Retrospective on the plate tectonic revolution focusing on K/Ar dating, linear volcanic chains and the geomagnetic polarity time scale

Abstract

Notions of migration of volcanism in the Hawaiian Islands were contemplated for decades, but no quantitative measurements were made until the early 1960s. Collections of volcanic rocks from the accessible high islands were undertaken by the author in 1961. Surprisingly, K/Ar dating was possible, owing to much lower atmospheric argon in the samples than anticipated, allowing radiogenic argon to be detected. It became evident that there had been a progression of volcanism from Kauai (~4.4 Ma) in the WNW of the archipelago to the Island of Hawaii in the ESE, where there is active volcanism, with a rate of migration of volcanism averaging ~13-16 cm/year. These results gave strong support to J Tuzo Wilson's hotspot model for the origin of certain island chains, proposed in 1963. Later, rates determined in other Pacific island chains were concordant with Hawaii as indicators of direction and rate of plate motions. Concurrently with the successful Hawaiian age measurements, palaeomagnetic results were obtained by Don Tarling and combined with the K/Ar ages these showed that normal and reversed polarity zones were time related, and a geomagnetic polarity time scale was rapidly developed. Marine magnetic measurements across mid ocean ridges showed symmetry of parallel zones of magnetized crust, interpreted by Fred Vine and Drummond Matthews as reflecting normal and reversely magnetized new ocean floor, that was soon calibrated against the geomagnetic polarity time scale, providing persuasive evidence in favour of plate tectonics and seafloor spreading. But K/Ar dating was crucial to the major changes in tectonic hypotheses.