Chapter 1

Flipping fields: Earth’s magnetic reversals

Our magnetic field comes from the flow of conducting material – largely iron – in the Earth’s outer core. The exact nature of the geodynamo is uncertain, but it reverses every few tens or hundreds of thousands of years. VineVineFred Vine (1939-present) started as a graduate student at Madingley Rise in 1962. He worked with Drum Matthews on the magnetic fluctuations recorded in the rocks of the Indian Ocean floor. Fred Vine modelled the linear patterns of normal and reversed magnetism, parallel to the mid ocean ridge and with mirror symmetry across it. They combined Harry Hess’s suggestion of sea floor spreading like a conveyor belt with the newly-discovered sequence of past reversals of the Earth’s magnetic field to produce a simple model of crustal formation at mid ocean ridges. Vine and Matthews published their model – independently discovered by Lawrence Morley at about the same time – in 1963. and MatthewsMatthewsDrum Matthews (1931–1997) was a fellow of Kings College Cambridge when McKenzie was an undergraduate. He was young and involved in College life. Drum was originally a geologist, with a PhD on the petrology of seafloor rocks. He worked extensively at sea, collecting gravity, magnetic and depth data, plus some seabed cores and samples, including on the International Indian Ocean Experiment in 1961-63. He, with Maurice Hill, influenced McKenzie to go into geophysics as a graduate student. Matthews continued in marine geophysics, and was the driving force behind the British Institutes Reflection Profiling Syndicate which obtained the first deep seismic profiles around the UK. suggested in 1963 that reversals accounted for the linear magnetic anomalies in the sea floor, in combination with Harry HessHarry HessHarry Hess (1906-1969) began his career at Princeton in the 1930s, undertaking submarine gravity experiments with Vening Meinesz. After war service and promotion to an Admiral of the US Navy, he returned to Princeton and his interests in marine geology. His proposal of seafloor spreading and the conveyor belt carrying new ocean crust away from the mid-ocean ridges was influential. He had also worked on the gravity anomalies that became associated with subduction zones, encouraging him to consider mantle convection as a part of plate tectonics. ’s idea of seafloor spreading, but neither concept was widely accepted. By 1968, the sequence of normal and reversed magnetism had been dated from the radioactive decay of minerals in the rocks and the conveyor belt patterns in the seafloor could be run backwards to reconstruct how the plates had moved in the past.