formation of scoria cone during explosive eruption at izu oshima volcano japan 1 scoria cones have been regarded to be formed by accumulation of ballistic bombs ejected by mild eruptions however more recent geological investigations show that some scoria cones could be formed during explosive eruptions here we demonstrate how the scoria cone of the 1986 izu oshima eruption was formed during the explosive eruption we measured particle fractionation of the cone and propose a theoretical model to explain the observation the model considers lateral transport of particles by turbulent eddies particles that reached characteristic column radius l are laterally transported to l where they starts to free fall we obtained 1 2 and 2 5 for larger and smaller particles respectively which is consistent to the observation in the model extensive fallout takes place at the base of the column where it expands rapidly we suggest that lateral particle projection and the rapid column expansion are the key processes to form the cones

complexities of plinian fall deposition at vent an example from the 1912 novarupta eruption alaska abstract an extremely proximal ejecta ring with exposures to within 100 m of vent was deposited during later stage plinian fall activity during the 1912 novarupta eruption in alaska one bed in the ejecta ring bed s contains predominantly andesitic clasts which serve to delineate the striking contrast in thinning rates along dispersal axis of the ejecta ring pyle bt values of 70 m bed s alone or 190 m whole ejecta ring and the coeval dacitic plinian fall deposits pyle bt values of 4 km proximal and 37 km medial distal the locally deposited andesitic and dacitic clasts of the ejecta ring are interpreted as products of an irregular collar of low fountaining ejecta partially sheathing the core of higher velocity dacitic ejecta that fed the stable converting 23 km high column the presence of such an extremely proximal accumulation of ejecta appears to be a feature common to several other historic eruptions that generated widespread fall deposits this feature in part accounts for conflicts between measured and calculated values for thickness maxima in plinian fall deposits and suggests that modifications may be required of existing models for plinian eruption columns

crystal settling in a vigorously converting magma chamber there has been much debate concerning the mechanism of fractional crystallization in magma chambers the traditional hypothesis of crystal settling has been widely replaced by the concept of in situ crystallization coupled with compositional convection observations from layered intrusions however are equivocal1 4 doubts have been raised about crystal settling on theoretical grounds because convective velocities in magma chambers are often much greater than the crystal settling velocities predicted by stokes law5 but there has been no experimental study of crystal settling in such vigorous convection here we present physical considerations and laboratory experiments which show that the phenomenon of particle settling in these conditions can be accounted for by a simple theory application of this theory to crystal settling in magma chambers suggests that crystal settling may be an efficient differentiation mechanism at least in basaltic magma chambers despite large convective velocities

a pseudo adakite derived from partial melting of tonalitic to granodioritic crust kyushu southwest japan arc abstract cretaceous peraluminous granitic intrusions occur in kyushu in the southwest japan arc as batholiths or stocks accompanied by metaluminous tonalite to granodiorite the peraluminous intrusions are classed as adakitic based on sr y y ppm relationships but they contain abundant k feldspar a phase not common in typical adakites to consider their significance in the arc system we investigated the tsutsugatake stock as representative of the intrusions exposed in central kyushu the tsutsugatake rocks are mainly garnet bearing two mica leuco granitoids their sr y ratios and rare earth element concentrations are appropriate for adakite but k 2 o na 2 o ratios 0 53 1 43 and al 2 o 3 13 7 15 0 wt mgo 0 20 0 55 wt sr 207 462 ppm cr 12 ppm and ni 8 ppm contents differ from those of typical adakite geochemical modeling shows that the petrogenesis of the tsutsugatake stock can be explained by partial melting of arc type tonalite or adakitic granodiorite under middle to lower crustal conditions leaving amphibole and plagioclase as residual phases the differences between the tsutsugatake stock and classical adakites lie in their source rocks and melting conditions the tsutsugatake source was not basaltic and the melting pressure was insufficient to stabilize garnet the tsutsugatake rocks are here designated pseudo adakites such pseudo adakites may be found in both modern and ancient arcs and would play an important role in crustal evolution as a member of the recycled constituent

basaltic pyroclastic flows of fuji volcano japan characteristics of the deposits and their origin fuji volcano is the largest active volcano in japan and consists of ko fuji and shin fuji volcanoes although basaltic in composition small volume pyroclastic flows have been repeatedly generated during the younger stage of shin fuji volcano deposits of those pyroclastic flows have been identified along multiple drainage valleys on the western flanks between 1 300 and 2 000 m a s l and have been stratigraphically divided into the shin fuji younger pyroclastic flows syp 1 to 4 downstream debris flow deposits are found which contain abundant material derived from the pyroclastic flow deposits the new14c ages for syp1 to syp4 are 3 2 3 0 2 9 and 2 5 ka respectively and correspond to a period where explosive summit eruptions generated many scoria fall deposits mostly toward the east the syp1 to syp4 deposits consist of two facies the massive facies is about 2 m thick and contains basaltic bombs of less than 50 cm in size scoria lapilli and fresh lithic basalt fragments supported in an ash matrix the surge facies is represented by beds 1 to 15 cm thick consisting mainly of ash with minor amount of fine lapilli the bombs and scoria are 15 to 30 in volume within the massive facies the ashes within the syp deposits consist largely of comminuted basalt lithics and crystals that are derived from the middle stage lava flows exposed at the western flanks syp1 to syp4 were only dispersed down the western flanks the reason for this one sided distribution is the asymmetric topography of the edifice the western slopes of the volcano are the steepest over 34 degrees most pyroclastic materials cannot rest stably on the slopes steeper than 33 degrees therefore ejecta from the explosive summit eruptions that fell on the steep slopes tumbled down the slopes and were remobilized as high temperature granular flows these flows consisted of large pyroclastics and moved as granular avalanches along the valley bottom furthermore the avalanching flows increased in volume by abrasion from the edifice and generated abundant ashes by the collision of clasts the large amount of the fine material was presumably available within the transport system as the basal avalanches propagated below the angle of repose taking the typical kinetic friction coefficient of small pyroclastic flows such flows could descend the western flanks where scattered houses are below 1 000 m a s l a similar type of pyroclastic flow could result if explosive summit eruptions occur in the future

high sr y and la yb ratios the meaning of the adakitic signature abstract the name of adakite is used to describe a far too large group of rocks whose sole common feature is high sr y and la yb ratios however such a signature can be achieved via different processes melting of a high sr y and la yb source deep melting with abundant residual garnet fractional crystallization or afc or interactions of felsic melts with the mantle causing selective enrichment in lree and sr over hree a database of the compositions of adakitic rocks including high silica and low silica adakites martin h smithies r h rapp r p moyen j f champion d c 2005 an overview of adakite tonalite trondhjemite granodiorite ttg and sanukitoid relationships and some implications for crustal evolution lithos 79 1 2 1 24 continental adakites and archaean adakites was assembled geochemical modeling of the potential processes is used to interpret it and reveals that 1 the classical model of slab melting provides the best explanation for the genesis of high silica adakites 2 low silica adakites are explained by garnet present melting of an adakite metasomatized mantle i e at depths greater than 2 5 gpa 3 continental adakites is a term encompassing a huge range of rocks with a corresponding diversity of petrogenetic processes and most of them are different from both low and high silica adakites 4 archaean adakites show a bimodal composition range with some very high sr y examples similar to part of the ttg suite reflecting deep melting 2 0 gpa of a basaltic source with a relatively high sr y while lower sr y rocks formed by shallower 1 0 gpa melting of similar sources comparison with the archaean ttg suite highlights the heterogeneity of the ttgs whose composition spreads the whole combined range of hsa and archaean adakites pointing to a diversity of sources and processes contributing to the ttg suite

transition from oceanic to continental lithosphere subduction in southern tibet evidence from the late cretaceous early oligocene 91 30 ma intrusive rocks in the chanang zedong area southern gangdese little is known about the detailed processes associated with the transition from oceanic to continental lithosphere subduction in the gangdese belt of southern tibet gbst here we report zircon u pb age major and trace element and sr nd hf isotopic data for late cretaceous early oligocene similar to 91 30 ma intermediate acid intrusive rocks in the chanang zedong area immediately north of the yarlung tsangpo suture zone these rocks represent five magmatic episodes at similar to 91 similar to 77 similar to 62 similar to 48 and similar to 30 ma respectively the 91 48 ma rocks have slightly lower initial sr 87 sr 86 0 7037 to 0 7047 and higher epsilon nd t 1 8 to 4 3 and epsilon hf t 3 5 to 14 7 values in comparison with those 0 7057 to 0 7062 3 3 to 2 5 and 2 2 to 6 6 of the similar to 30 ma intrusive rocks the similar to 91 similar to 62 and similar to 30 ma rocks are geochemically similar to slab derived adakites the similar to 91 ma somka adakitic granodiorites were likely derived by partial melting of the subducting neo tethyan oceanic crust with minor oceanic sediments and the similar to 91 ma somka dioritic rocks with a geochemical affinity of adakitic magnesian andesites likely resulted from interactions between adakitic magmas and overlying mantle wedge peridotite the similar to 77 ma luomu diorites were probably generated by partial melting of juvenile basaltic lower crust the similar to 62 ma naika and zedong adaldtic diorites and granodiorites were likely generated mainly by partial melting of thickened juvenile mafic lower crust but the source region of the zedong adakitic rocks also contained enriched components corresponding to indian continental crust the similar to 48 ma lamda granites were possibly generated by melting of a juvenile basaltic crust the younger similar to 30 ma chongmuda adakitic quartz monzonites and minor granodiorites were most probably derived by partial melting of early oligocene north ward subducted indian lower crust beneath the southern lhasa block taking into account the regional tectonic and magmatic data we suggest that the gangdese belt of southern tibet gbst underwent a tectonodynamic transition from oceanic subduction to continental subduction between 100 and 30 ma it evolved through four stages 100 65 ma roll back of subducted neo tethyan oceanic lithosphere 65 60 ma initial collision between indian and asian continents 60 40 ma breakoff of subducted neo tethyan oceanic lithosphere and similar to 30 ma northward subduction of the indian continent c 2014 elsevier b v all rights reserved

morphometry of scoria cones and their relation to geodynamic setting a dem based analysis abstract the morphometry of a great number of scoria cones belonging to volcanic fields of various geodynamic settings has been measured and analyzed addressing the question whether there is a relation between the prevalent cone shape in a given field and the geodynamic setting of the field itself morphometric analysis was carried out on freely downloadable digital elevation models dems the accuracy of the used dems and the associated error in scoria cone morphometry were determined by cross comparing high resolution lidar derived dems usgs ned tinitaly dem and aster gdem the 10 m tinitaly 01 and usgs ned dems are proven to be suitable for scoria cone morphometry whereas aster gdem can be used reliably for cones with volume greater than 30 10 6 m 3 according to a detailed morphometry of all scoria cones we propose that the cones related to subductional setting show relatively higher values of h c o w co and lower values of w cr w co than the cones related to extensional setting the detected differences can be imputable to peculiar eruption dynamics resulting in slight but systematic changes in shape and differences in lithological and sedimentological characteristics that govern post eruptive erosion to constrain the pathway of scoria cone erosion the detected morphometric changes were also interpreted using a simple linear degradation model utilizing the obtained simulation results the inferred initial cone base and the age of scoria cones we calculated a diffusion coefficient k for several dated cones which are related to the prevalent climate our results despite the high error associated allow to assess the median k for all volcanic fields due to the complexity of the factors behind it is not easy to understand if the prevalent shape characterizing a certain volcanic field is due mainly to sin eruptive or post eruptive mechanisms however our distinction between the two main geodynamic settings may be the first step to decipher these factors

implications of crustal property variations for models of tibetan plateau evolution shear coupled teleseismic p waves sampling the interior of the tibetan plateau provide evidence of systematic variations in crustal structure the crust thins by up to 20 km from south to north with a concomitant increase in poisson s ratio from normal values in the south to unusually high values in the north this suggests that the crust of the northern plateau is partially melted due to high temperatures these changes imply spatial and perhaps temporal variations in the way the elevation of the high plateau is created and maintained

3he 4he ratios in well gases in the kinki district sw japan surface appearance of slab derived fluids in a non volcanic area in kii peninsula abstract a total of 27 well gas samples collected from sw japan were subjected to precise analysis of he ne and ar isotopic compositions the sampling area covers localities where mantle derived helium emanates in the fore arc of the kii peninsula and is known as the kinki spot e g sano and wakita j geophys res 90 1985 8728 8741 the kinki spot apparently is located within the fore arc with no obvious magmatic activity which contrasts to the occurrence of similarly high 3 he 4 he ratios along the volcanic front of ne japan our high density sampling revealed more clearly that the high 3 he 4 he ratio wells are distributed in the kii peninsula above the area where the young and hot philippine sea plate is subducting with a relatively steeper dip than in the adjacent area this situation favors the near trench side of the subducting slab entering eclogite facies conditions involving slab dehydration without inducing melting of the mantle wedge the aqueous fluids derived from the slab acquire mantle he during their passage through the mantle wedge thus the observed association of mantle he and the fore arc dehydration of the philippine sea plate indicates that fluid derived from the subducting slab is contributing to the occurrence of mantle he in well gases of the kii peninsula the high 3 he 4 he region of kii peninsula also coincides with the occurrence of long period tremors these are caused by the movement of fluids derived from the subducting slab localized stresses cause significant uplift in the area interpreted as indicating that intra crustal fractures provide pathways through the crust for the fluids derived from depth thus magma is not necessarily required as the carrier of mantle derived helium to the surface of the fore arc region of kii peninsula

sequence and eruptive style of the 1783 eruption of asama volcano central japan a case study of an andesitic explosive eruption generating fountain fed lava flow pumice fall scoria flow and forming a cone the 3 month long eruption of asama volcano in 1783 produced andesitic pumice falls pyroclastic flows lava flows and constructed a cone it is divided into six episodes on the basis of waxing and waning inferred from records made during the eruption episodes 1 to 4 were intermittent vulcanian or plinian eruptions which generated several pumice fall deposits the frequency and intensity of the eruption increased dramatically in episode 5 which started on 2 august and culminated in a final phase that began on the night of 4 august lasting for 15 h this climactic phase is further divided into two subphases the first subphase is characterized by generation of a pumice fall whereas the second one is characterized by abundant pyroclastic flows stratigraphic relationships suggest that rapid growth of a cone and the generation of lava flows occurred simultaneously with the generation of both pumice falls and pyroclastic flows the volumes of the ejecta during the first and second subphases are 0 21 km3 dre and 0 27 km3 dre respectively the proportions of the different eruptive products are lava cone pumice fall 84 11 5 in the first subphase and lava cone pyroclastic flow 42 2 56 in the second subphase the lava flows in this eruption consist of three flow units l1 l2 and l3 and they characteristically possess abundant broken phenocrysts and show extensive welding texture these features as well as ghost pyroclastic textures on the surface indicate that the lava was a fountain fed clastogenic lava a high discharge rate for the lava flow up to 106 kg s may also suggest that the lava was initially explosively ejected from the conduit the petrology of the juvenile materials indicates binary mixing of an andesitic magma and a crystal rich dacitic magma the mixing ratio changed with time the dacitic component is dominant in the pyroclasts of the first subphase of the climactic phase while the proportion of the andesitic component increases in the pyroclasts of the second subphase the compositions of the lava flows vary from one flow unit to another l1 and l3 have almost identical compositions to those of pyroclasts of the first and second subphases respectively while l2 has an intermediate composition suggesting that the pyroclasts of the first and second subphases were the source of the lava flows and were partly homogenized during flow the complex features of this eruption can be explained by rapid deposition of coarse pyroclasts near the vent and the subsequent flowage of clastogenic lavas which were accompanied by a high eruption plume generating pumice falls and or pyroclastic flows

repeated crustal thickening and recycling during the andean orogeny in north chile 21 26 s posttectonic granitoids 2 lower crustal p wave velocities of 7 3 7 7 km s 1 compatible with underplated mafic crust and 3 results from recent experimental petrologic work showing garnet stability in mafic mineralogies 12 kbar 40 km crustal thickness analogous to older andean magmatic episodes in north chile newly underplated basaltic crust may account for the remaining deficit in neogene crustal thickening similar evolutionary patterns in geochemistry and initial sr and nd isotopic characteristics of andean 200 ma to present magmatic rocks suggest that the andean orogeny in this region evolved by a combination of processes of repeated arc migration tectonic and magmatic crustal thickening and igneous recycling which was controlled by periodically changing plate convergence rates and obliquity and corresponding changes in the rheologic behavior of the continental crust index terms 1020 geochemistry composition of the crust 1030 geochemistry geochemical cycles 0330 8159 evolution of the earth rheology crust and lithosphere 9360 information related to geographic region south america keywords andean orogeny continental crust recycling adakite ttg crustal thickening

sedimentation from turbulent jets and plumes theoretical models are developed for the sedimentation from the margins of a particle laden axisymmetric turbulent buoyant plume in a still environment and for an axisymmetric turbulent momentum jet the models assume that the mass of each individual size fraction of sediment carried in a parcel of fluid decreases exponentially with time for relatively coarse particles the fallout models predict that the sediment deposition beyond a distance r on the ground expressed in log units should decay linearly with distance away from the vent for the momentum jet and should decrease with r1 3 for the buoyant plume the exponential decay constant j is proportional to the terminal fall velocity vt of the particles in both cases and inversely proportional to the square root of the initial momentum flux m0 for the jet fallout jj vtmo 1 2 and to the third power of the initial buoyancy flux fo for the plume fallout jp vtfo 1 3 smaller particles are affected by reentrainment caused by the turbulent eddies sweeping ambient fluid back into the plume or jet and thus reincorporating some particles that were released from the flow at greater heights this is taken into account by introducing a reentrainment coefficient into the theoretical models with the assumption that the coefficient has a constant value for a plume of given strength in new experiments fallout occurs from the margins of particle laden fresh water buoyant jets and plumes in a tank of salty water and sedimentation is measured on the tank floor two experiments were weakly affected by reentrainment and show excellent agreement with the simple theory for smaller particles and increasingly buoyant plumes and strong jets particle reentrainment is important the experimental data are fitted by the new reentrainment theory confirming that values of the reentrainment coefficient are approximately constant for a given flow a settling number is defined as the ratio of the characteristic velocity of the jet or plume to the particle settling velocity for 1 reentrainment seems to reach an equilibrium state for which the reentrainment coefficient is a constant of value 0 1 for jets and 0 4 for plumes irrespective of flow strength or particle size the plume experiments indicate that the value of the reentrainment coefficient is strongly dependent on plume strength and particle size for slightly less than 1 the general principles of sedimentation from turbulent plumes and jets are applied to the fallout of pumice from volcanic eruption columns and of metalliferous particles from black smokers on the ocean floor for volcanic eruptions the results provide an explanation for the near vent overthickening of tephra fall deposits and imply that lithic and pumice fragments from small lapilli up to at least 1 m diameter blocks are efficiently reentrained into eruption columns the size of particles reentrained in hydrothermal plumes is predicted to vary from less than 100 m in weakly buoyant plumes up to over 1000 m in megaplumes

epeirogenic uplift above a detached slab in northern central america p wave tomographic images reveal that the northern central america highlands east of the modern volcanic arc overlie a detached slab hypsometric analysis of the highlands in honduras demonstrates that the region is a dissected plateau that is disrupted by normal faults near the north american caribbean plate margin the dissected central american plateau contains a network of superimposed rivers with meanders cut into bedrock such a geomorphic character indicates that the regional uplift occurred in the absence of tilting we propose that the epeirogenic uplift of northern central america is the buoyant upper plate response to the influx of mantle asthenosphere following the break off and sinking of the slab

geochemical and nd pb isotopic characteristics of the tethyan asthenosphere implications for the origin of the indian ocean mantle domain abstract it is unclear why the pb nd and sr isotopic composition of the modern mid ocean ridge basalts morb from the indian ocean is different from that of the north atlantic and pacific oceans a possible explanation for this is that the indian morb type isotopic signature is a long lived regional feature of the mantle as evidently shown by the isotopic composition of the 350 ma morb like mian lue northern ophiolite which was formed in the same region presently occupied by the indian ocean however this hypothesis is in conflict with the lack of indian morb type isotopic signature in a number of 150 ma tethyan and indian ocean crusts to further constrain the origin of the indian morb type isotopic signature we analyze the geochemical and pb nd and sr isotopic composition of representative mafic rocks from four tethyan ophiolites ranging in age from 90 to 360 ma the sr isotopic composition of the samples is unreliable due to alteration but the age corrected nd and pb isotopic ratios and geochemical data indicate that these tethyan rocks were derived from a geochemically depleted asthenospheric source that had a clear indian morb type isotopic signature we therefore conclude that the bulk of the indian suboceanic mantle was most probably inherited from the tethyan asthenosphere a few regions in both the tethyan and indian oceans however are most probably underlain by pacific and north atlantic morb type mantle and vice versa because of the flow of the asthenosphere in response to tectonic plate reorganizations that lead to openings and closures of ocean basins the indian morb type isotopic signature of the western pacific marginal basin crusts could be due to either flow of the indian ocean mantle into the western pacific or to endogenous production of such an isotopic signature from delaminated east asian sublithospheric materials during closure of the tethys ocean

evaluation of morphometry based dating of monogenetic volcanoes a case study from bandas del sur tenerife canary islands morphometry based dating provides a first order estimate of the temporal evolution of monogenetic volcanic edifices located within an intraplate monogenetic volcanic field or on the flanks of a polygenetic volcano two widely used morphometric parameters namely cone height width ratio hmax wco and slope angle were applied to extract chronological information and evaluate their accuracy for morphometry based ordering based on these quantitative parameters extracted from contour based digital elevation models dems two event orders for the bandas del sur in tenerife canary islands were constructed and compared with the existing k ar paleomagnetic and stratigraphic data the results obtained suggest that the commonly used hmax wco ratio is not reliable leading to inappropriate temporal order estimates while the slope angle gives slightly better results the overall performance of such descriptive parameters was however generally poor i e there is no strong correlation between morphometry and age the geomorphic morphometric mismatches could be the result of 1 the diversity of syn eruptive processes i e diverse initial morphologies causing geomorphic morphometric variability 2 contrasting edifice specific degradation that depends partly upon the inner facies architecture of the volcanic edifices 3 various external environmental controls e g tephra mantling from pyroclastic density currents unrelated to the edifice evaluated and 4 differences in the scale resolution of input data the observed degradation trend and changes in morphometric parameters over time do not support a simple degradation model for monogenetic scoria cones volcanoes

total grain size distribution of a mafic subplinian tephra tb 2 from the 1986 izu oshima eruption japan an estimation based on a theoretical model of tephra dispersal abstract total grain size distribution was estimated using field data and theoretical modelling isopleth maps of each size were constructed at intervals of 1 phi based on the detailed sampling and analysis of a subplinian deposit tb 2 a product of the climatic phase of the 1986 eruption of the izu oshima volcano japan the umbrella spreading of the eruptive plume is analysed from isopleth maps based on a theoretical model of tephra dispersal bursik m i sparks r s j gilbert j s carey s n 1992 sedimentation of tephra by volcanic plumes i theory and its comparison with a study of the fogo a plinian deposit sao miguel azores bull volcanol 54 329 344 the dispersal of particles in the diameter range of 32 4 mm agrees with the model and the deduced column height is consistent with the visual observation using field data and the theoretical model the erupted mass of each size fraction is estimated and the total grain size distribution and total mass were obtained the observed mass loading decay as a function of the area enclosed by the isopleth is revised using the calculated terminal fall velocity and flux of the plume in the umbrella region the total grain size distribution of tb 2 indicates an intermediate nature between plinian and hawaiian eruption styles

assessing eruption column height in ancient flood basalt eruptions a buoyant plume model is used to explore the ability of flood basalt eruptions to inject climate relevant gases into the stratosphere an example from the 1986 izu oshima basaltic fissure eruption validates the model s ability to reproduce the observed maximum plume heights of 12 16 km above sea level sustained above fire fountains the model predicts maximum plume heights of 13 17 km for source widths of between 4 16 m when 32 by mass of the erupted magma is fragmented and involved in the buoyant plume effective volatile content of 6 wt assuming that the miocene age roza eruption part of the columbia river basalt group sustained fire fountains of similar height to izu oshima 1 6 km above the vent we show that the roza eruption could have sustained buoyant ash and gas plumes that extended into the stratosphere at 45 n assuming 5 km long active fissure segments and 9000 mt of so2 released during explosive phases over a 10 15 year duration the 180km of known roza fissure length could have supported 36 explosive events phases each with a duration of 3 4 days each 5 km fissure segment could have emitted 62 mt of so2 per day into the stratosphere while actively fountaining the equivalent of about three 1991 mount pinatubo eruptions per day each fissure segment could have had one to several vents which subsequently produced lava without significant fountaining for a longer period within the decades long eruption sensitivity of plume rise height to ancient atmospheric conditions is explored although eruptions in the deccan traps 66ma may have generated buoyant plumes that rose to altitudes in excess of 18 km they may not have reached the stratosphere because the tropopause was substantially higher in the late cretaceous our results indicate that some flood basalt eruptions such as roza were capable of repeatedly injecting large masses of so2 into the stratosphere thus sustained flood basalt eruptions could have influenced climate on time scales of decades to centuries but the location i e latitude of the province and relevant paleoclimate is important and must be considered

complex proximal sedimentation from plinian plumes the example of tarawera 1886 the 1886 eruption of tarawera new zealand was unusual for a plinian eruption because it involved entirely basaltic magma originated in a 17 km long fissure and produced extremely overthickened proximal deposits with a complex geometry this study focuses on an 8 km long segment cutting across mount tarawera where over 50 point source vents were active during the 5 5 h eruption a detailed characterization of the proximal deposits is developed and used to interpret the range of styles and intensities of the vents including changes with time we identify the four vents that contributed most heavily to the plinian fall and evaluate the extent to which current volcanic plume models are compatible with the depositional patterns at tarawera three proximal units are mapped that have phreatomagmatic magmatic and phreatomagmatic characteristics respectively within the magmatic proximal unit beds of like character are grouped into packages and delineated on scaled cross sections package dispersal is quantified by measuring the linear thickness half distance t1 2 in the planes of the fissure walls most packages have localized dispersals low t1 2 indicating that strombolian style activity dominated most vents the more widely dispersed packages high t1 2 reflect contributions from additional transport regimes that were more vigorous but still contributed considerable material to the proximal region we conclude that the geometry of the observed proximal deposits requires three modes of fall transport 1 fallout from the upper portions of the plinian plumes produced principally by vents in four craters 2 sedimentation from the margins of the lower portions of the plinian plumes including the jets and possibly the lower convective regions and 3 ejection by weak strombolian style explosions from vents that did not contribute significant volumes of particles to the high plume we suggest that the curvature of the velocity profile across the jet region of each plume 1 4 km height was important and that the lower velocity at the margins allowed proximal deposition of a large volume of material with a wide grain size range

early eocene crustal thickening in southern tibet new age and geochemical constraints from the gangdese batholith abstract the lhasa terrane southern tibet is where the thickest part of the earth s crust occurs as a result of the india asia collision this paper reports zircon u pb age and geochemical data of samples collected from the gangdese batholith southern lhasa terrane a subset of which has already been dated at 65 41 ma using zircon u pb geochronology ji w q wu f y chung s l li j x liu c z 2009a zircon u pb chronology and hf isotopic constraints on the petrogenesis of gangdese batholiths southern tibet chemical geology 262 229 245 to constrain the timing and mechanism s responsible for crustal thickening beneath southern tibet our new data indicate that the gangdese magmatism lasted continuously from ca 65 to 34 ma during which geochemical evidence for crustal thickening is observed crustal thickening is best illustrated by the progressive increase in la yb and sr y of the early to latest eocene magmatic rocks associated with the decrease in hf isotopic ratios of paleocene to eocene magmatic zircons we also identify in this study the oldest ca 51 ma post collisional adakitic granitoids in the gangdese batholith interpreted as products from partial melting of underplated basaltic lower crust therefore a thickened crust likely existed by that time at least locally in the southern lhasa terrane tibet

petrogenesis of the early eocene adakitic rocks in the napuri area southern lhasa partial melting of thickened lower crust during slab break off and implications for crustal thickening in southern tibet abstract cenozoic adakitic rocks in the lhasa block southern tibet have been widely used to trace the lateral extent of crustal thickening however their petrogenesis remains controversial here we report geochronological and geochemical data for the napuri intrusive rocks in the core area of the quxu batholith southern lhasa zircon u pb dating suggests that they were generated at approximately 48 ma the studied samples show significant geochemical variations manifested by the coexistence of three types of igneous rocks groups i and ii rocks exhibit variable and high sio 2 66 4 73 9 wt high al 2 o 3 14 0 17 4 wt k 2 o 3 9 5 3 wt sr 273 718 ppm and sr y 18 3 to 81 3 values and low y 3 6 to 16 ppm heavy rare earth element ree e g yb 0 48 to 1 8 ppm mgo 0 4 1 0 wt cr 2 9 7 4 ppm and ni 1 6 4 5 ppm contents which are similar to those of thickened lower crust derived adakitic rocks the group i rocks show higher sr y 77 5 81 3 ratios and lower total ree 55 5 63 2 ppm contents with clearly positive eu and sr anomalies whereas the group ii rocks have relatively lower sr y 18 3 65 7 ratios and higher total ree 115 375 ppm contents with negligible or slightly negative eu and sr anomalies group iii rocks have the highest sio 2 74 5 76 0 wt y 17 0 23 7 ppm and yb 2 91 3 30 ppm contents and the lowest al 2 o 3 12 5 13 2 wt sr 81 3 141 ppm and sr y 4 8 5 9 values with distinctly negative eu and sr anomalies compared with the jurassic cretaceous granitoids in southern lhasa the relative enrichment in sr nd hf isotopic compositions 87 sr 86 sr i 0 7049 0 7055 e nd t 0 3 0 7 and e hf t zircon 3 6 11 4 for the napuri intrusive rocks indicates that they likely contained indian continental components the group i and group ii rocks most probably originated from thickened mafic lower crust amphibolite eclogites or garnet amphibolites with garnet rutile plagioclase as residual minerals in the source at 1 5 gpa corresponding to depths of 50 km and group iii rocks were probably generated by fractional crystallization of plagioclase from the adakitic magmas taking into account the narrow linear nature of the eocene magmatic belt and reported synchronous asthenosphere derived basaltic rocks in southern lhasa we suggest that upwelling asthenosphere triggered by the break off of subducted neo tethyan slab probably provided the required thermal conditions for lower crustal melting the identification of indian continental components in the napuri intrusive rocks probably indicates that the asia india collision had taken place prior to their emplacement the dramatic changes in the la yb n ratios and e nd t and e hf t values of magmatic rocks in the gangdese area at ca 51 46 ma indicate that the cenozoic crustal thickening associated with the indentation of the indian continent began in the early eocene ca 51 46 ma at the latest

reconstruction of eruption column dynamics on the basis of grain size of tephra fall deposits 2 application to the pinatubo 1991 eruption the granulometric methods to reconstruct eruption column dynamics developed in paper 1 are applied to the tephra fall deposits of the climactic plinian phase of the 1991 eruption at pinatubo the tephra fall deposits are composed of two units layer c1 which corresponds to the first half of the climactic phase and layer c2 the second half the granulometric estimates of the expansion rate of the umbrella cloud for layers c1 and c2 are 7 1010 and 3 1010 m3 s respectively which agree with the observations of satellite images these estimates indicate that the magma discharge rate decreased from 9 108 to 3 108 kg s during the eruption the grain size distribution at the top of the eruption column is characterized by depletion of coarse clasts suggesting a distinct deceleration in the gas thrust region down to several tens of meters per second particularly during the second half of the climactic phase the total amount of layers c1 and c2 is estimated to be 3 1012 kg and the very fine particles which did not deposit in the accessible on land area may occupy up to 60 of the total ejecta the effective duration of the eruption estimated by the present methods a few hours or less is substantially shorter than a previous estimate from the real time observation based on infrasonic data 10 hours this discrepancy may be attributed to the overestimation of the magma discharge rate due to the entrainment of the ambient air at the cloud top and or to the underestimation of the total amount of the ejecta due to the effect of the contemporaneous plinian activity and generation of pyroclastic flows it is also suggested that the intensity of eruption was fluctuating with time so that the instantaneous observations such as cloud height in satellite images do not always show a quantitative agreement with the time averaged features predicted by the present methods

petrologic case for eocene slab breakoff during the indo asian collision the greater himalayan sequence is the metamorphic core of the himalaya and has been a focus of considerable study yet its petrologic evolution remains controversial pre oligocene metamorphism was nearly obliterated by miocene metamorphism and melting which many workers ascribe to shear heating unusually high concentrations of radioactive elements accompanying burial and or several kilometers of miocene exhumation due to extensional faulting sparse oligocene to eocene ages are often assigned to a tectonically unspecified event an alternative slab breakoff model subduction of greater himalayan rocks to 100 km depth in the eocene followed by buoyant extrusion due to decoupling of the oceanic lithosphere has also been proposed based on rare eocene eclogites but without explanation of miocene melting or metamorphic petrogenesis we argue that slab breakoff readily explains the eocene eclogites miocene partial melts and late eocene k rich magmas in southeastern tibet and that metamorphic and plutonic ages help define the timing and rates of breakoff and extrusion this model implies that 1 much of the greater himalayan sequence was subducted to depths greater than commonly considered 2 fluid absent decompression melting at 30 35 km depth was the consequence of as much as 100 km of extrusion rather than radioactive heating or a smaller crustal level extensional or erosional event and 3 eruption of eocene k rich tibetan plateau lavas has no implication for topography of the tibetan plateau

particle segregation of an eruption plume as revealed by a comprehensive analysis of tephra dispersal theory and application abstract a new way to deduce particle segregation in an uprising eruption column is developed this method subdivides an eruption column into horizontal slices and particle dispersal from each column slice is calculated using an advection diffusion model named tephra2 a grid search is employed to obtain the particle segregation from each column slice and other unknown parameters vent position diffusion coefficient and particle density the initial conditions for the calculation were mass loading data collected on the ground wind profile and minimum empirical parameters this method is applied to the 1986b eruption of izu oshima volcano japan which was a sub plinian eruption with a vertically uprising column of 10 km high a previous study of this eruption employed a gravity current model which assumes that most of the tephra segregation took place at the column umbrella transition and at the base of the umbrella cloud the gravity current model was used to calculate the base height of the umbrella cloud from the mass loading distribution on the ground and the calculated base height 7 9 km is consistent with the observed column height however the present study shows that most of the particles segregated from the lower parts of the column 1 3 km high and no significant fallout from the upper part of the column was observed even though the segregation pattern is different between the present and past studies the total mass and total grain size distribution are similar this coincidence is attributed to the use of the same decay function of mass loading in both models both assume the exponential decay of mass loading as a function of the area enclosed by the isopleth although the physical meaning of the decay rate is different in the two models this conclusion also implies that the shape of the decay function itself cannot be used to determine which model is appropriate in describing the source column however the applicability of the gravity current model should be justified when the decay rate is low and when an unreasonably high diffusion coefficient is obtained by inversion techniques that use advection diffusion models the new approach of this study also enables the calculation of the total mass and total grain size distribution without hand drawn isopleth maps and uses all of the point data with error evaluations for each point data the calculation results e g the erupted mass are shown with associated errors

a numerical study of turbulent mixing in eruption clouds using a three dimensional fluid dynamics model 1 the dynamics of eruption clouds in explosive volcanic eruptions are governed by entrainment of ambient air into eruption clouds by turbulent mixing we develop a new numerical pseudo gas model of an eruption cloud by employing three dimensional coordinates a third order accuracy scheme and a fine grid size in order to investigate the behavior of entrainment due to turbulent mixing the quantitative features of entrainment are measured by a proportionality constant relating the inflow velocity at the edge of the flow to the average vertical velocity i e the entrainment coefficient our model has successfully reproduced the quantitative features of entrainment observed in the laboratory experiments as well as fundamental features of the dynamics of eruption clouds such as the generation of eruption columns and or pyroclastic flows the value of the entrainment coefficient for eruption clouds is estimated from the column height and critical condition for column collapse by comparing results of our model with those of previous one dimensional models it is suggested that the value of the entrainment coefficient for an eruption cloud is approximately constant although the value estimated from the critical condition for column collapse k 0 07 is slightly smaller than that based on the column height k 0 1 this difference reflects the vertical change of flow structure in the eruption cloud the eruption cloud in the upper region exhibits a meandering instability which leads to efficient mixing whereas the cloud near the vent maintains a concentric structure with an inner dense core surrounded by an outer shear region our model is consistent with previous one dimensional models for steady eruption clouds supported by the laboratory experiments and it is also applicable to unsteady and transient features of actual eruption clouds

gpr investigation of tephra fallout cerro negro volcano nicaragua a method for constraining parameters used in tephra sedimentation models most tephra fallout models rely on the advection diffusion equation to forecast sedimentation and hence volcanic hazards here we test the application of the advection diffusion equation to tephra sedimentation using data collected on the proximal 350 to 1 200 m from the vent to medial greater than 1 200 m from the vent tephra blanket of a basaltic cinder cone cerro negro volcano located in nicaragua our understanding of tephra depositional processes at this volcano is significantly improved by combination of sample pit data in the medial zone and high resolution ground penetrating radar gpr data collected in the near vent and proximal zones if the advection diffusion equation applies then the thickness of individual tephra deposits should have gaussian crosswind profiles and exponential decay with distance away from the vent at cerro negro steady trade winds coupled with brief eruptions of relatively low energy vei 2 3 create relatively simple deposits gpr data were collected along three crosswind profiles at distances of 700 1 600 m from the vent sample pits were used to estimate thickness of the 1992 tephra deposit up to 13 km from the vent horizons identified in proximal gpr profiles exhibit gaussian distributions with a high degree of statistical confidence with diffusion coefficients of 500 m2 s 1 estimated for the deposits confirming that the advection diffusion equation is capable of modeling sedimentation in the proximal zone the thinning trend downwind of the vent decreases exponentially from the cone base 350 m to 1 200 m from the vent beyond this distance deposit overthickening occurs identified in both gpr and sample pit datasets the combined data reveal three depositional regimes 1 a near vent region on the cone itself where fallout remobilizes in granular flows upon deposition 2 a proximal zone in which particles fall from a height of less than 2 km and 3 a medial zone in which particles fall from 4 to 7 km and the deposit is thicker than expected based on thinning trends observed in the proximal zone of the deposit this overthickening of the tephra blanket defining the transition from proximal to medial depositional facies is indicative of transition from sedimentation dominated by fallout from plume margins to that dominated by fallout from the buoyant eruption cloud a feature of deposits previously identified in larger volume eruptions we interpret this change to represent a change in diffusion law occurring at total particle fall times the fall time threshold of numerical models of 400 s thus the detailed gpr profiles and pit data collected at cerro negro help to validate current numerical models of tephra sedimentation

formation of clastogenic lava flows during fissure eruption and scoria cone collapse the 1986 eruption of izu oshima volcano eastern japan the 1986 eruption of b fissure at izu oshima volcano japan produced among other products one andesite and two basaltic andesite lava flows locally the three flows resemble vent effused holocrystalline blocky or aa lava however remnant clast outlines can be identified at most localities indicating that the flows were spatter fed or clastogenic the basaltic andesite flows are interpreted to have formed by two main processes a reconstitution of fountain generated spatter around vent areas by syn depositional agglutination and coalescence followed by extensional non particulate flow and b syn eruptive collapse of a rapidly built spatter and scoria cone by rotational slip and extensional sliding these processes produced two morphologically distinct lobes in both flows by a earlier non particulate flow of agglutinate and coalesced spatter which formed a thin lobe of rubbly aa lava ca 5 m thick with characteristic open extension cracks revealing a homogeneous holocrystalline interior and b later scoria cone collapse which created a larger lobe of irregular thickness 20 m made of large detached blocks of scoria cone interpreted to have been rafted along on a flow of coalesced spatter the source regions of these lava flows are characterized by horseshoe shaped scarps 30 m high with meso blocks ca 30 m in diameter of bedded scoria at the base one lava flow has a secondary lateral collapse zone with lower ca 7 m scarps backward tilted meso blocks are interpreted to be the product of rotational slip and forward tilted blocks the result of simple toppling squeeze ups of coalesced spatter along the leading edge of the meso blocks indicate that coalescence occurred in the basal part of the scoria cone this low viscosity coalesced spatter acted as a lubricating layer along which basal failure of the scoria cone occurred rotational sliding gave way to extensional translational sliding as the slide mass spread out onto the present caldera floor squeeze ups concentrated at the distal margin indicate that the extensional regime changed to one of compression probably as a result of cooling of the flow front sliding material piled up behind the slowing flow front and coalesced spatter was squeezed up from the interior of the flow through fractures and between rafted blocks the andesite flow although morphologically similar to the other two flows has a slightly different chemical composition which corresponds to the earliest stage of the eruption it is a much smaller lava flow emitted from the base of the scoria cone 2 days after the eruption had ceased this lava is interpreted to have been formed by post depositional coalescence of spatter under the influence of the in situ cooling rate and load pressure of the deposit extrusion occurred through the lower part of the scoria cone and subsequent non particulate flow of coalesced material produced a blocky and aa lava flow the mechanisms of formation of the lava flows described may be more common during explosive eruptions of mafic magma than previously envisaged

sedimentation of tephra by volcanic plumes i theory and its comparison with a study of the fogo a plinian deposit sao miguel azores sedimentation of ejecta from volcanic plumes has been studied as a function of distance from the source in the fogo a plinian deposit sao miguel azores the fogo a trachytic pumice deposit is reversely graded and can be divided into two parts on the basis of pumice colour abundance of syenite accessory lithic clasts and distribution the lower syenite poor part was dispersed to the south and was clearly influenced by wind the upper syenite rich part is coarsegrained and has a nearly symmetrical distribution around the vent elongation of isopachs to the east indicate a weak wind influence the grain size variations of lithic and crystal components in the upper coarse part were studied total accumulation and accumulation per unit area expressed in kg m2 show good fits to a gaussian function at distances greater than 7 km for grain diameters less than 2 cm these results agree with a theoretical model for a radially spreading turbulent current moving over a quiescent fluid the gaussian coefficient is shown to be a function of grain size and the flow rate of material into the umbrella region of the eruption column the coefficient is therefore also a function of column height the column height deduced from these data is 21 km which is in broad agrrement with the column height of 27 km deduced from maximum clast dispersal using the method of carey and sparks 1986 the accumulation of clasts larger than 2 cm agrees with a theory for the fallout of clasts from the margins of the ascending eruption column which treats the plume as a succession of large eddies that decrease their mass of particles as an exponential function of time calculations are also presented for the influence of the radial inflow of surrounding air into the column on the deposition of clasts these calculations constrain the wind speed during the later part of the fogo a eruption to be at most a few metres per second the study has allowed four different dynamic categories of clast behaviour to be recognised in eruption columns

mid eocene high sr y granites in the northern himalayan gneiss domes melting thickened lower continental crust abstract within the himalayan collisional belt granotoids occur along two sub parallel belts the northern himalayan gneiss domes nhgd and the high himalayan crystalline series hhcs in the yardoi area of nhgd two mica granite a new type granite occurs in the core of the yardoi gneiss dome ygd dala and quedang from north to south and extends at least 50 km long these granites have similar mineral composition elemental and radiogenic isotope geochemistry and age of formation shrimp zircon u pb dating indicates that the yardoi and the quedang two mica granites formed at 42 6 1 1 ma and 42 8 0 6 ma respectively similar to the dala pluton these two mica granites have 1 high sio2 68 wt al2o3 15 wt and a cnk 1 0 2 relatively high sr and lree but low y 40 and up to 250 and la yb 30 4 very weak or no eu anomalies and 5 as compared with those in the himalayan leucogranites low initial sr 87sr 86sr i

late cretaceous crustal growth in the gangdese area southern tibet petrological and sr nd hf o isotopic evidence from zhengga diorite gabbro abstract recent studies of gangdese granitic magmatism demonstrate a mantle contribution to crustal growth in southern tibet during the jurassic early eocene however the specific mechanism for adding such juvenile crust has been disputed owing to a lack of reliable evidence for contemporaneous deep mantle geodynamic processes here we report on the zhengga diorite gabbro suite from the gangdese area they consist of plagioclase labradorite and anorthite amphibole clinopyroxene biotite and minor magnetite epidote zircon and apatite la icp ms zircon u pb dating for two samples gives a ca 94 ma age for the zhengga intrusive rocks i e the late cretaceous apart from one high sio 2 52 2 wt diorite sample with slightly high k 2 o 1 72 wt and initial 87 sr 86 sr 0 7068 and low e nd t 5 6 values the gabbro samples are geochemically characterized by low sio 2 39 8 50 1 wt and k 2 o 0 3 1 1 wt strongly negative nb ta and positive sr anomalies and uniform initial 87 sr 86 sr 0 7043 0 7048 the gabbros can be divided into two groups group i gabbros with relatively low total rare earth element ree rb and sio 2 contents and positive eu anomalies and group ii gabbros with slightly higher total ree rb and sio 2 contents and negative eu anomalies the group i gabbros have e nd t 1 7 to 4 1 and zircon e hf t 6 5 to 11 1 and 18 o 5 89 to 7 24 values which are slightly different to those of the group ii gabbros 0 2 to 2 0 2 9 to 6 5 and 6 24 to 7 05 trace element compositions of amphibole and clinopyroxene grains suggest that the zhengga mafic magmas contained a significant fluid transported component probably released from subducted oceanic lithosphere we suggest that the parental magmas of the zhengga gabbros were generated by the hydrous partial melting of lithospheric mantle metasomatized by sediment melts fluids the group i gabbros were likely generated by the fractional crystallization of olivine or clinopyroxene from such parental magmas with insignificant crustal contamination whereas the group ii gabbros were probably produced by assimilation and fractional crystallization afc processes from mafic magmas that were geochemically similar to the group i gabbros pre collisional underplating of mantle derived mafic magmas likely played an important role in crustal growth and supplied the source materials for some late late cretaceous cenozoic granitoids of the gangdese batholiths this study also demonstrates that the hydrous partial melting of mantle wedge triggered by the dehydration in a subduction setting has a capacity to create significant volumes of juvenile continental crust

highly explosive eruption of the monogenetic 8 6 ka bp la vache et lassolas scoria cone complex chaine des puys france abstract the eruption of the trachy basaltic la vache and lassolas cone complex was the youngest eruption ca 8 6 ka bp and one of the most violent in the chaine des puys france here we present field data and results of grain size componentry and clast density measurements of different layers of the widespread tephra deposit that is associated with this cone forming eruption our data indicates five main eruption phases comprising a vent opening phase a second sustained highly explosive phase a third and fourth violent strombolian phase and a fifth dominantly effusive phase the layer formed by the opening phase is rich in lithic material which was previously considered to be the result of phreatomagmatic activity the data presented here on the componentry and textures of the pyroclastic material contradict this hypothesis we propose instead that the material of the basal layer results from fragmentation caused by the explosion of a first arriving gas dominated phase the variations in eruption intensity during the main eruption phases are interpreted here to be the result of gas segregation within the plumbing system and fluxes in the magma ascent rate during the eruption significant amount of gas segregation is indicated by the deposition of both gas poor and gas rich material and by the presence of plate tephra this is also supported by the simultaneous ejection of tephra and lava from both cones during most of the explosive activity we suggest that gas segregation occurred within shallow intrusions and that fresh ascending material in the main conduit mixed with degassed material that flow back into the conduit from the intrusion before fragmentation the interaction of the ascending magma and the opening of intrusions may have controlled the evolution and explosivity of the eruption the high explosivity at the beginning of the eruption and the wide dispersal area demonstrate that scoria cone eruptions in monogenetic fields can impose a major threat to the population and infrastructures nearby as these events may occur with little warning and therefore research on this kind of eruptions is of a major importance to better understand the processes driving these events

the origin and pre cenozoic evolution of the tibetan plateau different hypotheses have been proposed for the origin and pre cenozoic evolution of the tibetan plateau as a result of several collision events between a series of gondwana derived terranes e g qiangtang lhasa and india and asian continent since the early paleozoic this paper reviews and reevaluates these hypotheses in light of new data from tibet including 1 the distribution of major tectonic boundaries and suture zones 2 basement rocks and their sedimentary covers 3 magmatic suites and 4 detrital zircon constraints from paleozoic metasedimentary rocks the western qiangtang amdo and tethyan himalaya terranes have the indian gondwana origin whereas the lhasa terrane shows an australian gondwana affinity the cambrian magmatic record in the lhasa terrane resulted from the subduction of the proto tethyan ocean lithosphere beneath the australian gondwana the newly identified late devonian granitoids in the southern margin of the lhasa terrane may represent an extensional magmatic event associated with its rifting which ultimately resulted in the opening of the songdo tethyan ocean the lhasa northern australia collision at similar to 263 ma was likely responsible for the initiation of a southward dipping subduction of the bangong nujiang tethyan oceanic lithosphere the yarlung zangbo tethyan ocean opened as a back arc basin in the late triassic leading to the separation of the lhasa terrane from northern australia the subsequent northward subduction of the yarlung zangbo tethyan ocean lithosphere beneath the lhasa terrane may have been triggered by the qiangtang lhasa collision in the earliest cretaceous the mafic dike swarms ca 284 ma in the western qiangtang originated from the panjal plume activity that resulted in continental rifting and its separation from the northern indian continent the subsequent collision of the western qiangtang with the eastern qiangtang in the middle triassic was followed by slab breakoff that led to the exhumation of the qiangtang metamorphic rocks this collision may have caused the northward subduction initiation of the bangong nujiang ocean lithosphere beneath the western qiangtang collision related coeval igneous rocks occurring on both sides of the suture zone and the within plate basalt affinity of associated mafic lithologies suggest slab breakoff induced magmatism in a continent continent collision zone this zone may be the site of net continental crust growth as exemplified by the tibetan plateau c 2012 international association for gondwana research published by elsevier b v all rights reserved

pyroclastic edifices record vigorous lava fountains during the emplacement of a flood basalt flow field roza member columbia river basalt province usa the 1300 km 3 tholeiitic lava flow field of the 14 98 ma roza member of the columbia river basalt province has the best preserved vent system of any known continental flood basalt detailed geological mapping and logging of the exposed pyroclastic rocks along the 180 km long vent system enable the reconstruction of pyroclastic edifices partial cones built around vents the pyroclastic cones differ from those constructed during typical basaltic effusive eruptions and are characterized by low to moderate slope angles 1 km fountains enhanced by fallout from the lower parts of convective plumes that rose above the fountains