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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. Plain Language Summary: 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. Key Points: 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 [ABSTRACT FROM AUTHOR]

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 h is 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. Plain Language Summary: 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. Key Points: 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 [ABSTRACT FROM AUTHOR]

The oldest‐known rocks of the eroded Dunedin Volcano (DV), New Zealand, active 16.0–11.0 Ma, were emplaced by penecontemporaneous explosive eruptions and intrusions of magmas with a remarkable range of compositions, including basanite, alkali basalt, trachyte and phonolite. The compositional diversity of magmas emplaced in rapid succession during the earliest development of the volcano signify the existence of a complex magmatic system and highly evolved magmas, typical of those attributed to mature magmatic systems; here it developed in concert with the volcano's surface eruptions. We infer that multiple deep and shallow reservoirs were suddenly tapped as volcanism commenced. DV repeatedly erupted the same range of magmas over 5 Myr, suggests that these distributed reservoirs failed to significantly integrate through millions of years of slow volcano growth. [ABSTRACT FROM AUTHOR]

Theoretical source models of underwater explosions are often applied in studying tsunami hazards associated with subaqueous volcanism; however, their use in numerical codes based on the shallow water equations can neglect the significant dispersion of the generated wavefield. A non-hydrostatic multilayer method is validated against a laboratory-scale experiment of wave generation from instantaneous disturbances and at field-scale subaqueous explosions at Mono Lake, California, utilising the relevant theoretical models. The numerical method accurately reproduces the range of observed wave characteristics for positive disturbances and suggests a relationship of extended initial troughs for negative disturbances at low-dispersivity and high-non-linearity parameters. Satisfactory amplitudes and phase velocities within the initial wave group are found using underwater explosion models at Mono Lake. The scheme is then applied to modelling tsunamis generated by volcanic explosions at Lake Taupō, New Zealand, for a magnitude representing an ejecta volume of 0.1 km 3. Waves reach all shores within 15 min with maximum incident crest amplitudes around 0.2 m at shores near the source. This work shows that the multilayer scheme used is computationally efficient and able to capture a wide range of wave characteristics, including dispersive effects, which is necessary when investigating subaqueous explosions. This research therefore provides the foundation for future studies involving a rigorous probabilistic hazard assessment to quantify the risks and relative significance of this tsunami source mechanism. [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. Plain Language Summary: 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. Key Points: 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 [ABSTRACT FROM AUTHOR]

Theoretical source models of underwater explosions are often applied in studying tsunami hazards associated with submarine volcanism; however, their use in numerical codes based on the shallow water equations can neglect the significant dispersion of the generated wavefield. A non-hydrostatic multilayer method is validated against a laboratory-scale experiment of wave generation from instantaneous disturbances and at field-scale submarine explosions at Mono Lake, California, utilising the relevant theoretical models. The numerical method accurately reproduces the range of observed wave characteristics for positive disturbances and suggests a previously unreported relationship of extended initial troughs for negative disturbances at low dispersivity and high nonlinearity parameters. Satisfactory amplitudes and phase velocities within the initial wave group are found using underwater explosion models at Mono Lake. The scheme is then applied to modelling tsunamis generated by volcanic explosions at Lake Taupō, New Zealand, for a magnitude range representing ejecta volumes between 0.04–0.4 km3. Waves reach all shores within 15 minutes with maximum incident crest amplitudes around 4 m at shores near the source. This work shows that the multilayer scheme used is computationally efficient and able to capture a wide range of wave characteristics, including dispersive effects, which is necessary when investigating submarine explosions. This research therefore provides the foundation for future studies involving a rigorous probabilistic hazard assessment to quantify the risks and relative significance of this tsunami source mechanism. [ABSTRACT FROM AUTHOR]

The 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 × 107 kg s−1 for the northern Mt. Tarawera segment of the fissure and 1.4–5.7 × 106 kg s−1 for the Rotomahana segment. [ABSTRACT FROM AUTHOR]

Ruapehu (150 km3 cone, 150 km3 ring-plain) and Tongariro (90 km3 cone, 60 km3 ring-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 (>104 years) 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). [ABSTRACT FROM AUTHOR]

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. Plain Language Summary: 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. Key Points: 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 [ABSTRACT FROM AUTHOR]

Volcaniclastic 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 km2 but offshore volcanic rocks occurring over an additional ∼3500 km2 may 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. [ABSTRACT FROM AUTHOR]

The 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 km2 until ∼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. [ABSTRACT FROM AUTHOR]

The 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. [ABSTRACT FROM AUTHOR]

The diversion of magma is an important mechanism that may lead to the relocation of a volcanic vent. Magma diversion is known to occur during explosive volcanic eruptions generating subterranean excavation and remobilization of country and volcanic rocks. However, feedbacks between explosive crater formation and intrusion processes have not been considered previously, despite their importance for understanding evolving hazards during volcanic eruptions. Here, we apply numerical modeling to test the impacts of excavation and subsequent infilling of diatreme structures on stress states and intrusion geometries during the formation of maar–diatreme complexes. Explosive excavation and infilling of diatremes affects local stress states which inhibits magma ascent and drives lateral diversion at various depths, which are expected to promote intra-diatreme explosions, host rock mixing, and vent migration. Our models demonstrate novel mechanisms explaining the generation of saucer-shaped sills, linked with magma diversion and enhanced intra-diatreme explosive fragmentation during maar-diatreme volcanism. Similar mechanisms will occur at other volcanic vents producing crater-forming eruptions. [ABSTRACT FROM AUTHOR]

Fragmentation processes in eruptions are commonly contrasted as phreatomagmatic or magmatic; the latter requires only fragmentation of magma without external intervention, but often carries the connotation of disruption by bubbles of magmatic gas. Phreatomagmatic fragmentation involves vaporization and expansion of water as steam with rapid cooling and/or quenching of the magma. It is common to assess whether a pyroclast formed by magmatic or phreatomagmatic fragmentation using particle vesicularity, shape of particles, and degree of quenching. It is widely known that none of these criteria is entirely diagnostic, so deposit features are also considered; welding and/or agglomeration, particle aggregation, lithic fragment abundance, and proportion of fines. Magmatic fragmentation yields from rhyolite pumice to obsidian to basaltic achneliths or carbonatitic globules, making direct argument for magmatic fragmentation difficult, so many have taken an alternative approach. They have tested for phreatomagmatism using the fingerprints listed above, and if the fingerprint is lacking, magmatic fragmentation is considered proven. We argue that this approach is invalid, and that the criteria used are typically incorrect or incorrectly applied. Instead, we must consider the balance of probabilities based on positive evidence only, and accept that for many deposits it may not be possible with present knowledge to make a conclusive determination. [ABSTRACT FROM AUTHOR]

It is established that Surtseyan eruptions involve extensive magmawater interaction, but the specific volumes, geometries, and dynamic consequences of such interaction have not been precisely characterized. Textural studies seeking to understand phreatomagmatism have mainly focused on fine ash—an approach that is intuitive given the abundance of fine particles in Surtseyan deposits, but that neglects additional information preserved in coarser particles. Virtually without exception, scoria bombs from Surtsey (Iceland) and similar volcanoes show composite textures, with host material having entrained smaller clasts. Entrained clasts commonly show evidence of post-entrapment groundmass crystallization, and always are surrounded by void space indicating that they were wet at the time of entrapment. The composite textures—ubiquitous in bombs but also common in lapilli—support a classical model that describes Surtseyan volcanism as being driven by mingling of magma and water-saturated slurry in periodically flooded vents. We use textures, eruption observations, and basic thermodynamics to expand the magma-slurry model and relate it directly to the vapor dynamics that characterize Surtseyan jets and plumes. [ABSTRACT FROM AUTHOR]

Melt segregation is the extraction of residual melt from the rigid but permeable network of growing crystals with which it co-evolved. It is recognized as an effective mechanism of igneous differentiation that acts over many geologic time and length scales. Here we present evidence for rapid melt segregation in subaqueous basaltic pyroclasts. Segregation produced intravesicular extrusions, i.e., partly hollow balloons of glass penetrating the walls of earlier formed vesicles set in a microcrystalline groundmass. The segregation process can be described as a form of gas filter pressing, where microlite crystallization produced local gradients in volatile supersaturation and vapor pressure that drove melt extrusion into adjacent vesicles. Unlike in previously described segregation features, the structures presented here are shown to have formed after the extruded melt became enriched in fast-diffusing H2O, but before it became measurably enriched or depleted in slower diffusing major elements. We show that melt segregation of this embryonic type must occur in seconds, rather than in the days to centuries required for similar processes to occur in lava flows or magma chambers, and that it occurs within the short time scales that characterize explosive fragmentation of basaltic melt. [ABSTRACT FROM AUTHOR]

SUMMARY We analyse 10 representative intrusions from two sets of inclined diabase (Ferrar Dolerite) sheets exposed at Allan Hills (South Victoria Land, Antarctica), using petrographic and rock magnetic methods to determine microfabrics and infer magma flow directions. At least one diabase sample was collected at the margins of each intrusion. Magnetite and pyrrhotite contribute to magnetic fabrics of the samples. Thirty-six magnetic fabric directions, inferred from the mutual arrangement of either the magnetic lineation, or the magnetic foliation plane and local macroscopic flow indicators (e.g. horn-shaped apophyses and kinks) at the tips and margins of each intrusion reveal composite (i.e. both lateral and vertical) flow paths recorded along each intrusive segment. Petrographic textures and multiple flow directions inferred at sheet-segment tips reveal that 'passive' injection of magma via hydrofracturing produced the local shallow large igneous province plumbing as a sill-dominated intrusive complex very close to, or intersecting the palaeosurface. This contrasts with 'classic' arrays of either vertically or laterally injected blade-like dykes. [ABSTRACT FROM AUTHOR]

Much remains unknown about submarine explosive eruptions. Their deposits are found to great depths in all the world's oceans, but eruptions are typically described by analogy to a subaerial nomenclature that ignores the substantial and inevitable influences of hydrostatic pressure and magma-water interaction at submerged edifices. Here we explore magmatic volatile exsolution and magma-water interaction for a pyroclastic cone-forming eruption at ~1 km depth on Loihi Seamount, Hawaii. We examine vesicle textures in lapilli-the physical manifestation of degassing; dissolved volatiles in matrix glasses and olivine-hosted glass inclusions-the geochemical record of ascent and volatile exsolution; and fine ash morphology-the evidence for if and how external water assisted in fragmentation. This approach allows a submarine explosive eruption style to be defined: the magma achieved ~40% vesicularity through almost perfectly closed-system volatile exsolution from ~3 km below the vent, which accelerated and weakened the melt, allowing it to be fragmented by explosive magma-water interaction. We introduce the name "Poseidic" for this end-member style of submarine basalt explosivity. Poseidic eruptions are identifiable from measurable features in pyroclasts, and are possible at all subaqueous basaltic volcanoes. [ABSTRACT FROM AUTHOR]

This study focuses on a numerical model of turbidity currents with reversing buoyancy, i.e., flows that are rendered heavier than the ambient water due to the presence of suspended sediment but that as sedimentation progresses become lighter than the ambient water due to a difference in temperature or salinity. The flows considered here may be either pulsed events or continuous flows. It is well known that these flows are subject to a buoyancy reversal. Flows issuing upstream with a sufficient load of suspended sediment are heavier than the ambient water and thus form a bottom underflow. As sediment deposits in the downstream direction, however, the flow gradually loses its density excess, and eventually reverses its buoyancy, detaching upward from the bed. The nature of the sediment deposit emplaced near and downstream of the point of lofting can thus differ significantly from that emplaced upstream. A novel aspect of the work reported here is a mechanistic description of the tendency for the current to sort in the downstream direction a sediment mixture containing a wide range of grain sizes. The numerical model, which is based in the − closure for turbulence, is verified with a unique set of experimental data intended to model sediment sorting associated with turbidity currents created by explosive undersea eruptions. Both cases with nonreversing buoyancy and reversing buoyancy are considered. As such, the model not only provides a detailed description of flows with reversing buoyancy, but also provides a tool to aid sedimentologists in back-calculating the flow emplaced by turbidity currents from the downstream variation in the grain size distribution of the bed deposit. [ABSTRACT FROM AUTHOR]

The 1963–1967 shallow-to-emergent eruption in Iceland's Vestmannaeyjar earned a place in the history of volcanology by creating the island of Surtsey while under close observation of volcanologist Sigurdur Thorarinsson (Sigurður Þórarinsson in Icelandic). This is an example of what is now called Surtseyan volcanism, and it included explosive and effusive phases from multiple vents that formed the island of Surtsey itself, as well as one fully subaqueous pyroclastic edifice and two additional, but ephemeral, islands. Sigurdur Thorarinsson identified tephra jetting and continuous uprush as characteristic types of subaerial explosive activity of Surtseyan volcanism. Subaerial cone-forming deposits of Surtseyan volcanism are typically poorly sorted, with fine-grained beds rich in sideromelane ash fragments, punctuated by larger, ubiquitously composite bombs, whereas deposits sampled by coring deep into the submarine edifice include fines-poor horizons dominated by vesicular coarse sideromelane ash. Here, we present new textural data and highlight the diversity of pyroclasts and microtextures from Surtsey (Surtur I and Surtur II) and its satellite vents (Surtla, Syrtlingur and Jolnir), in the context of Surtseyan volcanism. We used several sample sets. Some were collected during the 3.5-year long eruption and were conserved in the Icelandic Natural History Museum, including one sample from the core drilled into Surtsey in 1979. Other samples were collected during more recent field campaigns on Surtsey Island. In closing, we discuss the implications of this diversity for the range of activity and products produced by Surtsey. [ABSTRACT FROM AUTHOR]

We describe and interpret deposits associated with the final Ubehebe Crater-forming, phreatomagmatic explosive phase of the multivent, monogenetic Ubehebe volcanic center. Ubehebe volcano is located in Death Valley, California, USA. Pyroclastic deposits occur in four main facies: (1) lapilli- and block-dominated beds, (2) thinly bedded lapilli tuff, (3) laminated and cross-laminated ash, and (4) massive lapilli ash/tuff. Lapilli- and block-dominated beds are found mostly within several hundred meters of the crater and transition outward into discontinuous lenses of lapilli and blocks; they are interpreted to have been deposited by ballistic processes associated with crater-forming explosions. Thinly bedded lapilli tuff is found mainly within several hundred meters, and laminated and cross-laminated ash extends at least 9 km from the crater center. Dune forms are common within ~2 km of the crater center, while finer-grained, distal deposits tend to exhibit planar lamination. These two facies (thinly bedded lapilli tuff and laminated and cross-laminated ash) are interpreted to record multiple pyroclastic surges (dilute pyroclastic currents). Repeated couplets of coarse layers overlain by finer-grained, laminated horizons suggest that many or most of the surges were transient, likely recording individual explosions, and they traveled over complex topography in some areas. These two factors complicate the application of classical sediment-transport theory to quantify surge properties. However, duneform data provide possible constraints on the relationships between suspended load sedimentation and bed-load transport that are consistent using two independent approaches. Massive lapilli ash/tuff beds occur in drainages below steep slopes and can extend up to ~1 km onto adjacent valley floors beneath large catchments. Although they are massive in texture, their grainsize characteristics are shared with laminated and cross-laminated ash facies, with which they are locally interbedded. These are interpreted to record concentrated granular flows sourced by remobilized pyroclastic surge deposits, either during surge transport or shortly after, while the surge deposits retained their elevated initial pore-gas pressures. Although similar surge-derived concentrated flows have been described elsewhere (e.g., Mount St. Helens, Washington, USA, and Soufriére Hills, Montserrat, West Indies), to our knowledge Ubehebe is the first case where such processes have been identified at a maar volcano. These concentrated flows followed paths that were independent of the pyroclastic surges and represent a potential hazard at similar maar volcanoes in areas with complex terrain. [ABSTRACT FROM AUTHOR]

The tsunamigenic potential of underwater volcanic eruptions is not well understood, even though eruption-generated tsunamis can be devastating. To address how erupted steam bursts from underwater volcanoes generate tsunamis, we present the experiments, using pressurized steam injected vertically into a water tank. Results over various eruption conditions identify three eruption regimes, namely, shallow-, intermediate-, and deep-water eruptions, according to the combined effects of water depths, source strengths, and source durations. The transition between shallow and intermediate eruptions is characterized by critical depths maximizing tsunami wave heights, while the transition between intermediate and deep eruptions is characterized by containment depths inhibiting surface disturbances. Water depth and source intensity are the dominant factors controlling maximum wave amplitudes, more so than aspects of jet duration and condensation. These experiments and supporting dimensional analysis improve our understanding of how underwater volcanic eruptions form tsunamis, while also providing a complete dataset for advancing tsunami generation models. [ABSTRACT FROM AUTHOR]

Rocks exposed at Punta Cono include very fine-grained to coarse-grained tuffs, lapilli tuffs, and tuff breccias deposited in a deep marine environment. Syndepositional basaltic intrusive activity was common. In one locality a hyaloclastite-peperite complex formed. Slumped sections with fluidal basalt 'clasts', derived from intrusions that entered the sediment pile from below, are present elsewhere. Abundant soft- sediment folds in fine-grained laminated subaqueous fall-out turf suggest steep gradients; these are cut by shallow channels filled with coarse-grained tuff, lapilli tuff, and rare turf breccia. The combination of marine fossils, extreme textural immaturity, abundant slump features, and syndepositional magmatism indicates deposition upon the submarine flanks of an active volcano. Recognition of magma-wet sediment interaction is hampered in volcaniclastic rocks because of the similarity between host and intrusive fragments. Products of magma-water-sediment interactions at Punta Maria include: (1) jigsaw-puzzle hyaloclastite, formed by non-explosive hydroclastic fragmentation of magma upon contact with water and water-bearing sediment; (2) peperites, produced by mixing of magma with sediment; and (3) an unusual turf breccia unit, the result of non-explosive mixing of 'wisps' of lava with sediment during remobilization of an unconsolidated section. Low-explosivity magma-water-sediment interactions are favoured by relatively high hydrostatic pressures in sub-wave base settings. [ABSTRACT FROM AUTHOR]