Varying processes, similar results: How composition influences fragmentation and subsequent feeding of large pyroclastic density currents

Unlike their silicic counterparts, mafic eruptions are known for being on the low-end of explosivity spectrum with eruption styles commonly ranging from effusive to Hawaiian fire fountaining. However, there increasing discoveries large Plinian eruptions, sometimes generating ignimbrites, suggesting that this phenomenon might not be so uncommon. So, what processes lead a magma fragment violently enough generate extensive ignimbrites? We sampled pumices ignimbrites and PDCs compositional range basaltic-andesite (Curacautín ignimbrite, Volcàn Llaima, Chile), andesite (Marapi, Indonesia) trachyte (Gunungkawi Batur, Indonesia). use SEM imagery X-ray Microtomography pyroclasts these deposits characterize phenocryst, microlite vesicle textures. From number densities we estimate fragmentation decompression rates in 0.4–1.6 MPa/s three deposits. With combination EPMA SIMS analyses characterise pre-eruptive storage conditions. Based bulk groundmass compositions, temperature (1,050–1,100°C), pressure (50–100 MPa) phenocryst content (1.0–2.5 vol%), conclude basaltic-andesitic Curacautín was at sub-liquidus conditions, which allowed fast widespread disequilibrium matrix crystallization (0–80 vol%) during ascent surface. Combined important rate, intense led viscosity jump 10 3 up >10 7 Pa s it brittlely. Conversely, Marapi PDC Gunungkawi similar coupled larger initial viscosities 5 –10 6 were sufficient The latter two slightly different however, mostly driven by overpressure, while bubble overpressure strain-rate cause ignimbrite. can form due peculiar conditions strongly non-linear feedback conduit, particularly very short timescales rates. high determined including volatile-content, key controlling formation more evolved magmas PDCs'.