Observations of Venus and Africa, an analogous terrestrial plate, indicate that upwellings a few hundred kilometers across are pervasive in the ambient upper mantle of both Venus and Earth. Because of their small size, when they reach the lithosphere the upwellings affect the surface only under special circumstances. On Venus they form volcano-tectonic structures called coronae where the lithosphere has been thinned by rifting. On Earth the small upwellings produce topographic swells where they impinge on a plate stationary with respect to the hot-spot reference frame, as is Africa. The small upwellings' presence throughout the mantle explains both the corona paradox, in which coronae are associated with rifts on Venus but do not appear to drive rifting, and the basin and swell topography of the African plate. The morphology of coronae requires that the small upwellings have some diapiric properties. Their pervasiveness, transient nature, and generally small size make them fundamentally different from traditional Hawaiian-style plumes, and they may be detached diapirs within a mantle convecting in the hard-turbulence regime. ¿ 1999 American Geophysical Union