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

two component auroral electrojet importance for substorm studies this paper attempts to synthesize the diverse number of observations of electric fields and currents in the high latitude ionosphere during substorms by demonstrating that there are often spatial shifts among regions of high ionospheric conductivity large electric fields and intense currents in the auroral electrojet it is shown that substorm time variations of the current patterns over the entire polar region consist of two basic components the first is related to the two cell convection pattern and the second to the westward electrojet in the dark sector which is in turn related to the three dimensional wedge current system these two components result from the relative strength of electric fields and conductivities in the intensification of the auroral electrojet and are identified as the signatures for directly driven and the unloading components in solar wind magnetosphere interactions we contend that disturbed intervals do not necessitate the presence of substorm expansion phase activity and that the vast number of earlier complex results concerning the auroral electrojet can be ascertained from the high degree of variability of the two components depending on substorm events substorm phases and their own spatial temporal scale sizes it is demonstrated that several major issues that have remained controversial are now accounted for reasonably well in terms of this two component electrojet model we also predict specific features of the substorm auroral electrojet that have not yet been observed

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

hotspots mantle plumes flood basalts and true polar wander persistent long lived stationary sites of excessive mantle melting are called hotspots hotspots leave volcanic trails on lithospheric plates passing across them the global constellation of fixed hotspots thus forms a convenient frame of reference for plate motions through the orientations and age distributions of volcanic trails left by these melting anomalies hotspots appear to be maintained by whole mantle convection in the form of upward flow through narrow plumes evidence suggests that plumes are deflected little by horizontal flow of the upper mantle mantle plumes are largely thermal features and arise from a thermal boundary layer most likely the mantle layer just above the core mantle boundary experiments and theory show that gravitational instability drives flow beginning with the formation of diapirs such a diapir will grow as it rises fed by flow through the trailing conduit and entrainment of surrounding mantle the structure thus develops a large spherical plume head and a long narrow tail on arrival at the base of the lithosphere the plume head flattens and melts by decompression producing enormous quantities of magma which erupt in a short period these are flood basalt events that have occurred on continents and in ocean basins and that signal the beginning of major hotspot tracks the plume supported hotspot reference frame is fixed in the steady state convective flow of the mantle and is independent of the core generated axial dipole paleomagnetic reference frame comparison of plate motions measured in the two frames reveals small but systematic differences that indicate whole mantle motion relative to the earth s spin axis this is termed true polar wander and has amounted to some 12 since early tertiary time the direction and magnitude of true polar wander have varied sporadically through the mesozoic probably in response to major changes in plate motions particularly subduction zone location that change the planet s moments of inertia

interaction of mantle plumes and migrating mid ocean ridges implications for the galapagos plume ridge system we investigate the three dimensional interaction of mantle plumes and migrating mid ocean ridges with variable viscosity numerical models numerical models predict that along axis plume width w and maximum distance of plume ridge interaction xmax scale with q u where q is plume source volume flux and u is ridge full spreading rate both w and xmax increase with buoyancy number b which reflects the strength of gravitational versus plate driven spreading scaling laws derived for stationary ridges in steady state with near ridge plumes are consistent with those obtained from independent studies of ribe 1996 in the case of a migrating ridge the distance of plume ridge interaction is reduced when a ridge migrates toward the plume because of the excess drag of the faster moving leading plate and enhanced when a ridge migrates away from the plume because of the reduced drag of the slower moving trailing plate given the mildly buoyant and relatively viscous plumes investigated here the slope of the lithospheric boundary and thermal erosion of the lithosphere have little effect on plume flow from observed plume widths of the galapagos plume migrating ridge system our scaling laws yield estimates of galapagos plume volume flux of 5 16 106km3 m y 1 and a buoyancy flux of 2 103 kg s 1 model results suggest that the observed increase in bathymetric and mantle bouguer gravity anomalies along cocos plate isochrons with increasing isochron age is due to higher crustal production when the galapagos ridge axis was closer to the plume several million years ago the anomaly amplitudes can be explained by a plume source with a relatively mild temperature anomaly 50 100 c and moderate radius 100 200 km predictions of the along axis geochemical signature of the plume suggest that mixing between the plume and ambient mantle sources may not occur in the asthenosphere but instead may occur deeper in the mantle possibly by entrainment of depleted mantle as the plume ascends from its source

plume capture by divergent plate motions implications for the distribution of hotspots geochemistry of mid ocean ridge basalts and estimates of the heat flux at the core mantle boundary abstract the coexistence of stationary mantle plumes with plate scale flow is problematic in geodynamics we present results from laboratory experiments aimed at understanding the effects of an imposed large scale circulation on thermal convection at high rayleigh number 106 ra 109 in a fluid with a temperature dependent viscosity in a large tank a layer of corn syrup is heated from below while being stirred by large scale flow due to the opposing motions of a pair of conveyor belts immersed in the syrup at the top of the tank three regimes are observed depending on the ratio v of the imposed horizontal flow velocity to the rise velocity of plumes ascending from the hot boundary and on the ratio of the viscosity of the interior fluid to the viscosity of the hottest fluid in contact with the bottom boundary when v 1 and 1 large scale circulation has a negligible effect on convection and the heat flux is due to the formation and rise of randomly spaced plumes when v 10 and 100 plume formation is suppressed entirely and the heat flux is carried by a sheet like upwelling located in the center of the tank at intermediate v and depending on established plume conduits are advected along the bottom boundary and ascending plumes are focused towards the central upwelling heat transfer across the layer occurs through a combination of ascending plumes and large scale flow scaling analyses show that the bottom boundary layer thickness and in turn the basal heat flux q depend on the peclet number pe and when 10 q pe1 2 and when 1 q pe 1 3 consistent with classical scalings when applied to the earth our results suggest that plate driven mantle flow focuses ascending plumes towards upwellings in the central pacific and africa as well as into mid ocean ridges furthermore plumes may be captured by strong upwelling flow beneath fast spreading ridges this behavior may explain why hotspots are more abundant near slow spreading ridges than fast spreading ridges and may also explain some observed variations of mid ocean ridge basalt morb geochemistry with spreading rate moreover our results suggest that a potentially significant fraction of the core heat flux is due to plumes that are drawn into upwelling flows beneath ridges and not observed as hotspots

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

the dynamics of off axis plume ridge interaction in the uppermost mantle two decades of geochemical and geophysical observations have led to the plume channel model whereby buoyant off axis mantle plumes feed and interact with diverging plates at mid ocean ridge axes which act as sinks of upper mantle material here we present results from two dimensional 2 d numerical experiments which incorporate the essential physics and fluid dynamic aspects of the plume ridge upper mantle system in order to test the feasibility of this plume ridge interaction model specifically experiments test the relative importance of physical effects such as strong viscosity variations and thermal and chemical buoyancy forcing in plume ridge dynamics results indicate that both transient and steady state connections may be established between off axis plumes and ridges for a range of realistic mantle conditions the presence of a strongly sloping rheological boundary layer rbl is a necessary condition for long term communication between an off axis buoyant upwelling and a spreading ridge the flux of buoyant material to the ridge is also shown to increase with increasing plume to mantle density contrast decreasing plume viscosity and smaller plume ridge separation distances plume ridge interaction regimes are defined based on the competing effects of plate driven and buoyancy driven flow thermal erosion of the viscous lithosphere strongly inhibits long term plume ridge interaction by enhancing the plates ability to deflect the plume both head and conduit away from the ridge axis

reply to kamide and richmond detailed empirical models of the ionospheric convection electric field and auroral latitude conductance are used to show that at 04 magnetic local time mlt the electric field magnitude peaks at least 5 poleward of the hall conductance and is the stronger contributor to the poleward half of the westward electrojet current in the morning sector at 20 mlt the eastward electrojet current is centered on the electric field and conductance profiles which have a similar peak latitude and extent

substorm changes of the electrodynamic quantities in the polar ionosphere cdaw 9 on the basis of ground magnetometer data from 75 northern hemisphere stations and the ionospheric conductivity distribution estimated from viking satellite observations of auroral images various electrodynamic quantities in the polar ionosphere are calculated for the april 1 1986 coordinated data analysis workshop cdaw 9 substorm since the scandinavia and russia chains of magnetometers were located in the premidnight midnight sector during this interval and the estimated conductivity distribution is instantaneous our data set provides us with a unique opportunity to examine some long standing problems associated with the substorm expansion onset several important findings of this study are summarized as follows 1 before the expansion onset of the substorm intensifications of ionospheric currents or the cross polar cap potential are very weak in this particular example both quantities begin to increase notably only with the initiation of the substorm expansion onset 2 the intensified westward electrojet flows along the poleward half of the enhanced ionospheric conductivity belt in the midnight sector during the expansion phase while its equatorward half is occupied by a weak eastward electrojet 3 the joule heating rate and the energy input rate of auroral particles are quite comparable preceding the expansion onset during the expansion phase of the substorm however joule heating shows a marked intensification but the latter increases only moderately indicating that the joule dissipation is more effective than auroral particle energy input during substorm times 4 the hall currents are not completely divergence free the corresponding field aligned currents show highly localized structures during the maximum epoch of the substorm with the upward current being located in the region of the steepest conductivity gradient on the poleward side of the westward electrojet in the midnight sector this is indirect evidence that the so called imperfect cowling channel is effective behind the westward traveling surge

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

a dynamic model for the iceland plume and the north atlantic based on tomography and gravity data summary the north atlantic around iceland is characterized by a large geoid high and an anomalous shallow ocean floor both observations are presumably due to upwelling of hot material in the mantle we extracted this region from a global tomography data set to study the structure of the mantle in more detail the tomography data reveal a confined low velocity structure from the core mantle boundary cmb to the upper mantle stretching from a location at the cmb below southwest greenland towards the upper mantle in a strongly eastward inclination in the present study we compare the observed gravity potential field in the north atlantic with a modelled field based on mantle temperature variations estimated from tomography seismic traveltime residuals are converted to temperature variations assuming a linear relation between seismic velocity and density and a pressure dependent thermal expansivity we found a maximum excess temperature in the plume conduit at the cmb of 250 c weakly decreasing towards the phase transition zone ptz in the ptz a temperature rise of 50 70 c is found which is in agreement with the latent heat release by the olivine phase transitions the 3 d temperature field is then used as the driving force for viscous flow in a cartesian convection model the model dimensions are chosen four times as large as the tomographic section to allow resolution for long wavelengths and convective return flow for a number of constant viscosity and temperature and depth dependent viscosity cases the dynamic topography gravity anomaly and geoid undulations are calculated and compared with the egm96 potential field coefficients in the wavelength range of 400 to 4000 km the observation data were also corrected for ocean lithosphere cooling and isostatic compensation of continental crust the best agreement between observation and modelled data 78 per cent fit for geoid and 47 per cent for gravity is obtained for a temperature dependent viscosity of about one order of magnitude for 500 c temperature variation and an increase of viscosity with depth by no more than a factor of 50 from the upper to the lower mantle the generally good spatial agreement supports the tomographic model at least for the upper mantle and indicates that the east greenland margin as well as the outer faroer ridge are dynamically supported low density material west of the kolbeinsey ridge might be linked to low density anomalies below the greenland shield the presence of lower mantle anomalies causes a large scale geoid high of 3 to 5 m in agreement with observations but our approach cannot further constrain the spatial distribution of anomalies in the lower mantle

the scales of mantle convection seismic topographic and gravity data show that there are two important scales of mantle convection these are associated with spherical harmonic degrees l 2 and 6 the l 2 pattern corresponds to the pattern of subduction cooling since the breakup of pangea most hotspots and ridges occur in the half of the globe unaffected by this cooling and all large igneous provinces were generated over this part of the mantle the l 2 distribution of upwellings and downwellings is likely to be a long lived feature of the earth cold regions of the mantle repeatedly attract continents and subduction reinforces the coldness an l 1 pattern of convection is probably related to supercontinents and their breakup degree 6 convection shows up in the spectrum of hotspots and upper mantle tomography and in the correlation of tomography and topography with the geoid cratons with their deep cold keels control the l 6 pattern and may even cause it by their role in establishing lateral temperature gradients and relief at the top of the convecting mantle subduction zones reinforce the craton pattern downwellings preferentially occur under cratons upwellings and hotspots occur at complementary locations supercontinents and their associated subduction zones l 1 constantly assemble and reassemble in the african atlantic hemisphere and the continental fragments in the dispersed state periodically settle into the polar band of geoid lows l 2 and 6 that now includes the americas antarctica australia and india if cratons control the l 6 pattern of convection and many patterns of l 6 are possible e g sectoral zonal checker board then upper mantle convection may reorganize roughly every 30 ma as cratons move about the hotspot spectrum is probably related to lithospheric extension as well as to broad upwellings a third and smaller scale of convection order 400 1000 km in dimension is just below the resolution of global tomography but shows up in the gravity field geoid and topography this scale is controlled by the depth of an endothermic phase change which tends to stratify mantle convection and the thickness of the upper mantle low viscosity zone convective domains of this dimension are also implied by the scales of chemical homogeneity lengths of rifts ridges and seamount chains fracture zone spacing and mid ocean ridge segmentation the temperature geochemistry and fertility of these upper mantle scale domains is controlled by their previous history of subduction continental insulation or refrigeration and processing by ridges fertile or volatile rich or hot cells can be mistaken for plumes hotspot swells are typically of the dimension that we argue is a characteristic upper mantle scale rather than a deep mantle plume scale geochemical domains of various sizes exist in the upper mantle they are broad scale features rather than point sources as in plume theories lithospheric dynamics and geometric focusing not mantle dynamics control the dimensions of so called hotspot eruptives

estimate of electromagnetic quantities in space from ground magnetic records magnetic field records at the earth s surface contain the effects of a variety of different source currents flowing in outer space during the last decade several computational techniques for the use of magnetometer data have been proposed to obtain the global pattern of the electric fields and currents and joule heating in the ionosphere the numerical algorithm has now been enormously improved such that simultaneous more direct or in situ measurements by satellites and radars can be incorporated into the scheme in particular owing to recent progress in satellite viewed auroral images it is possible to estimate instantaneous patterns of the ionospheric electrodynamic quantities over the entire polar region with a time resolution of several minutes this report demonstrates that contrary to our conventional views large ionospheric currents can flow in regions of low auroral activity

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

upper mantle heterogeneities and dynamics the width of geochemical and residual elevation anomalies along the mid ocean ridge system decrease as both the distance between migrating ridge segments and related hotspots and the local geochemical dispersion in the anomalies increase these observations provide further evidence that flows of mantle plumes in the upper mantle are deflected by migrating ridge sinks once they override hotspots

the strength of the iceland plume a geodynamical scaling approach abstract an important measure of the strength of a mantle plume is its buoyancy flux b defined as the integral over a horizontal plane of the product of the vertical velocity and the density deficit within the plume in the case of the iceland plume which currently rises directly beneath the mid atlantic ridge published estimates of b cover a range of a factor of 37 to reconcile these diverse estimates we study a simple fluid mechanical model of a ridge centered plume in which plume fluid with spreadability buoyancy over viscosity is supplied at a volumetric rate q from a plume conduit located directly beneath a ridge with a half spreading rate u the plume fluid spreads laterally to form a thin pool beneath a lithosphere whose thickness increases as the square root of age application of scaling and dimensional analysis to this model leads to a general scaling law for the waist width w w the length of the plume induced elevation anomaly along the ridge the law has the form w w w 0 f 2 b s where w 0 q 4 u 5 1 6 is the fundamental length scale for plume ridge interaction b q u 2 1 3 is of the order of the aspect ratio width thickness of the plume pool s 2 u 3 1 4 measures the effect on the pool of the sloping base of the lithosphere is the thermal diffusivity and f 2 is an unknown function we determine f 2 using a suite of 32 numerical solutions of a three dimensional thermomechanical model implemented in the code stagyy tackley 2008 to apply our scaling law to iceland we invert it to estimate the buoyancy flux b required to produce a waist width w w 2300 300 km after correction for the effect of ridge migration we find b 2 3 0 6 mg s 1 this is comparable within uncertainty to the buoyancy flux b 3 0 0 8 mg s 1 of the hawaiian plume estimated using a 3 d dynamical model by ribe and christensen 1999

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

the allure of rock crystal in copper age southern iberia technical skill and distinguished objects from valencina de la concepcion seville spain abstract rock crystal appears relatively frequently in late prehistoric iberian sites especially in the form of micro blades and knapping debris with some exceptions however these finds have seldom been looked into in any detail and therefore little is known about the technology involved in the use of this material its social and economic relevance or its symbolic significance in this paper we examine a collection of rock crystal artefacts recently found at valencina de la concepcion seville spain one of the largest 3rd millennium bc sites in western europe among the objects included in this study are a long dagger blade twenty five arrowheads and a core all of which form the most technically sophisticated and esthetically impressive collection of rock crystal material culture ever found in prehistoric iberia through the analysis of the procedures and techniques applied in the production of these objects the chemical characterisation of the raw materials through raman spectroscopy and rti image processing and the careful assessment of the archaeological contexts in which they were found this paper makes a robust contribution towards the study of the role of rock crystal in copper age technology and society recent research suggest that valencina was a major node in the circulation of exotic materials such as ivory amber cinnabar or flint in copper age iberia which provides a very good background to assess the relevance of rock crystal as a traded commodity in addition we discuss the role of rock crystal as a marker of status in large megalithic monuments as well as its possible symbolic connotations

mantle plume heads and the initiation of plate tectonic reorganizations we present results of numerical simulations which examine the plausibility of a reorganization of plate motions brought about by the interaction of a hot low viscosity plume head with the underside of a tectonic plate a numerical model of highly viscous fluid flow driven by thermal buoyancy is employed to examine the interaction of mantle plumes with surface plates in a two dimensional cartesian geometry plate like boundary conditions applied to the upper surface consist of a large active plate with fixed speed plus a smaller passive plate whose speed and direction of motion are permitted to react to tractions induced by the underlying mantle flow driven by the active plate the passive plate is also subject to applied end loads intended to simulate incipient subduction the mantle plume is initiated with a hot patch on the lower boundary and viscosity varies with temperature following an arrhenius rheology the calculations performed with fixed aspect ratio 4 1 and fixed rayleigh number ra 106 where ra refers to the strength of the starting plume were found to depend on three parameters the strength of the viscosity variation the size of the applied end load and the speed of the active plate these calculations indicate that plate motions can be sharply influenced by the lubricating effects of mantle plume heads even to the point of causing a reversal in the direction of plate motion

the dynamics of plume ridge interaction iii the effects of ridge migration summary to investigate how the migration of ridges influences their interaction with nearby mantle plumes we study a thin layer model in which a point source analogous to a plume stem releases buoyant fluid at a rate q into a viscous corner flow driven by a ridge like surface boundary condition the ridge has a half spreading rate u and migrates relative to the point source at a constant speed um and azimuth from the plate spreading direction numerical solutions of the governing differential equation yield predictions of the lateral extent w of plume fluid along the ridge waist width which we compare with observed waist widths for 11 on and near ridge hotspots the main dynamical effect of ridge migration is to increase the maximum interaction distance for a ridge receding from a plume and to decrease this distance for a ridge approaching a plume models including normal age thickening and plume induced thinning of the lithosphere show that both effects have only a small maximum 20 per cent influence on the dynamics our results suggest that the volume fluxes q of five plumes far from ridges sala y gomez st helena great meteor reunion and kerguelen may be two to three times higher than previously estimated