volatile element b cl f behaviour in the roof of an axial magma chamber from the east pacific rise abstract understanding the behaviour of volatile elements at mid ocean ridges is important for reasons ranging from their influence on mantle viscosity through to their role as a food source for the deep biosphere with the aim of constraining what processes control the distribution of volatiles in the ocean crust at fast spreading ridges we present a detailed study of the compositional variability in magmatic amphibole formed in the upper part of the plutonic sequence at the east pacific rise epr these amphiboles are massively enriched in chlorine by more than an order of magnitude and moderately enriched in boron with respect to magmatic amphiboles in cumulates from the mid atlantic ridge mar similar enrichments have been reported for basaltic glasses from the epr and are interpreted as indicative of assimilation the greater enrichments observed in the plutonic section suggest both that assimilation occurs at the roof of the axial magma chamber amc and that lava compositions may record minimum amounts of exogenic contamination amphiboles with compositions indicative of crystallisation from a contaminated magma occur to depths of 800 m beneath the sheeted dyke complex this is interpreted to indicate that at least this upper portion of the plutonic section forms via crystallisation within the amc followed by subsidence of a crystal mush amphibole boron isotope compositions show that assimilation of altered sheeted dykes plus hydrothermal fluids drives amc magmas to heavier 11b values up to 5 8 subsequent degassing within a solidifying crystal mush leads to a negative trend in 11b b with the most degassed magma having 11b as low as 21 2 this degassing was associated with hydrofracturing of the partially molten crystal mush and could have facilitated a temporal link with the overlying hydrothermal system

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

contribution of grain size and compositional data from the bengal fan sediment to the understanding of toba volcanic event abstract toba tephra layers in cores from the bengal fan have been studied to characterize their granulometric mineralogic and geochemical features the layers are silt sized sediments with no significant non volcanic contamination the samples are very homogeneous glass shards have moderate hydration about 6 water a calc alkaline character and a rhyolitic composition the major and trace element compositions indicate an orogenic affinity and coincide with literature data for the youngest toba tephra level ytt 75 ka x ray diffraction xrd data indicate that the glass is the dominant phase with minor feldspar biotite and quartz the grain size analyses of the beds show discontinuous vertical variation and no linear trends from south to north over about 1500 km the vertical granulometric gap of the layers probably represents a single pyroclastic emplacement unit subjected to pre depositional particle aggregation

seismic evidence for variations in axial magma chamber properties along the southern juan de fuca ridge abstract multichannel seismic data collected along the cleft segment on the southern juan de fuca ridge show that this intermediate spreading center is underlain by a mid crustal reflector interpreted as the top of an axial magma chamber amc the amc reflection is present along most of the segment and deepens gently from 2 0 km near the southern end of the segment beneath the ridge cleft observatory site to 2 3 km at the northern end beneath the site of the mid 1980s submarine eruption we analyzed the one dimensional seismic structure of the amc at two locations with contrasting lava chemistry beneath two different hydrothermal vent fields at the northern site waveform modeling in the time intercept slowness p domain indicates that the amc is 100 m thick and it is characterized by a decrease in p wave velocity from 6 km s to 3 7 km s in contrast the p wave velocity within the shallower 100 m thick amc at the southern site is higher 5 0 km s the decrease in seismic velocity within the amc indicates that it is partially molten and that it is not a cracking front as previously suggested for other intermediate spreading segments the data show a coherent seismic phase interpreted as the p to s wave conversion at the amc pamcs stacking of this event shows that the pamcs is only present along the northern part of the segment our results thus suggest along axis variations in the crystallinity of the amc the amc along cleft varies from a high crystal content

hierarchy of sea level cycles in the medial silurian siliciclastic succession of pennsylvania the medial silurian siliciclastic succession in central pennsylvania records two hierarchically superimposed cycles of sea level fluctuations the larger of these cycles controlled lithofacies architecture at the magnitude of formations and members five alternations of transgression and regression with an average cycle period of about 2 5 m y produced wedges of coarser shallow water facies pointing basinward northwest and wedges of deeper water basin center mudrock facies extending marginward southeast superimposed on these are smaller scale shallowing upward cycles typically 1 3 m thick that occur in three different contemporaneous facies basin margin tidal flats mid shelf offshore bars and below wave base in the storm influenced transition to the basin center average recurrence interval for such smaller cycles was about 100 ka their period was regular and was caused by sea level fluctuations of about 1 3 m

newly discovered eastern dispersal of the youngest toba tuff volcanic glasses with minor mafic mineral fragments such as biotite and hornblende found in deep sea sediments of the south china sea basin scsb have been clearly identified as eruptive products of the youngest toba tuff ytt northern sumatra indonesia the tephra layer occurs between marine oxygen isotopic event 5 1 79 3 ka and event 4 22 64 1 ka with an interpolated age of 74 0 ka which is in good consistence with previous radiometric dating 73 75 ka and ice core dating 71 5 ka of the ytt the tephra consists predominantly of bubble wall shards with minor elongated vesicles of pumice fragments geochemical characteristics of the tephra such as high total alkali content high 87 sr 86 sr ratio and uniformity of their compositions all suggest that the recovered tephra is of the youngest toba tuff this finding supports an extended dispersal of coarse 63 mm glass shards over 1500 km northeast of the toba caldera a direction opposite to what previously conceived while providing a better documentation of the distribution extent of the toba ash this report points to the need to reestimating the eruptive volume of the ytt and re evaluating its environmental impact q 2000 elsevier science b v all rights reserved

temporal and spatial cyclicity of accretion at slow spreading ridges evidence from the reykjanes ridge summary a unifying model of oceanic crustal development at slow spreading rates is presented in which accretion follows a cyclic pattern of magmatic construction and tectonic destruction controlled by along axis variation in melt supply and coupled to along axis variation in spreading rate and across axis asymmetry in spreading this study focuses on the reykjanes ridge mid atlantic ridge south of iceland which is divided along its entire length into numerous axial volcanic ridges avr five adjacent avrs have been analysed located between 57 30 n and 58 30 n and south of any strong iceland hotspot influence the seabed morphology of each avr is investigated using sidescan sonar data to determine relative age and eruptive history along axis gravity profiles for each avr are modelled relative to a seismically derived crustal reference model to reveal the underlying crustal thickness and density structure correlating these models with seabed features crustal structure ridge segment morphology and relative ages a model of cyclic ridge segmentation is developed in which accretion results in adjacent avrs with a range of crustal features which when viewed collectively reveal that second order segments on the reykjanes ridge have an along axis length of 70 km and comprise several adjacent avrs which in turn reflect the pattern of third order segmentation tectono magmatic accretion is shown to operate on the scale of individual avrs as well as on the scale of the second order segment as a whole

the quantum event of oceanic crustal accretion impacts of diking at mid ocean ridges seafloor diking eruptive events represent the irreducible quantum events of upper oceanic crustal accretion they record events by which a large portion of the oceanic crust has formed through geological history since 1993 the u s navy s real time sound surveillance system has allowed location of ongoing acoustic signatures of dike emplacement and basalt eruptions at ridge crests in the northeast pacific these diking eruptive events trigger a sequence of related rapidly evolving physical chemical and biological processes magmatic volatiles released during these events may provide nutrients for communities of subseafloor microorganisms some of which thrive in high temperature anaerobic environments many of the organisms identified from these systems are archaea if microorganisms can thrive in the water saturated pores and cracks within deep volcanically active portions of our planet other hydrothermally active planets may harbor similar life forms

quaternary tephrochronology and deposition in the subsurface sacramento san joaquin delta california u s a abstract we document characteristics of tephra including facies and geochemistry from 27 subsurface sites in the sacramento san joaquin delta california to obtain stratigraphic constraints in a complex setting analyzed tephra deposits correlate with 1 an unnamed tephra from the carlotta formation near ferndale california herein informally named the ash of wildcat grade 0 780 ma 2 the rockland ash bed 0 575 ma 3 the loleta ash bed 0 390 ma and 4 middle pleistocene volcanic ash deposits at tulelake california and pringle falls bend and summer lake oregon herein informally named the dacitic ash of hood 0 180 ma all four tephra are derived from cascades volcanic sources the rockland ash bed erupted from the southern cascades and occurs in up to 7 m thick deposits in cores from 40 m subsurface in the sacramento san joaquin delta tephra facies and tephra age constraints suggest rapid tephra deposition within fluvial channel and overbank settings likely related to flood events shortly following volcanic eruption such rapidly deposited tephra are important chronostratigraphic markers that suggest varying sediment accumulation rates in quaternary deposits below the modern sacramento san joaquin delta this study provides the first steps in a subsurface quaternary stratigraphic framework necessary for future hazard assessment

the toba volcanic super eruption of 74 000 years ago climate change environments and evolving humans the toba volcanic super eruption of 74 000 years ago climate change environments and evolving humansmichael d petraglia ravi korisettar j n palpii s1040 6182 11 00674 4doi 10 1016 j quaint 2011 12 001reference jqi 3091to appear in quaternary internationalreceived date 1 december 2011accepted date 1 december 2011please cite this article as petraglia m d korisettar r pal j n the toba volcanic super eruptionof 74 000 years ago climate change environments and evolving humans quaternary international 2011 doi 10 1016 j quaint 2011 12 001this is a pdf file of an unedited manuscript that has been accepted for publication as a service toour customers we are providing this early version of the manuscript the manuscript will undergocopyediting typesetting and review of the resulting proof before it is published in its final form pleasenote that during the production process errors may be discovered which could affect the content and alllegal disclaimers that apply to the journal pertain

grain size distribution analysis of sediments containing younger toba tephra from ghoghara middle son valley india abstract the toba super eruption in northern sumatra 74 ka was the largest eruption of the quaternary period terrestrial deposits of distal toba tephra have been found across the indian subcontinent although few localities have been adequately described with respect to stratigraphy and sedimentary history this study provides the first detailed description of the ghoghara 1a ash section middle son valley india the sediment analyses provide insights into the depositional processes in the valley before and after the initial period of ash deposition grain size distribution analysis of sediments from ghoghara 1a indicates that 4 5 cm of primary air fall ash was laid down at the time of the toba super eruption this study has implications concerning previous isotopic studies in the middle son valley concluding that reconstructions demonstrative of dramatic ecological changes are questionable

impacts of explosive volcanism on distal alluvial sedimentation examples from the pliocene holocene volcaniclastic successions of japan abstract volcanic activities can create cataclysmic hazards to surrounding environments and human life not only during the eruption but also by hydrologic remobilisation lahar processes after the cessation of eruptive activity although there are many studies dealing with the assessment and mitigation of volcanic hazards these are mostly concentrated on primary eruptive processes in areas proximal to active volcanoes however the influence of volcaniclastic resedimentation may surpass the impacts of primary eruptive activity in terms of both extent and persistence and can ultimately result in severe hazards in downstream areas examination of the volcaniclastic successions of non marine pliocene holocene sedimentary basins in japan has revealed hydrological volcaniclastic sedimentation in fluvial and lacustrine environments hundreds of kilometres from the inferred source volcano impacts on these distal and often spatially separated basins included drastic changes in depositional systems caused by sudden massive influxes of remobilised pyroclastic material typical volcaniclastic beds comprise centimetre to decimetre thick primary pyroclastic fall deposits overlain by metre to 10s of metres thick resedimented volcaniclastic deposits intercalated in sedimentary successions of non volcanic provenance the relatively low component of primary pyroclastic fall deposits in the volcaniclastic beds suggests that 1 potential volcanic hazards would be underestimated on the basis of primary pyroclastic fall events alone and 2 the majority of resedimented material was likely derived from erosion of non welded pyroclastic flow deposits in catchment areas rather than remobilisation of local fallout deposits from surrounding hillslopes the nature distribution and sequence of facies developed by distal volcaniclastic sediments reflect the influence of 1 proximity to ignimbrite but not directly with the distance to the eruptive centre 2 ignimbrite nature non welded or welded and volume 3 temporal changes in sediment flux from the source area 4 the physiography and drainage patterns of the source area and the receiving basin and any intervening areas and 5 the formation of ephemeral dam lakes and intra caldera lakes whose potential catastrophic failure can impact distal areas models of the styles and timing of distal volcaniclastic resedimentation are thus more complicated than those developed for proximal settings of stratovolcanoes and their volcaniclastic aprons and hence present different challenges for hazard assessment and mitigation

elastic thickness control of lateral dyke intrusion at mid ocean ridges abstract magmatic accretion at slow spreading mid ocean ridges exhibits specific features although magma supply is focused at the centre of second order segments melts are episodically distributed along the rift toward segment ends by lateral dyke intrusions it has been previously suggested that an along axis downward topographic slope away from the magma source is sufficient to explain lateral dyke propagation however this cannot account for the poor correlation between dyke opening and surface elevation in the 2005 2010 series of 14 dyke intrusions of afar ethiopia using mechanical arguments constrained by both geodetic and seismological observations we propose that the large dykes that initiate near the mid segment magma source are attracted toward segment ends as a result of a thickening of the elastic brittle lithosphere in the along rift direction this attraction arises from the difference of elastic resistance between the segment centre where the lithosphere is thermally weakened by long term focusing of melts and comparatively colder hence stronger segment ends the axial topographic gradient in magmatic rifts may be more likely explained as an incidental consequence of these variations of along axis elastic brittle thickness rather than the primary cause of lateral dyke injections

morphology and tectonics of the mid atlantic ridge 7 12 s 1 we present swath bathymetric gravity and magnetic data from the mid atlantic ridge between the ascension and the bode verde fracture zones where significant ridge hot spot interaction has been inferred the ridge axis in this region may be divided into four segments the central two segments exhibit rifted axial highs while the northernmost and southernmost segments have deep rift valleys typical of slow spreading mid ocean ridges bathymetric and magnetic data indicate that both central segments have experienced ridge jumps since 1 ma mantle bouguer anomalies mbas derived from shipboard free air gravity and swath bathymetric data show deep subcircular lows centered on the new ridge axes suggesting that mantle flow has been established beneath the new spreading centers for at least 1 myr inversion of gravity data indicates that crustal thicknesses vary by 4 km along axis with the thickest crust occurring beneath a large axial volcanic edifice once the effects of lithospheric aging have been removed a model in which gravity variations are attributed entirely to crustal thickness variations is more consistent with data from an axis parallel seismic line than a model that includes additional along axis variations in mantle temperature both geophysical and geochemical data from the region may be explained by the melting of small 200 km mantle chemical heterogeneities rather than elevated temperatures therefore there may be no ascension circe plume index terms 1517 geomagnetism and paleomagnetism magnetic anomaly modeling 3010 marine geology and geophysics gravity 3040 marine geology and geophysics plate tectonics 8150 8155 8157 8158

volcanic eruptions on mid ocean ridges new evidence from the superfast spreading east pacific rise 17 19 s uniform sediment cover were recovered from lava that buries older faulted terrain the boundary in lava composition coincides with a change in depth to the top of an axial magma lens seismic reflector consistent with magmas from two separate reservoirs being erupted in the same event chemical compositions from throughout the area indicate that lavas with identical compositions can be emplaced in separate volcanic eruptions within individual segments a comparison of our results to global data on submarine mid ocean ridge eruptions suggests consistent dependencies of erupted volume activated fissure lengths and chemical heterogeneity with spreading rate consistent with expected eruptive characteristics from ridges with contrasting thermal properties and magma reservoir depths index terms 3035 marine geology and geophysics midocean ridge processes 8414 volcanology eruption mechanisms 8439 volcanology physics and chemistry of magma bodies 3655 mineralogy and petrology major element composition keywords lava flow chemical heterogeneity erupted volume lava morphology side scan sonar

depths of partial crystallization of h2o bearing morb phase equilibria simulations of basalts at the mar near ascension island 7 11 s phase equilibria simulations were performed on naturally quenched basaltic glasses to determine crystallization conditions prior to eruption of magmas at the mid atlantic ridge mar east of ascension island 7 11 s the results indicate that mid ocean ridge basalt morb magmas beneath different segments of the mar have crystallized over a wide range of pressures 100 900 mpa however each segment seems to have a specific crystallization history nearly isobaric crystallization conditions 100 300 mpa were obtained for the geochemically enriched morb magmas of the central segments whereas normal n morb magmas of the bounding segments are characterized by polybaric crystallization conditions 200 900 mpa in addition our results demonstrate close to anhydrous crystallization conditions of n morbs whereas geochemically enriched morbs were successfully modeled in the presence of 0 4 1 wt h2o in the parental melts these estimates are in agreement with direct fourier transform ir measurements of h2o abundances in basaltic glasses and melt inclusions for selected samples water contents determined in the parental melts are in the range 0 04 0 09 and 0 30 0 55 wt h2o for depleted and enriched morbs respectively our results are in general agreement within 200 mpa with previous approaches used to evaluate pressure estimates in morb however the determination of pre eruptive conditions of morbs including temperature and water content in addition to pressure requires the improvement of magma crystallization models to simulate liquid lines of descent in the presence of small amounts of water

fluvial sediment transport and deposition following the 1991 eruption of mount pinatubo the 1991 eruption of mount pinatubo generated extreme sediment yields from watersheds heavily impacted by pyroclastic flows bedload sampling in the pasig potrero river one of the most heavily impacted rivers revealed negligible critical shear stress and very high transport rates that reflected an essentially unlimited sediment supply and the enhanced mobility of particles moving over a smooth fine grained bed dimensionless bedload transport rates in the pasig potrero river differed substantially from those previously reported for rivers in temperate regions for the same dimensionless shear stress but were similar to rates identified in rivers on other volcanoes and ephemeral streams in arid environments the similarity between volcanically disturbed and arid rivers appears to arise from the lack of an armored bed surface due to very high relative sediment supply in arid rivers this is attributed to a flashy hydrograph whereas volcanically disturbed rivers lack armoring due to sustained high rates of sediment delivery this work suggests that the increases in sediment supply accompanying massive disturbance induce morphologic and hydrologic changes that temporarily enhance transport efficiency until the watershed recovers and sediment supply is reduced d 2002 elsevier science b v all rights reserved

the volcanic tectonic cycle of the famous and amar valleys mid atlantic ridge 36 47 n evidence from basalt glass and phenocryst compositional variations for a steady state magma chamber beneath the valley midsections amar 3 the 1978 amar expedition extended the investigation of the mid atlantic ridge that was begun by the 1973 1974 famous expedition from the northern end of the famous rift the original famous area to the narrow central and southern end of the famous rift narrowgate region and into the broad amar valley south of fracture zone b available field and geochemical data allow us to characterize in detail the volcanic tectonic cycle for these two segments of the mid ocean ridge system a dynamic model of the crust forming processes within these two rift valley segments is deduced from 1 variations in the chemical composition of the erupted basaltic liquids as preserved in chilled glassy margins 2 composition and petrographie relationships of megacrysts and phenocrysts that represent cumulus crystallization onto the floor of a shallow magma chamber of a primitive magma that has reached pyroxene saturation early in its evolution and 3 detailed stratigraphie and regional observations that characterize the periodicity of volcanic construction and superimposed tectonism related to the ongoing extension of the valley floor each volcanic cycle is composed of several eruptive episodes that are initially typified by rapidly extruded sheet flows of primitive composition the decreasing extrustion rates at the end of an eruptive event are associated with more evolved liquid compositions that reflect fractionation within the shallow conduits intermixing of magmas is indicated by the relatively limited variability in glass composition not explicable in terms of simple fractionation processes the common evidence of crystal resorption and the apparent disequilibrium between megacrysts phenocrysts and their enclosing glass this indicates control by a crustal magma chamber liquid and crystal compositions and textural relations can best be explained by a three stage crystallization history 1 crystallization of olivine plagioclase and clinopyroxene onto the floor of a magma chamber 2 partial resorption of mineral phases by a superheated undersaturated liquid above the floor 3 intratelluric crystallization of olivine and plagioclase clinopyroxene during rise of melt to the surface through the conduit system above the magma chamber a magma chamber model is suggested for the amar famous rift valleys that satisfies both seismic and geological constraints we propose that a small steady state magma chamber is maintained beneath the topographically higher midsections of each valley segment amar and narrowgate regions but that these thin and terminate near the intersecting fracture zones expansion and contracton of the central magma body is controlled by the fluctuating imbalance between magma supply and chamber crystallization the famous narrowgate rift is currently in a contraction period so that in the northern famous region primitive liquid has erupted at mount venus and mount pluto without intercepting and mixing into the steady state chamber that supplies the mixed magmas to the narrowgate region

the 74 ka toba super eruption and southern indian hominins archaeology lithic technology and environments at jwalapuram locality 3 hominins living in southern india 74 000 years ago faced a deteriorating environment as the global climate moved from interglacial into full glacial conditions at the same time south asian populations witnessed the widespread deposition of tephra from the sumatran toba super eruption the largest explosive volcanic event of the past two million years here we report new data on the lithic technology and environmental context for a southern indian site with hominin occupation in association with toba tephra deposits jwalapuram locality 3 in the jurreru valley sedimentological and isotopic studies demonstrate that a cooling trend was in effect in this part of southern india prior to the eruption and that thick deposits of ash in the jurreru valley supported grassland communities before more wooded conditions were re established detailed technological analyses of an expanded lithic sample from locality 3 suggest cultural continuity after the eruptive event and comparisons with lithic core technologies elsewhere indicate that homo sapiens cannot be ruled out as the creator of these middle palaeolithic assemblages

crustal thickness on the mid atlantic ridge bull s eye gravity anomalies and focused accretion spreading segments of the mid atlantic ridge show negative bull9s eye anomalies in the mantle bouguer gravity field seismic refraction results from 33 s indicate that these anomalies can be accounted for by variations in crustal thickness along a segment the crust is thicker in the center and thinner at the end of the spreading segment and these changes are attributable to variations in the thickness of layer 3 the results show that accretion is focused at a slow spreading ridge that axial valley depth reflects the thickness of the underlying crust and that along axis density variations should be considered in the interpretation of gravity data

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

magmatic processes at slow spreading ridges implications of the ramesses experiment at 57 45 n on the mid atlantic ridge summary this paper is the first in a series of three this issue which present the results of the ramesses study reykjanes axial melt experiment structural synthesis from electromagnetics and seismics ramesses was an integrated geophysical study which was carefully targeted on a magmatically active axial volcanic ridge avr segment of the reykjanes ridge centred on 57 45aen it consisted of three major components wide angle seismic profiles along and across the avr using ocean bottom seismometers together with coincident seismic reflection profiles controlled source electromagnetic sounding csem and magnetotelluric sounding mt supplementary data sets included swath bathymetry gravity and magnetics analyses of the major components of the experiment show clearly that the sub axial geophysical structure is dominated by the presence and distribution of aqueous and magmatic fluids the avr is underlain by a significant crustal magma body at a depth of 2 5 km below the sea surface the magma body is characterized by low seismic velocities constrained by the wide angle seismic data a seismic reflection from its upper surface and a region of anomalously low electrical resistivity constrained by the csem data it includes a thin ribbon like melt lens at the top of the body and a much larger region containing at least 20 per cent melt in a largely crystalline mush zone which flanks and underlies the melt lens ramesses is the first experiment to provide convincing evidence of a significant magma body beneath a slow spreading ridge the result provides strong support for a model of crustal accretion at slow spreading rates in which magma chambers similar to those at intermediate and fast spreading ridges play a key role in crustal accretion but are short lived rather than steady state features the magma body can exist for only a small proportion of a tectono magmatic cycle which controls crustal accretion and has a period of at least 20 000 years these findings have major implications for the temporal patterns of generation and migration of basaltic melt in the mantle and of its delivery into the crust beneath slow spreading mid ocean ridges

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

quaternary geology of the middle son valley north central india implications for prehistoric archaeology abstract the late quaternary alluvial history of the son is very similar to that of the nile the amazon and the murray murrumbridgee during the terminal pleistocene cros bedded sands were laid down as bedload by a highly seasonal low sinuosity stream early holocene deposition of horizontally stratified silts and clays is the suspended load of regular overbank flooding by a less seasonal more sinuous stream the upward fining sequence of the late quaternary son coincides with a global change in climate in the intertropical zone from cold and dry in the terminal pleistocene 25 000 b p to warm and wet in the early holocene two earlier alluvial formations are present in the middle son valley one directly overlying eroded bedrock comprises fanglomerates and clays of a mantled pediment sequence developed when river level was low and the climate dry lower palaeolithic artifacts of upper acheulian aspect are common near the fanglomerate clay contact the clay may be reworked aeolian dust the other younger formation is fluviatile it represents an influx of allocthonous gravels under high flow regime during the late pleistocene 75 000 b p middle palaeolithic implements fashioned from the exotic gravels are found in situ in this formation produced by a return to wetter conditions earlier two dimensional models which equate the north central indian lower middle and upper palaeolithic respectively with gravels i ii and iii of the belan valley sequence now require amendment the three dimensional model proposed here to account for the quaternary alluvial formations in the middle son valley also has important implications for archaeological prospecting primary context microlithic sites are generally confined to the outer margin of the early holocene floodplain along the piedmont zone of the kaimur hills derived middle palaeolithic flakes and tools are scattered throughout the terminal pleistocene cross bedded sands of the son relatively undisturbed middle palaeolithic assemblages are stratified within the preceding alluvial formation

river response to quaternary climatic fluctuations evidence from the son and belan valleys north central india abstract the last glacial period was cold and dry in peninsular india in north central india the interval from 39 9 to 16 3 ka was associated with widespread and prolonged aggradation in the son and belan valleys the aggradation ended with sustained vertical incision after 16 3 ka and reflects a return to warmer wetter conditions in this region it would appear that terminal pleistocene to holocene river incision was broadly synchronous with a strong summer monsoon regime and higher levels of river discharge and the preceding river aggradation with lower discharge and a weaker or more variable summer monsoon regime two older phases of prolonged aggradation followed by vertical incision are evident in the son and belan valleys before 39 ka one of these phases is centred towards 73 4 ka when ash from the toba mega eruption in sumatra was deposited across peninsular india the following phase of aggradation has yielded infrared stimulated luminescence ages of 58 6 and 45 12 ka the youngest phase of aggradation began towards 5 5 ka and seems to mark a return to a weaker summer monsoon regime

segmentation and crustal structure of the western mid atlantic ridge flank 25 25 27 10 n and 0 29 m y we conducted a detailed geological geophysical survey of the west flank of the mid atlantic ridge between 25 25 n and 27 10 n and from the ridge axis out to 29 ma crust acquiring hydrosweep multibeam bathymetry hawaii mr1 sidescan sonar imagery gravity magnetics and single channel seismic reflection profiles the survey covered all or part of nine spreading segments bounded by mostly nontransform right stepping discontinuities which are subparallel to flow lines but which migrated independently of one another some discontinuities alternated between small right and left stepping offsets or exhibited zero offset for up to 3 4 m y despite these changes the spreading segments have been long lived and extend 20 m y or more across isochrons a large shift 9 in relative plate motion about 24 22 ma caused significant changes in segmentation pattern the nature of this plate boundary response together with the persistence of segments through periods of zero offset at their bounding discontinuities suggest that the position and longevity of segments are controlled primarily by the subaxial position of buoyant mantle diapirs or focused zones of rising melt within segments there are distinct differences in seafloor depth morphology residual mantle bouguer gravity anomaly and apparent crustal thickness between inside corner and outside corner crust this demands fundamentally asymmetric crustal accretion and extension across the ridge axis which we attribute to low angle detachment faulting near segment ends cyclic variations in residual gravity over the crossisochron run of segments also suggest crustal thickness changes of at least 1 2 km every 2 3 m y these are interpreted to be caused by episodes of magmatic versus relatively amagmatic extension controlled by retention and quasiperiodic release of melt from the upwelling mantle detachment faulting appears to be especially effective in exhuming lower crust to upper mantle at inside corners during relatively amagmatic episodes creating crustal domes analogous to turtleback metamorphic core complexes that are formed by low angle detachment faulting in subaerial extensional environments

new insights into mid ocean ridge volcanic processes from the 2005 2006 eruption of the east pacific rise 9 46 n 9 56 n digital seafloor imagery collected on 37 camera tows and alvin dives in which we identify 186 contacts between new and old lava are used to create the most detailed map of a mid ocean ridge mor eruption to date lava flows erupted in 2005 2006 at the east pacific rise epr covered an area of 14 6 km2 along 18 km of the epr crest between 9 46 and 9 56 n the 2005 2006 lava is characterized by inflated lobate and sheet morphologies in the flow interiors and pillow forms at terminal flow fronts numerous lava channels 10 50 m wide and 1 5 m deep trending approximately east west served as distributory pathways eruptions were sourced from fissures within the epr axial summit trough as well as fissures located on an off axis fissure mound 600 m east of the epr axis between 9 52 and 9 56 n portions of the lava flow reached as far as 2 km east of the axis near 9 51 2 n using a conservative estimate of 1 5 m for the average flow thickness implies that the 2005 2006 eruptions produced 22 106 m3 of lava 4 5 times larger than estimated volumes of 1991 1992 epr lava flows estimated lava volume for the 2005 2006 eruptions represents 15 of the magma available in the axial magma chamber

testing a model of alluvial deposition in the middle son valley madhya pradesh india irsl dating of terraced alluvial sediments and implications for archaeological surveys and palaeoclimatic reconstructions abstract over the past three decades the middle son valley madhya pradesh india has been the focus of archaeological geological and palaeoenvironmental investigations that aim to reconstruct regional climate changes in the late pleistocene and to understand the effects of the 74 ka toba super eruption on ecosystems and human populations in northern india the most recently published model of alluvial deposition for the middle son valley subdivides its alluvium into five stratigraphic formations each associated with a specific artefact assemblage in this study new cross valley topographic profiles field observations and infrared stimulated luminescence irsl age estimates are used to refine this model south of the rehi son river confluence these data not only provide insights into the fluvial history of the son river and its response to changes in palaeoclimate but will also inform future archaeological surveys by constraining the geomorphic context of surficial and excavated artefacts in the area

palaeozoic oolitic ironstones on the north american craton abstract more than 40 mostly minor palaeozoic oolitic ironstones oi accumulated on low latitude cratonic north america almost entirely in usa a few middle cambrian oi among the oldest anywhere were deposited on the western cratonic shelf of usa widely scattered late cambrian ones developed on the southwestern southeastern and northeastern flanks of the transcontinental arch middle ordovician oi were deposited on the cratonic interior and on the southern and southeastern cratonic margins in latest ordovician time neda oi spread across north central usa east of the arch and south of the laurentian upland minor ones developed in the southern part of the taconian foreland basin after major orogeny early and middle silurian clinton oi flourished throughout the foreland basin in eastern usa producing the largest oi deposit on the north american craton minor early late silurian oi accumulated in the central part of the basin middle and late devonian oi in the acadian foreland basin in northeastern cratonic usa developed progressively westward of the encroaching catskill delta an isolated middle devonian one accumulated in southwestern cratonic usa and latest devonian ones in north central usa east of the arch during this palaeozoic episode sites of oi deposition shifted eastward across the craton most of the oi were deposited during a hiatus in normal sedimentation accumulating in the upper part of shoaling upward sequences some early palaeozoic ones occur in glauconitic siliciclastic carbonate facies and contain mostly spherical hematitic ooids these suggest derivation from iron rich soils developed on glauconitic deposits oi in wholly siliciclastic facies containing distorted chamositic ooids with cores of mud peloids were common in later palaeozoic time these oi like many other phanerozoic ones suggest a synsedimentary to early diagenetic origin

uranium series age constraints on lavas from the axial valley of the mid atlantic ridge mark area abstract mass spectrometric measurements of 230 th 226 ra 235 u 231 pa and 238 u 230 th disequilibria are used to determine eruption ages for four mid ocean ridge basalts from the median valley of the mid atlantic ridge south of the kane fracture zone mark area three samples were collected across axis on the axial volcanic ridge i e the neovolcanic ridge near the snake pit hydrothermal mound and one sample was collected near the crest of serocki volcano 50 km south of snake pit 226 ra 230 th and 230 th 238 u activity ratios are low and uniform for all four samples while 231 pa 235 u activity ratios are elevated and somewhat more variable age constraints suggest that these lavas from the most robust volcanic edifices in the mark area are 10 000 20 000 yr old the age data are used to evaluate the efficacy of commonly used age estimate scales based on qualitative indicators e g sediment cover glass quality and to begin to quantify the temporal and spatial dependence of volcanic tectonic and hydrothermal processes at slow spreading oceanic ridges

eruptive timing and 200 year episodicity at 92 w on the hot spot influenced galapagos spreading center derived from geomagnetic paleointensity eruptive timing in mid ocean ridge systems is relatively poorly constrained despite being an important variable in our understanding of many mid ocean ridge processes including volcanic construction magma recharge flux and storage and the stability of hydrothermal systems and biological communities only a handful of absolute eruption chronologies exist yet they are essential in understanding how eruptive timing varies with important controlling variables to construct an eruptive history at one location on the galapagos spreading center we present age determinations derived from geomagnetic paleointensity to aid interpretation of the paleointensity data we also present results from on bottom magnetic anomaly measurements and forward modeling of topographic induced magnetic anomalies anomalies may lead to a 1 2 t bias in flow mean paleointensities which does not significantly affect the overall interpretation paleointensity results for the three youngest sampled units are indistinguishable consistent with the flows being emplaced in relatively rapid succession comparisons with models of geomagnetic field behavior suggest these flows were erupted sometime in the past 100 200 years the fourth sampled unit has a significantly higher paleointensity consistent with an age of roughly 400 years the possible bias in paleointensity data allows for ages as young as 50 years for the youngest three flows and 200 400 years for the oldest flow this age distribution demonstrates an episodicity in the emplacement of the largest flows at this location with a 200 300 year period of relative quiescence between emplacement of the oldest unit and the three youngest units

roof assimilation at fast spreading ridges an investigation combining geophysical geochemical and field evidence 1 the role of assimilation at the roof of the axial magma chamber amc at fast spreading mid ocean ridges is investigated using field geophysical and geochemical data field observations from ophiolites indicate that roof assimilation is a common process but provide little constraint on the amount of material assimilated arguments based on geophysical data that include the subsidence of the layer 2a 2b boundary across the width of the amc and the depth of large axial summit troughs require at least 50 100 m of assimilation however significant variations in the depth of the amc on the east pacific rise epr over short along axis distances suggest up to 500 m fluctuations in the depth of the amc may be common on short timescales geochemical modeling of the over enrichment of chlorine in epr basalts suggests that 20 of the oceanic crust may go through a cycle of crystallization alteration then assimilation this is far more than can be accounted for by roof subsidence and suggests that fluctuations in the level of the roof must account for the majority of the assimilation assimilation most likely occurs on a number of different timescales ranging from decadal due to perturbations in the thermal regime related to diking to longer period fluctuations controlled by magma supply from the mantle large scale roof assimilation has implications for the accretion of the lower crust due to heat losses associated with assimilation and for the interpretation of the geochemistry of the oceanic crust

en echelon axial volcanic ridges at the reykjanes ridge a life cycle of volcanism and tectonics abstract new deep towed sidescan records and multibeam bathymetric data along the reykjanes ridge allow us to assess the varying thermal effect of the icelandic hotspot the increasing predominance of faulting speculated to be derived from mantle upwelling and outflow over simple extensional neotectonics is traced from northeast to southwest numerous en echelon volcanic ridges built along the plate boundary are seen in various stages of growth and decay and a cycle of alternating construction and dismemberment is proposed unfaulted short narrow lensoid ridges occasionally in small groups are interpreted as immature and developing axial volcanic ridges broad convex flanked ridges appear to be mature sites of volcanic construction where the magma supply has been maintained narrow bow form ridges occasionally with rectilinear flanks represent axial volcanic ridges which have ceased construction and are in initial stages of disaggregation isolated polygonal blocks generally seen off axis represent the most advanced stage of dismantling of the surface crustal accretionary features while this cyclicity is speculated to be related to periodic magmatic and tectonic focusing and defocusing along the axis due to the restricted magma supply at this slow spreading ridge it is recognised that this needs modelling although the oblique spreading of the reykjanes ridge allows one to clearly distinguish between structures attributable to plate spreading from those related to small scale accretionary processes we propose that the major processes identified in this study are directly applicable to all slow spreading sections of mid ocean ridges

early diagenetic alteration of lateritic particle coatings in amazon continental shelf sediment abstract intense chemical weathering in the tropical amazon river basin strips crustal material of alkali and alkaline earth metals leaving a residue composed predominantly of oxyhydroxides of fe hydroxides of al and cation poor clays this lateritic residue often coats soil particles within the marine environment lateritic particle coatings on sand undergo or have the capacity to undergo rapid reaction and mineralogical alteration 1 at least 20 of fe in coatings can be readily liberated during short term incubation in anoxic sediment 2 coatings change color from red white to green yellow during burial implying fe3 fe2 3 average mg cation content of coatings increases net addition from 1 85 near the amazon s mouth to 4 76 120 km from the mouth or with burial 4 a narrowing of the measured range and a slight increase of si al occurs near mouth to offshore 5 mg fe al layer silicates are probably forming by transformation of kaolinite or other cation stripped material of similar composition in coatings mg uptake by lateritic particle coating reactions assuming comparable involvement of all particle sizes could account for up to 67 of the mg discharged in solution by the amazon river evidence for mg fe al layer silicate formation supports the hypothesis that ancient ironstone minerals formed in some cases by direct alteration of lateritic material

ytt ash from quaternary sediments of kapileshwar area purna alluvial basin central india abstract a thick pocket of volcanic ash 2 5 m represented by various shades of gray powdery massive to laminated in nature hosted in yellowish brown thinly bedded laminated silty clay lithocolumn has been studied for its preservational and geochemical aspects the ash column shows preservations of both primary and reworked ashes which are distinguishable on the basis of their field criteria including colors bedding geometries and thicknesses petrological study of the same reveals high representation of glass shards with bubble wall blocky and pumice morphologies minor occurrences of light and heavy minerals and unidentifiable admixture of fine grained sediments geochemistry of both bulk ash and glass shards through icp aes and epma respectively suggest high percentages of sio2 al2o3 na2o and k2o ree data generated through icp ms show strong negative eu anomaly the effects of weathering or aqueous solution on the geochemistry are also noticeable as evident by the variations in the values of both mobile and immobile elements however the data obtained from both bulk ash and glass shards are within the limits of ytt composition calm to very low energy environment of deposition is suggested for their deposition paleoclimatic proxies represented by rhizoliths rhizospheres rhizolith balls ant traces and vertebrate remains preserved in the pre tephra succession have also been commented

sources of uncertainty in the mazama isopachs and the implications for interpreting distal tephra deposits from large magnitude eruptions estimating the area of tephra fallout and volume of large magnitude eruptions is fundamental to interpretations of the hazards posed by eruptions of this scale this study uses the tephra from the caldera forming eruption of mount mazama crater lake or usa to demonstrate the challenges faced when working with large prehistoric tephra deposits and outlines the methodologies required to determine eruption volume and magnitude we combine 250 mazama tephra occurrences reported by a range of disciplines including archaeology paleoclimatology and volcanology with new field studies to better understand the extent of the distal tephra we find that the mazama tephra has been remobilised to varying degrees over the past 7000 years so each tephra locality was appraised for the likelihood that it records primary tephra fallout we designated 45 of the distal 100 km from source tephra sites as suitable for use in the production of isopachs using a spline fitting method the new distal isopachs were then integrated with proximal fallout data and estimates of the ignimbrite volume from previous studies to revise the estimated bulk erupted volume from the climactic mazama eruption to 176 km3 61 km3 dense rock equivalent dre this study demonstrates the importance of collating tephra localities from a range of disciplines and that even remobilised deposits provide valuable information about the extent of the deposit interpreting remobilised deposits can provide insight into post eruptive processes that could potentially pose secondary hazards following large magnitude eruptions we also show that in some circumstances remobilised deposits preserve important physical properties such as grain size

new occurrence of youngest toba tuff in abyssal sediments of the central indian basin abstract volcanic glass and pumice found in siliceous abyssal sediments of the central indian basin south of the equator have previously been assigned various origins including intra basin volcanism indonesian arc and krakatau rhyolitic glass shards dispersed in sediments from 8 cores that we have examined are compositionally identical to those of fallout deposits of the youngest toba tuff erupted at 74 ka from northern sumatra the correlation extends the distribution of coarse 63 m glass shards some 1500 km south of the previously known fallout zone and into the southern hemisphere reaching latitudes of ca 14 s this provides evidence for bi hemispheric dispersal of the ash cloud and supports global dispersal of gas and aerosols from the eruption such dispersal could have facilitated the global impact of the eruption

phanerozoic oolitic ironstones geologic record and facies model oolitic ironstonesl composed predominantly of ooidsl of ferric oxide and chamosite accumulated throughout much of phanerozoic time figure 1 and some of them have been important ore deposits they are built of two very common products of weathering iron oxide and clay but they constitute a distinctive and subordinate sedimentary facies for which there is no simple modem analogue as a result considerable disagreement has attended more than 125 years of discussion about their origin early analyses of these rocks for example the minette are of france focused mainly on their chemical and mineral composition and on processes of diagenesis more recent work has described their sedimentary textures and structures as well as their general facies and depositional environments ad eleye 1973 brookfield 1973 corbin 1980 hallam 1975 karpov et a11967

youngest toba tuff in the son valley india a weak and discontinuous stratigraphic marker abstract investigation of the climatic and environmental impacts of the youngest toba tuff ytt 74 ka bp eruption of toba volcano sumatra is crucial for understanding the consequences of the eruption for contemporaneous human populations the middle son valley in india was the first locality on the indian subcontinent where the ytt was reported the ash bed forms a discontinuous layer stretching for over 30 km along the river here we report on the stratigraphic contexts of ytt ash layers in alluvial deposits of the middle son valley in order to reconstruct the taphonomy of the ash deposits and the dynamic of their deposition although the distal ash has been studied since the 1980s its stratigraphic integrity and the mechanisms and pathways involved in its transport and deposition have bit previously been assessed we find that the ytt occurrences in the middle son valley may not be reliable chronostratigraphical markers for millennial scale palaeoenvironmental reconstruction

three dimensional seismic structure of the mid atlantic ridge 35 n evidence for focused melt supply and lower crustal dike injection 1 we gathered seismic refraction and wide angle reflection data from several active source experiments that occurred along the mid atlantic ridge near 35 n and constructed three dimensional anisotropic tomographic images of the crust and upper mantle velocity structure and crustal thickness the tomographic images reveal anomalously thick crust 8 9 km and a low velocity bull s eye from 4 to 10 km depth beneath the center of the ridge segment the velocity anomaly is indicative of high temperatures and a small amount of melt up to 5 and likely represents the current magma plumbing system for melts ascending from the mantle in addition at the segment center seismic anisotropy in the lower crust indicates that the crust is composed of partially molten dikes that are surrounded by regions of hot rock with little or no melt fraction our results indicate that mantle melts are focused at mantle depths to the segment center and that melt is delivered to the crust via dikes in the lower crust our results also indicate that the segment ends are colder receive a reduced magma supply and undergo significantly greater tectonic stretching than the segment center

storm effects on siliciclastic and carbonate shelf sediments in the medial silurian succession of pennsylvania abstract upper llandoverian through wenlockian strata in the pennsylvania part of the northern appalachian basin formed on a gentle epeiric ramp that extended from marginal tidal flats through coarser grained mid shelf shoals to a muddy basin center over approximately 10 million years the composition of the accumulating sediment evolved from siliciclastic to carbonate while the ramp maintained a relatively similar configuration and position below sea level and while storm generated processes continued as the major producers of sediment characteristics in both siliciclastic and carbonate parts of the succession sedimentary structures and their sequences are generally similar with only a small number of dissimilar features resulting from the differences in composition small scale coarsening and shallowing upward sequences are present in both mid shelf bar and basin transition deposits although specific sequence characteristics are different in siliciclastic and carbonate parts such sequences record cyclic fluctuations of relative sea level while the composition of the sediment evolved over this extended period the gentle ramp gradient was maintained largely by the processes generated by storms such processes were more effective because of the fluctuations of relative sea level

compositional variation and 226ra 230th model ages of axial lavas from the southern mid atlantic ridge 8 48 s we present geological observations and geochemical data for the youngest volcanic features on the slow spreading mid atlantic ridge at 8 48 s that shows seismic evidence for a thickened crust and excess magma formation young lava flows with high sonar reflectivity cover about 14 km2 in the axial rift and were probably erupted from two axial volcanic ridges each of about 3 km in length three different lava units occur along an about 11 km long portion of the ridge and lavas from the northern axial volcanic ridge differ from those of the southern axial volcanic ridge and surrounding lava flows basalts from the axial rift flanks and from a pillow mound within the young flows are more incompatible element depleted than those from the young volcanic field lavas from this volcanic area have 226ra 230th disequilibria model ages of 1 000 and 4 000 years whereas the older lavas from the rift flank and the pillow mound but also some of the lava field are older than 8 000 years glasses from the northern and southern ends of the southern lava unit indicate up to 100 c cooler magma temperatures than in the center and increased assimilation of hydrothermally altered material the compositional heterogeneity on a scale of 3 km suggests small magma batches rising vertically from the mantle to the surface without significant lateral flow and mixing the observations on the 8 48 s lava field support the model of low frequency eruptions from single ascending magma batches that has been developed for slow spreading ridges

lithofacies of the silurian keefer sandstone east central appalachian basin usa abstract fourteen lithofacies of the keefer sandstone in the area of northeastern west virginia and western maryland were identified depositional environments of the lithofacies include storm dominated marine shelf basin barrier island and backbarrier tidal flat and lagoon coastal lithofacies document a dominant basinward progradation with some sea level rise which placed backbarrier lithofacies above barrier island lithofacies shelf lithofacies as stacked coarsening upward sequences document as many as four fluctuations within the major progradation coarsening upward sequences are typically capped by transgressive sands which represent material eroded from relatively landward environments and transported basinward during coastal retreat deposition of cycles within the keefer occurred under conditions of tectonic quiescence dominant steady or falling sea level followed by rise in sea level and in a climate which was warm and humid

minimum speed limit for ocean ridge magmatism from 210pb 226ra 230th disequilibria although 70 per cent of global crustal magmatism occurs at mid ocean ridges where the heat budget controls crustal structure hydrothermal activity and a vibrant biosphere the tempo of magmatic inputs in these regions remains poorly understood such timescales can be assessed however with natural radioactive decay chain nuclides because chemical disruption to secular equilibrium systems initiates parent daughter disequilibria which re equilibrate by the shorter half life in a pair here we use 210pb 226ra 230th radioactive disequilibria and other geochemical attributes in oceanic basalts less than 20 years old to infer that melts of the earth s mantle can be transported accumulated and erupted in a few decades this implies that magmatic conditions can fluctuate rapidly at ridge volcanoes 210pb deficits of up to 15 per cent relative to 226ra occur in normal mid ocean ridge basalts with the largest deficits in the most magnesium rich lavas the 22 year half life of 210pb requires very recent fractionation of these two uranium series nuclides relationships between 210pb deficits 226ra 230th activity ratios and compatible trace element ratios preclude crustal magma differentiation or daughter isotope degassing as the main causes for the signal a mantle melting model can simulate observed disequilibria but preservation requires a subsequent mechanism to transport melt rapidly the likelihood of magmatic disequilibria occurring before melt enters shallow crustal magma bodies also limits differentiation and heat replenishment timescales to decades at the localities studied

regolith production and transport at the susquehanna shale hills critical zone observatory part 2 insights from meteoric 10be 1 regolith mantled hillslopes are ubiquitous features of most temperate landscapes and their morphology reflects the climatically biologically and tectonically mediated interplay between regolith production and downslope transport despite intensive research few studies have quantified both of these mass fluxes in the same field site here we present an analysis of 87 meteoric 10be measurements from regolith and bedrock within the susquehanna shale hills critical zone observatory ssho in central pennsylvania meteoric 10be concentrations in bulk regolith samples n 73 decrease with regolith depth comparison of hillslope meteoric 10be inventories with analyses of rock chip samples n 14 from a 24 m bedrock core confirms that 80 of the total inventory is retained in the regolith the systematic downslope increase of meteoric 10be inventories observed at ssho is consistent with 10be accumulation in slowly creeping regolith 0 2 cm yr 1 regolith flux inferred from meteoric 10be varies linearly with topographic gradient determined from high resolution light detection and ranging based topography along the upper portions of hillslopes at ssho however regolith flux appears to depend on the product of gradient and regolith depth where regolith is thick near the base of hillslopes meteoric 10be inventories at the north and south ridgetops indicate minimum regolith residence times of 10 5 3 7 and 9 1 2 9 ky respectively similar to residence times inferred from u series isotopes in ma et al 2013 the combination of our results with u series derived regolith production rates implies that regolith production and erosion rates are similar to within a factor of two on ssho hillcrests

diagenesis of quartz in the upper proterozoic kaimur sandstones son valley central india abstract the upper proterozoic kaimur sandstones in central india are quartz sublithic and lithic arenites cemented by quartz illite and hematite diagenetic quartz occurs in five modes syntaxial overgrowths fracture healings aggregates of small euhedral crystals quartz resulting from the alteration of detrital silicates and from the recrystallization of quartz intergranular pressure dissolution is suggested as the main source of silica with smaller contribution from other sources such as silica dissolved in meteoric waters stylolitization clay mineral diagenesis and the alteration of detrital silicates studies on the fluid inclusions and oxygen isotopes of diagenetic quartz suggest that meteoric water modified by diagenetic reactions has mediated the quartz cementation

mid ocean ridge eruptive vent morphology and substructure evidence for dike widths eruption rates and evolution of eruptions and axial volcanic ridges high resolution side scan sonar data of mid atlantic ridge mar inner valley floor axial volcanic ridges reveal details of their architectural elements we develop quantitative models for basaltic eruptions from dikes and compare the predicted products of these eruptions with the observed morphologic features inhibition of gas exsolution lack of magma disruption and hydrothermal effects combine to decrease the rise speed of magma in submarine dikes and enhance cooling leading to more rapid centralization of eruptions along the widest places in the dike dikes are predicted to initially feed eruptions from fissure vents producing lines of hummocky ridges centralization of activity to several adjacent vents produces chains of hummocky bulbous mounds and can enhance the effusion rate at a single vent to produce small seamounts the widest dikes are predicted to produce smooth flows up to several kilometers in length which should pond in adjacent lows edifice sizes predicted on the basis of volume fluxes and flow lengths implied by the widths of dikes observed on the seafloor in iceland and in ophiolite complexes are in quantitative agreement with the dimensions of observed features each ridge is made up of the products of a variety of these individual dike emplacement and extrusive events involving various dike widths cooling times and eruption durations on the basis of typical mar spreading rates about one dike emplacement event would be expected every 40 years since not all dikes reach the surface the actual mean interval between eruptions will exceed this mar dikes should solidify between average dike emplacement events dike emplacement events are more frequent on the east pacific rise epr and thus new dikes are more likely to reoccupy the sites of incompletely solidified older dikes differences between the morphology of the mar axial volcanic ridge and the sheet flow dominated epr are attributed to on average wider dikes erupting with greater frequency along the epr

building the crust at the mid atlantic ridge the morphology of the sea floor at mid ocean ridge spreading centres provides a key to understanding how ocean crust is constructed images of the axial zone of the slow spreading mid atlantic ridge obtained at a range of spatial scales show that crustal construction is complex and highly variable reflecting an underlying three dimensional variability in magmatic tectonic and hydrothermal processes

fluvial responses to volcanism resedimentation of the 1800a taupo ignimbrite eruption in the rangitaiki river catchment north island new zealand abstract the potential for the generation of dangerous and damaging lahars and floods in response to the eruption of voluminous pyroclastic debris has become increasingly appreciated in recent years the style and tempo of this response varies both between eruptions and between individual catchments impacted by a single eruption so that an understanding of the factors controlling this variation is necessary for precise hazard assessment the 1800a taupo eruption from the taupo volcanic centre in the central north island of new zealand devastated an area of 20 000 km 2 during eruption of a climactic ignimbrite impacting the headwaters of all major rivers draining radially from this region the rangitaiki river the subject of this paper differs from other catchments in that the taupo ignimbrite buried an essentially flat land surface inherited from a suite of welded ignimbrite sheets erupted between 320 340 and 230 ka the middle reaches of the catchment are characterised by narrow steep gorges alternating with low gradient basins developed in tectonic half grabens initially remobilisation of pyroclastic material in the headwaters was dominated by hyperconcentrated sheet flows resulting in shallow reworking in higher gradient areas reintegration of drainage networks was achieved by incision of deep channels and gullies assisted by breakouts from ephemeral lakelets developed in ignimbrite dammed depressions braided and later meandering streams superseded this pattern as rill and gully systems stabilised and sediment yields fell leading to a decline in drainage density gorge reaches acted as efficient conduits for remobilised material while the basins acted as local depocentres for the temporary storage of volcaniclastic sediments mediating the transfer of pyroclastic debris to the bay of plenty coast 100 km to the north reworking and resedimentation of pyroclastic debris began immediately after the eruption peaking early and then rapidly declining so that most geomorphic adjustments occurred within one or two decades of the initial disturbance

evidence from gravity data for focused magmatic accretion along the mid atlantic ridge gravity data from the mid atlantic ridge between latitudes 27 50 n and 30 40 n show that the accretion of magma at the ridge is focused at discrete centres along the spreading axis large positive gravity anomalies equivalent to reductions of almost 50 in crustal thickness are observed over non transform discontinuities bounding spreading segments

ridge jumps and propagations in the south atlantic ocean the normal pattern of mid ocean ridge divergent plate boundaries comprises straight spreading segments with narrow zones of crustal accretion bounded by transform faults where these segments are offset normally the configuration of spreading segments and transform faults is stable for long periods but in places on fast and medium rate spreading ridges the transform offsets have propagated along the ridge axis as some spreading segments have grown at the expense of adjacent ones1 7 although such behaviour has hitherto not been well documented on slow spreading ridges we have mapped multiple ridge jumps and propagating rifts in two areas of the slow spreading 18 mm yr 1 half rate south atlantic ridge crest we attribute this anomalous behaviour in the south atlantic to the presence of hotspots near our study areas causing elevated asthenospheric mantle temperatures and lateral mantle flow with consequent excess volcanism and disruption of the normally well focused melt delivery processes at the ridge crest

the toba volcanic event and interstadial stadial climates at the marine isotopic stage 5 to 4 transition in the northern indian ocean abstract the toba volcanic event one of the largest eruptions during the quaternary is documented in marine sediment cores from the northeastern arabian sea on the crest of the murray ridge and along the western indian continental margin we detected distinct concentration spikes and ash layers of rhyolithic volcanic shards near the marine isotope stage 5 4 boundary with the chemical composition of the youngest toba tuff time series of the uk 37 alkenone index planktic foraminiferal species magnetic susceptibility and sediment accumulation rates from this interval show that the toba event occurred between two warm periods lasting a few millennia using toba as an instantaneous stratigraphic marker for correlation between the marine and ice core chronostratigraphies these two arabian sea climatic events correspond to greenland interstadials 20 and 19 respectively our data sets thus depict substantial interstadial stadial fluctuations in sea surface temperature and surface water productivity we show that variable terrigenous eolian sediment supply played a crucial role in transferring and preserving the productivity signal in the sediment record within the provided stratigraphic resolution of several decades to centennials none of these proxies shows a particular impact of the toba eruption however our results are additional support that toba despite its exceptional magnitude had only a minor impact on the evolution of low latitude monsoonal climate on centennial to millennial time scales

geologic history of the summit of axial seamount juan de fuca ridge 1 multibeam 1 m resolution and side scan data collected from an autonomous underwater vehicle and lava samples radiocarbon dated sediment cores and observations of flow contacts collected by remotely operated vehicle were combined to reconstruct the geologic history and flow emplacement processes on axial seamount s summit and upper rift zones the maps show 52 post 410 ce lava flows and 20 precaldera lava flows as old as 31 2 kyr the inferred age of the caldera clastic deposits 1 2 m thick accumulated on the rims postcaldera between 31 ka and 410 ce there are no known lava flows near the summit the oldest postcaldera lava 410 ce is a pillow cone se of the caldera two flows erupted on the w rim between 800 and 1000 ce from 1220 to 1300 ce generally small eruptions of plagioclase phyric depleted mafic lava occurred in the central caldera and on the east rim larger post 1400 ce eruptions produced inflated lobate flows of aphyric less depleted and less mafic lava on the upper rift zones and in the n and s caldera all caldera floor lava flows and most uppermost rift zone flows postdate 1220 ce activity shifted from the central caldera to the upper s rift outside the caldera to the n rift and caldera floor and then to the s caldera and uppermost s rift where two historical eruptions occurred in 1998 and 2011 the average recurrence interval deduced from the flows erupted over the last 800 years is statistically identical to the 13 year interval between historical eruptions

ridge plume interactions or mantle heterogeneity near ascension island between the ascension and bode verde fracture zones an 200 km length of the axis of the mid atlantic ridge has a large positive residual depth anomaly and an axial high morphology that suggest a plume influence new wide angle seismic and gravity data from this area show that 1 the crustal thickness increases from 6 km at the propagating rift marking the northern limit of the anomalous zone to 10 km at the center of the zone and 2 the residual depth anomalies may be largely explained by airy isostatic support for these crustal thickness variations these observations combined with the lack of long wavelength gravity and residual depth anomalies associated with the proposed ascension plume suggest either the presence of a weak and intermittent plume or melting of a series of small mantle heterogeneities

evolution of solution structures in a deformed quartz arenite geometric changes related to permeability changes abstract quartz arenite of the silurian keefer sandstone at a site in west virginia u s a records an early pervasive solution related deformation by grain interpenetration volume losses measured by the pss method show shortening directions from both diagenetic compaction and early tectonic layer parallel shortening later solution related tectonic deformation during folding formed transgranular stylolites along pre existing faults or in rock volumes with abundant microfractures thus the geometry of solution related deformation changed progressively from pervasive at grain scale to focused along transgranular surfaces we propose that this geometric change resulted from intense porosity and permeability reduction during diagenetic cementation and early layer parallel shortening which required fluids to exploit tectonically created pathways along faults or networks of microfractures during later deformation

mappability stratigraphic variation and diagenetic problems in sedimentary map unit definition and field mapping mapping recognizing and defining sedimentary formations in poorly exposed terrains especially in units with significant stratigraphic variations and disparate definitive diagenetic characteristics focuses attention on both the problems of field mapping and the contrast between classical field mapping practices and modern stratigraphic and sedimentological analyses field mappers must contend with issues of scale and mappability in defining formations diagenetic versus primary depositional characteristics in delineating contacts and facies changes while making standard 1 24 000 1 25 000 scale maps formations were defined as mappable units even before codification of stratigraphic practice and pre twentieth century legacy formations remain as widely recognized units many modern formations recognized and defined in the late twentieth century also meet stratigraphic code definitions however postlegacy analyses of exceptionally well exposed sections using new techniques and insights while yielding knowledge of stratigraphic variability may yield newly named but not necessarily mappable formations resulting problems of formation recognition formation definition and field mapping of units and contacts on 1 24 000 or 1 25 000 scale maps are faced repeatedly by statemap and other mappers in some cases mapping of hybrid formations composed of two or more previously defined units is required in other cases stratigraphic position and general character must override details of type section stratigraphy in mapping formations we recommend latitude in correlation with type sections and suggest that while well exposed sections provide important insights into paleoenvironments and stratigraphic variations unless units are demonstrably mappable at the 1 24 000 or 1 25 000 scale these variations should not be used to subdivide previously defined and mappable formations into new unmappable formations

did the 73 ka toba super eruption have an enduring effect insights from genetics prehistoric archaeology pollen analysis stable isotope geochemistry geomorphology ice cores and climate models abstract the potential impact of the 73 ka toba super eruption upon global and regional climate terrestrial ecosystems and prehistoric human populations remains unclear evidence from genetics prehistoric archaeology pollen analysis stable isotope geochemistry geomorphology ice cores and climate models has provided some useful working hypotheses further progress requires a substantial improvement in the accuracy precision and resolution of the chronologies of each of the marine and terrestrial proxy records used to reconstruct the environmental impact of this extreme event

structure and development of an axial volcanic ridge mid atlantic ridge 45 n we describe the most comprehensive and detailed high resolution survey of an axial volcanic ridge avr ever conducted at 45 n on the mid atlantic ridge we use 3 m resolution sidescan sonar deep towed magnetic field measurements video observations from eleven rov dives and two very high resolution bathymetry and magnetic surveys the most recently active avr has high topographic relief high acoustic backscatter high crustal magnetization and little faulting it is sharp crested 25 4 km in extent and 500 m high and is covered by approximately 8000 volcanic hummocks whose detailed nature is revealed for the first time each is an individual volcano 450 m in diameter and 200 m high ranging from steep sided 45 cones to low domes many have suffered significant flank collapse hummocks tend to align in rows parallel to the avr axis parallel to its ne trending spurs or on its lower flanks sub normal to the avr trend these latter are spaced 1 2 km apart and comprise 1 2 km long rows of single volcanoes we infer that their emplacement is controlled by down flank magma transport possibly via lava tubes the avr contains only one large flat topped seamount the flanking median valley floor consists of either older hummocky volcanic terrain or flat lying mostly sediment covered lavas these typically have low relief lobate surfaces inflation and collapse structures and occasional lava tubes and tumuli the avr displays open fissures mostly along its crest there is direct evidence for only a few small faults on the avr though steep outward facing slopes draped by elongate pillows may be small normal faults covered by lava the surrounding median valley floor is heavily fissured normal faults cut it and an older avr the latter displaying significant outward facing faults high crustal magnetization an approximate proxy for crustal age within the brunhes is confined to the active avr magnetic palaeointensity measurements are consistent with ages up to 12 ka for several samples from the active avr and 12 ka for one sample from the median valley floor this is much less than the predicted spreading age implying distribution of melt off axis or episodic avr growth

mid ocean ridge basalt generation along the slow spreading south mid atlantic ridge 5 11 s inferences from 238u 230th 226ra disequilibria u series disequilibria have provided important constraints on the physical processes of partial melting that produce basaltic magma beneath mid ocean ridges here we present the first 238u 230th 226ra isotope data for a suite of 83 basalts sampled between 5 s and 11 s along the south mid atlantic ridge this section of the ridge can be divided into 5 segments a0 a4 and the depths to the ridge axis span much of the global range varying from 1429 to 4514 m previous work has also demonstrated that strong trace element and radiogenic isotope heterogeneity existed in the source regions of these basalts accordingly this area provides an ideal location in which to investigate the effects of both inferred melt column length and recycled materials 226ra 230th disequilibria indicate that the majority of the basalts are less than a few millennia old such that their 230th values do not require any age correction the u th isotope data span a significant range from secular equilibrium up to 32 230th excess also similar to the global range and vary from segment to segment however the 230th 238u ratios are not negatively correlated with axial depth and the samples with the largest 230th excesses come from the deepest ridge segment a1 two sub parallel and positively sloped arrays for segments a0 2 and a3 and a4 between 230th 238u and th u ratios can be modelled in various ways as mixing between melts from peridotite and recycled mafic lithologies despite abundant evidence for source heterogeneity there is no simple correlation between 230th 238u and radiogenic isotope ratios suggesting that at least some of the trace element and radiogenic isotope variability may have been imparted to the source regions 350 kyr prior to partial melting to produce the basalts in our preferred model the two 230th 238u versus th u arrays can be explained by mixing of melts from one or more recycled mafic lithologies with melts derived from chemically heterogeneous peridotite source regions

discovery of youngest toba tuff localities in the sagileru valley south india in association with palaeolithic industries the indian subcontinent contains a number of volcanic ash deposits representing the youngest toba tuff ytt volcanic eruption of 75 000 years ago though relatively few localities have been reported in detail here we identify tephra deposits in the sagileru valley south india in association with palaeolithic industries the glass shard and biotite composition of the sagileru tephra matches that of the ytt from other terrestrial sites in india and from the toba caldera and are distinct from earlier large eruptions from toba moreover our survey identified rare associations between lithic artefacts and ytt deposits making the sagileru valley one of the few globally identified locations with both ash and archaeology the identification of ash deposits and stone tool assemblages in the sagileru valley provides another source of information for understanding late pleistocene climate change depositional environments and hominin occupations of south asia

environmental impact of the 73 ka toba super eruption in south asia abstract the cooling effects of historic volcanic eruptions on world climate are well known but the impacts of even bigger prehistoric eruptions are still shrouded in mystery the eruption of toba volcano in northern sumatra some 73 000 years ago was the largest explosive eruption of the past two million years with a volcanic explosivity index of magnitude 8 but its impact on climate has been controversial in order to resolve this issue we have analysed pollen from a marine core in the bay of bengal with stratified toba ash and the carbon isotopic composition of soil carbonates directly above and below the ash in three sites on a 400 km transect across central india pollen evidence shows that the eruption was followed by initial cooling and prolonged desiccation reflected in a decline in tree cover in india and the adjacent region carbon isotopes show that c 3 forest was replaced by wooded to open c 4 grassland in central india our results demonstrate that the toba eruption caused climatic cooling and prolonged deforestation in south asia and challenge claims of minimal impact on tropical ecosystems and human populations

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

sequences cycles and basin dynamics in the silurian of the appalachian foreland basin abstract field stratigraphic analysis of the early and medial silurian strata of ontario new york and pennsylvania permits recognition of six major sequences reflecting 3rd order cycles in the niagaran llandoverian ludlovian of new york and adjacent regions these sequences correspond approximately to the medina lower clinton middle clinton upper clinton two sequences and lockport groups each is bounded by widespread unconformities produced by the interplay of eustatic sea level drop and local tectonic uplift in general erosional sequence boundaries and transgressive surfaces are merged with no record of the lowstand wedge each sequence is subdivisible into two to five sub sequences that are marked by sharp though generally non erosional basal discontinuities investigation of the clinton and lockport groups corroborates earlier suggestions that carbonate and siliciclassic tongues extended basinward from opposite margins synchronously in response to sea level drops analysis of the thickness and basinward extent of the progradational tongues reveals the existence of multi scale sedimentary cycles sequences and component sub sequences are correlative basinwide and are synchronous within the resolution of bio and event stratigraphy major drops in relative sea level are represented by the sequence bounding discontinutiies these surfaces are overlain by siliciclastic wedges near terrigenous source areas or winnowed carbonate pack and grainstones on the tectonically passive margin overlying highstand deposits typically shales calcareous mudstones and limestones are separated from lowstand or transgressive deposits by thin condensed intervals commonly marked by phosphatic or glauconitic material and or surfaces of maximum sediment starvation the relative highstand deposits of sequences and sub sequences are developed in relatively deep water facies and are at most slightly progradational hence most sequences and sub sequences are sharply bounded roughly upward deepening to slightly shallowing successions of strata in turn sub sequences are further divisible into widespread minor parasequences and parasequence sets probably corresponding to 6th and 5th order cycles respectively the smaller scale units 0 2 5 mthick commonly display an upward shallowing motif the nested relationship of these cycles supports the hierarchical model of transgressive regressive allocycles proposed by busch and rollins 1984 niagaran cycles discussed herein are equivalent to third fourth and fifth order cycles and are traceable circumbasinally depocenters of successive sequences and sub sequences display a pattern of eastward westward eastward basin axis migration through the silurian during early llandoverian the taconic queenston deltaic complex began to subside and the basin axis depocenter and eastern shoreline shifted eastward approximately 200 km over a 6 million year period as the basin axis migrated eastward the western basin ramp forebulge was repeatedly upwarded and eroded resulting in the generation of regionally angular unconformities at the bases of sequences ii iv and vi the pattern of unconformity greatest to the west suggests rise of the algonquin arch concomitant with eastward basin axis migration during the late llandoverian to early wenlockian the algonquin arch subsided slightly concurrently with a reversal in direction of basin axis migration during deposition of upper clinton and lockport vernon strata sequences v vi the basin axis shifted approximately 300 km back to the west eastward migration of the basin axis corresponds with a time of tectonic quiescence and probable thrust load relaxation the abrupt reversal of migration in the late llandoverian may be a signal of renewed thrusting in the hinterland at the onset of the salinic disturbance

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

evidence from gabbro of the troodos ophiolite for lateral magma transport along a slow spreading mid ocean ridge the lateral flow of magma and ductile deformation of the lower crust along oceanic spreading axes has been thought to play a significant role in suppressing both mid ocean ridge segmentation and variations in crustal thickness direct investigation of such flow patterns is hampered by the kilometres of water that cover the oceanic crust but such studies can be made on ophiolites fragments of oceanic crust accreted to a continent in the oman ophiolite small scale radial patterns of flow have been mapped along what is thought to be the relict of a fast spreading mid ocean ridge here we present evidence for broad scale along axis flow that has been frozen into the gabbro of the troodos ophiolite in cyprus thought to be representative of a slow spreading ridge axis the gabbro suite of troodos spans nearly 20 km of a segment of a fossil spreading axis near a ridge transform intersection we mapped the pattern of magma flow by analysing the rocks magnetic fabric at 20 sites widely distributed in the gabbro suite and by examining the petrographic fabric at 9 sites we infer an along axis magma flow for much of the gabbro suite which indicates that redistribution of melt occurred towards the segment edge in a large depth range of the oceanic crust our results support the magma plumbing structure that has been inferred indirectly from a seismic tomography experiment on the slow spreading mid atlantic ridge

depositional processes of reworked tephra from the late pleistocene youngest toba tuff deposits in the lenggong valley malaysia two fundamental issues for tephrostratigraphic work are the differentiation of primary from reworked tephra and the characterization of reworking mechanisms we study the depositional processes of four deposits of youngest toba tuff in the lenggong valley malaysia we focus on site stratigraphy particle size distributions magnetic susceptibility and mineralogical associations reworked tephra display variable sedimentological characteristics including polymodal and unimodal very fine to coarse grained distributions and variable concentrations of ash particle size distributions from this study are similar to published analyses for primary deposits demonstrating that particle size alone cannot distinguish primary from secondary tephra the tephra sequences are associated with fluvial and colluvial deposition three facies are identified flood flow mudflow and slumping the ash accumulated rapidly over a period of a few days to months in this valley the ideal site for paleoenvironmental reconstructions is kampung luat 3 where ash accumulated at least in two distinct phases despite the rapid accumulation the lenggong sites are not well suited for paleoenvironmental studies of the ytt impact the time lag between the primary deposition and the floods is unknown and the records could have been modified by site specific characteristics such variables should be considered when proposing paleo environmental reconstructions based on reworked tephra

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

origin of hematite carrying a late paleozoic remagnetization in a quartz sandstone bed from the silurian rose hill formation virginia usa abstract paleomagnetic and petrographic studies of a quartz sandstone bed from the silurian rose hill formation of the central appalachians show that hematite carrying a late paleozoic remagnetization is authigenic comparison of paleopole positions from the rose hill formation with late paleozoic poles for north america suggests a synfolding remagnetization the sandstone cement is characterized by pore lining hematite pore filling berthierine and intergrowths of hematite berthierine and quartz these cements postdate pressure solution of detrital quartz grains late paleozoic cementation is a consequence of diagenesis associated with alleghanian deformation detrital fe mg silicate grains are believed to have been a local source of fe for some authigenic hematite and berthierine although mass balance calculations suggest that an external source is also required some berthierine aggregates are pseudomorphic after detrital fe bearing silicate grains trace amounts of detrital tourmaline are the only remnants of the detrital fe mg silicates

palaeoenvironmental response to the 74 ka toba ash fall in the jurreru and middle son valleys in southern and north central india distal deposits of rhyolitic volcanic ash from the 74 ka supervolcanic eruption of toba in northern sumatra are preserved in numerous river valleys across peninsular india the toba eruption is hypothesized to have resulted in climate change and the devastation of ecosystems and hominin populations this study reports the results of the analysis of sediments and stratigraphical sequences from sites in the jurreru and middle son valleys in southern and north central india the aim of the study is to determine the extent of palaeoenvironmental change in both valleys as a result of the ash fall inferences based on evidence from the jurreru valley are more detailed where pre and post toba palaeoenvironmental changes are divided into seven phases the results indicate that ash fall deposits in both valleys underwent several phases of reworking that possibly lasted for several years indicating that ash was mobile in the landscape for a considerable period of time prior to burial this could have enhanced and lengthened the detrimental effects of the ash on vegetation and water sources as well as animal and hominin populations

individual glass shard trace element analyses confirm that all known toba tephra reported from india is from the c 75 ka youngest toba eruption uncertainty over the identity and age of toba tephras across peninsular india persists with radiometric age dates contradicting earlier compositional data which have been used to identify this important stratigraphic marker as the youngest toba tuff ytt to address this issue new single glass shard analyses have been performed for samples from morgaon and bori north western india which have recently been dated at c 800 ka these and indeed all toba tephra samples thus far analysed from india show the presence of four populations of glass shards defined by their ba y ratio which uniquely identifies them as products of the c 75 ka youngest toba eruption confirmation that the ytt fingerprint is characteristic comes from new analyses of oldest toba tuff ott glass shards from five sites in the indian ocean these are compositionally identical to layer d from the odp site 758 sediment core c 800 ka and belong to a single low ba population clearly different from ytt these analyses show that there is essentially no reworked ott material in the ytt eruption and indicate unequivocally that all known toba tephra occurrences in india belong to the c 75 ka youngest toba eruption