Explore the vast range of titles available.

To constrain the age and duration of large-scale caldera-forming eruptions, we measured paleomagnetic directions of pyroclastic deposits from the 30 ka Aira caldera eruption sequence, and the 60–45 ka Iwato pyroclastic flow deposits around Aira caldera in southern Kyushu, Japan. The 30 ka Aira caldera eruption sequence consists of Osumi pumice fall (OS), Tarumizu pyroclastic flow deposit (TM), Ito ignimbrite (IT) and co-ignimbrite ash (AT), in ascending order. Oriented samples were collected by drilling for OS, TM and welded ignimbrites, and by cubing for non-welded (unconsolidated) pyroclastic flow deposit. We systematically sampled Ito ignimbrite with the degree of welding as: non-welded (IT1), moderately welded (IT2), and strongly welded (IT3) facies that is limitedly distributed in north of Aira caldera. Mean paleomagnetic directions of OS, TM, IT1 and IT2 are consistent with those previously reported for AT and welded facies of IT. Although OS samples were collected from multiple pumice clasts at proximal locations, we obtained well-defined mean paleomagnetic directions corresponding to those of co-eruptive pyroclastic flow (TM). This suggests that even clast-supported and non-welded pumice fall can retain thermoremanent magnetization at the time of deposition. Significance tests for our mean paleomagnetic directions showed that of all sequential units of the Aira caldera-forming eruption deposits, only IT3 has a different paleomagnetic direction. Based on reported paleosecular variation changing rates in Japan, and on the angular difference associated with error of the two paleosecular directions, we estimate the time gap between IT3 and IT2, to be 24.3 ± 16.3 years. A conductive cooling model explains this time difference as due to differences in cooling time between quickly, and slowly cooled parts of a thick single ignimbrite unit. Although the possibility that multiple flow units occurred within tens of years of each other cannot be excluded, there are no observations of clear flow unit boundaries in the Ito ignimbrite to support the suggestion that the Aira caldera-forming eruption sequence was deposited within a shorter time than years. In addition, the age of Iwato pyroclastic flow deposit, which has been situated between 60 and 45 ka, was estimated by calculating the angular distance between the mean paleomagnetic direction and the modeled secular variation curve from the GGF100 ka. As the result, we determine the most probable age to be around 56 ka. Graphical abstract

Gravity-driven, ground-hugging gas-pyroclast mixtures produced during explosive volcanic eruptions define a full spectrum of particle concentration, flow regime and particle support mechanisms. To describe these phenomena, the term “pyroclastic density current” (PDC) has become increasingly popular in the last few tens of years. Here, I question the general application of the term PDC to the whole flow spectrum and, instead, I propose the simpler term “pyroclastic current”.

Volcanoes can produce tsunamis by means of earthquakes, caldera and flank collapses, pyroclastic flows or underwater explosions 1 – 4 . These mechanisms rarely displace enough water to trigger transoceanic tsunamis. Violent volcanic explosions, however, can cause global tsunamis 1 , 5 by triggering acoustic-gravity waves 6 – 8 that excite the atmosphere–ocean interface. The colossal eruption of the Hunga Tonga–Hunga Ha’apai volcano and ensuing tsunami is the first global volcano-triggered tsunami recorded by modern, worldwide dense instrumentation, thus providing a unique opportunity to investigate the role of air–water-coupling processes in tsunami generation and propagation. Here we use sea-level, atmospheric and satellite data from across the globe, along with numerical and analytical models, to demonstrate that this tsunami was driven by a constantly moving source in which the acoustic-gravity waves radiating from the eruption excite the ocean and transfer energy into it by means of resonance. A direct correlation between the tsunami and the acoustic-gravity waves’ arrival times confirms that these phenomena are closely linked. Our models also show that the unusually fast travel times and long duration of the tsunami, as well as its global reach, are consistent with an air–water-coupled source. This coupling mechanism has clear hazard implications, as it leads to higher waves along land masses that rise abruptly from long stretches of deep ocean waters. By analysing sea-level, atmospheric and satellite data captured after eruption of the Hunga Tonga–Hunga Ha’apai volcano, as well as numerical and analytical models, it is shown that global tsunamis can be triggered by acoustic-gravity waves.

Pozzolanic reaction of volcanic ash with hydrated lime is thought to dominate the cementing fabric and durability of 2000-year-old Roman harbor concrete. Pliny the Elder, however, in first century CE emphasized rock-like cementitious processes involving volcanic ash (pulvis) \"that as soon as it comes into contact with the waves of the sea and is submerged becomes a single stone mass (fierem unum lapidem), impregnable to the waves and every day stronger\" (Naturalis Historia 35.166). Pozzolanic crystallization of Al-tobermorite, a rare, hydrothermal, calcium-silicate-hydrate mineral with cation exchange capabilities, has been previously recognized in relict lime clasts of the concrete. Synchrotron-based X-ray microdiffraction maps of cementitious microstructures in Baianus Sinus and Portus Neronis submarine breakwaters and a Portus Cosanus subaerial pier now reveal that Al-tobermorite also occurs in the leached perimeters of feldspar fragments, zeolitized pumice vesicles, and in situ phillipsite fabrics in relict pores. Production of alkaline pore fluids through dissolution-precipitation, cation-exchange and/or carbonation reactions with Campi Flegrei ash components, similar to processes in altered trachytic and basaltic tuffs, created multiple pathways to post-pozzolanic phillipsite and Al-tobermorite crystallization at ambient seawater and surface temperatures. Long-term chemical resilience of the concrete evidently relied on water-rock interactions, as Pliny the Elder inferred. Raman spectroscopic analyses of Baianus Sinus Al-tobermorite in diverse microstructural environments indicate a cross-linked structure with Al3+ substitution for Si4+ in Q3 tetrahedral sites, and suggest coupled [Al3++Na+] substitution and potential for cation exchange. The mineral fabrics provide a geoarchaeological prototype for developing cementitious processes through low-temperature rock-fluid interactions, subsequent to an initial phase of reaction with lime that defines the activity of natural pozzolans. These processes have relevance to carbonation reactions in storage reservoirs for CO2 in pyroclastic rocks, production of alkali-activated mineral cements in maritime concretes, and regenerative cementitious resilience in waste encapsulations using natural volcanic pozzolans.

Our new paleomagnetic data provide the timescale for the eruptive sequence of the 45 ka Shikotsu caldera-forming eruption of VEI 7. The duration of the entire sequence is estimated to be centuries, which is considerably longer than previously thought. The studied volcanic sequence, located at ~ 10 km from the caldera rim, includes five units of pyroclastic flow deposits (units B3, C1, C2, D, and E in ascending order) and an uppermost pyroclastic surge deposit (unit F2). The ash matrix samples of the pyroclastic deposits, predominantly composed of juvenile material, were collected into aluminum and plastic cubes, which were precisely oriented using an originally designed tool set. As a result, the obtained paleomagnetic directions have high precision parameters (k = 200–1400) and small 95% confidence intervals (α 95  = 2–4°). These paleomagnetic directions determined from the sequence of the six units demonstrate a curve of paleomagnetic secular variation, which has a total angular distance of 14.4 ± 4.1°. The observed distinguishable paleomagnetic directions indicate that the earlier four units (B3, C1, C2, and D) were formed by four distinct eruptions over a period of 240 ± 70 years with repose times of decades between the eruptions. The indistinguishable paleomagnetic directions indicate that the three late-stage units (D, E, and F) were erupted in a short period, decades or less. Our new paleomagnetic data, combined with the reported petrological change in pumice, suggest that the extractions of crystal-poor rhyolitic melt from the magma chamber occurred in multiple eruptions over a considerable period (more than a hundred years). Graphical Abstract

Granular flows of angular rock fragments such as rock avalanches and dense pyroclastic flows are simulated numerically by means of the discrete element method. Since large-scale flows generate stresses that are larger than those generated by small-scale flows, the purpose of these simulations is to understand the effect that the stress level has on flow mobility. The results show that granular flows that slide en mass have a flow mobility that is not influenced by the stress level. On the contrary, the stress level governs flow mobility when granular flow dynamics is affected by clast agitation and collisions. This second case occurs on a relatively rougher subsurface where an increase of the stress level causes an increase of flow mobility. The results show also that as the stress level increases, the effect that an increase of flow volume has on flow mobility switches sign from causing a decrease of mobility at low stress level to causing an increase of mobility at high stress level. This latter volume effect corresponds to the famous Heim’s mobility increase with the increase of the volume of large rock avalanches detected so far only in the field and for this reason considered inexplicable without resorting to extraordinary mechanisms. Granular flow dynamics is described in terms of dimensionless scaling parameters in three different granular flow regimes. This paper illustrates for each regime the functional relationship of flow mobility with stress level, flow volume, grain size, channel width, and basal friction.

Volcanic tsunamis are generated by a variety of mechanisms, including volcano-tectonic earthquakes, slope instabilities, pyroclastic flows, underwater explosions, shock waves and caldera collapse. In this review, we focus on the lessons that can be learnt from past events and address the influence of parameters such as volume flux of mass flows, explosion energy or duration of caldera collapse on tsunami generation. The diversity of waves in terms of amplitude, period, form, dispersion, etc. poses difficulties for integration and harmonization of sources to be used for numerical models and probabilistic tsunami hazard maps. In many cases, monitoring and warning of volcanic tsunamis remain challenging (further technical and scientific developments being necessary) and must be coupled with policies of population preparedness.

Research on the hydrogeology of andesitic volcanic aquifers in subduction areas is reviewed. Andesitic aquifers are of high interest in volcanic arc islands and subduction zones, where they constitute a strategic water resource. This review gathers a compilation of worldwide results and case studies to propose a generic hydrogeological conceptual model (GHCM). It is based on the geological conceptual model splitting the volcanic edifice, from upstream to downstream, into central, proximal, medial and distal zones. In this geological structure, the GHCM identifies where the main aquifer types (fractured lava, pyroclastic flows, and the volcano-sedimentary basins downstream) and the typical aquitards (lahars, fine pyroclastic falls and surges, indurated pyroclastic flow, and weathered rocks) are structured and organized. To integrate the evolution of volcanoes and some specific volcanic activities, a specific GHCM for old andesitic volcanoes or andesitic shield volcanoes is detailed. The paper also describes how the GHCM results are of use to hydrogeologists in terms of scale (from the lithological units to the regional scale), to effectively site water wells, and to sustainably manage groundwater resources in such aquifers. Among these various scales, the volcanic “flank continuum” is presented as the most adapted to support groundwater resources management. Several ways to improve this GHCM are suggested, notably to better consider the geological complexity of these aquifers.

This review summarizes what the volcanology community has learned thus far from studying the deposits of pyroclastic currents (PCs) from the 1980 eruption sequence at Mount St. Helens. The review includes mass flow events during the May 18 eruption, including the lateral blast, the afternoon column collapse and boil-over PC activity, and some aspects of the debris avalanche. We also include a summary of PCs generated in the smaller eruptions following the climactic May 18 event. Our objective is to summarize the state of our understanding of PC transport and emplacement mechanisms from the combination of field and laboratory observations, granular flow experiments, and numerical modeling techniques. Specifically, we couple deposit characteristics, experiments, and numerical modeling techniques to critically address the problems of (1) constraining conditions in the flow boundary zone at the time of deposition; (2) the influence of substrate roughness and topography on PC behavior; (3) the prevalence, causes, and consequences of substrate erosion by PCs; and (4) the reconstruction of PC transportation and sedimentation processes from a combination of geophysical and sedimentological observations. We conclude by providing opportunities for future research as our field, experimental, and numerical research techniques advance.

Pyroclastic material in the form of altered volcanic ash or tephra has been reported and described from one or more stratigraphic units from the Proterozoic to the Tertiary. This altered tephra, variously called bentonite or K-bentonite or tonstein depending on the degree of alteration and chemical composition, is often linked to large explosive volcanic eruptions that have occurred repeatedly in the past. K-bentonite and bentonite layers are the key components of a larger group of altered tephras that are useful for stratigraphic correlation and for interpreting the geodynamic evolution of our planet. Bentonites generally form by diagenetic or hydrothermal alteration under the influence of fluids with high-Mg content and that leach alkali elements. Smectite composition is partly controlled by parent rock chemistry. Studies have shown that K-bentonites often display variations in layer charge and mixed-layer clay ratios and that these correlate with physical properties and diagenetic history. The following is a review of known K-bentonite and related occurrences of altered tephra throughout the timescale from Precambrian to Cenozoic.