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The formation, evolution and recycling of oceanic lithosphere are well understood within the framework of plate tectonics. However, no unified physical framework has yet explained the long-term mechanical stability, characteristic thickness, and secular evolution of the continental lithosphere. Here we show that continental lithosphere behaves as a self-regulating thermomechanical system with a preferred equilibrium determined by feedbacks between crustal thickness, radiogenic heat production, lithospheric strength and mantle temperature. This equilibrium corresponds to a preferred continental crustal thickness near the present-day global mean, where lithospheric strength is maximized and intraplate deformation is minimized. Departures from this equilibrium activate predictable restoring responses: crust thinner than the equilibrium preferentially accommodates compressional shortening and thickening, whereas thicker crust becomes gravitationally unstable and undergoes extensional thinning. Plate-boundary forces provide the principal external forcing that displaces continental lithosphere from equilibrium, whereas internal thermomechanical feedbacks govern the long-term mechanical response. As Earth's mantle cools, the equilibrium progressively shifts toward greater crustal thickness, providing a physical mechanism for quasi-monotonic continental growth and the long-term persistence of continental lithosphere. |