Ash-fall Deposits Research Articles

Integrated Monitoring of Volcanic Ash and Forecasting at Sakurajima Volcano, Japan

The Sakurajima volcano is characterized by frequent vulcanian eruptions at the Minamidake or Showa crater in the summit area. We installed an integrated monitoring system for the detection of volcanic ash (composed of remote sensing sensors XMP radars, lidar, and GNSS with different wave lengths) and 13 optical disdrometers on the ground covering all directions from the crater to measure drop size distribution and falling velocity. Campaign sampling of volcanic ash supports the conversion of particle counts measured by the disdrometer to the weight of volcanic ash. Seismometers and tilt/strain sensors were used to estimate the discharge rate of volcanic ash from the vents. XMP radar can detect volcanic ash clouds even under visual difficulty because of weather such as fog or clouds. A vulcanian eruption on November 13 was the largest event at the Sakurajima volcano in 2017; however, the volcanic plume was not visible due to clouds covering the summit. Radar revealed that the volcanic plume reached an elevation of 4.2–6.2 km. Post-fit phase residuals (PPR) from the GNSS analysis increased suddenly after the eruption, and large-PPR paths from the satellites to the ground-based receivers intersected each other at an elevation of 4.2 km. The height of the volcanic plume was also estimated from the discharge rate of volcanic ash to be 4.5 km, which is empirically related to seismic energy and the deflation volume obtained via ground deformation monitoring. Using the PUFF model, the weight of the ash-fall deposit was accurately forecast in the main direction of transport of the volcanic ash, which was verified by disdrometers. For further advances in forecasting of the ash-fall deposit, we must consider high-resolution wind field, shape of volcanic plume as the initial value, and the particle number distribution along the volcanic plume.

Sedimentary evolution of the marine Agua de la Mula Member (Agrio Formation, Lower Cretaceous) in the central Neuquén Basin: Source areas and paleogeographic considerations from a distal setting

The Agua de la Mula Member (Upper Hauterivian–Lower Barremian) of the Agrio Formation represents the infill of a mixed siliciclastic–carbonate low-gradient ramp system comprising both hydrocarbon sand reservoirs and a source rock interval considered as potential unconventional shale play. The present study analyzes the unit in offshore settings (Loma La Torre section) where high-quality, laterally continuous exposures allowed the recognition of muddy subaqueous clinoforms prograding northwestward. The muddy-dominated facies throughout the entire section represent an overall evolution from distal outer-ramp to inner-ramp settings associated with tidal flat deposits. Based on previous sedimentological studies and paleogeographic reconstruction, four facies belts are refined for the unit in the central Neuquén Basin: (1) shallow marine and deltaic facies to fluvial deposits documented in the subsurface (mainly siliciclastic); (2) mid-ramp to inner-ramp with subordinated outer-ramp settings (mainly siliciclastic); (3) outer-ramp to distal mid-ramp settings with muddy subaqueous clinoforms (mixed siliciclastic–carbonate); and, (4) basinal to outer-ramp areas (carbonate-dominated). In addition to the main sediment input from the south and southeastern margin of the basin, sedimentological and stratigraphic approaches suggest a sediment supply from the eastern and northeastern regions. Sporadic ash-fall deposits also indicate that the coetaneous magmatic arc was active. In fact, LA-ICP-MS U–Pb zircon ages from two tuff layers recorded in the study section yield an age interval of less than 1 m.yr., which allowed us to estimate the sedimentation and progradation rates of a clinoform set. Finally, four asymmetrical sequences are recognized comprising thin (carbonate–siliciclastic) transgressive systems tracts and thick regressive (highstand) systems tracts with stacked parasequences of shallowing-upward siliciclastic-dominated facies. These sequences are correlated with the four (4th-order) sequences previously defined in proximal settings. The observations suggest that sea-level changes were relatively synchronous events and, according to the present knowledge, controlled not only by orbital mechanisms but also by tectonism.

Palaeoenvironmental use of silicophytoliths in soils and palaeosols associated with Holocene volcanic ash deposits in north-western Argentina

We compare the silicophytolith record of current soils and Holocene palaeosols from two sedimentary sequences in a region between the Chaco-Pampean Plain and the Puna (Tolombón section in Santa María Valley, and Tafí del Valle section in Tafí Valley), NW Argentina. These two geographic areas are separated by the Aconquija-Cumbres Calchaquíes ranges, a climatic barrier to the wet Atlantic winds. These sequences contain volcanic ash deposits recording two large eruptions occurred during Holocene in the Southern Puna in the Central Volcanic Zone of the Andes, from the Cueros de Purulla volcano (ca. 7820 BP) and the Cerro Blanco Volcanic Complex (ca. 4200 cal BP). These ash-fall deposits buried soils and preserved their silicophytolith record, allowing the use of this palaeoenvironmental proxy to clarify the vegetation prior these two volcanic events. The silicophytolith assemblages yield evidence for a desert shrubland when Tolombón palaeosol was formed, similar to the present-day environment in the Santa María Valley, with presence of C4 grasses (Chloridoideae and Panicoidae). Instead, the climate was cooler and wetter ca. 4200 cal BP than currently in the Tafí Valley, increasing over time aridity and abundance of C4 grasses (Chloridoidae, Panicoideae), and decreasing the quantity of C3 grasses (Pooideae). Prehistorical and historical land use also contributed to this change. The impact of these large volcanic ash-falls on vegetation did not translate in an abrupt change on phytodiversity. Results point out a stable arid environment during Holocene in the Santa María Valley whereas the Tafí Valley was more sensitive to environmental changes due to its location in the eastern slopes of Aconquija-Cumbres Calchaquíes ranges, more exposed to the wet Atlantic winds.

La gran erupción de hace 4.2 ka cal en Cerro Blanco, Zona Volcánica Central, Andes: nuevos datos sobre los depósitos eruptivos holocenos en la Puna sur y regiones adyacentes

La erupción del Complejo Volcánico Cerro Blanco en el sur de la Puna, noroeste de Argentina (4410–4150 a BP) se investigó para obtener nueva información sobre estratigrafía, geomorfología, volcanología física, dataciones por radiocarbono, petrografía y geoquímica. La caracterización de los productos en relación a la evolución de la caldera de Cerro Blanco permitió estimar la distribución de los depósitos de ceniza de la fase paroxísmica Plineana de la erupción. Estos novedosos resultados evidencian una gran erupción explosiva riolítica que generó depósitos cineríticos en un área de aproximadamente 500.000 km2, acumulando > 100 km3 de tefra (volumen total). Este último valor supera el umbral inferior del Índice de Explosividad Volcánica (IEV) de 7. Los depósitos de caída de ceniza cubrieron la región, llegando a más de 400 km desde el Complejo Volcánico de Cerro Blanco, y los potentes depósitos de flujos piroclásticos rellenaron los valles vecinos alcanzando una distancia de 35 km. Esta erupción es la más grande documentada durante los últimos cinco milenios en la Zona Volcánica Central de los Andes y es probablemente una de las mayores erupciones explosivas holocenas del mundo. Además, se han identificado otras dos erupciones riolíticas en la región procedentes de otros dos centros eruptivos: una durante el Holoceno temprano y otra en el Holoceno tardío. La identificación y caracterización de estos grandes eventos volcánicos proporcionan nuevas guías para los registros geológicos y arqueológicos regionales del Holoceno, siendo marcadores cronostratigráficos de aplicación a una extensa área geográfica de América del Sur.

U-Pb Zircon Dating of Ash Fall Deposits from the Paleozoic Paraná Basin of Brazil and Uruguay: A Reevaluation of the Stratigraphic Correlations

AbstractAsh fall layers and vitroclastic-carrying sediments distributed throughout the entire Permian stratigraphic range of the Parana Basin (Brazil and Uruguay) occur in the Tubarao Supergroup (R...

In situ terminal settling velocity measurements at Stromboli volcano: Input from physical characterization of ash

Ash particle terminal settling velocity is an important parameter to measure in order to constrain the internal dynamics and dispersion of volcanic ash plumes and clouds that emplace ash fall deposits from which source eruption conditions are often inferred. Whereas the total Particle Size Distribution (PSD) is the main parameter to constrain terminal velocities, many studies have empirically highlighted the need to consider shape descriptors such as the sphericity to refine ash settling velocity as a function of size. During radar remote sensing measurements of weak volcanic plumes erupted from Stromboli volcano in 2015, an optical disdrometer was used to measure the size and settling velocities of falling ash particles over time, while six ash fallout samples were collected at different distances from the vent. We focus on the implications of the physical parameters of ash for settling velocity measurements and modeling. Two-dimensional sizes and shapes are automatically characterized for a large number of ash particles using an optical morpho-grainsizer MORPHOLOGI G3. Manually sieved ash samples show sorted, relatively coarse PSDs spanning a few microns to 2000 μm with modal values between 180 and 355 μm. Although negligible in mass, a population of fine particles below 100 μm form a distinct PSD with a mode around 5–20 μm. All size distributions are offset compared to the indicated sieve limits. Accordingly, we use the diagonal of the upper mesh sizes as the upper sieve limit. Morphologically, particles show decreasing average form factors with increasing circle-equivalent diameter, the latter being equal to 0.92 times the average size between the length and intermediate axes of ash particles. Average particle densities measured by water pycnometry are 2755 ± 50 kg m−3 and increase slightly from 2645 to 2811 kg m−3 with decreasing particle size. The measured settling velocities under laboratory conditions with no wind, <3.6 m s−1, are in agreement with the field velocities expected for particles with sizes <460 μm. The Ganser (1993) empirical model for particle settling velocity is the most consistent with our disdrometer settling velocity results. Converting disdrometer detected size into circle equivalent diameter shows similar PSDs between disdrometer measurements and G3 analyses. This validates volcanological applications of the disdrometer to monitor volcanic ash sizes and settling velocities in real-time with ideal field conditions. We discuss ideal conditions and the measurement limitations. In addition to providing sedimentation rates in-situ, calculated reflectivities can be compared with radar reflectivity measurements inside ash plumes to infer first-order ash plume concentrations. Detailed PSDs and shape parameters may be used to further refine radar-derived mass loading retrievals of the ash plumes.

Dynamics of shallow hydrothermal eruptions: new insights from Vulcano’s Breccia di Commenda eruption

Understanding the dynamics and effects of hydrothermal eruptions is crucial to the hazard assessment in both volcanic and geothermal areas. Eruptions from hydrothermal centres may occur associated with magmatic phases, but also as isolated events without magmatic input, with the most recent examples being those of Te Maari (Tongariro, New Zealand) in 2012 and Ontake (Japan) in 2014. The most recent caldera of the Island of Vulcano (southern Italy) hosts in its centre the La Fossa cone, active since 5.5 ka and now characterised by continuous fumarolic degassing. In historical times, La Fossa cone has experienced several hydrothermal eruptions, with the most violent event being the Breccia di Commenda eruption that occurred during the thirteenth century ad. Based on analysis of 170 stratigraphic logs, we show that the Breccia di Commenda eruption occurred in three main phases. After an opening, low-intensity ash emission phase (phase 1), the eruption energy climaxed during phase 2, when a series of violent explosions produced an asymmetric shower of ballistic blocks and the contemporaneous emplacement of highly dispersed, lithic-rich, blast-like pyroclastic density currents (PDCs). The tephra units emplaced during phase 2, ranging in volume from 0.2 to 2.7 × 105 m3, were covered in turn by thin ash fall deposits (phase 3). The dynamics of the most violent and intense stage of the eruption (phase 2) was investigated by numerical simulations. A three-dimensional numerical model was applied, describing the eruptive mixture as a Eulerian–Eulerian, two-phase, non-equilibrium gas-particle fluid (plus a one-way coupled Lagrangian ballistic block fraction). At the initial simulation time, a mass of about 109 kg, with initial overpressure above 10 MPa, and a temperature of 250 °C, was suddenly ejected from a 200-m-long, eastward inclined, NNE–SSW trending fissure. The mass release formed blast-like PDCs on both sides of the fissure and launched ballistic blocks eastwards. Field investigations and numerical simulations confirm that hydrothermal explosions at La Fossa cone include intense ballistic fallout of blocks, emission of PDCs potentially travelling beyond the La Fossa caldera and significant ash fallout. The hazard associated with both ballistic impact and PDC ingress, as associated with hydrothermal eruption, is significantly larger with respect to that associated with Vulcanian-type events of La Fossa.

Patagonia-Antarctica Early Paleozoic conjugate margins: Cambrian synsedimentary silicic magmatism, U-Pb dating of K-bentonites, and related volcanogenic rocks

This study describes the stratigraphic features, petrology and geochemistry and geochronology constraints of K-bentonites, ignimbrites and related volcanogenic rocks interbedded in the marine sedimentary sequence of the El Jagüelito Formation from northern Patagonia basement, southwestern Gondwana margin (41°33′S-65°15′W, South America). Six SHRIMP and ICP-MS U-Pb zircon ages from pyroclastic and volcanogenic sedimentary rocks indicate two stages of synsedimentary volcanism at c. 530 and c. 515 Ma which constraint the whole volcano-sedimentary pile to Early-Middle Cambrian. Distal volcanic ash fall deposits of K-bentonites and proximal pyroclastic flows of ignimbrites were erupted from three subaerial to subaqueous eruptive centers and are associated with volcanogenic facies and sub-volcanic intrusive equivalents. The volcanic rocks were deposited synchronously with on-going dynamic siliciclastic sedimentation into an actively subsiding basin.Dacitic K-bentonites and high-silica rhyolitic ignimbrites belong to the same high-K calc-alkaline/shoshonitic magma series and have a similar peraluminous signature. Their HFSE patterns with relative depletion in Nb, Sr, P, and Ti and the LREE-enriched patterns with a negative Eu anomaly indicate characteristics of subduction-related magmas associated with active continental arc magmatism. The geochemical discrimination diagrams together with geological features suggest an extensional tectonic setting for K-rich magmas within the overall convergent-margin system. Silicic, more potassic magmas with the inherited subduction-related character of the El Jagüelito Formation erupted on a back-arc basin. Their volcanic rocks are products of a mixture of fractionated mafic magmas and partial crustal melts. The integration of the Cambrian synsedimentary explosive volcanism of the El Jagüelito Formation with coeval magmatism of northern Patagonia led the interpretation to the continental scale of the paleo-Pacific margin of Gondwana by Early Paleozoic. The El Jagüelito Formation has provided consistent lithological, stratigraphic, geochemical and geochronological arguments to assess continental comparisons and a paleogeographic reconstruction between the eastern North Patagonian Massif and East Antarctica that fit in well the hypothesis that the northern Patagonia basement once occupied a position adjacent to East Antarctica. A series of geodynamic stages is proposed that allow for providing a Cambrian framework for understanding the magmatic arc-back arc system modified by episodic pulses of extension associated with slab-roll back which terminated in detachment of northern Patagonia from the East Antarctica continental margin by Late Cambrian. The several lines of geological evidence discussed in this paper point to a parautochthonous origin of the eastern North Patagonian Massif as an outboard assemblage that represents the conjugate margin of the Pensacola-Queen Maud-Ellsworth-Whitmore Mountains of Antarctica.

Facies analysis of tuffaceous volcaniclastics and felsic volcanics of Tadpatri Formation, Cuddapah basin, Andhra Pradesh, India

The felsic volcanics, tuff and volcaniclastic rocks within the Tadpatri Formation of Proterozoic Cuddapah basin are not extensively studied so far. It is necessary to evaluate the extrusive environment of felsic lavas with associated ash fall tuffs and define the resedimented volcaniclastic components. The spatial and temporal bimodal association were addressed, but geochemical and petrographic studies of mafic volcanics are paid more attention so far. The limited exposures of eroded felsic volcanics and tuffaceous volcaniclastic components in this terrain are highly altered and that is the challenge of the present facies analysis. Based on field observation and mapping of different lithounits a number of facies are categorized. Unbiased lithogeochemical sampling have provided major and selective trace element data to characterize facies types. Thin-section studies are also carried out to interpret different syn- and post- volcanic features. The facies analysis are used to prepare a representative facies model to visualize the entire phenomenon with reference to the basin evolution. Different devitrification features and other textural as well as structural attributes typical of flow, surge and ash fall deposits are manifested in the middle, lower and upper stratigraphic levels. Spatial and temporal correlation of lithologs are also supportive of bimodal volcanism. Felsic and mafic lavas are interpreted to have erupted through the ~ N-S trending rift-associated fissures due to lithospheric stretching during late Palaeoproterozoic. It is also established from the facies model that the volcaniclastics were deposited in the deeper part of the basin in the east. The rifting and associated pressure release must have provided suitable condition of decompression melting at shallow depth with high geothermal gradient and this partial melting of mantle derived material at lower crust must have produced mafic magmas. Such upwelling into cold crust also caused partial heat transfer and associated melting of the surrounding shallow crustal rocks to generate felsic magmas.

Distribution and mass of tephra-fall deposits from volcanic eruptions of Sakurajima Volcano based on posteruption surveys

We estimate the total mass of ash fall deposits for individual eruptions of Sakurajima Volcano, southwest Japan based on distribution maps of the tephra fallout. Five ash-sampling campaigns were performed between 2011 and 2015, during which time Sakurajima continued to emit ash from frequent Vulcanian explosions. During each survey, between 29 and 53 ash samplers were installed in a zone 2.2–43 km downwind of the source crater. Total masses of erupted tephra were estimated using several empirical methods based on the relationship between the area surrounded by a given isopleth and the thickness of ash fall within each isopleth. We obtained 70–40,520 t (4.7 × 10−8–2.7 × 10−5-km3 DRE) as the minimum estimated mass of erupted materials for each eruption period. The minimum erupted mass of tephra produced during the recorded events was calculated as being 890–5140 t (5.9 × 10−7–3.6 × 10−6-km3 DRE). This calculation was based on the total mass of tephra collected during any one eruptive period and the number of eruptions during that period. These values may thus also include the contribution of continuous weak ash emissions before and after prominent eruptions. We analyzed the meteorological effects on ash fall distribution patterns and concluded that the width of distribution area of an ash fall is strongly controlled by the near-ground wind speed. The direction of the isopleth axis for larger masses is affected by the local wind direction at ground level. Furthermore, the wind direction influences the direction of the isopleth axes more at higher altitude. While a second maximum of ash fall can appear, the influence of rain might only affect the finer particles in distal areas.

Sr 同位体比による姶良 Tn 火山灰 (AT) の同定

Sr 同位体比による姶良 Tn 火山灰 (AT) の同定

The September 14, 2015 phreatomagmatic eruption of Nakadake first crater, Aso Volcano, Japan: Eruption sequence inferred from ballistic, pyroclastic density current and fallout deposits

An explosive eruption occurred at Nakadake first crater, Aso Volcano in central Kyushu, southwestern Japan, on September 14, 2015. The sequence and causes of the eruption were reconstructed from the distribution, textures, grain-size, component and chemical characteristics of the related deposits, and video record. The eruptive deposits are divided into ballistics, pyroclastic density current and ash-fall deposits. A large number of ballistic clasts (mostly <10cm in diameter; maximum size 1.6m) are scattered within about 500m from the center of the crater. Almost half of the ballistics appear as fresh and unaltered basaltic andesite rocks interpreted to be derived from a fresh batch of magma, while the rest is weakly to highly altered clasts. A relatively thin ash derived from pyroclastic density currents covered an area of 2.3km2 with the SE-trending main axis and two minor axes to the NE and NW. The pyroclastic density current deposit (maximum thickness<10cm even at the crater rim) is wholly fine grained, containing no block-sized clasts. Based on the isopach map, the mass of the pyroclastic density current deposit was estimated at ca. 5.2×104tons. The ash-fall deposit is finer grained and clearly distributed to about 8km west of the source crater. The mass of the ash-fall deposit was calculated at about 2.7×104tons. Adding the mass of the pyroclastic density current deposit, the total discharged mass of the September 14, 2015 eruption was 7.9×104 tons. The September 14 pyroclastic density current and ash-fall deposits consist of glass shards (ca. 30%), crystals (20–30%) and lithic (40–50%) grains. Most glass shards are unaltered poorly crystallized pale brown glasses which probably resulted from quenching of juvenile magma. This suggests that the September 14, 2015 event at the Nakadake first crater was a phreatomagmatic eruption. Similar phreatomagmatic eruptions occurred at the same crater on September 6, 1979 and April 20, 1990 whose eruptive masses were one order larger than that of the September 14, 2015 eruption. These events highlight the potential hazard from phreatic or phreatomagmatic eruptions at Nakadake first crater, and provide useful information that will assist in preventing or mitigating future disasters at other similar volcanoes worldwide.

The Elusive Evidence of Volcanic Lightning

Lightning strikes are known to morphologically alter and chemically reduce geologic formations and deposits, forming fulgurites. A similar process occurs as the result of volcanic lightning discharge, when airborne volcanic ash is transformed into lightning-induced volcanic spherules (LIVS). Here, we adapt the calculations used in previous studies of lightning-induced damage to infrastructure materials to determine the effects on pseudo-ash samples of simplified composition. Using laboratory high-current impulse experiments, this research shows that within the lightning discharge channel there is an ideal melting zone that represents roughly 10% or less of the total channel radius at which temperatures are sufficient to melt the ash, regardless of peak current. The melted ash is simultaneously expelled from the channel by the heated, expanding air, permitting particles to cool during atmospheric transport before coming to rest in ash fall deposits. The limited size of this ideal melting zone explains the low number of LIVS typically observed in volcanic ash despite the frequent occurrence of lightning during explosive eruptions.

High-precision U-Pb zircon ages for explosive volcanism calibrating the NW European continental Autunian stratotype

Volcanism was intense in western European basins during the Late Carboniferous–Early Permian transition, related to the late Variscan extension and associated magmatism. In the Autun Basin, explosive eruptions, expressed as highly weathered distal ashfall deposits (tonsteins), were preserved in lacustrine to swamp environments. The analysis of eleven tonsteins, confirmed as non-reworked volcanic horizons, indicates a highly explosive acidic calc-alkaline volcanism, with active volcanic centres probably linked to Massif Central or Vosges–Black Forest magmatism. New accurate CA-ID-TIMS U–Pb ages are obtained from four of these tonsteins, recorded within the lower Autunian (Igornay and Muse Fms), which defines the lower part of the Autunian stratotype in the Autun Basin. The middle part of the Igornay Fm is dated at 299.9±0.38Ma, the Lally oil-shale bed is dated at 298.91±0.08Ma and the upper part of the Muse oil-shale bed is dated between 298.05±0.19 and 298.57±0.16Ma. For the first time, a numerical calibration of the regional continental Autunian stage in its stratotype is proposed, thus allowing correlation with global marine stages defined in the Urals, and highlighting a late Gzhelian/early Asselian age for the lower Autunian. The tonstein in the Lally oil-shale appears to demarcate the Carboniferous–Permian boundary within the ±100kyr resolution of the U-Pb ages in both the Autun and southern Uralian basins. Comparison with other European domains confirms that these new ages are consistent with well-known macrofloral assemblages, characteristic of the Autunian.

Causes of complexity in a fallout dominated plinian eruption sequence: 312 ka Fasnia Member, Diego Hernández Formation, Tenerife, Spain

The 312ka Fasnia eruption from the Las Cañadas Caldera on Tenerife, Canary Islands, Spain, produced a complex sequence of twenty-two intercalated units, including 7 pumice fall, 7 ignimbrite and 8 ash surge and fall deposits that define two distinct eruption sequences (Lower and Upper Fasnia sequences). The fallout units themselves are internally complex, reflecting waxing and waning of the eruption column, while many of the ignimbrites reflect multiple intra-plinian partial column collapse events associated with the injection of lithic clasts into the eruption column. The Lower and Upper Fasnia eruption phases were each terminated by caldera collapse and complete column collapse events. Probable blockage of the conduit and vent system during Lower Fasnia caldera collapse event briefly terminated the eruption, resulting in a short-lived period of erosion and sedimentation prior to the onset of the Upper Fasnia phase. The transition to the Upper Fasnia eruption phase coincided with the eruption of more geochemically homogeneous pyroclasts. In total, 62km3 of tephra were erupted, including 49km3 of juvenile clasts and >12km3 of lithic clasts. The DRE volume of magma erupted was 13km3 (Lower Fasnia>5km3, Upper Fasnia>8km3), two thirds of which (~9–10km3) was deposited purely by fallout. The Fasnia Member is one of the most complex plinian sequences known.

40Ar/39Ar geochronology and revised stratigraphy of the late Eocene Taxco volcanic field, southern Mexico

The late Eocene Taxco volcanic succession constitutes an important record of explosive silicic volcanism in the north-central Sierra Madre del Sur of southern Mexico. Detailed stratigraphic studies with age dating of individual units in this area were lacking in spite of it being an important mining district. Re-examination of the stratigraphy together with geochronologic studies were used to define the eruptive style and constrain the age of the main episode of silicic activity. Major element data show a rhyolitic composition for all the units analyzed. The volcanic succession records silicic explosive eruptions during which ash-flows, surge and ash-fall deposits, lava flows and domes were emplaced. Earliest activity was contemporary with the end of sediment accumulation of the continental Balsas Group. The first main episode of volcanic activity produced massive, moderately welded, crystal-poor lithic ignimbrites and ash-fall deposits, which evolved into crystal-rich, densely welded ignimbrites with flattened pumice clasts (San Gregorio ignimbrite) thought to represent erupted mush related to caldera collapse during emptying of the magma chamber. A second episode of non-welded, vapor-phase, indurated ignimbrites and surge deposits (Sombrerito ignimbrite), which are rich in pumice but crystal-poor, suggest volatile-rich magmatism and replenishment of the magma chamber by evolved magmas. Rhyolite emplaced in volcanic units of both episodes point to volatile-poor magmas during which rhyolite flows and domes formed.40Ar/39Ar dating of the units, from the crystal-rich San Gregorio ignimbrite near the base of the succession to rhyolites at the top, has constrained the age of the Taxco volcanic field to a relatively short period between ∼36 and 34.5 Ma. Most of the sanidine ages obtained are indistinguishable within error: the densely welded San Gregorio ignimbrite (34.75 ± 0.26, 35.22 ± 0.26 Ma) and the overlying Peral ignimbrite (35.29 ± 0.41, 35.68 ± 0.20). Plagioclase of the Tenería Formation rhyolites gave the same age within error (34.45 ± 1.10, 34.94 ± 0.34 Ma), documenting the end of the main volcanic episodes. A younger plagioclase age of 30.89 ± 1.45 Ma of a dome near Tetipac gives evidence of later silicic magmatism in the area.The source of the ignimbrites of the Taxco region is inferred to be located in the Tenería area, where abundant rhyolite dome structures seem to obliterate the main caldera structure. Differences in age, mineralogy and geochemistry between the Taxco succession and nearby volcanic zones where caldera structures have been identified preclude sources in these areas.

Hydrovolcanic ash emission between August 14 and 24, 2015 at Cotopaxi volcano (Ecuador): Characterization and eruption mechanisms

Cotopaxi is an active, hazardous and ice-covered stratovolcano 60km southeast of Quito, (Ecuador) whose last major eruption occurred in 1877. During 2001–2002, volcanic unrest characterized by volcanic seismicity and deformation ended without eruptive activity. On April 2015, a new increase of seismicity, SO2 emissions, thermal anomalies and edifice deformation, evolved into the onset of a new eruptive cycle, beginning August 14. We sampled and measured the ash fall deposits to the west of Cotopaxi between August 14 and 24, 2015. The ash collected was analyzed using grain size, X-Ray fluorescence, X-Ray diffraction and scanning electron microscope (SEM-EDS), revealing the eruptive products to be compound of dense fragments (mostly lithics), diverse types of scoria, pumice, free fractured crystals, glassy particles and aggregates. Most of hydrothermal alteration is observed during the initial stage of the eruption (14–15 August; including Cu oxides and Fe minerals in the lithics). The glassy particles were blocky morphology, and textural changes were recognized over 10days of eruption, varying from null or low vesicularity to low-to-moderate vesicularity, occasionally exhibiting molten or subrounded textures. The bulk ash has a basaltic-andesitic composition (~55.67wt% of SiO2), while clusters of selected particles (likely juvenile) analyzed through SEM+EDS reveal dacitic composition (65.67 and 65.8wt% SiO2). Plagioclase, clinopyroxene and orthopyroxene are the main minerals present, with accessory anhydrite, melanterite and pyrite (these typically observed during the initial stage of eruption). These variations in addition to the geophysical background, led us to interpret this eruption as the result of the volcano's hydrothermal system disruption due to a shallow, low-volume magma input, which initially evolved into phreatic activity at surface level. Further activity up to 24 August was triggered by the indirect interaction between magma and the depleted hydrothermal system, generating a magmatic-hydrothermal eruption. The issue is important for evaluating unrest periods at active stratovolcanoes, and the impact of their initial, low-volume ash falls in neighboring communities.

Changing fluvial styles in volcaniclastic successions: A cretaceous example from the Cerro Barcino Formation, Patagonia

The Cretaceous Puesto La Paloma (PLPM) and Cerro Castaño (CCM) members (Cerro Barcino Formation, Chubut Group) are pyroclastic-rich, alluvial successions deposited in the Somuncurá-Cañadón Asfalto Basin during sag and endorheic conditions. The PLPM comprises sheet-like tuffaceous sandstone strata, whereas the overlying CCM includes sheet-to ribbon-channel sandstone bodies intercalated within tuffaceous and fine-grained sediments. In this context, the goals of this contribution were: i) to make a detailed documentation of the contrasting sedimentary palaeonvironments; and ii) to infer the allocyclic controls that governed the sedimentation of both units. The study area is located in the western sector of the basin, where six localities, which were studied.Six facies associations were defined including ash-falls, sheet-floods, shallow lakes, aeolian, fluvial channel-belts, and reworked debris-flows. We defined four stratigraphic intervals for the studied sections, denominated 1 to 4 in chronological order of deposition, which increase their thicknesses toward the Puesto Mesa-Cerro León site. The interval 1 (18–42 m thick) corresponds to the PLPM and includes numerous pedogenized sheet-flood deposits, carbonate-rich lacustrine, aeolian sandy facies, and ash-fall beds. The interval 1 is interpreted as an ephemeral and unconfined alluvial system that interacted with aeolian dunes and dry interdune zones. The interval 2 (20–47 m thick) represents the lower part of the CCM. It shows an alternation of fluvial channel-belt deposits and vegetated floodplain facies with sediments originated from sheet-floods, lakes, and few ash-falls and debris-flows. The mean palaeoflow was toward E-SE, except in the northernmost locality where the drainage was towards SW. Proportion of channel-belt bodies ranges from 10 to 36%, reaching higher values in the northern part of the study area, where they are also thicker. The interval 2 represents a permanent, meandering or locally low-sinuosity, fluvial system, and displays both an increase of lacustrine facies and a decrease of ash-fall deposits. The interval 3 (7.5–27 m thick) corresponds to the middle part of the CCM, and lacks channel-belt bodies. It has the highest participation of sheet-flood and ash-fall deposits. This interval entirely records a pedogenized floodplain setting. In relation to the interval 2, participation of debris-flow deposits remains constant and lacustrine facies subtly increases. The interval 4 (18–148 m thick) represents the upper part of the CCM. It comprises an alternation of channel-belt bodies and pedogenized floodplain facies, the last characterized by sheet-flood, lake, debris-flow, and volcanic ash rain deposits. The mean palaeoflow was toward E-SE, except in the two localities positioned further north where the drainage was towards NE and SSE. Proportion of channel-belt deposits ranges from 6 to 32%. It represents channelized and perennial fluvial systems with meandering and locally low-sinuosity styles. Increase in channel proportion and thicker channel bodies are in the northern part of the study area. Particularly, in Puesto Mesa-Cerro León locality this interval is the thickest and has the highest proportion of thicker channel-belt bodies.We interpret these changes in facies architecture as the response to alternated periods of high (intervals 1 and 3) and low (intervals 2 and 4) primary pyroclastic sediment supply. Moreover, there was a climatic change to wetter conditions (intervals 1 to 2–4); as well as intrabasinal tectonic activity in northern area for intervals 2 and 4 inferred from palaeocurrent data.

P22 大規模火砕流堆積物とco-ignimbrite ashの量比

P22 大規模火砕流堆積物とco-ignimbrite ashの量比

Characteristics of the syneruptive-spouted type lahar generated by the September 2014 eruption of Mount Ontake, Japan

Mount Ontake erupted at 11:52 am on September 27, 2014, which generated pyroclastic density currents, ballistic projectiles, ash falls, and a small-scale lahar that spouted directly from craters formed by the eruption. Because this lahar may have been generated by water released from within these craters, we refer to this lahar as a “syneruptive-spouted type lahar” in this study. The lahar of the 2014 eruption was small relative to the other syneruptive type lahars reported in the past that were snowmelt type or crater lake breakout type lahars. Nevertheless, in the 2014 event, the syneruptive-spouted type lahar extended approximately 5 km downstream from the Jigokudani crater via the Akagawa River, with an estimated total volume of ~1.2 × 105 m3. We have reviewed other representative syneruptive-spouted type lahars that have been reported in Japan. The syneruptive-spouted type lahar attributed to the September 2014 eruption had the longest runout distance and largest volume of all cases studied. The mineral assemblage identified from samples of the lahar deposits is similar to that of ash-fall deposits from the same eruption. Previous workers deduced that the ash was derived mainly from shallow depths (within 2 km of the surface). The syneruptive-spouted type lahar deposits are therefore also considered to have originated from shallow depths. A syneruptive-spouted type lahar is a small-scale phenomenon that causes little direct damage to infrastructure, but has long-term influence on water quality. Increases in turbidity and decreases in pH are expected to occur in the Mount Ontake area downstream of Nigorisawa after heavy rainfall events in the future. Therefore, the potential indirect (but long term) damage of syneruptive-spouted type lahars should be considered for hazard mapping and planning volcanic disaster prevention measures.