Most Cited Journal of Volcanology and Geothermal Research Articles
The most cited articles published since 2011, extracted from Scopus.
Maar-diatreme volcanoes: A reviewJ. D L White | P. S. Ross
Volume 201, Issues 1-4, April 2011, Pages 1-29
Maar-diatreme volcanoes are produced by explosive eruptions that cut deeply into the country rock. A maar is the crater cut into the ground and surrounded by an ejecta ring, while the diatreme structure continues downward and encloses diatreme and root zone deposits. Here we attempt an evenhanded review of maar-diatreme volcanology that extends from mafic to kimberlitic varieties, and from historical maar eruptions to deeply eroded or mined diatreme structures. We conclude that maar-diatreme eruptions are episodic. Ejecta rings provide invaluable insight into eruption processes and sequence, but are incomplete records of diatreme formation. Deposits within the diatreme structure include, in varying proportions, lower unbedded deposits sometimes typified by subvertical contacts among domains of debris emplaced sequentially, and upper bedded deposits formed by sedimentation on surfaces open to the atmosphere. A basal root zone comprises the transition from coherent magmatic feeder dike to clastic deposits formed by fragmentation of magma and enclosing country rock; root zones are irregular in form, and the clastic deposits are typically intruded by contorted dikes. Irregular root zone-like chaotic breccias cut by contorted dikes are also present within diatreme deposits, where they represent intra-diatreme fragmentation zones and record changes in the location of the explosion locus during eruption. © 2011 Elsevier B.V.
The 2010 explosive eruption of Java's Merapi volcano-A '100-year' eventSurono | Philippe Jousset | John Pallister | Marie Boichu | M. Fabrizia Buongiorno | Agus Budisantoso | Fidel Costa | Supriyati Andreastuti | Fred Prata | David Schneider | Lieven Clarisse | Hanik Humaida | Sri Sumarti | Christian Bignami | Julie Griswold | Simon Carn | Clive Oppenheimer | Franck Lavigne
Volume 241-242, Issue , October 2012, Pages 121-135
Merapi volcano (Indonesia) is one of the most active and hazardous volcanoes in the world. It is known for frequent small to moderate eruptions, pyroclastic flows produced by lava dome collapse, and the large population settled on and around the flanks of the volcano that is at risk. Its usual behavior for the last decades abruptly changed in late October and early November 2010, when the volcano produced its largest and most explosive eruptions in more than a century, displacing at least a third of a million people, and claiming nearly 400 lives. Despite the challenges involved in forecasting this 'hundred year eruption', we show that the magnitude of precursory signals (seismicity, ground deformation, gas emissions) was proportional to the large size and intensity of the eruption. In addition and for the first time, near-real-time satellite radar imagery played an equal role with seismic, geodetic, and gas observations in monitoring eruptive activity during a major volcanic crisis. The Indonesian Center of Volcanology and Geological Hazard Mitigation (CVGHM) issued timely forecasts of the magnitude of the eruption phases, saving 10,000-20,000 lives. In addition to reporting on aspects of the crisis management, we report the first synthesis of scientific observations of the eruption. Our monitoring and petrologic data show that the 2010 eruption was fed by rapid ascent of magma from depths ranging from 5 to 30km. Magma reached the surface with variable gas content resulting in alternating explosive and rapid effusive eruptions, and released a total of ~0.44Tg of SO2. The eruptive behavior seems also related to the seismicity along a tectonic fault more than 40km from the volcano, highlighting both the complex stress pattern of the Merapi region of Java and the role of magmatic pressurization in activating regional faults. We suggest a dynamic triggering of the main explosions on 3 and 4 November by the passing seismic waves generated by regional earthquakes on these days. © 2012 Elsevier B.V.
A multi-decadal view of seismic methods for detecting precursors of magma movement and eruptionBernard A. Chouet | Robin S. Matoza
Volume 252, Issue , February 2013, Pages 108-175
With the emergence of portable broadband seismic instrumentation, availability of digital networks with wide dynamic range, and development of new powerful analysis techniques made possible by greatly increased computer capacity, volcano seismology has now reached a mature stage where insights are rapidly being gained on the role played by magmatic and hydrothermal fluids in the generation of seismic waves. Volcanoes produce a wide variety of signals originating in the transport of magma and related hydrothermal fluids and their interaction with solid rock. Typical signals include (1) brittle failure earthquakes that reflect the response of the rock to stress changes induced by magma movement; (2) pressure oscillations accompanying the dynamics of liquids and gases in conduits and cracks; and (3) magma fracturing and fragmentation. Oscillatory behaviors within magmatic and hydrothermal systems are the norm and are the expressions of the complex rheologies of these fluids and nonlinear characteristics of associated processes underlying the release of thermo-chemical and gravitational energy from volcanic fluids along their ascent path. The interpretation of these signals and quantification of their source mechanisms form the core of modern volcano seismology. The accuracy to which the forces operating at the source can be resolved depends on the degree of resolution achieved for the volcanic structure. High-resolution tomography based on iterative inversions of seismic travel-time data can image three-dimensional structures at a scale of a few hundred meters provided adequate local short-period earthquake data are available. Hence, forces in a volcano are potentially resolvable for periods longer than ~. 1. s. In concert with techniques aimed at the interpretation of processes occurring in the fluid, novel seismic methods have emerged that are allowing the detection of stress changes in volcanic structures induced by magma movement. These methods include (1) ambient noise interferometry, in which the ambient seismic noise is used to probe temporal changes in volcanic structures; (2) the measurement of seismic anisotropy, where changes in the alignment of fluid-filled microcracks and pore space are monitored to assess the response of the crust to pressurization of a magmatic system; and (3) the detection of systematic changes in fault plane solutions of volcano-tectonic earthquakes caused by local stress perturbations during conduit pressurization. As new seismic methods refine our understanding of seismic sources and behavior of volcanic structures, we face new challenges in elucidating the physico-chemical processes that cause volcanic unrest and its seismic and gas-discharge manifestations. Future important goals toward meeting those challenges must include a better understanding of the key types of magma movement, degassing and boiling events that produce characteristic seismic phenomena, along with a quantitative understanding of multiphase fluid behavior under dynamic volcanic conditions. Realizing these goals will be essential for the development of an integrated model of volcanic behavior and will require multidisciplinary research involving detailed field measurements, laboratory experiments, and numerical modeling. © 2012 .
Magma chambers: Formation, local stresses, excess pressures, and compartmentsAgust Gudmundsson
Volume 237-238, Issue , September 2012, Pages 19-41
An existing magma chamber is normally a necessary condition for the generation of a large volcanic edifice. Most magma chambers form through repeated magma injections, commonly sills, and gradually expand and change their shapes. Highly irregular magma-chamber shapes are thermo-mechanically unstable; common long-term equilibrium shapes are comparatively smooth and approximate those of ellipsoids of revolution. Some chambers, particularly small and sill-like, may be totally molten. Most chambers, however, are only partially molten, the main part of the chamber being crystal mush, a porous material. During an eruption, magma is drawn from the crystal mush towards a molten zone beneath the lower end of the feeder dyke. Magma transport to the feeder dyke, however, depends on the chamber's internal structure; in particular on whether the chamber contains pressure compartments that are, to a degree, isolated from other compartments. It is only during large drops in the hydraulic potential beneath the feeder dyke that other compartments become likely to supply magma to the erupting compartment, thereby contributing to its excess pressure (the pressure needed to rupture a magma chamber) and the duration of the eruption.Simple analytical models suggest that during a typical eruption, the excess-pressure in the chamber decreases exponentially. This result applies to a magma chamber that (a) is homogeneous and totally fluid (contains no compartments), (b) is not subject to significant replenishment (inflow of new magma into the chamber) during the eruption, and (c) contains magma where exsolution of gas has no significant effect on the excess pressure. For a chamber consisting of pressure compartments, the exponential excess-pressure decline applies primarily to a single erupting compartment. When more than one compartment contributes magma to the eruption, the excess pressure may decline much more slowly and irregularly.Excess pressure is normally similar to the in-situ tensile strength of the host rock, 0.5-9. MPa. These in-situ strength estimates are based on hydraulic fracture measurements in drill-holes worldwide down to crustal depths of about 9. km. These measurements do not support some recent magma-chamber stress models that predict (a) extra gravity-related wall-parallel stresses at the boundaries of magma chambers and (b) magma-chamber excess pressures prior to rupture of as much as hundreds of mega-pascals, particularly at great depths.General stress models of magma chambers are of two main types: analytical and numerical. Earlier analytical models were based on a nucleus-of-strain source (a 'point pressure source') for the magma chamber, and have been very useful for rough estimates of magma-chamber depths from surface deformation during unrest periods. More recent models assume the magma chamber to be axisymmetric ellipsoids or, in two-dimensions, ellipses of various shapes. Nearly all these models use the excess pressure in the chamber as the only loading (since lithostatic stress effects are then automatically taken into account), assume the chamber to be totally molten, and predict similar local stress fields. The predicted stress fields are generally in agreement with the world-wide stress measurements in drill-holes and, in particular, with the in-situ tensile-strength estimates.Recent numerical models consider magma-chambers of various (ideal) shapes and sizes in relation to their depths below the Earth's surface. They also take into account crustal heterogeneities and anisotropies; in particular the effects of the effects of a nearby free surface and horizontal and inclined (dipping) mechanical layering. The results show that the free surface may have strong effects on the local stresses if the chamber is comparatively close to the surface. The mechanical layering, however, may have even stronger effects. For realistic layering, and other heterogeneities, the numerical models predict complex local stresses around magma chambers, with implications for dyke paths, dyke arrest, and ring-fault formation. © 2012 Elsevier B.V.
Volcano infrasound: A reviewJeffrey Bruce Johnson | Maurizio Ripepe
Volume 206, Issues 3-4, September 2011, Pages 61-69
Exploding volcanoes, which produce intense infrasound, are reminiscent of the veritable explosion of volcano infrasound papers published during the last decade. Volcano infrasound is effective for tracking and quantifying eruptive phenomena because it corresponds to activity occurring near and around the volcanic vent, as opposed to seismic signals, which are generated by both surface and internal volcanic processes. As with seismology, infrasound can be recorded remotely, during inclement weather, or in the dark to provide a continuous record of a volcano's unrest. Moreover, it can also be exploited at regional or global distances, where seismic monitoring has limited efficacy. This paper provides a literature overview of the current state of the field and summarizes applications of infrasound as a tool for better understanding volcanic activity. Many infrasound studies have focused on integration with other geophysical data, including seismic, thermal, electromagnetic radiation, and gas spectroscopy and they have generally improved our understanding of eruption dynamics. Other work has incorporated infrasound into volcano surveillance to enhance capabilities for monitoring hazardous volcanoes and reducing risk. This paper aims to provide an overview of volcano airwave studies (from analog microbarometer to modern pressure transducer) and summarizes how infrasound is currently used to infer eruption dynamics. It also outlines the relative merits of local and regional infrasound surveillance, highlights differences between array and network sensor topologies, and concludes with mention of sensor technologies appropriate for volcano infrasound study. © 2011 Elsevier B.V.
Influence of the substrate on maar-diatreme volcanoes - An example of a mixed setting from the Pali Aike volcanic field, ArgentinaPierre Simon Ross | Séverine Delpit | Miguel J. Haller | Károly Németh | Hugo Corbella
Volume 201, Issues 1-4, April 2011, Pages 253-271
The morphologic parameters, pyroclastic deposits and evolution of maar-diatreme volcanoes are affected by the type of environment in which they are emplaced. End-member cases are a hard substrate (rocks) and a soft substrate (unconsolidated volcaniclastic or sedimentary deposits). In this paper, we present an example of a volcanic complex emplaced in a mixed hard-soft setting from the Pali Aike volcanic field (PAVF) near the Argentina-Chile border. The Plio-Pleistocene PAVF is an alkaline, mafic, back-arc monogenetic field which contains over 100 phreatomagmatic volcanoes. The studied volcanic complex contains two large coalescent maars overlain by scoria and spatter. The 1.4 × 1.3. km East Maar has better exposures than the shallower, 1.9. km-wide West Maar and seems to have been less modified by post-eruptive processes. The tephra rim of the East Maar was studied in detail and we infer it was produced mostly by base surges from phreatomagmatic eruption columns, with rare instances of intercalated scoria fall layers. Based on regional information, the general pre-maar stratigraphy is dominated by sedimentary and volcaniclastic rocks of the Magallanes Basin, including a thick poorly consolidated upper unit dating from the Miocene. These are overlain by Plio-Pleistocene fluvio-glacial deposits and PAVF lavas, some of which are exposed in the East Maar just below the phreatomagmatic deposits. All of these units are represented as lithic clasts in the tephra rim of the East Maar, the most abundant being the clasts from the earlier basaltic lavas and rock fragments derived from the glacial deposits. There is no specific evidence for a deep diatreme under the East Maar, and in this particular case, the mixed environment seems to have produced a maar-diatreme volcano typical of a soft substrate. © 2010 Elsevier B.V.
Degassing processes during lava dome growth: Insights from Santiaguito lava dome, GuatemalaA. S Peter Holland | I. Matthew Watson | Jeremy C. Phillips | Luca Caricchi | Marika P. Dalton
Volume 202, Issues 1-2, April 2011, Pages 153-166
Eruptions of intermediate magma may be explosive or effusive. The development of open system degassing has been proposed as a pre-requisite for effusion of intermediate magma, however processes leading to open system degassing are poorly understood. To better understand degassing processes during lava dome extrusion we report high temporal-resolution SO2 emission rate measurements collected with an ultra violet imaging camera at Santiaguito, Guatemala. Santiaguito is an ideal case study as the dome lava is compositionally very similar to products of the 1902 Plinian eruption of the parental Santa María volcano. We find that degassing is weak (0.4-1kgs-1) but continuous, and explosions are associated with small increases in emission rates (up to 2-3kgs-1). Continuous repose degassing occurs through a shallow cap rock which likely represents a proto-crust on the block lava flow which is extruded from the same vent. The continual permeability of the upper conduit argues against a mechanism of explosion triggering in which gas pressure builds beneath a viscous cap rock or plug. Rather, we consider degassing data better consistent with a model of shear-fracturing at the conduit margins. Using field constraints, we model the viscosity of Santiaguito magma as a function of depth and show that conditions for shear-fracturing are met from 150-600m to the surface. This is in line with independent estimates of explosion initiation depth. We show that repose timescales are orders of magnitude longer than the timescale for shear fracture, and suggest that explosions are triggered when a continuous network of smaller-scale fractures develops, at which point decompression occurs and an explosion is triggered. Fracture healing occurs by viscous relaxation however near to the surface where viscosity is highest, an unconsolidated gouge layer may develop. Our model implies that the observed explosions are a by-product of extrusion. Shear-fracturing can drive open system degassing of crystal rich intermediate magma at shallow levels in the conduit, as high magma viscosity is able to overcome the low strain rates associated with slow ascent of magma. © 2011 Elsevier B.V.
Payenia volcanic province in the Southern Andes: An appraisal of an exceptional Quaternary tectonic settingVictor A. Ramos | Andrés Folguera
Volume 201, Issues 1-4, April 2011, Pages 53-64
The Southern Volcanic Zone of the Andes has a Quaternary basaltic province along the retroarc which has a unique tectonic setting. The Payenia volcanic province covers an area larger than 40,000km2 between 33°30' and 38° South latitudes, with an estimated volcanic volume of about 8387km3 erupted through more than 800 volcanic centers in the last ~2Ma. The mainly basaltic province developed above the San Rafael Block is subdivided in three segments characterized by the Cerro Nevado, Llancanelo, Payún Matru, Tromen and Auca Mahuida volcanic fields, together with hundreds of minor monogenetic basaltic centers. The analysis of the different segments shows the formation of a common basalt plateau with intraplate signature from south to north between 2.0 and 1.7Ma, which reached the 35°S to the north. Above this plateau monogenetic centers as Nihuil Vn. 1.433Ma and Cerro Chato at 1.352Ma are developed, followed by the large polygenetic center of Cerro Nevado (3980ma.s.l.) at 1.320Ma. This plateau was broken by a series of normal faults that produced volcanic cone alignments such as the NNW-trending Mancha Jarilla lineament in the central part at about 1.0Ma. Extension shifted to the eastern margin of the San Rafael Block, which concentrates tens of monogenetic centers between 0.9 and 0.7Ma. Extension then migrated towards the foothills in the west, where many monogenetic cones were erupted through NW-trending normal faults between 0.5 and 0.435Ma. The collapse of the large Diamante Caldera at 0.445Ma coincides with that period. Subsequent volcanism was concentrated in (1) the Payún Matru volcanic field, with the eruption of Cerro Payén between 0.272 and 0.261Ma; the Payún Matru shield volcano, with polygenetic eruptions at least since the last 0.233Ma and with the caldera formation bracketed between 0.168±0.004Ma and 0.082±0.001Ma, followed by several eruptions until 7000yrs, and even historical ones; and in (2) the Tromen volcano, where younger than 0.2Ma eruptions took place and historical eruptions were reported. The understanding of these eruptions in time and space, combined with geophysical data, indicates the geometry of an important crustal attenuation beneath Payenia, associated with a hot sublithosphere. The Late Miocene uplifted San Rafael Block collapsed in the Early Pleistocene as a consequence of the steepening of the subducted slab, and the injection of hot asthenosphere produced the Quaternary Payenia volcanic province. Melts of the lower crust along the Principal Cordillera at these latitudes are responsible for the Quaternary calderas, ignimbritic flows and rhyolitic volcanism that express the crustal delamination of the Andes. The Payún Matru volcanic field concentrates this asthenospheric flow in the Present. © 2010 Elsevier B.V.
The rheology of two-phase magmas: A review and analysisH. M. Mader | E. W. Llewellin | S. P. Mueller
Volume 257, Issue , May 2013, Pages 135-158
We consider the current state of our understanding of the rheology of two-phase magmas, that is suspensions of either bubbles or crystals in a viscous silicate melt. The discussion is restricted to strain-rates at which the suspending melt can be considered Newtonian. We start by considering the range of textures found in magmas and the bubble deformation and particle motions caused by shearing. We then review proposed models for suspensions, focussing on those functions of the form ηr=f(φ) or τ=fγ̇ that have been most widely used to describe magmatic systems (ηr is the relative apparent viscosity of the suspension, φ is the volume fraction of the suspended phase, τ is the driving stress, and γ̇ is the strain-rate). Both theoretical and empirical methods are presented and then compared against the available analogue (i.e. non-magmatic) and magmatic data. The paper contains new data and significant re-analysis of previously published data. We present a new semi-empirical constitutive model for bubble-bearing magmas that is valid for steady and unsteady flow and large strains and strain-rates. This equation utilises a new parameter, the capillarity Cx, that encapsulates the combined effect of shearing and unsteadiness on bubble suspensions. We also present a new scheme for dealing with polydispersivity of bubble suspensions. New data on the rheology of particle suspensions undergoing forced-oscillations are presented. These data show that the Cox-Merz rule only holds for dilute particle suspensions φ≲0.25. A re-analysis of all available experimental data that relate rheology to particle aspect ratio provides distinct curves of maximum packing as a function of aspect ratio for smooth and rough particles with magmatic data lying on the curve appropriate for rough particles. We analyse several rheological datasets of crystal-bearing basaltic magmas and find that they are in good agreement with the constitutive equations derived from analogue data. By contrast, the same equations do not agree well with data for high-viscosity, haplogranitic melts. This may be an effect of fracturing or viscous dissipation within these samples. The paper concludes with a practical 'rheological recipes' section giving a step-by-step method for calculating a constitutive equation for a two-phase magmatic suspension and assessing its likely accuracy. © 2013 Elsevier B.V.
An overview of volcano infrasound: From hawaiian to plinian, local to globalDavid Fee | Robin S. Matoza
Volume 249, Issue , January 2013, Pages 123-139
Volcano infrasound is an increasingly useful technique for detecting, locating, characterizing, and quantifying eruptive activity, and can be used to constrain eruption source parameters. In recent years, studies of infrasound data from active volcanoes have shown clear progress towards mitigating volcanic hazards and understanding volcanic source processes. Volcano acoustic sources are shallow or aerial, thus volcano infrasound data provide valuable information on eruption dynamics and are readily combined with direct and remote observations of gas, ash, and other eruptive phenomena. The infrasound signals produced by volcanoes are indicative of the eruption style and dynamics. Here we review the diversity of infrasound signals generated by a wide variety of volcanic eruptions, from hawaiian to plinian, and the physical processes inferred to produce them. We place particular emphasis on regional (15-250. km distance) and global (>250 km distance) volcano infrasound studies, as recent work in this area has made significant advances in monitoring and characterizing remote and difficult-to-monitor eruptions. Long-range infrasonic detection of explosive volcanic eruptions is possible due to the energetic source mechanisms involved, minor atmospheric attenuation at low frequencies, and the existence of waveguides in the atmosphere. However, accurate characterization of the atmosphere and its spatiotemporal variability is required for reliable long-range sound propagation modeling and correct interpretation of global infrasound recordings. Conversely, because volcanic explosions are energetic and sometimes repetitive infrasound sources, they can be used to validate atmospheric and acoustic propagation models. © 2012 Elsevier B.V.
A quantitative study of the mechanisms governing dike propagation, dike arrest and sill formationF. Maccaferri | M. Bonafede | E. Rivalta
Volume 208, Issues 1-2, November 2011, Pages 39-50
Dikes and sills are the moving building blocks of the plumbing system of volcanoes and play a fundamental role in the accretionary processes of the crust. They nucleate, propagate, halt, resume propagation, and sometimes change trajectory with drastic implications for the outcome of eruptions (. Sigmundsson et al., 2010). Their dynamics is still poorly understood, in particular when different external influencing factors are interacting. Here we apply a boundary element model to study dike and sill formation, propagation and arrest in different scenarios. We model dikes as finite batches of compressible fluid magma, propagating quasi-statically in an elastic medium, and calculate their trajectories by maximising the energy release of the magma-rock system. We consider dike propagation in presence of density layering, of density plus rigidity layering, of a weakly welded interface between layers, under the action of an external stress field (of tectonic or topographic origin). Our simulations predict sill formation in several situations: i) when a horizontal weak interface is met by a propagating dike; ii) when a sufficiently high compressive tectonic environment is experienced by the ascending dike and iii) in case a dike, starting below a volcanic edifice, propagates away from the topographic load with a low dip angle. We find that dikes halt and stack when they become negatively buoyant and when they propagate with low overpressure at their upper tip toward a topographic load. Neutral buoyancy by itself cannot induce dikes to turn into sills, as previously suggested. © 2011 Elsevier B.V.
Reconstructing the architectural evolution of volcanic islands from combined K/Ar, morphologic, tectonic, and magnetic data: The Faial Island example (Azores)A. Hildenbrand | F. O. Marques | A. C G Costa | A. L R Sibrant | P. F. Silva | B. Henry | J. M. Miranda | P. Madureira
Volume 241-242, Issue , October 2012, Pages 39-48
The morpho-structural evolution of oceanic islands results from competition between volcano growth and partial destruction by mass-wasting processes. We present here a multi-disciplinary study of the successive stages of development of Faial (Azores) during the last 1. Myr. Using high-resolution digital elevation model (DEM), and new K/Ar, tectonic, and magnetic data, we reconstruct the rapidly evolving topography at successive stages, in response to complex interactions between volcanic construction and mass wasting, including the development of a graben. We show that: (1) sub-aerial evolution of the island first involved the rapid growth of a large elongated volcano at ca. 0.85. Ma, followed by its partial destruction over half a million years; (2) beginning about 360. ka a new small edifice grew on the NE of the island, and was subsequently cut by normal faults responsible for initiation of the graben; (3) after an apparent pause of ca. 250. kyr, the large Central Volcano (CV) developed on the western side of the island at ca 120. ka, accumulating a thick pile of lava flows in less than 20. kyr, which were partly channelized within the graben; (4) the period between 120. ka and 40. ka is marked by widespread deformation at the island scale, including westward propagation of faulting and associated erosion of the graben walls, which produced sedimentary deposits; subsequent growth of the CV at 40. ka was then constrained within the graben, with lava flowing onto the sediments up to the eastern shore; (5) the island evolution during the Holocene involves basaltic volcanic activity along the main southern faults and pyroclastic eruptions associated with the formation of a caldera volcano-tectonic depression. We conclude that the whole evolution of Faial Island has been characterized by successive short volcanic pulses probably controlled by brief episodes of regional deformation. Each pulse has been separated by considerable periods of volcanic inactivity during which the Faial graben gradually developed. We propose that the volume loss associated with sudden magma extraction from a shallow reservoir in different episodes triggered incremental downward graben movement, as observed historically, when immediate vertical collapse of up to 2. m was observed along the western segments of the graben at the end of the Capelinhos eruptive crises (1957-58). © 2012 Elsevier B.V.
Paroxysmal dome explosion during the Merapi 2010 eruption: Processes and facies relationships of associated high-energy pyroclastic density currentsJean Christophe Komorowski | Susanna Jenkins | Peter J. Baxter | Adrien Picquout | Franck Lavigne | Sylvain Charbonnier | Ralf Gertisser | Katie Preece | Noer Cholik | Agus Budi-Santoso | Surono
Volume 261, Issue , July 2013, Pages 260-294
An 11-minute sequence of laterally-directed explosions and retrogressive collapses on 5 November 2010 at Merapi (Indonesia) destroyed a rapidly-growing dome and generated high-energy pyroclastic density currents (PDCs) spreading over 22km2 with a runout of 8.4km while contemporaneous co-genetic valley-confined PDCs reached 15.5km. This event formed Stage 4 of the multi-stage 2010 eruption, the most intense eruptive episode at Merapi since 1872. The deposits and the widespread devastating impact of associated high-energy PDCs on trees and buildings show striking similarities with those from historical volcanic blasts (Montagne Pelée, Martinique, Bezymianny, Russia, Mount St. Helens, USA, Soufrière Hills, Montserrat). We provide data from stratigraphic and sedimentologic analyses of 62 sections of the first unequivocal blast-like deposits in Merapi's recent history. We used high resolution satellite imagery to map eruptive units and flow direction from the pattern of extensive tree blowdown. The stratigraphy of Stage 4 consists of three depositional units (U0, U1, U2) that we correlate to the second, third and fourth explosions of the seismic record. Both U1 and U2 show a bi-partite layer stratigraphy consisting each of a lower L1 layer and an upper L2 layer. The lower L1 layer is typically very coarse-grained, fines-poor, poorly-sorted and massive, and was deposited by the erosive waxing flow head. The overlying L2 layer is much finer grained, fines-rich, moderately to well-sorted, with laminar to wavy stratification. L2 was deposited from the waning upper part and wake of the PDC. Field observations indicate that PDC height reached ~330m with an internal velocity of ~100ms-1 within 3km from the source. The summit's geometry and the terrain morphology formed by a major transversal ridge and a funneling deep canyon strongly focused PDC mass towards a major constriction, thereby limiting the loss of kinetic energy. This favored elevated PDC velocities and high particle concentration, promoted overspilling of PDCs across high ridges into other river valleys, and generated significant dynamic pressures to distances of 6km that caused total destruction of buildings and the forest. The Merapi 2010 eruption highlights that explosive and gravitational disintegration of a rapidly growing dome can generate devastating high-energy, high-velocity PDCs. This constitutes a credible high impact scenario for future multi-stage eruptions at Merapi and at other volcanoes that pose particular monitoring, crisis response, and risk reduction challenges. © 2013 Elsevier B.V.
Shallow plumbing systems for small-volume basaltic volcanoes, 2: Evidence from crustal xenoliths at scoria cones and maarsGreg A. Valentine
Volume 223-224, Issue , April 2012, Pages 47-63
Monogenetic basaltic volcanoes record complex eruption processes and the relationships between those processes and shallow plumbing are poorly understood. This paper explores these relationships, building upon earlier studies of exposed shallow plumbing (in the upper hundreds of meters of crust) beneath volcanoes produced by magmatic eruption processes and those produced by phreatomagmatic processes, such as scoria cones and maars, respectively. Eruptive facies and xenolith abundances are described at three scoria cones and at tephra rings around two maars in the San Francisco Volcanic Field (Arizona, USA). Well-constrained subvolcanic sedimentary stratigraphy provides constraints on the depths of origin of xenoliths of different types. Sedimentary xenolith contents at scoria cones are <10 -3 and commonly <10 -4 (volume fraction of xenoliths) and are composed almost entirely of fragments from the uppermost sedimentary formation (Kaibab Formation, depth interval ~50-200m). These xenolith contents are consistent with conduits or dikes that widen mainly in the uppermost tens of meters of the crust, as observed at exposed plumbing systems of eroded scoria cones. Sedimentary xenolith contents in tephra ring deposits at one of the studied maar volcanoes also are typically <10 -3 (volume fraction) and the remaining fraction is dominated by clasts of pre-maar volcanic rocks that formed an ~50 thick surface layer over the sedimentary formations; the second studied maar has much higher xenolith contents but this also appears to be dominated by the shallowest unit. The maars' tephra ring deposits contain xenoliths from all of the major sedimentary units beneath the volcanoes (to depths of ~1200m) but the abundances and proportions of xenoliths are not consistent with the volumes of sub-volcanic units that would be disrupted assuming dimensions that are commonly observed in exposed maar plumbing systems (diatremes). These differences illustrate the different mechanisms for conduit/dike widening in magmatic versus phreatomagmatic eruptions. Namely, ascending eruptive mixtures driven by magmatic volatiles widen their relatively shallow conduits by erosion and mechanical failure of the walls and ejection of the resulting xenoliths. Maar-forming eruptions produce wide and deep diatremes mainly by mechanical disruption of country rock during many discrete magma-water explosions at varying depths in the subsurface. Deep explosions cause debris jets that may not erupt, and the disrupted country rock and juvenile material gradually churns and mixes within the diatreme. Only a small fraction of the debris is ejected from the crater by especially strong and/or shallow explosions, to form tephra ring deposits. © 2012 Elsevier B.V.
Origin of Late Permian Emeishan basaltic rocks from the Panxi region (SW China): Implications for the Ti-classification and spatial-compositional distribution of the Emeishan flood basaltsJ. G. Shellnutt | B. M. Jahn
Volume 199, Issues 1-2, January 2011, Pages 85-95
Basalts and a mafic dyke collected from the city of Panzhihua show characteristics of high-Ti and low-Ti Emeishan basalts respectively. The dyke yielded a SHRIMP zircon U-Pb mean age of 261±5Ma making it contemporaneous with the eruption of Emeishan basalts. The basalts have ISr ranging from 0.7059 to 0.7062 with εNd(T) ranging from -1.1 to +0.7 whereas the dyke has ISr ranging from 0.7056 to 0.7064 with εNd(T) ranging from +0.3 to +0.5. Trace element modeling shows that the two rock types can be generated by different degrees of partial melting from the same garnet-bearing source. Assimilation of crustal material is required in order to produce the depletion of some trace elements (e.g. Nb and Ta) of the dyke however crustal assimilation is not required to produce the basalts. Trace element modeling and isotopic data of the Emeishan basalts suggest that, in general, the high- and low-Ti basaltic rocks are likely derived from the same source and represent different degrees of partial melting with or without crustal assimilation. The location and geological relationships of the 'high-Ti' basalts indicate they erupted relatively early and within the central part of the ELIP, casting doubt on the previous spatial-compositional distribution of the Emeishan basalts. © 2010 Elsevier B.V.
Infrasonic observations of the June 2009 Sarychev Peak eruption, Kuril Islands: Implications for infrasonic monitoring of remote explosive volcanismRobin S. Matoza | Alexis Le Pichon | Julien Vergoz | Pascal Herry | Jean Marie Lalande | Hee il Lee | Il Young Che | Alexander Rybin
Volume 200, Issues 1-2, February 2011, Pages 35-48
Sarychev Peak (SP), located on Ostrov Matua, Kurils, erupted explosively during 11-16 June 2009. Whereas remote seismic stations did not record the eruption, we report atmospheric infrasound (acoustic wave ~. 0.01-20. Hz) observations of the eruption at seven infrasound arrays located at ranges of ~. 640-6400. km from SP. The infrasound arrays consist of stations of the International Monitoring System global infrasound network and additional stations operated by the Korea Institute of Geoscience and Mineral Resources. Signals at the three closest recording stations IS44 (643. km, Petropavlovsk-Kamchatskiy, Kamchatka Krai, Russia), IS45 (1690. km, Ussuriysk, Russia), and IS30 (1774. km, Isumi, Japan) represent a detailed record of the explosion chronology that correlates well with an eruption chronology based on satellite data (TERRA, NOAA, MTSAT). The eruption chronology inferred from infrasound data has a higher temporal resolution than that obtained with satellite data. Atmosphere-corrected infrasonic source locations determined from backazimuth cross-bearings of first-arrivals have a mean centroid ~. 15. km from the true location of SP. Scatter in source locations of up to ~. 100. km result from currently unresolved details of atmospheric propagation and source complexity. We observe systematic time-variations in trace-velocity, backazimuth deviation, and signal frequency content at IS44. Preliminary investigation of atmospheric propagation from SP to IS44 indicates that these variations can be attributed to solar tide variability in the thermosphere. It is well known that additional information about active volcanic processes can be learned by deploying infrasonic sensors with seismometers at erupting volcanoes. This study further highlights the significant potential of infrasound arrays for monitoring volcanic regions such as the Kurils that have only sparse seismic network coverage. © 2010 Elsevier B.V.
Catastrophic caldera-forming eruptions: Thermomechanics and implications for eruption triggering and maximum caldera dimensions on EarthP. M. Gregg | S. L. De Silva | E. B. Grosfils | J. P. Parmigiani
Volume 241-242, Issue , October 2012, Pages 1-12
Approximately every 100,000years the Earth experiences catastrophic caldera-forming "supereruptions" that are considered to be one of the most hazardous natural events on Earth. Utilizing new temperature-dependent, viscoelastic numerical models that incorporate a Mohr-Coulomb failure criterion, we find that eruptive failure of the largest magma chambers is a function of the geometry of the overlying roof and the location of the brittle-ductile transition. In particular, the ductile halo created around the hot magma chamber buffers increasing overpressures and prevents pressure relief via magmatic injection from the magma chamber. The numerical results indicate that as chamber volume increases, the higher temperatures in the host rock and the decrease in the roof aspect ratio cause a shift from reservoir-triggered eruption to an external roof-triggered mechanism. Specifically, as overpressure increases within the largest magma chambers, extensive uplift in the overlying roof promotes the development of through-going faults that may trigger eruption and caldera collapse from above. We find that for magma chamber volumes>10 3km 3, and roof aspect ratios (depth/width)<0.3, moderate magma chamber overpressures (<30MPa) will cause extensive through-going fault development in the overlying roof. This result indicates an external mechanism, caused by fault propagation in the roof, is a likely trigger for the largest caldera forming eruptions. The thermomechanical models also provide an estimate of the maximum size of magma chamber growth in a pristine host material and, thus, an estimate of the maximum size of the resultant caldera. We find a maximum reservoir volume range of 10 4-10 5km 3 for shallow crustal magma chambers emplaced at depths to the top of the magma chamber of 3-7km. These volumes produce maximum caldera areas of 10 3-10 4km 2, comparable to the largest calderas observed on Earth (e.g., Toba). These thermomechanical models offer critical new insight into the mechanics of catastrophic caldera collapse and provide a numerical construct for predicting how eruption is triggered in the largest crustal magma chambers. © 2012 Elsevier B.V.
A review of tephra transport and dispersal models: Evolution, current status, and future perspectivesA. Folch
Volume 235-236, Issue , August 2012, Pages 96-115
Tephra transport models try to predict atmospheric dispersion and sedimentation of tephra depending on meteorology, particle properties, and eruption characteristics, defined by eruption column height, mass eruption rate, and vertical distribution of mass. Models are used for different purposes, from operational forecast of volcanic ash clouds to hazard assessment of tephra dispersion and fallout. The size of the erupted particles, a key parameter controlling the dynamics of particle sedimentation in the atmosphere, varies within a wide range. Largest centimetric to millimetric particles fallout at proximal to medial distances from the volcano and sediment by gravitational settling. On the other extreme, smallest micrometric to sub-micrometric particles can be transported at continental or even at global scales and are affected by other deposition and aggregation mechanisms. Different scientific communities had traditionally modeled the dispersion of these two end members. Volcanologists developed families of models suitable for lapilli and coarse ash and aimed at computing fallout deposits and for hazard assessment. In contrast, meteorologists and atmospheric scientists have traditionally used other atmospheric transport models, dealing with finer particles, for tracking motion of volcanic ash clouds and, eventually, for computing airborne ash concentrations. During the last decade, the increasing demand for model accuracy and forecast reliability has pushed on two fronts. First, the original gap between these different families of models has been filled with the emergence of multi-scale and multi-purpose models. Second, new modeling strategies including, for example, ensemble and probabilistic forecast or model data assimilation are being investigated for future implementation in models and or modeling strategies. This paper reviews the evolution of tephra transport and dispersal models during the last two decades, presents the status and limitations of the current modeling strategies, and discusses some emergent perspectives expected to be implemented at operational level during the next few years. Improvements in both real-time forecasting and long-term hazard assessment are necessary to loss prevention programs on a local, regional, national and international level. © 2012 Elsevier B.V.
Andesites of the 2009 eruption of Redoubt Volcano, AlaskaMichelle L. Coombs | Thomas W. Sisson | Heather A. Bleick | Sarah M. Henton | Chris J. Nye | Allison L. Payne | Cheryl E. Cameron | Jessica F. Larsen | Kristi L. Wallace | Katharine F. Bull
Volume 259, Issue , June 2013, Pages 349-372
Crystal-rich andesites that erupted from Redoubt Volcano in 2009 range from 57.5 to 62.5wt.% SiO2 and have phenocryst and phenocryst-melt relations consistent with staging in the upper crust. Early explosive products are low-silica andesites (LSA, <58wt.% SiO2) that ascended from deeper crustal levels during or before the 6months of precursory activity, but a broad subsequent succession to more evolved and cooler products, and predominantly effusive dome growth, are interpreted to result from progressive mobilization and mixing with differentiated magmas tapped from pre-2009 Redoubt intrusions at ~3-6km depth. Initial explosions on March 23-28 ejected predominantly LSA with a uniform phenocryst assemblage of high-Al amphibole, ~An70 plagioclase, ortho- and clinopyroxene, FeTi oxides (890 to 960°C), and traces of magmatic sulfide. Melt in the dominant microlite-poor LSA was compositionally uniform dacite (67-68wt.% SiO2) but ranged to rhyolite with greater microlite growth. Minor amounts of intermediate- to high-silica andesite (ISA, HSA; 59-62.5wt.% SiO2) also erupted during the early explosions and most carried rhyolitic melt (72-74wt.% SiO2). A lava dome grew following the initial tephra-producing events but was destroyed by an explosion on April 4. Ejecta from the April 4 explosion consists entirely of ISA and HSA, as does a subsequent lava dome that grew April 4-July 1; LSA was absent. Andesites from the April 4 event and from the final dome had pre-eruptive temperatures of 725-840°C (FeTi oxides) and highly evolved matrix liquids (77-80wt.% SiO2), including in rare microlite-free pyroclasts. ISA has mixed populations of phenocrysts suggesting it is a hybrid between HSA and LSA. The last lavas from the 2009 eruption, effused May 1-July 1, are distinctly depleted in P2O5, consistent with low temperatures and high degrees of crystallization including apatite.Plagioclase-melt hygrometry and comparison to phase equilibrium experiments are consistent with pre-eruptive storage of all three magma types at 100-160MPa (4-6km depth), if they were close to H2O-saturation, coincident with the locus of shallow syn-eruptive seismicity. Deeper storage would be indicated if the magmas were CO2-rich. Relatively coarse-grained clinopyroxene-rich reaction rims on many LSA amphibole phenocrysts may result from slow ascent to, or storage at, depths shallow enough for the onset of appreciable H2O exsolution, consistent with pre-eruptive staging in the uppermost crust. We interpret that the 2009 LSA ascended from depth during the 8 or more months prior to the first eruption, but that the magma stalled and accumulated in the upper crust where its phenocryst rim and melt compositions were established. Ascent of LSA through stagnant mushy intrusions residual from earlier Redoubt activity mobilized differentiated magma pockets and interstitial liquids represented by HSA, and as LSA-HSA hybrids represented by ISA, that fed the subsequently erupted lava domes. © 2011.
Character, mass, distribution, and origin of tephra-fall deposits from the 2009 eruption of Redoubt Volcano, Alaska-Highlighting the significance of particle aggregationKristi L. Wallace | Janet R. Schaefer | Michelle L. Coombs
Volume 259, Issue , June 2013, Pages 145-169
The 2009 eruption of Redoubt Volcano included 20 tephra-producing explosions between March 15, 2009 and April 4, 2009 (UTC). Next-Generation radar (NEXRAD) data show that plumes reached heights between 4.6km and 19km asl and were distributed downwind along nearly all azimuths of the volcano. Explosions lasted between <1 and 31min based on the signal duration at a distal seismic station (86km). From Moderate Resolution Imaging Spectroradiometer (MODIS) imagery and field data, we estimate that over 80,000km2 received at least minor ash fall (>0.8mm thick), including communities along the Kenai Peninsula (80-100km) and the city of Anchorage (170km). Trace ash (<0.8mm) was reported as far as Fairbanks, 550km NNE of the volcano. We estimate the total mass of tephra-fall deposits at 54.6×109kg with a total DRE volume of 20.6×106m3.On March 15, a small (4.6. km asl) phreatic explosion containing minor, non-juvenile ash, erupted through the summit ice cap. The first five magmatic explosions (events 1-5) occurred within a 6-hour period on March 23. Plumes rose to heights between 5.5. km and 14.9. km asl during 2- to 20-minute-duration explosions, and were dispersed mainly along a NNE trajectory. Trace ash fall was reported as far as Fairbanks. Owing to a shift in wind direction and heavy snowfall during these events, field discrimination among many of these layers was possible. All deposits comprise a volumetrically significant amount of particle aggregates, yet only event 5 deposits contain coarse clasts including glacier ice. The most voluminous tephra fall was deposited on March 24 (event 6) from a 15. minute explosion that sent a plume to 18.3. km asl, and dispersed tephra to the WNW. Within 10. km of the vent, this deposit contains 1-11. cm pumice clasts in a matrix of 1-2. mm aggregate lapilli. A small dome was presumably emplaced between March 23 and March 26 and was subsequently destroyed during 1-14. minute magmatic explosions of events 7-8 (March 26) that sent plumes between 8.2. km and 19. km asl. Ash fell along a broad swath to the ESE, covering communities along the Kenai Peninsula with up to 1. mm of ash. Proximal deposits are largely composed of aggregate lapilli of 1-2. mm with very little coarse juvenile material. Events 9-18 (March 27) sent plumes between 5.2. km and 15.5. km asl during <. 1-11-minute-long explosions. Ash clouds dispersed along trajectories to the NE, ENE and N and event 17 deposited up to 1. mm of ash on upper Kenai Peninsula and Anchorage. A moderate-size dome was emplaced between March 29 and April 4 and was subsequently destroyed during event 19 on April 4 which lasted 31. min and sent ash to 15.2. km asl. The proximal deposit is principally composed of dense dome rock, unlike earlier events, indicating that event 19 was likely caused by dome failure. The cloud dispersed to the SE along a narrow trajectory and up to 1-2. mm of ash fell on the lower Kenai Peninsula.Particle size data showing a preponderance of fine ash, even in the most proximal locations, along with the abundance of aggregate lapilli documented in most samples, confirms that particle aggregation played a significant role in the 2009 eruption and induced premature fallout of fine ash. © 2012.
Sequential eruption of alkaline and sub-alkaline magmas from a small monogenetic volcano in the Auckland Volcanic Field, New ZealandA. J. Needham | J. M. Lindsay | I. E M Smith | P. Augustinus | P. A. Shane
Volume 201, Issues 1-4, April 2011, Pages 126-142
Rangitoto Volcano is the youngest and largest eruptive centre in the monogenetic intraplate Auckland Volcanic Field (AVF). The stratigraphy of Rangitoto pyroclastic deposits that have been preserved in swamps on nearby Motutapu Island and in Lake Pupuke on the mainland reveals that the volcano erupted twice; radiocarbon dating of 10 samples from the two tephra units in the swamps indicates eruption ages of 553 ± 7 and 504 ± 5. Cal years BP, for the lower and upper tephra layers, respectively. Geochemistry of the lava field and various scoria cones on Rangitoto Island itself reveals two distinct compositional groups: an alkaline olivine basalt group (that correlates geochemically with the lower tephra layer) and a group that is sub-alkaline and transitional to tholeiite (that correlates geochemically with the upper tephra layer). Based on this data, we infer that, following a phreatomagmatic vent-clearing phase, the early magmatic eruption of Rangitoto Volcano was Strombolian in character and produced an alkaline olivine basalt scoria cone and an associated thick ash deposit on nearby Motutapu Island. This was followed by a time gap of up to several decades, after which a second eruptive phase built the current summit scoria cone together with an encircling lava field. We suggest this later, sub-alkaline eruptive period was associated with the deposition of the thin upper tephra layer on Motutapu Island. The two suites of Rangitoto samples are chemically quite distinct, and each is associated with a distinct parental composition. Trace element modelling indicates the alkaline and sub-alkaline parental melts could have been derived by ~. 1 and 6. wt.% partial melting of an anhydrous garnet peridotite source at ~. 80 and 65. km depth, respectively. The compositional range within each suite is similar, and can be explained by mainly olivine together with minor clinopyroxene fractionation within a relatively simple conduit system in which mixing and mingling were not important. Significant olivine fractionation (< 25%) suggests that the magma may have spent some time in the upper conduit during ascent. This contrasts with a recently published model for the Crater Hill centre in the AVF, in which deep-seated fractionation of clinopyroxene followed by relatively rapid ascent to the surface has been invoked to explain the compositions seen there. The polycyclic nature of Rangitoto and in particular the reuse of the conduit system after a period of quiescence have implications for the concept of monogenetic volcanism. There are also implications for hazard assessment, such that when a future eruption occurs in the AVF, it will be necessary to consider the conduit a possible pathway for another eruption for up to several decades afterwards. © 2010 Elsevier B.V.
Insights into extensional processes during magma assisted rifting: Evidence from aligned scoria conesTyrone O. Rooney | Ian D. Bastow | Derek Keir
Volume 201, Issues 1-4, April 2011, Pages 83-96
Mechanical and magmatic processes exert first-order control on the architecture and evolution of rifts. As a continental rift develops towards a new oceanic spreading centre, extension that is initially accommodated in a broad zone of faulting and ductile stretching must transition towards a narrow zone of focused magmatic intrusion. The Main Ethiopian Rift (MER), part of the East African Rift System, is an ideal location to study this transition because it captures rifting processes during continental breakup. In this contribution we synthesise geochemical data from scoria cones in the Wonji Fault Belt (WFB) and Silti-Debre Zeyit Fault Zone (SDFZ) in the MER to provide new constraints on the development of mantle melting columns and magmatic plumbing systems since the onset of rifting. We utilize the extensive geophysical and geochemical databases, collected in the Ethiopian Rift, to show that geochemical evidence of heterogeneity in the depth of the mantle melting column which produced Quaternary rift basalts correlates with lithospheric structure. When combined with existing observations of asymmetry across the rift in terms of depth of melting column and magmatic plumbing systems, it is evident that the mechanical structure of the rift, defined during the initial stages of breakup, has played a dominant role in the initial development of magma assisted rifting in the MER. Surface structures and crustal-scale geophysical studies have suggested the WFB is analogous to a sea-floor spreading centre. However, the geochemical characteristics of rift basalts are consistent with mantle tomography that shows no evidence beneath the MER for passive magmatic upwelling beneath discrete rift segments as is observed in the ocean basins. Collectively, the Ethiopian data show that the distribution of mantle melts during the initiation of magma assisted rifting is fundamentally influenced by lithospheric structures formed during earlier syn-rift stretching. © 2010 Elsevier B.V.
The 2011-2012 summit activity of Mount Etna: Birth, growth and products of the new SE craterBoris Behncke | Stefano Branca | Rosa Anna Corsaro | Emanuela De Beni | Lucia Miraglia | Cristina Proietti
Volume 270, Issue , January 2014, Pages 10-21
Between January 2011 and April 2012, the Southeast Crater (SEC) on Mount Etna was the site of 25 episodes of lava fountaining, which led to the construction of a new pyroclastic cone on the eastern flank of the SEC. During these episodes lava overflows reached 4.3km in length with an area of 3.19km2 and a volume of 28×106m3. The new cone, informally called New Southeast Crater (NSEC), grew over a pre-existing subsidence depression (pit crater), which had been formed in 2007-2009. The evolution of the NSEC cone was documented from its start by repeated GPS surveys carried out both from a distance and on the cone itself, and by the acquisition of comparison photographs. These surveys reveal that after the cessation of the lava fountains in April 2012, the highest point of the NSEC stood 190m above the pre-cone surface, while the cone volume was about 19×106m3, representing 38% of the total (bulk) volume of the volcanic products including pyroclastic fallout erupted in 2011-2012, which is 50×106m3 (about 33×106m3 dense-rock equivalent). Growth of the new cone took place exclusively during the paroxysmal phases of the lava fountaining episodes, which were nearly always rather brief (on the average 2h). Overall, the paroxysmal phases of all 25 episodes represent 51h of lava fountaining activity - the time needed to build the cone. This is the fastest documented growth of a newborn volcanic cone both in terms of volume and height. Mean effusion rates during the lava fountaining episodes on 20 August 2011 (E11), as well as 12 and 24 April 2012 (E24 and E25) exceeded 500m3/s (with maximum rates of 980m3/s during E11) and thus they are among the highest effusion rates ever recorded at Etna. The composition of the erupted products varies in time, reflecting different rates of magma supply into the shallow feeding system, but without notable effects on the eruptive phenomenology. This implies that the dynamics leading to the episodic lava fountaining was largely, though not entirely, controlled by the repeated formation and collapse of a foam layer in the uppermost portion of the magmatic reservoir of the NSEC. © 2013 Elsevier B.V.
Petrological insights into the storage conditions, and magmatic processes that yielded the centennial 2010 Merapi explosive eruptionFidel Costa | Supriyati Andreastuti | Caroline Bouvet de Maisonneuve | John S. Pallister
Volume 261, Issue , July 2013, Pages 209-235
To understand the processes that made the 2010 eruption of Merapi much larger and more explosive than most dome-forming eruptions of the past century, we investigated the geochemistry, petrology, and pre-eruptive conditions of magmas erupted in 2006 and 2010. The juvenile rocks of 2010 are plagioclase, two-pyroxene basaltic andesites with seriate textures and minor amounts of reaction-free amphibole, Fe-Ti oxides, and rare crystals of olivine and biotite. The bulk-rock composition, mineral paragenesis, and textures are similar to those of juvenile blocks from the much less explosive eruption of 2006. One of the key differences is that most amphiboles in 2010 don't have breakdown reaction rims, whereas those of 2006 are largely reacted. We acquired >80 X-ray distribution maps of major and minor elements of large areas (>1cm2) and single crystals, backscattered electron images, electron microprobe analyse, and compositional traverses across crystals. The data reveal that both the 2006 and 2010 samples are heterogeneous at various spatial scales, with numerous reaction textures between pyroxenes and amphiboles, dissolution textures, and large variations of crystal sizes, morphologies, and compositions. These features record open-system magmatic processes involving the assimilation of carbonate rocks, and interactions between various parts of Merapi's plumbing system, including a degassed shallow magma system and deep hotter and more volatile rich magma intrusions.The petrological complexity of the samples makes unraveling the pre-eruptive conditions of Merapi magmas a petrological puzzle. We applied five different geothermobarometers and performed thermodynamic modeling with the MELTS algorithm, and we propose that there are at least three crystallization zones or environments below Merapi. A deep reservoir at about 30 (+/-3) km depth is suggested by some amphiboles and high-Al clinopyroxenes. Here is where the high-Al basaltic andesites from Merapi are generated probably by water-rich fractionation of more primitive magmas. Such deep magmas are volatile-rich and at near-liquidus conditions (≥4-6wt.% H2O, ≥0.15wt.% SO2, and an undetermined amount of CO2, at about 1050°C) when they start moving towards the surface. A second crystallization zone is recorded by another type of amphibole at about 13 (+/-2) km. Here high-Al clinopyroxene may also grow together with Ca-rich plagioclase. Assimilation of limestone may also occur at this level as recorded by the very Ca-rich plagioclases found in the cores of some crystals. At this location the water content of the melt must remain high enough to stabilize amphibole (4-6wt.% H2O) but CO2 and SO2 are probably already degassing and contribute to gas changes observed by the monitoring system at the surface. Finally, a shallower part of the system (<10km) is recorded by the lower anorthite plagioclase and low-Al in clinopyroxene, and perhaps also in orthopyroxene. This part of the system is probably crystal-rich and largely degassed, and is the likely source of the high-temperature fumaroles and the volcanic gas plumes that are commonly seen at Merapi.We propose that the 2006 and 2010 eruptions were driven by basically the same processes and magma types. The main difference is the much larger size of the deep and volatile-rich magma replenishment that took place in 2010, which had large effects on the kinetics and dynamics of the plumbing system and processes. In 2006, and perhaps also in most of the typical small dome-forming historical eruptions at Merapi, the direct ascent of deep and gas-rich magmas towards the surface is slowed down and partially arrested by the shallower crystal-rich zones of left-over magma from previous events. However, this was not possible in 2010, where the much larger (up to 10 times) size of the magma intrusions overwhelmed the crystal-rich eruption filter. In 2010 the deep magma probably resided for only a short time at intermediate to shallower depths which allowed it to proceed to the surface still carrying most of its deep gas cargo. The larger magma intrusion probably induced higher rates of crustal carbonate assimilation and production of additional CO2 gas at shallow depths. This contributed to the much faster than usual ascent rates and larger explosivities in 2010 than in 2006. These inferences are supported by the shorter interaction times calculated from the diffusion models of clinopyroxene compositions for the 2010 magmas, by the fact that most amphiboles are not broken down in 2010 as opposed to 2006, and also by the much shorter times of escalating monitoring signals (seismicity and deformation) in 2010 compared to 2006.A puzzling observation is that despite the multiple explosive phases of the 2010 eruption, pumiceous materials are rare, and were only found in the last part of the eruption. This contrasts with the abundant tephra layers and vesiculated deposits of older historical explosive events like 1872, and suggest that syn-eruptive processes in 2010 were also different from standard models. The rarity of expanded pumices in 2010 may be due to rapid degassing and re-welding of magma as it ascended from intermediate depths. Given the near constant bulk composition of Merapi magmas erupted in the last decades, and the similarity of textures and minerals in 2006 and 2010, our study suggests that most Merapi magmas are intrinsically capable of explosive eruptions. Here we propose that whether they do so or not mainly depends on the degree of interaction and magma mass proportions between the upper crystal-rich parts of the system (including carbonates) and the deeper and more gas-rich replenishing magmas. Older historical explosive eruptions at Merapi such as in 1872 were driven by more mafic magmas than those erupted in 2006 and 2010 and thus might be caused by different processes from those discussed here. The still unanswered and vexing questions remain as to why in 2010 a much larger amount of magma was segregated from depths and whether this will happen again in the near future. © 2013 Elsevier B.V.
A model for calculating eruptive volumes for monogenetic volcanoes - Implication for the Quaternary Auckland Volcanic Field, New ZealandGábor Kereszturi | Károly Németh | Shane J. Cronin | Javier Agustín-Flores | Ian E M Smith | Jan Lindsay
Volume 266, Issue , October 2013, Pages 16-33
Monogenetic basaltic volcanism is characterised by a complex array of behaviours in the spatial distribution of magma output and also temporal variability in magma flux and eruptive frequency. Investigating this in detail is hindered by the difficulty in evaluating ages of volcanic events as well as volumes erupted in each volcano. Eruptive volumes are an important input parameter for volcanic hazard assessment and may control eruptive scenarios, especially transitions between explosive and effusive behaviour and the length of eruptions. Erosion, superposition and lack of exposure limit the accuracy of volume determination, even for very young volcanoes. In this study, a systematic volume estimation model is developed and applied to the Auckland Volcanic Field in New Zealand. In this model, a basaltic monogenetic volcano is categorised in six parts. Subsurface portions of volcanoes, such as diatremes beneath phreatomagmatic volcanoes, or crater infills, are approximated by geometrical considerations, based on exposed analogue volcanoes. Positive volcanic landforms, such as scoria/spatter cones, tephras rings and lava flow, were defined by using a Light Detection and Ranging (LiDAR) survey-based Digital Surface Model (DSM). Finally, the distal tephra associated with explosive eruptions was approximated using published relationships that relate original crater size to ejecta volumes. Considering only those parts with high reliability, the overall magma output (converted to Dense Rock Equivalent) for the post-250 ka active Auckland Volcanic Field in New Zealand is a minimum of 1.704km3. This is made up of 1.329km3 in lava flows, 0.067km3 in phreatomagmatic crater lava infills, 0.090km3 within tephra/tuff rings, 0.112km3 inside crater lava infills, and 0.104km3 within scoria cones. Using the minimum eruptive volumes, the spatial and temporal magma fluxes are estimated at 0.005km3/km2 and 0.007km3/ka. The temporal-volumetric evolution of Auckland is characterised by an increasing magma flux in the last 40ky, which is inferred to be triggered by plate tectonics processes (e.g. increased asthenospheric shearing and backarc spreading of underneath the Auckland region).© 2013 Elsevier B.V.