Walker, G.P.L.
Laugardagur, 27. september 2014 22:18

GEORGE PATRICK LEONARD WALKER’s research contributions in New Zealand and worldwide arose from his dedication to measuring, rather than simply describing, volcanic eruptions and their deposits and applying his exceptionally keen intuition to generate major advances in understanding. His fame rested on his ability to merge simple data sets with novel conceptual models to yield fundamentally new insights. His work was characterized by its extreme originality and broad scope, and forms the underpinning of most modern understanding of how volcanoes erupt. He, more than any other individual worldwide, played a role in turning volcanology from its previous descriptive style into a modern quantitative science.

walker


George was born on March 2, 1926, and was brought up in London and, after the outbreak of World War 2, in Northern Ireland. As a teenager he had decided that he knew nothing about botany or geology, so he bought a book on each subject and became rapidly captivated by the latter. He subsequently studied for his Bachelor’s (1948) and Master’s (1949) degrees in Geology at Queens University, Belfast, and then moved to the University of Leeds to study the amygdale-filling minerals in the Tertiary basalt lavas of the Antrim Plateau under W.Q. Kennedy, completing his Ph.D. in 1956. He was appointed to an Assistant Lectureship at Imperial College in 1951, and was subsequently a Lecturer (1954-1964) and Reader (1964-1978).

George’s research contributions focussed around two areas, initially mineralogy, specializing in studies of the zeolite minerals that had developed in ancient sequences of basalt lavas, then subsequently in physical volcanology. His first major contributions arose from his recognition that different combinations of the many zeolite species (about 60 of which he was able to recognise casually in the field) were consistently present in specific rock layers and thus showed how far below the original ground surface any particular piece of lava had been buried. He initially used these simple observations in studies of volcanic rocks in Antrim and the Inner Hebrides to reconstruct the largely eroded shapes of ancient volcanoes of which, for example, the islands of Mull and Skye are the remnants. More notably he then mapped huge areas of otherwise monotonous basalt lava sequences in eastern Iceland and used the patterns of zeolite zonation to make fundamental inferences about the structure of the upper parts of the Earth’s crust there. These observations, gathered over months of painstaking fieldwork, were critical in providing geological evidence for the process of sea-floor spreading during the development of the revolutionary ideas in Earth Sciences that have since become known as plate tectonics.

In 1963-64 the eruption of Surtsey occurred off the south coast of Iceland, and a visit to see this live volcano captured George’s interest to the extent that he changed research directions from old, cold rocks into the products of young volcanism. From the mid 1960s his research efforts were focussed on young volcanic eruptions and their products.

George’s simple but novel techniques and wide-ranging studies provided a framework of systematic field documentation and interpretation that form the foundation of modern physical volcanology. In the period from 1967 to 1978, his studies covered a wide range of volcano types and eruption styles, from active lava flows on Mount Etna through huge ancient lava flows in the Deccan Traps of India, to the products of explosive eruptions in the Azores, Canary Islands, Italy and elsewhere. In his work on lavas, he recognized the importance of non-newtonian rheology in controlling the morphology of lava flows, and of effusion rate in controlling travel distances. The Deccan work paved the way for an understanding of the ways in which lava flows are constructed according to their eruption rate as single ‘simple’ flows versus compound, multiple-flow fields. With pyroclastic fall deposits he devised criteria for recognising deposits from different eruption styles and produced a classification that quantified relationships between deposit characteristics, such as grain size, thickness and bedding structures, and the parental volcanic processes. In the case of pyroclastic flow deposits (ignimbrites) with his students (notably Steve Self and Steve Sparks) he established and systematically documented the major characteristics and grain size variations of these voluminous and poorly understood deposits. During this period also he began a collaboration with Lionel Wilson that established innovative approaches, now greatly extended and widely used, to the physics of explosive eruptive processes.

In the mid 1970s George grew very disillusioned with the UK science scene and his position at Imperial College. An opportunity opened up in 1977, when he was awarded the fourth Captain James Cook Fellowship from the Royal Society of New Zealand. He chose to take this up at the University of Auckland and moved with his family to Auckland in February 1978. Initially he took leave of absence from Imperial College, but by selling up in London it was clear that he had no intention of returning and he eventually resigned his position there. New Zealand gave George a new surge of creative energy and he set about studying the explosive volcanism of the Taupo Volcanic Zone with great gusto. In his time here, he made important contributions to concepts of the eruptive styles and vigour of explosive eruptions, largely based around deposits from 4 events from Taupo and Okataina volcanoes.

First, he studied the fall deposits from a complex early eruption from Okataina, the Rotoiti eruption. These fall deposits are remarkable for containing voluminous beds (>1-10 km3) that are composed of 70 to 80 weight % crystals, and George was interested in the processes that could cause such a concentration of crystals over the ca. 20 weight % found in the pumice fragments in the same deposits. He proposed a two-stage model, whereby crystal enrichment occurred firstly as pyroclastic flows were generated, then secondly as selective winnowing occurred in vast littoral explosions as the flows entered the sea at the Bay of Plenty coastline. Although the stratigraphic basis for the model was demonstrated to be flawed by the (unpublished) fieldwork of Ian Nairn, a better explanation for the crystal enrichment process in these and other similar deposits has yet to be found and the work has remained very influential.

Second, he launched into a comprehensive study of the fall deposits from two of the youngest, largest and most explosive eruptions from Taupo volcano (Taupo, Waimihia) to create a host of new data and ideas for explosive volcanism. Using data from pumice fall deposits produced in these eruptions, he devised a mass balance method of calculating their volumes that utilized his observation that dense crystals in the deposits on land had to be counterbalanced by a calculable volume of vitric ash material that was ‘missing’, blown away out to sea. This method was and is very labour intensive but yielded volume estimates for these deposits that were 2 to 3 times greater than those inferred from the on-land deposits alone. Parts of the Taupo eruption involved the large-scale interaction of magma with water in a pre-existing Lake Taupo, and George documented the products of this ‘wet’ volcanism, showing how flushing of ash by water gave rise to a complex spectrum of depositional processes and yielded deposits that were remarkably unchanging in their grain-size characteristics from source to distal extremities. The work on these young fall deposits, and subsequently on older fall deposits of the Mangaone Subgroup from Okataina (with Zinzuni Jurado-Chichay), clarified the quantitative concepts of magnitude, intensity, dispersive power and destructive potential applied to the products of prehistoric eruptions.

Third, the Taupo eruption culminated in a remarkable pyroclastic flow event that produced the deposit known as the Taupo ignimbrite. George had seen this deposit during an earlier trip to New Zealand in 1965, and had decided even then that it was unusual and that he wanted to study it. In this work, together with Colin Wilson and other collaborators, he derived many new ideas including recognition of the importance of the aspect ratio (thickness to area) of ignimbrites to their emplacement conditions, definition of the landscape-draping veneer facies in low-aspect-ratio ignimbrites, and recognition of several depositional lithofacies of ignimbrites and relating them to the structure of individual pyroclastic flows.

Fourth, unable to resist the opportunity, George worked on deposits from the 1886 basaltic eruption of Tarawera. Basaltic explosive eruptions of such power and size are rare, and with Steve Self and Lionel Wilson he documented the grainsize characteristics of the scoria fall deposit (complementary to Ian Nairn’s studies of the phreatomagmatic ejecta from Rotomahana) and proposed a model for the physical processes that gave rise to such an unusual deposit.

Almost as a byproduct of his deposit-specific studies, George also melded his New Zealand work into innovative overviews of processes and products of explosive volcanism. He documented systems like Taupo as ‘inverse volcanoes’, proposing that their deposits were so widespread that the co-eruptive caldera collapse was not compensated for and the volcano ended up as a large downsag in the Earth’s surface. Similarly, he incorporated his New Zealand studies into several overview papers on the nature of ignimbrites. Although resident for less than three years in New Zealand, George and his studies gave an immense fillip to volcanological research in this country and put it indelibly on the volcanological map.

Unable to stay in New Zealand, George’s final career move took him to the University of Hawai’i at M?noa in early 1981 to take up the newly established Gordon A. Macdonald Chair in Volcanology, a post he held until retirement in 1996. In Hawai’i, his interests naturally turned once again to basaltic volcanoes and eruptive processes. His contributions included masterful descriptions of dyke swarms in eroded volcanoes in the Pacific, detailed insightful studies of lava dynamics based on the young or active lavas of Kilauea volcano in the 1980s and 1990s (with Scott Rowland), recognising the importance of inflation of lava flows during emplacement (his so-called lava-rise mechanism) and the accompanying surface deformation structures such as tumuli. Other important studies were on Toba volcano, where he worked on the huge ignimbrites formed by the three eruptions that formed this caldera, and on anisotropy of magnetic susceptibility.  He did this first in order to locate source vents for the Toba deposits, and later to understand the flow of magma in dykes and of basaltic lavas on the surface (with Michael Knight, Emilio Herrero-Bervera and Edgardo Cañon-Tapia).

As well as his exceptional contributions to volcanology, George was a brilliant teacher, supportive of anyone who wished to learn, at all levels from schoolchildren to postgraduate students. In the field he was utterly in his element while there was still light in the sky (or car headlights available), and he had an unequalled ability in showing students how to understand complex volcanic processes using simple systematic observations and analogue models allied with logical deductive thinking. A major legacy is in the great number of people who he helped, encouraged and inspired to work in volcanology. In parallel with his ability to inspire came a shyness and an avoidance of fame and publicity. He was genuinely happiest in the field making observations and furthering understanding of volcanoes and their eruptions.

George’s achievements in research were recognised worldwide by elections as a Fellow of the Royal Society of London in 1975, an Honorary Fellow of the Royal Society of New Zealand  in 1987, a Fellow of the Geological Society of America also in 1987, and as a Fellow of the American Geophysical Union in 1988. For his work in Iceland he was elected to honorary membership of the Iceland Science Society in 1968, received the Icelandic Order of the Falcon in 1980 (a rare honour for a foreign national) and in 1988 received an Honorary D.Sc. from the University of Iceland. From the UK, he received a D.Sc. from the University of London in 1982, and the Lyell and Wollaston Medals of the Geological Society of London in 1982 and 1995, respectively. In New Zealand, he won the McKay Hammer Award of the Geological Society of New Zealand in 1982. Most fittingly, for one with George’s career history, he was awarded the Thorarinsson Medal (the highest award in volcanology) from the International Association of Volcanology and Chemistry of the Earth’s Interior in 1989.

George’s success as a scientist also owed much to the support given to him for over 40 years by his wife Hazel, who not only raised their children, but typed and retyped manuscripts (in the days before word processors) and did large amounts of laboratory work for him. He is survived by Hazel, their children Alison and Leonard, and a grandson Matthew.