Explosive Behavior of Intermediate Magmas: The Example of Cotopaxi Volcano (Ecuador)

The variability in intensity and style shown by explosive volcanism has been traditionally explained by a complex interplay among melt composition and pre-eruptive volatile content, which modulate magma ascent and conduit dynamics. However, magmas having similar compositions may be characterized by subtle textural changes affecting magma rheology and eventually explosive dynamics. Here we study five eruptions occurred at Cotopaxi volcano (Ecuador) in the last 2000years characterized by a small variation in magma composition but spanning a wide range of intensity to investigate how these parameters control variations in eruption dynamics. We combined eruption source parameters (ESPs), obtained from the application of recent models to all the available field data, with new textural data and state-of-the-art conduit dynamics modeling. We found that, despite having variable microlite content and texture, the effect of microlite on magma rheology is partly counterbalanced by variable phenocryst abundance, resulting in a comparable total crystal content. The combination of modeling results with textural data and ESPs suggests that subtle variability in crystal content and magma composition may be accompanied by strong feedback effects among crystallization, changes in melt/magma viscosity and volatile exsolution, with microlite crystallization resulting in a rapid change of magma rheology and modifications in the explosive dynamics. By combining ESPs with quantitative textural data (i.e. melt normalized vesicle number density) and conduit modeling, we also show how general observed correlations between composition and texture of juvenile products with eruption intensity are not evident when applied to eruptions characterized by a small compositional range.