International Workshop on Caldera Volcanism: Analysis, Modelling and Response” Tenerife

16th – 21st October, 2005

Ray Cas, Adrian Pittari and Jill Middleton

Forty participants from all over the world and many different disciplines gathered at the Parador Hotel in the centre of the Las Cañadas Caldera Complex, Tenerife, Canary Islands, Spain, from the 16th to the 21st October, to discuss the many unclear issues related to the origin and behaviour of calderas. Although some of the “usual suspects” were present, it was refreshing to see many of the new generation of volcanologists and other geoscientists attend to present new insights. The workshop program consisted of three days of presentations and discussions, interspersed with 2 days of fieldtrips.

Day 1 of presentations was dominated by presentations on the geology, stratigraphy, facies architecture, petrology and geochemistry of calderas, with a view to better understanding eruption processes, the configuration of calderas and their development. Presentations by A. Pittari, J. Middleton, J. Wolff, P. Olin and R. Cas clarified many aspects of the complex evolution of the Las Cañadas Caldera Complex of Tenerife, its eruptive behaviour, products and magma evolution. G. Aguirre-Diaz summarised the fascinating geology of the graben calderas of the Tertiary Sierra Madre Occidental, of Mexico. S. Self and J. Wolff, as a well-oiled duet, presented ideas on the origin of the dual, nested Valles Caldera system, the Bandelier ignimbrites and their magma chambers, including an interesting assessment of the ratio of the volume of erupted products to the magma chamber volume. H. Schmincke reviewed the volcanic geology of Gran Canaria. R. Sulpizio presented an overview of the flow dynamics of small volume pyroclastic density currents, emphasising the role of granular flow.

Day 2 focussed on magma properties, hazards, risk and monitoring. D. Dingwell began by considering how magma properties can change during an eruption. The rates at which phenocrysts form in magma chambers and conduits were discussed by J. Cabato, and the role of CO2 and H2O in explosive eruptions was considered by H. Gonnermann. Several papers (L. Christiansen, M. Battaglia, M. Todesco) considered the influence of the movement of magmas, gas and/or water (aquifer, hydrothermal, groundwater) in controlling short-term ground deformation in Long Valley and Campi Flegrei calderas. A. Newman reviewed the geophysical constraints on understanding the subsurface caldera architecture of the Long Valley Caldera, including its magma chamber. G. De Natale presented an in depth understanding of the complex Campi Flegrei Caldera, the nature of its period of unrest, the hazard issues and the implications of the patterns of seismicity for understanding the interplay of magma dynamics, geothermal activity, caldera structures and regional tectonic effects. J. Marrero discussed the 2004 seismic crisis on Tenerife. R. Sulpizio discussed the role of the topography of the Monte Somma caldera wall, Vesuvio volcano, in controlling the dispersal patterns of pyroclastic density currents from several eruptions, whereas M. Todesco presented numerical simulations of the way in which topography in the Campi Flegrei calderas could influence runout distances and dispersal of pyroclastic density currents of several magnitudes, and how such modelling could be used to develop hazard and risk planning.

Day 3 of presentations highlighted the role of modelling the structural development of calderas. V. Acocella presented a very clear review of the role of caldera collapse analogue experiments in clarifying the factors that influence different styles of caldera collapse and the physical scaling issues. A. Folch and A. Geyer followed with further considerations of these issues, the relationship between analogue experiments and numerical modelling, and accounts of specific experiments. A. Gudmundsson emphasised the role of the stress field in caldera collapse, and discussed the nature of the faults that form and the pressure state of the magma chamber during caldera collapse. T. Walter summarised the origins and timing of radial and ring fractures and dykes in the basaltic San Fernandina caldera in the Galapagos Islands. R. Carniel and R. Ortíz considered the role of using pre-eruptive earthquake patterns to help predict volcanic eruptions. N. Coppo presented results of audio magneto-telluric studies of the Las Cañadas Caldera Complex of Tenerife, which indicated the existence of discrete aquifer and hydrothermal aqueous systems within the Guajara and Diego Hernández calderas. J. Gottsman reviewed the different geophysical methods used the image subsurface structure of calderas, noting the need for greater effort in this regard. W. Mueller summarised the complex geology of deformed Archaean submarine calderas of the Abitibi Belt in Canada.

One of the very successful aspects of the workshop was the plentiful opportunity for discussion, both after each presentation and at the end of each day of formal presentations. Between an hour and a half and two hours were dedicated to open discussion of significant issues. Although these were led by nominated individuals, there was plenty of scope for all participants to provide input, which generated many lively discussions and a variety of views. Some of the “hot topics” discussed include the factors that control the shapes and orientations of calderas, the nature of the caldera faults, the nature and mechanics of the collapse process, the nature of magma chambers (size, shape, constitution, formation, longevity), the pressure and deformational regimes operating during caldera collapse, the nature of pressures in the magma chamber before, during and after caldera collapse (over-pressure versus under-pressure), the mechanical behaviour of the roof block during the collapse, the rates of collapse, the role of geophysics, the roles of analogue experiments/modelling and numerical modelling. Last but not least was the universal recognition that field studies and data must continue to underpin all interpretations and modelling of calderas, their behaviour and eruptions.

The two field days focussed on the contrasting geology of the caldera wall and outflow areas on the Bandes del Sur based on the past and current research of the Monash University Volcanology Research Group of R. Cas, the Washington State University Geochemistry Research Group of J. Wolff, and the CSIC Barcelona Volcanology and Modelling Research Group of J. Martí.

The first field day focussed on the discontinuous stratigraphy, dispersal patterns and deposit characteristics on the Bandes del Sur, in the south. A variety of pyroclastic fallout, flow and surge deposits were examined. Discussions focussed on criteria for recognising caldera collapse events in the stratigraphic record, including deposit volumes and lithic clast content and abundance. The nature of the source eruptions, the timing and nature of eruption column collapse (early fountain style collapse, late climactic plinian collapse, episodic, repeated intra-plinian collapse), the problems of calculating pyroclastic flow deposit volumes and the implications of variations in juvenile clast compositions for magma generation and magma chamber dynamics were also discussed.

The second field day was devoted to the stratigraphy and volcanology of part of the caldera wall rock succession, with the focus being the predominance of welded fallout deposits, and the difficulty of correlations between the caldera wall succession and the more distal stratigraphy of the Bandes del Sur succession.

In summary, this was an extremely stimulating and thought provoking workshop, made so by excellent organization by the workshop convenors Joan Martí and Jo Gottsman, a fantastic venue and the willingness of a multi-disciplinary, multi-national and multi-cultural group of participants to share their varied experiences and perspectives in a friendly, constructive but challenging way. Two outcomes of the workshop will be a proposal to establish a multi-disciplinary IAVCEI working group on calderas and a special publication on calderas. Another outcome was that the participants drank the Parador Hotel bar dry of draught beer, a sure sign that everyone truly got into the swing of the occasion and exchanged many informal impressions about calderas and other things after hours.

Last Update: Nov. 5, 2005