sub volcanic crystallization at stromboli aeolian islands southern italy preceding the sciara del fuoco sector collapse evidence from monzonite lithic suite we describe texture mineralogy and whole rock composition of cognate monzonite sub volcanic clasts within debris flow deposits related to the 5000 years catastrophic phreatomagmatic eruption probably linked to the sciara del fuoco sector collapse the debris flows are at the top of accretionary lapilli rich ash deposits overlying potassic ks shoshonites lavas of the neostromboli period the monzonites are inferred to be crystallized in situ at low p at the side walls and or roof margins of a shallow magma chamber and to be cogenetic with the ks neostromboli extrusives they can be considered ideal orthocumulates since they approximately retain a bulk liquid composition and possibly represent slowly cooled equivalents of their ks shoshonite host rock the closure temperature of final solidification of the monzonite lithic suite was estimated through ternary feldspar geothermobarometry plagioclase k feldspar and k feldspar biotite equilibria and is in the range of 750 790 c with a maximum logfo2 around 15 1 15 3 the estimated pressure of crystallization is 0 5 kbar potassic lavas and dikes previously emplaced during the neostromboli period also resemble the monzonites in both major trace elements and mineral chemistry the cogenetic relationship between ks neostromboli extrusives and the monzonite host rock magma from which the sub volcanic clasts were derived is clear evidence that a shallow magma chamber existed between the caldera collapse of the vancori period and the sciara del fuoco sector collapse i e between 13 000 and 5000 years the monzonite clasts were derived from crystallization at very shallow depth ca 1 km and strongly support the hypothesis of violent decompression of the shallow magmatic plumbing system during the sciara del fuoco sector collapse climax of the regressive landslide event with maximum disruption of the chamber walls took place during emplacement of the debris flows i e during the late stage of the neostromboli phreatomagmatic eruption

variable parent magmas and recharge regimes of the parinacota magma system n chile revealed by fe mg and sr zoning in plagioclase abstract zoning patterns of an content and fe mg and sr concentrations in plagioclase phenocrysts in andesites from parinacota volcano n chile reflect alternating recharge events with two chemically distinct mafic magmas these magmas are characterized by low and high sr contents similar to two recent mafic flank eruptions one end member basaltic andesite shows large sr enrichment and heavy rare earth depletions and thus equilibrated with lower crustal rocks at depth where plagioclase high sr is unstable and garnet high hree y is stable a second end member magma is lower in sr ba contents and has ree patterns typical for parent magmas elsewhere in the central andes the number of recorded recharge events increases after a catastrophic sector collapse and during the subsequent rebuilding of the stratocone variations of an fe and mg contents and morphology of zones suggest also changes in water pressure including decompression under water under saturated and water saturated conditions evidence for decompression is more present in post collapse samples suggesting that the change in the volcano dynamics involves changes in magma chamber location this shows the importance of the cone collapse event in the volcano s magmatic evolution we propose that both end members are only seen at the surface of parinacota volcano because of the particular dynamics of this volcanic system and that similar processes might occur in other volcanic centres of the central volcanic zone

postshield volcanism and catastrophic mass wasting of the waianae volcano oahu hawaii the 3 9 to 2 9 ma waianae volcano is the older of two volcanoes making up the island of oahu hawaii exposed on the volcanic edifice are tholeiitic shield lavas overlain by transitional and alkalic postshield lavas the postshield alkalic cap consists of aphyric hawaiite of the palehua member of the waianae volcanics overlain unconformably by a small volume of alkalic basalt of the kolekole volcanics kolekole volcanics mantle erosional topography including the uppermost slopes of the great lualualei valley on the lee side of the waianae range twenty new k ar dates combined with magnetic polarity data and geologic relationships constrain the ages of lavas of the palehua member to 3 06 2 98 ma and lavas of the kolekole volcanics to 2 97 2 90 ma the geochemical data and the nearly contemporaneous ages suggest that the kolekole volcanics do not represent a completely independent or separate volcanic event from earlier postshield activity thus the kolekole volcanics are reduced in rank becoming the kolekole member of the waianae volcanics magmas of the palehua and kolekole members have similar incompatible element ratios and both suites show evidence for early crystallization of clinopyroxene consistent with evolution at high pressures below the edifice however lavas of the kolekole member are less fractionated and appear to have evolved at greater depths than the earlier palehua hawaiites postshield primary magma compositions of the palehua and kolekole members are consistent with formation by partial melting of mantle material of less than 5 10 relative to waianae shield lavas within the section of palehua member lavas an increase with respect to time of highly incompatible to moderately incompatible element ratios is consistent with a further decrease in partial melting by approximately 1 2 this trend is reversed with the onset of eruption of kolekole member lavas where an increase in extent of partial melting is indicated the relatively short time interval between the eruption of palehua and kolekole member lavas appears to date the initial formation of lualualei valley which was accompanied by a marked change in magmatic conditions we speculate that the mass wasting event separating lavas of the palehua and kolekole members may be related to the formation of a large submarine landslide west and southwest of waianae volcano enhanced decompression melting associated with removal of the equivalent volume of this landslide deposit from the edifice is more than sufficient to produce the modeled increase of 1 2 in extent of melting between the youngest palehua magmas and the posterosional magmas of the kolekole member the association between magmatic change and a giant landsliding event suggests that there may be a general relationship between large mass wasting events and subsequent magmatism in hawaiian volcano evolution

a facies model for a quaternary andesitic composite volcano ruapehu new zealand ruapehu composite volcano is a dynamic volcanic sedimentary system characterised by high accumulation rates and by rapid lateral and vertical change in facies four major cone building episodes have occurred over 250 ka from a variety of summit flank and satellite vents eruptive styles include subplinian strombolian phreatomagmatic vulcanian and dome related explosive eruptions and extrusion of lava flows and domes the volcano can be divided into two parts a composite cone of volume 110 km3 surrounded by an equally voluminous ring plain complementary portions of ruapehu s history are preserved in cone forming and ring plain environments cone forming sequences are dominated by sheet and autobrecciated lava flows which seldom reach the ring plain the ring plain is built predominantly from the products of explosive volcanism both the distal primary pyroclastic deposits and the reworked material eroded from the cone much of the material entering the ring plain is transported by lahars either generated directly by eruptions or triggered by the high intensity rain storms which characterise the region ring plain detritus is reworked rapidly by concentrated and hyperconcentrated streams in pulses of rapid aggradation immediately following eruptions and more gradually in the longer intervals between eruptions

what controls sheet intrusion in volcanoes structure and petrology of the stromboli sheet complex italy abstract a multidisciplinary study has been conducted on 109 intrusive sheets of stromboli the relation between sheet injection conditions with the magmatic and structural variations of the volcano and the parameters governing the preferential intrusion geometry inside the volcano in a time perspective have been determined during its subaerial activity the last 100 ka stromboli experienced periods of extrusive growth and sheet intrusions alternated to caldera and lateral collapses of the nw sciara del fuoco flank the sheet intrusions similar to the extrusive rocks span a large compositional range from calc alkaline to potassic through high k calc alkaline and shoshonitic series the most evolved compositions of the effusive rocks however have not been found as intrusions whereas some mafic and basic magmas of sheets were never erupted the repeated time change of location of the intrusions from strombolicchio to the ne sw trend passing trough the inferred paleostromboli summit cone and the southern flank could represent the expression of the shift of the magma chamber towards the southeast and the subsequent onset of a clear ne sw elongated magma chamber located along the main stromboli weakness zone the stromboli evolution in the last 100 ka has been characterized by the persistent n 10 55 e intrusion direction which reflects the influence of the edifice s state of stress on the magma feeding paths after 13 ka ago sheets mainly intruded along to the sector collapse scarps in a circum amphitheatre geometry which has been demonstrated to be influenced by the new cone morphostructure and not by different magma rheological characters

unusual lapilli tuff ejecta erupted at stromboli during the 15 march 2007 explosion shed light on the nature and thermal state of rocks forming the crater system of the volcano abstract textural and mineralogical study of high temperature angular blocks erupted during the stromboli explosion of 15 march 2007 was used to make inferences on the nature and thermal state of rocks forming the subsurface of the volcano summit crater terrace the studied ejecta consist of lapilli tuff that formed as a result of the transformation and high temperature induration sintering of the basaltic scoriae lapilli and ash originally accumulated as loose tephra during the current activity of the volcano the main processes leading to the tephra transformation were investigated through microstructural observations mineral and glass analyses sem eds and emp analyses investigations revealed that subsolidus reactions and partial melting of the tephra occurred at temperatures higher than 600 c and under variable fo 2 conditions from qfm to hm buffering curves in some blocks evidence of high t reheating and partial melting at the expense of secondary hydrothermal minerals was also observed in order to track the subsolidus reheating history of the basaltic pyroclasts a detailed study of the pseudomorphic phases and reactions after olivine driven by iron oxidation under high t conditions was performed the observed mineralogical transformation suggests that the lapilli tuff material originating from the burial of tephra routinely accumulated by persistent strombolian explosions within the crater terrace were in some cases altered by the circulation of acidic fluids and were in any case reheated due to isotherm rise forced by high heat flux and gas streaming delivered by the underlying magma system it is worth noting that the ejection of these unusual volcanic lithotypes was possible because a few days before the 15 march 2007 event the craters were clogged with lapilli tuff material that slid into the crater bottom between 7 and 9 march findings of this study suggest that the scattered permanently active vents and shallow conduits of stromboli are surrounded sideways and underneath the crater terrace by a fairly large volume of high temperature rocks with variable degree of compaction sintering up to partially melted such a spectrum of rock types is in good agreement with the conceptual model of prominent thermal zoning all around sideway and upwards the active magmatic system we speculate that continuous migration upwards of isotherms led to transformation and partial melting of the normal strombolian tephra

lyttelton volcano banks peninsula new zealand primary volcanic landforms and eruptive centre identification abstract lyttelton volcano banks peninsula new zealand has historically been viewed as a simple volcanic cone this paper uses digital terrain models dtm and primary volcanic landforms to reinterpret lyttelton volcano as having multiple eruptive centres primary volcanic landforms are features produced during active volcanism classified as constructional hypabyssal and erosional volcanic features constructional volcanic features are lava flows scoria cones and domes hypabyssal volcanic features are dykes and sills and erosional volcanic features are valley and ridge patterns and orientations lava flow trends are recognised from aerial photograph analysis and supported by field observations highlighting radiating lava trends around specific locations within lyttelton harbour scoria cones and domes occur on the outer flanks of volcanic cones and are used as such in the identification of remnant cone surfaces dyke orientations are plotted and then projected to the interior of the volcano defining 13 zones of convergence the projected arrays of these orientations highlight defined regions along the erosional crater rim each indicating a radial dyke swarm from which the projected trends of the associated dykes indicate an eruptive centre valley and ridge orientations are projected from the longest valley or ridge segment towards the inner harbour radiating erosive patterns are incepted during the growth and degradation of a volcanic cone with the resulting trends orienting to the summit zones of convergence eruptive centres are identified from lava flow orientations onlapping lava sequences scoria cones and intrusive locations the summit of a volcanic edifice can be identified from the orientations of valleys and ridges while radial dyke systems determine whether this summit was a volcanic centre or simply a local topographic high volcanic landforms are used to identify cone sectors the preserved sector associated with a particular eruptive centre cone sector limits are defined by a basal footprint and an erosional crater rim with similar arcuate features remnant cone features being exposed in the interior of the volcano lyttelton volcano comprises fifteen volcanic cones with vent locations controlled by underlying fault lineaments the growth and erosion of each volcanic cone is reflected in primary volcanic landforms with the preserved features of cones confined to cone sectors and cone artefacts

groundmass crystallization of mount st helens dacite 1980 1986 a tool for interpreting shallow magmatic processes the 1980 1986 eruption of mount st helens volcano provides an unprecedented opportunity to observe the evolution of a silicic magma system over a short time scale groundmass plagioclase size measurements are coupled with measured changes in matrix glass plagioclase and fe ti oxide chemistry to document increasing groundmass crystallinity and thus to better constrain proposed physical models of the post may 18 1980 magmatic reservoir measurements of plagioclase microlite and microphenocryst sizes demonstrate that relatively rapid growth approximately 10 9 cm s of groundmass plagioclase occurred immediately subsequent to may 18 relatively rapid plagioclase growth continued through the end of 1980 at an average rate of 3x10 11 cm s plagioclase growth rates then decreased to 1x10 11 cm s through 1986 changes in groundmass crystallinity are reflected in changes in both matrix glass and plagioclase microphenocryst rim chemistry although the matrix glass composition appears to have remained approximately constant from 1981 1986 after a rapid compositional change from may 18 until the end of 1980 plagioclase microphenocrysts show increasingly more complex zoning patterns with time microphenocryst core compositions are commonly positively correlated with crystal size both of these observations indicate continuous groundmass plagioclase growth through 1986 magmatic temperatures estimated from fe ti oxide pairs are approximately constant through 1981 at eruption temperatures of 930 c and at log fo2 of 10 8 by 1985 1986 oxide temperatures decreased to 870 c chemical and textural changes can be explained by 1 rapid degassing and crystallization in response to the intrusion of magma into a shallow 4 5 km reservoir toward the end of the may 18 1980 eruption 2 continued crystallization at a much reduced rate through 1986 due to slow cooling of the shallow magma reservoir growth rates and consequent chemical changes appear to decrease at the end of 1980 this is coincident with the change in eruption style from explosive eruptions sometimes followed by dome growth to solely extrusive dome building events and can be explained by the expected viscosity increase of both degassing and increasing crystallinity the model of twostage crystallization of magma in a shallow reservoir is consistent with conclusions from gas studies casadevall et al 1983 gerlach and casadevall 1986 a b patterns of crater deformation chadwkck et al 1988 and post 1980 seismicity endo et al 1990 although it does not explain the experimental data of hill and rutherford 1989 on the growth rate of amphibole reaction rims textural measurements on mount st helens dacite can also be used to evaluate crystallization kinetics in silicic magmas systems for which experimental data is almost non existent plagioclase growth rates are 5 10 times slower than estimated plagioclase growth rates in basaltic systems a result consistent with the higher viscosity of a more silicic melt furthermore patterns of textural change both average crystal size and number density are similar to those observed during the 1984 mauna loa eruption by lipman and banks 1987 suggesting that the only modification to the crystallization behavior of plagioclase required in extrapolation from basaltic systems is a moderate decrease in rates such that the rate of crystallization scales with the melt viscosity

origin of high silica liquids at stromboli volcano aeolian islands italy inferred from crustal xenoliths high silica igneous xenoliths granophyre and obsidian fragments with sio2 ca 75 wt and high silica glass sio2 between ca 64 and 75 wt within glass bearing tonalitic and dioritic xenoliths have been discovered at stromboli they are well beyond the silica range of the volcanic rocks erupted during the subaerial activity of the volcano the granophyre and the obsidian fragments occur within the petrazza pyroclastics high k andesite pumiceous scoriae of the paleostromboli i period 61 ka whereas the glass bearing tonalites and diorites are hosted in the omo basaltic andesite lava flows of the paleostromboli ii period 64 55 ka the obsidian represents an extremely evolved liquid derived from low pressure fractional crystallization of high k calc alkaline magmas of stromboli coupled with minor assimilation of upper crust or terrigenous sediments the other studied high silica products have an anatectic origin the granophyre composition is compatible with a genesis by low degree melting of leucotonalites similar to some calabrian arc lithotypes partial melting of crustal rocks from the stromboli basement is also demonstrated by interstitial glass in the tonalite and diorite xenoliths textural evidence and the presence of variable glass compositions suggest that in these plutonic xenoliths the melt chemistry was controlled by dehydration melting of biotite and non modal melting of local mineralogy high degree undercooling of granophyre melt and of the partially melted xenoliths is consistent with rapid rise of the host magmas leading to eruption whether partial melting was initiated during stoping and transport of xenoliths within the rising magma or in situ during magma storage within the crust has not been unequivocally established

growth after collapse the volcanic and magmatic history of the neostromboli lava cone island of stromboli italy this study focuses on the holocene neostromboli lava cone which is part of the stromboli volcano aeolian islands italy it represents an important example of the structure and behaviour of lava cone re growth shortly after a large scale volcano sector collapse neostromboli s eruptive style is dominated by extrusion of sheet like a a lava flows and associated autoclastic breccias the lava cone developed during five phases of activity phases a to e representing discrete episodes of lava effusion characterised by rapid changes in magma composition spatial development of the feeder system and physical properties of erupted lavas through time although they all belong to the potassic series ks with sio2 49 55 wt and mgo 2 50 6 25 wt the neostromboli lavas have distinct petrogenetic characteristics ranging from leucite free and low sr shoshonitic basalts phases a and b to leucite bearing shoshonitic basalts with high phase c and intermediate phase d sr contents and to leucite free biotite bearing shoshonites phase e three different batches of mantle derived relatively primitive basic magmas fed the activity of phases a c and d the evolution phases a to b is inferred to have occurred in a continuously replenished tapped and crystallizing rtf magma chamber whereas phase e products were dominantly generated by fractional crystallization from phase d magmas the neostromboli cone shows summit flank and satellite vents and sheet intrusions with different geometry and location through time reflecting the interplay of regional tectonics precursory instability processes and volcano load

glass bearing crustal xenoliths buchites erupted during the recent activity of stromboli aeolian islands abstract three xenoliths erupted as ejecta during recent violent explosion of stromboli volcano aeolian islands were investigated in this paper they consist of high temperature mineral association cordierite hercynite spinels sillimanite plagioclase mullite corundum and abundant glass 10 70 vol and may be classified as buchites the peraluminous composition of the xenoliths their trace element distribution and ree patterns support their origin from granulite and amphibolite facies metapelites of the calabrian continental crust which is considered the crystalline basement beneath stromboli buchites have an isotopic composition comparable to that of stromboli extrusives and significantly different from that of the calabrian basement the glass is generally colourless and has a si al alkali rich composition except for one sample where a fe mg ca rich reddish brown glass also occurs these two kinds of glass show complicated textures where patches of reddish brown glass are often surrounded by plagioclase and or cordierite or forms streaks and swirls with the colourless glass cordierite plagioclase and oxides have different compositions according to their position in the xenoliths ca rich plagioclase an72 95 mg poor cordierite mg values 47 66 and al rich spinels are in the inner portions of the xenoliths and associated with colourless glass on the contrary close to the contact with the host lava or associated with coloured glass cordierite shows higher mg values ti fe bearing oxides occur and plagioclase is chemically similar to the basalt phenocrysts an66 71 the abundant and fresh glass and the idiomorphic shape of the high temperature minerals suggest that the xenoliths were hold in the basaltic magma before its extrusion for a significant time to allow their partial or nearly complete melting and subsequent nucleation and growth of new phases during this stage the interaction between the anatectic liquid and the basaltic magma affected the original isotopic composition of the xenoliths and in some cases produced glass and mineral phases cordierite plagioclase and oxides with different composition

the effects of flank collapses on volcano plumbing systems the growth of large volcanoes is commonly interrupted by episodes of flank collapse that may be accompanied by catastrophic debris avalanches explosive eruptions and tsunamis el hierro the youngest island of the canary archipelago has been repeatedly affected by such mass wasting events in the last 1 ma our field observations and petrological data suggest that the largest and most recent of these flank collapses the el golfo landslide likely influenced the magma plumbing system of the island leading to the eruption of higher proportions of denser and less evolved magmas the results of our numerical simulations indicate that the el golfo landslide generated pressure changes exceeding 1 mpa down to upper mantle depths with local amplification in the surroundings and within the modeled magma plumbing system stress perturbations of that order might drastically alter feeding system processes such as degassing transport differentiation and mixing of magma batches

nature and origin of cone forming volcanic breccias in the te herenga formation ruapehu new zealand volcanic breccias form large parts of composite volcanoes and are commonly viewed as containing pyroclastic fragments emplaced by pyroclastic processes or redistributed as laharic deposits field study of cone forming breccias of the andesitic middle pleistocene te herenga formation on ruapehu volcano new zealand was complemented by paleomagnetic laboratory investigation permitting estimation of emplacement temperatures of constituent breccia clasts the observations and data collected suggest that most breccias are autoclastic deposits five breccia types and subordinate coherent lava flow cores constitute nine unconformity bounded constructional units two types of breccia are gradational with lava flow cores red breccias gradational with irregularly shaped lava flow cores were emplaced at temperatures in excess of 580 c and are interpreted as aa flow breccias clasts in gray breccia gradational with tabular lava flow cores and in some places forming down slope dipping avalanche bedding beneath flows were emplaced at varying temperatures between 200 and 550 c and are interpreted as forming part of block lava flows three textural types of breccia are found in less intimate association with lava flow cores matrix poor well sorted breccia can be traced upslope to lava flow cores encased in autoclastic breccia unsorted boulder breccia comprises constructional units lacking significant exposed lava flow cores clasts in both of these breccia types have paleomagnetic properties generally similar to those of the gray breccias gradational with lava flow cores they indicate reorientation after acquisition of some or all magnetization and ultimate emplacement over a range of temperatures between 100 and 550 c these breccias are interpreted as autoclastic breccias associated with block lava flows matrix poor well sorted breccia formed by disintegration of lava flows on steep slopes and unsorted boulder breccia is interpreted to represent channel floor and levee breccias for block lava flows that continued down slope less common matrix rich stratified tuff breccias consisting of angular blocks minor scoria and a conspicuously well sorted ash matrix were generally emplaced at ambient temperature although some deposits contain clasts possibly emplaced at temperatures as high as 525 c these breccias are interpreted as debris flow and sheetwash deposits with a dominant pyroclastic matrix and containing clasts likely of mixed autoclastic and pyroclastic origin pyroclastic deposits have limited preservation potential on the steep proximal slopes of composite volcanoes likewise these steep slopes are more likely sites of erosion and transport by channeled or unconfined runoff rather than depositional sites for reworked volcaniclastic debris autoclastic breccias need not be intimately associated with coherent lava flows in single outcrops and fine matrix can be of autoclastic rather than pyroclastic origin in these cases and likely many other cases the alternation of coherent lava flows and fragmental deposits defining composite volcanoes is better described as interlayered lava flow cores and cogenetic autoclastic breccias rather than as interlayered lava flows and pyroclastic beds reworked deposits are probably insignificant components of most proximal cone forming sequences

transient processes in stromboli s shallow basaltic system inferred from dolerite and magmatic breccia blocks erupted during the 5 april 2003 paroxysm we describe the mineralogy geochemistry and mesomicrostructure of fresh subvolcanic blocks erupted during the 5 april 2003 paroxysm of stromboli aeolian islands italy these blocks represent 50 vol of the total erupted ejecta and consist of fine to medium grained basaltic lithotypes ranging from relatively homogeneous dolerites to strongly or poorly welded magmatic breccias the breccia components are represented by angular fragments of dolerites entrapped in a matrix of vesiculated lava like to scoriae crystal rich cr basalt all of the studied blocks are cognates with the cr basalt of the normal strombolian activity or lavas and they are often coated by a few centimeter thick layer of crystal poor cp basaltic pumice erupted during the paroxysm we suggest that they result from the rapid increase of pressure and related subvolcanic rock failure that occurred shortly before the 5 april 2003 explosion when the uppermost portion of the edifice inflated and suffered brecciation as the result of the sudden rise of the gas rich cp basalt that triggered the eruption dolerites and magmatic matrix of the breccias show major and trace element compositions that match those of the cr basalts erupted during normal strombolian activity and effusive events at stromboli volcano dolerites consist of a phenocrysts normally found in the cr basalts and b late stage magmatic minerals such as sanidine an60 28 plagioclase fe mn rich olivines fo68 48 phlogopite apatite and opaque mineral pairs magnetite and ilmenite most of which are never found both in lava flows and scoriae erupted during the persistent explosive activity that characterizes typical strombolian behavior subvolcanic crystallization of the stromboli cr magma leading to slowly cooled equivalents of basalts could result from transient drainage of the magma from the summit craters to lower levels fingering and engulfing of the material that collapsed from the summit crater floor into the shallow basaltic system during the late evening of 28 december 2002 coupled with the short break in the summit persistent explosions between december 2002 and march 2003 permitted the cr magma pockets to solidify as dolerites which were confined to the uppermost portion of the system and thus not involved in the ongoing flank effusive activity crystal size distribution of the basaltic blocks and crystallization of the finer grained 0 1 mm mafic minerals of the dolerites over a time interval of 100 days closely agrees with the above interpretation vesicle filling miarolitic cavities locally found in some dolerites with minerals deposited as vapor phase crystallization is a result of continuous gas percolation through the rocks of the uppermost portion of the volcanic system poorly welded magmatic breccias formed during syn eruptive processes of 5 april 2003 when the paroxysm strongly shattered the shallow subvolcanic system and many dolerite fragments were entrapped in the cr magma in contrast the high degree of welding between the dolerite clasts and the cr basaltic matrix in the strongly welded magmatic breccias provides a snapshot of subvolcanic intrusions of the cr basalt into the dolerite when after a 2 month break in activity cr magmas started to rise again to the summit craters blocks similar to these subvolcanic ejecta of 5 april 2003 were also erupted during previous paroxysms e g 1930 suggesting that changes in the usual strombolian activity e g short breaks in the persistent mild explosions and or flank effusive activity lead to transient crystallization of dolerites in the shallow plumbing system

physical forcing of phytoplankton dynamics in the southwestern ross sea coastal zone color scanner czcs imagery of phytoplankton pigments and passive microwave imagery of sea ice distributions in the southwestern ross sea are presented for three different seasons 1978 1989 1979 1980 and 1981 1982 and were analyzed in conjunction with meteorological data obtained from a series of automatic weather stations aws dynamics of the phytoplankton bloom in terra nova bay differed from those in the ross sea owing to spatial differences in katabatic wind fields which determine when the surface waters stratify interannual variation in the timing of formation of the ross sea polynya appears to be controlled by winter temperatures which determine sea ice thickness and integrity rather than variability or intensity in wind stress together czcs aws and passive microwave data suggest that when the ross sea polynya forms early stronger and more frequent katabatic winds result in increased advective losses of phytoplankton in surface waters and a delay in the phytoplankton bloom if polynya formation is delayed until after the winds diminish in frequency the phytoplankton bloom will develop earlier the observation that diatoms dominate both the marginal ice zone and terra nova bay which are hydrographically similar while phaeocystis antarctica is found in unstable waters north of the ross ice shelf suggests that stratification plays an important role in determining species composition in the ross sea

on the conditions of sheet injections and eruptions in stratovolcanoes nearly all eruptions in stratovolcanoes composite volcanoes central volcanoes are supplied with magma through fractures consequently a primary physical condition for an eruption to occur in a stratovolcano is that a magma driven fracture is able to propagate to the surface magma filled fractures frozen or fluid are referred to as sheet intrusions more specifically they are named dykes when subvertical and inclined or cone sheets when inclined field observations indicate that most sheet intrusions do not reach the surface to feed eruptions but rather become arrested at various crustal depths for this reason periods of volcanic unrest with sheet injections are much more common than volcanic eruptions whether a sheet intrusion becomes arrested or alternatively propagates to the surface depends primarily on the stress field in the stratovolcano a stratovolcano normally consists of layers of contrasting mechanical properties such as soft low young s modulus pyroclastic units and stiff high young s modulus lava flows we present numerical models indicating that volcanoes composed of such layers commonly develop stress fields encouraging sheet and dyke arrest the models indicate that a necessary condition for a sheet intrusion to reach the surface and feed a volcanic eruption is that the stress field along the sheet pathway becomes homogenised we propose that much of the activity in a stratovolcano during a volcanic cycle encourages stress field homogenisation field studies show that the sheet intrusions in individual stratovolcanoes have various dips some are vertical dykes others inclined sheets and still others horizontal sills analytical models indicate that the dip of a sheet reaching the surface can have great effects on the magma transport during an eruption this effect is normally greater for a flat volcano such as a collapse caldera than for a stratovolcano that forms a topographic high we conclude that the shallower the dip of a sheet intrusion the less will be its volumetric magma transport to the surface of a stratovolcano

some consequences of volcanic edifice destruction for eruption conditions abstract destruction of a volcanic edifice by landslides or phreatic explosions unloads the upper crust induced changes of stress field around and of magmatic pressure within a magma reservoir are investigated with an analytical model for the deformation of a liquid filled cavity within an elastic half space unloading affects the reservoir pressure and hence the net result depends on how the liquid filled reservoir responds to a change of remote stress magma compressibility is taken into account and may dampen changes of internal pressure in small volatile rich reservoirs the main consequence of edifice destruction is a decrease of magmatic pressure and stresses on the reservoir walls in some cases this may be responsible for dyke closure at the reservoir walls which stops magma withdrawal and may prevent eruption if an eruption does occur edifice destruction affects the volume of magma erupted depending on edifice size magma reservoir size and depth the erupted volume may be smaller or larger than that which would be erupted with no damage to the edifice these results suggest that major phreatic explosions may prevent magmatic eruptions

volcanological and magmatological evolution of stromboli volcano aeolian islands the roles of fractional crystallization magma mixing crustal contamination and source heterogeneity the present paper reports the results of a detailed stratigraphical petrological and geochemical investigation on the island of stromboli aeolian arc southern tyrrhenian sea major and trace element data determined on a large quantity of samples from well established stratigraphic positions indicate that the magmatological evolution of the island through time was more complex than previously known the activity of the exposed part of stromboli which occurred over a time span of about 100 000 years started with the emission of high k calc alkaline hkca volcanics which were covered by calc alkaline ca shoshonitic sho high k calc alkaline hkca and potassic ks products the most recent activity consists of hkca lavas and the present day sho basaltic volcanics emitted by mildly explosive strombolian activity most of the products are lavas with minor amounts of pyroclastic rocks emplaced mainly during the early stages of activity the transition from the sho to the ks cycle was associated with the collapse of the upper part of the volcanic apparatus the transition from ks to the present day sho activity has been found to have occurred at the time of the sliding of the western portion of the volcano that generated the sciara del fuoco depression the rock series cropping out at stromboli show variable enrichment in potassium incompatible trace elements and radiogenic sr which increase from ca through hkca and sho up to ks rocks major trace element and sr isotopic data agree in indicating that the hkca and sho series evolved by crystal liquid fractionation starting from different parental liquids whereas crustal assimilation appears to have been the leading process during the evolution of ks volcanics mixing processes also played a role although they can be well documented only when they occurred between magmas with different isotopic and geochemical characteristics geochemical modelling based on trace element and isotopic data indicates that the mafic magmas of the different volcanic series may be generated by melting of an upper mantle heterogeneously enriched in incompatible elements and radiogenic sr by addition via subduction of different amounts of crustal material geochemical data however are also in agreement with the alternative hypothesis that the most mafic magmas of the different series have been generated by combined processes of fractional crystallization assimilation and mixing of a ca magma in a deep sited magma chamber the mafic magmas formed by these complex processes were successively emplaced in a shallow reservoir where they evolved by simple fractional crystallization hkca and sho series and by assimilation of crustal material ks the occurrence of changes in the geochemical signatures of the magmas at the time of the structural modification of the volcano is believed to favour the hypothesis that the variable composition observed in the volcanic rocks of stromboli is the result of processes occurring within the volcanic system

persistent polybaric rests of calc alkaline magmas at stromboli volcano italy pressure data from fluid inclusions in restitic quartzite nodules a fluid inclusion study has been performed on quartzite nodules of stromboli volcano hosted by calc alkaline lavas of both the strombolicchio 200 ka and paleostromboli ii 60 ka periods the nodules are mainly composed of quartz crystals with subordinate plagioclase and k feldspar small interstitial minerals such as plagioclase k feldspar clinopyroxene biotite and quartz are also found together with glass muscovite epidote and zircon occur as accessory minerals different quartzite nodules occur 1 equigranular polygonal granoblastic quartzite nodules forming a polygonal texture with clear triple points 2 inequigranular polygonal granoblastic quartzite nodules and 3 break up nodules with strongly resorbed quartz these quartzites are restites from partial melting involving felsic crustal rocks at the magma wall rock contact restitic quartz re crystallises at variable and generally high temperatures leading to the formation of quartzites with different textures quartz grains contain five types of fluid inclusions distinguished on the basis of both fluid type and textural phase relationships at room temperature type i are two phase liquid vapour co2 rich fluid inclusions they are primary and subordinately pseudosecondary in origin and have undergone re equilibration processes type ii mono phase two phase vapour liquid vapour co2 rich fluid inclusions are the most common and based on their spatial distribution and shape they can be divided into two subclasses type iia and type iib type ii inclusions are secondary or pseudosecondary and they are assumed to have formed after decrepitation of type i inclusions and cracking of the host quartz type iii inclusions are mono phase vapour they possibly contain co2 at very low density and surround the inner rims of quartz grains type iv two phase silicate melt inclusions contain glass co2 rich fluid some of them are cogenetic with type ii inclusions finally type v two phase liquid vapour aqueous inclusions are both vapour rich and liquid rich aqueous inclusions microthermometric experiments were performed on both type i and ii inclusions type i inclusions homogenise to liquid between 20 and 30 5 c type iia inclusions homogenise to vapour in the 24 to 30 c range with a maximum peak of frequency at 29 c type iib inclusions also homogenise to vapour between 14 and 25 c there appears to be no difference in homogenisation temperature distribution between the strombolicchio and paleostromboli ii samples the trapping pressures of the fluid inclusions have been obtained by combining the microthermometric data of the strombolicchio and paleostromboli ii samples with the pressure temperature volume i e density characteristics for a pure co2 system the data on the early inclusions type i suggest an important magma rest at a pressure of about 290 mpa i e about 11 km depth type iia co2 inclusions suggest that a second magma rest occurred at a pressure of about 100 mpa i e about 3 5 km depth whereas type iib inclusions were trapped later at a shallower depth during the final magma upwelling no pressure depth differences seem to occur between the strombolicchio and paleostromboli ii periods indicating the same polybaric rests for the calc alkaline magmas of stromboli despite their significantly different ages this persistence in magma stagnation conditions from 200 to 60 ka suggests a similar plumbing system for the present day strombolian activity

faulting assisted lateral collapses and influence on shallow magma feeding system at ollague volcano central volcanic zone chile bolivia andes abstract the role played by the regional tectonics and relationships between volcanic structures and their shallow magmatic feeding system are of paramount importance in understanding multiple lateral collapse events and thus for hazard assessment in active volcanoes ollague 21 18 s 68 11 w andean central volcanic zone chile bolivia border is an active andesite dacite composite volcano developed on a regional nw striking extensional fault system that bisects the volcanic edifice geological and stratigraphic studies on the summit part of the cone together with new 40ar 39ar dating and petrographic and geochemical major trace elements analyses have allowed us to delineate the volcanic history of ollague and recognize multiple deformation and lateral collapse events the evolution of ollague appears to be characterized by four stages of volcano building separated by three main events of deformation of the cone these deformation events developed progressively by downthrow of the sw sector of the volcano 800 ka along the nw striking normal fault propagating from the substrate to a final catastrophic failure 300 ka of the sw sector and debris avalanche emplacement both nw trending regional tectonics and weakness of the edifice caused by the successive deformation and collapse events are responsible for both the geometric variations of the shallow magma feeding system and the coexistence of summit and parasitic dacitic vents central and lateral vents shifted in a sw trending direction i e perpendicular to the tectonic trend and along the axis of the collapse movement and developed along a nw trending alignment i e parallel to the main tectonic trend and collapse scarps

kaguyak dome field and its holocene caldera alaska peninsula abstract kaguyak caldera lies in a remote corner of katmai national park 375 km sw of anchorage alaska the 2 5 by 3 km caldera collapsed 5 8 0 2 ka 14c age during emplacement of a radial apron of poorly pumiceous crystal rich dacitic pyroclastic flows 61 67 sio2 proximal pumice fall deposits are thin and sparsely preserved but an oxidized coignimbrite ash is found as far as the valley of ten thousand smokes 80 km southwest postcaldera events include filling the 150 m deep caldera lake emplacement of two intracaldera domes 61 5 64 5 sio2 and phreatic ejection of lakefloor sediments onto the caldera rim co2 and h2s bubble up through the lake weakly but widely geochemical analyses n 148 including pre and post caldera lavas 53 74 sio2 define one of the lowest k arc suites in alaska the precaldera edifice was not a stratocone but was instead nine contiguous but discrete clusters of lava domes themselves stacks of rhyolite to basalt exogenous lobes and flows four extracaldera clusters are mid to late pleistocene but the other five are younger than 60 ka were truncated by the collapse and now make up the steep inner walls the climactic ignimbrite was preceded by 200 years by radial emplacement of a 100 m thick sheet of block rich glassy lava breccia 62 65 5 sio2 filling the notches between the truncated dome clusters the breccia now makes up three segments of the steep caldera wall which beheads gullies incised into the breccia deposit prior to caldera formation they were probably shed by a large lava dome extruding where the lake is today

a past giant lateral collapse and present day flank instability of fogo cape verde islands fogo island is a large and extremely steepsided oceanic island volcano in the cape verde archipelago it has a large ca 9 km across east facing summit collapse structure the monte amarelo collapse with a probable volume of at least 150 200 km3 for most of its history the monte amarelo volcano had a small but productive central vent complex and radial rift zones fed by laterally propagating dykes shortly before the collapse the latter were replaced by north south trending arrays of en echelon vertically propagating dykes since the monte amarelo collapse the scar has partly filled with a new volcano the cha das caldeiras volcano the summit cone of this volcano the pico do fogo is a very young feature but has been abandoned in the most recent phase of activity from the 18th century onwards the same period has also seen the abandonment of earlier radial rift zones with laterally propagating dykes and their replacement with en echelon arrays of vents fed by vertically propagating dykes these form an n s trending array within the older collapse structure and are associated with seaward displacement of the eastern flank of the volcano within the old collapse structure the most recent eruptions those of 1951 and 1995 appear to be associated with episodes of flank instability manifested in n s surface fissuring and east facing normal faults these recent structural changes in the volcano parallel those which took place in the monte amarelo volcano prior to its collapse

the magmatic feeding system of el reventador volcano sub andean zone ecuador constrained by texture mineralogy and thermobarometry of the 2002 erupted products abstract after a 26 years long quiescence el reventador an active volcano of the rear arc zone of ecuador entered a new eruptive cycle which lasted from 3 november to mid december 2002 the initial sub plinian activity vei 4 with andesite pyroclastic falls and flows shifted on 6 and 21 november to an effusive stage characterized by the emission of two lava flows andesite to low silica andesite lava 1 and basaltic andesite lava 2 containing abundant gabbro cumulates the erupted products are medium to high k calc alkaline and were investigated with respect to major element oxides mineral chemistry texture and thermobarometry inferred pre eruptive magmatic processes are dominated by the intrusion of a high t mafic magma possibly up to 1165 15 c into an andesite reservoir acting as magma mixing and trigger for the eruption before this refilling the andesite magma chamber was characterized by water content of 5 3 1 0 high oxygen fugacity nno 2 and temperatures in the upper and lower part of the reservoir of 850 and 952 65 c respectively accurate amphibole based barometry constrains the magma chamber depth between 8 2 and 11 3 km 2 2 km the 6 october 2002 seismic swarm hypocenters from 10 to 11 km preceding el reventador eruption supports the intrusion of magmas at these depths the widespread occurrence of disequilibrium features in most of the andesites e g complex mineral zoning and phase overgrowths indicates that convective self mixing have been operating together with fractional crystallization inferred from the cognate gabbro cumulates before the injection of the basic magma which then gave rise to basaltic andesite and low silica andesite hybrid layers magma mixing in the shallow chamber is inferred from the anomalous sio 2 al 2 o 3 whole rock pattern and strong olivine disequilibria both lavas show three types of amphibole breakdown rims mainly due to heating mixing processes and or relatively slow syn eruptive ascent rate decompression of the magmas the lack of any disequilibrium textures in the pumices of the 3 november fall deposit suggest that pre eruptive mixing did not occur in the roof zone of the chamber a model of the subvolcanic feeding system of el reventador consistent with the intrusion of a low al 2 o 3 crystal rich basic magma into an already self mixed andesite shallow reservoir is here proposed it is also inferred that before entering the shallow chamber the basaltic magma underwent a polybaric crystallization at deeper crustal levels

regional stress field in the area of stromboli italy insights into structural data and crustal tectonic earthquakes abstract crustal tectonic seismicity on stromboli is here discussed along with the regional tectonic and geodynamic context of the aeolian archipelago italy the aim of this paper is to contribute to the reduction of the crucial gap in information concerning the tectonic activity originating at the volcano stromboli in the last decade it is indeed well known that strombolian eruptions related to the persistent state of activity on this volcano allow a fast collection of seismic data which are mainly represented by explosion quakes and tremor however only some sporadic information concerns tectonic earthquakes due to their low occurrence rate a catalog of 150 crustal tectonic earthquakes recorded at stromboli from 1985 to 1996 is presented here the analyses of the seismograms were carried out on the basis of the records of the station str located in the southwestern part of the island the magnitude considered ranged from 1 5 to 3 7 hypocentral location indicates foci at less than 12 km although for the majority of the earthquakes which are not recorded at the seismic stations deployed on the other islands of the aeolian archipelago a shallower within a few kilometers depth can be hypothesized the strain release associated with the 150 tectonic earthquakes of our catalog was also evaluated the low levels of occurrence and seismic energy release reached throughout the ca twelve years analyzed here confirm the minor role played by processes of brittle failure of rocks this aspect of seismic activity is nevertheless important as it can provide a contribution to the knowledge of the dynamics of this volcano the focal mechanisms available integrated with data from the literature show the t axes oriented about ne sw and nw se these findings are consistent with the deformative and tectonic characteristics of this volcanic edifice and of the aeolian archipelago which shows the same two directions of extension this is in agreement with subduction which is very old or which passively evolves towards the final stages and is controlled by the transform activity of the aeolian tindari giardini fault system

eruptive history of south sister oregon cascades abstract south sister is southernmost and highest of the three sisters three geologically dissimilar stratovolcanoes that together form a spectacular 20 km reach along the cascade crest in oregon north sister is a monotonously mafic edifice as old as middle pleistocene middle sister a basalt andesite dacite cone built between 48 and 14 ka and south sister is a basalt free edifice that alternated rhyolitic and intermediate modes from 50 ka to 2 ka largely contemporaneous with middle sister detailed mapping 330 chemical analyses and 42 radioisotopic ages show that the oldest exposed south sister lavas were initially rhyolitic 50 ka by 37 ka rhyolitic lava flows and domes 72 74 sio2 began alternating with radially emplaced dacite 63 68 sio2 and andesite 59 63 sio2 lava flows construction of a broad cone of silicic andesite dacite 61 64 sio2 culminated 30 ka in a dominantly explosive sequence that began with crater forming andesitic eruptions that left fragmental deposits at least 200 m thick this was followed at 27 ka by growth of a steeply dipping summit cone of agglutinate dominated andesite 56 60 5 sio2 and formation of a summit crater 800 m wide this crater was soon filled and overtopped by a thick dacite lava flow and then by 150 m of dacitic pyroclastic ejecta small volume dacite lavas 63 67 sio2 locally cap the pyroclastic pile a final sheet of mafic agglutinate 54 56 sio2 the most mafic product of south sister erupted from and drapes the small 300 m wide present day summit crater ending a summit building sequence that lasted until 22 ka a 20 kyr long hiatus was broken by rhyolite eruptions that produced 1 the rock mesa coulee tephra and satellite domelets 73 5 sio2 and 2 the devils chain of 20 domes and short coulees 72 3 72 8 sio2 from n s vent alignments on south sister s flanks the compositional reversal from mafic summit agglutinate to recent rhyolites epitomizes the frequently changing compositional modes of the south sister locus throughout its lifetime south sister is part of a reach of the cascades unusually active in the last 50 kyr characterized by high vent density n s vent alignments and numerous eruptive units of true rhyolite 72 sio2 that distinguishes it from much of the quaternary cascade arc these are eruptive expressions of the complex confluence of arc and intraplate magmatic tectonic regimes

volcano tectonic and geochemical evolution of an oceanic intra plate volcano tahiti nui french polynesia abstract geological mapping accurate k ar dating and geochemical analyses of lavas allow a detailed reconstruction of the geological history of tahiti nui island french polynesia the exposed volcanic activity is first characterized by the construction of a main shield from 1 4 ma to 870 ka with a maximum aerial eruptive rate around 2 km3 kyr lavas from this early building stage are alkaline slightly silica undersaturated with 87sr 86sr and 143nd 144nd compositions rather constant and close to the enriched mantle ii type vent locations were first concentrated along a main e w rift zone which was responsible for the lateral collapse of the northern and southern flanks of the main shield around 0 87 ma ago the subsequent activity was first restricted to the northern depression corresponding to an eruptive rate of about 5 km3 kyr in the period 850 760 ka significant variations in la sm 87sr 86sr and 143nd 144nd occur in lavas erupted immediately after the main northern landslide indicating a sudden increase in the extent of partial melting likely caused by the decompression subsequent to collapse however the later activity declined and the lavas exhibit a gradual change toward strongly silica undersaturated basanites likely indicating a decreasing extent of partial melting of the upper mantle the evolution of radiogenic isotope ratios over time indicates a change in the source toward more depleted compositions until around 500 ka post erosion volcanic activity following an apparent hiatus of 240 kyr exhibits similar major and trace element and isotope compositions the volcano structural and geochemical evolution of tahiti nui and the overall alkaline character of the lavas from other eruptive complexes of the society alignment suggest a relative weakness low temperature and low eruptive rate of the society plume compared to the hawaiian hot spot

volcano load control on dyke propagation and vent distribution insights from analogue modeling 1 the spatial distribution of eruptive vents around volcanoes can be complex and evolve as a volcano grows observations of vent distribution at contrasting volcanoes from scoria cones to large shields show that peripheral eruptive vents concentrate close to the volcano base we use analogue experiments to explore the control of volcano load on magma ascent and on vent location results show that the local loading stress field favors eruption of rising magma away from the volcano summit if a central conduit is not established or is blocked two sets of scaled experiments are developed with contrasting rheological properties to analyze similarities and differences in simulated magma rise below a volcano 1 golden syrup magma analogue is injected into a sand plaster mixed layer crust analogue under a cone 2 water or air magma analogues is injected into gelatin under a sand cone rising dykes approaching the cone stress field are stopped by the load compressive stress with continued intrusion dyke overpressure builds up dykes extend laterally until their tips are able to rise vertically again and to erupt in the flank or at the base of the volcano lateral offset of the extrusion point relative to the edifice summit depends on substratum thickness volcano slope and dyke overpressure the 3d geometry of golden syrup intrusions varies with experimental parameters from cylindrical conduits to dyke and sill complexes experimental results are compared with illustrative field cases and with previously published numerical models this comparison enables applications and limitations of the analogue models to be highlighted and allows us to propose a conceptual model for the evolution of vent distribution with volcano growth