Repose Period Research Articles

Single-crystal 40Ar/39Ar dating of the Montagna Grande-Monte Gibele trachytic shield volcano, Pantelleria (Strait of Sicily rift zone), Italy

Montagna Grande and Monte Gibele represent the northwestern and southeastern portions of a shield volcano (hereafter “MGV”) located entirely within the Cinque Denti caldera structure on Pantelleria. The Cinque Denti caldera formed during the 45.7 ± 1.0 ka eruption of the pantelleritic Green Tuff. Subsequent isostatic compensation was achieved by the eruption within the caldera of ∼3 km3 of trachyte lavas that comprise the MGV and are thought to represent renewed tapping of the Green Tuff magma reservoir. The time interval between the eruption of the Green Tuff and the MGV trachytes has not been well constrained, with previous KAr ages from alkali feldspar yielding ages between 44 ± 8 and 28 ± 16 ka. In this study we report the results of new single-crystal anorthoclase incremental heating and total fusion 40Ar/39Ar ages collected from three trachyte lava flows collected at different sites and at different stratigraphic heights that provided ages of 26.2 ± 2.0, 22.5 ± 0.8 ka, and 22.3 ± 2.8 ka (uncertainties at 2σ). The results indicate that there was a repose period of up to ∼20,000 years between the eruption of the caldera-forming ignimbrite and the caldera-filling trachytes. The timing of our oldest dated sample is coincident with renewed basaltic volcanism in the northwest part of the island at ∼28 ka and suggests that the eruption of the trachyte was promoted by mafic recharge. We briefly review the petrogenetic processes operative in the reservoir during the repose period. Modelling of the thermal and compositional evolution of the fractionating system indicates an eruption rate of 7.5 × 10−4 km3/yr over the ∼4 ka eruption period.

Prolonged near-solidus and steady-state magma storage for the Youngest Toba Tuff: Evidence from TitaniQ thermometry and diffusion chronometry

Super-eruptions are among the most threatening natural hazards on Earth. The storage and thermal status of magmas within the crust are fundamental to understand the triggering mechanisms of explosive eruptions. This study presents cathodoluminescence (CL) images and Ti contents of quartz crystals from the ∼75 ka Youngest Toba Tuff (YTT), Indonesia. More than half of the quartz crystals did not display compositional zoning, whereas others contained simple zoning with bright rims (100 to 150 μm in width) or fine oscillatory zoning. The crystallization temperatures of unzoned quartz crystals were mostly near or below the eruptive temperatures estimated by Fe-Ti oxides, and similar to those of CL-dark zones in zoned quartz crystals. This indicates that the majority of quartz crystals were crystallized at relatively low and constant temperatures during the repose period prior to the YTT eruption. In comparison, the CL-bright zones of zoned quartz crystals are commonly thin and crystallized within or slightly beyond the range of eruptive temperatures, indicating progressive thermal maturation of the crustal magma reservoir. Combined with greyscale intensity and Ti-in-quartz diffusion modeling, the residence time of quartz crystals within the crustal magma reservoir was estimated to vary from ∼100 to 400 kyrs, which is of similar orders of magnitude with those obtained by zircon U-Pb and allanite U-Th ages. Our results suggest that the large YTT crustal magma reservoir was generally stored at near-solidus temperatures in discrete and thermally heterogeneous magma batches for a prolonged period of hundreds of kyrs. We argue that sustained and stable magma influx, instead of pre-eruptive magma reheating, is essential to maintain the thermally steady-state YTT crustal magma reservoir and leads to rapid aggregation of different magma batches prior to the YTT super-eruption.

Assessment of the Relationship Between Repose Period and Eruption Magnitude of Gamalama Volcano for Community Preparedness in Ternate Island – Indonesia

Assessment of the Relationship Between Repose Period and Eruption Magnitude of Gamalama Volcano for Community Preparedness in Ternate Island – Indonesia

Volume, Effusion Rate, and Lava Transport During the 2021 Fagradalsfjall Eruption: Results From Near Real‐Time Photogrammetric Monitoring

AbstractThe basaltic effusive eruption at Mt. Fagradalsfjall lasted from 19 March to 18 September 2021, ending a 781‐year repose period on Reykjanes Peninsula, Iceland. By late September 2021, 33 near real‐time photogrammetric surveys were completed using satellite and airborne images, usually processed within 3–6 hr. The results provide unprecedented temporal data sets of lava volume, thickness, and effusion rate. This enabled rapid assessment of eruption evolution and hazards to populated areas, important infrastructure, and tourist centers. The lava flow field has a mean lava thickness exceeding 30 m, covers 4.8 km2 and has a bulk volume of 150 ± 3 × 106 m3. The March–September mean bulk effusion rate is 9.5 ± 0.2 m3/s, ranging between 1 and 8 m3/s in March–April and increasing to 9–13 m3/s in May–September. This is uncommon for recent Icelandic eruptions, where the highest discharge usually occurs in the opening phase.

The eruptive history and magma composition of Pleistocene Cerro Negro volcano (Northern Chile): Implications for the complex evolution of large monogenetic volcanoes

Monogenetic volcanoes are small-volume landforms (< 1 km3) which can develop complex eruption histories. Their genesis may include variations in their primary magma sources and petrogenetic evolution, eruption styles transitions, changes in their emplacement conditions. The Central Andes (Northern Chile) contains >900 small-volume volcanoes, most of which lack a detailed knowledge of their eruption histories due to the limited volcanological studies available. In this paper, we present a detailed volcanostratigraphic, morphometric and petrologic study are used to reconstruct the geology and eruptive history of Cerro Negro volcano. It is a Pleistocene pyroclastic cone which stands out from all monogenetic volcanoes of this region due to its larger size (~1 km3) and location along the axis of the orogen. The activity of Cerro Negro began with a phreatomagmatic phase that produced breccias and a lithic-rich tuff, followed by magmatic activity with lava fountaining and Strombolian styles. This latter magmatic phase produced short lava flows, but mostly spatter agglutination and tephra fallout layers. The composition of erupted materials spans from calk-alkaline basaltic andesite to dacite. The least evolved magmas of Cerro Negro are derived from mantle wedge's partial melts without significant crustal assimilation. However, changes in the composition through the eruption suggest crystal fractionation and shallow magma storage, followed by mixing and crustal contamination during the late stage of the eruption. The absence of significant repose period between the deposition of the distinct products and their total volume together with the observed compositional variations imply that Cerro Negro represents a large volume polycyclic monogenetic volcano. These results are important to understand the mafic magma evolution in thickened crusts and the eruptive styles of long-lasting monogenetic eruptions.

Linking proximal ignimbrites and coeval distal tephra deposits to establish a record of voluminous Early Quaternary (2.4–1.9 Ma) volcanism of the Tauranga Volcanic Centre, New Zealand

The Tauranga Volcanic Centre (TgaVC) of the North Island, New Zealand, was active from 2.95 to 1.9 Ma. It lies temporally and spatially between the currently active Taupō Volcanic Zone, one of the most productive regions of Quaternary silicic volcanism globally, and its predecessor, the Coromandel Volcanic Zone. This study provides an enhanced chronology for pyroclastic volcanism of the TgaVC with four locally widespread units named and defined as follows: Welcome Bay and Wharo ignimbrites (together formerly known as Lower Pāpāmoa Ignimbrite); Otawera Ignimbrite; and Arateka Ignimbrite (formerly known as Upper Pāpāmoa Ignimbrite). These eruptives were followed by the eruption of the large-volume Waiteariki Ignimbrite at 2.1 Ma. In northern Hawke's Bay (150–170 km southeast of TgaVC), several distal tephra-fall horizons and an ignimbrite (Hikuroa Pumice Member) are preserved in marine-hosted sediments that are stratigraphically equivalent to the five ignimbrites of the TgaVC. Using new major- and trace-element data on glass in conjunction with new zircon-derived U-Pb ages, a correlation between the Hawke's Bay tephras and the TgaVC ignimbrites is established and the Hikuroa Pumice Member is confirmed as the distal deposit of the Waiteariki Ignimbrite. The integrated proximal and distal records show that at least eight eruptions occurred in the TgaVC between 2.4 and 1.9 Ma and provide a maximum repose period between explosive eruptions of ca. 50 kyrs.

Petrological evidence of rapid evolution of the magma plumbing system of Bezymianny volcano in Kamchatka before the December 20th, 2017 eruption

We used petrological data on samples of the December 20th, 2017 eruption of the Bezymianny volcano (Kamchatka, Russia) to investigate the evolution of the magmatic system during the repose period and the following reactivation of the volcano. The eruptive products are diverse in bulk rock and matrix glass compositions, but are similar in phenocryst proportion, composition, and zoning. Three rock types are distinguished by the presence of either silica-phase (cristobalite or tridymite) or their absence. Rocks without silica phase (Silica-phase Free Host Rocks) are characterized by ~55–56 wt% bulk SiO2 content and have predominantly glassy groundmass with rare microlites in dacitic matrix glass (65.5–70.5 wt% SiO2 and 2.5–3.5 wt% K2O). Cristobalite-bearing rocks are characterized by ~56.6–57.5 wt% bulk silica content and highly crystalline groundmass with many small microlites and rhyolitic matrix glass (76.5–78 wt% SiO2 and 3.8–4.2 wt% K2O) between them. Tridymite-bearing rocks are characterized by ~58.8 wt% bulk silica content and glassy groundmass with rare microlites in rhyolitic matrix glass (77–77.5 wt% SiO2 and ~3.6 wt% K2O).Rhyolite-MELTS simulation and mineral thermobarometry indicate that silica-phase-bearing rock last equilibrated under low-pressure conditions (~15 Mpa) corresponding to the conduit depths, whereas rocks without silica phase crystallized at higher pressure corresponding to shallow magma storage depths (>40–60 Mpa). Bulk rock mass-balance calculations indicate that silica-phase-bearing magmas were produced by 15% of crystal fractionation from the magma without silica phase. Based on this petrological data, we reconstruct the evolution of the magmatic system as follows. During the volcano repose period, undisturbed magma fractionated and stratified due to buoyant ascent of fluid and melt to the chamber roof. Rejuvenation of magmatic system caused pushing the differentiated magma from the top magmatic reservoir to the conduit, where silica phases crystallized. We argue that cristobalite-bearing rocks are likely the remnants of conduit plug, whereas tridymite-bearing rocks represent magma trapped in the conduit under the plug. The renewal of explosive activity in December 2017 was triggered by magma influx from deeper levels of the plumbing system shortly before the explosion, which is supported by the presence of amphibole-bearing mafic enclaves of mid-crustal origin (520–850 Mpa).

A petrological and conceptual model of Mayon volcano (Philippines) as an example of an open-vent volcano

Mayon is a basaltic andesitic, open-vent volcano characterized by persistent passive degassing from the summit at 2463 m above sea level. Mid-size (< 0.1 km3) and mildly explosive eruptions and occasional phreatic eruptions have occurred approximately every 10 years for over a hundred years. Mayon’s plumbing system structure, processes, and time scales driving its eruptions are still not well-known, despite being the most active volcano in the Philippines. We investigated the petrology and geochemistry of its crystal-rich lavas (~ 50 vol% phenocrysts) from nine historical eruptions between 1928 and 2009 and propose a conceptual model of the processes and magmatic architecture that led to the eruptions. The whole-rock geochemistry and mineral assemblage (plagioclase + orthopyroxene + clinopyroxene + Fe-Ti oxide ± olivine) of the lavas have remained remarkably homogenous (54 wt% SiO2, ~ 4 wt% MgO) from 1928 to 2009. However, electron microscope images and microprobe analyses of the phenocrysts and the existence of three types of glomerocrysts testify to a range of magmatic processes, including long-term magma residence, magma mixing, crystallization, volatile fluxing, and degassing. Multiple mineral-melt geothermobarometers suggest a relatively thermally buffered system at 1050 ± 25 °C, with several magma residence zones, ranging from close to the surface, through reservoirs at ~ 4–5 km, and as deep as ~ 20 km. Diffusion chronometry on > 200 orthopyroxene crystals reveal magma mixing timescales that range from a few days to about 65 years, but the majority are shorter than the decadal inter-eruptive repose period. This implies that magma intrusion at Mayon has been nearly continuous over the studied time period, with limited crystal recycling from one eruption to the next. The variety of plagioclase textures and zoning patterns reflect fluxing of volatiles from depth to shallower melts through which they eventually reach the atmosphere through an open conduit. The crystal-rich nature of the erupted magmas may have developed during each inter-eruptive period. We propose that Mayon has behaved over almost 100 years as a steady state system, with limited variations in eruption frequency, degassing flux, magma composition, and crystal content that are mainly determined by the amount and composition of deep magma and volatile input in the system. We explore how Mayon volcano’s processes and working model can be related to other open-vent mafic and water-rich systems such as Etna, Stromboli, Villarrica, or Llaima. Finally, our understanding of open-vent, persistently active volcanoes is rooted in historical observations, but volcano behavior can evolve over longer time frames. We speculate that these volcanoes produce specific plagioclase textures that can be used to identify similar volcanic behavior in the geologic record.

Morphological aspects of epididymal microscopy and rete testis in greater Rhea Americana

ABSTRACT: The purpose of this research was to study the histology and describe the microscopy findings of the epididymis epithelium of greater Rhea americana at three time periods: November 2005 (n=14), December 2006 (n= 20), and May 2007 (n= 20), to observe and compare the differences that occurred. We studied the epididymis from 54 rheas, bred in Santa Maria, Rio Grande do Sul, Brazil. The epididymis were collected during commercial slaughter and fixed in bouin. Optical microscopy was used to measure the cellular structure, types of cells, tubules, and stereological values like the epididymis epithelium diameters, lumen, thickness, and relative volume of the tissue structure. Additionally, electron microscopy was studied. In December 2006 and May 2007, the means of the epididymis tubular diameter were: 79.1 and 58.1 µm, epithelium thickness: 24.0 and 52.2 µm, and lumen diameter: 55.0 and 5.8 µm, respectively. Regarding the volumetric proportion, we reported the following values: epithelium volume 36.2 and 80.4%, lumen without spermatozoon 19.6 and 3.0%, lumen with spermatozoon 5.4 and 0.0%, interstitium 35.4 and 12.0%, blood vessels 3.5 and 4.6%, structures in cellular superficies 1.4 and 0%, lamina 1.4 and 3.2%, and artifacts 0.3 and 1.3%, respectively. The epididymis ducts had a circular form in transverse sections with spermatozoon only in November 2005 and December 2006. The Rhea’s epididymis morphology was found to be similar to ostriches, roosters, and Japanese quail. Here, we present data from stereological microscopy (tubular diameter, epithelium thickness, and lumen diameter), volumetric proportion (epithelium, lumen without spermatozoon, lumen with spermatozoon, interstitium, blood vessels, structures in cellular superficies; cilium, estereocilium, and lamina) in this species during the repose and sexual activity period (reproductive season).

Volcanically Induced Transient Atmospheres on the Moon: Assessment of Duration, Significance, and Contributions to Polar Volatile Traps

AbstractA transient lunar atmosphere formed during a peak period of volcanic outgassing and lasting up to about ~70 Ma was recently proposed. We utilize forward‐modeling of individual lunar basaltic eruptions and the observed geologic record to predict eruption frequency, magma volumes, and rates of volcanic volatile release. Typical lunar mare basalt eruptions have volumes of ~102–103 km3, last less than a year, and have a rapidly decreasing volatile release rate. The total volume of lunar mare basalts erupted is small, and the repose period between individual eruptions is predicted to range from 20,000 to 60,000 years. Only under very exceptional circumstances could sufficient volatiles be released in a single eruption to create a transient atmosphere with a pressure as large as ~0.5 Pa. The frequency of eruptions was likely too low to sustain any such atmosphere for more than a few thousand years. Transient, volcanically induced atmospheres were probably inefficient sources for volatile delivery to permanently shadowed lunar polar regions.

Eruption history and petrogenesis of rocks from Nyos volcano (NW Cameroon): Evidence from lithostratigraphy and geochemistry

Even though ca 1746 people and over 3000 cattle were killed in 1986 by sudden release of about 800 million m3 of CO2 from Lake Nyos volcano in northwest Cameroon, the lake's formation history is not known. Here we report results of a lithostratigraphic and petrogenetic study of Nyos volcano that allows us to confirm its phreatomagmatic (maar) origin. Eruptive products are divided into 2 units, the phases of each of which are closely related in time. Unit A comprises 4 phases, the last (A-4) represents the cataclysmic phreatomagmatic formation of Nyos maar and may have been triggered by collapse of the vent system of the previous (A-1 to A-3) phases, and/or reactivation of basement faults. After a repose period, activity shifted NE of the maar to form Unit B products. The B-1 phase of this unit deposited the scoria now covering most of the Lake Nyos area. Unit B-2 constructed the Fon's cone. A small lava flow (Unit B-3) represents the latest phase of volcanic activity of Nyos volcano. Based on area-thickness estimation methods, a total of ca 0.8 km3 of eruptive material was produced from the volcano. Geochemical data suggest that magma of the 2 units probably formed by melting of the same garnet-bearing OIB-like asthenospheric mantle source, and evolved mainly by crustal contamination and fractional crystallization in independent magma batches that erupted without mixing. Our results provide a basis for advice on general hazard mitigation in the Lake Nyos area.

Eruptive history of the Late Quaternary Ciomadul (Csomád) volcano, East Carpathians, part I: timing of lava dome activity

Located at the southern tip of the Intra-Carpathian Volcanic Range in Romania, and composed of a dozen dacitic lava domes, the Ciomadul (Csomad) volcanic complex is the youngest eruptive centre of the Carpatho-Pannonian Region. Whereas, in the last decade, the explosive history of Ciomadul since 50 ka has been well constrained by numerous studies, the chronology of the dome sequence still lacks robust chronological constraints and an extended analysis of all available data. Here, we apply a detailed K-Ar dating approach to refine the chronology of the lava dome eruptions, using the unspiked K-Ar Cassignol-Gillot technique. Our dating focused on the most voluminous central part of the lava dome complex. New eruption ages were determined following a strict separation (of 10 g) of groundmass from about 3 kg of unaltered sample rocks, thereby isolating material whose cooling was contemporaneous with the eruption. The newly applied methodology, mainly consisting of a double full preparation, first at larger grain size (~ 0.4 mm) and then at < 100 μm, provides an appropriate procedure to separate suitable material to obtain the K-Ar age of the eruption, i.e. the sample’s groundmass, in which there is no risk of the presence of older, inherited crystals. Our new geochronological data set gives an improved insight into the temporal construction of the Ciomadul volcanic complex, where (due to the method applied here) all ages are younger than those from previous studies that used whole-rock K-Ar ages. Our new results show that Ciomadul’s volcanic activity began with the construction of the south-eastern, peripheral domes from ca. 850 ka to 440 ka. After a ca. 250-ky-long repose period, the activity resumed in the northern part at around 200 ka, with subsequent domes emplaced between 200 and 130 ka, aligned roughly north-south in the western-central part of the complex. Following a 30-ky-long quiescence period, the eastern-central domes formed between 100 and 60 ka. In addition to the chronological history of lava dome volcanism, we also investigated the sequence of crystallization of mineral phases present in the lavas with respect to the modification of eruption ages. Ages obtained on pure minerals (plagioclase, amphibole, and biotite) are systematically older than those obtained on groundmass, showing that most of them formed up to 1.85 Myr before eruption in a long-lived, pre-Ciomadul magmatic system. Crystal size distribution (CSD) data support the age contrasts between juvenile groundmass and older inherited minerals. After injection of new magma and convective mixing with crystal clots, ascent of the resulting mixture led to eruptions of material representing contrasting ages.

Eruption Pattern and a Long-Term Magma Discharge Rate over the Past 100 Years at Kelud Volcano, Indonesia

Kelud Volcano is among the most active volcanoes in Indonesia, with repeated explosive eruptions throughout its history. Here, we reconstructed the relationship between the repose period and the cumulative volume of erupted material over the past 100 years and estimated the long-term magma discharge rate and future eruptive potential and hazards. Tephra data and eruption sequences described in historical documents were used to estimate the volume and mass discharge rate. The volumes of the 1901, 1919, 1951, 1966, 1990, and 2014 eruptions were estimated as 51–296 × 106m3. The mass discharge rates were estimated to be on the order of 107kg/s for the 1919, 1951, and 2014 eruptions and the order of 106kg/s for the 1966 and 1990 eruptions. Based on a linear relationship between the repose period and cumulative erupted mass, the long-term mass discharge rate was estimated as ∼ 1.5 × 1010kg/year, explaining the features of the larger eruptions (1919, 1951, and 2014) but not those of the smaller eruptions (1966 and 1990). This estimate is relatively high compared to other typical basaltic-andesitic subduction-zone volcanoes. This result provides important insights into the evolution of magmatic systems and prediction of future eruptions at Kelud Volcano.

Thermal model of successive dike injections and implications for the development of intraplate volcanoes

Temperatures in the root zones of volcanoes play a critical role in the development and persistence of shallow-level magmatic reservoirs in the crust. Here, we present a 1D thermal model allowing evaluation of the thermal impact of magma travelling in conduits to the surface on the root zone of a volcano. This thermal model has been developed to better understand the formation of a vertical intrusion located in the root zone of a dismembered Miocene volcano on Fuerteventura, Canary Archipelago. This intrusion, named PX1, constitutes an almost pure amalgamation of dikes of either clinopyroxenitic or gabbroic composition. Both types of dikes display cumulate textures and are interpreted as resulting from the protracted crystallization of a mafic magma. The formation of clinopyroxenitic, in contrast to gabbroic dikes, requires that the residual melt was extracted at high temperature (>1050°) to avoid plagioclase crystallization.Simulations of multiple dike injections show that the temperature in the root zone increases significantly with the addition of dikes, but the maximum temperature reached in the system depends on the duration of magma flow in the conduits and the time interval between dike injections (i.e., repose period). Active flow is the critical parameter that distinguishes instantaneous dike injection from a magmatic conduit. Without significant magma flow (>1 month), high-temperature conditions (>1000 °C) cannot be maintained in the pluton unless dikes are very thick and the repose period is extremely small. On the other hand, magma flow times of one to several months, combined with short time intervals between dike injections (<25 years), which are conditions comparable to those recorded for historical eruptions of oceanic island volcanoes, allow the production and preservation of temperatures above the plagioclase liquidus for significant durations, as required to generate clinopyroxenitic dikes such as those observed in the PX1 pluton.Persistent high temperature in the vicinity of magma conduits limits the differentiation of melts in transit to the surface, providing a potential explanation for why lavas of mafic to intermediate composition predominate in intraplate volcanoes such as Fuerteventura or Fogo Island (Cape Verde Archipelago). In extreme cases, when temperatures over 1000–1050 °C in the central part of the feeding zone are maintained for years, the remaining magma in the conduit does not solidify but is preserved in a mushy state. New pulses of magma would not be able to cross this zone but would rather amalgamate in the incipient magma reservoir. The present model differs from previous models of sill intrusion in that magmas do not need to pond at depth to create a reservoir but merely supply heat while travelling to the surface. Depending on the time interval between dike injections and the duration of magma flow through the crust, magma rising in vertical conduits could directly feed the volcanic edifice or could lead to the formation of magma reservoirs. This process may explain why some volcanoes erupt mafic or differentiated magmas during distinct periods of activity.

Timescales of water accumulation in magmas and implications for short warning times of explosive eruptions

Water plays a key role in magma genesis, differentiation, ascent and, finally, eruption. Despite the recognized crucial function of water, there are still several issues that continue to blur our view about its role in magmatic systems. What are the timescales of H2O accumulation in crystallizing magmas? What are the ascent rates of water-rich residual melts leading to explosive eruptions? Here, we track the timescale of water accumulation in a residual melt resulting from crystallization of a hydrous CO2-bearing magmatic mass stored at mid- to deep-crustal levels in a subduction-related geodynamic setting. Our results indicate that, after a repose period ranging from few to several thousand years, water-rich melts with water concentrations larger than 6–9 wt.% can migrate towards the Earth surface in very short timescales, on the order of days or even hours, possibly triggering explosive eruptions with short warning times and devoid of long-term geophysical precursors.

Influence of Exsolved Volatiles on Reheating Silicic Magmas by Recharge and Consequences for Eruptive Style at Volcán Quizapu (Chile)

AbstractThe two most recent eruptions of Volcán Quizapu (southern Andes, Chile), only 85 years apart, were both triggered by magma recharge and extruded the same volume (about 5 km3) of the same volatile‐rich dacitic magma, but showed a remarkable shift from effusive (1846–1847) to explosive (1932) behavior. We demonstrate, using a newly developed model, that the presence or absence of an exsolved volatile phase in the reservoir strongly influences its mechanical and thermal response to new inputs of magma. We propose that, prior to the 1846–1847 effusive eruption, gas bubbles damped the build‐up of excess pressure and allowed recharge of a significant volume of magma before triggering the 1846–1847 eruption. The strong temperature increase that resulted enhanced syneruptive outgassing leading to an effusive eruption. In contrast, during the repose period between the 1847 and 1932 eruptions, new recharges found a much less compressible host reservoir as the exsolved gas phase was largely removed in response to the prior eruption, yielding rapid pressurization, minor reheating, and comparatively less syneruptive outgassing. The combination of these effects culminated in an explosive eruption.

Volcaniclastic stratigraphy of Gede Volcano, West Java, Indonesia: How it erupted and when

Gede Volcano, West Java (Indonesia), is located 60km south of Jakarta within one of the regions with highest population density in the world. Therefore, knowledge of its eruption history is necessary for hazard evaluation, because even a small eruption would have major societal and economic consequences. Here we report the results of the investigation of the stratigraphy of Gede (with the focus on its volcaniclastic deposits of Holocene age) and include 23 new radiocarbon dates. We have found that a major part of the volcanic edifice was formed in the Pleistocene when effusions of lavas of high-silica basalt dominated. During this period the volcano experienced large-scale lateral gravitational failure followed by complete reconstruction of the edifice, formation of the summit subsidence caldera and its partial refilling. After a repose period of >30,000years the volcanic activity resumed at the Pleistocene/Holocene boundary. In the Holocene the eruptions were dominantly explosive with magma compositions ranging from basaltic andesite to rhyodacite; many deposits show heterogeneity at the macroscopic hand specimen scale and also in the minerals, which indicates interactions between mafic (basaltic andesite) and silicic (rhyodacite) magmas. Significant eruptions of the volcano were relatively rare and of moderate violence (the highest VEI was 3–4; the largest volume of erupted pyroclasts 0.15km3). There were 4 major Holocene eruptive episodes ca. 10,000, 4000, 1200, and 1000yr BP. The volcanic plumes of these eruptions were not buoyant and most of the erupted products were transported in the form of highly concentrated valley-channelized pyroclastic flows. Voluminous lahars were common in the periods between the eruptions. The recent eruptive period of the volcano started approximately 800years ago. It is characterized by frequent and weak VEI 1–2 explosive eruptions of Vulcanian type and rare small-volume extrusions of viscous lava. We estimate that during last 10,000years, Gede erupted less than 0.3km3 DRE (Dense Rock Equivalent) of magma. Such small productivity suggests that the likelihood of future large-volume (VEI≥5) eruptions of the volcano is low, although moderately strong (VEI 3–4) explosive eruptions capable of depositing pyroclastic flows and lahars onto the NE foot of the volcano are more likely.

The 600 yr eruptive history of Villarrica Volcano (Chile) revealed by annually laminated lake sediments

Lake sediments contain valuable information about past volcanic and seismic events that have affected the lake catchment, and they provide unique records of the recurrence interval and magnitude of such events. This study uses a multilake and multiproxy analytical approach to obtain reliable and high-resolution records of past natural catastrophes from ~600-yr-old annually laminated (varved) lake sediment sequences extracted from two lakes, Villarrica and Calafquen, in the volcanically and seismically active Chilean Lake District. Using a combination of micro–X-ray fl uorescence (µXRF) scanning, microfacies analysis, grain-size analysis, color analysis, and magnetic-susceptibility measurements, we detect and characterize four different types of event deposits (lacustrine turbidites, tephra-fall layers , runoff cryptotephras, and lahar deposits) and produce a revised eruption record for Villarrica Volcano, which is unprecedented in its continuity and temporal resolution. Glass geochemistry and mineralogy also reveal deposits of eruptions from the more remote Carran–Los Venados volcanic complex, Quetrupillan Volcano, and the Huanquihue Group in the studied lake sediments. Time-series analysis shows 112 eruptions with a volcanic explosivity index (VEI) ≥2 from Villarrica Volcano in the last ~600 yr, of which at least 22 also produced lahars. This signifi cantly expands our knowledge of the eruptive frequency of the volcano in this time window, compared to the previously known eruptive history from historical records. The last VEI ≥2 eruption of Villarrica Volcano occurred in 1991. Based on the last ~500 yr, for which we have a complete record from both lakes, we estimate the probability of the occurrence of future eruptions from Villarrica Volcano and statistically demonstrate that the probability of a 22 yr repose period (anno 2013) without VEI ≥2 eruptions is ≤1.7%. This new perspective on the recurrence interval of eruptions and historical lahar activity will help improve volcanic hazard assessments for this rapidly expanding tourist region, and it highlights how lake records can be used to signifi cantly improve historical eruption records in areas that were previously uninhabited.

An unusual syn-eruptive bimodal eruption: The Holocene Cuicuiltic Member at Los Humeros caldera, Mexico

The Cuicuiltic Member (CM) at Los Humeros Caldera, eastern Mexican Volcanic Belt is a Holocene (6.4kaB.P.) succession of alternated fallout deposits of contrasting composition (trachydacite pumice and basaltic andesite scoria). The CM covers approximately 250km2 on its proximal facies and its thickness ranges from 1.5m to 8.0m. It postdates two caldera-forming ignimbrites (Xaltipan and Zaragoza) and numerous Plinian successions. It is subdivided in 9 units (C1 to C9) according to its textural and chemical characteristics. Sub-horizontal, topography-draping layers of trachydacite pumice lapilli, andesitic pumice lapilli and basaltic-andesite scoria lapilli with sporadic one-meter blocks are common lithofacies. The base is formed by coarse trachydacite pumice lapilli (C1 and C2), overlain by a layer with banded pumice (C3). Thin layers of ash and ash-tuff are intermittent on lower units, whilst continuous at the base of C4. The middle units, C4 and C6 are basaltic-andesite pumice, and scoria lapilli to blocks; C5 is in-between the two mafic units and it is represented by a layer of pale grey pumice lapilli. Units C7 and C8 are a mixture of white trachydacite pumice, scoria lapilli and banded pumice. The uppermost layer, C9, is a brown to grey andesitic pumice lapilli. Extensive fieldwork allowed a close and reliable correlation of layers that helps to understand the complexity of stratigraphic relations and sources for those layers. The distribution of these units is varied across the caldera, with the trachydacite layers dispersal from the centre towards the NW, whilst the andesitic units have maximum thicknesses over the SE and NE sectors of the caldera. Isopach and isopleth maps, combined with detailed mapping of near-vent spatter facies, orientation of local bomb sags and variation of mean clasts size for some layers were very useful to determine the vent location, particularly for the andesitic–basaltic layers.The CM represents the last explosive event registered at Los Humeros caldera, combining simultaneously both Plinian and Strombolian activities. The eruption was fed from one central vent and at least two independent but simultaneously active vents up to 6km apart. The eruption began with a trachydacite explosive eruption depositing pristine white pumice in the centre of the caldera in two stages. After a short repose period, at least two andesitic to basaltic fissure vents erupted ejecting pumice and scoria over the southeast and northeast sector, mainly along a weakness structural plane parallel to the Potreros scarp. These eruptions became less energetic with time until reaching classic Strombolian style with sudden fluxes generating local and discrete pyroclastic currents. Trachydacite pumice kept falling intermittently accompanied by mingling at depth. The end of the Cuicuiltic eruption is marked by the well-graded layer C9, which registers lapilli-sized pumice-fall clasts from a hybridized magma chamber. The CM may represent (a) either a trachydacite magma chamber disturbed by a basaltic intrusion triggering the eruption of the evolved material first or, (b) a heterogeneously zoned magma reservoir tapping the evolved magma first and due to contrasting density and viscosity conditions, the basaltic and andesitic materials were extruded through adjacent weakness planes up to the surface.

Numerical simulation of tephra transport and deposition of the 1982 El Chichón eruption and implications for hazard assessment

El Chichón volcano, Chiapas, Mexico, erupted explosively on March 29th, 1982, after a repose period of about 550years. Amongst ten eruptive episodes documented between March 29th and April 4th, only the three that occurred on March 29th and April 4th produced significant pyroclastic tephra deposits. Here we use analytical (HAZMAP) and numerical (FALL3D) tephra transport models to reconstruct the deposits and the atmospheric plume dispersal associated with the three main fallout units of the 1982 eruption. On the basis of such a reconstruction, we produce hazard maps of tephra fallout associated to a Plinian eruption and discuss the implications of such a severe eruption scenario.