McKenzie’s great insight in his 1967 Journal of Geophysical Research paper was to see the thermal structure of the mid-ocean ridges as an essentially passive result of the crustal separation there. “If I was right, ridges did not need to have any deep structure; they were simply like ice floes,” he says. “They move apart and the water wells up between them and freezes.” The mantle below is passive; upwelling of hot material does not cause the ridges. It also demonstrates that large free air gravity anomalies measured from satellites do not reflect high temperatures in the mantle, but may hold information about mantle convection – that they are, in fact, better indicators of convection than the positions of the ridges. This exploration of thermal evolution resulting from tectonics informed McKenzie’s later analysis of lithosphere extension to produce sedimentary basins, his modelling of the maturation of oil-producing sediments and his work on melt generation.