Abstract
Open-vent, persistently degassing volcanoes—such as Stromboli and Etna (Italy), Villarrica (Chile), Bagana and Manam (Papua New Guinea), Fuego and Pacaya (Guatemala) volcanoes—produce high gas fluxes and infrequent violent strombolian or ‘paroxysmal’ eruptions that erupt very little magma. Here we draw on examples of open-vent volcanic systems to highlight the principal characteristics of their degassing regimes and develop a generic model to explain open-vent degassing in both high and low viscosity magmas and across a range of tectonic settings. Importantly, gas fluxes from open-vent volcanoes are far higher than can be supplied by erupting magma and independent migration of exsolved volatiles is integral to the dynamics of such systems. The composition of volcanic gases emitted from open-vent volcanoes is consistent with its derivation from magma stored over a range of crustal depths that in general requires contributions from both magma decompression (magma ascent and/or convection) and iso- and polybaric second boiling processes. Prolonged crystallisation of water-rich basalts in crustal reservoirs produces a segregated exsolved hydrous volatile phase that may flux through overlying shallow magma reservoirs, modulating heat flux and generating overpressure in the shallow conduit. Small fraction water-rich melts generated in the lower and mid-crust may play an important role in advecting volatiles to subvolcanic reservoirs. Excessive gas fluxes at the surface are linked to extensive intrusive magmatic activity and endogenous crustal growth, aided in many cases by extensional tectonics in the crust, which may control the longevity and activity of open-vent volcanoes. There is emerging abundant geophysical evidence for the existence of a segregated exsolved magmatic volatile phase in magma storage regions in the crust. Here we provide a conceptual picture of gas-dominated volcanoes driven by magmatic intrusion and degassing throughout the crust.
Highlights
Open-vent volcanoes are characterised by their persistent outgassing and mildly explosive activity between major eruptions (Andres and Kasgnoc 1998; Francis et al 1993; Rose et al 2013; Vergniolle and Métrich 2021)
A high seismic velocity zone with a lateral dimension of ~ 6 km has been imaged beneath the summit at 9–18-km depth (Hirn et al 1997). This body probably comprises cumulates produced from intrusive magmas, fragments of which are occasionally erupted as cognate xenoliths (Corsaro et al 2014). This cumulate body likely contains significant volumes of volatile-rich melts generated through second boiling as well as regions dominated by an exsolved volatile phase
Traditionally it has been assumed that magma is the ‘carrier’ for advecting volatiles—requiring mass balance in the upper crust to account for open-vent outgassing fluxes—we have shown instead that large volumes of intruded magma at depth, stored at multiple levels throughout the crust, provide a potential source of segregated exsolved volatiles, which inevitably must contribute to the large outgassing fluxes at open-vent volcanoes
Summary
Open-vent volcanoes are characterised by their persistent outgassing and mildly explosive activity between major eruptions (Andres and Kasgnoc 1998; Francis et al 1993; Rose et al 2013; Vergniolle and Métrich 2021). Volcanoes that transition from being ‘open vent’ to ‘closed vent’ over years to decades timescales may be classified as ‘persistently restless’. Telica Volcano, Nicaragua, transitions between a ‘weak seal’ and a ‘destabilised’ state, which may produce phreatomagmatic eruptions
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