The earth-moon system forms a gravitational couple which diminishes the earth's spin. The internal distribution of density and anelasticity, determined seismologically, requires that the body (core and mantle) of the earth below the low-velocity zone is decelerated by a westward-directed surface-force system acting on its top. The magnitude of the tangential system is estimated to be between 3 10²³ and 6.5 10²³ dyne cm. This system is present whether the seat of the couple is the seas or the outer part of the solid earth. Because such a system cannot perpetually affect a layer characterized by partial melting and materia transference without causing secular distortion, the lithosphere should undergo westward displacement.

Theoretically, the lithosphere should not rotate intact about the mantle but should segment, because the force system and the resistance to it vary regionally. The segments should rotate and interact at their edges. As westward progression is not uniform, the edges of some segments should override or underride others; elsewhere, a separation should develop between segments. These theoretical effects are in accord with the observed concentration of seafloor spreading in low latitudes, with its orientation, and with the poleward disappearance of deep seismicity. The paleomagnetic record is interpreted to suggest a westward displacement rate of the litrosphere of 5 cm/year at the equator.

In a model presented as a working hypothesis, secular tidal distortion acts as the primary tectonic mechanism. In a second model, this system imposes a bias on convective processes of internal origin.