The Oruanui Eruption: Insights into the Generation and Dynamics of the World's Youngest Supereruption

<p>This work investigates the pre- and syn-eruptive magmatic processes that culminated in the world’s youngest supereruption – the ~25.4 ka, 530 km³ Oruanui eruption from Taupo volcano, New Zealand – from the perspective of crystals contained in single parcels of frozen magma (pumice). The eruption is unusual in its variety of magmatic compositions. About 98-99 % by mass of the juvenile material is high-SiO₂ rhyolite (HSR; >74 wt% SiO₂), with lesser volumes of tholeiitic and calc-alkaline mafic magmas (total 3-5 km³; basaltic andesite to andesite: 53-63 % SiO₂), low-silica rhyolite (LSR: 0.1-0.5 km³; <74 wt% SiO₂) and a ‘foreign’ biotite-bearing rhyolite from an adjacent magma source (0.03 km³; ~74 wt% SiO₂). Detailed textural and chemical data from amphibole, plagioclase, and orthopyroxene are placed within the context of an established time-stratigraphic, volcanological and petrographic framework, of unrivalled detail globally for an eruption of this age and magnitude. Other previously published information from zircon and quartz is also incorporated. This unique contextual information is used to constrain observations and inferences regarding the processes that moved the Oruanui magma from a largely uneruptible crystal-rich progenitor at depth (where an eruption was possible), to a highly eruptible melt-rich magma at shallow crustal levels (where eruption was inevitable). A thermally and compositionally stratified crystal mush body, with an upper SiO₂-saturated and quartz bearing cap at ~3.5 km depth and quartz-free roots extended down to at least ~10 km. This inference is made on three bases. 1) That the quartz cores contain trapped melt that is more evolved than the melt component of the immediately pre-eruptive magma body, indicating their growth within mush from a more evolved interstitial melt. 2) The majority of plagioclase, amphibole, and orthopyroxene cores, in contrast to quartz have compositions that indicate growth from less evolved melts than that encountered in the final melt-dominant magma body. 3) Barometric estimates from amphibole core compositions indicate derivation from a range of depths (~3.5 to 10 km). The spatial and temporal transitions from mush to melt-dominant magma body are recorded in the textural and compositional zonations within the crystal phases. Crystals from all levels of the zoned mush body were entrained during the melt extraction process resulting in a diversity of crystal compositions being brought together in the melt-dominant magma body. Textural disequilibrium features in the cores of orthopyroxene and plagioclase crystals reflect their temporary departure from stability during the accompanying significant decompression (recorded in the amphibole model pressures). Counterpart chemical signatures, reflecting this partial orthopyroxene and plagioclase dissolution, are recorded in the amphiboles which show no textural evidence for destabilisation during ascent. Crystal chemical and textural zonation in the rim growths of the plagioclase, orthopyroxene, and amphibole record further crystallisation in the accumulating melt-dominant magma body, and reflect cooling and compositional evolution of the body towards its final pre-eruptive conditions. The timing of growth of the melt dominant magma body is constrained by Fe-Mg diffusion modelling of key boundaries in orthopyroxene crystals. Accumulation of this body began only ~1600 years and peaked at 230 years prior to the eruption, as vast volumes of melt and entrained crystals were drained from the mush body and began to accumulate at shallower levels (~3.5 to 6.0 km depth). Within the thin, sill-like melt-dominant magma body, significant heat loss drove vigorous convection. Textural and chemical zonation patterns within the rim-zones of plagioclase, orthopyroxene and amphibole, inferred to have grown solely in the melt-dominant magma body, depict a secular cooling and melt evolution trends towards final uniform thermal (~770 °C) and compositional conditions inferred for the HSR magma. Despite the rapid accumulation of a vast volume of crystal-poor HSR magma at shallow crustal levels, the apparent gas-saturated nature of that magma, and vigorous convection within the melt-dominant magma body itself, the chronologies from HSR orthopyroxene imply that the magma underwent a period of stasis of about 60 years. The presence of 3-16 wt% of ‘foreign’ biotite-bearing juvenile pumices in the early Oruanui fall deposits (phases 1 and 2) show that coincident with the onset of the Oruanui eruption, magma was transported laterally in a dike from an adjacent independent magma system 10-15 km to the NNE to intersect the active Oruanui conduit. Consideration of the tectonic stress orientations associated with this lateral transport imply that an external tectonic influence through a major rifting event was a critical factor in the initiation of the Oruanui eruption. Only the presence of the foreign magma, and linkages to detailed field-based and geochemical constraints enables the tectonic influence to be identified. During the eruption itself, minor quantities of Oruanui LSR magma were erupted , and with a crystal cargo, reflecting derivation from deeper (mostly >6 km), hotter (~820 °C) sources in the crystal mush roots to the system. Comparisons of LSR crystal compositions with cores to many HSR crystals for plagioclase, orthopyroxene and amphibole imply that the LSR magma was derived from pockets in the mush zone ruptured during escalation of the eruption vigour during phase 3. The LSR and its crystals are inferred to be closely similar in their characteristics to the feedstock magma that generated the melt-dominant body and evolved through subsequent cooling and fractionation to form the HSR. In overall terms, the evidence from the crystal phases demonstrates that a super-sized rhyolite magma body can be physically created in a geologically very short period of time. The compositional textures and data for all the mineral phases, both previously published and newly presented in this work, yield a consistent story of extraordinarily rapid extraction of LSR melt and entrained crystals into a rapidly evolving and cooling HSR body. When coupled with field constraints these data establish a central role for extensional tectonics in regulating the pre-and syn-eruptive processes and their timings in the Oruanui system.</p>

The Oruanui Eruption: Insights into the Generation and Dynamics of the World's Youngest Supereruption

<p>This work investigates the pre- and syn-eruptive magmatic processes that culminated in the world’s youngest supereruption – the ~25.4 ka, 530 km³ Oruanui eruption from Taupo volcano, New Zealand – from the perspective of crystals contained in single parcels of frozen magma (pumice). The eruption is unusual in its variety of magmatic compositions. About 98-99 % by mass of the juvenile material is high-SiO₂ rhyolite (HSR; >74 wt% SiO₂), with lesser volumes of tholeiitic and calc-alkaline mafic magmas (total 3-5 km³; basaltic andesite to andesite: 53-63 % SiO₂), low-silica rhyolite (LSR: 0.1-0.5 km³; <74 wt% SiO₂) and a ‘foreign’ biotite-bearing rhyolite from an adjacent magma source (0.03 km³; ~74 wt% SiO₂). Detailed textural and chemical data from amphibole, plagioclase, and orthopyroxene are placed within the context of an established time-stratigraphic, volcanological and petrographic framework, of unrivalled detail globally for an eruption of this age and magnitude. Other previously published information from zircon and quartz is also incorporated. This unique contextual information is used to constrain observations and inferences regarding the processes that moved the Oruanui magma from a largely uneruptible crystal-rich progenitor at depth (where an eruption was possible), to a highly eruptible melt-rich magma at shallow crustal levels (where eruption was inevitable). A thermally and compositionally stratified crystal mush body, with an upper SiO₂-saturated and quartz bearing cap at ~3.5 km depth and quartz-free roots extended down to at least ~10 km. This inference is made on three bases. 1) That the quartz cores contain trapped melt that is more evolved than the melt component of the immediately pre-eruptive magma body, indicating their growth within mush from a more evolved interstitial melt. 2) The majority of plagioclase, amphibole, and orthopyroxene cores, in contrast to quartz have compositions that indicate growth from less evolved melts than that encountered in the final melt-dominant magma body. 3) Barometric estimates from amphibole core compositions indicate derivation from a range of depths (~3.5 to 10 km). The spatial and temporal transitions from mush to melt-dominant magma body are recorded in the textural and compositional zonations within the crystal phases. Crystals from all levels of the zoned mush body were entrained during the melt extraction process resulting in a diversity of crystal compositions being brought together in the melt-dominant magma body. Textural disequilibrium features in the cores of orthopyroxene and plagioclase crystals reflect their temporary departure from stability during the accompanying significant decompression (recorded in the amphibole model pressures). Counterpart chemical signatures, reflecting this partial orthopyroxene and plagioclase dissolution, are recorded in the amphiboles which show no textural evidence for destabilisation during ascent. Crystal chemical and textural zonation in the rim growths of the plagioclase, orthopyroxene, and amphibole record further crystallisation in the accumulating melt-dominant magma body, and reflect cooling and compositional evolution of the body towards its final pre-eruptive conditions. The timing of growth of the melt dominant magma body is constrained by Fe-Mg diffusion modelling of key boundaries in orthopyroxene crystals. Accumulation of this body began only ~1600 years and peaked at 230 years prior to the eruption, as vast volumes of melt and entrained crystals were drained from the mush body and began to accumulate at shallower levels (~3.5 to 6.0 km depth). Within the thin, sill-like melt-dominant magma body, significant heat loss drove vigorous convection. Textural and chemical zonation patterns within the rim-zones of plagioclase, orthopyroxene and amphibole, inferred to have grown solely in the melt-dominant magma body, depict a secular cooling and melt evolution trends towards final uniform thermal (~770 °C) and compositional conditions inferred for the HSR magma. Despite the rapid accumulation of a vast volume of crystal-poor HSR magma at shallow crustal levels, the apparent gas-saturated nature of that magma, and vigorous convection within the melt-dominant magma body itself, the chronologies from HSR orthopyroxene imply that the magma underwent a period of stasis of about 60 years. The presence of 3-16 wt% of ‘foreign’ biotite-bearing juvenile pumices in the early Oruanui fall deposits (phases 1 and 2) show that coincident with the onset of the Oruanui eruption, magma was transported laterally in a dike from an adjacent independent magma system 10-15 km to the NNE to intersect the active Oruanui conduit. Consideration of the tectonic stress orientations associated with this lateral transport imply that an external tectonic influence through a major rifting event was a critical factor in the initiation of the Oruanui eruption. Only the presence of the foreign magma, and linkages to detailed field-based and geochemical constraints enables the tectonic influence to be identified. During the eruption itself, minor quantities of Oruanui LSR magma were erupted , and with a crystal cargo, reflecting derivation from deeper (mostly >6 km), hotter (~820 °C) sources in the crystal mush roots to the system. Comparisons of LSR crystal compositions with cores to many HSR crystals for plagioclase, orthopyroxene and amphibole imply that the LSR magma was derived from pockets in the mush zone ruptured during escalation of the eruption vigour during phase 3. The LSR and its crystals are inferred to be closely similar in their characteristics to the feedstock magma that generated the melt-dominant body and evolved through subsequent cooling and fractionation to form the HSR. In overall terms, the evidence from the crystal phases demonstrates that a super-sized rhyolite magma body can be physically created in a geologically very short period of time. The compositional textures and data for all the mineral phases, both previously published and newly presented in this work, yield a consistent story of extraordinarily rapid extraction of LSR melt and entrained crystals into a rapidly evolving and cooling HSR body. When coupled with field constraints these data establish a central role for extensional tectonics in regulating the pre-and syn-eruptive processes and their timings in the Oruanui system.</p>

Dynamics of Large, Wet Volcanic Clouds : The 25.4 ka Oruanui Eruption of Taupo Volcano, New Zealand

<p>This work investigates the dynamics of large-scale, ‘wet’ volcanic eruption clouds generated by the interaction of silicic magma with external water. The primary case study draws from a detailed record of non-welded pyroclastic deposits from the ~25.4 ka Oruanui eruption of Taupo volcano, New Zealand, one of the largest phreatomagmatic eruptions documented worldwide. This research uses a three-pronged approach, integrating results from (i) field observations and textural data, (ii) mesoscale numerical modeling of volcanic plumes, and (iii) analogue laboratory experiments of volcanic ash aggregation. This interdisciplinary approach provides a new understanding of dynamic and microphysical interactions between collapsing and buoyant columns, and how this behavior controls the large-and small-scale nature of phreatoplinian eruption clouds. Stratigraphic field studies examine the styles of dispersal and emplacement of deposits from several phases of the Oruanui eruption (primarily phases 2, 3, 5, 6, 7 and 8). Detailed stratigraphic observations and laser diffraction particle size analysis of ash aggregates in these deposits clarify the evolution of aggregation mechanisms with time through the relevant eruption phase, and with distance from vent. Deposits of the wettest phase (3) show the key role of turbulent lofting induced by pyroclastic density currents in forming aggregates, particularly those with ultrafine ash rims (30-40 vol.% finer than 10 μm) which are uniquely formed in the ultrafine ash-dominated clouds above the currents. Drier deposits of phases 2 and 5, which also saw lower proportions of material emplaced by pyroclastic density currents, contain fewer aggregates that are related to low water contents in the medial to distal plume. Discovery and documentation of high concentrations of diatom flora in the Oruanui deposits indicates efficient fragmentation and incorporation of paleo-lake Taupo sediments during the eruption. This highlights the potential for incidental contamination of volcanic deposits with broader implications for correlation of distal tephras and possible contamination of paleoenvironmental records due to incorporation of diachronous populations of volcanically-dispersed diatoms. The impact of extensive surface water interaction on large-scale volcanic eruptions (>108 kg s-1 magma) is examined by employing the first 2-D large-eddy simulations of ‘wet’ volcanic plumes that incorporate the effects of microphysics. The cloud-resolving numerical model ATHAM was initialized with field-derived characteristics of the Oruanui case study. Surface water contents were varied from 0-40 wt.% for eruptions with equivalent magma eruption rates of c. 1.3 x108 and 1.1 x109 kg s-1. Results confirm that increased surface water has a pronounced impact on column stability, leading to unstable column behavior and hybrid clouds resulting from simultaneous ascent of material from stable columns and pyroclastic density currents (PDCs). Contrary to the suggestion of previous studies, however, abundant surface water does not systematically lower the spreading level or maximum height of volcanic clouds, owing to vigorous microphysics-assisted lofting of PDCs. Key processes influencing the aggregation of volcanic ash and hydrometeors (airborne water phases) are examined with a simple and reproducible experimental method employing vibratory pan agglomeration. Aggregation processes in the presence of hail and graupel, liquid water (<30 wt.%), and mixed water phases are investigated at temperatures from 18 to -20 °C. Observations from impregnated thin sections, SEM images and x-ray computed microtomography of these experimental aggregates closely match natural examples from phreatomagmatic phases of the ~25.4 ka Oruanui and Eyjafjallajökull (May 2010) eruptions. These experiments demonstrate that the formation of concentric, ultrafine rims comprising the outer layers of rim-type accretionary lapilli requires recycled exposure of moist, preexisting pellets to regions of volcanic clouds that are relatively dry and dominated by ultrafine (<31 μm) ash. This work presents the first experimentally-derived aggregation coefficients that account for changing liquid water contents and sub-zero temperatures, and indicates that dry conditions (<10 wt.% liquid) promote the strongly size-selective collection of sub-31 μm particles into aggregates (given by aggregation coefficients >1). These quantitative relationships may be used to predict the timescales and characteristics of aggregation, such as aggregate size spectra, densities and constituent particle size characteristics, when the initial size distribution and hydrometeor content of a volcanic cloud are known. The integration of numerical modeling, laboratory experimentation and field data lead to several key conclusions. (1) The importance of the microphysics of ash-water interactions in governing the eruption cloud structure, boosting the dispersal power of the cloud and controlling aggregate formation in response to differing water contents and eruption rates. (2) Recognition of the contrasting roles of differential aggregation versus cloud grain size in controlling the formation and nature of aggregate particles, notably those with characteristic ultrafine outer rims. (3) The importance of pyroclastic density currents as triggers for convection and aggregation processes in the eruption cloud.</p>

Dynamics of Large, Wet Volcanic Clouds : The 25.4 ka Oruanui Eruption of Taupo Volcano, New Zealand

<p>This work investigates the dynamics of large-scale, ‘wet’ volcanic eruption clouds generated by the interaction of silicic magma with external water. The primary case study draws from a detailed record of non-welded pyroclastic deposits from the ~25.4 ka Oruanui eruption of Taupo volcano, New Zealand, one of the largest phreatomagmatic eruptions documented worldwide. This research uses a three-pronged approach, integrating results from (i) field observations and textural data, (ii) mesoscale numerical modeling of volcanic plumes, and (iii) analogue laboratory experiments of volcanic ash aggregation. This interdisciplinary approach provides a new understanding of dynamic and microphysical interactions between collapsing and buoyant columns, and how this behavior controls the large-and small-scale nature of phreatoplinian eruption clouds. Stratigraphic field studies examine the styles of dispersal and emplacement of deposits from several phases of the Oruanui eruption (primarily phases 2, 3, 5, 6, 7 and 8). Detailed stratigraphic observations and laser diffraction particle size analysis of ash aggregates in these deposits clarify the evolution of aggregation mechanisms with time through the relevant eruption phase, and with distance from vent. Deposits of the wettest phase (3) show the key role of turbulent lofting induced by pyroclastic density currents in forming aggregates, particularly those with ultrafine ash rims (30-40 vol.% finer than 10 μm) which are uniquely formed in the ultrafine ash-dominated clouds above the currents. Drier deposits of phases 2 and 5, which also saw lower proportions of material emplaced by pyroclastic density currents, contain fewer aggregates that are related to low water contents in the medial to distal plume. Discovery and documentation of high concentrations of diatom flora in the Oruanui deposits indicates efficient fragmentation and incorporation of paleo-lake Taupo sediments during the eruption. This highlights the potential for incidental contamination of volcanic deposits with broader implications for correlation of distal tephras and possible contamination of paleoenvironmental records due to incorporation of diachronous populations of volcanically-dispersed diatoms. The impact of extensive surface water interaction on large-scale volcanic eruptions (>108 kg s-1 magma) is examined by employing the first 2-D large-eddy simulations of ‘wet’ volcanic plumes that incorporate the effects of microphysics. The cloud-resolving numerical model ATHAM was initialized with field-derived characteristics of the Oruanui case study. Surface water contents were varied from 0-40 wt.% for eruptions with equivalent magma eruption rates of c. 1.3 x108 and 1.1 x109 kg s-1. Results confirm that increased surface water has a pronounced impact on column stability, leading to unstable column behavior and hybrid clouds resulting from simultaneous ascent of material from stable columns and pyroclastic density currents (PDCs). Contrary to the suggestion of previous studies, however, abundant surface water does not systematically lower the spreading level or maximum height of volcanic clouds, owing to vigorous microphysics-assisted lofting of PDCs. Key processes influencing the aggregation of volcanic ash and hydrometeors (airborne water phases) are examined with a simple and reproducible experimental method employing vibratory pan agglomeration. Aggregation processes in the presence of hail and graupel, liquid water (<30 wt.%), and mixed water phases are investigated at temperatures from 18 to -20 °C. Observations from impregnated thin sections, SEM images and x-ray computed microtomography of these experimental aggregates closely match natural examples from phreatomagmatic phases of the ~25.4 ka Oruanui and Eyjafjallajökull (May 2010) eruptions. These experiments demonstrate that the formation of concentric, ultrafine rims comprising the outer layers of rim-type accretionary lapilli requires recycled exposure of moist, preexisting pellets to regions of volcanic clouds that are relatively dry and dominated by ultrafine (<31 μm) ash. This work presents the first experimentally-derived aggregation coefficients that account for changing liquid water contents and sub-zero temperatures, and indicates that dry conditions (<10 wt.% liquid) promote the strongly size-selective collection of sub-31 μm particles into aggregates (given by aggregation coefficients >1). These quantitative relationships may be used to predict the timescales and characteristics of aggregation, such as aggregate size spectra, densities and constituent particle size characteristics, when the initial size distribution and hydrometeor content of a volcanic cloud are known. The integration of numerical modeling, laboratory experimentation and field data lead to several key conclusions. (1) The importance of the microphysics of ash-water interactions in governing the eruption cloud structure, boosting the dispersal power of the cloud and controlling aggregate formation in response to differing water contents and eruption rates. (2) Recognition of the contrasting roles of differential aggregation versus cloud grain size in controlling the formation and nature of aggregate particles, notably those with characteristic ultrafine outer rims. (3) The importance of pyroclastic density currents as triggers for convection and aggregation processes in the eruption cloud.</p>