MAS March Monthly Meeting

Prof. John Valley


Analysis of terrestrial zircons yields radiometric ages nearly as old as the Earth, but these ages (4 – 4.4 billion years old) have been challenged. Questions around Uranium-Lead (U-Pb) radiometric dating geochronology have been in play for over 100 years, and were first resolved in 2014 for a 4.374 billion year old zircon. These new results based on oxygen isotopes show that most of the Hadean Eon (ca. 4 – 4.4 billion years ago) was not “hell-like” as commonly believed and implied by the name. The earliest Earth was indeed hot, violent and inhospitable, but by 4.3 billion years ago its surface had cooled and the steam atmosphere condensed to form habitable oceans. Thus, it’s possible that life emerged almost 1 billion years earlier than the oldest known microfossils suggest.

John Valley retired from teaching in July 2019 after spending nearly four decades as a professor in the Department of Geoscience at UW. In April, he was elected to the National Academy of Sciences.

In Oct of 2019, Valley received the Arthur L. Day Medal from the Geological Society of America (GSA) at the association’s annual meeting in Phoenix. The medal, one of the GSA’s highest honors, recognizes “outstanding distinction in contributing to geologic knowledge through the application of physics and chemistry to the solution of geologic problems.”

Valley has spent much of the past two decades developing new techniques to explore and quantify isotope compositions from ultra-small samples. This led to his groundbreaking theory that there were oceans on Earth much earlier than previously thought—which opens the possibility that life may have existed much earlier than the oldest known micro-fossils—and a major shift in thinking and a change in geology textbooks.

Location: UW Space Place, 2300 N. Park St., Madison, WI