Open-vent volcanoes fuelled by depth-integrated magma degassing

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.

An interpretative view of open-vent volcanoes

“…Their erupted magmas span wide ranges of chemical composition from basalts to nephelinite or trachyandesite and phonolite (Fig. 1), temperature and viscosity (see reviews of Rose et al 2013 andEdmonds et al 2022). Low to moderate viscosity systems are characterized by permanent plumes and gas emissions, eventually explosive activity at the craters and/or emissions of lava flows, whereas high viscosity, silica-rich magmas able to retain gas bubbles promote lava dome growth and possibly vulcanian eruptions.…”

“…The recent comparison between thermal and SO 2 fluxes suggests that the magma supply rate at Masaya and associated with SO 2 degassing may be 5.5 more than the amount of magma which reaches the surface from thermal fluxes estimated by satellites (Aiuppa et al 2018). Edmonds et al (2022) provide a review of the degassing and volcanic activity at open-vent volcanoes producing a wide range of magma compositions from basalts to dacites and rhyolites.…”

New insights on the active degassing system of the Lipari–Vulcano complex (South Italy) inferred from Local Earthquake Tomography

“…44 ). In fact, a medium saturated with gas behaves as a sponge and thus has a low compression modulus that causes low P-wave velocity values, whereas the decrease in S-wave velocity is not as substantial, determining a low Vp/Vs ratio 8,45 . Thus, it is widely believed that low Vp and low Vp/Vs anomalies are indicative of a gas-filled porous medium [46][47][48] .…”

Pre-eruptive Conditions of the 3 March 2015 Lava Fountain of Villarrica Volcano (Southern Andes)

“…In this respect, experimental petrology may provide further clues as to the drivers of such paroxysmal events, as well as textural studies focused on ascent rates and diffusion chronometry. Moreover, monitoring of gas emissions might provide key precursory signals for future paroxysmal events (Aiuppa et al 2007;Burton et al 2007;Allard 2010;Aiuppa et al 2017;Edmonds et al 2022).…”