ALBERT

Description: A major challenge in volcanology is determining the factors that control the frequency and magnitude of eruptions at hazardous caldera volcanoes. Understanding the critical sequence of events that may lead to future eruptions is vital for volcanic monitoring and risk assessment. Here we use magma chemistry and mineral diffusion modeling to interpret the magmatic processes and time scales involved in the youngest three eruptions (2.15–1.7 ka) from Taupo volcano (New Zealand), which peaked with the voluminous A.D. 232 eruption. Of the rhyolites erupted since ca. 12 ka, the 〈2.15 ka magmas have the lowest whole-rock SiO 2 content and reversely zoned crystals, yet with high-SiO 2 melt inclusions. Mineral zonations and compositional shifts reflect a 30–40 °C temperature increase over the immediately preceding (〉2.75 ka) rhyolites that were tapped from the same magma reservoir. Orthopyroxene Fe-Mg diffusion time scales indicate that the onset of rapid heating and priming of the host silicic mush occurred 〈120 yr prior to the 〈2.15 ka eruptions, with subsequent melt accumulation occurring in only decades. Elevated mafic magma supply to the silicic mush pile, rapid melt accumulation, and high differential tectonic stress built up and culminated in the ~105 km 3 A.D. 232 eruption, one of the largest and most violent Holocene eruptions globally. These youngest eruptions demonstrate how Taupo’s magmatic system can rapidly change behavior to generate large eruptible melt bodies on time scales of direct relevance to humans and monitoring initiatives.

Description: Explosive eruptions create a transient bridge between the solid Earth and atmosphere, frequently injecting volcanic aerosols to stratospheric levels. Although known to disrupt terrestrial and aquatic ecosystems at the surface, the role of explosive volcanism in airborne transport of microscopic organisms has never been characterized. This study documents abundant freshwater diatoms (microskeletons of siliceous algae) in widespread tephra from the 25.4 ka Oruanui eruption of Taupo volcano, New Zealand. By matching the tephra-hosted species assemblages to those in coerupted clasts of lacustrine sediment, we demonstrate that ~0.6 km 3 of diatom remains were incorporated during magma-water interaction with a lake system overlying the vents, and were dispersed along with fine ash particles hundreds of kilometers downwind. One of the dominant species, Cyclostephanos novaezeelandiae , is endemic to New Zealand’s North Island and serves as a unique identifier of the eruptive source region. Our results suggest that dispersal of microorganisms may be an overlooked feature of a number of ancient and modern eruptions, and indicate a novel pathway of microbe transport in airborne volcanic plumes. We conclude that the biogenic signatures contained within distal tephras have potential application in the characterization of eruption dynamics, location, and environmental settings of volcanic source areas.

Description: We here explore the temporal and spatial relationships between the contrasting sources for two eruptive episodes that collectively represent the Whakamaru Group, the largest ignimbrite-forming sequence in the ~2 m.y. history of the Taupo Volcanic Zone in New Zealand. At 349 ± 4 ka (weighted mean at 2), the 〉1500 km 3 widespread Whakamaru Group ignimbrites and ~700 km 3 Rangitawa Tephra fallout were erupted in association with collapse of the 40 km long by 25 km wide rectilinear Whakamaru caldera. New 40 Ar/ 39 Ar age data presented here show that the co-magmatic 〉110 km 3 Paeroa Subgroup ignimbrites, previously included as part of the Whakamaru Group, are slightly younger and were erupted at 339 ± 5 ka (weighted mean at 2). New field evidence also presented here demonstrates that the Paeroa Subgroup ignimbrites came from a source geographically separated from vents for the widespread Whakamaru Group ignimbrites. The presence of co-ignimbrite lag breccias, sizes of vent-derived lithic clasts, thicknesses of exposed and subsurface deposits, and morphologies of deposits imply that eruptions of the Paeroa Subgroup occurred from a linear source (the Paeroa linear vent zone), coinciding with the present-day northeast-striking Paeroa fault, and outside (northeast) of the earlier Whakamaru caldera collapse area. No separate caldera has been recognized, although three nearby areas may have undergone eruption-related subsidence. Residual magma from the Whakamaru or adjacent Kapenga caldera areas may have migrated toward the Paeroa linear vent zone during eruptive episodes, resulting in subsidence in either, or both, of these areas. Shallow plutons are also inferred to lie beneath near source fault blocks (Paeroa and Te Weta) on each side of the fault, and eruption-related subsidence may have been expressed as movement across the Paeroa fault and localized subsidence in the southern Paeroa fault block. Subsequent secular, rift-related displacement along the Paeroa fault has obscured the Paeroa linear vent zone.

Description: A major challenge in volcanology is determining the factors that control the frequency and magnitude of eruptions at hazardous caldera volcanoes. Understanding the critical sequence of events that may lead to future eruptions is vital for volcanic monitoring and risk assessment. Here we use magma chemistry and mineral diffusion modeling to interpret the magmatic processes and time scales involved in the youngest three eruptions (2.15–1.7 ka) from Taupo volcano (New Zealand), which peaked with the voluminous A.D. 232 eruption. Of the rhyolites erupted since ca. 12 ka, the 〈2.15 ka magmas have the lowest whole-rock SiO 2 content and reversely zoned crystals, yet with high-SiO 2 melt inclusions. Mineral zonations and compositional shifts reflect a 30–40 °C temperature increase over the immediately preceding (〉2.75 ka) rhyolites that were tapped from the same magma reservoir. Orthopyroxene Fe-Mg diffusion time scales indicate that the onset of rapid heating and priming of the host silicic mush occurred 〈120 yr prior to the 〈2.15 ka eruptions, with subsequent melt accumulation occurring in only decades. Elevated mafic magma supply to the silicic mush pile, rapid melt accumulation, and high differential tectonic stress built up and culminated in the ~105 km 3 A.D. 232 eruption, one of the largest and most violent Holocene eruptions globally. These youngest eruptions demonstrate how Taupo’s magmatic system can rapidly change behavior to generate large eruptible melt bodies on time scales of direct relevance to humans and monitoring initiatives.

Description: The spatial and temporal distributions of volcaniclastic deposits in arc-related basins reflect a complex interplay between tectonic, volcanic, and magmatic processes that is typically difficult to unravel. We take advantage of comprehensive geothermal drill hole stratigraphic records within the Taupo-Reporoa Basin (TRB), and integrate them with new 40 Ar/ 39 Ar age determinations, existing age data, and new mapping to develop a four-dimensional model of basin evolution in the central Taupo Volcanic Zone (TVZ), New Zealand. Here, exceptional rhyolitic productivity and high rates of extensional tectonism have resulted in the formation of at least eight calderas and two subparallel, northeast-trending rift basins, each of which is currently subsiding at 3 to 4 mm/yr: the Taupo fault belt (TFB) to the northwest and the TRB to the southeast (the main subject of this paper). The basins are separated in the northeast by a high-standing, fault-controlled range termed the Paeroa block, which is the focus of mapping for this study, and in the southwest by an along strike alignment of smaller scale faults and an associated region of lower relief. Stratigraphic age constraints within the Paeroa block indicate that a single basin (~120 km long by 60 km wide) existed within the central TVZ until 339 ± 5 ka (Paeroa Subgroup eruption age), and it is inferred to have drained to the west through a narrow and deep constriction, the present-day Ongaroto Gorge. Stratigraphic evidence and field relationships imply that development of the Paeroa block occurred within 58 ± 26 k.y. of Paeroa Subgroup emplacement, but in two stages. The northern Paeroa block underwent uplift and associated tilting first, followed by the southern Paeroa block. Elevations (〉500 m above sea level) of lacustrine sediments within the southern Paeroa block are consistent with elevations of rhyolite lavas in the Ongaroto Gorge, the outlet to the paleolake in which these sediments were deposited, and indicate that the Paeroa block has remained relatively stable since development. East of the Paeroa block, stratigraphic relationships show that movement along the Kaingaroa Fault zone, the eastern boundary of the central TVZ, is associated with volcano-tectonic events. Stratigraphic and age data are consistent with rapid formation of the paired TRB and TFB at 339 ± 5 ka, and indicate that gradual, secular rifting is punctuated by volcano-tectonic episodes from time to time. Both processes influence basin evolution.