the time averaged geomagnetic field global and regional biases for 0 5 ma summary palaeodirectional data from lava flows and marine sediments provide information about the long term structure and variability in the geomagnetic field we present a detailed analysis of the internal consistency and reliability of global compilations of sediment and lava flow data time averaged field models are constructed for normal and reverse polarity periods for the past 5 ma using the combined data sets nonzonal models are required to satisfy the lava flow data but not those from sediments alone this is in part because the sediment data are much noisier than those from lavas but is also a consequence of the site distributions and the way that inclination data sample the geomagnetic field generated in the earth s core different average field configurations for normal and reverse polarity periods are consistent with the palaeomagnetic directions however the differences are insignificant relative to the uncertainty in the average field models thus previous inferences of non antipodal normal and reverse polarity field geometries will need to be re examined using recently collected high quality palaeomagnetic data our new models indicate that current global sediment and lava flow data sets combined do not permit the unambiguous detection of northern hemisphere flux lobes in the 0 5 ma time averaged field highlighting the need for the collection of additional high latitude palaeomagnetic data anomalous time averaged field structure is seen in the pacific hemisphere centred just south of hawaii the location of the anomaly coincides with heterogeneities in the lower mantle inferred from seismological data the seismic observations can be partly explained by lateral temperature variations however they also suggest the presence of lateral compositional variations and or the presence of partial melt the role of such heterogeneities in influencing the geomagnetic field observed at the earth s surface remains an unresolved issue requiring higher resolution time averaged geomagnetic field models along with the integration of future results from seismology mineral physics and numerical simulations

geomagnetic field intensity and reversals during the past four million years a long and continuous record of the geomagnetic field intensity shows that intensity variations are dominated by two modes major episodes of field regeneration prevail on timescales of a few thousand years immediately after most reversals whereas stable polarity states are characterized by a slow 0 5 myr relaxation process reversals can be seen as the consequence of a progressive degradation of the dipole field

electromagnetic core mantle coupling and paleomagnetic reversal paths calculations of electromagnetic torques between the fluid outer core and the heterogeneous lower mantle demonstrate that the geomagnetic field can lock with the mantle during polarity transitions we show that the virtual geomagnetic pole vgp is attracted toward mantle regions with low electrical conductivity and repelled from mantle regions with high conductivity using seismically determined structure of the lower mantle to infer the pattern of conductivity variations at the base of the mantle we find two preferred reversal routes for the vgp one beneath the americas and another beneath asia these paths particularly the one beneath the americas lie close to the vgp paths seen in some sedimentary and lava records of polarity reversals suggesting that the core and mantle are electromagnetically coupled during reversals

geographical distribution of transitional vgps evidence for non zonal equatorial symmetry during the matuyama brunhes geomagnetic reversal abstract paleomagnetic records of the matuyama brunhes polarity reversal obtained recently from the northern equatorial and southern latitudes of the atlantic sector display a distinct symmetry with respect to the equator the virtual geomagnetic pole vgp paths from the mid latitude sites in the northern and southern hemispheres dsdp site 609 and core v16 58 are nearly coincident tracking northward through the americas the vgp path from the equatorial site odp site 664 tracks northward through eastern asia nearly antipodal to the other two paths modeling results indicate that superimposing an h 1 3 geometry onto a reversing axial dipole explains the sense of equatorial symmetry exhibited by these three records these results are consistent with the bulk of the data available from other records of this polarity transition suggesting that the transitional vgps during this reversal tend to fall along two distinct longitudinal bands the grouping of transitional vgps suggests that there is a strong geographical control over the reversal process an apparent correlation between the longitudinal bands of transitional vgps and the location of the major flux concentrations in the radial component of the historical geomagnetic field at the core mantle boundary 1 suggests that the processes which cause these flux concentrations may have influenced the geometry of the matuyama brunhes transitional field

simulated geomagnetic reversals and preferred virtual geomagnetic pole paths summary the question of whether virtual geomagnetic poles vgps recorded during reversals and excursions show a longitudinal preference is a controversial one amongst palaeomagnetists one possible mechanism for such vgp clustering is the heterogeneity of heat flux at the core mantle boundary cmb we use 3 d convection driven numerical dynamo models with imposed non uniform cmb heat flow that show stochastic reversals of the dipole field we calculate transitional vgps for a large number of token sites at the earth s surface in a model with a simple heat flux variation given by a y22 harmonic the vgp density maps for individual reversals differ substantially from each other but the vgps have a tendency to fall around a longitude of high heat flow the mean vgp density for many reversals and excursions shows a statistically significant correlation with the heat flow in a model with an imposed heat flux pattern derived from seismic tomography we find maxima of the mean vgp density at american and east asian longitudes roughly consistent with the vgp paths seen in several palaeomagnetic studies we find that low latitude regions of high heat flow are centres of magnetic activity where intense magnetic flux bundles are generated they contribute to the equatorial dipole component and bias its orientation in longitude during reversals the equatorial dipole part is not necessarily dominant at the earth s surface but is strong enough to explain the longitudinal preference of vgps as seen from different sites

infrasonic harmonic tremor and degassing bursts from halema uma u crater kilauea volcano hawaii 1 the formation evolution collapse and subsequent resurrection of a vent within halema uma u crater kilauea volcano produced energetic and varied degassing signals recorded by a nearby infrasound array between 2008 and early 2009 after 25 years of quiescence a vent clearing explosive burst on 19 march 2008 produced a clear complex acoustic signal near continuous harmonic infrasonic tremor followed this burst until 4 december 2008 when a period of decreased degassing occurred the tremor spectra suggest volume oscillation and reverberation of a shallow gas filled cavity beneath the vent the dominant tremor peak can be sustained through helmholtz oscillations of the cavity while the secondary tremor peak and overtones are interpreted assuming acoustic resonance the dominant tremor frequency matches the oscillation frequency of the gas emanating from the vent observed by video tremor spectra and power are also correlated with cavity geometry and dynamics with the cavity depth estimated at 219 m and volume 3 106 m3 in november 2008 over 21 varied degassing bursts were observed with extended burst durations and frequency content consistent with a transient release of gas exciting the cavity into resonance correlation of infrasound with seismicity suggests an open system connecting the atmosphere to the seismic excitation process at depth numerous degassing bursts produced very long period 0 03 0 1 hz infrasound the first recorded at kilauea indicative of long duration atmospheric accelerations kilauea infrasound appears controlled by the exsolution of gas from the magma and the interaction of this gas with the conduits and cavities confining it

dipolar reversal states of the geomagnetic field and core mantle dynamics palaeomagnetic records from lavas suggest that at least two specific inclined dipolar field configurations have dominated the reversal process for the past ten million years these long lived states provide a way to explain directional rebounds aborted reversals and the recording in sediments of what appear to be preferred longitudinal paths of the virtual geomagnetic pole the polar orientations correlate with near radial flux concentrations recognizable when today s field is stripped of its axial dipole and with lower mantle seismic anomalies suggesting a tie to deep earth dynamics

longitudinally confined geomagnetic reversal paths from non dipolar transition fields it has long been thought that conditions at the boundary between the core and mantle influence the earth s magnetic field but the supporting evidence is rather indirect1 3 recent palaeomagnetic results suggesting that there are persistent preferred longitudinal paths for the virtual geomagnetic pole vgp during reversals4 would provide the first direct evidence of the solid mantle s influence on the core although their statistical significance has been disputed5 6 the results are potentially exciting because the preferred paths lie close to the pacific rim where the present geomagnetic secular variation changes character2 7 here we present a simple model based on an extension of a previous theory8 that produces reversals with vgp paths confined within relatively narrow longitude bands despite the transition field having a substantially non dipolar structure thus although longitude bias of the vgp paths is definitive evidence for core mantle interaction simple vgp paths are not evidence of near dipolar transition fields

global geomagnetic field models for the past 3000 years transient or permanent flux lobes psvmod1 0 is a compilation of globally distributed palaeodirectional data from archaeomagnetic artefacts lava flows and lake sediments at 24 sites evaluated at 100 year intervals from 1000 bc to ad 1800 we estimate uncertainty in these measures of declination and inclination by comparison with predictions from standard historical models in time intervals of overlap and use the 100 year samples and their associated uncertainties to construct a sequence of minimum structure global geomagnetic field models global predictions of radial magnetic field at the coremantle boundary cmb as well as inclination and declination anomalies at the earth9s surface provide an unprecedented view of geomagnetic secular variations over the past 3000 years and demonstrate a consistent evolution of the field with time resolution of the models is poorest in the southern hemisphere where only six of the 24 sites are located several with incomplete temporal coverage low flux regions seen in the historical field near the north pole are poorly resolved but the northern hemisphere flux lobes are clearly visible in the models these lobes are not fixed in position and intensity but they only rarely venture into the pacific hemisphere the pacific region is seen to have experienced significant secular variation a strong negative inclination anomaly in the region like that seen in 0 5 ma models persists from 1000 bc until ad 1000 and then gradually evolves into the smaller positive anomaly seen today on average between 1000 bc and ad 1800 the non axial dipole contribution to the radial magnetic field at the core mantle boundary is largest in the north central pacific and beneath central asia with clear non zonal contributions at the earth9s surface average inclination anomalies are large and negative in the central pacific and most positive slightly to the east of central africa inclination anomalies decrease with increasing latitude average declinations are smallest in equatorial regions again with strong longitudinal variations largest negative departures are centred over australia and eastern asia secular variation at the earth9s surface is quantified by standard deviation of inclination and declination about their average values and at the cmb by standard deviation in radial magnetic field all three show significant geographical variations but appear incompatible with the idea that secular variation in the pacific hemisphere is permanently attenuated by greatly enhanced conductivity in d 00 beneath the region

high resolution record of the upper olduvai transition from po valley italy sediments support for dipolar transition geometry abstract a detailed record of the upper olduvai polarity transition composed of 100 transitional directions has been obtained from the crostolo section in northern italy a careful examination of the rock magnetic properties of the sediments using standard paleomagnetic techniques thermomagnetic scanning electron microscopy microprobe x ray diffraction and mossbauer analysis shows that greigite is the main magnetic carrier of the remanence the correlation of a shift in the magnetic record to a small scale sedimentary feature observed in the section indicates that the magnetization was acquired at deposition or very shortly after the virtual geomagnetic pole vgp path is largely confined along a great circle over north and south america 90 west of the site and consists of three stages first the vgp moves to southern mid latitudes then after a period of standstill it comes back to almost true north and finally the south pole is reached in a third step although different from a record from the southern indian ocean this path is virtually identical to those obtained for the same transition from north pacific deep sea cores and partly coincides with a north atlantic record which suggests that a dipolar component may be present in the transitional field during the upper olduvai reversal a review of the recently obtained records of various transitions shows that in more than two thirds of the cases the vgp paths are similarly confined along a meridian over the americas or antipodal to them irrespective of the sampling site and of the sense of the transition although not deterministic this tendency suggests that a similar dipolar component might be present in the transitional fields of other reversals of different ages

element partitioning between magnesium silicate perovskite and ferropericlase new insights into bulk lower mantle geochemistry n this study we investigated iron magnesium exchange and transition metal trace element partitioning between magnesium silicate perovskite mg fe sio3 and ferropericlase mg fe o synthetised under lower mantle conditions up to 115 gpa and 2200 k in a laser heated diamond anvil cell recovered samples were thinned to electron transparency by focused ion beam and characterized by analytical transmission electron microscopy atem and nanometer scale secondary ion mass spectroscopy nanosims iron concentrations in both phases were obtained from x ray energy dispersive spectroscopy measurements and nanosims our results are the first to show that recently reported spin state and phase transitions in the lower mantle directly affect the evolution of fe mg exchange between both phases mg perovskite becomes increasingly iron depleted above 70 80 gpa possibly due to the high spin low spin transition of iron in ferropericlase conversely the perovskite to post perovskite transition is accompanied by a strong iron enrichment of the silicate phase ferropericlase remaining in the fe rich phase though nanoparticles of metallic iron were observed in the perovskite bearing runs suggesting the disproportionation of ferrous iron oxide but were not observed when the post perovskite phase was present implications on the oxidation state of the earth and core segregation will be discussed transition trace element ni mn concentrations determined with the nanosims show similar trends and could thus be used to trace the origin of diamonds generated at depth this study provides new results likely to improve the geochemical and geophysical models of the earth s deep interiors

the time averaged geomagnetic field as recorded by lava flows over the past 5 myr summary a recently compiled lava flow data base spanning the last 5 million years is used to investigate properties of the time averaged geomagnetic field more than 90 per cent of the power in the palaeofield can be accounted for by a geocentric axial dipole however there are significant second order structures in the field declination and inclination anomalies for the new data base indicate that the main second order signal is the far sided effect and there is also evidence for non zonal structure vgp virtual geomagnetic pole latitude distributions indicate that over the last 5 million years normal and reverse polarity morphologies are different and that any changes in the normal polarity field morphology are undetectable given the present data distribution regularized non linear inversions of the palaeomagnetic directions support all these observations we test the hypothesis that zonal models for the time averaged field are adequate to describe the data and find that they are not non zonal models are needed to fit the data to within the required tolerance level normal and reverse polarity field models obtained are significantly different field models obtained for the brunhes epoch data alone are much smoother than those obtained from combining all the normal polarity data simulations indicate that these differences can be explained by the less extensive data distribution for the brunhes epoch the field model for all of the normal polarity data ln1 contains features observed in the historical field maps although the details differ ln1 suggests that although the two northern hemisphere flux lobes observed in the historical field are stationary to a first order approximation they do show changes in position and amplitude a third less pronounced flux lobe is observed in ln1 over central europe the lack of structure in the southern hemisphere is due in part to the paucity of data jackknife estimates of the field models for different subsets of the data suggest that a few sites contribute significant structure to the final field models more conservative estimates of the time averaged field morphology are obtained by removing these sites

can heterogeneous core mantle electromagnetic coupling control geomagnetic reversals abstract an analytical model is developed for the electromagnetic torques exerted on the mantle by the poloidal magnetic field of the core interacting with a laterally heterogeneous conducting layer at the base of the mantle torques due to changes in both orientation and intensity of a dipolar poloidal field are included contrary to earlier suggestions our calculations predict that the trajectory of the magnetic pole during a polarity reversal is not strongly affected by heterogeneity in mantle electrical conductivity

the core mantle boundary the largest compositional discontinuity within the earth is the core mantle boundary cmb at a depth of 2889 km this boundary and the adjacent transition zones in the lowermost mantle and outermost core play a critical role in the earth s thermal and chemical evolution estimates of the heat flux out of the core indicate that the 200 km thick d region at the base of the mantle is a major thermal boundary layer the d region like its counterpart boundary layer in the lithosphere is laterally heterogeneous and possibly chemically stratified this complexity obscures the exact dynamic behavior of the d region however it is clear that the large density contrast 4 3 g em 3 across the cmb provides favorable con ditions for density stratification on both sides of the boundary the d region may be the refuse pile for heavy mantle heterogeneities while the outermost core may have a concentration of light components boundary layer instabilities in d have been invoked as sources of both localized mantle plumes and major upwelling currents in whole mantle convection systems dynamically supported topography on the cmb and lateral temperature gradients in d may produce coupling between the core and mantle influencing the excitation of the geodynamo the outer most core is generally believed to be radially and laterally homogeneous although chemical stratification has not been ruled out seismological geodynamical and geomagnetic investigations are unveiling the three dimensional complexity of the deep earth in the vicinity of the cmb

earth s enigmatic interface at the boundary between earth s mantle and its core physical properties such as density change dramatically in their perspective garnero and jeanloz 1 discuss the competing models of this boundary s structure distinguishing between the models will require more high quality seismic data 1 http www sciencemag org cgi content full 289 5476 70

paleomagnetic volcanic data and geometric regularity of reversals and excursions mostly on the basis of paleomagnetic sedimentary data it has been suggested that maps of virtual geomagnetic poles vgps corresponding to directions of the magnetic field at each site tend to fall along american and asian longitudes during reversals and excursions such geometric regularity in transitional fields may indicate that the core and mantle are dynamically coupled however studies of paleomagnetic lava data have thus far failed to show any pattern in transitional fields in this paper we examine a paleomagnetic lava database covering reversals and excursions which have occurred over the last 20 myr volcanic eruptions occur sporadically thus we normalize the data to account for the fact that reversal and excursional events at the various sites are recorded by different numbers of intermediate directions but we prefer not to use averaging methods of previous investigators who discarded or combined directions when they appeared to be similar we find that volcanic data give intermediate vgps which tend to fall along american and asian longitudes roughly consistent with the sedimentary data this result is not an apparent artifact arising from the nonuniform geographic distribution of volcanic sites provided the appropriate polarity is assigned to intermediate vgps we find that icelandic vgps tend to fall along asian longitudes other patterns in the data for example latitudinal clustering of vgps or distinguishing longitudinal preferences of excursions from reversals are not resolved however it appears that in general transitional fields are nondipolar

seismic precursors of the february 10 1990 eruption of kelut volcano java abstract kelut volcano eastern java indonesia erupted on february 10 12 1990 seismic activity and an increase of temperature and acoustic noise in the crater lake appeared three months before the eruption and were the main precursory signs of this eruption during the pre eruptive stage the number of events was eight per day on average and the cumulative seismic energy released was 5 6 10 15 erg these low values are uncommon for an andesitic volcano seismic activity mainly consisted of swarms of very shallow volcano tectonic earthquakes many of which were part of seismic multiplets in the last three weeks before the eruption seismic activity remained at a very low level this quiescent period was followed 24 hours prior to the eruption initiation by an intense swarm of low energy events which progressively became continuous spasmodic tremor most of the data obtained during the pre eruptive stage are consistent with a process of fracturation of a shallow and low strength plug blocking the eruptive conduit before the eruption several observations suggest that water is an important element in the preeruptive and eruptive phenomena at kelut volcano in particular the monochromatic events observed at kelut are characterized by one narrow peak in their spectra the estimations of q value and impedance contrast for these events suggest that the fluid which fills the source resonator could be water

the last two geomagnetic polarity reversals recorded in high deposition rate sediment drifts high resolution records of geomagnetic polarity transitions are rare owing to the difficulty of preserving in the geological record details of a process with a duration of only a few thousand years lavas can record the instantaneous field on cooling but lava records are themselves discontinuous and therefore polarity transition fields are seldom captured sedimentary records are more continuous but typical deep sea sedimentation rates of a few cm kyr 1are too low to record transition fields adequately here we report transition records from north atlantic drift deposits with high sedimentation rates 12 cm kyr 1 at the ocean drilling program sites 983 and 984 virtual geomagnetic polar vgp paths are similar within and between sites for the last two geomagnetic reversals attesting to a memory effect spanning back to back reversals the complex vgp paths feature two vgp clusters one located in northeastern asia and the other in the south atlantic ocean off south america and provide a link between the longitudinally constrained transitional vgps often recorded in sediments1 2 which represent highly smoothed renditions of transition fields and the scattered transitional vgps3 and vgp clusters4 observed in lava records

monotonic infrasound and helmholtz resonance at volcan villarrica chile 1 monotonic infrasound with stable peaked frequency of 0 77 hz was recorded at volcan villarrica in january 2010 similar monotonic infrasound had been previously reported at villarrica e g ripepe et al 2010 using joint infrasound and visual observations from a suspended camera we demonstrate that the likely source of infrasound is helmholtz resonance produced from a cavity with volume 105 m3 that separates the active convecting lava lake from an overhanging spatter roof spatter roof dimension 65 m diameter and vent diameter 10 m in the roof are constrained from video observations assuming a cylindrical cavity we infer a cavity height of 31 m that is corroborated by video records of spatter drips the drips take as long as 2 2 s to fall from the roof into the lake corresponding to a height of more than 24 m which is in good agreement with the observed resonance frequency

asthenospheric viscosity inferred from correlated land sea earthquakes in north east japan the viscosity of the earth s mantle has been estimated from studies of post glacial rebound1 2 post seismic deformations of the ground following large earthquakes3 4 and aftershock sequences5 7 here we derive a value for the viscosity of the asthenosphere from a correlation found in the historical catalogue of subduction induced seismicity between the intraplate land and interplate sea earthquakes in north east japan the correlation persists since the time of reliably reported earthquakes in ad 1600 land events precede sea events by 36 yr with a mean distance between land sea pairs of 200 km because of the visocoelastic coupling of the lithosphere to the asthenosphere a plausible mechanism to explain the correlation is stress migration governed by the viscosity of the asthenosphere large land shocks generate diffuse like stress pulses which sweep past and unlock the thrust fault in the subduction zone thus triggering the sea events the correlation time and distance provide a measure of the speed of diffusion 5 6 km yr 1 and hence an estimate of the viscosity 7 1018 pa s

effect of mantle plumes on the growth of d by reaction between the core and mantle the core mantle boundary is a fundamental compositional discontinuity in the earth where molten iron alloy from the core meets solid silicate minerals from the mantle heat flow from the core to the mantle creates a thermal boundary layer at the base of the mantle at the same time chemical reactions may create a layer of different composition and density than the overlying mantle using a finite element model of convection in a spherical shell that includes formation of dense material at the base of the shell we investigate how a layer forms at the base of the mantle development of a layer at the base of the mantle by this method depends both on the composition of the material forming at the core mantle boundary and on the rate at which the material diffuses into the mantle if the material is less than 3 6 denser than the overlying mantle assuming reasonable choices of lower mantle thermodynamic parameters the reactant material will be swept away by upwelling plumes more dense material on the other hand forms an internally convecting layer that entrains material from above the varying thickness and composition of the layer is consistent with geophysical observations of the characteristics of the deep mantle

preferred reversal paths caused by a heterogeneous conducting layer at the base of the mantle 1 time variations in the geomagnetic field induce secondary electric currents in the mantle which contribute to the magnetic field at the surface we explore the possibility that currents in a conducting layer at the base of the mantle disturb the transition field during magnetic reversals and alter the path of the virtual geomagnetic pole vgp we calculate the magnitude of the magnetic perturbation during a reversal and quantify the influence of this perturbation on the vgp reversal path the calculations show that the vgp reversal path depends on the geographic position of the observation site a histogram of reversal paths obtained using a randomly distributed set of sites reveals a preferred path through the americas when the histogram of reversal paths is evaluated with a limited number of sites based on locations in existing lava and sediment databases we find that the paths are confined to asian and american longitudes which is compatible with reversal path distributions inferred from paleomagnetic records

absence of preferred longitude sectors for poles from volcanic records of geomagnetic reversals according to a recent compilation of sedimentary records from the past 12 myr the reversing geomagnetic field displays a marked long term longitudinal organization which seems to be correlated with the thermal structure of the lower mantle1 however the crucial palaeomagnetic observation that intermediate virtual geomagnetic poles are preferentially confined to longitudes over the americas or antipodal to this sector2 3 might be an artefact caused by distortion and smoothing of the geomagnetic signal by sediments4 6 here we present a compilation of 400 intermediate poles from 121 volcanic records of excursions and reversals less than 16 myr old sampled at various longitudes and latitudes which does not confirm previous inferences1 and reinforces doubts4 6 about the ability of sediments to properly record a rapidly varying field in contrast to virtual poles from excursions those from reversals are uniformly distributed in longitude which indicates that the reversing field is statistically axisymmetrical thus we conclude that there is no evidence for control of transitional fields by the temperature distribution in the lowermost mantle

recurring transitional geomagnetic field geometries evidence from sediments and lavas records of the geomagnetic polarity transitions bounding the nunivak subchron have been obtained from the suva marl fiji 18 2 s 178 5 e the transitional virtual geomagnetic pole vgp positions cluster near southern south america and are remarkably similar to the vgp paths for these same reversals obtained from sicily the lower nunivak vgp path and relative intensity record are also very similar to a lower pliocene reverse to normal r to n polarity transition recorded in lavas on kauai the similarities in these transition records from both lava and sediment records suggests that an inclined near dipolar state of the transitional field recurred in these back to back reversals

unusual low frequency volcanic seismic events with slowly decaying coda waves observed at galeras and other volcanoes abstract events with slowly decaying coda waves are observed in active andesitic volcanoes associated mainly with vulcanian type eruptions or large gas emissions however these signals are also recorded in some quiescent volcanoes these unusual signals are considered to be related to magmatic activity and generally occur beneath the active crater the signals have been observed to be a short term precursor galeras volcano colombia 1992 1993 asama yama volcano japan 1983 after eruption tokachi dake volcano japan 1989 during seismic swarms meakan dake volcano japan 1982 and during quiescence puracevolcano colombia 1994 1995 tarumai volcano japan 1970 1971 1975 spectral analysis reveals common characteristics for this type of signal the spectrum is characterized by one or several sharp frequency peaks fundamental frequencies are not affected by epicentral distance azimuth or travel time indicating a source effect the damping coefficient for coda waves ranges from 0 002 to 0 02 and is related to large values of the quality factor q c ranging from 250 to 25 respectively these parameters may be the result of large amounts of gas bubbles in the magma body

inclination shallowing and preferred transitional vgp paths transitional vgp paths recorded in sediments cluster into two antipodal preferred longitude bands that tend to lie 90 away from their site longitudes the latter also being clustered vgp paths obtained from lava flow sequences though much fewer appear not to show these biases suggesting a rock magnetic influence on vgp paths recorded in sediments inclination shallowing of detrital magnetic remanence enhanced under low transitional field strengths is the most likely candidate we illustrate the effects of inclination shallowing by applying a simple shallowing model tan ir f tan ia where ia is the inclination of the magnetic remanence and ia is the inclination of the ambient field with field variation to hypothetical data sets shallowing induced clustering increases as f decreases and becomes extreme as f approaches 0 1 we have used the model to de shallow the available set of transitional vgp sediment records for various values of f the probability that the observations arise from inclination shallowing of a uniform random distribution of paths increases as f decreases when f drops to 0 13 there is a 50 chance of getting at least as much grouping as observed to decide if inclination shallowing is a dominant factor in the clustering we need to know whether such extreme shallowing is widespread in sedimentary records under transitional field conditions field and laboratory redeposition data are not yet adequate to resolve this question

tornillo type seismic signals at galeras volcano colombia 1992 1993 abstract unusual low frequency seismic events called tornillos screws at the observatorio vulcanologico y sismologico de pasto ovsp have been observed at galeras volcano during 1992 1993 of six eruptions that occurred between july 1992 and june 1993 five were preceded by episodes of tornillo signals these signals are characterized by 1 a waveform having a homogeneous distribution of frequencies 2 a long coda lasting up to several minutes 3 a small amplitude compared to the duration and 4 a slow decay of the coda the tornillo signals have been grouped into thirteen principal forms the 1992 1993 tornillos showed four main periods of occurrence each of which was followed by an eruption episode i occurred from 11 to 16 july 1992 and consisted of nine events episode ii lasted from 23 december 1992 to 14 january 1993 with twenty events episode iii was observed from 13 february to 23 march 1993 with 74 events episode iv lasted from 10 april to 7 june 1993 with 109 events the seismic activity at galeras in april may 1993 was characterized mainly by the occurrence of tornillo signals the behavior of these signals was similar to that before previous episodes ending with eruptions these observations permitted us to forecast the 7 june 1993 eruption several days to weeks beforehand the occurrence of tornillos is the most important criterion for determining the probability of an eruption at galeras in the current period of reactivation the maximum daily number of tornillos and the longest duration of individual events occur near the end of each episode within an individual episode the durations of single events increase progressively near to the point of eruption a positive correlation is observed between the total number of pre eruptive tornillo signals during an episode and the volume of material ejected by the eruption these observations may suggest that the presence of tornillos is an indication of variations in the physical conditions between the fluid and the surrounding solid material within the volcano and they constitute an important tool for forecasting future eruptions at galeras

electromagnetic core mantle coupling iii laterally varying mantle conductance abstract we consider the implications of lateral variations in deep mantle conductance for electromagnetic core mantle coupling and the consequent decadal variations in length of day large scale variations in conductance can alter long term trends in the torque but do not on average enhance its magnitude or alter its decadal variation many previous estimates of the torque predict an unobserved negative trend during this century this trend may result from modelling assuming an incorrect conductance structure by suitable choice of this structure the trend can be removed but there is no correlation between such a structure and seismological observations of lower mantle heterogeneity runcorn argued that a highly conducting layer in the deep mantle beneath the pacific could account for features of the observed secular variation and also preferred palaeomagnetic vgp reversal paths we argue that the conductance customarily advocated for this mechanism is inconsistent with core mantle angular momentum exchange on decadal time scales

statistical properties of paleomagnetic directions in kerguelen lava flows implications for the late oligocene paleomagnetic field 40 ar 39 ar ages are reported to aid in the calibration of the paleomagnetic results the primary contribution of this report however is a compilation of these new data with those already published in order to describe statistically the characteristics of the paleomagnetic field as recorded by the kerguelen flood basalts in total 258 paleomagnetic directions sampled at 13 stratigraphic sections through the lava pile are available and span an approximately 5 ma window from 25 to 30 ma the composite section represents at least 11 polarity zones that are correlated to the reference geomagnetic polarity timescale our approach is to investigate the average normal and reversed polarity field directions over this 5 ma window we calculated a paleomagnetic pole found to be located at l 85 5n f 189 3 e a95 2 3 k 16 5 n 233 this pole is in close agreement with the coeval paleomagnetic poles obtained from different worldwide places when analyzed in the indo atlantic hot spot reference frame in the statistical analysis we tackle the specific question is the secular variation isotropic this question is directly related to the occurrence of a longitudinal confinement of the virtual geomagnetic poles which is still a matter of debate among paleomagnetists by means of statistical tests we show that the paleomagnetic data from kerguelen agree with an isotropic model for paleosecular

palaeomagnetic constraints on the geometry of the geomagnetic field during reversals palaeomagnetic records of the path of the pole during reversals of the earth s magnetic field provide a test of the hypothesis that dipolar or low order axisymmetric components of the field dominate during reversals multiple records of reversals during the past 12 myr show no simple or consistent geographical pattern although a more robust analysis of the transitional field awaits a greater number of well distributed sampling sites the present data are not inconsistent with the simplest models in which a field reminiscent of the non dipole component of the present day field becomes dominant

tornillos beneath tongariro volcano new zealand abstract unusual low frequency seismic events have recently begun to occur beneath tongariro volcano new zealand because the seismic waveforms appear similar to waveforms called tornillos which have been recorded at galeras volcano in colombia and have been found to precede small to medium sized ash eruptions there by several weeks to months we are monitoring them closely at tongariro these events are characterised by small amplitude monochromatic or multichromatic waveforms that decay slowly with time features that suggest a source involving magmatic or hydrothermal fluids within a zone surrounded by hard rock the frequency content of monochromatic events is typically between 1 and 3 hz and is often found to be peaked at f 1 2 hz event durations range from about 10 to 80 s the appearance of the shorter duration events is identical to so called lp or long period events the lps and tornillos appear to be co located so that the only difference between lps and tornillos is the extended duration of the latter these events have been identified in the seismograms of the permanent monitoring station kavz since at least january 2001 occurring at a rate of about one per month during june august 2001 the rate of occurrence gradually increased peaking in september 2001 at several events per day we deployed two broadband and several short period seismometers at close distances to the expected source location the resulting seismograms show a complex mixture of seismic waves originating from shallow depth beneath the area of the te maari craters located approximately 0 5 km from the nearest station the close range broadband records reveal some interesting details about the tornillo source the tornillo waveform is found to consist of two parts a low frequency f 1 2 hz part and a higher frequency f 3 hz part the high frequency energy travels at body wave speeds c 3 0 km s while the low frequency energy travels at the speed of shear or surface waves in the shallow crust c 1 km s interestingly for some events the low frequency waves arrive before the high frequency body waves at the closest station this and comparison with several lp events suggests a spatial separation of the high and low frequency sources with the low frequency energy originating at shallow depth and acting as a trigger for the release of tectonic stress at greater depth in the form of high frequency body waves

deciphering records of geomagnetic reversals polarity reversals of the geomagnetic field are a major feature of the earth s dynamo questions remain regarding the dynamical processes that give rise to reversals and the properties of the geomagnetic field during a polarity transition a large number of paleomagnetic reversal records have been acquired during the past 50 years in order to better constrain the structure and geometry of the transitional field in addition over the past two decades numerical dynamo simulations have also provided insights into the reversal mechanism yet despite the large paleomagnetic database controversial interpretations of records of the transitional field persist they result from two characteristics inherent to all reversals both of which are detrimental to an ambiguous analysis on the one hand the reversal process is rapid and requires adequate temporal resolution on the other hand weak field intensities during a reversal can affect the fidelity of magnetic recording in sedimentary records this paper is aimed at reviewing critically the main reversal features derived from paleomagnetic records and at analyzing some of these features in light of numerical simulations we discuss in detail the fidelity of the signal extracted from paleomagnetic records and pay special attention to their resolution with respect to the timing and mechanisms involved in the magnetization process records from marine sediments dominate the database they give rise to transitional field models that often lead to overinterpret the data consequently we attempt to separate robust results and their subsequent interpretations from those that do not stand on a strong observational footing finally we discuss new avenues that should favor progress to better characterize and understand transitional field behavior

some dynamical consequences of partial melting in earth s deep mantle we use regional scale numerical models of mantle convection to investigate the simple hypothesis that seismically anomalous thin patches of earth s lowermost mantle termed ultralow velocity zones or ulvz are derived from partial melting of ordinary mantle the models span the lower 500 km of earth s mantle employ both temperature and melting related contributions to buoyancy and include a cold randomly moving downwelling introduced from above to maintain a thermal boundary layer partial melting of ordinary mantle introduces a ubiquitous partially molten layer above an isothermal core mantle boundary as a consequence of its isothermal and isobaric conditions although it naturally develops variations in thickness greater than two orders of magnitude with the thickest portions occurring at the base of upwelling plumes and a thin layer elsewhere we find that only a dense partially molten mixture produces partial melt distributions that are compatible with seismic observations of ulvz however if such a melt percolates downward a dense basal liquid layer accumulates above the core mantle boundary the apparent requirement of a volumetrically dense and non percolating melt phase in the lowermost mantle presents serious problems for the hypothesis that ulvz arise from melting of ordinary mantle and suggests that such features likely form as a consequence of more complex processes furthermore these considerations suggest that the solidus of ordinary mantle is a reasonable upper bound on the present day temperature of the cmb 2007 published by elsevier b v

growing the earth s d layer effect of density variations at the core mantle boundary the d region of the mantle has been interpreted as a variable thickness layer of hot chemically heterogeneous material at the base of the mantle this paper reports the results of numerical simulations testing whether growth of a chemically distinct layer at the base of the mantle is possible by influx of molten iron alloy from the core to the mantle a finite element model of thermo chemical mantle convection was used to simulate exchange of material between the mantle and core growth of a significant layer at the base of the mantle depends on several factors we focus on the effect of density of the material in the chemical boundary layer relatively low density material was swept away from the core mantle boundary by mantle upwellings very high density material in contrast was not swept away but flowed into the mantle too slowly to form a layer as thick as d for moderate density contrasts between material in a chemical boundary layer and the overlying mantle a laterally heterogeneous layer formed the stable layer results in an elevated temperature contrast between the core and the mantle high temperatures in this layer are associated with broad regions of elevated temperature in the overlying mantle

volcanic eruptions observed with infrasound 1 infrasonic airwaves produced by active volcanoes provide valuable insight into the eruption dynamics because the infrasonic pressure field may be directly associated with the flux rate of gas released at a volcanic vent infrasound also enhances the efficacy of volcanic hazard monitoring and continuous studies of conduit processes here we present new results from erebus fuego and villarrica volcanoes highlighting uses of infrasound for constraining quantitative eruption parameters such as eruption duration source mechanism and explosive gas flux

blast waves from violent explosive activity at yasur volcano vanuatu 1 infrasonic and seismic waveforms were collected during violent strombolian activity at yasur volcano vanuatu averaging 3000 seismic events showed stable waveforms evidencing a low frequency 0 1 0 3 hz signal preceding 5 6 s the explosion infrasonic waveforms were mostly asymmetric with a sharp compressive 5 106 pa onset followed by a small long lasting rarefaction phase regardless of the pressure amplitude the ratio between the positive and negative phases was constant these waveform characteristics closely resembled blast waves infrared imagery showed an apparent cold spherical front 20 m thick which moved between 342 and 405 m s before the explosive hot gas fragments cloud we interpret this cold front as that produced by the vapor condensation induced by the passage of the shock front we suggest that violent strombolian activity at yasur was driven by supersonic dynamics with gas expanding at 1 1 mach number inside the conduit

cross sectional stability of tidal inlets a comparison between numerical and empirical approaches article i nfo a process based morphodynamic model is used to simulate the evolution of tidal inlets towards equilibrium when subjected to a range of tide and wave conditions specific attention is given to the evolution of the cross sectional area of the inlet channel equilibrium values of cross sectional areas are found by approaching the equilibrium from two initial conditions i e by starting an experiment with an initial value of the cross sectional area that is smaller than the estimated equilibrium value and an initial value that is larger than the estimated equilibrium value similar to nature a good correlation between cross sectional area and tidal prism is found this suggests that the physical mathematical formulations in the model used to describe the hydrodynamic and sedimentary process are a good representation of the morphodynamics in the real world and that the conceptual idea of escoffier on the cross sectional stability of inlets is valid

detailed jaramillo field reversals recorded in lake sediments from armenia lower mantle influence on the magnetic field revisited abstract while it is well established that the earth s magnetic field is generated by a self sustaining dynamo that reversed its polarity at irregular intervals in the geological past the very mechanisms causing field reversals remain obscure paleomagnetic reconstructions of polarity transitions have been essential for physically constraining the underlying mechanisms in terms of time scale but thus far remain ambiguous with regard to the transitional field geometry here we present new paleomagnetic records from a rapidly deposited lacustrine sediment sequence with extraordinarily stable paleomagnetic signals which has captured in unprecedented detail the bottom reverse to normal r n and top normal to reverse n r transitions of the jaramillo subchron at 1 072 ma and at 0 988 ma the obtained virtual geomagnetic pole vgp path indicates an oscillatory transitional field behavior with four abrupt transequatorial precursory jumps across the pacific the distribution of vgp positions indicates regions of preferred occurrence our results are in agreement with previously proposed bands of transitional vgp occurrence over the americas and australia northwest pacific additionally our vgp positions seem to avoid large low shear velocity provinces llsvps above the core mantle boundary cmb thus our data supports the idea that the transitional field geometry is controlled by heat flux heterogeneities at the cmb linked to llsvps

insights into mt etna s shallow plumbing system from the analysis of infrasound signals august 2007 december 2009 previous studies performed on mt etna on short and discontinuous time intervals indicate the north east crater nec as the most active source of infrasound the source mechanism of nec infrasound events was modeled as a double resonance this lead to infer the connection between the nec and both the southeast crater sec and the eruptive fissure ef that opened at the beginning of the 2008 2009 eruption nevertheless there are still several open questions that need to be addressed for instance the steadiness of nec event features should be studied as well as the orderliness of spectral changes of nec events time related to eruptive activity of other vents the investigation of such topics is strongly enhanced by the possibility of analysing infrasound signals during year long time periods with this aim about 40 000 infrasound events recorded at mt etna from august 2007 to december 2009 were analysed by using spectral and location techniques it was noted in particular that the nec events featured periods with very steady waveforms and spectral characteristics lasting from days to months with slow or sudden variations the most important eruptive episodes occurring at the sec or the ef were accompanied by significant spectral changes in nec events in light of such systematic behaviour the connection between the nec and the sec ef plumbing systems was not considered temporary but rather stable even during a relatively long time interval 2006 2009 moreover study of nec event spectral features and their changes over multiple years supports the double resonance source model such a model together with the inferred connections between nec and sec ef feeding systems implies that level fluctuations of a magma column inside the nec conduit correspond to magmastatic pressure decrease increase inside the main plumbing system these findings open up new and interesting possibilities for monitoring magma pressure changes inside the mt etna plumbing system