![]() They are estimated to be responsible for 28.1% of the casualties of volcanic eruptions documented between 1500 AD and 2017 ( Brown et al., 2017) with the last disaster to date being the 3 June 2018 Volcàn Fuego (Guatemala) PDCs that killed between 332 and 2,900 people ( Naismith et al., 2020). Pyroclastic Density Currents (PDCs) are the most hazardous direct consequence of explosive volcanism. Conversely, the high viscosity determined by pre-eruptive storage conditions, including temperature and volatile-content, are key in controlling the formation of more evolved magmas PDCs'. ![]() We conclude that mafic ignimbrites can form due to a combination of peculiar storage conditions that lead to strongly non-linear feedback processes in the conduit, particularly intense microlite crystallization on very short timescales coupled with intense decompression rates. ![]() The fragmentation processes for these latter two deposits were slightly different however, with the Marapi PDC fragmentation being mostly driven by vesicle overpressure, while a combination of bubble overpressure and intense strain-rate were the cause of fragmentation for the Gunungkawi ignimbrite. Conversely, for the Marapi PDC and Gunungkawi ignimbrite, similar decompression rates coupled with larger initial bulk viscosities of 10 5–10 6 Pa s were sufficient to fragment the magma brittlely. Combined with the important decompression rate, this intense crystallization led to a magma bulk viscosity jump from 10 3 up to >10 7 Pa s and allowed it to fragment brittlely. Based on the bulk and groundmass compositions, the storage temperature (1,050–1,100☌), pressure (50–100 MPa) and phenocryst content (1.0–2.5 vol%), we conclude that the basaltic-andesitic Curacautín magma was at sub-liquidus conditions, which allowed fast and widespread disequilibrium matrix crystallization (0–80 vol%) during ascent to the surface. With a combination of EPMA and SIMS analyses we characterise pre-eruptive storage conditions. From vesicle number densities we estimate fragmentation decompression rates in the range of 0.4–1.6 MPa/s for the three deposits. We use SEM imagery and X-ray Microtomography on pyroclasts from these deposits to characterize phenocryst, microlite and vesicle textures. We sampled pumices from ignimbrites and PDCs with a compositional range from basaltic-andesite (Curacautín ignimbrite, Volcàn Llaima, Chile), andesite (Marapi, Indonesia) to trachyte (Gunungkawi ignimbrite, Batur, Indonesia). So, what processes lead a mafic magma to fragment violently enough to generate extensive ignimbrites? However, there are increasing discoveries of large mafic Plinian eruptions, sometimes generating ignimbrites, suggesting that this phenomenon might not be so uncommon. Unlike their silicic counterparts, mafic eruptions are known for being on the low-end of the explosivity spectrum with eruption styles commonly ranging from effusive to Hawaiian fire fountaining.
0 Comments
Leave a Reply. |
AuthorWrite something about yourself. No need to be fancy, just an overview. ArchivesCategories |