Convection plumes, anchored in the lower mantle of the Earth, were proposed by W. J. Morgan in 1970 to explain the existence of oceanic islands such as Hawaii, their relative fixity with respect to one another over geological time, and the “tracks” of volcanoes left by the lithospheric plate sliding over such a “hotspot.” The basalts found on oceanic islands differ in their isotopic composition from basalts found at midocean ridges, which lent geochemical support to the notion that these islands tap into a different reservoir for basaltic material, presumably the lower mantle, the silicate shell of the Earth below about 660 km (410 mi) depth where the upper-mantle minerals olivine, garnet, and pyroxene break down to very high pressure phases of perovskite and magnesiowuestite. But while the geochemical observations give some strong arguments in favor of the plume hypothesis, only seismology can pinpoint the source location of such plumes. Until recently, direct seismic evidence was missing. This allowed a small but vocal minority to oppose the plume hypothesis, advocating instead shallow lithospheric processes to be responsible for the observed phenomena.