Decoupling of small-volume pyroclastic flows and related hazards at Merapi volcano, Indonesia

The November 1994 eruption at Merapi volcano provided good evidence of decoupling of dome-collapse pyroclastic flows and of large-scale detachment of an ash-cloud surge (ACS) component from the basal block-and-ash flow (BAF). Timing and stratigraphic relationships of the largest 1994 ACS indicate that this escaped from the valleys, travelled well ahead of the BAF, arrived at the termination tens of seconds before it and deposited a discrete ACS deposit beneath the BAF unit. This suggests that the ACS detachment mostly occurred relatively high on the volcano slope, likely at the foot of the proximal cone. Later pyroclastic flow eruptions in January 1997 and July 1998 also showed evidence of ACS detachment, although to a lesser extent, suggesting that ACSs could be a frequent hazard at Merapi volcano. Based on an extensive review of the available literature and on field investigations of historical deposits, we show here that flow decoupling and ACS detachment in the way inferred from the 1994 eruption is a common process at Merapi. The ACS-related destructions outside valleys were frequently reported in the recent past activity of the volcano, i.e. in at least 16 pyroclastic flow eruptions since 1927. Destruction occurred systematically in eruptions where maximum runout of the BAFs was 6.5 km or more, and occurred rarely for BAF runouts of 4.5 km or less. The ACS deposits have been recognized beneath some valley-filling BAF units we attribute to some recent destructive eruptions, i.e. the 1930, 1954, 1961 and 1969 eruptions. Topographic conditions at Merapi volcano favouring ACS detachment include: (a) the high slope (30°) of the proximal cone, leading to high proximal velocities of the pyroclastic flows and thus to the transfer of large amounts of particles into the ash cloud; (b) the strong break in slope at the foot of the proximal cone, where the velocity of the basal BAF is strongly reduced and a major ACS component is thought to form and detach by shearing over the BAF; and (c) the small depth of most valleys in the first kilometres beyond the foot of the cone, which allows minor ACS components to escape from the valleys during travel of the BAF; however, flow decoupling and ACS detachment occur for only some of the numerous pyroclastic flows that follow the same path in a given eruption. This indicates that topography alone cannot lead to flow decoupling. We suggest two factors that control flow decoupling and its extent. The main one is flow volume (and thus flux, as both are correlated in almost instantaneous, dome-collapse events), as suggested by the observed relationship between flow decoupling and the travel distance of the pyroclastic flows. The second factor is the amount of available ash in the flow at its early stage, which influences the mass and thus momentum of the ash cloud. The amount of ash in the pyroclastic flows of Merapi may depend on several factors, among which are (a) the physical and thermal state of the part of the active dome that collapses, and (b) the proportion of older, cold rocks incorporated in the flow, either by undermining of surrounding summit rocks by the current pyroclastic flow activity or by erosion on the upper slopes

Stratigraphy and textural characteristics of the 1982–83 tephra of Galunggung volcano (Indonesia): implications for volcanic hazards

91.40.RsThe Galunggung volcano in western Java (Indonesia) was the site of historical activity in 1822, 1894, 1918, and 1982–83, located in a pre-historical horseshoe-shaped caldera. In 1982–83, a nine-month-long eruption generated successively (1) ash-and-scoria flows channeled in two valleys and extending 6 km from the vent (vulcanian phase 1), (2) surges and ash falls related to the excavation of a wide maar crater, with ash columns 20 km high (phreatomagmatic phase 2), and (3) ash and scoria falls that built a small cone inside the maar crater (strombolian phase 3). During phreatomagmatic phase 2, there was a significant increase of explosivity. Paradoxically, the magma composition had evolved from andesite to primitive magnesian basalt. Jet-plane incidents were recorded during this period: on the June 24 and July 13, 1982, two Boeing 747 aircraft experienced engine power loss when passing through the plume. The vertical variations of grain sizes and xenolith contents of pyroclasts were measured in the 1982–83 eruptive deposits. We show that a progressive increase of the ratio of xenolith versus juvenile magma before the end of vulcanian phase 1 heralded the increase of explosivity leading towards phreatomagmatic phase 2. In the same way, the decrease of the same ratio at the end of the phreatomagmatic phase 2 heralded the decrease of explosivity and the onset of strombolian phase 3. The transition from phase 1 to phase 2 is also marked by a slight but continuous decrease of the vesicularity index of juvenile clasts. We emphasize the increasing efficiency of groundwater/magma interaction during the eruption. The increasing interaction and renewed explosive activity occurred after a period of rest, during which additional groundwater was supplied in the vicinity of the magma column. The data suggest that it would have been possible to predict as soon as April–May 1982 the transition from vulcanian to phreatomagmatic activity, and consequently the corresponding increase in explosivity

The zone 1440 AD eruption of the soufriere of Guadeloupe (Lesser Antilles) : an experimental determination of the pre-eruptive consitions of the andesitic magma body.

The 1440 AD eruption of the Soufriere of Guadeloupe (Lesser Antilles arc) is characterized by a dramatic change of the chemical composition of erupted products, from andesitic (clear pumices, 61-62 wt% SiO2) to basaltic andesite (dark scoria, 55-56 wt% SiO2). This compositional sequence is interpreted to reflect the tapping of a silicic magma body remobilised following the arrival of a mafic magma batch. The conditions of equilibrium ( T, P, fO2, melt H2O content) within the andesite body prior to the mixing event were determined from a coupled ineralogical/petrological and experimental approach

Decompression-induced crystallization in hydrated silica-rich melts: Empirical models of experimental plagioclase nucleation and growth kinetics

International audienceIsothermal and isobaric crystallization of plagioclase in a water-saturated synthetic rhyolitic melt is investigated through time series of decompression experiments. The experimental variables are the rate at which samples are initially decompressed (30, 150, and 1200 MPa/h) from 200 MPa and 875°C, final pressure (25 to 160 MPa), and holding time at final pressure (up to 17 days). Through textural measurements of the crystals, plagioclase crystallization kinetics has been characterized in terms of nucleation lag and rates of nucleation and growth. Plagioclase crystallization is markedly dependent on effective undercooling, Teff, and holding time at crystallization pressure. With Teff increasing from 55 to 110°C, (i) nucleation lag decreases from 1-2 days to ~15 min, (ii) maximum nucleation rates increase from ~10-3 to 10-2 mm-2.s-1, and (iii) maximum growth rates decrease from ~10-6 to 5x10-7 mm.s-1. The initial decompression rate (30, 150, and 1200 MPa/h) has no systematic control on crystallization at final pressure, except for the 1200-MPa/h series in which samples show nucleation difficulties. From the experimental data of Teff-constrained plagioclase number density, proportion, and morphology, we provide means to assess conditions of nucleation and growth of natural plagioclase microlites from rapidly-ascended rhyolitic melts, through the determination of plagioclase liquidus curve and Teff prevailing during crystallization

A new geodynamic interpretation for the South Portuguese Zone (SW Iberia) and the Iberian Pyrite Belt genesis.

International audienceThe South Portuguese Zone (SPZ) constitutes the southernmost segment of the Variscan Iberian Massif. It is bounded to the north by the Beja-Acebuches Ophiolitic Complex and related accretionary wedge. To the south lie the Iberian Pyrite Belt (IPB) and flysch deposits forming the southern extent of the zone. Structural analysis within the Spanish side of the SPZ supports continuous south propagating deformation, evolving from early synmetamorphic thrusting in the internal zone to thin-skinned tectonics in the southern external domain. The accretion of the SPZ to the Ossa Morena Zone is also witnessed by the presence of various mélanges, observed throughout the investigated area. Part of the mélanges observed in the IPB are related to the volcanics and mineralizations setting. A key point to understand the IPB mineralizations genesis is to constrain the volcanogenic model. One underestimated feature is the large amount of submarine calc-alkaline ignimbritic facies, implying the presence of caldera structures within the province. Such correlation between caldera environment and ore deposits strongly suggests that the IPB developed in a continental arc. Our geodynamic model proposes an early north directed subduction associated with the obduction of the oceanic crust toward the south. Southward, this episode is immediately followed by the development of the accretionary prism, while farther south, a second subduction zone responsible for the arc setting of the IPB initiates. Subsequent Visean continental collision is associated with the deposit of the south propagating flysch and the present geometry of the SPZ

Pre-explosive conduit conditions of the 1997 Vulcanian explosions at Soufrière Hills Volcano, Montserrat: I. Pressure and vesicularity distributions

International audienceAn authoritative case of Vulcanian eruptive dynamics is the series of 88 explosions that occurred between August and October 1997 at Soufrière Hills volcano on Montserrat Island. The state of the magmatic column just before a Vulcanian explosion is still poorly understood, but conditions the eruptive style. This study establishes such a pre-explosive stratigraphy by 1) documenting the textures covering the range of the 1997 products, 2) quantitative analysis of H2O content in interstitial glass measured by Karl–Fischer Titration, and 3) combining these data with a simple model linking pre- and post-explosive vesicularities. The model shows that syn-explosive degassing affects greatly the way porosity evolves by decompression during an explosion. The stratigraphy reconstruction shows a three-part vertical layering of the conduit prior to explosion with overall denser values than those previously suggested. A dense and strongly degassed plug caps the column. It is underlain by a shallow transition zone featuring complex mingling between vesicular and dense magma up to 10 MPa. At higher pressure, up to 80 MPa, lies a more homogeneous zone of relatively dense (10–20 vol.%) magma, which was emplaced under partly open-system degassing. This conduit stratigraphy gives the vision of a strongly heterogeneous magma column immediately prior to its disruption. Our analysis suggests that fragmenting such a composite magma cannot happen in a single coherent pulse, but rather as stages. The transition zone contains heterogeneous amounts of exsolved gas that could explain the pulsatory nature of the Vulcanian jets at the beginning of the explosions. This contrasts with the nearly constant vesicularities of the deeper part of the pre-explosive magma column, which are propitious to a general, short-lived disruption

Stratigraphy and age of the Cappadocia ignimbrites, Turkey: reconciling field constraints with paleontologic, radiochronologic, geochemical and paleomagnetic data

The stratigraphy and age of the Neogene Cappadocia ignimbrites (Central Turkey) have been inferred in previous studies from fieldwork and K–Ar age determinations. The resulting stratigraphic schemes, however, differed from each other, suggesting that further studies were required to produce a reliable succession. In this paper, we examine the chronostratigraphy of mammalian remains recovered in the continental sediments interbedded with the Cappadocia ignimbrites. Using recent advances in mammalian chronostratigraphy, we evaluate selected taxa and faunal associations to place new and independent constraints on the ignimbrite ages. The biostratigraphically bracketed ages concur with some published radiometric dates, but they disagree with others, principally at localities where major stratigraphic discrepancies have arisen in the literature. In order to reconcile these apparent inconsistencies, we combine, at selected sites, our field observations with the biostratigraphic and radiometric age limits, and we compare these with the available geochemical and magnetic data. This allows us to present revised age estimates, and a revised stratigraphy which includes the correlation of the local Sofular deposits with the large-volume Gördeles ignimbrite. The issues faced in this study apply to other ignimbrite provinces in the world. For instance, ignimbrite eruption frequency in Cappadocia is higher than the resolution of many published K–Ar ages. Furthermore, different K–Ar ages have led to the description of individual and distinct ignimbrites that fieldwork and geochemical data allow to merge into a single ignimbrite. Argon loss from pumice samples leading to radiometric “rejuvenation” provides a likely explanation for most stratigraphic discrepancies. Cappadocia is the only documented ignimbrite field in which the chronostratigraphy of vertebrate remains provides better constraints on some ignimbrite ages than scattered K–Ar dates. We further argue that K–Ar dates from the Cappadocia area are too imprecise to establish a reliable magnetostratigraphic scheme for the ignimbrite succession, with the exception of the not, vert, similar2.8 Ma Valibaba Tepe ignimbrite

Trachyte phase relations and implication for magma storage conditions in the Chaîne des Puys (French Massif Central).

International audiencePetrological data have been acquired on the natural trachytes from the Chaîne des Puys, French Massif Central, and on experimental products from phase equilibria in order to (i) constrain the storage conditions of the trachytic magmas that lead to explosive eruptions (dome destructions as block-and-ash flows or pumice-and-ash flows) and (ii) provide phase relationships and chemical compositions for differentiated alkaline liquids in intraplate continental context. Phase assemblages, proportions, and compositions have been determined on six trachytes with SiO2 contents varying from 62 to 69 wt % and alkali contents of 10.5-12.0 wt %. The samples contain up to 30 % of phenocrysts, mainly consisting of feldspar (15-17 %; plagioclase and/or alkali-feldspar), biotite (2-6 %; except in the SiO2-poorest sample), Fe-Ti oxides (1-3 %) ± amphibole (< 5 %), ± clinopyroxene (~1 %). All samples have apatite and zircon as minor phases and titanite has been found in one sample. Pristine glasses (melt inclusions or residual glasses) in pumice from explosive events are trachytic to rhyolitic (65-73 wt % SiO2 and 10.5-13.0 wt % alkali). H2O dissolved in melt inclusions and the biotite+alkali feldspar+magnetite hygrobarometer both suggest pre-eruptive H2O contents up to 8 wt %. These are so far the highest H2O contents ever reported for alkaline liquids in an intraplate continental context. Melt inclusions also contain ~3400 ppm chlorine, ~700 ppm fluorine, and ~300 ppm sulphur. Crystallisation experiments of the six trachytes have been performed between 200 and 400 MPa, 700 and 900°C, H2O saturation, and oxygen fugacity of NNO +1. The comparison between the natural and experimental phase assemblage, proportion, and composition suggests magma storage conditions at a pressure of 300-350 MPa (~10-12 km deep), melt H2O content ~8 wt % (close to saturation), an oxygen fugacity close to NNO~0.5, and temperatures increasing from 700 to 825°C with decreasing bulk SiO2 of the trachyte. The high H2O contents of the trachytes show that wet conditions may prevail during the differentiation of continental alkaline series. Regardless of the size of the magma reservoir assumed to have fed the trachyte eruptions, calculation of the thermal relaxation timescales indicates that the tapped reservoir(s) are likely to be still partially molten nowadays. The four northernmost edifices may correspond to a single large reservoir with a lateral extension of up to 10 km, which could be possibly reactivated in weeks to months if intercepted by new rising basalt batches

Effects of fro and HzO on andesite phase relations between 2 and 4 kbar

International audienceExperimentalp hasee quilibriah ave been investigatedo n three medium-K silicic andesite (60-61 wt % SiO2) samples from Mount Pelhe at 2-4 kbar, 850-1040øC, under both vapor-saturatedC O2-freea nd vapor-saturatedC O2-bearingc onditions.M ost experiments were crystallizatione xperimentsu singd ry glassesp reparedf rom the naturalr ocks.B oth normal-a nd rapidq uenche xperimentsw ere performed.T wo rangeso f oxygenf ugacity( fo2) were investigatedN: NO (Ni-NiO buffer) to NNO + 1 and NNO + 2 to NNO + 3. At 2 kbar for moderatelyo xidizing conditionsp, lagioclase( pl) and magnetite( mt) are the liquidus phases, followed by low-Ca pyroxene (opx); these three phases coexist over a large temperature( T)-H20 range( 875-950øC and 5-7 wt % H20 in melt). Amphibole (am) is stable undern ear vapor-saturatedC O2-freec onditionsa t 876øC. At 900øC, ilmenite (ilm) is found only in experimentsle sst hano r equalt o NNO. Upon increasingp ressure( P) underv aporsaturatedC O2-freec onditionsp, l + mt is replacedb y am + mt on the liquidusa bove3 .5 kbar. For highly oxidizingc onditionsm, t is the solel iquidusp hasea t 2 kbar, followed by pl and opx, except in the most H20-rich part of the diagram at 930øC, where opx is replaced by Carich pyroxene( cpx) and am. Compositionso f ferromagnesianp hasess ystematicallyc orrelate with changingfo2E xperimentalg lassesr angef rom andesitict hroughd acitict o rhyolitic, showings ystematicc ompositionavl ariationsw ith pl + opx + mt fractionation( increaseo f SiO2a ndK 20,d ecreasoef A1203C, aO,F eOt, andM gO).F eO*/MgOm oderateilnyc reases with increasingS iO2.F orfo2 conditionst ypical of calk-alkalinem agmatism( approximately NNO + 1), magnetite is either a liquidus or a near-liquidus phase in hydrous silicic andesite magmas,a nd this shoulds timulater eexaminationfo r the mechanismso f generationo f andesitesb y fractionationf rom basalticp arents

Géologie du volcanisme

International audienceCet ouvrage couvre la plupart des aspects centraux de la volcanologie, depuis les processus physiques de la dynamique éruptive jusqu'aux différents types de volcans et de structures volcaniques, en passant notamment par les caractéristiques des produits volcaniques. Il suit les différentes échelles d'observation - l'échantillon (les roches volcaniques), l'affleurement (les formations volcaniques effusives et explosives et leurs remaniements), la carte (les volcans et structures volcaniques) - afin de donner aux étudiants les outils de description et d'interprétation des objets, depuis le microscope pétrographique jusqu'au camp de terrain.Ainsi sont présentés les textures des roches volcaniques et leurs relations avec la dynamique éruptive, les critères d'identification et d'interprétation des produits pyroclastiques, ou encore les principes de cartographie en terrain volcanique. Enfin, il traite des aspects appliqués de la géologie volcanique comme l'évaluation des aléas volcaniques et la cartographie en terrain volcanique