The plate tectonics theory established 50 years ago has formed a solid framework for understanding how the earth works on all scales. In this theory, movement of the tectonic plates relative to the subjacent asthenosphere is one of the fundamental tenets. However, the nature of the boundary between the lithosphere and asthenosphere (LAB) beneath ocean basins remains under debate. The current consensus is that the oceanic lithosphere thickens with age by accreting asthenosphere material from below, and reaches its full thickness (L) similar to 90 km at the age (t) similar to 70 Ma. This lithospheric thickening fits the relation L alpha t(1/2), consistent with conductive cooling to the seafloor. A puzzling observation is that although conductive cooling continues, the oceanic lithosphere ceases to grow any thicker than similar to 90 km when t > 70 Ma. Small scale convection close beneath the LAB has been generally invoked to explain this puzzle, but why such convection does not occur until L similar to 90 km at t > 70 Ma has been a matter of conjecture. In this paper, we summarize the results of many years of experimental petrology and petrological studies of oceanic basalts, which indicate consistently that the LAB is a petrological phase boundary marking the intersection of the geotherm with the solidus of amphibole (pargasite)bearing peridotite. That is, petrologically, the LAB is an isotherm of similar to 1100 degrees C with L alpha t(1/2) for t < 70 Ma and an isobar of similar to 3 GPa (similar to 90 km) fort > 70 Ma. This unifying concept explains why the LAB depth increases with age for t < 70 Ma and maintains constant (similar to 90 km) for t > 70 Ma. The LAB, that is intrinsically determined by petrological phase equilibria, does not require small-scale convection. However, because the mantle above the LAB is the conductive lithosphere (pargasite-bearing peridotite) and below the LAB is the viscosity -reduced convective asthenosphere (peridotite + incipient melt), the small-scale convection in the asthenosphere close beneath the LAB (similar to 90 km) under older (t > 70 Ma) seafloors becomes possible, whose convective heat supply balances the conductive heat loss, maintaining the constant heat flow, seafloor depth and lithosphere thickness.