ABSTRACT Lateral accelerations require lateral forces. We propose that force imbalances in the unique Earth-Moon-Sun system cause large-scale, cooperative tectonic motions. The solar gravitational pull on the Moon, being 2.2× terrestrial pull, causes lunar drift, orbital elongation, and an ~1000 km radial monthly excursion of the Earth-Moon barycenter inside Earth’s mantle. Earth’s spin superimposes an approximately longitudinal 24 h circuit of the barycenter. Because the oscillating barycenter lies 3500–5500 km from the geocenter, Earth’s tangential orbital acceleration and solar pull are imbalanced. Near-surface motions are enabled by a weak low-velocity zone underlying the cold, brittle lithosphere: The thermal states of both layers result from leakage of Earth’s internal radiogenic heat to space. Concomitantly, stress induced by spin cracks the lithosphere in a classic X-pattern, creating mid-ocean ridges and plate segments. The inertial response of our high-spin planet with its low-velocity zone is ~10 cm yr –1 westward drift of the entire lithosphere, which largely dictates plate motions. The thermal profile causes sinking plates to thin and disappear by depths of ~200–660 km, depending on angle and speed. Cyclical stresses are effective agents of failure, thereby adding asymmetry to plate motions. A comparison of rocky planets shows that the presence and longevity of volcanism and tectonism depend on the particular combination of moon size, moon orbital orientation, proximity to the Sun, and rates of body spin and cooling. Earth is the only rocky planet with all the factors needed for plate tectonics.