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

ice divide flow at hans tausen iskappe north greenland from surface movement data surface strain rates around the southeastern dome of hans tausen iskappe in peary land north greenland 82 5 n 27 5 w are determined from global positioning system surveys of a strain net average longitudinal surface strain rate increases towards the dome from 1 4 0 2 x 10 4 a 1 at 5 10 ice thicknesses from the divide to 2 4 1 0 x 10 4 a within 1 ice thickness from the divide analysis of the data shows that the ice cap is presently building up within the strain net with an average rate of 0 04 0 02 m a assuming a uniform thickening the shape factor of the horizontal velocity the ratio between the vertically averaged horizontal velocity and the horizontal surface velocity decreases towards the dome from 0 9 at a distance of 10 ice thicknesses from the dome to 0 5 at the dome based on application of the continuity equation our results indicate that a region with anomalous flow is formed around the dome supporting recent indications reported by vaughan and others 1999 it is not possible from our data to constrain parameters of the flow law because there is no independent estimate of the significant present thickening of the central part of the ice cap and its pattern around the dome

anisotropic behavior of grip ices and flow in central greenland abstract mechanical tests have been performed on strongly textured ice samples coming from a wide range of depths from 1328 down to 2868 m of the greenland ice core project grip for different sample orientations with respect to the prescribed stress in this way two directional viscosities corresponding to the easy glide and to the hard glide orientations were determined along the core the viscoplastic anisotropy gradually increases down to a depth of 2600 m and slightly decreases below revealing a clear correlation between rheology and texture the experimental mechanical response compares well with that predicted by the viscoplastic self consistent vpsc model the vpsc model is also applied to ice samples that exhibit an axisymmetric texture to show in more detail the sensitivity of the rheology to specific texture parameters this leads to a number of recommendations for future mechanical tests on anisotropic samples a large scale ice flow model is finally used to estimate the influence of ice anisotropy on the flow along the grip gisp2 flow line the particular mechanical behavior of deep grip ices in the stress regime corresponding to an ice divide leads to deformation rates that are highly sensitive to bedrock topography and texture pattern this feature is likely to favour the formation of stratigraphic disturbances in deep ice layers as observed in the last 300 m of both grip and gisp2 cores

the application of icp ms methods to tephrochronological problems pearce nicholas westgate j a perkins w t preece s j 2004 the application of icp ms methods to tephrochronological problems applied gechemistry 19 3 pp 289 322 rae2008

relation between neighbouring grains in the upper part of the northgrip ice core implications for rotation recrystallization abstract we propose a new method to investigate the relationships between neighbouring crystals and apply it to the textures measured along the upper 900 m of the northgrip ice core this method shows unexpected correlations between neighbours in the so called normal grain growth regime questioning the classical view on the onset of rotation recrystallization in ice sheets moreover the fractionation rate associated to the rotation recrystallization appears constant through time finally grains with low angle boundaries do not present a special distribution pattern of their c axes this suggests that rotation recrystallization is an isotropic process not affected by the direction of the macroscopic strain

fresh pumice from the central indian basin a krakatau 1883 signature the occurrence of pumice in the central indian basin cib has been a controversy some are of the opinion that these pumices are erupted from a volcano along the ninetyeast ridge while others suggest the indonesian arc volcanism as their probable source recently it is also suggested that the pumice are the products of in situ silicic volcanism in the cib this study was initiated in order to resolve this controversy about the origin of the fresh pumice from the cib the fresh pumice pieces of different sizes and shapes were collected from the surficial sediments of the cib from an average depth of about 5000 m 12 s 76 e glass is the main phase comprising about 95 volume and resembles the matrix glass in krakatau 1883 dacitic pumice which is also very poor 5 in the modal mineral composition the mineral and glass analyses were carried out with the help of the electron microprobe the orthopyroxenes clinopyroxenes plagioclases and spinels are the mineral phases present in these pumices the pyroxenes have similar wo en and fs components to those previously reported from the 1883 krakatau eruption products geochemically these fresh pumices have a high sio2 content 71 and high total alkali content 8 similar to 1883 eruption products of krakatau thus based on the similarities in mineralogy and glass chemical composition it is inferred that these fresh pumices are the products of the fierce 1883 eruption of the krakatau volcano in the sunda strait of the indonesian arc

thermo mechanically coupled ice sheet response cold polythermal temperate classical mixture concepts are the appropriate vehicle for describing the dynamics of ice masses containing some water we review and derive respectively the theoretical formulations of cold polythermal and temperate ice masses emphasize the peculiarities of the model equations and point to difficulties that were encountered with the proposed models the focus is both on the adequate physical motivation of the models and the consistency of their mathematical representation

petrologic evolution of krakatau indonesia implications for a future activity abstract krakatau volcano is located along a n35e volcanic lineament running through the sunda straits indonesia its last activity has been characterized by successive phases each beginning with the construction of a cone and ending with its destruction and the formation of a caldera the two last pre and post 1883 cycles are well known but the more ancient ones are not so clearly defined lavas of krakatau belong to an andesitic series in which fractional crystallization plays the most important role the petrologic evolution is characterized by a cyclicity in good agreement with the structural evolution the succession is regular basalts basic andesites acid andesites dacites a gap between acid and basic andesites occurs in each cycle the destructive stages correspond to the occurrence of dacitic terms the anak cycle was characterized from 1927 to 1979 by basalts and basic andesites the 1981 eruption involved a more differentiated magma close to dacitic detailed study of the petrologic evolution since 1883 emphasizes the predominant role of fractional crystallization this process occurred during a very short period between 1979 and 1981 separation of labradorite augite olivine and magnetite from parental basic andesite may generate the dacitic descendant in a shallow reservoir p h 2 o estimated about 0 5 kbar implications for a future activity are considered

deformation in the vicinity of ice divides calculs numeriques aux elements finis de la variation de la vitesse horizontale avec la profondeur au voisinage d une difference de la glace symetrique isotherme et ou glissement sur un lit plat

the shallow ice approximation for anisotropic ice formulation and limits ice sheet flow modeling is generally based on the shallow ice approximation sia developed for isotropic ice we extend this approximation to anisotropic ice and check its validity for both the isotropic and anisotropic cases by comparing the results of up to second order sia with those obtained from a model solving the full set of mechanical equations the theory is developed for a constitutive relation describing newtonian and non newtonian behavior but numerical results are shown only for a newtonian behavior the results are compared for plane flow of isothermal ice under steady state conditions sia gives an excellent representation of the newtonian flow of an isotropic ice sheet and a good description for the anisotropic case the zero order approximation is sufficient to describe ice flow precisely over flat bedrock for the anisotropic and isotropic cases even close to an ice divide for uneven bedrock the second order sia gives excellent results for isotropic ice and acceptable results for anisotropic ice finally we quantify the error introduced by using an enhancement factor to represent the anisotropy when longitudinal stresses are taken into account

on the effects of anisotropic rheology on ice flow internal structure and the age depth relationship at ice divides we use numerical modeling with a full system stokes solver to elucidate the effects of nonlinear rheology and strain induced anisotropy on ice flow at ice divides we find that anisotropic rheology profoundly affects the shape of both isochrone layering and surface topography anisotropic effects cause the formation of a downward curving fold i e a syncline in isochrones in the lower central area beneath the ice divide when the resulting syncline is superimposed on the well known raymond anticline a double peaked raymond bump is formed furthermore to each side of the raymond bump flanking synclines are formed in addition anisotropic effects are found to give rise to a subtle concavity in the surface profile to both sides of the summit the lower center syncline the flanking synclines and the near summit surface concavity have all previously been observed in nature but hitherto no explanation for the genesis of these features has been given we compare modeling results with radiograms collected from fuchs ice piedmont and kealey ice rise antarctica good overall agreement is found in particular we are able to reproduce all observed qualitative features of surface geometry and internal layering by including and only by including the effects of induced nonlinear rheological anisotropy on flow rheological anisotropy has the potential to profoundly affect the age distribution with depth and caution must be exercised when estimating age of ice from ice cores with an isotropic model the occurrence of linear features parallel to the ridge of ice divides often seen in satellite imagery is indicative of long term stability rather than signs of ongoing ice divide migration as previously suggested such ice divides are ideal locations for extracting ice cores

multiscale modeling of ice deformation behavior understanding the flow of ice in glaciers and polar ice sheets is of increasing relevance in a time of potentially significant climate change the flow of ice has hitherto received relatively little attention from the structural geological community this paper aims to provide an overview of methods and results of ice deformation modeling from the single crystal to the polycrystal scale and beyond to the scale of polar ice sheets all through these scales various models have been developed to understand describe and predict the processes that operate during deformation of ice with the aim to correctly represent ice rheology and self induced anisotropy most of the modeling tools presented in this paper originate from the material science community and are currently used and further developed for other materials and environments we will show that this community has deeply integrated ice as a very useful model material to develop and validate approaches in conditions of a highly anisotropic behavior this review by no means exhaustive aims at providing an overview of methods at different scales and levels of complexity

abyssal sediment erosion in the central indian basin evidence from radiochemical and radiolarian studies abstract radiochemical analyses of three spade cores collected around 78 e between 10 s and 12 s from the abyssal depths in the central indian basin yield an average accumulation rate of 2 mm ka the ratios of 230th flux in the sediments to its production rate in the overlying water column fa fp are extremely low 0 2 this fact and the absence of collosphaera invaginata first appearance datum 150 200 ka an index radiolarian species of neogene radiolarian zone 1 indicate intense erosion and lateral transport of younger sediments from this region the radiochemical and radiolarian biostratigraphic evidence for nearly 175 ka of erosion of the chronological record of the sediments in this region has been attributed to the effect of turbulent antarctic bottom water entering the central indian basin through the northern saddles of the ninety east ridge

textures and fabrics in the grip ice core a comprehensive study of textures and fabrics has been carried out on the greenland ice core project grip ice core crystal sizes and c axis orientations have been measured on thin sections with conventional techniques yielding new information on the growth rotation and recrystallization of ice crystals in the greenland ice sheet normal grain growth is found to persist to a depth of 700 m in the core where the onset of polygonization due to increasing strain prevents a further increase in grain size in the holocene ice smaller crystals are observed in the wisconsin ice larger crystals are found in the eemian ice and the crystal size is found to vary with climatic parameters in these periods this dependence which probably results from variable impurity content in the ice persists to a depth of 2930 m coarse grained ice probably resulting from rapid growth of crystals at comparatively high temperatures is found in the lowest 100 m of the core the data on c axis orientations reveal a steady evolution of the fabric from random near the surface to a strong single maximum in the lower part of the ice sheet a significant strengthening is not observed at the holocene wisconsin transition the fabric development indicates that vertical compression at the ice divide is the main mode of deformation down to a depth of 2850 m the evolution toward a single maximum fabric hardens the ice against vertical compression but softens it against simple shear evidence of simple shear deformation is clearly observed between 2850 m and 2950 m depth stretched fabrics in coarse grained ice in the lowest 100 m could be due to tensional stresses this ice is unlikely to be undergoing any significant horizontal deformation at the present time

sonic methods for measuring crystal orientation fabric in ice and results from the west antarctic ice sheet wais divide we describe methods for measuring crystal orientation fabric with sonic waves in an ice core borehole with special attention paid to vertical girdle fabrics that are prevalent at the wais divide the speed of vertically propagating compressional waves in ice is influenced by vertical clustering of the ice crystal c axes shear wave speeds particularly the speed separation between fast and slow shear polarizations are sensitive to azimuthal anisotropy sonic data from the wais divide complement thin section measurements of fabric thin sections show a steady transition to strong girdle fabrics in the upper 2000 m of ice followed by a transition to vertical pole fabrics below 2500 m depth compressional wave sonic data are inconclusive in the upper ice due to noise as well as the method s inherent insensitivity to girdle fabrics compared with available thin sections sonic data provide better resolution of the transition to pole fabrics below 2500 m notably including an abrupt increase in vertical clustering near 3000 m our compressional wave measurements resolve fabric changes occurring over depth ranges of a few meters that cannot be inferred from available thin sections but are sensitive only to zenithal anisotropy future logging tools should be designed to measure shear waves in addition to compressional waves especially for logging in regions where ice flow patterns favor the development of girdle fabrics

a flow law for anisotropic ice and its application to ice sheets a new formulation of the flow law of polycrystalline ice was derived for an rbitrary c axis orientation fabric pattern the rate factor of the flow law is given by a geometrical factor that can be calculated with c axis fabric data and stress condition a theoretical calculation combining this flow law with a lattice rotation model by intracrystalline slip in the constituent grains predicts the development of characteristic fabric patterns observed in many laboratory experiments and in the field according to this flow model of polycrystalline ice if significant recrystallization does not occur uniaxial compression makes the ice body harder with increasing strain whereas simple shear makes the ice body softer with strain this may significantly affect ice sheet dynamics

the origin of ferro manganese oxide coated pumice from the central indian ocean basin pumice clasts partially and fully coated with ferro manganese oxide from the central indian ocean basin ciob were analysed for major trace and rare earth elements and glass and mineral grain chemistry to assess their possible source these pumice clasts have 95 glassy matrix with 5 of plagioclase ortho pyroxene amphibole and spinel mineral grains pumice is rhyolitic with high content of silica and total alkalis and show strong light rare earth element enrichment with la 100 times enriched compared to chondritic abundance and exhibits a pronounced negative eu anomaly both the partially and fully coated pumice clasts have a similar composition but this and their glass and mineral orthopyroxene chemistry differs from all tuffs known to have erupted from the toba caldera complex the partial to complete fe mn oxide coating suggests the pumice probably predates activity from the toba caldera their general composition is consistent with a volcanic arc origin and show similarities to rhyolitic eruptives from sumatra the distribution of partially and fully coated pumice in the ciob overlaps with that of uncoated pumice from the voluminous ytt eruption suggesting that other large rhyolitic calderas active in sumatra indonesia similarities in composition to ash layers from the ciob suggest a possible late miocene late pleistocene age for these pumices

glacial interglacial contrast in mbt cbt proxies in the south china sea implications for marine production of branched gdgts and continental teleconnection abstract two proxies derived from branched glycerol dialkyl glycerol tetraethers br gdgts the methylation index of branched tetraethers mbt and the cyclization ratio of branched tetraethers cbt are often used to reconstruct paleo mean annual air temperature maat and soil ph based on the premise that br gdgts in the marine environment are mainly of terrigenous origin however mounting evidence indicates that br gdgts can be produced in situ in oceanic settings which may affect maat reconstruction and the use of other related paleoenvironmental proxies we have determined br gdgt distributions in a sedimentary core md05 2896 7 from the southern south china sea which provided high resolution profiles of mbt and cbt indices as well as the branched and isoprenoid tetraether bit index bit varied systematically with glacial interglacial cycles reaching much lower

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

a possible submarine volcano near the central part of ninety east ridge indian ocean abstract floating pumice masses have been observed several times near the central part of ninety east ridge indian ocean in 1879 and in 1883 the author is of the opinion that the source of these pumice masses cannot have been the volcano krakatau in the sunda strait as has been suggested earlier by other authors but that they originated in situ examples of floating pumice observed over submarine volcanoes lying at depths of 1500 m or even more are mentioned briefly by other authors suggesting that eruptions in deep water may indeed lead to its formation the suspected submarine volcano may coincide with an elongated but rather flat elevation a little north of the central part of ninety east ridge the center of this elevation is located approximately at 6 05 s 89 10 e a roughly nw se profile of this area shows the presence of this shield like feature very near the site where floating pumice was seen on three occasions the inferred submarine volcanic center is lying within the belt of heat flow anomaly called belt i a particular hot spot with an unusually high heat flow value 7 61 hfu is located southeast of the feature in question the heat flow values measured in belt i are well above the world average this belt may be the result of the presence of linearly arranged magma chambers at a relatively shallow depth probably within the oceanic crust this belt has a length of 1100 km the seismic activity of the area investigated is similarly remarkable numerous powerful shocks have taken place there belt i represents the axis of the seismic zone extending from points b to c fig 5 the seismic belt is regarded as a zone of weakness within which fractures can originate permitting rock melts to emerge from depth towards the surface

method for anisotropic crystal orientation fabrics detection using radio wave depolarization in radar sounding of mars polar layered deposits the polar layered deposits plds provide a wealth of information about the past climate evolution of mars surface mass fluxes and ice flow mainly governed topography and layering of the pld china s mars probe including an orbiter and a landing rover will be launched by 2020 a new type satellite borne mars penetrating radar instrument has been selected to be a part of the payloads on the orbiter its main scientific objectives are to map the distribution of water water ice and to detect the soil characteristics at global scale on the martian crust compared with mars advanced radar for subsurface and ionospheric sounding and shallow radar the biggest difference is that the antenna system of this mars penetrating radar consists of two dipole antennas mutually perpendicular this special configuration enables the investigation of the ice flow of pld by detecting and analyzing the features of anisotropic crystal orientation fabric cof thus relying on the fact that the radio waves are depolarized while passing through an anisotropic cof layer in this paper a method for anisotropic cof detection based on this radar system is proposed the radar echo formulation of anisotropic cof is derived and the ratio of the signals measured by the two perpendicular antennas is used to analyze the anisotropy of cof we demonstrate that the ratio is an ideal criterion for the detection and analysis of cof since it contains all parameters about the anisotropy feature of cof and it is independent of the attenuation in the propagation path and the reflection coefficient in order to verify the validity of the derived analytical expression of the ratio for the detection of cof finite different time domain simulations are carried out based on a simple model of the subsurface of pld which contains an anisotropic cof layer the advantages of this method the potential application scenarios and the effects of the martian environment are also discussed

flow law for ice in polar ice sheets theories of glacier flow are based commonly on the assumption that ice is not a newtonian fluid but has a non linear stress dependent viscosity1 5 here we re examine the spreading of antarctic ice shelves and suggest that the data cannot define a unique flow law tilt measurements in four boreholes in both the arctic and antarctic seem to show that a linear flow law e a1 where a1 10 15 s 1 pa 1 at 28 c where e is the effective strain rate and the effective shear stress may be just as appropriate for describing the flow of polar ice sheets

a new fully three dimensional numerical model for ice dynamics the problem of describing ice dynamics has been faced by many researchers in this paper a fully three dimensional model for ice dynamics is presented and tested using an approach followed by other researchers ice is considered a non linear incompressible viscous fluid so that a fluid dynamic approach can be used the model is based on the full three dimensional stokes equations for the description of pressure and velocity fields on the saint venant equation for the description of the free surface time evolution and on a constitutive law derived from glen s law for the description of ice viscosity the model computes the complete pressure field by considering both the hydrostatic and hydrodynamic pressure components it is time evolutive and uses high order numerical approximation for equations and boundary conditions moreover it can deal with both constant and variable viscosity three theoretical tests and two applications to priestley glacier antarctica are presented in order to evaluate the performance of the model and to investigate important phenomena of ice dynamics such as the influence of viscosity on pressure and velocity fields basal sliding and flow over perturbed bedrocks all these applications demonstrate the importance of treating the complete pressure and stress fields

ash layers and pumice in the central indian basin relationship to the formation of manganese nodules abstract manganese nodules in areas of biosiliceous sedimentation in the central indian basin cib have three main types of nuclei 1 fragments of altered ash layer 2 felsic pumice and 3 indurated residual sediments red clay type morphological variations among nodules are due to the form of the nuclei the type of accretionary process diagenetic or hydrogenetic accretion and thickness of the crust piston cores in the same areas sampled up to four tephra layers interstratified in red clay all of which underlie condensed series of pliocene to recent siliceous oozes the in situ tephra layers and fragments of tephra in the core of the nodules are similar in composition they are completely altered to smectite and smectite zeolite tephra horizons are 10 35 mm thick and have sharp contacts with the red clay rare earth element ree patterns zr hf and nb content and ti al ratios strongly suggest that this altered volcaniclastic material originated as distal fallout from the indonesian volcanic arc iva and was deposited in late neogene time positive ce ce anomalies indicate that alteration of this tephra may have occurred at the sediment water interface prior to burial under hydrogenous conditions rhyolite pumicelapilli are water laid and are variably replaced by fe mn oxides inducing a higher metal content in the bulk nodules pumice constitutes one of the main types of nuclei of diagenetic nodules that are found in the deep sea troughs and plains where biosiliceous deposits are thick petrological characteristics of the pumice suggest that volcanic eruptions from toba in northern sumatra are the most likely source tectonic deformation of the indian plate may have affected the sedimentary sequence causing uplift and outcrop of ash layers on the topographic highs and slopes eroded fragments of these ash layers and waterlaid pumice fragments have contributed significantly to the large extent of mn nodules in the cib

the role of crystal fabric in flow near an ice divide polycrystalline ice near an ice divide typically shows a crystal fabric crystal preferred orientation with c axes clustered vertically we explore the effect of this fabric on the large scale flow pattern near an ice divide we incorporate an analytical formulation for anisotropy into a non linear flow law within a finite element ice sheet flow model with four different depth profiles of crystal fabric we find that the effect of fabric is significant only when a profile has a minimum cone angle of less than 25 for a steady state divide the shape and size of the isochrone arch can depend as much on the crystal fabric as it does on the non linearity of ice flow a vertically oriented fabric tends to increase the size of the isochrone arch never to reduce it also non random fabric has little effect on the ice divide flow pattern when ice is modeled as a linear newtonian fluid finally when we use a crystal fabric profile that closely approximates the measured profile for siple dome west antarctica the model predicts concentrated bed parallel shearing 300 m above the bed

chemical aspects of alteration of acidic tuffs and their application to siliceous deposits in order to evaluate fundamental chemical characteristics of acidic tuffs and to determine which of these characteristics in volcanogenic sediments is preserved during weathering processes we have analysed 41 pairs of ash samples each pair consists of 1 raw samples obtained from ash layers which consist of a mixture of volcanic glasses and clays and 2 glass shard aggregations extracted from the raw samples the migration of elements in the samples during alteration was examined using the analysed values of these pairs the results showed that certain compositional ratios of the glass shards do not basically change during alteration several diagrams showing these essentially constant ratios in acidic 65 sio2 volcanic glasses are presented these diagrams are helpful in identifying altered siliceous deposits derived from acidic tuffs and in discriminating such deposits from biogenically derived cherts

two orthotropic models for strain induced anisotropy of polar ice as polar ice descends from the free surface to depth in a large ice sheet it undergoes deformations which give rise to the formation and subsequent evolution of a fabric and associated anisotropy in this paper two orthotropic models of such strain induced anisotropy are considered model a is based on analysis ofthe microscopic behaviour of an individual ice crystal with transversely isotropic response and assumed uniform stress in a polycrystal the macroscopic response of the ice aggregate is then derived by applying the concept of an orientation distribution function and the resulting viscous law relates the strain rate to the stress and three structure tensors in model b it is assumed that the macroscopic response of ice is determined by the fabric induced entirely by macroscopic deformations and all microprocesses taking place at the grain level are ignored a constitutive relation is derived from a general frame indifferent law for orthotropic materials and expresses the stress in terms of the strain rate strain and three structure tensors the two models are applied to determine the viscous response of ice to continued uniaxial compression and simple shearing in order to compare the predictions of both theories

a numerical study of anisotropic low reynolds number free surface flow for ice sheet modeling few ice sheet flow models have been developed that solve the complete set of mechanical equations until now these models were limited to isotropic conditions we present here a two dimensional finite difference method capable of solving the equations for the steady flow of a viscous incompressible anisotropic fluid with a free surface under isothermal conditions it is not a standard method especially with respect to the time discretization of the numerical scheme and converges for very low reynolds numbers this method is applied here to the planar flow of anisotropic ice over flat or irregular bedrock with no slip boundary conditions at the ice bedrock interface the results are presented here for newtonian behavior in the vicinity of an ice divide the ice is assumed to be isotropic at the ice sheet surface with continuous and prescribed development of anisotropy with increasing depth going from isotropic to anisotropic situations our results indicate that the free surface becomes flatter and the shear strain rates larger and more concentrated near the bedrock the flow is less sensitive to variations of the bedrock topography in the anisotropic case than in the isotropic case furthermore a new phenomenon appears in the anisotropic case the partial stagnation of ice in the holes of the bedrock these effects have significant consequences when dating the ice the isochrones obtained in the anisotropic case are flatter and the anisotropic ice is more than 10 younger above the bumps and more than 100 older within the holes than for the isotropic ice

flow law hypotheses for ice sheet modeling ice flow modeling requires a flow law relating strain rates to stresses in situ but a flow law cannot be measured directly in ice sheets microscopic processes such as dislocation glide and boundary diffusion control both the flow law for ice and the development of physical properties such as grain size and c axis fabric these microscopic processes can be inferred from observations of the physical properties and the flow law can then be estimated from the microscopic processes a review of available literature shows that this approach can be imperfectly successful interior regions of large ice sheets probably have depth varying flow law constants with the stress exponent n for power law creep less than 3 in upper regions and equal to 3 only in deep ice n probably equals 3 through most of the thickness of ice shelves and ice streams

modelling of kealey ice rise antarctica reveals stable ice flow conditions in east ellsworth land over millennia flow at ice divides their shape size and internal structure depend not only on local conditions but also on the flow regimes and past histories of the surrounding ice masses here we use field data from kealey ice rise ellsworth land west antarctica in combination with flow modelling to investigate any possible signs of transients in the flow of the surrounding ice masses kealey ice rise shows linear surface features running parallel to its ridge in satellite imagery and a conspicuous layering in the ground penetrating radar data known as double peaked raymond bumps through numerical modelling by using an anisotropic full stokes thermomechanical flow solver we analyse the evolution of kealey ice rise and the timescales involved we conclude that the features observed in the stratigraphy of kealey ice rise require at least 3 ka of near stationary flow conditions however we cannot exclude the possibility of a recent flow reorganization in the last century we stress that the signs of stationary flow in radar stratigraphy observed in kealey ice rise have been observed in other ice divides in the east ellsworth land area suggesting stationary flow conditions over a millennial timescale in the region

an analytical approach to deformation of anisotropic ice crystal aggregates deformation rates of single hexagonal crystals deforming by glide on the basal plane are described as a function of stress state and crystal orientation these results are used to infer the deformation rate of crystal aggregates assuming that the stress distribution within the crystal aggregate is homogeneous analytical equations for the deformation rate of anisotropic ice aggregates are derived for vertically symmetric girdle fabric this type of fabric is approximated by a uniform distribution of c axis orientations between a cone angle and a smaller girdle angle relative to the symmetry axis for simple shear stress acting on a single maximum fabric there is a slight de enhancement for cone angles of 60 90 in uniaxial compression a maximum enhancement of 1 7 occurs at a cone angle of 57 a pure shear stress state has similar features with the additional complication that it causes a non zero transverse strain rate except for perfect vertical alignment of crystals and isotropic fabric in combined states of stress the contribution of each stress component to the strain rate depends on fabric a single enhancement factor is not adequate to describe the effects of anisotropy for complex stress states

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

drift pumice at christmas island and hawaii evidence of oceanic dispersal patterns the use of pumice as a geological tracer for oceanic dispersal patterns was evaluated by comparing the elemental signatures of drift pumice collected from christmas island and hawaii in relation to prevailing oceanic currents and pumice source areas both sites lie in the middle of the pacific ocean far from sources of volcanic pumice hawaii lies in the persistent westward flowing north equatorial current christmas island in contrast is influenced by the highly variable westward flowing south equatorial current and the eastward flowing equatorial counter current pumice is very abundant at christmas island in the beach drift pumice from christmas island is derived from the western pacific ocean krakatau southwestern pacific ocean tonga trench east pacific ocean mexico south atlantic ridge and an unknown source in contrast pumice is rare in hawaii pumice from hawaii originates primarily from the south sandwich islands mexico isla san benedicto and krakatau the currents that control dispersal of pumice also control dispersal of larvae and rafted organisms christmas island has a higher coral diversity 31 genera 81 species than hawaii 17 genera 50 species hawaii receives only small amounts of pumice drift from a limited area to the east and has a more restricted coral diversity while christmas receives massive amounts of pumice and presumably larvae and rafted organisms from the area of high coral diversity to the west

isothermal flow of an anisotropic ice sheet in the vicinity of an ice divide simulations of glacier flow are commonly based on the assumption that ice has an isotropic viscosity here we examine the plane flow of ice in the special region of an ice divide using a constitutive relation for an anisotropic incompressible viscous body that is orthotropic and transversally isotropic ice is assumed to be isotropic at the ice sheet surface with the continuous development of a vertical single maximum c axis fabric with increasing depth we consider the theoretical case of an isothermal ice sheet over a horizontal bedrock with no slip at the ice bedrock interface the ice sheet surface elevation is imposed and the flow corresponding to the steady state is calculated using a two dimensional finite difference model based on the resolution of a pressure poisson equation in this model all components of the stress and strain rate tensor are calculated the main conclusion is that for a fixed surface elevation the general flow pattern accelerates when the anisotropic behavior of the ice is taken into account due to the greater fluidity with respect to shear stress the downward motion of the ice is faster despite a higher resistance to vertical deformation as a result the dominance of shear strain rate in the flow of polar ice is stronger in the anisotropic case than in the isotropic case the shear stresses are slightly relaxed while the longitudinal stresses are significantly increased in the anisotropic case

past precipitation rates derived from ice core measurements methods and data analysis ice cores in which annual layers can be identified or particular horizons dated provide a record of past precipitation rates if measured layer thicknesses can be corrected for thinning by flow since deposition total strain is usually calculated from steady state flow models because of lack of information about the ice cap s history an assumption of uniform vertical strain rate is valid at best in the upper half of the ice column complex models are needed to take account of uneven bedrock and variations in precipitation rate upstream of the borehole drilling at an ice divide avoids these complications new data analyses suggest that precipitation at camp century has fluctuated by only plus or minus 10 for the past 8500 years whereas at devon island on the opposite side of baffin bay precipitation has been uniform only for the past 1300 years with significantly higher values before that estimation of pre holocene precipitation rates is especially difficult because that ice may have an effective viscosity different from holocene ice three published estimates of pre holocene precipitation at camp century are unreliable the authors did not allow for thinning and their time scales differ appreciably data from three antarctic stations show some inconsistencies measuring concentrations of 10be in ice cores may be an alternative method of determining past precipitation rates