understanding multi vent plinian eruptions at mt tongariro volcanic complex new zealand the andesitic tongariro volcanic complex in the central north island of new zealand comprises tens of vents along an actively extending graben past work has hypothesised that multiple vents may operate during a single explosive eruption episode providing an unprecedented hazard challenge for this region the stratigraphic record suggests an irregular clustering of eruption episodes over time including an unusual 200 year long period from 11 3 11 1 ka when several large explosive eruptions occurred we carried out new field mapping stratigraphy and sedimentology of the resulting pyroclastic deposits to identify seven episodes during this period locating the vent sites and calculating their climactic dynamic properties five of the episodes involved two or more vent locations that produced separate sub plinian plinian eruption columns eruption columns ranged between 16 and 24 km high and erupted volumes of 0 05 to 0 76 km3 indicating magnitudes of 4 to 5 each of these episodes show north to south unzipping of the tongariro graben with episodes beginning in northerly vents before being joined by vents south of these this included two scenarios northern edifice vents followed by central edifice ones or central edifice vents followed by southern ones in both cases the northerly vents generally display the greatest peak eruption magnitudes and the steadiest eruptions by contrast the southernmost vents formed along a fault controlled fissure like system these produced unsteady plumes and pyroclastic density currents the northward widening tectonic graben structure thus influences the pattern and style of these plinian style eruptions this study highlights a real possibility for an extreme hazard scenario involving successive large scale eruptions at volcanic systems in slowly rifting environments and sheds new light on the volcano tectonic interaction in such settings

the caldera forming eruption of ksudach volcano about cal a d 240 the greatest explosive event of our era in kamchatka russia the largest plinian eruption of our era and the latest caldera forming eruption in the kuril kamchatka region occurred about cal a d 240 from the ksudach volcano this catastrophic explosive eruption was similar in type and characteristics to the 1883 krakatau event the volume of material ejected was 18 19 km3 8 km3 dre including 15 km3 of tephra fall and 3 4 km3 of pyroclastic flows the estimated height of eruptive column is 22 30 km a collapse caldera resulting from this eruption was 4 6 5 km in size with a cavity volume of 6 5 7 km3 tephra fall was deposited to the north of the volcano and reached more than 1000 km pyroclastic flows accompanied by ash cloud pyroclastic surges extended out to 20 km the eruption was initially phreatomagmatic and then became rhythmic with each pulse evolving from pumice falls to pyroclastic flows erupted products were dominantly rhyodacite throughout the eruption during the post caldera stage when the shtyubel cone started to form within the caldera basaltic andesite and andesite magma began to effuse the trigger for the eruption may have been an intrusion of mafic magma into the rhyodacite reservoir the eruption had substantial environmental impact and may have produced a large acidity peak in the greenland ice sheet

coupled caldera subsidence and stirring inferred from analogue models analogue modelling of caldera forming eruptions suggests that sinking of the magma chamber roof is variable in space and time leading to substantial stirring and mixing of magma this can explain the common occurrence of geochemical zonation and magma mingling in deposits erupted from calderas

the plinian fallout associated with quilotoa s 800 yr bp eruption ecuadorian andes abstract large volcanic eruptions at dacitic or rhyolitic volcanoes often generate exceptional volumes of fine ash that mantles an area up to a million km 2 these eruptions are characterized by extreme fragmentation of the magma and hence extraordinary dispersal of ash and are categorized as plinian ultraplinian or phreatoplinian events large volume co ignimbrites or co plinian ashes are often produced by such eruptions high fragmentation indices of 90 are attributed to the violent eruption of silicic magma especially if augmented by fuel coolant reactions produced when abundant external water interacts with the magma the present study documents a case where the fine ash 1 mm diameter fall deposit related to the plinian phase of the eruption comprises the overwhelming bulk about 87 wt of the eruptive products this is another example demonstrating the predominance of a widespread fine grained co plinian ash which follows the initial coarser lapilli fall historical eruptions at two other andean volcanoes quizapu chile and huaynaputina peru and at santa maria guatemala and novarupta alaska produced similar ash fall sequences quilotoa s 800 yr bp eruption in the ecuadorian andes is an example of a powerful plinian eruption at a small dacitic volcano it produced a crystal rich fine grained ash layer which mantled an extrapolated 810 000 km 2 with 1 mm thickness and has a bulk magma volume of about 18 3 km 3 stratigraphic and petrographic studies as well as granulometric and density analyses were employed to investigate these eruptive products particularly the distal co plinian fall deposits the eruptive cycle consisted of a phreatomagmatic triggering the establishment of a powerful sustained plinian column and subsequently partial column collapses leading to widespread surge and ash flow activity quilotoa s 800 yr bp eruption has a vei volcano explosivity index 6 and generated a maximum column height of about 35 km and a mass discharge rate of 2 10 8 kg s in comparison pinatubo s 1991 eruption had similar column heights and discharge rates but smaller erupted volumes factors that possibly contributed to the extreme explosiveness and resulting fine grained ash production include inferred volatile overpressures in the crystal dominated dacite magma a rapid eruption onset a forceful and sustained plinian column possibly through a crater lake thus promoting extreme fragmentation of the magma but without leaving evidence of wet phreatoplinian type deposits

the bad step tuff a lava like rheomorphic ignimbrite in a calc alkaline piecemeal caldera english lake district lava like tuffs are lithologically indistinguishable from lavas and form part of a temperature and composition controlled continuum from low grade tuffs which are non welded or slightly welded through high grade densely welded tuffs extremely high grade tuffs which may be agglutinated right to their upper and lower contacts to spatter fed lava flows in some high grade tuffs a component of nonparticulate flow may postdate emplacement and deposition but in extremely high grade tuffs non particulate deformation normally occurs during emplacement and deposition in such cases syn depositional non particulate deformation previously called primary welding and non particulate slumping previously called secondary flowage processes overlap and are continuous one into the other so that distinction between them and their resultant structures is unrealistic and inapplicable therefore the term rheomorphism should be used to embrace all types of non particulate flow the bad step tuff is the most lava like of a sequence of rheomorphic calc alkaline rhyolitic ignimbrites emplaced during a climactic caldera forming eruption episode in the english lake district it is a ponded sheet 40 to 400 m thick which comprises a basal crudely stratified heterolithic breccia a thick flow laminated and locally vesicular central part which beomes increasingly flow folded upwards and an upper autobreccia despite an absence of vitroclastic textures within the main laminated part field relations show it to be a tuff diagnostic criteria are 1 a gradation within a lithophysal zone from unambiguous vitroclastic matrix of the basal lithic breccia upwards into the central flow laminated tuff 2 only rate autobreccia at the base of the sheet but ubiquitous autobreccia at the top of the sheet and 3 close textural similarity with localized intensely rheomorphic parts of associated ignimbrites that widely display unequivocal vitroclastic textures where their rheomorphism is less marked the extremely high grade character of the bad step tuff may reflect its proximal setting in a piecemeal type caldera high emplacement temperatures resulted from high rate but low velocity vent emission from fissures along numerous cross cutting calderafloor faults producing very low boil over eruption columns and proximal ponding

lithic breccia and ignimbrite erupted during the collapse of crater lake caldera oregon abstract the climactic eruption of mount mazama 6845 y b p vented a total of 50 km3 of compositionally zoned rhyodacitic to basaltic magma from a a single vent as a plinian pumice fall deposit and the overlying wineglass welded tuff and b ring vents as ignimbrite and coignimbrite lithic breccia accompanying the collapse of crater lake caldera new field and grain size data for the ring vent products are presented in this report the coarse grained poorly bedded clast supported lithic breccia extends as far as 18 km from the caldera center like the associated ignimbrite the breccia is compositionally zoned both radially and vertically and silicic mixed and mafic types can be recognized based on the proportion of rhyodacitic pumice matrix fractions in silicic breccias are depleted of fines and are lithic and crystal enriched relative to silicic ignimbrite due to vigorous gas sorting during emplacement ignimbrite occurs as a proximal veneer deposit overlying the breccia a medial 8 to 25 km from the caldera center compositionally zoned valley fill as much as 110 m thick and an unzoned distal 20 km facies which extends as far as 55 km from the caldera breccia within 9 km of the caldera center is interpreted as a coignimbrite lag breccia formed within the deflation zone of the collapsing ring vent eruption columns expanded pyroclastic flows of the deflation zone were probably vertically graded in both size and concentration of blocks as recently postulated for some turbidity currents an inflection in the rate of falloff of lithic clast size within the lithic breccia at 9 km may mark the outer edge of the deflation zone or may be an artifact of incomplete exposure the onset of ring vent activity at mt mazama was accompanied by a marked increase in eruptive discharge pyroclastic flows were emplaced as a semicontinuous stream as few ignimbrite flow unit boundaries are evident as eruption from the ring vents progressed flow runout distance and the extent of breccia deposition decreased due to a greater internal flow friction and b decreasing eruption column heights effect b probably resulted from a progressive decrease in magmatic gas content and discharge rate waning discharge may have been promoted by the tapping of more viscous crystal rich magma collapse of conduit walls and declining caldera collapse rate

explosive volcanic eruptions from linear vents on earth venus and mars comparisons with circular vent eruptions 1 conditions required to support buoyant convective plumes are investigated for explosive volcanic eruptions from circular and linear vents on earth venus and mars vent geometry linear versus circular plays a significant role in the ability of an explosive eruption to sustain a buoyant plume on earth linear and circular vent eruptions are both capable of driving buoyant plumes to equivalent maximum rise heights however linear vent plumes are more sensitive to vent size for analogous mass eruption rates linear vent plumes surpass circular vent plumes in entrainment efficiency approximately when lo 3ro owing to the larger entrainment area relative to the control volume relative to circular vents linear vents on venus favor column collapse and the formation of pyroclastic flows because the range of conditions required to establish and sustain buoyancy is narrow when buoyancy can be sustained however maximum plume heights exceed those from circular vents for current atmospheric conditions on mars linear vent eruptions are capable of injecting volcanic material slightly higher than analogous circular vent eruptions however both geometries are more likely to produce pyroclastic fountains as opposed to convective plumes owing to the low density atmosphere because of the atmospheric density profile and water content on earth explosive eruptions enjoy favorable conditions for producing sustained buoyant columns while pyroclastic flows would be relatively more prevalent on venus and mars these results have implications for the injection and dispersal of particulates into the planetary atmosphere and the ability to interpret the geologic record of planetary volcanism

new insights on entrainment and condensation in volcanic plumes constraints from independent observations of explosive eruptions and implications for assessing their impacts abstract the turbulent entrainment of atmosphere and the condensation of water vapor govern the heights of explosive volcanic plumes these processes thus determine the delivery and the lifetime of volcanic ash and aerosols into the atmosphere predictions of plume heights using one dimensional integral models of volcanic plumes however suffer from very large uncertainties related to parameterizations for entrainment and condensation in particular the wind entrainment coefficient which governs the contribution of crosswinds to turbulent entrainment is subject to uncertainties of one order of magnitude leading to relative uncertainties of the order of 50 on plume height in this study we use a database of 94 eruptive phases with independent estimates of mass eruption rate and plume height to constrain and evaluate four popular 1d models we employ re sampling methods to account for observational uncertainties we show that plume height predictions are significantly improved when i the contribution of water vapor condensation to the plume buoyancy flux is excluded and ii the wind entrainment coefficient is held constant between 0 1 and 0 4 we explore implications of these results for predicting the climate impacts of explosive eruptions and the likelihood that eruptions will form stable umbrella clouds or devastating pyroclastic flows last we discuss the sensitivity of our results to the definition of plume height in the model in light of a recent set of laboratory experiments and draw conclusions for improving future databases of eruption parameters

evolution of lascar volcano northern chile lascar volcano in the central andes is an ese wnw elongate composite stratocone stage i 3 at 26 45 ka producing the soncor ignimbrite no caldera formed laharic and alluvial fan deposits and a debris avalanche formed from 26 45 to 19 2 ka indicating a humid climatic period the dacitic and andesitic western stratocone stage iii developed over the soncor vent stage iii culminated in the 9 2 ka tumbres eruption which formed andesitic scoria flow fans and a summit collapse crater stage iv activity switched to the east forming an andesite lava three nested summit craters which repeatedly filled with lava and collapsed absence of significant erosion since 9 2 ka is due to the much drier holocene climate activity since 1984 displays cycles of lava dome formation in the summit crater lava dome subsidence with crater collapse vulcanian to plinian explosive eruptions culminating in the major explosive eruption of 19 20 april 1993 the total volume of magma erupted from lascar over a 43 ka period is estimated at 30 to 40 km 3

effects of particle mixtures and nozzle geometry on entrainment into volcanic jets efficient turbulent entrainment causes otherwise dense volcanic jets to rise high into the atmosphere as buoyant plumes classical models suggest that the inflow of air is 10 15 of the axial velocity giving predictions for the height of the plume and in turn the composition and structure of the resulting umbrella clouds crucially entrainment is assumed independent of source geometry and mechanically unaffected by the pyroclastic mixture properties we show that particle inertia and vent geometry act to modify the shape of the largest eddies defining the jet s edge and thus entrainment of the ambient whereas particle free flows are essentially unaffected by vent shape entrainment into particle laden flows is enhanced for flared vents and reduced for cylindrical vents our results predict that vent erosion during an explosive eruption reduces the height of volcanic jets alters the structure and sedimentation regime of the umbrella cloud and the resulting deposit

estimating the eruptive volume of a large pyroclastic body the otowi member of the bandelier tuff valles caldera new mexico the 1 60 ma caldera forming eruption of the otowi member of the bandelier tuff produced plinian and coignimbrite fall deposits outflow and intracaldera ignimbrite all of it deposited on land we present a detailed approach to estimating and reconstructing the original volume of the eroded partly buried large ignimbrite and distal ash fall deposits dense rock equivalent dre volume estimates for the eruption are 89 33 10 km3 of outflow ignimbrite and 144 72 km3 of intracaldera ignimbrite also there was at least 65 km3 dre of plinian fall when extrapolated distally and 107 40 12 km3 of coignimbrite ash was lost from the outflow sheet to form an unknown proportion of the distal ash fall the minimum total volume is 216 km3 and the maximum is 550 km3 hence the eruption overlaps the low end of the super eruption spectrum vei 8 0 despite an abundance of geological data for the otowi member the errors attached to these estimates do not allow us to constrain the proportions of intracaldera ic outflow o and distal ash a to better than a factor of three we advocate caution in applying the ic o a 1 1 1 relation of mason et al 2004 to scaling up mapped volumes of imperfectly preserved caldera forming ignimbrites

calderas and caldera structures a review abstract calderas are important features in all volcanic environments and are commonly the sites of geothermal activity and mineralisation yet it is only in the last 25 years that a thorough three dimensional study of calderas has been carried out utilising studies of eroded calderas geophysical analysis of their structures and analogue modelling of caldera formation as more data has become available on calderas their individuality has become apparent a distinction between caldera caldera complex cauldron and ring structure is necessary and new definitions are given in this paper descriptions of calderas based on dominant composition of eruptives basaltic peralkaline andesitic dacitic rhyolitic can be used and characteristics of each broad group are given styles of eruption may be effusive or explosive with the former dominant in basaltic calderas and the latter dominant in andesitic dacitic rhyolitic and peralkaline calderas four end member collapse styles occur plate or piston piecemeal trapdoor and downsag but many calderas have multiple styles features of so called funnel and chaotic calderas proposed in the literature can be explained by other collapse styles and the terms are considered unnecessary ground deformation comprises subsidence or collapse essential characteristics of a caldera and uplifting doming and fracturing due to tumescence and or resurgence frequent but not essential collapse may occur on pre existing structures such as regional faults or on faults created during the caldera formation and the shape of the collapse area will be influenced by depth size and shape of the magma chamber the final morphology of a caldera will depend on how the caldera floor breaks up whether collapse takes place in one event or multiple events whether vertical movement is spasmodic or continuous throughout the eruptive sequence and whether blocks subside uniformly or chaotically at one or more collapse centres a meaningful description of any caldera should therefore include number of collapse events presence or absence of resurgence caldera floor coherency caldera floor collapse geometry and dominant composition of eruptives

volcanology volcanic bipolar disorder explained eruptions come in a range of magnitudes numerical simulations and laboratory experiments show that rare giant super eruptions and smaller more frequent events reflect a transition in the essential driving forces for volcanism

complex dynamics of small moderate volcanic events the example of the 2011 rhyolitic cordon caulle eruption chile after decades of repose puyehue cordon caulle volcano chile erupted in june 2011 following a month of continuously increasing seismic activity the eruption dispersed a large volume of rhyolitic tephra over a wide area and was characterized by complex dynamics during the initial climactic phase of the eruption 24 30 h on 4 5 june 11 14 km high plumes dispersed most of the erupted tephra eastward towards argentina reaching as far as the atlantic ocean this first eruptive phase was followed by activity of lower intensity leading to the development of a complex stratigraphic sequence mainly due to rapid shifts in wind direction and eruptive style the resulting tephra deposits consist of 13 main layers grouped into four units each layer was characterized based on its dispersal direction sedimentological features and on the main characteristics of the juvenile fraction texture density petrography chemistry the lowest part of the eruptive sequence unit i corresponding to the tephra emitted between 4 and 5 june is composed of alternating lapilli layers with a total estimated volume of ca 0 75 km3 these layers record the highest intensity phase during which a bent over plume dispersed tephra towards the southeast east with negligible up wind sedimentation products emitted during 5 6 june unit ii signaled an abrupt shift in wind direction towards the north leading to the deposition of a coarse ash deposit in the northern sector ca 0 21 km3 in volume followed by a resumption of easterly directed winds a third phase unit iii began on 7 june and resulted in tephra deposits in the eastern sector and ballistic bombs around the vent area a final phase unit iv started after 15 june and was characterized by the emission of fine grained white tephra from ash charged plumes during low level activity and the extrusion of a viscous lava flow timing and duration of the first eruptive phases were constrained based on comparison of the dispersal of the main tephra layers with satellite images showing that most of the tephra was emitted during the first 72 h of the event the analyzed juvenile material tightly clusters within the rhyolitic field with negligible chemical variations through the eruptive sequence textural observations reveal that changes in eruption intensity and consequently in magma ascent velocity within the conduit and complex interactions between gas rich and gas depleted magma portions during ascent resulted in vesicular clasts with variable degrees of shear localization and possibly in the large heterogeneity of the juvenile material

volcanic winter and accelerated glaciation following the toba super eruption the eruption of toba in sumatra 73 500 years ago was the largest known explosive volcanic event in the late quaternary1 it could have lofted about 1015 g each of fine ash and sulphur gases to heights of 27 37 km creating dense stratospheric dust and aerosol clouds here we present model calculations that investigate the possible climatic effects of the volcanic cloud the increase in atmospheric opacity might have produced a volcanic winter 2 a brief pronounced regional and perhaps hemispheric cooling caused by the volcanic dust followed by a few years with maximum estimated annual hemispheric surface temperature decreases of 3 5 c the eruption occurred during the stage 5a 4 transition of the oxygen isotope record a time of rapid ice growth and falling sea level3 we suggest that the toba eruption may have greatly accelerated the shift to glacial conditions that was already underway by inducing perennial snow cover and increased sea ice extent at sensitive northern latitudes as the onset of climate change may have helped to trigger the eruption itself4 we propose that the toba event may exemplify a more general climate volcano feedback mechanism

transport and deposition of pyroclastic the complex eruption sequence from the 1000 a d caldera forming eruption of volcan ceboruco known as the jala pumice offers an exceptional opportunity to examine how pyroclastic material is transported and deposited from pyroclastic density currents over variable topography three main pyroclastic surge deposits sl s2 and s3 and two pyroclastic flow deposits marquesado and north flank pfds were emplaced during this eruption pyroclastic surge deposits are massive planar or cross bedded poor to well sorted and display fluctuations in thickness median diameter sorting and lithology as a function of distance topography and flow dynamics marquesado pyroclastic flow deposits reveal lateral variations from massive poorly sorted deposits located within 5 km of ceboruco to planar bedded moderately well sorted deposits located 15 km away over the nearly horizontal topography to the south of ceboruco north flank pyroclastic flow deposits also reveal lateral variations from massive poorly sorted deposits located within 4 km of ceboruco to planar bedded moderately well sorted deposits located 8 km away atop an escarpment that steeply rises 230 m from the northern valley floor field observations granulometric analyses component analyses and crystal sedimentation calculations along flow parallel sampling transects all suggest that both surges and flows were density stratified currents where deposition occurred from a basal region of higher particle concentration that was supplied from an overlying dilute layer that transports particles in suspension this supports the idea of a transition between flow and surge end members with variations in particle concentration topography greatly affects the transport and depositional capacity of the pyroclastic density currents as a result of blocking either by topographic obstacles or by abrupt breaks at the base of volcano slopes whereas the origin of jala pumice surge deposits phreatomagmatic versus magmatic appears to have little impact on their flow dynamics

using electron backscatter diffraction patterns to examine recrystallization in polar ice sheets the fabric of polycrystalline ice is typically described using the c axis orientation alone but this is insufficient for a full description of grain orientations in this hexagonal material electron backscatter diffraction ebsd provides full c and a axis orientation of individual grains and is used here to study greenland ice sheet project 2 gisp2 ice specimens from depths of 1551 1642 and 1745 m complete orientation data are used to compare nearest neighbor relationships to overall fabric and to differentiate between recrystallization mechanisms changes in orientation between grains and subgrains in gisp2 specimens were correlated with the appearance of grain boundaries on thin sections and used to identify grain sets that were probably produced by polygonization comparison of grain misorientations that take into account both c and a axis differences with those derived from c axis directions alone reveals the presence of polygonization and illustrates the usefulness of this technique

particle fallout thermal disequilibrium and volcanic plumes we develop a model of a volcanic eruption column that includes the effects of fallout of pyroclasts and thermal disequilibrium we show that clast fallout with no thermal disequilibrium has only a small effect upon the column however disequilibrium changes column behaviour significantly and can even induce collapse our results may explain the lower plume heights and less widely dispersed fallout of cone forming eruptions contrasted with sheet forming eruptions the model also predicts that the transition from the gas thrust to the convective region in a column results in an inflection in dispersal curves that some features of the stratigraphy common to many fall deposits may result from column velocity structure and that there may exist a region near the volcanic vent in which maximum pyroclast size does not decrease significantly with distance

the bishop tuff new insights from eruptive stratigraphy the 0 76 ma bishop tuff from long valley caldera in eastern california consists of a widespread fall deposit and voluminous partly welded ignimbrite the fall deposit f exposed over an easterly sector below and adjacent to the ignimbrite is divided into nine units f1 f9 with no significant time breaks except possibly between f8 and f9 maximum clast sizes are compared with other deposits where accumulation rates are known or inferred to estimate an accumulation time for f1 f8 as ca 90 hrs the ignimbrite ig is divided into chronologically and or geographically distinct packages of material earlier packages ig1 were emplaced mostly eastward are wholly intraplinian coeval with fall units f2 f8 lack phenocrystic pyroxenes and contain few or no glass mountain derived rhyolite lithic fragments earlier packages ig2 were erupted mostly to the north and east are at least partly intraplinian interbedded with fall unit f9 to the east contain pyroxenes and have lithic fractions rich in g

the recent plinian explosive activity of mt pelee volcano lesser antilles the p1 ad 1300 eruption plinian explosive eruptions represent a major volcanic hazard in the lesser antilles arc that must be carefully assessed based on reconstructions of past activity the present study focusses on a detailed time evolution of the p1 eruption ad 1300 at mt pelee volcano martinique after an initial dome forming stage a plinian phase commenced the p1 plinian style sequence is mostly a pumice fall deposit with an inversely graded base interbedded with a surge deposit and overlain by final flow surge deposit field data on deposit dispersal thickness and grain size distribution are used together with physical models to reconstruct the dynamical evolution of this eruption we find that the mass eruption rate increased from 2 107 to 9 107 kg s 1 producing a 19 22 km high plinian plume initially stable but which ultimately collapsed to form a 1 3 km high fountain empirical models of deposit thinning suggest that the minimum volume of pyroclastic deposits is 0 15 km3 about 25 that previously estimated published data on magmatic water contents in glass inclusions are used together with mass discharge rates to elucidate the mechanisms leading to column collapse conditions at the base of the column were close to the plume fountain transition soon after the pelean plinian style transition due to the competing effects of increase in both gas content and mass discharge rate after a short stage of partial collapse the column underwent a total collapse due to an increasing discharge rate

event stratigraphy of a caldera forming ignimbrite eruption on tenerife the 273 ka poris formation the 273 ka poris formation in the bandas del sur group records a complex compositionally zoned explosive eruption at las canadas caldera on tenerife canary islands the eruption produced widespread pyroclastic density currents that devastated much of the se of tenerife and deposited one of the most extensive ignimbrite sheets on the island the sheet reaches 40 m thick and includes plinian pumice fall layers massive and diffuse stratified pumiceous ignimbrite widespread lithic breccias and co ignimbrite ashfall deposits several facies are fines rich and contain ash pellets and accretionary lapilli eight brief eruptive phases are represented within its lithostratigraphy phase 1 comprised a fluctuating plinian eruption in which column height increased and then stabilized with time and dispersed tephra over much of the southeastern part of the island phase 2 emplaced three geographically restricted ignimbrite flow units and associated extensive thin co ignimbrite ashfall layers which contain abundant accretionary lapilli from moist co ignimbrite ash plumes a brief plinian phase phase 3 again dispersing pumice lapilli over southeastern tenerife marked the onset of a large sustained pyroclastic density current phase 4 which then waxed phase 5 covering increasingly larger areas of the island as vents widened and or migrated along opening caldera faults the climax of the poris eruption phase 6 was marked by widespread emplacement of coarse lithic breccias thought to record caldera subsidence this is inferred to have disturbed the magma chamber causing mingling and eruption of tephriphonolite magma and it changed the proximal topography diverting the pyroclastic density current s down the guimar valley phase 7 phase 8 involved post eruption erosion and sedimentary reworking accompanied by minor down slope sliding of ignimbrite this was followed by slope stabilization and pedogenesis the fines rich lithofacies with abundant ash pellets and accretionary lapilli record agglomeration of ash in moist ash plumes they resemble phreatomagmatic deposits but a phreatomagmatic origin is difficult to establish because shards are of bubble wall type and the moisture may have arisen by condensation within ascending thermal co ignimbrite ash plumes that contained atmospheric moisture enhanced by that derived from the evaporation of seawater where the hot pyroclastic currents crossed the coast ash pellets formed in co ignimbrite ash clouds and then fell through turbulent pyroclastic density currents where they accreted rims and evolved into accretionary lapilli

experimental constraints on the distinct effects of ash lapilli and larger pyroclasts on entrainment and mixing in volcanic plumes the rate at which atmosphere is entrained and mixed into relatively dense explosive volcanic jets of gas and pyroclasts ash lapilli determines whether they rise into the atmosphere as buoyant plumes or collapse to form pyroclastic flows here we use analog experiments on grid stirred turbulence to isolate particle inertial effects related to the motions of ash and lapilli on the quantitative dynamical conditions for the onset of turbulent entrainment and mixing from energetic considerations we estimate the distinct effects of ash and lapilli on the entrainment rate we find that whereas large ash and lapilli sized pyroclasts contribute angular momentum to entraining eddies and may enhance entrainment into volcanic jets by 37 compared to expectations without inertial particles ash is dissipative and may reduce entrainment by 14 we also show that the internal turbulent mixing properties are largely insensitive to particle inertial effects our results predict that highly explosive events involving water or ice should for example feed plumes that rise higher than eruption columns that are enriched in larger pyroclasts although these events are also more likely to produce pyroclastic flows

thermohaline variability in the upper ocean the main goal of this exploratory study is to determine how the temperature salinity relationship changes with horizontal length scale and depth in the ocean temperature and salinity were measured on a range of scales from 4 m to 1000 km towing a seasoar along isobars and isopycnals in the subtropical gyre of the north pacific during the winter of 1997 the wavelet transform technique is used to compute the horizontal density ratio and thermohaline variability as a function of scale and location measurements along an isobar in the mixed layer show that the horizontal density ratio is 1 at all scales observed that is horizontal temperature and salinity gradients tend to cancel each other in their effect on density thermohaline variability at small scales is intermittent and clusters around large scale thermohaline anomalies below the base of the mixed layer horizontal gradients of temperature are only partially opposed by salinity and the density ratio is close to 2 in the thermocline the distribution of thermohaline variability is uniform along isobars but intermittent and colocated at different scales along isopycnals density compensated variability ubiquitous in the mixed layer is reduced along deeper isopycnals compensation of horizontal temperature and salinity gradients supports recent theoretical ideas that mixing in the winter mixed layer depends on horizontal density gradients

volcanological study of the great tambora eruption of 1815 the 1815 eruption of tambora is one of the largest explosive volcanic events of the past 10 000 yr by a conservative estimate 175 km3 of nepheline normative trachyandesitic pyroclastic material equivalent to about 50 km3 of dense rock was erupted in 24 h major activity began with a brief plinian phase followed by pyroclastic flows much of the ignimbrite that was produced shows evidence of deposition by turbulent flows a 6 7 km caldera about 1 km deep formed at the end of the pyroclastic flow phase caldera volume closely matches that of magma ejected plinian and co ignimbrite ash fall 1 cm thick covered 500 000 km2 of the java sea and surrounding islands accounting for more than two thirds of the magma volume erupted this distal tephra is known only from 1815 reports indicating that some great eruptions especially those from island volcanoes may be virtually undetectable in the geologic record without supporting information such as deep sea core data land based volume estimates may be insufficient to characterize eruption magnitude in many cases

silver creek caldera the tectonically dismembered source of the peach spring tuff sanidine 40 ar 39 ar geochronology confirms that silver creek caldera which straddles the eastern edge of the colorado river extensional corridor near oatman arizona united states is the source of the peach spring tuff eight new dates five from outflow three from caldera fill are analytically indistinguishable and combined with the most precise previously published date give a weighted mean average age of 18 78 0 02 ma a fragment of the caldera identified in the midst of the extensional corridor is structurally juxtaposed with mesozonal plutons of identical age the implied extension direction 182 225 is compatible with abundant previously published structural data for the region

addressing complexity in laboratory experiments the scaling of dilute multiphase flows in magmatic systems the kinematic and dynamic scaling of dilute multiphase mixtures in magmatic systems is the only guarantee for the geological verisimilitude of laboratory experiments we present scaling relations that can provide a more complete framework to scale dilute magmatic systems because they explicitly take into account the complexity caused by the feedback between particles crystal bubble or pyroclast and the continuous phase liquid or gas we consider three canonical igneous systems magma chambers volcanic plumes and pyroclastic surges and we provide estimates of the proposed scaling relations for published experiments on those systems dilute magmatic mixtures can display a range of distinct dynamical regimes that we characterize with a combination of average eulerian properties and instantaneous lagrangian variables the eulerian properties of the mixtures yield the reynolds number re which indicates the level of unsteadiness in the continuous phase the lagrangian acceleration of particles is a function of the viscous drag and gravity forces and from these two forces are derived the stokes number st and the stability number rt two dimensionless numbers that describe the dynamic behavior of the particles within the mixture the compilation of 17 experimental studies relevant for pyroclastic surges and volcanic plumes indicates that there is a need for experiments above the mixing transition ren10 4 and for scaling st and rt among the particle dynamic regimes present in surges and plumes some deserve special attention such as the role of mesoscale structures on transport and sedimentary processes or the consequences of the transition to turbulence on particle gathering and dispersal the compilation of seven experimental studies relevant to magma bodies indicates that in the laminar regime crystals mostly follow the motion of the melt and thus the physical state of the system can be approximated as single phase in the transition to turbulence magmas can feature spatially heterogeneous distributions of laminar regions and important velocity gradients this heterogeneity has a strong potential for crystals sorting in conclusion the re st rt framework demonstrates

fluid dynamics in explosive volcanic vents and craters abstract explosive volcanic jets can transition to buoyant plumes or collapse to form pyroclastic density currents depending on their ability to entrain and heat the ambient air recent one dimensional 1d analysis shows that fluid acceleration through volcanic vents and craters changes the velocity and pressures within these jets sufficiently enough to be a first order control on plume dimensions and therefore air entrainment and column stability koyaguchi et al 2010 these 1d studies are only applicable to craters and vents with angles of less than about 30 to vertical using analytical formulations and numerical simulations this study describes 2d effects of shallowly dipping vents and craters on volcanic eruptions the effect of vents on acceleration and expansion of eruptive mixtures of ash and gas is described as a force imparted on the fluid by the vent wall the wall force fw this force is a measure of the momentum coupling between an eruption and the solid earth that takes place in the vent rapid divergence of supersonic eruptive fluid within shallowly dipping vents occurs via prandtl meyer expansion which results in different pressure and velocity fields than those predicted by 1d analysis this expansion decreases fw and the vertical acceleration experienced by the eruptive fluid in the vent for jets predicted by 1d analysis to exit the vent at supersonic velocities and at atmospheric pressure this decrease in fw will cause an increase in the predicted plume area decreasing column stability the complex 2d shape of volcanic vents can change jet structure presence and location of shock waves and preclude the development of jets that exit the vent supersonically with no internal standing shock waves i e perfectly expanded or pressure balanced jets these significant complications in jet structure and increase in plume radius may result in changes to air entrainment plume stability and tephra distribution

effects of the crater on eruption column dynamics 1 during explosive eruptions a mixture of pyroclasts and volcanic gas forms a buoyant eruption column or a pyroclastic flow we systematically investigate how the condition that separates these two eruption styles column collapse condition depends on crater shape and magma chamber conditions by integrating the theoretical models for conduit flow flow inside a crater and eruption column dynamics the results show that previous model predictions of column collapse condition based on the relationship between magma discharge rate and water content nf strongly depend on crater shape depth d and opening angle when a crater is present the decompression and or compression of gas pyroclast mixture inside and just above the crater result in two distinct types of column collapse collapse with increasing hm side collapse and that with decreasing lm side collapse hm side collapse is caused by an increase in conduit radius during the waxing stage of an eruption lm side collapse is associated with a decrease in magma chamber pressure during the waning stage the value of for hm side collapse varies by two orders of magnitude depending on crater shape for fixed nf this estimate also depends on assumed models for decompression into the atmosphere the value of for lm side collapse is 108 kg s 1 when d tan 102 m these results are consistent with the field observations from the st helens 1980 and pinatubo 1991 eruptions

an algorithm for the detection and characterisation of volcanic plumes using thermal camera imagery volcanic plumes are turbulent mixtures of particles and gas which are injected into the atmosphere during a volcanic eruption depending on the intensity of the eruption plumes can rise from a few tens of metres up to many tens of kilometres above the vent and thus present a major hazard for the surrounding population currently however few if any algorithms are available for automated plume tracking and assessment here we present a new image processing algorithm for segmentation tracking and parameters extraction of convective plume recorded with thermal cameras we used thermal video of two volcanic eruptions and two plumes simulated in laboratory to develop and test an efficient technique for analysis of volcanic plumes we validated our method by two different approaches first we compare our segmentation method to previously published algorithms next we computed plume parameters such as height width and spreading angle at regular intervals of time these parameters allowed us to calculate an entrainment coefficient and obtain information about the entrainment efficiency in strombolian eruptions our proposed algorithm is rapid automated while producing better visual outlines compared to the other segmentation algorithms and provides output that is at least as accurate as manual measurements of plumes

a stress controlled mechanism for the intensity of very large magnitude explosive eruptions abstract large magnitude explosive eruptions are the result of the rapid and large scale transport of silicic magma stored in the earth s crust but the mechanics of erupting teratonnes of silicic magma remain poorly understood here we demonstrate that the combined effect of local crustal extension and magma chamber overpressure can sustain linear dyke fed explosive eruptions with mass fluxes in excess of 10 10 kg s from shallow seated 4 6 km depth chambers during moderate extensional stresses early eruption column collapse is facilitated with eruption duration of the order of few days with an intensity of at least one order of magnitude greater than the largest eruptions in the 20th century the conditions explored in this study are one way in which high mass eruption rates can be achieved to feed large explosive eruptions our results corroborate geological and volcanological evidences from volcano tectonic complexes such as the sierra madre occidental mexico and the taupo volcanic zone new zealand

a new view of the dynamics stability and longevity of volcanic clouds abstract powerful explosive volcanic eruptions inject ash high into the atmosphere which spreads to form umbrella clouds identifying key physical processes governing the dynamics stability and longevity of umbrella clouds is central to assessing volcanic hazards as well as the nature of volcanic forcings on climate here we present a series of laboratory experiments producing turbulent particle laden jets and subsequent axisymmetric intrusive gravity currents into a stratified environment our experiments reproduce many of the main dynamical regimes observed during the formation of an explosive volcanic column and highlight new dynamics for the umbrella cloud theoretical predictions of column collapse from a simple model of a turbulent jet are in good agreement with experimental observations as well as previous studies depending on the flow intensity the strength of the initial environmental density stratification and the particle concentration at the source resulting umbrella clouds can however evolve through a series of new regimes as a result of the dynamics of particle sedimentation within these flows as well as from their bases using scaling theory we show that during cloud spreading internal sedimentation drives the growth and intermittent overturn of thin gravitationally unstable particle boundary layers pbls as particle rich plumes this pbl driven convection can have remarkable effects ranging from progressive dilution of clouds to their catastrophic overturn and collapse in natural eruptions whether the dynamics of pbls play a major role in particle sedimentation depends on the grain size distribution inside the cloud and on eruption column height in general particles larger than 60 m 1 mm are expected to settle individually whereas finer particles will accumulate pbls resulting in the formation of armless mammatus clouds or dangerous gravity currents at much larger distances from the volcanic vent than ever considered before such dynamics is apparent in observations of numerous modern eruptions and is inferred from the deposits of historic and prehistoric eruptions for where there exist appropriate data consideration of the consequences of these phenomena for problems such as volcanic hazards to humans and climate change may thus be very important in the assessment of future eruptions

geometrical and mechanical constraints on the formation of ring fault calderas ash flow plate subsidence piston like calderas are bounded by a set of arcuated sub vertical collapse faults named ring faults experimental studies on caldera formation performed mostly using spherical or cylindrical magma chamber geometries find that the resulting ring faults correspond to steeply outward dipping reverse faults and show that pre existing fractures developed during pre eruptive phases of pressure increase may play a major role in controlling the final collapse mechanism a situation that should be expected in small to medium sized ring fault calderas developed on top of composite volcanoes or volcanic clusters on the other hand some numerical experiments indicate that large sill like elongated magma chambers may induce collapse due to roof bending without fault reactivation as seems to occur in large plate subsidence calderas formed independently of pre existing volcanoes also numerical experiments allow the formation of nearly vertical or steeply inward dipping normal ring faults in contrast with most of the analogue models using a thermoelastic model we investigate the geometrical and mechanical conditions to form ring fault calderas in particular the largest ones without needing a previous crust fracturing results are given in terms of two dimensionless geometrical parameters namely and e the former is the chamber extension to chamber depth ratio whereas the latter stands for the chamber eccentricity we propose that the e pair determinates two different types of ring fault calderas with different associated collapse regimes ring fault region a is related to large plate subsidence calderas i e andean calderas or western us calderas for which few depressurisation is needed to set up a collapse initially governed by flexural bending of the chamber roof in contrast ring fault region b is related to small to moderate sized calderas i e composite volcano calderas for which much depressurisation is needed our opinion is that collapse requires in the latter case reactivation of pre existing fractures and it is therefore more complex and history dependent

experimental study of caldera formation scaled experiments have been carried out on caldera collapse mechanisms using silicone as analogue magma and dry sand as analogue rock experiments were carried out in two and three dimensions using a range of roof aspect ratios thickness width 0 2 to 4 5 appropriate for caldera collapse they reveal a general mechanism of collapse only weakly dependent on the shape of the reservoir for low roof aspect ratios 1 subsidence starts by flexure of the roof and the formation of outward dipping reverse ring faults which in turn trigger formation of peripheral inward dipping normal ring faults the subsidence always occurs asymmetrically in cross section the reverse faults delimit a coherent piston bounded on each side by an annular zone of inwardly tilted strata located between the reverse and normal ring fault sets the surface depression consists of a nondeformed area piston surrounded by an annular extensional zone tilted strata for high aspect ratios 1 multiple reverse faults break up the roof into large pieces and subsidence occurred as a series of nested wedges 2 d or cones 3 d the extensional zone dominates the surface depression in the case where preexisting regional faults do not play a major role the collapse mechanics of calderas probably depends strongly on the roof aspect ratio calderas with low roof aspect ratios are predicted to collapse as coherent pistons along reverse faults the annular extensional zone might be the source of the large landslides that generate intracaldera megabreccias collapse into magma reservoirs with high roof aspect ratios may be the origin of some funnel calderas where explosive reaming is not dominant

particle recycling and oscillations of volcanic eruption columns we study the dynamics of sedimentation and reentrainment of particles from the umbrella cloud above an axisymmetric turbulent particle laden buoyant plume we develop a model to show that the reentrainment of particulate material into the uprising plume will cause the particle flux to increase by a factor of e between the plume source and the umbrella cloud a buoyant plume rising in an environment of uniform density may thereby become negatively buoyant if its particle loading becomes sufficiently high we compare the predictions of the model to a series of laboratory experiments and show that at high particle loadings the plume undergoes an oscillatory collapse this periodic collapse generates dense gravity flows down the flanks of the plume in the context of volcanic eruptions such an instability associated with particle recycling may lead to the formation of interleaved fall and flow deposits such as have been observed near the collapse horizon in a number of pyroclastic deposits

fissure ignimbrites fissure source origin for voluminous ignimbrites of the sierra madre occidental and its relationship with basin and range faulting the sierra madre occidental is mostly composed of middle tertiary large volume ignimbrites from the united states mexico border 318n the sierra madre occidental extends southward to its intersection with the mexican volcanic belt 218n ignimbrites of equivalent age extend into southern mexico as discontinuous outcrops considering the average thickness of 1000 m for these ignimbrites based on representative measured sections a conservative estimate of their total volume is 393 000 km 3 fewer than 15 calderas have been identified in this province and the source of most of these ignimbrites has been an unsolved problem we present geologic evidence indicating that fissures most of them with the regional trend of basin and range faults served as conduits for the ignimbrites these fissures can be several kilometers long and are represented by pyroclastic ignimbrite dikes rhyolitic lava dikes linearly aligned lava domes and elongated coignimbrite lithic lag breccias adjacent to basin and range faults considering that the basin and range extension overlapped in time and space with the ignimbrite flare up we propose a model in which batholith sized magma chambers reached shallow crustal levels and erupted their contents when they reached basin and range normal faults the faults acted as vents and caused fast decompression when the system was opened and large volumes of silicic magmas were explosively erupted finally devolatilized rhyolitic magmas were emplaced as domes or dikes we propose the term fissure ignimbrites for ignimbrites formed in this way

the flow dynamics of an extremely large volume pyroclastic flow the 2 08 ma cerro galan ignimbrite nw argentina and comparison with other flow types the 2 08 ma cerro galan ignimbrite cgi represents a 630 km3 dense rock equivalent vei 8 eruption from the long lived cerro galan magma system 6 ma it is a crystal rich 35 60 pumice 10 generally and lithic poor 5 generally rhyodacitic ignimbrite lacking a preceding plinian fallout deposit the cgi is preserved up to 80 km from the structural margins of the caldera but almost certainly was deposited up to 100 km from the caldera in some places only one emplacement unit is preserved in proximal to medial settings and in most distal settings suggesting constant flow conditions but where the pyroclastic flow moved into a palaeotopography of substantial valleys and ridges it interacted with valley walls resulting in flow instabilities that generated multiple depositional units often separated by pyroclastic surge deposits the cgi preserves a widespread sub horizontal fabric defined by aligned elongate pumice and lithic clasts and minerals e g biotite a sub horizontal anisotropy of magnetic susceptibility fabric is defined by minute magnetic minerals in all localities where it has been analysed the cgi is poor in both vent derived accessory lithics and locally derived lithics from the ground surface accidental lithics locally derived lithics are small 20 cm and were not transported far from source points all data suggest that the pyroclastic flow system producing the cgi was characterised throughout by high sedimentation rates resulting from high particle concentration and suppressed turbulence at the depositional boundary layer despite being a low aspect ratio ignimbrite based on these features we question whether high velocity and momentum are necessary to account for extensive flow mobility it is proposed that the cgi was deposited by a pyroclastic flow system that developed a substantial high particle concentration granular under flow which flowed with suppressed turbulence high particle concentration and fine ash content hindered gas loss and maintained flow mobility in order to explain the contemporaneous maintenance of high particle concentration high sedimentation rate at the depositional boundary layer and a high level of mobility it is also proposed that the flow s was continuously supplied at a high mass feeding rate it is also proposed that internal gas pressure within the flow directed downwards onto the substrate over which the flow was passing reduced the friction between the flow and the substrate and also enhanced its mobility the pervasive sub horizontal fabric of aligned pumice lithic and even biotite crystals indicates a consistent horizontal shear force existed during transport and deposition in the basal granular flow consistent with the existence of a laminar shearing granular flow regime during the final stages of transport and deposition

are eruptions from linear fissures and caldera ring dykes more likely to produce pyroclastic flows turbulent volcanic jets are produced by highly energetic explosive eruptions and may form buoyant plumes that rise many tens of kilometres into the atmosphere to form umbrella clouds or collapse to generate ground hugging pyroclastic flows ash injected into the atmosphere can be transported for many hundreds of kilometres with the potential to affect climate disrupt global air travel and cause respiratory health problems pyroclastic flows by contrast are potentially catastrophic to populations and infrastructure close to the volcano key to which of these two behaviours will occur is the extent to which the mechanical entrainment and mixing of ambient air into the jet by large entraining eddies forming the jet edge changes the density of the air ash mixture low entrainment rates lead to pyroclastic flows and high entrainment rates give rise to buoyant plumes recent experiments on particle laden multi phase volcanic jets from flared and straight sided circular openings suggest that the likelihood for buoyant plumes will depend strongly on the shape and internal geometry of the vent region this newly recognised sensitivity of the fate of volcanic jets to the structure of the vent is a consequence of a complex dynamic coupling between the jet and entrained solid particles an effect that has generally been overlooked in previous studies building on this work here we use an extensive series of experiments on multi phase turbulent jets from analogue linear fissures and annular ring fractures to explore whether the restrictive vent geometry during cataclysmic caldera forming ccf eruptions will ultimately lead a relatively greater frequency of pyroclastic flows than eruptions from circular vents on stratovolcanoes our results understood through scaling analyses and a one dimensional theoretical model show that entrainment is enhanced where particle motions contribute angular momentum to entraining eddies however because the size of the entraining eddies scales approximately with vent width the extent of entrainment is reduced as the vent width becomes small in comparison to its length consequently our work shows that for specified mass eruption rates the high length to width ratio vents typical of ccf events are more likely to produce pyroclastic flows we suggest that the enigmatic trend in the geological record for the largest ccf eruptions to produce pyroclastic flows is an expected consequence of their being erupted through continuous or piece wise continuous caldera ring fractures

onset of caldera collapse during ignimbrite eruptions the rapid depressurisation and discharge of volatile rich silicic magma chamber and the onset of caldera collapse associated with ignimbrite eruption is investigated using combined theoretical and experimental approaches we first present a scaling analysis of the force balance on the roof of a depressurising magma chamber this provides a failure criterion for the chamber roof and shows that the chamber underpressure required to trigger coherent piston collapse along a vertical or steeply outward dipping reverse ring fault increases with the roof aspect ratio r thickness width the failure criterion is validated by a series of experiments that reveal a transition with increasing roof aspect ratio from piston collapse to either 1 no collapse low reservoir underpressure or 2 non coherent collapse high reservoir underpressure then taking into account the failure criterion obtained with the scaling analysis a theoretical model allows calculation of the erupted chamber volume fraction required to trigger caldera collapse we consider a two stage model with an initially overpressured chamber that becomes progressively underpressured as the eruption proceeds the main input parameters are the chamber depth vertical extent and ellipticity in plan view the magma water content the cohesion and coefficient of internal friction of the roof and the ring fault dip in the case of a mechanically homogeneous roof and a single large and rapid eruption the erupted volume fraction required to trigger piston collapse increases with the roof aspect ratio until the chamber is totally emptied before initiation of any collapse at r 1 1 4 the exact value depending mainly on the ring fault dip and to a lesser extent on the coefficient of internal friction of the roof as shown by experiments collapse into reservoirs with r 1 1 4 probably cannot collapse as a piston and if a surface caldera forms at all collapse is more likely to occur non coherently data for the onset of caldera collapse during seven well documented major ignimbrite eruptions are in broad agreement with the model

volcanic and structural evolution of the okataina volcanic centre dominantly silicic volcanism associated with the taupo rift new zealand article the okataina volcanic centre ovc contains the northeasternmost caldera complex in onshore taupo volcanic zone tvz new zealand sited largely within the taupo rift pyroclastic fall deposits and ignimbrites deposited in the bay of plenty coast area between 420 and 625 ka were probably erupted from ovc and these provide a maximum age for the centre the earliest ignimbrite for which there is good evidence for eruption from ovc is the c 550 ka quartz biotite ignimbrite exposed to the west of lake okataina and to se of ovc this ignimbrite probably correlates with one of the early ignimbrites found in the kawerau geothermal wells and is large enough to have been accompanied by caldera collapse it was followed by extensive rhyolitic explosive eruptions of the murupara subgroup culminating with eruption of the matahina ignimbrite at c 325 ka this c 160 km 3 magma volume eruption was accompanied by caldera collapse to form the southern part of the present day okataina caldera complex a long duration sequence of rhyolite lavas and pyroclastics was then erupted on the southern and western sides of ovc before eruption of the 100 km

tarawera 1886 new zealand a basaltic plinian fissure eruption abstract new zealand s biggest and most destructive volcanic eruption of historical times was that of tarawera in 1886 the resulting scoria fall has a dispersal very similar in extent to that of the vesuvius a d 79 pumice fall and is one of the few known examples of a basaltic deposit of plinian type a new estimate of the volume 2 km 3 is significantly greater than previous estimates the basalt came mainly from a 7 km length of fissure and emission and exit velocity were fairly uniform along at least 4 km of it this is one of the few documented examples of a plinian eruption from a fissure vent primary welding of the scoria fall resulted where the accumulation rate exceeded about 250 mm min 1 a model of the eruption dynamics is proposed which leads to an estimate of 28 km for the height of the eruption cloud and implies a magma volatile fraction of 1 5 3 violent phreatic explosions occurred in the southwestern extension of the fissure across the rotomahana geothermal field and it is thought that some of the water responsible for the power of the plinian eruption came from this source though its amount was not sufficient to turn the eruption into a phreatoplinian one

revised estimates for the volume of the late bronze age minoan eruption santorini greece the late bronze age minoan eruption of santorini greece occurred from within an existing caldera low temperature pyroclastic flow emplacement on shallow slopes outside the caldera can only be consistent with the caldera being filled with eruption products that are not preserved field observations and seismic reflection surveys suggest that this missing material has been downfaulted the volume of the caldera infill is estimated as 18 26 km 3 dense rock equivalent dre increasing the total of the minoan eruption deposits to 78 86 km 3 dre and making it the largest known holocene eruption this study highlights a general mechanism whereby caldera eruption volumes could be significantly underestimated

the eruption pyroclastic flow behaviour and caldera in filling processes of the extremely large volume 1290 km3 intra to extra caldera permian ora ignimbrite formation southern alps italy abstract the permian ora formation 277 274 ma preserves the products of the ora caldera super eruption northern italy the stratigraphic architecture of the exceptionally well preserved intra caldera succession provides evidence for caldera collapse at the onset of the eruption a multiple discharge point fissure eruption style and progressive incremental caldera in filling by numerous pyroclastic flow pulses within the caldera the ignimbrites of the ora formation are voluminous 1290 km3 crystal rich 25 to 55 and ubiquitously welded the ora formation has been divided into four members a d which also define the principal eruption phases the eruption proceeded in four main stages 1 early caldera collapse and vent opening producing locally distributed basal co ignimbrite lithic breccia member a 2 vent clearing which produced the eutaxitic lithic rich ignimbrite and minor thin ground and ash cloud surge deposits member b 3 waxing and steady eruption which produced the dominant eutaxitic coarse crystal rich ignimbrite with local lithic rich and fine crystal rich ignimbrite and minor surge deposits member c and 4 waning eruption recorded by the eutaxitic fine crystal rich ignimbrite with local lithic rich ignimbrite deposits member d the incremental filling and late stage outpouring of pyroclastic material from the caldera is recorded by vertical and lateral lithofacies deposit variation and some correlation between stratigraphic sections these findings reveal a structure to the outwardly monotonous 1300 m thick intra caldera fill and thinner

release characteristics of overpressurised gas from complex vents implications for volcanic hazards many explosive volcanic eruptions produce underexpanded starting gas particle jets the dynamics of the accompanying pyroclast ejection can be affected by several parameters including magma texture gas overpressure erupted volume and geometry with respect to the latter volcanic craters and vents are often highly asymmetrical here we experimentally evaluate the effect of vent asymmetry on gas expansion behaviour and gas jet dynamics directly above the vent the vent geometries chosen for this study are based on field observations the novel element of the vent geometry investigated herein is an inclined exit plane 5 15 30 slant angle in combination with cylindrical and diverging inner geometries in a vertical setup these modifications yield both laterally variable spreading angles as well as a diversion of the jets where inner geometry cylindrical diverging controls the direction of the inclination both the spreading angle and the inclination of the jet are highly sensitive to reservoir conduit pressure and slant angle increasing starting reservoir pressure and slant angle yield 1 a maximum spreading angle up to 62 and 2 a maximum jet inclination for cylindrical vents up to 13 our experiments thus constrain geometric contributions to the mechanisms controlling eruption jet dynamics with implications for the generation of asymmetrical distributions of proximal hazards around volcanic vents

the 26 5 ka oruanui eruption new zealand an introduction and overview abstract the 26 5 ka oruanui eruption from taupo volcano in the central north island of new zealand is the largest known wet eruption generating 430 km3 of fall deposits 320 km3 of pyroclastic density current pdc deposits mostly ignimbrite and 420 km3 of primary intracaldera material equivalent to 530 km3 of magma erupted magma is 99 rhyolite and 90 pdc deposits range from mm to cm thick ultra thin veneers enclosed within fall material to 200 m thick ignimbrite in proximal areas the farthest travelled 90 km most energetic pdcs velocities 100 m s 1 occurred during phase 8 but the most voluminous pdc deposits were emplaced during phase 10 grain size variations in the pdc deposits are complex with changes seen vertically in thick proximal accumulations being greater than those seen laterally from near source to most distal deposits modern lake taupo partly infills the caldera generated during this eruption a 140 km2 structural collapse area is concealed beneath the lake while the lake outline reflects coeval peripheral and volcano tectonic collapse early eruption phases saw shifting vent positions development of the caldera to its maximum extent indicated by lithic lag breccias occurred during phase 10 the oruanui eruption shows many unusual features its episodic nature wide range of depositional conditions in fall deposits of very wide dispersal and complex interplay of fall and pdc activity

temperature evolution during magma ascent in basaltic effusive eruptions a numerical application to stromboli volcano the dynamics of magma ascent are controlled by the complex interdependent processes of crystallisation rheological evolution gas exsolution outgassing non ideal gas expansion and temperature evolution temperature changes within the conduit in particular play a key role on ascent dynamics since temperature strongly controls the crystallisation process which in turn has an impact on viscosity and thus on magma ascent rate the cooling produced by gas expansion is opposed by the heat produced by crystallisation and therefore the temperature profile within the conduit is quite complex this complexity means that unravelling the dynamics controlling magma ascent requires a numerical model unfortunately comprehensive integrated models with full thermodynamic treatment of multiple phases and rheological evolution are challenging to produce due to the numerical challenges involved until now models have tended to focus on aspects of the problem without a holistic approach in which petrological thermodynamic rheological and degassing processes and their interactions were all explicitly addressed and quantified here we present a new multiphase steady state model for magma ascent in which the main physical and chemical processes such as crystallisation degassing outgassing rheological evolution and temperature variations are quantitatively calculated basaltic magma s crystallisation and flow are sensitive to initial temperature and volatile content and therefore we investigate temperature variations during magma ascent in a basaltic system with a range of volatile contents as a test case we use one of the most well studied recent basaltic effusive eruptions the 2007 eruption of stromboli italy assuming equilibrium crystallisation and exsolution we compare the solutions obtained both with and without an isothermal constraint finding that temperature variations within the conduit have a significant influence on the ascent dynamics and therefore cannot be ignored when modelling basaltic volcanism furthermore we find good agreement between model results and volcanological observations when the non isothermal condition is assumed we investigate through a sensitivity analysis the role of magma chamber temperature on mass flow rate and crystal content we find that a temperature variation of 30 k at the base of the conduit has a bigger effect on mass discharge rate than an increase of 1 wt in water content finally we find that whilst variations in initial co2 concentration can affect the temperature pressure and viscosity profiles along the conduit they do not alter the macroscopic behaviour of the system c 2015 elsevier b v all rights reserved

global imaging of polar cap patches with dual airglow imagers during a 2 h interval from 2240 to 2440 ut on 12 november 2012 regions of increased 630 0 nm airglow emissions were simultaneously detected by dual all sky imagers in the polar cap one at longyearbyen norway 78 1 n 15 5 e and the other at resolute bay canada 74 7 n 265 1 e the resolute bay incoherent scatter radar observed clear enhancements of the f region electron density up to 1012 m 3 within these airglow structures which indicates that these are optical manifestations of polar cap patches propagating across the polar cap during this interval of simultaneous airglow imaging the nightside dawnside dayside duskside half of the patches was captured by the imager at longyearbyen resolute bay this unique situation enabled us to estimate the dawn dusk extent of the patches to be around 1500 km which was at least 60 70 of the width of the antisunward plasma stream seen in the super dual auroral radar network convection maps in contrast to the large extent in the dawn dusk direction the noon midnight thickness of each patch was less than 500 km these observations demonstrate that there exists a class of patches showing cigar shaped structures such patches could be produced in a wide range of local time on the dayside nearly simultaneously and spread across many hours of local time soon after their generation

the october 1902 plinian eruption of santa maria volcano guatemala abstract the october 1902 eruption of santa maria volcano guatemala was one of the largest this century it was preceded by a great earthquake on april 19 centered at the volcano as well as numerous other major earthquakes the 18 20 hour long plinian eruption on october 25 produced a column at least 28 km high reaching well into the stratosphere the airfall pumice deposit covered more than 1 2 million km2 with a trace of ash and was only two meters thick at the vent white dacitic pumice dark gray scoriaceous basalt with physically and chemically mixed intermediate pumice and loose crystals of plagioclase hornblende hypersthene biotite and magnetite make up the juvenile components of the deposit lithic fragments are of volcanic plutonic and metamorphic origin the plinian deposit is a fine grained crystal rich single pumice fall unit and shows inverse grading mapping of the deposit gives a volume of 8 3 km3 within the one mm isopach crystal concentration studies show that the true volume erupted was at least 20 km3 equivalent to 8 5 km3 of dense dacite and that 90 of the ejecta was less than 2 mm in diameter the plinian volume eruption rate averaged 1 2 105 m3s 1 and the average gas muzzle velocity of the column exceeded 270 ms 1 a total of 8 3 1018 j of energy were released by the eruption a knowledge of both theoretically derived eruption parameters and contemporary information allows a detailed analysis of eruption mechanisms this eruption was the major stratospheric aerosol injection in the 1902 1903 period however mid to low latitude northern hemisphere temperature deviation data for the years following the eruption show no significant temperature decrease this may be explained by the sulfur poor nature of dacite magmas suggesting that volatile composition rather than mass of volatiles is the controlling parameter in climatic response to explosive eruptions