Stratovolcano Research Articles

Chapter 3.1a Antarctic Peninsula and South Shetland Islands: volcanology

Abstract The voluminous continental margin volcanic arc of the Antarctic Peninsula is one of the major tectonic features of West Antarctica. It extends from the Trinity Peninsula and the South Shetland Islands in the north to Alexander Island and Palmer Land in the south, a distance of c. 1300 km, and was related to east-directed subduction beneath the continental margin. Thicknesses of exposed volcanic rocks are up to c. 1.5 km, and the terrain is highly dissected by erosion and heavily glacierized. The arc was active from Late Jurassic or Early Cretaceous times until the Early Miocene, a period of climate cooling from subtropical to glacial. The migration of the volcanic axis was towards the trench over time along most of the length of the arc. Early volcanism was commonly submarine but most of the volcanism was subaerial. Basaltic–andesitic stratocones and large silicic composite volcanoes with calderas can be identified. Other rock associations include volcaniclastic fans, distal tuff accumulations, coastal wetlands and glacio-marine eruptions. Other groups of volcanic rocks of Jurassic age in Alexander Island comprise accreted oceanic basalts within an accretionary complex and volcanic rocks erupted within a rift basin along the continental margin that apparently predate subduction.

Chapter 5.4a Marie Byrd Land and Ellsworth Land: volcanology

Abstract Nineteen large (2348–4285 m above sea level) central polygenetic alkaline shield-like composite volcanoes and numerous smaller volcanoes in Marie Byrd Land (MBL) and western Ellsworth Land rise above the West Antarctic Ice Sheet (WAIS) and comprise the MBL Volcanic Group (MBLVG). Earliest MBLVG volcanism dates to the latest Eocene (36.6 Ma). Polygenetic volcanism began by the middle Miocene (13.4 Ma) and has continued into the Holocene without major interruptions, producing the central volcanoes with 24 large (2–10 km-diameter) summit calderas and abundant evidence for explosive eruptions in caldera-rim deposits. Rock lithofacies are dominated by basanite and trachyte/phonolite lava and breccia, deposited in both subaerial and ice-contact environments. The chronology of MBLVG volcanism is well constrained by 330 age analyses, including 52 new 40 Ar/ 39 Ar ages. A volcanic lithofacies record of glaciation provides evidence of local ice-cap glaciation at 29–27 Ma and of widespread WAIS glaciation by 9 Ma. Late Quaternary glaciovolcanic records document WAIS expansions that correlate to eustatic sea-level lowstands (MIS 16, 4 and 2): the WAIS was +500 m at 609 ka at coastal Mount Murphy, and +400 m at 64.7 ka, +400 m at 21.2 ka and +575 m at 17.5 ka at inland Mount Takahe.

Volcanic processes and magmatic evolution of Tianchi volcano, Changbaishan

Abstract The Changbaishan volcanic field located on the Gaima (Gaema, Gaiman) Plateau has witnessed plateau-forming eruptions along with the uplift of the Gaima Plateau. The Tianchi basaltic lava shield volcano was formed at the main peak of Changbaishan, with cone construction eruptions that formed a huge and steep trachytic composite cone on the gentle lava shield. At the peak of the Millennium Eruption (ME), the height of the eruption column reached 25 km and the bulk volume of tephra was about 120 km 3 . The ME eventually formed Tianchi caldera, after which several eruptions occurred, albeit of a much smaller scale. The magmas involved in the shield-forming eruptions are characterized by both alkalic series trachybasalt and basaltic trachyandesite and subalkalic tholeiite and basaltic andesite. In the cone-construction and ignimbrite-forming eruption stages, the magma is completely composed of alkalic series trachyte and comendite. The largest negative Eu anomalies observed in ME magmas indicate that plagioclase was strongly crystallized and differentiated.

Evidences of an unknown debris avalanche event (

Azufral volcano is an active composite volcano located in SW Colombia, whose edifice is mostly formed by pyroclastic deposits and lava flows. Recent road works in the region, NE of the volcano, revealed unknown deposits, one of them related to a debris avalanche event. This deposit, herein named San Roque debris avalanche deposit, outcrops along a 13 km section in the Túquerres - Samaniego road. Using this long exposure, the deposit was studied through sedimentological, grain size, microtextural and mineralogical analyses. The deposit is massive, varicolored, poorly sorted, with a predominance of gravel and sand-sized clasts; it contains angular megaclasts some of them with jigsaw cracks, and exhibits injections, faults, and horsts-and-graben structures. Surface microtextures in some clasts evidence collisions and shearing interactions among them, while mineralogy evidences the collapsed material involved in the avalanche. Based on stratigraphic relationships, the collapse occurred between 0.58 Ma and ~18 ka. The San Roque debris avalanche deposit covers an area of 56.01 km2, has an estimated volume of 0.51 km3, an approximated runout of 11 km, and an apparent friction coefficient H/L of 0.109; these parameters are indicative of a large and very mobile flow. Its main transport mechanism responded to a dynamic disintegration of the larger clasts and a subsequent interaction by contact or collision (inertial-granular flow model), although some zones moved in a cushioned way remaining coherent over long distances (multi-shear model).

Guidelines for digital geological maps of Pliocene-Holocene composite volcanoes: A contribution from Colombia

Volcanic geological maps provide significant data for volcanic hazard assessment, exploration of mineral-energetic resources, physical volcanology and petrological studies, as well as geophysical models. However, the lack of consensus about the best graphic representation of volcanic geological maps and the most efficient organizational structure of spatial data contained in these maps, has resulted in a variability of representations and data storage. Here we propose some guidelines to unify the representation of composite volcanoes geological maps that may facilitate the communication within a particular country and across political boundaries by reaching a common language.In this paper, we share the advances obtained in Colombia by the Servicio Geológico Colombiano (Colombian Geological Survey), through searching for improved cartographic strategies and efficient representations of mapped volcanogenic units in the North-Andean setting. The main objective of this paper is to provide a guideline for the digital representation of Pliocene-Holocene composite volcanoes geological maps, applicable elsewhere, that displays a comprehensive presentation based on an accurate and qualified geodatabase package. This contribution offers: (i) a catalogue of symbols for the representation of geological and geomorphological elements; (ii) a catalogue of colors and patterns for the representation of the different lithostratigraphic units; (iii) a graphic distribution prototype for the information within the resulting map sheet, and (iv) a geodatabase structure with its corresponding catalogue of objects. With this initiative, we aim to begin an international collaborative effort on reaching a common understanding of best practices in the representation and systematic digital organization of thematic maps of composite volcanoes.

Forced vibration analysis of composite-geometrically exact elliptical cone helices via mixed FEM

In this pioneering study, the cross-sectional warping included transient response and normal/shear stress components of composite elliptical and elliptical cone helices over exact axis geometry are investigated using a mixed FEM. The transient analysis is performed using the Newmark time integration algorithm with or without the amplitude decay factor. The constitutive equations of composite curved rods are derived from three-dimensional elasticity theory. A displacement-type finite element formulation computing the warping-included torsional rigidity is incorporated with the mixed finite element formulation. The curvatures and displacement-type finite elements are used to estimate the normal and shear stress distributions on the respective cross-sections. The maximum normal/shear stresses of a composite straight beam are compared with the literature. An excellent agreement is obtained for the results of an exact elliptical cone helix under dynamic loads compared to the results of 3D solid finite elements. During the implementation of the time integration scheme, the first and second time derivatives of forces and moments are preserved, and their time histories are discussed. Finally, the influences of helix geometry, lamination, and the ratios of material constants on the transient response besides the stresses are investigated. All the numerical examples in this paper are original for the literature.

Contrasting volcano spacing along SW Japan arc caused by difference in age of subducting lithosphere

The SW Japan arc built by subduction of the Philippine Sea (PHS) plate exhibits uneven distribution of volcanoes: thirteen Quaternary composite volcanoes form in the western half of this arc, Kyushu Island, while only two in the eastern half, Chugoku district. Reconstruction of the PHS plate back to 14 Ma, together with examinations based on thermal structure models constrained by high-density heat flow data and a petrological model for dehydration reactions suggest that fluids are discharged actively at depths of 90–100 km in the hydrous layer at the top of the old (> 50 Ma), hence, cold lithosphere sinking beneath Kyushu Island. In contrast, the young (15–25 Ma) oceanic crust downgoing beneath Chugoku district releases fluids largely at shallower depths, i.e. beneath the non-volcanic forearc, to cause characteristic tectonic tremors and low-frequency earthquakes (LFEs) and be the source of specific brine springs. Much larger amounts of fluids supplied to the magma source region in the western SW Japan arc could build more densely-distributed volcanoes.

Unraveling the electrical conductivity structure to decipher the hydrothermal system beneath the Mt. Hasan composite volcano and its vicinity, SW Cappadocia, Turkey

To comprehend the hydrothermal potential beneath the Mt. Hasan composite volcano and its vicinity, three-dimensional analyses of magnetotelluric data collected at 27 sounding locations were utilized to develop electrical conductivity (or its reciprocal resistivity) images in southwest Cappadocia, Central Anatolia, Turkey. A low resistivity anomaly associated with a clay cap was observed in the resulting numerical models that followed the characteristics of the earlier theoretical models developed for the electromagnetic interpretation of hydrothermal systems. The observed anomaly appeared in the form of a deep smectite, smectite–illite, and illite–chlorite sequence buried under the cover layers made up of welded ignimbrites, contemporaneous sediments, and monogenetic Quaternary volcanism products. Further evidence was observed that indicated the presence of a relatively deep (~4–6 km), single magmatic chamber beneath Mt. Hasan and the surrounding system. The Tuz Gölü Fault and some other auxiliary faults within this system, present a high correlation with the low-to-high resistivity interfaces in the vicinity of Mt. Hasan.

Hydrothermal Alteration on Composite Volcanoes: Mineralogy, Hyperspectral Imaging, and Aeromagnetic Study of Mt Ruapehu, New Zealand

AbstractProlonged volcanic activity can induce surface weathering and hydrothermal alteration that is a primary control on edifice instability, posing a complex hazard with its challenges to accurately forecast and mitigate. This study uses a frequently active composite volcano, Mt Ruapehu, New Zealand, to develop a conceptual model of surface weathering and hydrothermal alteration applicable to long‐lived composite volcanoes. The alteration on Mt Ruapehu was classified using ground samples as non‐altered, supergene argillic, intermediate argillic, and advanced argillic. The first two classes have a paragenesis that is consistent with surficial infiltration and circulation of low‐temperature (<40°C) neutral to mildly acidic fluids, inducing chemical weathering and formation of weathering rims on rock surfaces. The intermediate and advanced argillic alteration formed from hotter (≥100°C) hydrothermal fluids with lower pH, interacting with the andesitic to dacitic host rocks. The distribution of weathering and hydrothermal alteration has been mapped with airborne hyperspectral imaging through image classification, while aeromagnetic data inversion was used to map alteration to up to 500‐m depth. The joint use of hyperspectral imaging complements the geophysical methods since it can spectrally identify hydrothermal alteration mineralogy. This study established a conceptual model of hydrothermal alteration history of Mt Ruapehu, exemplifying a long‐lived and nested active and ancient hydrothermal system. This study's combination approach can be used to indicate the most likely sources of future debris avalanches, which are a significant hazard on Ruapehu.

Fabrication of the carbon fiber reinforced plastic (CFRP) cone tube through the laboratory-scale 3-axis winding machine

Filament winding process is one of the composite fabrication methods, which has relative lower manufacturing costs, higher efficiency and automation. It is commonly used to manufacture axisymmetric composite products, such as tubes, vessels, and domes, which is mainly used in aerospace, military and defense technology. However, it is a challenging task to fabricate a composite cone structure with the high winding angle through a laboratory-scale 3-axis winding machine. This paper aims to design and fabricate the carbon fiber reinforced plastic (CFRP) cone tube by using a low-cost filament winding machine. The cone mandrel was designed and prepared using additive printing technique. Dry and wet winding processes were conducted with yarn and 3K carbon fiber tow, respectively. The CFRP cone tube was successfully designed and fabricated with a winding angle of 75.11°±0.12°. It can be concluded that the wet winding process provides better winding quality and higher surface smoothness compared to the dry winding process. Moreover, the compressive modulus was 1.62 GPa, and the maximum compressive stress was 16.29 MPa under quasi-static compression loading.

Alluvial fans on volcanic islands: A morphometric perspective (São Vicente, Cape Verde)

The distribution, morphology and development of volcanic island alluvial fans are investigated using São Vicente from the Cape Verde volcanic chain (east-central Atlantic Ocean). This is a tectonically and volcanically inactive dryland setting, comprising an eroded Plio-Pleistocene composite volcano overlying a submarine shield volcano. We mapped 228 alluvial fans, quantifying fan (F) and catchment (C) morphological properties (area [A], gradient [G], relief [R], length [L]). Fans and catchments of varying size and gradient are distributed around the island coastal margins (6%) and along interior valley sides (94%). Regression results (i.e. +ve/−ve slopes of best fit lines) conformed to broader fan research but with weak correlations, and coefficient-exponent values outside, or at the lower end of values reported from non-volcanic settings. Analysis of fan area and gradient vs catchment area regression residuals revealed fans were dominated by small-steep gradient (28%) and large-low gradient forms (39%). Fan building space is a fundamental control, specifically linked to volcano structure, but with interplay between rock strength, base level, and climate. Fans around the volcanic edifice flanks are uncommon due to steep flank slopes. Where flank margin fans occur they are typically small and steep forms due to marine erosion. Fans along flank incised valleys are common. Large low gradient fans are associated with a major volcanic rift-aligned valley structure, with extensive erosion into altered and weathered edifice-core lithologies. Small-steep gradient fans are associated with edifice flank valleys. Steeper and narrower valley forms reflect incision into stronger lithologies of the composite volcano flanks, restricting fan building space. Fans located within the volcano central depression are amongst the largest and lowest gradient types, building into the eroded and unconfined edifice core space. Coalescence, apex-toe incision, toe erosion and catchment stream channel backfilling displayed by most fans suggest landscape erosion dominates during more arid periods (e.g. interglacials), with the dryland setting generating limited sediment and insufficient flood runoff for transport and fan deposition. Fan building likely occurs during wetter periods linked to Quaternary climate variability. Quaternary eustatic variations are relevant for the volcano edifice margins where sea level changes control fan building space and erosion.

Volcanoes buried in Te Riu-a-Māui/Zealandia sedimentary basins

ABSTRACT Many volcanoes are buried within Te Riu-a-Māui/Zealandia sedimentary basins. Based on a large collection of seismic reflection and drillhole data, this paper outlines the morphology, location and age of over 400 volcanoes buried and preserved in offshore New Zealand basins. We discuss the relationships of these volcanoes to the timing and locations of the main tectonic events that shaped Zealandia in the last 105 Myr. Late Cretaceous volcanism mainly formed large (>20 km3) composite cones and volcanic complexes that erupted along rift faults related to the break-up of Zealandia from south-eastern Gondwana. Cenozoic intraplate volcanic activity was widespread, typically forming clusters of scattered small-volume (<1 km3) craters and cones. In contrast, large Cenozoic volcanic complexes, composite cones and shield volcanoes erupted along linear arcs associated with subduction along the Pacific-Australian plate boundary. In detail, the presence of pre-existing structures and types of host rocks also influence the passage of magma in the shallow (<5 km) crust and the location of eruptive centres, regardless the tectonic setting of eruption. Insights from this work demonstrate the value of characterising volcanoes buried in sedimentary basins for improved understanding of the interplay between volcanism and plate tectonics in Zealandia and further afield.

Morphometry of glaciovolcanic edifices from Iceland: Types and evolution

The morphology of glaciovolcanoes is considered one of the distinctive characteristics of their ice-confining eruption environment. However, a thorough morphometric analysis of a large number of glaciovolcanic edifices has never been performed. Based on semi-automatic geomorphometric mapping, we present a morphometric database of 155 glaciovolcanic edifices within the Icelandic neovolcanic zones, formed during the last 0.78 Ma. This database enables a comprehensive analysis of the morphometric diversity of a large suite of glaciovolcanic edifices. Sheet-like formations, however, are not considered due to lack of data. Using three planimetric measurements (basal length, average basal width and average summit plateau width) and their ratios, three main morphometric groups can be distinguished in a ternary diagram: 1) conical edifices with no or small summit plateaus, 2) linear ridges and 3) flat-topped edifices (subdivided into equidimensional and elongated). All three groups contain edifices with and without lava caps. These morphometric groups can be fitted to the commonly accepted terminology for cone/mound, tindar and tuya. However, since lava caps occur in all morphometric groups, a grouping based on its existence is not practical. This suggests that by adding the descriptor “lava-capped” to any of the three classes may be a useful way to refine the classification. Based on the ternary diagram, ridges are the most morphometrically distinct glaciovolcanic edifice, because of their extreme elongation, followed by flat-topped edifices and finally conical edifices. However, morphometric parameters cannot be used singlehanded to identify glaciovolcanic edifices from other types of volcanic edifices such as composite volcanoes, shields or submarine volcanoes, and should always be complemented with other observations. The glaciovolcanic edifice volumes range from 0.15 · 10−2 km3 to 32 km3. Conical edifices are the smallest (<0.1 km3) and the group of flat-topped edifices has the largest edifices (>1 km3). Overall, with growth three morphological evolutions can be considered: (A) an initial eruptive fissure concentrates into one vent generating an equidimensional edifice, that either can be a conical or a flat-topped edifice; (B) the edifice maintains its elongation, suggesting that a fissure is the dominating vent structure, and during continued eruption develops significant summit plateaus, generating elongated flat-topped edifices; and (C) the edifice increases in elongation because the fissure lengthens during the eruption, and a plateau-building stage does not occur, producing increasingly elongated ridges.

Lithosedimentological and tephrostratigraphical characterisation of small-volume, low-intensity eruptions: The 1800 years Tufa Trig Formation, Mt. Ruapehu (New Zealand)

Low to moderate-intensity eruptions (VEI ≤ 3) constitute the most frequent eruptions on historical time scales and can last from days to years. Direct observations of historical eruptions (e.g. Ruapehu 1995–1996, Nakadake 2003–2005, Etna 2002–2003) have highlighted the complexity of these eruptions, which often involve multiple phases and sudden changes in eruption behaviour. Eruptive products associated with low to moderate-intensity eruptions are typically characterised by small erupted volumes ≪1 km3 and a significant amount of ash-sized material, making their preservation subject to local environmental conditions. Accordingly, long-term (1000s of years) eruptive records tend to be biased towards the usually less frequent but better-preserved, more intense eruptions with VEI > 4 and volumes >1 km3, leading to an underestimation of the complexity and frequency of small-scale eruptions.This research presents a high-resolution tephrostratigraphic framework for the Tufa Trig Formation, formed during the last 1800 years of activity of the 2797 m high andesitic composite volcano Mt. Ruapehu in New Zealand. Systematic mapping and characterisation of macroscopic lithosedimentological features and tephra dispersal are combined with further studies on selected characteristic sequences to add complexity to the 1800 years tephra record of one of New Zealand's historically most active volcanoes. Tephra deposits display a range of deposit textures, componentry and dispersal, with eruptive activity showing a time-variant distribution. Based on lithosedimentological deposit features, three main eruption types can be distinguished to have formed the Tufa Trig Formation: (1) ash-dominated low-intensity eruptions having volumes of ~1 × 106 m3 and leading to the deposition of single bed ash units; (2) small-volume (5–10 × 106 m3) moderate intensity eruptions, which distributed lapilli-sized material up to distances of 20 km; and (3) multi-phase eruptions that were associated with multi bed ash sequences, involved a magnitude larger tephra volumes (15.19–68.89 × 106 m3), and whose tephra dispersal shows multiple thickness maxima in different directions. The here presented framework extends existing knowledge on the complexity of tephra deposits associated with low to moderate-intensity eruptions and provides the fundamental base for further, more detailed studies at Mt. Ruapehu and similar volcanoes worldwide. To guide future studies on similar records of detailed VEI ≤ 3 volcanic sequences elsewhere, several key lessons and challenges in the characterisation of such fall units are summarised also.

Pinched between the plates: Armenia's voluminous record of volcanic activity

Located in the heart of the Lesser Caucasus mountains, where the Arabian and Eurasian tectonic plates collide, Armenia occupies an exceptional geological position shaped through millions of years of subduction and collision. It is a unique place on the Earth recording extensive intrusive and volcanic activity related to the long‐standing continental convergence. The volcanoes of Armenia provide a rare opportunity to study the sources and processes involved in this unusual type of magmatism. More than 500 Quaternary volcanoes have been mapped in Armenia, most of them formed from single eruptive episodes. Among several large composite volcanoes, the mighty Aragats stands out as the largest volcano in Armenia and the region altogether. Volcanic deposits testify to the range of eruptive styles—from the ignimbrites formed in eruptions as explosive and voluminous as any seen globally in the modern era to the enormous fissure‐fed lava flows that form the Southern Caucasus flood basalt province, the smallest and youngest Large Igneous Province in the world. Several pre‐historical and historical eruptions have been documented, highlighting the potential for future volcanic activity in the region. In recent years, research has focused on the volcanic hazards associated with the Armenian Nuclear Power Plant, located in the foothills of Aragats volcano. This article highlights some of the extraordinary volcanic and intrusive features observed in Armenia and summarizes aspects of recent volcanological and petrological research.

The glaciovolcanic evolution of the Copahue volcano, Andean Southern Volcanic Zone, Argentina-Chile

Glaciovolcanism produces distinctive features that are useful paleoclimate proxies for the distribution of past ice sheets and glacier extent. The Copahue volcano located in the Andean Southern Volcanic Zone, Argentina-Chile, is an active composite volcano known to have glaciovolcanic features such as lava bodies with glassy margins and anomalous cooling fractures. However, the emplacement conditions of these products and the influence of Pleistocene glaciations on the evolution of the Copahue volcano remains poorly understood. In this contribution, we propose a model for glaciovolcanic evolution of the Copahue volcano based on the analysis, interpretation, and mapping of its products. Ten lithofacies are described on the eastern flank of Copahue volcano exhibiting several examples of glaciovolcanism. The evolution of the Copahue volcano can be divided into two main sequences: the Ancient Sequence (S1) and the Young Sequence (S2), separated by a major erosive phase. The S1 (early-middle Pleistocene-late Pleistocene) consists of an initial subaerial effusive stage followed by a major glaciovolcanic stage, during which a thick ice cap existed and the edifice grew beneath an englacial lake with the eventual formation of a lava-fed delta. The S2 (late Pleistocene-Present) is defined by mainly effusive activity during periods of glacial advance and retreat recorded by an alternation of unconfined subaerial lavas and ice-confined lavas. The evolution of the Copahue volcano therefore indicates two glaciations in the Copahue-Caviahue area during the late Pleistocene, in contrast to a single glaciation. Based on the glacial history in the area we associate the first glaciation with the end of Marine Isotope Stage 3 (57–29 ka) and/or the Last Glacial Maximum period (26.5–19.0 ka), and the second less-extensive glacial period with the Antarctic Cold Reversal (14.5–12.9 ka).

Deep and rapid thermo-mechanical erosion by a small-volume lava flow

We document remarkably efficient thermo-mechanical erosion by a small-volume lava flow. Downcutting by a basaltic-andesite lava flow on the steep-sided Momotombo volcano, Nicaragua, occurred at 100 times the rate commonly reported for thermal erosion in lava flow fields, even though this flow was small-volume (0.02km3) and effused at a low rate for <1 week. The lava flow incised into the pyroclastic substrate up to 30m, with erosion depth controlled primarily by thermal reduction of substrate hardness. We show that incision depth decreases, approximately exponentially, with distance along the flow path, until erosion stopped and the flow became constructional. This transition occurs 650m from the vent on a slope averaging a 32∘ incline. Results indicate that syn-eruptive erosion is an important morphological process on some steep-sided volcanoes that are predominantly composed of layered pyroclasts. Rapid erosion and incision increased flow run-out for the 1905 flow, which in turn directed the flow and run-out of the 2015 lava flow. Mapping and understanding these features is critical for improving lava flow hazard assessments and provides insight into the construction and growth of composite cones.

Hydrothermal Activity and Seismicity at Teahitia Seamount: Reactivation of the Society Islands Hotspot?

Along mid-ocean ridges, submarine venting has been found at all spreading rates and in every ocean basin. By contrast, intraplate hydrothermal activity has only been reported from five locations, worldwide. Here we extend the time series at one of those sites, Teahitia Seamount, which was first shown to be hydrothermally active in 1983 but had not been revisited since 1999. Previously, submersible investigations had led to the discovery of low-temperature (≤30°C) venting associated with the summit of Teahitia Seamount at ≤1500 m. In December 2013 we returned to the same site at the culmination of the US GEOTRACES Eastern South Tropical Pacific (GP16) transect and found evidence for ongoing venting in the form of a non-buoyant hydrothermal plume at a depth of 1400 m. Multi-beam mapping revealed the same composite volcano morphology described previously for Teahitia including four prominent cones. The plume overlying the summit showed distinct in situ optical backscatter and redox anomalies, coupled with high concentrations of total dissolvable Fe (≤186 nmol/L) and Mn (≤33 nmol/L) that are all diagnostic of venting at the underlying seafloor. Continuous seismic records from 1986-present reveal a ∼15 year period of quiescence at Teahitia, following the seismic crisis that first stimulated its submersible-led investigation. Since 2007, however, the frequency of seismicity at Teahitia, coupled with the low magnitude of those events, are suggestive of magmatic reactivation. Separately, distinct seismicity at the adjacent Rocard seamount has also been attributed to submarine extrusive volcanism in 2011 and in 2013. Theoretical modeling of the hydrothermal plume signals detected suggest a minimum heat flux of 10 MW at the summit of Teahitia. Those model simulations can only be sourced from an area of low-temperature venting such as that originally reported from Teahitia if the temperature of the fluids exiting the seabed has increased significantly, from ≤30°C to ∼70°C. These model seafloor temperatures and our direct plume observations are both consistent with reports from Loihi Seamount, Hawaii, ∼10 year following an episode of seafloor volcanism. We hypothesize that the Society Islands hotspot may be undergoing a similar episode of both magmatic and hydrothermal reactivation.

Evidence for welding of a block and ash pyroclastic flow deposit: the case of Cerro Bravo Volcano, Colombia

Cerro Bravo is an active composite volcano located in the San Diego—Cerro Machin Volcano-Tectonic Province (SCVTP), the northernmost volcanic chain in the Colombian Andes. Among the products associated with the volcano, there is a peculiar “indurated” deposit emplaced between 7 and 2.9 ka, which resembles a block and ash pyroclastic flow deposit. Through field observations, physical property analyses, petrography and scanning electron microscopy (SEM), we demonstrate that this induration is the result of welding processes. High density, deformed glassy fragments with a common orientation, low porosity, fiamme and sintering, are the most visible characteristics. The evidence indicates that welding reached up to rank IV in a I to VI classification scheme. We suggest that welding is associated with (1) high emplacement temperature related to the syneruptive destruction of a growing endogenous dome and (2) a low glass transition temperature related to the influence of upward water diffusion after emplacement of the flow over wet ground. Thus, with emplacement temperatures between 670 and 540 °C, and water contents between 0.5 and 1.2 wt%, the welding process was unusually efficient. Calculations made indicate that for the deposit to present the observed characteristics of welding, the emplacement temperature needed to exceed the threshold of the glass transition temperature for a time of 5–6 days and have an effective viscosity of 1012 Pa s. As a result of the welding, the deposit decreased its thickness by between 15 and 7 m. This work contributes to the knowledge of welding processes and stresses the unusual occurrence of welding in this type of pyroclastic flow deposit which thus requires special emplacement conditions in terms of temperature and water content.

Geochemical evolution of the Quaternary Chachimbiro Volcanic Complex (frontal volcanic arc of Ecuador)

The Chachimbiro volcanic complex (CVC) is a composite volcano located in the frontal part of the Ecuadorian Quaternary Arc.Four periods of activity can be differentiated in Chachimbiro ∼400 ka-long volcanic history, named CH1 (405.7 ± 20.0–298.6 ± 32.9 ka), CH2 (121.75 ± 23.2–36.08 ± 2.8 ka), CH3 (36080 ± 280–22730 ± 120 ybp) and CH4 (5.760 ± 30 ybp) respectively. The magmatic suite ranges in composition from andesites to rhyodacites, and displays systematic increases of SiO2, Sr/Y, La/Yb, incompatible elements (e.g., LREE, MREE, LILE) and depletion of HREE with time. These geochemical features, coupled with radiogenic isotope data and textural observations (e.g., several resorption and overgrowth cycles in plagioclase phenocrysts), suggest an open system evolution for CVC including typical crustal magmatic processes such as recharge, assimilation and fractional crystallisation.To better constrain the magmatic crustal processes of the CVC, we use a multi-model approach including three open system models: RFC (Recharge, Fractional Crystallisation), AFC (Assimilation, Fractional Crystallisation), and EC-RAFC (Energy Constrained – Recharge Assimilation, Fractional Crystallisation). Different simulations have been developed to best fit CVC’s geochemistry (including trace elements, Sr/Y ratios and Sr isotopes), which suggest predominant fractional crystallisation and variable assimilation as the main processes explaining the bulk of the geochemical data of CVC. This is consistent with an overall thermal decline of the magmatic system through time.The genetic model here presented involves the input of a magmatic recharge component matured in the low-mid crust through dominant fractionation of amphibole (±garnet), into intermediate crustal reservoir(s) where magma undergoes extensive FC.