Contrasting Volcanic Deformation in Arc and Ocean Island Settings Due To Exsolution of Magmatic Water

Abstract

Two of the most widely observed co‐eruptive volcanic phenomena—Ground deformation and volcanic outgassing—Are fundamentally linked via the mechanism of magma degassing and the development of compressibility, which controls how the volume of magma changes in response to a change in pressure. Here we use thermodynamic models—Constrained by petrological data—To reconstruct volatile exsolution and the consequent changes in magma properties. We use the fraction of SO2 exsolved during decompression to predict co‐eruptive SO2 flux and magma compressibility to predict co‐eruptive surface deformation (both normalized by erupted volume). We conduct sensitivity tests using properties of typical basalts to assess how varying magma volatile content, crustal properties, and chamber geometry affect co‐eruptive deformation and degassing. We find that magmatic H2O content has the most impact on both SO2 flux and volume change. Our findings have general implications for typical basaltic systems in arc and ocean island settings. The higher water content of arc magmas makes them more compressible than ocean island magmas and leads to muted or non‐existent deformation being observed during arc eruptions. Our models are consistent with observation: Deformation has been detected during 48% of basaltic eruptions in ocean island settings (16/33) during the satellite era (2005–2020), but only 11% of basaltic eruptions in arc settings (7/61).

The effects of oxygen fugacity and sulfur on the pressure of vapor-saturation of magma

“…and Fe 3+ /Fe T and/or S 6+ /S T to constrain f O2 . However, few such calculations of P v sat include the effects of dissolved S or f O2 (e.g., Carroll and Webster 1994;Newman and Lowenstern 2002;Lesne et al 2015;Iacovino et al 2021;Yip et al 2022).…”

“…Most efforts to calculate P v sat using this approach have assumed that the vapor contains only ) and/or CO 2 molecules (CO 2,mol ) (e.g., VolatileCalc/MIMiC by Newman and Lowenstern, 2002;Rasmussen et al, 2020; MagmaSat by Ghiorso and Gualda, 2015; VESIcal by Iacovino et al, 2021; EVo by Liggins et al 2020Liggins et al , 2022Yip et al 2022; MafiCH by Allison et al, 2022; and various unnamed models such as those presented by Papale et al 2006;Iacono-Marziano et al 2012;and Duan 2014). These calculations do not generally consider the effects on P v sat and the corresponding vapor composition of the presence of reduced C-O-H (e.g., H 2 , CO, CH 4 ) and S-bearing species in the vapor and melt.…”

High‐Resolution InSAR Reveals Localized Pre‐Eruptive Deformation Inside the Crater of Agung Volcano, Indonesia

“…However, we propose a shallow hydrothermal system as the source of deformation based on: (a) the lack of magmatic gases emitted prior to eruption, pointing to extensive scrubbing by a hydrothermal system (Syahbana et al., 2019; Symonds et al., 2001), (b) the increased activity of the fumarolic field located in the crater during September‐October 2017, (c) the shallow depth of the modeled source (<200 m) and the lack of temperature anomalies at the summit until late 2017, which exclude the presence of a persistent high‐melt‐fraction magma body (possibly remnant from the 1963 eruption) in the shallow subsurface, (d) migration of seismic activity from October‐November, attributed to magma ascent (Sahara et al., 2021; Wellik et al., 2021), lagging behind the intra‐crater deformation starting in September, and (e) the difference in volume change associated with the September‐October deformation (∼1 × 10 4 m 3 ) and the erupted magma from 25 November to 18 December 2017 (27 × 10 6 m 3 ) (Andaru et al., 2021). While sub‐surface volume change and erupted volume are not expected to be equal (Kilbride et al., 2016; Yip et al., 2022), the orders of magnitude difference points to differing sources/mechanisms. The volume change of the September‐October source is therefore not related to magma transport, but rather to a perturbation of the shallow hydrothermal system.…”

“…Petrological studies based on the 1963 eruption identified a shallow magma storage zone at depths of 3–7 km (Geiger et al., 2018; Self & King, 1996). Either this reservoir was exhausted during the 1963 eruption and no longer exists, or the magma is so gas‐rich (Syahbana et al., 2019) that it is highly compressible and the deformation is below the detection limit (Albino et al., 2019; Kilbride et al., 2016; Yip et al., 2022). Deformation associated with magma ascent is rarely observed and probably indicates that the ascent was rapid and occurred a few days before the onset of eruption (Wellik et al., 2021).…”

“…This could indicate that a long‐lived, highly compressible magma reservoir was already present at depths shallower than 10 km. The compressibility, caused by the exsolution of volatiles from silicate melt (Kilbride et al., 2016; Wong & Segall, 2020), allows the reservoir to change its volume in response to pressure perturbations, reducing the magnitude of deformation observed at the surface (Kilbride et al., 2016; Yip et al., 2022). Alternatively, magma ascent to the summit could have been too localized to be detected by distant GPS stations and too rapid to be detected with satellite InSAR.…”