Caldera Rim Research Articles

La Fossa Caldera breaching and submarine erosion (Vulcano island, Italy)

La Fossa Caldera (LFC) on Vulcano island, in the Aeolian archipelago (Italy), is a multi-collapse caldera whose north-eastern part lies underwater. Through the integration of marine geophysical surveys and sampling carried out in the last decade, its submarine extension and offshore areas have been reconstructed in unprecedented detail. Strong erosive processes breached the submarine portion of LFC and gradually dismantled the caldera rims and infill, leading to the emplacement of a wide volcaniclastic fan from the base of LFC down to over 1000m. The bathy-morphologic setting, seismoacoustic facies and analyses of sediment samples from the fan, together with volumetric considerations, suggest that this feature is the pathway/deposit of submarine unconfined gravity flows and turbidity currents deriving from the gradual dismantling of LFC. Submarine erosive processes are still active and the progressive erosion of the northern caldera sector threatens the stability of the active La Fossa Cone and the nearby coastal settlements.

Remanent magnetization of maghemitized basalts from Krafla drill cores, NE-Iceland

We investigated the natural remanent magnetization (Jr) of hydrothermally altered basalts from two drill cores KH1 (200 m) and KH3 (400 m) situated at the rim of the Krafla caldera in NE Iceland, where a geothermal field (>150°C) is still active. Low temperature oxidation along with mineral reactions in the chlorite zone (<350°C) is the prevailing cause for the maghemitization and a strong decrease of Jr to occur in our study. Despite a significant decrease of Jr with respect to fresh basalts in surface outcrops of the same area, the stepwise demagnetization analyses of Jr show the presence of a stable magnetic component with the expected inclination of 77° in Iceland. Because the alteration temperature (<350°C) is above the Curie temperatures of most of the original titanomagnetite (40°–350°C), we suggest that a normal direction of remanence is chemically acquired during the low temperature alteration. We observed only one reliable negative inclination at 293.2 m in the KH3 core, which we rather interpret to be acquired during a geomagnetic excursion with reverse polarity than caused by a self-reversal mechanism.

Estimation of the near-surface velocity structure of the Yasur-Yenkahe volcanic complex, Vanuatu

Small-aperture array measurements of seismic noise at seven sites around Yasur volcano (Vanuatu) are performed to estimate the VP and VS velocities of the shallow structure. The spatial autocorrelation (SPAC) and the frequency–wavenumber (f–k) methods are used to determine Rayleigh phase velocity dispersion curves. Phase velocities computed with the SPAC method vary between 580m/s and 960m/s at 1Hz and between 270m/s and 420m/s at 15Hz. F–k dispersion curves show velocities of 300–340m/s and 800–940m/s at 1Hz and 200–230m/s at 15Hz. An inversion technique based on the use of the neighbourhood algorithm has been used to calculate the one-dimensional velocity model at each site. Velocity models reach 200m deep and mainly contain two layers and a half-space. For sites close to the Siwi caldera rims, comparisons with geology and hydrothermal system studies suggest that the two layers highlighted in models may correspond to two large pyroclastic sequences related to caldera collapses based on the flank of an old volcano. Results obtained for the other three sites, located inside the caldera, show the influence of the hydrothermal system on P- and S-wave velocities. For these sites, fluid circulation inside the volcanic deposits causes lower velocities at depth. To obtain a near-surface velocity model of the volcanic structure, each 1D velocity model is spatially extrapolated according to the surface geology. Results highlight four distinct areas, the Siwi caldera edges with high velocities and the resurgent block, the ash plain and the Yasur edifice with lower velocities at depth.

Holocene vegetation history based on phytolith records in Asodani Valley, northern part of the Aso caldera, Japan

Vegetation history during the Holocene has been reconstructed using phytolith records obtained from three tephra sections in the Asodani Valley basin, northern part of Aso caldera, southwestern Japan. The topography of the Asodani Valley basin is divided into four types: somma (caldera rim and pyroclastic-flow plateau), caldera wall, caldera floor and post-caldera central cone slope. The Holocene vegetation transitions in the basin vary between these topographic types. At the northern caldera rim, Sasa (cool-temperature dwarf bamboo) grassland existed prior to 8 ka, and thereafter Pleioblastus (warm-temperature dwarf bamboo) grassland dominated to the present. At the steep caldera wall, forest with understory of Bambusoideae plants (Sasa and Pleioblastus) existed during the Holocene, and mixed forests composed of evergreen trees, deciduous trees and conifers appeared after 6 ka. On the post-caldera central cone slope, grassland vegetation composed of Sasa existed prior to 10.2 ka, and Miscanthus (pampas grass) grassland with sparse trees established around 13 ka and flourished in the last 4 ka. The center of the Asodani Valley was occupied by the last intra-caldera lake prior to 8.9 ka, and thereafter changed to swampy and fluvial environments. Phragmites occurred continuously in and around the intra-caldera lake and the subsequent marshes and swamps in the last 11.4 ka. The difference of vegetation transition in the Holocene between topographic types can be explained as difference of human activities in and around the Aso caldera. Grassland dominated by Sasa, Pleioblastus and Miscanthus occurred on pyroclastic-flow plateau around the caldera and on the central cone slope, which were strongly affected by human activities including burning for a long time, and forest with understory Bambusoideae plants was preserved on the caldera wall, which is unapproachable because of the steep topography.

Reinvestigation of the 1989 Mammoth Mountain, California seismic swarm and dike intrusion

During the period of unrest and inflation beneath Long Valley, east central California, between 1988 and 1992, a swarm of seismic activity occurred beneath Mammoth Mountain, on the caldera rim in 1989 followed by the onset of diffuse CO 2 degassing. These events, combined with elevation changes along a leveling line north of Mammoth Mountain suggested a dike intrusion. An early inversion of deformation data resulted in a dike model with an unrealistically small amount of opening (13 cm) for the proposed dimensions. The current reinvestigation of the 1989 seismic event at Mammoth Mountain, CA incorporates relocated earthquakes ( Prejean et al., 2003) and ellipsoidal inflation sources beneath the caldera to the east ( Langbein, 2003) to better constrain the location and dimensions of a proposed dike intrusion beneath Mammoth Mountain. Assuming that the dike coincides with a “keel” of seismicity at 7–9 km depth, most of the dislocation parameters can be constrained. Fixing all dike parameters except opening to coincide with the keel, inversion of the local leveling data yields a dike with 3.3 m of opening. Allowing the dike height to vary as well improves the fit to the data significantly and results in a dike 1 m wide and nearly 7 m high. These results indicate that the intruded dike had significantly more opening than previously modeled. Magma volumes for reasonable models center around 0.015 km 3, suggesting a small, likely basaltic, intrusion.

90,000-year phytolith record from tephra section at the northeastern rim of Aso caldera, Japan

Vegetation history during the last 90,000 years has been reconstructed using the phytolith record obtained from a tephra section located at the northeastern rim of Aso caldera, southwestern Japan. The phytolith assemblage from the section revealed that grassland vegetation dominated by Gramineae consistently occurred for 90,000 years. Sparse vegetation composed of Sasa sect. Crassinodi (cool-temperate dwarf bamboo) and Zoysia (lawn) was considered to be established within 1000 years after the catastrophic Aso-4 eruption (89 ka). The Sasa grassland dominated by Sasa sect. Crassinodi existed continuously at the northeastern caldera rim between 89 and 13.5 ka. The dominance of Sasa sect. Crassinodi in the grassland suggests that the Aso caldera rim during the period was under a cooler and drier climate. Plants other than Sasa dwarf bamboo declined during the period of 75–66 ka (MIS4) whereas Pleioblastus sect. Nezasa (warm-temperate dwarf bamboo), Andropogoneae (pampas grass) and Zoysia existed in the Sasa grassland between 66 and 30 ka (MIS3). During the period from 30 to 13.5 ka (MIS2) corresponding to the Last Glacial Maximum (LGM), phytoliths from plants other than Sasa dwarf bamboo and fern are only present in small numbers, suggesting that they declined in response to a cool climate during the LGM. Sasa grassland, which had continued since 89 ka, existed after 13.5 ka at the northeastern rim of Aso caldera. In the earlier stage of the Holocene, phytoliths of Sasa sect. Crassinodi and Sasa sect. Sasa dominated, but Pleioblastus dwarf bamboo became a constituent of the grassland around 8 ka. The proportion of Pleioblastus dwarf bamboo (mainly Pleioblastus sect. Nezasa) in the grassland increased and continued flourishing until the present, although Sasa gradually decreased. In addition, arboreal phytoliths were recognized at several Holocene horizons. These phenomena indicate that grassland vegetation composed mainly of Sasa and Pleioblastus dwarf bamboos with sparse trees dominated at the northeastern rim of the Aso caldera in Holocene time.

Rocas piroclásticas asociadas conla formación de la caldera de Guayabo, Cordillera de Guanacaste, Costa Rica

The volcanic depression of the Guayabo calderas was emplaced during several collapses between 1.5 to 0.6 m.y. ago. These eruptions are recorded in a series of pumice-ash flows, pyroclastic surges and air-falls tuffs, called Guayabo Formation, that are exposed in the western and southern part of the Guayabo caldera rim. The pyroclastic-flow sheets represent more than 30 km3 of dacitic to rhyolitic magma associated to crystal fractionation processes. The volumen and also the energy of the eruptions decrease in the time. Microprobe and geobarometric analyses of amphiobole suggest that the level of the magma chamber was less than 5 km.

Modification of the magnetic mineralogy in basalts due to fluid-rock interactions in a high-temperature geothermal system (Krafla, Iceland)

SUMMARY Active high-temperature (>150 °C) geothermal areas like the Krafla caldera, NE-Iceland, often show distinct magnetic lows in aeromagnetic anomaly maps suggesting a destruction of magnetic minerals by hydrothermal activity. The main alteration processes in such an environment are low-temperature oxidation (<350 °C, maghemitization) and fluid–rock interactions. We investigated the rock magnetic properties [natural remanent magnetization (NRM) magnetic susceptibility and their temperature and field variation] and the mineralogy, using X-ray diffraction, microscopic methods and electron microprobe analyses, of two drill cores (KH1 and KH3) from the rim of the Krafla caldera. The drill cores have distinctly lower NRM values (average <3 A m–1) compared to younger surface basalts (average 20 A m–1) along with a large variation in magnetic susceptibility (1.3 × 10−7– 4.9 × 10−5 m3 kg–1). The secondary mineral assemblage (sulphides, sphene, rutile and chlorite) indicates an alteration within the chlorite–smectite zone for both cores without depth zoning. Optical miscroscopy in combination with the Bitter technique and backscatter electron microscopy along with the thermomagnetic analyses allow distinguishing two different magnetomineralogical groups of titanomaghemite: (1) titanomaghemite with intermediate titanium concentration and probably high vacancy concentration, and (2) titanomaghemite with low titanium concentration and low vacancy concentration. The mineral assemblages, textures and magnetic properties deduced from the mentioned magnetic measurements indicate two-stage transformation mechanism: (1) Dissolution of titanium at low pH under oxidizing conditions. The ulvospinel component of titanomagnetite and ilmenite forms rutile or sphene, and Fe2 + migrates out of the spinel lattice forming titanomaghemite. (2) Formation of pyrite and dissolution of remaining titanomaghemite under reducing and acidic conditions. The latter mechanism produces ghost textures (all titanomaghemite is transformed and only their former grain shapes are preserved), with only paramagnetic minerals left and ferrimagnetic minerals nearly dissolved. This mechanism could explain the significant magnetization loss, which is seen in many local magnetic anomaly lows within the oceanic crust and volcanic islands like Iceland or Hawaii. The production of nanoporous textures in titanomaghemites is suggested as a mechanism for the enhancement of the magnetic susceptibility values related to the hydrothermal alteration of Krafla.

Vegetation changes after the late period of the Last Glacial Age based on pollen analysis of the northern area of Aso Caldera in central Kyushu, Southwest Japan

Within the Aso Caldera in central Kyushu, Japan, the Asodani valley is located in the northern part of the inner area, encircled by a wide caldera rim plain consisting largely of grassland. Pollen analyses and radiocarbon dating of samples from the Senchomuta core of the upper part of the basal sediments in the inner area revealed a change in vegetation after around 14,000 cal BP. According to the analytic results from the Hosenbashi core, the forest was mainly formed under cool-temperate to sub-arctic conditions at about 24,000–17,000 cal BP, followed by a change to temperate conditions with predominance of deciduous trees, and subsequently followed by warm-temperate conditions to the present consisting largely of evergreen trees.Herb pollen abundance indicated that the grassland on the broad caldera rim plain 300 to 500 m above Asodani developed at the Last Glacial Age. During the climatic warming of the Post Glacial Age, the grassland basically continued with trees until just after the K-Ah ash layer (7280 cal BP) was deposited. At about 6500 to 5000 cal BP, the herb pollen rate decreased, and the grassland changed to some scattered low-density forest vegetation composed of pine, deciduous and/or evergreen Quercus, and willow. After that period, it is suspected that the grassland was restored by human activity.

A scattering image of Campi Flegrei from the autocorrelation functions of velocity tomograms

SUMMARY We propose a new quantitative approach for the joint interpretation of velocity and attenuation tomography images, performed through the lateral separation of scattering and intrinsic attenuation. The horizontal P-wave scattering attenuation structure below Campi Flegrei Caldera (CFC) is imaged using the autocorrelation functions (ACF) of P-wave vertical velocity fluctuations. Cluster analysis (CA) is then applied to interpret the images derived from ACF and the available P-wave total attenuation images at 2000 m quantitatively. The analysis allows the separation of intrinsic and scattering attenuation on a 2-D plane, adding new geophysical constraints to the present knowledge about this volcanic area. The final result is a new, quantitative image of the past and present tectonic and volcanological state of CFC. P-wave intrinsic dissipation dominates in an area approximately located under the volcanic centre of Solfatara, as expected in a region with a large presence of fluids and gas. A north–south scattering attenuation region is mainly located below the zone of maximum uplift in the 1982–1984 bradiseismic crisis, in the sea side of the Pozzuoli bay, but also extending below Mt Nuovo. This evidence favours the interpretation in terms of a hard but fractured body, contoured by strong S-wave scatterers, corresponding to the Caldera rim: the region is possibly a section of the residual magma body, associated with the 1538 eruption of Mt Nuovo.

A gravity study of silicic domes in the Krafla area, N-Iceland

Silicic rocks in Iceland are generally associated with central volcanoes and are often emplaced as domes on or around caldera rims. Some of these domes were formed subglacially while others were erupted under ice-free conditions. A gravity survey was carried out in the area of Krafla in 2007 and 2008 to determine the mean bulk densities of three silicic domes; essential data for meaningful modelling of the emplacement of cryptodomes and lava domes. Such data are scarce. Profiles were measured over three formations: Hlíðarfjall, made of rhyolite, 310 m high, 2 km long and formed under ice 90 000 years BP, Hrafntinnuhryggur, made of rhyolite, 80 m high, 2.5 km long and formed subglacially 24 000 years BP and Hraunbunga made of dacite, 125 m high and 1.8 km long, formed under ice-free conditions 10 000 years BP. Mean bulk density for each formation was obtained by the Nettleton method. Mean bulk density and volumes obtained were; Hlíðarfjall: 1600–1800 kg/m³, 0.143±0.014 km³; Hrafntinnuhryggur: 1575–1875 kg/m³, 0.021±0.002 km³; Hraunbunga: 1750–1775 kg/m³, 0.040±0.004 km³. The results show that all the domes have low densities, reflecting both low grain-density and high porosity. The density values are significantly lower than those of the surroundings, creating a density contrast possibly sufficient to drive the ascent of silicic magma. Furthermore, results from forward gravity modelling demonstrate that these formations are neither buried by younger volcanic eruptives nor are any roots detected. The domes studied were therefore emplaced by a dike to the surface.

The May 2005 eruption of Fernandina volcano, Galápagos: The first circumferential dike intrusion observed by GPS and InSAR

The May 2005 eruption of Fernandina volcano, Galapagos, occurred along circumferential fissures parallel to the caldera rim and fed lava flows down the steep southwestern slope of the volcano for several weeks. This was the first circumferential dike intrusion ever observed by both InSAR and GPS measurements and thus provides an opportunity to determine the subsurface geometry of these enigmatic structures that are common on Galapagos volcanoes but are rare elsewhere. Pre- and post- eruption ground deformation between 2002 and 2006 can be modeled by the inflation of two separate magma reservoirs beneath the caldera: a shallow sill at ~1 km depth and a deeper point-source at ~5 km depth, and we infer that this system also existed at the time of the 2005 eruption. The co-eruption deformation is dominated by uplift near the 2005 eruptive fissures, superimposed on a broad subsidence centered on the caldera. Modeling of the co-eruption deformation was performed by including various combinations of planar dislocations to simulate the 2005 circumferential dike intrusion. We found that a single planar dike could not match both the InSAR and GPS data. Our best-fit model includes three planar dikes connected along hinge lines to simulate a curved concave shell that is steeply dipping (~45–60°) toward the caldera at the surface and more gently dipping (~12–14°) at depth where it connects to the horizontal sub-caldera sill. The shallow sill is underlain by the deep point source. The geometry of this modeled magmatic system is consistent with the petrology of Fernandina lavas, which suggest that circumferential eruptions tap the shallowest parts of the system, whereas radial eruptions are fed from deeper levels. The recent history of eruptions at Fernandina is also consistent with the idea that circumferential and radial intrusions are sometimes in a stress-feedback relationship and alternate in time with one another.

Borobudur, a basin under volcanic influence: 361,000 years BP to present

Borobudur basin is located in Central Java (Indonesia), 30 km to the North of Yogyakarta City. The basin is famous for its UNESCO world heritage temple and for one of the world's most active volcanoes, Merapi, located to the East of Borobudur basin. Merapi is one of the three andesitic volcanoes that surround the basin: Merapi, Merbabu and Sumbing volcanoes. Therefore, volcanic activity has strongly influenced the evolution of Borobudur basin. The object of this contribution is to present the evolution of Borobudur basin over the last 161,000 years in the light of volcanic influence. The methodology and tools developed for this research span over different areas of expertise, from geochemistry, geology and geomorphology to remote sensing, GIS and archeology. Results highlight the following points: 1. Two major volcanic events deposited volcaniclastic materials up to tens of meters thick ~ 119,000 years BP and ~ 31,000 years BP. in the Southern part of the Borobudur basin. The second volcanic event could correspond to the collapse of the older Ancient Merapi ( Camus et al., 2000) or Proto-Merapi Stage ( Newhall et al., 2000). 2. There is no trace in the Borobudur basin of a large debris avalanche < 31,000 BP, indicating that the young debris avalanche inferred in the literature for Merapi Volcano was either too small to reach 20 km from the actual summit of Merapi; or, despite the orientation of the avalanche caldera rim on Merapi Volcano, the debris avalanche was deposited more towards the South, completely eroded or covered by younger deposits. 3. There are several generations of paleolakes in the Borobudur basin. The latest one has shrunk until historical times, corroborating the theory of Newhall et al. (2000) and Murwanto et al. (2004) that Borobudur Temple was standing by a water body. Most of these paleolakes were impounded following volcanic events. Paleolakes most probably originated from the blockage of the hydrographic network by volcanic material. 4. Borobudur temple was never buried under volcanic material during historic times.

Characteristics and management of the 2006–2008 volcanic crisis at the Ubinas volcano (Peru)

Ubinas volcano is located 75 km East of Arequipa and ca. 5000 people are living within 12 km from the summit. This composite cone is considered the most active volcano in southern Peru owing to its 24 low to moderate magnitude (VEI 1–3) eruptions in the past 500 years. The onset of the most recent eruptive episode occurred on 27 March 2006, following 8 months of heightened fumarolic activity. Vulcanian explosions occurred between 14 April 2006 and September 2007, at a time ejecting blocks up to 40 cm in diameter to distances of 2 km. Ash columns commonly rose to 3.5 km above the caldera rim and dispersed fine ash and aerosols to distances of 80 km between April 2006 and April 2007. Until April 2007, the total volume of ash was estimated at 0.004 km 3, suggesting that the volume of fresh magma was small. Ash fallout has affected residents, livestock, water supplies, and crop cultivation within an area of ca. 100 km 2 around the volcano. Continuous degassing and intermittent mild vulcanian explosions lasted until the end of 2008. Shortly after the initial explosions on mid April 2006 that spread ash fallout within 7 km of the volcano, an integrated Scientific Committee including three Peruvian institutes affiliated to the Regional Committee of Civil Defense for Moquegua, aided by members of the international cooperation, worked together to: i) elaborate and publish volcanic hazard maps; ii) inform and educate the population; and iii) advise regional authorities in regard to the management of the volcanic crisis and the preparation of contingency plans. Although the 2006–2008 volcanic crisis has been moderate, its management has been a difficult task even though less than 5000 people now live around the Ubinas volcano. However, the successful management has provided experience and skills to the scientific community. This volcanic crisis was not the first one that Peru has experienced but the 2006–2008 experience is the first long-lasting crisis that the Peruvian civil authorities have had to cope with, including attempts to utilize a new alert-level scheme and communications system, and the successful evacuation of 1150 people. Lessons learned can be applied to future volcanic crises in southern Peru, particularly in the case of reawakening of El Misti volcano nearby Arequipa.

Quantitative hazard assessment of phreatomagmatic eruptions at Vulcano (Aeolian Islands, Southern Italy) as obtained by combining stratigraphy, event statistics and physical modelling

The detailed analysis of stratigraphy allowed the reconstruction of the complex volcanic history of La Fossa di Vulcano. An eruptive activity mainly driven by superficial phreatomagmatic explosions emerged. A statistical analysis of the pyroclastic Successions led to the identification of dilute pyroclastic density currents (base surges) as the most recurrent events, followed by fallout of dense ballistic blocks. The scale of events is related to the amount of magma involved in each explosion. Events involving about 1 million cm 3 of magma occurred during recent eruptions. They led to the formation of hundreds of meters thick dilute pyroclastic density currents, moving down the volcano slope at velocities exceeding 50 m/s. The dispersion of density currents affected the whole Vulcano Porto area, the Vulcanello area. They also overrode the Fossa Caldera's rim, spreading over the Piano area. For the aim of hazard assessment, deposits from La Fossa Cone and La Fossa Caldera were studied in detail, to depict the eruptive scenarios at short-term and at long-term. By means of physical models that make use of deposit particle features, the impact parameters have been calculated. They are dynamic pressure and particle volumetric concentration of density currents, and impact energy of ballistic blocks. A quantitative hazard map, based on these impact parameters, is presented. It could be useful for territory planning and for the calculation of the expected damage.

Morphological and geochemical analysis of the Laguna Blanca/Zapaleri obsidian source in the Atacama Puna

AbstractThis paper describes a large obsidian deposit located along the southern banks of Laguna Blanca, on the eastern slope of the Jarellón volcanic caldera near the Chilean–Bolivian border. The obsidian at this site occurs in flows or sheets of deflated black or reddish to brown pebbles, redeposited on the shores of a lake. Blocks of obsidian are only found around the caldera rim. Here we analyze the shape of the obsidian pebbles and their geochemistry, comparing them with previously published data. The results indicate that the geochemical composition of the samples strongly matches previous analyses of obsidian from cultural contexts. This obsidian source was one of the most important sites of obsidian procurement since at least the Formative Period in the Atacama Puna region. © 2010 Wiley Periodicals, Inc.

Rockfall susceptibility map for Athinios port, Santorini Island, Greece

This article analyzes rockfall susceptibility in the steep caldera slopes upstream of Athinios port, Santorini Island, Greece. The study area is situated in the internal rim of a submarine caldera where the most important problem that is recorded is the frequent rockfalls that not only cause damages to roads and vehicles but also pose a threat to people that are transported or located on the port. As a result, a methodology which combines information relatively to surficial and engineering geology, geomorphological processes, and structural analysis was adopted. The methodology incorporates evenly a maximum runout map generated by means of reach probability of rock block analysis, using the empirical model of “reach angle”. Additionally volumes of rockfall events categorized and presented through a map to assist the compilation of rockfall susceptibility map which allows us to identify areas and human activities exposed to these incidents and set up several protection meters.

Magmatic Evolution and plumbing system of ring-fault volcanism: the Vulcanello Peninsula (Aeolian Islands, Italy)

The Vulcanello peninsula is situated north of Vulcano, the southernmost island of the Aeolian Arc. It was built at the rim of La Fossa Caldera between 1000 and 1650 A.D. Erupted products are mafic to intermediate in composition, while the coeval products erupted inside the caldera are mainly rhyolitic. Therefore, Vulcanello’s activity represents an anomalous mafic post-caldera volcanism in a convergent setting. A petrographic and geochemical study was carried out on lavas and pyroclastic rocks representing the entire eruptive history of the volcanic centre. New data (major and trace elements and Sr isotope ratios on whole rocks, and major element compositions on mineral phases) and geochemical models were used to investigate shallow level differentiation processes ( i.e. , fractional crystallisation, fractional crystallisation plus crustal assimilation, degassing, magma mixing/recharge). The study suggests that the entire Vulcanello activity can be considered as the uninterrupted expulsion of a single deep magma batch of shoshonitic composition emitted from a NE–SW ring fault of La Fossa Caldera. The magma is genetically related to the shoshonitic basalts found as melt inclusions in the olivine crystals erupted in the products of the 1888–1890 “ La Fossa” activity. This points to a possible single deep plumbing system for both La Fossa Cone and Vulcanello centres, strongly controlled by NW–SE to N– S regional structures. The shoshonitic magma, undergoing fractional crystallisation, partly rose directly to the surface where two strombolian cones were constructed, while residual magma remained at depth, and, partially degassed and crystallised, it subsequently erupted both effusively to form a lava platform and explosively to form a third pyroclastic cone. The remaining magma evolved to latite by AFC process and was erupted both as a lava flow (Punta del Roveto) and in the form of pyroclastic products ( i.e. , the upper part of the third cone), controlled by shallow ring faults of La Fossa Caldera. Therefore the Vulcanello plumbing system is controlled by tectonic structures at depth and by shallower volcano-tectonic (caldera) fractures.

New 40Ar/39Ar ages reveal contemporaneous mafic and silicic eruptions during the past 160,000 years at Mammoth Mountain and Long Valley caldera, California

We undertook a 40 Ar/ 39 Ar study of young mafic and silicic lavas at Mammoth Mountain and the Long Valley caldera (east-central California) to better understand the frequency of these eruptions and the magmatic plumbing system that drives them. Our results show that most of Mammoth Mountain, a lava-dome complex straddling the southwestern topographic rim of the caldera, consists of trachydacite lavas erupted at ca. 68 ka. These ages and new 29- and 41 ka ages for trachydacite lavas in the northwest quadrant of the caldera indicate that these silicic lavas are considerably younger than previously thought. Mafic lavas vented widely in the western third of the caldera in the past 190,000 years, suggesting that this area has not been underlain by large bodies of silicic magma during this interval, as such magma would have prevented the rise of the denser basaltic magma. We identify four eruptive sequences over the past 190,000 years: the western moat sequence (∼190–160 ka), the Mammoth sequence (∼120–58 ka), the northwest caldera sequence (∼41–29 ka), and the Inyo chain sequence (∼9 ka–present). In each eruptive sequence mafic and silicic lavas erupted contemporaneously from spatially associated vents. This suggests that intrusion of alkali basalt into the shallow crust led to the silicic eruptions. If the seismic unrest and deformation of the past three decades is a result of basalt injected beneath Mammoth Mountain and perhaps the western third of the caldera, then there is the possibility of spatially associated small-volume silicic eruptions, which would typically be considerably more explosive. In the past 40,000 years, eruptions have occurred along a N-S linear trend less than 10 km wide, limiting the zone subject to volcanic hazards. Our data bear on Pleistocene glaciation in the region. Ages of 162 ± 2 ka and 99 ± 1 ka for bracketing mafic lava flows better constrain the age of the Casa Diablo till. Our results provide equivocal support for a suggested anticorrelation between volcanism and glaciation for the past 800,000 years in eastern California (Glazner et al., 1999).

Characteristics of the summit lakes of Ambae volcano and their potential for generating lahars

Abstract. Volcanic eruptions through crater lakes often generate lahars, causing loss of life and property. On Ambae volcano, recent eruptive activities have rather tended to reduce the water volume in the crater lake (Lake Voui), in turn, reducing the chances for outburst floods. Lake Voui occupies a central position in the summit caldera and is well enclosed by the caldera relief. Eruptions with significantly higher magnitude than that of 1995 and 2005 are required for an outburst. A more probable scenario for lahar events is the overflow from Lake Manaro Lakua bounded on the eastern side by the caldera wall. Morphology and bathymetry analysis have been used to identify the weakest point of the caldera rim from which water from Lake Manaro Lakua may overflow to initiate lahars. The 1916 disaster described on south-east Ambae was possibly triggered by such an outburst from Lake Manaro Lakua. Taking into account the current level of Lake Manaro Lakua well below a critical overflow point, and the apparently low potential of Lake Voui eruptions to trigger lahars, the Ambae summit lakes may not be directly responsible for numerous lahar deposits identified around the Island.