Textural and geochemical constraints on andesitic plug emplacement prior to the 2004\u20132010 vulcanian explosions at Galeras volcano, Colombia

Amelia A Bain,Susan C Loughlin,Joaquín A Cortés,Gloria Patricia Cortés,Eliza S Calder

doi:10.1007/s00445-018-1260-y

Amelia A Bain, Susan C Loughlin + Show 3 more

Open Access

https://doi.org/10.1007/s00445-018-1260-y

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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 (~ 105 m3) 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 (~ 106 m3) should be associated with shorter repose times (tens of days) and produce ballistics characterised by high numbers of small, more tabular plagioclase microlites.

Highlights

Dense, degassed and highly crystalline plugs are often emplaced in the shallow conduit of arc volcanoes when intermediateEditorial responsibility: L
The correlation of glass volatile content with bomb morphology and texture is consistent with the findings of Hoblitt and Harmon (1993) at Mount St Helens, who found that pyroclasts of 1980 blast dacite containing > 0.4 wt% H2O in the groundmass glass vesiculated on eruption timescales, whereas pyroclasts containing < 0.4 wt% H2O showed no evidence of syn-eruptive vesiculation and remained dense
Second-order variations between plugs are associated with evolving eruption dynamics throughout the 2004–2010 sequence of vulcanian explosions

Summary

Introduction

Dense, degassed and highly crystalline plugs are often emplaced in the shallow conduit of arc volcanoes when intermediateEditorial responsibility: L. The efficiency of magma outgassing during further crystallisation is thought to control the transition from effusive to explosive behaviour through the build-up of pore overpressure (Sparks 1997). In this paradigm, repetitive vulcanian explosions with numerous interspersed gas and ash venting events represent explosive activity that is embedded within the overarching process of extrusion of degassed, highly crystalline intermediate magmas. Repetitive vulcanian explosions with numerous interspersed gas and ash venting events represent explosive activity that is embedded within the overarching process of extrusion of degassed, highly crystalline intermediate magmas This process of extrusion often results in the construction of lava domes, the intimate association of lava dome construction and vulcanian activity

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