Abstract
We have coupled three independent methods to investigate the time evolution of eruptive intensity during the sub-Plinian and Plinian phases of the 3600-year BP Late Bronze-Age eruption of Santorini Volcano: (1) mass eruption rate based on new lithic isopleth maps for multiple layers of the fall deposit, (2) magma decompression rate calculated from vesicle number densities, and (3) magma decompression rate calculated from H2O gradients in melt reentrants, with methods 2 and 3 measured on the same suite of pyroclasts. Mass eruption rate increased by two orders of magnitude, reaching 210 × 106 kg s−1 at the peak of the Plinian phase (plume height 28.4 ± 1.0 km); it then declined in the final stage of fallout emplacement following the first generation of pyroclastic surges. Decompression rates from melt reentrants (0.008 to 0.25 MPa s−1) are two to three orders of magnitude lower than those from vesicle number densities, assuming heterogeneous vesicle nucleation (2 to 19 MPa s−1). Melt reentrants are thought to record slow decompression in the deep feeder conduit, whereas vesicles record much higher rates of decompression in the shallow conduit due to the steep, nonlinear pressure gradients associated with magma vesiculation and fragmentation. Upwardly converging flow from a dike-like, deep conduit to a more cylindrical, shallow conduit may also have played a role in causing upwardly accelerating flow. Variations in deep decompression rate recorded by melt reentrants are decoupled from mass eruption rate, whereas those recorded by vesicles lie in between. Taken with the transition from unsteady to steady Plinian eruption, this may reflect the existence of transient flow conditions in the conduit system due to widening and lengthening of a deep feeder dike as Plinian eruption progressed. As the mass eruption rate rose to its peak value, the fragmentation level fell in the conduit due to increasing rates of magma strain and decompression.
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Acknowledgements
The authors would like to thank Bruce Houghton for conversations surrounding isopleth/isopach data, and helpful reviews by the editor, one anonymous reviewer and Alessandro Vona that led to an improved manuscript. We also thank Christophe Constantin for thin sections, Jean-Marc Hénot for assistance with the SEM, and Erica Duncan, Emma Kerins, Megan Saalfeld, and Crystal Christensen for processing SEM images. This is Laboratory of Excellence ClerVolc Contribution Number 448.
Funding
This work was funded through a Clermont Auvergne University Postdoctoral grant to Druitt and an NSF grant EAR-1922513 to Myers.
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Editorial responsibility: R. Cioni; Deputy Executive Editor: J. Tadeucci
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Myers, M.L., Druitt, T.H., Schiavi, F. et al. Evolution of magma decompression and discharge during a Plinian event (Late Bronze-Age eruption, Santorini) from multiple eruption-intensity proxies. Bull Volcanol 83, 18 (2021). https://doi.org/10.1007/s00445-021-01438-3
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DOI: https://doi.org/10.1007/s00445-021-01438-3