Plumbing system architecture and dynamics beneath an island arc caldera (Santorini, Greece)
Professor Tim Druitt, University Clermont-Auvergne, France
Santorini is one of the most explosive arc calderas in the world, having had over ten plinian eruptions, and at least four caldera collapses, over the last 350,000 years. It lies on continental crust about 25 km thick. This paper reviews what we have learned about the plumbing system of Santorini through a combination of sample characterisation, phase equilibria experimentation, melt inclusion volatile barometry, and multi-mineral diffusion chronometry. Mantle-derived basalt intrudes into the lower to middle crust, where it fractionates to andesitic, then silicic, melts in gabbroic to dioritic mush layers. Immediately prior to a plinian eruption, a large volume of evolved melt is extracted from melt lenses within these mushes and ascends to the top of the subcaldera pluton through high permeability channels. The bubbly melt (plus entrained antecrysts) then intrudes to form a large, sill-shaped reservoir that in turn feeds the plinian eruption less than a few centuries to decades later. These upper crustal reservoirs are transient holding chambers that possibly form where vertical gradients in crustal rheology cause the ascending melt to stall. During ascent from the middle to upper crust, the silicic melts grow phenocrysts by vapour-saturated decompression upon entrained, deeper-derived antecrysts. Melt extraction from depth is probably triggered by destabilisation of mush layers, due either to tectonic forces or to spontaneous breakdown of the solids framework once the melt fraction reaches a critical fraction. The smaller silicic eruptions of interplinian periods appear to be characterised by similar processes and timescales. Interplinian eruptions preceding and following plinian eruptions can tap mid-crustal mush layers different from those feeding the plinian eruptions themselves, as shown by trace element and isotopic signatures of the magmas. We favour a model in which the rheological behaviour of the subcaldera crustal column modulates magma fluxes and storage levels through the crust, and influences eruption styles.
Beyond the blobs: advances in geophysical imaging of magmatic systems
Dr Michele Paulatto, Imperial College London, UK
Constraining magma reservoirs is important for understanding eruptive processes and the formation of continental crust. Geophysical imaging is a particularly powerful tool as it provides information on 3D structure, which most other methods cannot address. Geophysics however, is far from a silver bullet and has limitations arising mostly from finite resolution and non-uniqueness of geological interpretations. Recent advances in geophysical inversion and interpretation promise to revolutionize the amount of information and level of detail that they can gain from geophysical data. Michele will discuss the work that his group are doing at Imperial College to advance the state of the art in volcano tomography. The group focus on three aspects: 1) joint inversion of multiple geophysical observables (e.g. seismic and gravity data); 2) full-waveform inversion of seismic data; 3) quantitative interpretation. Seismic velocities are affected by many factors (composition, porosity, temperature, melt content), therefore interpretation of P-wave velocity models can rarely provide robust estimates on melt distribution in the crust. Joint inversion of independent geophysical datasets allows one to recover multiple parameters and start reducing the non-uniqueness of interpretations. Full-waveform inversion (FWI) exploits the information contained in the full record of natural or man-made seismic sources, resulting in 3-5 times better resolution than traditional seismic inversion techniques. FWI also allows the recovery of a wider range of elastic parameters including attenuation, anisotropy and S-wave velocities. Quantitative interpretation uses these multiple parameters to place stronger constraints on melt content and distribution and the composition of the host rock. These techniques are being used to image Santorini Volcano, Greece, using data from the NSF-funded PROTEUS experiment (Plumbing Reservoirs Of The Earth Under Santorini). The experiment was designed for high-density spatial sampling of the seismic wavefield to allow us to apply state-of-the-art 3D inversion methods, and includes 89 ocean bottom seismometers, 65 land stations and 13,300 airgun shots.
Architecture and dynamics of the Altiplano-Puna transcrustal magmatic system (central Andes) from the PLUTONS project
Professor Matt Pritchard, Cornell University, USA
The central Andes is a key global location to study the architecture of transcrustal magmatic systems as it currently hosts the world's largest zone of silicic partial melt in the form of the Altiplano-Puna Magma or Mush Body (APMB). Further, the APMB is dynamic, with a ground deformation pattern 150 km in diameter that has been ongoing for several decades, centered on Uturuncu volcano, Bolivia. I will discuss results from the recently completed international PLUTONS project, which focused a suite of geophysical, petrological, and geomorphological techniques at Uturuncu with numerical modeling to infer the subsurface distribution, quantity, and movements of partial melts. We find geophysical anomalies extending from the base of the crust to the surface indicating multiple distinct reservoirs of magma and/or hydrothermal fluids with different physical properties. The characteristics of the geophysical anomalies differ somewhat depending on the technique used - reflecting the different sensitivity of each method to subsurface melt (or fluid) of different compositions, temperature, connectivity, and volatile content, and highlighting the need for integrated, multi-disciplinary studies. We offer a new interpretation of the cause of ground deformation related to reorganization of the magma mush that causes no net ground deformation over hundred to thousands of years that is consistent with the lack of tilted shorelines around Uturuncu.
The geometry of active magma systems
Dr Juliet Biggs, University of Bristol, UK
The orders of magnitude difference in viscosity between crystal-rich regions (mush) and melt-rich regions (chambers) controls the behaviour of vertically-extensive magmatic systems. High-resolution satellite geodesy provides a unique insight into the patterns of magma movement over short (weeks-decades) timescales, reflecting the geometric distribution of the mobile, eruptible component of the system. In this talk, Juliet will discuss the insights provided by 25 years of InSAR observations of deformation patterns at volcanoes around the world. To the first order, co-eruptive deflation and pre-eruptive inflation have been seen but typically the deformation is temporally variable, suggesting pulsed magma supply within viscoelastic rheology. In addition, there are numerous examples of co-eruptive uplift, typically attributed to endogenous growth of the edifice, and inter-eruptive subsidence, attributed to cooling, degassing or hydrothermal processes. The spatial pattern of deformation reveals that many systems are composed of multiple, interacting magma chambers, which can be tens of kilometers from the eruptive vent. The spatial patterns of subsurface magma chambers relates to surface alignments suggesting that pre-existing structures, such as faults, play an important role on the growth of magma reservoirs. This suggests a revised model of the development of shallow magma bodies based on growth and coalescence along fault structures.
Time dependence of volcano inflation: mass influx or viscoelastic relaxation?
Professor Paul Segall, Stanford University, USA
Distinguishing time dependent magma chamber pressurization from relaxation of a viscoelastic aureole surrounding the chamber using geodetic measurements has remained challenging. Elastic models with mass inflow proportional to the pressure difference between the chamber and a deep reservoir predict exponential re-inflation. For a spherical chamber surrounded by a Maxwell viscoelastic shell with pressure dependent recharge, the surface deformation is the sum of two exponentials, dependent on refilling and Maxwell times.
GPS displacements following eruptions of Grímsvötn, Iceland in 2004 and 2011 exhibit rapid post-eruptive inflation, followed by inflation with a much longer time constant. Markov Chain Monte Carlo inversion with the viscoelastic model shows the time series can be fit with viscosity of ~ 2e16 Pa-s, and a relatively incompressible magma. The latter appears to conflict with the ratio of erupted volume to geodetically inferred source volume change of 10, obtained for the best fitting Mogi source, which implies a very compressible magma.
Re-examination of the co-eruptive GPS and tilt data with a more general ellipsoidal model reveals that the best fitting source are oblate and deeper, consistent with seismic tomography, with much larger volume changes, relative to spherical models, consistent with incompressible melt. This talk will also show that non-Newtonian magma rheology can, at least qualitatively, explain the data.
Why are thermal aureoles thinner or thicker than expected? The role of pluton emplacement dynamics
Dr Catherine Annen, Univeristy of Bristol, UK
The transfer of heat between a cooling magma body and its host rock induces metamorphic thermal aureoles. In the case of tabular intrusions, simple analytical models predict that the thickness of thermal aureoles scales linearly with the thickness of magma intrusions. However, some aureoles are much thicker or much thinner than predicted by analytical models. With numerical simulations, several factors affecting the size of thermal aureoles can be examined. A contrast in the rate of heat transfer between the magma and the country rock significantly impacts the maximum temperatures reached in the aureole. This contrast in heat transfer can be due to differences in conductivities or to the effect of convection, either in the magma or in the country rock. The emplacement dynamics of the magma also control the size of an aureole. Incremental emplacement, where magma injection is focused closed to one contact produces asymmetric aureoles. In comparison with a similar intrusion that is instantaneously emplaced, fast incremental emplacement induces a thicker aureole whereas slow incremental emplacement induces a thinner aureole. Inversion of aureole sizes can shed light on the associated intrusion dynamics provided that all other parameters affecting heat transfer are taken into account.