Caldera Wall Research Articles

The Eruption of Submarine Rhyolite Lavas and Domes in the Deep Ocean – Havre 2012, Kermadec Arc

Silicic effusive eruptions in deep submarine environments have not yet been directly observed and very few modern submarine silicic lavas and domes have been described. The eruption of Havre caldera volcano in the Kermadec arc in 2012 provided an outstanding database for research on deep submarine silicic effusive eruptions because it produced 15 rhyolite (70–72 wt.% SiO2) lavas and domes with a total volume of ∼0.21 km3 from 14 separate seafloor vents. Moreover, in 2015, the seafloor products were observed, mapped and sampled in exceptional detail (1-m resolution) using AUV Sentry and ROV Jason2 deployed from R/V Roger Revelle. Vent positions are strongly aligned, defining NW-SE and E-W trends along the southwestern and southern Havre caldera margin, respectively. The alignment of the vents suggests magma ascent along dykes which probably occupy faults related to the caldera margin. Four vents part way up the steeply sloping southwestern caldera wall at 1,200-1,300 m below sea level (bsl) and one on the caldera rim (1,060 m bsl) produced elongate lavas. On the steep caldera wall, the lavas consist of narrow tongues that have triangular cross-section shapes. Two of the narrow-tongue segments are connected to wide lobes on the flat caldera floor at ∼1,500 m bsl. The lavas are characterized by arcuate surface ridges oriented perpendicular to the propagation direction. Eight domes were erupted onto relatively flat sea floor from vents at ∼1,000 m bsl along the southern and southwestern caldera rim. They are characterized by steep margins and gently convex-up upper surfaces. With one exception, the domes have narrow spines and deep clefts above the inferred vent positions. One dome has a relatively smooth upper surface. The lavas and domes all consist of combinations of coherent rhyolite and monomictic rhyolite breccia. Despite eruption from deep-water vents (most > 900 m bsl), the Havre 2012 rhyolite lavas and domes are very similar to subaerial rhyolite lavas and domes in terms of dimensions, volumes, aspect ratio, textures and morphology. They show that lava morphology was strongly controlled by the pre-existing seafloor topography: domes and wide lobes formed where the rhyolite was emplaced onto flat sea floor, whereas narrow tongues formed where the rhyolite was emplaced on the steep slopes of the caldera wall.

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.

Subaqueous strombolian eruptions, and eruption-fed pyroclastic deposits in a Paleoproterozoic rifted-arc: Hidden formation, Flin Flon, Canada

The Hidden formation of the Paleoproterozoic Flin Flon arc assemblage comprises the stratigraphic hanging wall to the volcanogenic massive sulfide (VMS) deposits at Flin Flon, Manitoba, Canada. It represents a basaltic volcanic edifice erupted within a large subsidence structure developed in a rifted-arc environment. It is predominately pillow lavas with subordinated amounts volcaniclastic rocks.Rocks that stratigraphically overly the Schist and Mandy VMS deposits ∼5 km southeast of the main Flin Flon deposits have been correlated through litho- and chemo-stratigraphy with the Hidden formation. They display evidence of pyroclastic volcanism and concomitant subsidence. Pillow lavas, and hyaloclastite are unequivocal evidence of subaqueous eruption.The Hidden formation in the Schist – Mandy area comprises mafic tuff, mafic megabreccia, basalt lava, fluidal-clast breccia, and plagioclase crystal-rich volcaniclastic lithofacies, intruded by synvolcanic dikes and sills. The megabreccia lithofacies is characterized by angular basalt clasts (1–20 m) in a matrix of finer basalt clasts and is interpreted to have been deposited by mass flows due to slumping of the caldera wall. A within flow vertical transition in the lavas from massive to pillowed flows suggests a slowing of emplacement velocity, decrease in temperature, and an increase in the viscosity during eruption. Based on the monolithic nature of the fluidal-clast breccias, and the fluidal shapes of clasts, with delicate tails and intact glassy margins, they are interpreted to be the result of strombolian eruptions. An increase in the ingestion of water in the vent(s) lead to a change from magmatic to phreatomagmatic fragmentation. Planar and cross laminations, graded beds and the abundance of broken plagioclase crystals, in combination with scoria and lithic clasts suggest the mafic tuff and plagioclase crystal-rich volcaniclastic lithofacies were deposited by subaqueous, eruption-fed density currents derived from a phreatomagmatic eruption.Detailed mapping of the volcanic lithofacies in the Schist – Mandy area has facilitated the identification of synvolcanic faults defining inner caldera margins and volcanic vents. These structures when traced downwards through stratigraphy define structural corridors corresponding with the Schist and Mandy VMS deposits, indicating they represent long-lived structures that also acted as magmas and fluid pathways.

Volcanic and hydrothermal processes in submarine calderas: The Kulo Lasi example (SW Pacific)

The study area is located at the transition between the northern end of the Tonga Trench and the North Fiji fracture zone, where tectonic movements are reputed to be the fastest in the world. To the southeast of Futuna Island, a broad area of volcanism occurs within a region characterized by a change in the tectonic fabric between a NE-SW oriented volcanic graben and the N-S oriented Alofi ridge. In 2010, the active volcano Kulo Lasi, which represents the most recent volcanic episode in the Futuna area, was discovered in the center of this extensive volcanic zone. Kulo Lasi is a 20 km diameter shield volcano that rises 400 m above the seafloor. It is composed of basaltic to trachy-andesitic lava with no obvious geochemical affinity with the Tonga subduction that occurs 500 km to the east. The central caldera is 5 km in diameter and 300 m deep and is located at a water depth of 1500 m. Diving operations with the submersible Nautile and high-resolution AUV mapping, have revealed the presence of numerous active and inactive hydrothermal fields on the floor and the walls of the caldera. Four tectono-volcanic stages can be distinguished at Kulo Lasi caldera. In stage 1, the shield volcano is built. Annular reverse faults develop at the summit and control circulation of water/rock-dominated hydrothermal fluids and high-temperature alteration of rocks along the nascent normal faults. Mixing of hydrothermal fluids with seawater is favored along normal superficial faults, leading to the formation of low-temperature Fe/Mn mineralization at the summit of the volcano. During stage 2, the caldera collapse, gradually revealing outcrops of the altered and mineralized zones formed during Stage 1. As the magma chamber cools and collapses, less heat is available. As a result, medium to low-temperature (<100 °C) Fe/Si deposits form on the floor of the caldera. In stage 3, refilling and ascent of the magma chamber at depth promote the uplift of a central resurgent dome devoid of recent lava emission. The depth of the magma chamber under the dome is estimated to be 1.6 km. Annular dykes feeding the new lava flows are controlled by the deep reverse faults at the outer rim of the caldera floor. The eruptive events are accompanied by short-lived emission of magmatic fluids, rich in SO2, materialized by the presence of native sulfur depositions on the surface of the most recent lava flows. During stage 4 (present day), black smoker sulfide chimneys, controlled by the deep reverse faults, form at the surface of the most recent lava flows at the outer part of the caldera floor. The fluids emitted result from the mixing of a deep fluid of the water/rock reaction type and of shallow seawater superheated in contact with the hot dykes. Hydrogen is generated during reaction of seawater with the hot dikes and H2 is enriched in the mixed fluids. Active (43 °C) siliceous chimneys at the base of the caldera walls and low-temperature vents (6 °C) perched on the caldera walls suggests a decrease in the venting temperature at a distance from the floor of the caldera. In chimneys, the sphalerite/pyrrhotite/isocubanite/barite paragenesis, probably linked to the high H2 concentrations in the fluids, indicate unusual reducing conditions for a back-arc. The sulfide mineralization also has unusual trace element concentrations. The concentration in Pb and Ba appears characteristic of back-arc environments while the enrichment in Co, Se, and Sn is more common to mineralization associated with basalt in mature back-arcs and associated with ultramafic rocks on mid ocean ridges.

Dynamics of the Askja caldera July 2014 landslide, Iceland, from seismic signal analysis: precursor, motion and aftermath

Abstract. Landslide hazard motivates the need for a deeper understanding of the events that occur before, during, and after catastrophic slope failures. Due to the destructive nature of such events, in situ observation is often difficult or impossible. Here, we use data from a network of 58 seismic stations to characterise a large landslide at the Askja caldera, Iceland, on 21 July 2014. High data quality and extensive network coverage allow us to analyse both long- and short-period signals associated with the landslide, and thereby obtain information about its triggering, initiation, timing, and propagation. At long periods, a landslide force history inversion shows that the Askja landslide was a single, large event starting at the SE corner of the caldera lake at 23:24:05 UTC and propagating to the NW in the following 2 min. The bulk sliding mass was 7–16 × 1010 kg, equivalent to a collapsed volume of 35–80 × 106 m3. The sliding mass was displaced downslope by 1260 ± 250 m. At short periods, a seismic tremor was observed for 30 min before the landslide. The tremor is approximately harmonic with a fundamental frequency of 2.3 Hz and shows time-dependent changes of its frequency content. We attribute the seismic tremor to stick-slip motion along the landslide failure plane. Accelerating motion leading up to the catastrophic slope failure culminated in an aseismic quiescent period for 2 min before the landslide. We propose that precursory seismic signals may be useful in landslide early-warning systems. The 8 h after the main landslide failure are characterised by smaller slope failures originating from the destabilised caldera wall decaying in frequency and magnitude. We introduce the term “afterslides” for this subsequent, declining slope activity after a large landslide.

Structure, Seismicity, and Accretionary Processes at the Hot Spot‐Influenced Axial Seamount on the Juan de Fuca Ridge

AbstractAxial Seamount is the most volcanically active site of the northeast Pacific, and it has been monitored with a growing set of observations and sensors during the last two decades. Accurate imaging of the internal structure of volcanic systems is critical to better understand magma storage processes and to quantify mass and energy transport mechanisms in the crust. To improve the three‐dimensional velocity structure of Axial Seamount, we combined 469,891 new traveltime arrivals, from 12 downward extrapolated seismic profiles, with 3,962 existing ocean‐bottom‐seismometers traveltime arrivals, into a joint tomographic inversion. Our approach reveals two elongated magma reservoirs, with melt fraction up to 65%, representing an unusually large volume of melt (26–60 km3), which is likely the result of enhanced magma supply from the juxtaposition of the Cobb hot spot plume (0.26–0.53 m3/s) and the Axial spreading segment (0.79–1.06 m3/s). The tomographic model also resolves a subsided caldera floor that provides an effective trap for ponding lava flows, via a “trapdoor” mechanism. Our model also shows that Axial's extrusive section is thinnest beneath the elevated volcano, where anomalously thick (11 km) oceanic crust is present. We therefore suggest that focused and enhanced melt supply predominantly thickens the crust beneath Axial Seamount through diking accretion and gabbro crystallization. Lastly, we demonstrate that our three‐dimensional velocity model provides a more realistic starting point for relocating the local seismicity, better resolving a network of conjugate outward and inward dipping faults beneath the caldera walls.©2018. The Authors.

Thermal power of Grímsvötn, Iceland, from 1998 to 2016: Quantifying the effects of volcanic activity and geothermal anomalies

Grímsvötn lies beneath the centre of the Vatnajökull ice cap, Iceland, and has for centuries been one of the most geothermally active calderas in the world as well as being Iceland's most active volcano. Calorimetric studies of its heat output have suggested long-term heat release of 2–4 GW. We have performed a detailed study of the heat release at Grímsvötn over the period 1998–2016, which includes the eruptions of 1998, 2004 and 2011. Annual mapping of the ice surface is used to monitor ice volume changes, combined with results of mass-balance monitoring over this same period. We estimate an average heat release of 1800 ± 200 MW, whereof about ~1200 MW is the base geothermal heat flux. The remaining ~600 MW is an average over the study period, composed of peaks above the base flux from melting during eruptions and geothermal anomalies. The most intense melting occurred at eruption sites during eruptions. Less intense signals lasting several months to years were due to increased geothermal melting creating new ice cauldrons and deepening pre-existing ones. Such thermal anomalies were observed both as precursors to the eruptions, and in the 2–3 years following. The strongest signal followed the 1998 eruption which produced an average of 650 MW in the two years following. After the 2004 eruption a thermal anomaly of 500 MW was observed in the following years, and an average signal of 450 MW followed the 2011 eruption. These thermal signals demonstrate a post-eruption link to increased geothermal activity at the caldera walls.

Characteristics of debris avalanche deposits inferred from source volume estimate and hummock morphology around Mt. Erciyes, central Turkey

Abstract. Debris avalanches caused by volcano sector collapse often form characteristic depositional landforms such as hummocks. Sedimentological and geomorphological analyses of debris avalanche deposits (DADs) are crucial to clarify the size, mechanisms, and emplacement of debris avalanches. We describe the morphology of hummocks on the northeastern flank of Mt. Erciyes in Kayseri, central Turkey, likely formed in the late Pleistocene. Using a remotely piloted aircraft system (RPAS) and the structure-from-motion and multi-view stereo (SfM–MVS) photogrammetry, we obtained high-definition digital elevation model (DEM) and orthorectified images of the hummocks to investigate their geometric features. We estimated the source volume of the DAD by reconstructing the topography of the volcano edifice using a satellite-based DEM. We examined the topographic cross sections based on the slopes around the scar regarded as remnant topography. Spatial distribution of hummocks is anomalously concentrated at a certain distance from the source, unlike those that follow the distance–size relationship. The high-definition land surface data by RPAS and SfM revealed that many of the hummocks are aligned toward the flow direction of the debris avalanche, suggesting that the extensional regime of the debris avalanche was dominant. However, some displaced hummocks were also found, indicating that the compressional regime of the flow contributed to the formation of hummocks. These indicate that the flow and emplacement of the avalanche were constrained by the topography. The existing caldera wall forced the initial eastward flow to move northward, and the north-side caldera wall forced the flow into the narrow and steepened outlet valley where the sliding debris underwent a compressional regime, and out into the unconfined terrain where the debris was most likely emplaced on an extensional regime. Also, the estimated volume of 12–15 × 108 m3 gives a mean thickness of 60–75 m, which is much deeper than the reported cases of other DADs. This suggests that the debris avalanche must have flowed further downstream and beyond the current DAD extent. Assessments of the DAD incorporating the topographic constraints can provide further insights into the risk and mitigation of potential disasters in the study area.

Proposal of a new idea for designating hazard areas based on travel distance of sediment and woody debris discharged from rainfall－induced landslides in Aso caldera walls

Proposal of a new idea for designating hazard areas based on travel distance of sediment and woody debris discharged from rainfall－induced landslides in Aso caldera walls

Probabilistic-numerical assessment of pyroclastic current hazard at Campi Flegrei and Naples city: Multi-VEI scenarios as a tool for "full-scale" risk management.

The Campi Flegrei volcanic field (Italy) poses very high risk to the highly urbanized Neapolitan area. Eruptive history was dominated by explosive activity producing pyroclastic currents (hereon PCs; acronym for Pyroclastic Currents) ranging in scale from localized base surges to regional flows. Here we apply probabilistic numerical simulation approaches to produce PC hazard maps, based on a comprehensive spectrum of flow properties and vent locations. These maps are incorporated in a Geographic Information System (GIS) and provide all probable Volcanic Explosivity Index (VEI) scenarios from different source vents in the caldera, relevant for risk management planning. For each VEI scenario, we report the conditional probability for PCs (i.e., the probability for a given area to be affected by the passage of PCs in case of a PC-forming explosive event) and related dynamic pressure. Model results indicate that PCs from VEI<4 events would be confined within the Campi Flegrei caldera, PC propagation being impeded by the northern and eastern caldera walls. Conversely, PCs from VEI 4–5 events could invade a wide area beyond the northern caldera rim, as well as part of the Naples metropolitan area to the east. A major controlling factor of PC dispersal is represented by the location of the vent area. PCs from the potentially largest eruption scenarios (analogous to the ~15 ka, VEI 6 Neapolitan Yellow Tuff or even the ~39 ka, VEI 7 Campanian Ignimbrite extreme event) would affect a large part of the Campanian Plain to the north and the city of Naples to the east. Thus, in case of renewal of eruptive activity at Campi Flegrei, up to 3 million people will be potentially exposed to volcanic hazard, pointing out the urgency of an emergency plan. Considering the present level of uncertainty in forecasting the future eruption type, size and location (essentially based on statistical analysis of previous activity), we suggest that appropriate planning measures should face at least the VEI 5 reference scenario (at least 2 occurrences documented in the last 10 ka).

High-resolution K-Ar dating of a complex magmatic system: The example of Basse-Terre Island (French West Indies)

For the first time in the Lesser Antilles volcanic arc, we combine for one island, 123K-Ar ages, with 133 major and trace elements geochemical analyses, in order to better constrain the volcanic history of Basse-Terre Island. In this study, nine new ages have been obtained from the southern part of the island, and complemented with eighty-three new major and trace element analyses of samples collected across the island. The southern part of Basse-Terre Island has been the loci of volcanic activity since the last 200kyr. It is characterized by the construction of the Grande-Découverte Volcanic Complex (GDVC) composed by the Grande-Découverte – Soufrière (GDS) and the Trois-Rivières Madeleine Field (TRMF). After the onset of construction at least at 205±28ka, the GDVC displays strikingly continuous activity between 140±13 and 56±3ka, followed by a 30kyr volcanic hiatus, which is coeval with the hiatus also observed for the TRMF activity. Two new ages of 125±14 and 140±13ka obtained on a lava flow from the Grande-Découverte caldera wall suggest the presence of a depression, resulting of a major flank collapse and/or explosive event, before 140ka. Finally, a new age of 9±6ka, obtained from outcrops exposed on the edge of the Class River, in the north of the GDVC, allows us to calculate channel incision rates between 11 and 56mm/yr. These values are consistent with incision rates determined on other volcanic islands with similar climates.In a broad sense, the petrology and geochemistry of Basse-Terre Island rocks appear fairly homogeneous, with mainly andesite and basaltic-andesite rocks and typical features of volcanic-arc lavas. Nevertheless, in detail, various magmatic processes can be discerned. Most variations are principally controlled by crystal-melt fractionation-accumulation, but major and trace elements also highlight episodic magmatic recharge, involving magma mixing. There are also indications for assimilation of crustal rocks with continental affinity, as well as mantle input of slab-derived fluids. Trace element ratios suggest the presence of at least two different magmatic sources characterized by different partial melting rates and different continental contributions for Basse-Terre Island. Different massifs show a bimodal behavior, with the Basal Complex, the Axial Chain (Piton de Bouillante and Southern Axial Chain), the Monts-Caraïbes volcanoes and the Sans-Toucher volcano in the first group, and the Septentrional Chain, and the Grande-Découverte Volcanic Complex (GDS and TRMF) in the second. Given the unique amount of time-constrained geochemical data, this study provides a complete and detailed investigation of volcanic evolution in the central part of the Lesser Antilles active arc.

Eigenvector of gravity gradient tensor for estimating fault dips considering fault type

The dips of boundaries in faults and caldera walls play an important role in understanding their formation mechanisms. The fault dip is a particularly important parameter in numerical simulations for hazard map creation as the fault dip affects estimations of the area of disaster occurrence. In this study, I introduce a technique for estimating the fault dip using the eigenvector of the observed or calculated gravity gradient tensor on a profile and investigating its properties through numerical simulations. From numerical simulations, it was found that the maximum eigenvector of the tensor points to the high-density causative body, and the dip of the maximum eigenvector closely follows the dip of the normal fault. It was also found that the minimum eigenvector of the tensor points to the low-density causative body and that the dip of the minimum eigenvector closely follows the dip of the reverse fault. It was shown that the eigenvector of the gravity gradient tensor for estimating fault dips is determined by fault type. As an application of this technique, I estimated the dip of the Kurehayama Fault located in Toyama, Japan, and obtained a result that corresponded to conventional fault dip estimations by geology and geomorphology. Because the gravity gradient tensor is required for this analysis, I present a technique that estimates the gravity gradient tensor from the gravity anomaly on a profile.

浅間前掛火山山頂部と黒斑火山崩壊カルデラ壁に記録された火砕噴火による安山岩質溶結火砕丘の形成(＜特集＞火山噴火史解明のための露頭データベース構築の検討(2))

浅間前掛火山山頂部と黒斑火山崩壊カルデラ壁に記録された火砕噴火による安山岩質溶結火砕丘の形成(＜特集＞火山噴火史解明のための露頭データベース構築の検討(2))

A Major Change in the Stratigraphy of the Santorini Volcano in Greece

Two prominent and similar pumice series were described on Thera in 1879— the Upper and Lower Pumice Series (UPS and LPS). Since then, geologists have treated the two series separately because they seemingly occurred at distinct levels and had different ages. Here we show that these two pumice series are identical; there is no LPS on Santorini. All stratigraphic conclusions based on the LPS from Santorini should therefore be discarded. The water-filled Santorini caldera with its steep inner slopes existed before the eruption. Volcano-tectonic effects in connection with caldera formation created concentric terraces that were mantled by the products of the Late Bronze Age (LBA) eruption. Subsequent erosion only left remnants of the mantle behind. Topographic effects followed by slumping during sedimentation caused confusion of the stratigraphy on the caldera wall. Our results are supported by geological, paleontological and archaeological evidence. Furthermore, the caldera with its minerals, pigments, harbours and hot springs was accessible for the Thereans. This reinterpretation opens new perspectives for archaeological research. The catastrophic LBA eruption (previously called the Minoan eruption) destroyed a flourishing culture on Santorini and impacted neighbouring cultures around 1613 BC.

Melt-rock interaction near the Moho: Evidence from crystal cargo in lavas from near-ridge seamounts

The Taney Seamounts are a NW-SE trending linear, near mid-ocean ridge chain consisting of five volcanoes located on the Pacific plate 300km west of San Francisco, California. Taney Seamount-A, the largest and oldest in the chain, is defined by four well-exposed calderas, which expose previously infilled lavas. The calderas can be differentiated in time by their cross-cutting relationships, creating a relative chronology. The caldera walls and intracaldera pillow mounds were sampled systematically by a remotely operated vehicle (ROV) to obtain stratigraphically-controlled samples, a unique aspect of this study.The geochemistry of the seamount varies from more differentiated to more primitive with time (6.2–8.6wt.% MgO), suggesting that the sub-caldera reservoir is open and undergoes periodic collapse, replenishment, crystallization, and eruption. The youngest and least differentiated lavas entrained a crystal cargo of plagioclase (An80–90) with melt inclusion volatile saturation pressures indicating entrapment in the lower oceanic crust and upper mantle (6–12km, with 45% between 8 and 10km below the sea floor). Melt inclusions exhibit high Al2O3, low SiO2, positive Sr and Eu anomalies and negative Zr and Nb anomalies when normalized to typical Pacific mid-ocean ridge basalt (MORB). In comparison, the host lavas exhibit positive Sr anomalies, but no concurrent Zr, and Nb anomalies. Based on thermodynamic modeling using alphaMELTS, we develop a melt-rock interaction model defined by melting and assimilation of plagioclase-rich cumulates by hot, primitive mantle-derived melts. Significantly, the variability of the negative Zr and Nb anomalies cannot be explained by either cumulate melting or AFC alone. We propose that the melt inclusions record the interaction between cumulate partial melts and the assimilating melt, demonstrating the importance of cumulate melting during the assimilation process. Later percolating melts underwent diffusive interaction with, and entrained, recrystallized plagioclase cumulates resulting in the positive Sr signal without the concurrent Eu, Zr, or Nb anomalies observed in the host lavas. These results demonstrate that melt-rock interaction at the lower crust or upper mantle is an important process at Taney Seamount-A, and potentially other magmatic systems associated with seamount chains and ridge axes.

Postcaldera volcanism and hydrothermal activity revealed by autonomous underwater vehicle surveys in Myojin Knoll caldera, Izu‐Ogasawara arc

AbstractMyojin Knoll caldera, one of the submarine silicic calderas lying on the volcanic front of the northern Izu‐Ogasawara arc, has attracted increasing attention since the discovery of a large hydrothermal field called the Sunrise deposit. Although numerous submersible surveys have been conducted in Myojin Knoll caldera, they have not sufficiently explored areas to produce a complete picture of the caldera and understand the origin of the Sunrise deposit. We conducted comprehensive deep‐sea surveys using an autonomous underwater vehicle and obtained high‐resolution bathymetric and magnetic data and sonar images from ~70% of the caldera. The detailed bathymetric map revealed that faulting and magma eruptions, possibly associated with an inflation‐deflation cycle of the magma reservoir during postcaldera volcanism, had generally occurred in the caldera wall. The main dome of the central cone was covered with lava flows and exhibits exogenous growth, which is unusual for rhyolitic domes. The magnetization distribution in the central cone indicates preferential magma intrusion along a NW‐SE direction. It is presumed that magma migrated along this direction and formed a rhyolite dome at the foot of the southeastern caldera wall, where the Sunrise deposit occurs. The Sunrise deposit is composed mainly of three ridges extending in slope directions and covers ~400 × ~400 m. Magnetization reduction in the deposit area is small, indicating that the alteration zone beneath the Sunrise deposit is slanting rather than vertical. It is presumed that several slanting and near‐vertical volcanic vents serve as pathways of hydrothermal fluid in Myojin Knoll caldera.

Reconstructing the evolution of an eroded Miocene caldera volcano (Yamanlar volcano, İzmir, Turkey)

The Miocene Yamanlar composite volcano is located in the central part of a shear zone in western Turkey. The volcano's deeply-eroded interior provides excellent three-dimensional exposure of a faulted caldera-floor and caldera-fill rocks as well as surrounding extracaldera ignimbrites. We present a much-revised stratigraphy and geological map of Yamanlar in order to quantify the evolutionary stages of the volcano. The Yamanlar volcanic cone was composed of >800m of basaltic-andesite to andesite lavas and lava domes. The volcano underwent at least one phase of caldera formation associated with an explosive eruption that deposited an ignimbrite sheet within and outside the caldera. Lithofacies architecture analysis is applied to the proximal and medial exposures of the Early-Middle Yamanlar Formation, which occurs outside of the caldera. Field evidence of the succession indicates a caldera-forming eruption. Our results indicate that the formation of the Yamanlar caldera resulted from one major catastrophic eruption that generated several sustained pyroclastic density currents (PDCs) subdivided by fall deposits with sharp contacts. The ignimbrite sheet is composed of four flow units. The presence of numerous coarse-grained lithic-rich horizons within the ignimbrite sheet is consistent with caldera subsidence. Post-caldera volcanism is indicated by intrusions and lava domes erupted along the inferred caldera-bounding faults, some of which record ~90m of displacement. Widespread, coarse-grained breccias that overlie the ignimbrite sheet are interpreted as debris avalanche deposits resulting from gravitational failure of the flanks of the volcano or the caldera wall during or after caldera subsidence.

平成28年（2016年）熊本地震によって南阿蘇村周辺域で発生した斜面災害

The Kumamoto earthquake (Mj 7.3) on April 16, 2016 triggered numerous landslides in and around Minamiaso Village, which is located at the western part of Aso caldera, southwestern Japan. The landslides were divided into two types: landslides occurring at steep caldera walls and landslides generated on the slopes of post-caldera central cones of Aso Volcano. Several landslides occurred on slopes steeper than 25° at the northwestern to western caldera walls, which comprise pre-Aso volcanic rocks (lavas and pyroclastics). The largest landslide (ca. 300 m high, 130–200 m wide) occurred on the western caldera wall, damaging National Route 57 and the Hohi line of the Japan Railway. Because a clear rupture surface could not be observed, unstable blocks which had been divided by cracks, were likely to collapse due to the intense earthquake on April 16. At the post-caldera central cones of Aso Volcano, earthquake-induced landslides were generated not only on steep slopes but also on slopes gentler than 10°. They occurred in unconsolidated superficial tephra deposits overlying lavas and agglutinates, and the thickness of the slides usually ranged from 4 to 8 m. The sliding masses traveled long distances (＜600 m), compared to small differences in elevation. The deposits were composed of tephra blocks of a few meters and there was no evidence that they were transported by water. These facts suggest that some landslides mobilized rapidly into debris avalanches, traveling a few hundred meters. The associated debris avalanche resulted in five casualties and severe damage to houses at the foot of the Takanoobane lava dome. The characteristics of the April 16, 2016 earthquake-induced landslides differ from those of rainfall-induced landslides in July 2012, June 2001, and July 1990 at Aso Volcano, and provide important information for preventing or mitigating future landslide disasters in the Aso caldera region.

Chapter 7 Eruptive history and evolution of Sete Cidades Volcano, São Miguel Island, Azores

Abstract Sete Cidades is an active central volcano on the western part of São Miguel. The geological record reveals that subaerial activity started more than 250 ka ago. Stratigraphic units defined for Sete Cidades deposits reflect major events in the history of the volcano and are organized into two main groups: the Inferior Group and the Superior Group. Caldera formation resulted from three major paroxysmal events that occurred at about 36, 29 and 16 ka ago. Analysis of the eruptive history of Sete Cidades shows that effusive or moderately explosive eruptions, of Hawaiian and/or Strombolian styles, were located on the slopes of the central volcano. Conversely, trachytic explosive activity is mostly centred inside the caldera involving, in a first stage, predominantly Plinian and sub-Plinian phenomena, changing about 5 ka ago to a dominant hydromagmatic style. Trachytic effusive eruptions are represented by domes and associated lava flows that crop out in the inner caldera walls and on the western slopes of the volcano. Offshore submarine activity is represented by the historic Surtseyan eruptions of 1638 and 1811. In the last 5 ka Sete Cidades was the most active central volcano in the Azores with 17 explosive eruptions predominantly with hydromagmatic character.

Tavui Volcano: neighbour of Rabaul and likely source of the Middle Holocene penultimate major eruption in the Rabaul area

A major, geologically youthful, submarine caldera volcano, Tavui, was discovered in the Rabaul area of Papua New Guinea in 1986. Tavui Volcano has lateral dimensions of 9 to 10 km, slightly smaller than those of Rabaul Volcano, but Tavui’s caldera is much deeper than the nested caldera complex at Rabaul, and in general, its escarpment walls are very steep. The two caldera systems are essentially silicic and are separated by a zone of dominantly basalt-andesite stratovolcanoes. Rock samples dredged from Tavui have low- to medium-K contents, in contrast to the medium- to high-K rocks of Rabaul, indicating that the two systems have evolved separately and that the chemical, and perhaps physical, conditions within these neighbouring systems are different. Tavui is the likely source of the 6.9 ka BP Raluan Ignimbrite, the penultimate major eruption deposit in the Rabaul area. The Raluan Ignimbrite is rhyolitic and has geochemical characteristics incompatible with those of products from Rabaul Volcano. On the other hand, there is a close match between the geochemistry of the Raluan Ignimbrite and that of rhyolitic samples dredged from Tavui Caldera. The much older (≈79 ka) Tokudukudu Ignimbrite, which is also rhyolitic, is slightly more K-rich than both the Raluan Ignimbrite and rhyolites dredged from Tavui Caldera, but in general, its geochemical characteristics are similar to those of Tavui rhyolites and, therefore, is considered to be a possible product of Tavui. The recognition that Tavui was the likely source of the penultimate major eruption and of at least one other significant eruption in the Rabaul area markedly changes the perceptions of local volcanic hazard. In addition, Tavui’s potential for generation of tsunami is acknowledged, not just in association with volcanic eruptions but also from earthquake-related and possibly spontaneous collapse of parts of the steep caldera walls. The presence of Tavui greatly increases the net geologic hazard in the Rabaul area.