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Institute of Advanced Study

Professor Richard Arculus

IAS Fellow at St Mary's College, Durham University (October - December 2009)

Professor Richard Arculus is an igneous petrologist and currently Professor of Geology in the Research School of Earth Sciences at the Australian National University. He completed a doctorate at the University of Durham in the early 1970s, and subsequently held post-doctoral fellowships at the Carnegie Institution of Washington and the Australian National University. Through the 1970s, 80s, and early 90s, he had academic positions at Rice University, University of Michigan, and the University of New England.

The prime focus of Professor Arculus' research has been island arc systems such as those forming the Pacific "Ring of Fire". His most highly cited research stems from this focus. The oceanic lithosphere created at divergent boundaries formed by ocean ridges is returned to Earth's interior at subduction zones, bearing the imprint of chemical exchange with the oceans including extensive hydration, and a superstrate of sediment derived predominantly from biological activity in the oceans plus detritus transported from continents. Some of these materials and especially water, are heavily processed and returned to the Earth's surface via island and continental arc volcanoes. In fact, the most explosive volcanic eruptions, largest earthquakes, most profound ocean deeps, greatest gravity anomalies, and highest mountains all occur at zones of plate convergence and subduction. Volcanic arc systems are the surface manifestation of a complex spectrum of magmatic and tectonic processes triggered by plate subduction at zones of convergence. An international effort is currently focusing on processes in and above subduction zones because magmatic products of supra-subduction zone activity seem to be the major building blocks of the continental crust, at least through the last billion years.

Professor Arculus was actively involved with the international Ocean Drilling Program's efforts in the 1980s in the arcs of the western Pacific. Since moving back to the ANU in the mid-1990s, he has led the successful effort of Australia to become a member of the successor program (Integrated Ocean Drilling Program). Professor Arculus has also been a participant in or led a number of research voyages with Australia's Marine National Facility (RV Franklin and RV Southern Surveyor) studying arc-backarc systems in waters of Papua New Guinea, Solomon Islands, Vanuatu, Fiji, Tonga, and New Zealand. His other primary research interest is in examples of rocks that have traveled to great depths in subduction zones and been fortuitously exhumed, and the nature of petrologic and geochemical changes experienced by these rocks during the subduction process. Examples include outcrops in New Caledonia and the Qilian Mountains of China.

Professor Arculus is currently the editor of Journal of Geophysical Research (Solid Earth), and has previously been editor of a number of other high-citation impact, international journals. He has given keynote talks at many international conferences. During his time at the IAS, he will be collaborating particularly with members of the staff of the Department of Earth Sciences on research topics related to island arcs, and writing a book for Cambridge University Press entitled "Volcanic Arc Systems".

Fellow's Home Page

Resources

Professor Richard Arculus Publications

Arculus, R. (2008) Volcanic Arc Systems. Cambridge: Cambridge University Press.


IAS Insights Paper

Abstract

Volcanoes and their eruptions were familiar phenomena to homo sapiens in the species' African development, but among the first recorded speculations, Empedocles identified volcanic activity as a manifestation of fire, one of the four roots responsible for all structures in the world. Impressed by the maritime proximity of many European volcanoes, a belief developed that interaction between the sea and magma is critical in triggering explosive volcanism - nowadays this is termed phreatomagmatic activity. Following the eighteenth-century triumph of the plutonists over the neptunists with respect to the origin of igneous rocks, several nineteenth-century geologists advocated exsolution of H2O, previously dissolved at depth in the magma rather than via shallow interactions with seawater, as the main driving force for explosions. The enormous expansion accompanying transformation of liquid to gaseous H2O provides the energy for this explosive activity. We know considerable amounts of H2O (and other volatile gases such as helium) were accreted during formation of the Earth. Magmas transport these gases towards the surface, and lowered pressure results in exsolution of volatiles.

Igneous petrology and volcanology made little contribution generally to the development of plate tectonic theory, but pioneers such as Arthur Holmes, Harry Hess and Robert Coats were early advocates of crustal recycling, the importance of H2O in serpentinising the oceanic crust and global recycling of H2O via subduction zones respectively. Release of H2O from a subducted plate triggers melting and magma formation in the overlying mantle. Magmas subsequently erupted in arcs developed over subduction zones are much richer in H2O than at ridge or hot-spot settings. It appears the most important continental crustbuilding blocks are the basalt-andesite-dacite-rhyolite suites formed in arcs. The bulk composition of the continental crust is known to be equivalent to andesite, and the distinctive trace element geochemical characteristics of the continental crust are only matched by arc magma types. The aphorism 'no water, no granites - no oceans, no continents' summarises the importance of H2O in global recycling and the genesis of continent-forming arc magmas. Earth seems to be the only planet with plate tectonics and a fundamental question is whether a liquid hydrosphere (unique to Earth among the terrestrial planets) is a necessary condition for this style of tectonism.

In addition to H2O, other potentially volatile and fluid-mobilised elements and compounds (including ore-forming metals) are transported in supercritical fluids exsolved from arc magmas. Beneath the sea floor, these fluids become mixed with recycled seawater to emerge in hydrothermal vent ('blacksmoker') systems.  On large scales, precipitation of metallic sulfides from hydrous fluids in the crust leads to large, mineable Au-Cu-rich ore deposits known as 'porphyry coppers.'  In the past century, the possible scale of explosive eruptions driven primarily by exsolution of H2O, has become recognised. We can extrapolate from observed small events such as Mt St Helens of the Oregon Cascades, through Taupo in New Zealand (50 km3, 1.8 Ka BP; 500 km3, 26.5 Ka BP), to the Oligocene (30 Ma) 5000 km3 of the Fish Canyon Tuff in Colorado. These large eruptions can have significant environmental effects.

Insights Paper