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: Silicic volcanic systems provide timed snapshots at the Earth's surface of the magmatic processes that also build complementary plutons in the crust. Links between these two realms are considered here using three Quaternary (〈2.6 Ma) examples from New Zealand and the USA. In these systems, magmatic processes can be timed and the changes in magmatic conditions can be followed through the sequence of quenched volcanic eruption products. Before an eruption, magma accumulation processes can occur on timescales as short as decades, and whole magma systems can be rebuilt in millennia. Silicic volcanic processes, in general, act on timescales that are too rapid to be effectively measured in the exposed plutonic record.

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 links between large-scale silicic volcanism and plutonism offer insights into the dynamics of crustal magmatic systems and growth of continental crust. In Hong Kong, voluminous silicic ignimbrites and linked plutons record a ~26 Myr period of magmatism from ~164 to 138 Ma. We present data from these linked volcanic-plutonic assemblages at the Lantau and High Island caldera complexes, with an emphasis on the ~143–138 Ma activity from the latter. To track the evolution of these magmatic systems, U-Pb dating and trace element analyses using secondary-ion mass spectrometry (SIMS) were carried out on zircons from 21 samples from both volcanic and plutonic samples. The SIMS age data sets divide into two groups across volcanic and plutonic origins: (1) seven samples with unimodal age spectra [five of which have the same mean value as the published Isotope Dilution Thermal Ionization Mass Spectrometry (ID-TIMS) age from the same sample]; and (2) 14 samples yielding multiple age components. Age patterns from both groups suggest that the previously separated ~143 Ma Repulse Bay (RBVG) and ~141–140 Ma Kau Sai Chau volcanic groups (KSCVG) instead represent activities over a single ~5 Myr period. Direct linkages previously proposed between some volcanic and plutonic units for this period (e.g., High Island Tuff, Kowloon Granite) are no longer supported, and magmatism represented by exposed plutons continued until 137.8 ± 0.8 Ma (Mount Butler Granite). Under CL imagery, a wide range of zircon textures identified in both volcanic and plutonic samples is indicative of complex processes, some of which are identified through trace element data coupled with textural characteristics. Overall, intra-grain (cores vs. rims; sector-zonation) and intra-sample variations in trace element abundances and ratios are larger than those between samples. Zircon chemistries in both volcanic and plutonic samples fall into two groups during the ~5 Myr history of the High Island caldera magmatic system. One group (RBVG and "cold" granites) includes inherited grains back to 164 Ma and wider ranges in Hf, Y, total trivalent elements, Th and U concentrations and Th/U, Yb/Gd, and U/Yb ratios than the other (KSCVG and "hot" granites). Two possible evolutionary models of the High Island caldera magmatic system are: (1) the system randomly tapped a single crustal domain that fluctuated in temperature as a result of varying interactions of hotter melts, or (2) the volcanic and plutonic records reflect the interplay of two crustal domains with contrasting "low-" and "high-temperature" characteristics. In Hong Kong, some plutonic bodies were comagmatic with large-scale volcanism, while others were emplaced at shallow crustal levels independently of volcanism, matching the current two end-member views of the volcanic-plutonic relationship.

Description: The geological record of volcanic eruptions suggests that scientists are some way from being able to forecast eruptions at many of the world's volcanoes. There are three reasons for this. First, continuing geological discoveries show that our knowledge is incomplete. Second, knowledge is limited about why, how, and when volcanic unrest turns into eruptions, and over what timescales. Third, there are imbalances between the studies of past eruptions, and the geophysical techniques and observations on modern events, versus the information needed or demanded by society. Scientists do not yet know whether there are other, presently unknown, factors that are important in controlling eruptions, or if there is an inherent unknowability about some volcanic systems.

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.