Degassing conditions and effusive-explosive transitions at Guagua Pichincha volcano, Ecuador

Abstract

<p>Stratovolcanoes are often characterized by cyclic eruptive activity marked by effusive-explosive transitions between dome-forming and Plinian eruptions. Guagua Pichincha volcano (Ecuador) has been a location of such cyclicity for the past ~2000 years with recurrent Plinian eruptions in the 1st century, 10th century, and CE 1660 which were separated by dome-forming eruptions such as the recent 1999-2001 eruption. Short time-scale shifts in activity between dome-forming, Vulcanian and Subplinian eruptions also occurred repeatedly in the period September to December 1999. These cycles are therefore a prominent example of effusive-explosive transitions at varying timescales. Here, we investigate the reasons for such shifts in activity, by focusing on degassing and outgassing processes within the conduit, and their roles in pressurization and ensuing eruption dynamics. We have coupled a petrophysical and vesicle textural analysis of dacites from the CE 1660 Plinian eruption and the dome-forming, Vulcanian and Subplinian 1999-2001 eruptions, with percolation models in order to better understand the role of degassing on eruptive style. We demonstrate that the transition from dome-forming to Plinian activity correlates with differences in phenocryst content and consequently in inferred bulk viscosity. A lower initial viscosity is inferred for the Plinian case, consistent with faster ascent, closed-system degassing, explosive fragmentation and eruption. In contrast, dome-forming phases are promoted by a higher magma viscosity, with slower ascent enhancing gas escape and microlite crystallization, yielding effusive activity.</p><p> </p>