a semi quantitative method for the reconstruction of eustatic sea level history from seismic profiles and its application to the southern south china sea abstract a quantitative model is proposed to estimate the magnitude of eustatic sea level fluctuations on the basis of definitions of relative and eustatic sea level in this model factors such as erosion sedimentation compaction basement tectonic and loading subsidence and paleo bathymetry are considered to estimate incremental changes in eustatic sea level we developed methods to solve for the variables involved in the model on the basis of interpretation of seismic profiles we reconstructed the onlap offlap points removed by erosion by fitting the effective thickness in areas without erosion with a piecewise defined function and extrapolating this function to zero thickness we used decompaction and backstripping algorithms to remove the effect of compaction and to calculate loading subsidence of the basement regional data however are needed to estimate the paleo water depths and the tectonic subsidence for this reason our methodology is considered semi qualitative as an application of this model an eustatic sea level curve since the pliocene 5 33 my was deduced from interpretation of our high resolution seismic data from the northern sunda shelf south china sea scs on this curve 36 fourth order sea level cycles were recognized with periods ranging from 0 08 to 0 29 my the curve matches well with the smoothed deep sea stable oxygen isotope curves from benthic foraminifera

caldera formation in the rum central igneous complex scotland the northern marginal zone of the rum central igneous complex in nw scotland represents part of the early felsic phase of the volcano the marginal zone is a relic of the early caldera floor and the infilling of sedimentary and igneous rocks its formation has been explored through field examination of the ring fracture system of the complex and its pyroclastic and epiclastic intracaldera facies a sequence of magmatic tumescence and chamber growth caused initial doming followed by the formation of a collapse structure without accompanying volcanism this collapse structure circular in plan is akin in origin to a salt basin formed by crustal stretching above a rising diapir we call this the proto caldera collapse breccias which represent the slumping and sliding of megablocks blocks and boulders of the torridonian sandstones which form the walls of the basin were the original infilling logs of these deposits reveal considerable variation in thickness of the breccias from 80 170 m in the complex indicating an uneven floor to the proto caldera consistent with piecemeal collapse following accumulation of up to 70 m thickness of breccia thin interbedded rhyodacitic crystal tuffs 10 30 cm record the earliest eruptions of felsic magma in the caldera caldera formation was then interrupted by a period of quiescence recorded by the presence of an epiclastic sandstone of locally several metres thickness formed by washout of fines from the breccias subsequent resurgence created a fracture pattern characteristic of doming along which rhyodacite magma rose in dykes and erupted up to perhaps 10 km3 of rhyodacitic intracaldera ignimbrites this major eruption caused further incremental subsidence of the caldera floor into a now partly emptied magma chamber mafic inclusions in the ignimbrites point to the eruption being triggered by multiple injections of basic magma into a chamber occupied by felsic magma

on the effect of crustal layering on ring fault initiation and the formation of collapse calderas collapse calderas can be attributed to subsidence of the magma chamber roof along bounding sub vertical normal faults ring faults after a decompression of the magma chamber it has previously been shown that for ring faults to initiate and thus facilitate collapse the stress field both at the surface and around the magma chamber must satisfy specific critical conditions here we present new numerical models that use a finite element method to investigate the effects of crustal layering on local stress field distribution results are compared with existing criteria for ring fault initiation different subsurface scenarios were simulated by varying the stiffness young s modulus of layers placed above the magma chamber and the host rock in which the chamber is seated we consider depressurisation of a magma chamber so as to simulate magma withdrawal results indicate that mechanical layering is a further first order variable in the rare achievement of stress conditions required for ring fault formation and may be influential in facilitating or inhibiting caldera collapse we show that for a given geometrical set up the magnitude and position of maximum tensional stress at the earth s surface are influenced by the occurrence and relative distribution of mechanically stiff or soft lithologies above the magma chamber for example tensional stress at surface may be reduced by the presence of stiff layers e g lavas or increased by soft layers e g pyroclastic units compared to generic simulations using a homogeneous background medium overall we find that the presence of mechanically soft material promotes surface fracture initiation in addition we suggest that the position of peak tensional stress at surface derived by the numerical models does not represent that related to the position of the bounding ring fault but is instead related to the position of initial tensional fractures appearing prior to the collapse faults

a proximal record of caldera forming eruptions the stratigraphy eruptive history and collapse of the palaeogene arran caldera western scotland caldera forming volcanic eruptions are among the most dangerous and can generate extensive pyroclastic deposits and deliver ash into global atmospheric circulation systems as calderas collapse the eruptions can deposit thick proximal ignimbrite sequences and thinner ignimbrites more distally however the proximal record of caldera collapse is often obscured by later intrusions volcanism faults alteration water and sediments which significantly limits our understanding of these eruptions a palaeogene caldera system in central arran western scotland preserves a rare proximal caldera fill succession the arran volcanic formation this caldera largely comprises highly heterogeneous ignimbrites and minor intra caldera sedimentary rocks the current level of erosion and the general absence of faults intrusions and sediments allows a complex stratigraphy and collapse history to be determined which can be linked to changing eruptive styles at a constantly evolving volcano the first recorded phase was eruption of a homogeneous rhyolitic lava like tuff deposited from high temperature high mass flux pyroclastic density currents generated from low fountaining columns that retained heat a succeeding phase of highly explosive plinian eruptions marked by a thick blanket of massive lapilli tuffs was then followed by piston like caldera collapse and erosion of steep caldera walls volcanism then became generally less explosive with predominantly lava like and eutaxitic tuffs and cognate spatter rich agglomerates interbedded with non homogenous lapilli tuffs high topographic relief between distinct units indicate long periods of volcanic quiescence during which erosive processes dominated these periods are in several places marked by sedimentary rocks and evidence for surface water which includes a localised basaltic andesitic phreatomagmatic tuff the caldera forming eruptions recorded by the arran volcanic formation provide an important insight into caldera collapse processes and proximal ignimbrite successions the lack of thick autobreccias and lithic rich lapilli and block layers indicates that subsidence was relatively gradual and incremental in this caldera and not accompanied by catastrophic wall collapse during eruption the relatively horizontal nature of the caldera fill units and paucity of intra caldera faulting indicate that piston subsidence was the dominant method of collapse with a relatively coherent caldera floor bounded by a steeply dipping ring fault possible resurgence may have caused later doming of the floor and radial distribution of subsequent ignimbrites and sedimentary rocks our work emphasises the continued need for field studies of caldera volcanoes

controls on caldera structure results from analogue sandbox modeling we conducted scaled analogue sandbox models of caldera formation in order to understand the effects of chamber depth and orientation on the spatial and temporal development of calderas dry sand contained in a 1 m diameter cylinder served as a crustal rock analogue and a water filled 0 6 m diameter rubber bladder served as an analogue magma chamber scaling parameters included a length ratio l of 2 5 10 5 and a stress ratio of 1 8 2 4 10 5 in contrast to some previous analogue models the viscosity of the fluid in the chamber and its withdrawal rate were properly scaled generally deformation began with broad sagging followed by an arcuate or linear outward dipping fault that formed on one side of the caldera this fault propagated laterally around the caldera in both directions sometimes joining other faults and typically forming an overall polygonal structure as subsidence continued the caldera grew incrementally outward and progressively formed a series of concentric outward dipping faults lastly a peripheral zone of extension and pronounced sagging and commonly an inward dipping outer fault related to extension developed at the surface as the depth of the chamber increased 1 the area of faulting decreased 2 the symmetry of the caldera was affected and 3 the coherence of the subsiding block decreased tilting the chamber caused highly asymmetric subsidence to occur in this case faults formed first where the bladder was shallowest subsidence then shifted rapidly to where the bladder was deepest producing an elongate trapdoor caldera that was deepest where the bladder was deepest our experiments highlight the roles of sagging and faulting during caldera subsidence surface fault patterns both in our experiments and at natural calderas are frequently not circular the aspect ratio of the block above the magma chamber controls the shape of the caldera which is frequently polygonal the faults at natural calderas determine locations and migration of eruptive vents the degree of subsidence the style of postcal dera resurgent magmatism and the extent of hydrothermal circulation our experiments reveal details of how calderas grow outward incrementally and demonstrate that asymmetric subsidence along linear and arcuate faults is common to many calderas

sea level changes in the tectonically stable malay thai peninsula abstract the malay thai peninsula is part of sundaland a geological region of relative crustal stability therefore the positions of old shorelines of the past 20 000 years or so can be considered to indicate positions of actual past sea levels over 200 shoreline indicators at positions below as well as above present sea level have been radiocarbon dated these dated positions indicate that prior to 6 ka the sea rose from at least 90 m below present sea level at rates that initially amounted to 15 mm year and later became 6 mm year at ca 6 ka the rising sea surpassed its current position and at ca 5 ka it reached 5 m during this maximum marine inundation the peninsula was narrower since the low coastal plains did not yet exist in the period following this maximum transgression the available shoreline data indicate that regional sea level subsided through a series of fluctuations that were characterized by amplitudes of 2 m and periods of about 2000 years the peaks and troughs of the fluctuations became progressively lower the latest peak was at about 1 7 m or 0 7 m above present high tide a few hundred years ago the cause of these fluctuations is not yet known but the trend suggests that in the near future regional sea level may subside at annual rates between 1 5 mm and 2 mm this rate of descent is 25 to 30 of the estimated rate of sea level rise that may result from global warming in other words for the malay thai peninsula the effect of sea level rise on account of manmade greenhouse gases will be partly compensated by actual subsidence of regional sea level

a geophysical interpretation of the 1883 krakatau eruption abstract from the present submarine topography in the vicinity of the krakatau islands it is concluded that the focus of the large explosions was situated to the northwest of the present rakata island the channel between krakatau and sebesi islands was completely blocked by banks of volcanic material immediately after the eruptions and it is suggested that this material was mainly lithic fragments the explosion sequences of krakatau are deduced from the records of sea waves and air waves observed at jakarta the large tsunami was caused by the most violent explosion simultaneously with the largest air waves it is inferred that the origin of the tsunami was a sudden upheaval of the seawater due to a violent explosion and that the height of the tsunami near its source was 30 40 m energy of the explosion is estimated from analyses of the air waves as one order of magnitude greater than that of the 1956 bezymianny eruption thus the 1883 krakatau eruption may have caused explosive removal of more than 10 km3 of material the subsurface structure of krakatau islands after the 1883 eruption is deduced from gravity anomalies it is concluded that at the bottom of the caldera there are deposits of low density in the shape of an inverted cone 8 km in diameter and 1 km in depth from the residual gravity anomaly observed over the caldera one can estimate the mass deficiency there this allows estimates to be made as to the amount of ejecta although large uncertainties remain these data indicate that explosive removal of material was the main process responsible for the disappearance of the northern half of the former rakata krakatau island in the 1883 eruption

formation of oskjuvatn caldera at askja north iceland mechanism of caldera collapse and implications for the lateral flow hypothesis the oskjuvatn caldera at askja volcano north iceland was formed as a result of an explosive eruption at askja on 28th 29th march 1875 oskjuvatn is one of the youngest collapse calderas on earth and its initiation and growth were observed and documented by explorers and geologists working in the askja region between 1875 and 1932 it is an example of caldera formation where the volume of the caldera collapse far exceeds the volume of associated erupted products the discrepancy between the collapse volume and associated erupted products has been used by previous workers as a justification for the hypothesis that the injection of lateral dykes from askja fed the february to october 1875 n jahraun fissure eruptions at the sveinagja graben some 60 km north of askja however historical accounts documenting the caldera formation in sufficient detail show clearly that oskjuvatn formed by piecemeal collapse taking over 40 years to reach its current form we use these accounts to undertake a detailed examination of the stages of caldera collapse and to compare them with the known magmatic output of askja in the years following the 1875 eruption arcgis software was used to calculate the volume of oskjuvatn during the various stages of collapse and the volume of material erupted after 1875 while a dyke extending between askja and sveinagja may be implied to account for the missing volume our results demonstrate that the volume of oskjuvatn in july 1875 closely corresponds to the volume of rhyolitic material erupted on 28th 29th march 1875 in addition geochemical evidence indicates that the n jahraun magmas were sourced from a deeper reservoir rather than by lateral injection from askja s shallow crustal magma chamber we therefore suggest that the injection of sills and dykes in a coherent intrusion complex beneath askja central volcano combined with background deflation of the magma storage zone beneath oskjuvatn caldera over 30 to 40 years following 1875 satisfactorily accommodates the volume required to form oskjuvatn caldera

high temperature emplacement and liquefaction of shallow water caldera forming eruption products early miocene tateyamazaki dacite in the oga peninsula ne japan abstract the early miocene tateyamazaki dacite infills a 3 2 km diameter caldera it comprises poorly sorted massive biotite bearing dacite pumice lapilli tuff in which huge blocks of densely welded dacite lapilli tuff basaltic andesite lava and other lithologies are commonly set dense blocks are variably cracked and intruded by the host lapilli tuff sparse blocks of bedded lapilli tuff and tuff are variably disaggregated to intermingle with the host rocks or are plastically deformed into irregular shapes rootless tuff veins millimeters to 30 cm thick are developed within the host rocks mainly dipping at 10 30 and are locally branched and mutually cut to form a network where thicker they are stratified and locally carry accidental fragments accidental lapilli up to 2 or 3 cm wide and 30 cm long are locally set in near vertical and variably sinuous arrays although poorly defined they are reminiscent of fluid escape structures the host pumice lapilli tuff however retains in part a thermal remnant magnetization trm vector stable at temperatures above 280 c blocks in the caldera fill also retain trm but the vectors are rotated significantly from those of the host pumice lapilli tuff and the adjacent volcanic rocks tateyamazaki dacite is thus likely to have been emplaced at high temperatures and intermingled with shattered basement rocks and ambient water to be partly liquefied within the caldera immediately after or during the caldera forming eruption

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

potentially active volcanoes of peru observations using landsat thematic mapper and space shuttle imagery landsat thematic mapper tm images and space shuttle color photograph have been used to make a synoptic study of the volcanoes of southern peru 14 17 s the northernmost portin of the central volcanic zone cvz 14 28 s of the andes apart from providing consistent coverage the chief merit of the tm for this study has been the spatial resolution provided by the 30 m pixel size the optimal 20 m resolution variable lighting and viewing geometry and stereo capability of the shuttle photography provided an invaluable ancilliary data set at the resolution available subtle glacial morphological features such as valley and terminal moraines can be confidently identified and these features have been used to determine the relative ages of volcanoes volcanoes have been classified as potentially active if they have i a well preserved summit crater ii pristine lava flow texture and morphology iii flank lava flows with low albedo and iv evidence of postglacial 10 000 years activity eight major volcanoes are postulated to be potentially active most are large dominantly andesitic composite cones with edifice heights of up to 2500 m some of which threaten nearby settlements one of them sabancaya was active as recently as july 1988 other little known postglacial volcanic features include huaynaputina site of a major explosive rhyolitic eruption in 1600 a d and several fields of monogenetic scoria cones and lava flows the active volcanic front is some 200 km east of the peru chile trench and the volcanoes lie on a trenchparallel trend oblique to the ew subduction this narrow volcanic zone is thought to reflect the steep dip of the nazca plate through the zone of magma generation the break in the trend of the volcanic front in the northern extremity of the volcanic zone is thought to reflect the complexity of the crustal stress field above a major segment boundary in the subducting plate the fields of mafic monogenetic centers also occur in this region in comparison with the southern part of the cvz the general paucity of older volcanic edifices north of 17 s suggests a more recent onset of volcanism north of this latitude probably resulting from the oblique subduction of the nazca ridge and the consequent northward migration of its intersection with the pere chile trench this coupled with the lack of any large silicic caldera systems and youthful dacite domes like those found further south suggest that there are real differences between the volcanic evolution of different parts of the cvz

reconstruction of the ad 1600 huaynaputina eruption based on the correlation of geologic evidence with early spanish chronicles the largest historical eruption vei 6 in the andes began on february 19 and continued until march 6 or 15 ad 1600 at huaynaputina a dacitic stratovolcano located on a high volcanic plateau in south peru tephra falls pyroclastic flows and surges disrupted life in an area of 4900 km2 around the volcano and ash fall was reported 200 500 km away in south peru west bolivia and north chile the aftermath of the large scale eruption was severe and protracted for the people and colonial economy of south peru by linking up the series of events inferred from spanish chronicles with the lithofacies and composition of the tephra bulk volume 11 4 12 1 km3 dense rock equivalent dre 4 6 4 95 km3 we distinguish five eruptive phases 1 during the plinian phase a sustained plinian column 27 35 km high on february 19 20 delivered a dacitic pumice fall of 3 1 km3 dre volume the plinian pumice formed a widespread lobe of 95 000 km2 within the 1 cm isopach strong winds carried fine ash 500 km to the west and west northwest into the pacific ocean the computed volumetric eruption rate was in the range of 5 4 6 6 104 to 1 105 m3 s and the mass eruption rate 1 3 1 6 108 kg s the onset and high discharge of the sustained plinian eruption was fueled by the disruption of an active hydrothermal system enclosed in the pre ad 1600 amphitheater the plinian column shut off as the vent was choked when the fragmentation focus deepened to beneath the weathered bedrock 1600 m below the vent area 2 during the second phase a dwindling column sent ash falls on proximal to medial areas and possibly pyroclastic surges on proximal slopes 3 during the third ignimbrite forming phase with interspersed hydromagmatic events pyroclastic flows 1 5 2 km3 in volume were channeled into the rio tambo canyon and tributaries the flows with interbedded base surge deposits in proximal tributaries probably produced vigorous columns over high rugged relief around the huaynaputina plateau winds winnowing the columns dispersed a widespread co ignimbrite ash probably mixed with co plinian ash over an area of 265 000 km2 4 during the fourth phase an unusual crystal ash fall was deposited when the residual magma with a crystal content as high as 80 was tapped near the end of the eruption 5 during the fifth phase ash flows produced surge deposits and lag fall breccias near vent small volume ash flow deposits in proximal catchments and a thin ash fall layer in medial to distal areas the proximal deposits were also produced by diluted flows able to surmount ridges 1000 m high as far as 15 km east from the vent the ignimbritic and hydromagmatic phases greatly modified the 400 m diameter plinian vent tapping of the crystal rich magma and ash flows towards the end of the eruption led to the formation of two youthful vents in domes geochemistry and mineralogy of the plinian and post plinian units point to an unusual zoned magma sequence the ignimbrite forming phase tapped a magma batch richer in silica than the less differentiated plinian magma the crystal rich magma of unit 4 was fed by crystal mush in a layered magma reservoir and or from two magma reservoirs at distinct depths the geochemical and mineralogical trend throughout the eruption and preliminary measurements of geobarometers suggest a complex model linking a shallow 6 7 km magma reservoir to a deeper 15 km reservoir the total dre volume 4 6 4 95 km3 of erupted tephra did not lead to caldera collapse ring fractures cutting multiple vents are associated with a dyke swarm intruding the weathered volcanic bedrock this suggests the onset of a funnel type or piecemeal collapse

how were the ditches filled sedimentological and micromorphological classification of formation processes within graben like archaeological objects mezi nejcast ji kladene otazky pat i zp sob y zapl ovani archeologick ch objekt vhodn mi objekty pro studium formativnich proces archeologick ch objekt jsou hrotite p ikopy z mladsiho neolitu a z doby imske

present day soil distribution explained by prehistoric land use podzol arenosol variation in an ancient woodland in denmark abstract archaeological evidence has rarely been used by soil scientists to explain the distribution of present day soils yet recent human influence is known to be important for all soils sites with abandoned prehistoric fields are commonly found throughout western europe but the impact on present day soils is not fully known the studied site alstrup krat is an intact bronze and iron age site with a well preserved system of fields infields and common areas common it was abandoned around 2000 bp and has been beech wood since this paper concerns the connection between the present day soil distribution and this former land use the methods used in the field were soil survey intensive ditching and profile descriptions and these were combined with soil physical and chemical laboratory analyses texture oc n p fe al optical density on oxalate extracts odoe and base ions this revealed two distinct soil types whose borders coincided with the prehistoric common infield border on the dominant parent material sandy and gravelly tills hyperdystric arenosols are found in the infields and haplic podzols in the common area entic podzols or spodic arenosols are developed in the common infield transition zone and in infields that were furthest from the dwellings during cultivation podzols had been expected in the infields as iron age agriculture is believed to have exhausted the soil and caused podzolization chemical and physical differences between infields and the common area did not explain the observed soil distribution the only difference between the two adjacent areas was prehistoric land use until 2000 years ago the main factor is probably agriculture during the iron age which physically rejuvenated the soil in the infields archaeological information thus provided a powerful tool for explaining soil spatial distribution where standard soil science variables and methods would have failed

early human occupation and land use changes near the boundary of the orinoco and the amazon basins se venezuela palynological evidence from el pauji record this paper shows a holocene paleoecological reconstruction based on a peat bog sequence el pauji 4 28 n 61 35 w 865 m elevation located in the transition zone between the gran sabana se venezuela savannas and the amazon rainforests paleoecological trends are based on the analysis of pollen and pteridophyte spores algal and animal remains fungal spores and charcoal particles the whole record embraces the last ca 8000 cal yr bp and was subdivided into five pollen zones representing the following vegetation succession savanna rainforest mosaic 8250 7715 yr bp dense rainforests 7715 5040 yr bp savanna rainforest mosaic 5040 2690 yr bp secondary dry forests 2690 1440 yr bp and peat bog in an open savanna landscape 1440 yr bp present these vegetation changes have been attributed to the action of climate and or land use changes as well as the corresponding synergies between them fire has been determinant in the landscape evolution based on the reconstructed fire and vegetation shifts a changing land use pattern could have been recognized between the early and the mid holocene ca 8 3 5 0 kyr bp land use practices seem to have been more linked to shifting agriculture in a rainforest landscape as is usual in amazon cultures with medium fire incidence affecting only local forest spots or surrounding savannas more extensive forest burning was recorded between ca 5 0 and 2 7 kyr bp followed by land abandonment and the dominance of drier climates between 2 7 and 1 4 yr bp the modern indigenous culture which prefers open environments and makes extensive use of fire thus preventing forest re expansion seem to have established during the last 1500 yr therefore a significant cultural replacement has been proposed for the region leading to the present day situation changing human activities have been instrumental for ecological evolution in this savanna rainforest transitional region as well as for the shaping of modern landscapes

organic geochemical evidence for the origin of ancient anthropogenic soil deposits at tofts ness sanday orkney lipid biomarker components of soils constituting three orcadian archaeological fossil soil profiles were analysed the combined assessment of lipid distributional and compound specific stable carbon isotope data enabled the identification of grass turves as the most probable material used in the formation of the anthropogenic soil deposits appraisal of 5 stanol components indicated a faecal input to one of the soils which on considering distributional evidence was ascribed a human porcine origin additional study of polar bile acids from this profile revealed a distribution exhibiting a predominance of deoxycholic acid indicating the primary faecal input to be mainly derived from humans although the minor occurrence of hyodeoxycholic acid a characteristic component of pig faeces attested to a limited porcine input

sedimentological analysis of the tephra from the 12 15 august 1991 eruption of hudson volcano the tephra fallout from the 12 15 august 1991 explosive eruption of hudson volcano cordillera de los andes 45 54 s 72 58 w chile was dispersed on a narrow elongated ese sector of patagonia covering an area on land of more than 100 000 km2 the elongated shape of the deposit together with the relatively coarse mean and median values of the particles at a considerable distance from the vent were the result of strong winds blowing to the southeast during the eruption the thickness of the fall deposit decreases up to 250 km ese from hudson volcano where it begins to thicken again secondary maxima are well developed at approximately 500 km from the vent secondary maxima together with grainsize bimodality in individual layers and in the bulk deposit suggest that particle aggregation played an important role in tephra sedimentation the fallout deposit is well stratified with alternating fine grained and coarsegrained layers which is probably a result of strong eruptive pulses followed by relatively calm periods and or changes in the eruptive style from plinian to phreatoplinian the tephra is mostly composed of juvenile material the coarse mode mostly pumice shifts to finer sizes with distance from the volcano the fine mode mostly glass shards is always about 5 6 phi glass shards and pumice are mostly light gray to colorless however considerable amounts of dark poorly vesiculated blocky shards suggest a hydromagmatic component in the eruption a land based tephra volume of 4 35 km3 was estimated and a total volume of 7 6 km3 arose from an extrapolation which took into account the probable volume sedimented in the sea bulk density ranges from 0 9 to 1 10 gr cm3 beyond 110 km from the vent rather uniform density values measured in crushed samples 2 45 2 50 gr cm3 at all distances from the vent reveal a relatively homogeneous composition mean and median sizes decrease rapidly up to 270 km from the vent beyond that point they are more or less constant whereas the maximum size 1 phi shows a steady decrease up to 550 km a concomitant improvement in sorting is observed this is attributed to sorting due to wind transport combined with particle aggregation at different times and distances from the vent the hudson tephra fallout shares some strikingly similar features with the mount st helens 18 may 1980 and quizapu 1932 eruptions

stress evolution during caldera collapse abstract the mechanics of caldera collapse are subject of long running debate particular uncertainties concern how stresses around a magma reservoir relate to fracturing as the reservoir roof collapses and how roof collapse in turn impacts upon the reservoir we used two dimensional distinct element method models to characterise the evolution of stress around a depleting sub surface magma body during gravity driven collapse of its roof these models illustrate how principal stress orientations rotate during progressive deformation so that roof fracturing transitions from initial reverse faulting to later normal faulting they also reveal four end member stress paths to fracture each corresponding to a particular location within the roof analysis of these paths indicates that fractures associated with ultimate roof failure initiate in compression i e as shear fractures we also report on how mechanical and geometric conditions in the roof affect pre failure unloading and post failure reloading of the reservoir in particular the models show how residual friction within a failed roof could without friction reduction mechanisms or fluid derived counter effects inhibit a return to a lithostatically equilibrated pressure in the magma reservoir many of these findings should be transferable to other gravity driven collapse processes such as sinkhole formation mine collapse and subsidence above hydrocarbon reservoirs

sea level induced variations in clay mineral composition in the southwestern south china sea over the past 17 000 yr variations in clay mineral composition of sediment cores from the margin and continental slope of the sunda shelf southern south china sea se asia covering the past 17 000 yr reflect changing influences of sediment sources together with clay mineral partitioning processes in shallow waters we identify the deglacial sea level rise as the principal factor driving these changes during the late glacial high values of kaolinite are interpreted to reflect a higher contribution of clays from soils that have formed on the exposed sunda shelf and in the southern archipelagos of indonesia at this time core sites were located in close proximity to the mouths of the sunda shelf palaeo drainage systems on the emerged shelf sundaland the progressive landward displacement of the coastline and breakdown of these vast drainage systems during deglaciation led to a decrease in influence of the kaolinite rich southern sources when the coastline had retreated closely to its present day position in mid holocene times the former dominance of southern sources was replaced by a stronger influence of illite rich sources e g borneo the overriding control of sea level changes on the clay mineral distribution patterns precludes a definite climatic interpretation of clay mineral data in terms of climatic monsoonal changes in such highly dynamic sedimentary environment

the mechanics of large volcanic eruptions abstract the mechanical conditions for a volcanic eruption to occur are conceptually simple a magma driven fracture normally a dyke must be able to propagate from the source to the surface the mechanics of small to moderate eruptions eruptive volumes less than 10 km3 is reasonably well understood whereas that of large eruptions eruptive volumes of 10 1000 km3 is poorly understood here i propose that while both large and small eruptions are primarily driven by elastic energy and may come from the same magma chambers and reservoirs the mechanisms by which the elastic energy is transformed or relaxed in these eruptions are different more specifically during small to moderate eruptions the excess pressure in the source the primary pressure driving the eruption falls exponentially until it approaches zero whereby the feeder dyke closes at its contact with the source and the eruption comes to an end under normal conditions the ratio of the eruptive and intrusive material of the eruption to the volume of a totally molten shallow basaltic crustal magma chamber at the common depth below the surface of 1 5 km is about 1400 and that of a partially molten deep seated basaltic magma reservoir in the lower crust or upper mantle is about 5000 many magma chambers are partially molten in which case the ratio could be close to that of reservoirs most magma chambers are estimated to be less than about 500 km3 for which the maximum eruptive volume would normally be about 0 4 km3 an eruptive volume of 1 km3 would require a totally molten chamber of about 1400 km3 while chambers of this size certainly exist witness the volumes of the largest eruptions large eruptions of 10 1000 km3 clearly require a different mechanism namely one whereby the excess pressure is maintained during much of the eruption i suggest that the primary excess pressure maintenance mechanism is through caldera subsidence for shallow magma chambers and graben subsidence for deep seated magma reservoirs in this mechanism it is the subsidence of tectonic origin and associated volume reduction shrinkage of the magma source that drives out an exceptionally large fraction of the magma in the source thereby generating the large eruption most explosive eruptions that exceed volumes of about 25 km3 and many smaller are associated with caldera collapses the data presented suggest that many large effusive basaltic eruptions in iceland in the united states and elsewhere are associated with large graben subsidences in terms of the present mechanism successful forecasting of large eruptions requires understanding and monitoring of the volcanotectonic conditions that trigger large caldera and graben subsidences

downsag and extension at calderas new perspectives on collapse geometries from ice melt mining and volcanic subsidence structures at calderas may form as a result of precursory tumescence subsidence due withdrawal of magmatic support resurgence and regional tectonism structural reactivation and overprinting are common to explore which types of structures may derive directly from subsidence without other factors evidence is reviewed from pits caused by the melting of buried ice blocks mining subsidence scaled subsidence models and from over 50 calderas this review suggests that complex patterns of peripheral deformation with multiple ring and arcuate fractures both inside and outside caldera rims topographic embayments arcuate graben and concentric zones of extension and compression may form as a direct result of subsidence and do not require a complex subsidence and inflation history downsag is a feature of many calderas and it does not indicate subsidence on an inward dipping ring fault as has been inferred previously where magmatic inflation is absent or slight initial arcuate faults formed during collapse are likely to be multiple and dip outwards to vertical associated downsag causes the peripheries of calderas undergo radial centripetal extension and this accounts for some of the complex peripheral fractures arcuate crevasses graben and some topographic moats the structural boundary of a caldera defined here as the outermost limits of subsidence and related deformation including downsag commonly lies outside ring faults and outside the embayed topographic wall it is likely to be funnel shaped i e inward dipping even though ring and arcuate fractures within it may dip outward inward dipping arcuate normal faults at shallow levels and steep inward dipping contacts between a caldera s fill and walls may both occur at a caldera that has initially subsided on outward dipping ring faults they arise due to peripheral surficial extension gravitational spreading and scarp collapse topographic enlargement at some calderas and the formation of embayments may reflect general progressive downsag and localized downsag respectively these processes may occur in addition to surficial degradation of oversteep ring fault scarps

explosive volcanism vei 6 without caldera formation insight from huaynaputina volcano southern peru through examination of the vent region of volcan huaynaputina peru we address why some major explosive eruptions do not produce an equivalent caldera at the eruption site here in 1600 more than 11 km3 dre vei 6 were erupted in three stages without developing a volumetrically equivalent caldera fieldwork and analysis of aerial photographs reveal evidence for cryptic collapse in the form of two small subsidence structures the first is a small non coherent collapse that is superimposed on a cored out vent this structure is delimited by a partial ring of steep faults estimated at 0 85 by 0 95 km collapse was non coherent with an inwardly tilted terrace in the north and a southern sector broken up along a pre existing local fault displacement was variable along this fault but subsidence of approximately 70 m was found and caused the formation of restricted extensional gashes in the periphery the second subsidence structure developed at the margin of a dome the structure has a diameter of 0 56 km and crosscuts the non coherent collapse structure subsidence of the dome occurred along a series of up to seven concentric listric faults that together accommodate approximately 14 m of subsidence both subsidence structures total 0 043 km3 in volume and are much smaller than the 11 km3 of erupted magma crosscutting relationships show that subsidence occurred during stages ii and iii when 2 km3 was erupted and not during the main plinian eruption of stage i 8 8 km3

an early holocene sea level jump and delta initiation 1 early holocene sea level change controlled the evolution of clastic coastal depositional systems radiocarbon dated borehole cores obtained from three incised valley fill systems in asia changjiang song hong and kiso river record very similar depositional histories especially between about 9000 and 8500 cal bp sedimentary facies changes from estuarine sand and mud to shelf or prodelta mud suggest that the marine influence in the incised valleys increased during this period in addition large decreases in sediment accumulation rates occurred a sea level jump causes an estuarine system and its depocenter to move rapidly landward it is possible that the final collapse of the laurentide ice sheet accompanied by catastrophic drainage of glacial lakes at approximately 8500 cal bp caused such a jump the jump was followed immediately by a period of decelerated sea level rise that promoted delta initiation

very long period seismic signals and caldera formation at miyake island japan over a period of roughly 40 days starting on 8 july 2000 a caldera structure 1 7 kilometers in diameter developed by means of gradual depression and expansion of the summit crater at miyake island japan at the same time very long period vlp seismic signals were observed once or twice a day source mechanism analyses of the vlp signals show that the moment tensor solutions are smooth step functions over a time scale of 50 seconds with dominant volumetric change components we developed a model to explain the caldera and the vlp signals in which a vertical piston of solid materials in the conduit is intermittently sucked into the magma chamber by lateral magma outflow this model offers potential for making quantitative estimations of the characteristic physical properties of magma systems

variation in collagen stable nitrogen values in black tailed jackrabbits lepus californicus in relation to small scale differences in climate soil and topography abstract longstanding observations about the relationship between increasing aridity and 15 n enrichment in mammalian collagen values have led to an interest in their use as a paleoclimatic marker here we report on variability in collagen nitrogen values from five modern and two archaeological samples of black tailed jackrabbits lepus californicus from the eastern great basin of the united states n 178 individuals nitrogen 15 n values were highly variable within all samples modern samples showed significant differences despite similarities in average annual precipitation and temperature archaeological samples were indistinguishable from each other or from modern samples taken from the same area despite independent evidence for differences in precipitation for the two prehistoric periods considered differences between modern samples were most strongly associated with soil characteristics we discuss these results in light of their relationship with topography and vegetation and highlight their implications for archaeological applications of stable nitrogen analyses in several contexts

neutral carbohydrate geochemistry of particulate material trap and core sediments in an eutrophic lake aydat france carbohydrate compositions were determined on sinking particles and core samples from eutrophic lake aydat carbohydrate fluxes indicate that phytoplanktonic production is the main organic matter om source whereas the monosaccharides concentrations normalised to oc contents timage mostly reflect degradation the abundance of rhamnose in all samples its increasing fluxes together with fucose with increasing water depth and its strong decrease in the top 5 cm of the bottom sediments mark the intense activity of the microbiota the almost complete removal of sugars in the water and top sediment indicates very little contribution to the preserved om stock

is it possible to determine formation processes of organic deposits through polysaccharides pilot study from the prehistoric site in brandýs nad labem czech republic this paper deals with a possible interpretation value of biochemical methods in comparison with the classic tools of geoarchaeology for the evaluation of formation processes organic rich layers fr

tectonically controlled piecemeal caldera collapse a case study of glencoe volcano scotland caldera collapse at glencoe scotland was incremental and involved complex movements of numerous fault blocks before formation of the ring fault for which the volcano is renowned intracaldera depocenters had the form of grabens or half grabens locally with downsag and bounding monoclinal flexures and these changed form and location for each of the five major ignimbrite eruptions that represent the early volcanic history this piecemeal caldera collapse is in contrast to previous interpretations in which all subsidence was thought to have involved a coherent crustal block moving on the ring fault collapse and magmatic plumbing were profoundly influenced by preexisting tectonic faults trending northwest and northeast a dominant northwest trending graben controlled the general location and form of major caldera depocenters and repeatedly channeled a major river through glencoe the main graben was transected orthogonally by two cross grabens each of the first three caldera eruption cycles involved initial phreatomagmatic explosivity which built tuff cones followed by magmatic fountaining that produced lava like silicic ignimbrites the three lava like ignimbrites total more than 300 m thick two later eruptions produced eutaxitic silicic ignimbrites together more than 300 m thick and contemporaneous progressive downsag was associated with the formation of extensional fractures crevasses hundreds of meters deep sills up to 100 m thick of mingled andesite and rhyolite between the intracaldera ignimbrites were accommodated by increments of subsidence unconformities and sedimentary layers within the stratigraphic succession record fluvial erosion and abrupt switches to alluvial and or lacustrine deposition between each ignimbrite eruption the changes in drainage and sedimentation and the development of coarse debris avalanche breccias reflect tectonic faulting during the periods between major eruptions the duration of the period including the five caldera forming eruptions was probably 0 5 m y and the magnitude of the tectonic faulting that occurred prior to and during the caldera developments 0 5 km m y normal displacement is similar to that of the most actively subsiding sedimentary basins glencoe shows that the piecemeal nature of calderas may be plainly evident only in deeply dissected systems which with knowledge that caldera volcanoes commonly overlie faults suggests that piecemeal calderas may be more common than previously recognized the research also shows that the emplacement of granites s l was episodic rather than synchronous throughout the magmatic province and it suggests that numerous plutons in the province e g in donegal ireland may have formed at sites of central volcanoes that are no longer preserved

multidisciplinary approach of the hyperarid desert of pampas de la joya in southern peru as a new mars like soil analog abstract the distribution of living organisms organic matter and chemical properties in mars like environments on earth can be used as a model to guide the investigation of possible habitable environments on mars this work aims to demonstrate that the place known informally as the mar de cuarzo sea of quartz in the pampas de la joya desert southern peru between 16 s and 17 s latitude contains soils with characteristics similar to those found on the martian surface using a multidisciplinary approach we studied the environmental data geology organic matter content oxidant activity and microbiology of this area our data show that 1 mar de cuarzo is a hyper arid area with a lower concentration of organic matter than those found in the mars like soils from yungay area atacama desert in chile while at the same time having comparable extreme environmental conditions and very low levels of microorganisms 2 the detrital components of the soils come essentially from the andean volcanic chain and local outcrops of precambrian gneisses and cretaceous granitic batholiths 3 the presence of microclimates geomorphological features and the high influence of the el nino southern oscillation enso allowed the formation of exotic and heterogeneous chemical deposits in these soils including iron oxides sulfates and other evaporites 4 thermal volatilization in these soils using methods similar to the viking and phoenix instruments shows high oxidant activity 5 labeled release experiment similar to the viking instrument shows high degradation of nutrients added in these soils altogether the mar de cuarzo area in the pampas de la joya is an interesting place for astrobiological studies as a new analog to mars and for comparative analyses with other hyperarid analogs as yungay

sedimentation re sedimentation and chronologies in archaeologically important caves problems and prospects excavations in the photic zones of caves have provided cornerstone archaeological sequences in many parts of the world before the appearance of modern dating techniques cave deposits provided clear evidence for the antiquity relative ages and co occurrence of ancient human remains material culture and fauna earlier generations of archaeologists had generally rather limited understanding of taphonomic and depositional processes but the twentieth century saw considerable improvement in excavation and analytical techniques the advent of modern dating and chronological methodologies offers very powerful tools for the analysis of cave fill deposits and this has resulted in the recognition of chronological incoherence in parts of some sites with consequent re evaluation of previous archaeological disputes obtaining multiple dates per context provides a means to assess the integrity and coherence of the archaeological and environmental records from cave fills in the case of the haua fteah libya this technique allowed the recognition of chronological coherence in low energy depositional environments and limited recycling in high energy contexts we provide a conceptual model of the relationship between recycling sedimentation rate and process energy high resolution investigation enables recognition of the complexity of the formation of cave sequences thus an increasingly sophisticated understanding of human behaviour and environmental relationships in the past and potentially gives a new life to old data

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

formation of caldera periphery faults an experimental study changing stresses in multi stage caldera volcanoes were simulated in scaled analogue experiments aiming to reconstruct the mechanism s associated with caldera formation and the corresponding zones of structural weakness we evaluate characteristic structures resulting from doming chamber inflation evacuation collapse chamber deflation and cyclic resurgence inflation and deflation and we analyse the consequential fault patterns and their statistical relationship to morphology and geometry doming results in radial fractures and subordinate concentric reverse faults which propagate divergently from the chamber upwards with increasing dilation the structural dome so produced is characterised bysteepening in the periphery whereas the broadening apex subsides pure evacuation causes the chamber roof to collapse along adjacent bell shaped reverse faults the distribution of concentric faults is influenced by the initial edifice morphology steep and irregular initial flanks result in a tilted or chaotic caldera floor the third set of experiments focused on the structural interaction of cyclic inflation and subsequent moderate deflation following doming caldera subsidence produces concentric faults that characteristically crosscut radial cracks of the dome the flanks of the edifice relax resulting in discontinuous circumferential faults that outline a structural network of radial and concentric faults the latter form locally uplifted and tiltedwedges half grabens that grade into horst and graben structures this superimposed fault pattern also extends inside the caldera we suggest that major pressure deviations in magma chamber s are reflected in the fault arrangement dissecting the volcanoflanks and may be used as a first order indication of the processes and mechanisms involved in caldera formation

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

the physical volcanology of the 1600 eruption of huaynaputina southern peru volcan huaynaputina is a group of four vents located at 16 36 s 70 51 w in southern peru that produced one of the largest eruptions of historical times when 11 km3 of magma was erupted during the period 19 february to 6 march 1600 the main eruptive vents are located at 4200 m within an erosion modified amphitheater of a significantly older stratovolcano the eruption proceeded in three stages stage i was an 20 h sustained plinian eruption on 19 20 february that produced an extensive dacite pumice fall deposit magma volume 2 6 km3 throughout medial distal and distal parts of the dispersal area a fine grained plinian ashfall unit overlies the pumice fall deposit this very widespread ash magma volume 6 2 km3 has been recognized in antarctic ice cores a short period of quiescence allowed local erosion of the uppermost stage i deposits and was followed by renewed but intermittent explosive activity between 22 and 26 february stage ii this activity resulted in intercalated pyroclastic flow and pumice fall deposits 1 km3 the flow deposits are valley confined whereas associated co ignimbrite ash fall is found overlying the plinian ash deposit following another period of quiescence vulcanian type explosions of stage iii commenced on 28 february and produced crudely bedded ash lapilli and bombs of dense dacite 1 km3 activity ceased on 6 march compositions erupted are predominantly high k dacites with a phenocryst assemblage of plagioclase hornblende biotite fe ti oxides apatite major elements are broadly similar in all three stages but there are a few important differences stage i pumice has less evolved glass compositions 73 sio2 lower crystal contents 17 20 lower density 1 0 1 3 g cm3 and phase equilibria suggest higher temperature and volatile contents stage ii and stage iii juvenile clasts have more evolved glass 76 sio2 compositions higher crystal contents 25 35 higher densities up to 2 2 g cm3 and lower temperature and volatile contents all juvenile clasts show mineralogical evidence for thermal disequilibrium inflections on a plot of log thickness vs area1 2 for the fall deposits suggest that the pumice fall and the plinian ash fall were dispersed under different conditions and may have been derived from different parts of the eruption column system the ash appears to have been dispersed mainly from the uppermost parts of the umbrella cloud by upper level winds whereas the pumice fall may have been derived from the lower parts of the umbrella cloud and vertical part of the eruption column and transported by a lower altitude wind field thickness half distances and clast half distances for the pumice fall deposit suggests a column neutral buoyancy height of 24 32 km and a total column height of 34 46 km the estimated mass discharge rate for the 20 h long stage i eruption is 2 4 108 kg s and the volumetric discharge rate is 3 6 105 m3 s the pumice fall deposit has a dispersal index hildreth and drake 1992 of 4 4 and its index of fragmentation is at least 89 reflecting the dominant volume of fines produced of the 11 km3 total volume of dacite magma erupted in 1600 approximately 85 was evacuated during stage 1 the three main vents range in size from 70 to 400 m alignment of these vents and a late stage dyke parallel to the nnw sse trend defined by older volcanics suggest that the eruption initiated along a fissure that developed along pre existing weaknesses during stage i this fissure evolved into a large flared vent vent 2 with a diameter of approximately 400 m this vent was active throughout stage ii at the end of which a dome was emplaced within it during stage iii this dome was eviscerated forming the youngest vent in the group vent 3 a minor extra amphitheater vent was produced during the final event of the eruptive sequence recharge may have induced magma to rise away from a deep zone of magma generation and storage subsequently vesiculation in the rising magma batch possibly enhanced by interaction with an ancient hydrothermal system triggered and fueled the sustained plinian eruption of stage i a lower volatile content in the stage ii and stage iii magma led to transitional column behavior and pyroclastic flow generation in stage ii continued magma uprise led to emplacement of a dome which was subsequently destroyed during stage iii no caldera collapse occurred because no shallow magma chamber developed beneath this volcano

sulfur yield of the 1600 eruption of huaynaputina peru contributions from magmatic fluid phase and hydrothermal sulfur abstract while major explosive eruptions of anhydrite bearing silicic magma have been known to influence earth s climate through the release of sulfate aerosols quantifying the atmospheric loading of these eruptions remains an elusive goal the eruptions that have had the most impact on climate typically erupted oxidized and anhydrite bearing magma the 9 km3 of dacite magma erupted during the vei 6 eruption of huaynaputina peru in 1600 is oxidized o2 nno 1 4 but does not contain magmatic anhydrite yet it is thought to have a significant impact on climate herein we follow the approach that s contents in magmatic apatite grains can be used to provide an independent estimate of the magnitude and source of the atmospheric sulfur loading apatite so3 concentrations of 0 09 to 0 17 wt yield an estimate of 0 8 mt of total erupted sulfur derived from degassing of the melt this is considerably less than the stratospheric sulfur input of 16 55 mt of s estimated from ice core data and requires that much of the stratospheric sulfur load from huaynaputina was from other sources than the melt while a fluid phase in equilibrium with the melt is a likely source a significant contribution from combustion of sulfur from a fossil hydrothermal system excavated during eruption is evidenced by comminuted hydrothermal debris and lithics that make up to 2 5 to 10 wt of erupted material we calculate that up to 22 of the total s budget of the eruption could reasonably be attributed to combusted hydrothermal sulfur thus three sources of sulfur the melt a coexisting fluid phase and external hydrothermal sulfur appear to explain the atmospheric input from this eruption our work confirms that unraveling the s budget of large silicic eruptions is extremely complex and requires a multi faceted approach based on a detailed understanding of the eruption

new insights into eruption source parameters of the 1600 ce huaynaputina plinian eruption peru in the central andes large plinian eruptions volcanic explosivity index 5 occur at a relatively high frequency i e average one every 2000 to 4000 years over the past 50 000 years in peru such recurring explosive activity represents a significant challenge for regions typically hosting several million people e g southern peru western bolivia and northern chile with vei 6 the 1600 ce huaynaputina eruption is considered the largest historical eruption in south america we have re examined the first plinian phase of this eruption in order to better assess critical eruption source parameters i e erupted volume plume height mass eruption rate eruption duration the revised bulk volume of the tephra fall deposit associated with the plinian phase is approximately 13 14 km3 almost twice the previous estimate 7 8 km3 within the 1 cm isopach based on methods including power law weibull function and bayesian linear regression tephra was dispersed by strong winds to the wnw as far as 400 km on peruvian territory and then in the pacific ocean seven villages were buried killing 1500 people the revised plume height estimate 32 2 2 5 km is consistent with the early estimations as a result the huaynaputina 1600 ce first eruption phase lies in the upper part of the plinian field close to the ultra plinian transition making this event one of the largest in the past millennium which coincides with results from recent studies on palaeoclimatic impacts

caldera subsidence in areas of variable topographic relief results from analogue modeling calderas form in volcanic areas commonly associated with topographic relief pre existing topography plays an important role in the style of the caldera subsidence topography increases the load and affects the principle stress trajectories located between the roof of the magma chamber and the surface the morphology internal structure and temporal evolution of calderas are therefore sensitive to the local topography we carried out scaled analogue experiments to investigate the effect of pre existing topographic relief on caldera subsidence by modeling the presence of stratocones and plateaus of variable mass diameter and positions prior to collapse we induced collapse in sandbox experiments by withdrawing water from a rubber bladder to simulate caldera collapse into a large shallow reservoir deformation was first manifested by sagging of the sand at the surface followed by the upward propagation of a set of subsidence controlling faults from the bladder in experiments with no topography these faults usually reached the surface near the centre of the cylinder as evacuation and incremental growth progressed a second outer set of subsidence controlling faults developed this outer set dominated the subsidence by increasing the topographic load by 6 the subsidence efficiency increased producing calderas up to approximately 20 deeper the inner set of subsidence controlling faults steepened and controlled most of the collapse by contrast to the experiments without topography adding load also reduced outward growth of peripheral sagging and development of extensional faults leading to a diameter of sagging 20 smaller than with no topography a comparative analysis of our results with mount mazama at crater lake oregon and glass mountain at long valley california supports the interpretation that the addition of topographic load modifies the evolution of the major caldera faults

changes in the tectonic regime above a subduction zone of andean type the andes of peru and bolivia during the pliocene pleistocene this paper adresses the changes in the tectonic regime in the peruvian and bolivian andes that have occurred since the upper miocene when the present day elevation of the cordillera above sea level has been almost reached the stress patterns are deduced essentially from a field study of fault kinematics and a numerical inversion of the slip vector data measured on the fault planes the cuzco fault system in southern peru is chosen as an example to illustrate the methodology used in this region striations on both active and holocene faults are in agreement with a n s extension but faults affecting early pleistocene deposits exhibit two families of striations the younger results from the previous n s extension the older involving reverse motions results from either an e w or a n s compression faults affecting pliocene formations often show an oldest family of striations resulting from a ne sw or an e w trending extension thus three tectonic regimes are demonstrated which are also supported by regional unconformities and sedimentological data 1 a pliocene extensional regime 2 a lower pleistocene compressional regime and 3 a mid pleistocene present day extensional regime similar analyses conducted in the pacific and sub andean lowlands allow sketching of the successive pliocene pleistocene stress patterns in the central andes the quaternary and present day stress pattern is characterized by a n s extension in the high andes and in the pacific lowlands and by an e w compression in the sub andean lowlands and at the contact between the nazca and south american plates this stress pattern is interpreted at a large wavelength 100 km as an effect of compensated topography this model supposes that the vertical lithospheric stress zz increases with the topography the crustal thickness and the low density mantle beneath and that the lithospheric maximum compressional horizontal stress hmax trending e w roughly parallel to the convergence is fairly constant on both edges of the andes the tectonics being compressional zz is 3 and hmax is 1 in the high andes zz becomes 1 then the e w trending hmax is 2 and hmin trending n s is 3 allowing extension to occur in this direction the pliocene stress pattern was characterized by a ne sw or an e w trending extension in the high andes in the pacific lowlands and possibly in the sub andean lowlands this stress pattern was clearly different from the present day one because the e w trending stress was hmin this required a weak push or eventually tractional boundary forces acting on the south american lithosphere it is suggested that this might result from a strong slab pull due to a long steeply dipping slab which decreased the value of the xx stress transmitted to the overriding plate the early pleistocene state of stress was compressional since the elevation of the andes had not markedly decreased during this period this required an increase of the e w trending stress value this resulted from a strong coupling between the two lithospheric plates possibly due to a rupture of a long slab under its own weight other spatial changes in the stress pattern are related to the particular situation of the forearc to the subduction of the buoyant nazca ridge and to the different dips of the slab extension in the high andes is of small magnitude of the order of 1 during the last 1 2 m y in a few basins it may have attained 40 during the pliocene 5 3 m y

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

prehistoric alteration of soil in the lower rhine basin northwest germany archaeological 14c and geochemical evidence in early holocene chernozems were assumed to have covered the entire loess landscape of the lower rhine basin today mirrored by the distribution of luvic phaeozems these luvic phaeozems have characteristic dark brown bht horizons accumulating clay and humus inherited and translocated from their precursors chernozem black humic a horizons we examined luvic phaeozems along a 33 km long and 2 0 2 5 m deep gas pipeline trench in the lower rhine basin west of cologne along this transect we discovered clusters of hundreds of regularly shaped pits these pits were always connected to the bht horizons of adjacent luvic phaeozems the luvic phaeozem horizons and the pits were investigated by combining methods from geo archaeology geographical distribution within the landscape shape of the pits soil texture geochemistry content of carbon nitrogen and black carbon palaeobotany species determination of charcoals and ams 14 c measurements we found that the luvic phaeozems occurred not only in the loess covered landscape but also in the sandy holocene floodplain and their distribution could not be limited to certain slope positions or parent material carbon and nitrogen concentrations in the luvic phaeozem horizons and pits were larger than in the surrounding luvisols whereas the c n ratios were small 10 material found in the luvic phaeozem pits was clearly different from material found in prehistoric settlements the pits investigated here never contained artifacts and carbon and nitrogen concentrations and c n ratios were smaller we found charcoal particles and black carbon contributed up to 46 of the total organic carbon the ams 14 c ages of charcoals and black carbon indicated that fire occurred from mesolithic 9500 5500 bc to the medieval ages 500 1500 ad and mainly in the late end neolithic period 4400 2200 bc we conclude that i the luvic phaeozem pits and horizons are man made formed during several archaeological epochs between mesolithic and middle ages ii these pits must have been formed outside the actual prehistoric settlements offsite and may serve as a novel archaeological feature iii the purpose of these pits at present is not clear and iv human activity has altered and ultimately formed the investigated soils of the lower rhine basin in prehistoric time

formation and fate of sedimentary depocentres on southeast asia s sunda shelf over the past sea level cycle and biogeographic implications abstract sea level variations are the major factor controlling sedimentation as well as the biogeographic patterns at continental margins over late quaternary times fluctuations on millennial time scales produce locally complex deposits in coasts and on shelves associated with short term influence on species development this article reviews the sedimentary and biogeographic history of the tropical siliciclastic sunda shelf as an end member of continental shelves regarding extreme width an enormous sediment supply and highest biodiversity in response to rapid sea level fluctuations we describe particular depositional segments as part of a genetic succession of zones from land to the deep sea based on literature data field observations and calculation of hydro isostatic adjustment effects on changing relative sea level these segments are characterized by individual sedimentary processes and deposits and by a specific potential for material storage and re mobilization long term regressive intervals led to overall sigmoidal promoting extremely thick and wide succeeding units in contrast rapid lateral shifts of defined depocentres over long distances took place in response to short term sea level fluctuations fully isolated small scale sediment bodies formed when sea level changed at exceptionally high rates as a result of the high availability of organic rich sediments mangrove and freshwater peats formed frequently over late quaternary times the appearance of thick massive and widespread peats is mainly linked to time intervals of a sea level rise at slow rates whilst organic matter appears much more dispersely in the sediments during episodes of rapidly changing sea level the preservation potential of the regressive units is generally high due to highest initial sediment supply stabilizing soil formation during exposure and rapid subsidence preservation of depositional elements from other periods is more exceptional and either restricted to local morphological depressions or to episodes of rapid sea level change besides complex channel incision an overall lowering of the sediment surface related to erosion as deep as 20 m or more over wide areas took place mainly during sea level lowering the final export of shelf material is documented by enormous mass wasting packages on the associated continental slope from a palaeogeographic perspective the rapid formation or disappearance of special habitat zones such as mangrove fringes and extended mud flats led to species establishment or truncation in distribution in addition the opening or closure of ocean passages as narrow bridges allowing limited species crossing or as fully colonized corridors had severe impact on eco fragmentation and the expansion or contraction of species independent of such particular conditions sea level changes have been too rapid over the past climatic cycle to allow full regeneration and mature development of coast related ecosystems

k ar geochronology of the late cenozoic volcanic rocks of the cordillera occidental southernmost peru abstract twenty four k ar radiometric ages are presented for late cenozoic continental volcanic rocks of the cordillera occidental of southernmost peru lat 16 57 17 36 s rhyodacitic ignimbrite eruptions began in this transect during the late oligocene and continued episodically through the miocene the development of andesitic dacitic strato volcanoes was initiated in the pliocene and continues to the present the earliest ignimbrite flows 25 3 22 7 ma are intercalated in the upper coarsely elastic member of the moquegua formation and demonstrate that this sedimentary unit accumulated in a trough parallel to andean tectonic trends largely in the oligocene more voluminous ash flow eruptions prevailed in the early miocene 22 8 17 6 ma and formed the extensively preserved huaylillas formation this episode was coeval with a major phase of andean uplift and the pyroclastics overlie an erosional surface of regional extent incised into a paleogene volcano plutonic arc terrain an age span of 14 2 8 9 ma mid late miocene is indicated for the younger chuntacala formation which again comprises felsic ignimbrite flows largely restricted to valleys incised into the pre huaylillas formation lithologies and at lower altitudes an extensive aggradational elastic facies the youngest areally extensive ignimbrites constituting the sencca formation were extruded during the late miocene in the earliest pliocene the ignimbrites were succeeded by more voluminous calcalkaline intermediate flows which generated numerous large and small stratovolcanoes these range in age from 5 3 to 1 6 ma present day or holocene volcanism is restricted to several large stratovolcanoes which had begun their development during the pleistocene by 0 7 ma the late oligocene early miocene ca 22 23 ma reactivation of the volcanic arc coincided with a comparable increase in magmatic activity throughout much of the cordilleras occidental and oriental of the central andes

caldera collapse during the 2000 eruption of miyakejima volcano japan a collapsed caldera 1 6 km in diameter and 450 m in depth was formed at the summit of miyakejima volcano during the 2000 eruption the collapsed caldera appeared on 8 july with a minor phreatic eruption 12 days after seismic activity and magma intrusion occurred northwest of the volcano growth of the caldera took from 8 july to the middle of august with seismic swarms associated with the continuous intrusion of magma northwest of the volcano the growth rate of the caldera was about 1 4 107 m3 day and the final volume of the collapsed caldera was about 6 108 m3 major phreatomagmatic eruptions produced a total of about 1 6 1010 kg 1 1 107 m3 of volcanic ash after caldera growth the caldera structure and the nature of the eruptive materials of the first collapse on 8 july suggest that the surface subsidence was caused by the upward migration of a steam filled cavity with stoping of the roof rock above the magma reservoir the diameter of the stoping column was estimated to be 600 700 m from circumferential faults that developed in the caldera floor and the collapse of the caldera wall enlarged the diameter of the caldera to 1 6 km the total volume of the caldera and the horizontal diameter of the stoping column gave a subsidence of the caldera floor of 1 6 2 1 km

timing of the last glacial maximum from observed sea level minima during the last glacial maximum ice sheets covered large areas in northern latitudes and global temperatures were significantly lower than today but few direct estimates exist of the volume of the ice sheets or the timing and rates of change during their advance and retreat here we analyze four distinct sediment facies in the shallow tectonically stable bonaparte gulf australia each of which is characteristic of a distinct range in sea level to estimate the maximum volume of land based ice during the last glaciation and the timing of the initial melting phase we use faunal assemblages and preservation status of the sediments to distinguish open marine shallow marine marginal marine and brackish conditions and estimate the timing and the mass of the ice sheets using radiocarbon dating and glacio hydroisostatic modelling our results indicate that from at least 22 000 to 19 000 calendar years before present land based ice volume was at its maximum exceeding today s grounded ice sheets by 52 5 x 10 exp 6 cu km a rapid decrease in ice volume by about 10 percent within a few hundred years terminated the last glacial maximum at 19 000 250 years

geoarchaeology of upper palaeolithic loess sites located within a transect through moravian valleys czech republic abstract the middle and upper palaeolithic sites situated within the system of moravian and south silesian valleys are key localities for understanding the patterns of seasonal mobility that enabled humans to exploit the north european plain and a possibility to distinguish the importance of the weather and climate for their subsistence practises loess accumulations that have covered the best preserved open air palaeolithic sites in central europe display the climatic record covering at least 30 000 years the sedimentological microstratigraphical and geochemical record of three studied upper palaeolithic loess sites show significant changes documenting increased precipitation towards the north a progressive coarsening of the loess deposits during the upper pleniglacial contrasting with the progressive fining toward the north european glaciation was detected this methodological approach explains more precisely the context of formation processes connected also with human activity within the corridor between the north european plain and the danube basin through which a wide range of organisms including humans and their prey species were channelled

climatic and human controls on holocene floodplain vegetation changes in eastern pennsylvania based on the isotopic composition of soil organic matter abstract a paleoenvironmental time series spanning the holocene was constructed using 29 radiocarbon ages and 149 standardized 13 c som values from alluvial terrace profiles along the middle delaware river valley there is good agreement between increasing 13 c som and panicoideae phytolith concentrations suggesting that variations in c 4 biomass are a major contributor to changes in the soil 13 c a measurement error deconvolution curve over time reveals two isotope stages ii i with nine sub stages exhibiting variations in average 13 c som average c 4 stage ii 10 7 4 3 ka shows above average 13 c som increase c 4 values with evidence of an early holocene warming and dry interval sub stage iib 9 8 8 3 ka that coincides with rapid warming and cool dry abrupt climate change events sub stage iid 7 0 4 3 ka is an above average 13 c som increase c 4 interval associated with the mid holocene warm dry hypsithermal the stage ii i shift at 4 3 ka documents a transition toward below average 13 c som decrease c 4 values and coincides with decreasing insolation and hydroclimatic change sub stages ib and id above average c 4 coincide with the first documented occurrence of maize in the northeastern usa and a substantial increase in human population during the late woodland these associations suggest that people influenced 13 c som during the late holocene

effects of wall rock elasticity on magma flow in dykes during explosive eruptions abstract magma flow during explosive volcanic eruptions has been described assuming rigid conduits with simple cylindrical or planar geometries here we study the dynamics of explosive volcanic flows to take account of the role of elastic deformation of the conduit influenced by local magmatic pressure three cases are investigated a dyke with elliptical cross section a cylindrical conduit and a deep dyke connected to a shallow cylinder the model cpiuc macedonio et al 2005 was used for simulations and generalized to account for elastic deformations of the conduit cross section area due to magmatic overpressure fragmentation level is typically deeper in a dyke than in a cylinder for flows in wide dykes pressure at the fragmentation depth can be lower than the surrounding lithostatic pressure by several tens of mpa indicating that the wall rocks of the dyke will be unstable constraining the dyke width and eventually blocking the eruption on the other hand when the fragmentation level is shallow the corresponding lithostatic pressure is not large enough to close the dyke and eruptions from wide dykes are possible the behaviour changes drastically when we assume the conduit is a dyke at depth that evolves to a cylinder near the surface in this case even very wide dykes can be stable because the fragmentation level moves into the cylindrical region where deformation is negligible

archaeological sediments from settlement mounds of the sagzabad cluster central iran human induced deposition on an arid alluvial plain abstract we outline results of a pilot geoarchaeological study of three settlement mounds tepe in the qazvin plain near tehran ca 7350 to 2450 cal bp conducted as part of a wider investigation of the interaction between prehistoric human communities and their environments in western iran the aim of the geoarchaeological study was to assess the potential of settlement sediments as records of past human practices and to guide future systematic sampling for geoarchaeological analyses here we integrate bulk sediment sediment texture organic matter with thin section based thin section micromorphology sem analyses in order to characterize human inputs in settlement deposits and infer human practices that drove sediment deposition plinth degradation and various kinds of burning byproducts and domestic and manufacturing waste furnished distinctive inputs to settlement sediments different types of fuel wood perhaps tamarix straw dung may reflect diverse domestic and manufacturing contexts of fire use

the stratigraphy structure and evolution of zengmu basin abstract the zengmu basin is a large tertiary marine sedimentary basin in the southern region of the south china sea it is considered to have multiple combinations of source rocks reservoirs and caprock making it a favourable prospect for oil and gas seismic investigations have shown that this has three tectonic sequences separated by two unconformities the upper sequence consists of oligocene to recent marine sediments the middle sequence is thought to be composed of paleogene to eocene shelf neritic clastic deposits and the lower sequence is interpreted as being pre tertiary metamorphic or igneous rocks the development of the basin has been controlled by faults especially by northwest trending faults leading to gradual stepped block subsidence to the northeast three fault systems trending nw ne and s n separate the basin into ten secondary tectonic units with five depressions and five highs taking into account the seismic reflection data collected during the 1987 survey by the shiyan ii and the geological information available from earlier work this paper elucidates our understanding of the sequences division and tectonic subdivision and of the formation evolution and oil and gas prospects of the zengmu basin

analogue models of collapse calderas and resurgent domes collapse calderas and resurgent domes are a common association related to inflation deflation processes in volcanic systems the structure of calderas and domes depends upon the permitted relative movements of crustal volumes at depth the so called space problem in order to study the structures of collapse calderas and resurgent domes and to take the space problem into account several analogue models were made dry quartz sand was used to simulate the rheology of the brittle crust while newtonian silicone putty located at the base of the sand pack simulated the ductile behaviour of the magma a piston moved the silicone putty downward or upward inducing collapse and doming within the sand three separate sets of experiments simulated 1 caldera collapse 2 resurgence and 3 superimposition of resurgence on collapse and vice versa collapse experiments are characterized by the development of two concentric depressions the first formed depression is bordered by outward dipping reverse ring faults the subsequent outer concentric depression is bordered by inward dipping normal ring faults the deformation pattern during resurgence is a function of the overburden thickness t and the dome diameter d for higher t d ratios a dome forms bordered by inward dipping high angle reverse ring faults outward dipping normal ring faults develop at late stages for lower t d ratios the dome shows at late stages a crestal depression accompanied by radial fractures subsequently an apical extrusion of silicone occurs the superimposition of resurgence over collapse and vice versa is characterized by the complete reactivation with opposite kinematics of all the pre existing ring faults during inversion both in caldera and resurgence reverse ring faults form in the early stages due to differential uplift extensional structures subsequently form to accommodate gravitational collapse during the activity of the reverse faults the experiments and the overall similarities with nature suggest that the activity of both reverse and normal faults constitutes a possible solution to the space problem during major collapses or resurgences

sub surface structures and collapse mechanisms of summit pit craters abstract summit pit craters are found in many types of volcanoes and are generally thought to be the product of collapse into an underpressured reservoir caused by magma withdrawal we investigate the mechanisms and structures associated with summit pit crater formation by scaled analogue experiments and make comparisons with natural examples models use a sand plaster mixture as analogue rock over a cylinder of silicone simulating an underpressured magma reservoir experiments are carried out using different roof aspect ratios roof thickness roof width of 0 2 2 they reveal two basic collapse mechanisms dependant on the roof aspect ratio one occurs at low aspect ratios 1 as illustrated by aspect ratios of 0 2 and 1 outward dipping reverse faults initiated at the silicone margins propagates through the entire roof thickness and cause subsidence of a coherent block collapse along the reverse faults is accommodated by marginal flexure of the block and tension fractures at the surface aspect ratio of 0 2 or by the creation of inward dipping normal faults delimiting a terrace aspect ratio of 1 at an aspect ratio of 1 overhanging pit walls are the surface expressions of the reverse faults experiments at high aspect ratio 1 2 reveal a second mechanism in this case collapse occurs by stopping which propagates upwards by a complex pattern of both reverse faults and tension fractures the initial underground collapse is restricted to a zone above the reservoir and creates a cavity with a stable roof above it an intermediate mechanism occurs at aspect ratios of 1 1 1 2 in this case stopping leads to the formation of a cavity with a thin and unstable roof which collapses suddenly the newly formed depression then exhibits overhanging walls surface morphology and structure of natural examples such as the summit pit craters at masaya volcano nicaragua have many of the features created in the models indicating that the internal structural geometry of experiments can be applied to real examples in particular the surface area and depth of the underpressured reservoir can be roughly estimated we present a morphological analysis of summit pit craters at volcanoes such as kilimanjaro tanzania san cristobal telica and masaya nicaragua and ubinas peru and indicate a likely type of subsidence and possible position of the former magma reservoir responsible for collapse in each case

magmatic storage conditions decompression rate and incipient caldera collapse of the 1902 eruption of santa maria volcano guatemala abstract phase equilibria experiments and analysis of natural pumice and phenocryst compositions indicate the 1902 santa maria dacite was stored at 140 170 mpa and 840 850 c prior to eruption h2o saturated cold seal experiments conducted in vessels with an intrinsic log fo2 of nno 1 0 5 show that the natural phase assemblage melt plagioclase amphibole orthopyroxene fe ti oxides apatite is stable from approximately 115 140 mpa at temperatures below 825 c to 840 860 c at 150 mpa to 850 and 50 mpa and 50 c greater than experimental run conditions precise estimates of magmatic conditions based solely upon amphibole composition are likely inaccurate the experimental results and analysis of natural crystals suggest that although the natural amphiboles likely record a broad range of magmatic conditions only the lower bounds of that range reflect pre eruptive storage conditions comparison of santa maria microlite abundances with decompression experiments examining other silicic systems from the literature suggests that the 1902 dacite decompressed at the rate of 0 005 to 0 01 mpa s during the eruption applying the decompression rate with the previously described eruption rate of approximately 2 3 108 kg s williams and self 1983 carey and sparks 1986 to the conduit model conflow reveals that the eruption conduit was dike like with an along strike length 1 km despite depositing 20 km3 of dacite tephra equivalent to 8 5 km3 magma the 1902 eruption did not form an obvious caldera this work suggests that collapse of the dike like conduit terminated the eruption preventing full caldera collapse

new structural limits on magma chamber locations at the valley of ten thousand smokes katmai national park alaska new structural data from the novarupta basin katmai national park alaska site of the largest volcanic eruption of this century 1912 provide limits for the location of magma chambers associated with this eruption to investigate the subsurface structure of the 1912 vents and to support an interdisciplinary study of this young volcanic system a project of geologic mapping of surficial and bedrock structures in the vent region of the 1912 eruption has been undertaken landslide scarps arcuate grabens a monoclinal fold and truncated ridges circumscribe the novarupta basin marking the inferred outer rim of the vent a set of radial fissures crosses the southern margin of the basin striking approximately 140 degree subparallel to the dominant bedrock joint set these fissures and joints along with the local plate motion vector and the inferred regional stress orientation are consistent with a feeder dike propagating from a reservoir beneath trident volcano to the eruptive vent

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