Caldera System Research Articles

The Talili Pyroclastics eruption sequence: VEI 5 precursor to the seventh century CE caldera-forming event at Rabaul, Papua New Guinea

Studies of the products of the activity precursory to caldera-forming events are essential in revealing how caldera systems evolve in the prelude to climactic eruptions. Here, we investigate the deposits formed by the Talili Pyroclastics eruption sequence, which preceded the youngest major eruptive event, the seventh century CE (1400 BP) Rabaul Pyroclastics VEI 6 caldera-forming eruption, at Rabaul in northeastern Papua New Guinea. The Talili Pyroclastics deposits are essentially a bimodal (dacite-basalt) tephra sequence that was emplaced during the 2.8 ky leading to the Rabaul Pyroclastics eruption. The dominant activity during the Talili period, starting at 4200 BP, involved strong (up to VEI 5), water-modified explosive eruptions of dacite from intra-caldera vents. This activity generated > 4.4 km3 of unstratified, bedded and laminated fine-grained fall deposits, pyroclastic surges and a small ignimbrite. In addition, all three of the young stratovolcanoes on the eastern and northeastern fringes of Rabaul Caldera were active during the Talili era, producing > 0.7 km3 of basaltic scoria fall and flow deposits. The main phase of Talili era basalt production occurred at 4100 BP and involved sequential eruptions from two of the stratovolcanoes, Palangiangia and Kabiu. The sequence of dacitic Talili eruptives shows a progressive decline then late-stage increase in SiO2 contents, with corresponding changes in Fe2O3, MgO and CaO. Early and late products have compositions similar to those of the most-evolved products of the major eruptions at Rabaul (intermediate to high-SiO2 dacites), while middle-stage products are similar to those of the historical eruptions (lower-SiO2 dacites and andesites). The basaltic eruptives have three different compositions, attributed to three different (stratovolcano) sources. There is no evidence of mixing or mingling of basalt and dacite. However, andesitic scoria inclusions in the early-erupted dacitic ignimbrite represent mingling of andesitic and dacitic magmas. As with the Talili Pyroclastics, at least two other earlier tephra sequences at Rabaul probably resulted from phases of VEI 5 volcanism and directly underlie the deposits of major eruptions. This leads to the inference that cycles of escalating eruptive activity have prevailed prior to some of the major eruptions, and suggests that the strength of volcanism at Rabaul may rise from the lower levels (maximum of VEI 4) evident since the Rabaul Pyroclastics event as the system moves towards the next major eruption.

A proximal record of caldera-forming eruptions: the stratigraphy, eruptive history and collapse of the Palaeogene Arran caldera, western Scotland

Caldera-forming volcanic eruptions are among the most dangerous, and can generate extensive pyroclastic deposits and deliver ash into global atmospheric circulation systems. As calderas collapse, the eruptions can deposit thick proximal ignimbrite sequences and thinner ignimbrites more distally. However, the proximal record of caldera collapse is often obscured by later intrusions, volcanism, faults, alteration, water and sediments, which significantly limits our understanding of these eruptions. A Palaeogene caldera system in central Arran, western Scotland, preserves a rare proximal caldera-fill succession, the Arran Volcanic Formation. This caldera largely comprises highly heterogeneous ignimbrites and minor intra-caldera sedimentary rocks. The current level of erosion, and the general absence of faults, intrusions and sediments, allows a complex stratigraphy and collapse history to be determined, which can be linked to changing eruptive styles at a constantly evolving volcano. The first recorded phase was eruption of a homogeneous rhyolitic lava-like tuff, deposited from high temperature, high mass-flux pyroclastic density currents generated from low fountaining columns that retained heat. A succeeding phase of highly explosive Plinian eruptions, marked by a thick blanket of massive lapilli tuffs, was then followed by piston-like caldera collapse and erosion of steep caldera walls. Volcanism then became generally less explosive, with predominantly lava-like and eutaxitic tuffs and cognate spatter-rich agglomerates interbedded with non-homogenous lapilli tuffs. High topographic relief between distinct units indicate long periods of volcanic quiescence, during which erosive processes dominated. These periods are, in several places, marked by sedimentary rocks and evidence for surface water, which includes a localised basaltic-andesitic phreatomagmatic tuff. The caldera-forming eruptions recorded by the Arran Volcanic Formation provide an important insight into caldera collapse processes and proximal ignimbrite successions. The lack of thick autobreccias and lithic-rich lapilli- and block-layers indicates that subsidence was relatively gradual and incremental in this caldera, and not accompanied by catastrophic wall collapse during eruption. The relatively horizontal nature of the caldera-fill units and paucity of intra-caldera faulting indicate that piston subsidence was the dominant method of collapse, with a relatively coherent caldera floor bounded by a steeply dipping ring fault. Possible resurgence may have caused later doming of the floor and radial distribution of subsequent ignimbrites and sedimentary rocks. Our work emphasises the continued need for field studies of caldera volcanoes.

Using Gravity and Magnetic Data for Insights into the Mesoproterozoic St. Francois Terrane, Southeast Missouri: Implications for Iron Oxide Deposits

Gravity and magnetic data in conjunction with available drill hole data were used to decipher the nature of the volcanic lithologies and their relationship to iron oxide ore deposits within the northwest portion of the Mesoproterozoic St. Francois Terrane in southeast Missouri. Analyses of the data included creating a deep-seated gravity anomaly map by removing the gravitational effect of the Paleozoic sedimentary units, residual gravity and magnetic anomaly maps, and two and one-half dimensional gravity and magnetic models. The residual gravity and magnetic anomaly maps highlighted a series of short-wavelength maxima and minima anomalies that could be interpreted to be granitic within collapsed caldera systems surrounded later by ring intrusions. The gravity and magnetic models illustrate that the sources of the anomalies are situated mainly within the upper 5 km of the crust and help to constrain the location of suspected calderas. The iron oxide deposits, some of which contain significant quantities of REE minerals, are rarely associated with both residual gravity and magnetic maxima but may be on the edge of these maxima and/or occur over gravity minima. The REE-rich Pea Ridge deposit is located over a gravity minimum where modeling shows the main ore body extending down to 1.5 km with a low-density region extending to 5 km below the ore body. The cause of this low-density body is unknown but may be associated with the formation of the ore deposit. Potential iron oxide deposits may occur to the south and southwest of Pea Ridge based on our analysis while to the northeast of the Pea Ridge deposit an anomaly pattern similar to that over Pea Ridge was found.

Development of a mixed seawater-hydrothermal fluid geochemical signature during alteration of volcanic rocks in the Archean (∼2.7 Ga) Abitibi Greenstone Belt, Canada

We investigated a group of silicified volcanic rocks from the ∼2.72 Ga Hunter Mine Group (HMG), Abitibi Greenstone Belt, Canada, in order to document progressive compositional change associated with alteration in a subaqueous caldera system. Rocks of the HMG divide into three groups based on mineralogy and texture for petrographic and geochemical analyses. Volcanic features (phenocrysts, pseudomorphs after primary glass shards, lapilli, volcanic clasts) are preserved in all groups, despite changing mineralogy from primarily quartz, feldspar, chlorite (Groups 1 and 2), to quartz, hematite and carbonate (Groups 2 and 3). Compositionally, Group 1 rocks resemble volcanic rocks in the region, while Group 2 and 3 rocks show a change in mineralogy to iron, silica, and carbonate minerals, which is associated with depletion of many major and trace elements associated with volcanic rocks (Al2O3, Na2O, K2O, Zr). In addition, rare earth elements display a clear progression from volcanic signatures in Group 1 (PrSN/YbSN = 1.7–2.96, EuSN/EuSN∗ = 0.84–1.72, Y/Ho = 25.20–27.41, LaSN/LaSN∗ = 0.97–1.29, and Zr/Hf = 38.38–42.09) to transitional mixed volcanic, hydrothermal, and seawater signatures in Group 2 (PrSN/YbSN 1.33–2.89, EuSN/EuSN∗ 1.33–2.5, Y/Ho = 23.94–30, LaSN/LaSN∗ 0.93–1.34, and Zr/Hf = 40–70), to mixed hydrothermal and seawater signatures in Group 3 (PrSN/YbSN 0.62–2.88, EuSN/EuSN∗ 1.30–7.15, LaSN/LaSN∗ 1.02–1.86, Y/Ho = 25.56–55, and Zr/Hf = 35–50).We interpret that silicification of volcanic rocks (Group 1) produced transitional altered volcanic rocks (Group 2), and siliceous and jaspilitic rocks (Group 3), based on preservation of delicate volcanic features. Building on this explanation, we interpret that major, trace- and rare-earth element mobility occurred during the process of silicification, during which siliceous and jaspilitic rocks (Group 3) acquired aspects of the rare-earth element geochemical signatures of marine chemical precipitates. We conclude that seafloor silicification in hydrothermal depositional settings is capable of producing rocks that resemble marine chemical precipitates such as banded iron formation, and could be a process that is widespread in the Archean. Consequently, because silicified volcanic rocks from the HMG possess mixed seawater and hydrothermal rare-earth element characteristics similar to Archean iron formations and cherts, we suggest caution must be exercised when interpreting the geochemical information preserved in metamorphosed rocks where original genesis is unknown.

Volcanism and tectonism in the southern Central Andes: Tempo, styles, and relationships

An important objective of volcanic research is to establish a cause-and-effect relationship between the age of fault kinematics and volcanic arc evolution based on structural and stratigraphic evidence. The predominant hyperarid climate in the Central Andes since Miocene times makes it a world-class area for investigating the evolution of a volcanic arc. This region records the complete development of the late Cenozoic Andean volcanic arc. This study focuses on the interpretation of volcanism in the context of recently dated tectonic structures along the southern Central Andes Volcanic Zone between 24.5° and 27° S. This segment of the arc has had a complex evolution and consists of hundreds of volcanoes, including constructional (monogenetic and polygenetic) and caldera volcanoes. By reviewing and reevaluating the geological maps in the literature, we are able to better constrain the temporal evolution of the central Chilean volcanic arc, including timing and kinematics of regional faults. Recognition of 15 Oligocene to Pleistocene ignimbrites and their sources has allowed us to define 11 caldera systems contemporaneous with effusive constructional volcanoes. The extent of the ignimbrite deposits allows them to be correlated between isolated outcrops that preserve different stratigraphic sequences, enabling the construction of a more complete and accurate volcanic stratigraphy. Two main NE-SW– and N-S–oriented thrust systems dominate the structural architecture of this segment of the arc. The first, located in the Precordillera, was active between 25 and 14 Ma and extends over 200 km to the northeast through the Pedernales-Arizaro thrust fault. Parallel to this thrust, the east-vergent Antofalla thrust fault system developed during Oligocene–Miocene times. The second system, located within the volcanic arc, includes sinuous N-S contractional structures that developed in pulses between the middle and late Miocene. There appears to be a cause-and-effect relationship between tectonic pulses and the development of volcanism, whereby changes in the upper crustal stress field lead to the generation of extensional domains. These conditions favor magma storage at upper crustal levels, thus promoting a suction-pump effect. The coexistence of both dominantly effusive constructional volcanism and explosive caldera volcanism results from the same tectonic conditions that produced shortening, as a consequence of the maximum compressive stress and conjugated extensions. In this paper, we suggest a new model that integrates the coexistence and contemporaneity of compressive structures and the widespread development of effusive constructional volcanism and explosive caldera volcanism along the Andean Oligo-Miocene volcanic arc.

Historic hydrovolcanism at Deception Island (Antarctica): implications for eruption hazards

Deception Island (Antarctica) is the southernmost island of the South Shetland Archipelago in the South Atlantic. Volcanic activity since the eighteenth century, along with the latest volcanic unrest episodes in the twentieth and twenty-first centuries, demonstrates that the volcanic system is still active and that future eruptions are likely. Despite its remote location, the South Shetland Islands are an important touristic destination during the austral summer. In addition, they host several research stations and three summer field camps. Deception Island is characterised by a Quaternary caldera system with a post-caldera succession and is considered to be part of an active, dispersed (monogenetic), volcanic field. Historical post-caldera volcanism on Deception Island involves monogenetic small-volume (VEI 2–3) eruptions such forming cones and various types of hydrovolcanic edifices. The scientific stations on the island were destroyed, or severely damaged, during the eruptions in 1967, 1969, and 1970 mainly due to explosive activity triggered by the interaction of rising (or erupting) magma with surface water, shallow groundwater, and ice. We conducted a detailed revision (field petrology and geochemistry) of the historical hydrovolcanic post-caldera eruptions of Deception Island with the aim to understand the dynamics of magma-water interaction, as well as characterise the most likely eruptive scenarios from future eruptions. We specifically focused on the Crimson Hill (estimated age between 1825 and 1829), and Kroner Lake (estimated age between 1829 and 1912) eruptions and 1967, 1969, and 1970 events by describing the eruption mechanisms related to the island’s hydrovolcanic activity. Data suggest that the main hazards posed by volcanism on the island are due to fallout, ballistic blocks and bombs, and subordinate, dilute PDCs. In addition, Deception Island can be divided into five areas of expected activity due to magma-water interaction, providing additional data for correct hazard assessment on the island.

Geochemical constraints on the link between volcanism and plutonism at the Yunshan caldera complex, SE China

The Yunshan caldera complex is part of a larger scale, ca. 2000-km-long volcanic–plutonic complex belt in the coastal region of SE China. The volcanic rocks in the caldera complex are characterized by high-silica peraluminous and peralkaline rhyolites associated with an intracaldera porphyritic quartz monzonite pluton. In this study, we present zircon U–Pb, Hf and stable O isotopes along with geochemical data of both volcanic and plutonic rocks to evaluate the potential petrogenetic link between volcanism and plutonism in the Yunshan caldera complex. SHRIMP zircon U–Pb geochronology of both volcanic and plutonic rocks yields almost identical ages ranging from 95.6 to 93.1 Ma. The peraluminous and peralkaline rhyolites show negative anomalies of Sr, P, Ti and Ba and to a lesser extent negative Nb and Ta anomalies, along with positive Rb anomalies and ‘seagull-like’ rare earth element (REE) patterns with negative Eu anomalies and low (La/Yb)N ratios. The intracaldera porphyritic quartz monzonite displays minor negative Rb, Nb, Ta, Sr, P and Ti anomalies and a positive Ba anomaly with REE patterns characterized by relatively high (La/Yb)N ratios and lack significant Eu anomalies. The peraluminous and peralkaline rhyolites and the porphyritic quartz monzonite exhibit consistent e Nd(t) of − 3.7 to − 2.2 and display zircon e Hf(t) values of − 2.1 to 3.7. They further have similar, mantle-like, zircon oxygen isotopic compositions (δ18OVSMOW mainly = 4.63 to 5.76‰). We interpret these observations to be in agreement with a crystal mush model in which the parental magma of the volcanic and plutonic rocks of the Yunshan caldera complex was likely produced by interaction of asthenosphere melts with subduction-influenced enriched mantle wedge. The peralkaline rhyolites are interpreted to represent the most differentiated magma that has subsequently experienced significant fluid–melt interactions, whereas the porphyritic quartz monzonite may be representative of the residual crystal mush. The Yunshan rhyolites typically match the geochemical characteristics of ‘hot-dry-reduced’ rhyolites indicating that, during the late Cretaceous, the tectonic setting of SE China changed from a compressional environment to an extensional environment, i.e., from an arc into a back-arc setting. Our results imply that volcanic and plutonic rocks in caldera systems may provide unique constraints on the evolution of the magmatic system in which both the erupting melt and the residual crystalline material are being preserved.

Hydrothermal activity and subsoil complexity: implication for degassing processes at Solfatara crater, Campi Flegrei caldera

The Solfatara area and its fumaroles are the main surface expression of the vigorous hydrothermal activity within the active Campi Flegrei caldera system. At depth, a range of volcanic and structural processes dictate the actual state of the hydrothermal system below the crater. The presence of a large variety of volcanic products at shallow depth (including pyroclastic fallout ash beds, pyroclastic density current deposits, breccias, and lavas), and the existence of a maar-related fault system appears to exert major controls on the degassing and alteration behavior. Adding further to the complexity of this environment, variations in permeability and porosity, due to subsoil lithology and alteration effects, may further influence fluid flow towards the surface. Here, we report results from a field campaign conducted in July 2015 that was designed to characterize the in situ physical (temperature, humidity) and mechanical (permeability, strength, stiffness) properties of the Solfatara crater subsoil. The survey also included a mapping of the surficial hydrothermal features and their distributions. Finally, laboratory measurements (porosity, granulometry) of selected samples were performed. Our results enable the discrimination of four main subsoils around the crater: (1) the Fangaia domain located in a topographic low in the southwestern sector, (2) the silica flat domain on the western altered side, (3) the new crust domain in the central area, and (4) the crusted hummocks domain that dominates the north, east, and south parts. These domains are surrounded by encrusted areas, reworked material, and vegetated soil. The distribution of these heterogeneous subsoils suggests that their formation is mostly related to (i) the presence of the Fangaia domain within the crater and (ii) a system of ring faults bordering it. The subsoils show an alternation between very high and very low permeabilities, a fact which seems to affect both the temperature distribution and surficial degassing. A large range of surface temperatures (from 25 up to 95 degrees C) has been measured across these surfaces, with the hottest spot corresponding to the mud pools, the area of new crust formation, and the crusted hummocks. In the subsoil, the distribution of temperature is more complex and controlled by the presence of coarser, and more permeable, sandy/pebbly levels. These act as preferential pathways for hot hydrothermal fluid circulation. In contrast, low permeability, fine-grained levels act as thermal insulators that remain relatively cold and hinder fluid escape to the surface. Hot gases reach the surface predominantly along (vertical) fractures. When this occurs, mound-like structures can be formed by a cracking and healing process associated with significant degassing. It is anticipated that the results presented here may contribute to an improved understanding of the hazard potential associated with the ongoing hydrothermal activity within the Solfatara crater. At this site the permeability of the near-surface environment and its changes in space and time can affect the spatial and temporal distribution of gas and heat emission. Particularly, in areas where reduction in permeability occurs, it can produce pore pressure augmentation that may result in explosive events.

Experimental and petrological constraints on long-term magma dynamics and post-climactic eruptions at the Cerro Galán caldera system, NW Argentina

Cerro Galán in NW Argentina records >3.5Myr of magmatic evolution of a major resurgent caldera complex. Beginning at 5.72Ma, nine rhyodacitic ignimbrites (68–71wt% SiO2) with a combined minimum volume of >1200km3 (Dense Rock Equivalent; DRE) have been erupted. The youngest of those ignimbrites is the eponymous, geochemically homogenous, caldera-forming 2.08±0.02Ma Cerro Galán Ignimbrite (CGI; >630km3 DRE). Following this climactic supereruption, structural and magmatic resurgence led to the formation of a resurgent dome and post-climactic lava domes and their associated pyroclastic deposits. A clear transition from amphibole to sanidine-bearing magmas occurred during the evolution of Cerro Galán and is inferred to represent a shallowing of the magma system. We test this hypothesis here using experimental phase equilibria.We conducted a series of phase equilibria experiments on the post-climactic dome lithologies under H2O-saturated conditions using cold seal Waspaloy pressure vessels with an intrinsic log fO2 of NNO+1±0.5 across a temperature-pressure range of 750–900°C and 50–200MPa (PH2O=Ptotal), respectively. Petrologic and geochemical analysis of the post-climactic lithologies shows that the natural phase assemblage (plagioclase+quartz+biotite+sanidine+Fe-Ti oxides±apatite±zircon) is stable at <50MPa (PH2O) and 805–815°C. Applying experimental results to the CGI pumice, which has the same phenocryst phase assemblage and modal abundance, whole rock and phenocryst chemistry, and overlapping temperature and fO2 as the post-climactic deposits, suggests that these pre-eruptive conditions (PH2O<50MPa) are relevant for the magmas that sourced the climactic CGI supereruption as well.Amphibole in the early Cerro Galán ignimbrites (Toconquis Group; 5.72–4.51Ma, and the Cueva Negra Ignimbrite, 3.77±0.08Ma) records crystallization across a range of pressures (500 to 200±60MPa). In the interval between the eruption of the Cueva Negra ignimbrite and the CGI (2.08±0.02Ma) the complex magma system shallowed and stalled at low pressures (<100MPa), resulting in a more simple magma reservoir configuration represented by a large-volume, geochemically homogenous magma body. The shallowing of the Cerro Galán magma system during this time explains the marked transition from amphibole to sanidine-bearing magmas and seems to characterize many large silicic caldera-forming magma systems that erupt over million year timescales to generate long-lived volcanic complexes.The post-climactic history of Cerro Galán is informed through a detailed investigation of the textural differences among the post-climactic dome lithologies, and a comparison of those textures with previously published decompression experiments. These suggest that the highly vesiculated, pumiceous clasts with rare microlites represent magma stored within the core of the lava dome that decompressed relatively rapidly (0.003–0.0003MPas−1) and evolved via closed system degassing. Resulting over-pressure of the dome may have triggered superficial explosion. In contrast, dense clasts with abundant crystalline silica precipitates represent more typical dome-forming magmas that decompressed more slowly (<0.00005MPas−1), evolved via open system degassing, and form the outer carapace of a lava dome. Integrating decompression histories with results from new phase equilibria experiments suggests that during post-climactic volcanic activity at Cerro Galán, remnant CGI dome-forming magmas ascended from the shallow magma reservoir (<4km) to motivate resurgent uplift and erupt as lava domes either explosively as vesiculated clasts or effusively as dense clasts that make up the outer structure of lava domes.

Reconnaissance survey of the Duolun ring structure in Inner Mongolia: Not an impact structure

AbstractThe Duolun basin, which is located in Inner Mongolia, China, has been proposed to be an impact structure with an apparent rim diameter of about 70, or even 170 km. The designation as an impact structure was based on its nearly circular topography, consisting of an annular moat that surrounds an inner hummocky region, and the widespread occurrences of various igneous rocks, polymict breccias, and deformed crustal rocks. Critical shock metamorphic evidence is not available to support the impact hypothesis. We conducted two independent reconnaissance field surveys to this area and studied the lithology both within and outside of the ring structure. We collected samples from all lithologies that might contain evidence of shock metamorphism as suggested by their locations, especially those sharing similar appearances with impact breccias, suevites, impact melt rocks, and shatter cones. Field investigation, together with thin‐section examination, discovered that the suspected impact melt rocks are actually Early Cretaceous and Late Jurassic lava flows and pyroclastic deposits of rhyolitic to trachytic compositions, and the interpreted impact glass is typical volcanic glass. Petrographic analyses of all the samples reveal no indications for shock metamorphic overprint. All these lines of evidence suggest that the Duolun basin was not formed through impact cratering. The structural deformation and spatial distribution pattern of the igneous rocks suggest that the Duolun basin is most likely a Jurassic–Cretaceous complex rhyolite caldera system that has been partly filled with sediments forming an annular basin, followed by resurgent doming of the central area.

Timescale of magma chamber processes revealed by U–Pb ages, trace element contents and morphology of zircons from the Ishizuchi caldera, Southwest Japan Arc

AbstractU–Pb geochronology and trace element chemistry of zircons in a microscale analysis were applied to the Ishizuchi caldera in the Outer Zone of Southwest Japan in order to estimate the timescale of the magma process, in particular, the magma differentiation. This caldera is composed mainly of ring fault complexes, major pyroclastic flow deposits, and felsic intrusion including central plutons. Using SHRIMP‐IIe, our new U–Pb zircon ages obtained from the major pyroclastic flow deposits (Tengudake pyroclastic flow deposits), granitic rocks from central plutons (Soushikei granodiorite and Teppoishigawa quartz monzonite), and rhyolite from the outer ring dike (Tenchuseki rhyolite) and the inner ring dike (Bansyodani rhyolite) are 14.80 ±0.11 Ma, 14.56 ±0.10 Ma, 14.53 ±0.12 Ma, 14.55 ±0.11 Ma and 14.21 ±0.19 Ma, respectively. Based on the U–Pb ages, the Hf contents and the REE patterns of the zircons, three stages are recognized in the evolutionary history of the magma chamber beneath the Ishizuchi caldera: (i) climactic Tengudake pyroclastic flow eruption; (ii) Tenchuseki rhyolite intrusion into the outer ring dike and central pluton intrusion; and (iii) Bansyodani rhyolite intrusion in the inner ring dike. These results indicate a magma evolution history of the Ishizuchi caldera system which took at least ca 600 kyr from the climatic caldera‐forming eruption to the post‐caldera intrusions. Our new geochronological data suggest that the Ishizuchi caldera formed as part of the voluminous and episodic magmatism that occurred in the wide zone along the Miocene forearc basin of Southwest Japan during the inception of the young Philippine Sea Plate subduction.

Zircon U–Pb dating using LA-ICP-MS: Quaternary tephras in Yakushima Island, Japan

Zircon U–Pb dating using LA-ICP-MS was applied to three Quaternary tephras in Yakushima Island, southern Japan: the Anbo Tephra, the Koseda pyroclastic flow deposit (Ksd), and the Kikai-Tozurahara (K-Tz) Tephra. The obtained U–Pb ages were 0.73±0.04Ma (error shown as 95% confidence level) for the Anbo, 0.63±0.04Ma for the Ksd, and 0.17±0.05Ma for the K-Tz. These ages are consistent with or slightly older than those reported previously, which demonstrates the applicability of the U–Pb method to date zircons as young as ~0.1Ma. The K-Tz contains ~0.7–0.6Ma zircons, assumed to be derived from the Anbo and Ksd tephras. Because the K-Tz was originated from the Kikai Caldera, ~30km north of Yakushima Island, both the Anbo and Ksd were likely also derived from the same caldera system. Therefore our data and interpretation will contribute to elucidate the magmatic history of the Kikai Caldera whose latest eruption devastated prehistoric human settlements of southern Kyushu, Japan, and provide an improved chronostratigraphic constraint for Japan.

Geothermal systems in volcanic arcs: Volcanic characteristics and surface manifestations as indicators of geothermal potential and favorability worldwide

This paper brings a global perspective to volcanic arc geothermal assessments by evaluating trends and correlations of volcanic characteristic and surface manifestation data from world power production sites in subduction zone volcanic settings. The focus of the work was to evaluate volcanic centers individually and as a group in these arcs by correlating various geologic characteristics with known potential to host electricity grade geothermal systems at the volcanic centers. A database was developed that describes key geologic factors expected to be indicative of productive geothermal systems in a global training set, which includes all 74 subduction zone volcanic centers world-wide with current or proven power production capability. Importantly, this data set only contains data from subduction zone volcanoes and contains no negative cases, limiting the populations of any statistical groups. Regardless, this is the most robust geothermal benchmark training set for magmatic-heated systems to date that has been made public. The work reported here is part of a larger project that included data collection, evaluation, correlations and weightings, fairway and favorability modeling and mapping, prediction of blind systems, and uncertainty analysis to estimate errors associated with model predictions. This first paper describes volcano characteristics, compositions and eruption ages and trends along with surface manifestation observations and temperatures as they relate to known power producing systems. Our findings show a strong correlation between the presence and size of active flank fumarole areas and installed power production. Additionally, the majority of volcanic characteristics, including long-held anecdotal correlations related to magmatic composition or size, have limited to no correlation with power production potential. Notable exceptions are correlations between greater power yield from geothermal systems associated with older (Pleistocene) caldera systems than systems hosted by Holocene calderas or non-caldera volcanic centers. Power-hosting volcanic centers that have erupted within the last 160years supply 50% of the global installed geothermal power in subduction zones, and nearly all of these systems are generally mafic (basaltic or andesitic) in average composition. Volcanic centers erupting between 160 and 900years ago are dominated by felsic volcanic systems, and provide 47% of the global power from volcanic arcs. Only 3% of geothermal power produced in subduction zones are hosted by volcanic center erupting more than 900years ago. We anticipate that these results may be able to help guide future geothermal exploration efforts.

Simultaneous eruptions from multiple vents at Campi Flegrei (Italy) highlight new eruption processes at calderas

Volcanic eruptions are typically characterized by the rise and discharge of magma at the surface through a single conduit-vent system. However, in some cases, the rise of magma can be triggered by the activation of eruptive fissures and/or vents located several kilometers apart. Simultaneous eruptions from multiple vents at calderas, not related to caldera collapse (e.g., ring faults), are traditionally regarded as an unusual phenomenon, the only historically reported examples occurring at Rabaul caldera, Papua New Guinea. Multiple venting within a caldera system is inherently difficult to demonstrate, owing partly to the infrequency of such eruptions and to the difficulty of documenting them in time and space. We present the first geological evidence that at 4.3 kyr B.P., the Solfatara and Averno vents, 5.4 km apart, erupted simultaneously in what is now the densely populated Campi Flegrei caldera (southern Italy). Using tephrostratigraphy and geochemical fingerprinting of tephras, we demonstrate that the eruptions began almost at the same time and alternated with phases of variable intensity and magnitude. The results of this study demonstrate that multi-vent activity at calderas could be more common than previously thought and volcanic hazards could be greater than previously evaluated. More generally we infer that the simultaneous rise of magma and gas along different pathways (multiple decrepitation of chamber[s]) could result in a sudden pressure rise within the sub-caldera magmatic system.

Modeling of shallow structures in the Cappadocia region using gravity and aeromagnetic anomalies

In this study, shallow structures and bodies creating gravity and magnetic anomalies in the Cappadocia Volcanic Complex region in central Anatolia were investigated in order to determine the tectonic origin and structural setting of young volcanic units. The shallow geological structures in the region are depressions filled with mainly low-density, loose volcano-clastics and ignimbrite sheets associated with the continental Neogene deposits. These units together with other volcanic products are originated from the large Neogene and Quaternary volcanoes of the central Anatolia, particularly in the Cappadocia region. At first, spectral analysis to obtain the cut-off frequencies for the high-pass filter was performed in this investigation. Then, gravity and magnetic data were high-pass filtered to remove the deep and regional effects on anomalies and to unveil only shallow structures’ effects. Subsequently, upward and downward continuations were carried out to determine how these shallow structures influence the total anomalies and their contribution in the confining total potential field. In addition, three and two dimensional gravity models (3D and 2D) of the study area were also constructed to obtain the bottom depth of shallow bodies.According to spectral analysis results, shallow structures could be separated into two groups from the power spectrums and bottom depth of deeper structure was commonly determined about 2km in gravity and magnetic spectrum, both. More shallow structure is at the depth around 0.317km according to the gravity power spectrum. Obviously, 3D and 2D models are consistent with the spectral analysis results for the deeper unit depth. A circular, large depression (70×50km2) surrounds Mount Melendiz with a 1–2.7km depth range (2km in average). Because the depressions around the central volcanoes of Mount Melendiz and Mount Hasan cover very large areas in the basin scale, the shallow and low-density volcanic units can hardly be claimed that they are the products of a large caldera system in contrast to the expectations. However, this idea, in any case, is still worth to pondering over and should be investigated with further volcanological methods.

Relationship of pyroclastic volcanism and lake-water acidification to Jehol Biota mass mortality events (Early Cretaceous, northeastern China)

Geochemical analysis of the 14.4-m-thick lacustrine succession of the Lower Cretaceous Yixian Formation (Jehol Group) has yielded new insights concerning vertebrate mass mortality events in the Lake Sihetun volcanic caldera in western Liaoning Province (northeastern China) that produced the Jehol Biota fossil lagerstätten. The long-term evolution of the caldera system resulted in a shift from felsic to mafic magma chemistry, accompanied by a reduced frequency of pyroclastic eruptions, declining hydrothermal activity, and lower lacustrine productivity. The basal Tetrapod Beds exhibit strong hydrothermal influence, as indicated by enrichments of boron (B), certain alkalis (Rb, Cs), rare-earth elements (REEs), yttrium (Y), and many metals (e.g., Co, Cr, Cu, Ge, Mo, Ni, Sb, U, V, and W), and by strongly negative molybdenum isotope compositions (δ98Mo to –2.50‰) that may record large fractionations between molybdate and thiomolybdate species in the Sihetun caldera hydrothermal system. In contrast, the overlying Fish Beds and Non-Fossiliferous Beds have an elemental and Mo-isotopic composition similar to calc-alkaline basalts (δ98Mo=–0.29±0.04‰) in the surrounding watershed, suggesting weathering of Yixian Formation volcanic rocks as the major source of sediment. During its <700-kyr-long history, Lake Sihetun was affected by four environmental cycles, each commencing with a series of pyroclastic eruptions that triggered systematic changes in lakewater chemistry. Following each eruption interval, enhanced weathering of volcanic ash in the surrounding watershed caused lakewater pH to decrease and lacustrine productivity to increase. Continued weathering of bases from basement volcanic rocks subsequently produced alkaline conditions in the lake, leading to precipitation of authigenic carbonate layers and lower lacustrine productivity. Analysis of geochemical redox proxies strongly suggests that the Lake Sihetun water column was completely oxic, in contrast to earlier inferences of a stratified anoxic water column, and that ubiquitous lamination in the lacustrine succession was due to other factors such as widespread microbial mats and/or rapid sediment deposition.

An ignimbrite caldera from the bottom up: Exhumed floor and fill of the resurgent Bonanza caldera, Southern Rocky Mountain volcanic field, Colorado

Among large ignimbrites, the Bonanza Tuff and its source caldera in the Southern Rocky Mountain volcanic field display diverse depositional and structural features that provide special insights concerning eruptive processes and caldera development. In contrast to the nested loci for successive ignimbrite eruptions at many large multicyclic calderas elsewhere, Bonanza caldera is an areally isolated structure that formed in response to a single ignimbrite eruption. The adjacent Marshall caldera, the nonresurgent lava-filled source for the 33.9-Ma Thorn Ranch Tuff, is the immediate precursor for Bonanza, but projected structural boundaries of two calderas are largely or entirely separate even though the western topographic rim of Bonanza impinges on the older caldera. Bonanza, source of a compositionally complex regional ignimbrite sheet erupted at 33.12 ± 0.03 Ma, is a much larger caldera system than previously recognized. It is a subequant structure ∼20 km in diameter that subsided at least 3.5 km during explosive eruption of ∼1000 km 3 of magma, then resurgently domed its floor a similar distance vertically. Among its features: (1) varied exposure levels of an intact caldera due to rugged present- day topography—from Paleozoic and Precambrian basement rocks that are intruded by resurgent plutons, upward through precaldera volcanic floor, to a single thickly ponded intracaldera ignimbrite (Bonanza Tuff), interleaved landslide breccia, and overlying postcollapse lavas; (2) large compositional gradients in the Bonanza ignimbrite (silicic andesite to rhyolite ignimbrite; 60%–76% SiO 2 ); (3) multiple alternations of mafic and silicic zones within a single ignimbrite, rather than simple upward gradation to more mafic compositions; (4) compositional contrasts between outflow sectors of the ignimbrite (mainly crystal-poor rhyolite to east, crystal-rich dacite to west); (5) similarly large compositional diversity among postcollapse caldera-fill lavas and resurgent intrusions; (6) brief time span for the entire caldera cycle (33.12 to ca. 33.03 Ma); (7) an exceptionally steep-sided resurgent dome, with dips of 40°–50° on west and 70°–80° on northeast flanks. Some near-original caldera morphology has been erosionally exhumed and remains defined by present-day landforms (western topographic rim, resurgent core, and ring-fault valley), while tilting and deep erosion provide three-dimensional exposures of intracaldera fill, floor, and resurgent structures. The absence of Plinian-fall deposits beneath proximal ignimbrites at Bonanza and other calderas in the region is interpreted as evidence for early initiation of pyroclastic flows, rather than lack of a high eruption column. Although the absence of a Plinian deposit beneath some ignimbrites elsewhere has been interpreted to indicate that abrupt rapid foundering of the magma-body roof initiated the eruption, initial caldera collapse began at Bonanza only after several hundred kilometers of rhyolitic tuff had erupted, as indicated by the minor volume of this composition in the basal intracaldera ignimbrite. Caldera-filling ignimbrite has been largely stripped from the southern and eastern flank of the Bonanza dome, exposing large areas of caldera-floor as a structurally coherent domed plate, bounded by ring faults with locations that are geometrically closely constrained even though largely concealed beneath valley alluvium. The structurally coherent floor at Bonanza contrasts with fault-disrupted floors at some well-exposed multicyclic calderas where successive ignimbrite eruptions caused recurrent subsidence. Floor rocks at Bonanza are intensely brecciated within ∼100 m inboard of ring faults, probably due to compression and crushing of the subsiding floor in proximity to steep inward-dipping faults. Upper levels of the floor are locally penetrated by dike-like crack fills of intracaldera ignimbrite, interpreted as dilatant fracture fills rather than ignimbrite vents. The resurgence geometry at Bonanza has implications for intracaldera-ignimbrite volume; this parameter may have been overestimated at some young calderas elsewhere, with bearing on outflow-intracaldera ratios and times of initial caldera collapse. Such features at Bonanza provide insights for interpreting calderas universally, with respect to processes of caldera collapse and resurgence, inception of subsidence in relation to progression of the ignimbrite eruption, complications with characterizing structural versus topographic margins of calderas, contrasts between intra- versus extracaldera ignimbrite, and limitations in assessing volumes of large caldera-forming eruptions. Bonanza provides a rare site where intact caldera margins and floor are exhumed and exposed, providing valuable perspectives for understanding younger similar calderas in some of the world9s most active and dangerous silicic provinces.

A complex magma reservoir system for a large volume intra- to extra-caldera ignimbrite: Mineralogical and chemical architecture of the VEI8, Permian Ora ignimbrite (Italy)

Intra-caldera settings record a wealth of information on caldera-forming processes, yet field study is rarely possible due to lack of access and exposure. The Permian Ora Formation, Italy, preserves >1000m of vertical section through its intra-caldera succession. This provides an excellent opportunity to detail its mineralogical and geochemical architecture and gain understanding of the eruption evolution and insight into the pre-eruptive magma system. Detailed juvenile clast phenocryst and matrix crystal fragment point count and image analysis data, coupled with bulk-rock chemistry and single mineral compositional data, show that the Ora ignimbrite succession is rhyolitic (72.5–77.7% SiO2), crystal-rich (~25–57%; average 43%) and has a constant main mineral population (volcanic quartz+sanidine+plagioclase+biotite). Although a seemingly homogeneous ignimbrite succession, important subtle but detectable lateral and vertical variations in modal mineralogy and bulk-rock major and trace elements are identified here.The Ora Formation is comprised of multiple lithofacies, dominated by four densely welded ignimbrite lithofacies. They are crystal-rich, typically lithic-poor (<2%), and juvenile clast-bearing (average 20%). The ignimbrite lithofacies are distinguished by variation in crystal fragment size and abundance and total lithic content. The intra-caldera stratigraphic architecture shows both localised and some large-scale lithofacies correlation, however, it does not conform to a ‘layer-cake’ stratigraphy. The intra-caldera succession is divided into two depo-centres: Southern and Northern, with proximal extra-caldera deposits preserved to the south and north of the system.The Southern and Northern intra-caldera ignimbrite successions are discriminated by variations in total biotite crystal abundance. Detailed mineralogical and chemical data records decreases across the caldera system from south to north in biotite phenocrysts in the groundmass of juvenile clasts (average 12–2%), matrix biotite (average 7.5–2%) and plagioclase crystal fragments (average 18–6%), and total crystal fragment abundance in the matrix (average 47–37%); a biotite compositional change to iron-rich (0.57–0.78 Fe); and bulk-rock element decreases in Fe2O3, MgO, P2O5, Ce, Hf, V, La and Zr, and increases in SiO2, Y and Nb, with TiO2. Together, the changes enable subtle distinction of the Southern and Northern successions, indicating that the Northern deposits are more evolved. Furthermore, the data reveals discrimination within the Northern succession, with the northwestern extra-caldera fine-crystal-rich lithofacies, having a distinct texture, componentry and composition.The componentry variation, mineralogical and chemical ranges identified here are consistent with an eruption from a heterogeneous magma system. Our results suggest that the Ora magma was likely stored in multiple chambers within a genetically related magma reservoir network. The mineralogical and chemical architecture together with stratigraphic relationships, enable interpretation of eruption sequence. Caldera eruption is proposed to have commenced in the south and progressed to the north, forming the two pene-contemporaneous caldera depressions. Moreover, this data illustrates heterogeneity and local zonation from base-to-top of the main intra-caldera and extra-caldera successions. These variations together with crystal fragment size variations between ignimbrite lithofacies support the hypothesis of a multi-vent eruption process, incremental caldera in-filling by subtly compositionally different pyroclastic flow pulses, and a lower intensity eruption style (Willcock et al., 2013, 2014).

CALDERA ACTIVITIES IN NORTH BALI, INDONESIA

One of the most serious natural hazards is largescaled Plinian eruption that forms caldera. Although the probability to have such large-scaled eruption is very low, the result will be catastrophic if it happens. Thus, it is important to clarify features of caldera system including eruption timing, scale of eruption, precursor activity, etc. With enough scientific information, we may be able to mitigate such very rare but extremely large-scaled geohazard. In Bali Island, Indonesia, there are two caldera systems; Batur caldera and Buyan-Bratan caldera. Batur caldera was previously well studied (e.g. Sutawijaya, 2009), whilst Buyan-Bratan caldera’s geological and volcanological features have not been examined at all. The Buyan-Bratan caldera is about 6 km × 11 km in size and contains three caldera lakes. Wellformed several post-caldera cones are covered by thick soil and vegetation and developed from central to southern part of the caldera. BuyanBratan caldera and post-caldera cones are thought to be older than Batur caldera activities. Geothermal manifestation is confirmed within the caldera. Geological features of the Buyan-Bratan caldera and post-caldera cones were clarified by petrographic and petrochemical analyses. Also, KAr ages were obtained for each volcanic edifice in Buyan-Bratan caldera system.&#x0D; &#x0D; Keywords: Caldera, Plinian eruption, Buyan, Batur, volcano hazards

First recorded eruption of Nabro volcano, Eritrea, 2011.

We present a synthesis of diverse observations of the first recorded eruption of Nabro volcano, Eritrea, which began on 12 June 2011. While no monitoring of the volcano was in effect at the time, it has been possible to reconstruct the nature and evolution of the eruption through analysis of regional seismological and infrasound data and satellite remote sensing data, supplemented by petrological analysis of erupted products and brief field surveys. The event is notable for the comparative rarity of recorded historical eruptions in the region and of caldera systems in general, for the prodigious quantity of SO2 emitted into the atmosphere and the significant human impacts that ensued notwithstanding the low population density of the Afar region. It is also relevant in understanding the broader magmatic and tectonic significance of the volcanic massif of which Nabro forms a part and which strikes obliquely to the principal rifting directions in the Red Sea and northern Afar. The whole-rock compositions of the erupted lavas and tephra range from trachybasaltic to trachybasaltic andesite, and crystal-hosted melt inclusions contain up to 3,000 ppm of sulphur by weight. The eruption was preceded by significant seismicity, detected by regional networks of sensors and accompanied by sustained tremor. Substantial infrasound was recorded at distances of hundreds to thousands of kilometres from the vent, beginning at the onset of the eruption and continuing for weeks. Analysis of ground deformation suggests the eruption was fed by a shallow, NW–SE-trending dike, which is consistent with field and satellite observations of vent distributions. Despite lack of prior planning and preparedness for volcanic events in the country, rapid coordination of the emergency response mitigated the human costs of the eruption.