result, a plate carrying old, cold, dense ocean lithosphere begins to sink downward into the asthenosphere under a more buoyant plate edge, creating a convergent plate boundary.

ssss1 World map showing the age of oceanic crust; such maps confirmed the origin of oceanic crust by sea floor spreading.

Source: From Lamont Doherty Earth Observatory.