Rapid heat discharge during deep-sea eruptions generates megaplumes and disperses tephra

Deep-marine volcanism drives Earth's most energetic transfers of heat and mass between the crust and the oceans. While magmatic activity on the seafloor has been correlated with the occurrence of colossal enigmatic plumes of hydrothermal fluid known as megaplumes, little is known of the primary source and intensity of the energy release associated with seafloor volcanism. As a result, the specific origin of megaplumes remains ambiguous. By developing a mathematical model for the dispersal of submarine tephras, we show that the transport of pyroclasts requires an energy discharge that is sufficiently powerful (~1-2 TW) to form a hydrothermal plume with characteristics matching those of observed megaplumes in a matter of hours. Our results thereby directly link megaplume creation, active magma extrusion, and tephra dispersal. The energy flux at the plume source required to drive the dispersal is difficult to attain by purely volcanogenic means, and likely requires an additional input of heat, potentially from rapid evacuations of hot hydrothermal fluids triggered by dyke intrusion. In view of the ubiquity of submarine tephra deposits, our results demonstrate that intervals of rapid hydrothermal discharge are likely commonplace during deep-ocean volcanism.

Deep sea explosive eruptions may be not so different from subaerial eruptions

The dynamics of deep sea explosive eruptions, the dispersion of the pyroclasts, and how submarine eruptions differ from the subaerial ones are still poorly known due to the limited access to sea environments. Here, we analyze two ash layers representative of the proximal and distal deposits of two submarine eruptions from a 500 to 800 m deep cones of the Marsili Seamount (Italy). Fall deposits occur at a distance of more than 1.5 km from the vent, while volcanoclastic flows are close to the flanks of the cone. Ash shows textures indicative of poor magma-water interaction and a gas-rich environment. X-ray microtomography data on ash morphology and bubbles, along with gas solubility and ash dispersion models suggest 200–400 m high eruptive columns and a sea current velocity <5 cm/s. In deep sea environments, Strombolian-like eruptions are similar to the subaerial ones provided that a gas cloud occurs around the vent.

Interpretation of detections of volcanic activity at Ioto Island obtained from in situ seismometers and remote hydrophones of the International Monitoring System

In-situ seismic observations identified that volcanic activity of Ioto (formerly Iwojima), a volcanic island offshore Japan, increased in early September 2018. Observations of discolored nearshore waters and a splash reported by a local flyover provided evidence for a connection between undersea eruptions and recorded seismic activity. However there remain uncertainties as to when the undersea eruption series commenced and how much of the in-situ seismic activity recorded on the island was associated with volcanic earthquakes versus undersea eruptions. During this period, a large number of underwater acoustic (hydroacoustic) signals were recorded by the Comprehensive Nuclear-Test-Ban Treaty (CTBT) International Monitoring System (IMS) hydroacoustic station HA11, at Wake Island (U.S. Territory), in the northwestern Pacific Ocean with signals with directions of arrival consistent with sources located at Ioto. The analysis presented here interprets signal features of the remote hydroacoustic recordings provided by HA11 in order to attempt to distinguish between volcanic earthquake signals and undersea eruption signals originating from Ioto. Histograms of hydroacoustic events interpreted as originating from Ioto correlate well with the in-situ seismic observations at Ioto in the early stage of volcanic activity. The results presented suggest that around 75% of the signals detected at HA11 with directions of arrival consistent with Ioto as their origin could be associated with undersea eruptions, supporting the conclusion that the IMS hydroacoustic stations can contribute to volcanic event remote monitoring.

Using Soundscapes to Assess Deep-Sea Benthic Ecosystems

Targets of deep-sea mining commonly coincide with biodiversity hotspots, such as hydrothermal vents. The resilience of these ecosystems relies on larval dispersal, which may be directed by habitat-specific soundscapes. We urge for a global effort to implement soundscape as a conservation tool to assess anthropogenic disruption to deep-sea benthic ecosystems.

Rapid crystallization during recycling of basaltic andesite tephra: timescales determined by reheating experiments

Microcrystalline inclusions within microlite-poor matrix are surprisingly common in low intensity eruptions around the world, yet their origin is poorly understood. Inclusions are commonly interpreted as evidence of crystallization along conduit margins. Alternatively, these clasts may be recycled from low level eruptions where they recrystallize by heating within the vent. We conducted a series of experiments heating basaltic andesite lapilli from temperatures below the glass transition (~690 °C) to above inferred eruption temperatures (>1150 °C) for durations of 2 to >60 minutes. At 690 °C < T < 800 °C, crystallization is evident after heating for ~20 minutes; at T > 800 °C, crystallization occurs in <5 minutes. At T ≥ 900 °C, all samples recrystallize extensively in 2–10 minutes, with pyroxenes, Fe-oxides, and plagioclase. Experimental crystallization textures closely resemble those observed in natural microcrystalline inclusions. Comparison of inclusion textures in lapilli from the active submarine volcano NW Rota-1, Mariana arc and subaerial volcano Stromboli suggest that characteristic signatures of clast recycling are different in the two environments. Specifically, chlorine assimilation provides key evidence of recycling in submarine samples, while bands of oxides bordering microcrystalline inclusions are unique to subaerial environments. Correct identification of recycling at basaltic vents will improve (lower) estimates of mass eruption rate and help to refine interpretations of eruption dynamics.

Aspects of marine geoscience: a review and thoughts on potential for observing active processes and progress through collaboration between the ocean sciences

Much progress has been made in the UK in characterizing the internal structures of major physiographic features in the oceans and in developing understanding of the geological processes that have created or shaped them. UK researchers have authored articles of high impact in all areas described here. In contrast to terrestrial geoscience, however, there have been few instrumented observations made of active processes by UK scientists. This is an area that could be developed over the next decades in the UK. Research on active processes has the potential ability to engage the wider public: Some active processes present significant geo-hazards to populations and offshore infrastructure that require monitoring and there could be commercial applications of technological developments needed for science. Some of the suggestions could involve studies in shallow coastal waters where ship costs are much reduced, addressing tighter funding constraints over the near term. The possibilities of measuring aspects of volcanic eruptions, flowing lava, turbidity currents and mass movements (landslides) are discussed. A further area of potential development is in greater collaboration between the ocean sciences. For example, it is well known in terrestrial geomorphology that biological agents are important in modulating erosion and the transport of sediments, ultimately affecting the shape of the Earth's surface in various ways. The analogous effect of biology on large-scale geomorphology in the oceans is also known but remains poorly quantified. Physical oceanographic models are becoming increasingly accurate and could be used to study further the patterns of erosion, particle transport and deposition in the oceans. Marine geological and geophysical data could in turn be useful for further verification of such models. Adapting them to conditions of past oceans could address the shorter-period movements, such as due to internal waves and tides, which have been barely addressed in palaeoceanography.

Mapping the sound field of an erupting submarine volcano using an acoustic glider

An underwater glider with an acoustic data logger flew toward a recently discovered erupting submarine volcano in the northern Lau basin. With the volcano providing a wide-band sound source, recordings from the two-day survey produced a two-dimensional sound level map spanning 1 km (depth) × 40 km(distance). The observed sound field shows depth- and range-dependence, with the first-order spatial pattern being consistent with the predictions of a range-dependent propagation model. The results allow constraining the acoustic source level of the volcanic activity and suggest that the glider provides an effective platform for monitoring natural and anthropogenic ocean sounds.

Isolated communities of Epsilonproteobacteria in hydrothermal vent fluids of the Mariana Arc seamounts

Low-temperature hydrothermal vent fluids represent access points to diverse microbial communities living in oceanic crust. This study examined the distribution, relative abundance, and diversity of Epsilonproteobacteria in 14 low-temperature vent fluids from five volcanically active seamounts of the Mariana Arc using a 454 tag sequencing approach. Most vent fluids were enriched in cell concentrations compared with background seawater, and quantitative PCR results indicated that all fluids were dominated by bacteria. Operational taxonomic unit-based statistical tools applied to 454 data show that all vents from the northern end of the Mariana Arc grouped together, to the exclusion of southern arc seamounts, which were as distinct from one another as they were from northern seamounts. Statistical analysis also showed a significant relationship between seamount and individual vent groupings, suggesting that community membership may be linked to geographical isolation and not geochemical parameters. However, while there may be large-scale geographic differences, distance is not the distinguishing factor in the microbial community composition. At the local scale, most vents host a distinct population of Epsilonproteobacteria, regardless of seamount location. This suggests that there may be barriers to exchange and dispersal for these vent endemic microorganisms at hydrothermal seamounts of the Mariana Arc.