on estimating crystal shape for crystal size distribution analysis abstract the accurate determination of three dimensional crystal habit long intermediate short axis is a requirement to enable correct stereological conversion of 2d crystal size measurements to true 3d crystal size distributions csds in this contribution we introduce a database and spreadsheet program which provides an objective estimate of true crystal habit from raw 2d measurements the database compares the sample s 2d measurements with 2d shape curves for random sections through 703 different habits the output gives the five best match curves and corresponding crystal habits based on a least squares fit between sample and database the minimum sample size required to give good shape estimates from 2d data is tested using random runs of sections through known shapes with different population densities at minimum 75 crystal sections are required to robustly determine crystal habit for csd measurements if crystals are tabular in shape with more acicular shapes requiring a minimum of 250 sections suggesting a sample size of 250 sections to be used for shape determination in natural examples where the true 3d shape is unknown the crystal habit of the best match shape can then be used to provide a more robust csd analysis which in turn can be used to investigate magmatic processes with more certainty

correlations of volcanic ash texture with explosion earthquakes from vulcanian eruptions at sakurajima volcano japan abstract we compare the texture of volcanic ash with the maximum amplitude of explosion earthquakes a eq for vulcanian eruptions from sakurajima volcano we analyze the volcanic ash emitted by 17 vulcanian eruptions from 1974 to 1987 using a stereoscopic microscope we classify the glassy particles into smooth surface particles s type particles and non smooth surface particles ns type particles according to their surface conditions gloss or non gloss appearance as an indicator of the freshness of the particles s type particles are further classified into v type particles those including vesicles and nv type particles those without vesicles by means of examinations under a polarized microscopic of polished thin sections cross correlated examinations against seismological data show that 1 the number fraction of s type particles s fraction has a positive correlation with a eq 2 the number ratio of nv type particles to v type particles the n v number ratio has a positive correlation with a eq and 3 for explosions accompanied with bl type earthquake swarms the n v number ratio has a negative correlation with the duration of the bl swarms bl swarms refer to the phenomenon of numerous bl type earthquakes occurring within a few days prior to an increase in explosive activity kamo k 1978 some phenomena before the summit crater eruptions at sakura zima volcano bull volcanol soc japan 23 53 64 the positive correlation between the n v number ratio and a eq could indicate that a large amount of separated gas from fresh magma results in a large a eq plagioclase microlite textual analysis of nv type particles from five explosive events without bl swarms shows that the plagioclase microlite number density mnd and the l w length width ratio have a positive correlation with a eq a comparison between textural data mnd l w ratio crystallinity and the result of a decompression induced crystallization experiment couch s sparks r s j carroll m r 2003 the kinetics of degassing induced crystallization at soufriere hills volcano montserrat j petrol 44 1477 1502 suggests that a plagioclase microlite texture of volcanic ash from eruptions without bl swarms could be generated by a decompression of 100 160 mpa if the mnd is controlled by the water exsolution rate from melt the positive correlation between the mnd and a eq may suggest that a eq becomes large when the effective decompression is large and the water exsolution rate is high from 6 2 10 5 to 1 9 10 4 wt s the estimated magma ascent rate ranges from 0 11 to 0 35 m s which is one order of magnitude faster than that of an effusive eruption and one to three orders slower than those for a sub plinian eruption this suggests that the ascent rate of magma plays an important role in the occurrence of vulcanian eruptions we propose a simple model for vulcanian eruptions at sakurajima volcano that takes into account the correlation between the s fraction and a eq

expanding the tephrostratigraphical framework for the south shetland islands antarctica by combining compositional and textural tephra characterisation abstract tephra layers preserved in lake sedimentary sequences provide valuable isochrons with which to synchronise palaeoclimatic records however in regions where tephra inputs are dominated by a single volcanic source overlapping chemical compositions can preclude unambiguous correlation of tephra layers in this study we characterise multiple visible macrotephra layers within sedimentary sequences from three lakes in byers peninsula antarctica by combining compositional analyses with additional constraints from textural componentry we identify three distinct tephra isochrons t1 t2 and t3 each with distinct textural properties the relative proportion of glassy compared to crystal rich grains varies from 50 t3 to 3 t1 of the total sample although the proportion of dense to vesicular grains differs only slightly between all sampled tephra layers the dominant vesicle shape varies from spherical t3 to irregular and polylobate t1 t2 these textural differences can be related to variations in the eruptive processes occurring at the deception island source volcano this study highlights the efficacy of a correlative approach based on both chemical and physical tephra properties for deconvolving the tephra stratigraphy in regions where chemical compositions are non unique

deep sea fragmentation style of havre revealed by dendrogrammatic analyses of particle morphometry in 2012 the eruption of deep sea volcano havre produced an abundance of fine ash at a depth of 1000 m below sea level in this study the 2d shapes of havre ash grains retrieved from the seafloor were compared quantitatively with those of particles generated in a suite of different fragmentation experiments which used remelted rhyolitic rock and pumice from the eruption site a new statistical data analysis technique denoted as dendrogrammatic analysis of particle morphology dapm is introduced it is designed to compare large numbers of morphometric data sets containing shape information for a set of ash particles to group them by morphological similarities and to visualize these clusters in a dendrogram further steps involve t tests and equivalence tests and reveal morphometric differences as well as matching features the dapm suggests that the majority of havre ash was thermohydraulically produced by induced fuel coolant interaction a subset of ash particles features an elongated tube morphology their morphometry matches that of particles that were experimentally produced by a combination of shearing and quenching and we infer that the natural particles were formed by synextrusive ash venting

ash erupted during normal activity at stromboli aeolian islands italy raises questions on how the feeding system works ash fallout collected during 4 days of sampling at stromboli confirms that a crystal rich hp degassed magma erupts during the strombolian explosions that are characteristic of the normal activity of this volcano we identified 3 different types of juvenile ash fragments fluidal spongy and dense which formed through different mechanisms of fragmentation of the low viscosity physically heterogeneous in terms of the size and spatial distribution of bubbles shoshonitic magma a small amount less than 3 vol of volatile rich magma with low porphyricity lp erupted as highly vesicular ash fragments has been collected together with the hp magma during normal strombolian explosions laboratory experiments and the morphological textural and compositional investigations of ash fragments reveal that the lp ash is fresh and not recycled from the last paroxysm 15 march 2007 we suggest that small droplets of lp magma are dragged to the surface by the time variable but persistent supply of deep derived co2 rich gas bubbles this coupled ascent of bubbles and lp melts is transient and does not perturb the dynamics of the hp magma within the shallow reservoir this finding provides a new perspective on how the stromboli volcano works and has important implications for monitoring strategies

an oceanographic flip flop deep water exchange in fortune bay newfoundland observations are presented of deep water exchange in a large coastal embayment on the south coast of newfoundland the bay has several outer sills not unlike those found in fjords outside the bay two submerged channels on the continental shelf are reservoirs for two different deep water masses which may flow over separate sills to replace deep water within the bay the deep water is renewed approximately twice a year on average exchange occurs both during the half year period centered on midwinter and during the half year period centered on late summer with the water properties of the inflow in each period being dominated by one of the two source water masses in the shelf channels the system therefore occupies one of two renewal states for much of the year the transition between states appears to be induced largely by seasonal variations in the wind forcing over the shelf

mauna loa s submarine western flank landsliding deep volcanic spreading and hydrothermal alteration four new remotely operated vehicle dives carried out by monterey bay aquarium research institute mbari reveal a heterogeneous distribution of lithologies and compositions along a transect across the submarine west flank of mauna loa from the outer scarp of the frontal bench to the upper flank the frontal bench is composed predominantly of volcaniclastic sediments ranging from very fine grained monomictic hyaloclastites to coarse grained compositionally mixed volcaniclastic breccias the predominance of subaerially derived clasts suggests accumulations of landslide deposits probably emplaced along a regional shear plane preserved in cataclastic breccias with local foliations and grain trails repeated packages of inversely graded strata are interpreted to reflect thrust imbrication of the resulting volcaniclastic apron during volcanic spreading of mauna loa s western flank similar to that now documented along k lauea s south flank many of the rocks from the bench show evidence for alteration ranging from low grade burial diagenesis to higher grade hydrothermal alteration including phases never before observed in submarine hawaiian rocks including epidote talc sphene and corrensite alteration is concentrated in deformed zones denoting pathways for fluid flow into or out of the volcanic edifice formed at depth the altered rocks were subsequently transported along low angle thrust faults into the bench and exposed along high angle fractures and faults the upper submarine flanks are draped by subaerially erupted submarine emplaced pillow lavas and interbedded hyaloclastites generated by shoreline crossing lava flows basalt glasses indicate mauna loa origin but imply earlier compositions than present day lavas consistent with ar ar ages suggesting eruption 0 28 0 10 ma late stage detachment of a nearshore slump produced the lika 2 debris avalanche that broke through the frontal bench perhaps portending the evolution of the active hilina slump now present on k lauea volcano s south flank

giant blocks in the south kona landslide hawaii a large field of blocky sea floor hills up to 10 km long and 500 m high are gigantic slide blocks derived from the west flank of mauna loa volcano on the island of hawaii these megablocks are embedded in the toe of the south kona landslide which extends 80 km seaward from the present coastline to depths of nearly 5 km a 10 15 km wide belt of numerous smaller 1 3 km long slide blocks separates the area of giant blocks from two submarine benches at depths of 2600 and 3700 m depth that terminate seaward 20 to 30 km from the shoreline similar giant blocks are found on several other major submarine hawaiian landslides including those north of oahu and molokai but the south kona blocks are the first to be examined in detail using high resolution bathymetry dredging and submersible diving dredging of two of the giant blocks brought up pillowed tholeiitic lava observations from the u s navy submersible sea cliff on the asymmetrically steep eastern flank of one block 10 km long and 300 m high revealed a succession of fractured massive basalt laminar lava flows hyaloclastite and pillow lavas chemical analyses of dredged lava identified 19 units that overlap compositionally with lavas from the south rift zone ridge of mauna loa sulfur content indicates that most of the lavas were erupted in subaerial and shallow submarine

petrologic constraints on the decompression history of magma prior to vulcanian explosions at the soufriere hills volcano montserrat abstract a series of 88 vulcanian explosions occurred at the soufriere hills volcano montserrat between august and october 1997 conduit conditions conducive to creating these and other vulcanian explosions were explored via analysis of eruptive products and one dimensional numerical modeling of magma ascent through a cylindrical conduit the number densities and textures of plagioclase microlites were documented for twenty three samples from the events the natural samples all show very high number densities of microlites and 50 by number of microlites have areas 2 pre explosion conduit conditions and decompression history have been inferred from these data by comparison with experimental decompressions of similar groundmass compositions our comparisons suggest quench pressures 13 75 mpa each during ascent from chamber to surface values are consistent with field studies of the same events and statistical analysis of explosion time series data the microlite volume number density trend with depth reveals an apparent transition from growth dominated crystallization to nucleation dominated crystallization at pressures of 7 mpa and lower a concurrent sharp increase in bulk density marks the onset of significant open system degassing apparently due to a large increase in system permeability above 70 vesicularity this open system degassing results in a dense plug which eventually seals the conduit and forms conditions favorable to vulcanian explosions the corresponding inferred depth of overpressure at 250 700 m near the base of the dense plug is consistent with depth to center of pressure estimated from deformation measurements here we also illustrate that one dimensional models representing ascent of a degassing crystal rich magma are broadly consistent with conduit profiles constructed via our petrologic analysis the comparison between models and petrologic data suggests that the dense conduit plug forms as a result of high overpressure and open system degassing through conduit walls

towards fast and routine analyses of volcanic ash morphometry for eruption surveillance applications the morphometry of volcanic ash produced by explosive eruptions yields ample information on fragmentation processes e g magmatic vs magma water interactions and on transport and sedimentation mechanisms most previous works on volcanic clast morphometry focused on the apparent 2d projected shape of ash grains here termed apash to infer processes and eruptive styles however textural analyses of ash grains has remained a long and tedious task that made such approaches inappropriate for eruption surveillance duties in this work we show that new technological advances on automated dispersion of granular materials imaged with a camera coupled microscope and enhanced computer capabilities enable fast and high resolution image acquisition of thousands of ash grains that resolve this limitation with a morpho grainsizer designed for such fast and routine measurements we perform a series of apash analyses on selected ash fractions of tephra deposits from known eruptive styles we record the size aspect ratio circularity and convexity of apash images and assess resolution reproducibility minimum population size and total analytical duration and offer recommendations for the reporting of apash data for inter laboratory comparisons to avoid fractal geometry concerns our analyses are carried out at constant size range 250 300 m and optical magnification 5 on 3000 grains per samples collected from homogenized samples results from the andesitic 1999 ongoing eruption of tungurahua volcano ecuador show that ash particles from the moderate 2001 phase are relatively equant and convex in shape while the stronger 2006 subplinian phase produced ash grains with more elongated less circular and less convex apash signatures ash grains from a basaltic scoria cone forming eruption show even more ragged apash characteristics overall our protocol allows obtaining accurate and reproducible morphometric measurements that reveal subtle variations of the morphological signature and the short duration 1 8 hours of the whole analytical process renders high resolution analyses of ash shape achievable for volcano surveillance applications this research ultimately aims to set up a morphometric database of apash results for well defined eruptive styles in order to interpret on a short term basis any apash data from active volcanoes for monitoring purposes

rheological change and degassing during a trachytic vulcanian eruption at kilian volcano chaine des puys france magma ascent during silicic dome forming eruptions is characterized by significant changes in magma viscosity permeability and gas overpressure in the conduit these changes depend on a set of parameters such as ascent rate outgassing and crystallization efficiency and magma viscosity which in turn may influence the prevailing conditions for effusive versus explosive activity here we combine chemical and textural analyses of tephra with viscosity models to provide a better understanding of the effusive explosive transitions during vulcanian phases of the 9 4 ka eruption of kilian volcano chaine des puys france our results suggest that effusive activity at the onset of vulcanian episodes at kilian volcano was promoted by i rapid ascent of initially crystal poor and volatile rich trachytic magma ii a substantial bulk and melt viscosity increase driven by extensive volatile loss and crystallization and iii efficient degassing outgassing in a crystal rich magma at shallow depths trachytic magma repeatedly replenished the upper conduit and variations in the amount of decompression and cooling caused vertical textural stratification leading to variable degrees of crystallization and outgassing outgassing promoted effusive dome growth and occurred via gas percolation through large interconnected vesicles fractures and tuffisite veins fostering the formation of cristobalite in the carapace and talus regions build up of overpressure was likely caused by closing of pore space bubbles and fractures in the dome through a combination of pore collapse cristobalite formation sintering in tuffisite veins and limited pre fragmentation coalescence in the dome or underlying hot vesicular magma sealing of the carapace may have caused a transition from open to closed system degassing and to renewed explosive activity we generalize our findings to propose that the broad spectrum of eruptive styles for trachytic magmas may be inherited from a combination of characteristics of trachytic melts that include high water solubility and diffusivity rapid microlite growth and low melt viscosity compared to their more evolved subalkaline dacitic and rhyolitic equivalents we show that trachytes may erupt with a similar style e g vulcanian but at significantly higher ascent rates than their andesitic dacitic and rhyolitic counterparts this suggests that the periodicity of effusive explosive transitions at trachytic volcanoes may differ from that observed at the well monitored andesitic dacitic and rhyolitic volcanoes which has implications for hazard assessment associated with trachytic eruptions

furthering the investigation of eruption styles through quantitative shape analyses of volcanic ash particles abstract volcanic ash morphology has been quantitatively investigated for various aims such as studying the settling velocity of ash for modelling purposes and understanding the fragmentation processes at the origin of explosive eruptions in an attempt to investigate the usefulness of ash morphometry for monitoring purposes we analyzed the shape of volcanic ash particles through a combination of 1 traditional shape descriptors such as solidity convexity axial ratio and form factor and 2 fractal analysis using the euclidean distance transform edt method we compare ash samples from the hydrothermal eruptions of iwodake japan in 2013 tangkuban perahu indonesia in 2013 and marapi sumatra indonesia in 2015 the dome explosions of merapi java indonesia in 2013 the vulcanian eruptions of merapi in 2010 and tavurvur rabaul papaua new guinea in 2014 and the plinian eruption of kelud indonesia in 2014 particle size and shape measurements were acquired from a particle size analyzer with a microscope camera attached to the instrument clear differences between dense blocky particles from hydrothermal or dome explosions and vesicular particles produced by the fragmentation of gas bearing molten magma are well highlighted by conventional shape descriptors and the fractal method in addition subtle differences between dense blocky particles produced by hydrothermal explosions dome explosions or quench granulation during phreatomagmatic eruptions can be evidenced with the fractal method the combination of shape descriptors and fractal analysis is therefore potentially able to distinguish between juvenile and non juvenile magma which is of importance for eruption monitoring

the ninole basalt implications for the structural evolution of mauna loa volcano hawaii lava flows of the ninole basalt the oldest rocks exposed on the south side of the island of hawaii provide age and compositional constraints on the evolution of mauna loa volcano and the southeastward age progression of hawaiian volcanism although the tholeiitic ninole basalt differs from historic lavas of mauna loa volcano in most major element contents e g variably lower k na si higher al fe ti ca ree and other relatively immobile minor elements are similar to historic and prehistoric mauna loa lavas and the present major element differences are mainly due to incipient weathering in the tropical environment new k ar whole rock ages from relatively fresh roadcut samples suggest that the age of the ninole basalt is approximately 0 1 0 2 ma although resolution is poor because of low contents of k and radiogenic ar originally considered the remnants of a separate volcano the ninole hills are here interpreted as faulted remnants of the old south flank of mauna loa deep canyons in the ninole hills eroded after massive landslide failure of flanks of the southwest rift zone have been preserved from burial by younger lava due to westward migration of the rift zone landslide induced depressurization of the southwest rift zone may also have induced phreatomagmatic eruptions that could have deposited widespread basaltic ash that overlies the ninole basalt subaerial presence of the ninole basalt documents that the southern part of hawaii island had grown to much of its present size above sea level by 0 1 0 2 ma and places significant limits on subsequent enlargement of the south flank of mauna loa

crustal subsidence rate off hawaii determined from 234u 238u ages of drowned coral reefs a series of submerged coral reefs off northwestern hawaii was formed during largely glacial intervals when the rate of local sea level rise was less than the maximum upward growth rate of the reefs mass spectrometric sup 234 u sup 238 u ages for samples from six such reefs range from 17 to 475 ka and indicate that this part of the hawaiian ridge has been subsiding at a roughly uniform rate of 2 6 mm yr for the past 475 ka the sup 234 u sup 238 u ages are in general agreement with model ages of reef drowning based on estimates of paleo sea level stands derived from oxygen isotope ratios of deep sea sediments but there are disagreements in detail the high attainable precision plus minus 10 ka or better on samples younger than approximately 800 ka large applicable age range relative robustness against open system behavior and ease of analysis for this technique hold great promise for future applications of dating of 50 1 000 ka coral

comment on field and seismic evaluation of the block and ash flows emplaced from eruption columns of the 2005 vulcanian explosions at volcan de colima mexico by zobin et al bull volcanol 2016 78 27

origin and depositional environment of clastic deposits in the hilo drill hole hawaii volcaniclastic units cored at depths of about 87 164 178 226 and 246 m below sea level and carbonate units located between depths of 27 and 53 m below sea level in the hilo drill core were found to be deposited at or near sea level four of these units are hydroclastic deposits formed when subaerially erupted mauna loa lava flows entered the ocean and fragmented to produce quenched glassy fragments during hydrovolcanic explosions ash units 24 and 26 at 178 m depth accumulated at sea level in a freshwater bog they contain pyroxenes crystallized from tholeiitic magma that we infer erupted explosively at the summit of kilauea volcano two carbon rich layers from these ashes have a weighted average radiocarbon age of 38 6 0 9 ka the ashes probably correlate with the oldest and thickest part of the pahala ash ash unit 44 at the transition from mauna kea to mauna loa lava flows was probably nearly 3 2 m thick and is inferred to be equivalent to the lower thick part of the composite homelani ash mapped in hilo and on the flanks of mauna kea the age of this part of homelani ash is between 128 33 and 200 10 ka it may have erupted subglacially during the pohakuloa glacial maxima on mauna kea beach sand units 12 and 22 were derived from nearby mauna loa and mauna kea lava flows the middle of beach sand unit 38 was derived mainly from lava erupted near the distal end of the subaerial east rift zone of kilauea volcano these sands were transported about 33 km northwest to hilo bay by prevailing longshore currents combined age depth and sea level markers in the core allow us to determine that lava flow recurrence intervals averaged one flow every 4 kyr during the past 86 kyr and one flow every 16 kyr between 86 and 200 ka at the drill site and that major explosive eruptions that deposit thick ash in hilo have occurred only twice in the last 400 kyr these recurrence intervals support the moderate lava flow hazard zonation zone 3 for coastal hilo previously determined from surficial mapping

an experimental study of ostwald ripening of olivine and plagioclase in silicate melts implications for the growth and size of crystals in magmas we carried out an experimental study to characterize the kinetics of ostwald ripening in the forsterite basalt system and in the plagioclase an65 andesite system eight experiments were done in each system to monitor the evolution of mean grain size and crystal size distribution csd with time t the experiments were performed in a 1 atmosphere quench furnace at 1 250 c for plagioclase and 1 300 c for olivine very contrasted coarsening kinetics were observed in the two series in the plagioclase series the mean grain size increased as log t from 3 m to only 8 7 m in 336 h the kinetic law in log t means that ostwald ripening was rate limited by surface nucleation at plagioclase liquid interfaces in the olivine series the mean grain size increased as t1 3 from 3 m to 23 2 m in 496 h a kinetic law in t1 3 is expected when ostwald ripening is rate limited either by diffusion in the liquid or by grain growth dissolution controlled by a screw dislocation mechanism the shape of olivine csds in particular their positive skewness indicates that grain coarsening in the olivine experiments was controlled by a screw dislocation mechanism not by diffusion as the degrees of undercooling t or supersaturation involved in ostwald ripening are essentially 1 c the mechanisms of crystal growth identified in our experiments are expected to be those prevailing during the slow crystallisation of large magma chambers we extrapolated our experimental data to geological time scales to estimate the effect of ostwald ripening on the size of crystals in magmas in the case of plagioclase ostwald ripening is only efficient for mean grain sizes of a few microns to 20 m even for a time scale of 105 years it can however result in a significant decrease of the number of small crystals per unit volume and contribute to the development of convex upwards csds for olivine the mean grain size increases from 2 3 m to 70 m in 1 year and 700 m in 103 years a mean grain size of 3 mm is reached in 105 years accordingly the rate of grain size dependent processes such as compaction of olivine rich cumulates or melt extraction from partially molten peridotites may significantly be enhanced by textural coarsening

dispersal of volcaniclasts during deep sea eruptions settling velocities and entrainment in buoyant seawater plumes abstract we use tank experiments to measure settling rates of deep sea volcaniclastic material recovered from the arctic 85 e gakkel ridge and pacific juan de fuca ridge loihi seamount oceans we find that clast size and shape exert a strong influence on settling velocity with velocities of 30 cm s for large 8 mm blocky clasts compared to velocities of 2 5 cm s for small

the 40ar 39ar and k ar dating of lavas from the hilo 1 km core hole hawaii scientific drilling project mauna kea lava flows cored in the hilo hole range in age from 200 ka to about 400 ka based on 40ar 39ar incremental heating and k ar analyses of 16 groundmass samples and one coexisting plagioclase the lavas all subaerially deposited include a lower section consisting only of tholeiitic basalts and an upper section of interbedded alkalic transitional tholeiitic and tholeiitic basalts the lower section has yielded predominantly complex discordant 40ar 39ar age spectra that result from mobility of 40ar and perhaps k the presence of excess 40ar and redistribution of 39ar by recoil comparison of k ar ages with 40ar 39ar integrated ages indicates that some of these samples have also lost 39ar nevertheless two plateau ages of 391 40 and 400 26 ka from deep in the hole combined with data from the upper section show that the tholeiitic section accumulated at an average rate of about 7 to 8 m kyr and has a mean recurrence interval of 0 5 kyr flow unit samples from the upper section yield relatively precise 40ar 39ar plateau and isotope correlation ages of 326 23 241 5 232 4 and 199 9 ka for depths of 415 7 m to 299 2 m within their uncertainty these ages define a linear relationship with depth with an average accumulation rate of 0 9 m kyr and an average recurrence interval of 4 8 kyr flow unit the top of the mauna kea sequence at 280 m must be older than the plateau age of 132 32 ka obtained for the basal mauna loa flow in the corehole the upward decrease in lava accumulation rate is a consequence of the decreasing magma supply available to mauna kea as it rode the pacific plate away from its magma source the hawaiian mantle plume the age depth relation in the core hole may be used to test and refine models that relate the growth of mauna kea to the thermal and compositional structure of the mantle plume

the 40ar 39ar dating of core recovered by the hawaii scientific drilling project phase 2 hilo hawaii 1 the hawaii scientific drilling project phase 2 hsdp 2 recovered core from a 3 1 km thick section through the eastern flanks of mauna loa and mauna kea volcanoes we report results of 40ar 39ar incremental heating by broad beam infrared laser of 16 basaltic groundmass samples and 1 plagioclase separate mostly from k poor tholeiites the tholeiites generally have mean radiogenic 40ar enrichments of 1 3 and some contain excess 40ar however isochron ages of glass poor samples preserve stratigraphic order in all cases a 246 m thick sequence of mauna loa tholeiitic lavas yields an isochron age of 122 86 kyr all errors 2 at its base beneath the mauna loa overlap sequence lie mauna kea s postshield and shield sequences a postshield alkalic lava yields an age of 236 16 kyr in agreement with an age of 240 14 kyr for a geochemically correlative flow in the nearby hsdp 1 core hole where more complete dating of the postshield sequence shows it to have accumulated at 0 9 0 4 m kyr from about 330 to 200 ka mauna kea s shield consists of subaerial tholeiitic flows to a depth of 1079 m below sea level then shallow submarine flows hyaloclastites pillow lavas and minor intrusions to core bottom at 3098 m most subaerial tholeiitic flows fail to form isochrons however a sample at 984 m yields an age of 370 180 kyr consistent with ages from similar levels in hsdp 1 submarine tholeiites including shallow marine vitrophyres clasts from hyaloclastites and pillow lavas were analyzed however only pillow lava cores from 2243 2614 and 2789 m yield reliable ages of 482 67 560 150 and 683 82 kyr respectively a linear fit to ages for shield samples defines a mean accumulation rate of 8 6 3 1 m kyr and extrapolates to 635 kyr at core bottom alternatively a model relating mauna kea s growth to transport across the hawaiian hot spot that predicts downward accelerating accumulation rates that reach 20 m kyr at core bottom depaolo and stolper 1996 is also consistent with all reliable ages except the deepest

crystal size distribution analysis of plagioclase in experimentally decompressed hydrous rhyodacite magma article i nfo this study presents crystal size distributions csd of plagioclase forming during decompression experiments of hydrous rhyodacite magma samples were annealed at 130 mpa subjected to continuous decompression at either 2 mpa h 1 or 0 5 mpa h 1 and then quenched along their respective decompression paths at 20 mpa intervals samples display concave up curved csd plots which result from a combination of two crystal populations those which formed during the anneal period and those which formed during decompression the csds also display a decrease in crystal number density at 3 7 m a size range that is easily resolvable with the imaging strategy employed the downturn at small sizes is presumably due to insufficient compensation for the intersection probability effect in converting 2d measurements to 3d size distributions crystal nucleation and growth rates derived from csds using standard assumptions are compared with values obtained using 2d measurements of bulk crystal populations batch methods csd calculated nucleation rates are substantially low relative to the batch values in fact csd derived volumetric nucleation rates may underestimate actual nucleation rates by up to two orders of magnitude in contrast growth rates from csds are consistently higher than batch rates although plagioclase growth rates are relatively constant during decompression at a given rate the average growth rate in the rapidly decompressed series is approximately five times faster than the crystal growth rate in the slowly decompressed series because crystal growth rate depends on decompression rate csds are incapable of revealing decompression timescales or magma ascent rates without independent knowledge of crystal growth rate published by elsevier b v

temporal variation in volcanic ash texture during a vulcanian eruption at the sakurajima volcano japan abstract we discuss the process of vulcanian eruptions on the basis of temporal variation of the texture of andesitic volcanic ash ejected from the sakurajima volcano japan falling ash samples were collected every 2 50 min for petrological analysis these ash samples contained juvenile particles altered rock fragments and fragments of crystals here we subdivide the juvenile particles into blocky vesicular and fluidal particles because the crystallinities of the vesicular and fluidal particles are lower than those of the blocky particles we refer to the vesicular and fluidal particles as low crystallinity particles lcps and the blocky particles as high crystallinity particles hcps the temporal variation observed in ash texture reveals that the ratio of lcps to hcps lcp hcp was small at the initiation of the eruption and increased as the eruption progressed the higher microlite crystallinity in the hcps indicates crystallization under lower pressures for longer durations relative to the lcp magma the increase of the lcp hcp ratio during a single vulcanian eruption suggests that the emission of less viscous gas rich magma under a cap rock increases as the eruption progresses such temporal variation in ash texture reflects the transition from a sudden explosion to continuous ash emission during a vulcanian eruption

insights on tephra settling velocity from morphological observations in this study we present a systematic and detailed morphological characterization of tephra particles from different eruptions fontana lapilli masaya nicaragua keanak ko i formation kilauea usa recent dome explosions of soufriere hills volcano montserrat and the calculation of their terminal fall velocity tfv as obtained based on different drag prediction models i e wilson and huang 1979 haider and levenspiel 1989 ganser 1993 and dellino et al 2005 in particular particle sphericity and therefore particle surface area is essential for the calculation of tfv of irregular shape particles but is of complex determination various attempts have been proposed according to our results 2d morphological characterization of volcanic particles is a fast and simple application for a wide range of particle size and provides consistent sphericity and settling velocity values 3d scanning also provides a promising tool for lapilli sized tephra 2 cm in contrast gas adsorption derived surface area is not suitable for the calculation of tfv of volcanic particles mostly because it mainly describes the surface contribution of nanometric pores that are not expected to affect significantly tfv and because bulk sample analysis is representative of neither individual particles nor of the whole particle population settling velocities calculated using values of surface area derived from gas adsorption analyses are up to two orders of magnitude lower than the values obtained through 2d analysis in addition our results also show how the influence of particle shape on tfv increases with particle size in particular calculated tfv converges at small particle sizes 3 regardless of the model applied suggesting that the spherical assumption is appropriate for this size fraction discrepancies with the spherical model are within 10 discrepancies with the spherical model increase with particle size up to about 50 and depend on the choice of both the tfv model and the morphological parameterization used in particular the drag prediction model of ganser 1993 is sensitive to the effect of particle morphology on tfv and is well suited for all sizes and reynolds numbers of typical tephra particles finally our results show how individual size categories whole and half are not associated with individual tfv values but with a range of values which increases with class size nonetheless the half system is associated with a smaller standard deviation than the whole system and is therefore more appropriate for the modeling of tephra dispersal in any case for dispersal modeling purposes it is more appropriate to indicate a range of settling velocities for each size class rather than giving an average value

viscosity of high crystal content melts dependence on solid fraction 1 the rheological properties of suspensions containing high solid fractions are investigated attention is focused on viscosity of silicate and magmatic melt systems a general empirical equation which describes the relative viscosity of suspensions as a function of suspended solid fraction is proposed in the limit of very dilute solid concentrations it reduces to the einstein equation the proposed relationship is satisfactorily applied to reproduce available experimental data relative to silicate melts moreover the extrapolation of the model to very high concentrations is compared with experimental observations on partially melted granite

decompression rate of magma at fragmentation inference from broken crystals in pumice of vulcanian eruption abstract decompression rate of magma at fragmentation immediately before or after fragmentation was determined using broken crystals found in the pumices ejected during the vulcanian explosion of sakurajima volcano japan the combined analysis of textural data obtained from the natural pumice samples and a simple model for the crystal fracturing in vesiculating viscous magma indicated a decompression rate 7 0 10 3 7 8 10 4 pa s for the ejecta produced during the later phase of vulcanian explosion this result suggests that the short duration of the vulcanian explosion is controlled by the rapid decrease of the magma ascent rate to a vent an understanding of the control mechanism of the termination of an eruption by precise prediction of the eruption process is essential for both volcanology and hazard mitigation

estimating rates of decompression from textures of erupted ash particles produced by 1999 2006 eruptions of tungurahua volcano ecuador persistent low to moderate level eruptive activity of andesitic volcanoes is difficult to monitor because small changes in magma supply rates may cause abrupt transitions in eruptive style as direct measurement of magma supply is not possible robust techniques for indirect measurements must be developed here we demonstrate that crystal textures of ash particles from 1999 to 2006 vulcanian and strombolian eruptions of tungurahua volcano ecuador provide quantitative information about the dynamics of magma ascent and eruption that is difficult to obtain from other monitoring approaches we show that the crystallinity of erupted ash particles is controlled by the magma supply rate msr ash erupted during periods of high magma supply is substantially less crystalline than during periods of low magma supply this correlation is most easily explained by efficient degassing at very low pressures 50 mpa and degassing driven crystallization controlled by the time available prior to eruption our data also suggest that the observed transition from intermittent vulcanian explosions at low msr to more continuous periods of strombolian eruptions and lava fountains at high msr can be explained by the rise of bubbles through strombolian or trapping of bubbles beneath vulcanian vent capping variably viscous and crystalline magma

initial phases of explosion earthquakes accompanying vulcanian eruptions at lokon empung volcano indonesia abstract we examine the initial phases of explosion earthquakes accompanying vulcanian eruptions at lokon empung volcano in indonesia to reveal the triggering process of explosive eruptions in 2012 2013 56 vulcanian eruptions at lokon empung were observed by our temporary observation network being comprised of four broadband seismometers and two infrasound microphones at 1 6 6 8 km from the active vent the seismic records of each explosion earthquake share almost the same waveform characteristics of initial phases consisting of a small compressional onset p phase and a subsequent large dilatational phase d phase particle orbits of both phases show straight motion from beneath the active vent which suggests that these phases are composed of a longitudinal body wave for each explosion the origin times of the p phase precedes 0 8 2 5 s before the occurrence of an explosion at the vent that are detected by infrasound data since the signal to noise ratio of the p phase is insufficient for a quantitative analysis we analyze the d phase dominating the initial phases our analysis for the signals of 0 2 1 0 hz shows the d phase are well explained by a cylindrical contraction source with a half cosine shaped time function located at 1 0 1 3 km depth beneath the active vent we also recognize that some explosions are followed by a prominent tremor that coincides with continuous ash emission et the seismic amplitudes and intensity of the d phase of events in et are larger than the explosions without accompanying tremor ex the frequency distribution of the time interval from the previous eruption is also different in the events in et and ex the implosion source in the initial phases of explosion earthquakes at several km beneath the active vent has been reported at sakurajima volcano since our result shows considerable agreement with the previous works at sakurajima both lokon empung and sakurajima may share similar initial processes of vulcanian eruptions

color measurements of volcanic ash deposits from three different styles of summit activity at sakurajima volcano japan conduit processes recorded in color of volcanic ash abstract we measured quantitatively colors of volcanic ash deposits erupted from three different styles of summit activity strombolian activity vulcanian explosions and continuous ash venting activity at sakurajima volcano from 1974 to 1985 colors of strombolian ash samples have larger yellow components of their visible spectra b values than those of explosion and continuous venting ash samples colors of explosion ash samples show larger variation in both red and yellow components of their visible spectra a and b values respectively while colors of continuous venting ash samples are in the narrow ranges within colors of explosion ash samples colors of components with lower densities than 3 1 g cm 3 groundmass and phenocrystic plagioclase obtained by magnetic and heavy liquid separation methods are similar to the unseparated bulk ash samples this result suggests that the color variations of ash deposits are mainly originated from the particles composed of groundmass the particles can be classified into three different types of particles with different vesicularity and crystallinity vesicular particle vp dense particle with vesicles dpv and dense particle without vesicles dp analytical results of component proportions chemical compositions of groundmass glasses ferrous iron contents and surface ferric materials show that 1 vp has larger yellow components of the visible spectrum b values and high ferrous iron content and is less crystallized than the dp and dpv 2 dp has larger red and yellow components of its visible spectrum a and b values respectively and involves ferric materials on the surfaces produced by oxidation process and 3 dpv has smaller red and yellow components of its visible spectrum a and b values respectively and involves less ferric materials on the ash surfaces color differences of ash deposits from three different activity styles can be explained by the different mixing ratios of vp dpv and dp during the strombolian activity the vp is a main component in the ash which is formed from relatively less degassed and crystallized magma in the vulcanian explosion and continuous ash venting activity the proportions of dpv and dp in ash are larger than that in the strombolian activity the highly crystallized dp may correspond to a vent cap and dpv to a magma below the cap the color measurements of ash deposits provide information on the pre eruptive processes at the shallower levels of a conduit

juvenile magma recognition and eruptive dynamics inferred from the analysis of ash time series the 2015 reawakening of cotopaxi volcano forecasting future activity and performing hazard assessments during the reactivation of volcanoes remain great challenges for the volcanological community on august 14 2015 cotopaxi volcano erupted for the first time in 73 years after approximately four months of precursory activity which included an increase in seismicity gas emissions and minor ground deformation here we discuss the use of near real time petrological monitoring of ash samples as a complementary aid to geophysical monitoring in order to infer eruption dynamics and evaluate possible future eruptive activity at cotopaxi twenty ash samples were collected between august 14 and november 23 2015 from a monitoring site on the west flank of the volcano these samples contain a range of grain types that we classified as hydrothermal altered lithic juvenile and free crystals the relative proportions of theses grains evolved as the eruption progressed with increasing amounts of juvenile material and a decrease in hydrothermally altered material in samples from the initial explosion juvenile grains are glassy microlite poor and contain hydrothermal minerals opal and alunite the rising magma came in contact with the hydro thermal system under confinement causing hydro magmatic explosions that cleared the upper part of the plumbing system subsequently the magmatic column produced a thermal aureole in the conduit and dried out the hydrothermal system allowing for dry eruptions magma ascent rates were low enough to allow for efficient outgassing and microlite growth constant supply of magma from below caused quasi continuous disruption of the uppermost magma volume through a combination of shear deformation and gas expansion the combination of increasing crystallinity of juvenile grains and high measured so2 flux indicate decreasing integrated magma ascent rates and clearing of the hydrothermal system along transport pathways in a system open to gas loss the near real time monitoring of ash samples combined with traditional geophysical monitoring techniques during the reawakening of cotopaxi allowed us to gain a much clearer understanding of events than when using traditional geophysical monitoring alone c 2016 elsevier b v all rights reserved

models of hawaiian volcano growth and plume structure implications of results from the hawaii scientific drilling project the shapes of typical hawaiian volcanoes are simply parameterized and a relationship is derived for the dependence of lava accumulation rates on volcano volume and volumetric growth rate the dependence of lava accumulation rate on time is derived by estimating the eruption rate of a volcano as it traverses the hawaiian plume with the eruption rate determined from a specified radial dependence of magma generation in the plume and assuming that a volcano captures melt from a circular area centered on the volcano summit the timescale of volcano growth is t 2 r plate where r is the radius of the melting zone of the circular plume and plate is the velocity of the pacific plate the growth progress of a volcano can be described by a dimensionless time t t plate 2r where t 0 is chosen to be the start of volcano growth and t 1 approximates the end of shield growth using a melt generation rate for the whole plume of 0 2 km 3 yr a plume diameter of 50 km and a plate velocity of 10 cm yr we calculate that the lifetime of a typical volcano is 1000 kyr for a volcano that traverses the axis of the plume the standard dimensions are a volume of 57 000 km 3 a summit thickness of 18 km a summit elevation of 3 6 km and a basal radius of 60 km the volcano first breaches the sea surface at t 0 22 when it has attained only 5 of its eventual volume 80 of the volume accumulates between t 0 3 and t 0 7 typical lava accumulation rates start out over 50 m kyr in the earliest stages of growth from the seafloor and level out at 35 m kyr from t 0 05 until t 0 4 from t 0 4 to t 0 9 the submarine lava accumulation rates decrease almost linearly from 35 m kyr to 0 subaerial accumulation rates are about 30 lower the lava accumulation rate is a good indicator of volcano age a volcano that passes over the plume at a distance 0 4r off to the side of the plume axis is predicted to have a volume of about 60 of the standard volcano a lifetime about 8 shorter and lava accumulation rates about 15 20 smaller the depth age data for mauna kea lavas cored by the hawaii scientific drilling project are a good fit to the model parameters used given that mauna kea appears to have crossed the plume about 15 20 km off axis the lifetime of mauna kea is estimated to be 920 kyr mauna loa is predicted to be at a stage corresponding to t 0 8 kilauea is at t 0 6 and loihi is at t 0 16 the model also allows the subsurface structure of the volcanoes the interfaces between lavas from different volcanoes to be modeled radial geochemical structure in the plume may be blurred in the lavas because the volcanoes capture magma from a sizeable cross sectional area of the plume this inference is qualitatively born out by available isotopic data the model predicts that new hawaiian volcanoes are typically initiated on the seafloor near the base of the next older volcano but generally off the older volcano s flank

characterization of the luminance and shape of ash particles at sakurajima volcano japan using ccd camera images we develop a new method for characterizing the properties of volcanic ash at the sakurajima volcano japan based on automatic processing of ccd camera images volcanic ash is studied in terms of both luminance and particle shape a monochromatic ccd camera coupled with a stereomicroscope is used to acquire digital images through three filters that pass red green or blue light on single ash particles we measure the apparent luminance corresponding to 256 tones for each color red green and blue for each pixel occupied by ash particles in the image and the average and standard deviation of the luminance the outline of each ash particle is captured from a digital image taken under transmitted light through a polarizing plate also we define a new quasi fractal dimension d qf to quantify the complexity of the ash particle outlines we examine two ash samples each including about 1000 particles which were erupted from the showa crater of the sakurajima volcano japan on february 09 2009 and january 13 2010 the apparent luminance of each ash particle shows a lognormal distribution the average luminance of the ash particles erupted in 2009 is higher than that of those erupted in 2010 which is in good agreement with the results obtained from component analysis under a binocular microscope i e the number fraction of dark juvenile particles is lower for the 2009 sample the standard deviations of apparent luminance have two peaks in the histogram and the quasi fractal dimensions show different frequency distributions between the two samples these features are not recognized in the results of conventional qualitative classification criteria or the sphericity of the particle outlines our method can characterize and distinguish ash samples even for ash particles that have gradual property changes and is complementary to component analysis this method also enables the relatively fast and systematic analysis of ash samples that is required for petrologic monitoring of ongoing activity such as at the sakurajima volcano

reef growth and volcanism on the submarine southwest rift zone of mauna loa hawaii a marine sampling program utilizing the pisces 5 submersible operated by the hawaii undersea research laboratory noaa has confirmed the presence of a major submerged coral reef offshore from ka lae south point hawaii the top of the reef is now 150 160 m below sea level radiocarbon and useries dating indicates that it drowned about 13 9 ka by the combined effects of island subsidence 2 5 mm year and the rapid rise of sea level at the end of the last glaciation so that the relative submergence rate of more than 10 mm year exceeded the upward growth rate of the reef the submerged reef caps the offshore part of the southwest rift zone ridge of mauna loa which has apparently undergone little volcanic activity offshore since 170 ka and possibly since 270 ka this fact suggests that rift zone activity is becoming increasingly restricted toward the upper part of the volcano a condition possibly heralding the end of the shield building stage

prodigious submarine landslides on the hawaiian ridge the extensive area covered by major submarine mass wasting deposits on or near the hawaiian ridge has been delimited by systematic mapping of the hawaiian exclusive economic zone using the side looking sonar system gloria these surveys show that slumps and debris avalanche deposits are exposed over about 100 000 km2 of the ridge and adjacent seafloor from kauai to hawaii covering an area more than 5 times the land area of the islands some of the individual debris avalanches are more than 200 km long and about 5000 km3 in volume ranking them among the largest on earth the slope failures that produce these deposits begin early in the history of individual volcanoes when they are small submarine seamounts culminate near the end of subaerial shield building and apparently continue long after dormancy consequently landslide debris is an important element in the internal structure of the volcanoes the dynamic behavior of the volcanoes can be modulated by slope failure and the structural features of the landslides are related to elements of the volcanoes including rift zones and fault systems the landslides are of two general types slumps and debris avalanches the slumps are slow moving wide up to 110 km and thick about 10 km with transverse blocky ridges and steep toes the debris avalanches are fast moving long up to 230 km compared to width and thinner 0 05 2 km they commonly have a well defined amphitheater at their head and hummocky terrain in the lower part oceanic disturbance caused by rapid emplacement of debris avalanches may have produced high level wave deposits such as the 365 m elevation hulopoe gravel on lanai that are found on several islands most present day submarine canyons were originally carved subaerially in the upper parts of debris avalanches subaerial canyon cutting was apparently promoted by the recently steepened and stripped slopes of the landslide amphitheaters

rift zone abandonment and reconfiguration in hawaii mauna loa s ninole rift zone a new onshore offshore three dimensional seismic velocity model for the island of hawaii reveals a massive buried rift zone within mauna loa9s southeast flank introduced here as the ninole rift zone this feature extends more than 60 km south of mauna loa9s summit spans a depth range of 2 14 km below sea level and is the probable source of the 100 200 ka ninole volcanics in several prominent erosional hills the ancient rift zone may stabilize mauna loa9s southeast flank focusing recent volcanic activity and deformation onto the unbuttressed west flank the upper portion of the ninole rift zone appears to have migrated westward over time possibly triggered by landsliding causing its eventual abandonment in preference to mauna loa9s present day southwest rift zone subsequently the lower southwest rift zone broke away tracking rift intrusions along the trace of the kahuku detachment fault similar rift zone migration is thought to be under way at kilauea volcano and may one day lead to the abandonment of the east rift zone such rift zone reconfiguration is a reflection of changing stress conditions within growing volcanoes it is probably much more common than previously assumed and may enable the growth of large volcanic edifices such as mauna loa

stratigraphic constraints on the timing and emplacement of the alika 2 giant hawaiian submarine landslide abstract previous work has found evidence for giant tsunami waves that impacted the coasts of lanai molokai and other southern hawaiian islands tentatively dated at 100 and 200 ka by u series methods on uplifted coral clasts seafloor imaging and related work off hawaii island has suggested the alika phase 2 debris avalanche as the source of the 100 ka giant wave deposits although its precise age has been elusive more recently a basaltic sand bed in odp site 842 300 km west of hawaii estimated at 100 20 ka has been suggested to correlate with this or another large hawaiian landslide our approach to the timing and linkage of giant submarine landslides and paleo tsunami deposits is a detailed stratigraphic survey of pelagic deposits proximal to the landslide feature beginning with a suite of seven piston gravity and box cores collected in the vicinity of the alika 2 slide we used u series dating techniques including excess 230 th and 210 pb profiling high resolution paleomagnetic stratigraphy including continuous u channel analysis 18 o stratigraphy visual and x ray sediment lithology and the petrology and geochemistry of the included turbidites and ash layers minimum ages for the alika phase 2a slide from detailed investigation of two of the cores are 112 15 ka and 125 24 ka 2 based on excess 230 th dating a less precise age for the alika phase 1 and or south kona slide is 242 80 ka 2 consistent with previous geological estimates oxygen isotope analyses of entrained planktonic foraminifera better constrain the alika phase 2a maximum age at 127 5 ka which corresponds to the beginning of the stage 5e interglacial period it is proposed that triggering of these giant landslides may be related to climate change when wetter periods increase the possibility of groundwater intrusion and consequent phreatomagmatic eruptions of shallow magma chambers our study indicates the contemporaneity of the alika giant submarine landslides and distal deposits from enormous turbidity currents as well as coral clasts reported to be tsunami deposits on lanai and molokai through direct dating and compositional analysis of the landslide deposits

drowning of the 150 m reef off hawaii a casualty of global meltwater pulse 1a we present evidence that the drowning of the 2150 m coral reef around hawaii was caused by rapid sea level rise associated with meltwater pulse 1a mwp 1a during the last deglaciation new u th and 14 c accelerator mass spectrometry dates combined with reinterpretation of existing radiometric dates constrain the age of the coral reef to 15 2 14 7 ka u th age indicating that reef growth persisted for 4 3 k y following the end of the last glacial maximum at 19 ka the drowning age of the reef is roughly synchronous with the onset of mwp 1a between 14 7 and 14 2 ka dates from coralline algal material range from 14 to 10 cal ka calibrated radiocarbon age 1 4 k y younger than the coral ages a paleoenvironmental reconstruction incorporating all available radiometric dates high resolution bathymetry dive observations and coralgal paleobathymetry data indicates a dramatic rise in sea level around hawaii ca 14 7 ka paleowater depths over the reef crest increased rapidly above a critical depth 30 40 m drowning the shallow reef building porites corals and causing a shift to deep water coralline algal growth preserved as a crust on the drowned reef crest

crystallization kinetics in continuous decompression experiments implications for interpreting natural magma ascent processes decompression experiments were performed to study the kinetics of plagioclase crystallization during ascent of hydrous rhyodacite magma these experiments differ from previous studies in that they employ a continuous rather than stepwise pressure time trajectory four series of experiments were performed at rates of 0 5 10mpa h corresponding to ascent rates of 0 007 0 14 m s experiments quenched along each decompression path allow snapshot style monitoring of progressive crystallization as expected rates of plagioclase nucleation and growth depend strongly on decompression rate regardless of decompression rate approximately equal volumes of feldspar crystallize during a given decompression interval even when the observed crystallinity is lower than the equilibrium crystallinity apparently a constant degree of solidification is maintained by a crystallization mechanism striking a shifting balance between nuclei formation and growth of existing crystals other key results pertain to the efficacy with which experimental results allow interpretation of the decompression histories of natural rocks feldspar microlites do not maintain chemical equilibrium with melt they are more anorthite rich than equilibrium plagioclase suggesting that interpretations of magma ascent processes in nature require comparisons with dynamic rather than static phase equilibrium experiments although decompression rate is an important element controlling final textures in volcanic rocks no single compositional or textural parameter uniquely records this rate of the criteria examined microlite number density and morphology are the best indicators of magma decompression rate based on a small number of comparisons between continuous and multi step decompression style at the same time integrated decompression rate 1mpa h decompression path evidently influences crystal texture with stepwise decompression yielding textures correlative with faster decompression than indicated by the time integrated decompression rate thus consideration of magma ascent style in addition to rate will undoubtedly strengthen the interpretive power of experimental studies for constraining natural magma ascent processes

rapid passage of a small scale mantle heterogeneity through the melting regions of kilauea and mauna loa volcanoes recent kilauea and mauna loa lavas provide a snapshot of the size shape and distribution of compositional heterogeneities within the hawaiian mantle plume here we present a study of the pb sr and nd isotope ratios of two suites of young prehistoric lavas from these volcanoes 1 kilauea summit lavas erupted from ad 900 to 1400 and 2 14 c dated mauna loa flows erupted from 2580 140 yr before present relative to ad 1950 these lavas display systematic isotopic fluctuations and the kilauea lavas span the pb isotopic divide that was previously thought to exist between these two volcanoes for a brief period from ad 250 to 1400 the 206 pb 204 pb and 87 sr 86 sr isotope ratios and end values of kilauea and mauna loa lavas departed from values typical for each volcano based on historical and other young prehistoric lavas moved towards an intermediate composition and subsequently returned to typical values this is the only known period in the eruptive history of these volcanoes when such a simultaneous convergence of pb sr and nd isotope ratios has occurred the common isotopic composition of lavas erupted from both kilauea and mauna loa during this transient magmatic event was probably caused by the rapid passage of a small scale compositional heterogeneity through the melting regions of both volcanoes this heterogeneity is thought to have been either a single body 35 km long based on the distance between the summits of these volcanoes or the plume matrix itself which would be expected to be present beneath both volcanoes the time scale of this event centuries is much shorter than previously noted for variations in the isotopic composition of hawaiian lavas due to the upwelling of heterogeneities within the plume thousands to tens of thousands of years calculations based on the timing of the isotopic convergence suggest a maximum thickness for the melting region and thus the heterogeneity of 5 10 km the small size of the heterogeneity indicates that melt can be extracted from small regions within the hawaiian plume with minimal subsequent chemical modification beyond the effects of crystal

40ar 39ar geochronology of submarine mauna loa volcano hawaii 1 new geochronologic constraints refine the growth history of mauna loa volcano and enhance interpretations of the petrologic geochemical and isotopic evolution of hawaiian magmatism we report results of 40ar 39ar incremental heating experiments on low k tholeiitic lavas from the 1 6 km high kahuku landslide scarp cutting mauna loa s submarine southwest rift zone and from lavas in a deeper section of the rift obtaining precise40ar 39ar ages from young tholeiitic lavas containing only 0 2 0 3 wt k2o is challenging due to their extremely low radiogenic 40ar contents analyses of groundmass from 45 lavas yield 14 new age determinations 31 success rate with plateau and isochron ages that agree with stratigraphic constraints lavas collected from a 1250 m thick section in the landslide scarp headwall were all erupted around 470 10 ka implying an extraordinary period of accumulation of 25 mm yr possibly correlating with the peak of the shield building stage this rate is three times higher than the estimated vertical lava accumulation rate for shield building at mauna kea 8 6 3 1 mm yr based on results from the hawaii scientific drilling project between 470 and 273 ka the lava accumulation rate along the southwest rift zone decreased dramatically to 1 mm yr we propose that the marked reduction in lava accumulation rate does not mark the onset of post shield volcanism as previously suggested but rather indicates the upward migration of the magma system as mauna loa evolved from a submarine stage of growth to one that is predominantly subaerial thereby cutting off supply to the distal rift zone prior to 250 ka lavas with loihi like isotopic signatures were erupted along with lavas having typical mauna loa values implying greater heterogeneity in the plume source earlier in mauna loa s growth in addition to refining accumulation rates and the isotopic evolution of the lavas erupted along the southwest rift zone our new40ar 39ar results constrain the eruption of the ninole basalts from 227 to 108 ka and provide maximum estimates on the timing of the ka lae and south kona landslides

cooling induced crystallization of microlite crystals in two basaltic pumice clasts microlites in pumice fragments can record the rate of magma decompression and ascent but only if none grow while those fragments cool in the atmosphere for highly viscous silicate melts such crystallization is unlikely but more basic melts are known to crystallize rapidly and thus could partially crystallize during cooling and overprint decompression textures to examine whether post fragmentation crystallization can occur we examined two basaltic pumice clasts from the sub plinian april 1999 eruption of shishaldin volcano alaska radial sectioning shows that microlite content doubles from rim to core in both mainly from growth of plagioclase dendrite magnetite also increases greatly in content but only within the larger pumice clast such radial textures demonstrate that crystallization occurred after fragmentation and before deposition no welding occurred using a conductive cooling model coupled with a model for temperature in the eruption column we estimate that rims of the pumice clasts cool to their glass transition temperature in 100 200 s but their cores take 500 2000 s to cool which translates into cooling rates of 0 2 to 2 5 c s using a conservative assumption that all plagioclase nucleated before cooling began we estimate that both short and long axes grew at 4 8 2 7 x 10 7 cm s such rates match those determined experimentally for basaltic melts at similar cooling rates magnetite grew only at the slowest cooling rates and the rate of bulk magnetite crystallization equals that of plagioclase our results demonstrate that groundmass crystallization can occur in basic melts on the timescale of explosive eruptions and so pumice clasts from such eruptions must be viewed with caution before being used to infer eruption dynamics