Summit Of Volcano Research Articles

In situ observations and sampling of volcanic emissions with NASA and UCR unmanned aircraft, including a case study at Turrialba Volcano, Costa Rica

Abstract Scientific knowledge of transient and difficult-to-access airborne volcanic emissions comes primarily from remote sensing observations, and a few in situ data from sporadic heroic or inadvertent airborne encounters. In the past, patchy knowledge of the composition and behaviour of such plumes from explosive volcanic eruptions, and associated drifting ash and gas clouds, have centrally contributed to unwanted and dangerous aircraft encounters that have put crews at risk and, in some cases, greatly damaged aircraft. Thus, improved knowledge of boundary conditions and plume composition, as inputs to both mass retrieval and predictive models for cloud trajectories , would be of benefit. In this paper, we describe how small robotic unmanned aerial vehicles (sUAVs) can address a variety of measurements that are typically beyond the reach of, and sometimes too dangerous for, manned aircraft. The direct measurements and sampling that can be achieved by sUAVs address serious gaps in knowledge of volcanic processes, and provide important validation data for estimations of volcanogenic ash and gas concentrations gleaned using remote sensing techniques. These data, in turn, constrain key proximal and distal boundary conditions for aerosol and gas transport models on which are based a number of decisions and evaluations by hazard responders and regulatory agencies. We briefly describe a case study from our ongoing field study at Turrialba Volcano in Costa Rica, where we are conducting an international campaign of systematic airborne in situ measurements of volcanogenic SO 2 and other gases, as well as aerosols, with sUAVs and aerostats (e.g. tethered balloons and kites), in conjunction with data acquisitions by the Advanced Spaceborne Thermal Emission and Reflection (ASTER) radiometer onboard the NASA Terra Earth orbital platform. To our knowledge, this is the first such systematic in situ UAV- and aerostat-based observation programme for SO 2 and particulates in a volcanic plume for correlation with orbital data. We preliminarily report good agreement between our UAV/aerostat and ASTER SO 2 retrievals within a 5 km radius of the volcano summit, at altitudes of up to 12 500 ft ( c. 3850 m) above sea level (asl) for concentrations within the range of 5–20 ppmv (ppm by volume). Additional work continues.

DMODELS: A MATLAB software package for modeling crustal deformation near active faults and volcanic centers

We have developed a MATLAB software package for the most common models used to interpret deformation measurements near faults and active volcanic centers. The emphasis is on analytical models of deformation that can be compared with data from the Global Positioning System (GPS), InSAR, tiltmeters and strainmeters. Source models include pressurized spherical, ellipsoidal and sill-like magma chambers in an elastic, homogeneous, flat half-space. Dikes and faults are described following the mathematical notation for rectangular dislocations in an elastic, homogeneous, flat half-space. All the expressions have been checked for typographical errors that might have been present in the original literature, extended to include deformation and strain within the Earth's crust (as opposed to only the Earth's surface) and verified against finite element models. A set of GPS measurements from the 2006 eruption at Augustine Volcano (Alaska) is used to test the software package. The results show that the best fit source to the GPS data is a spherical intrusion (ΔV=5×10−4km3), about 880m beneath the volcano's summit.

Volcanic deformation and flank instability due to magmatic sources and frictional rheology: the case of Mount Etna

SUMMARY The overall picture of Mount Etna deformation emerging since a couple of decades of geodetic surveys shows effects of magma accumulation, characterized by inflation/deflation cycle, accompanied by a sliding instability of the southeast flank, whose manifestation is an increase in the horizontal deformation away from the volcano summit. This is a very interesting case to test whether advanced models, taking into account topography, internal structure and frictional rheology, may contribute to a better understanding of the complex interplay among mechanical response, magmatic activity and gravitational load occurring in a volcanic system. Using finite element numerical models we make predictions of surface displacements associated with a simple expansion source and with a dike-like vertical discontinuity. A new methodology is developed to initialize the lithostatic stress field according to the material and geometrical complexities of the models considered. Our results show that, while an amplification of the horizontal displacement can be easily obtained up to a maximum distance of 10 km from the source, we have not been able to find any configuration to extend further this signal. For the case of Mount Etna this suggests that the large horizontal displacements observed in the east flank along the coast cannot be directly related to magma accumulation below the volcano’s summit.

First evidence of hydromagmatism at Colima volcano (Mexico)

Here we report for the first time evidences of hydromagmatism at Colima volcano (western sector of Trans-Mexican Volcanic Belt). The studied location exposes 9m of fine ash deposits dated at around 15calkyrBP by 14C measurement on organic matter collected at the very base of the stratigraphic succession. The ash deposits rest directly on the top of a debris avalanche deposit. The morphology and grain size distribution of ash samples from the basal part of the stratigraphic succession testify for an explosive magma–water interaction, which produced finely bedded layers with blocky fragments characterized by adhering particles, quenching cracks and chemical pitting. The availability of ground water was hypothesized to be in close relationship with the amelioration of climate conditions at the end of the Last Glacial period, with rising temperatures that induced melting of glaciers and snow previously accumulated on the volcano summit slopes. This recognition demonstrates how, even a “dry” volcano like Colima, can experience hydromagmatism under adequate climatic conditions.

Flux measurements of explosive degassing using a yearlong hydroacoustic record at an erupting submarine volcano

The output of gas and tephra from volcanoes is an inherently disorganized process that makes reliable flux estimates challenging to obtain. Continuous monitoring of gas flux has been achieved in only a few instances at subaerial volcanoes, but never for submarine volcanoes. Here we use the first sustained (yearlong) hydroacoustic monitoring of an erupting submarine volcano (NW Rota‐1, Mariana arc) to make calculations of explosive gas flux from a volcano into the ocean. Bursts of Strombolian explosive degassing at the volcano summit (520 m deep) occurred at 1–2 min intervals during the entire 12‐month hydrophone record and commonly exhibited cyclic step‐function changes between high and low intensity. Total gas flux calculated from the hydroacoustic record is 5.4 ± 0.6 Tg a−1, where the magmatic gases driving eruptions at NW Rota‐1 are primarily H2O, SO2, and CO2. Instantaneous fluxes varied by a factor of ∼100 over the deployment. Using melt inclusion information to estimate the concentration of CO2 in the explosive gases as 6.9 ± 0.7 wt %, we calculate an annual CO2 eruption flux of 0.4 ± 0.1 Tg a−1. This result is within the range of measured CO2 fluxes at continuously erupting subaerial volcanoes, and represents ∼0.2–0.6% of the annual estimated output of CO2from all subaerial arc volcanoes, and ∼0.4–0.6% of the mid‐ocean ridge flux. The multiyear eruptive history of NW Rota‐1 demonstrates that submarine volcanoes can be significant and sustained sources of CO2 to the shallow ocean.

How do volcanic rift zones relate to flank instability? Evidence from collapsing rifts at Etna

Volcanic rift zones, characterized by repeated dike emplacements, are expected to delimit the upper portion of unstable flanks at basaltic edifices. We use nearly two decades of InSAR observations excluding wintertime acquisitions, to analyze the relationships between rift zones, dike emplacement and flank instability at Etna. The results highlight a general eastward shift of the volcano summit, including the northeast and south rifts. This steady‐state eastward movement (1–2 cm/yr) is interrupted or even reversed during transient dike injections. Detailed analysis of the northeast rift shows that only during phases of dike injection, as in 2002, does the rift transiently becomes the upper border of the unstable flank. The flank's steady‐state eastward movement is inferred to result from the interplay between magmatic activity, asymmetric topographic unbuttressing, and east‐dipping detachment geometry at its base. This study documents the first evidence of steady‐state volcano rift instability interrupted by transient dike injection at basaltic edifices.

Seismic observations of Redoubt Volcano, Alaska — 1989–2010 and a conceptual model of the Redoubt magmatic system

Seismic activity at Redoubt Volcano, Alaska, has been closely monitored since 1989 by a network of five to ten seismometers within 22km of the volcano's summit. Major eruptions occurred in 1989–1990 and 2009 and were characterized by large volcanic explosions, episodes of lava dome growth and failure, pyroclastic flows, and lahars.Seismic features of the 1989–1990 eruption were 1) weak precursory tremor and a short, 23-hour-long, intense swarm of repetitive shallow long-period (LP) events centered 1.4km below the crater floor, 2) shallow volcano-tectonic (VT) and hybrid earthquakes that separated early episodes of dome growth, 3) 13 additional swarms of LP events at shallow depths precursory to many of the 25 explosions that occurred over the more than 128day duration of eruptive activity, and 4) a persistent cluster of VT earthquakes at 6 to 9km depth.In contrast the 2009 eruption was preceded by a pronounced increase in deep-LP (DLP) events at lower crustal depths (25 to 38km) that began in mid-December 2008, two months of discontinuous shallow volcanic tremor that started on January 23, 2009, a strong phreatic explosion on March 15, and a 58-hour-long swarm of repetitive shallow LP events. The 2009 eruption consisted of at least 23 major explosions between March 23 and April 5, again accompanied by shallow VT earthquakes, several episodes of shallow repetitive LP events and dome growth continuing until mid July. Increased VT earthquakes at 4 to 9km depth began slowly in early April, possibly defining a mid-crustal magma source zone.Magmatic processes associated with the 2009 eruption seismically activated the same portions of the Redoubt magmatic system as the 1989–1990 eruption, although the time scales and intensity vary considerably among the two eruptions. The occurrence of precursory DLP events suggests that the 2009 eruption may have involved the rise of magma from lower crustal depths. Based on the evolution of seismicity during the 1989–1990 and 2009 eruptions the Redoubt magmatic system is envisioned to consist of a shallow system of cracks extending 1 to 2km below the crater floor, a magma storage or source region at roughly 3 to 9km depth, and a diffuse magma source region at 25 to 38km depth. Close tracking of seismic activity allowed the Alaska Volcano Observatory to successfully issue warnings prior to many of the hazardous explosive events that occurred in 2009.

Pre-eruptive conditions of dacitic magma erupted during the 21.7 ka Plinian event at Nevado de Toluca volcano, Central Mexico

The Nevado de Toluca volcano in Central Mexico has been active over the last ca. 42ka, during which tens of km3 of pyroclastic material were erupted and two important Plinian-type eruptions occurred at ca. 21.7ka (Lower Toluca Pumice: LTP) and ca. 10.5ka (Upper Toluca Pumice: UTP). Samples from both the LTP and UTP contain plagioclase, amphibole, iron-titanium oxides, and minor anhedral biotite, set in a vesicular, rhyolitic, glassy matrix. In addition, UTP dacites contain orthopyroxene. Analysis of melt inclusions in plagioclase phenocrysts yields H2O contents of 2–3.5wt.% for LTP and 1.3–3.6wt.% for UTP samples. Ilmenite–ulvospinel geothermometry yields an average temperature of ~868°C for the LTP magma (hotter than the UTP magma, ~842°C; Arce et al., 2006), whereas amphibole–plagioclase geothermometry yields a temperature of 825–859°C for the LTP magma. Water-saturated experiments using LTP dacite suggest that: (i) amphibole is stable above 100MPa and below 900°C; (ii) plagioclase crystallizes below 250–100MPa at temperatures of 850–900°C; and (iii) pyroxene is stable only below pressures of 200–100MPa and temperatures of 825–900°C. Comparison of natural and experimental data suggests that the LTP dacitic magma was stored at 150–200MPa (5.8–7.7km below the volcano summit). No differences in pressure found between 21.7ka and 10.5ka suggest that these two magmas were stored at similar depths. Orthopyroxene produced in lower temperature LTP experiments is compositionally different to those found in UTP natural samples, suggesting that they originated in two different magma batches. Whole-rock chemistry, petrographic features, and mineral compositions suggest that magma mixing was responsible for the generation of the dacitic Plinian LTP eruption.

Recent unrest of Changbaishan volcano, northeast China: A precursor of a future eruption?

Over 12 years of continuous monitoring of Changbaishan volcano in the border region of China and North Korea by means of volcanic seismicity, ground deformation, and volcanic gas geochemistry yields new evidence for magmatic unrest of the volcano between 2002 and 2006. In this so‐called “active period,” the frequency of volcanic earthquakes increased by about 2 orders of magnitude compared to that of the background “inactive periods.” The active period was also accompanied by ground inflation, high values of CO2, He, H2, and high ratios of N2/O2and3He/4He in volcanic gases released from three hot springs near the caldera rim. The monitoring evidence implies pressurization of the magma chamber, possibly caused by incremental magma recharge. The ground deformation data from both GPS and precise leveling are modeled to suggest the corresponding deformation source is at 2–60 km depth beneath the volcano's summit, where earthquake swarms were detected in 2002 and 2003. Our findings suggest that the magma chamber beneath Changbaishan volcano has awakened and resumed activity after remaining dormant since AD 1903. There is an urgent need to keep close watch on this active and very hazardous volcano in northeast China.

Internal structure and building of basaltic shield volcanoes: the example of the Piton de La Fournaise terminal cone (La Réunion)

In April 2007, a caldera collapsed at the Dolomieu summit crater of Piton de La Fournaise (La Réunion Island, Indian Ocean) revealing new outcrops up to 340 m high along the crater walls. The lithostratigraphic interpretation of these new exposures allows us to investigate the most recent building history of a basaltic shield volcano. We present the history of the Piton de La Fournaise terminal cone, from the building of a juvenile cone during which periods of explosive activity dominated, to the most recent effusive period. The changes in eruptive dynamics are the cause of successive summit crater/pit–crater collapses. In April 2007, such an event occurred during rapid emptying of the shallow plumbing system feeding a large effusive lateral eruption. During the most recent effusive period, an eastward migration of the eruptive crater was observed and was linked to the successive destructions of the shallow magma reservoir during each collapse. The resulting changes in the local stress field favor the formation of a new reservoir and thus the migration of activity. Internal structures reveal that the building of the upper part of the terminal cone was predominantly by exogenous growth and that the hydrothermal system is confined at a depth > 350 m. These observations on Piton de La Fournaise provide new insights into construction of the summits of other basaltic shield volcanoes.

Kinematic analysis of vertical collapse on volcanoes using experimental models time series

Volcanoes are often associated with vertical collapse, due to deeper magma withdrawal. Calderas are the most notable type of vertical collapse, on the summit of volcanoes. Caldera collapse has been observed and monitored only at Miyakejima (Japan; 2000), Dolomieu (Reunion; 2007) and Fernandina (Galapagos; 1968), highlighting our limited knowledge on its kinematic behavior. Here we use experimental models to investigate the kinematic evolution of calderas and vertical collapses in general. We extract velocity and strain fields using the Particle Image Velocimetry (PIV) technique, generating time series. Experimental vertical collapses undergoing constant subsidence velocity show three main kinematic behaviors: (1) continuous collapse, whose velocity is similar to the source subsidence velocity; (2) incremental collapse, with episodic (stepwise) accelerations along pre‐existing ring structures; (3) sudden collapse, resulting from the upward migration of a cavity, only for T/D > 2 (T and D are the depth and width of the magma chamber, respectively) and without ring structures. The velocity in the collapsing column may increase up to four orders of magnitude with regard to the constant subsidence velocity of the source. Comparison to nature suggests that: (1) there are close kinematic similarities with monitored collapse calderas, explaining their incremental subsidence after the development of ring structures; (2) sudden pit crater formation is induced by the upward propagation of cavities, due to magma removal at depth and in absence of ring structures; (3) all these types of vertical collapses have a consistent mechanism of formation and kinematic behavior, function of T/D and the presence/absence of ring structures.

Understanding Etna flank instability through numerical models

As many active volcanoes, Mount Etna shows clear evidence of flank instability, and different mechanisms were suggested to explain this flank dynamics, based on the recorded deformation pattern and character. Shallow and deep deformations, mainly associated with both eruptive and seismic events, are concentrated along recognised fracture and fault systems, mobilising the eastern and south-eastern flank of the volcano. Several interacting causes were postulated to control the phenomenon, including gravity force, magma ascent along the feeding system, and a very complex local and/or regional tectonic activity. Nevertheless, the complexity of such dynamics is still an open subject of research and being the volcano flanks heavily urbanised, the comprehension of the gravitative dynamics is a major issue for public safety and civil protection. The present research explores the effects of the main geological features (in particular the role of the subetnean clays, interposed between the Apennine–Maghrebian flysch and the volcanic products) and the role of weakness zones, identified by fracture and fault systems, on the slope instability process. The effects of magma intrusions are also investigated. The problem is addressed by integrating field data, laboratory tests and numerical modelling. A bi- and tri-dimensional stress–strain analysis was performed by a finite difference numerical code (FLAC and FLAC3D), mainly aimed at evaluating the relationship among geological features, volcano-tectonic structures and magmatic activity in controlling the deformation processes. The analyses are well supported by dedicated structural–mechanical field surveys, which allowed to estimate the rock mass strength and deformability parameters. To take into account the uncertainties which inevitably occur in a so complicated model, many efforts were done in performing a sensitivity analysis along a WNW–ESE section crossing the volcano summit and the Valle del Bove depression. This was mainly devoted to evaluate the effect of topography, geometry and rheological behaviour of the structural units. The 3D numerical model, extended 40×60km, was implemented to simulate the volcano deformation pattern. First, the role of the Pleistocene subetnean clays was investigated, then, two “structural weakness zones” – the Pernicana Fault system and the NE rift – were introduced and their effects on the flank instability evaluated. Two extreme hydrogeological conditions, drained and undrained, were analysed. The results are expressed in terms of stress–strain field, displacement pattern, plasticity states and shear strain increments. Two main instability mechanisms were identified: one at shallow depth, with the sliding surface located inside the subetnean Quaternary clay, and another deep-seated mechanism with a not continuous and less evident sliding surface, developed inside the Apennine–Maghrebian Chain flysch, bordered by active structures. Both mechanisms contribute to explain the present deformation pattern and some of the main structures of the Etna flank. The effect of magma pressure exerted on the active dyke walls during eruptions was then simulated and relations between magmatic activity and flank instability were preliminarily investigated.

Rapid rates of growth and collapse of Monowai submarine volcano in the Kermadec Arc

Most of Earth’s volcanoes are under water. As a result of their relative inaccessibility, little is known of the structure and evolution of submarine volcanoes. Advances in navigation and sonar imaging techniques have made it possible to map submarine volcanoes in detail, and repeat surveys allow the identification of regions where the depth of the sea floor is actively changing. Here we report the results of a bathymetric survey of Monowai submarine volcano in the Tonga–Kermadec Arc, which we mapped twice within 14 days. We found marked differences in bathymetry between the two surveys, including an increase in seafloor depth up to 18.8 m and a decrease in depth up to 71.9 m. We attribute the depth increase to collapse of the volcano summit region and the decrease to growth of new lava cones and debris flows. Hydroacoustic T-wave data reveal a 5-day-long swarm of seismic events with unusually high amplitude between the surveys, which directly link the depth changes to explosive activity at the volcano. The collapse and growth rates implied by our data are extremely high, compared with measured long-term growth rates of the volcano, demonstrating the pulsating nature of submarine volcanism and highlighting the dynamic nature of the sea floor.

Geodetic observations during the 2009 eruption of Redoubt Volcano, Alaska

In March 2009 Redoubt Volcano, about 160km to the SW of Anchorage, Alaska, began its most recent explosive eruption. Deformation induced by this event was recorded by a GPS campaign network consisting of 14 benchmarks, which had been established in 1991 after the previous eruption. The network was partially reoccupied in 2001 and 2008 and no volcanic deformation was detected during that period. In response to precursory unrest starting in January 2009, the Alaska Volcano Observatory temporarily deployed continuously recording GPS instruments at four of the campaign benchmarks only days before the onset of explosive activity in March 2009.The only GPS instrument recording continuously during the months prior to the eruption was the Plate Boundary Observatory (PBO) station AC17, about 28km northeast of the volcano's summit. Data from this station reveals subtle motion radially outward from the volcano beginning as early as May 2008, which reversed with the onset of explosive activity.Using simple analytical models we link the precursory activity to a point source intrusion of 0.0194 0.03400.0092km3 in volume at 13.50 17.3310.17kmbelowsealevel (bsl, superscripts and subscripts refer to upper and lower ends of confidence intervals at the 95% level). During the explosive phase about 0.05 >0.10.028km3 of magma was evacuated from a prolate spheroid with its centroid at 9.17 15.176.92kmbsl, a semimajor axis of 4.50 >10.001.25km length and a semi-minor axis of 0.475 >4.00.3km. The effusive activity is inferred to come from the same source, decreasing in volume by 0.0167 0.02280.0106km3.Including observations from seismology and petrology, we hypothesize a mid-crustal two reservoir system with material sourced from >20km flowing in at about 13.5km depth and reheating residual material in the proposed spheroid. The mixture migrated to shallower depth (2–4.5kmbsl) and reheated material there. As this residual magma erupted, it was replaced by the material from the spheroidal reservoir at 7–11.5km depth, which renders the shallow source undetectable for geodetic instruments.In addition to long term displacements we investigate sub-daily kinematic positioning solutions and find that large, short-term positioning offsets correlate with large explosive events. Spikes in phase residuals plotted along the sky tracks of individual satellites can be related to individual plumes given favorable satellite–station-geometry, which may be of use in volcano monitoring.

The transition of spatter to lava-like body in lava fountain deposits: features and examples from the Cabezo Segura volcano (Calatrava, Spain)

The Cabezo Segura II volcanic cone (Calatrava volcanic province, Iberian microplate) comprises proximal wall deposits with a well defined crater wall unconformity and crater-fill deposits. The complex volcanic succession, that shows evidence of several eruptive episodes, was built by magmatic and hydrovolcanic explosions of different styles (Strombolian, Hawaiian, sub-Plinian and phreato-Strombolian) generated from a multiple feeder ultrabasic dyke. Intra-crater rock units at the volcano summit include spatter deposits together with up to 10m thick and more than 200m long lava-like bodies. Geological logs for the main lava-like bodies define a characteristic facies model that involves a central lava-like mass which grades vertically into a transition zone of apparently coherent spatter, then dense spatter and, finally, into vuggy spatter deposits. These units are inferred to have formed during pulsating lava fountain-type explosive eruptions; the depicted facies distribution being the result of progressive increase in welding grade and densification of the spatter in response to variations in the accumulation rate. Their field features may be used as a guide for the precise identification of vent sites in deposits of Hawaiian eruptions. Also, structures like those here recognised, that might have survived in lava-like flows, could be of help to identify when lava-producing eruptions represented an explosive Hawaiian event (lava fountains) and not a purely effusive event.

Barometry of lavas from the 1951 eruption of Fogo, Cape Verde Islands: Implications for historic and prehistoric magma plumbing systems

Fogo is one of the most active oceanic volcanoes in the world. The island was affected by a prehistoric giant lateral collapse that decapitated the summit of the former Monte Amarelo volcano. Subsequent volcanism has partly filled the collapse scar and built up the present-day Cha das Caldeiras plain and the Pico do Fogo stratovolcano. We have conducted a thermobarometric study of historic and prehistoric, basanitic to tephritic rocks in order to gain insight into Fogo's magma plumbing system and the impact of the collapse event on fractionation depths. A main focus was the penultimate 1951 eruption, which produced basanites to tephrites (5.0–8.2wt.% MgO) at two sites south and northwest of Pico do Fogo. Clinopyroxene–melt barometry of phenocrysts yields a well-confined pressure range of 480–650MPa for the final crystallization level. Microthermometric data of CO2-dominated fluid inclusions in olivine and clinopyroxene phenocrysts yield systematically lower pressures of 250–430MPa. Inclusions in cumulate xenoliths yield pressures of 100–290MPa. The combined data indicate pre-eruptive magma storage in the uppermost mantle between 17 and 22km depth and syn-eruptive short-term magma stalling within the lower crust at 8–13km depth. The lower pressures revealed by fluid inclusions in xenoliths may indicate that they originate from pre-1951 magma pulses that stalled and crystallized at variable levels in the crust. There is, however, no petrologic evidence for persistent crustal magma chambers. Clinopyroxene–melt barometric data of other historic and prehistoric eruptions indicate that magma storage and differentiation occurred in the uppermost mantle at pressures between 420 and 870MPa (15–30km depth) throughout the subaerial evolution of Fogo. Our data indicate that fractionation depths decreased significantly during a period of about 100ka representing a strong growth phase of Fogo edifice leading up to the giant Monte Amarelo flank collapse at 123–62ka. This trend seems to have been interrupted by the flank collapse, as early post-collapse magmas stagnated at deeper levels than late pre-collapse ones. Historic eruptions show shallower and broader pressure ranges, indicating an increase in complexity of the magma storage systems. Thus petrologic data indicate that flank collapse events may significantly influence deep-seated magma plumbing systems beneath ocean islands.

Submarine volcanoes of the Kolumbo volcanic zone NE of Santorini Caldera, Greece

The seafloor northeast of Santorini volcano in Greece consists of a small, elongated rifted basin that has been the site of recent submarine volcanism. This area lies within the Cyclades back-arc region of the present Hellenic subduction zone where the seafloor of the eastern Mediterranean Sea is descending beneath the Aegean microplate. The Cycladic region and the Aegean Sea as a whole are known to be regions of north–south back-arc extension and thinning of continental crust. Nineteen submarine volcanic cones occur within this small rift zone, the largest of these being Kolumbo which last erupted explosively in 1650AD, causing significant damage and fatalities on the nearby island of Santorini. Previous SEABEAM mapping and seismic studies from HCMR indicate that many of the smaller v'olcanic cones have been built above the present seafloor, while others are partly buried, indicating a range of ages for the activity along this volcanic line. None of the cones to the northeast of Kolumbo had been explored in detail prior to a cruise of the E/V Nautilus (NA007) in August 2010. The ROV Hercules was used to explore the slopes, summits and craters of 17 of the 19 centers identified on multibeam maps of the area. Water depths of the submarine volcano's summits ranged from 18 to 450m. In general, the domes/craters northeast of Kolumbo were sediment covered and showed little evidence of recent volcanic activity. Outcrops of volcanic rock were found in the crater walls and slopes of some of the cones but they typically consisted of volcanic fragments of pumice and lava that have been cemented together by biological activity, indicative of the lack of recent eruptions. Geochemical analysis of samples collected on the northeast cones showed evidence of low temperature hydrothermal circulation on the summit and upper flanks in the form of stream-like manganese precipitates emanating from pits and fractures.

飛騨山脈焼岳火山周辺における東北地方太平洋沖地震後の群発地震活動

Seismic activity near the Yake-dake (Mt. Yake) volcano in the Hida mountain range that took place immediate after the 2011 off the Pacific Coast of Tohoku earthquake was investigated. It initiated about ten minutes after the mainshock of the Tohoku earthquake and lasted for about one month. At the beginning, two active swarms were observed. One is at the northern flank of the Yake-dake volcano and the other is located between Yake-dake and Mt. Norikura volcanoes. The latter activity decreased by March 20, and the former activity lasted until early April. It includes two M≥4.5 earthquakes and we could locate more than 9,600 events in the study area during March and April. We mainly focused on the activity near the Yake-dake volcano in this paper. Near the Yake-dake volcano, seismic activity began with M4.7 (JMA) earthquake at 14:57 JST on March 11. This M4.7 event is located 3 km north to the volcano and seismicity increased between the summit of the Yake-dake volcano and the hypocenter. On March 21, an M4.8 (JMA) event took place at 13:15 JST also at 3 km north to the volcano. After this second M≥4.5 earthquake, seismic activity migrated to the north about 1 km. Focal mechanism solutions of these swarm earthquakes show NW-SE compression stress field, which coincides with regional stress field indicated by previous studies. No temporal changes of focal mechanisms are shown during March and April, which probably indicates no magmatic activity such as dyke intrusion related to the Yake-dake volcano took place in this time period.

Geologic interpretation of the near-infrared images of the surface taken by the Venus Monitoring Camera, Venus Express

We analyze night-time near-infrared (NIR) thermal emission images of the Venus surface obtained with the 1-μm channel of the Venus Monitoring Camera onboard Venus Express. Comparison with the results of the Magellan radar survey and the model NIR images of the Beta-Phoebe region show that the night-time VMC images provide reliable information on spatial variations of the NIR surface emission. In this paper we consider if tessera terrain has the different NIR emissivity (and thus mineralogic composition) in comparison to the surrounding basaltic plains. This is done through the study of an area SW of Beta Regio where there is a massif of tessera terrain, Chimon-mana Tessera, surrounded by supposedly basaltic plains. Our analysis showed that 1-μm emissivity of tessera surface material is by 15–35% lower than that of relatively fresh supposedly basaltic lavas of plains and volcanic edifices. This is consistent with hypothesis that the tessera material is not basaltic, maybe felsic, that is in agreement with the results of analyses of VEX VIRTIS and Galileo NIMS data. If the felsic nature of venusian tesserae will be confirmed in further studies this may have important implications on geochemical environments in early history of Venus. We have found that the surface materials of plains in the study area are very variegated in their 1-μm emissivity, which probably reflects variability of degree of their chemical weathering. We have also found a possible decrease of the calculated emissivity at the top of Tuulikki Mons volcano which, if real, may be due to different (more felsic?) composition of volcanic products on the volcano summit.

Hydrogen in the gas plume of an open-vent volcano, Mount Etna, Italy

[1] We report here on the first hydrogen determinations in the volcanic gas plume of Mount Etna, in Italy, which we obtained during periodic field surveys on the volcano's summit area with an upgraded MultiGAS. Using a specific (EZT3HYT) electrochemical sensor, we resolved H2 concentrations in the plume of 1–3 ppm above ambient (background) atmosphere and derived H2-SO2 and H2-H2O plume molar ratios of 0.002–0.044 (mean 0.013) and 0.0001–0.0042 (mean 0.0018), respectively. Taking the above H2-SO2 ratios in combination with a time-averaged SO2 flux of 1600 Gg yr−1, we evaluate that Etna contributes a time-averaged H2 flux of ∼0.65 Gg yr−1, suggesting that the volcanogenic contribution to the global atmospheric H2 budget (70,000–100,000 Gg yr−1) is marginal. We also use our observed H2-H2O ratios to propose that Etna's passive plume composition is (at least partially) representative of a quenched (temperatures between 750°C and 950°C) equilibrium in the gas-magma system, at redox conditions close to the nickel-nickel oxide (NNO) mineral buffer. The positive dependence between H2-SO2, H2-H2O, and CO2-SO2 ratios suggests that H2 is likely supplied (at least in part) by deeply rising CO2-rich gas bubbles, fluxing through a CO2-depleted shallow conduit magma.