Degassing during quiescence as a trigger of magma ascent and volcanic eruptions

Abstract

Understanding the mechanisms that control the start-up of volcanic unrest is crucial to improve the forecasting of eruptions at active volcanoes. Among the most active volcanoes in the world are the so-called persistently degassing ones (e.g., Etna, Italy; Merapi, Indonesia), which emit massive amounts of gas during quiescence (several kilotonnes per day) and erupt every few months or years. The hyperactivity of these volcanoes results from frequent pressurizations of the shallow magma plumbing system, which in most cases are thought to occur by the ascent of magma from deep to shallow reservoirs. However, the driving force that causes magma ascent from depth remains unknown. Here we demonstrate that magma ascent can be triggered by the passive release of gas during quiescence, which induces the opening of pathways connecting deep and shallow magma reservoirs. This top-down mechanism for volcanic eruptions contrasts with the more common bottom-up mechanisms in which magma ascent is only driven by processes occurring at depth. A cause-effect relationship between passive degassing and magma ascent can explain the fact that repose times are typically much longer than unrest times preceding eruptions, and may account for the so frequent unrest episodes of persistently degassing volcanoes.

Deposits, petrology and mechanism of the 2010–2013 eruption of Kizimen volcano in Kamchatka, Russia

“…Degassing causes notable depressurization and increase in viscosity of the silicic magma in the chamber (Girona et al 2014(Girona et al , 2015. This process decreases the dynamic of the dacitic magma chamber, acting as a density filter.…”

“…In cases where there is no evidence for pre-eruptive degassing, explosive-effusive transition can be also controlled by injection and admixing of recharge magma into the shallow silicic magmatic system (Ruprecht and Bachmann 2010). Recently, it has also been suggested that persistent passive degassing itself could foster magma replenishment and even trigger new eruptions (Girona et al 2014(Girona et al , 2015.…”

Volatile Degassing From Magma Chambers as a Control on Volcanic Eruptions

“…In this study, we will explore how the two-way coupling of the hydrothermal magmatic system affects the eruption dynamics and the thermohydromechanical response of the crust. We note that some recent studies (Girona et al, 2014(Girona et al, , 2015 have shown, using primarily mechanical models, that passive degassing has an essential effect on the pressure buildup of magmatic reservoirs in the context of open-vent conduit systems. Our study extends this analysis to more a general magmatic architecture and also incorporates a more comprehensive thermomechanical model for the temporal evolution of melt, especially the release of magmatic volatiles as the magma crystallizes or ascends to shallower depths.…”

“…Finally, from a magma reservoir perspective, previous studies have shown that magmatic volatiles play a vital role in modulating the temporal evolution of magma and the buildup of overpressure required for eruption (Degruyter & Huber, 2014;Karlstrom & Richards, 2011). However, few studies have incorporated a direct coupling of magmatic and hydrothermal systems, modulated by the magmatic volatiles (Black & Manga, 2017;Girona et al, 2015;Iguchi et al, 2002;Simakin & Botcharnikov, 2001). The loss of volatiles will significantly affect the fluid pressure within the volcanic system, leading to depressurization of the magma reservoir over time as well as affecting the stress state of the background crust.…”

“…Although, we have mostly discussed volatile loss as a negative influence on the likelihood of eruptions, continuous volatile loss from deeper systems can in turn flux volatiles into the magma reservoirs at shallower depths and enhance their likelihood of eruption, either by providing a buoyancy source or by changes in mineral assemblages and volatile solubility in melts (Giuffrida et al, 2018). Analogously, Girona et al (2015) suggested that volatile loss by passive degassing can trigger an eruption by promoting melt flow toward shallower depths by maintaining a differential pressure.…”