eruptive history of earth s largest quaternary caldera toba indonesia clarified single grain laser fusion sup 40 ar sup 39 ar analyses of individual sanidine phenocrysts from the two youngest toba indonesia tuffs yield mean ages of 73 plus minus 4 and 501 plus minus 5 ka in addition glass shards from toba ash deposited in malaysia were dated at 68 plus minus 7 ka by the isothermal plateau fission track technique these new determinations in conjunction with previous ages for the two oldest tuffs at toba establish the chronology of four eruptive events from the toba caldera complex over the past 1 2 m y ash flow tuffs were erupted from the complex every 0 34 to 0 43 m y culminating with the enormous 2500 3000 km sup 3 youngest toba tuff eruption caldera formation and subsequent resurgence of samosir island timing of this last eruption at toba is coincident with the early wisconsin glacial advance the high precision sup 40 ar sup 39 ar age eruption of such magnitude may provide an important marker horizon useful as a baseline for research and modeling of the worldwide climatic impact of exceptionally large explosive eruptions

a melt inclusion study of the toba tuffs sumatra indonesia the toba caldera in northern sumatra indonesia is the site of earth s largest quaternary volcanic eruption this eruption dated at 74 ka produced the 2800 km3 youngest toba tuff ytt and ash fall quartz bearing silicic pumices ranging from 68 to 77 wt sio2 indicate that the ytt magma was zoned compositionally and mineralogically prior to the ytt eruption two other silicic quartz bearing tuffs known as the middle toba tuff mtt and the oldest toba tuff ott were erupted from toba at 0 501 ma and 0 840 ma respectively although the volatile contents of the toba magmas are poorly constrained aerosols generated by the ytt eruption are generally thought to have caused a global volcanic winter an evaluation of the pre eruptive dissolved gas contents of the ytt magma is fundamental to understanding the global effects of this eruption we used melt inclusions in quartz crystals to determine the dissolved h2o co2 s cl and f contents of the ytt mtt and ott magmas quartz crystals selected from pumice blocks and welded tuffs that spanned the compositional ranges of the three units were chosen for study major and trace element analyses of melt inclusions were also conducted to characterize melt evolution and provide context to the volatile data melt inclusions from the ytt mtt and ott are rhyolitic in composition 73 77 wt sio2 and have overlapping indistinguishable major element trends inclusions from the large eruptions ytt and ott have similar geochemical relationships where the most evolved melt inclusions occur in the least silicic bulk rock samples and are more evolved than their matrix glasses in contrast the least evolved melt inclusions occur in the most silicic samples and either overlap with or are less evolved than their matrix glasses these patterns can be explained if quartz crystals in the least silicic magmas were inherited from more evolved melt while those in the most silicic melt formed in situ several lines of evidence suggest that crystal settling of early formed quartz crystals in long lived magma bodies was an important process in the ytt and ott magmas trace element data from the ytt melt inclusions have wide ranges rb 250 830 ppm ba 10 485 ppm and mostly support these models however incompatible trace elements are enriched 2 3 over their matrix glass compositions in all samples in the smaller mtt melt inclusions overlapped with the compositional range of the bulk rock samples and are consistent with in situ crystallization ftir analyses indicate that the ytt mtt and ott melts contained about 4 0 5 5 2 0 5 5 and 2 0 5 5 wt h2o respectively co2 contents in all units are generally 100 ppm and cl is 2000 ppm water gradients occur in all 3 tuffs cl gradients are evident only in the ytt and the least silicic samples from all units had no detectable co2 sulfur contents in melt inclusions are low in all samples 32 ppm and overlap with matrix glass s contents applying broad degassing constraints about 1014 g of h2so4 aerosols were likely loaded into the atmosphere during the ytt eruption 2 orders of magnitude less than previous petrologic estimates considerably more cl and f about 1015 g of each may have been part of the ytt aerosol cloud

potential field inversion for a layer with uneven thickness the tyrrhenian sea density model abstract inversion of large scale potential field anomalies aimed at determining density or magnetization is usually made in the fourier domain the commonly adopted geometry is based on a layer of constant thickness characterized by a bottom surface at a fixed distance from the top surface we propose a new method to overcome this limiting geometry by inverting in the usual iterating scheme using top and bottom surfaces of differing but known shapes randomly generated synthetic models will be analyzed and finally performance of this method will be tested on real gravity data describing the isostatic residual anomaly of the southern tyrrhenian sea in italy the final result is a density model that shows the distribution of the oceanic crust in this region which is delimited by known structural elements and appears strongly correlated with the oceanized abyssal basins of vavilov and marsili

global marine gravity from retracked geosat and ers 1 altimetry ridge segmentation versus spreading rate 1 three approaches are used to reduce the error in the satellite derived marine gravity anomalies first we have retracked the raw waveforms from the ers 1 and geosat gm missions resulting in improvements in range precision of 40 and 27 respectively second we have used the recently published egm2008 global gravity model as a reference field to provide a seamless gravity transition from land to ocean third we have used a biharmonic spline interpolation method to construct residual vertical deflection grids comparisons between shipboard gravity and the global gravity grid show errors ranging from 2 0 mgal in the gulf of mexico to 4 0 mgal in areas with rugged seafloor topography the largest errors of up to 20 mgal occur on the crests of narrow large seamounts the global spreading ridges are well resolved and show variations in ridge axis morphology and segmentation with spreading rate for rates less than about 60 mm a the typical ridge segment is 50 80 km long while it increases dramatically at higher rates 100 1000 km this transition spreading rate of 60 mm a also marks the transition from axial valley to axial high we speculate that a single mechanism controls both transitions candidates include both lithospheric and asthenospheric processes

three dimensional estimation of elastic thickness under the louisville ridge a three dimensional approach to estimating elastic thickness is presented which uses dense satellite altimetry and sparse ship bathymetry this technique is applied to the louisville ridge system to study the tectonic history of the region the inversion is performed as both a first order approximation and a nonlinear relationship between gravity and topography based on parker s 1973 equation while the higher order effect on the gravity anomaly is nearly zero for most of the region the magnitude is significant over the summits of the ridge nevertheless the inclusion of the nonlinear terms has only a minor influence on the elastic thickness estimate within each region lowering the value by 1 2 km compared with the linear result the incorrect assumption of two dimensionality for circular features exhibits a marked effect on the gravitational anomaly resulting in false sidelobe structure of nearly 20 mgal for large seamounts our elastic thickness estimates are compared with the contradictory values obtained in previous studies by cazenave and dominh 1984 and watts et al 1988 we find an increasing elastic thickness along the chain from southeast to northwest with a discontinuity along the wishbone scarp the jump in elastic thickness values northwest of the scarp appears to be an indication of an age discontinuity caused by an extinct spreading center north of the ridge

p s velocity and vp vs ratio beneath the toba caldera complex northern sumatra from local earthquake tomography summary in this paper we investigate the crustal and uppermost mantle structure beneath toba caldera which is known as the location of one of the largest cenozoic eruptions on earth the most recent event occurred 74 000 yr bp and had a significant global impact on climate and the biosphere in this study we revise data on local seismicity in the toba area recorded by a temporary passcal network in 1995 we applied the newest version of the lotos 07 algorithm which includes absolute source location optimization of the starting 1 d velocity model and iterative tomographic inversion for 3 d seismicp s or thevp vs ratio and source parameters special attention is paid to verification of the obtained results beneath the toba caldera and other volcanoes of the arc we observe relatively moderate for volcanic areas negative p and s velocity anomalies that reach 18 per cent in the uppermost layer 10 12 per cent in the lower crust and about 7 per cent in the uppermost mantle much stronger contrasts are observed for the vp vs ratio that is a possible indicator of dominant effect of melting in origin of seismic anomalies at a depth of 5 km beneath active volcanoes we observe small patterns 7 15 km size with a high vp vs ratio that might be an image of actual magmatic chambers filled with partially molten material feeding the volcanoes in the mantle wedge we observe a vertical anomaly with low p and s velocities and a high vp vs ratio that link the cluster of events at 120 140 km depth with toba caldera this may be an image of ascending fluids and melts released from the subducted slab due to phase transitions however taking into account poor vertical resolution these results should be interpreted with prudence although the results show clear signatures that are quite typical for volcanic areas low velocity and high vp vs ratio beneath volcanoes we do not observe any specific features in seismic structure that could characterize toba as a super volcano

a gravity and volcanostratigraphic interpretation of the lake toba region north sumatra indonesia abstract in northern and central sumatra a wide distribution of ignimbrite and volcanic sediments is observed some geologists consider that these volcanic products were erupted from lake toba the subduction of the indian ocean plate along the trench of central sumatra started at least as long ago as the late miocene fitch s model 1972 obtained the plate convergence vector 67 mm yr decomposed into its components perpendicular 58 mm yr and parallel 34 mm yr to the trench axis tears seaward of the mountain range created the sumatran fault about 2 ma ago at that time the edge of the plate reached a depth of almost 100 km and a large amount of dacite magma was generated the tuk tuk dacite a kind of ignimbrite was erupted along the sumatran fault and covered a broad area the age of this ignimbrite was obtained as 1 9 0 4 ma k arage and 2 0 0 3 ma fission track age since this eruption the hinterland of this fault has been changed to a tensional field forming a graben structure at the northern end of this graben west of harranggaoul the ignimbrite haranggaoul formation was erupted 1 3 0 3 ma ago forming a caldera the lacustrine sediments of samosir island have a normal magnetization and overlie the tuk tuk dacite unconformably from this we can conclude that the graben structure has been covered with water lake toba at 0 7 0 6 ma ago this tectonic movement has continued so that it is difficult to distinguish the caldera structure on the gravity anomaly map of the northern part of lake toba from the results of the gravity survey it can be concluded that at 0 1 ma ignimbrite flowed down along the asahan river and filled the old river along the lowland between parapat and porsea forming a caldera 30 000 years ago dacite and and esite eruptions have occurred around lake toba forming the volcanic cones

relationship between tectonism and volcanism along the great sumatran fault zone deduced by spot image analyses abstract satellite images provide evidence for numerous stepovers pull apart grabens and volcanic structures along the nw trending right lateral great sumatran fault zone high resolution spot images permit us to analyze the relationship between volcanic and tectonic structures this analysis reveals that releasing stepovers and pull aparts are ephemeral structures along the great sumatran fault zone and that the geometry of the strike slip fault evolves permanently through time the interpretative structural maps obtained from spot image analysis reveal that the formation of huge peculiarly shaped volcanic calderas has occurred in large releasing stepover fault zones and that the bounding faults of rectangular pull apart basins are analogous to the circular ring faults of calderas in particular detailed study of the segments at the southernmost 150 km long termination of the great sumatran fault zone from northwest of ranau lake to semangka bay allows us to describe the development of the ranau collapse caldera this caldera is located within a pull apart basin bounded by a stepover its peculiar rectangular shape and its relatively large size about 200 km 2 are controlled by the evolution of the extinct ranau releasing stepover similarly the toba elliptical caldera one of the largest volcanic caldera in the world is elongated parallel to the present trace of the great sumatran fault and appears to be related to a releasing stepover associated with a wide pull apart basin that is not active at present

probing the accumulation history of the voluminous toba magma the age and compositional zonation in crystals from the youngest toba tuff record the prelude to earth9s largest quaternary eruption we used allanite crystals to date and decipher this zoning and found that the crystals retain a record of at least 150 000 years of magma storage and evolution the dominant subvolcanic magma was relatively homogeneous and thermally stagnant for 110 000 years in the 35 000 years before eruption the diversity of melts increased substantially as the system grew in size before erupting 75 000 years ago

high resolution depth and coastline over major atolls of south china sea from satellite altimetry and imagery abstract current bathymetric models for the south china sea scs are largely based on predicted depths from gravity and sparse single beam echo sounder measurements such models lack high resolution coastlines and shallow water bottom features around atolls and islands this study refines the gravity field of the scs using sea surface heights from measurements of satellite altimeter geosat gm ers 1 gm jason 1 gm and the original cryosat 2 a new one minute gravity anomaly grid is determined the modeled gravity anomalies show a 6 mgal rms discrepancy with shipborne measurements in shallow waters an altimeter only bathymetric model is derived from the new gravity grid by the gravity geological method that uses the latest global and regional models of the ocean depth and marine gravity as a priori knowledge the new model outperforms current scs bathymetric models and is accurate to 100 m based on comparison with multi beam depth measurements optical images from ikonos 2 quickbird 2 geoeye 1 worldview 1 2 and 3 are rectified and digitized to derive the zero coastline and 20 m depth contours reef lines around 44 atolls which are integrated with the altimeter only depths giving significantly improved accuracies and spatial resolutions in modeled depths the improvement percentages of coastlines by the satellite imagery range from 50 to 97 at 41 of the 44 atolls we establish a webpage for free access to the optical and depth images and the depth and gravity grids we will continue to update satellite images altimeter derived gravity grids and bathymetric models over major atolls of the scs

mineralogy of yamato 791073 with reference to crystal fractionation of the howardite parent body shock textures and mineral chemistry indicate that yamato 791073 is a type of meteorite intermediate between diogenite and cumulate eucrite and is similar to the yamato 75032 type achondrites the presence of vesicles in shock melt glass and swirly glasses of plagioclase composition as in the suevite in the matrix indicates that it is the most intensely shocked sample among this group of achondrites pyroxene compositions in y791073 fall in a limited range from mg mg fe 0 7 to 0 6 and are compatible with the continuum of rock types produced presumably by differentiation on the howardite parent body the chemical variation of pyroxene is wider than that of the monomict y75032 group in addition to fe rich diogenite like pyroxene ca2mg66fe31 common in y75032 y791073 contains clasts of pyroxene texturally like those in moore county the plagioclase glass with an89 may originally have been associated with this pyroxene as a cumulate eucrite such clasts are uncommon in the y75032 type achondrites y791073 also contains a very differentiated si and k rich clast y791073 shows a more polymict nature than other y75032 type achondrites but the lithic variation more limited than normal howardites suggests that it sampled two or three layers of the layered crust or pluton produced by the crystal fractionation

the regional geology and evolution of the toba volcano tectonic depression indonesia the late quaternary toba volcano tectonic depression in the sunda arc is the largest resurgent cauldron in the world and lies in one of the largest ignimbrite fields the toba tuffs although composed of at least 3000 km 3 of acid tuffs spread over 20000 km 2 of mainland sumatra the greater part of the toba tuffs is believed to be a single ignimbrite cooling unit formed about 100 000 years ago the toba tuffs also include a 30000 year old airfall tuff which was erupted from a centre just n of the toba depression the toba depression developed through the same stages as other resurgent cauldrons it formed after lithification of the toba tuffs by collapse along regional faults crossing an eastwards bulge in the sumatran geanticline resurgent uplift raised lake sediments within the depression by about 500 m the uplift which was not confined to the depression was probably caused by continuing influx of magma and it is possible that the toba depression is still in the stage of post resurgence volcanism the eruption of the toba tuffs and of all post ignimbrite volcanism took place on the line of the western marginal fault of the depression this marginal fault once extended northwards offshore into a zone of miocene back arc rifting

no short reversals of brunhes age recorded in the toba tuffs north sumatra indonesia paleomagnetic and 40ar 39ar data indicate that two tuffs are present at siguragura north sumatra indonesia a reversely magnetized tuff the earliest eruptive unit of the toba caldera of matuyama age 0 84 ma and a normally magnetized tuff the last eruptive unit of the toba caldera of late brunhes age no reversely magnetized tuffs of late brunhes age are present at this location as suggested by previous investigators

all toba tephra occurrences across peninsular india belong to the 75 000 yr b p eruption abstract a controversy currently exists regarding the number of toba eruptive events represented in the tephra occurrences across peninsular india some claim the presence of a single bed the 75 000 yr old toba tephra others argue that dating and archaeological evidence suggest the presence of earlier toba tephra resolution of this issue was sought through detailed geochemical analyses of a comprehensive suite of samples allowing comparison of the indian samples to those from the toba caldera in northern sumatra malaysia and importantly the sedimentary core at odp site 758 in the indian ocean a core that contains several of the earlier toba tephra beds in addition two samples of toba tephra from western india were dated by the fission track method the results unequivocally demonstrate that all the presently known toba tephra occurrences in peninsular india belong to the 75 000 yr b p toba eruption hence this tephra bed can be used as an effective tool in the correlation and dating of late quaternary sedimentary sequences across india and it can no longer be used in support of a middle pleistocene age for associated acheulian artifacts

stratigraphy paleomagnetism and magnetic fabric of the toba tuffs constraints on the sources and eruptive styles the toba depression in north central sumatra is a complex of several overlapping calderas resulting from three major ignimbrite forming eruptions within the depression the upland masses of samosir and northern uluan consist of welded ignimbrite capped by coarse breccia and lacustrine sediment hitherto interpreted to be two parts of a single resurgent dome this study has demonstrated that the welded tuffs of samosir and uluan have different magnetic polarities and therefore at least two different ignimbrites are present the samosir uluan massif may consist of parts of two resurgent domes the first ignimbrite eruption occurred at 0 84 ma and produced a very thick 400 m densely welded unit having a reversed polarity anisotropy of magnetic susceptibility ams flow direction and lithic size data indicate that the source lies in the southern part of the toba depression and the thick deposit of uluan is thought to have ponded in a 40 km wide caldera the second ignimbrite is normally magnetized ams flow direction data indicate two separate source vents one to the north in the haranggaol caldera and another to the south the thick deposit at samosir is thought to have ponded in the southern caldera coarse sediments then accumulated over samosir and northern uluan and were capped by lacustrine deposits a renewed episode of resurgence then uplifted samosir island and possibly the northern part of uluan at approximately 0 075 ma the last and apparently largest ignimbrite eruption occurred from calderas in the north and south parts of the toba depression this ignimbrite is mostly nonwelded and normally magnetized part of the uluan dome was destroyed by collapse of the sibandung caldera and latung graben and concomitant with renewed subsidence of the haranggaol and porsea calderas

is the slip rate variation on the great sumatran fault accommodated by fore arc stretching in oblique convergent subduction areas an obliquity increase should produce a slip rate increase on trench parallel strike slip faults and thus the fore arc platelet should be stretched along the great sumatran fault gsf that is associated with the oblique convergent sunda arc a northward increase of the gsf slip rate occurs parallel with the convergence obliquity transpressional back arc deformation accommodates part of the gsf slip rate variation while no significant fore arc stretching is observed the sumatran case shows that oblique convergence may be accommodated by deformation of a 500 km wide zone from the fore arc to the back arc domains

mechanisms of activity and unrest at large calderas large caldera collapses represent catastrophic natural events second only to large meteoritic impacts in addition some calderas are densely populated making the risk extreme even for moderate eruptions understanding caldera mechanisms unrest and the danger of eruption is therefore a crucial challenge for earth sciences several key features of caldera behaviour have yet to be fully understood through a combination of case studies and theoretical modelling the following topics are addressed in this volume the conditions required to produce and to release large volumes of magma erupted during caldera formation how magmatic feeding systems evolve before and after a caldera has formed the processes that limit the behaviour of precursors to eruptions how pre emptive precursors can be distinguished from those that drive unrest without an eruption and given that post collapse eruptions may occur across a wide area the optimum procedures for designing hazard maps and mitigation strategies

tectonic discrimination of basalts with classification trees traditionally geochemical classification of basaltic rocks of unknown tectonic affinity has been performed by discrimination diagrams although easy to use this method is fairly inaccurate because it only uses bi or trivariate data furthermore many popular discrimination diagrams are statistically not very rigorous because the decision boundaries are drawn by eye and they ignore closure thus violating the rules of compositional data analysis classification trees approximate the data space by a stepwise constant function and are a more rigorous and potentially more effective way to determine tectonic affinity trees allow the simultaneous use of an unlimited number of geochemical features while still permitting visualization by an easy to use two dimensional graph two classification trees are presented for the discrimination of basalts of mid ocean ridge ocean island and island arc affinities the first tree uses 51 major minor and trace elements and isotopic ratios and should be used for the classification of fresh basalt samples a second tree only uses high field strength element analyses and isotopic ratios and can also be used for basalts that have undergone alteration the probability of successful classification is 89 for the first and 84 for the second tree as determined by 10 fold cross validation even though the trees presented in this paper use many geochemical features it is not a problem if some of these are missing in the unknown sample classification trees solve this problem with surrogate variables which give more or less the same decision as the primary variables the advantages of the classification tree approach over discrimination diagrams are illustrated by a comparative test on a sample dataset of known tectonic affinities although arguably better than discrimination diagrams classification trees are not perfect and the limitations of the method are illustrated on a published dataset of basalts from the pindos basin greece

rheomorphism of welded tuffs abstract peralkaline welded tuffs from the islands of gran canaria canary islands and pantelleria italy show abundant evidence for post depositional flow it is demonstrated that rheomorphism or secondary mass flowage can occur in welded tuffs of ignimbrite and air fall origin the presence of a linear fabric is taken as the diagnostic criterion for the recognition of the process deposition on a slope is an essential condition for the development of rheomorphism after compaction and welding internal structures produced during rheomorphic flow can be studied by the methods of structural geology and show similar dispositions to comparable features in sedimentary slump sheets it is shown that secondary flowage can occur in welded tuffs emplaced on gentle slopes provided that the apparent viscosity of the magma is sufficiently low compositional factors favor the development of rheomorphism in densely welded tuffs of peralkaline type

bathymetric prediction from dense satellite altimetry and sparse shipboard bathymetry the southern oceans south of 30 s are densely covered with satellite derived gravity data track spacing 2 4 km and sparsely covered with shipboard depth soundings hundreds of kilometers between tracks in some areas flexural isostatic compensation theory suggests that bathymetry and downward continued gravity data may show linear correlation in a band of wave lengths 15 160 km if sediment cover is thin and seafloor relief is moderate at shorter wave lengths the gravity field is insensitive to seafloor topography because of upward continuation from the seafloor to the sea surface at longer wavelengths isostatic compensation cancels out most of the gravity field due to the seafloor topography we combine this theory with wiener optimization theory and empirical evidence for gravity noise to signal ratios to design low pass and band pass filters to use in predicting bathymetry from gravity the prediction combines long wavelengths 160 km from low pass filtered soundings with an intermediate wavelength solution obtained from multiplying downward continued band pass filtered 15 160 km gravity data by a scaling factor s s is empirically determined from the correlation between gravity data and existing soundings in the 15 160 km band by robust regression and varies at long wave lengths we find that areas with less than 200 m of sediment cover show correlation between gravity and bathymetry significant at the 99 level and s may be related to the density of seafloor materials in these areas the prediction has a horizontal resolution limit of 5 10 km in position and is within 100 m of actual soundings at 50 of grid points and within 240 m at 80 of these in areas of very rugged topography the prediction underestimates the peak amplitudes of seafloor features images of the prediction reveal many tectonic features not seen on any existing bathymetrie charts because the prediction relies on the gravity field at wavelengths 160 km it is insensitive to errors in the navigation of sounding lines but also cannot completely reproduce them therefore it may be used to locate tectonic features but should not be used to assess hazards to navigation the prediction is available from the national geophysical data center in both digital and printed form

global gravity bathymetry and the distribution of submarine volcanism through space and time 1 the seafloor is characterized by numerous seamounts and oceanic islands which are mainly volcanic in origin relatively few of these features 0 1 however have been dated and so little is known about their tectonic setting one parameter that is sensitive to whether a seamount formed on near or far from a mid ocean ridge is the elastic thickness te which is a proxy for the long term strength of the lithosphere most previous studies are based on using the bathymetry to calculate the gravity anomaly for different values of te and then comparing the calculated and observed gravity anomaly the problem with such an approach is that bathymetry data are usually limited to single beam echo sounder data acquired along a ship track and these data are too sparse to define seamount shape we therefore use the satellite derived gravity anomaly to predict the bathymetry for different values of te by comparing the predicted bathymetry to actual shipboard soundings in the vicinity of each locality in the wessel global seamount database we have obtained 9758 te estimates from a wide range of submarine volcanic features in the pacific indian and atlantic oceans comparisons where there are previous estimates show that bathymetric prediction is a robust way to estimate te and its upper and lower bounds te at sites where there is both a sample and crustal age show considerable scatter however and there is no simple relationship between te and age nevertheless we are able to tentatively assign a tectonic setting to each te estimate the most striking results are in the pacific ocean where a broad swath of on ridge volcanism extends from the foundation seamounts and ducie island easter island ridge in the southeast across the equator to the shatsky and hess rises in the northwest interspersed among the on ridge volcanism are flank ridge and off ridge features the indian and atlantic oceans also show a mix of tectonic settings off ridge volcanism dominates in the eastern north atlantic and northeast indian oceans while flank ridge volcanism dominates the northeastern indian and western south atlantic oceans we have been unable to assign the flank ridge and off ridge estimates an age but the on ridge estimates generally reflect we believe the age of the underlying oceanic crust we estimate the volume of on ridge volcanism to be 1 1 106 km3 which implies a mean seamount addition rate of 0 007 km3 yr 1 rates appear to have varied through geological time reaching their peak during the late early cretaceous and then declining to the present day

new geochemical evidence for the youngest toba tuff in india abstract new geochemical data on tephra samples from a layer present at several archeological sites in india support correlation of this layer to the youngest toba tuff erupted from northern sumatra about 74 000 yr ago the data show that the indian tephra layer is not a correlative of older tephra erupted from toba as has been suggested on the basis of artifact assemblages previously published geochemical data on the indian tephra beds was based on bulk ash samples containing mineral and clay contaminants and the resulting variability in analyses did not allow identification or discrimination of individual eruptive events our new data were collected on individual glass shards and small purified glass separates which have greater resolving power in fingerprinting acheulian and paleolithic artifacts found at some of the indian tephra sites do not reflect the antiquity of the tephra bed as they occur in fluvial sediments and may be reworked

some remarks on resolving seamounts in satellite gravity 1 we examine peak to trough gravity anomaly amplitudes over seamounts comparing gravity derived from satellite altimetry with gravity measured by ships we also compare amplitudes from linear and higher order non linear computations and conclude that two term forward modeling of seamounts should be adequate altimetric amplitudes are within 90 of ship amplitudes at seamounts greater than 14 km in characteristic radius at smaller seamounts altimetric amplitude is reduced probably due to smoothing filters used to process the altimeter data perhaps fortuitously this amplitude loss nearly matches the amplitude under estimate that occurs if one assumes the forward model should contain only a linear term thus bathymetric prediction by linear filtering may do a fairly good job of resolving seamounts much smaller than 14 km in characteristic radius

first toba supereruption revival little has been known about the earliest toba eruptive episodes that created the largest known caldera complex of quaternary age here we report evidence for the eastward dispersal of the oldest toba tuff in south china sea sediments to 2500 km away from the source the tephra deposits occur below the brunhes matuyama geomagnetic boundary 778 ka and slightly above the australasian microtektite layer 793 ka calibrated by astronomically tuned oxygen isotope stratigraphy the middle pleistocene toba eruption occurred during the deglaciation at 788 2 2 ka according to the tephra occurrence between marine isotope stages 20 and 19 this refined age is in good agreement with the 4 0 ar 3 9 ar date of 800 20 ka for the toba tephra layer d from ocean drilling program odp site 758 but significantly younger than the commonly cited ar ar age of 840 30 ka the eruption expelled at least 800 1000 km 3 dense rock equivalent of rhyolitic magma on the basis of the widespread tephra fall deposit in the basins of the indian ocean and the south china sea in spite of its exceptional magnitude the timing of this major eruption does not indicate a causal linkage between this event and a long term global climatic deterioration

new analytic solutions for modeling vertical gravity gradient anomalies modern processing of satellite altimetry for use in marine gravimetry involves computing the along track slopes of observed sea surface heights projecting them into east west and north south deflection of the vertical grids and using laplace s equation to algebraically obtain a grid of the vertical gravity gradient vgg the vgg grid is then integrated via overlapping flat earth fourier transforms to yield a free air anomaly grid because of this integration and associated edge effects the vgg grid retains more short wavelength information e g fracture zone and seamount signatures that is of particular importance for plate tectonic investigations while modeling of gravity anomalies over arbitrary bodies has long been a standard undertaking similar modeling of vgg anomalies over oceanic features is not commonplace yet here we derive analytic solutions for vgg anomalies over simple bodies and arbitrary 2 d and 3 d sources we demonstrate their usability in determining mass excess and deficiency across the mendocino fracture zone a 2 d feature and find the best bulk density estimate for jasper seamount a 3 d feature the methodologies used herein are implemented in the generic mapping tools available from gmt soest hawaii edu

flexure modelling at seamounts with dense cores summary the lithospheric response to seamounts and ocean islands has been successfully described by deformation of an elastic plate induced by a given volcanic load if the shape and mass of a seamount are known the lithospheric flexure due to the seamount is determined by the thickness of an elastic plate te which depends on the load density and the age of the plate at the time of seamount construction we can thus infer important thermomechanical properties of the lithosphere from te estimates at seamounts and their correlation with other geophysical inferences such as cooling of the plate whereas the bathymetry i e shape of a seamount is directly observable the total mass often requires an assumption of the internal seamount structure the conventional approach considers the seamount to have a uniform density e g density of the crust this choice however tends to bias the total mass acting on an elastic plate in this study we will explore a simple approximation to the seamount s internal structure that considers a dense core and a less dense outer edifice although the existence of a core is supported by various gravity and seismic studies the role of such volcanic cores in flexure modelling has not been fully addressed here we present new analytic solutions for plate flexure due to axisymmetric dense core loads and use them to examine the effects of dense cores in flexure calculations for a variety of synthetic cases comparing analytic solutions with and without a core indicates that the flexure model with uniform density underestimates te by at least 25 per cent this bias increases when the uniform density is taken to be equal to the crustal density we also propose a practical application of the dense core model by constructing a uniform density load of same mass as the dense core load this approximation allows us to compute the flexural deflection and gravity anomaly of a seamount in the wavenumber domain and minimize the limitations recognized from the analytic tests then the dense core model is applied to predict the lithospheric flexure beneath howland island in the tokelau seamount chain these results are compared with the predictions of the uniform density model based on age dating of howland and the age of the seafloor traditional te versus age curves predict the elastic plate thickness beneath the seamount to be around 20 km which is comparable to the best dense core model of te 26 km however the best uniform density model is found at te 12 km which is significantly less than the predicted from our investigations of synthetic and real seamount cases we conclude that the dense core model approximates the true mass distribution of a seamount better than the uniform density model finally we suggest that the role of underplating in flexure modelling may need to be reexamined because the dense core model predicts substantially less deflections than the uniform density model without requiring additional buoyancy caused by underplated material

subducted seafloor relief stops rupture in south american great earthquakes implications for rupture behaviour in the 2010 maule chile earthquake great subduction earthquakes cause destructive surface deformation and ground shaking over hundreds of kilometres their rupture length is limited by the characteristic strength of the subduction plate interface and by lateral variations in its mechanical properties it has been proposed that subduction of topographic features such as ridges and seamounts can affect these properties and stop rupture propagation but the required relief and physical mechanisms of topographic rupture limitation are not well understood here we show that the rupture limits of thirteen historic great earthquakes along the south america nazca plate margin are strongly correlated with subducted topography with relief 1000 m including the juan fernandez ridge the northern limit of rupture in the mw 8 8 maule chile earthquake of 27 february 2010 is located where this ridge subducts analysis of intermediate magnitude earthquakes shows that in most places the subduction of high seafloor relief creates weak aseismic zones at the plate interface which prevent rupture propagation but that the juan fernandez ridge is associated with a locally strong plate interface the maximum rupture length and thus magnitude of great subduction earthquakes is therefore determined by the size and lateral spacing of topographic features where they are present on the subducting plate

the toba caldera complex abstract the toba caldera in indonesia is one of the most remarkable volcanic features formed during quaternary geologic time its rich history of research for over a century has yielded important information on the physical volcanology of silicic calderas and super eruptions geochemical evolution of silicic magma bodies and geophysical imaging of active sub volcanic systems during the past 1 3 my the toba area has erupted intermediate composition lavas followed by intermediate pyroclastics three quartz bearing silicic tuffs and most recently intermediate to silicic lavas this pattern represents the incremental assembly and periodic eruption of a crustal magma body of batholithic proportions the apparent migration of activity to the west may have implications for the next toba super eruption

lake toba volcano magma chamber imaged by ambient seismic noise tomography 1 for the first time ambient noise tomography is used to clearly image the magma chamber beneath lake toba caldera one of the largest quaternary calderas on earth using data from 40 seismic stations deployed between may and october 2008 around lake toba empirical green s functions are extracted from long term cross correlations of continuous records these functions are dominated by rayleigh waves whose group velocities can be measured in the period range from 2 5 to 12 seconds arrival times of these waves are picked for a given period and inverted using 2 d tomography to calculate lateral variations in velocity for the given period this was done for six different periods which all correspond to different sampling depths thus the six 2 d models presented together provide information on velocity variations with depth the results show a low velocity body coincident with the lake toba caldera representing the magma chamber under the volcano the chamber is observed to have a complex 3 d geometry with at least two separate sub chambers underlying the caldera other results include a deep low velocity body possibly another magma chamber south west of the lake with an upper limit of 7 km depth the maximum depth to which this body reaches could not be resolved the sumatra fault marks a velocity contrast but only down to depths not greater than 5 km the reliability of the results was further confirmed by checkerboard recovery tests

distribution of magma beneath the toba caldera complex north sumatra indonesia constrained by three dimensional p wave velocities seismicity and gravity data 1 we estimate the one and three dimensional p wave velocity structure beneath the toba caldera complex a 30 100 km topographic depression in north sumatra using arrival time data of local earthquakes recorded by a 40 station seismic network that operated for 4 months inversions reveal the presence of p velocities up to 37 below normal that likely map the distribution of magma within this subduction related volcanic system considered the world s largest in the upper 10 km of crust the largest low velocity region underlies the southern two thirds of the depression and coincides with a gravity low centered over the resurgent dome a smaller volume of low velocities is observed in the upper crust under the north end of the depression separating the two regions is a zone of locally high velocities indicating that the shallow subcaldera magma system is composed of two separate reservoirs not a single one that extends the entire length of the caldera complex above each low velocity region is a postcollapse volcano that erupted mostly mafic lavas after the last major caldera collapse 74 kyr ago a low velocity column below one of these volcanoes can be traced into the uppermost mantle and corresponds with a planar distribution of low frequency earthquakes in the 20 to 40 km depth range the low frequency earthquakes apparently record the migration of melt in the mafic roots of this large volume crustal magma system

retracking ers 1 altimeter waveforms for optimal gravity field recovery summary we have reprocessed ers 1 radar altimeter waveforms using an algorithm designed to minimize sea surface slope error and decouple it from significant wave height swh error standard waveform retracking estimates three parameters arrival time swh and amplitude we show that errors in retracked estimates of arrival time and swh are inherently correlated because of the noise characteristics of the returned waveform this suggests that some of what is called sea state bias in the literature may be caused by correlated errors rather than true electromagnetic or skewness bias we have developed a retracking algorithm that reduces this error correlation and makes the resolution of sea surface slope signals independent of sea state the main assumption is that the swh varies smoothly along the satellite track over wavelengths of 90 km this approach reduces the rms error in sea surface slope to only 62 per cent of that of standard retracking methods while our method is optimized for gravity field recovery it may also improve the resolution of sea surface height signals of interest to physical oceanographers

converted wave imaging of the toba caldera indonesia 1 receiver functions calculated from data recorded by the 1995 passcal toba seismic experiment in northern sumatra indonesia and the permanent station psi reveal the presence of a low seismic velocity zone at between 8 and 14 km depth which is interpreted to be the top of a magma reservoir the moho under the toba caldera lies near 30 km with a localized thickened region that reaches 39 km beneath the eastern edge of the caldera with an abrupt shallowing to the northeast this thickened crust may be where hot gabbroic material underplates the base of the crust to serve as the heat source for shallower volcanism alternatively the thickened crust and moho offset may be due to localized crustal thickening when this region was situated in a sharp restraining bend of a now extinct segment of the sumatran fault

limited global change due to the largest known quaternary eruption toba 74 kyr bp abstract the 74 kyr bp super eruption of toba volcano in sumatra is the largest known quaternary eruption on the basis of preserved deposits the eruption magnitude has been estimated at 7 10 15 kg 2800 km 3 of dense magma the largest sulphate anomaly in the greenland ice sheet project 2 core has been identified as fallout from toba s stratospheric aerosol veil correlation of the sulphate and oxygen isotope stratigraphy of the ice core suggests that the toba eruption might have played a role in triggering a millennium of cool climate prior to dansgaard oeschger event 19 although a comparable stadial preceded event 20 a possible 6 yr duration volcanic winter immediately following the eruption has also been proposed as the cause of a putative bottleneck in human population supporting in a general way the garden of eden model for the origin of modern humans however along with counter arguments regarding the timing of any demographic crash there remain major gaps in our understanding of the 74 kyr bp toba eruption that hinder attempts to model its global atmospheric and climatic and hence human consequences the tephra record reveals basic aspects of the eruption style but calculations of the duration and hence intensity and plume height of the event are poorly constrained furthermore estimates of the sulphur yield of the erupting magma central to predictions of its atmospheric and climatic impacts vary by two orders of magnitude 3 5 330 10 10 kg previous estimates of globally averaged surface cooling of 3 5 c after the eruption are probably too high a figure closer to 1 c appears more realistic the volcanological uncertainties need to be appreciated before accepting arguments for catastrophic consequences of the toba super eruption

stratigraphy of the toba tuffs and the evolution of the toba caldera complex sumatra indonesia during the past 1 2 m y a magma chamber of batholithic proportions has developed under the 100 by 30 km toba caldera complex four separate eruptions have occurred from vents within the present collapse structure which formed from eruption of the 2800 km3 youngest toba tuff ytt at 74 ka eruption of the three older toba tuffs alternated from calderas situated in northern and southern portions of the present caldera the northern caldera apparently developed upon a large andesitic stratovolcano the calderas associated with the three older tuffs are obscured by caldera collapse and resurgence resulting from eruption of the ytt samosir island and the uluan block are two sides of a single resurgent dome that has resurged since eruption of the ytt samosir island is composed of thick ytt caldera fill whereas the uluan block consists mainly of the oldest toba tuff ott in the past 74000 years lava domes have been extruded on samosir island and along the caldera s western ring fracture this part of the ring fracture is the site of the only current activity at toba updoming and fumarolic activity the toba eruptions document the growth of the laterally continuous magma body which eventually erupted the ytt repose periods between the four toba tuffs range between 0 34 and 0 43 m y and give insights into pluton emplacement and magmatic evolution at toba

toba ash layers in the south china sea evidence of contrasting wind directions during eruption ca 74 ka two cores from the southern south china sea contain discrete ash layers that mainly consist of rhyolithic glass shards on the basis of the specmap time scale the ash layers were dated to ca 74 ka the age of the youngest toba eruption in northern sumatra this link is supported by the chemical composition of the glass which is distinct from volcanic glass supplied from the philippines and the northern south china sea but is almost identical with the chemistry of the toba ash the youngest toba ash layers in the south china sea expand the previously known ash fall zone over more than 1800 km to the east the dispersal of ashes from sumatra in both western and eastern directions indicates two contrasting wind directions and suggests that 1 the toba eruption probably happened during the southeast asian summer monsoon season and 2 the volume of erupted magma was larger than previously interpreted

toba ash on the indian subcontinent and its implications for correlation of late pleistocene alluvium the toba ash occurs extensively in the indian subcontinent and marks a ca 74 000 yr old event in the bay of bengal and indian ocean it is about 10 cm thick whereas in several alluvial basins it is usually 1 3 m thick the latter occurs in a partly reworked state but as nearly chemically pure first cycle sediments the ash has a broad northwesterly dispersal pattern samples of ash from the indian subcontinent compare closely with the youngest 74 000 yr b p toba tuff and the deep sea toba ash in bulk chemical composition ree signature and bubble wall shard morphology however a more proximally located and thicker 2 5 m ash bed from the alluvial basins in the gneissic area and close to east coast has a lower magnitude negative eu anomaly possibly because of minor contamination by feldspathic silt quaternary sediments in the central narmada and middle son basins contain rich late and middle pleistocene mammalian and cultural records based on the presence of the ash layer marker and stratigraphic relations late pleistocene sediments within the subcontinent can be correlated with those from central india and the deep sea

phenocrysts versus xenocrysts in the youngest toba tuff implications for the petrogenesis of 2800 km3 of magma the petrogenesis of the 2800 km 3 of magma erupted as the youngest toba tuff has been investigated using experimental petrology and 40 ar 39 ar dating of biotite sanidine hornblende and plagioclase from the tuff we find that hornblende does not crystallize experimentally from the magma at temperatures and pressures indicated by the natural mineral assemblage hornblende is also not in isotopic equilibrium with biotite and sanidine both of which grew experimentally hornblende thus appears xenocrystic despite being a major phase in the tuff some plagioclase is also xenocrystic on the basis of ar isotopes but others are probably phenocrystic because plagioclase grew experimentally crystal clots of hornblende plagioclase observed in the tuff suggest that the xenocrysts came from a common source which was at least 1 5 ma the oldest hornblende 40 ar 39 ar age our results suggest that the toba tuff magma resided at nearly water saturated pressures of 100 150 mpa and that xenocrysts were entrained as recently as 10 yr before the eruption the ubiquitous presence of hornblende in the tuff indicates that entrainment occurred throughout the 2800 km 3 of magma