Downward-propagating eruption following vent unloading implies no direct magmatic trigger for the 2018 lateral collapse of Anak Krakatau
K. S. CutlerSebastian WattMike CassidyAmber Madden-NadeauSamantha EngwellMirzam AbdurrachmanMuhammad Edo Marshal NurshalDavid R. TappinSteven N CareyAlessandro NovellinoCatherine HayerJames E. HuntSimon DayStéphan T. GrilliIdham Andri KurniawanNugraha Kartadinata
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The lateral collapse of Anak Krakatau volcano, Indonesia, in December 2018 highlighted the potentially devastating impacts of volcanic edifice instability. Nonetheless, the trigger for the Anak Krakatau collapse remains obscure. The volcano had been erupting for the previous six months, and although failure was followed by intense explosive activity, it is the period immediately prior to collapse that is potentially key in providing identifiable, pre-collapse warning signals. Here, we integrate physical, microtextural and geochemical characterisation of tephra deposits spanning the collapse period. We demonstrate that the first post-collapse eruptive phase (erupting juvenile clasts with a low microlite areal number density and relatively large microlites, reflecting a crystal-growth dominated regime) is best explained by instantaneous unloading of a relatively stagnant upper conduit. This was followed by the second post-collapse phase, on a timescale of hours, which tapped successively hotter and deeper magma batches, reflected in increasing plagioclase anorthite content and more mafic glass compositions, alongside higher calculated ascent velocities and decompression rates. The characteristics of the post-collapse products imply downward propagating destabilisation of the magma storage system as a response to collapse, rather than pre- collapse magma ascent triggering failure. Importantly, this suggests that the collapse was a consequence of longer-term processes linked to edifice growth and instability, and that no indicative changes in the magmatic system could have signalled the potential for incipient failure. Therefore, monitoring efforts may need to focus on integrating short- and long-term edifice growth and deformation patterns to identify increased susceptibility to lateral collapse. The post-collapse eruptive pattern also suggests a magma pressurisation regime that is highly sensitive to surface-driven perturbations, which led to elevated magma fluxes after the collapse and rapid edifice regrowth. Not only does rapid regrowth potentially obscure evidence of past collapses, but it also emphasises the finely balanced relationship between edifice loading and crustal magma storage.Mercury
Disequilibrium
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Most examples of magma mingling described in the literature result from the intrusion of hot mafic magma into colder felsic magma. This paper describes a small body (100 X 30 m) in the Ladakhi Himalayas, northwest India, where mingling occurred when granite magma intruded and disrupted a pool of partially molten quartz-diorite that formed fine-grained pillow-like enclaves. The mingled body is surrounded by coarse-grained quartz-diorite that was effectively solid during granite emplacement and, within a short distance from the body, was brecciated by the granite. Because the enclaves are virtually in situ, their shapes retain details related to their disruption and to the relative motion between the two magmas. Whereas this seems to be a rare description of mingling and formation of mafic enclaves by intrusion of felsic into mafic magma, this paper argues that, because many batholiths evolve from mafic to felsic, this may be more common in nature than generally realized and not simply an extraordinary feature of that particular locality in the Himalayas.
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Quartz monzonite
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Explosive volcanic eruptions generate plumes of hot gas and quenched molten rock that has been fragmented by the expansion of gas as the magma exits the vent. These fragments are called pyroclasts . The tephra layers are comprised of volcanic glass, crystals, and lithic material. Given that tephra is dispersed over wide areas and forms a geologically instantaneous layer, these tephra layers can be particularly useful for chronology – providing a relative chronology between sites and age if the eruption has been dated using radiometric methods. Correlating volcanic ash layers between sites and to specific eruption deposits preserved at their source volcanoes can be achieved using the composition of the volcanic glass shards. The major and trace element glass compositions remain the same for a specific eruption deposit irrespective of the distance from the vent, and they constitute the chemical fingerprint of the tephra. Volcanic deposits can be dated using many commonly employed radiometric dating methods.
Tephrochronology
Chronology
Volcanic ash
Peléan eruption
Volcanic glass
Radiometric dating
Volcanic hazards
Phreatomagmatic eruption
Vulcanian eruption
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Regolith
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Anorthite
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Phreatomagmatic eruption
Scoria
Isopach map
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Peléan eruption
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Peléan eruption
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Magma chamber
Igneous differentiation
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Plagioclase phenocrysts from mafic magmatic enclaves and plagioclase crystals from host granitoids of some plutons of the central part of the Sierra Nevada Batholith are complexly zoned and commonly divided into three neatly distinct parts: an oscillatory, locally patchy zoned, andesine or more calcic core, a ring with dusty calcic plagioclase, and a normally zoned rim of sodic plagioclase. Although aspect of the calcic rings and width and zoning of the rims may slightly vary from the enclaves to the hosts, cores of both phenocrysts and large plagioclase crystals show similar zoning and composition. The andesine or more calcic cores are interpreted to have crystallized in the felsic or the mafic magma, respectively, and been incorporated into the coeval magma when the two magmas mixed. Introduction of the xenocrystic cores into a magma where they were not in equilibrium resulted in partial dissolution, development of abundant patchy zoning, and coating with dusty calcic plagioclase. In both granitoids and mafic magmatic enclaves, composition and zoning contrast between cores and rims of the plagioclase crystals reflect drastic changes in conditions of crystallization before and after the mechanical mixing event. Mixing of two magmas with contrasted compositions is suggested to be the major mechanism for generating complexly zoned plagioclase xenocrysts in granitoids and mafic magmatic enclaves. This hypothesis is consistent with many recent models in which mixing of two contrasting components is proposed to play a fundamental part in the generation of calc‐alkaline granitoids and mafic magmatic enclaves of the Sierra Nevada Batholith. Plagioclase xenocrysts may also provide information on the timing of the different mixing processes and on the magmatic evolution of the plutons.
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Igneous differentiation
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