a 56 million year rhythm in north american sedimentation during the phanerozoic abstract long term 10 myr fluctuations in climate sea level and sedimentation have been documented in the stratigraphic record but the lack of well constrained data series has made it difficult to rigorously evaluate cyclic periodic or quasi periodic changes at this scale here we utilize a new compilation of the coverage area of sedimentary rocks in north america to investigate the dominant modes orders of stratigraphic variability and to evaluate potential long period cyclic changes in sedimentation on the continent during the phanerozoic our analysis resolves two principal temporal modes of variability 1 a strongly sinusoidal mode with a periodicity of 56 myr 3 myr and 2 a longer term phanerozoic mode the m curve linked to the wilson cycle which is indistinguishable from a stochastic autoregressive process the newly identified 56 myr cycle in sedimentation delineates most of the cratonic sequences that have previously been identified qualitatively in north america but here we propose a quantitative redefinition that includes nine distinct units and two mega sequences the timing of the 56 myr beat in sedimentation is consistent with an orogenic oscillator source or an oscillatory dynamic in mantle convection and its tempo is statistically similar to a known rhythm in number of marine animal genera in the global fossil record thus the identification of a significant periodic signal in the sedimentary record of north america provides new evidence for an important tectonic and or mantle scale cyclic process that links both large scale biological evolution and physical environmental change

the proximity of hotspots to convergent and divergent plate boundaries an analysis of four different hotspot distributions ranging from morgan s 1972 original list of 19 to vogt s 1981 list of 117 reveals that the hotspots are preferentially located near divergent plate boundaries the probability of this proximity occurring by chance alone is quite remote less than 0 01 for all four hotspot distributions the same analysis also reveals that the hotspots are preferentially excluded from regions near convergent plate boundaries the probability of this exclusion occurring by chance alone is 0 1 or less for three out of the four distributions examined we interpret this behavior as being a consequence of the effects of large scale convective circulation on ascending mantle plumes mantle thermal plumes the most probable source of hotspots arise from instabilities in a basal thermal boundary layer plumes are suppressed from regions beneath convergent boundaries by descending flow and are entrained into the upwelling flow beneath spreading centers plate scale convective circulation driven by subduction may also advect mantle thermal plumes toward spreading centers

a rapid mechanism to remobilize and homogenize highly crystalline magma bodies the largest products of magmatic activity on earth the great bodies of granite and their corresponding large eruptions have a dual nature homogeneity at the large scale and spatial and temporal heterogeneity at the small scale this duality calls for a mechanism that selectively removes the large scale heterogeneities associated with the incremental assembly of these magmatic systems and yet occurs rapidly despite crystal rich viscous conditions seemingly resistant to mixing here we show that a simple dynamic template can unify a wide range of apparently contradictory observations from both large plutonic bodies and volcanic systems by a mechanism of rapid remobilization unzipping of highly viscous crystal rich mushes we demonstrate that this remobilization can lead to rapid overturn and produce the observed juxtaposition of magmatic materials with very disparate ages and complex chemical zoning what distinguishes our model is the recognition that the process has two stages initially a stiff mushy magma is reheated from below producing a reduction in crystallinity that leads to the growth of a subjacent buoyant mobile layer when the thickening mobile layer becomes sufficiently buoyant it penetrates the overlying viscous mushy magma this second stage rapidly exports homogenized material from the lower mobile layer to the top of the system and leads to partial overturn within the viscous mush itself as an additional mechanism of mixing model outputs illustrate that unzipping can rapidly produce large amounts of mobile magma available for eruption the agreement between calculated and observed unzipping rates for historical eruptions at pinatubo and at montserrat demonstrates the general applicability of the model this mechanism furthers our understanding of both the formation of periodically homogenized plutons crust building and of ignimbrites by large eruptions

interaction of rising and sinking mantle plumes the frequency of plume formation in fully developed thermal convection is determined experimentally because the fluid has a temperature dependent viscosity the cold and hot thermal boundary layers have different thicknesses and viscosities as a result plumes are released from these layers with different frequencies there also appears to be a direct temporal interaction between the cold and hot boundary layers in particular whereas there is a single frequency for cold plume formation hot plumes form with multiple frequencies one of which is similar to the frequency of cold plumes the measurements thus suggest that cold sinking plumes may induce hot rising plumes

synthetic seismic signature of thermal mantle plumes abstract the first seismic images of mantle plumes have been a source of significant debate to interpret these images it is useful to have an idea of a plume s expected seismic signature we determined a set of dynamic thermal whole mantle plumes with parameters appropriate for the earth s mantle and shallow mantle temperature contrasts compatible with surface observations we explore the sensitivity of amplitude and width of thermal plume anomalies to model parameters the conversion of thermal to seismic structure accounts for effects of temperature pressure an average mantle composition including phase transitions and anelasticity with depth dependent expansivity and temperature and depth dependent viscosity these relatively weak plumes have lower mantle diameters of 300 600 km at one half of the maximum temperature anomaly to attain the narrow upper mantle plumes inferred from surface observations and tomography viscosity reduction by a factor 30 100 is necessary either as a jump or as a strong gradient all model plumes had buoyancy fluxes 4 mg s and it seems difficult to generate whole mantle thermal plumes with fluxes much lower due to changing seismic sensitivity to temperature with depth and mineralogy variations in the plumes seismic amplitude and width do not coincide with those in their thermal structure velocity anomalies of 2 4 are predicted in the uppermost mantle reduced sensitivity in the transition zone as well as complex velocity anomalies due to phase boundary topography may hamper imaging continuous whole mantle plumes in the lower mantle our plumes have seismic amplitudes of only 0 5 1 unlike seismic velocities anelasticity reflects thermal structure closely and yields plume anomalies of 50 100 in dln 1 qs

modeling lava lake heat loss rheology and convection 1 measurements at erta ale s lava lake and theoretical equations for lake rheology density driven convection and thermally driven plume ascent allow the constraint of lake dynamics cooling and crystallization expected from surface heat losses imply a viscosity increase from 150 pa s to 300 1800 pa s for cooled surface layers convection is expected to proceed vigorously under low viscosity conditions driving rapid 0 1 0 4 m s 1 surface motions and sluggishly under moderate to high viscosity conditions to drive slower motions 0 08 m s 1 convection is likely driven by small 6 kg m 3 density differences where surface cooling can influence lake rheology and explain variable rates of surface convective motion

global distribution and characteristics of coronae and related features on venus implications for origin and relation to mantle processes coronae on venus range from 60 to over 2000 km across and are characterized by a complex range of morphologies the annuli around coronae range from about 10 to 150 km across and have tectonic features ranging from extensional to compressional to a combination of both topographically coronae are domes plateaus plateaus with interior lows and rimmed depressions a subset of features classified here as coronae corresponds to depressions and is interpreted to consist of large scale calderas a number of features have been identified with many of the basic characteristics of coronae similar interior deformation associations with volcanism high topography but lacking a distinct tectonic annulus these features tend to be somewhat smaller than coronae and may represent failed coronae or coronae in an early stage of evolution the size distribution of coronae and coronalike features with maximum widths greater than about 250 km is well represented by a power law of the form n d kd where n is the number of coronae with maximum widths greater than d km and 3 05 the spatial distribution of coronae is not random the features are concentrated in a few groups and along several chains coronae are similar in many morphologic characteristics to major volcanic shield structures and volcanic rises such as western eistla regio the largest corona artemis is actually larger than several volcanic rises on venus coronae and volcanic rises appear to be surface manifestations of mantle plumes there is no evidence of any systematic variation in age along chains of coronae as occurs in hotspot chains on earth instead a number of multiple and overlapping coronae may indicate limited movement of the surface above a hotspot or mantle plume the morphology and size distribution of coronae highlands and major shields suggest that mantle upwelling on venus operates either on several spatial scales with coronae representing smaller scale upflows and major volcanic rises representing larger convective upwellings or on several temporal scales with coronae representing shorter duration upflows and major volcanic rises representing long term upwellings

the influence of interior mantle temperature on the structure of plumes heads for venus tails for the earth 1 to form mantle plumes with large heads and narrow trailing conduits requires large 102 viscosity variations within the hot thermal boundary layer at the plume source however new and published data from laboratory experiments show that if a layer of fluid with a temperature dependent viscosity is heated from below and cooled from above convection occurs beneath a stagnant lid and the interior fluid temperature remains close to that of the hot boundary consequently the viscosity reduction across the hot thermal boundary layer is always 10 and upwellings take the form of discrete plume heads thermals subduction of the stagnant layer equivalent to recycling lithospheric plates on the earth will cool the interior of the mantle thus increasing the viscosity ratio hence the existence of earth like mantle plumes may be a consequence of plate tectonics an absence of plate tectonics on venus may lead to upwellings composed of thermals the surface expression of which are coronae as well as a smaller core heat flux than on earth

hotspots mantle plumes and core heat loss abstract the heat flux at the core mantle boundary cmb is a key parameter for core dynamics since it controls its cooling however it is poorly known and estimates range from 2 tw to 10 tw the lowest bound comes from estimates of buoyancy fluxes of hotspots under two assumptions that they are surface expression of mantle plumes originating from the base of the mantle and that they are responsible for the totality of the heat flux at the cmb using a new procedure to detect plumes in a numerical model of rayleigh benard convection convection between isothermal horizontal planes with internal heating it is shown that many hot plumes that start from the bottom boundary do not reach the top surface and that the bottom heat flux is primarily controlled by the arrival of cold plumes hot plumes easily form at the bottom boundary but they are mostly due to the spreading of cold plume heads that allow the concentration of hot matter these plumes are generally not buoyant enough to cross the whole system and the hot plumes that reach the top surface result from an interaction between several hot plumes according to this simple dynamical behavior the heat flux at the bottom boundary is shown to be strongly correlated with the advection due to cold plumes and not with advection by hot plumes that arrive at the surface it is then inferred that the heat flux out of hotspots can only give a lower bound to the heat flow at the cmb and that knowing the advection by subducted plates would give a better estimate

transient mantle convection on venus the paradoxical coexistence of highlands and coronae in the bat region thecoexistenceofvenusianhighlands attributedtolong livedaxisymmetricmantleplumes anduncompensatedcoronae attributed totransientdiscretemantle thermals isdifficulttoreconcilewithmodelsofmantleconvectionunderthermallysteady stateconditions however cratering and geological studies indicate a uniformly young surface age 700 myr as well as a comparable timescale for resurfacing 100 to 400 myr possibly consistent with a recent lithospheric overturn and a transient mantle thermal regime we use laboratory experiments on free and forced thermal convection at high rayleigh number ra 10 7 in a variable viscosity fluid to investigate the steady state and transient thermal regimes preceding and following such an overturn from analyses of shadowgraph images andtimeseries of global andlocal variations intemperature basal heat fluxandviscosity weestablishsteady statestagnant and active lid states and characterize two intermediate transient regimes flow in steady state stagnant lid is in the form of intermittent thermals consistent with published work during the transition to active lid convection the stagnant lid isstirred into the interior using a conveyorbelt spreadingofthiscoldfluidalongthehotboundaryleadstoatransitiontoa mixedmode offlowfromthehotboundary approximatelyisoviscous thermalsrise from the thermal boundarylayer aheadofthe advancingcoldfrontandlow viscosityplumes rise from behind the front as a result of an enhanced temperature contrast the longevity of this regime and the timescale for the transient depends on the rate of overturn pe and the aspect ratio of the system a the magnitude of local temperature viscosity and heat flux variationsincreaseswithpeandcanexceedsteady statevaluesforactive lidconvection additionalnumericalsimulationsshowthatthe mixed mode regime will occur in the presence of internal heating and for no and free slip boundaries in contrast the transition from active lid to stagnant lid convection is marked by a change from a flow composed of plumes and large scale overturning motions to a regime dominated by risingand sinking thermals on a timescale ofthermal diffusion applied tovenus our resultssupport a hypothesis that the contemporaneous coexistence of the atla and beta highlands regions with interspersed uncompensated coronae is consistent withatransientthermalregimefollowingalithosphericoverturn itisalsoexpectedthatsuchcoronaeformedn250myraftertheupliftof the highlands implications of the thermal origin of coronae for venusian mantle structure are also explored 2007 elsevier b v all rights reserved

resurfacing events on venus implications on plume dynamics and surface topography 1 global resurfacing events explain the relatively young and uniformly aged surface of venus in numerical models featuring these events a variety of plume classes occur ranging from strong stable to smaller wind driven upwellings we investigate this plume dynamics in a 3d cartesian geometry paying special attention to the surface topography while the stable upwellings of the stagnant lid phase correlate with an elevation of the surface the wind driven upwellings of the resurfacing phase do not necessarily correlate with a positive topography signal we rather observe that a thick plate can dominate the topography above upwellings

geothermal anomalies in the crust and upper mantle along southern rocky mountain transitions heat flow temperature gradient and gravity data are analyzed along five profiles crossing from the colorado plateau or great plains to the high topographic areas of the southern rocky mountains these profiles cross large negative bouguer gravity anomalies associated with the high topography in the region and are within the area studied by two recent seismic investigations the heatflow and gravity profiles typically cross from maximum to minimum values in the region allowing an fft fast fourier transform algorithm to be applied to the data so that high frequencies can be filtered the resulting smoothed data can be fitted by the first few terms of a sine series to estimate mid depths of crustal and upper mantle step sources although anomalies could also be caused by different source geometries at much shallower or much deeper depths the analysis provides a depth comparison for thermal and gravity anomaly sources modeled with similar geometries and a comparison of thermal source depths with seismic velocity anomalies there is considerable similarity along the profiles between the gravity and heat flow data and anomaly sources appear to be both in the crust and upper mantle there often appears to be spatial coincidence between thermal and gravity anomaly sources and seismic velocity anomalies proposed in the upper mantle implying thermal influence to these gravity and seismic anomalies thermal anomalies proposed to have sources in the crust in some areas are consistent with gravity and shear wave anomalies and support studies in those areas suggesting high crustal radiogenic concentrations in relatively less dense granitic crustal rocks

coronae on venus and mars implications for similar structures on earth complex volcano tectonic structures referred to as coronae had not been described until the exploration of the surface of venus these large generally circular structures are characterized by an elevated surface concentric and radial fracture systems and extensive volcanism thought to be unique to venus rare circular features on mars bear a close resemblance to coronae the most prominent corona like feature on mars is alba patera a broad low relief plateau shaped volcano tectonic center surrounded by an annulus of concentric fractures 600 km in diameter a geophysical model for the formation of venusian coronae involving uplift due to an ascending mantle diapir followed by gravitationally driven relaxation is applied to mars the results indicate that alba patera could have formed by such a mechanism the formation of coronae and corona like features on venus and mars from mantle diapirs suggests that similar structures may have formed in earth9s lithosphere

transitions in the style of mantle convection at high rayleigh numbers the pattern and style of mantle convection govern the thermal evolution internal dynamics and large scale surface deformation of the terrestrial planets in order to characterize the nature of heat transport and convective behaviour at rayleigh numbers ra appropriate for planetary mantles between 104 and 108 we perform a set of laboratory experiments convection is driven by a temperature gradient imposed between two rigid surfaces and there is no internal heating as the rayleigh number is increased two transitions in convective behaviour occur first we observe a change from steady to time dependent convection at ra 105 a second transition occurs at higher rayleigh numbers ra 5 106 with large scale time dependent flow being replaced by isolated rising and sinking plumes corresponding to the latter transition the exponent in the power law relating the nusselt number nu to the rayleigh number nu ra is reduced both rising and sinking plumes always consist of plume heads followed by tails there is no characteristic frequency for the formation of plumes

early transient superplumes and the origin of the martian crustal dichotomy 1 the large temperature difference between the core and mantle of mars at the end of planetary accretion creates a hot internally convecting thermal boundary layer at the base of the mantle whose viscosity is several orders of magnitude lower than the viscosity of the mantle above it theoretical analysis and numerical simulations of the instability of this thermal boundary layer show that it is likely that only one large plume forms this superplume may play a role in the formation of crustal dichotomy and generation of the magnetic field in the early history of mars

an 60 million year periodicity is common to marine 87sr 86sr fossil biodiversity and large scale sedimentation what does the periodicity reflect abstractwe find that the marine 87sr 86sr record shows a significant periodicity of m yr the 87sr 86sr record is out of phase with a 62 3 m yr periodicity previously reported in the record of marine animal diversity these periodicities are close to 58 4 m yr cycles found for the number of gap bounded sedimentary carbonate packages of north america we propose that these periodicities reflect the operation of a periodic pulse of the earth in large scale earth processes these may be linked to mantle or plate tectonic events possibly uplift which affects the earth s climate and oceans and thus the geochemistry sedimentation and biodiversity of the marine realm

pulsations in seafloor spreading rates and transit time dynamics correlation studies and theoretical models suggest the major plate driving forces slab pull and ridge push are closely linked to seafloor age at subduction zones transit time and hereafter denoted based on this fact it has been proposed that seafloor spreading rate fluctuations should be stably damped because a speedup should lead to rejuvenation of and hence a weakening of driving forces if true such stability could imply the first order cycle in pacific spreading rates during the mesozoic cenozoic the pulse of the earth was externally forced by mantle dynamics however we show that a crucial assumption in this stability argument is invalid increasing spreading rates do not necessarily cause transit time to decrease the transit time dynamics of seafloor are complex simple plate models suggest that rather than stabilizing spreading rate dependent plate driving forces may be destabilizing leading to self sustained and aperiodic oscillations these results raise questions about the cause of spreading rate cycles most importantly are the plates themselves through age dynamics actively contributing to the pulse of the earth

a conceptual model for the relationship between coronae and large scale mantle dynamics on venus 1 among the most enigmatic features on venus are the more than 500 quasi circular structures collectively referred to as coronae associated volcanism and tectonism suggest that coronae result from small transient mantle plumes although only about 20 have clearly associated positive gravity anomalies indicative of current activity coronae are concentrated within a region defined by the dynamically active highland areas of beta atla and themis regiones bat coronae avoid the lowest or highest topography and geoid subsets of the corona population indicate that the spatial distribution of coronae has evolved from a previously less concentrated global distribution to the current bat concentration recent laboratory experiments on thermal convection at high rayleigh number using fluids with temperature dependent viscosity suggest a straightforward interpretation of these relationships small scale upwellings from a hot lower thermal boundary layer tend to be excluded by longer wavelength downwelling structures from the cold upper boundary layer the relative paucity of coronae in the broad lowland plains regions is consistent with the inference of mantle downwellings beneath the plains coronae are largely excluded from compensated highlands perhaps because the thickened crust is not easily disrupted in these regions laboratory experiments indicate that small transient boundary layer instabilities can coexist with and be focused toward large scale upwellings possibly explaining the spatial concentration of coronae in the bat region thus coronae provide important insights into interacting scales of convective flow in venus s mantle and into the evolution of venus s mantle dynamics direct tests of our model are possible through geological mapping

the influence of boundary heterogeneity in experimental models of mantle convection recent global seismological observations have revealed lateral variations in the thickness of the d 00 layer we have performed laboratory experiments to explore how undulations of various sizes in the d 00 layer aect convection patterns we nd that topography on the lower boundary can induce plumes and that there is a critical height above which topography controls convection patterns observed undulations in the d 00 layer exceed this critical height sug gesting that they may control mantle convection patterns

coldspots and hotspots global tectonics and mantle dynamics of venus magellan radar images and altimetry data and pioneer venus gravity data of major highlands and lowlands on venus are examined with the objective of relating these tectonic and volcanic landforms to convection within venus mantle two roughly circular lowlands atalanta and lavinia planitiae are bowl shaped depressions which contain compressional tectonic features and lack hotspot related features such as coronae and shield volcanoes they are identified as surface expressions of young mantle coldspots or regions of mantle downwelling highlands can be divided into two distinct groups volcanic rises and plateau shaped highlands volcanic rises are domical circular to elongate regions characterized by volcanic construction extensional tectonism large positive gravity and geoid anomalies centered on the highlands and large apparent depths of compensation volcanic rises include beta bell atla and western eistla regiones and are identified as surface expressions of large mantle hotspots or plumes plateau shaped highlands are steep sided elevated regions that range from circular alpha regio to elongate or irregular ovda regio tellus regio in shape their surfaces are dominated by complex ridged terrain rather than the shield volcanoes and flows that dominate volcanic rises most plateau shaped highlands contain compressional structures some of which commonly lie along their margins and have small gravity and geoid anomalies and small apparent depths of compensation compared to volcanic rises plateau shaped highlands which include those mentioned above and western ishtar terra thetis regio and phoebe regio are identified as surface expressions of mantle coldspots or regions of downwelling the highland coldspot features are elevated as a consequence of crustal thickening crustal thickening is absent or minor at lowland coldspots the importance of mantle downwelling in the tectonic deformation of venus suggests that its mantle like the mantle of earth is strongly heated from within the major venusian hotspots attest to the existence of large mantle plumes carrying heat upward from the planet s core

the shear strength of partly saturated soils a direct shear apparatus is described which could be used to study the shear strength of partly saturated soils under controlled conditions the principle of this device is described and the test results are presented and discussed with this apparatus it is also possible to determine the effect on the shear strength of a change in solute suction

small mantle upwellings are pervasive on venus and earth observations of venus and africa an analogous terrestrial plate indicate that upwellings a few hundred kilometers across are pervasive in the ambient upper mantle of both venus and earth because of their small size when they reach the lithosphere the upwellings affect the surface only under special circumstances on venus they form volcano tectonic structures called coronae where the lithosphere has been thinned by rifting on earth the small upwellings produce topographic swells where they impinge on a plate stationary with respect to the hot spot reference frame as is africa the small upwellings presence throughout the mantle explains both the corona paradox in which coronae are associated with rifts on venus but do not appear to drive rifting and the basin and swell topography of the african plate the morphology of coronae requires that the small upwellings have some diapiric properties their pervasiveness transient nature and generally small size make them fundamentally different from traditional hawaiian style plumes and they may be detached diapirs within a mantle convecting in the hard turbulence regime

onset of mantle plumes in the presence of preexisting convection the onset of plumes where broad scale convection already exists is investigated along with the thermal history of the earth geological evidence relating to the onset time of mantle plumes is not definitive no well defined hotspot tracks older than mesozoic have been identified but it is difficult to tell whether this absence is a reliable indication of a late onset of mantle plumes since only continental crust of this age exists using the hotspot track distribution for the pacific the absence of pre mesozoic hotspot tracks on the canadian shield cannot easily be attributed to chance however young continental hotspot tracks are controversial and it is not clear whether tracks such as the monteregian hills white mountain magma series would be recognized as such without considering the contributing evidence of the adjacent oceanic crust and its well defined seamount chains our simple models yield potentially testable predictions the instability of plume formation in a system where broad scale convection already exists depends on horizontal conduit flow in the basal boundary layer toward the plume source geographically plumes are most likely to form where the boundary layer is thickest and where broad scale ascending flow already exists parameterized convection indicates that the heat transport by conduit flow in the boundary layer toward plumes depends linearly on the ratio of effective viscosity for broad scale convection to effective boundary layer viscosity in terms of material properties this ratio is the ratio of viscosity across the boundary layer to about the 0 45 power for the case where the broad scale convection is heated from below the viscosity contrast across the boundary layer depends on the temperature contrast across the layer a late onset of mantle plumes is implied if the temperature contrast at the base of the mantle has increased with time that is if the core has cooled more slowly than the interior of the mantel in one scenario before about 3 b y b p the mantle temperature varied slowly because surface heat flow was nearly balanced by internal radioactivity during this time the mantle was hotter than present and its internal viscosity lower hie temperature contrast at the base of the mantel was thus small after 3 b y internal radioactivity no longer balanced surface heat flow and the mantle cooled about a few hundred degrees celsius as indicated by petrological observations the core cooled slower than the mantle and the temperature contrast across the bottom boundary layer increased with time leading to a late onset of plumes phenomenological inferences are obtained as venus may have cooled more rapidly than the earth its mantle may be too cool for rapid plate tectonics and the gravity field and the convective pattern are dominated by hot upwelling regions

modulation of mantle plumes and heat flow at the core mantle boundary by plate scale flow results from laboratory experiments we report results from analog laboratory experiments in which a large scale flow is imposed upon natural convection from a hot boundary layer at the base of a large tank of corn syrup the experiments show that the subdivision of the convective flow into four regions provides a reasonable conceptual framework for interpreting the effects of large scale flow on plumes region i includes the area of the hot thermal boundary layer tbl that is thinned by the large scale flow thereby suppressing plumes region ii encompasses the critically unstable boundary layer where plumes form region iii is the area above the boundary layer that is devoid of plumes region iv comprises the area of hot upwelling and plume conduits quantitative analysis of our experiments results in a scaling law for heat flux from the hot boundary and for the spatial extent of plume suppression when applied to the earth s core mantle boundary cmb our results suggest that large scale mantle flow due to sinking lithospheric plates can locally thin the tbl and suppress plume formation over large fractions of the cmb approximately 30 of heat flow from the core may be due to increased heat flux from plate scale flow furthermore cmb heat flux is non uniformly distributed along the cmb with large areas where heat flux is increased on average by a factor of 2 as a consequence the convective flow pattern in the outer core may be affected by cmb heat flux heterogeneity and sensitive to changes in plate scale mantle flow becauseofplumesuppressionanddfocusingtofhotmantlefromthecmbintozonesofupwellingflow plumeconduits hotspots

at least three scales of convection in a mantle with strongly temperature dependent viscosity abstract we studied the convective patterns developing at high rayleigh numbers and intermediate viscosity ratios in a fluid with a strongly temperature dependent viscosity within this sluggish lid regime three different scales of convection develop the largest convective scale is cellular with cold downwelling sheets of viscous fluid encasing hotter less viscous parts of the tank within each of those cells develop several typically 3 7 hot 3d upwelling plumes upon impinging under the cold thermal boundary layer each plume in turn generates locally a small ring of cold material which does not reach the bottom of the tank applying those results to the earth s mantle we suggest that the large scale features of mantle convection and the co existence of several scales of convection i e slabs and plumes are produced by thermal convection in a mantle with high rayleigh number and a strongly temperature dependent viscosity material this generates a convective pattern in two large scale cells the pacific and the indo atlantic boxes our experiments further suggest that what has been named the two hot superplumes i e the two seismically slow regions encased within the subduction rings are in fact each constituted of several hot instabilities moreover plumes impacts under the lithosphere should be surrounded by cold rings of small extent the asthenosphere appears therefore as the graveyard of both small scale convection and hot plumes generated in the system