Effects of volatile behaviour on dome collapse and resultant pyroclastic surge dynamics: Gunung Merapi 2010 eruption

<p>In 2010, Gunung Merapi (Central Java, Indonesia) generated two violent eruption sequences on 26 October and 5 November culminating in widespread pyroclastic density currents (PDCs) associated with the destruction of lava domes. Tephra from PDC deposits were analysed to examine pre-dome collapse volatile behaviour and evidence of carbonate assimilation. Secondary-ion mass spectroscopy (SIMS) depth profiles of plagioclase phenocrysts reveal that the <sup>7</sup>Li/<sup>30</sup>Si ratios in 26 October products are higher in the glass compared to the crystal, indicating a build-up of Li in the groundmass not observed in the 5 November samples. Higher Li in the groundmass suggests gas accumulation and rapid development of conduit overpressure in the shallow plumbing system prior to the initial 26 October explosion, which was only captured through the behaviour of quickly diffusing Li and not H<sub>2</sub>O. Following the explosion-induced decompression, juvenile magma rapidly ascended in great volume to generate extremely destructive PDCs following subsequent dome collapses, particularly on 5 November. Additionally, 26 October tephras contain carbonate grains in the ash component and abundant CO<sub>2</sub> within the lava lapilli groundmass glass, which supports previous studies indicating assimilation of calc-silicate lithologies by the Merapi magma at depth in the plumbing system prior to the onset of 2010 activity. </p>