Textural and geochemical constraints on andesitic plug emplacement prior to the 2004–2010 vulcanian explosions at Galeras volcano, Colombia

Abstract

Hazardous sequences of vulcanian explosions are thought to result from the repeated emplacement and destruction of degassed, highly crystalline magma plugs in the shallow conduit of arc volcanoes. The processes governing the timing and magnitude of these explosions are thought to be related to magma ascent rate and efficiency of degassing and crystallisation. We study a rare suite of time-constrained ballistic bombs from the 2004–2010 period of activity of Galeras volcano to reconstruct magma plug architecture prior to six individual vulcanian explosions. We find that each plug was vertically stratified with respect to crystallinity, vesicularity and melt volatile content, melt composition and viscosity. We interpret this structure as resulting from multiple bubble nucleation events and degassing-driven crystallisation during multi-step ascent of the magma forming the plug, followed by spatially variable crystallisation within the plug under contrasting conditions of effective undercooling created by degassing. We propose that the shallow conduit evolved from more open degassing conditions during 2004–2008 to more closed conditions during 2009–2010. This resulted in explosions becoming smaller and less frequent over time during 2004–2008, then larger and more frequent over time during 2009–2010. This evolution was controlled by changing average ascent rates and is recorded by systematic changes in plagioclase microlite textures. Our results suggest that small volume vulcanian explosions (~ 10 5 m 3 ) should generally be associated with longer repose times (hundreds of days) and produce ballistics characterised by small numbers of large, prismatic plagioclase microlites. Larger volume vulcanian explosions (~ 10 6 m 3 ) should be associated with shorter repose times (tens of days) and produce ballistics characterised by high numbers of small, more tabular plagioclase microlites. Electronic supplementary material The online version of this article (10.1007/s00445-018-1260-y) contains supplementary material, which is available to authorized users.

Evolution of Mechanical Properties of Lava Dome Rocks Across the 1995–2010 Eruption of Soufrière Hills Volcano, Montserrat

“…Such relationships of strengthening with increasing crystallinity have been noted in partially crystalline polymers (e.g., Brady, 1976). The crystallinity-strength relationship at a dome-building volcano was discussed by Bain et al (2019), where low crystallinity samples were associated with low repose times between volcanic explosions, and therefore low residency times within the upper conduit and dome. We speculate that a longer residence time at elevated temperature within the volcano leads to increased densification of material as well as increased crystallization.…”

Constraints on the porosity, permeability and porous micro-structure of highly-crystalline andesitic magma during plug formation

“…These spontaneous explosions result from the emplacement of high-crystallinity, low-permeability plugs of degassed magma in the shallow conduit (<500 m, Fig. 1) (Bain et al, 2019;Clarke et al, 2007;Giachetti et al, 2010;Hammer et al, 1999;Sparks, 1997;Stix et al, 1997;Voight, 1999;Wright et al, 2007). Further degassing beneath these low-permeability plugs triggers explosions when the strain rate exerted by overpressure in gas bubbles overcomes the structural relaxation rate of the magma (Clarke, 2013;Clarke et al, 2007;Coats et al, 2018;Dingwell, 1996;Lavallée et al, 2012;Sparks, 1997;Stix et al, 1993).…”

“…Further degassing beneath these low-permeability plugs triggers explosions when the strain rate exerted by overpressure in gas bubbles overcomes the structural relaxation rate of the magma (Clarke, 2013;Clarke et al, 2007;Coats et al, 2018;Dingwell, 1996;Lavallée et al, 2012;Sparks, 1997;Stix et al, 1993). The formation of such degassed, low-permeability plugs is intimately linked to the processes of melt degassing, crystallisation and magma outgassing during magma ascent (Bain et al, 2019;Cashman and Blundy, 2000;Hammer et al, 2000Hammer et al, , 1999Lavallée et al, 2012;Preece et al, 2016). Tracking the evolution of porosity and permeability in ascending magmas is therefore fundamentally important for building understanding of eruption dynamics, improving ascent models and successful eruption forecasting.…”

Triggering of the powerful 14 July 2013 Vulcanian explosion at Tungurahua Volcano, Ecuador

“…Pl, CPx, OPx, and Amp. are plagioclase, clinopyroxene, orthopyroxene, and amphibole, respectively (Hammer et al, 1999), Mt Pelee 1902, various eruptions of Merapi 1986(Preece et al, 2016Hammer et al, 2000), Galeras 2004(Bain et al, 2018 and Soufrière Hills Volcano 1997 (Clarke et al, 2007). (b) Experimental decompression data from continuous decompression experiments (Riker et al, 2015;Brugger and Hammer, 2010b;Martel, 2012), single step decompression experiments (Riker et al, 2015;Couch et al, 2003;Brugger and Hammer, 2010b) and multistep decompression experiments (Brugger and Hammer, 2010b Shea and Hammer (2013) and Andujar et al (2017) and from the bulk composition analysis of ash from the 14 July 2013 explosion.…”

“…The origin of other Vulcanian events have also been estimated as 1-2 km below the vent (Battaglia et al, 2019;Burgisser et al, 2011;Clarke et al, 2007) or shallower (Bain et al, 2018). Do such depths mark, as at Tungurahua, the lower bound of gas accumulation?…”