A Textural Perspective on the Magmatic System and Eruptive Behaviour of Merapi Volcano

Quantitative textural analysis of crystals, including their number density, shapes, sizes, overall abundance and size distribution can be used to shed light on magmatic processes and the timescales over which they operate. At Merapi, textural analysis of phenocrysts in dome lavas, lava flows, tephra, and in plutonic cumulates has revealed that open system steady state conditions prevail throughout the crustal magma plumbing system over short time periods, with non-steady state conditions prevailing over the longer term. Phenocryst crystallisation likely takes place over tens to hundreds of years prior to eruption. Quantitative textural analysis of feldspar microlites, in conjunction with compositional data, elucidate magma ascent and degassing processes within the conduit during dome forming eruptions, and additionally reveal the driving forces behind transitions between effusive and explosive eruptive behaviour. For example, microlite textures from different stages of the 2010 eruption show that transitions between explosive and effusive activity in 2010 were driven primarily by the dynamics of magma ascent in the shallow conduit.