Buoyant plume material ponds at the base of oceanic lithosphere. It is a natural probe for the relief on the base of the lithosphere owing to plate age because it flows laterally along an upside-down drainage pattern. Geomorphic and geochemical studies of near-ridge and on-ridge hot spots indicate, as expected, that the oceanic lithosphere thickens as it ages. Corridors of thin lithosphere channel plume material along the axis, while the thick lithosphere at the end of ridge segments dams this along-axis flow. Channel hot spots extend from plumes toward ridge axes. However, a good correlation between local lithospheric thickness and local age is not evident at fracture zones within the old ocean basins. Such a correlation would be expected if the lithosphere cooled only by conduction. Then pressure-release melting and volcanoes would occur preferentially near fracture zones where plume material cascades from thick old to thin young lithosphere. This concentration of volcanic edifices does not occur. This lack of correlation between lithosphere thickness and plate age is an expected consequence of small-scale convection at the base of the lithosphere. Scaling relationships and numerical models indicate that modestly vigorous convection of several milliwatt per meter squared is sufficient to perturb local lithospheric relief beneath fracture zones without changing the gross correlation of lithospheric thickness with plate age. Typically, the step in lithospheric thickness moves toward the old side of the fracture zone.