V11C-2048
Volcanic and Structural History of the NE Rift Zone of Tenerife, Canary Islands, Spain
The NE Rift of Tenerife is an excellent example of a persistent, recurrent rift, providing important evidence on the origin and dynamics of these major volcanic features. The rift developed in three successive, intense and relatively short eruptive stages (a few hundred ka), separated by longer periods of quiescence or reduced activity: A Miocene stage (7203+/-155ka), apparently extending the central Miocene shield of Tenerife towards the Anaga massif; an Upper Pliocene stage (2710+/-58ka) and the latest stage, with the main eruptive phase, in the Pleistocene. Detailed geological (GIS) mapping, geomagnetic reversal mapping and stratigraphic correlation, and radioisotopic (K/Ar) dating of volcanic formations allowed the reconstruction of the latest period of rift activity. In the early phases of this stage the majority of the eruptions grouped tightly along the axis of the rift and show reverse polarity (corresponding to the Matuyama). Dykes are of normal and reverse polarities. In the final phase of activity, eruptions are more disperse and lavas and dykes are consistently of normal polarity (Brunhes). Volcanic units of normal polarity crossed by dykes of normal and reverse polarities yield ages apparently compatible with normal events (M-B Precursor and Jaramillo) in the Upper Matuyama epoch. Three lateral collapses successively mass-wasted the rift: The Micheque collapse, completely concealed by subsequent nested volcanism, and the Güímar and La Orotava collapses, that are only partially filled. Pre- collapse and nested volcanism is predominantly basaltic, except in the Micheque collapse, where magmas evolved towards intermediate and felsic (trachytic) compositions. Rifts in the Canary Islands are long-lasting, recurrent features, probably related to primordial, plume-related fractures acting throughout the entire growth of the islands. Basaltic volcanism forms the bulk of the islands and rift zones. However, collapses of the flanks of the rifts disrupt their established fissural feeding system, frequently favouring magma ascent and shallow emplacement, leading to differentiation of magmas, and intermediate to felsic nested eruptions. Rifts and their collapse may therefore act as an important factor in providing petrological variability to oceanic volcanoes. Conversely, the possibility exists that the presence of important felsic volcanism may indicate lateral collapses in oceanic shields and ridge-like volcanoes, even if they are concealed by post-collapse volcanism or partially mass-wasted by erosion.
V11C-2049
Stratigraphic, Granulometric and Geochemical Studies of a Major Plinian Eruption on Dominica, Lesser Antilles
The island of Dominica, located in the center of the Lesser Antilles island arc has witnessed, probably within the last 100,000 years, three large volume Plinian eruptions. One of these, associated with the Morne Diablotins center, forms the Grande Savane pyroclastic flow fan, that extends off shore as a distinctive submarine feature for a distance of at least 14 km. Stratigraphical studies of road cuts and well-exposed sea cliffs indicate the fan is composed of an older unit composed of reworked deposits at the base followed by at least four sequences, based on the presence of paleosols, of block and ash flow deposits. The upper unit of block and ash flows is overlain, with no evidence of an intervening paleosol, by a sequence of ignimbrites and pumiceous surges (representing the Plinian eruption). There is no evidence of an initial Plinian fall deposit, so the lowest bed in the succession is an ignimbrite with a highly irregular base that cuts into the underlying block and ash flow deposits, the upper parts of which are colored red due to thermal effects. This lowest ignimbrite is welded (minimum porosity of 15%) throughout its thickness (maximum thickness of greater than 21 m), although a few outcrops near the margins show a thin (20-30 cm) non-welded but lithified zone beneath the welded zone. The remainder of the sequence is composed of lithified ignimbrite that can be subdivided into three units separated by pumiceous surge layers. The ignimbrite succession is overlain, with no obvious break, by a thin fall deposit containing accretionary lapilli and gas cavities, followed by three pumiceous surge deposits (lower and upper show planar stratification and the middle surge shows massive bedding); towards the north the upper two surge deposits are separated by thin pumiceous lapilli fall and ash fall deposits. This surge sequence extends laterally outside of the main area of ignimbrite deposition. The pumice clasts from the ignimbrites are andesitic in composition and show essentially no variation up stratigraphy. In contrast, the surges are more variable in composition, ranging from andesite to dacite. Modeling of these data will provide information on the dynamics of this major Plinian eruption including the effects of water/magma interaction.
V11C-2050
Potential atmospheric controls on the widespread tephra dispersal from the Youngest Toba Tuff eruption, 74 ka BP
Tephra fall deposits from the youngest Toba eruption (74 ka) in Sumatra extend northwest into the Arabian Sea, northeast into the South China Sea and south into the Indian Ocean, covering an area > 5 million km2. The Lagrangian tephra dispersal model PUFF and monthly averaged modern reanalysis winds have been used to simulate a Toba eruption cloud that could generate the observed bilobate fall deposit. Constraints on the necessary position of the simulated ash cloud were inferred from the position of the observed fall deposit, recovered predominantly from deep-sea cores. Simulations indicate that northward ash dispersal can be achieved primarily by winds near the level of the tropopause (100 hPa). Anticyclonic wind cells are present at altitudes of 14 to 19 km (150-70 hPa) to the north and south of Toba during Northern Hemisphere winter/spring. These cells could be responsible for depositing ash to the north within the Bay of Bengal and to the south in the Indian Ocean. During summer/fall these cells shift west and to higher latitudes resulting in a cessation of the northward transport of tephra near Toba. During all seasons, higher altitude ash (< 50 hPa) is either transported directly to the east or to the west depending on the phase of the Quasi-biennial Oscillation (QBO). The westerly phase of the QBO transports ash eastward where lower altitude winds then advect the ash north over the South China Sea. The easterly phase of the QBO transports ash further westward over India. These PUFF simulations suggest that to generate an ash cloud over the observed deposit, the majority of the Toba ash had to be advected to the north by near tropopause winds during the winter or spring.
V11C-2051
Decadal-Scale, Time-Varying Lags Between Short Period Strombolian Eruption and Very Long Period Seismic Signals at Erebus Volcano, Antarctica
Erebus volcano constitutes an accessible natural laboratory for long-term monitoring and investigation of
Strombolian eruptive behavior. We examined broadband seismograms of Strombolian eruptions from the
Erebus lava lake spanning nine years of recordings of more than 5000 individual events in a new
comprehensive catalog. Although Erebus is noted for its persistent activity, the frequency of Strombolian
eruptions is highly variable. Detailed cross-correlation analysis of the generally similar eruption waveforms
shows systematically time-varying lags (changes of up to several seconds) between the onset of short period
signals resulting from eruptions and accompanying very long period (VLP) signals associated with conduit
disturbance and re-equilibration. Changes in lag time are measures of differences between the two primary
features of the seismic field, where the short-period signal is generated by the explosive decompression of
the gas slug at or near the surface of the lava lake, and the VLP signal is driven primarily by flow processes
due to the rise and expulsion of the gas slug and subsequent post-eruptive refilling of the conduit system.
Time-varying lags are examined for secular trends at two long-running broadband stations and are compared
with variations in eruption size, power spectral density characteristics, eruption frequency, short period and
VLP event similarity, and lava lake morphology. We show the results of this comprehensive temporal study
and overview mechanisms potentially responsible for the observed time varying lags.
http://erebus.nmt.edu
V11C-2052
Simulations of dike intrusion and ground deformation by SPH and DEM
Precise seismic and geodetic observation networks reveal the detail migration beneath volcano. For example, successive dike intrusions in several years were observed in 1990s at East-off of Izu-Peninsula, Japan, revealed by hypocenter distribution (Morita et al., 2006) and magma intrusion and lateral migration was observed at Miyakejima volcano, Japan, in 2000 (Ueda et al., 2005). To detect the locations and volumetric changes of the source of ground deformation, Mogi-model (1958) and Okada-model (1985, 1992) have been widely used and these enables us to identify the source quickly, in some cases, automatically. However these models assume isotropic and homogeneous crust, it is necessary to evaluate the effect of anisotropy and heterogeneity of volcanic body, as well as the fractures due to dike intrusion. We will discuss multi-scale phenomena, i.e. microscopic fracture to macroscopic ground deformation, so we adapt the smoothed particle hydrodynamics method (SPH) and discrete element method (DEM). 1. SPH simulation: SPH method defines the kernel function to characterize the coupling between particles as a smoothing scheme. The algorithm and discretization procedures are easy. In our preliminary analysis (both 2D and 3D), we assume a fluid magma chamber / dike in an elastic medium, and triggers magma particle to move, then we obtain some fractured region around intruded magma and ground deformations. 2. DEM simulation: DEM defines the spring and dash-pot coefficient between particles and introduces failure criterion. Our preliminary analysis (2D) simulates the rectangular magma (dike) expansion and the result shows some nearby fractures and ground deformation of inflation. (The SPH and DEM programs are originally coded by Takashi Matsushima, Tsukuba Univ.)
V11C-2053
Record of Miocene sea-floor dome volcanism, Cabo de Gata, Spain
The Cabo de Gata volcanic field comprises a 200 km2 succession of andesitic and dacitic dome complexes, together with possibly caldera-related facies and minor . Detailed investigation of exposures in the Vela Blanca to Cala Genoveses area in the southwest of the field, together with new 40Ar/39Ar groundmass ages, reveal a complex succession of andesitic to rhyolitic facies erupted as domes on a relatively shallow sea floor to emergent setting within ~1 m.y. The succession youngs from southwest to northeast. At Vela Blanca, an andesitic complex is represented by strongly columnar jointed lava; other evidence of marine emplacement is weakest in this area, suggesting an at least partially subaerial setting. The andesites are in uncertain stratigraphic relation with rhyolitic pumice breccia: more recent debris avalanches have disrupted original stratigraphy. Andesite is overlain by a dacitic succession that is the oldest well-dated unit in the area (13.36±0.13 Ma). These rocks represent a growing dacitic dome that disrupted ambient, fine-grained shallow-marine sedimentation in the area. The basal flow from the dome has a pseudo-eutaxitic texture that may represent an upper carapace; the non-vesicular nature of "fiamme" suggests a complex attenuation of flow bands. The dacite is overlain by a highly variable, intermixed hyaloclastite and flow unit called the andesite of el Barronal. Vertically flow-banded and columnar-jointed andesite grades into hyaloclastite that comprises brecciated columnar-jointed fragments. Elsewhere, flow banding in lava is sub-horizontal, grading laterally, in the seaward direction, into hyaloclastite and shoreward into dense dome material. We interpret this succession as representing a dome that spread through lava flows on the seafloor, but that may have had a subaerial component as well. Farther east, andesite crops out as radially columnar-jointed lava that represents mega-pillows or lava tubes. The pillowed horizon grades into a zone of increasing hyaloclastic contribution – hyaloclastite intrudes the lava flow succession as irregular patches that increase in area at the expense of lava. These zones are ~50% vesicles compared to very poorly vesiculated "host" rock. The farthest east exposures comprise vertically intercalated lava and hyaloclastite. These deposits may represent the result of water streaming through cracks in the lava flow that paralleled flow bands. Streaming water caused hyaloclastic fragmentation and simultaneously resedimented clasts. Finally, at Cala Genoveses, a small rhyolitic dome complex intrudes andesite lava flow along strongly vitrophyric margins, and, in the extrusive facies, is represented by a pumice flow and intricate relations with overlying sediment, as pumice blocks spalled off the flow. These deposits are overlain by andesite dated at 12.19±0.08 Ma.
V11C-2054
GPR Facies Analysis of Block-and-ash Flows at Merapi Volcano, Central Java, Indonesia
Merapi, a 2911-m-high basaltic andesite volcanic complex in Central Java is one of the most frequently erupting volcanoes in Indonesia and best known for its nearly persistent volcanic activity characterized by the extrusion of viscous lava domes and collapse of these domes to produce block-and-ash flows (BAFs). During the most recent eruptive episode in 2006, BAFs affected the densely populated areas on the volcano's southern flank and were the first major flows in this area for over a century. More importantly, the flows were not confined to the existing river valleys but spilled over the valley sides to create overbank flows that resulted in fatalities in the village of Kaliadem about 5 km away from Merapi. In August 2008, a four-week field programme of ground penetrating radar (GPR) data collection was carried out on the 2006 deposits aiming to improve traditional models of BAF transport and deposition on the basis of observational and interpretational evidence, which are often complicated by a combination of poor exposure, rapid lateral facies variations and unknown palaeo-topography. The GPR survey focused on the large-scale (deposit) and small-scale (intra-deposit) structures of (a) the valley-filling BAF deposits in the proximal, medial and distal regions of the Gendol river valley and (b) the overbank deposits in the Kaliadem area. The GPR sections of the valley-filling deposits are characterized by a high density of diffraction hyperbolae from large, decimetre- to metre-sized blocks within the deposits, either randomly distributed or concentrated in particular horizons within the deposits, which allow larger scale variations in grain size and deposit thickness as well as flow unit boundaries to be traced over areas where exposure is limited or absent. Similar patterns are observed in the GPR sections of the proximal, wedge-shaped overbank deposits, though the deposits lack the abundance of large blocks typical of the valley-filling deposits. Distinct, coherent and traceable reflections are evident in the GPR sections of the more distal overbank deposits that relate to defined, sharp changes in matrix grain size, distinct blocky horizons and internal architectures that can be related to observable units in exposed deposit sections. These variations, indicative of subtle, yet important, changes in the nature and dynamics of the overbank flows across interfluves, occur over relatively short flow distances (100 metres or more) that are clearly mapable in the GPR sections at a metre scale. Given the unpredictable behaviour and potential for loss of life of these overbank flows, not only at this volcano but also at similar locations around the world, it is important that these particular deposits are studied in detail. This study shows that GPR is a useful, non-invasive tool for developing new, improved interpretations on the transport and emplacement dynamics associated with BAF deposits.
V11C-2055
Mt. Teide and Mt. Etna Volcanic Rocks Characterization (VIS-IR) at Different Scales and its Organization in a Spectral Library
In order to calibrate and validate remote sensing data, information derived from laboratory and in situ measurements are needed. Periodically campaigns are organized to perform measurements and samplings to retrieve spectral and geochemical information. Information are very important to improve the theoretical model and to realize a classification. During 16-22 September 2007 a field campaign was performed on Mt. Teide planned in the frame of the European Community project (Preview). Surface characteristics of rocks in the caldera area were measured to validate satellite images acquired during the same campaign time window by ASTER sensor. Besides a Micro-FTIR (Fourier Thermal Infrared spectrometer) was used to measure the surface emissivity of the surface material during the early hours of the day and of the night. Also a Field Spec Pro instrument was used to acquire reflectance spectra in the VIS-SWIR spectral range. Subsequently, laboratory spectral measurements were performed on many representative samples. In the present work, we present the spectral library implemented to show and access the results.
V11C-2056
GIS-based statistical mapping technique for block-and-ash pyroclastic flow and surge hazards
Assessments of pyroclastic flow (PF) hazards are commonly based on mapping of PF and surge deposits and estimations of inundation limits, and/or computer models of varying degrees of sophistication. In volcanic crises a PF hazard map may be sorely needed, but limited time, exposures, or safety aspects may preclude fieldwork, and insufficient time or baseline data may be available for reliable dynamic simulations. We have developed a statistically constrained simulation model for block-and-ash PFs to estimate potential areas of inundation by adapting methodology from Iverson et al. (1998) for lahars. The predictive equations for block-and-ash PFs are calibrated with data from many volcanoes and given by A = (0.05-0.1)V2/3, B = (35-40)V2/3 , where A is cross-sectional area of inundation, B is planimetric area and V is deposit volume. The proportionality coefficients were obtained from regression analyses and comparison of simulations to mapped deposits. The method embeds the predictive equations in a GIS program coupled with DEM topography, using the LAHARZ program of Schilling (1998). Although the method is objective and reproducible, any PF hazard zone so computed should be considered as an approximate guide only, due to uncertainties on coefficients applicable to individual PFs, DEM details, and release volumes. Gradational nested hazard maps produced by these simulations reflect in a sense these uncertainties. The model does not explicitly consider dynamic behavior, which can be important. Surge impacts must be extended beyond PF hazard zones and we have explored several approaches to do this. The method has been used to supply PF hazard maps in two crises: Merapi 2006; and Montserrat 2006- 2007. We have also compared our hazard maps to actual recent PF deposits and to maps generated by several other model techniques.
V11C-2057
Subaqueous non-vesicular to poorly-vesicular shards: hydroclastic fragmentation on seamounts and summit calderas
Recognizing pyroclastic deposits that originate directly from magmatic and phreatomagmatic explosions in a subaqueous setting is based upon sedimentary structures, such as massive, stratified, and graded beds as well as (pyro)clast size. Ideally such deposits form ordered fining-and thinning-upward sequences. Pumice, scoria, glass shards, euhedral and broken crystals, and lithic fragments are constituents that support an explosive heritage. Recent deep-sea ROV and submersible dives have retrieved non-vesicular to vesicle- poor, mm-scale, mafic shards in 5-15 cm-thick massive and/or graded (stratified) deposits, for which a subaqueous explosive origin has been inferred. These sheet hyaloclastites with variable shard shapes were first documented on Seamount 6 as deep-sea Limu O Pele at water depths > 1000 m. We identified in Seamount 6 samples equant to blocky shards with angular to subrounded terminations, but also subordinate hair-like and contorted glassy filaments, warped shards and irregular shards. Shards display internal laminations (flow-banding?) and have local perlitic fractures. Bubble wall shards derived from scoria burst were rare. In combination with all the above and a poor shard vesicularity (< 2%), a magmatic explosive origin seems improbable. Such small-volume deposits have been reported from seamounts and summit calderas associated with subaqueous drainage tubes and ponded magma in depths > 1000 m. We envision that hydrostatic pressure commensurate with water depth played a significant role. The deposits can be readily explained by a hydroclastic process whereby fragmentation occurred at the milli-second (Limu) to second scale (hyaloclastite). Hence, hyperquenched glass shards or thread-like glass filaments need not require magmatic explosivity. Constant surface interaction between aphyric, low-viscosity, high temperature, magma-lava at depth with seawater causes fragmentation (granulation) that can generate such delicate shards. The transfer of heat to the ambient medium, seawater, favours turbulent convection causing strong water movement that strips glassy rinds and lofts the fine-grained shards and Limu O Pele into the water column. Once entrained, shards are deposited after water turbulence abates. Congestion of the water column causes deposition from low-density turbidity currents and subaqueous fallout. In this manner delicate textures would remain intact even if removed from the site of hydroclastic fragmentation.
V11C-2058
Magmatic activity inferred from tree activity at Mt. Etna
Volcanic activity can be detected using optical imagery focused on signal tree-growth anomalies (i.e., NDVI) (Houlié et al., 2006). Theses anomalous tree-growth signals maybe caused by changes in environmental conditions, but their actual causes still remain challenging to identify. We hypothesize that the possible factors involved can only be: 1) carbon dioxide degassing from the soil, 2) increase in soil temperature, 3) increase in water availability. As the correlation between tree growth seems established during the pre- eruptive activity for selected areas, we still don;t know if wether the trees have the capability to store to information and preserve it for a long time or not.We sampled cores from trees growing along the North Eastern rift zone flank of Etna, near Piano Provenzana (Sicily, Italy). Trees close to the fracture show larger activity while all trees might respond to a single stimulating source . We present here the results of this study and discuss the results under the light of the eruptive activity catalog during the last century.
V11C-2059
Remarkably Gaussian Tephra Fallout from Basaltic Eruptions
Tephra fallout models used to forecast volcanic hazards rely on the advection-diffusion equation to forecast hazards. If the advection-diffusion equation applies, then the thickness of tephra blanket deposits should show Gaussian crosswind profiles and exponential decay with distance from the vent. Complications may arise due to factors such as particle size distributions, particle density, and atmospheric effects not incorporated in the advection-diffusion model. Continuous profiles derived from GPR surveys collected on the tephra blanket of Cerro Negro Volcano, Nicaragua allow us to test the advection-diffusion model. Steady trade winds coupled with eruptions that tend to be brief and relatively low energy create relatively simple deposits. Data was collected for cross wind profiles at varying distances from the vent. Horizons identified in these profiles exhibit Gaussian distributions with a high degree of statistical confidence. Additionally, the shape of one continuous profile leading from the crater rim out onto the tephra blanket is examined.
V11C-2060
The ~14 ka Plinian-type eruption at Tacana Volcanic Complex, Mexico- Guatemala
Tacana Volcanic Complex (TVC), located in the northwestern end of the Central America Volcanic Arc, in the Mexico-Guatemala border, has erupted in Plinian fashion at least three times during the past 30,000 years. The youngest of these events occurred just ~14,000 yr. B.P., which deposited a pumice-rich fallout, informally named the Tacana Pumice. Although it is a relatively recent eruption, the most proximal outcrop of the deposit appears up to 9 km from the vent because of intense erosion and emplacement of younger deposits. The fallout is massive and composed of ~85 vol. % of andesitic pumice with ~18 vol. % crystals consisting of plagioclase (An60 to An42), clinopyroxene (augite), orthopyroxene (hypersthene), ilmenite, and titanomagnetite, all immersed in a vesicular glass. Isopach and isopleth maps indicate that the main dispersal axis of the fallout is to the north, with the 60-cm isopach covering an area of 266 km2. The total volume of the fallout is 8 km3 (~3 km3 of magma), and was dispersed by a 28-km-high eruption column. Ilmenite-titanomagnetite compositions preserve a homogeneous temperature of ~876°C at a log oxygen fugacity of -11.08. FTIR analyses of melt inclusions of plagioclase crystals from Tacana Pumice samples yielded preliminary values of only 0.2 wt.% H2O and no CO2 was found. Regardless of the scarcity of exposed outcrops, either close or even further from the vent, this deposit stands as the largest Plinian eruption of andesitic magma that has occurred at the Tacana Volcanic Complex. Because the Tacana summit has had phreatic eruptions in 1950 and 1986, it is regarded as an active volcano and an eruption similar to the Tacana Pumice must be considered as a very real hazard in the future.
V11C-2061
Magma periodic bursts into shallow volcanic reservoir. Implication for the quantification of intruded magma volumes.
Broadband seismic data enable us to test whether Ultra Long Period (ULP) signals can be used to determine the magma chamber pressure state and to forecast volcanic eruptions. In a systematic investigation of seismic signals recorded at the GEOSCOPE station RER in la Réunion Island near the Piton de la Fournaise, we show that ULP signals from the past two decades are typically associated with eruptions of Piton de la Fournaise. Some of these unusual signals were previously detected and interpreted in terms of tilt. We tentatively propose to interpret these signals as signatures of the activity of the upper magma feeding system of the Piton de la Fournaise volcano chamber. They are detected using STS-1 seismometers sensitive in the lower part of the bandwidth 0.001 to 0.01 Hz. Assuming geometrical parameters of the source and magma injection rates in the upper reservoir, the collection of all seismic events allowed us to build a time-series of absolute pressure in the upper magma chamber. The absolute pressure framework provides new constraints to parametrize a wide range of geophysical and geochemical models together. This alternative model should provide new insights in the eruptive processes that characterize Piton de la Fournaise volcano.
V11C-2062
Does the Inversion of Surface Deformation Data Allow Retrieving the Characteristics of Pressurized Ellipsoidal Cavities?
When modelling the deformation source as a pressurized ellipsoid, inversion of measured surface displacements allows to estimate the product between the source volume (V) and overpressure (P). The volume change of the source is of great importance, e. g. for estimating the intrusion density from gravity data, but its link to VP is not trivial. We present exact expressions for the volume change of a pressurized ellipsoidal cavity in an infinite homogeneous elastic medium. The expressions can be used as approximate solutions also for a homogeneous half-space. We show that previously-published widely-used expressions are correct for spheres, but underestimate the ratio of the volume change to the product of pressure and volume in any other case. We discuss the capability to infer the shape of a single ellipsoidal cavity from far-field deformation measurements and our results indicate that source axis ratios may often be hard to estimate, whereas it may be easier to infer the volume change of the source. We also consider the case of a source region consisting of interconnected pressurized ellipsoidal cavities, and discuss the capability to infer both the total volume change and the shape and orientation of the source region and cavities. We find that (i) if all the cavities share the same shape and orientation, but have different volumes, the ensemble of cavities is seen in the far field as a single ellipsoidal cavity sharing the same shape and orientation, and it is possible to compute the total volume change from the inversion of surface displacements, but the shape of the apparent single ellipsoidal cavity does not necessarily reflect the shape of the source region; (ii) if all the cavities share the same shape but have different orientations, it is possible to compute the total volume change only if the cavity shape is assumed a priori; (iii) if the cavities are different both in shape and orientation, it is possible to compute the total volume change only if the product of pressure and total volume is assumed a priori.
V11C-2063
Geochemical evidence of the renewal of volcanic activity: the 2007 Stromboli (Italy) eruption.
Geochemical surveillance has been carried out at Stromboli since 1999 using discrete and continuous monitoring of thermal waters and soil degassing. On 27 February 2007 a new eruption began which lasted until 2 April; it was characterized by effusive activity on the Sciara del Fuoco and also by a paroxistic event (15 March). This crisis represented an opportunity to refine the model developed previously and to improve our understanding of the relationship between the magmatic dynamics of the volcano and geochemical variations. The main aim of this research was to evaluate the level of criticality of the volcanic activity. The SO2 fluxes of the degassing plume and the CO2 fluxes emitted from the soil at Pizzo Sopra la Fossa are herein presented. Furthermore, we propose a refined geochemical model of fluids circulation, including plume and summit fumarolic soil degassing. Noteworthy geochemical signals of volcanic unrest were also clearly identified (before, during and after the effusive activity) in the degassing plume as well as in the degassing from the soil at the summit.
V11C-2064
New preliminary geological map of Pacaya Volcano, Guatemala
Orthophoto coverage acquired in 2006 was used to construct a new geologic map for Pacaya, a basaltic cone located south of Guatemala City, on the south rim of the Amatitlán Caldera. Pacaya has been in eruption since 1961 and has erupted about 0.21 km3 of magma since then (K Durst et al, BV in review). The shape of Pacaya has markedly changed as the MacKenney cone, formed during the eruption has grown to overlap the earlier summit cone. The new map shows the pattern of surficial lavas and a GIS archive (including airphotos, satellite imagery and field sketchmaps) constructed at MTU records the pattern of growth of Pacaya by mapping the covered lava units as well as those now at the surface. We are seeking ideas of how to design the format of the subsurface information of the map. At this time we seek comments from the geophysical community about how to design the map for optimum use.
V11C-2065
Flow Dynamics of the Pitchstone Plateau Obsidian Flow in Yellowstone National Park, WY
Extremely large volume rhyolite flows are known to exist, but have never been witnessed, and so little is known about how such massive, high viscosity lava flows erupt. To better elucidate their behavior, we focus on the 70 km3 Pitchstone Plateau obsidian flow, erupted 72 ka from the Yellowstone caldera (Christiansen, 2007). We have examined samples collected relatively evenly from near vent to the flow front, characterizing the number densities and sizes of groundmass microlites. Phenocryst content and a phenocryst assemblage of >95% quartz and sanidine do not change within the flow, indicating that the magma is homogenous. We find that microlite number densities remain relatively constant in the flow, with the exception of the flow front, which has a density several orders of magnitude higher. Total densities range from 9.65x108 cm-3 to 1.43x1011 cm-3. Average size of all crystal types range from 1.6 um3 to 47 um3. To constrain viscosity and kinetics of groundmass crystallization of the magma, we investigated the pre-eruptive storage conditions of the magma. The temperature was about 700-740 degrees Celsius based on [Ti] in quartz phenocryst rims and fayalite-augite-magnetite equilibrium. Volatiles dissolved in glass inclusions range up to 3.5 wt.% water and, when detected, up to 200 ppm CO2, which implies fluid saturated pressures of up to 100 MPa. Hydrothermal phase equilibrium experiments confirm that quartz and sanidine co-precipitate at those temperatures only when water pressures are below 125 MPa. Groundmass water contents of 0.13 wt.% show that the melt degassed completely, which led to an increase in viscosity from <107 Pa s in the chamber to 1011.6-1012.2 Pa s during extrusion. We will combine our estimates of viscosity with size and number data to investigate the kinetics of groundmass crystallization and estimate flow dynamics of the lava. 1Christiansen, 2007 USGS Open-file Report
V11C-2066
Estimating Thermal Energy Emission and Eruption Rates at Guatemalan Volcanoes Using Thermal Data From a FLIR Camera, ASTER and MODIS Data Sources
Analysis of thermal images taken with a Forward-Looking Infrared camera has allowed us to establish a baseline data set for three open vent volcanoes in Guatemala that vary in composition from dacite (Santiaguito) to basalt (Fuego and Pacaya). This allows for the evaluation of eruption rates using remote sensing and provides satellite thermal remote sensing validations. The field data were collected during two field trips in 2008. The Santiaguito data have been atmospherically corrected and analyzed to allow estimates of the emitted thermal energy and also the equivalent eruption rate (Rose, et al 2008). Using similar techniques, data from Pacaya volcano were analyzed to obtain estimated emission of thermal energy along with observations of vent morphology. The long term goal is to employ a variety of thermal remote sensing tools, including data comparison from ASTER and MODIS sources, in order to closely monitor eruption rates at open vent volcanoes, such as Santiaguito, Fuego and Pacaya. Ultimately, eruption rate estimates at these volcanoes may lead to improved hazard forecasts.
V11C-2067
Volcanic flow deposits on the flanks of Long Island, Papua New Guinea: lavas or pyroclastics?
Volcanic eruptions on Long Island, Papua New Guinea, occurred 16,000, 4,000, and 300 years ago (Newhall and Dzurisin, 1988). The most recent eruption coincides with widespread local legends that referred to a "The Time of Darkness" (Blong, 1982), and Long Island was the likely source of the widespread Tibito tephra of that age, with an air-fall volume in excess of 11 km3 (Blong, 1982). Based on sidescan and multibeam bathymetric imagery, we mapped a large volcanic flow deposit with numerous flow fronts off the coast of Long Island, with dimensions 70 km long and up to 24 km wide. We estimate the volume of the flow to be 14 km3. The flow may be composed of pumiceous or pyroclastic material derived from one of these major eruptions, or may be a lava flow. The geometry of the flow indicates that it is not associated with a turbidity- current, unlike the well mapped submarine pyroclastic flow on the flanks of Montserrat in the Lesser Antilles Arc, and therefore may have been a pumiceous or a lava flow. The flow has a relatively high amplitude back scatter in side-scan imagery and appears to cover much of the sedimentary deposits, including an adjacent field of giant sediment waves. Based on estimates of sedimentation rates in this area, we conclude that the flow is not pre-Holocene in age, and may be associated with the last caldera-forming eruption of ~300 year ago, but could be even younger.
V11C-2068
Caldera types and collapse styles
Three main types of collapse calderas can be defined, 1) summit caldera, 2) classic caldera, and 3) graben caldera. Summit calderas are those formed at the top of large volcanoes and are related to relatively small- volume pyroclastic products that include plinian fallouts and ignimbrites, such as Crater Lake, Las Cañadas, and Somma-Vesuvio. Classic calderas are semi-circular to irregular-shaped large structures, several km in diameter that are related to relatively large-volume pyroclastic products including pumice fallouts and widespread ignimbrites, such as Long-Valley, Campi Flegrei, and Los Humeros. Graben calderas are explosive volcano-tectonic collapse structures from which large-volume, ignimbrite-forming eruptions occurred through several vents along the graben walls and the intra-graben block faults causing the collapse of the graben or of a sector of the graben. The main products of graben calderas are surge-deposits and large-volume widespread ignimbrite sheets. Pumice fallouts are practically absent. Examples include the Sierra Madre Occidental in Mexico, La Pacana (Andes), Catalan Pyrenees, and perhaps Scafell (United Kingdom). Any of the three caldera types mentioned above could have collapsed at least in three different ways, 1) piston, when the collapse occurs as a single crustal block; 2) trap-door, when collapse occurs unevenly along one side while the opposite side remains with no collapse; 3) piece-meal, when collapse occurs as broken pieces of the crust on top of the magma chamber.
V11C-2069
Crystallinities of the Lovejoy Basalt: A Comparison of Proximal and Distal Flow Units
The Lovejoy Basalt is a series of basaltic lavas that erupted from vents located at Thompson Peak near Susanville, California. Lavas of the Lovejoy Basalt flowed nearly 250 km southwest to Oroville, Chico, across the Sacramento Valley to Orland, and to Putnam Peak near Vacaville. Lavas have 4% MgO and calcium rich plagioclase crystals (up to 16% CaO, An 82). Recently published 40Ar/39Ar ages cluster at approximately 15.4 Ma (1). To understand how the Lovejoy Basalt flows were able to stay hot enough to flow long distances across northern California, we investigate the groundmass crystallinities to extract physical parameters of lava flow emplacement. SEM- Back Scatter Electron images are used to map and quantify groundmass crystallinities of Lovejoy Basalt flows at four locations, Red Clover Creek, Table Mountain, near the area of Little Grass Valley Reservoir, and Putnam Peak. Red Clover Creek is located approximately 46 km from the source and has a sequence of 7 flows. Quantification of crystallinities helps constrain the localized cooling regime at flow boundaries. Observations of quenched crystals at the base of one flow unit suggest that the lava was emplaced on a cold surface, which signifies a gap in time between the emplacement of flow units. Samples from Table Mountain, located approximately 119 km from the source, were collected from a single, thick flow unit approximately 30 m thick. The complex patterns of crystallinities from the base to the top of the flow suggest that pulses of new lava were injected into the inflating flow as groundmass crystallization occurred. Crystal sizes in the Table Mountain samples are five times larger than groundmass crystals in the more proximal Red Clover Creek flow units. This reflects that the flow emplacement temperature of the Table Mountain flow unit was low enough for plagioclase crystallization to continue but high enough for the flow to advance 70 km greater than that traveled by the Red Clover Creek sequence of flows. Additional sampling sites compare groundmass crystallinities of flows across northern California. (1) Garrison, et al., 2008
V11C-2070
Compositional Zonations of the Arico Ignimbrite, Tenerife, Spain: Implications for Depositional Processes
We present preliminary results of detailed sampling and EMP analyses of glassy matrix within populations of essential pyroclasts for the classic phonolitic Arico Ignimbrite (Schmincke and Swanson 1967). Purpose of this work is to define compositional zonations within the deposit in the Bandas del Sur region of southern Tenerife, and to correlate those zonation patterns between spatially separated areas. We test the possible use of compositional horizons as depochrons for the deposit. Data indicate that the ignimbrite is weakly normally zoned. At the medial Magua sampling location the lower 4 m of the deposit has MgO and CaO contents of 0.35 and 0.55% respectively (type-1 phonolite). The upper 7 m of the deposit (type-2 phonolite) appears to be continuously zoned from 0.40 and 0.55% at the lower end, to 0.6 and 0.95% (MgO and CaO) at the top. Distal (coastal) facies were observed to contain a heterogeneous mixture of types 1&2 phonolite + minor rhyolite. One interpretation of these data is that deposition from the original density current began in more distal areas prior to more proximal areas.
V11C-2071
A Bathymetric Survey of Lake Toba, Indonesia: Further Results
Lake Toba, the largest caldera lake in the world, formed following a supervolcano eruption 74,000 years ago in northern Sumatra. Explosive eruption of ~2800 km3 of silicic magma from a batholith sized magma body produced a 100 x 30 x 2 km deep caldera. Approximately 2/3 of the caldera floor is now covered by Lake Toba, obscuring volcanic landforms and geologic features critical to understanding the history of the caldera. In September 2005, we initiated a bathymetric survey of Lake Toba using a GPS linked sonar device mounted to a small boat. Traveling at a speed of 3-10 km/h, and using a bottom depth sampling rate of 1 sounding/sec, sonar data was collected along almost 200 km of transect lines. This survey produced lake-bottom profiles with detailed bathymetric expression, indicating that it would be possible to generate a high resolution bathymetric map of Lake Toba with additional surveying. The second phase of our data collection was done in May 2008. Collectively, the survey now consists of about 90 separate transect lines over nearly 600 km of lake bottom. Results from both surveys allow several first order observations concerning caldera formation, resurgent doming, lava dome emplacement, and large scale erosional events as follows. Samosir Island is a partly submerged resurgent dome whose overall size (~60 x 20 km) and shape can now be accurately determined. The dome retains its asymmetrical shape below the water line with gentle westward slopes that merge into the caldera wall. Its steep east face rapidly descends to water depths of >400 m near the eastern caldera margin. The Uluan block appears to be a smaller, symmetrical equivalent of the Samosir dome. However, its extremely steep western face is more similar to the abrupt caldera collapse scar than the east face of Samosir. The deepest parts of the lake are found at the base of the ring fracture fault, approximately 0.5 km from the shoreline, with depths of 500 m recorded along the northeastern ring fracture. Most of the 400-1200 m steep caldera walls extend uninterrupted another 400-500 m below lake level. Collapse mega-blocks are common features near the ring fracture. Beyond the base of the steep ring fracture slopes and the Samosir dome, vast featureless plains are found at depths of 450-490 m and dominate the large open regions of the northern and southern portions of the lake. The shallowest portions of the lake are found in Porsea Bay, a fingerlike projection that leads to the present outlet of the lake. Sonar profiles suggest the bay may drown a paleo-river valley that cut through the Uluan block. Other significant sub-lacustrine features revealed by our survey include a post- caldera collapse debris avalanche deposit, a ring fracture lava dome, and additional faults in the Samosir dome. Based upon thick (~100 m) lacustrine sediments exposed on Samosir Island and apparent echoes on some bottom profiles, considerable thicknesses of lacustrine sediments are expected to mantle the welded ignimbrites on the caldera floor.
V11C-2072
Volcanic Threat in Central America: Assessment and Comparison of Volcanic Hazards and Associated Vulnerability in Guatemala, El Salvador, Nicaragua and Costa Rica.
Within the Central American volcanic arc, that runs through Guatemala, El Salvador, Nicaragua and Costa Rica, there are 72 volcanoes that have exhibited historical (>26) or Holocene (>37) activity. Some of these volcanoes are immediately adjacent to large cities with several thousand inhabitants. A striking example is San Salvador volcano, in El Salvador, with more than 500,000 people living within 10 km from its summit. Other presently active volcanoes, such as Fuego and Santiaguito in Guatemala, continually threaten local communities established nearby, where appropriate monitoring strategies and mitigation plans have only recently started to develop. This contribution presents an assessment of volcanic threat in Central America based on the historic volcanic activity registered at each volcano, recognition of the hazards most likely to occur, and exposure of inhabitants and infrastructure near volcanic vents, next to potential lahar paths and downwind. Parameters used to quantify the compound hazard and vulnerability, include those presented by Ewert et al. (2005) as part of the Framework for a National Volcano Early Warning System in the US, NVEWS (USGS open-file report 2005-1164). New parameters are devised to improve threat assessment by including evaluation of the enormous variability in depth and extent of information, the differing social factors that influence vulnerability (Wisner et al, 2003, At Risk) and the sophistication of monitoring efforts. The volcano dataset used was compiled from the Smithsonian's Global Volcanism Program's website, as well as other relevant publications. Information about population and infrastructure was obtained from the LandScan population database and census data compiled by local governments.
V11C-2073
Commonalities and Contrasts in Location, Morphology and Emplacement of Large-volume Evolved Lava Flows
Observations of active dacite domes and evolved (SiO2 wt.% >65) plinian-style eruptions are considered to reveal typical behaviors of Si-rich volcanic systems. However, despite lack of mention in modern volcanology textbooks, large-volume (>4 km3) evolved lava flows exist globally. These large- volume evolved lava flows have many characteristics in common regardless of location and precise tectonic setting: they are associated with other large-volume deposits (both lava flow units and ignimbrites); are commonly found with large silicic systems; regionally, they are associated with bimodal volcanism and eruption of these large-volume evolved flows does not generate a caldera. Large-volume evolved lava flows have low aspect ratios, tend to be uniform in thickness from the vent to the distal margins and abruptly decrease in thickness at the flow front where they may form enormous pahoehoe-like lobes. A lack of pyroclastic textures such as bubble wall shards, pumice fragments, broken phenocrysts and lithics is taken as evidence for their lava flow origin rather than an ignimbrite origin despite their high SiO2 contents. Presence of a pervasive basal breccia and lobate distal margins also suggest a lava flow emplacement origin, that only the most intensely rheomorphic ignimbrite could potentially mimic. Our own studies and those from the literature suggest high eruption temperatures and peralkaline chemistries may be responsible for producing unusually low viscosities to account for large lateral extents; emplacement via fissure vents and insulations of the flow may also be key in attaining great volumes.
V11C-2074
Comparison of High Temporal Resolution SO2 Emission Rates and Geophysical Data at Central American Volcanoes
Recently developed ultraviolet cameras present a significant improvement in the field of volcanic SO2 measurements. Older instruments utilized for measurement of SO2 emissions in volcanic plumes (e.g., COSPEC, mini-DOAS, FLYSPEC) are often limited in their accuracy by plume and volcano geometry, uncertainties in plume velocity, plume variability over the course of a single measurement, and low temporal resolution (generally at least 1 minute per measurement, more often <10 measurements per day). In contrast, UV cameras, with their data's synoptic nature, allow for investigation of the high degree of variability in plumes and offer the opportunity to measure SO2 flow rates (as a proxy for volcanic emission rates) at any number of locations within the image, including at or very near the vent. Plume velocities can be derived from UV camera data time series, eliminating the often erroneous estimations of plume velocity based on wind speed. Temporal resolution of the SO2 dataset is also much improved with the camera able to yield SO2 emission rates at sampling frequencies approaching 1 Hz. The superior temporal resolution facilitates comparison to other volcanic datasets, such as seismicity, microgravity, and deformation. UV camera time series from Fuego volcano, Guatemala, and Masaya volcano, Nicaragua indicate that SO2 flow rates at these volcanoes may double over the course of <10 minutes, and subsequently return to low levels on the same time scale. Though some of the variations in SO2 flow rate are apparently due to wind eddies or other small scale atmospheric variations near the vent, other variations are likely to be real variations in emission rate that are directly related to volcanic processes. These fluctuations are evaluated in relation to simultaneous acoustic and seismic datasets in an attempt to understand the nature of such fluctuations. We present the latest improvements to current algorithms for the processing of UV camera data, including advances in automatic derivations of plume velocity, as well as preliminary integration of camera-derived SO2 emission rates with other geophysical datasets from Fuego, Masaya, and other Central American volcanoes. The high temporal resolution of the UV camera's dataset offers a new opportunity for evaluation, alongside concurrent seismic and acoustic datasets, of variations in SO2 emission rate on a short time scale.
V11C-2075
Toward speleothem tephrochronology: Experimental simulation of volcanic ash weathering in a karst setting
Speleothem studies are beginning to identify various geochemical signatures of tephra layers related to known explosive volcanic events. Frappier previously analyzed the trace elemental composition of a recent calcite stalagmite from Belize. A large increase in diverse trace elemental impurities was found to correspond to the regional ashfall from the April 1982 V.E.I. 5 eruption of El Chichon in Chiapas, Mexico. To establish a reliable speleothem tephrochronological absolute dating tool, researchers must develop a mechanistic understanding of the processes that govern the deposition, preservation, and geochemical composition of speleothem tephra marker horizons. In tracking the processes between ashfall and speleothem deposition, it is particularly vital to characterize the initial tephra weathering products and their modification in karst soils and carbonate seepage waters. Here we present results from an experimental simulation of early stage weathering of trachyandesitic tephra. Volcanic ash from the 1982 El Chichon eruption was obtained from the Smithsonian Institution for leaching experiments. Using a factorial design, aliquots of a weak nitric acid solution were exposed to fine (less than 64 micron) and bulk ash samples, both alone and in the presence of karst soil from Belize. Mixtures were filtered to separate the solid fraction from the leachate, and ICP-MS analysis was performed on the suite of leachate samples at Boston University. We discuss the experimental results with respect to the geochemical composition of the tephra and observed stalagmite cryptotephra layer. We also address implications for development of an absolute dating tool for Quaternary geochronolgy and paleoclimatology of speleothems.