variations in magmatic processes along the east greenland volcanic margin summary seismic velocities and the associated thicknesses of rifted and igneous crust provide key constraints on the rifting history the differentiation between non volcanic and volcanic rifted margins the driving force of magmatism at volcanic margins that is active or passive upwelling and the temperature anomaly in the lithosphere this paper presents two new wide angle seismic transects of the east greenland margin and combines the velocity models with a compilation of 30 wide angle seismic velocity models from several publications along the entire east greenland margin compiled maps show the depth to basement depth to moho crustal thickness and thickness of high velocity lower crust hvlc with velocities above 7 0 km s 1 first we present two new wide angle seismic transects which contribute to the compilation at the northeast greenland margin and over the oceanic crust between shannon island and the greenland fracture zone velocity models produced by ray tracing result in total traveltime rms misfits of 100 120 milliseconds and 2 values of 3 7 and 2 3 for the northern and southern profiles with respect to the data quality and structural complexity 2 d gravity modelling is used to verify the structural and lithologic constraints the northernmost profile awi 20030200 reveals a magma starved break up and a rapidly thinning oceanic crust until magnetic anomaly c21 47 1 ma the southern seismic transect awi 20030300 exhibits a positive velocity anomaly associated with the shannon high and a basin of up to 15 km depth beneath flood basalts between shannon island and the continent ocean boundary break up is associated with minor crustal thickening and a rapidly decreasing thickness of oceanic crust out to anomaly c21 the continental region is proposed to be only sparsely penetrated by volcanism and not underplated by magmatic material at all compared to the vast amount of magmatism further south break up is proposed to have occurred at the seaward boundaries of the continent ocean transition zones at between 50 and 54 ma propagating from north to south based on a joint analysis incorporating transects from the kejser franz joseph fjord and godthab gulf secondly the variation of the hvlc along the east greenland margin from 60 to 77 n and from transects of its conjugate margin shows inverted emplacement of prominent landward and seaward hvlc thickness portions from north to south in a distribution chart the differences in the hvlc distribution are attributed to one or more of the following three models in the first model it is inferred that a transfer zone detachment acts as a barrier to northward magma flow in the second model underplating results in thicker and highly intruded lower crust with several small scale feeder dykes that locally increase the lower crustal velocities in the third model a second magmatic event associated with the separation of the jan mayen microcontinent is considered lithospheric scale inhomogeneities might be responsible for the heterogeneous melt generation the inversion of the hvlc distribution in continental and oceanic domains and differences in its velocities

provenance of plumes in global convection models in global convection models constrained by plume motions and subduction history over the last 230 myr plumes emerge preferentially from the edges of thermochemical structures that resemble present day large low shear velocity provinces llsvps beneath africa and the pacific ocean it has been argued that large igneous provinces lips erupting since 200 ma may originate from plumes that emerged from the edges of the llsvps and numerical models have been devised to validate this hypothesis although qualitative assessments that are broadly in agreement with this hypothesis have been derived from numerical models a quantitative assessment has been lacking we present a novel plume detection scheme and derive monte carlo based statistical correlations of model plume eruption sites and reconstructed lip eruption sites we show that models with a chemically anomalous lower mantle are highly correlated to reconstructed lip eruption sites whereas the confidence level obtained for a model with purely thermal plumes falls just short of 95 a network of embayments separated by steep ridges form in the deep lower mantle in models with a chemically anomalous lower mantle plumes become anchored to the peaks of the chemical ridges and the network of ridges acts as a floating anchor adjusting to slab push forces through time the network of ridges imposes a characteristic separation between conduits that can extend into the interior of the thermochemical structures this may explain the observed clustering of reconstructed lip eruption sites that mostly but not exclusively occur around the present day llsvps

the mesoscale and microscale structure and organization of clouds and precipitation in midlatitude cyclones viii a model for the seeder feeder process in warm frontal rainbands abstract previous field studies have indicated that warm frontal rainbands form when ice particles from a seeder cloud grow as they fall through a lower level feeder cloud in this paper we present results from a parameterized numerical model of the growth processes that can lead to the enhancement of precipitation in a seeder feeder type situation the model is applied to two types of warm frontal rainbands in the first type 1 situation the vertical air motions are typical of those associated with slow widespread lifting in the vicinity of warm fronts in the second type 2 situation the vertical air motions are stronger and more characteristic of the mesoscale the model simulations show that in the type 1 situations the growth of the seed ice crystals within the feeder zone is due to vapor deposition the feeder zone in this case is slightly sub saturated with respect to water due to the presence of the seed crystals in regions where the feeder zone is not seeded from aloft snow cryst

variable crustal structure along the juan de fuca ridge influence of on axis hot spots and absolute plate motions multichannel seismic and bathymetric data from the juan de fuca ridge jdfr provide constraints on axial and ridge flank structure for the past 4 8 ma within three spreading corridors crossing cleft northern symmetric and endeavour segments along axis data reveal south to north gradients in seafloor relief and presence and depth of the crustal magma lens which indicate a warmer axial regime to the south both on a regional scale and within individual segments for young crust cross axis lines reveal differences between segments in moho two way traveltimes of 200 300 ms which indicate 0 5 1 km thicker crust at endeavour and cleft compared to northern symmetric moho traveltime anomalies extend beyond the 5 15 km wide axial high and coincide with distinct plateaus 32 and 40 km wide and 200 400 m high found at both segments on older crust moho traveltimes are similar for all three segments 2100 100 ms indicating little difference in average crustal production prior to 0 6 and 0 7 ma the presence of broad axis centered bathymetric plateau with thickened crust at cleft and endeavour segments is attributed to recent initiation of ridge axis centered melt anomalies associated with the cobb hot spot and the heckle melt anomaly increased melt supply at cleft segment upon initiation of axial volcano and southward propagation of endeavour segment during the brunhes point to rapid southward directed along axis channeling of melt anomalies linked to these hot spots preferential southward flow of the cobb and heckle melt anomalies and the regional scale south to north gradients in ridge structure along the jdfr may reflect influence of the northwesterly absolute motion of the ridge axis on subaxial melt distribution

constraints on the lateral strength of slabs from three dimensional dynamic flow models if the viscosity of subducted lithosphere is described purely by temperature and pressure it should remain considerably more viscous than the surrounding material as it descends through the upper mantle many dynamic models of the long wavelength geoid associated with slabs require an increase in viscosity from the upper to the lower mantle but assume the perturbation to the lateral viscosity structure arising from the slabs themselves can be ignored previous studies indicate that strong and localized viscosity variations should have a dramatic influence on the geoid we present 3d finite element models of the regional geoid of the western pacific subduction zones slab buoyancies and viscosities are defined using the distribution of seismicity the geoid is very sensitive to the lateral strength of the slab very viscous slabs penetrating a low viscosity mantle generate significant 10 50 m long wavelength geoid lows opposite to the geoid high which is observed over slabs to obtain a geoid high comparable to that observed the lower mantle viscosity must be 60 200 times greater than the upper mantle viscosity and the slab must be in contact with the lower mantle these strict requirements suggest that slabs have been weakened and cannot act as stress guides from the deep mantle to the surface

preconditioning and ozone hole filling in the antarctic spring the temporal variations in mean zonal wind horizontal temperature gradient at 30 mb and total ozone in antarctic spring 1 sept 30 nov for nine seasons 1979 1987 were examined the ozone hole filling commenced when the zonal flow decelerated to 50 58 m sec 1 at 30 mb our calculation of rossby critical wave number with vertical shear suited for antarctic spring indicated that flow is preconditioned for vertical propagation of rossby critical wave number two at this range of zonal flow this preconditioning can be attributed to the diabatic heating in the antarctic spring since no sudden minor warmings coolings have occurred during the period

channeling of plume flow beneath mid ocean ridges we investigate the interaction of mantle plumes with mid ocean ridges in 3 d numerical convection models using a finite volume multigrid technique with local grid refinement we are able to incorporate a much stronger temperature dependence of viscosity than was possible in previous models this allows lateral viscosity variations of more than six orders of magnitude which is realistic in a ridge situation in models of ridge centered plumes with constant flux we determine how the shape and the along axis width of the plume head depends on various model parameters the plume material is significantly channeled along the axis of a slow spreading ridge if the plume viscosity is in the order of 1017 pa s the plume material spreads radially below the ridge if the spreading rate is fast or the viscosity contrast between plume and ambient mantle is small in time dependent calculations we investigate the temporal evolution of short term variations of the plume flux at slow spreading ridges these perturbations propagate in the form of pulses along the ridge with a speed much larger than the spreading rate our results support the hypothesis that pulsations of the iceland plume are the cause for v shaped topography and gravity anomalies at the reykjanes ridge

slow ridge hotspot interactions from global gravity seismic tomography and 87sr 86sr isotope data summary among the mantle hotspots present under oceanic areas a large number are located on or close to active oceanic ridges this is especially true in the slow spreading atlantic and indian oceans the recent availability of worldwide gravity grids and the increasing coverage of geochemical data sets along active spreading centres allow a fruitful comparison of these data with global geoid and seismic tomography models and allow one to study interactions between mantle plumes and active slow spreading ridges the observed correlations allow us to draw preliminary conclusions on the general links between surficial processes which shape the detailed morphology of the ridge axes and deeper processes active in the upper mantle below the ridge axial domains as a whole the interactions are first studied at the scale of the atlantic the mid atlantic ridge from iceland to bouvet island from the correlation between the zero age free air gravity anomaly which reflects the zero age depth of the ridge axis and sr isotopic ratios of ridge axis basalts the study is then extended to a more global scale the slow ridges from iceland to the gulf of aden by including geoid and uppermantle tomography models the interactions appear complex ranging from the evect of large and very productive plumes almost totally overprinting the long wavelength segmentation pattern of the ridge to that of weaker hotspots barely marking some of the observables in the ridge axial domain intermediate cases are observed in which hotspots of medium activity or whose activity has gradually decreased located at some distance from the ridge axis produce geophysical or geochemical signals whose variation along the axis can be correlated with the geometry of the plume head in the upper mantle such observations tend to preclude the use of a single hotspot ridge interaction model and stress the need for additional observations in various plume ridge configurations

geologic and kinematic constraints on late cretaceous to mid eocene plate boundaries in the southwest pacific abstract starkly contrasting tectonic reconstructions have been proposed for the late cretaceous to mid eocene 85 45 ma evolution of the southwest pacific reflecting sparse and ambiguous data furthermore uncertainty in the timing of and motion at plate boundaries in the region has led to controversy around how to implement a robust southwest pacific plate circuit it is agreed that the southwest pacific comprised three spreading ridges during this time in the southeast indian ocean tasman sea and amundsen sea however one and possibly two other plate boundaries also accommodated relative plate motions in the west antarctic rift system wars and between the lord howe rise lhr and pacific relevant geologic and kinematic data from the region are reviewed to better constrain its plate motion history during this period and determine the time dependent evolution of the southwest pacific regional plate circuit a model of 1 west dipping subduction and basin opening to the east of the lhr from 85 55 ma and 2 initiation of northeast dipping subduction and basin closure east of new caledonia at 55 ma is supported west dipping subduction and basin opening were not driven by convergence as has previously been proposed our plate circuit analysis suggests that between at least 74 ma and subduction initiation at 55 ma there was little net relative motion between the pacific plate and lhr

thermomechanical erosion of the lithosphere by mantle plumes a simplified semianalytic theory is presented that describes the process of lithosphere thinning through thermomechanical erosion by a hot mantle upwelling or plume the theory compares well with published numerical models but predicts that the rate of thinning is very dependent on the lateral scale of the upwelling and on the viscosity through the lithosphere asthenosphere transition a plume tail is likely to cause rapid local thinning reducing the lithosphere thickness substantially within a few million years plume heads or broadly spreading plume tail material are unlikely to cause significant thinning by this mechanism alone these results are consistent with geophysical evidence for localized thinning under the hawaiian volcanic chain but not under the broader hawaiian swell they are also qualitatively consistent with the distribution of basaltic volcanism in oceanic and continental hotspot tracks the theory predicts that thin continental lithosphere may be further thinned sufficiently by a plume tail to cause crustal melting within about 20 m y thus this mechanism may account for some delayed silicic magmatism on some slow continental plates higher thinning rates would have occurred in the archean

constraints on the dynamics of mantle plumes from uplift of the hawaiian islands the v0 2 mm yr uplift of hawaiian islands lanai and molokai and hawaiian swell topography pose important constraints on the structure and dynamics of mantle plumes we have formulated 3 d models of mantle convection to investigate the effects of plume plate interactions on surface vertical motions and swell topography in our models the controlling parameters are plume radius excess plume temperature and upper mantle viscosity we have found that swell height and swell width constraints limit the radius of the hawaiian plume to be smaller than 70 km the additional constraint from the uplift at lanai requires excess plume temperature to be greater than 400 k if excess plume temperature is 400 k models with plume radius between 50 and 70 km and upper mantle viscosity between 10 20 and 3u10 20 pa s satisfy all the constraints our results indicate that mantle plume in the upper mantle may be significantly hotter than previously suggested this has important implications for mantle convection and mantle melting in addition to constraining plume dynamics our models also provide a mechanism to produce the observed uplift at lanai and molokai that has never been satisfactorily explained before 9 2002 elsevier science b v all rights reserved

time dependence of mantle plumes some simple theory dynamical calculations indicate that plate tectonics are likely to have a major effect on the initiation of plumes from the basal boundary layer in the mantle and their subsequent evolution plume start because the boundary layer is hotter than the overlying mantle and hence unstable to perturbations the growth rate of perturbations is too weakly dependent on wavelength to control the geographic distribution of plumes instead the distribution of plumes is strongly influenced by the shape of large perturbations associated with slabs and surface plates plumes evolve because downward return flow is needed to replace the material tapped from the boundary layer this flow downwarps the top of the boundary layer several hundred kilometers from the plume and thus restricts further flow of hot material toward an active plume the geometry of this downwarp is strongly influenced by the initial relief of the top of the boundary layer and hence by plate processes the actual history of plates and slabs is thus needed in numerical models for comparison with real data on plume fluxes

the fluid dynamics of plume ridge and plume plate interactions an experimental investigation abstract a number of hotspots occur at or near mid ocean ridges e g iceland causing elevated bathymetry gravity anomalies and geochemical enrichment for hundreds of kilometers along otherwise normal ridge axes this is commonly presumed to be the result of hot plume material from the deep mantle flowing outward from a narrow plume conduit here we explore the dynmics of plume ridge and plume plate interaction using laboratory experiments we model chemically buoyant plumes as opposed to thermally buoyant plumes so we ignore the effects of thermal diffusion while concentrating on the strictly fluid dynamical aspects of the interaction we find that the spreading of a continuously fed plume beneath a fixed plate is described by a simple expression derived from a balance between plume buoyancy and viscous resistance to spreading for plumes spreading beneath ridges we find two remarkably distinct phases of flow at first the plume spreads approximately as it would beneath a fixed plate then a rapid transition occurs to a steady state condition in which the plume width along the ridge waist is constant this width is found to be proportional to a natural length scale that increases with the square root of the plume volumetric flux and the ratio of the mantle plume viscosities and decreases with the square root of the plate spreading velocity although our experimental conditions with uniform mantle viscosity and no thermal diffusion are perhaps oversimplified compared to the real earth the results scale reasonably well to observation our model suggests that sufficient time has elapsed 60 myr for the iceland plume to reach steady state in its interaction with the mid atlantic ridge using independent estimates for the plume volumetric flux at the iceland hotspot and assuming a viscosity ratio of one the model predicts a minimum waist width of about 850 km in close agreement with the observed value of about 920 km data from other hotspots jan mayen azores galapagos are also consistent with the experimental results for average mantle plume viscosity ratios of 1 3 using estimated plume fluxes with a simple conservation of mass equation yields reasonable estimates of plume thickness ranging from 70 to 180 km for these hotspots thus the experimental model appears to be reasonably consistent with observation given that thermal diffusion has been disregarded

evolution of the walvis ridge rio grande rise hot spot system implications for african and south american plate motions over plumes crystallization ages of volcanic rocks dredged or drilled from the walvis ridge ten sites and the rio grande rise one site have been determined by the 40ar 39ar incremental heating technique the fundamentally age progressive distribution of these basement ages suggests a common hot spot source for volcanism on the island of tristan da cunha along the walvis ridge and rio grande rise and for the formation of the continental flood basalts located in namibia africa and brazil south america the walvis ridge rio grande rise volcanic system evolved along a section of the south atlantic spreading axis as the african and south american plates migrated apart astride or in close proximity to an upwelling plume reconstructions of the spatial relationship between the spreading axis the tristan hot spot and the evolving walvis ridge rio grande rise volcanic feature show that at about 70 ma the spreading axis began to migrate westward away from the hot spot the resulting transition to intraplate hot spot volcanism along the walvis ridge and associated termination of rio grande rise formation also involved a northward migration of previously formed african seafloor over the hot spot rotation parameters for african motion over fixed hot spots i e absolute motion have been recalculated such that the predicted trail of the tristan hot spot agrees with the distribution of radiometric and fossil basement ages along the walvis ridge african absolute motion has been extended to the south and north american plates by the addition of relative motion reconstruction poles

observations of geometry and ages constrain relative motion of hawaii and louisville plumes abstract the classic view of linear island chains as volcanic expressions of interactions between changing plate tectonic motions and fixed mantle plumes has come under renewed scrutiny in particular observed paleolatitudes from the emperor seamounts imply that the hawaii hotspot was 5 15 further north during formation of these seamounts and that rapid retardation of its southward migration was the primary agent forming the angular hawaii emperor bend supporting this view are predictions from fluid dynamic experiments that suggest the general mantle circulation may displace narrow mantle plumes consequently the surface locations of hotspots are not fixed and may have varied considerably in the past however the locations and ages of available rock samples place fundamental limits on the relative motion between the hawaii and louisville hotspots here we use such data to estimate empirical age progression curves for separate chains and calculate the continuous variations in hotspot separations through time while the data are sparse the inferred inter hotspot motion for ages 55 myr appears significant but the observed relative motion is only about half of what is predicted by mantle dynamics models to reconcile the observed paleolatitudes with our observed relative motion requires either a larger contemporaneous southward motion of the louisville hotspot than previously suggested or a moderate component of true polar wander

lithospheric thinning by midplate mantle plumes and the thermal history of hot plume material ponded at sublithospheric depths mantle plumes supply hot material to the asthenospheric channel which spreads beneath initially normal oceanic lithosphere at midplate hotspots the fate of this material and the thermal structure of the lithosphere depend on the vigor of secondary convection through the steep thermal gradient above the ponded plume material it has been proposed either that 1 secondary convection is so vigorous that material hotter than the normal mantle adiabat is cooled a short distance downstream of the hotspot such that the geotherm resembles that of younger lithosphere or 2 that the plume material retains its excess temperature for a significant time and the lithosphere thins slowly a parameterized method of modeling transient secondary convection is developed to examine this question calculations indicate that secondary convection is likely to be sluggish unless partial melting greatly lowers viscosity in addition vigorous secondary convection would greatly retard upstream asthenospheric flow of plume material beneath the nose of the hawaiian swell and asthenospheric flow of hot material away from hotspots on slowly moving plates like cape verde this conclusion is compatible with limited seismic evidence that indicates that the lithosphere is not detectably thinned beneath the hawaiian swell

surface subsidence caused by mantle plumes and volcanic loading in large igneous provinces abstract large igneous provinces form with massive flood basalts being erupted over a large region within a few million years a prevailing model for their formation is a mantle plume model in which a plume head originates from the core mantle boundary ascends through the mantle and produces topographic uplift and eventually massive melting at the surface however many geological observations indicate surface subsidence in the central region before or during flood basalt eruptions in this study we demonstrate that a plume head temporarily ponding below the 660 km phase change boundary causes significant subsidence at the earth s surface over an extended period before the eruption and that the loading from erupted basalts causes surface subsidence at the periphery of the eruption area that affects the environment for subsequent episodes of basalt eruptions our studies therefore demonstrate that the observed subsidence history in many flood basalt provinces is characteristic of the dynamics of mantle plumes and may be used as diagnostics for identifying plume induced flood basalt events

numerical investigations of the mantle plume initiation model for flood basalt events recent studies have linked both the eruption of continental flood basalts and the emplacement of large oceanic plateaus to the initial phase of mantle plume activity i e the plume head model here we develop this concept further through numerical models of a large thermal diapir rising through the mantle and impinging upon the base of the lithosphere a finite element analysis in axisymmetric geometry is used to model the dynamics of solid state convective flow and heat transport in the mantle and an anhydrous batch melting model is used to estimate melt volumes and compositions obtained from partial melting in the shallow mantle we explore the effects of a number of model variables including mantle viscosity structure plume temperature and diameter and the mechanical response of the lithosphere our study emphasizes cases in which the lithosphere does not undergo significant extension prior to large scale melting a standard model is found to be consistent with some observed characteristics of flood basalt plume initiation events this model includes an initial diapir of radius 400 km and initial excess temperature 350oc rising through a mantle of viscosity 102 pa s and impinging beneath a model oceanic lithosphere 100 ma in age if lithospheric extension does not occur melting is almost entirely sublithospheric the melt volume produced is 3 5x106 km 3 and its composition has high mgo content 15 20 wt a time sequence of precursory central axial uplift of several kilometers followed by major melting volcanism is a robust feature of all the models regardless of mantle viscosity structure or lithospheric response low viscosity in the upper mantle compresses the timescale for these events and results in a constriction of the plume head initially impinging on the lithosphere the radial extent of the melting region about 500 km at the time of maximum melt production is only slightly affected by the upper mantle viscosity for nonrifting lithosphere no significant melt generation occurs for initial plume excess temperatures less than about 300oc the initial plume radius has only a modest effect upon the volume of melt obtained the most important aspect of lithospheric response is whether or not the uppermost crust and lithosphere above 40 km depth undergo extension otherwise the ductile strength of the lower lithosphere has little effect if the lithosphere is allowed to extend freely the melt volume increases by about an order of magnitude and it is comparable with some of the largest oceanic plateaus such as ontong java if continental or oceanic flood basalt events occur on old nonrifting lithosphere the primary melts are likely to be much more mgo rich than the basalts erupted at the surface this is consistent with abundant evidence for olivine fractionation in many flood basalt provinces along with the occurrence of picrite lavas and we hypothesize that fractionation occurs at a density trap at the crust mantle boundary moho at this stage and following it is likely that significant lower crustal and mantle lithospheric contamination of mantle plume derived magmas may occur both isotopically and in the trace elements the sequence and timing of both precursory and post magmatic events in the vast well exposed oceanic flood basalt terrane known as wrangellia se alaska british columbia are consistent with the predictions of our model the uplift predicted by our models is of order 2 4 km if we require that mantle plumes are hot enough to melt beneath unrifted lithosphere effects not modeled here such as small scale convection and hydrous melting will tend to reduce the model excess temperature and uplift in order to better understand the relation between mantle plumes and large igneous provinces several important advances are required implementation of a fractional melting model some realistic assessment of melt transport through the mantle and crust and a fully three dimensional convection model

compensation of swells and plateaus in the north pacific no direct evidence for mantle convection at intermediate and long wavelengths the ratio of geoid height to topography is sensitive to the depth and mode of compensation a low geoid topography ratio 8 m km in conjunction with a poor correlation between geoid height and topography is evidence of mantle convection after subtracting a reference geoid from the observed geoid previous studies have found a regular pattern of geoid highs and lows with a characteristic wavelength of 3000 4000 km since these geoid highs and lows were poorly correlated with topography and resulted in very high geoid topography ratios 10 20 m km they were believed to reflect the planform of mantle convection we show that the regular pattern of geoid highs and lows is an artifact caused by truncating the reference geoid at spherical harmonic degree 10 since the geoid spectrum is red the residual geoid is dominated by degree 11 when the harmonics of the reference geoid are rolled off gradually the regular pattern of geoid highs and lows disappears in the northeast pacific the new residual geoid reflects the lithosphere age offsets across the major fracture zones in the northwest pacific the residual geoid corresponds to isostatically compensated swells and plateaus we have calculated the geoid topography ratio for 10 swells and plateaus and have found a range of compensation depths the highest geoidaopography ratio of 5 5 m km occurs on the flanks of the hawaiian swell intermediate ratios occur in four areas including the midway swell these intermediate ratios reflect a linear combination of the decaying thermal swell and the increasing volume of airy compensated seamounts low geoid topography ratios occur over the remaining five areas e g emperor seamounts reflecting the absence of a thermal swell our findings do not support the hypothesis that the planform of mantle convection is evident in the geoid we see only indirect evidence of thermal plumes reheating the lower lithosphere

mantle temperature anomalies along the past and paleoaxes of the galapagos spreading center as inferred from gravity analyses to better understand the effects of hot spots on mid ocean ridge thermal structure we investigate the subsurface density structure of the galapagos spreading center and nearby lithosphere using shipboard gravity and bathymetry data we obtain maps of mantle bouguer anomalies mba by removing from the free air gravity the attractions of seafloor topography and a 6 km thick model crust comparison of observed and theoretical mba profiles along isochrons for ages 0 0 7 7 ma suggests that seafloor topography is isostatically compensated by mass anomalies primarily in the upper 100 km of the mantle this result is consistent with the notion that seafloor topography along the galapagos spreading center is supported by lateral changes of crustal thickness and upper mantle density both of which are controlled by temperatures in the upper mantle where decompression melting occurs along the ridge axis the mba decreases from the east and west toward the galapagos hot spot by 90 mgal reaching a minimum nearest the hot spot at 91 w seafloor topography mirrors the mba along axis increasing by 1 1 km toward the hot spot these variations in mba and bathymetry can be explained by crustal thickening and mantle density variations resulting from a gradual axial temperature increase of 50 25 c toward the hot spot the predicted crustal thickening of 2 4 km nearest the hot spot accounts for 70 75 of the along axis mba and bathymetry anomalies mantle density variations account for the rest of the anomalies from the crustal isochron of age 7 7 ma to the present day axis the along isochron amplitudes of mba decrease from 150 to 90 mgal the corresponding along isochron bathymetry anomalies decrease from 1 7 to 1 1 km these observations along the paleoaxes of the galapagos spreading center indicate that the axial temperature anomaly was 70 hotter in the past 86 25 c and has steadily decreased to 50 25 c as the ridge axis migrated away from the galapagos hot spot these along isochron temperature anomalies however have remained well below that estimated for the hot spot itself 200 c indicating that the lateral temperature gradient between the hot spot and the ridge axis has remained 10 20 times greater than that along the ridge axis over the past 7 7 m y

on the resolution of radial viscosity structure in modelling long wavelength postglacial rebound data summary studies of postglacial rebound pgr constrain the viscosity of the mantle in an absolute sense and have also been used to constrain the radial variation in mantle viscosity such radial variations probably have important effects on many aspects of solid earth dynamics including the existence and style of plate tectonics rotational dynamics thermal convection and mixing of chemical species by convection most studies of pgr as well as other studies constraining viscosity structure e g the geoid agree that the average viscosity of the upper mantle beneath the lithosphere is smaller than that of the lower mantle however there is marked disagreement regarding the amount of viscosity contrast and the thickness of an upper mantle low viscosity zone lvz with estimates for the viscosity contrast ranging from a mere factor of 2 to several orders of magnitude here we seek to both quantify and clearly illustrate the fundamental modelling trade off between viscosity contrast and layer thickness d and show that if only long wavelength constraints are available pgr models are largely indistinguishable for constant values of d 3 characterizing an lvz models employing both relative sea level rsl data from hudson bay and time variations in gravity from the grace satellite illustrate this principle models with lvz viscosity contrasts ranging from 3 orders of magnitude fit the data equally well depending on the thickness of the lvz other traditional data sets constraining pgr e g fennoscandian strand lines appear no more capable of independently resolving this ambiguity although some studies that incorporate both long wavelength rsl type constraints and shorter wavelength constraints such as tilting of continental margins appear to favour a relatively thin high viscosity contrast lvz new gps data from north america and scandinavia may be able to resolve this question even further however at the present time pgr studies do not preclude and perhaps favour a relatively thin lvz of viscosity contrast at least one and perhaps as much as 3 orders of magnitude beneath the lithosphere in some regions this conclusion has significant implications for understanding the phenomenon of plate tectonics and the evolution and dynamics of the mantle

two station measurements of rayleigh wave group velocity along the hawai ian swell two station measurements of rayleigh wave group velocity between midway atoll and o ahu provide a useful constraint on lithospheric thickness along the hawai ian swell comparison of the observed dispersion curve with age dependent regionalized dispersion curves suggests that the swell acts as 50 110 m y old lithosphere and not as 20 50 m y old lithosphere as is predicted by the currently favored hypothesis for the swell s formation a preliminary isotropic shear velocity model for the swell suggests that the lithosphere may be as thick as 100 km this value is difficult to reconcile with the lithosphere being substantially thinned during its passage over the hawai ian hotspot

history of rift propagation and magnetization intensity for the cocos nazca sspreading center analysis of magnetic anomaly profiles collected nearly parallel to tectonic flow lines allows detailed interpretation of the complicated tectonic history of the cocos nazca spreading center forward models of the magnetic anomalies accounting for spreading rate variations ridge jumps asymmetric spreading magnetization intensity variations and bathymetry show excellent agreement with observed anomalies spreading rates can be constrained to a common finite rotation history through anomaly 4a with three changes in rates rate changes at about 1 5 ma and 4 1 ma correspond to changes in rate gradients and occur during the well calibrated part of the reversal timescale so they can unquestionably be identified as true changes in plate motion a 15 rate decrease at about 5 2 ma could be interpreted either as a change in plate motion or as an artifact of poor calibration of the older part of the timescale the change at 4 1 ma is especially important because many timescales are based on the assumption of constant spreading rate for this plate pair for 0 6 ma rift propagation has played a dominant role in the continuous reorganization of the geometry of the ridge axis propagation has been predominantly away from the hotspot with jumps predominantly south ward propagation rates have ranged from 30 to 120 mm yr commonly near 70 mm yr origin of most propagation sequences is difficult to interpret but many appear to involve discrete southward ridge jumps forming a new segment near the hotspot magnetic anomaly amplitude appears to be a reliable tracer of fe content of lavas several generalizations can be drawn about along axis variations in magnetization intensities since 8 ma high magnetizations are only observed at the far ends relative to the galapagos hotspot of segments at least 150 km long offset at the end of a high magnetization segment is at least 15 km and there are no offsets larger than 30 45 km between high magnetization segments and the reconstructed position of the hotspot we interpret these patterns to indicate that fractionated lavas erupt where gradients in magma supply cause along axis flow of evolved magma the gradients in supply result from subaxial flow of hotspot derived asthenosphere in a narrow conduit this flow is only partly obstructed by an offset of 20 30 km but entirely blocked by an offset of 50 km

microphysical properties of convectively generated fall streaks within the stratiform comma head region of continental winter cyclones abstractthis paper presents analyses of the microphysical structure of comma head stratiform precipitation in 14 continental cyclones focusing on fall streaks of hydrometeors produced by cloud top convective generating cells data were obtained at temperatures between 4 and 45 c using in situ instrumentation and the w band university of wyoming cloud radar all operated aboard the national science foundation national center for atmospheric research c 130 analyses are presented first for a case study of one cyclone followed by statistical analyses of the full dataset using radar based objective classifications the statistical percentile number concentrations averaged 1 9 times larger within the fall streaks compared to the regions between them and the corresponding ice water content and median mass diameter values averaged 2 2 and 1 1 times larger ice phase conditions were predominant within the stratiform precipitation with deposition and aggregation the primary ice growth mechanisms no distinc

oceanic spreading center hotspot interactions constraints from along isochron bathymetric and gravity anomalies we analyzed bathymetric and gravity anomalies along present and paleoaxes of oceanic spreading centers influenced by the iceland azores galapagos tristan and easter hotspots residual bathymetry up to 4 7 km and mantle bouguer gravity up to 340 km anomalies are maximum at on axis hotspots and decrease with increasing ridge hotspot separation distance d until becoming insignificant at d 500 km along isochron widths of bathymetric anomalies up to 2700 km depend inversely on paleo spreading rate reflecting the extent to which plume material will flow along axis before being swept away by the spreading lithosphere flux balance arguments suggest that the five hotspots feed material to ridges with comparable fluxes of 2 2 106 km3 m y assuming that the amplitudes of these geophysical anomalies reflect temperature dependent crustal thickness and mantle density variations we suggest that ridge temperature anomalies are maximum 150 225 c when plumes are ridge centered and decrease with increasing ridge hotspot distance due to cooling of the ridgeward migrating plume material

kaapvaal craton changing first and second order controls on sea level from c 3 0 ga to 2 0 ga abstract well preserved supracrustal basin fills including the oldest large depository witwatersrand known makes the kaapvaal craton an ideal example for studying controls on first and second order stratigraphic cyclicity the duration of the three basins studied witwatersrand ventersdorp and transvaal from c 3 0 to c 2 0 ga is almost twice that of the entire phanerozoic period from which the paradigms of classical sequence stratigraphy have been derived amongst these is an assumption that cyclicity at specific hierarchical levels is predictable and repetitive through earth history this does not appear to be borne out by the precambrian record studied here instead a hierarchical order of cycles based on relative importance of sequences allied to a more flexible approach to controls on stratigraphic cyclicity at first and second orders is preferred first order cycles within the c 3 0 2 0 ga period on kaapvaal are synonymous with the entire evolution of a supracrustal basin fill defined by a specific basin type second order cycles define basin fill packages of unique character which reflect large scale shifts in the accommodation sedimentation balance early to middle archaean evolution of the kaapvaal craton encompassed amalgamation of a mosaic of subdomains to produce an early south southeastern nucleus by c 3 1 ga accretion of composite terranes from both north and west followed from c 3 0 2 7 ga this was contemporaneous with evolution of the c 3 0 2 8 ga witwatersrand basin the high grade limpopo terrane was amalgamated with the kaapvaal craton between 2 95 and 2 69 ga although a younger age for this orogensis is also possible the witwatersrand first order depositional cycle was controlled largely by subduction related tectonic loading and had a duration of c 190 my the c 5 my first order cycle of the c 2 7 ga ventersdorp basin was essentially controlled by thermal uplift induced extensional subsidence and that of the c 650 my transvaal basin by extensional and subsequent thermal subsidence it is noteworthy in the three basins examined that first order cycles controlled by plate tectonics witwatersrand and transvaal basins were about two orders of magnitude longer relative to the ventersdorp plume tectonic cycle second order cycles also varied greatly from c 100 my to c 1 my within the witwatersrand and transvaal basins subsidence allowed extensive seaways to develop within which classical sequence stratigraphic systems tracts can be recognized reflecting different stages of shoreline shifts within the short lived ventersdorp continental basin system sedimentological criteria can be used to discriminate high and low accommodation systems tracts the transvaal first order cycle was terminated at c 2 0 ga by the bushveld complex plume event

the final warming and polar vortex disappearance during the southern hemisphere spring seven years 1979 1985 of nmc data are used to analyze southern hemisphere middle stratosphere winter to summer circulation transitions these transitions are classified into two extreme categories 1 a mid october final warming accompanied by a major mid october wave event 1979 and 1982 and 2 a mid november final warming with no strong october wave event 1980 and 1981 after the final warming a wave event in late november early december results in the disappearance of the polar vortex all of these wave events in conjunction with zonal mean mass motions act to reduce zonal mean gradients of potential vorticity and ozone through horizontal mixing and hence they act to destroy the vortex and fill the ozone hole additionally it is noted that the final warming and the vortex breakdown are two distinct events separated by 15 40 days finally past studies show that the time of these events has not radically changed

organization and structure of clouds and precipitation on the mid atlantic coast of the united states part ii the mesoscale and microscale structures of some frontal rainbands abstract the mesoscale and microscale structures of the clouds and precipitation associated with a frontal system on the mid atlantic coast of the united states are investigated using radar reflectivity and doppler velocity data surface mesonet conventional surface upper air and microphysical data the frontal structure showed similarities to a warm occlusion with code air advection aloft preceding cold air advection at lower levels six rainbands were observed in association with the frontal system all of these rainbands developed within the region of coverage of the nws wsr 57 radar at cape hatteras north carolina two were upper level features associated with a prefrontal surge of cold air and the main push of cold dry air aloft these rainbands were similar in structure to prefrontal surge and wide cold frontal rainbands respectively observed on the pacific northwest coast the microphysical and small mesoscale structures of the wide cold frontal rainband are examined three of the rainbands

mechanical erosion and reheating of the lithosphere a numerical model for hotspot swells it is currently debated if either thermal erosion of the lithosphere or dynamical support is the source of topography and geoid anomalies the origin of tins controversy lies probably in the difficulty to model simultaneously these two effects for this purpose we have studied the time dependent behavior of two dimensional convection with a temperature and pressure dependent viscosity the use of a control volume method allows us to define a rigid zone simulating the mechanical lithosphere the interface between the lithosphere and the convective mantle is determined by a viscosity cutoff first some experiments model the rise of a plume below the lithosphere in order to observe the evolution of the uplift and thus to appreciate the various processes involved in the swell formation before the plume reaches the base of the thermal lithosphere an uplift a few hundred meters in amplitude develops which can only be ascribed to a pure dynamical support the major uplift occurs when the ductile part of the lithosphere the convective boundary layer is squeezed by the plume the reheating of the mechanical lithosphere takes place after this transient stage of dynamical erosion however this late process is very slow but can magnify the amplitude of the swell if the lithosphere stays long enough above the plume these results shed some light on the different mechanisms occurring during the swell formation but the configuration modeled does not correspond to the one expected for actual hotspot swells they feature plume rising up to the lithosphere while natural situations correspond to lithosphere drifting above preexisting plumes an experiment with a moving lithosphere was run and shows that thermal erosion does not affect significantly a moving lithosphere even for relatively slow drifting velocities few centimeters year indeed the thermal structure of the lithosphere is not modified above the 800 c isotherm except for a motionless plate in this case the resulting swell should be greater this could explain why azores crozet or cap verde swells are so high on the other hand the shape of a swell over a moving lithosphere is strikingly reminiscent of the hawaiian swell

rheology of the upper mantle a synthesis rheological properties of the upper mantle of the earth play an important role in the dynamics of the lithosphere and asthenosphere however such fundamental issues as the dominant mechanisms of flow have not been well resolved a synthesis of laboratory studies and geophysical and geological observations shows that transitions between diffusion and dislocation creep likely occur in the earth s upper mantle the hot and shallow upper mantle flows by dislocation creep whereas cold and shallow or deep upper mantle may flow by diffusion creep when the stress increases grain size is reduced and the upper mantle near the transition between these two regimes is weakened consequently deformation is localized and the upper mantle is decoupled mechanically near these depths

vertical velocity and physical structure of generating cells and convection in the comma head region of continental winter cyclones abstractthe vertical motion and physical structure of elevated convection and generating cells within the comma heads of three continental winter cyclones are investigated using the wyoming w band cloud radar mounted on the national science foundation national center for atmospheric research nsf ncar c 130 supplemented by analyses from the rapid update cycle model and weather surveillance radar 1988 doppler wsr 88d data the cyclones followed three distinct archetypical tracks and were typical of those producing winter weather in the midwestern united states in two of the cyclones dry air in the middle and upper troposphere behind the pacific cold front intruded over moist gulf of mexico air at lower altitudes within the comma head separating the comma head into two zones elevated convection in the southern zone extended from the cold frontal surface to the tropopause the stronger convective updrafts ranged from 2 to 7 m s 1 and downdrafts ranged from 2 to 6 m s 1 the horizontal scale of the

regional uplift associated with continental large igneous provinces the roles of mantle plumes and the lithosphere the timing and duration of surface uplift associated with large igneous provinces provide important constraints on mantle convection processes here we review geological indicators of surface uplift associated with five continent based magmatic provinces emeishan traps 260 million years ago ma siberian traps 251 ma deccan traps 65 ma north atlantic phase 1 61 ma and phase 2 55 ma and yellowstone 16 ma to recent all five magmatic provinces were associated with surface uplift surface uplift can be measured directly from sedimentary indicators of sea level in the north atlantic and from geomorpholocial indicators of relative uplift and tilting in yellowstone in the other provinces surface uplift is inferred from the record of erosion in the deccan north atlantic and emeishan provinces transient uplift that results from variations in thermal structure of the lithosphere and underlying mantle can be distinguished from permanent uplift that results from the extraction and emplacement of magma transient surface uplift is more useful in constraining mantle convection since models of melt generation and emplacement are not required for its interpretation observations of the spatial and temporal relationships between surface uplift rifting and magmatism are also important in constraining models of lip formation onset of surface uplift preceded magmatism in all five of the provinces biostratigraphic constraints on timing of uplift and erosion are best for the north atlantic and emeishan provinces where the time interval between significant uplift and first magmatism is less than 1 million years and 2 5 million years respectively rifting post dates the earliest magmatism in the case of the north atlantic phase 1 and possibly in the case of siberia the relative age of onset of offshore rifting is not well constrained for the deccan and the importance of rifting in controlling magmatism is disputed in the emeishan and yellowstone provinces in these examples rifting is not a requirement for onset of lip magmatism but melting rates are significantly increased when rifting occurs models that attempt to explain emplacement of these five lips without hot mantle supplied by mantle plumes often have difficulties in explaining the observations of surface uplift rifting and magmatism for example small scale convection related to craton or rift boundaries edge driven convection cannot easily explain widespread 1000 km scale transient surface uplift emeishan deccan north atlantic and upper mantle convection initiated by differential incubation beneath cratons the hotcell

evolution of starting mantle plumes a comparison between numerical and laboratory models abstract laboratory experiments on the initiation and development of mantle plumes suggest the formation of a bulbous plume head that grows in time by entrainment of surrounding material followed by a thin feeder conduit this type of plume is generally not seen in numerical models of mantle convection to better determine the possible differences between numerical and laboratory experiments we have numerically reproduced a published laboratory experiment on plume initiation theoretical predictions for the growth of the plume head have been verified and the experiments allowed for a new determination of a similarity constant models with a constant viscosity fluid show much less entrainment simulations using a plume rising from a thermal boundary layer with an olivine type temperature dependence show that most of the source material is contained in the plume head and that the conduit is largely composed of entrained material simulations with a more realistic non newtonian rheology show that plumes can rise much faster through the mantle which will lead to higher temperatures of the plume head at the earth s surface compared to what experiments with newtonian rheology suggest

asthenosphere flow model of hotspot ridge interactions a comparison of iceland and kerguelen we develop a numerical model to test the asthenosphere flow paradigm in which hotspots feed the low viscosity asthenosphere and lithosphere growth consumes the asthenosphere the dynamics of this flow model are controlled by the relative position of the hotspot to the ridge absolute plate velocities lithosphere and asthenosphere rheologies and tectonic boundary conditions the model is applied to two distinct regions the iceland hotspot centered on the mid atlantic ridge and the kerguelen hotspot located near the southeast indian ridge the iceland model generates along ridge flow rates as high as 30 cm y for an asthenosphere viscosity of 5 1018 pa s these flow rates are consistent with previous interpretations of the origin of southward pointing v s south of iceland the preferred kerguelen model produces significant hotspot to ridge flow followed by along ridge flow velocities from the off axis hotspot to the ridge are 66 cm y for a model within which asthenosphere viscosity varies from 5 1018 to 5 1019 pa s the asthenosphere flow paradigm can explain major features of hotspot ridge interactions for both on axis and off axis hotspots

lateral flow and ponding of starting plume material many mantle plumes started with voluminous heads of hot material that ascended as diapirs through the mantle flood basalts radial dike swarms and frequently continental breakup are associated with the impingement of starting plume heads on the base of the lithosphere the buoyant plume head material ponds at the base of the lithosphere and then flows laterally relief on the base of the lithosphere acts as an upside down drainage pattern with enclosed catchments for this flow that is plume material preferentially collects beneath regions where the lithosphere is locally thin such as thermally subsiding sedimentary basins an example is uplift associated with the iceland starting plume which inverted the mesozoic irish sea basin additional relief at the base of the lithosphere is associated with rifting and seafloor spreading in the case of the northern north sea material from the iceland starting plume ponded beneath a basin with locally thin lithosphere rifting breached the enclosed catchment allowing the ponded plume material to drain across the new passive margin toward very thin lithosphere produced by seafloor spreading rifting also creates a conduit for the flow of plume material away from the region underlain by the starting plume head an example is the volcanic passive margin off nova scotia which is 2000 km from the center of the bahama starting plume head numerical and analytical calculations indicate that lateral flow of plume material consistent with this observation is expected if its viscosity is around 1018 pa s

the bent hawaiian emperor hotspot track inheriting the mantle wind bends in volcanic hotspot lineaments best represented by the large elbow in the hawaiian emperor chain were thought to directly record changes in plate motion several lines of geophysical inquiry now suggest that a change in the locus of upwelling in the mantle induced by mantle dynamics causes bends in hotspot tracks inverse modeling suggests that although deep flow near the core mantle boundary may have played a role in the hawaiian emperor bend capture of a plume by a ridge followed by changes in sub pacific mantle flow can better explain the observations thus hotspot tracks can reveal patterns of past mantle circulation

structure and dynamics of sheared mantle plumes 1 an extensive series of laboratory experiments is used to investigate the behavior of sheared thermal plumes the plumes are generated by heating a small circular plate on the base of a cylindrical tank filled with viscous fluid and then sheared by rotating a horizontal lid at the fluid surface the motion of passive tracers in the plumes is visualized by the release of several dye streams on the hot plate we systematically examine the dependence of the convective flow on four dimensionless numbers a velocity ratio a rayleigh number the viscosity ratio and an aspect ratio we identify and delineate two transitions in the convective behavior from a regime where the plume can spread upstream against the shear to a regime where the entire plume is advected downstream and from a regime of negligible cross stream circulation to a regime with significant cross stream circulation and thermal entrainment our analysis of the steady profiles of the plumes shows that they initially rise with a constant vertical rise velocity this rise velocity depends on the buoyancy flux and ambient viscosity but is almost independent of the centerline plume viscosity which suggests that most of the thermal plume has a viscosity that is much closer to the ambient viscosity than the centerline viscosity as the plumes approach the lid they decelerate as the viscous drag on them steadily increases the lateral spreading of the plumes under the lid is found to be well described by similarity solutions derived for the spreading of compositional plumes on a rigid surface if the effective viscosity of the thermal plumes is taken to be the ambient viscosity rather than the centerline viscosity a similar theoretical model is found to roughly predict the upstream spreading of thermal plumes at low shear but it breaks down at moderate to high shear where the entire plumes are advected downstream when our results are applied to the earth we find that mantle plumes are mostly divided into only two flow regimes in the upper mantle plumes under slow moving plates experience upstream flow and negligible cross stream circulation while plumes under faster moving plates including all pacific plumes experience significant cross stream circulation and are advected downstream we also demonstrate that geochemical heterogeneities in a plume s source region will result in an azimuthally zoned plume and in an asymmetric geographical distribution of geochemical heterogeneities in the erupted hot spot basalts as is seen in the hawaiian galapagos marquesas and tahiti society island chains for individual mantle plumes we determine their diameter and vertical rise velocity as well as the extent of upstream spreading and the rate of lateral spreading under the lithosphere

crustal thickness along the western galapagos spreading center and the compensation of the galapagos hotspot swell wide angle refraction and multichannel reflection seismic data show that oceanic crust along the galapagos spreading center gsc between 97 w and 91 25pw thickens by 2 3 km as the galapagos plume is approached from the west this crustal thickening can account for v52 of the 700 m amplitude of the galapagos swell after correcting for changes in crustal thickness the residual mantle bouguer gravity anomaly associated with the galapagos swell shows a minimum of 325 mgal near 92 15pw the area where the gsc is intersected by the wolf darwin volcanic lineament wdl the remaining depth and gravity anomalies indicate an eastward reduction of mantle density estimated to be most prominent above a compensation depth of 50 100 km melting calculations assuming adiabatic passive mantle upwelling predict the observed crustal thickening to arise from a small increase in mantle potential temperature of v30 c the associated thermal expansion and increase in melt depletion reduce mantle densities but to a degree that is insufficient to explain the geophysical observations the largest density anomalies appear at the intersection of the gsc and the wdl our results therefore require the existence of compositionally buoyant mantle beneath the gsc near the galapagos plume possible origins of this excess buoyancy include melt retained in the mantle as well as mantle depleted by melting in the upwelling plume beneath the galapagos islands that is later transported to the gsc our estimate for the buoyancy flux of the galapagos plume 700 kg s 31 is lower than previous estimates while the total crustal production rate of the galapagos plume 5 5 m 3 s 31 is comparable to that of the icelandic and hawaiian plumes

lithospheric controls on melt production during continental breakup at slow rates of extension application to the north atlantic rifted margins form from extension and breakup of the continental lithosphere if this extension is coeval with a region of hotter lithosphere then it is generally assumed that a volcanic margin would follow here we present the results of numerical simulations of rift margin evolution by extending continental lithosphere above a thermal anomaly we find that unless the lithosphere is thinned prior to the arrival of the thermal anomaly or half spreading rates are more than 50mmyr 1 the lithosphere acts as a lid to the hot material the thermal anomaly cools significantly by conduction before having an effect on decompression melt production if the lithosphere is thinned by the formation of extensional basins then the thermal anomaly advects into the thinned region and leads to enhanced decompression melting in the north atlantic a series of extensional basins off the coast of northwest europe and greenland provide the required thinning this observation suggests that volcanic margins that show slow rates of extension only occur where there is the combination of a thermal anomaly and previous regional thinning of the lithosphere

the occurrence of irregular ice particles in stratiform clouds abstract recent studies that have classified ice particles from airborne imaging probe data have concluded that the vast majority of ice particles in stratiform precipitation systems are of an irregular shape this conclusion stands in contrast to the findings from microscope observations of snow particles at the ground during the improvement of microphysical parameterization through observational verification experiment from november to december 2001 in the oregon cascade mountains improve 2 which show that most snow crystals either single crystals or the component crystals of snow aggregates are readily identified as regular types within established crystal classification systems this apparent contradiction is rectified by examining the definition of the term irregular as applied to ice particles and by considering limitations of different methods for observing ice particles it is concluded that the finding of the airborne probe based studies is a consequence of both limitations of the observin

a new inference of mantle viscosity based upon joint inversion of convection and glacial isostatic adjustment data we present new profiles of mantle viscosity derived on the basis of non linear occam style joint inversions of an extensive set of data associated with mantle convection and glacial isostatic adjustment gia the convection related observables include satellite derived free air gravity harmonics the geodetically inferred excess ellipticity of the cmb and tectonic plate motions the gia constraints involve two classes of observables previously shown to be relatively insensitive to errors in the late pleistocene ice history the so called fennoscandian relaxation spectrum frs and a set of site specific decay times determined from the postglacial sea level history in hudson bay and sweden the inverted viscosity profiles show a significant three orders of magnitude increase from the upper mantle mean value of f4 10 20 pa s to a high viscosity 10 23 pa s peak at

microphysical properties and radar polarimetric features within a warm front abstracton 21 january 2009 the warm front of an extensive low pressure system affected u k weather in this work macroscopic and microphysical characteristics of this warm front are investigated using in situ optical array probes temperatures sensors and radiosondes and s band polarimetric radar data from the aerosol properties processes and influences on the earth s climate clouds project the warm front was associated with a warm conveyor belt a zone of wind speeds of up to 26 m s 1 which played a key role in the formation of extensive mixed phase cloud mass by ascending significant liquid water lwc 0 22 g m 3 at a level 3 km and creating an ideal environment at temperatures 5 c for ice multiplication then generating cells which formed in the unstable and sheared layer above the warm conveyor belt influenced the structure of the stratiform cloud layer dividing it into two types of elongated and slanted ice fall streaks one depicted by large zdr values and the other by large zh

the seasonal evolution of planetary waves in the southern hemisphere stratosphere and troposphere seasonality in planetary wave structure in the southern hemisphere stratosphere and troposphere 1000 1 mb is studied based on eight years 1979 1986 of daily geopotential data emphasis is placed on the distinctive seasonality in the stratosphere along with tracing the stratospheric fluctuations into the troposphere time filtering is used to separate stationary and transient components maximum geopotential height variance is observed in the middle to upper stratosphere in late winter spring with a distinct secondary smaller maximum in late fall early winter these maxima sandwich the strongest mid winter zonal winds this sequence occurs approximately one month earlier in the upper stratosphere two corresponding peaks are observed in stratospheric zonal mean wind and temperature fluctuations the two maxima in stratospheric wave variance are not in agreement with observed seasonal changes in quasi geostrophic refractive index of the zonal mean flow the stratospheric geopotential variance maxima result from nearly equal amounts of stationary and transient zonal wave 1 fluctuations with transient wave 2 also contributing during late winter stationary wave 1 shows intriguing seasonal evolution the early winter maximum is predominantly equivalent barotropic whereas that of late winter spring is highly baroclinic in the stratosphere stratospheric poleward fluxes of heat and momentum both exhibit pronounced maxima during september october these result primarily from the stationary wave 1 contributions and the transients assume a secondary role although not as pronounced as the stationary wave seasonality seasonal differences in transient wave behaviour are also documented including three dimensional propagation characteristics and wave mean flow interaction diagnostics

numerical modeling of chemically buoyant mantle plumes at spreading ridges the geometry of spreading of plumes beneath midoceanic ridges is investigated by three dimensional numerical modeling with the goal of characterizing the width of the plume along the ridge or the waist width chemically buoyant plumes are modeled in order to compare to previously reported laboratory tank experiments the plume is generated near the bottom of the box and rises and forms a mushroom head with some entrainment of surrounding fluid the head flattens at a considerable depth beneath the ridge before rising to the surface once the plume reaches the surface the head is quickly divided by the diverging plates and the waist width is found to have reached a steady state value w the results show that w is proportional to the square root of the volumetric flux of the plume divided by the diverging plate speed which is consistent with previously reported experimental data our scaling law gives an independent method for estimating the volumetric flux of mantle plumes the calculated plume fluxes are two to four times larger than previous estimates if plume buoyancy is purely of thermal origin then excess temperatures can be estimated from the fluxes for iceland azores and the galapagos the calculated excess temperatures are 140 57 and 51 c respectively in agreement with recent independent estimates from modeling of gravity and bathymetry

the role of cloud top generating cells and boundary layer circulations in the finescale radar structure of a winter cyclone over the great lakes abstractdata from airborne w band radar are used in conjunction with thermodynamic fields from the weather research and forecasting model and air parcel back trajectories from the hysplit model to investigate the finescale reflectivity vertical motion and airmass structure of the comma head of a winter cyclone in the vicinity of the great lakes cloud top generating cells formed along an upper level frontal boundary vertically separating dry air which 48 h earlier was located in the upper troposphere over south central canada from moist air which was located in the lower troposphere over the southeast united states the stronger updrafts within the generating cells had vertical velocities ranging from 1 to 3 m s 1 the generating cells were important to precipitation production within the comma head precipitation trails formed within the generating cells could sometimes be followed to the boundary layer before merging boundary layer air beneath the cyclone s comma head exhibited convective circulati

insights into the evolving microphysical and kinematic structure of northeastern u s winter storms from dual polarization doppler radar abstractthe recent weather surveillance radar 1988 doppler wsr 88d network upgrade to dual polarization capabilities allows for bulk characterization of microphysical processes in northeastern u s winter storms for the first time in this study the quasi vertical profile qvp technique wherein data from a given elevation angle scan are azimuthally averaged and the range coordinate is converted to height is extended and applied to polarimetric wsr 88d observations of six northeast winter storms to survey their evolving bulk vertical microphysical and kinematic structures these analyses are supplemented using hourly analyses from the rapid refresh rap model regions of ascent inferred from qvps were consistently associated with notable polarimetric signatures implying planar crystal growth when near 15 c and riming and secondary ice production at higher temperatures the heaviest snowfall occurred most often when ascent and enhanced propagation differential phase shift occurred near 15 c

numerical evaluation of mantle plume spacing size flow rates and unsteadiness predictions of several characteristics of mantle plumes from boundary layer theory have been tested and calibrated with numerical models in cylindrical coordinates instability spacing in the initial stages of the numerical models is well predicted as are the lift off time of plume heads the size of plume heads at lift off and the heat flow of individual plume tails however since the prediction quality for the instability spacing and the lift off time of plume heads deteriorates in the presence of larger heterogeneities in the thermal boundary layer these predictions seem not very useful for mantle plumes predictions for the plume head size at lift off appear to be largely independent of the degree of boundary layer heterogeneity and therefore may provide reasonable estimates for mantle plumes the predictions for the heat flow of a plume tail seem to yield usable results for both stationary plume tails and those that show at least periods of quasi stationary heat flow assuming that mantle plumes are reasonably steady the prediction formula is used to derive estimates for the temperature difference t across the thermal boundary layer at the core mantle boundary this depends on the fraction of the core surface that is presumed to be occupied by plume feeding areas taking this fraction to be greater than 1 2 yields t 400 c too low to be compatible with petrological constraints on plumes smaller fractions yield larger t but this seems to be incompatible with the geophysical distribution of hotspots

observational constraints on models of the hawaiian hot spot swell a detailed analysis of the topography and geoid associated with the hawaii emperor seamount chain reveals that while geoid topography ratios are high over the southeastern part of the chain they fall off with distance away from the current hot spot location the topographic expression of the swell has two maxima near the intersections between the seamount chain and the murray and molokai fracture zones respectively both geoid and topography rapidly decrease westward of the intersection with the murray fracture zone it appears that the region of the pacific plate between the two fracture zones has been more susceptible to reheating and hot spot penetration than elsewhere along the chain alternatively the observed pattern may be indicative of time variation in the strength of the hawaiian hot spot

geochronology of age progressive volcanism of the oregon high lava plains implications for the plume interpretation of yellowstone 1 the high lava plains province hlp is a late cenozoic bimodal volcanic field at the northern margin of the basin and range province in southeastern oregon that hosts a westward younging trend of silicic volcanism that crudely mirrors northeastward migration of silicic volcanism along the yellowstone snake river plain ysrp trend we present 40ar 39ar ages for 19 rhyolite domes 5 rhyolite ash flow tuffs and 34 basaltic lavas from the hlp the previously identified trend of westward migration of hlp rhyolites is confirmed the rate of propagation is 33 km m y from 10 to 5 ma slowing to 13 km m y after 5 ma the duration of silicic volcanism at any locus is 2 m y three older hlp dacite domes yielded ages of 15 5 ma basalts are not age progressive we identify several episodes of increased basaltic activity at 7 5 7 8 5 3 5 9 and 2 3 ma with the younger episode likely continuing into the recent the hlp and ysrp trends emerged from the axis of middle miocene basaltic volcanism of the columbia river and steens basalts we propose a model in which 1 miocene flood basalts and widespread silicic rocks are the result of emplacement of a plume head near the craton margin enhanced by flow up a topographic gradient along the base of the lithosphere at the craton margin 2 the hlp trend is the result of westward flow originating at the craton margin and 3 the ysrp trend is the trace of the motion of the north american plate over the tail of the plume

temporal variation of the hawaiian plume flux abstract the magnitude of the hawaiian swell which surrounds the hawaiian volcanic chain does not decline monotonically and smoothly with distance along the chain the observed undulations are interpreted here as reflecting past variations in the buoyancy flux of the hawaiian plume rather than the monotonic thermal decline of the swell with age as has been inferred previously this interpretation is supported by seismic evidence that indicates there has been little thinning of the lithosphere which it is shown further implies that the time scale for thermal subsidence will be very long the largest change in flux was an increase in the plume flux by more than a factor of two about 25 30 myr ago the size of the volcanic edifices shows a broad correlation with the variations in the swell supporting this interpretation

lateral flow of hot plume material ponded at sublithospheric depths local buoyancy forces drive the flow of ponded plume material along the base of the lithosphere this processes complicates the geological effects of plumes beyond those of simple passage of plates over point heat sources variations in the depth to the base of the lithosphere significantly affect lateral flow the flow rate is modified to a moderate extent by locally thin lithosphere beneath midplate hotspots such as hawaii the effect is relevant when numerical models are rescaled to better fit the observed uplift and the observed distance from the hotspot to the nose of the swell variations in the depth to the base of the lithosphere have a much stronger effect on flow near ridge axes effectively forming an upside down drainage pattern plume material flows upslope toward ridge axes from off axis plumes and along ridge axes from on axis plumes at sufficiently low spreading rates thinner lithosphere exists above plume material than above normal asthenosphere the edges of normal lithosphere adjacent to plume material act to some extent as levees which limit the lateral spreading of plume material pressure release melt is associated with the upslope flow of plume material it is expected to cause volcanism away from the plume and to sometimes supply melt depleted plume material to ridge axes that are upslope from the region of voluminous melting

laboratory investigation of the interaction of off axis mantle plumes and spreading centres mantle plumes and mid ocean ridge spreading centres are the dominant phenomena through which mass and heat are transported from the mantle to the earth s surface it now seems that the dispersion of near ridge plumes beneath the lithosphere is modulated strongly by mid ocean ridges in particular geochemical and geophysical observations have suggested that rising plumes are diverted towards and feed nearby ridges 1 7 here we confirm the feasibility of this model with laboratory experiments that incorporate the essential physical and fluid dynamic aspects of a plume ridge upper mantle system our results indicate that an off axis plume may communicate thermally and chemically with a spreading ridge through a narrow sub horizontal conduit instead of a broader radially spreading plume head a necessary condition for this communication is the presence of a lithospheric or rheological boundary layer that thickens away from the ridge axis owing to conductive cooling interestingly we find that for high plume temperatures increasing the plume thermal buoyancy may inhibit rather than enhance plume ridge interaction as a result of increased erosion of the overlying lithosphere

shape of thermal plumes in a compressible mantle with depth dependent viscosity the mantle plume model has been invoked to explain the formation of large igneous provinces lip and associated age progressive hotspot tracks the shape of mantle plumes should be significantly altered by physical properties of the mantle and will influence how plume theory is used to interpret observational constraints based on theoretical analysis and numerical modeling we explore the parameters that control the shape of thermal plumes in a compressible mantle a theoretical analysis shows that the ambient mantle viscosity plays a dominant role in determining the radius of thermal plumes this analysis is verified by numerical solutions a continuously decreasing mantle viscosity from the cmb to the lithosphere can effectively reduce the radius of both plume head and tail a low viscosity zone between 100 and 660 km depths where viscosity decreases by a factor of 100 reduces the radius of a plume conduit by approximately a factor of 3 such a low viscosity zone can reduce the plume head radius impinging the lithosphere from larger than 500 km to 200 km when the low viscosity zone is confined to between 100 and 410 km depths the plume head size becomes even smaller to form large igneous provinces a small plume head implies time progressive volcanism from lip center to lip edge

dynamically supported geoid highs over hotspots observation and theory hotspots are associated with long wavelength geoid highs an association that is even stronger when the geoid highs associated with subduction zones are removed we quantify these associations by expanding the hotspot distribution in spherical harmonics and calculating correlation coefficients as a function of harmonic degree the hotspot distribution spectrum is nearly white except for peaks at degrees 2 and 6 it is correlated positively with the slab residual geoid for degrees 2 6 with low seismic velocity in the lower mantle at degree 2 and with low seismic velocity in the upper mantle at degree 6 we test fluid mechanical models for hotspots including lithospheric delamination and hot plumes by calculating their predicted dynamic geoid responses and comparing them to the observations these models include the effects of temperature dependent rheology our preferred hotspot model based on observations of the geoid and seismic tomography has plumes preferentially occurring in regions of large scale background temperature highs in a mantle which has a substantial viscosity increase with depth although other models are possible alternatively strong plumes in the lower mantle may neck down and attenuate in the upper mantle due to changes in plume viscosity the major low density anomalies causing the geoid highs associated with plumes appear to be in the lower mantle we hypothesize that cold lower mantle underlying the complementary geoid lows is due to deeply subducted oceanic lithosphere and that hotspot plumes tend to be excluded from these cold regions of the mantle

the kinetics of the reaction of majorite plus ferropericlase to ringwoodite implications for mantle upwellings crossing the 660 km discontinuity we have measured the kinetics of reaction between mgo and majoritic garnet at 20 gpa and 1773 2123 k as a proxy for the reaction between perovskite and ferropericlase during mantle upwelling across the 660 km seismic discontinuity ringwoodite forms a layer between mgo and garnet and in the case of aluminous garnets the interface between ringwoodite and garnet develops a fingering instability resulting in a complex intergrowth at this interface by contrast the mgo ringwoodite interface is always planar for an initial planar mgo garnet interface two thicknesses are therefore defined 1 a layer thickness x1x1 which is the maximum thickness of ringwoodite which forms a plane parallel bounded layer next to the mgo and 2 an interface thickness x2x2 which is the maximum extent of the intergrowth region away from the ringwoodite layer the growth of both of these regions can be described by apparent rate constants kiki which are arrhenius with view the mathml sourceln k10 6 36 0 25 m2 s and e1 456 40 kj mole1 456 40 kj mol for the ringwoodite layer and view the mathml sourceln k20 9 2 3 3 m2 s and e2 371 53 kj mole2 371 53 kj mol for the intergrowth region the fingering instability is caused by the incompatibility of aluminium in ringwoodite and its low chemical diffusivity in garnet which results in an increase of surface area at the ringwoodite garnet interface to minimise the aluminium concentration at the interface the intergrowth region contains a fine grained mixture of ringwoodite and garnet which coarsens very slowly with time this might result in a transient weakening of upwelling regions of mantle just above the 660 km seismic discontinuity allowing some viscous decoupling between the upper and lower mantle

the effect of a shallow low viscosity zone on the apparent compensation of mid plate swells abstract a simple model for mid plate swells is that of convection in a fluid which has a low viscosity layer lying between a rigid bed and a constant viscosity region finite element calculations have been used to determine the effects of the viscosity contrast the layer thickness and the rayleigh number on the flow and on the perceived compensation mechanism for the resulting topographic swell as the viscosity decreases in the low viscosity zone the effective local rayleigh number for the top boundary layer of the convecting cell increases also because the lower viscosity facilitates greater velocities in the low viscosity zone the low viscosity layer produces proportionally greater horizontal flow near the conducting lid causing the base of the conducting lid to appear like a free boundary the change in the local rayleigh number and in the effective boundary condition both cause the top boundary layer to thin through a green s function analysis we have found that the low viscosity zone damps the response of the surface topography to the temperature anomalies at depth whereas it causes the gravity and geoid response functions to change sign at depth counteracting the positive contributions from the shallower temperature variations by increasing the viscosity contrast the conbined effects of the thinning of the boundary layer and the behaviour of the response functions allow the apparent depth of compensation to become arbitrarily small therefore shallow depths of compensation cannot be used to argue against dynamic support of mid plate swells furthermore we compared the distribution of the effective compensating densities which is used to obtain the geoid to that of pratt compensation which is often used to calculate the depth of compensation from geoid and topography data for mid plate swells for all of our calculations including those with no low viscosity layer the effective gravitational mass distribution is more complex than assumed in simple pratt models so that the pratt models are not an appropriate gauge of the compensation mechanism

migrating cretaceous eocene magmatismin the serra do mar alkaline province se brazil melts from the deflected trindade mantle plume department of geological sciences university of durham south road durham dh1 3le uk department of earth sciences university of cambridge downing street cambridge cb2 3eq uk department of physics the university newcastle upon tyne ne1 7ru uk department of geology mcmaster university 1280 main street west hamilton ont canada l8s 4m1 departmento de geoquimica e recursos minerais instituto de geociencias universidade de brasilia

the topographic and gravitational expression of density anomalies due to melt extraction in the uppermost oceanic mantle abstract evidence suggests that basaltic extrusion can decrease the density of the depleted source region by up to 2 for 20 partial melting this hypothesis is tested by calculating the effect of the density anomaly between the depleted and undepleted mantle on the topography gravity and geoid fields and comparing the anomalies to the observed topography gravity and geoid anomalies over volcanic features in the oceans if the source region lies at the base of the lithosphere we find that the gravity and geoid anomalies are proportionally more sensitive to the density contrast than the topography anomaly this effect is most pronounced for wavelengths less than 1000 km since mid ocean ridge volcanism the largest source of volcanism in the oceanic plates is fairly uniform along the ridge the depleted material in the oceanic plates and the corresponding topography and gravity anomalies should also be somewhat uniform in extent any long wavelength anomalies will be obscured in the topography and geoid fields by other anomalies at similar wavelengths such as slab and deep mantle anomalies however at mid plate swells and seamounts if the density contrast is greater than 0 4 the topography gravity and geoid anomalies are measurable to be consistent with the wavelengths represented in the geoid and topography data at hawaii the depleted material must be spread into a thin layer by convective flow at mid plate swells even in the thin layer model the topography and geoid anomalies due to the depleted material would still be as large as 200 m and 2 m respectively after the convective flow producing the bulk of the swell is removed the anomaly due to the depleted material would remain and would be apparent in the geoid and topography fields at seamounts if the source region is near the base of the plate then the presence of depleted material should be distinguishable in the geoid field but be somewhat obscured by flexural effects therefore although a topographic or geoid anomaly due to depletion has not been observed the density anomaly cannot be discounted because only careful scrutiny and modelling of mid plate swells and seamounts would distinguish its effect

polar night vortex breakdown and large scale stirring in the southern stratosphere the present paper examines the vortex breakdown and large scale stirring during the final warming of the southern hemisphere stratosphere during the spring of 2005 a unique set of in situ observations collected by 27 superpressure balloons spbs is used the balloons which were launched from mcmurdo antarctica by the strateole vorcore project drifted for several weeks on two different isopycnic levels in the lower stratosphere we describe balloon trajectories and compare them with simulations obtained on the basis of the velocity field from the geos 5 and ncep ncar reanalyses performed with and without vorcore data to gain insight on the mechanisms responsible for the horizontal transport of air inside and outside the well isolated vortex we examine the balloon trajectories in the framework of the lagrangian properties of the stratospheric flow coherent structures of the flow are visualized by computing finite time lyapunov exponents ftle a combination of isentropic analysis and ftle distributions reveals that air is stripped away from the vortex s interior as stable manifolds eventually cross the vortex s edge it is shown that two spbs escaped from the vortex within high potential vorticity tongues that developed in association with wave breaking at locations along the vortex s edge where forward and backward ftle maxima approximately intersect the trajectories of three spbs flying as a group at the same isopycnic level are examined and their behavior is interpreted in reference to the ftle field these results support the concept of stable and unstable manifolds governing transport of air masses across the periphery of the stratospheric polar vortex

the dynamics of plume ridge interaction 2 off ridge plumes a variety of geophysical and geochemical evidence indicates that ascending mantle plumes can interact with ocean ridges located up to 1400 km away i investigate the dynamics of this interaction using a simple model in which a point source with volume flux q analogous to a plume stem releases buoyant fluid into a viscous corner flow driven by the divergence of rigid surface plates with thickness kx u 1 2 where u is the half spreading rate the point source is located at a distance xp from the ridge and ridge migration is neglected the buoyant fluid forms a thin sublithospheric layer whose thickness s x y satisfies a nonlinear advection diffusion equation describing the balance of advection by the corner flow buoyancy driven self spreading flow toward the ridge along the sloping base of the lithosphere and continuous accretion into the lithosphere numerical solutions of this equation yield scaling laws for the lateral extent w waist width of plume material along the ridge the fraction r of the plume flux that crosses the ridge and the maximum value of xp beyond which interaction ceases the sloping base of the lithosphere has only a minor few tens of percent influence on these quantities which are determined principally by the balance of advection and self spreading an extension of the model to include plume induced lithospheric thinning shows that this process increases the waist width by an amount of order 10 finally the model provides a new explanation for the observation that plumes interact primarily with ridges that are migrating away from them rather than toward them

crustal thickness and the average depth and degree of melting in fractional melting models of passive flow beneath mid ocean ridges in simple models of passive flow beneath mid ocean ridges fractional melting of the upwelling mantle produces unexpected relationships amongst crustal thickness degree of partial melting and the average depth of melting if the degree of melting is estimated on the basis of basalt composition then the average degree of melting is one third the maximum degree of melting and the average depth of melting is about two thirds the maximum depth in the simplest models the crustal thickness is about 1 5 times the product of the average fraction of melting and the height of the melting column if the average degree of melting represented in mid ocean ridge basalts were 10 and melt production continued to the base of the crust then only a 40 km high residual melting column would be required to produce a 6 km thick crust in more realistic models melting beginning at 60 km below the base of the crust and continuing to the top of the mantle would produce a 6 km thick crust with average degree of melting of 6 67 or melting beginning at 67 km and ending at 30 km below the base of the crust would produce a 6 km thick crust with an average degree of melting of only about 5 4 models of composition produced by fractional melting must consider the rate of melt production and the degree of melting throughout the melting region at the points where the melt is actually produced

surface microphysical observations within east coast winter storms on long island new york abstractsurface observations of ice habit and degree of riming were measured for 12 cyclone events over 3 winter seasons at stony brook new york on the northeast coast of the united states a total of 205 6 cm of snow accumulated during these storms with an average degree of riming of 1 25 out of 5 and snow to liquid ratio ranging from 3 1 to 17 1 there were consistent spatial patterns of habit and riming intensity relative to the cyclone structure cold type habits side planes and bullets commonly occurred within the outer comma head to the north and northeast of the cyclone center in the middle of the comma head moderately rimed dendrites plates and needles were observed close to the cyclone center heavy riming was observed with needles and graupel the western quadrant of the comma head had primarily plates and dendrites with little to no riming periods of light riming and high snow liquid ratios 13 1 are dominated by cold type habits dendrites and plates and have similar vertical m

observed microphysical evolution for two east coast winter storms and the associated snow bands abstractthis paper presents the observed microphysical evolution of two coastal extratropical cyclones 19 20 december 2009 and 12 january 2011 and the associated passage of heavy snowbands in the cyclone comma head the observations were made approximately 93 km east of new york city at stony brook new york surface microphysical measurements of snow habit and degree of riming were taken every 15 30 min using a stereo microscope and camera and snow depth and snow density were also recorded a vertically pointing ku band radar observed the vertical evolution of reflectivity and doppler vertical velocities there were rapid variations in the snow habits and densities related to the changes in vertical motion and depth of saturation at any one time a mixture of different ice habits was observed certain ice habits were dominant at the surface when the maximum vertical motion aloft occurred at their favored temperature for depositional growth convective seeder cells above 4 km msl resulted in relativel

ice crystal habits and growth processes in stratiform clouds with embedded convection examined through aircraft observation in northern china abstractice crystal habits and growth processes in two cases of stratiform clouds with embedded convection are investigated using data observed simultaneously from three aircraft on 18 april 2009 and 1 may 2009 as part of the beijing cloud experiment bce the results show that the majority of ice crystal habits found in the two cases at temperatures between 0 and 16 c included platelike needle column capped column dendrite and irregular a mixture of several ice crystal habits was identified in all of the clouds studied however the ice crystals recorded in the embedded convection regions contained more dendrites and possessed heavier riming degrees and the ice crystals identified in the stratiform clouds contained more hexagonal plate crystals both riming and aggregation processes played central roles in the broadening of particle size distributions psds and these processes were more active in embedded convection regions than in stratiform regions however riming was more prevalent in the

high t low p metamorphism in the palaeoproterozoic halls creek orogen northern australia the middle crustal response to a mantle related transient thermal pulse high t low p metamorphic rocks of the palaeoproterozoic central halls creek orogen in northern australia are characterised by low radiogenic heat production high upper crustal thermal gradients locally exceeding 40 c km 1 sustained for over 30 myr and a large number of layered mafic ultramafic intrusions with mantle related geochemical signatures in order to account for this combination of geological and thermal characteristics we model the middle crustal response to a transient mantle related heat pulse resulting from a temporary reduction in the thickness of the mantle lithosphere this mechanism has the potential to raise mid crustal temperatures by 150 400 c within 10 20 myr following initiation of the mantle temperature anomaly via conductive dissipation through the crust the magnitude and timing of maximum temperatures attained depend strongly on the proximity duration and lateral extent of the thermal anomaly in the mantle lithosphere and decrease sharply in response to anomalies that are seated deeper than 50 60 km maintained for 5 myr in duration and or have half widths 100 km maximum temperatures are also intimately linked to the thermal properties of the model crust primarily due to their influence on the steady state background thermal gradient the amplitudes of temperature increases in the crust are principally a function of depth and are broadly independent of crustal thermal parameters mid crustal felsic and mafic plutonism is a predictable consequence of perturbed thermal regimes in the mantle and the lowermost crust and the advection of voluminous magmas has the potential to raise temperatures in the middle crust very quickly although pluton related thermal signatures significantly dissipate within 10 myr even for very large high temperature intrusive bodies the interaction of pluton and mantle related thermal effects has the potential to maintain host rock temperatures in excess of 400 450 c for up to 30 myr in some parts of the mid crust the numerical models presented here support the notion that transient mantle related heat sources have the capacity to contribute significantly to the thermal budget of metamorphism in high t low p metamorphic belts especially in those characterised by low surface heat flow very high peak metamorphic geothermal gradients and abundant mafic intrusions

south arch volcanic field newly identified young lava flows on the sea floor south of the hawaiian ridge several young lava fields were imaged by gloria sidescan sonar along the hawaiian arch south of hawaii the largest 35 by 50 km across includes a central area characterized by high sonar backscatter and composed of several flow lobes radiating from a vent area reflection profiling and sea floor photography indicate that the central lobes are flat sheet flowsbounded by pillowed margins thin surface sediment and thin palagonite rinds on lava surfaces suggest ages of 1 10 ka vents are localized along the arch crest near bases of cretaceous seamounts two dredged flows are basanite and alkalic basalt broadly similar to rejuvenated stage and some pre shield alkalic lavas on the hawaiian ridge arch volcanism representsperipheral leakage of melt from the hawaiian hot spot over much larger areas than previously recognized

heat flow and the thermal origin of hot spot swells the hawaiian swell revisited we present 150 new heat flow measurements obtained at eight sites along a 1230 km long profile across the hawaiian swell about 700 km ese of midway island most of the measurements include in situ thermal conductivity determinations which helped to reduce the statistical uncertainties 95 confidence at all sites to 59 mw m 2 about 20 higher than the predicted heat flow for 100 ma seafloor based on simple lithospheric cooling models thus even though there is a small increase in heat flow with age along the swell when compared with the off swell heat flux the anomalous heat flow associated with the hawaiian swell is probably of the order of 5 10 mw m 2 and arguably may not exist at all a previous investigation von herzen et al 1982 may have overestimated the magnitude of the heat flow anomaly associated with the hawaiian swell by comparing heat flux measurements on the swell with values expected for simple lithospheric cooling models the heat flow anomalies associated with the bermuda rise and the cape verde rise may also be smaller than previously estimated given the uncertainties in the heat flux on the normal seafloor surrounding these swells the lack of a significant heat flow anomaly associated with the bathymetric expression of the hawaiian swell is inconsistent with simple models of lithospheric reheating dynamic support accompanied by modest temperature increases 80 ma oceanic lithosphere

a comprehensive habit diagram for atmospheric ice crystals confirmation from the laboratory airs ii and other field studies abstract recent laboratory experiments and in situ observations have produced results in broad agreement with respect to ice crystal habits in the atmosphere these studies reveal that the ice crystal habit at 20 c is platelike extending to 40 c and not columnar as indicated in many habit diagrams found in atmospheric science journals and texts these diagrams were typically derived decades ago from laboratory studies some with inherent habit bias or from combinations of laboratory and in situ observations at the ground observations that often did not account for habit modification by precipitation from overlying clouds of varying temperatures habit predictions from these diagrams often disagreed with in situ observations at temperatures below 20 c more recent laboratory and in situ studies have achieved a consensus on atmospheric ice crystal habits that differs from the traditional habit diagrams these newer results can now be combined to give a comprehensive description of ice crystal habits

mantle thermal structure and active upwelling during continental breakup in the north atlantic seismic reflection and refraction data acquired on four transects spanning the southeast greenland rifted margin and greenland iceland ridge gir provide new constraints on mantle thermal structure and melting processes during continental breakup in the north atlantic maximum igneous crustal thickness varies along the margin from s 30 km in the near hotspot zone 6 500 km from the hotspot track to v18 km in the distal zone 500 1100 km magmatic productivity on summed conjugate margins of the north atlantic decreases through time from 1800 300 to 600 50 km 3 km ma in the near hotspot zone and from 700 200 to 300 50 km 3 km ma in the distal zone comparison of our data with the british faeroe margins shows that both symmetric and asymmetric conjugate volcanic rifted margins exist joint consideration of crustal thickness and mean crustal seismic velocity suggests that along margin changes in magmatism are principally controlled by variations in active upwelling rather than mantle temperature the thermal anomaly vt at breakup was modest v100 125 c varied little along the margin and transient data along the gir indicate that the potential temperature anomaly 125 50 c and upwelling ratio v4 times passive of the iceland hotspot have remained roughly constant since 56 ma our results are consistent with a plume impact model in which 1 a plume of radius v300 km and vt of v125 c impacted the margin around 61 ma and delivered warm material to distal portions of the margin 2 at breakup 56 ma the lower half of the plume head continued to feed actively upwelling mantle into the proximal portion of the margin and 3 by 45 ma both the remaining plume head and the distal warm layer were exhausted with excess magmatism thereafter largely confined to a narrow 6 200 km radius zone immediately above the iceland plume stem alternatively the warm upper mantle layer that fed excess magmatism in the distal portion of the margin may have been a pre existing thermal anomaly unrelated to the plume fl 2001 elsevier science b v all rights reserved

three dimensional modeling of plume lithosphere interaction in order to understand better the dynamics of hot spots such as hawaii we present a three dimensional numerical model for the interaction of a thermal plume with a moving lithosphere the model domain is a rectangular box filled with fluid whose viscosity depends upon temperature and pressure the lithosphere is represented by a layer of cold highly viscous fluid moving with an imposed horizontal velocity u in the x direction and a thermal plume is generated by a circular temperature anomaly on the bottom of the box the steady flow is determined numerically using a hybrid spectral finite difference technique the flow is characterized by a stagnation streamline of width y x1 5 that represents the edge of the spreading plume material we illustrate the detailed behavior of the model using the example of the hawaiian plume our best fitting hawaiian model is obtained by adjusting the plume buoyancy flux b until the predicted topography anomaly matches the observed hawaiian swell topography we find b 4100 kg s 1 which implies a plume radius of 90 km for an assumed plume mantle temperature contrast of 300 c the predicted topography is supported primarily by density anomalies beneath the lithosphere and cannot be explained by lithospheric erosion we therefore conclude that the classical lithospheric reheating model is unable to account for hotspot swells the horizontal flux of buoyancy associated with the swell exceeds b by up to 80 suggesting that current estimates of b for mantle plumes are too high empirically derived scaling laws for the width of the stagnation streamline and for the topography anomaly exhibit power law dependencies on b and u that agree well with those predicted by the refracted plume model of olson 1990 the principal weakness of the model is that the predicted geoid topography ratio of 0 010 for the hawaiian swell is about twice the observed value of 0 004 0 006

anomalously young volcanoes on old hot spot traces i geology and petrology of cocos island cocos island is the summit of a seamount on the aseismic cocos ridge a proposed trace of the galapagos hot spot the island lies on a portion of the ridge that is middle miocene in age but k ar and paleomagnetic dates indicate that cocos is only about 2 m y old cocos thus offers a rare opportunity for an on land study of seamount volcanism superimposed on an early hot spot volcanism cocos island was built in three major stages that define three lithostratigraphic units 1 shield building 2 explosive volcanism and 3 post explosive volcanism stages all cocos rocks belong to the typical oceanic island alkali basalt to trachyte series and have fairly homogeneous sr 87 sr 86 sr 0 70299 0 70308 nd 143 nd 144 nd 0 512952 0 513001 and pb 206 pb 204 pb 19 214 19 251 207 pb 204 pb 15 553 15 596 206 pb 204 pb 38 899 39 036 isotopic ratios the cocos rock series was generated by fractional crystallization of olivine clinopyroxene plagioclase ilmenite and apatite from similar alkali basalt parental magmas sr nd and pb isotopic ratios indicate that cocos and galapagos volcanic rocks may have come from a common although heterogeneous mantle reservoir and this implies that the young cocos volcano is still a part of the galapagos hot spot signal