I was born in Hobart, Tasmania in the mid-thirties, went to the Friends’ School, and then onto the University of Tasmania to undertake a science degree in 1953. I studied primarily Geology and Chemistry, doing an honours year in Geology, in particular involving mapping just north of Hobart in the Bridgewater-Pontville area. Professor Carey was the head of the Geology Department at that time, so his enthusiasm and wide view of world geology and continental drift, were important to me as a young undergraduate. In 1957 I went to the Australian National University (ANU) to do a PhD under Dr Germaine Joplin in the Department of Geophysics, headed by Professor J.C. Jaeger. My topic was the mapping, petrology and geochemistry of the Red Hill Intrusion, south of Hobart, just west of Margate and Snug. This intrusion is one of many bodies of tholeiitic magma in the near flatlying Permo-Triassic sediments, now shown to be of Jurassic age. The main attraction of this body was that many years before a student mapping the area had discovered very silicic rocks in the upper part of Red Hill, and recognized that these rocks were related to a much more mafic intrusion. My studies, completed in 1960, showed conclusively that the granophyres in the upper part of Red Hill were in a high part of the dolerite intrusion and were undoubtedly differentiates of the mafic magma. A number of papers resulted from these investigations.
With the encouragement of Prof. Jaeger I received a CSIRO postdoctoral fellowship, with the intention to go to University of California at the Berkeley campus to become familiar with the techniques and applications of the K/Ar isotopic dating method under Drs Garniss Curtis and Jack Evernden in the Department of Geology. Curtis and Evernden had set up this laboratory with the assistance of Dr John Reynolds in the Department of Physics, who had designed and built a revolutionary mass spectrometer with a glass envelope. This enabled argon and other noble gas measurements to be made for the first time by closing off the machine to its pumps and then admitting the purified noble gas into the machine, now operated statically, for isotopic analysis. Thus, at Berkeley, a revolution in the dating technique was occurring, as much smaller samples than previously could be measured. In 1961, I was appointed as a Research Fellow in Jaeger’s department in Canberra, where I spent most of my career until my formal retirement at the end of year 2000. In 2001 I was appointed an Emeritus Professor in the ANU. The majority of my time was involved with the new K/Ar laboratory in ANU, originally set up by visitor Evernden and Dr John Richards, the latter a member of Jaeger’s department. I joined Richards, a chemist, in the K/Ar laboratory late in 1961, having spent a month collecting samples of volcanic rocks from the accessible high islands of the Hawaiian chain, from Kauai to the big island of Hawaii over a distance of ~530 km, on the way back to Australia. Most people at the time were very pessimistic about dating such young rocks. For many years it had been recognized that the volcanoes of the Hawaiian island chain were progressively older to the NNW, as current volcanism was confined to the SE part of the big island, with the oldest volcano, Kohala, at the northern part of the big island extinct, and the islands to the NW all extinct and progressively more deeply eroded. It turned out that many of the volcanic rocks yielded excellent, reproducible, K/Ar ages, and I was able to quantify the rate of migration of volcanism along the chain at ~10 cm/year, as well as providing considerable information on the age of individual edifices. This work helped to confirm Tuzo Wilson’s hotspot model for the origin of volcanic island chains. The ages ranged from about 0.4 Ma in Kohala to about 4.4 Ma in Kauai for the shield-building volcanism, the latter just over 500 km NNW of the active volcano of Kilauea to the SSE on the big island of Hawaii. The reason for this surprising ability to measure ages on these quite youthful volcanic rocks was simply because the amount of atmospheric argon in well-crystallized, unaltered igneous rocks was much lower than extrapolations from materials then used for dating, such as biotite mica, had suggested, allowing the radiogenic argon to be much more easily detected. Subsequent work on other Pacific island chains, including the Society, Austral and Marquesas island chains, done mainly through Robert Duncan’s efforts in my laboratory, showed a general concordance of rates and directions of migration of volcanism, providing evidence that the island chains were relatively good recorders of plate motions. Subsequent GPS measurements have shown that rates of motion recorded by volcanism in island chains is probably only an approximation, because the volcanic rocks used for the dating came from above sea level, near the summits of much larger edifices, so the ages may not truly reflect the time of the major building of the volcano. The results possibly are also only approximate, because the fixed nature of the source in the mantle below the moving plate may be somewhat questionable.
The Hawaiian K/Ar data were also combined with palaeomagnetic studies being undertaken concurrently by PhD student Don Tarling in ANU, yielding, among other things, the polarity of the magnetization. It quickly became apparent that there was a zonation of normal and reversed polarity with time, not only showing that reversed magnetization was real, but that a polarity time scale was possible. With the group at the US Geological Survey, our work quickly lead to the development of the geomagnetic time scale, which also was found to be applicable to the calibration of the age of magnetic ‘stripes’ on either side of mid-ocean ridges, and hence rates of seafloor spreading. Thus, this work became part of the foundation for the plate tectonic model of Earth behaviour.
In 1978 I became involved in trying to sort out the apparent inconsistencies in K/Ar ages assigned to the KBS Tuff in the Turkana Basin of northern Kenya. Ages for the tuff ranged from about 1.6 to 2.6 Ma, and it became important to resolve this issue as at the time one of the oldest known hominin fossils occurred in strata just below the KBS Tuff. We were able to show that the KBS Tuff was in fact about 1.86 ± 0.02 Ma old, using the conventional K/Ar method on anorthoclase phenocrysts in pumice clasts in the KBS Tuff, followed by step heating experiments by the 40Ar/39Ar dating technique, leading to essentially flat age spectra, and in more recent times by single crystal 40Ar/39Ar dating, confirming the previously determined ages. We now have over 35 levels well-dated in the sequences of the Omo-Turkana Basin, most ages completely consistent with the stratigraphic order, from just over 4.2 Ma to virtually the present day, with a significant gap in deposition, at least in the subaerial sections, between about 0.7 and 0.2 Ma. The studies have continued until this day in the Omo-Turkana Basin as new tephra beds, some of which have pumice clasts, have been found. A major driving force for the detailed geochronology has been the recovery from the sequences of numerous vertebrate fossils, including many hominins of various genera and species. Provided fossils can be placed relative to the known sequence of rhyolitic tuffs, then ages can be assigned to generally better than 0.1 Ma, without the need for further dating. Numerical time scales of this kind are important as they allow ages to be placed on vertebrate fossils independently of assumptions as to their evolutionary origins.
During this long career in K/Ar dating several mass spectrometers were employed to undertake the isotopic analyses, and increased precision of measurement was achieved as well as a major reduction in size of sample required, especially with the introduction of 40Ar/39Ar dating in the mid 1970s. With single crystal dating becoming possible we moved toward automation, so that more than a decade ago we began to make argon extractions, and isotopic measurements including data acquisition, by computer, without attendance of an individual. This improved the productivity of the laboratory quite dramatically. During this whole time, many significant geological studies were undertaken by myself and by PhD students and postdoctoral people, so the laboratory was very productive.
In formal retirement much of the time is spent writing up past work but doing some experimental study, mainly on the Omo-Turkana Basin sequences, at the University of Queensland, as much of the laboratory at ANU was dismantled several years ago.
Honours awarded include Fellow of the Geological Society of America (1978); Fellow of the Australian Academy of Science (1988); Fellow of the American Geophysical Union (1997); Hon DSc, University of Glasgow (2009); and Adjunct Professor or Honorary Professor, University of Queensland from 2007. I have been a Visiting Fellow in RSES, ANU since 2001.
In 1960 I married Pam Hodgson and we produced three children, all now grown up and leading successful lives. A special tribute goes to Pam for her support over all the years, especially as my field trips often meant extended periods away from home.
Geochronology by the K/Ar dating system as well as some noble gas geochemistry. In the dating, most recently by 40Ar/39Ar, McDougall has been concentrating on the East African sequences in northern Kenya and southern Ethiopia where a large number of hominin fossils have been found. We now have over 35 dated levels in the sequences over the last 4.25 Ma, including an age of ~195,000 years for the oldest presently known fossil of Homo sapiens at Kibish in southern Ethiopia.
The restricted bibliography, given below, shows most but not all of the fields I have worked in during my career.
- McDougall, I. (2014). K/Ar and 49Ar/39Ar isotopic dating techniques as applied to young volcanic rocks, particularly those associated with hominin localities. Pp 1-15 in Treatise on Geochemistry 2nd edition, Cerling T. (ed), http://dx.doi.org/10.1016/B978-0-08-095975-7.01201-8. Elsevier, Oxford, U.K.
- McDougall, I. (2013). Retrospective on the plate tectonic revolution focusing on K/Ar dating, linear volcanic chains and the geomagnetic polarity time scale. Earth Sciences History, 32, pp.313-331.
- Brown, F. H. & McDougall, I. (2011). Geochronolgy of the Turkana Depression of northern Kenya and southern Ethiopia. Evolutionary Anthropology, 20, pp. 217-227.
- McDougall, I., Brown, F. H. & Fleagle J. G. (2008). Sapropels and the age of hominins Omo I and II, Kibish, Ethiopia. Jourrnal of Human Evolution, 55, pp. 409-420.
- Honda, M., McDougall, I., Patterson, D. B., Doulgeris, A. & Clague, D. A. (1991). Possible solar noble-gas component in Hawaiian basalts. Nature, 349, pp.149-151.
- McDougall, I. (1985). K-Ar and 40Ar/39Ar dating of the hominid bearing Plio-Pleistocene sequence at Koobi Fora, Lake Turkana, northern Kenya. Bulletin of the Geological Society of America, 96, pp. 159-175
- McDougall, I., Kristjansson, L. & Saemundsson, K. (1984). Magnetostratigraphy and geochronology of northwest Iceland. Journal of Geophysical Research, 89, pp. 7029-7060.
- McDougall, I., Embleton, B. J. J. & Stone, D. B. (1981). Origin and evolution of Lord Howe Island southwest Pacific Ocean. Journal of the Geological Society of Australia, 28, pp. 155-176.
- McDougall, I. & Duncan R. A. (1980). Linear volcanic – recording plate motions? Tectonophysics, 63, pp. 275-295.
- McDougall, I. (1976). Geochemistry and origin of the Columbia River Group, Oregon and Washington. Bulletin of the Geological Society of America, 87, pp. 777-792.
- McDougall, I. (1971). The geochronology and evolution of the young volcanic island of Reunion, Indian Ocean. Geochimica et Cosmochimica Acta, 35, pp. 261-288.
- McDougall, I. & Chamalaun, F. H. (1966) Geomagnetic polarity scale of time. Nature, 212, pp. 1415-1418.
- McDougall, I. (1964). Potassium-argon ages from lavas of the Hawaiian Islands. Bulletin of the Geological Society of America, 75, pp. 107-128.
- McDougall, I. & Tarling, D. H. (1964). Dating geomagnetic polarity zones. Nature, 202, pp. 171-172.
- McDougall, I. (1962). Differentiation of the Tasmanian dolerites: Red Hill dolerite-granophyre association. Bulletin of the Geological Society of America, 73, pp. 279-316.
- McDougall, I. (1961). Determination of the age of a basic intrusion by the potassium-argon method. Nature, 190, pp. 1184-1186.