comparative lahar hazard mapping at volcan citlaltepetl mexico using srtm aster and dted 1 digital topographic data abstract in this study we evaluated and compared the utility of spaceborne srtm and aster dems with baseline dted 1 bald earth topography for mapping lahar inundation hazards from volcan citlaltepetl mexico a volcano which has had a history of producing debris flows of various extents in particular we tested the utility of these topographic datasets for resolving ancient valley filling deposits exposed around the flanks of the volcano for determining their magnitude using paleohydrologic methods and for forecasting their inundation limits in the future we also use the three datasets as inputs to a gis stream inundation flow model laharz and compare the results in general all three datasets with spatial resolution of 90 m or better were capable of resolving debris flow and lahar deposits at least 3 106 m3 in volume or larger canopy and slope related height errors in the aster and srtm dems limit their utility for measuring valley filling cross sectional area and deriving flow magnitude for the smallest deposits using a cross sectional area to volume scaling equation height errors in the aster and srtm dems also causes problems in resolving stream valley hydrography which controls lahar flow paths and stream valley morphology which controls lahar filling capacity however both of the two spaceborne dem datasets are better than dted 1 at resolving fine details in stream hydrography and erosional morphologies of volcaniclastics preserved in the valleys around the more humid eastern flanks of the volcanic range the results of laharz flow inundation modeling using all three dems as inputs are remarkably similar and co validate one another for example at citlaltepetl all lahar simulations show that the city of orizaba is the most vulnerable to flows similar in magnitude to or larger than one that occurred in 1920 many of the other cities and towns illustrated are built higher up on terrace deposits of older debris flows and are safe from all but the largest flows which occur less frequently finally asters 60 km swath width and 8 duty cycle presents a challenge for mapping lahar inundation hazards at e w oriented stream valleys in low latitude areas with persistent cloud cover however its continued operations enhances its utility as a means for updating the continuous but one time coverage of srtm and for filling voids in the srtm dataset such as those that occur along steep sided valleys prone to hazards from future lahars

grain size dependence of sediment composition and environmental bias in provenance studies abstract this article investigates both experimentally and theoretically the compositional changes associated with textural effects and hydraulic sorting during sediment transport and deposition which cause systematic distortion in quantitative provenance analysis environmental bias traditional procedures aimed at eliminating textural noise find limited success the gazzi dickinson method cannot remove hydrodynamic related modal variability multiple size window strategies are time consuming narrow or moving size window strategies represent misleading or impractical short cuts being less convenient options than simply analysing each sample in bulk new concepts introduced here unravel the superposed causes of compositional variability in modern sediments intrasample modal variability fundamentally explained by settling equivalence relationships can be accurately modelled mathematically intersample modal variability principally resulting from selective entrainment can be assessed and removed by a simple principle in absence of provenance changes and environmental bias the weighted average density of terrigenous grains srd index should be equal for each sample and each grain size class of each sample to the weighted average density of source rocks by correcting relative abundances of detrital minerals in proportion to their densities we can restore the appropriate srd index for any provenance and subprovenance type in each sample or grain size class modal variability is effectively reduced by this procedure which can be applied confidently to modern sediments deposited by tractive currents in any environment good results are obtained even for placer sands and finest classes where heavy mineral concentration is strongest such srd correction also successfully compensates for biased narrow window modes thus providing a numerical solution of general validity to the problem of environmental bias in sedimentary petrology after compensating for settling equivalence and selective entrainment effects residual size dependent compositional variability may be provenance related minor in ganga brahmaputra sediments provenance related effects are spectacularly displayed in the nile basin where volcaniclastic silt mixes with basement derived quartzofeldspathic sand and wind blown saharan quartz

morphology and emplacement of an unusual debris avalanche deposit at jocotitlan volcano central mexico a pre historic collapse of the northeastern flank of jocotitlan volcano 3950 m located in the central part of the trans mexican volcanic belt produced a debris avalanche deposit characterized by surficial hummocks of exceptional size and conical shape the avalanche covered an area of 80 km2 had an apparent coefficient of friction h l of 0 11 a maximum runout distance of 12 km and an estimated volume of 2 8 km3 the most remarkable features of the jocotitlan debris avalanche deposit are the several steep 29 32 conical proximal hummocks up to 165 m high large tansverse ridges up to 205 m high and 2 7 km long situated at the base of the volcano and the steep 15 50 m thick terminal scarp proximal conical hummocks and parallel ridges that can be visually fitted back to their pre collapse position on the mountain resulted from a sliding mode of emplacement steep primary slopes developed as a result of the accumulation of coarse angular clasts at the angle of repose around core clasts that are decameters in size distal hummocks are commonly smaller less conical and clustered with more diffuse outlines field evidence indicates that the leading distal edge of the avalanche spilled around certain topographic barriers and that the distal moving mass had a yield strength prior to stopping in the ne sector the avalanche was suddenly confined by topographically higher lacustrine and volcaniclastic deposits which as a result were intensely thrust faulted folded and impacted by large clasts that separated from the avalanche front post emplacement loading also induced normal faulting of these soft locally water rich sediments the regional tectonic pattern n ne direction of flank failure and the presence of a major normal fault which intersects the volcano and is parallel to the orientation of the acambay graben located 10 km to the n suggest a genetic relationship between the extensional tectonic stress regime and triggering of catastrophic slope failure the presence of a 3 m thick sequence of pumice and obsidian rich pyroclastic surge and fall tephra directly overlying the debris avalanche deposit indicates that magma must have been present within the edifice just prior to the catastrophic flank failure the breached crater left by the avalanche has mostly been filled by dacitic domes and lava flows the youngest pryroclastic surge deposits on the upper flanks of the volcano have an historical c14 age of 680 80 yearsbp ad 1270 80 thus jocotitlan volcano formerly believed to be extinct should be considered potentially active because of its close proximity to mexico city 60 km the most populous city in the world reactivation could engender severe hazards

geomorphological evidence of the influence of pre volcanic basement structure on emplacement and deformation of volcanic edifices at the cofre de perote pico de orizaba chain and implications for avalanche generation abstract pre volcanic structure of the basement influences volcanism distribution and avalanche generation in volcanic edifices therefore systematic studies of basement structure below volcanic chains are necessary to understand the deformation effects observed in the surface and vice versa based on a compilation of pre existing data interpretation of aerial photographs and satellite images and a collection of structural data we analyzed morphological and structural features of the cofre de perote pico de orizaba cp po volcanic chain and its basement we have identified three sets of regional lineaments that are related to basement trends 1 nw 55 se fractures are parallel to anticline folds observed in cretaceous rocks that originated during laramide shortening these folds present an abrupt morphology observed only in the eastern flank but that is likely to continue below the volcanic chain 2 ne 55 sw fractures are parallel to normal faults at the basement we infer that these basement faults confine the cp po chain within a stepped graben with a total normal displacement of about 400 m these faults have been active through time since they have affected volcanic deposits and induced the emplacement of monogenetic vents notably lineaments of monogenetic vents concentrate where the basement is relatively shallow 3 another set of faults oriented n s has been observed affecting the scarce basement outcrops at the western flank of the chain covered by lacustrine deposits lineaments measured in the volcanic edifice of pico de orizaba correlate with the regional trends in particular the ne 55 sw alignment of monogenetic vents and fractures at pico de orizaba suggest that the same dike trend exists within the volcanic edifice a normal fault with similar orientation was documented at the ne continuation of an alignment crossing the volcanic edifice along the jamapa canyon in the absence of magmatic activity related to collapses the displacement of ne 55 sw faults represents a potential triggering mechanism for generating avalanches at pico de orizaba volcano instability is enhanced by the presence of n s trending fractures crossing the entire volcanic edifice and e w fractures affecting only the present day cone we conclude that mechanical instability of the volcanic chain is influenced by the basement structure heterogeneity but further detailed studies are necessary at individual volcanoes to evaluate their effects on volcano deformation

the colima volcanic complex mexico part ii late quaternary cinder cones since the late pleistocene eleven cinder and lava cones have erupted on the floor of the southern colima graben ne and nw of the large active andesitic volcano colima scoria and lava samples from nine of the cones form a completely transitional basic alkalic series including basanites 9 leucite basanites 3 and minettes 15 the commonest variety of mica lamprophyre these samples represent primitive high temperature magmas with 47 6 50 3 sio2 7 4 15 3 mgo 2 5 4 4 k2o and 2 2 9 9 normative nepheline all members of this basic alkalic suite contain mg olivine fo75 94 chromite augite and late plagioclase and titanomagnetite the petrographic transition from basanite to minette is marked by the appearance of sanidine and the volatile bearing phases phlogopite apatite and analcime during late stages of crystallization as these phases increase in abundance presumably reflecting a rise in magmatic volatile content there are corresponding increases in the whole rock concentrations of 16 incompatible elements although these incompatible elements are relatively abundant even in the basanites many are highly concentrated in the minettes ba 4 200 ppm sr 3 100 ppm zr 550 ppm ce 190 ppm hf 18 ppm among the incompatible elements the degrees of enrichment in the minettes relative to the basanites decrease in the order h th ce la nd zr hf u ba sm eu pb p nb sr ti these enrichments may reflect the increasing importance of minor incompatible element rich mantle phases during partial melting the concentrations of alkali metals k na rb and cs do not correlate with these other elemental enrichments the leucite basanties have similar incompatible element contents to many minettes differing from them only in the presence of leucite rather than analcime and ti f rich groundmass phlogopite rather than hydrous phlogopite phenocrysts thus the leucite basanites represent relatively dry equivalents of minettes two of the eleven cinder cones are calc alkaline in nature and do not belong to the basanite minette group the easternmost cone is constructed of high al basalt and the northernmost of basaltic andesite the high al basalt 49 5 sio2 9 3 mgo 221 ppm ni closely approximates a parental magma to the post caldera andesitic suite of v colima 56 5 61 6 sio2 the basaltic andesite is relatively enriched in incompatible elements compared to the high al basalt v colima trend the ne normative basanite minette samples are highly enriched in incompatible elements while the contemporaneous hy qz normative calc alkaline suite encompassing the high al basalt and v colima s andesites is characterized by relatively low abundances of these elements no likely mineral assemblage can relate the alkaline and calc alkaline suites through crystal fractionation they probably represent fundamentally different melting events during the quaternary the main focus of andesitic volcanism in the southern colima graben has migrated southward with time volcan colima marks its present position 5 km south of the pleistocene volcano nevado de colima and another 15 km from the still older volcan cantaro the eruptions of basic alkalic magma probably occurred during the late stages of nevado s life and through the life of v colima they generally migrated from west to east with time towards v cantaro the most recent cone v apaxtepec is the only one east of the andesitic colima cantaro axis the oldest and the two youngest cones produced basanites while minettes dominated at cones of intermediate ages the cinder cone eruptions may have coincided with a phase of lamprophyre dike injection into plutons solidifying beneath the extinct volcanoes north of v colima the southern end of the colima graben can be viewed then as the volcanic analog of many classical post plutonic hypabyssal lamprophyre localities

sedimentary response to milankovitch type climatic oscillations and formation of sediment undulations evidence from a shallow shelf setting at gela basin on the sicilian continental margin abstract a multi proxy chronological framework along with sequence stratigraphic interpretations unveils composite milankovitch cyclicity in the sedimentary records of the last glacial interglacial cycle at ne gela basin on the sicilian continental margin chronostratigraphic data including foraminifera based eco biostratigraphy and 18 o records tephrochronological markers and 14 c ams radiometric datings was derived from the shallow shelf drill sites geob14403 54 6 m recovery and geob14414 27 5 m collected with both gravity and drilled mebo cores in 193 m and 146 m water depth respectively the recovered intervals record marine isotope stages and substages mis from mis 5 to mis 1 thus comprising major stratigraphic parts of the progradational deposits that form the last 100 ka depositional sequence calibration of shelf sedimentary units with borehole stratigraphies indicates the impact of higher frequency 20 ka sea level cycles punctuating this 100 ka cycle this becomes most evident in the alternation of thick interstadial highstand hst wedges and thinner glacial forced regression fsst units mirroring seaward shifts in coastal progradation albeit their relatively short lived depositional phase these subordinate hst units form the bulk of the 100 ka depositional sequence two mechanisms are proposed that likely account for enhanced sediment accumulation ratios sar of up to 200 cm ka during these intervals 1 intensified activity of deep and intermediate levantine intermediate water liw associated to the drowning of mediterranean shelves and 2 amplified sediment flux along the flooded shelf in response to hyperpycnal plumes that generate through extreme precipitation events during overall arid conditions equally the latter mechanism is thought to be at the origin of undulated features resolved in the acoustic records of mis 5 interstadials which bear a striking resemblance to modern equivalents forming on late holocene prodeltas of other mediterranean shallow shelf settings

stratovolcano stability assessment methods and results from citlaltepetl mexico citlaltpetl volcano is the easternmost strato volcano in the trans mexican volcanic belt situated within 110 km of veracruz it has experienced two major collapse events and subsequent to its last collapse rebuilt a massive symmetrical summit cone to enhance hazard mitigation efforts we assess the stability of citlaltpetl s summit cone the area thought most likely to fail during a potential massive collapse event through geologic mapping alteration mineralogy geotechnical studies and stability modeling we provide important constraints on the likelihood location and size of a potential collapse event the volcano s summit cone is young highly fractured and hydrothermally altered fractures are most abundant within 5 20 m wide zones defined by multiple parallel to subparallel fractures alteration is most pervasive within the fracture systems and includes acid sulfate advanced argillic argillic and silicification ranks fractured and altered rocks both have significantly reduced rock strengths representing likely bounding surfaces for future collapse events the fracture systems and altered rock masses occur non uniformly as an orthogonal set with n s and e w trends because these surfaces occur non uniformly hazards associated with collapse are unevenly distributed about the volcano depending on uncertainties in bounding surfaces but constrained by detailed field studies potential failure volumes are estimated to range between 0 04 0 5 km 3 stability modeling was used to assess potential edifice failure events modeled failure of the outer portion of the cone initially occurs as an intact block bounded by steeply dipping joints and outwardly dipping flow con tacts as collapse progresses more of the inner cone fails and the outer intact block transforms into a collection of smaller blocks eventually a steep face develops in the uppermost and central portion of the cone this modeled failure morphology mimics collapse amphitheaters pres ent at many of the world s stratovolcanoes that have experienced massive failure events

generation of pyroclastic flows and surges by hot rock avalanches from the dome of mount st helens volcano usa several hot rock avalanches have occurred during the growth of the composite dome of mount st helens washington between 1980 and 1987 one of these occurred on 9 may 1986 and produced a fan shaped avalanche deposit of juvenile dacite debris together with a more extensive pyroclastic flow deposit laterally thinning deposits and abrasion and baking of wooden and plastic objects show that a hot ash cloud surge swept beyond the limits of the pyroclastic flow plumes that rose 2 3 km above the dome and vitric ash that fell downwind of the volcano were also effects of this event but no explosion occurred all the facies observed originated from a single avalanche erosion and melting of craterfloor snow by the hot debris caused debris flows in the crater and a small flood that carried juvenile and other clasts north of the crater a second broadly similar event occured in october 1986 larger events of this nature could present a significant volcanic hazard

archaic settlement in el tebernal veracruz first insights into paleoenvironmental conditions and resource exploitation abstract we present first results about an archaic 9000 4000 bp site el tebernal in the eastern part of mexico at el tebernal a set of lithic artifacts was found associated with a paleosol lower paleosol and buried by an upper paleosol and a modern soil the artifacts are made from local limestone and chert selected paleosol properties were evaluated total organic carbon grain size distribution carbon stable isotope composition magnetic susceptibility and micromorphology that provide information about paleoenvironmental conditions the lower paleosol is composed of the following horizons 3e 3eb 3be 3btg1 3btg2 3c while the upper paleosol only has a 2a horizon the lower paleosol is very well developed with in situ clay coatings moderate weathering of primary minerals and a matrix free of carbonates although the alluvium in the lowermost part of the profile contains a mixture of limestone and volcanic materials the entire sequence is covered by the modern soil mantle which is formed by an a horizon radiocarbon ages determined on soil organic matter from the 3btg1 and 2a horizons were 5580 to 5320 cal bp and 2350 to 2320 cal bp respectively we interpret the age of 3btg1 as close to the final stage of soil formation because the organic matter content in this horizon is illuviated pedogenesis likely began sometime prior to the reported age in the early holocene the paleosol properties indicate a long period of landscape stability n 10 3 10 4 years under humid and temperate conditions it is probable that volcanic events from pico de orizaba located several kilometers from el tebernal affected this stability one of the last eruptions produced a block and ash flow of dacitic composition dated at 4100 bp that contributed to the end of pedogenesis the upper paleosol 2a horizon as well as the modern soil contain ceramics from the post classic period ad 900 1521 the profile is cut by a channel and infilled by a dark material that dates to 1140 to 970 cal bp ad 810 980 in consequence the area has been strongly affected by human occupations during the holocene we suggest these ceramic societies caused erosion and activation of geomorphic processes

sedimentologic and dynamic differences between hot and cold laharic debris flows of mayon volcano philippines numerous hot lahars produced during and immediately after the 1984 eruption of mayon volcano along with frequent cold lahars of the succeeding wet monsoon seasons have left widespread debris flow deposits fresh deposits of hot debris flows have thin crusts that are remarkably resistant to erosion and to vegetal growth these crusts contain clasts with uniform thermoremanent magnetization directions rare soft sediment deformation structures and gas escape tubes deposits of unchannelized hot and cold debris flows both have sheetlike or lobate geometry non erosional bases poor sorting relatively minor silt and only traces of clay sized ash either matrix or clast support and common out sized clasts at their tops it is difficult to distinguish between the deposits after burial but they have subtle significant sedimentologic differences the inversely graded bases of hot debris flow deposits are thick suggesting a lack of cohesion and considerable dispersive pressure they have long clast axes aligned parallel to bedding due to strongly sheared laminar flow and generally they are succeeded by gradual normal grading from waning flow conditions in cold debris flow beds only the basal few centimeters may be inversely graded the bulk being commonly ungraded indicating the active role of cohesive forces in the hot flows vaporization of water by heat from large clasts may have facilitated mobility by decreasing internal friction indirect velocity calculations indicate that they moved faster and traveled farther than did cold flows proximal hot debris flows were all supercritical with froude numbers decreasing enroute the most distal hot debris flows and all of the cold ones were subcritical cold debris flows show no correlation between froude number and distance of transport

a machine learning based surrogate model of mars thermal evolution constraining initial conditions and parameters of mantle convection for a planet often requires running several hundred computationally expensive simulations in order to find those matching certain observables such as crustal thickness duration of volcanism or radial contraction a lower fidelity alternative is to use 1 d evolution models based on scaling laws that parametrize convective heat transfer however this approach is often limited in the amount of physics that scaling laws can accurately represent e g temperature and pressure dependent rheologies or mineralogical phase transitions can only be marginally simulated we leverage neural networks to build a surrogate model that can predict the entire evolution 0 4 5 gyr of the 1 d temperature profile of a mars like planet for a wide range of values of five different parameters reference viscosity activation energy and activation volume of diffusion creep enrichment factor of heat producing elements in the crust and initial temperature of the mantle the neural network we evaluate and present here has been trained from a subset of 10 000 evolution simulations of mars ran on a 2 d quarter cylindrical grid from which we extracted laterally averaged 1 d temperature profiles the temperature profiles predicted by this trained network match those of an unseen batch of 2 d simulations with an average accuracy of 99 7per cent

effects of the november 13 1985 eruption on the snow pack and ice cap of nevado del ruiz volcano colombia abstract about twenty five to thirty percent of the ice on the ice cap of nevado del ruiz was fractured and destabilized by earthquakes and explosive activity on november 13 1985 comparison of aerial and ground photographs taken before and after the eruption allows calculation of the volume of snow and ice removed from each glacial basin during the eruption about 16 4 2 km 2 of the surface area of the ice and snow was lost the corresponding volume is estimated to be close to 0 06 km 3 or 9 of the total volume of ice and snow at least four distinct processes including explosions ice avalanches surficial erosion of snow and slush avalanches were responsible for transporting snow firn and ice downslope heat from pyroclastic flows and surges and their deposits was responsible for melting snow and ice both at the surface of the ice cap in situ and during its downslope transport voluminous lahars were triggered in 3 of the 4 main drainages on the volcano by the approximately 43 million m 3 of liquid water about 62 million m 3 of snow firn and ice produced by these combined processes thermal and mechanical effects of the pyroclastic flows and surges included scouring direct melting and entraining snow firn and ice which left several erosional features on the ice cap and deposits both on the ice and on the edges of the ice cap subsurface and englacial drainage of water through crevasses was deduced from depositional and morphological features at the ice margins some meltwater might have been trapped in reservoirs but some water was also released shortly after the eruption the estimated volume of released meltwater is about 43 million m 3 i e 2 4 times more than the approximate 11 12 million m 3 of water incorporated into the lahars thus roughly 21 million m 3 of water were released but did not contribute to lahar generation this non contributing water was 1 included in snow avalanches and sediment laden slush avalanches outside the lahars channels or bound up in the wet pyroclastic surge deposits 2 incorporated in the phreatic explosive products or saturated firn by meltwater which issued from the thermal effect of the tephra fall 3 sublimated into steam during the passage of the pyroclastic flows or 4 stored within the ice meltwater may still be be trapped within the ice cap and may be a potential source of water for future lahars

particle size fractionation of titanium and zirconium during weathering and pedogenesis of granitic rocks in nw spain abstract titanium and zr were analyzed in 10 weathering and pedogenetic soil profiles developed on granitic rocks from nw spain concentrations were obtained by xrf and the ti and zr bearing minerals were studied by petrographical sem and eds analysis both elements were measured in rock bulk soil and granulometric fractions sand 2000 50 m silt 50 2 m clay titanium concentrations in the rock varied between 0 5 and 4 2 g kg 1 and zr concentrations from 52 to 173 mg kg 1 concentrations in bulk soil for the different horizons remain almost unchanged and are strongly related to the element content of the parent rock in some epipedons ti content is relatively lower than that of the underlying horizons due to a dilution effect caused by the high organic matter content and the addition of fresh minerals from erosion upslope the mineralogical study revealed that zr is present exclusively in zircons while ti occurs in rutile ilmenite and titanite but mainly as a structural component of the biotite regarding the particle size fractions both elements behave differently zirconium is enriched in the silt fraction of all soil horizons with concentrations up to 5 times that of the parent rock while ti is enriched in the silt up to 5 times and clay fractions up to 12 times of the solum horizons and depleted in the sand fraction of the a horizons these results indicate that zr concentrations are mainly controlled by the abundance of zircons in the rock and their physical detachment from the host minerals upon weathering ti mainly depends on the release from the structure of biotites through chemical weathering and its reprecipitation as neoformed mineral phases this physicochemical fractionation of ti and zr in soils bears important implications for studies on soil genesis continuity of soil profiles and the calculation on enrichment factors which are based on the conservative behaviour of both elements

sedimentology and event timing of a catastrophic volcaniclastic mass flow volcan hudson southern chile eruption of volcan hudson aisen region southern chile on 12 august 1971 melted up to 80 of ice within the 9 km wide hudson caldera rapid mobilisation of abundant fluvio glacial sediment within the adjacent huemules valley led to the generation of a catastrophic mass flow which swept westwards along the valley eventually entering the fjord some 40 km downstream the mass flow developing from one source and being unaffected by tributary input was characterised by three events within the flood hydrograph 1 a precessional flood possibly linked to breakage of an early formed dammed lake 2 a main mass flow which varied in its rheological behaviour from plug flow to hyperconcentrated flow and 3 a recessional flood at the tail of the main flow which diluted towards a concentrated streamflow the main debris flow displayed characteristics of pulsing internal segregation density stratification significant downstream sediment assimilation cohesive frictional freezing with ensuing fluidisation and development of an advancing runout phase consideration of these sedimentary characteristics and spatial temporal flow transformations are used to propose a schematic model of event timing within this catastrophic volcaniclastic flow

swift snowmelt and floods lahars caused by great pyroclastic surge at mount st helens volcano washington 18 may 1980 the initial explosions at mount st helens washington on the moring of 18 may 1980 developed into a huge pyroclastic surge that generated catastrophic floods off the east and west flanks of the volcano near source surge deposits on the east and west were lithic sorted lacking in accretionary lapilli and vesiculated ash not plastered against upright obstacles and hot enough to char wood all attributes of dry pyroclastic surge material deposited at the surge base on steep slopes near the volcano transformed into high concentration lithic pyroclastic flows whose deposits contain charred wood and other features indicating that these flows were hot and dry stratigraphy shows that even the tail of the surge had passed the east and west volcano flanks before the geomorphically distinct floods lahars arrived this field evidence undermines hypotheses that the turbulent surge was itself wet and that its heavy components segregated out to transform directly into lahars nor is there evidence that meters thick snow slab avalanches intimately mixed with the surge to form the floods the floods must have instead originated by swift snowmelt at the base of a hot and relatively dry turbulent surge impacting hot pyroclasts probably transferred downslope momentum to the snow surface and churned snow grains into the surge base melting snow and accumulating hot surge debris may have moved initially as thousands of small thin slushflows as these flows removed the surface snow and pyroclasts newly uncovered snow was partly melted by the turbulent surge base this and accumulating hot surge debris in turn began flowing a self sustaining process feeding the initial flows the flows thus grew swiftly over tens of seconds and united downslope into great slushy ejecta laden sheetfloods gravity accelerated the floods to more than 100 km h as they swept down and off the volcano flanks while the snow component melted to form great debris rich floods lahars channeled into valleys

geochemistry and grain size distribution of surface sediments of a large semi enclosed lagoon connected to the western bay of bengal abstract batticaloa is a large 135 5 km2 semi enclosed micro tidal lagoon that formed by barrier deposits in eastern sri lanka and has a unique geographic position due to its tropical monsoonal climate and absence of tectonics the recent soft bottom sediments of batticaloa lagoon were studied on the basis of geochemistry and grain size distribution in order to interpret their characteristics and provenance poor to moderately sorted medium sands are the dominant sedimentary deposits for the lagoon therefore a medium energy condition is suggested for sediment deposition the average silt and clay fraction of the lagoon sediments is less than 3 indicating medium or high energy flows coarser deposits are restricted to channel and lagoon mouths the scatter plots of nb zr cr and sc vs th and cr v vs y ni show a felsic provenance for the sediments the normalized diagrams of major element oxides and trace elements for the regional rocks vijayan complex of sri lanka indicate high country rock influence for the sedimentation for the semi enclosed setting the enrichment and distribution of zr th nb cr and tio2 suggest the possibility of mineral sands that are fractionated with hydrodynamic setting of the lagoon

cerro grande volcano the evolution of a miocene stratocone in the early trans mexican volcanic belt abstract cerro grande volcano is a well preserved low angle composite volcano that evolved between 11 and 9 ma it represents the beginning of volcanism in the eastern sector of the early trans mexican volcanic belt extensive field stratigraphic and petrologic work revealed a complex volcanic evolution that can be summarized in six principal stages i construction of a shield like lava cone ii generation of unusual lithic rich ignimbrites apparently related to powerful eruptions that caused rapid and significant vent erosion these eruptions could have resulted from shallow magma water interactions iii eruption of peripheral fissural lava flows following a nnw sse trending fissure system iv period of dome growth and explosive collapse v short period of repose followed by open vent explosive magmatic and hydromagmatic eruptions vi radial eruption of lava flows related to a ring fissural system geochemistry data of late miocene volcanics show a continental arc magmatic origin and show that crystal fractionation contributed strongly to magma differentiation several lava flows were erupted with a persistent nnw sse trending orientation and the entire late miocene geologic record is affected by highly dislocated normal faults with the same orientation this may indicate that late miocene volcanism was under tectonic control finally late miocene volcanics and the inception of the early trans mexican volcanic belt could be related to a shallow subduction angle influenced by a significant increase in the convergence rate along the middle american trench at about 20 ma

geochemistry and tectonic setting of mafic rocks in western dronning maud land east antarctica implications for the geodynamic evolution of the proterozoic maud belt on the basis of new bulk major and trace element including ree as well as sm nd and rb sr isotope data used in conjunction with available geochronological data a post tectonic mafic igneous province and four groups of pre to syntectonic amphibolite are distinguished in the polymetamorphic maud belt of western dronning maud land east antarctica protoliths of the group 1 amphibolites are interpreted as volcanic arc mafic intrusions with archaean to palaeoproterozoic nd model ages and depletion in nb and ta isotopic and lithogeochemical characteristics of this earliest group of amphibolite indicate that the maud belt was once an active continental volcanic arc the most likely position of this arc for which a late mesoproterozoic age c 1140 ma is indicated by available u pb single zircon age data was on the southeastern margin of the kaapvaal grunehogna craton the protoliths of group 2 amphibolites are attributed to the 1110 ma borgmassivet umkondo thermal event on the basis of comparable nd model ages and trace element distributions group 3 amphibolite protoliths are characterized by mid ocean ridge basalt type ree patterns and low th yb ratios and they are related to neoproterozoic extension group 4 amphibolite protoliths are distinguished by high dy yb ratios and are attributed to a phase of syntectonic pan african magmatism as indicated by rb sr isotope data

downstream dilution of a lahar transition from debris flow to hyperconcentrated streamflow nearly instantaneous melting of snow and ice by the march 19 1982 eruption of mount st helens released a 4 106 m3 flood of water from the crater that was converted to a lahar volcanic debris flow through erosion and incorporation of sediment by the time it reached the base of the volcano over the next 81 km that it traveled down the toutle river the flood wave was progressively diluted through several mechanisms a transformation from debris flow to hyperconcentrated streamflow began to occur about 27 km downstream from the crater when the total sediment concentration had decreased to about 78 by weight 57 by volume the hyperconcentrated lahar runout flood wave transporting immense quantities of sand in suspension continued to experience progressive downstream dilution although turbulence was significantly dampened by the extremely high suspended load very large standing waves and antidune waves were observed the hyperconcentrated lahar runout flow deposited an unusual faintly stratified coarse sand which locally contained small isolated gravel lenses very similar deposits in the quaternary stratigraphy of mount st helens and other cascades volcanoes suggest that lahars may be more frequent than previously recognized

age nd hf isotopes and geochemistry of the vijayan complex of eastern and southern sri lanka a grenville age magmatic arc of unknown derivation abstract the ca 1 0 1 1 ga vijayan complex vc of eastern and southeastern sri lanka is one of three high grade metamorphic terranes making up the basement of the island and is in tectonic contact with the adjacent older highland complex it consists predominantly of granitoid gneisses ranging in composition from diorite to leucogranite with a distinct calc alkaline geochemical signature and is interpreted as a magmatic arc strong ductile deformation has obliterated almost all original intrusive relationships high grade metamorphism during the pan african event at ca 610 520 ma has produced widespread granulite facies assemblages that are now largely retrogressed and were affected by extensive late metamorphic k metasomatism in southern sri lanka the vijayan gneisses are tectonically interlayered with rocks of the highland complex in a so called mixed zone we report zircon ages whole rock nd and hf in zircon isotopic systematics and geochemical data for a large selection of vijayan gneissses in order to better characterize this complex and its tectonic setting the zircon ages are predominantly in the range 1100 1000 ma with a few early neoproterozoic intrusions and identify the vc as a grenville age magmatic arc many zircons experienced minor to significant lead loss at about 580 ma the nd and hf isotopic data confirm a generally primitive origin for most vijayan gneisses but significant variations in both isotopic systems argue for source heterogeneities and the possible involvement of minor amounts of older continental material in their genesis immobile trace and rare earth element distributions favour an origin of the gneiss protoliths through melting at relatively shallow depth and the chemical variation from diorite and tonalite via granodiorite to granite in the vc can be explained by increasing fractionation of plagioclase and biotite accompanied by amphibole and accessory phases the vc may be comparable in some respect to the composition and evolution of the kohistan arc in pakistan the unique composition age and isotopic characteristics of the vc find no convincing counterpart in other fragments of the gondwana supercontinent and its origin therefore remains exotic and enigmatic

mt pelee martinique may 8 and 20 1902 pyroclastic flows and surges abstract the distribution stratigraphic relationships and fragmental components of the may 8 and 20 1902 pyroclastic flows from mt pelee martinique together with eyewitness accounts suggest the following explanation for those eruptions the eruptions were vertically directed magmatic perhaps initiated phreatically and contained abundant juvenile lithics from congealed magma of the dome and neck this resulted in a two part eruption column having 1 a dense lithic charged part which collapsed into the crater and flowed out of a pre existing notch in its side giving rise to pyrochlastic flows and 2 a magmatically derived column containing gases juvenile vitric material and crystals which largely by passed the neck and dome and escaped into the atmosphere all of the energy of the flows was apparently focused through the notch they emerged fully turbulent and flowed down riviere blanche gravity segregation of large and abundant fragments soon resulted in a dense high concentration poorly fluidized block and ash flow confined to the valley leaving above a fully turbulent high energy ash cloud surge as the ash cloud surge moved down the mountain it continued to expand outward the process of gravity segregation continued as the ash cloud surge expanded resulting in secondary block and ash underflows toward st pierre the secondary block and ash flows developed on a gently sloping upland surface 100 m or more above the valley of riviere blanche the turbulent fragment depleted surges above the secondary block and ash flows maintained sufficient energy to devastate the landscape outward to about 3000 m including st pierre the surges refracted around obstacles and in one place moved up a small valley in a direction opposite to the main flows

le pico de orizaba mexique structure et evolution d un grand volcan andesitique complexe volcan pico de orizaba which marks the eastern end of the trans mexican volcanic belt is one of the largest andesitic composite volcanoes in america it is located above a series of crustal distensive faults making the boundary of the coast plains of the gulf of mexico from thealtiplano for this reason the volcano shows an asymmetry from the west its elevation is about 3 000 m whereas on the eastern side it reaches 4 000 to 4 500 m from its base

phreatomagmatic explosions of rhyolitic magma experimental and field evidence 1 experimental studies on explosive molten fuel coolant interaction mfci using basaltic melt compositions and water as the coolant have provided insight into the physical processes of basaltic and andesitic phreatomagmatic volcanism abundant field evidence indicates that rhyolitic and dacitic phreatomagmatism occurs in nature but it has not been possible until now to generate laboratory mfci explosions from the interaction between high silica melts and water under laboratory conditions the high viscosity of these melts apparently prevents formation of an effective hydrodynamic premix of melt and water the documented precursor of experimental explosive mfci caused by mafic melts our new experiments utilized samples from a rhyolitic tuff ring volcano in mexico tepexitl an experimental approach was developed in which premixing conditions were generated by mechanical deformation of the melt leading to brittle type fragmentation at the melt water interface physical measurements recorded during laboratory explosion provide quantitative evidence for rhyolitic explosive mfci additionally a comparison of experimentally produced particles with natural ones from tepexitl deposits show nearly identical chemical mineralogical composition grain size and grain morphology detailed textural analysis confirmed the presence of phreatomagmatically produced particles in both experimental and natural analog particles the results from this series of experiments indicate that under natural conditions stress induced magma fracturing can lead to a critical magma water interface growths and trigger phreatomagmatic explosions of high silica magma the water source for these eruptions may include shallow aquifers surface water bodies strong precipitation and intrusion into ice or wet unconsolidated sediments

nevado del ruiz volcano colombia an example of the state of the art of volcanology four years after the tragic november 13 1985 eruption

experimental study of olivine melt reaction and its petrological implications abstract reaction experiments between olivine and sio2 rich melt were carried out at 1 atm under controlled fo2 conditions with variables comprising the chemical compositions of olivine and melt temperature and time vermicular crystals of olivine or opaque zones composed of fine mixtures of ca poor pyroxene magnetite and melt are formed by reaction between olivine more fayalitic than the equilibrium olivine and melt partial dissolution irrespective of the compositions of olivine reaction rims of ca poor pyroxene are formed on the olivine crystals or on the opaque zones by the reaction with an andesite melt but not with a boninite melt the former melt cannot coexist with olivine while the latter can on an olivine pyroxene reaction surface the observation of the reaction rims suggests that many pyroxene reaction rims on olivine are not formed by simple monotonic cooling of magmas but by reaction between disequilibrium pairs of olivine and sio2 rich melt as the result of xenocryst incorporation and or magma mixing the variety of reaction textures present in volcanic rocks such as dusty plagioclase alkali feldspar and pyroxene and pyroxene reaction rims on olivine indicate that many of these rocks have experienced contamination by xenolith incorporation and or magma mixing

petrology of the younger andesites and dacites of iztaccihuatl volcano mexico ii chemical stratigraphy magma mixing and the composition of basaltic magma influx the younger andesites and dacites of iztaccihuatl volcano mexico constitute a medium k calcalkaline rock suite 58 66 wt per cent sio2 characterized by high afg numbers 100mg mg o85 ife 55 66 and relatively high abundances of mgo 2 5 6 6 wt per cent ni 17 158 p p m and cr 42 224 p p m chemical stratigraphy plots of eruptive sequences indicate the existence of a plexus of long lived dacite magma chambers periodically replenished by influxes of basaltic magma ascending from depth short term geochemical evolution after batch influx was dictated by magma mixing and eventual dilution of the basaltic component by quasi steady state hornblende dacite magma the chemical data support textural and mineralogical evidence for rapid homogenization of originally diverse magmas by convective blending of residual liquids accompanied by dynamic fractional crystallization nixon 1988 internally consistent mixing calculations used to derive the composition of basaltic magma influx incorporate analytical uncertainties and the observed range of salic end member compositions mafic end members are basalts to basaltic andesites 52 54 wt per cent sio2 with m numbers 73 76 mgo 9 11 wt per cent ni 250 p p m and cr 340 510 p p m concentrations and liquidus olivine compositions fo90 88 appropriate for unfractionated partial melts of mantle peridotite the majority of model compositions are ol hy normative similar to those of primitive basajtic lavas on the flanks of iztaccihuatl and in the valley of mexico however calculated magma batches range from weakly qz normative to strongly ne normative both calculated and analyzed basaltic compositions are distinguished by highly variable abundances of alkalies and incompatible trace elements notably rb ba sr p zr and y initial sr sr ratios for iztaccihuatl lavas 0 7040 0 7046 n 24 are comparable to those for primitive basaltic rocks 0 7037 o 7o45 n 4 and indicate that 1 mantle source regions are isotopically heterogeneous and 2 contamination of iztaccihuatl magma chambers by radiogenic crustal rocks was not a significant factor in the evolution of calc alkaline andesites and dacites the replenishment of iztaccihuatl dacite reservoirs by ne normative magmas late in the history of cone growth precludes exhaustion of mantle source regions by progressive partial melting trie waning stages of volcanic activity at iztaccihuatl appear to reflect the inability of dense basaltic influxes to successfully penetrate a large high level chamber of low density hornblende dacite magmai

fragmentation transport and deposition of a low grade ignimbrite the citlaltepetl ignimbrite eastern mexico the citlaltepetl ignimbrite records one of the largest explosive events during the holocene activity of citlaltepetl volcano pico de orizaba multiple pyroclastic flow units a fall deposit and some lahar units were emplaced between 8500 9000 y b p as a result of repetitive but discrete explosive events the whole ignimbrite resulted from discrete fluctuations in eruptive intensity that decreased with time the initial pyroclastic flow pulse was by far the most violent and widespread event and its deposits show conspicuous variations in structure and texture that could be associated with different mechanisms of transport and emplacement subpopulation sequential fragmentation transport sft analyses were carried out in order to determine the physical mechanisms that selectively concentrate or remove particles in the moving flows we suggest that lateral and temporal changes in the flow rheology in which fluidization yield strength entrainment of atmospheric air and sedimentation played a dominant role in flow propagation and emplacement may imprint a unique signature in the grain size spectra the lowermost unit of the citlaltepetl ignimbrite can be envisaged by a model in which progressive aggradation near the vent became replaced by en masse emplacement farther outward

geochemistry of fine grained clastic sediments in the archean abitibi greenstone belt canada implications for provenance and tectonic setting abstract archean sandstone and shale samples from turbiditic tf and alluvial facies af in the southern volcanic zone of the abitibi greenstone belt canada have been analyzed for major and trace elements the sandstones from tf are mainly greywackes whereas those from af are lithic arenite sio 2 ranges from 51 to 68 geochemically the clastic sediments cluster into mafic element mep and low mafic element lmep populations the mep sediments which include turbiditic facies sediment samples from the larder lake group in the kirkland lake area are characterized by elevated abundances of mgo 5 ni 200 ppm cr 400 ppm sc 22 ppm and feo 7 5 abundances lower contents of incompatible elements such as th u ta hf zr and total rees n ratios are 4 5 with eu eu 1 the lmep sediments which include samples from the beatty dome and three nations lake formations in timmins the kewagama group noranda area and timiskaming group in the kirkland lake area however show reverse abundance distributions the geochemical gaps shown by the major and trace elements define two populations of sediments suggesting contrasting provenances for them the mep sediments require a dominantly mafic and ultramafic 90 source with 10 felsic contribution whereas the lmep sediments require a mixture of felsic 75 and mafic 25 rocks this compositional distribution of the sediments is closely related to their tectonic environments the mep was formed in basins proximal to or within an active oceanic rift consequently having ultramafic and mafic rocks as the major source for the lmep calc alkaline volcanic sequences formed in an island arc like setting were the major source these two contrasting volcanic plutonic terranes were tectonically juxtaposed during late archean collisional tectonics

major holocene block and ash fan at the western slope of ice capped pico de orizaba volcano mexico implications for future hazards abstract a major block and ash fan extends more than 14 km westward from the summit of pico de orizaba volcano in the eastern part of the trans mexican volcanic belt radiocarbon dating of charcoal within the fan deposits yielded holocene ages that range between 4040 80 and 4660 100 y b p stratigraphical sedimentological geochemical and scanning electron microscope studies indicate that this fan originated within a relatively short time span by multiple volcanic explosions at the summit crater this activity produced a series of pyroclastic flows mainly block and ash flows and lahars which were channelized by a glacial cirque and connecting u shaped valleys as they descended toward the base of the volcano a recurrence of a similar eruption today would pose severe hazards to the population of more than 50 000 people who live in a potentially dangerous zone a detailed reconstruction of the sequence of events that led to the formation of the block and ash fan is presented to help mitigate the risk special attention is given to the effects of an ice cap and the role of pre existing glacial morphology on the distribution of products from such an eruption

eruption of a major holocene pyroclastic flow at citlaltepetl volcano pico de orizaba mexico 8 5 9 0 ka abstract multiple volcanic eruptions occurred between 8500 and 9000 yr b p from the central crater of citlaltepetl volcano generating a series of pyroclastic flows that formed a deposit with a total volume of about 0 26 km3 d r e the flows descended in all directions around the crater but they were mostly controlled by topography and deposited in valleys or local topographic depressions up to about 30 km from vent although the flows were apparently emplaced without much violence some features of the deposits reveal local turbulent conditions and an expanded fluidization that can be related to moderate flow velocities the deposit has two members the lower one consists of multiple flow units and the upper one includes a single flow unit and a thin basal pumice fall both members are lithologically similar and dominated by dense andesitic scoriae with minor amounts of different pumice types andesitic dacitic and banded and lithics in a silty matrix the eruption probably had a low pressure boiling over mechanism and was possibly triggered by mixing of dacitic and andesitic magmas juvenile material in the pyroclastic flow deposit is compositionally similar to that of holocene lava flows at citlaltepetl which have apparently resulted from magma homogenization of mafic and silicic end members because the system is continuously injected with new basaltic andesitic magma a recurrence of explosive activity is possible in future eruptions

geochemistry of archean shales from the witwatersrand supergroup south africa source area weathering and provenance with a few exceptions shales from the archean witwatersrand supergroup 2800 ma in south africa are depleted in na ca lile ree and hfse compared to phanerozoic shales cr co and ni are enriched in all witwatersrand shales and fe and mg are high in shales from the west rand groups wrg and lower central rand group crg shales from the crg and uppermost wrg are enriched in na al lile ree hfse and transition metals relative to shales from the lower wrg chondrite normalized ree patterns for all witwatersrand shales are enriched in light ree and exhibit small to moderate negative eu anomalies a positive correlation of ree and al2o3 contents in the shales suggests that ree are contained principally in clay minerals relative to shales from the crg shales from the wrg exhibit depletions of na ca and sr a feature probably reflecting intense chemical weathering of their source rocks cia indices in witwatersrand shales are variable chiefly 70 98 even within the same shale unit such variations reflect chiefly variable climatic zones or rates of tectonic uplift in source areas with perhaps some contribution from provenance and element remobilization during metamorphism compared to present day upper continental crust all but the orange grove roodepoort and k8 shales appear to have been derived from continental sources depleted in lile ree and hfse and enriched in transition metals computer mixing models based on six relatively immobile elements th hf yb la sc co and four source rocks indicate that the relative proportions of granite basalt and komatiite increased with time in sediment source areas at the expense of tonalite the contributions of basalt and komatiite appear to reach a maximum during deposition of the booysens shale and granite during deposition of the k8 shales and possibly during deposition of the orange grove shales

ophiolitic source rocks for taconic age flysch trace element evidence flysch generated along the appalachian orogen during the ordovician taconic orogeny ranges in age from late arenig to early caradoc allochthonous flysch and from late llanvirn to ashgill autochthonous flysch sandstones in these units are dominantly lithic arenites with a heavy mineral suite containing common to abundant mg rich chromite north of the canada united states border the only reasonable sources for the chromite were the lower ultramafic units of obducted ophiolites trace element analyses cr ni v cu ti zr y of flysch sandstones indicate a striking increase in cr and ni concentrations from the united states appalachians to the canadian appalachians in the united states units cr is in the range of 10 to 50 ppm canadian flysch units average an order of magnitude more cr these results suggest that ophiolite obduction either did not occur or was of minor importance during the taconic orogeny in the united states appalachians the difference in ophiolite abundance between the canadian and united states appalachians suggests a fundamental difference in tectonic style along the orogen a feature that must be accommodated by plate tectonic models new models will probably need to consider both obliquity of subduction and geological complexity of the ordovician collision zone in order to explain the spatial distribution of obducted ophiolite sheets

geochemical and isotopic profile of pico de orizaba citlaltepetl volcano mexico insights for magma generation processes abstract pico de orizaba or citlaltepetl volcano is the easternmost and highest stratovolcano of the subduction related mexican volcanic belt mvb located 400 km nne of the middle america trench this active volcano comprises four evolutionary stages ranging in age from 0 65 ma to the holocene and is surrounded by quaternary monogenetic scoria cones and maar volcanoes magmatic products of the stratocone range from basaltic andesites to rhyolites and the cinder cones erupted basalts and basaltic andesites all rock compositions form a continuous calc alkaline suite petrogenetic processes involved in magma generation and evolution include fractional crystallization and mid crustal assimilation trace element patterns with elevated ba nb positive pb spikes and th enrichments indicate contributions from subducted sediment low ba th ratios suggest melting of hydrous sediment without significant loss of fluid mobile elements prior to melting sr nd isotopic ratios of pico de orizaba and cinder cones are nearly chondritic and are located on a mixing curve between pacific morb and paleozoic crust of se mexico however vertical nd distributions in an 87sr 86sr vs end diagram cannot be explained by crustal assimilation and indicate contributions of a sedimentary component with unradiogenic nd in contrast to other eastern mvb volcanic centres pico de orizaba magmas are derived almost exclusively from a depleted mantle source compared to other mvb stratocones pico de orizaba shows the least radiogenic nd isotope ratios at nearly identical 87sr 86sr steep trends in a 206pb 204pb vs 207pb 204pb diagram favour the involvement of young 207pb enriched oceanic sediments in magma generation processes of pico de orizaba volcano the pb isotope data do not support any assimilation of lower crustal grenvillian basement

rock magnetic and geochemical proxies for iron mineral diagenesis in a tropical lake lago verde los tuxtlas east central mexico magnetic and non magnetic mineral analyses were conducted on a lacustrine sequence from lago verde in the tropical coast along the gulf of mexico that covers the last 2000 years the site witnessed the transformation of the environment since the early olmec societies until forest clearance in the last century through these analyses we investigated the processes that affected the magnetic mineralogy in order to construct a model of past environmental changes and compare this model with the archeological record and inferred climatic changes in the northern hemisphere of tropical america volcanic activity has played a major influence on sediment magnetic properties as a purveyor of ti magnetites ti maghemites and as a factor of instability in the environment anoxic reductive conditions are evident in most of lago verde s sedimentary record direct observations of magnetic minerals and ratios of geochemical fe ti and ferrimagnetic f and paramagnetic p susceptibility data are used as parameters for magnetite dissolution p fe f and precipitation f ti of magnetic minerals intense volcanic activity and anoxia are recorded before a d 20 leading to the formation of framboidal pyrite increased erosion higher evaporation rates lower lake levels anoxia and reductive diagenesis in non sulphidic conditions are inferred for laminated sediments between a d 20 850 this deposit matches the period of historical crisis and multiyear droughts that contributed to the collapse of the maya civilization dissolution of magnetite a high organic content and framboidal pyrite point to anoxic sulphidic conditions and higher lake levels after a d 850 higher lake levels in lago verde broadly coincide with the increased precipitation documented during the medieval warm period a d 950 1350 in the northern tropical and subtropical regions of the american continent for the little ice age a d 1400 1800 the relatively moist conditions inferred are in concordance with the glacial advances recorded in central mexico higher erosion rates reflect destruction of the rainforest over the last 40 years

late pleistocene to precolumbian behind the arc mafic volcanism in the eastern mexican volcanic belt implications for future hazards an area of widespread alkaline to subalkaline volcanism lies at the northern end of the cofre de perote citlaltepetl pico de orizaba volcanic chain in the eastern mexican volcanic belt mvb two principal areas were active about a dozen latest pleistocene to precolumbian vents form the 11 km wide e w trending cofre de perote vent cluster cpvc at 2300 2800 m elevation on the flank of the largely pleistocene cofre de perote shield volcano and produced an extensive lava field that covers s 100 km 2 more widely dispersed vents form the naolinco volcanic field nvf in the sierra de chiconquiaco north of the city of jalapa xalapa three generations of flows are delineated by cone and lava flow morphology degree of vegetation and cultivation and radiocarbon dating the flows lie in the behind the arc portion of the northeastern part of the mvb and show major and trace element chemical patterns transitional between intraplate and subduction zone environments flows of the oldest group originated from la joya cinder cone radiocarbon ages v42 000 yr bp at the eastern end of the cpvc this cone fed an olivine basaltic flow field of v20 km 2 that extends about 14 km southeast to underlie the heavily populated northern outskirts of jalapa the capital city of the state of veracruz the central cone group ccg of intermediate age consists of four morphologically youthful cinder cones and associated vents that were the source of a lava fields 27 km 2 of late pleistocene or holocene age the youngest group includes the westernmost flow from cerro colorado and a lava flow v2980 bp from the rincon de chapultepec scoria cone of the nvf the latest eruption from the compound el volcancillo scoria cone occurred about 870 radiocarbon years ago and produced two chemically and rheologically diverse lava flows that are among the youngest precolumbian flows in mexico and resemble paired aa pahoehoe flows from mauna loa volcano the el volcancillo eruption initially produced the high effusion rate short duration toxtlacuaya alkaline aa lava flow from the southeastern crater this 12 km long hawaiite average 50 5 sio2 flow was followed by extrusion of the calc alkaline r o naolinco lava flow from the northwestern crater this large volume v1 3 km 3 tube fed basaltic pahoehoe flow average sio2 49 traveled 50 km inferred effusion rates suggest emplacement over a decade long period flows of all three age groups are transected by highway 140 and the railway that form major transportation arteries between jalapa and puebla this area has not previously been considered to be at volcanic risk but volcanism here has continued into precolumbian time future eruptions of similar magnitude and location to those documented here could pose significant hazards to transportation corridors and to densely populated areas in and to the north of jalapa slight

petrology of the younger andesites and dacites of iztaccihuatl volcano mexico i disequilibrium phenocryst assemblages as indicators of magma chamber processes

the 1985 nevado del ruiz volcano catastrophe anatomy and retrospection abstract this paper seeks to analyze in an objective way the circumstances and events that contributed to the 1985 nevado del ruiz catastrophe in order to provide useful guidelines for future emergencies the paper is organized into two principal parts in the first part an anatomy of the catastrophe is developed as a step by step chronicle of events and actions taken by individuals and organizations during the period november 1984 through november 1985 this chronicle provides the essential background for the crucial events of november 13 this year long period is broken down further to emphasize important chapters the gradual awareness of the awakening of the volcano a long period of institutional skepticism reflecting an absence of credibility the closure of the credibility gap with the september 11 phreatic eruption followed by an intensive effort to gird for the worst and a detailed account of the day of reckoning the second part of the paper retrospection examines the numerous complicated factors that influenced the catastrophic outcome and attempts to cull a few lessons from armero in order to avoid similar occurrences in the future in a nutshell the government on the whole acted responsibly but was not willing to bear the economic or political costs of early evacuation or a false alarm science accurately foresaw the hazards but was insufficiently precise to render reliable warning of the crucial event at the last possible minute catastrophe was therefore a calculated risk and this combination the limitations of prediction detection the refusal to bear a false alarm and the lack of will to act on the uncertain information available provided its immediate and most obvious causes but because the crucial event occurred just two days before the armero emergency management plan was to be critically examined and improved the numerous circumstances which delayed progress of emergency management over the previous year also may be said to have contributed to the outcome thus the catastrophe was not caused by technological ineffectiveness or defectiveness nor by an overwhelming eruption or by an improbable run of bad luck but rather by cumulative human error by misjudgment indecision and bureaucratic shortsightedness armero could have produced no victims and therein dwells its immense tragedy

intra and extra caldera volcaniclastic facies and geomorphic characteristics of a frequently active mafic island arc volcano ambrym island vanuatu abstract ambrym is one of the most voluminous active volcanoes in the melanesian arc it consists of a 35 by 50 km island elongated east west parallel with an active fissure zone the central part of ambrym about 800 m above sea level contains a 12 kilometre wide caldera with two active intra caldera cone complexes marum and benbow these frequently erupting complexes provide large volumes of tephra lapilli and ash to fill the surrounding caldera and create an exceptionally large devegetated plateau ash plain as well as sediment choked fluvial systems leading outward from the summit caldera deposits from fall subordinate base surge and small volume pyroclastic scoria flows dominate the volcaniclastic sequences in near vent regions frequent and high intensity rainfall results in rapid erosion of freshly deposited tephra forming small scale debris flow and modified grain flow dominated deposits box shaped channel systems are initially deep and narrow on the upper flanks of the composite cones and are filled bank to bank with lapilli dominated debris flow deposits these units spill out into larger channel systems forming debris aprons of thousands of overlapping and anastomosing long narrow lobes of poorly sorted lapilli dominated deposits these deposits are typically remobilised by hyperconcentrated flows debris rich stream flows and rare debris flows that pass down increasingly shallower and broader box shaped valleys lenses and lags of fines and primary fall deposits occur interbedded between the dominantly tabular hyperconcentrated flow deposits of these reaches aeolian sedimentation forms elongated sand dunes flanking the western rim of the ash plain outside the caldera initially steep sided immature box canyons are formed again conveying dominantly hyperconcentrated flow deposits these gradually pass into broad channels on lesser gradients in coastal areas and terminate at the coast in the form of prograding fans of ash dominated deposits the extra caldera deposits are typically better sorted and contain other bedding features characteristic of more dilute fluvial flows and transitional hyperconcentrated flows these outer flank volcaniclastics fill valleys to modify restricted portions of the dominantly constructional landscape lava flows and satellite cones of ambrym apparent maturity of the volcanic system has resulted in the subsidence of the present summit caldera at a similar rate to its infill by volcaniclastic deposits

the controls on the major and trace element variation of shales siltstones and sandstones of pennsylvanian permian age from uplifted continental blocks in colorado to platform sediment in kansas usa shales siltstones and sandstones of pennsylvanian permian age from near the source in colorado to those in the platform in eastern colorado and kansas have been analyzed for major elements and a number of trace elements including the rees the near source sandstones are significantly more enriched student t test at better than the 99 confidence level in sio2 and na2o concentrations and more depleted in al2o3 fe2o3 total tio2 th hf sc cr cs rees y and ni concentrations and laco and lani ratios than the near source shales and siltstones most likely due to more plagioclase and quartz and less clay minerals in the sandstones than in the shales and siltstones there are no significant differences in k2o and sr concentrations and eueu lalu lase thsc thco and crth ratios between the near source sandstones and the near source shales and siltstones samples of the molas hermosa and cutler formations near the source that were formed in different environments in the same area contain no significant difference in eueu lalu lasc thsc thco and crth ratios so a generally silicic source and not the environment of deposition was most important in producing these elemental ratios for example crth ratios of near source shales siltstones and sandstones range from 2 5 to 17 5 and eueu range from 0 48 to 0 78 which are in the range of sources of sediments derived from mainly silicic and not basic sources near source shales and siltstones contain significantly higher student t test and more varied concentrations of most elements al2o3 fe2o3 mno tio2 ba th hf ta co sc rees nb y but significantly lower concentrations of na2o and eueu than platform shales and siltstones in kansas e g la 65 7 40 and eueu 0 55 0 07 in near source shales and siltstones and la 23 7 8 7 and eueu 0 64 0 08 in platform shales and siltstones the sio2 and cao concentrations are not significantly different in platform shales and siltstones compared to the near source shales and siltstones so dilution of other minerals by quartz and calcite is not the main reason for the lower concentration of most elements in the platform relative to the near source shales and siltstones rather the lesser concentrations of most elements in clay minerals of the platform shales and siltstones can account for the lower concentration of most elements compared to corresponding near source shales and siltstones the lower concentrations of many elements in clay minerals in the platform shales and siltstones may be a result of having been derived from recycling of clay minerals from older rocks the greater homogeneity of elemental concentrations of the platform shales and siltstones compared to those in the source is also consistent with homogeneous mixing of such recycled material also there is no significant difference in thsc laco thco lani and crth ratios of the near source sedimentary rocks in colorado to the platform shales and siltstones in kansas and the latter are also consistent with derivation from mostly silicic source rocks

the quetzalapa pumice a voluminous late pleistocene rhyolite deposit in the eastern trans mexican volcanic belt the study area is located in the east part of the trans mexican volcanic belt in the las cumbres volcanic complex lcvc which lies between two large stratovolcanoes pico de orizaba 5700 m a s l to the south and cofre de perote 4200 m a s l to the nne the most conspicuous structure of the lcvc is a 4 km diameter circular crater with a dacitic dome in the center which constitutes the remains of a destroyed stratovolcano the quetzalapa pumice qp was produced by a plinian eruption that was dated by the 14c method at 20 000 yr bp the eruptive sequence consists predominantly of pumice fall deposits and scarce intra plinian pyroclastic flow deposits which crop out on the west flank of the lcvc the absence of post plinian ignimbrite deposits is striking the deposits are well sorted clast supported with reverse grading at the base with a medium to high accessory lithics content the maximum average thickness of the deposit in the proximal areas is about 15 m and has been divided into three members the basal member bm 2 m thick with four submembers bmf1 bmf2 bmf3 and bmafl the intermediate member im 10 m thick with two submembers impf and imaf and the upper member um 3 m thick with four submembers umpl umsdf umwaf and umpls the predominant component of the fall deposits is a white highly vesiculated pumice with 71 sio2 content plagioclase is the most abundant mineral followed by 1 3 mm long biotite phenocrysts the accessory lithics are lavas mostly of andesitic composition their abundance increases toward the uppermost levels of the sequence we calculate a minimum volume of 8 4 km3 2 22 km3 dense rock equivalent for the entire qp deposit isopach and isopleth maps show that the im deposit has an elongated distribution with a nne ssw direction whereas the um deposit has a circular distribution we estimate a maximum eruptive column height for the im of 20 km field studies and isopach and isopleth maps indicate that the eruptive column was affected by a strong wind previous studies located the qp source in the las cumbres crater however based on the isopach and isopleth distribution and the lack of pumice fall deposits inside the las cumbres crater we consider that the qp emission center is located on the west flank of the lcvc and was buried by its own pumice fall deposits it coincides with an explosion crater called la capilla formed during the closing phase of the qp eruption a pumice fountain model is proposed to explain the observed sequence of deposits according to this model the material was emitted through a hose type conduit during a monogenetic eruption of rhyolitic composition this kind of volcanic activity is not extensively reported in the literature

a quaternary monogenetic volcanic field in the xalapa region eastern trans mexican volcanic belt geology distribution and morphology of the volcanic vents abstract the most abundant volcanic manifestations along the east west trending trans mexican volcanic belt tmvb are the scoria cones these have been grouped by other authors in extended monogenetic volcanic fields such as michoacan guanajuato chichinautzin apan and los tuxtlas here we present geological and morphological data of a relatively unknown group of monogenetic volcanoes located on the east flank of the cofre de perote volcano cp around the city of xalapa in the state of veracruz mexico within an area of about 2400 km2 the xalapa monogenetic volcanic field xmvf contains over 50 late quaternary volcanoes most of them are scoria cones but small shield volcanoes and tuff rings also occur the lava flows produced by these volcanoes are constrained by an abrupt topography and cover a great percentage of the surface on the eastern and northeastern flanks of cp between 3000 and 500 m a s l the representative rocks of the different volcanic centers include olivine basalt basaltic andesite with phenocrysts of plagioclase pyroxene and minor olivine and andesite with phenocrysts of plagioclase and pyroxene sio2 and al2o3 contents of the rocks vary between 45 and 62 wt and 15 to 18 wt respectively most of the basaltic rocks have mgo contents between 4 2 and 9 wt ni and cr concentrations between 23 and 180 and 10 to 380 ppm respectively with a typical calc alkaline behavior trace elements suggest two types of magmas the most abundant are characterized by an enrichment of lile and lree with negative anomalies of nb and ti which denote a calc alkaline affinity others are lile depleted and show high concentrations of mgo cr and ni which is typical of primary calc alkaline magmas the mean scoria cone morphological values are cone height hco 90 8 m cone diameter wco 686 38 m crater diameter wcr 208 49 m and 0 12 km3 for the cone volume we dated twelve different scoria cones using the 40ar 39ar method for the other cones we used morphologic indicators such as the ratio of the scoria cone height to basal diameter hco wco and maximum slope angle additionally vegetation type and cone and lava flow morphologies were also considered in order to determine rough relative ages in general the ages fall into three groups those older than 2 0 ma those between 0 25 0 40 ma and those less than approximately 0 1 ma

geochemical signature of provenance in sand size material in soils and stream sediments near the tobacco root batholith montana u s a abstract trace element geochemistry of sandstones are being used to determine provenance we have conducted preliminary and limited experiments to determine to what extent daughter sands retain the geochemical signature of parent rocks six sets of first order stream sediments soils from adjacent slopes and a variety of parent rocks were collected from southwestern montana u s a sampling in a low relief area ensured that climate and residence time of soils on slopes could be eliminated as variables sand size fractions of stream sediments and soils and the corresponding parent rocks granodiorite quartz monzonite granite gneiss biotite tonalite gneiss and amphibolite were analyzed for most major elements and selected trace elements petrologic modal analysis of the parent rocks and the 0 25 0 50 mm fraction of each sand was done to monitor major mineralogic control if any on chemical compositions of the samples our data show that the abundances of the si and al in sediments do not discriminate provenance abundances of ca mg fe and ti may broadly distinguish between sands derived from metamorphic and igneous source rocks at least in the area studied differences in abundances of the ba and th and the ratio of la lu between granitic tonalitic and amphibolitic parent rocks are preserved in the daughter sediments that we studied however the size of the eu anomaly in the ree patterns of different daughter sediments is not diagnostic of parent rocks abundances of co and sc distinguish between sediments derived from felsic and mafic rocks a better provenance discrimination is obtained if the ratios la sc th sc la co ba sc and ba co are used petrologic modal data show that mineral contents and chemical compositions of parent rocks are compatible with each other the chemical composition of the sands may be roughly correlated to the petrological modal data but the abundances of some minor and trace elements of sediments cannot be inferred from modal mineralogy this is expected because these elements may concentrate in accessory minerals and or may weather out into aqueous or clay mineral fractions it is also compatible with conclusions of previous studies that some of these elements do not reside in sand size fractions of siliciclastic sediments

field observations of pristine block and ash flow deposits emplaced april 16 17 1991 at volcan de colima mexico abstract for the first time since the effusive activity of 1981 1982 volcan de colima extruded scoriaceous andesitic lava on march 1 1991 this followed a period of increasing seismicity summit dome deformation and fumarolic activity that began in february of 1991 growth of the new lava dome proceeded until april 16 at 16 00 hrs local time gmt 6 hours when a series of gravitational failures of the upper edifice and the new lava dome flow triggered the emplacement of successive small volume non glowing block and ash flows to a distance of 4 km within the upper reaches of barranca el cordoban south southwest of the present summit aerial photos taken at the time of the event clearly show the lack of explosions at the summit dome efficient elutriation of the finest silt and clay sized fraction from the main body of the avalanching material produced large reddish yellowish and grayish ash clouds which rose to an altitude of 1 5 2 5 km for several hours and were carried southeast to northeast by predominant winds vegetation in the path of the debris was totally stripped sand blasted and bent in the sense of flowage but was not burnt no fatalities were reported we present a summary of our field observations of the pristine deposits which we studied prior the rainy season on april 20 27 between 2 200 and 2 500 m elevation

paraglacial and postglacial debris flows on a little ice age terminal moraine jamapa glacier pico de orizaba mexico the study area is located on the northern face of pico de orizaba mexico 5700 m asl on the terminal moraine of jamapa glacier which dates from the little ice age large debris flows are recurrent on the proglacial ramp the comparison of lichen colonies growing on the deposits of the flows reveals that two generations of flows are present an old one and a very recent one that occurred between 1994 and 1995 studies were made of the sedimentologic characteristics of the flows and ground temperatures were recorded to a depth of 70 cm on the floor of the channels comparison of the lichen growth on the exposed surfaces of the deposits led to the relative dating of the older debris flow which is associated with the beginning of the retreat of the glacier and the saturation of the terminal moraine deposits by meltwater the more recent flow has less transport capacity and is identified with the formation of an impermeable layer of permafrost that covers the bottom of the channel of the old debris flow the permafrost layer formed when snow accumulations on the bottom of the channel were covered by ash that fell from the slopes

initiation and flow behavior of the 1980 pine creek and muddy river lahars mount st helens washington two large high velocity lahars volcanic debris flows were triggered by a pyroclastic surge during the first few minutes of the may 18 1980 eruption of mount st helens the initial surge cloud evolved progressively by gravity segregation from a gas mobilized highly inflated density flow to a dense water mobilized basal debris flow lahar and accompanying ash cloud as it flowed down the east flank of the volcano the main source of the water for the lahars was probably from eroded snow and ice incorporated into the flow by turbulent mixing but ground water expelled together with the rock debris by the initiating volcanic explosions also may have contributed peak lahar discharge from the pine muddy fan upper smith creek and ape canyon probably exceeded 250 000 m 3 s initially but decreased exponentially in the downstream direction total volume of the lahars was in excess of 1 4 10 7 m 3 initial peak flow velocities in excess of 30 m s also decreased markedly downstream where flow was not impeded velocity was strongly related to the depth slope term r 2 3 s 1 2 from the manning uniform flow equation as a power law function during much of the route traveled lahar flow appears to have been supercritical deposits left in channels were generally thin relative to flow depth 0 to 2 5 m particles up to small boulder size were randomly distributed in the poorly sorted nonstratified matrix indicating complete suspension in a fully developed debris flow slurry however much larger clasts were transported as bed load computed sediment concentrations of matrix slurry samples ranged from 84 to 91 solids by weight and were similar for the two lahars two indirect methods for computing peak flow velocity previously only tentatively applied to debris flows were tested for accuracy by comparing computed lahar arrival times with recorded arrival times at swift reservoir the computed velocities appear to be 15 slower than the recorded velocities which is consistent with the restriction that the velocity formulas produce only minimum values

multiple edifice collapse events in the eastern mexican volcanic belt the role of sloping substrate and implications for hazard assessment abstract the citlaltepetl cofre de perote volcanic chain forms an important physiographic barrier that separates the central altiplano 2500 masl from the gulf coastal plain gcp 1300 masl the abrupt eastward drop in relief between these provinces gives rise to unstable conditions and consequent gravitational collapse of large volcanic edifices built at the edge of the altiplano eastward sloping substrate caused by the irregular configuration of the basement rocks is the dominant factor that controls the direction of collapsing sectors in all major volcanoes in the region to be preferentially towards the gcp these collapses produced voluminous debris avalanches and lahars that inundated the well developed drainages and clastic aprons that characterize the coastal plain large catastrophic collapses from citlaltepetl las cumbres and cofre de perote volcanoes are well documented in the geologic record some of the avalanches and transformed flows have exceptionally long runouts and reach the gulf of mexico traveling more than 120 km from their source so far no direct evidence has been found for magmatic activity associated with the initiation of these catastrophic flank collapses apparently instability of the volcanic edifices has been strongly favored by very intense hydrothermal alteration abrupt topographic change and intense fracturing in addition to the eastward slope of the substrate the reactivation of pre volcanic basement structures during the late tertiary and the e w to ene ssw oriented regional stress regimes may have played an important role in the preferential movement direction of the avalanches and flows in addition to magmatic hydrothermal processes high amounts of rainfall in the area is another factor that enhances alteration and eventually weakens the rocks it is very likely that seismic activity may be the principal triggering mechanism that caused the flank collapse of large volcanic edifices in the eastern mexican volcanic belt however critical pore water pressure from extraordinary amounts of rainfall associated with hurricanes or other meteorological perturbation cannot be ruled out particularly for smaller volume collapses there are examples in the area of small seismogenic debris flows that have occurred in historical times showing that these processes are not uncommon assessing the stability conditions of major volcanic edifices that have experienced catastrophic sector collapses is crucial for forecasting future events this is particularly true for the eastern mexican volcanic belt where in many cases no magmatic activity was associated with the collapse therefore edifice failure could occur again without any precursory warning

distributions of total inorganic and organic phosphorus in surface and recent sediments of the sub tropical and semi pristine guaratuba bay estuary se brazil this study addresses the distribution of total phosphorus tp and its inorganic ip and organic op fractions grain size and organic matter of surface and recent sediments coupled to the behavior of total and dissolved inorganic phosphorus tp and dip of the water column of the semi pristine guaratuba bay estuary se brazil surface sediment samples were taken at 43 sites spread along the estuarine gradient and recent sediments from 3 short 35 cm long cores from the upper central and lower portions of the estuary respectively highest tp and ip concentrations of surface sediments were detected within the upper sector and the transition zone between the upper and central sectors all characterized by fine sediments low salinities and water depths in contrast the lower sector and its narrow and deep tidal channel subject to more intense tidal forcing exhibited a higher fraction of sandy sediments with lower tp ip and op contents in spite of the spatial variability in sediment grain size ip corresponded to the major fraction of tp in all estuarine sectors and both tp and ip correlated significantly with the fine sedimentary silt clay grain size fraction the fine surface sediments acted as a trap for ip at the fresh water low salinity interface which also corresponded to the region of a dip sink in surface waters in general the short sediment cores showed that tp and ip contents increased from 15 cm depths to the top layer published sedimentation rates from additional cores taken at the sites of the short cores of this study implied that depositional alterations of tp and ip increased during the early 1970s which corresponded to the onset of anthropogenic disturbances from crop plantations in the lowland plains of the river end member and urbanization at the estuary s mouth and along the adjacent coast

geochemical evidence for the accumulation of vanadium in soils of chronic kidney disease areas in sri lanka chronic kidney disease ckd is a major health problem in the dry zone of sri lanka although the precise figures are not known number of patients suffering from ckd has reached a crisis level during the last decade this study describes soil geochemistry of the affected region to establish the etiology for the unresolved problem of ckd a field survey was carried out in 88 localities and concentrations of 16 major and trace elements from soil samples were determined using x ray fluorescence spectroscopy it was revealed that the number of ckd patients is high where elevated concentrations of soil vanadium were observed further v levels in nonagricultural soils of ckd reported sites have significantly exceeded the risk level 200 mg kg given by the canadian environmental council the strong enrichment of v in soil indicates that its bioavailable fraction is also high besides the v of soils studied fe and cr contents are also contained to be high however the values have not reached up to the threshold levels moreover the presence of lower concentrations of other harmful trace elements f as pb zn cu and ni in the hotspots does not exhibit a significant impact on ckd strong positive correlations of v with fe cr ti and ni indicate that they may have derived from mineral phases of weathered basement rocks further ratios of the immobile trace elements in soils studied illustrate that v is mainly derived from parent meta igneous rocks of basaltic composition

holocene block and ash flows from summit dome activity of citlaltepetl volcano eastern mexico a major eruption produced several block and ash flows about 4 100 years b p at citlaltepetl volcano pico de orizaba an ice capped 5670 m high andesitic active stratovolcano located at the eastern end of the mexican volcanic belt repetitive gravitational collapse of a dacitic dome at the summit crater produced a series of block and ash flows lahars and floods which were channeled through two main river valleys on the west and south flanks of the volcano the total erupted volume is estimated to be at least 0 27 km3 the deposits in both areas are similar in composition and size but they differ in the area covered distribution and structure the western deposits form a large fan cover a larger area and include numerous laharic and fluviatile deposits in contrast the southern deposits form prominent terraces where confined in narrow channels and have associated laharic units in distal areas where the flows reach a maximum distance of 30 km from the vent directed disruptions of a central summit dome occurred possibly first to the west and then to the southeast perhaps due to minor modifications of the summit dome morphology producing the voluminous block and ash flow deposits documented here the flows were strongly controlled by topography influencing the deposition of the moving particles grain size variations along the flow paths are hardly detectable suggesting no evident lateral downstream transformations because sudden changes in dome morphology may cause significant variations in the direction of future dome collapse specific areas of potential affectation cannot be predicted therefore about 350 000 inhabitants living within a radius of 35 km from the vent could be potentially impacted if catastrophic block and ash flows were to recur in the future from similar summit dome activity recognition of these deposits is therefore important for hazard assessment because some seemingly safe areas may be at high risk

depositional history of diamictites from the late paleoproterozoic dalma formation e india abstract the paleoproterozoic dalma formation situated on the singhbhum craton in eastern india consists almost entirely of mafic to ultramafic rocks that were deposited by effusive volcanic activity and as pyroclastic deposits the main focus of the present study however is on diamictites consisting like both other rock types exclusively of chlorite and actinolitic hornblende that are intercalated with these volcanic rocks these diamictites are interpreted as mass flow deposits the textures and mineralogical composition of the diamictites suggests a volcanic origin of the material most probably they are reworked pyroclastic deposits with the same mineralogy as the dalma lavas on this basis it is concluded that the diamictites represent the deposits of one or more volcanic mudflows they must have formed of material that had already travelled over a considerable distance in the form of grain by grain transport as indicated by the presence of both rounded and distinctly broken clasts and by the relatively small size of the clasts both the grain by grain and the mass transport must have occurred following pathways that were covered all over with volcanic material of mafic composition this transport must have taken place under terrestrial conditions as no marine or lacustrine clasts or matrix is included the results of the study suggest that the diamictites within the dalma formation cannot be seen as a possible equivalent of paleoproterozoic glacigenic sediments such as known from australia and south africa

recent changes in sediment delivery by the huanghe yellow river to the sea causes and environmental implications in its estuary summary over the past 60 years climate change and human activities in the huanghe yellow river basin have changed the river system in particular construction of large reservoirs and soil conservation practices within the river basin have reduced sediment flux to the sea by 90 and increased the grain size of suspended sediment delivered to the sea 30 m now versus 18 m before in median grain size scouring of the channel in the lower reaches has added a new sediment source to those derived from the loess region of the middle reaches before construction of the xiaolangdi reservoir at the downstream end of the middle reaches suspended sediment concentrations at station lijin the last hydrographic station before the river enters the sea were high enough 35 kg m 3 in the flood season to offset the density contrast between freshwater and ambient seawater thus hyperpycnal flows developed along the subaqueous slope at the river mouth during the flood season observations from two cruises near the huanghe river mouth one before and one after the operations of the xiaolangdi reservoir suggest that buoyant hypopycnal plumes rather than hyperpycnal plumes have occurred at the river mouth since the dramatic changes in concentration and grain size of suspended sediment discharged to the sea the rapid decrease of sediment discharge to the sea and the increase of grain size of suspended sediments not only changed the sediment dispersal pattern at the river mouth but also modified the shoreline and subaqueous slope the effects of climate change and human activities in the river basin have been transferred along the hydrological pathway to the estuary and delta and have changed estuarine sediment dynamics and delta morphology the huanghe presents an illustrative example of river basin coastal zone interactions in the context of climate change and human activities

biogenic precipitation of manganese oxides and enrichment of heavy metals at acidic soil ph abstract natural mn oxides are largely biogenic in origin formed via the microbial oxidation of mn ii these minerals are extremely efficient scavengers of heavy metals yet to date microbial mn oxide precipitation and subsequent heavy metal sorption have received little attention in mining impacted environments where heavy metal concentrations are elevated but bio geochemical conditions are typically unfavourable for both abiotic and biogenic mn oxide precipitation featuring acidic ph and low organic carbon contents here we investigate the formation of mn oxide bio geochemical barrier layers and the immobilisation of heavy metals in these layers in soil profiles from a former uranium mining site in ronneburg germany detailed soil profiling shows the site has an acidic soil ph that varies from 4 7 to 5 1 and eh values from 640 to 660 mv using synchrotron x ray diffraction and x ray absorption spectroscopy together with scanning electron microscopy and electron microprobe analysis we find that the dominant mn oxide present in the mn oxide layers is a poorly crystalline hexagonal birnessite akin to synthetic mno 2 covering and cementing quartz grains using phylogenetic analysis based on 16s rdna we identify and characterise six strains of manganese oxidising bacteria mob from the acidic mn oxide layers which we subsequently culture to produce poorly crystalline hexagonal birnessite akin to that found at the study site specifically we identify three gram positive spore forming firmicutes affiliated to bacillus safensis bacillus altitudinis and brevibacillus reuszeri which are able to oxidise mn after initiating spore formation two gram positive actinobacteria belonging to the genera arthrobacter and frondihabitans and one gram negative proteobacteria belonging to the genus sphingomonas geochemical thermodynamic speciation modelling indicates that the abiotic precipitation of mn oxides in the mn oxide layers is unfavourable and we suggest that the mn oxides in the bio geochemical barriers at our study site are biogenically precipitated in an acidic soil environment to our knowledge this is the first report to identify the above six bacterial strains and specifically identify spore forming bacteria as mob in an acidic soil environment we find that the poorly crystalline hexagonal birnessite precipitated in the mn oxide layers efficiently immobilises ba ni co cd zn and ce and as such we find that mob and biogenically precipitated mn oxides can exert a strong control on the fate and mobility of metals in mining impacted environments

a model for mixing basaltic and dacitic magmas as deduced from experimental data it has been demonstrated experimentally that basaltic and dacitic magmas can be easily mixed to form both banded dacite and homogeneous andesite in less than a few hours the presence of phenocrysts larger than 0 5 mm increased considerably the efficiency of mixing flow patterns in the experimental system were visualized using pt spheres which indicated that convection occurs in basalt melt but not in dacite melt the reynolds numbers of the basaltic and dacitic melts in the experimental system were calculated to be about 10 3 and 10 6 respectively mixing proceeds initially by mechanical mixing of the two magmas in a large scale but later by coupling interfacial convection and mutual diffusion thus depending on the depth where vesiculation and following disruption of the magma occurs banded pumice homogeneous pumice and homogeneous andesite lava are erupted the observed textures of mixed rocks of plinian type eruption and the limiting occurrence of banded pumice are satisfactorily accounted for on this model

build up and depositional dynamics of an arc front volcaniclastic complex the miocene tepoztlan formation transmexican volcanic belt central mexico volcanic terrains such as magmatic arcs are thought to display the most complex surface environments on earth ancient volcaniclastics are notoriously difficult to interpret as they describe the interplay between a single or several volcanoes and the environment the early miocene tepoztlan formation at the southern edge of the transmexican volcanic belt belongs to the few remnants of this ancestral magmatic arc and therefore is thought to represent an example of the initial phase of evolution of the transmexican volcanic belt based on geological mapping detailed logging of lithostratigraphic sections palaeocurrent data of sedimentary features and anisotropy of magnetic susceptibility mapping of two dimensional panels from outcrop to field scale and geochronological data in an area of ca 1000 km 2 three periods in the evolution of the tepoztlan formation were distinguished which lasted around 4 myr and are representative of a volcanic cycle edifice growth phases followed by collapse in a magmatic arc setting the volcaniclastic sediments accumulated in proximal to medial distances on partly coalescing aprons similar to volcanic ring plains around at least three different stratovolcanoes these volcanoes resulted from various eruptions separated by repose periods during the first phase of the evolution of the tepoztlan formation 22ae 8t o 22 ae2 ma deposition was dominated by fluvial sediments in a braided river setting pyroclastic material from small andesitic dacitic composite volcanoes in the near vicinity was mostly eroded and reworked by fluvial processes resulting in sediments ranging from cross bedded sand to an aggradational series of river gravels the second phase 22ae 2t o 21 ae3 ma was characterized by periods of strong volcanic activity resulting in voluminous accumulations of lava and tuff which temporarily overloaded and buried the original fluvial system with its detritus continuous build up of at least three major volcanic centres further accentuated the topography and in the third phase 21ae 3t o 18 ae8 ma mass flow processes represented by an increase of debris flow deposits became dominant marking a period of edifice destruction and flank failures