McKenzie did not need a doctorate to continue to work at Cambridge; he was already a Research Fellow. “At Cambridge in the early sixties, nobody took very much notice of that. But I wanted a postdoc in the States and I knew that I had better get my PhD.” Once he did so, he went to CaltechThe California Institute of Technology originated in 1891, but took its modern form (and name) in 1921; seismology was a concern in southern California and in 1937 the Seismological Laboratory moved to Caltech. Charles F Richter and Beno Gutenberg established their measurement scale for earthquakes there and by the 1960s the Seismo Lab was a focus for earthquake research. Frank Press, who was Director from 1957-65, was an innovator in quantitative research, and put an emphasis on computing in earthquake seismology. on a scholarship. “I had spent three years in Cambridge as an undergraduate and three years as a postgraduate and off I went. Nobody expected me to come back.” At Caltech he was able to perform the computations and produce the figures for his 1967 paper on heat flow and gravitational anomalies at ridges – his first work on plate tectonics. He also read widely, discovering earthquake seismology, reading and drawing together the strands of information from diverse papers.