About Michael McWilliams

Professor Michael McWilliams is a physicist whose undergraduate interests in understanding how the earth works led him to pursue graduate degrees in geophysics and a career in earth science.

He joined Stanford in 1977 and has taught undergraduate and graduate classes in geology, geochemistry, geophysics, computer science and ancient climate change. His favorite teaching experiences have been leading the very popular introductory geology class for undergraduates, leading undergraduate seminars on the Origin of the Universe, teaching in Stanford’s Continuing Studies Program, and leading educational trips to interesting places with the Stanford Alumni Association's Travel/Study Program.

McWilliams and his colleagues use mass spectrometric techniques to measure tiny amounts of solid and gaseous radioactive isotopes to determine the age of rocks and minerals. They use these ages to better understand earth’s evolution, the rate of geological processes, and the history of important earth events such as volcanism, earthquakes, mass extinctions and global climate change.  

Apart from teaching and research at Stanford, he has served in a variety of international science governance and leadership roles, including as Chief Executive of GNS Science [New Zealand’s geological survey], Chief of CSIRO’s Division of Earth Science and Resource Engineering [Australia, now reorganized as Mineral Resources and Energy], and Director of the DeLaeter Centre [Australia].

Argon–argon (sometimes called 40Ar/39Ar) thermochronology is one of the radiometric dating methods that I have used in my research career. The technique uses neutrons inside a nuclear reactor to convert a 39K (a stable isotope of potassium) to  radioactive 39Ar. If a standard mineral of known age is co-irradiated with unknown samples, it is possible to use a single measurement of argon gas isotopes to calculate the 40K/40Ar ratio, and thence to calculate the cooling age of the unknown sample. We  use these ages to determine the timing of important events in Earth history, for example pinpointing the date when an extraterrestrial impact likely caused the extinction of the dinosaurs 66 million years ago.