Showing papers in "Geophysical monograph in 2008"

Volcanology and Magma Geochemistry of the Present‐Day Activity: Constraints on the Feeding System

Abstract: Stromboli volcano is famous in the scientific literature for its persistent state of activity, which began about 1500 years ago and consists of continuous degassing and mild intermittent explosions (normal Strombolian activity). Rare lava emissions and sporadic more violent explosive episodes (paroxysms) also occur. Since its formation, the present-day activity has been dominated by the emission of two basaltic magmas, differing chiefly in their crystal and volatile contents, whose characteristics have remained constant until now. The normal Strombolian activity and lava effusions are fed by a crystal-rich, degassed magma, stored within the uppermost part of the plumbing system, whereas highly vesicular, crystal-poor light-colored pumices are produced during paroxysms testifying to the ascent of volatile-rich magma batches from deeper portions of the magmatic system. Mineralogical, geochemical, and isotopic data, together with data on the volatile contents of magmas, are presented here with the aim of discussing (1) the relationships between the different magma batches erupted at Stromboli, (2) the mechanisms of their crystallization and transfer, (3) the plumbing system and triggering mechanisms of Strombolian eruptions.

Crater Gas Emissions and the Magma Feeding System of Stromboli Volcano

Abstract: Quiescent and explosive magma degassing at Stromboli volcano sustains high-temperature crater gas venting and a permanent volcanic plume which constitute key sources of information on the magma supply and dynamics, the physical processes controlling the explosive activity and, more broadly, the volcano feeding system. The chemical composition and the mass output of these crater emissions (gases, trace metals, radioactive isotopes) were measured using different methodologies: within-plume airborne measurements, ground-based plume filtering, and/or in situ analysis, remote UV and open-path Fourier transform infrared absorption spectroscopy. The results obtained, summarized in this paper, demonstrate a primary control of the magmatic gas phase on the eruptive regime and the budget of the volcano. The large excess gas discharge, compared with the lava extrusion rate, and the source depth of slug-driven Strombolian explosions evidence extensive separate gas transfer across the volcano conduits, promoted by the high gas content (vesicularity) and then permeability of the shallow basaltic magma. Combined with data for volatiles dissolved in olivine-hosted melt inclusions, the results provide updated constraints for the magma supply rate (similar to 0.3 m(3) s(-1) average), the ratio of intrusive versus extrusive magma degassing (similar to 15), and the amount of unerupted degassed magma that should be convectively cycled back in conduits and accumulated beneath the volcano over time (similar to 0.25 km(3) in the last three decades). The results also provide insight into the possible triggering mechanism of intermittent paroxysmal explosions and the geochemical signals that might allow forecasting these events in the future.

Tectonics, Dynamics, and Seismic Hazard in the Canada―Alaska Cordillera

Abstract: The North America Cordillera mobile belt has accommodated relative motion between the North America plate and various oceanic plates since the early Mesozoic. The northern half of the Cordillera (Canada-Alaska Cordillera) extends from northern Washington through western Canada and central Alaska and can be divided into four tectonic domains associated with different plate boundary interactions, variable seismicity, and seismic hazard. We present a quantitative tectonic model of the Canada-Alaska Cordillera based on an integrated set of seismicity and GPS data for these four domains: south (Cascadia subduction region), central (Queen Charlotte-Fairweather transcurrent region), north (Yakutat collision region), and Alaska (Alaska subduction region). This tectonic model is compared with a dynamic model that accounts for lithosphere strength contrasts and intemal/ boundary force balance. We argue that most of the Canada―Alaska Cordillera is an orogenic float where current tectonics are mainly limited to the upper crust, which is mechanically decoupled from the lower part of the lithosphere. Variations in deformation style and magnitude across the Cordillera are mostly controlled by the balance between plate boundary forces and topography-related gravitational forces. In particular, the strong compression and gravitational forces associated with the Yakutat collision zone are the primary driver of the complex tectonics from eastern Yukon to central Alaska, resulting in crustal extrusion, translation, and deformation across a 1500 x 1000-km 2 region. This tectonic-dynamic model can be used to provide quantitative constraints to seismic hazard models. We present a simple example of mapping M w = 7 earthquake return periods throughout the Cordillera.

The paroxysmal event and its deposits

Abstract: The 5 April 2003 eruption of Stromboli volcano (Italy) was the most violent of the past 50 years. It was also the best documented due to the accurate geophysical monitoring of the ongoing effusive eruption. Detailed field studies carried out a few hours to a few months after the event provided further information that were coupled with visual documentation to reconstruct the explosive dynamics. The eruption consisted of an 8-min-long explosive event preceded by a short-lived precursory activity that evolved into the impulsive ejection of gas and pyroclasts. Meter-sized ballistic blocks were launched to altitudes of up to 1400 m above the craters falling on the volcano flanks and on the village of Ginostra, about 2 km far from the vent. The vertical jet of gas and pyroclasts above the craters fed a convective plume that reached a height of 4 km. The calculated erupted mass yielded values of 1.1―1.4 × 10 8 kg. Later explosions generated a scoria flow deposit, with an estimated mass of 1.0―1.3 × 10 7 kg. Final, waning ash explosions closed the event. The juvenile fraction consisted of an almost aphyric, highly vesicular pumice mingled with a shallow-derived, crystal-rich, moderately vesicular scoria. Resuming of the lava emission a few hours after the paroxysm indicate that the shallow magmatic system was not significantly modified during the explosions. Combination of volume data with duration of eruptive phases allowed us to estimate the eruptive intensity: during the climactic explosive event, the mass discharge rate was between 10 6 and 10 7 kg/s, whereas during the pyroclastic flow activity, it was 2.8―3.6 ― 10 5 kg/s. Strong similarities with other historical paroxysms at Stromboli suggest similar explosion dynamics.

Multiple equilibria and abrupt transitions in Arctic summer sea ice extent

Abstract: An application of bifurcation theory to the stability of Arctic sea ice cover is described. After reviewing past such efforts, a simple mathematical representation is developed of processes identified as contributing essentially to abrupt decreases in 21st century Arctic summer sea ice extent in climate model simulations of the Community Climate System Model, version 3 (CCSM3). The resulting nonlinear equations admit abrupt sea ice transitions resembling those in CCSM3 and also plausibly represent further gross aspects of simulated Arctic sea ice evolution such as the accelerating decline in summer ice extent in the late 20th and early 21st centuries. Equilibrium solutions to these equations feature multiple equilibria in a physically relevant parameter regime. This enables abrupt changes to be triggered by infinitesimal changes in forcing in the vicinity of the bifurcation or, alternatively, by finite perturbations some distance from the bifurcation, although numerical experiments suggest that abrupt transitions in CCSM3 may arise mainly from the increasing sensitivity of sea ice to fluctuations in ocean heat transport as ice thickness and extent diminish. A caveat is that behavior following a complete seasonal loss of ice cover is sensitive to aspects of the parameterization of ocean shortwave absorption.

The 5 April 2003 Explosion of Stromboli: Timing of Eruption Dynamics using Thermal Data

Abstract: Stromboli's 5 April 2003 explosion sent an ash plume to 4 km and blocks to 2 km, representing one of the most powerful events over the past 100 years. A thermal sensor 450 m east of the vent and a helicopter-flown thermal camera captured the event dynamics allowing detailed reconstruction. This review links previous studies providing a complete collation and clarification of the actual event chronology, while showing how relatively inexpensive thermal sensors can be used to provide great insight into processes that cannot be observed from locations outside the eruption cloud. The eruption progressed through four phases, comprised 29 discrete explosions, and lasted 373 s. The opening phase (phase 1) comprised ∼30 s of precursory ash emission, with stronger emission beginning after 17 s. This was abruptly terminated by the main blast of phase 2 that comprised emission of a rapidly expanding ash cloud followed, after 0.4 s, by a powerful jet with velocities of up to 320 m/s. A second explosive phase (phase 3) began 38 s later and involved ascent of a phoenix cloud and explosive emission above a lateral vent lasting 75 s. This was followed by a 175-s-long phase of weaker, pulsed emission. The eruption was terminated by a series of three explosions (phase 4) sending ash to ∼600 m at velocities of 27―45 m/s and lasting 87 s. Together, these results have shown that a low-energy opening phase was followed by the highest-energy phase. Each phase itself comprised groups of discrete explosions, with energy of the explosions diminishing during the two final phases.

The Eruptive Activity of 28 and 29 December 2002

Abstract: At 1820 UT of 28 December 2002, an eruptive vent opened on the NE flank of the Sciara del Fuoco (SdF) at 600 m above sea level, marking the onset of the 2002–2003 eruptive crisis of Stromboli volcano. The first eruptive hours were characterized by mild spattering and effusive activity from the new vent and the summit vent at crater 1. Gravitational instability processes also determined the partial collapse of NE walls of the summit cone (crater 1). Pyroclastic material partly accumulated on the NE part of the SdF and partly flowed downslope and reached the sea at Spiaggia dei Gabbiani, forming a ~4-m-thick, reddish avalanche, that was soon covered by a lava flow emitted in the following hours. In this paper, we describe the first hours of activity through eyewitnesses’ reports, geophysical monitoring, field and laboratory studies, of the erupted pyroclastic material and lava flows. Daily temperature measurements were carried out on the avalanche deposit formed by the flow of scoria along the SdF, using a handheld thermal camera mainly during helicopter surveys. A fast cooling rate was typical of the deposit surface, and a slow cooling rate was representative of its inner portion.

Mineralogical, Geochemical, and Isotopic Characteristics of the Ejecta from the 5 April 2003 Paroxysm at Stromboli, Italy: Inferences on the Preeruptive Magma Dynamics

Abstract: The 5 April 2003 explosive eruption at Stromboli emplaced typical basaltic scoria, pumice, and lithic blocks. This paper reports a detailed set of mineralogical, geochemical, and isotopic data on the juvenile ejecta and fresh subvolcanic blocks, including micro-Sr isotope analyses and major and dissolved volatile element contents in olivine-hosted melt inclusions. The juvenile ejecta have compositions similar to those of their analogs from previous paroxysms; the 2003 pumice, however, does not contain stable high-MgO olivine, usually typical of large-scale paroxysms and has lower compatible element contents. Texture, composition, and Sr isotope disequilibrium of crystals in pumice indicate that most of them are inherited from the shallow crystal-rich magma and/or crystal mush. The most primitive magma is recorded as rare melt inclusion in olivine Fo 85―86 . It has a typical S/Cl (1.1) and a total volatile content of 3.1 wt % from which the total fluid pressure was evaluated >240 MPa. Hence, moderate pressure conditions can be envisaged for the mechanism triggering the April 2003 paroxysm. The subvolcanic blocks are shoshonitic basalts with 45―50 vol % of phenocrysts (plagioclase + clinopyroxene + olivine). The late-stage crystallization of the crystal-rich magma lead to the formation ofNa-sanidine with plagioclase An 60―25 + olivine Fo 68―49 + Ti-magnetite ± apatite ± phlogopite ± ilmenite assemblage. Mineralogy, chemistry, and Sr―Nd isotopic signatures of the subvolcanic blocks indicate they represent the slowly cooled equivalents of batches of crystal-rich basaltic magma stored in the uppermost subvolcanic feeding system during the last few years. Cooling might be facilitated by short breaks in the summit crater activity.

Slope Failures Induced by the December 2002 Eruption at Stromboli Volcano

Abstract: We reconstruct the sequence of landslides that occurred soon after the beginning of the December 2002 eruption on the NW flank of Stromboli volcano. Landslides involved the northeastern part of the Sciara del Fuoco (SdF) slope, an old collapse scar filled by products of volcanic activity, producing tsunami waves that severely damaged the coast of the island of Stromboli. Volumes of the mass detached from the subaerial and submarine slope were quantified by comparing preslide and postslide slope surfaces obtained by aerophotogrammetric and bathymetric data, which also allowed, in conjunction with field observations and helicopter surveys, the reconstruction of geometry and kinematics of landslides. According to the reconstructed sequence, 2 days after the beginning of the eruption, the upper part of the NE sector of the SdF slope experienced major displacements (few tens of meters). Movements propagated downslope and affected the nearshore portion of the submerged slope without a rapid sliding of the displaced mass into the sea. The following hours were characterized by a progressive increase of deformations, localized along shear zones extending over two thirds of the subaerial slope. This phase proceeded until a submarine slide about 6 x 10 6 m 3 in volume occurred, causing a first tsunami wave. The subaerial mass delimited by the shear zones and unbuttressed at its foot, then slipped into the sea producing a second tsunami wave. The main landslide event (and the minor slumps which followed) removed a volume of about 10 x 10 6 m 3 of the infilling deposit, to a thickness of at least 65 m. Hypotheses were formulated on the mechanisms that controlled the different phases of the instability sequence. Since hydraulic and stress/strain conditions progressively changed during the slope evolution, the formulated mechanisms are also based on geotechnical analyses and considerations on the mechanical behavior of volcaniclastic materials. The process that led to the landslide events.

Rapid Ice Mass Loss: Does It Have an Influence on Earthquake Occurrence in Southern Alaska?

Abstract: The glaciers of southern Alaska are extensive, and many of them have undergone gigatons of ice wastage on time scales on the order of the seismic cycle. Since the ice loss occurs directly above a shallow main thrust zone associated with subduction of the Pacific-Yakutat plate beneath continental Alaska, the region between the Malaspina and Bering Glaciers is an excellent test site for evaluating the importance of recent ice wastage on earthquake faulting potential. We demonstrate the influence of cumulative glacial mass loss following the 1899 Yakataga earthquake (M=8.1) by using a two dimensional finite element model with a simple representation of ice fluctuations to calculate the incremental stresses and change in the fault stability margin (FSM) along the main thrust zone (MTZ) and on the surface. Along the MTZ, our results indicate a decrease in FSM between 1899 and the 1979 St. Elias earthquake (M=7.4) of 0.2 - 1.2 MPa over an 80 km region between the coast and the 1979 aftershock zone; at the surface, the estimated FSM was larger but more localized to the lower reaches of glacial ablation zones. The ice-induced stresses were large enough, in theory, to promote the occurrence of shallow thrust earthquakes. To empirically test the influence of short-term ice fluctuations on fault stability, we compared the seismic rate from a reference background time period (1988-1992) against other time periods (1993-2006) with variable ice or tectonic change characteristics. We found that the frequency of small tectonic events in the Icy Bay region increased in 2002-2006 relative to the background seismic rate. We hypothesize that this was due to a significant increase in the rate of ice wastage in 2002-2006 instead of the M=7.9, 2002 Denali earthquake, located more than 100km away.

Fluid Geochemistry of Stromboli

Abstract: An accurate description of the geochemical system is presented here based on a review of scientific work performed during the past decade. The surface manifestations of the volcanic system of Stromboli have been investigated using several measuring techniques. Studying the chemical composition of the volcanic plume and of fumarolic emissions has provided information on magma degassing processes. The total fluxes of the emitted gases from both the plume and the soil were found to vary with changes in volcanic activity (from normal Strombolian activity to effusive and/or paroxysmal activity). Thermal water results from the interaction between volcanic gases, host rock, seawater, and meteoric water and temporal changes observed in the chemical and the isotopic composition of the gases dissolved into thermal waters highlighted the rising of new magma batches. Combining modeling of gas―water―rock interactions with an understanding of the volcanic system allowed identifying preferential sampling sites and parameters for the geochemical monitoring of volcanic activity at Stromboli Island.

Volcanic and Seismic Activity at Stromboli Preceding the 2002-2003 Flank Eruption

Abstract: Regular surveys with a thermal camera from both ground- and helicopter-based surveys have been carried out on Stromboli since October 2001. This data set allowed us to detect morphological changes in Stromboli's summit craters produced by major explosions and to track an increase in volcanic activity associated with a heightened magma level within the main conduit that preceded the 2002―2003 effusive eruption. Together with thermal measurements, geophysical surveys performed in May and September/October 2002 highlighted clear increases in the amplitude of very long period (VLP) events, consistent with the ascent of the magma column above the VLP source region. The increased magma level was probably induced by elevated pressure in the deep feeding system, controlled by regional tectonic stress. This, in turn, pressurized the uppermost part of the crater terrace, producing greater soil permeability and soil degassing. Eventually, the magma loading caused the NW flank of the summit craters to fracture, allowing lava to flood out at high effusion rates on 28 December 2002, starting an approximately 6-month-long effusive eruption.

Geochemical Prediction of the 2002–2003 Stromboli Eruption From Variations in CO2 and Rn Emissions and in Helium and Carbon Isotopes

Abstract: Significant changes in both the chemistry of coastal thermal waters and the soil CO 2 and Rn emissions in the crater area were recorded at Stromboli prior to the eruption that began on 28 December 2002. The dissolved CO 2 contents and the δ 13 C and 3 He/ 4 He values were elevated in the thermal aquifer from July 2002. Synchronous variations in the same isotope ratios were recorded in the summit fumarolic gases, with both 3 He/ 4 He and δ 13 C values of gases released from a fumarole in the summit area increasing between May and November 2002. These variations are indicative of early degassing of a new gas-rich magma batch with a 13 C- and 3 He-rich signature. This magma recharge probably fed the intense Strombolian activity recorded during that period. The eruption began with a major explosion that produced a glowing avalanche, immediately followed by a fluid lava overflow from the NE crater and subsequent lava effusion from vents opened in the Sciara del Fuoco depression. Sharp increases in CO 2 soil flux and Rn emissions― to values never observed previously―were recorded in the summit crater area 10 d before the eruption onset. These CO 2 and Rn anomalies are indicative of a high gas-driven magma supply rate and gas overpressure within the conduit. The sudden depressurization of the magma filling the upper conduit probably caused the major explosion that occurred on 28 December, which heralded the effusive phase. These data demonstrate the importance of collecting a wide spectrum of geochemical data from different geological sites when monitoring a volcano.

Scientific Community and Civil Protection Synergy During the Stromboli 2002–2003 Eruption

Abstract: The eruption of Stromboli 2002―2003, thanks to its complex scenario (flank instability, tsunami, necessity to rapidly upgrade monitoring networks), has provided an important opportunity to verify the response of the national system of civil protection to volcanic emergencies. In particular, it has tested and validated the model of collaboration, in use by Italian law, between the Department of Civil Protection and the National Institute of Geophysics and Volcanology. This synergy has enabled a better understanding and ability to tackle the eruptive crisis from its first stages, as well as implement monitoring systems both dependably and swiftly. In this work, the numerous first monitoring tasks carried out during the critical initial stages of the eruption are described, and the activities and planned action are reported over the course of the eruption that has made Stromboli one of the best monitored volcanoes not only in Italy but throughout the world.

Movements of the Sciara Del Fuoco

Abstract: The Sciara del Fuoco (SdF) landslides that occurred at the end of December 2002 prompted researchers to install geodetic networks to monitor deformations related to potential new slope failures. With this aim, an integrated multiparametric monitoring system was designed and deployed. In particular, this complex monitoring system is composed of four single systems: an electronic distance measurement network, installed immediately after the landslide events, a real-time GPS network, a ground-based interferometric linear synthetic aperture radar (GB-InSAR), and an automated topographic monitoring system (named Theodolite Robotic Observatory of Stromboli, or THEODOROS); the three last systems provided a continuous monitoring of selected points or sectors of the SdF. Data acquired from different systems have been jointly analyzed to reach a better understanding of the SdF dynamics. Displacement data obtained from the topographic systems are compared with those obtained from GB-InSAR, and the results of the comparison are analyzed and discussed. Furthermore, in this chapter, an example of a warning system that can detect slope instability precursors on the SdF based on a statistical analysis of the data collected by the THEODOROS system is reported.

Gas Flux Rate and Migration of the Magma Column

Abstract: The 2002-2003 effusive eruption of Stromboli volcano represents an excellent opportunity to investigate the transition from effusive to explosive activity at an open-conduit basaltic system, when activity migrated from effusive vents, at the base of the craters, to summit explosions. The transition is investigated here through the analysis of very long period seismicity, delay times between infrasonic and thermal onsets of explosions, and SO2 flux recorded during a 1-year period. The synergy of the multiple geophysical observations points to a magma-driven migration of the magma column. Here the increased magma supply at the eruption onset lead to opening of effusive fissures, which draining the magma in the shallow conduit caused the decrease of the magma level. The decrease of the magma supply at the end of the effusion lead to sealing of effusive fissures, upraise of the magma level within the conduit, and reestablishment of explosive activity from the summit vents.

The 5 April 2003 paroxysm at Stromboli: a review of geochemical observations

Abstract: This paper reviews the published geochemical variations observed during the 2002―2003 eruption at Stromboli volcano. At the end of 2002, a new eruption began at Stromboli with a lava flow that lasted until the end of July 2003. In 5 April 2003, an explosive paroxysm occurred with the ejection of bombs that reached the village of Ginostra, about 4 km from the craters. During the eruption, specific variations in chemical composition of groundwaters and summit fumaroles were recorded before the explosion, most of them for the first time. The water pH decreased significantly (0.5 units), and the dissolved CO 2 increased in two thermal wells (Cusolito and Zurro) located near Stromboli harbor from March until 5 April. Peaks in the dissolved He were also observed at all the sampling sites. All of these changes in the thermal aquifer suggested a pressurization of the system due to the degassing of a volatile-rich magma at depth. In the summit area, the SO 2 /HCl and SO 2 /HF ratios in the plume increased suddenly between 1 and 3 April due to the degassing of an S-rich magma that was approaching the shallow levels of the plumbing system, and this was involved in the explosion that occurred a few days later. This eruption was the first at Stromboli to be analyzed using geochemical models. The variations observed in the basal aquifer and in the summit area occurred on very different timescales: a few weeks and a few days, respectively.

Assimilation of Observations with Models to Better Understand Severe Ionospheric Weather at Mid-Latitudes

Abstract: For many decades, the mid-latitude ionosphere was regarded as well characterized even if not well modeled As a result, the Federal Aviation Authority's Wide Area Augmentation System (WAAS) was developed to provide augmented GPS positioning information to correct for ionospheric variability However, over the past 5 years, recurrent superstorms in the ionosphere have forced the WAAS system to go offline for many hours at a time This report discusses present-day knowledge regarding these conditions and how they are associated with unexpectedly steep horizontal gradients in the mid-latitude ionosphere total electron content (TEC) In a general sense, the possible physical mechanisms are understood, but during a storm the distribution and evolution of the driving forces for these mechanisms are neither understood nor adequately observed, the two main driving forces being the convection electric field and the neutral wind In this paper a simplified convection electric field pattern is presented and used to drive a physics-based ionospheric model This demonstrates how the superstorm ionospheric condition could be generated Data assimilation is a new approach that could exceed present-day empirical and physical model limitations There are three main expectations for data assimilation: (1) combined with a good ionospheric background model, the data assimilation must provide realistic global specification of the ionosphere; (2) it must also provide additional information about the ionosphere that is not already evident in the observation, that is, altitude profiles of the electron density when only slant TEC integrals of the electron density are available; and (3) with full physics-based models in the assimilation procedure, data assimilation models must also provide the drivers, that is, the neutral wind and electric field patterns This paper examines the current status of these three expectations with regard to the future for the scientist and the space weather forecaster

Textural and Compositional Characteristics of Lavas Emitted During the December 2002 to July 2003 Stromboli Eruption (Italy): Inferences on Magma Dynamics

Abstract: Periodic lava sampling was carried out at the active vents during the entire duration ofthe 28 December 2002 to 22 July 2003 effusive eruption. Major and trace element bulk rock analyses were performed at different laboratories, thereby acquiring four independent sets of analysis. Nd and Sr isotope ratios were obtained on whole rocks and groundmasses, together with micro-Sr isotope analyses on plagioclase and clinopyroxene by microdrilling technique. Crystal size distribution, mineral, and glassy matrix chemistry were analyzed on selected samples. The products show a fairly homogeneous composition, close to that of the crystal-rich scoria that erupted in the previous years. Slight variations of trace elements and isotope ratios between products that erupted before and after the 5 April paroxysm are likely accounted for by limited mixing between the fresh, volatile-rich magma that erupted during the paroxysm and the volatile-poor magma feeding the lava flow. Micro-Sr isotope data show large isotopic disequilibria pointing to the persistence of highly Sr-radiogenic xenocrysts or crystal cores in the shallow magmatic system, probably recycled from the previous activity. Data rule out important changes in the dynamics of the plumbing system shortly before the eruption. A discrete input of deep magma into the lower part of the shallow system some months before the eruption may be at the origin of the increase of the magmatostatic pressure in the conduits, leading to the effusive eruption. An alternative hypothesis considers a nearly steady-state feeding system undergoing gradual, long-term pressure increase in its upper part, eventually leading to periodic lava effusions.