multiphase petrography of volcanic rocks using element maps a method applied to mount st helens 1980 2005 quantitative textural analyses including crystal size distributions csds provide insights into crystallisation kinetics of magmatic systems investigations of volcanic crystal textures often rely on greyscale variations on backscattered electron images to identify crystal phases which must then be thresholded and or traced manually a laborious task and investigations are typically restricted to a single crystal phase a method is presented that uses energy dispersive x ray element maps to generate textural data each pixel is identified as a crystal phase glass or vesicle according to relative chemical composition enabling concurrent acquisition of multiphase csd crystallinity and mineral mode data data processing is less time intensive for the operator but considerable instrument time is required to generate element maps the method is applied to 17 dacite samples from the 1980 1986 and 3 from the 2004 2005 eruptive periods of mount st helens volcano usa to provide quantitative insights into multiphase textural evolution all of the csds are curved and concave up in the standard csd plot with curvature increasing with plagioclase content to facilitate comparisons with previous studies csds for microlites 50 m length crystals are approximated as straight lines the line intercepts and slopes provide information on n 0 nucleation density and characteristic length or g the product of growth rate g and residence time respectively these parameters as well as the total groundmass crystallinity show distinct differences between explosive deposits from summer 1980 and post summer 1980 domes post summer 1980 microlite n 0 values are mostly at the lower end of the range of those measured for summer 1980 samples total groundmass crystallinities during summer 1980 are between 10 and 30 vol whereas post summer 1980 crystallinity increases to between 39 and 51 vol the range of n 0 values is similar to those previously published for mount st helens but g is consistently higher g of a may 1985 sample analysed in this study is approximately 2 m higher compared with previously published data for the same sample when processed using similar methodologies groundmass crystallinity data show similar trends to those previously published for the 1980 to 1986 eruption increasing sharply after summer 1980 then increasing more gradually during the dome building phase of the eruption the effects of varying l the apparent crystal size and crystal aspect ratio on resultant csds are also investigated whilst relative temporal variations in csds for a given dataset can be investigated absolute values from different studies cannot be compared unless methods of data acquisition and processing are exactly the same

experimental alluvial fans advances in understanding of fan dynamics and processes alluvial fans are depositional systems that develop because of a disparity between the upstream and downstream sediment transport capacity of a system usually at the base of mountain fronts as rivers emerge from the constrained mountain area onto the plain they are dynamic landforms that are prone to abrupt changes on a geomorphological decades to centuries time scale while also being long term deposition features that preserve sedimentary strata and are sensitive indictors of environmental change the complexity of interactions between catchment characteristics climate tectonics internal system feedbacks and environmental processes on field alluvial fans means that it is difficult to isolate individual variables in a field setting therefore the controlled conditions afforded by experimental models has provided a novel technique to overcome some of these complexities the use of experimental models of alluvial fans has a long history and these have been implemented over a range of different research areas utilising various experimental designs using this technique important advances have been made in determining the primary factors influencing fan slope understanding of avulsion dynamics identifying autogenic processes driving change on fan systems independent of any change in external conditions and the mechanics of flow and flood risk on alluvial fans to name a few however experiments cannot be carried out in isolation thus combining the findings from experimental alluvial fans with field research and numerical modelling is important and likewise using these techniques to inform experimental design if this can be achieved there is potential for future experimental developments to explore key alluvial fan issues such as stratigraphic preservation potential and simulating extra terrestrial fan systems

erosion by pyroclastic flows on lascar volcano chile pyroclastic flows generated in the 19 20 april 1993 eruption of lascar volcano chile produced spectacular erosion features scree and talus were stripped from the channels and steep slopes on the flanks of the volcano exposed bedrock and boulders suffered severe abrasion producing smoothed surfaces on coarse breccias and striations and percussion marks on bedrock and large boulders erosional furrows developed with wavelengths of 0 5 2 m and depths of 0 1 0 3 m furrows commonly nucleated downstream of large boulders or blocks which are striated on the upstream side and thereby produced crag and tail structures erosive features were produced where flows accelerated through topographic restrictions or where they moved over steep slopes the pyroclastic flows are inferred to have segregated during movement into lithic rich and pumice rich parts lithic rich deposits occur on slopes up to 14 whereas pumice rich deposits occur only on slopes less than 4 and mainly at the margins and distal parts of the 1993 fan the lithic rich deposits contain large up to 1 m lithic clasts eroded from the substrate and transported from the vent whereas pumice rich deposits contain only small typically 2 cm lithic clasts these observations suggest that lithic clasts segregated to the base of the flows and were responsible for much of the erosive phenomena the erosive features distribution of lithic clasts and deposit morphology indicate that the 1993 flows were highly concentrated avalanches dominated by particle interactions in some places the flows slid over the bedrock causing abrasion and long striations which imply that large blocks were locked in fixed positions for periods of about 1 s however shorter striae at different angles impact marks segregation of the deposits into pumice and lithic rich parts and mixing of bedrock derived lithic clasts throughout the deposits indicate that clasts often had some freedom of movement and that jostling of particles allowed internal mixing and density segregation to occur within the flows

vesiculation and microlite crystallization induced by decompression a case study of the 1991 1995 mt unzen eruption japan isothermal decompression experiments were performed to simulate magma ascent at unzen volcano from the depths of magma storage to shallow crustal levels corresponding to pressure decrease from 300 to 50mpa a partially crystallized synthetic rhyodacitic magma representing equilibrium conditions at 8508c and 300mpa was used as a starting material this has a composition identical to the groundmass of unzen rocks erupted in 1991 1995 decompression rates were varied from 0 0002 to 20mpa s experiments conducted with decompression rates 0 1mpa s 1 were decompressed continuously a multi step decompression approach was used at decompression rates 0 1mpa s the experiments were fluid saturated either containing only water as a fluid component h2o bearing or containing a water and carbon dioxide mixture h2o co2 initial mole fraction of h2o in the fluid 0 6 the experimental products of the h2o bearing experiments consist of amphibole pyroxenes oxides and glass plagioclase microlites nucleated and grew only in experiments with the two lowest decompression rates of 0 0005 and 0 0002mpa s the length of those plagioclases is up to 200 250 mm which is consistent with the size of plagioclase microlites observed in the natural samples the experimental products of the h2o co2 bearing system are composed of pyroxenes oxides glass and plagioclase plagioclase microlites in the h2o co2 system were already present in the starting assemblage and grew to a maximum size of 80 mm equilibrium concentrations of water in the residual glasses at the final pressure of 50mpa are reached at decompression rates 1mpa s 1 for the h2o co2 bearing system and 0 1mpa s 1 for the h2o bearing system the bubble number density bnd values range from 10 to 10 mm 3 in the h2o bearing system and from 10 4 6 to 10 mm 3 in the h2o co2 bearing systems in both systems bnd values decrease with decreasing decompression rate from 20 to 0 01mpa s and increase with decreasing decompression rates50 01mpa s which is interpreted to reflect predominant bubble growth and bubble nucleation respectively the onset of crystallization observed from changes in the chemical composition of the residual melt occurs at decompression rates 50 1mpa s at the lowest decompression rate 0 0002mpa s the chemical composition of the residual melt in the h2o co2 bearing system becomes similar to the natural matrix glass composition there is no significant variation of the microlite number density mnd value as a function of the decompression rate the mnd values for plagioclases only range from 10 to 10 mm whereas the mnd values for the other phases range from 10 to 10 mm our experimental mndpl values are in the range of those from natural samples 10 10 mm we show that the size of microlites nucleating and crystallizing during decompression plagioclase in our experimental dataset is useful to constrain magma ascent rates at the onset of the crystallization of the corresponding phase based on the size of plagioclase microlites and on the composition of the residual melts the average magma ascent rates of unzen magmas in the pressure range 200 to 50mpa is estimated to be 10 50 m h

linking mid scale distributive fluvial systems to drainage basin area geomorphological and sedimentological evidence from the endorheic gastre basin argentina

the declining but non rejuvenating base level the lisan lake the dead sea area israel laboratory experiments investigating the effects of a lowering base level do not simulate natural eustatic lowering along concave river profiles the field data on this issue are also quite limited in evaluating the control exercised by lowering base level on a drainage network distinction must be made between its influence and those of other hydromorphological processes operating within the basin field data on morphological relations based on photogrammetric mapping and longitudinal profiling have been gathered in the dead sea area israel where a rapid fall in base level has occurred during the holocene the study area is an entrenched fan delta with a sequence of 14 unpaired fan terraces which die out at intersection points within an alluvial fan system the results suggest that the intersection points did not function as base levels the receding base level played only a passive role allowing entrenchment without transmitting a head cutting feedback basinwards the arid environment caused a delay in transmission of information through the system the results support the model of short episodic and discontinuous erosional events inherent in the evolution of drainage basins it is suggested that base level effects in temperate and humid regions are not transferrable to arid zones

early to late pleistocene history of debris flow fan evolution in western death valley california using cosmogenic 10be and 26al abstract debris flow fans with depositional records over several 10 5 years may be useful archives for the understanding of fan construction by debris flows and post depositional surface modification over long timescales reading these archives however requires that we establish the temporal and spatial pattern of debris flow activity over time we used a combination of geomorphic mapping of fan surface characteristics digital topographic analysis and cosmogenic radionuclide dating using 10 be and 26 al to study the evolution of the warm springs fan on the west side of southern death valley california the 10 be concentrations yield dates that vary from 989 43 to 595 17 ka on the proximal fan and between 369 13 and 125 5 ka on distal fan surfaces the interpretation of these results as true depositional ages though is complicated by high inheritance with a minimum of 65 ka measured at the catchment outlet and of at least 125 ka at the distal fan results from the 26 al measurements suggest that most sample locations on the fan surfaces underwent simple exposure and were not affected by complex histories of burial and re exposure this implies that warm springs fan is a relatively stable landform that underwent several 10 5 years of fan aggradation before fan head incision caused abandonment of the proximal and central fan surfaces and deposition continued on a younger unit at the distal fan we show that the primary depositional debris flow morphology is eliminated over a time scale of less than 10 5 years which prevents the delineation of individual debris flows as well as the precise reconstruction of lateral shifts in deposition as we find it on younger debris flow fans secondary post depositional processes control subsequent evolution of surface morphology with the dissection of planar surfaces while smoothing of convex up interfluves between incised channels continues through time

controls on fan development evidence from fan morphometry and sedimentology sierra nevada se spain abstract we have studied the morphology and the morphometric relations between alluvial fans and drainage basins in a bajada system including more than 20 coalescent fans developed since the late pleistocene as a result of the recent uplift of the sierra nevada betic cordillera spain three allocycles of tectonic origin were recognised across which there is a clear evolution from debris flow to sheet flow dominated fans in connection with a decrease in the volume of fines available in the source areas the larger volume of accommodation space created by higher tectonic subsidence in the northern sector favoured vertical accumulation of sediment with the appearance of less elongated fans in the rest of the system where subsidence is less more elongated fans appear with the development of incised channels and irregular distribution of sediment in depositional lobes this involves the appearance of markedly asymmetric transverse profiles as well as lengthy recurrence between the sedimentation events on particular sectors of the fans where headward eroding gullies develop the lithology of the source area where intensely fractured rocks are found is responsible for an important sediment supply and a significant degree of clast sorting from the source area consequently a weak longitudinal trend in particle size can be recognised which influences the predominance of longitudinal constant slope profiles and anomalous relations between both fan area and fan slope and their drainage areas recent intense tectonic activity has caused the appearance of abnormally low slopes in upper sectors of some catchments where mass flows are trapped and the fans present a subsequently higher proportion of sheet flows recent piracy phenomena can be recognised in some of the drainage basins which indicate rejuvenation of the source area in which case the fan presents a rapid increase in the proportion of mass flows with the development of a segmented longitudinal profile due to the recent nature of the processes the morphometric relations in the fan fed by the basin affected by capture do not coincide with the other fans in the area

mechanisms of bubble coalescence in silicic magmas bubble coalescence is an important process that strongly affects magmatic degassing without coalescence bubbles remain isolated from one another in the melt severely limiting gas release despite this fact very little has been done to identify coalescence mechanisms from textures of magmatic rocks or to quantify the dynamics of bubble coalescence in melts in this paper we present a systematic study of bubble coalescence mechanisms and dynamics in natural and experimentally produced bubbly rhyolite magma we have used a combination of natural observations aided by high resolution x ray computed tomography petrological experiments and physical models to identify different types of bubble bubble interaction that lead to coalescence on the timescales of magma ascent and eruption our observations and calculations suggest that bubbles most efficiently coalesce when inter bubble melt walls thin by stretching rather than by melt drainage from between converging bubble walls orders of magnitude are more rapid than melt drainage bubble wall stretching produces walls thin enough that inter bubble pressure gradients may cause the melt wall to dimple further enhancing coalescence to put these results into volcanogical context we have identified magma ascent conditions where each coalescence mechanism should act and discuss the physical conditions for preserving coalescence structures in natural pumice the timescales we propose could improve volcanic eruption models which currently do not account for bubble coalescence although we do not address the effect of shear strain on bubble coalescence the processes discussed here may operate in several different eruption regimes including vesiculation of lava domes post fragmentation frothing of vulcanian bombs and bubbling of pyroclasts in conduits

river terraces and alluvial fans the case for an integrated quaternary fluvial archive abstract the fluvial archive literature is dominated by research on river terraces with appropriate mention of adjacent environments such as lakes despite modern sedimentary basins comprising a significant 88 volume of distributive fluvial systems of which alluvial fans 1 km

climatic versus tectonic controls of fan sequences lessons from the dead sea israel tectonic activity is commonly held responsible for abrupt coarsening of alluvial fan sequences and the role of climate is often relegated the pleistocene fans of the dead sea rift indicate clearly the problems of differentiating the sedimentary signatures of climatic and tectonic adjustments in a closed lake basin exposures are excellent the rivers are currently incising in response to the low lake levels of the holocene producing cliffs 50m high these reveal an interdigitation of lake deposits dominantly fine grained carbonates and the coarse river gravels which characterize the fan deposits the bounds of each facies are sharp river sediments are superimposed on lake sediments with little evidence of either reworking or loading in turn aragonite crusts are draped over alluvial gravels indicating the sudden return of still water conditions neither can be explained by gradual adjustment of the hinterland river system after tectonic shock instead it is clear that the coarse alluvium was deposited by discrete high magnitude floods that occurred only sporadically because of the infrequent nature of rainfall in this arid environment in interpreting basin development in ancient settings it is always tempting to assume that coarse deposits are indicative of tectonism however the dead sea fans demonstrate that climate is just as likely to be responsible for the conglomerates in a sedimentary sequence

relating vesicle shapes in pyroclasts to eruption styles vesicles in pyroclasts provide a direct record of conduit conditions during explosive volcanic eruptions although their numbers and sizes are used routinely to infer aspects of eruption dynamics vesicle shape remains an underutilized parameter we have quantified vesicle shapes in pyroclasts from fall deposits of seven explosive eruptions of different styles using the dimensionless shape factor omega a measure of the degree of complexity of the bounding surface of an object for each of the seven eruptions we have also estimated the capillary number ca from the magma expansion velocity through coupled diffusive bubble growth and conduit flow modeling we find that omega is smaller for eruptions with rm ca ll 1 than for eruptions with ca gg 1 consistent with previous studies we interpret these results as an expression of the relative importance of structural changes during magma decompression and bubble growth such as coalescence and shape relaxation of bubbles by capillary stresses among the samples analyzed strombolian and hawaiian fire fountain eruptions have rm ca ll 1 in contrast to vulcanian plinian and ultraplinian eruptions interestingly the basaltic plinian eruptions of tarawera volcano new zealand in 1886 and mt etna italy in 122 bc for which the cause of intense explosive activity has been controversial are also characterized by rm ca gg 1 and larger values of omega than strombolian and hawaiian style fire fountain eruptions we interpret this to be the consequence of syn eruptive magma crystallization resulting in high magma viscosity and reduced rates of bubble growth our model results indicate that during these basaltic plinian eruptions buildup of bubble overpressure resulted in brittle magma fragmentation

magmatic hydrothermal evolution and devolatilization beneath merapi volcano indonesia abstract at merapi volcano indonesia explosive eruptions lasting several months result from injections of reduced mafic magmas and are interspersed by periods of lava dome growth and collapse quiescent degassing assimilation of wall rock and fractional crystallization lasting a few years silicate melt inclusions and volcanic gases were analyzed to study processes of magmatic volatile exsolution and volcanic degassing during periods of quiescent degassing at merapi volcanic gases were sampled at merapi in 1994 during a phase of quiescent degassing and in 2006 at the end of a dome growth and collapse explosive eruption silicate melt inclusions were collected and analyzed from lavas and scorias for their major element and volatile components solubility relationships between h 2 o and co 2 in melt inclusions demonstrate that melt was being trapped beneath merapi to depths reaching 19 km host amphibole geobarometry indicates crystallization pressures reaching up to 26 km hence there is evidence of deep pooling and crystallization of magma beneath merapi at appreciable depths using a mass balance model of magmatic volatile exsolution we demonstrate that the magma degassing from 19 km to the surface approaches closed system behavior compared to the bulk magmatic volatiles analyzed in melt inclusions volcanic gases had similar amounts of co 2 were enriched in h 2 o and s and depleted in cl and f we thus propose that the magmatic volatile phase was initially exsolved from the magma as a supercritical fluid and that it subsequently exsolved into a h 2 o cl f rich brine and co 2 s rich vapor according to the h 2 o nacl model brine vapor exsolution occurred at 5 km in depth for a 5 wt nacl 900 c to 1000 c fluid although the presence of h 2 s so 2 and co 2 may cause the supercritical fluid to unmix at greater depths magma at shallower depths exsolved a brine and a vapor directly from the melt the volcanic gases at merapi thus represent the vapor fraction of a magmatic volatile phase as brine ponded below the surface we further suggest that carbon dioxide liberated by the magmatic assimilation of limestone as proposed by other studies fluxed the melt thereby promoting co 2 enrichment of the melt and h 2 o degassing finally we propose that merapi volcanic gases are enriched in sulfur compared to the modeled volatile phase composition because deep reduced mafic magma supplies sulfur to the shallow magmatic plumbing system

petrologic constraints on the decompression history of magma prior to vulcanian explosions at the soufriere hills volcano montserrat abstract a series of 88 vulcanian explosions occurred at the soufriere hills volcano montserrat between august and october 1997 conduit conditions conducive to creating these and other vulcanian explosions were explored via analysis of eruptive products and one dimensional numerical modeling of magma ascent through a cylindrical conduit the number densities and textures of plagioclase microlites were documented for twenty three samples from the events the natural samples all show very high number densities of microlites and 50 by number of microlites have areas 2 pre explosion conduit conditions and decompression history have been inferred from these data by comparison with experimental decompressions of similar groundmass compositions our comparisons suggest quench pressures 13 75 mpa each during ascent from chamber to surface values are consistent with field studies of the same events and statistical analysis of explosion time series data the microlite volume number density trend with depth reveals an apparent transition from growth dominated crystallization to nucleation dominated crystallization at pressures of 7 mpa and lower a concurrent sharp increase in bulk density marks the onset of significant open system degassing apparently due to a large increase in system permeability above 70 vesicularity this open system degassing results in a dense plug which eventually seals the conduit and forms conditions favorable to vulcanian explosions the corresponding inferred depth of overpressure at 250 700 m near the base of the dense plug is consistent with depth to center of pressure estimated from deformation measurements here we also illustrate that one dimensional models representing ascent of a degassing crystal rich magma are broadly consistent with conduit profiles constructed via our petrologic analysis the comparison between models and petrologic data suggests that the dense conduit plug forms as a result of high overpressure and open system degassing through conduit walls

an overview of lava dome evolution dome collapse and cyclicity at soufriere hills volcano montserrat 2005 2007 1 the third episode of lava dome growth at soufriere hills volcano montserrat was characterised by higher average magma discharge rates than either previous dome growth episode at this volcano and yet fewer collapses during sustained dome growth at moderate high average rates 6 m3 s we identified 2 6 week discharge pulses that each supplied c 20 mm3 magma from depth our observations are consistent with some existing models but we explain discrepancies by a combination of higher volatile contents and higher ascent rates cycles of c 11 16 days were evident in rockfall lp rockfall and shallow lp earthquake counts related to dome growth and degassing we speculate that degassing at the conduit margins together with stick slip conduit flow may drive these cycles only one major collapse 10 mm3 occurred during the third episode on may 20 2006 as a new magma pulse entered the dome and coincided with heavy rainfall

the impact of quaternary sea level and climatic change on coastal alluvial fans in the cabo de gata ranges southeast spain conventionally a fall in base level is seen as stimulating incision into the distal zones of alluvial fans in the cabo de gata ranges of southeast spain evidence exists to the contrary two sets of quaternary coastal alluvial fans demonstrate the interaction between climatically driven variations in the supply of sediment and eustatically driven changes in base level the fans are supplied from miocene volcanic terrain within which no evidence can be found for major tectonic deformation during the period of fan development the evolution of the east coast fans has been affected by variations in sediment supply and changes in sea level the west coast fans were buffered from the effects of changes in sea level by coastal barriers three phases of past sedimentation can be identified on the fans these can be differentiated on the basis of field observations of soil profiles particularly colour of the b horizons and accumulation of caco3 and laboratory analyses of sequential iron oxide extractions and magnetic mineral properties the two earlier major sedimentation phases were coincident with global glacials ca 135 ka and ca 85 10 ka based on the stratigraphy and uranium thorium dating of the coastal sediments high sea levels during the intervening interglacial and during the holocene caused erosion of the distal zones of the east coast fans which led to channel incision into the fan surfaces on the west coast fans no such incision occurred simply proximal incision by small fanhead trenches the youngest relatively minor phase of fan sedimentation has occurred during the holocene these contrasting contexts have produced differing styles of fans with telescopic fan morphology on the east coast and stacked morphology on the west coast fans the differences are reflected in the fan profiles with steeper gradients dominating the east coast fans and extensive lower gradient distal surfaces on the west coast fans fan morphometry based on analysis of the residuals from drainage area to fan area and gradient regressions also differentiates between the fan contexts the fan building phases appear to be controlled proximally by climatically driven pulses of sediment supplied to the fans these occurred during global glacials coincident with low sea levels and caused fan progradation onto the exposed foreshore the intervening global interglacials were times of little fan sedimentation and on the east coast where high sea levels were able to erode the fan toes deep through fan dissection ensued

mnd microlite number density water exsolution rate meter abstract microlites in effusive or pyroclastic rocks are possible indicators of water exsolution in particular the microlite number density mnd is considered to be a function of the rate of water exsolution from melt in this paper we have constructed a mnd water exsolution rate meter based on the recent results of theory experiments and the natural observation of crystallization kinetics the mnd method accounts for the effects of melt composition and water content on the diffusivity of crytallizing components in melt by using this meter we can estimate the water exsolution rate at the microlite nucleation depth from a mnd value provided the crystal phase plagioclase or clinopyroxene is known we applied the meter to the case of the 1991 1995 dome eruption at unzen and the 1986b subplinian eruption at izu oshima we obtained the water exsolution rates in the range of 6 1 10 6 to 2 8 10 5 wt s approximately at 70 mpa for unzen plagioclase mnd 1014 to 1015 m 3 and 1 1 10 3 to 1 1 10 1 wt s for izu oshima pyroxene mnd 1015 to 1017 m3 under the assumption of equilibrium vesiculation and steady state flow the corresponding decompression rate and ascent velocity are calculated in the range of 240 to 1100 pa s and 0 014 to 0 068 m s for unzen and 1 2 104 to 1 3 106 pa s and 1 3 to 133 m s for izu oshima this contrast in the ascent velocity at the microlite nucleation depth is closely related to the reason why unzen and izu oshima revealed the different eruption styles namely dome growth and explosive eruption respectively

a nanolite record of eruption style transition microlite characteristics have been successfully used as indicators of magma decompression rate however microlites do not record the explosive effusive transitions in sub plinian eruptions when such transitions are governed by the decrease of exsolved volatile pressure in the shallow conduit rather than by the magma ascent rate to overcome this limitation we studied the nanolites in the quenched products of the 2011 shinmoedake japan eruption nanolites are nanometer scale components of the groundmass minerals and exhibit a steeper slope of crystal size distribution than that of microlites in the 2011 eruption the style of activity had undergone transformation from sub plinian eruption to vulcanian explosion and intermittent effusion of lava we found that although the products formed by different eruptive activities have similar microlite characteristics such products can be distinguished clearly by their mineral assemblage of nanolites the samples of pumice of sub plinian eruptions and vulcanian explosions and the dense juvenile fragments of lava in descending order of explosivity contained respectively nanolites of low ca pyroxene low ca pyroxene plagioclase and low ca pyroxene plagioclase fe ti oxides nanolites crystallize when undercooling of the magma due primarily to dehydration increases rapidly near the surface the water contents of the interstitial glass indicate that the quenched depths did not differ greatly between eruption styles hence the different nanolite assemblages of each eruption style are assumed to have resulted from differences in magma residence time near the surface therefore we propose that nanolites have the potential to indicate the physicochemical conditions of magma at the transition points of eruption styles

shoreline processes and the age of the lake lahontan highstand in the jessup embayment nevada the well developed shoreline record of pluvial lake lahontan in the jessup embayment nevada is used to refine the history of late pleistocene lake level fluctuations and to assess controls on shoreline development and distribution controls on the strength and type of shorelines developed include local slope the amount and characteristics of sediment available for transport the availability of accommodation space and length of time the lake level resides at a particular shoreline elevation at the sehoo highstand and during the early part of the regression strong storm winds and waves from the south southeast set up a clockwise net shore drift pattern near the head of the embayment although significant differences in local slope geometry of the shoreline and wave energy existed in the embayment crestal heights of constructional shoreline features formed at the highstand vary 2 6 m in elevation and hence provide a relatively precise marker of the highstand elevation radiocarbon dating of a camel bone preserved in high shoreline deposits indicates that the lake reached its highest elevation of 1338 5 m in the embayment and receded from that elevation immediately prior to 13 070 60 yr b p similar and slightly older radiocarbon ages on gastropod shells preserved in barrier deposits at 1327 m 13 280 110 yr b p and 1331 m 13 110 110 yr b p suggest that the final rise to the highstand was very rapid and that the lake maintained its highest stand for a very brief period of time perhaps only for years or decades the brevity of the highstand is reasonable in light of the recent formation of similar barrier features in modern pyramid lake which formed in less than seven months due to a rapid increase in lake level

magnetite scavenging and the buoyancy of bubbles in magmas part 1 discovery of a pre eruptive bubble in bishop rhyolite little is known about the presence distribution and size of bubbles in rhyolitic magmas prior to eruption using x ray tomography to study pumice from early erupted bishop rhyolite we discovered a large vesicle with abundant magnetite crystals attached to its walls attachment of magnetite crystals to bubble walls under pre eruptive conditions can explain the cluster of magnetite crystals as a result of bubbles rising and collecting magnetite crystals alternatively bubbles may have nucleated on magnetite crystals and then coalesced to form one large bubble with multiple magnetite crystals attached to it we argue that the clusters of magnetite crystals could not have formed during or after eruptive decompression and conclude that this vesicle corresponds to a bubble present prior to eruptive decompression the inferred presence of pre eruptive bubbles in the bishop magma confirms the interpretation that the magma was volatile saturated prior to eruption the pre eruptive size of this bubble is estimated based on three independent approaches 1 the current size of the vesicle 2 the total cross sectional area of the magnetite crystals and 3 the bubble size required for the aggregate to be neutrally buoyant these approaches suggest a pre eruptive bubble 300 850 m in diameter with a preferred value of 600 750 m

crystallization of mount st helens 1980 1986 dacite a quantitative textural approach quantitative measurements of crystal size distributions csds have been used to obtain kinetic information on crystallization of industrial compounds randolph and larson 1971 and more recently on hawaiian basalts cashman and marsh 1988 the technique is based on a population balance resulting in a differential equation relating the population densityn of crystals to crystal sizel i e at steady staten no exp l itg whereno is nucleation density g is the average crystal growth rate is the average growth time and the nucleation ratej nog csd inn vsl plots of plagioclase phenocrysts in 12 samples of mount st helens blast dacite and 14 samples of dacite from the 1980 1986 mount st helens dome are similar and averageg 9 6 1 1 10 3 cm andno 1 2 106 cm 4 reproducibility of the measurements was tested by measuring csds of 12 sections cut from a single sample in three mutually perpendicular directions precision of the size distributions is good in terms of relative but not necessarily absolute values 10 growth and nucleation rates for plagioclase have been calculated from these measurements using time brackets of 30 150 years growth ratesg are 3 10 10 12cm s and nucleation ratesj are 5 21 10 6 cm3 s g andj for fe ti oxides calculated from csd data areg 2 13 10 13 cm sec andj 7 33 10 5 cm3 s respectively the higher nucleation rate and lower growth rate of oxides resulted in a smaller average crystal size than for plagioclase sizes of plagioclase microlites 0 01 mm in the blast dacite groundmass have been measured from backscatter sem photographs nucleation of these microlites was probably triggered by intrusion of material into the cone of mount st helens in spring 1980 this residence time of 52 days gives minimum crystallization estimates ofg 1 3 10 11 cm s andj 9 16 1o3 cm3 s the skeletal form of the microlites provides evidence for nucleation and growth at high values of undercooling t relative to the phenocrysts a comparison of nucleation and growth rates for the two crystal populations phenocrysts vs microlites suggests that while growth rate seems to be only slightly affected by changes in t nucleation rate is a very strong function of undercooling a comparison of plagioclase nucleation and growth rates measured in the mount st helens dacite and in basalt from makaopuhi lava lake in hawaii suggests that plagioclase nucleation rates are not as dependent on composition groundmass textures suggest that plagioclase microphenocrysts crystallized at depth rather than in the conduit in the dome or after extrusion onto the surface most of this crystallization appears to be in the form of crystal growth coarsening of groundmass microphenocrysts at small degrees of undercooling rather than extensive nucleation of new crystals this continuous crystallization in a shallow magmatic reservoir may provide the overpressurization needed to drive the continuing periodic domebuilding extrusions which have been the pattern of activity at mount st helens since december 1980

strain localization in vesicular magma implications for rheology and fragmentation the rheology of two or three phase magmas has been the focus of much interest because it controls magma ascent and eruption behavior research on magma rheology has typically considered homogeneous fl ow here we demonstrate based on natural examples that strain resulting from viscous fl ow preceding explosive fragmentation localizes into shear zones at a microscopic scale strain localization affects the rheological behavior of magmas modifying predictions based on experiments localization leads to high strain rate shear zones where elongated sheared vesicles and shear heating have a weakening effect surrounding regions of relatively low strain rate where subequant vesicles have a strengthening effect thus energy is dissipated more effi ciently into localized bands where strain rate increases through feedback effects and can lead to melt fragmentation

reconciling the roles of tectonism and climate in quaternary alluvial fan evolution tectonism and climate are the primary variables considered in conceptual models of alluvial fan evolution and basin fill architecture yet their roles and relative importance are far from resolved the madison range and madison river valley southwestern montana were affected during late quaternary time by varying degrees of tectonic activity as well as by climate change through multiple glaciations we are thus able to evaluate the impact of tectonism and climate on fan evolution two primary fan deposits were correlated along the range front they were deposited as proglacial outwash fans during glacial periods of increased sediment and water discharge during the present interglacial period these deposits were entrenched and secondary fans formed on lower fan areas temporal and stratigraphic relations indicate that climate was largely responsible for driving both fan aggradation and entrenchment in contrast stratigraphic relations between fan deposits and surface faults do not support a direct relation either spatial or temporal between local faulting and fan deposition because tectonism produces and maintains the relief necessary for fans to form its primary role is long term controlling the duration over which fan deposition may occur along a mountain front if quaternary alluvial fan evolution is representative of ancient alluvial fan evolution this study has important implications for interpreting primary controls on deposition of the ancient alluvial fan sequences

influence of the fragmentation process on the dynamics of vulcanian eruptions an experimental approach abstract the dynamics of magma fragmentation is a controlling factor in the behavior of explosive volcanic eruptions if porous magma is sufficiently decompressed a fragmentation front develops and travels through the magma at a certain speed fragmentation speed while the resulting particles are being ejected to investigate the influence of the fragmentation process on eruption dynamics we have performed fragmentation experiments in a shock tube apparatus using natural volcanic samples with diverse porosities and different applied pressures 4 20 mpa for each experiment we simultaneously measured the fragmentation speed and ejection velocities the results are consistent with a theoretical model based on a 1 d shock tube theory considering the conservation laws across the fragmentation front our results show that a certain pressure threshold has to be exceeded for fragmentation and ejection of the particles to take place and that the fragmentation speed determines the initial conditions of the expansion of the gas particle mixture the fragmentation process has a controlling influence on the velocity density and mass discharge rate per unit area of the gas particle mixture all factors which can affect eruption dynamics significantly the model presented herein may help describe the dynamics of vulcanian eruptions and improve hazard assessment

complex dynamics of small moderate volcanic events the example of the 2011 rhyolitic cordon caulle eruption chile after decades of repose puyehue cordon caulle volcano chile erupted in june 2011 following a month of continuously increasing seismic activity the eruption dispersed a large volume of rhyolitic tephra over a wide area and was characterized by complex dynamics during the initial climactic phase of the eruption 24 30 h on 4 5 june 11 14 km high plumes dispersed most of the erupted tephra eastward towards argentina reaching as far as the atlantic ocean this first eruptive phase was followed by activity of lower intensity leading to the development of a complex stratigraphic sequence mainly due to rapid shifts in wind direction and eruptive style the resulting tephra deposits consist of 13 main layers grouped into four units each layer was characterized based on its dispersal direction sedimentological features and on the main characteristics of the juvenile fraction texture density petrography chemistry the lowest part of the eruptive sequence unit i corresponding to the tephra emitted between 4 and 5 june is composed of alternating lapilli layers with a total estimated volume of ca 0 75 km3 these layers record the highest intensity phase during which a bent over plume dispersed tephra towards the southeast east with negligible up wind sedimentation products emitted during 5 6 june unit ii signaled an abrupt shift in wind direction towards the north leading to the deposition of a coarse ash deposit in the northern sector ca 0 21 km3 in volume followed by a resumption of easterly directed winds a third phase unit iii began on 7 june and resulted in tephra deposits in the eastern sector and ballistic bombs around the vent area a final phase unit iv started after 15 june and was characterized by the emission of fine grained white tephra from ash charged plumes during low level activity and the extrusion of a viscous lava flow timing and duration of the first eruptive phases were constrained based on comparison of the dispersal of the main tephra layers with satellite images showing that most of the tephra was emitted during the first 72 h of the event the analyzed juvenile material tightly clusters within the rhyolitic field with negligible chemical variations through the eruptive sequence textural observations reveal that changes in eruption intensity and consequently in magma ascent velocity within the conduit and complex interactions between gas rich and gas depleted magma portions during ascent resulted in vesicular clasts with variable degrees of shear localization and possibly in the large heterogeneity of the juvenile material

bnd bubble number density decompression rate meter for explosive volcanic eruptions abstract this decade has been witness to substantial progress in the understanding of bubble dynamics and its role in volcanic eruptions a comparison between the results of experiments and a model for bubble nucleation shows that bubble number density bnd can be calculated as a function of decompression rate for given physical properties such as the diffusivity of water in a silicate melt and interfacial tension within an acceptable margin of error conversely in this paper we propose a method hereafter referred to as bnd decompression rate meter to estimate the decompression rate by using bnd data of natural pumice samples from volcanic eruptions as a result of the application of the bnd decompression rate meter to pumice and scoria from explosive eruptions it is found that the decompression rates increase in the range from 10 6 to 10 8 pa s with the eruption column heights these rates are proportional to the 4th power of the eruption column heights and are linear to the discharge rates the absolute values of the estimated decompression rates are very high and cannot be explained by the existing models of conduit flows in order to explain such high rates of decompression and the correlation with discharge rate we propose a possible model according to which the bubble nucleation propagates downward as a rarefaction shock wave in the conduit

lithofacies and depositional environments of miocene deposits from tectonically controlled basins red river fault zone northern vietnam abstract the lao cai yen bai and bao yen basins connected with the red river fault zone rrfz are filled with over 1000 m thick neogene miocene clastic deposits of different type and origin coarse grained alluvial deposits predominate along the south western margin of the lao cai and yen bai basins while different types of conglomerates and sandstones even siltstones occur in their central and along their north eastern parts such asymmetry in the cross section basin fill points to the south western active margin of these basins three main facies associations composed of 12 lithofacies have been recognized in the miocene succession filling the studied basins 1 debris flow dominated alluvial fan association 2 sheetflood dominated alluvial fan association and 3 lacustrine association both alluvial fan facies associations are broadly spread in the studied basins especially along their south western margins the debris flow dominated deposits are represented by decimetre to several meters thick sand and mud supported disorganized pebble to cobble size occasionally boulder breccia and conglomerate beds the sheetflood dominated deposits are represented by alternating planar bedded pebble and cobble gravel couplets the occurrence of conglomerate bodies within and overlapping fine deposits of the lacustrine facies association may suggest deposition in a debris flow and sheetflood dominated alluvial fan setting associated with narrow elongated basins that could be occupied by lakes and bordered by active fault scarps the facies pattern of the particular basins is connected with the evolution of the rrfz the origin of these basins may be correlated with a sinistral transtensional regime which occurred in the early to middle miocene moreover the high grade metamorphic core of the day nui con voi massif presently the main structural element of the rrfz was probably not exposed during sedimentation of most of the strata in the studied basins

influence of lithologic erodibility on alluvial fan area western white mountains california and nevada differences in lithologic erodibility and sediment storage within a drainage basin affect the relationship between alluvial fan area and drainage basin area along the western white mountains large fans are produced by basins underlain by resistant rocks which have steep and narrow trunk stream canyons with little sediment in storage small fans are produced by basins composed of erodible lithologic units which have wider valley floors lower valley side slopes and considerably more sediment stored along trunk stream canyons than is the case in basins underlain by resistant rocks

subvolcanic plumbing systems imaged through crystal size distributions the configuration of subvolcanic magma storage regions exercises a fundamental control on eruptive style and hazard such regions can be imaged remotely using seismic geodetic or magnetotelluric methods although these are far from routine and rarely unambiguous the textures of erupted volcanic rocks as quantified through crystal size distributions csd provide space and time integrated information on subvolcanic plumbing systems although these data cannot be used readily for reconstruction of key parameters such as conduit geometry or magma chamber depth here we develop a numerical approach to interpretation of csd in products of steady eruptions based on crystallization kinetics and hydrodynamic flow simulation to image subvolcanic plumbing systems the method requires knowledge of magma properties crystal growth kinetics measured experimentally and discharge rate measured observationally the method is applicable to steady state eruptive regimes distributions of pressure temperature crystal content and conduit cross section area with depth are obtained from a csd from a sample erupted from mount st helens volcano usa values of average conduit diameter 30 m and magma chamber depth 14 km below the summit are in good agreement with independent estimates

development and maintenance of a telescoping debris flow fan in response to human induced fan surface channelization chalk creek valley natural debris flow laboratory colorado usa abstract channel change has been a constant theme throughout william l graf s research career graf s work has examined channel changes in the context of natural environmental fluctuations but more often has focused on quantifying channel change in the context of anthropogenic modifications here we consider how channelization of a debris flows along a bajada has perpetuated and sustained the development of telescoping alluvial fan two dimensional debris flow modeling shows the importance of the deeply entrenched channelized flow in the development of a telescoping alluvial fan gis analyses of repeat five different debris flows high resolution 5 cm digital elevation models dems generated from repeat terrestrial laser scanning tls data elucidate sediment and topographic dynamics of the new telescoping portion of the alluvial fan the embryonic fan flow constriction from channelization helps to perpetuate debris flow runout and to maintain the embryonic fan and telescoping nature of the alluvial fan complex embryonic fan development in response to five debris flows proceeds with a major portion of the flows depositing on the southern portion of the embryonic fan the third through the fifth debris flows also begin to shift some deposition to the northern portion of the embryonic the transfer of sediment from a higher portion of the embryonic fan to a lower portion continues currently on the embryonic fan while channelized flow has been shown to be critical to the maintenance of the telescoping fan the flow constriction has led to higher than background levels of sediment deposition in chalk creek a tributary of the arkansas river a majority of the sediment from each debris flow is incorporated into chalk creek as opposed to being stored on the embryonic fan

response of alluvial fan systems to the late pleistocene to holocene climatic transition contrasts between the margins of pluvial lakes lahontan and mojave nevada and california usa abstract dated shorelines of late pleistocene pluvial lakes lahontan great basin desert northwest nevada and mojave mojave desert eastern california provide timelines for the assessment of alluvial fan sedimentation at the lake margins during the late pleistocene to early holocene two sets of alluvial fan systems have been mapped the stillwater fans feeding lake lahontan and the zzyzx fans feeding lake mojave their contrasting morphologies suggest different responses of the two fan systems to late pleistocene to early holocene climatic change at the time the stillwater fan systems underwent minimal sedimentation with the catchment hillslopes apparently stable the zzyzx fans experienced major changes in water and sediment supply from the catchment hillslopes there was a major phase of hillslope debris flow activity followed by fanhead trenching and distal fan progradation both areas were wetter and colder in the late pleistocene than they are today but during the transition to the holocene the zzyzx area was more likely to experience intense rains associated with the monsoonal penetration of warm moist tropical air into the southwest vegetation reconstructions for the late pleistocene to the early holocene suggest that catchment hillslopes in the mojave supported a desert shrub vegetation but those in the stillwaters supported juniper woodland and grasses at low elevations and pine at higher elevations contrasts in hillslope vegetation cover together with storm activity may account for the different responses of the alluvial fans to climatic change during the pleistocene to holocene climatic transition after the falls in lake levels of lakes lahontan and mojave in the early holocene both areas underwent aridification resulting in reductions in hillslope vegetation cover increased storm runoff led to fanhead trenching and distal progradation of the alluvial fans variations in fan style at that time may relate primarily to base level conditions resulting from different gradients on the exposed lake shores

the role of base level change in the dissection of alluvial fans case studies from southeast spain and nevada abstract tectonics climate and base level are the underlying controls of alluvial fan development on many fans base level is stable but where base level change occurs it may itself be a response to tectonics or climatic change tectonically induced base level change will be temporally independent of climatic change and will show spatial variability depending on the rate of propagation of tectonically induced regional dissection this is illustrated by the tabernas fans southeast spain the timing of climatically induced base level change follows the timing of climatic change itself and will show more consistency of spatial relationships within groups of fans affected alluvial fans which toe out in coastal zones or at lake shorelines may be influenced by climatically controlled base level changes conventionally a fall in base level would be expected to cause dissection of the distal fan zone however the reverse may occur with fan progradation taking place in response to a base level fall and dissection occurring in response to a base level rise two contrasted case studies are considered based in part on previously published work from dry region alluvial fan zones the coastal cabo de gata fans in southeast spain and the stillwater and cold springs fans in nevada usa in these areas base level changes occurred in response to late quaternary eustatic changes of sea level and changes in the levels of pluvial lakes respectively within both areas there are fans not affected by base level change cabo de gata west coast and inland fans cold springs fans and others where base level change has been important in fan evolution cabo de gata east coast fans stillwater fans the differences in geomorphic regime between those fans influenced by base level changes and those not are expressed in morphometric contrasts and different fan profile and plan characteristics within those fans influenced by base level changes the different mechanisms have influenced the location and elevation of the zone of incision the differences can be attributed in part to the temporal interactions of base level change with climatically induced changes in sediment supply and in part to differences in gradients of the foreshore exposed by the falling sea or lake base levels

effect of syneruptive decompression path on shifting intensity in basaltic sub plinian eruption implication of microlites in yufune 2 scoria from fuji volcano japan abstract to constrain the timing and conditions of syneruptive magma ascent that are responsible for shifting eruption intensity we have investigated a basaltic sub plinian eruption that produced yufune 2 scoria in fuji volcano 2200 years ago we deduced magmatic decompression conditions from groundmass microlite textures including decompression path i e evolution in decompression rate and approximate decompression rate in order to relate them to eruption intensity the microlites revealed decompression conditions after water saturation at 700 1100 m depth the temporal change in scoria size indicates that the magma discharge rate and resultant eruption intensity increased from unit a to unit b and then declined toward ending units d and e the overall decompression rate in each eruptive unit has a positive correlation with eruption intensity the variation in decompression rate was enlarged in the final units where the maximum remained the same as the peak through the eruption 0 13 0 22 mpa s for units b and c while the minimum was 0 025 mpa s the large variation here is due to 1 variation in flow velocity across conduit and 2 part of the erupted magma in unit d experienced remarkably slow decompression 0 002 0 003 mpa s resulting from decreased overpressure in the reservoir following the major eruption of unit b furthermore crystal size distribution csd of microlites implied that the earliest erupted magma unit a had once been decompressed slowly 0 005 0 012 mpa s having been arrested by material in the conduit vent system which was followed by an increase in decompression rate due to removal of the material at the initiation of the eruption in addition the magma that had been ascending slowly before the unit d eruption may record the increase in decompression rate this increased rate resulted from being pushed up by the successive magma at the start of that eruption two factors had a major impact on eruption intensity first magma decompression rate determined the degree of gas phase separation from ascending magma judging from csd different decompression rates had been generated at least at the start of microlite crystallization the second factor is the conduit radius that in combination with magma ascent rate controlled the magma discharge rate before the major eruption of unit b the conduit radius likely increased as evidenced by xenoliths of basaltic lava and lithic fragments with the same petrography as the xenoliths in unit a in unit e the conduit radius decreased through inward development of high density magma from the conduit margin

differential features of alluvial fans controlled by tectonic or eustatic accommodation space examples from the betic cordillera spain abstract the creation of accommodation space in subaerial sedimentary environments is closely linked to uplift the source area along basin margin faults or to a rise in base level depending amongst others on the relative importance of these two factors the margins of terrestrial sedimentary basins where alluvial fans develop produce aggradational progradational or retrogradational stratigraphic architecture we studied a total of 43 quaternary alluvial fans in the granada and bajo segura basins betic cordillera spain the analysis of their stratigraphic architecture and sedimentary facies has revealed the existence of three groups characterised by distinct stratal stacking patterns which is the result of differences in eustasy and tectonics the qualitative and quantitative analysis of these alluvial fans and their drainage basins has allowed us to define certain characteristic geomorphological patterns related in each case to the main mechanism operative in the creation of accommodation space these patterns are described as follows 1 alluvial aggradation is the main result of high tectonic subsidence with the development of quite open but small fans with a constant steep slope lacking incised channels and headward eroding gullies 2 in contrast when the principal mechanism in creating accommodation space is low tectonic subsidence progradation occurs and the fans are narrower and larger moreover the slopes of such fans are less steep in relation to the drainage area than in the above case being constant or even concave in longitudinal profile they also have short deep incised channels culminating downfan in a depositional lobe in inactive zones long deep headward eroding gullies develop 3 areas of extremely low tectonic subsidence and base level rise produce very open fans in the sedimentary basin although with a pronounced mountain embayment which gives them a characteristic mushroom shape in plan view in the latter case retrogradation is the typical stratigraphic stacking pattern and the fans are quite extensive with respect to their drainage areas usually showing a segmented longitudinal profile with a low gradient the feeder channels do not surpass stream order 3 and various extremely long and shallow incised channels can appear 4 contrary to the case of fans developing under a regime of subsidence when the accommodation is due to base level rise the fan slopes bear no relation to the extent of the catchment basins in such a case the overall fan slope is strongly influenced by the extent of the mountain embayment because it determines the distributions of debris and sheet flows over the alluvial surface

the vesicular layer and carbonate collars of desert soils and pavements formation age and relation to climate change abstract the vesicular fine grained a horizon av is the widespread ubiquitous surficial horizon of desert soils in diverse landforms and parent materials of varying ages now known to form mostly through accumulation of eolian dust recent studies show that dust accumulation and concomitant soil development are genetically linked to stone pavement formation changes in the magnitude of eolian activity and effective leaching related to quaternary climatic changes are also hypothesized to have influenced the evolution of the av horizon numerical modeling geochronologic and field laboratory studies elucidate the nature of pedogenic processes controlling compositional evolution of av how the changing av horizon increasingly influences soil infiltration and carbonate translocation and accumulation and the control that clasts of the evolving pavement exert on pedogenic processes results of a model that determines soil bulk chemical composition based on mixing of estimated proportions of externally derived eolian material and parent materials imply that the evolution of the soil bulk composition is strongly influenced by av horizon formation the early development of a weakly to moderately developed av horizon directly over gravelly parent material in late and middle holocene soils moderately influences soil infiltration but significant leaching of very soluble materials and some carbonate in dust are permitted in older pleistocene soils however the texturally more mature av and underlying cumulic nongravelly horizons more strongly limit the rate and depth of leaching and soil bulk composition therefore more closely approximates a simple mixture of dust and parent material other aspects of av horizon development and its relations to the pavement are evaluated through studies of pavement clasts with coatings of soil carbonate referred to as carbonate collars development of a numerical model that integrates soil hydrology a co2 production diffusion model calcite kinetics and thermodynamic considerations composition and thermal characteristics of pavement clasts and the textural and structural properties of the surface horizon provides the basis for testing a hypothesis of collar formation model results combined with results of 13c and 18o analyses of collar carbonate demonstrate how precipitation of calcite on pavement clasts and within the av is favored at a depth much shallower than that indicated by the classic carbonate depth climate relationship of jenny and leonard jenny h j leonard c d 1935 functional relationships between soil properties and rainfall soil science 38 363 381 and arkley arkley r j 1963 calculations of carbonate and water movement in soil from climatic data soil science 96 239 248 or simulated by numerical models of carbonate accumulation simultaneous development of thick carbonate collars and the av horizon requires the sustained pavement clast av horizon coupling for at least centuries to possibly millennia new thermoluminescence ages also indicate that much of the av horizon formed in the holocene and that it is certainly much younger than the older pleistocene pavements this supports the previously proposed hypothesis that increased dust flux during the pleistocene to holocene transition triggered and or greatly accelerated av horizon development an understanding of the genesis of collars provides not just an understanding of how carbonate can accumulate in surface environments but it also provides important clues into processes of pavement evolution and preservation of av horizons during long glacial periods the av horizon is not merely an insignificant surficial zone of recent dust accretion instead its development profoundly influences the genesis of desert soils and pavements

a numerical modelling and experimental study of flow width dynamics on alluvial fans alluvial fans are dynamic landforms the evolution of which is controlled by both external environmental forcing climate tectonics and base level change and internal process form feedbacks the latter include changes in flow configuration between sheetflow and channelized flow states driven by aggradation and degradation which may in turn promote changes in sediment transport capacity recent numerical modelling indicates that such feedbacks may lead to dramatic and persistent fan entrenchment in the absence of external forcing however the parameterization of flow width within such models is untested to date and is subject to considerable uncertainty this paper presents results from an experimental study of flow width dynamics on an aggrading fan in which spatial and temporal patterns of fan inundation are monitored continuously using analysis of digital vertical photography observed flow widths are compared with results from a simple theoretical model developed for non equilibrium aggradational conditions results demonstrate that the theoretical model is capable of capturing the first order characteristics of width adjustment over the course of the experiment and indicate that flow width is a function of fan aggradation rate this illustrates that models of alluvial flow width derived for equilibrium conditions may have limited utility in non equilibrium situations despite their widespread use to date

experimental study on evolution of lacustrine shallow water delta abstract in the process of delta evolution the flow regime and sediment transport change with the development of topography flow is the dynamic factor of shaping topography while sediment scouring and silting is the direct cause of topography formation and they are constrained and influenced by topographic changes simultaneously in order to explore rules of water and sediment movement and topographic sedimentation of delta under microscale conditions we studied the phenomena rules and evolution mechanism of flow movement sediment transport particle sorting topographic sedimentation and flow path generation merger and recombination under microscale conditions from the perspective of physical experiment the influence of different external conditions on lacustrine shallow water delta evolution and the self organization of delta evolution are studied through comparative experiments under the condition of distinct sediment supplied rates or lake levels and repeatability experiments under the same conditions some rules of flow movement sediment transport and topographic sedimentation such as characteristics of each stage during the process of evolution of lacustrine shallow water delta the periodic change of flow regime and the prediction model the characteristics of sedimentary morphology under different conditions and the influence of stochastic factors on delta evolution are summarized the experiments suggest coupled linkage of interaction of water and sediment self organization of flow path evolution and the equilibrium tendency of delta structure under the action of coupling interaction of multiple factors of water sediment and topographic sedimentation

an experimental investigation of autogenic behaviour during alluvial fan evolution the findings from a series of laboratory experiments designed to examine the role of autogenic process form interactions as controls on alluvial fan evolution are presented the experimental approach follows the similarity of processes concept and is therefore not scaled to a real world prototype the experiments were conducted in the university of exeter sediment research facility using a 3 3 m fixed bed experimental table with controlled sediment and water supply fed to the fan apex and evacuation of all sediment that passes the distal boundary the evolution of the fan was recorded using high resolution time lapse overhead photography to permit analysis of fan topography flow patterns channel migration and avulsion on the fan surface over the course of the experimental run the qualitative observations from three experimental scenarios are discussed and evaluated in the context of current understanding of alluvial fan evolution these demonstrate that in the absence of any change in the extrinsic variables there is an autogenic transition from sheetflow to channelised flow as the experimental fans evolve this is consistent with theory that predicts this change in response to a decline in aggradation rate through time

textures water content and degassing of silicic andesites from recent plinian and dome forming eruptions at mount pelee volcano martinique lesser antilles arc previous petrological and phase equilibrium experimental studies on recent silicic andesites from mount pelee volcano have evidenced comparable pre eruptive conditions for plinian and dome forming pelean herein eruptions implying that differences in eruptive style must be primarily controlled by differences in degassing behaviour of the mount pelee magmas during eruption to further investigate the degassing conditions of plinian and pelean magmas of mount pelee we study here the most recent mount pelee s products p1 at 650 years b p 1902 and 1929 eruptions which cover a range of plinian and pelean lithologies for bulk rock vesicularities glass water contents glass inclusions in phenocrysts and matrix glasses and microtextures water contents of glass inclusions are scattered in the plinian pumices but on average compare with the experimentally deduced pre eruptive melt water content i e 5 3 6 3 wt whereas they are much lower in the dominant pelean lithologies crystalline poorly vesicular lithics and dome samples this indicates that the glass inclusions of the pelean products have undergone strong leakage and do not represent pre eruptive water contents the water content of the pyroclast matrix glasses are thought to closely represent the residual water content in the melt at the time of fragmentation determination of the water contents of both the pre eruptive melt and matrix glasses allows the estimation of the amount of water exsolved upon syn eruptive degassing we find the amount of water exsolved during the eruptive process to be higher in the pelean products than in the plinian ones typically 90 100 and 65 70 of the initial water content respectively the vesicularities calculated from the amount of exsolved water compare with the measured vesicularities for the plinian pumices consistent with a closed system near equilibrium degassing up to fragmentation by contrast the low residual water contents low groundmass vesicularities and extensive groundmass crystallization of the pelean products are direct evidence of open system degassing microtextural features including silica bearing and silica free voids in the pelean lithologies may represent a two stage vesiculation

lateral erosion toe cutting of alluvial fans by axial rivers implications for basin analysis and architecture abstract we document the neglected phenomenon of lateral erosion toe cutting of alluvial fans by non incising axial river channels field examples from the holocene of the big lost river basin idaho and the plio pleistocene of the rio grande rift new mexico help to establish architectural models with more general application to basin analysis the process of toe cutting can lead to complete fan destruction and may be a response to climate change tectonic tilting fault propagation or a combination of these variables it gives rise to near horizontal erosion surfaces cut in fan sediment steep fan margin scarps progressive up fan incision from the scarp by a network of channels soil formation up fan away from the incised channel network a deposit of axial alluvium that overlies the erosion surface and onlaps the scarp once avulsion occurs to take the axial channel away from the bajada margin distinctive healing wedges of fan alluvium prograde across abandoned axial river channel and floodplain deposits gradually onlapping the eroded scarp and its upstream network of incised channels toe cutting has important stratigraphic basin analysis and economic consequences bajada deposits subject to the process exhibit appreciable extra groundwater and petroleum reservoir potential in the intercalations of more porous and permeable axial fluvial sediments

heterogeneous bubble nucleation in highly viscous silicate melts during instantaneous decompression from high pressure abstract the nucleation of h 2 o bubbles in magmas has been proposed as a trigger for volcanic eruptions to determine how bubbles nucleate heterogeneously in silicate melts experiments were carried out in which high silica rhyolitic melts were hydrated at 800 c and either 50 or 125 mpa and then decompressed by 20 91 mpa at temperatures that ranged between 550 and 700 c and held at the lower pressures for 10 720 s before being quenched bubbles nucleated in number densities n b that vary between 3 10 7 and 2 10 8 cm 3 blocky shaped magnetite or the ends of needle shaped hematite acted as sites for nucleation but only if a minimum super saturation was exceeded which increases with increasing melt viscosity bubbles did not nucleate along the lengths of hematite needles nor on plagioclase both the beginning and ending times of the nucleation event increases with increasing melt viscosity using nucleation theory predictions neither the slower nucleation rates nor the changing activation of nucleation sites can be adequately explained by the differences in temperature water diffusivity or viscosity instead the variations in nucleation kinetics are best explained by changes in surface tension between melt and vapor resulting from the increasing polymerization of the melt at lower temperatures and water contents because only 10 8 bubbles cm 3 nucleate on magnetite in the rhyolite melt used almost regardless of experimental conditions results from this study may not be directly comparable to vesicle numbers in volcanic pumice of different compositions

autogenic avulsion channelization and backfilling dynamics of debris flow fans alluvial fans develop their semi conical shape by quasi cyclic avulsions of their geomorphologically active sector from a fixed fan apex on debris flow fans these quasi cyclic avulsions are poorly understood partly because physical scale experiments on the formation of fans have been limited largely to turbidite and fluvial fans and deltas in this study debris flow fans were experimentally created under constant extrinsic forcing and autogenic sequences of backfilling avulsion and channelization were observed backfilling avulsion and channelization were gradual processes that required multiple successive debris flow events debris flows avulsed along preferential flow paths given by the balance between steepest descent and flow inertia in the channelization phase debris flows became progressively longer and narrower because momentum increasingly focused on the flow front as flow narrowed resulting in longer run out and deeper channels backfilling commenced when debris flows reached their maximum possible length and channel depth as defined by channel slope and debris flow volume and composition after which they progressively shortened and widened until the entire channel was filled and avulsion was initiated the terminus of deposition moved upstream because the frontal lobe deposits of previous debris flows created a low gradient zone forcing deposition consequently the next debris flow was shorter which led to more in channel sedimentation causing more overbank flow in the next debris flow and resulting in reduced momentum to the flow front and shorter runout this topographic feedback is similar to the interaction between flow and mouth bars forcing backfilling and transitions from channelized to sheet flow in turbidite and fluvial fans and deltas debris flow avulsion cycles are governed by the same large scale topographic compensation that drives avulsion cycles on fluvial and turbidite fans although the detailed processes are unique to debris flow fans this novel result provides a basis for modelling of debris flow fans with applications in hazards and stratigraphy

abrupt transitions during sustained explosive eruptions examples from the 1912 eruption of novarupta alaska plinian ignimbrite activity stopped briefly and abruptly 16 and 45 h after commencement of the 1912 novarupta eruption defining three episodes of explosive volcanism before finally giving way after 60 h to effusion of lava domes we focus here on the processes leading to the termination of the second and third of these three episodes early erupted pumice from both episodes show a very similar range in bulk vesicularity but the modal values markedly decrease and the vesicularity range widens toward the end of episode iii clasts erupted at the end of each episode represent textural extremes at the end of episode ii clasts have very thin glass walls and a predominance of large bubbles whereas at the end of episode iii clasts have thick interstices and more small bubbles quantitatively all clasts have very similar vesicle size distributions which show a division in the bubble population at 30 m vesicle diameter and cumulative number densities ranging from 107 109 cm 3 patterns seen in histograms of volume fraction and the trends in the vesicle size data can be explained by coalescence signatures superimposed on an interval of prolonged nucleation and free growth of bubbles compared to experimental data for bubble growth in silicic melts the high 1912 number densities suggest homogeneous nucleation was a significant if not dominant mechanism of bubble nucleation in the dacitic magma the most distinct clast populations occurred toward the end of plinian activity preceding effusive dome growth distributions skewed toward small sizes thick walls and teardrop vesicle shapes are indicative of bubble wall collapse marking maturation of the melt and onset of processes of outgassing the data suggest that the superficially similar pauses in the 1912 eruption which marked the ends of episodes ii and iii had very different causes through episode iii the trend in vesicle size data reflects a progressive shift in the degassing process from rapid magma ascent and coupled gas exsolution to slower ascent with partial open system outgassing as a precursor to effusive dome growth no such trend is visible in the episode ii clast assemblages we suggest that external changes involving failure of the conduit vent walls are more likely to have effected the break in explosive activity at 45 h

decompression experiments as an insight into ascent rates of silicic magmas an experimental study of h2o exsolution bubble growth and microlite crystallisation during ascent decompression of silicic magmas in the volcanic conduit is presented isobaric and decompression experiments were performed on a rhyolitic melt at 860 c nno 1 h2o saturation and pressures between 15 and 170 mpa two sets of decompression experiments were performed with decompression rates varying between 0 001 and 960 mpa min 1 from 150 to 50 mpa high pressure decompression and 2 from 50 to 15 mpa low pressure decompression the experiments highlight incomplete h2o exsolution for decompression rates 100 mpa min incomplete bubble growth for decompression rates 0 1 mpa min crystal nucleation time lags and incomplete chemical re equilibration to final pressures the observed crystallisation process i e growth versus nucleation depends on the decompression range indeed decompression induced crystallisation during high pressure decompressions is dominated by growth of existing crystals whereas during low pressure decompressions crystal nucleation is the dominating process this study provides a means to infer magma ascent rates in eruptions of silicic magmas through a combined petrologic and experimental approach

porosity redistribution enhanced by strain localization in crystal rich magmas magma degassing characterized by changes in permeability and porosity distribution has a crucial control on the style of eruption during ascent magma might develop large porosities and crystallize while it is subjected to shear shear in turn enhances complex fabrics that result from the reorganization of the different phases crystals gas melt such fabrics have not yet been evaluated experimentally on a three phase system we performed torsion experiments on a synthetic crystal rich hydrous magma at subsolidus conditions with 11 vol porosity to establish a link between strain partitioning and porosity redistribution crystals induce non newtonian deformation resulting in localization of the shear strain three dimensional microtomography and two dimensional scanning electron microscope imaging show gas accumulation in local microstructures caused by shear induced crystal fabric our data show that strain localization is a mechanism that could enable magma degassing at very low vesicularity

eruption dynamics inferred from microlite crystallization experiments application to plinian and dome forming eruptions of mt pelee martinique lesser antilles decompression experiments have been conducted to simulate syn eruptive crystallization in volcanic conduits to infer magma ascent rates and conditions during dome forming and plinian eruptions of silicic arc volcanoes the experiments were carried out on starting material with the composition of mt pele e rhyolitic interstitial melt 76 wt sio2 and consisted of three consecutive steps hydration decompression and annealing hydration saturated and undersaturated was performed at 8508c and 200mpa and was followed by isothermal decompression either linearly or stepwise to a final pressure pf of 30 or 5 10mpa decompression rates range from 0 003 to 25mpamin 1 decompression durations of 15 min to 40 days two samples were cooled by 258c and 508c during a 3 day step at pf subsequent to decompression the samples were held for up to 15 days at pf the experiments generated three types of crystals as products of pre syn and post decompression crystallization the experiments basically differ from previous studies in that they are specifically designed to discriminate crystal nucleation from growth and to evaluate the influence of pre decompression crystals on the decompression induced crystallization the effects of pre decompression crystals decompression rate undercooling pf and terminal cooling have been determined on plagioclase nucleation growth morphology and composition the main results suggest a positive correlation between decompression rate and the number density of plagioclase crystals nucleated at pf and highlight the effect of pre decompression crystals in further decompression induced crystallization the relations between the decompression conditions and the plagioclase characteristics have been used to infer mt pele e eruption dynamics suggesting that 1 plinian magmas ascend from the reservoir within less than 1h 1 10 m s 2 dome and block and ash flow magmas ascend within more than 2 5 days giving time for syn decompression crystallization around pre existent microlites 3 dome magmas provide evidence for long stagnation and cooling at low pressure and 4 surge magmas ascend without significant crystallization within less than 4 days and massively nucleate plagioclase at very low pressure the extent and violence of dome destruction may depend on the size or age of the dome with large or old domes favouring mildly explosive block and ash flows whereas small or young protodomes may generate highly explosive surges

exploring the roles of climate and tectonics in the geomorphic evolution of the chitwan intermontane valley central himalaya abstract the himalayan front exhibits varying convergence rates along its length this study examines the morphotectonic evolution of the landforms in an intermontane valley viz chitwan dun present in a relatively rapidly converging segment of the himalaya using field investigations geomorphic analysis and geochronology we demonstrate that five geomorphic levels have developed in the past 112 ka four major phases of aggradation occurred in the chitwan i e 112 ka 112 70 ka 70 25 ka and 18 11 ka tectonic activity on the jharahi jht and belani thrusts bt at 112 ka and 70 ka respectively resulted in the uplift of the geomorphic units to level 5 and level 4 continuous aggradation in the valley occurred between 70 and 25 ka level 3 the tectonic activity during this period is marked by the reactivation of the west chitwan thrust wct and the central churia thrust cct the level 3 surfaces were abandoned during the lgm 25 18 ka and subsequently incised enhanced monsoon in the post lgm period favored aggradation of the level 2 and 1 geomorphic units the level 2 surfaces were abandoned due to activity on the danda dt and shaktikhor thrusts st whereas the level 1 landforms were incised due to climatic changes in the holocene comparing the results of this study with other parts of the himalaya we infer that despite variable tectonic convergence rates major aggradation and incision cycles are uniform this suggests that late quaternary climatic perturbations have strongly influenced the fluvial systems across the himalaya however the role of tectonics is also manifested in the form of multiple levels of surfaces and terraces

bubble nucleation growth and coalescence during the 1997 vulcanian explosions of soufriere hills volcano montserrat abstract soufriere hills volcano had two periods of repetitive vulcanian activity in 1997 each explosion discharged the contents of the upper 0 5 2 km of the conduit as pyroclastic flows and fallout frothy pumices from a deep gas rich zone lava and breadcrust bombs from a degassed lava plug and dense pumices from a transition zone vesicles constitute 1 66 vol of breadcrust bombs and 24 79 of pumices all those larger than a few tens of m being interconnected small vesicles few hundreds of m in pumices are interpreted as pre dating explosion implying pre explosive conduit porosities up to 55 about a sixth of large vesicles in pumices and all those in breadcrust bombs are angular voids formed by syn explosive fracturing of amphibole phenocrysts an intermediate sized vesicle population formed by coalescence of the small syn explosive bubbles bubble nucleation took place heterogeneously on titanomagnetite number densities of which greatly exceed those of vesicles and growth took place mainly by decompression development of pyroclast vesicle textures was controlled by the time interval between the onset of explosion decompression and surface quench in contact with air lava plug fragments entered the air quickly after fragmentation 10 s so the interiors continued to vesiculate once the rinds had quenched forming breadcrust bombs deeper gas rich magma took longer 50 s to reach the surface and vesiculation of resulting pumice clasts was essentially complete prior to surface quench this accounts for the absence of breadcrusting on pumice clasts and for the textural similarity between pyroclastic flow and fallout pumices despite different thermal histories after leaving the vent it also allowed syn explosive coalescence to proceed further in the pumices than in the breadcrust bombs uniaxial boudinage of amphibole phenocrysts in pumices implies significant syn explosive vesiculation even prior to magma fragmentation probably in a zone of steep pressure gradient beneath the descending fragmentation front syn explosive decompression rates estimated from vesicle number densities 0 3 6 5 mpa s 1 are consistent with those predicted by previously published numerical models

from vulcanian explosions to sustained explosive eruptions the role of diffusive mass transfer in conduit flow dynamics abstract magmatic explosive eruptions are influenced by mass transfer processes of gas diffusion into bubbles caused by decompression melnik and sparks melnik o e sparks r s j 2002 modelling of conduit flow dynamic during explosive activity at soufriere hills volcano montserrat in druitt t h kokelaar b p eds the eruption of soufriere hills volcano montserrat from 1995 to 1999 geological society london memoirs 21 307 317 proposed two end member cases corresponding to complete equilibrium and complete disequilibrium in the first case diffusion is fast enough to maintain the system near equilibrium and a long lived explosive eruption develops in the latter case pre existing bubbles expand under conditions of explosive eruption and decompression but diffusive gas transfer is negligible this leads to a much shorter eruption here we develop this model to consider the role of mass transfer by investigating transient flows at the start of an explosive eruption triggered by a sudden decompression the simulations reveal a spectrum of behaviours from sustained to short lived highly non equilibrium vulcanian style explosions lasting a few tens of seconds through longer lasting eruptions that can be sustained for tens of minutes and finally to eruptions that can last hours or even days behaviour is controlled by a mass transfer parameter which equals n 2 3 d where n is the bubble number density and d is the diffusivity the parameter is expected to vary between 10 5 and 1 s 1 in nature and reflects a time scale for efficient diffusion the spectrum of model behaviours is consistent with variations in styles of explosive eruptions of silicic volcanoes in the initial stages peak discharges occur over 10 20 s and then decline to low discharges if a critical bubble overpressure is assumed to be the criterion for fragmentation then fragmentation may stop and start several times in the declining period causing several pulses of high intensity discharge for the cases of strong disequilibria the fluxes can decrease to negligible values where other processes such as gas escape through permeable magma prevents explosive conditions becoming re established so that explosive activity stops and dome growth can start for cases closer to the equilibrium the eruption can evolve towards a quasi steady sustained flow never declining sufficiently for gas escape to become dominant

tectonic controls on the morphometry of alluvial fans around danehkhoshk anticline zagros iran article i nfo alluvial fans are important landforms where their morphology and morphometry reflect changes in tectonic climate base level and drainage basin characteristics along the margins of tectonically active mountain ranges like the zagros mountains alluvial fans are generally assumed to act as useful landforms for identify ing the level of tectonic activity the purpose of this paper is to evaluate the relationship between active tec tonics and morphometric characteristics of alluvial fans around danehkhoshk anticline in the simply folded belt of zagros morphometric characteristics of alluvial fans such as area fa slope sf length of base bf width length ratio w l radius r sweep angle sa and entrenchment e as well as valley floor width to height ratio vf and strata dips of anticline limbs dal were measured the study area was sub divided into eight tectonic zones and then the mean values of the above mentioned parameters were calculated in each zone result reveals that values of sa bf and e are directly proportional to dal the poor relationships between catchment characteristics slope and area and fan parameters are probably due to extensive karstic landforms of catchments having complex hydrologic systems and hence result in complex catchment fan relations the highly entrenched fans with high sweep angles and long bases are characteristic of tectonically active fronts of danehkhoshk anticline having v shaped valleys higher vf values steep triangular facets and more rotated limbs higher dal values apart from the tectonic control on fan development the fan head entrenchment and negative accumulation spaces on most alluvial fans can be attributed to decreased sediment load and discharge the drier the present day climate regime

numerical study of nucleation and growth of bubbles in viscous magmas the nucleation and growth processes of bubbles in viscous magmas with a constant decompression rate have been numerically investigated based on a formation which accounts for effects of viscosity as well as diffusivity interfacial tension and decompression rate the numerical solutions show two regimes in the nucleation and growth process a diffusion controlled regime and a viscosity controlled regime mainly depending on the decompression rate initial saturation pressure and viscosity the basic mechanism common to both regimes is that growth governs nucleation through depletion of degassing components in basaltic eruptions the vesiculation is essentially controlled by diffusion and the viscosity controlled regime is limited to very high decompression rate and very small water content when andesitic magma saturated by water at 10 mpa is decompressed through the propagation of rarefraction wave induced by a landslide as took place in the mount st helens 1980 eruption the vesiculation is controlled by the viscosity up to 100 m depth on the other hand in a rhyolitic magma for the same situation vesiculation is controlled by the viscosity over the whole depth of the magma column in the viscosity controlled regime the vesicularity may be 90 or less as seen in silicic pumice whereas in the diffusion controlledmore regime the vesicularity equals or exceeds 98 such as in reticulite in hawaiian basalt an observed variation of the number density of bubbles by several orders of magnitude in plinian eruptions and the correlation with the sio2 content can be attributed approximately to the dependence of diffusivity of viscosity on sio2 content and temperature assuming the apparent correlation between sio2 content and temperature of magma less

late quaternary paleodrainage reconstruction of the maros river alluvial fan abstract the surface of the extensive maros alluvial fan is densely 0 78 km km 2 covered by paleochannels the aim of the present study is to reconstruct the final late quaternary stage of the alluvial fan development by identifying paleodrainage directions and determining morphology and pattern of the paleochannels and the age of their activity on the surface of the alluvial fan 18 meandering braided anastomosing or misfit paleochannel zones were identified the existence of these channels indicates that the main process of the fan development was avulsion rather than lateral channel shift and the overbank floodplain accumulation played a minor role as the paleochannels were not buried the oldest paleodrainages a c shifted towards north from the east west axis of the alluvial fan draining water to the koros basin the similar age 18 7 2 3 ka 14 2 1 4 ka of the paleochannel zones indicates rapid avulsions or the coexistence of paleodrainages in the northern half of the alluvial fan the last large d paleodrainage change occurred 12 4 2 1 ka when the channel turned westward a large meandering channel functioned for a period 12 4 2 1 ka 9 6 1 3 ka and with the coexisting misfit channels these channels drained a considerable amount of water q b 2500 m 3 s in the next development phase e paleodrainages the maros flowed north but after a sharp bend it turned toward the south and it started to form the southern lobe of the alluvial fan these channels existed between 8 5 0 9 ka and 3 5 0 4 ka and had a bankfull discharge of 1000 2000 m 3 s the last f paleodrainage of the maros river existed for a short time 1 9 0 3 ka 1 6 0 3 ka and indicated decreasing discharge q b 1400 m 3 s in the final phase of the alluvial fan evolution the present day course of the river was developed and its discharge decreased further 680 m 3 s

phenocryst hosted melt inclusions record stalling of magma during ascent in the conduit and upper magma reservoir prior to vulcanian explosions soufriere hills volcano montserrat west indies the mechanics of explosive eruptions influence magma ascent pathways vulcanian explosions involve a stop start mechanism that recurs on various timescales evacuating the uppermost portions of the conduit during the repose time between explosions magma rises from depth and refills the conduit and stalls until the overpressure is sufficient to generate another explosion we have analyzed major elements cl s h2o and co2 in plagioclase hosted melt inclusions sampled from pumice erupted during four vulcanian events at soufriere hills volcano montserrat to determine melt compositions prior to eruption using fourier transform infrared spectroscopy we measured values up to 6 7 wt h2o and 80 ppm co2 of 42 melt inclusions 81 cluster between 2 8 and 5 4 wt h2o 57 to 173 mpa or 2 7 km suggesting lower conduit to upper magma reservoir conditions we propose two models to explain the magmatic conditions prior to eruption in model 1 melt inclusions were trapped during crystal growth in magma that was stalled in the lower conduit to upper magma reservoir and during trapping the magma was undergoing closed system degassing with up to 1 wt free vapor this model can explain the melt inclusions with higher h2o contents since these have sampled the upper parts of the magma reservoir however the model cannot explain the melt inclusions with lower h2o because the timescale for plagioclase crystallization and melt inclusion entrapment is longer than the magma residence time in the conduit in model 2 melt inclusions were originally trapped at deeper levels of the magma chamber but then lost hydrogen by diffusion through the plagioclase host during periodic stalling of the magma in the lower conduit system in this second scenario which we favor the melt inclusions record re equilibration depths within the lower conduit to upper magma reservoir

pre explosive conduit conditions of the 1997 vulcanian explosions at soufriere hills volcano montserrat i pressure and vesicularity distributions an authoritative case of vulcanian eruptive dynamics is the series of 88 explosions that occurred between august and october 1997 at soufriere hills volcano on montserrat island the state of the magmatic column just before a vulcanian explosion is still poorly understood but conditions the eruptive style this study establishes such a pre explosive stratigraphy by 1 documenting the textures covering the range of the 1997 products 2 quantitative analysis of h2o content in interstitial glass measured by karl fischer titration and 3 combining these data with a simple model linking pre and post explosive vesicularities the model shows that syn explosive degassing affects greatly the way porosity evolves by decompression during an explosion the stratigraphy reconstruction shows a three part vertical layering of the conduit prior to explosion with overall denser values than those previously suggested a dense and strongly degassed plug caps the column it is underlain by a shallow transition zone featuring complex mingling between vesicular and dense magma up to 10 mpa at higher pressure up to 80 mpa lies a more homogeneous zone of relatively dense 10 20 vol magma which was emplaced under partly open system degassing this conduit stratigraphy gives the vision of a strongly heterogeneous magma column immediately prior to its disruption our analysis suggests that fragmenting such a composite magma cannot happen in a single coherent pulse but rather as stages the transition zone contains heterogeneous amounts of exsolved gas that could explain the pulsatory nature of the vulcanian jets at the beginning of the explosions this contrasts with the nearly constant vesicularities of the deeper part of the pre explosive magma column which are propitious to a general short lived disruption

bubble nucleation in rhyolitic melts experiments at high pressure temperature and water content abstract we report the first measurement of bubble nucleation in hydrated rhyolitic melts in response to pressure release two rhyolitic obsidians one containing less than 1 of microlites of fe ti oxides and the other about 20 of various crystals were hydrated at 150 mpa and 780 850 c after saturation was reached 5 3 5 5 wt water pressure was lowered and the samples were allowed to nucleate and grow bubbles for various amounts of time before the final rapid quenching of the experiments the results demonstrate the importance of heterogeneous nucleation microlites of fe ti oxides are very efficient as sites for bubble nucleation in their presence modest nucleation was observed even after decompression by 6 10 8 bubbles cm 3 in the absence of microlites no nucleation occurred at p mpa at p 10 mpa bubbles also nucleated on crystals of biotite zircon and apatite modest nucleation 10 3 10 5 cm 3 took place even in crystal free samples but it was still heterogeneous when p exceeded 80 mpa nucleation in crystal free samples became extensive 10 5 10 7 cm 3 the lack of correlation of bubble density with either time or decompression suggests that nucleation was still heterogeneous nucleation rates were controlled mainly by the availability of sites rates were faster than 10 6 cm 3 s 1 when microlites were present and faster than 10 5 cm 3 s 1 in the absence of microlites at p 70 mpa narrow size distributions in most samples suggests that nucleation took place immediately after the pressure drop the experimental data we present here indicate that the presence or absence of efficient nucleation sites can lead to two distinct modes of bubble formation when a large number of efficient sites e g fe ti oxide are present bubble nucleation requires very little supersaturation and to a good approximation gas and magma are in equilibrium in magmas that are crystal free or contain crystals that are inefficient at nucleating bubbles very high degrees of supersaturation are required in order to initiate nucleation these two modes of exsolution may lead to contrasting styles of convection pressure build up and eruption

the role of oxides in the shallow vesiculation of ascending magmas abstract despite their generally low volume fraction fe ti oxides have the potential to greatly influence the eruptive style because they lower the supersaturation pressure for heterogeneous bubble nucleation once nucleated bubbles respond fast to pressure changes fostering rapid expansion and explosive behavior yet oxide microlite quantifications are often absent from data of explosive products we used new re analyzed and published data to build a compilation of oxide number densities onds and vesicle number densities vnds of trachytic and calc alkaline products four eruptive styles were selected 1 vulcanian explosions from soufriere hills volcano montserrat lascar volcano chile and kilian volcano france 2 blasts from mt pelee volcano lesser antilles mount st helens usa and merapi volcano indonesia 3 a sub plinian explosion from merapi volcano and 4 lava dome effusions with intermittent collapse from soufriere hills and mt pelee volcanoes natural samples were separated into two groups according to the dominant texture of the products of each event 1 vesicular pumice clasts from explosions with a strong vertical component and 2 dense clasts with diktytaxitic textures from dome collapse event and lateral blast group 1 samples are either distributed alongside the 1 1 trend between vnd and ond that spans from 1015 to 1017 m 3 or have a constant vnd of 1016 16 5 m 3 regardless of ond a large proportion of oxides 55 100 touch vesicles a more variable proportion of vesicles 16 72 are in contact with oxides because of syn explosive growth and coalescence group 2 samples have onds in the same broad range as group 1 samples we also used new and published data to build a compilation of onds and vnds of five series of experimental decompression of rhyolitic and phonolitic melts in samples with ond vnd most bubbles are in contact with more than one oxide and 64 88 of the oxides are in contact with bubbles such high levels of connectivity suggest that the role of oxides in controlling bubble nucleation has been underestimated when vnd ond nucleation densities of experimental vesicles can be reproduced by heterogeneous nucleation models which we used to calculate syn explosive decompression rates from vnds at merapi soufriere hills and kilian these rates and textural evidence suggest that the decompression front accompanying these vulcanian and sub plinian explosions is responsible for syn explosive bubble nucleation we calculated the average pre explosive ascent rates necessary to yield the observed onds at soufriere hills and merapi volcanoes the resulting rates 0 005 20 m s overlap considerably with the range of critical ascent rate inferred for the effusive explosive transition supporting the hypothesis that this transition is primarily controlled by oxide microlites in the conduit because oxides are a primer for explosive behavior when present in sufficiently high number densities focusing on the older eruption of kilian for which no observation is available we infer that pre explosive ascent rates of 7 10 3 m s were necessary for explosive behavior to occur

the lake lahontan highstand age surficial characteristics soil development and regional shoreline correlation abstract the lake lahontan basin has been the site of numerous pluvial lakes during the pleistocene we address the question of whether or not the highest remnant shoreline features around the perimeter of the lake were produced during the most recent sehoo highstand 13 ka the penultimate eetza highstand 140 280 ka or both to do so we document surficial characteristics morphologic preservation and soil development on multiple sehoo beach barriers in the jessup embayment to define the range in characteristics displayed by latest pleistocene beach features sehoo barriers generally exhibit original constructional morphology that has been little modified by erosion soils developed on sehoo barriers are generally thin and weakly developed and are strongly influenced by the introduction of eolian fines into the predominately clast supported coarse beach gravels similar observations from 13 other highstand barriers and from seven older than latest pleistocene paleosols located around the basin form the basis for a regional comparison based on similar characteristics including the degree of morphologic preservation and weak soil development we conclude that the widespread and nearly continuous high shoreline around the perimeter of lake lahontan dates from the most recent major lake cycle in all areas except in the walker lake subbasin in the walker lake subbasin isolated early to middle pleistocene lacustrine outcrops and landforms are elevated as much as 70 m above the late pleistocene limit but are differentiated by their degraded form and lack of continuity around the subbasin similar unambiguous landforms were not observed elsewhere and at similar elevations in the northern subbasins of lake lahontan

magnetite bubble aggregates at mixing interfaces in andesite magma bodies magnetite is a particularly favourable site for heterogeneous bubble nucleation in magma and yet only very rarely is evidence for this preserved due to the myriad of processes that act to overprint such an association the possibility of bubble magnetite aggregates in magmas carries with it interesting implications for the fluid mechanics of magma bodies and for the magma mixing process responsible for the formation of andesites we use image analysis and statistical methods to illustrate a spatial association between magnetite and bubbles in mafic enclaves there is a large range in magnetite contents in the enclaves up to 7 5 which is related to the porosity of the enclaves indicating a mechanism of enrichment of the mafic magma in magnetite in the andesite there is no spatial association between bubbles and magnetite and the magnetite content of the andesite is small we suggest a mechanism for enclave formation whereby in vapour saturated magma bubbles nucleate on magnetite upon intrusion into the base of an andesite magma body these bubble magnetite aggregates rise and sweep up other magnetites resulting in the accumulation of aggregates at the magma interface instabilities lead to the flotation of enclaves characterized by enrichment in magnetite and bubbles

experimental constraints on degassing and permeability in volcanic conduit flow this study assesses the effect of decompression rate on two processes that directly influence the behavior of volcanic eruptions degassing and permeability in magmas we studied the degassing of magma with experiments on hydrated natural rhyolitic glass at high pressure and temperature from the data collected we defined and characterized one degassing regime in equilibrium and two regimes in disequilibrium equilibrium bubble growth occurs when the decompression rate is slower than 0 1 mpa s 1 while higher rates cause porosity to deviate rapidly from equilibrium defining the first disequilibrium regime of degassing if the deviation is large enough a critical threshold of super saturation is reached and bubble growth accelerates defining the second disequilibrium regime we studied permeability and bubble coalescence in magma with experiments using the same rhyolitic melt in open degassing conditions under these open conditions we observed that bubbles start to coalesce at 43 vol porosity regardless of decompression rate coalescence profoundly affects bubble texture and size distributions and induces the melt to become permeable we determined coalescence to occur on a time scale 180 s independent of decompression rate we parameterized and incorporated our experimental results into a 1d conduit flow model to explore the implications of our findings on eruptive behavior of rhyolitic melts with low crystal contents stored in the upper crust compared to previous models that assume equilibrium degassing of the melt during ascent the introduction of disequilibrium degassing reduces the deviation from lithostatic pressure by 25 the acceleration at high porosities 50 vol by a factor 5 and the associated decompression rate by an order of magnitude the integration of the time scale of coalescence to the model shows that the transition between explosive and effusive eruptive regimes is sensitive to small variations of the initial magma ascent speed and that flow conditions near fragmentation may significantly be affected by bubble coalescence and gas escape

decompression rate of magma at fragmentation inference from broken crystals in pumice of vulcanian eruption abstract decompression rate of magma at fragmentation immediately before or after fragmentation was determined using broken crystals found in the pumices ejected during the vulcanian explosion of sakurajima volcano japan the combined analysis of textural data obtained from the natural pumice samples and a simple model for the crystal fracturing in vesiculating viscous magma indicated a decompression rate 7 0 10 3 7 8 10 4 pa s for the ejecta produced during the later phase of vulcanian explosion this result suggests that the short duration of the vulcanian explosion is controlled by the rapid decrease of the magma ascent rate to a vent an understanding of the control mechanism of the termination of an eruption by precise prediction of the eruption process is essential for both volcanology and hazard mitigation

petrology and fe ti oxide reequilibration of the 1991 mount unzen mixed magma a dacitic magma 64 5 wt sio2 a mixture of phenocryst rich rhyodacite and an aphyric mafic magma was erupted during the recent 1991 1995 mount unzen eruptive cycle the experimental and analytical results of this study reveal additional details about conditions in the premixing and postmixing magmas and the nature of the mixing process the preeruption rhyodacitic magma was at a temperature of 790 20 c according to fe ti oxide phenocryst cores and at a depth of 6 to 7 km 160 mpa according to al in hornblende geobarometry the mafic magma that mixed with the rhyodacite is found as andesitic 54 to 62 wt sio2 enclaves in the erupted magma and was essentially aphyric when intruded phase equilibria indicate that an aphyric andesite at 160 mpa is 1030 c h2o saturated and possibly as high as 1130 c 2 wt h2o the composition of the rhyodacite which was mixed with the andesite is estimated to lie between 67 and 69 wt sio2 using these compositions and temperatures the temperature of the unzen magma after mixing is estimated to be at least 850 to 870 c the groundmass fe ti oxide microphenocrysts and those in pargasite bearing reaction zones around biotite phenocrysts both give 890 20 c temperatures the oxide oxide contacts give temperatures of 910 20 c the 900 30 c postmixing temperatures are consistent with phase equilibria experiments which show that the magma was not above 930 c at 160 mpa our fe ti oxide reequilibration experiments suggest that the mixing of the two magmas began within a few weeks of the eruption which is a shorter time than is calculated using available diffusion data there is also evidence that some mixing took place much closer to the time of extrusion based on the presence of unrimmed biotite phenocrysts in the magma

magma storage region processes inferred from geochemistry of fe ti oxides in andesitic magma soufriere hills volcano montserrat w i analyses of fe ti oxides help constrain models of magma storage region processes for the soufriere hills volcano montserrat w i and provide clear evidence of the nature of transient heating events in the magma storage region to constrain timescales of magma heating and remobilization the tio2 zoning patterns in a time series of natural titanomagnetites were compared with those produced in controlled phase equilibrium experiments on the andesite most samples of andesite erupted from 1995 to 2002 contain titanomagnetite crystals with uniform core compositions tio278 wt many crystals are characterized by rimward increases in tio2 interpreted to be ti diffusion gradients caused by heating of the andesite by invading basaltic magma some andesites erupted during periods of the highest observed mass eruption rate however contain titanomagnetite with uniformly low tio2 contents from core to rim the observation that no ti diffusion gradients and no elevated core tio2 contents occur in the vast majority of titanomagnetite grains in magma batches that were erupted more than 2 years after the onset of the present eruption strongly suggests first that heating of the batches of andesite occurred just before eruption and second that injection of basaltic magma has continued throughout the eruption heat but little mass may be transferred from the invading basalt to the andesite in the magma storage region by injection of dikes or formation of sills ponding of basaltic magma at the base of a pre existing andesitic magma storage region is the simplest explanation consistent with observations the fe ti oxide data strongly suggest that an internal conduit within the andesitic magma storage region carries magma from the zone of heating to the overlying conduit which carries the magma through the upper arc crust in this model the magma chamber is being emptied from the bottom at the contact between pre existing andesite and newly intruded basalt

quantifying sediment transport dynamics on alluvial fans from spatial and temporal changes in grain size death valley california how information about sediment transport processes is transmitted to the sedimentary record remains a complex problem for the interpretation of fluvial stratigraphy alluvial fan deposits represent the condensed archive of sediment transport which is at least partly controlled by tectonics and climate for three coupled catchment fan systems in northern death valley california we measure grain size across 12 well preserved holocene and late pleistocene surfaces mapped in detail from field observations and remote sensing our results show that fan surfaces correlated to the late pleistocene are on average 30 50 coarser than active or holocene fan surfaces we adopt a self similar form of grain size distribution based on the observed stability of the ratio between mean grain size and standard deviation downstream using statistical analysis we show that fan surface grain size distributions are self similar we derive a relative mobility function using our self similar grain size distributions which describes the relative probability of a given grain size being transported we show that the largest mobile grain sizes are between 20 and 35mm a value that varies over time and is clearly lower in the holocene than in the pleistocene a change we suggest is due to a drier climate in the holocene these results support recent findings that alluvial fan sedimentology can record past environmental change and that these landscapes are potentially sensitive to climatic change over a glacial interglacial cycle we demonstrate that the self similarity methodology offers a means to explore changes in relative mobility of grain sizes from preserved fluvial deposits

kinetics of heterogeneous bubble nucleation in rhyolitic melts implications for the number density of bubbles in volcanic conduits and for pumice textures we performed decompression experiments to simulate the ascent of a phenocryst bearing rhyolitic magma in a volcanic conduit the starting materials were bubble free rhyolites water saturated at 200 mpa 800 c under oxidizing conditions they contained 6 0 wt dissolved h2o and a dense population of hematite crystals 8 7 2 105 mm 3 pressure was decreased from the saturation value to a final value ranging from 99 to 20 mpa at constant temperature 800 c the rate of decompression was either 1 000 or 27 8 kpa s in all experiments we observed a single event of heterogeneous bubble nucleation beginning at a pressure pn equal to 63 3 mpa in the 1 000 kpa s series and to 69 1 mpa in the 27 8 kpa s series below pn the degree of water supersaturation in the liquid rapidly decreased to a few 0 1 wt the nucleation rate dropped and the bubble number density bnd stabilized to a value strongly sensitive to decompression rate 80 mm 3 at 27 8 kpa s vs 5 900 mm 3 at 1 000 kpa s this behaviour is like the behavior formerly described in the case of homogeneous bubble nucleation in the rhyolite h2o system and in numerical simulations of vesiculation in ascending magmas similar degrees of water supersaturation were measured at 27 8 and 1 000 kpa s implying that a faster decompression rate does not result in a larger departure from equilibrium our experimental results imply that bnds in acid to intermediate magmas ascending in volcanic conduits will depend on both the decompression rate left left text d p text d t right right and the number density of phenocrysts especially the number density of magnetite microphenocrysts 1 100 mm 3 which is the only mineral species able to reduce significantly the degree of water supersaturation required for bubble nucleation very low bnds 1 mm 3 are predicted in the case of effusive eruptions left left text d p text d t right right 0 1 kpa s high bnds up to 107 mm 3 and bimodal bubble size distributions are expected in the case of explosive eruptions 1 a relatively small number density of bubbles 1 100 mm 3 will first nucleate in the lower part of the conduit left left text d p text d t right right 10 kpa s either at high pressure on magnetite or at lower pressure on quartz and feldspar or by homogeneous nucleation in the liquid and 2 then extreme decompression rates near the fragmentation level left left text d p text d t right right 103 kpa s will trigger a major nucleation event leading to the multitude of small bubbles typically a few micrometers to a few tens of micrometers in diameter which characterizes most silicic pumices

heterogeneous bubble nucleation on fe ti oxide crystals in high silica rhyolitic melts the nucleation of h2o bubbles in magmas has been proposed as a trigger for volcanic eruptions to determine how bubbles nucleate heterogeneously in silicate melts experiments were carried out in which high silica rhyolitic melts were hydrated at 740 800 c and 50 175 mpa decompressed by 20 70 mpa and held at the lower pressures for 10 s before being quenched the hydration conditions were subliquidus and all samples contain blocky magnetite needle shaped hematite plagioclase magnetite is abundant at 800 c and high pressures whereas hematite becomes more abundant at lower temperatures and pressures bubbles nucleated in a single event in all samples with the number density nt of bubbles varying between 2 107 and 1 109 cm 3 at low degrees of supersaturation one to a few bubbles nucleate on faces of magnetite but at medium to high degrees of supersaturation multiple bubbles nucleate on single magnetite grains on hematite one to a few bubbles nucleated at the ends of the needle shaped crystals at medium supersaturations but formed along their entire lengths at high supersaturations nt increases as water diffusivity decreases indicating that the number of bubbles nucleated is influenced by their growth which depletes the melt with respect to h2o and lowers supersaturation if volcanic eruptions are triggered by bubble formation in magmas stored in shallow level magma chambers then the supersaturations needed for heterogeneous nucleation suggest that only small amounts of crystallization are needed whereas homogeneous nucleation is unlikely to trigger eruptions

bubble growth during decompression of magma experimental and theoretical investigation a model of bubble growth during decompression of supersaturated melt was developed in order to explore the conditions for preservation of gas overpressure in bubbles or for maintaining supersaturation of the melt the model accounts for the interplay of three dynamic processes decompression rate of the magma deformation of the viscous melt around the growing bubble and diffusion of volatiles into the bubble generally these processes are coupled and the evolution of bubble radius and gas pressure is solved numerically for a better understanding of the physics of the processes we developed some analytical solutions under simplifying assumptions for cases where growth is controlled by viscous resistance diffusion or linear decompression rate we show that the solutions are a function of time and two dimensionless numbers which are the ratios of either the diffusive or viscous time scales over the decompression time scale the conditions for each growth regime are provided as a function of the two governing dimensionless parameters analytical calculations for some specific cases compare well with numerical simulations and experimental results on bubble growth during decompression of hydrated silicic melts the model solutions including the division to the growth regimes as function of the two parameters provide a fast tool for estimation of the state of erupting magma in terms of gas overpressure supersaturation and gas volume fraction the model results are in agreement with the conditions of plinian explosive eruption e g mount st helens 18 may 1980 where high gas overpressure is expected the conditions of effusion of lava domes with sudden onset of explosive activity are also in agreement with the model predictions mostly in equilibrium degassing and partly in overpressure conditions we show that in a situation of quasi static diffusion during decompression the diffusive influx depends on the diffusivity away from the bubble insensitive to the diffusivity profile

environmental controls on the evolution of alluvial fans in buena vista valley north central nevada during late quaternary time abstract alluvial fans in buena vista valley north central nevada formed during at least four time periods during the late quaternary as evidenced by four temporally and spatially constrained fan deposits qf1 qf2 qf3 and qf4 from oldest to youngest the stratigraphic relation between qf3 and remnant beach ridge deposits of pluvial lake lahontan the latter of which forms a paleoclimatic proxy for this region indicate that climate and climatic change are primary controls on the evolution of fans qf3 deposition commenced during or immediately following the highstand of lake lahontan between 13 5 and 12 5 ka and continued as lake levels plummeted to levels below the sill elevation of the buena vista subbasin we interpret qf3 to be the result of a dynamic interplay between hillslope vegetation soils and hydrology as they are conditioned by climate and climatic change accelerated pedogenesis because of aerosolic silts and clays from exposed lake sediments coupled with decreased density of vegetation reduced infiltration capacity of the hillslope soils in interscrub areas in contrast to fans in more arid climates the increased runoff was not associated with increased coarse grained sediment yield and major fan aggradation discharge from sediment deficient basins entrenched older proximal fan deposits and transported the sediment downfan to form secondary fans the process response of qf3 to climatic change in north central nevada is especially illustrative of the impact of climate on landscapes in that 1 it results from changes in basin sediment and or water yield that occur in less than 500 years and 2 it differs from responses in arid climates to the south and more semiarid climates to the north the magnitude of vegetation change including type and density of vegetation and the relative proportion of interscrub area is particularly critical to water and sediment yield unit qf2 an early late pleistocene deposit may also have been deposited during a transition from a wetter to drier climate the synchronism of aggradation stability and entrenchment on all fans along the perimeter of buena vista valley in the late quaternary is further support for the dominance of climatic control on fan process on the other hand the impact of other extrinsic variables on fan process including tectonism base level and basin lithology all of which vary along the perimeter is negligible

use of multiparameter relative age methods for age estimation and correlation of alluvial fan surfaces on a desert piedmont eastern mojave desert california numerical and calibrated age determinations of the late quaternary alluvial fan deposits of the soda mountains piedmont in the mojave desert provide an opportunity to study the utility of the multiparameter relative age ra method for distinguishing and mapping geomorphic surfaces on a desert piedmont most ra parameters could not discriminate between deposits of holocene age although pavements have formed over locally significant parts of surfaces as young as middle holocene several parameters including lithologic composition particle size soil development and varnish cover permit distinguishing between holocene surfaces and late pleistocene surfaces statistically significant differences in initial particle size and lithology of the deposits inferred to be the result of complex interaction among hillslope alluvial fan and eolian processes and climatic change create conditions unfavorable for use of most ra techniques in contrast soil profile development and varnish cover data are successful in discrimination among deposits of holocene and pleistocene age this is attributed to the development of pedogenic features and varnish that are strongly dependent on dust influx and to the relatively minor dependence of these features on differences in the depositional character of the fan

reconstructing the timing of flash floods using 10be surface exposure dating at leidy creek alluvial fan and valley white mountains california nevada usa large alluvial fans characterize the piedmonts of the white mountains california nevada usa with large boulders strewn across their surfaces the boulders are interpreted as flash floods deposits with an unclear trigger for the transport process several triggers are possible including glacial lake outburst floods glofs thunderstorms or rainfall on snow cover from a paleoenvironmental perspective the origin of the flash floods is of fundamental importance the alluvial fans that flank the white mountains at leidy creek display particularly impressive examples of these deposits the boulder deposits and the source catchment at leidy creek were examined using 10be terrestrial cosmogenic nuclide tcn surface exposure dating to help elucidate their age and origin all boulders dated on the alluvial fans date to the holocene this is in accordance with the geomorphic analyses of the leidy creek catchment and its terraces and sediment ridges which were also dated to the holocene using optically stimulated luminescence osl and 10be surface exposure the results suggest that the boulders on the alluvial fan were deposited by flash floods during thunderstorm events affecting the catchment of the leidy creek valley paleomonsoonal induced mid holocene flash floods are the most plausible explanation for the discharges needed for these boulder aggradations but a regional dataset is needed to confirm this explanation

differential effects of base level tectonic setting and climatic change on quaternary alluvial fans in the northern great basin nevada usa

desert pavement characteristics on wadi terrace and alluvial fan surfaces wadi al bih u a e and oman abstract in arid mountain areas the dating and correlation of alluvial depositional surfaces is often uncertain especially in regions where the geomorphology is not well known surface modification by the development of soil and desert pavement may allow the correlation of geomorphic surfaces and estimation of at least their relative ages pleistocene wadi terraces and associated alluvial fans occur in wadi al bih u a e and oman for which correlations and age relationships are not known three age related groups of fans and terraces have been identified and mapped on the basis of their morphostratigraphic relationships deposition of the oldest terrace sediments and associated fans followed a long period of sustained incision after miocene uplift of the region the younger two groups of terraces and fans are inset within the older group to identify the gross effects of pavement development comparisons have been made between terrace surface and subsurface particle size distributions the older terraces have finer surface sediments and a greater contrast between finer surface and subsurface sediments than the younger terraces this reflects the degree of pavement development particle size on the fan surfaces is comparable with that on the equivalent terrace surfaces criteria for the classification of pavements were developed based on clast fracturing and angularity size sorting packing and surface texture from which a simple index of pavement development has been derived other properties rock varnish and weathering characteristics were also recorded but these proved to be less discriminatory than pavement characteristics the pavement data have been augmented by observations on soils detailed studies of pavements on terraces 8 sites 12 samples covering the three age groups and fans 5 sites 10 samples covering the three age groups allow differentiation between age groups the three terraces show three different age related pavement types expressed by differences in the pavement development index weakly developed pavements little fracturing sub rounded clasts some modification of the depositional fabric incipient soil development stage i caco3 accumulation occur on the youngest terrace and fan surfaces moderately developed pavements clast fracturing sub angular clasts moderate sorting and packing deeper soil development stage ii caco3 accumulation occur on the middle terrace and fan surfaces well developed pavements complete clast fracturing into small angular fragments mature sorting and packing of the pavement surface deep soil development with strong horizonation stage iii caco3 accumulation occur on the highest terrace and oldest fan surfaces there are minor differences between the youngest pavements on terraces and fans which reflect initial sedimentological differences these differences become less as the pavements develop on the basis of comparative studies the oldest terrace is estimated to date from sometime prior to ca 100 ka bp the second terrace and the most extensive fan surface from the late pleistocene and the youngest terrace and fan phase from the latest pleistocene or early holocene

magma degassing and intermittent lava dome growth 1 after its 1980 explosive eruption mount st helens developed a lava dome that grew intermittently for several years each growth episode was followed by a long repose suggesting that the magma column above the reservoir was in hydrostatic equilibrium a mechanism allowing an increasingly thicker dome is proposed loading of the crater floor by the dome acts to prevent gas leakage from magma by closing fractures around the volcanic conduit fractures get closed down to a depth that increases as the dome grows calculations of dome thickness as a function of dome radius are in good agreement with observations renewed growth is triggered by the spreading of the dome gas retention over a larger depth extent allows smaller magma densities and a taller magma column above the reservoir according to this model small domes can in fact promote explosive volcanic conditions and be unstable

paroxysmal dome explosion during the merapi 2010 eruption processes and facies relationships of associated high energy pyroclastic density currents an 11 minute sequence of laterally directed explosions and retrogressive collapses on 5 november 2010 at merapi indonesia destroyed a rapidly growing dome and generated high energy pyroclastic density currents pdcs spreading over 22 km2 with a runout of 8 4 km while contemporaneous co genetic valley confined pdcs reached 15 5 km this event formed stage 4 of the multi stage 2010 eruption the most intense eruptive episode at merapi since 1872 the deposits and the widespread devastating impact of associated high energy pdcs on trees and buildings show striking similarities with those from historical volcanic blasts montagne pelee martinique bezymianny russia mount st helens usa soufriere hills montserrat we provide data from stratigraphic and sedimentologic analyses of 62 sections of the first unequivocal blast like deposits in merapi s recent history we used high resolution satellite imagery to map eruptive units and flow direction from the pattern of extensive tree blowdown the stratigraphy of stage 4 consists of three depositional units u0 u1 u2 that we correlate to the second third and fourth explosions of the seismic record both u1 and u2 show a bi partite layer stratigraphy consisting each of a lower l1 layer and an upper l2 layer the lower l1 layer is typically very coarse grained fines poor poorly sorted and massive and was deposited by the erosive waxing flow head the overlying l2 layer is much finer grained fines rich moderately to well sorted with laminar to wavy stratification l2 was deposited from the waning upper part and wake of the pdc field observations indicate that pdc height reached 330 m with an internal velocity of 100 m s 1 within 3 km from the source the summit s geometry and the terrain morphology formed by a major transversal ridge and a funneling deep canyon strongly focused pdc mass towards a major constriction thereby limiting the loss of kinetic energy this favored elevated pdc velocities and high particle concentration promoted overspilling of pdcs across high ridges into other river valleys and generated significant dynamic pressures to distances of 6 km that caused total destruction of buildings and the forest the merapi 2010 eruption highlights that explosive and gravitational disintegration of a rapidly growing dome can generate devastating high energy high velocity pdcs this constitutes a credible high impact scenario for future multi stage eruptions at merapi and at other volcanoes that pose particular monitoring crisis response and risk reduction challenges

aspects of dynamics of the middle atmosphere inferred by using data from a satellite and from a numerical model the data used in this study are from a stratospheric sounding unit ssu on a polar orbiting satellite and from a numerical model of the middle atmosphere based on the primitive equations they are used to discuss the dynamics of the major stratospheric warming of 1984 85 the warming is of note because the westerly circulation started to break down rapidly when the westerly vortex was larger than might be expected of a state pre conditioned for a major warming at the height of the warming there is a sudden loss of high values of ertel s potential vorticity on isentropic maps derived from the ssu ground based measurements and a simulation of the warming with the numerical model indicate that strong gradients of temperature arise in westward sloping fronts which the ssu is unable to resolve fully the simulated stratospheric fronts have features in common with observed fronts that form in the upper troposphere although there may turn out to be dynamical differences between the two cases