Lateral ﬂank collapse is one of the main causes of destruction of oceanic basalticvolcanoes [Holcomb and Searle, 1991]. The consequences of such processes, i.e debris avalanches [Moore et al., 1989] and tsunamis [Keating and McGuire, 2000; Kelfoun et al., 2010], make this issue of primary importance for risk mitigation. Flank movements can be driven by gravity alone, the edi ﬁce being affected by spreading [Borgia et al., 1992; Merle and Borgia, 1996; van Wyk de Vries and Francis, 1997], or by the combined effects of gravity and forceful magma injections [Swanson et al., 1976; Borgia, 1994; Lundgren et al., 2004]. Forceful magma injection models assume that the volcano ﬂank slides on a low angle fault, pushed by the recurrent injection of magma into vertical rift zones [Dieterich, 1988]. Frictional resistance of the fault can be reduced by pore ﬂuid pressure [Thomas et al., 2004]. However, suprahydrostatic ﬂuid pressure is required to explain the occurrence of catastrophic failures [Iverson, 1995; Elsworth and Day, 1999]. Such pressure can be reached for unrealistically thick clay layers (several hundred of meters) or in the case of extremely low hydraulic diffusivity [Iverson, 1995]. Another mechanism involves earthquake shaking, which induces ground liquefaction through elevated pore ﬂuid pressure, possibly enhanced by gases [Papatheodorou and Ferentinos, 1997; Thomas et al., 2004], but this mechanism only concerns the shallow subsurface. Thus, the origin of large-scale ﬂank failure remains enigmatic.
system of frozen conduits that delivered the basaltic magmas from the mantle to the surface during the activity of Nikolka.
Note that the high-velocity anomalies are associated not only with basalticvolcanoes, but also observed beneath some active volcanoes with felsic compositions. For example, beneath the southwestern part of the Shiveluch, a prominent high-velocity anomaly is observed in the upper crust down to ~10 km depth. This is a typical andesitic volcano with violent explosive eruptions strongly disturbing the edifice and forming extensive deposits of pyroclasts in the surrounding areas (e.g., Belousov et al., 1999). Therefore, low velocity at shallow layers would be more expected than the high-velocity ones. Nevertheless, the existence of high-velocity bodies within volcanic edifices having predominant silicic composition is not uncommon. Similar features were observed in a number of andesitic volcanoes in the world: Mt. Vesuvius in Italy (Zollo et al., 1998), Redoubt volcano in Alaska (Kasatkina et al., 2014), Popocatépetl volcano in Mexico (Kuznetsov et al., 2014) and others. It means that beneath such volcanoes the magmatic intrusions may form voluminous
a b s t r a c t
Piton des Neiges basaltic volcano (La Réunion) has been deeply dissected by erosion, exposing large volumes of debris avalanche deposits. To shed light on the factors that led to volcano ﬂank destabilizations, we studied the structure, the crystallographic and magnetic fabrics of the substratum of a debris avalanche unit. This substratum is a complex of N50 seaward-dipping sills that has been exposed by the avalanche. Structural observations show that the sill plane in contact with the avalanche is one of the latest intrusions in the sill complex. In this uppermost sill, the anisotropy of magnetic susceptibility (AMS) is correlated to the crystallographic preferred orientation of magmatic silicate minerals, allowing us to use AMS as a proxy to infer the magmatic ﬂow. The AMS fabric across the intrusion is strongly asymmetric, which reveals that the contact sill was emplaced with a normal shear displacement of its hanging wall. The shear displacement and the magma ﬂow in the intrusion are both directed toward the NNE, i.e. toward the sea, which is also the direction of the slope and of the debris avalanche runout. Because all the sills in the intrusion complex have a similar dip and dip direction, it is likely that several of them also underwent a cointrusive slip toward the NNE. We conclude that this cointrusive normal slip, repeated over many intrusions of the sill complex, increased the ﬂank instability of the volcano. This incre- mental instability may have ended up into the observed debris avalanche deposit. At Piton de la Fournaise, the active volcano of La Réunion, sill intrusion and cointrusive ﬂank displacement have been inferred from geophys- ical studies for the April 2007 eruption. By providing direct evidence of sheared sills, our study substantiates the idea that repeated sill intrusions may eventually trigger ﬂank destabilizations in basalticvolcanoes.
Though endogenous growth is not predominant at Piton de La Fournaise, signs of endogenous construction give information on the stress field inside the edifice. Dikes propagate parallel to the maximum compression axes σ1 within cone; σ1 is vertical at depth favoring vertical magma migration, as observed in the craters walls of many basaltic shields. Toward the surface, stress can favor dike arrest. Observations on caldera walls of basalticvolcanoes show that many dikes become arrested at shallow depths (Geshi et al. 2010 ). Because of the lack of the third dimension in outcrop, it is difficult to recognize arrested intrusions at Piton de La Fournaise. In 2008, however, the renewal of activity at Piton de La Fournaise began with numerous non- erupting intrusions (Peltier et al. 2010 ), suggesting that a change in the stress field after the April 2007 collapse subsequently favored the arrest of dikes at depth.
1. Illustrations of features typical of maar-diatreme volcanoes. (a) A dry Holocene maar: Ubehebe Crater, California (700 to 800 m-wide, 235 m-deep; Crowe and Fisher, 1973). The dark gray material is the phreatomagmatic tephra, and the crater cuts deeply into the pre-existing sandstones and conglomerates. (b) A water-filled Quaternary crater: the Weinfeld maar in the West Eifel volcanic field, Germany (700 m diameter, 90 m deep; Lorenz and Zimanowski, 2008). (c) A quarry wall in the tephra rim of the Quaternary Pulvermaar (West Eifel), showing the extreme abundance of country rock fragments in these surge deposits. Juvenile clasts (~20% by volume) are too small to be seen clearly at this scale. (d) The lower part of diatremes is typically occupied by non-bedded, poorly sorted volcaniclastic deposits such as these lapilli-tuffs in the Jurassic Coombs Hills complex (Ross, 2005). A narrow dike of basaltic andesite with a zig-zag pattern cuts the vent fill. (e)-(f) Two photos from Coombs Hills showing that syn-eruptive pyroclastic deposits originally deposited on the ground surface can end-up as (e) rafts or (f) large remnants of a volcanic edifice slumped within diatremes. The raft has steeply dipping lapilli-tuff and tuff beds and is surrounded by heterolithic, non-bedded vent fill. The well-layered tephra ring remnant forms a quarter-circle shape in map view; the sub-vertical exposure (f) shows a dike of juvenile-rich tuff-breccia invading the tephra ring beds.
4.43–4.13 Ga-old granodioritic and/or tonalitic crustal component on early Mars ( Sautter et al., 2015 ; Sautter et al., 2016 ). These quartz-normative rocks, which are believed to represent the early Martian continental crust, cannot be produced from partial melting of the mantle and subsequent fractional crystallization ( Sautter et al., 2015 ; Sautter et al., 2016 ). By contrast, recent modeling using the MELTS software ( Udry et al., 2018 ) suggests that the felsic rocks of Gale crater were produced by the accumulation/fractionation of feldspar from basaltic melt; however, these authors make no comparison with modern TTG (plagiogranite) rocks. Our data demonstrated on Figures 5, 6 are also in line with our model and suggest that the Noachian granodiorite (felsic) rocks on Mars may have been produced by aqueous ﬂuid-assisted partial melting of peridotite induced by reaction with basaltic melts. Survival of the primary intermediate to felsic liquids and formation of a solid crust may have been possible if the effects of gravitational overturn were limited ( Elkins-Tanton, 2012 ; Bouvier et al., 2018 ). Thus, the primordial environment of the peridotite protocrust in the presence of mantle-derived (e.g., basaltic) melt at shallow depth is favorable for the production of silica- enriched melts provided liquid water is present in sufﬁcient amounts. The mechanism suggested in our study may thus have led to formation of large volumes of the earliest felsic crust.
In this paper, we describe the tropicalization of ÆR+ radon probes in order to make them suitable for deployment at high altitudes (>3000 m) and in acidic environments on Mt. Etna Volcano.
1.3. IoT Network Technologies toward Large-Scale Deployment of Small Sensors on Active Volcanoes
Volcano monitoring implies a data transfer from measuring stations to an observatory where the information can be treated and analyzed in real time. In a case of pending eruption, data transfer has to be as fast and as robust as possible in order to allow the earliest alert as possible. In a perspective of monitoring oriented toward real-time crisis management, the data transfer step has a predominant importance, since it will condition the quantity of information available to experts. What is more, the complexity of volcanic systems often requires geographically distributed measurements rather than punctual records in order to draw relevant scientific conclusions. For this reason, sensors are deployed in large networks covering important geographical areas, making data transfer even more challenging. The recent development of small sensor technologies relevant to many domains of volcano monitoring (mostly geodesy [ 19 ] and gas composition [ 20 ]) is opening the way to the deployment of networks with unprecedented density. Such deployments require innovative data transfer solutions able to manage efficiently a very high number of stations.
Travel-time tomography was used to determine the deep structure of the volcanoes. The resulting cross-sections show roughly triangular, high-velocity cores beneath the calderas and rifts. Plots of P-wave velocities in the south flank of KV along a N-S horizontal profile (Figure 2c) showed that velocity falls substantially with distance from the rift axis. Strikingly, the norm of the velocity gradient is largest near the intersection of the reverse fault plane and the decollement plane. This maximum is reached for a ~6 km/s P-wave velocity, which is the limit used (6) to differentiate between intrusive rocks and lava flows in Hawaiian volcanoes. Hence, an accurate analysis of seismological data tends to show that reverse faulting occurs in the south flank of KV. It initiates near the core/cover boundary, at the depth of the decollement plane, and propagates through the oceanic crust.
Keywords: colonization module, basalt alteration, Guaymas basin, organic-rich sediment, hydrothermal systems
Alteration of the oceanic crust by seawater is one of the most important processes controlling the global fluxes of many ele- ments at mid-oceanic ridges and ridge flanks (e.g., Staudigel and Hart, 1983; Wheat and Mottl, 2000 ) and the mineralogical and chemical composition of the aging oceanic crust ( Alt, 1995 ). Since sub-seafloor basaltic crust represents the largest habitable zone by volume on Earth, microbes may play a significant role in the alteration process ( Bach and Edwards, 2003 ). Microorganisms exploiting these reactions are known from basalt exposed at the seafloor, where the oxidation of reduced sulfur (S) and iron (Fe) compounds from basalt with dissolved oxygen and nitrate from seawater supports high microbial biomass and diversity ( Mason et al., 2008; Santelli et al., 2008a; Orcutt et al., 2011b ). It has been
therefore highly depolymerized and thus very soluble.
Since the altered layer of the basaltic glass is less dense (1.26 g.cm- 3 ) than the altered ternary
glass, it should dissolve in the same way, in spite of the initial silicon saturation of the solution. The solution would then become oversaturated with respect to amorphous silica, which would precipitate, and the alteration of the glass would then proceed under steady-state conditions. Figure 11 summarizes the mechanism we propose to explain how the basaltic glass is altered under silicon saturation conditions. Alteration is mainly driven by the precipitation of clays, which consume Si and other elements from the glass such as Mg, Fe, Al and possibly Ti. The release of these elements makes the remnant material easier to dissolve, and the additional influx of silica leads to the precipitation of amorphous silicon on the surface of the glass. The glass dissolution process appears congruent overall.
6.6. Temporal and spatial evolution of magma
In post-collisional volcanisms, generally calcalkaline products erupted before alkaline products, whereas the majority of alkaline and calcalkaline products from the studied area had formed in the same time interval. The majority of the alkali basalts and basaltic andesites collected from HS, OZ and K, and ES basaltic andesites have particularly the same age range. The temperature and pressure values calculated using the microprobe analysis data revealed the existence of more than one magma chamber, which previous researchers had also indicated this information (Aydar, 1992; Aydar and Gourgaud, 1997; Güçtekin, 1997). The age data show that spatial evolution was more effective in these four regions than temporal evolution. The analogous age ranges of the samples and the fact that the ages of some samples varied only by a few thousand years hindered the ascertainment of temporal evolution. The data sets of the present study and tomographic images of recent studies indicate that it is not really possible to speak of the existence of a present-day plume in Central Anatolia (Biryol et al., 2011). The geochemical data of the present study suggest that these four regions had mainly evolved from the lithospheric mantle.
The Re-Os data for macroalgae presented here have been successfully used to trace the in ﬂuence of basaltic weathering on the Re and Os isotope systematics of Icelandic coastal waters and show the in ﬂuence of sea- water in an estuarine setting, providing further evidence to support the use of the 187 Os/ 188 Os of macroalgae as a proxy for the 187 Os/ 188 Os of seawater. As such, the latter presents a more amenable option to direct sea- water analysis, potentially becoming a useful tool for tracing a variety of Earth system processes. Additionally, in keeping with previous work, we have shown that Re and Os is taken up by macroalgae from the dissolved load of the seawater it inhabits. However, unlike previous work, we also suggest that additional uptake of both Re and Os could occur from the bed load. However, experimental studies similar to those done for Re and Os uptake from dissolved load (Racionero-Gómez et al., 2016; Racionero-Gómez et al., 2017) are needed for both bed and suspended particulate load to assess the in ﬂuence of these phases on the Re-Os systematics of macroalgae.
An analysis and interpretation of sidescan sonar imagery, coupled with that of optical seaﬂoor imagery, informs us of the constructional and deformational history of Lucky Strike seamount. This provides a model to explain the characteristics and temporal evolution of other central volcanoes found along mid-ocean ridges. The Lucky Strike central volcano was emplaced within a preexisting rift valley, rising 500 m or more above surrounding seaﬂoor, as a result of a phase of high effusive rates. In general, volcanic output during central volcanic construction is sufﬁcient to bury any tectonic features (faults, ﬁssures) associated with plate separation, as indicated by the unfaulted nature of volcano ﬂanks preserved on and off axis. Based on volumes of these ediﬁces (from a few to more than a hundred cubic kilometers), we infer that this phase of volcanism must involve either a continuous melt supply, or the succession of numerous replenish- ment of and eruption events from an underlying magma chamber. The conic nature of these ediﬁces and the evidence from volcanic acoustic facies of a concentric structure around the summit of Lucky Strike indi- cates that the focusing of melt is efﬁcient. Decrease in the rate of melt supply may promote the formation
shallow crustal reservoir (Shcherbakov et al. 2011 ; Turner et al. 2013 ).
Sample descriptions and petrography
We collected a time-series suite of erupted volcanic prod- ucts from Bezymianny and Klyuchevskoy volcanoes (sam- ple descriptions in Online Resource 1). Samples were col- lected during field seasons following observed eruptions (Bezymianny 2006–2010; 1956 lateral blast eruption is eas- ily identifiable) or by collaborators at the Institute of Vol- canology and Seismology in Petropavlovsk-Kamchatsky (Klyuchevskoy). Bezymianny samples include five sam- ples from older extrusive domes that surround the modern edifice of the volcano as well as 13 samples from modern eruption products ranging in age from the 1956 directed- blast eruption to the June 1, 2010 pyroclastic flow eruption. Most modern eruptive products from Bezymianny are from bombs with obvious cooling textures (breadcrusted outer surfaces or slumping over larger dense blocks), which were emplaced in pyroclastic flows.
*: Corresponding author : firstname.lastname@example.org
The Mediterranean Sea constitutes a unique environment to study cold-seep ecosystems due to the presence of different geodynamic settings, from an active margin along the Mediterranean Ridge (MR) to a passive margin in the Nile Deep-Sea Fan (NDSF). We attempted to identify the structure of benthic communities associated with the Napoli and Amsterdam mud volcanoes (MVs) located on the MR and to establish the links between faunal distribution and environmental conditions at different spatial scales. Comparison between the 2 MVs revealed that the faunal distribution seemed to be mainly controlled by the characteristics of the microhabitats. On both geological structures, the variability between the different microhabitats was higher than the variability observed between replicates of the same microhabitat, and the distribution of macrofauna was apparently linked to gradients in physico-chemical conditions. The peripheral sites from Napoli were generally more oxygenated and harboured lower species richness than the active sites. The reduced sediment microhabitat from Amsterdam presented the highest methane concentrations and was mainly colonised by symbiont-bearing vesicomyid bivalves and heterotrophic dorvilleid polychaetes. Overall, a higher taxonomic diversity was observed on Napoli. Substratum type was hypothesised to be the second factor influencing faunal distribution. The results of this study highlight the high heterogeneity of faunal communities associated with seep ecosystems within this region and the need to pursue investigations at various spatial and temporal scales.
Distributed under a Creative Commons Attribution| 4.0 International License
Tephra Dispersal Model: PLUME-MOM/HYSPLIT Simulations Applied to Andean Volcanoes
M. Tadini, Olivier Roche, P. Samaniego, A. Guillin, N. Azzaoui, M. Gouhier, M De' Michieli, F. Pardini, J. Eychenne, B. Bernard, et al.