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ABSTRACTRuapehu (150 km3cone, 150 km3ring-plain) and Tongariro (90 km3cone, 60 km3ring-plain) are iconic stratovolcanoes, formed since ∼230 and ∼350 ka, respectively, in the southern Taupo Volcanic Zone and Taupo Rift. These volcanoes rest on Mesozoic metasedimentary basement with local intervening Miocene sediments. Both volcanoes have complex growth histories, closely linked to the presence or absence of glacial ice that controlled the distribution and preservation of lavas. Ruapehu cone-building vents are focused into a short NNE-separated pair, whereas Tongariro vents are more widely distributed along that trend, the differences reflecting local rifting rates and faulting intensities. Both volcanoes have erupted basaltic andesite to dacite (53–66 wt.% silica), but mostly plagioclase-two pyroxene andesites from storage zones at 5–10 km depth. Erupted compositions contain evidence for magma mixing and interaction with basement rocks. Each volcano has an independent magmatic system and a growth history related to long-term (>104years) cycles of mantle-derived magma supply, unrelated to glacial/interglacial cycles. Historic eruptions at both volcanoes are compositionally diverse, reflecting small, dispersed magma sources. Both volcanoes often show signs of volcanic unrest and have erupted with a wide range of styles and associated hazards, most recently in 2007 (Ruapehu) and 2012 (Tongariro).

ABSTRACTThe cataclysmic basaltic eruption of Mt. Tarawera in 1886 represents a significant cultural and scientific event for New Zealand. This review utilises published and new observations, to reinterpret eruptive parameters encompassing the entirety of the eruption. The ∼17 km eruptive fissure, active for 4+ hours, extends across Mt. Tarawera to the hydrothermally active Waimangu region. Correlating published observations of bed thickness, componentry and microtextures from Mt. Tarawera to new bed descriptions and granulometry for the Rotomahana-Waimangu rift segment allows for a re-assessment of eruption variations along the length of the fissure. Variably thick pyroclastic fall sequences at Mt. Tarawera contrast with the pyroclastic surges and an eruption plume that together deposited the ‘Rotomahana Mud’ erupted along the Rotomahana-Waimangu segment. Providing insight into pre-eruptive conditions, new mineral chemistry from Mt. Tarawera provides the first constraints on crystallisation pressures (<2 kbar), temperatures (<1100°C), and magmatic water content (<2.8 wt%). Recalculated volumes indicate a bulk eruptive volume of 1.1–1.3 km3, and a juvenile basalt volume of up to 0.67 km3, which then lead to calculated discharge rates of 3.7 × 107–7.8 × 107kg s−1for the northern Mt. Tarawera segment of the fissure and 1.4–5.7 × 106kg s−1for the Rotomahana segment.

ABSTRACTThe intraplate rocks of the Dunedin Volcanic Group (DVG) in New Zealand’s South Island erupted in two discrete areas between 25 and 21 Ma before becoming distributed over > 7,800 km2until ∼9 Ma. Although most eruptive centres were of small volume and mainly vented alkaline basanite, the largest centre–the 16–11 Ma composite Dunedin Volcano–discharged basanite and basalt through to trachyte and phonolite. DVG components were mainly derived from mantle sources with 87Sr/86Sr = ∼0.7029, 143Nd/144Nd = ∼0.5129, 206Pb/204Pb = ∼20.0, 207Pb/204Pb = ∼15.65, 208Pb/204Pb = 39.5 and εHf = +3.5 to + 10.1 that extended to anomalously light δ26Mg (−0.47). Exceptions are some potassic basalts in NW of the field with elevated 207Pb/204Pb and more radiogenic Sr. The DVG Sr-Nd-Pb isotopes mostly overlap with metasomatised anhydrous mantle peridotite xenoliths but have less radiogenic Hf, meaning that equivalent anhydrous mantle rock-types cannot be the sole magma sources. Although there is debate regarding whether DVG was derived from the lithospheric or asthenospheric mantle, intermittent melting of a middle lithospheric mantle metasomatised by hydrous asthenosphere-derived melts could account for: (1) the widely distributed magmatism for ∼16 Myr during which time Otago lithosphere shifted NW ∼ 870 km over the asthenosphere; (2) the small chemical range of the least evolved magmas; (3) the Sr-Nd-Pb-Hf isotopic range; and (4) an absence of lower lithosphere mantle xenoliths. This process could account for other occurrences of isotopically restricted Zealandia alkaline intraplate volcanism.

ABSTRACTVolcaniclastic deposits, pillow lavas, dikes and sills of the intraplate Waiareka-Deborah Volcanic Field in North Otago were emplaced into and onto the Zealandia Eocene-Early Oligocene continental shelf. The on-land extent is ∼890 km2but offshore volcanic rocks occurring over an additional ∼3500 km2may be related. Examination of the on-land volcaniclastic deposits indicates volcanism was dominated by short-lived clustered surtseyan-style eruptions. Pyroclast glass and sill and lava bulk rock chemistries show that the magmas were mainly sub-alkaline basalt to basaltic andesite. Minor alkaline centres are best represented by basanitic-melanephelinitic volcaniclastic deposits at Kakanui, which also contain an array of megacrysts plus mantle and crustal xenoliths. The sub-alkaline and alkaline source reservoirs had similar Sr and Nd isotope ratios (87Sr/86Sr34 Ma = 0.70346 ± 40, 143Nd/144Nd34 Ma = 0.51282 ± 4) but the sub-alkaline rocks tend to be slightly less radiogenic in 206Pb/204Pb or 208Pb/204Pb versus 207Pb/204Pb space. The dominant sub-alkaline nature and the isotopic compositions distinguish this volcanic field from the nearby Dunedin Volcanic Group and Alpine Dike Swarm. As the Waiareka-Deborah isotopic compositions are poorly represented in the Otago mantle lithosphere, the magmas may have been derived from the asthenosphere. There were multiple modes of Cenozoic intraplate volcanism in Otago.

ABSTRACTThe Karitane Suite of East Otago marks the northern extent of evolved magmas erupted contemporaneously on the periphery of the Miocene Dunedin Volcano. Compositions range from relatively primitive basalts to highly evolved trachyandesites and phonolites with strong geochemical affinity to Dunedin Volcano magmas and their fractionation trends. Karitane trachyandesites are abundant with fluidal, hornblende-rich mafic inclusions interpreted as remnant features of magma mingling between basanitic dikes and evolved magma reservoirs dispersed in the crust. Mineral data reveals discrete populations of Mg-rich diopside within evolved members that suggest they were inherited during mixing with basanite magmas prior to eruption. It is proposed that the overall distribution and concentration of evolved magmas within the Dunedin volcanic system is source-controlled and a function of mantle productivity. The Karitane Suite is therefore the product of low-volume, dispersed peripheral magma reservoirs recharged and mixed with rapidly ascending basanitic dikes of the surrounding Waipiata volcanic field.

Hazard assessments in monogenetic volcanic fields require estimates of the runout of pyroclastic surges that result from phreatomagmatic explosive activity. Previous assessments used runout distances of 1–4 km, with large cases up to 6 km. Surge deposits at Ubehebe Crater (∼2100 y.b.p., Death Valley, California) have been traced ∼9 km from the crater center, and likely originally extended 1–3 km farther. There is no evidence that the Ubehebe Crater activity was unusually energetic; rather, its distal deposits are better preserved than those at most maar volcanoes because of its young age and the arid environment. Numerical simulations illustrate how low temperatures facilitate long runout of phreatomagmatic surges due to reduced expansion of entrained air compared to hot surges, allowing cool surges to retain higher densities than ambient air. We suggest that hazard assessments for volcanic fields with phreatomagmatic, maar‐forming eruptions should consider runout distances in the range of 10–15 km. Assessments of volcanic hazards in areas prone to small‐volume basaltic volcanoes includes consideration of the lateral extent of devastating pyroclastic surges from eruptions that are caused by magma‐water explosions. Previous assessments considered surge distances up to 6 km, but the deposits from Ubehebe Crater (Death Valley, California, USA) have been traced to 9 km and likely originally extended farther. This was not an unusual eruption, but its deposits are exceptionally preserved due to the arid environment. Pyroclastic surges from magma‐water explosions are likely to be cooler than other volcanic flows, and this facilitates long travel distances and hence far‐reaching potential impacts if the area is inhabited. We suggest that future assessments should consider pyroclastic surge distances of 10–15 km from the vent. Hazard assessments in monogenetic volcanic fields have considered pyroclastic surge runout up to 6 km based upon preserved deposits of previous eruptionsSurge deposits from Ubehebe Crater extend to 9 km; this may not be unusual and such surges may have long runout facilitated by their low temperaturesWe suggest that future hazard assessments in monogenetic volcanic fields consider pyroclastic surge runout up to 10–15 km Hazard assessments in monogenetic volcanic fields have considered pyroclastic surge runout up to 6 km based upon preserved deposits of previous eruptions Surge deposits from Ubehebe Crater extend to 9 km; this may not be unusual and such surges may have long runout facilitated by their low temperatures We suggest that future hazard assessments in monogenetic volcanic fields consider pyroclastic surge runout up to 10–15 km

Volcanic tsunamis can expand the radius of hazards posed by a volcano well beyond the reach of the eruption itself; however, their source mechanisms are poorly understood. The tsunamigenic potential of pyroclastic density currents was studied experimentally by releasing a fluidized column of glass beads from a reservoir; the beads then ran down an inclined ramp into a water‐filled flume and generated waves. The effects of the particle size distribution on the generated waves were analyzed by comparing the waves generated by flows comprising different proportions of particles of diameters 63–90 μm and 600–850 μm. The flows comprising higher proportions of large particles travel more slowly down the ramp; however, all the flows impact the water at velocities greater than the shallow water wave speed, gh$\sqrt{gh}$, where his the still‐water depth. The entrance of the fluidized flow into the water generates a solitary‐like leading wave followed by a smaller trough and trailing waves. Upon impact, the flow separates into a part advected with the wave crest and a part which turbulently mixes with water and propagates along the bottom of the flume, forming an underwater gravity current. A higher proportion of large particles makes the flows more porous, allowing the water to penetrate through the flows more easily, slightly decreasing the efficiency of the energy transfer. While this affects the celerity of the waves, the results show that, over the studied range of particle size distributions, all the flows can generate waves of similar amplitudes regardless of the particle size distribution. Volcanic eruptions can produce mixtures of volcanic particles and hot air that travel horizontally driven by gravity, which are called pyroclastic density currents (PDCs). PDCs can travel large distances at high speeds. When PDCs reach the sea, they displace the water and can generate large waves, which are called tsunamis. This paper explores PDC generated tsunamis by performing experiments of the entrance of a granular flow into water. The granular material, made up of various proportions of glass beads of diameters 63–90 μm and 600–850 μm, was released from a reservoir. The beads then ran down the porous ramp into a water‐filled flume and generated waves. A compressed air supply was introduced below the porous ramp to fluidize the material. The fluidization causes granular mixtures to behave like a fluid. Flows with higher proportions of large particles travel more slowly down the ramp, but all the flows are fast enough to generate waves. The flows generate a large wave followed by smaller waves. Higher proportions of large particles result in slightly slower waves because water can penetrate through the spaces between the particles more easily, dissipating the energy. However, our results show that all the flows analyzed here can generate waves of similar amplitudes. Tsunami generation by pyroclastic density currents was studied by modeling the entrance of granular fluidized flows into the waterNon‐intrusive methods were used to characterize the fluidized flow properties, the flow entry into the water and the generated wavesThe particle size distribution of the flows has a negligible effect on the amplitude of the waves and underwater gravity current's velocity Tsunami generation by pyroclastic density currents was studied by modeling the entrance of granular fluidized flows into the water Non‐intrusive methods were used to characterize the fluidized flow properties, the flow entry into the water and the generated waves The particle size distribution of the flows has a negligible effect on the amplitude of the waves and underwater gravity current's velocity

Tuff dykes of unusual size invade coarser-grained volcaniclastic rocks of the Jurassic Mawson Formation at Coombs Hills and Allan Hills, Antarctica. We infer that the material for the largest dykes was elutriated from their host's matrix. To our knowledge, this mode of genesis has not been invoked before. In this volcanic setting elutriation was driven by magmatic heat, but the production of relatively finer-grained clastic dykes by elutriation of material from coarser ill-sorted sources may be significant in many settings. [ABSTRACT FROM AUTHOR]

Submarine volcanic eruptions can perturb the water surface and subsequently generate localized tsunamis. Herein waves are produced in analogous laboratory experiments by injections of non‐condensable expanding air into fresh water. By varying source pressures, jet durations and water depths, we obtain two key results. There exists a saturation duration above which the maximum wave height will not increase as the eruption continues. Apart from in extremely shallow or deep water depths, this saturation duration is positively correlated with the source intensity and negatively with the water depth. There also exists a critical water depth where an eruption with a given source intensity can generate the largest waves. When an eruption lasts less than the saturation duration, increasing the eruption duration will increase the critical water depth and the corresponding maximum wave height. The saturation duration and critical water depth are the primary controls on tsunami size and hazard. When volcanoes erupt underwater, they can generate waves that are dangerous even at coastlines far away from the eruption itself. In this study, we investigated whether the wave heights changed when the underwater eruption lasted a different amount of time. Our experiments modeled an underwater eruption by releasing compressed air into a tank of water. We carefully controlled the duration of the eruption and measured the size of the waves generated in the tank. Although at first longer eruptions generated larger wave heights, eventually the height reached a maximum value. Further increases in the duration did not lead to larger wave heights. Changing the water depth also affected the wave heights; there is a water depth where a given eruption can generate the largest wave heights. Our experiments show how this “critical” water depth changes depending on the eruption duration. Because both the depth and duration of eruption are important in determining the wave height, they should be considered in future hazard assessments of tsunamis generated by underwater volcanic eruptions. We investigate the effect of underwater volcanic eruption duration on the properties of generated waves, using analogous gas jet experimentsThere is a saturation duration above which the maximum wave amplitude does not increase with increasing eruption durationFor durations shorter than the saturation duration, longer eruptions increase the critical water depths associated with the largest waves We investigate the effect of underwater volcanic eruption duration on the properties of generated waves, using analogous gas jet experiments There is a saturation duration above which the maximum wave amplitude does not increase with increasing eruption duration For durations shorter than the saturation duration, longer eruptions increase the critical water depths associated with the largest waves

Peperites formed by mixing of magma and wet sediment are well exposed along Punta China, Baja California, Mexico, where two sills intrude a section of lava flows, limestones, and volcaniclastic rocks. Irregular lobes and dikes extend from the sills several meters into host sediments, including highly comminuted flow top breccias (lithic lapilli tuff breccias) and shelly micrites, whereas intrusive contacts with lava flows are sharp and planar. Where one sill intruded both coarse-grained volcaniclastic rock and fine-grained limestone, textural differences between the hosts produced strikingly different styles of peperite. Blocky masses of the basaltic intrusions up to 1 m in size were dispersed for distances up to 3 m into host lithic lapilli tuff breccias; the blocks consequently underwent in situ fragmentation as they were rapidly quenched. The high degree of dispersion resulted from steam explosions as the magma enveloped pockets of water in the coarse-grained permeable host. Elutriation of fine-grained material from vertical pipes in tuff breccia above the lower sill provides evidence for meter-scale fluidization of the host. The contact zone between the basaltic magma and the shelly micrite host resembles a mixture of two viscous, immiscible fluids (fluidal peperite). Intrusion occurred behind a stable vapor film which entrained lime mud particles and carried them off “grain by grain” as magma advanced into the host. Thin-section-scale elutriation pipes formed. Microglobular peperite represents a “frozen” example of a fuel-coolant interaction (FCI) between basaltic magma and fluidized micrite host. The intimate intermixing of magma and host at the submillimeter level is attributed to fluid instabilities developed along the magma-vapor-host interface. Such intimate intermixing of magma and water-bearing fragmental debris is commonly a precursory step toward explosive hydrovolcanism.

Abstract:  The subaqueous phases of an eruption initiated approximately 85 m beneath the surface of Pleistocene Lake Bonneville produced a broad mound of tephra. A variety of distinctive lithofacies allows reconstruction of the eruptive and depositional processes active prior to emergence of the volcano above lake level. At the base of the volcano and very near inferred vent sites are fines-poor, well-bedded, broadly scoured beds of sideromelane tephra having local very low-angle cross-stratification (M1 lithofacies). These beds grade upward into lithofacies M3, which shows progressively better developed dunes and cross-stratification upsection to its uppermost exposure approximately 10 m below syneruptive lake level. Both lithofacies were emplaced largely by traction from relatively dilute sediment gravity flows generated during eruption. Intercalated lithofacies are weakly bedded tuff and breccia (M2), and nearly structureless units with coarse basal layers above strongly erosional contacts (M4). The former combines products of deposition from direct fall and moderate concentration sediment gravity flows, and the latter from progressively aggrading high-concentration sediment gravity flows. Early in the eruption subaqueous tephra jetting from phreatomagmatic explosions discontinuously fed inhomogeneous, unsteady, dilute density currents which produced the M1 lithofacies near the vent. Dunes and crossbeds which are better developed upward in M3 resulted from interaction between sediment gravity flows and surface waves triggered as the explosion-generated pressure waves and eruption jets impinged upon and occasionally breached the surface. Intermingling of (a) tephra emplaced after brief transport by tephra jets within a gaseous milieu and (b) laterally flowing tephra formed lithofacies M2 along vent margins during parts of the eruption in which episodes of continuous uprush produced localized water-exclusion zones above a vent. M4 comprises mass flow deposits formed by disruption and remobilization of mound tephra. Intermittent, explosive magma–water interactions occurred from the outset of the Pahvant eruption, with condensation, entrainment of water and lateral flow marking the transformation from eruptive to "sedimentary" processes leading to deposition of the mound lithofacies.

Submarine volcanic eruptions have the potential to generate tsunamis, which can cause destruction well beyond the range of the eruption itself. Here, we present a series of underwater eruption experiments in which a non‐condensing gas was injected into a water tank with a range of water depths and applied pressures. This study proposes an effective scaled water depth and categorizes underwater eruptions into three types: deep‐water eruptions, intermediate‐water eruptions, and shallow‐water eruptions. In deep‐water eruptions, most of the energy is dissipated within the water column before the plume reaches the surface, and negligible waves are generated. In intermediate‐water eruptions, reductions in water depth reduce the loss of energy to the water column, leaving more energy available for wave generation. This causes an increase in wave heights as water shallows, up to a point. In sufficiently shallow‐water cases, the water depth is so small that almost all of the energy from the eruptive jet or plume passes through the water and is dissipated into the air, so there is only small wave‐making potential, even with relatively intense source strength. Therefore, there exists a critical water depth at which an eruption with a given source intensity will generate the largest waves. That depth lies at the boundary between the intermediate‐ and shallow‐depth regimes, where the energy available for wave generation is at a maximum. This research reveals fundamental wave generation mechanisms related to underwater gas eruptions, thereby extending our understanding of submarine volcanic tsunami generation and providing a foundation for future hazard assessment. Tsunamis are most often generated by seafloor motion due to earthquakes. However, tsunamis generated by underwater volcanic eruptions are less common and are often underestimated by researchers. In order to study the waves generated by short‐lived underwater volcanic eruptions, we injected compressed air from a submerged vent into a tank filled with water in the laboratory. We find that for an eruption of a given source intensity, the maximum wave heights first increase and then decrease as water depths increase from shallow to deep. This is because the ejected jet passes through the free surface and its energy is lost in air in shallow‐water depths, while most of the energy is lost underwater during the plume motion in deep‐water depths. Therefore, there is an optimal depth where an eruption with a given source intensity can generate the largest waves. These results are significant because they show the most dangerous conditions in tsunamis generated by submarine volcanic eruptions. The riskiest conditions need to be better accounted for in the future tsunami risk assessments. We present new physical experiments to model waves generated by underwater gas eruptionsBased on an effective scaled depth, underwater eruptions are divided into three types: deep‐, intermediate‐, and shallow‐water eruptionsWe hypothesize that there is a critical water depth at which a subaqueous eruption of a given intensity will generate the largest waves We present new physical experiments to model waves generated by underwater gas eruptions Based on an effective scaled depth, underwater eruptions are divided into three types: deep‐, intermediate‐, and shallow‐water eruptions We hypothesize that there is a critical water depth at which a subaqueous eruption of a given intensity will generate the largest waves