Michael John O’Hara

2007 Harry H. Hess Medal Winner

Michael John O’Hara was awarded the 2007 Harry H. Hess Medal at the AGU Fall Meeting Honors Ceremony, held on 12 December 2007 in San Francisco, Calif.


The Harry H. Hess Medal recognizes “outstanding achievements in research in the constitution and evolution of Earth and other planets.” It is thus most appropriate that Mike O’Hara receive the 2007 Hess Medal of the American Geophysical Union.

Mike started as a field geologist studying the Scourie Gneisses in Scotland. As a pioneer, Mike used experimental petrology to study the petrogenesis of mantle rocks and rocks derived from the mantle that build the ocean crust. Mike’s talent is also marked by his insights into the petrogenesis of lunar basalts. It is fair to say that modern igneous petrology and geochemistry would not be the same without Mike’s many discoveries, creative efforts, and deep insights.

Garnet peridotites cropping out along orogenic belts are not of crustal origin but are deep mantle rocks tectonically brought to the surface. This was, however, not understood before O’Hara and Mercy [1963]. Harzburgites are melting residues after basalt extraction from the more fertile lherzolite, but this was not obvious before O’Hara and Yoder [1963].

The plagioclase/spinel and spinel/garnet peridotite facies boundaries in P-T space are well-known concepts when discussing mantle melting, but it was Mike who correctly established the topologies (curva-ture) of these boundaries in the 1960s.

Mantle melting residues are compositionally depleted and physically buoyant, but this concept and its geodynamic significance were not widely recognized until O’Hara [1973], who showed the difficulties of “mantle plume” models invoking undepleted fertile mantle that is too dense to ascend.

Mike demonstrated in the 1960s how changing pressure affects the peridotite-basalt phase equilibria. He understood back then that none of the erupted basalts is a primary magma, but most of us are too slow to appreciate this fact.

Mike predicted in 1965 that wet peridotite melting can produce silica-rich liquid resembling andesite, which was verified later by others.

Mike invented in 1968 the CMAS projection to analyze phase relationships and compositional paths of basaltic magma generation and evolution. This powerful tool has been used ever since in various modi-fied forms in research papers and in classrooms.

Mike quantified elegantly that trace element characteristics in basalts are not straightforward source signatures, but also record complex mantle melting and crustal magma chamber processes.

Mike stimulated much debate on alkali volatilization, parental magmas and sources, and lunar composition and evolution. His 107-page-long paper [2000] is a classic account of lunar and terrestrial basaltic petrogenesis history in the twentieth century.

Mike’s scientific success lies in his vision, insights, and approach—an approach that does not follow bandwagons, but challenges tradition and authority. It is his ability to debate and challenge that has rapidly advanced our field. Besides, Mike is a nice and caring man who has helped many Earth scientists in their endeavors with encouragement and advice. His enthusiasm continues to affect a new generation of scientists with the wonder of our planet.

Mr. President, members of the committee, and the Union, it is my great pleasure to present Mike O’Hara to you as the Harry Hess medalist for 2007.

—YAOLING NIU, Durham University, Durham, U.K.


My first contribution to Earth science came as a first-year student during the 1953 Cambridge University Spitsbergen expedition: the serendipitous discovery of lower Cambrian fossils during a comfort stop, observation sharpened by polar bear tracks just encounteredand the expedition rifle 20 miles away. Education was thorough before Health and Safety got into the act! Proposed subsequently for election to the student geological society (yes, it was that stuffy), my attitude was deemed frivolous. Four years later I was invited to join, but I had a lot of fun in the mountaineering club meanwhile. Climbing enthusiasts approach an objective by the least probable, while still possible, route, another facet of multiple working hypotheses that maybe explains a few aspects of my career.

Stuart Agrell was pivotal in diverting me from a career as climbing instructor, C. E. Tilley in pointing me at the first high-P, high-T granulite facies terrain at Scourie for my Ph.D. At Edinburgh from 1958, Arthur Holmes and Fred Stewart fostered my interest in garnet-peridotites, and mutual interest in fermented fluids was pursued with Keith Cox, Ian Dalziel, and Brian Upton. There followed a wildly stimulating year at the Geophysical Laboratory with Frank Schairer and Hat Yoder. “Well, if it isn’t the late Dr. O’Hara again!” was Frank’s greeting, at 8:00 in the morning with umpteen overnight runs freshly quenched. Setting up an experimental labora-tory back at Edinburgh, I was joined by Gordon Biggar, a former student of Peter Wyllie (one of my heroes). Claude Herzberg and Ed Stolper were memorable research students.

Vigorous debates, terrestrial and lunar, ensued with Dave Green, Ted Ringwood (“I’ll get your passport revoked” on finding that I was Australian-born), Ian Carmichael, Paul Gast, and the “Harvard Mafia” of Tim Grove, John Longhi, and Dave Walkerjust who won that cocktail party?

Then I went to Aberystwyth in 1978, where I had the chance discovery of the most extensive Archaean metasediments in the North Atlantic craton at Stoer with my wife, Sue, in 1981–1982, and a thorough grounding in geopolitics with David Bowen and Keith O’Nions during the U.K. Earth Science Review 1986–1988 and its aftermath. Two brilliant years followed at Sultan Qaboos University, 1988–1990, with Samir Hanna, teaching very rewarding students primarily in the field. Then an outstanding late-career opportunity at Cardiff, masterminded by Dave Rickard, brought me into the association with Yaoling Niu that has illuminated my last decade.

I am greatly honored by the Hess Medal. Chance has often found me opposing the bulk of American scientific opinion. In that context, this is a very generous gift indeed. Thank you all. Harry himself commented on my work in 1968: “O’Hara’s paragraphs and even his sentences…are difficult to read.” He may have written my epitaph!

I leave you with four questions: Previous intellectual and technological cultures have flourished, and then perished. Why? What are we doing to avoid the same fate? Would life have evolved to its present state if each genetic experiment had required prior approval from six referees and a committee? Could Harry Hess have made his great contributions in today’s environment?

—MICHAEL JOHN O’HARA University of Wales, Aberystwyth, U.K.