effects of a perovskite post perovskite phase change near core mantle boundary in compressible mantle convection 1 numerical simulations of compressible isochemical mantle convection are used to investigate the effect of the perovskite to post perovskite phase transition at around 2700 km depth which has recently been discovered by high pressure experiments and ab initio calculations on the convective planform temperature and heat transport characteristics of the mantle the usual phase transitions at 410 km olivine spinel and 660 km spinel perovskite are also included the exothermic post perovskite phase change at 2700 km depth destabilizes the lower thermal boundary layer increasing the heat flow increasing interior mantle temperature and increasing the number and time dependence of upwelling plumes the resulting weak highly time dependent upwellings also have a smaller horizontal spacing than the plumes that occur in the absence of the phase transition while the influence of post perovskite phase change may be smaller than that of some other complexities such as compositional stratification it appears to have an important enough effect that it should not be ignored in dynamical studies of mantle convection

cooling of the earth urey ratios and the problem of potassium in the core the compatibility of mantle urey ratios um calculated from thermal evolution models with those calculated from geochemical models is considered the initial heat generation rate in the mantle used for the thermal evolution models was derived from two geochemical models of the present day primitive mantle with concentrations of u of 20 ppb 2 7 times chondritic and 26 ppb 3 5 times chondritic respectively the depletion of the mantle through continental crust production was allowed for the effect of including the continental crust on um is a reduction of its value by about 0 1 to balance the observed average value of the surface heat flow qs it was assumed that the core may contain k up to cosmochemically constrained quantities for 26 ppb u in the present primitive mantle no core heat sources are required the present value of um for this model is 0 57 and the core heat flow is about 12 of qs with 20 ppb u in the present primitive mantle heat production equivalent to one to two silicate earth budgets of k is required in the core to balance qs and um is between 0 35 and 0 4 the core heat flow is then between 25 and 33 of qs

geothermal control on flow patterns in the last glacial maximum ice sheet of iceland because it is located both on the mid atlantic ridge and on a mantle plume iceland is a region of intense tectonics and volcanism during the last glaciation the island was covered by an ice sheet approximately 1000 m thick a reconstruction of the ice flow lines based on glacial directional features shows that the ice sheet was partly drained through fast flowing streams fast flow of the ice streams has been recorded in megascale lineations and flutes visible on the currently deglaciated bedrock and is confirmed by simple mass balance considerations locations of the major drainage routes correlate with locations of geothermal anomalies suggesting that ice stream activity was favoured by lubrication of the bed by meltwater produced in regions of high geothermal heat flux similar control of ice flow by geothermal activity is expected in ice sheets currently covering tectonically and volcanically active area such as the west antarctic ice sheet copyright 2000 john wiley sons ltd

morphometry of subaerial shield volcanoes and glaciovolcanoes from reykjanes peninsula iceland effects of eruption environment abstract we present a morphometric study of 33 basaltic volcanic edifices from the reykjanes peninsula iceland using a 20 m resolution digital elevation model dem slope values distinguish subaerial from intraglacial eruption environments with glaciovolcanic edifices having average slope values that are 5 higher than subaerial shields the 26 analyzed glaciovolcanic edifices are separated into 3 groups based on size and are also categorized following the new classification scheme of tuyas by russell et al 2014 into 15 tindars 1 conical tuya 3 flat topped tuyas and 7 complex tuyas the glaciovolcanic edifices show a continuum of landforms ranging from small elongated tindars to large equidimensional flat topped tuyas the smaller edifices 3 are all tindars and the larger edifices 0 1 km 3 are flat topped tuyas the mid sized edifices 0 01 0 1 km 3 show a wide variety of shapes and classify either as tindars or as complex tuyas with only one edifice classifying as a conical tuya edifice elongation tends to decrease with volume suggesting that small edifices are primarily fissure controlled whereas larger edifices are mainly controlled by a central vent the mid sized complex tuyas are transitional edifices suggesting that some intraglacial eruptions start as fissure eruptions that subsequently concentrate into one or more central vents whereas the mid sized tindars suggest a sustained fissure eruption there is a tectonic control on the orientation of the edifices evidenced by a strong correlation between edifice elongation azimuth and mapped faults and fractures most edifice elongations cluster between 020 and 080 coinciding with the strike of normal faults within and at the boundary of regional volcanic systems but some edifices have elongations that correlate with n s striking book shelf faults this implies that intraglacial eruptions are controlled by pre existing pathways in the crust as has been previously observed for subaerial fissure eruptions in terms of classification quantification of the limits between the four tuya types proposed by russell et al 2014 is difficult because of the transitional nature shown by several edifices a threshold of 1 8 in ellipticity index e i values can be used to distinguish tindars from the other three types flat topped tuyas are distinguished by their greater overall size their large and relatively flat summit regions reflected in bimodal slope distributions and their low e i and low to intermediate irregularity index i i values the only analyzed conical tuya has very low e i and i i values very small summit regions and very steep flank slopes the complex tuyas have variable morphometries but are in general characterized by high i i values and very irregular slope distributions no correlation is observed between edifice scale morphology and lithology e g pillow dominated or hyaloclastite dominated

the influence of post perovskite strength on the earth s mantle thermal and chemical evolution abstract we have investigated the influence of post perovskite ppv viscosity on mantle convective dynamics and stirring efficiency using numerical modeling and simple analytical theory our results show that the strength of ppv has a dramatic influence on convective dynamics the presence of a weak ppv enhances heat transfer across the bottom thermal boundary layer resulting in higher temperatures lower mantle viscosities and considerably larger convective velocities this leads to a significant increase in stirring efficiencies with decreasing the ppv strength by at least one order of magnitude in addition using a simple parameterized convection evolution that includes the influence of ppv coupled to a mixing model we show that during the long term history of the earth s mantle the presence of ppv yields systematically hotter thermal evolution and more efficient convective stirring such a strong effect of ppv strength on mantle stirring efficiency suggests that the influence of ppv phase must be considered when interpreting both geochemical and geophysical observations

effect of post perovskite rheology on the thermal evolution of the earth abstract secular cooling of the earth mantle is a complex process affected by many factors here we present the results of a modelling study focused on efficiency of cooling in the presence of rheologically distinct post perovskite we evaluate combined effects of variable thermal expansivity and diffusivity initial thermal condition and heat source model and concentrate on the effects of rheologically weak post perovskite cooling of the core is included in the model core is assumed to be an isothermal heat reservoir with temperature controlled by heat flux through core mantle boundary our 2d axisymmetric convection model has pressure temperature and phase dependent viscosity and includes the effects of an endothermic phase transition at 660 km depth and an exothermic perovskite post perovskite phase transition in the lowermost mantle in agreement with previous studies we conclude that depth dependent material parameters tend to delay secular cooling presence of the weak post perovskite on the other hand significantly enhances core cooling and its effect on core temperature is opposite and comparable in magnitude

is the long wavelength geoid sensitive to the presence of postperovskite above the core mantle boundary 1 the analysis of seismic data represents today the primary tool in the search for the presence of postperovskite in the lowermost mantle d 00 this work aims at testing whether the inversion of gravitational data can also contribute to the detection of postperovskite in d 00 w e assume that the transition from perovskite to postperovskite is accompanied by a reduction in viscosity and test the effects of such viscosity change on the prediction of the dynamic geoid with a numerical model of subducted lithosphere our results show that the long wavelength component of the geoid is very sensitive to the presence of postperovskite areas in d 00 especially if their viscosity is significantly lower than the viscosity of the surrounding perovskite and if these areas are located close to density

silicic volcanism at ljosufjoll iceland insights into evolution and eruptive history from ar ar dating abstract ljosufjoll volcano is the largest outcrop of silicic volcanic material in the volcanic snaefellsnes flank non rifting zone of iceland the silicic eruptives range from trachytes to alkaline and peralkaline comenditic rhyolites and show evidence for eruption in both subaerial and subglacial environments thirteen silicic eruptive units have been identified and mapped by a combination of field observations and geochemical correlation the trachytes probably formed by fractional crystallisation of a basaltic magma to form an alkali feldspar rich trachytic mush with continued fractionation of the interstitial melt producing low ba and sr rhyolite magma ar ar dating of feldspars and matrix glass or holocrystalline groundmass has been carried out on twelve of the units many units have a complex ar isotopic system with many samples showing evidence for inherited 40ar in the form of feldspar xenocrysts the deduced eruption ages span an age range from 650ka the eruptive history of ljosufjoll probably extends further back than this but the products of previous eruptions have since been removed by erosion or buried a syenite xenolith was dated at 1 1ma indicating that silicic alkaline magmatism has been occurring at ljosufjoll for over one million years eruptions of silicic material at ljosufjoll appear to be more common during times of rapid climate change and fluctuating ice volume

morphometry of glaciovolcanic edifices from iceland types and evolution abstract the morphology of many glaciovolcanoes is considered one of the distinctive characteristics of their ice confining eruption environment however a thorough morphometric analysis of a large number of glaciovolcanic edifices has never been performed based on a semi automatic geomorphometric mapping of glaciovolcanic edifices of the icelandic neovolcanic zones we present a morphometric database of 155 glaciovolcanic edifices formed during the last 0 78 ma this database enables a comprehensive analysis of the morphometric diversity of a large suite of glaciovolcanic edifices sheet like formations however are not considered due to lack of data using three planimetric measurements basal length average basal width and average summit plateau width and their ratios three main morphometric groups can be distinguished in a ternary diagram 1 conical edifices with no or small summit plateaus 2 linear ridges and 3 flat topped edifices subdivided into equidimensional and elongated all three groups contain edifices with and without lava caps these morphometric groups can be fitted to the commonly accepted terminology for cone mound tindar and tuya however since lava caps occur in all morphometric groups a grouping based on its existence is not practical this suggests that by adding the descriptor lava capped to any of the three classes may be a useful way to refine the classification based on the ternary diagram ridges are the most morphometrically distinct glaciovolcanic edifice because of their very extreme elongation followed by flat topped edifices and finally conical edifices however morphometric parameters cannot be used singlehanded to identify glaciovolcanic edifices from other types of volcanic edifices such as composite volcanoes shields or submarine volcanoes and should always be complemented with other observations the glaciovolcanic edifice volumes range from 0 15 10 2km3 to 32 km3 conical edifices are the smallest 1 km3 overall with growth three morphological evolutions can be considered a an initial eruptive fissure concentrates into one vent generating an equidimensional edifice that either can be a conical or a flat topped edifice b the edifice maintains its elongation suggesting that a fissure is the dominating vent structure and during continued eruption develops significant summit plateaus generating elongated flat topped edifices and c the edifice increases in elongation because the fissure lengthens during the eruption and a plateau building stage does not occur producing increasingly elongated ridges

phase transition in mgsio 3 perovskite in the earth s lower mantle a new polymorph of mgsio3 more stable than the pbnm perovskite phase has been identified by first principles computations it has the cairo3 structure with cmcm symmetry and consists of sio3 layers intercalated by eightfoldcoordinated mg ions high temperature calculations within the quasiharmonic approximation give a volume change of f1 5 and a clapeyron slope of f7 5f0 3 mpa k at f2750 k and f125 gpa these pressure temperature p t conditions are close to those in which a phase transition in mgsio3 perovskite has been observed by in situ angle dispersive x ray diffraction measurements this transition appears to be associated with the du discontinuity d 2004 elsevier b v all rights reserved

the potential influence of radiative heat transfer on the formation of megaplumes in the lower mantle abstract recent seismic tomographic models have revealed broad low velocity anomalies in the lower mantle beneath africa and the central pacific which suggest a break in the symmetry between hot and cold regions in lower mantle dynamics we have considered the possible impact from radiative heat transfer which can be described by a nonlinear temperature dependent coefficient in the thermal conductivity in 2d numerical simulations results for rayleigh numbers up to 106 show a strong stabilizing influence from radiative heat transfer on mantle upwellings and the production of extremely hot thermal anomalies in the interior this nonlinearity is responsible for producing a strong attractor in the mantle convective system which greatly simplifies its time dependent dynamics the possible link of the simplified lower mantle time dependent dynamics with polar wander is discussed the main point here is that slow time dependence of the huge anomalies in the lower mantle can be the main mechanism controlling long term rotational dynamics

lateral variations in cmb heat flux and deep mantle seismic velocity caused by a thermal chemical phase boundary layer in 3d spherical convection article i nfo numerical simulations of thermo chemical multi phase compressible mantle convection in a three dimensional spherical shell are used to investigate the relationship between lateral variations in seismic shear wave velocity vs above the core mantle boundary cmb and lateral variations in heat flux across the cmb qcmb when compositional variations and the post perovskite phase transition are included for simple thermal convection the vs qcmb relationship is reasonably but not perfectly linear the post perovskite transition introduces a non linearity that amplifies fast vs anomalies in cold regions but there is still a unique mapping between vs and qcmb lateral variations in composition such as piles of dense material introduce another non linearity that affects hot upwelling regions and introduces a non uniqueness in vs qcmb if the dense material e g morb is seismically fast compared to the surrounding material in this case dense piles are ringed by sharp low vs anomalies if the cmb is covered by a global dense layer than variations in vs and qcmb are reduced but so is the mean value of qcmb in all cases the peak to peak lateral variation in qcmb is similar to or larger than twice the mean value which might create problems for generating a dynamo according to existing numerical dynamo simulations analytical scalings are developed to explain the observed trends

influence of initial cmb temperature and other parameters on the thermal evolution of earth s core resulting from thermochemical spherical mantle convection 1 here we present a wide ranging parameter study of the effects of initial core mantle boundary cmb temperature concentration of radioactive potassium in the core and density difference between harzburgite and mid ocean ridge basalt morb in a coupled spherical model of thermochemical mantle convection and parameterized core heat balance the initial cmb temperature is expected to be much higher than the solidus temperature of silicates at the base of the mantle the results indicate that as with previous purely thermal convection models the final state of the system is only weakly dependent on initial cmb temperature unless the cmb becomes blanketed by a global layer of dense material fully 3 d spherical cases have a very similar core evolution to cases in a 2 d spherical annulus giving confidence in the applicability of 2 d spherical annulus geometry for modeling earth s evolution obtaining a successful thermal evolution in the sense of obtaining the correct present day inner core size and maintaining a geodynamo over geological time is helped by the accumulation of piles of dense material at the cmb subducted morb in the present calculations and a concentration of radiogenic k in the core in the range 400 800 ppm the present day cmb heat flow is predicted to be around 9 tw while this is lower than estimates based on calculating temperature gradients in regions where the postperovskite transition is seismically imaged these tend to be areas of higher than average heat flux and thus likely overestimate the global heat flow

effects of low viscosity post perovskite on thermo chemical mantle convection in a 3 d spherical shell 1 numerical simulations of thermo chemical multi phase mantle convection in a 3 d spherical shell are performed to determine how a low viscosity of post perovskite affects dynamics and structures in the deep mantle low viscosity post perovskite weakens the deepest part of slabs allowing them to more effectively spread over the core mantle boundary cmb and it also results in a greater volume of basalt segregating both of which increase the size of dense chemical piles the horizontal lengthscale of regions of pooled slab material and the steepness of piles edges in composition and phase consistent with the existence of steep sharp sided edges found in seismic analyses cmb heat flux is strongly enhanced in regions of low viscosity post perovskite consistent with a theoretical prediction and both cmb and surface heat flux are increased on average by a low viscosity of post perovskite which could have important implications for the evolution of earth s core and mantle

the importance of radiative heat transfer on superplumes in the lower mantle with the new post perovskite phase change abstract a new post perovskite phase has been identified recently from both x ray observations and first principles calculations this novel phase change occurs at a depth about 200 km above the core mantle boundary for temperatures of around 2800 k using a two dimensional extended boussinesq model we have studied the dynamical consequences of this deep mantle phase transition on mantle convection with particular emphasis on the effects on lower mantle plume structures we have employed a depth dependent viscosity with a viscosity maximum in the mid lower mantle and two phase transitions one at 670 km depth and the other at a depth of 2650 km the phase transition at 670 km is endothermic while we have examined both exothermic and endothermic possibilities for the new phase transition our results show the following situations favorable for the development of superplumes 1 endothermic phase transition for both constant and radiative thermal conductivities and 2 exothermic phase transition and radiative thermal conductivity smaller unstable plumes are found for exothermic phase transition and constant thermal conductivity extremely high lateral temperature increases exceeding 1500 degrees are found inside the lower mantle plume for constant thermal conductivity this lateral temperature contrast is mitigated by the presence of radiative thermal conductivity in order for superplumes to prevail in the lower mantle with a deep exothermic phase change we must invoke some form of radiative thermal conductivity for stabilizing and promoting large upwellings with a width exceeding at least 500 km strong lateral heterogeneities in both the seismic velocity and density fields can be produced in the plumes and the surrounding mantle for this new phase transition with a large clapeyron slope close to 10 mpa k

the impact of variability in the rheological activation parameters on lower mantle viscosity stratification and its dynamics the recently discovered spin state crossover in iron in major mantle minerals at high pressures should exert dramatic influences on transport properties such as the activation creep parameters in the deep mantle wentzcovitch et al 2009 have computed the elastic parameter of ferropericlase that is affected by this crossover the bulk modulus at high realistic conditions using first principles density functional theory plus hubbard u as a consequence of the spin crossover there is a significant softening of the bulk modulus and an attendant reduction of the activation energy in the creep law using a parameterized model capturing the basic physics we have studied the dynamical consequences within the framework of a 2 d cartesian convection model our models reveal a series of asthenospheres and are characterized first by a low viscosity channel below the 670 km boundary second asthenosphere caused by an increase of temperature and decrease in grain size reduction superplascity with the post spinel transition which results in partially layered convection further the variability of the rheological parameters leads to the prevalent formation of viscosity minimum at a depth of about 1600 km third asthenosphere caused by spin crossovers followed by a viscosity hill in the bottom half of the lower mantle the latter due to the increase of the activation energy in some low spin irons at the bottom of the mantle our numerical simulations reveal a tendency to the formation of small scale convection below the 670 km boundary and bigger stabilized plumes and or plume clustering in the deep lower mantle other material properties at the bottom of the mantle decrease of thermal expansivity radiative thermal conductivity also exert significant influence on the multi scale lower mantle plume dynamics

subglacial and ice contact volcanism at the oraefajokull stratovolcano iceland eruptions of oraefajokull have produced mafic and silicic magmas and have taken place in both glacial and interglacial periods the geology of the volcano records the differing response of magmas of contrasting composition to interaction with ice of variable thickness and gives insight into the development of a long lived ice covered stratovolcano vatnafjall a ridge on the southeast flank of oraefajokull is the first area of the volcano to have been mapped in detail and the geological map is presented here alongside descriptions of each erupted unit the oldest units comprise pillow lavas hyaloclastite and jointed lava flows that were formed during subglacial basaltic eruptions involving abundant meltwater the products of a subsequent explosive initially phreatomagmatic subglacial rhyolite eruption were confined by ice to form a tephra pile over 200 m thick that was intruded by dense rhyolite magma towards the end of the eruption confinement by ice caused a later trachydacite lava flow to form buttresses and a steep pillar whilst some of the meltwater produced infiltrated the lava to generate red and black glassy breccias and cause localised steam explosions it is likely that much of it drained down the steep topography the most recently erupted units are subaerial basaltic lava flows the oldest of which were erupted during an interglacial period and have subsequently been partially eroded and scoured by advancing ice ice has been important in shaping the edifice by confining eruptive products to form constructional features and by later eroding parts of them to form deep valleys reconstructions of volcano ice interaction allowed the local thickness of the glacier at the time of each eruption to be estimated and demonstrates that the upper surface of the ice has varied in elevation by over 700 m

tuyas a descriptive genetic classification we present a descriptive genetic classification scheme and accompanying nomenclature for glaciovolcanic edifices herein defined as tuyas positive relief volcanoes having a morphology resulting from ice confinement during eruption and comprising a set of lithofacies reflecting direct interaction between magma and ice melt water the combinations of lithofacies within tuyas record the interplay between volcanic eruption and the attending glaciohydraulic conditions although tuyas can range in composition from basaltic to rhyolitic many of the characteristics diagnostic of glaciovolcanic environments are largely independent of lava composition e g edifice morphology columnar jointing patterns glass distributions pyroclast shapes our classification consolidates the diverse nomenclature resulting from early isolated contributions of geoscientists working mainly in iceland and canada and the nomenclature that has developed subsequently over the past 30 years tuya subtypes are first recognized on the basis of variations in edifice scale morphologies e g flat topped tuya then on the proportions of the essential lithofacies e g lava dominated flat topped tuya and lastly on magma composition e g basaltic lava dominated flat topped tuya these descriptive modifiers potentially supply additional genetic information and we show how the combination of edifice morphologies and lithofacies can be directly linked to general glaciohydraulic conditions we identify nine distinct glaciovolcanic model edifices that potentially result from the interplay between volcanism and glaciohydrology detailed studies of tuya types are critical for recovering paleo environmental information through geological time including ice sheet locations extents thicknesses and glaciohydraulics such paleo environmental information represents a new innovative underutilized resource for constraining global paleoclimate models

the role of solid solid phase transitions in mantle convection abstract with changing pressure and temperature conditions downwelling and upwelling crustal and mantle rocks experience several solid solid phase transitions that affect the mineral physical properties owing to structural changes in the crystal lattice and to the absorption or release of latent heat variations in density together with phase boundary deflections related to the non null reaction slope generate important buoyancy forces that add to those induced by thermal perturbations these buoyancy forces are proportional to the density contrast between reactant and product phases their volume fraction the slope and the sharpness of the reaction and affect the style of mantle convection depending on the system composition in a homogeneous pyrolitic mantle there is little tendency for layered convection with slabs that may stagnate in the transition zone because of the positive buoyancy caused by post spinel and post ilmenite reactions and hot plumes that are accelerated by phase transformations in the 600 800 km depth range by adding chemical and mineralogical heterogeneities as on earth phase transitions introduce bulk rock and volatiles filtering effects that generate a compositional gradient throughout the entire mantle with levels that are enriched or depleted in one or more of these components phase transitions often lead to mechanical softening or hardening that can be related to a different intrinsic mechanical behaviour and volatile solubility of the product phases the heating or cooling associated with latent heat and the transient grain size reduction in downwelling cold material strong variations in viscosity would enhance layered mantle convection causing slab stagnation and plume ponding at low temperatures and relatively dry conditions reactions are delayed due to the sluggish kinetics so that non equilibrium phase aggregates can persist metastably beyond the equilibrium phase boundary survival of low density metastable olivine ringwoodite pyroxene and pyrope garnet in the transition zone and uppermost lower mantle produces positive buoyancy forces that decrease the subduction velocity and may lead to slab stagnation in the transition zone the presence of deep metastable portions is still debated and should not be associated a priori with a completely dry slab as field observations suggest that heterogeneously hydrated oceanic plates could contain metastable dry portions surrounded by transformed wet rocks

the emplacement of an obsidian dyke through thin ice hrafntinnuhryggur krafla iceland an eruption along a 2 5 km long rhyolitic dyke at krafla volcano northern iceland during the last glacial period formed a ridge of obsidian hrafntinnuhryggur the ridge rises up to 80 m above the surrounding land and is composed of a number of small volume lava bodies with minor fragmental material the total volume is 0 05 km3 the lava bodies are flow or dome like in morphology and many display columnar jointed sides typical of magma ice interaction quench fragmented lower margins indicative of interaction with meltwater and pumiceous upper surfaces typical of subaerial obsidian flows the fragmental material compromises poorly sorted perlitic quench hyaloclastites and poorly exposed pumiceous tuffs lava bodies on the western ridge flanks are columnar jointed and extensively hydrothermally altered at the southern end of the ridge the feeder dyke is exposed at an elevation 95 m beneath the ridge crest and flares upwards into a lava body using the distribution of lithofacies we interpret that the eruption melted through ice only 35 55 m thick which is likely to have been dominated by firn hrafntinnuhryggur is therefore the first documented example of a rhyolitic fissure eruption beneath thin ice firn the eruption breached the ice leading to subaerial but ice firn contact lava effusion and only minor explosive activity occurred the ridge appears to have been well drained during the eruption aided by the high permeability of the thin ice firn which appears not to have greatly affected the eruption mechanisms we estimate that the eruption lasted between 2 and 20 months and would not have generated a significant jokulhlaup 70 m3s 1

evidence age and thickness of a frozen paleolake in utopia planitia mars abstract recent mapping studies west of elysium mons mars have pinpointed subice features that suggest the existence of a frozen paleolake in utopia planitia as recently as 1 8 billion years ago the subice features are interpreted to be hyaloclastic ridges and hills table mountains associated jokulhlaup deposits and fluvial channels photoclinometric studies of these features and of a basal scarp around the northwest flank of elysium mons interpreted to have been an ice sheet boundary indicate that the maximum thickness of ice within the basin may have been about 180 m this thickness of ice during a relatively late stage of martian geologic history would have important implications concerning the atmospheric the climatic and possibly the exobiologic history of the planet

seismic imaging of the d and constraints on the core heat flux abstract recent seismic imaging of the deep mantle has revealed lens shaped layers near the bottom of the mantle the morphology of this layering has been attributed to the crossing of the post perovskite phase boundary by the local geotherm we have developed a technique for quantifying the relationships between the depths of the double crossings of the post perovskite transition developed in 3d spherical mantle convection as a means for testing the viability for using the clapeyron slope of the phase transition to put bounds on the heat flux from the core we have argued from the probability distribution of the local heat flow at the core mantle boundary that there are intrinsic problems in using error function from half space cooling curves for constraining the core heat flux because of the non gaussian shape of the probability distribution of the core heat flow in realistic 3d convection models in order to constrain better the core heat flux with the above arguments we need to know much better the mantle viscosity stratification and the pressure dependence of thermal conductivity in the deep mantle only then can we put together enough constraints to discuss with greater certainty about the core heat flux

geodynamical modeling and multiscale seismic expression of thermo chemical heterogeneity and phase transitions in the lowermost mantle the d region at the base of the mantle is characterized by seismologically inferred 3d heterogeneity including multiple interfaces localized low velocity zones and anisotropy the occurrence of the postperovskite ppv phase transition with a steep clapeyron slope of 11 5 13 mpa k close to the core mantle boundary is a prime candidate for explaining observed seismic layering in the d to examine the effect of the ppv phase transition on seismic structure we have carried out finite element simulations with high resolution up to 3 km in a cylindrical geometry the rheology of the mantle has both newtonian diffusion and non newtonian components with a much greater propensity to non newtonian for ppv from the temperature output we computed the 2d variations in shear wavespeed using a seismic equation of state based on mineral physics data we then use a wave packet decomposition of the wavespeed variations which accounts for the events to stations illumination to obtain the seismic expressions of the geodynamically modeled structures the results reveal lens shaped ppv structures much like the patterns obtained from seismic imaging with scs data a similar analysis of thermo chemical anomalies from a subducting slab with crustal material shows that structures with a spatial scale of morb crustal thickness produce characteristic features reminiscent of the small scale detail in the seismic imaging results these experiments illustrate the high sensitivity of the seismic expression near the cmb to the wavespeed coefficients of the oceanic crust under high pressure conditions

physical controls and resulting morphological forms of quaternary ice contact volcanoes in western canada abstract volcanism in association with large quantities of ice manifests itself in a variety of morphological forms developed under differing physical conditions these physical conditions include the location amount thickness and type of confining ice the location and quantity of trapped water and the surrounding topography ice in the form of thick continental ice sheets thinner alpine glaciers or even perennial accumulations of snow can influence the resulting morphological form of the volcano the chemical composition of the erupting magma effusion rate and total erupted volume will also affect the shape of the resulting edifice quaternary volcanoes influenced by many of these factors are found in western canada the morphological forms range from tuyas to subglacial mounds to stratovolcanoes this paper provides a review of ice contact volcanism and volcanic products formed in these environments using canadian examples for illustration

lower mantle dynamics with the post perovskite phase change radiative thermal conductivity temperature and depth dependent viscosity the new post perovskite phase near the core mantle boundary has important ramifications on lower mantle dynamics we have investigated the dynamical impact arising from the interaction of temperature and depth dependent viscosity with radiative thermal conductivity up to a lateral viscosity contrast of 10 4 on both the ascending and descending flows in the presence of both the endothermic phase change at 670 km depth and an exothermic post perovskite transition at 2650 km depth the phase boundaries are approximated as localized zones we have employed a two dimensional cartesian model using a box with an aspect ratio of 10 within the framework of the extended boussinesq approximation our results for temperature and depth dependent viscosity corroborate the previous results for depth dependent viscosity in that a sufficiently strong radiative thermal conductivity plays an important role for sustaining superplumes in the lower mantle once the post perovskite phase change is brought into play this aspect is especially emphasized when the radiative thermal conductivity is restricted only to the post perovskite phase these results revealed a greater degree of asymmetry is produced in the vertical flow structures of the mantle by the phase transitions mass and heat transfer between the upper and lower mantle will deviate substantially from the traditional whole mantle convection model streamlines revealed that an overall complete communication between the top and lower mantle is difficult to be achieved 2005 elsevier b v all rights reserved

comparison of the dynamic and parameterized models of mantle convection including core cooling the two dimensional cooling model of mantle convection including core cooling was studied the core temperature is assumed to be constant in space but it changes with time because of the convective retrieval of heat the viscosity is also constant in space but it is a function of the area averaged mean temperature in order to check the possible effects of temperature dependent viscosity on the cooling the rate of cooling is controlled by three factors the temperature change of the core by a unit change in the core heat flux the viscosity law governed by the mean temperature and the contribution of the internal heating several different types of temperature field were used as the initial conditions some results showed considerable fluctuations in the relation between the temporal rayleigh number and the nusselt numbers which are appropriately defined for both the upper and lower thermal boundary layers a parameterized model of the cooling history of the mean and bottom temperatures was calculated assuming the power law relation between the rayleigh and the nusselt numbers which is determined by the least square fit of the results of the dynamic calculations for the parameterized models we integrated backward in time the agreement between the dynamic and parameterized calculations is generally good except for the cases when the rate of cooling is large or the decay in internal heating is small when the rate of the cooling is large the lower thermal boundary layer disappears this results in unpredictability of the bottom temperature however this case is not applicable for the earth when the decay in internal heating is small we find that the agreement between the dynamic and parameterized models is sensitive to the change in the amount of internal heating assumed in parameterized models this is because the internal heat source directly affects the rate of cooling the value which is the power index of the nu ra relation varies from 0 3 to 0 4 when the bottom thermal boundary layer becomes weak because of the fast cooling of the core deviates from 0 3 considerably

numerical models of mantle convection with secular cooling summary a 2 d time dependent finite difference numerical model is used to investigate the thermal character and evolution of a convecting layer which is cooling as it convects two basic cooling modes are considered in the first both upper and lower boundaries are cooled at the same rate while maintaining the same temperature difference across the layer in the second the lower boundary temperature decreases with time while the upper boundary temperature is fixed at 0 c the first cooling mode simulates the effects of internal heating while the second simulates planetary cooling as mantle convection extracts heat from and thereby cools the earth s core the mathematical analogue between the effects of cooling and internal heating is verified for finiteamplitude convection it is found that after an initial transient period the central core of a steady but vigorous convection cell cools at a constant rate which is governed by the rate of cooling of the boundaries and the viscosity structure of the layer for uppermantle models the transient stage lasts for about 30 per cent of the age of the earth while for the whole mantle it lasts for longer than the age of the earth consequently in our models the bulk cooling of the mantle lags behind the cooling of the coremantle boundary models with temperature dependent viscosity are found to cool in the same manner as models with depth dependent viscosity the rate of cooling is controlled primarily by the horizontally averaged variation of viscosity with depth if the earth s mantle cools in a similar fashion secular cooling of the planet may be insensitive to lateral variations of viscosity

possible links between long term geomagnetic variations and whole mantle convection processes the earth s internal magnetic field varies on timescales of months to billions of years the field is generated by convection in the liquid outer core which in turn is influenced by the heat flowing from the core into the base of the overlying mantle much of the magnetic field s variation is thought to be stochastic but over very long timescales this variability may be related to changes in heat flow associated with mantle convection processes over the past 500 myr correlations between palaeomagnetic behaviour and surface processes were particularly striking during the middle to late mesozoic era beginning about 180 myr ago simulations of the geodynamo suggest that transitions from periods of rapid polarity reversals to periods of prolonged stability such as occurred between the middle jurassic and middle cretaceous periods may have been triggered by a decrease in core mantle boundary heat flow either globally or in equatorial regions this decrease in heat flow could have been linked to reduced mantle plume head production at the core mantle boundary an episode of true polar wander or a combination of the two

three dimensional structures and dynamics in the deep mantle effects of post perovskite phase change and deep mantle layering 1 three dimensional numerical simulations of thermo chemical mantle convection with multiple phase transitions olivine spinel perovskite post perovskite are used to investigate the morphology of compositional and post perovskite ppv boundary structures above the core mantle boundary cmb compositionally dense subducted morb partially segregates above the cmb forming ridge like dense piles morphologically similar to structures in previous spherical or cartesian studies in which a dense layer was inserted a priori an anti correlation is observed between regions where a thick post perovskite layer occurs and regions containing piles of dense material as in our previous two dimensional study lateral variations in the thickness or occurrence of a ppv layer cause very strong seismic shear wave velocity anomalies which may dominate thermal or composition related seismic heterogeneity in the depth range where the variable ppv boundary occurs and also generate sharp sided structures as observed seismologically both compositional and ppv effects may be necessary to simultaneously explain large scale heterogeneity and the global distribution of the discontinuity atop d

the combined influences of variable thermal conductivity temperature and pressure dependent viscosity and core mantle coupling on thermal evolution abstract most convection studies of thermal history have not considered explicitly the thermal interaction between the mantle flow and the core we have investigated the influences of variable thermal conductivity and variable viscosity temperature and pressure dependent on the boundary layer and thermal characteristics of the d layer and the evolution of the thermo mechanical profiles of horizontally averaged viscosity and thermal conductivity viscosity contrast due to temperature dependence of up to 30 000 has been considered our results show clearly that variable thermal conductivity though small in magnitude as compared to variations in the viscosity does exert a significant delaying influence on mantle cooling thereby keeping the urey ratio low reducing the growth of the bottom thermal boundary layer and changing the viscosity profiles over time a higher temperature at the core mantle boundary increases the overall time dependent behavior of the thermal boundary layers enhanced radiative conductivity results in faster cooling opposite to the effect of the phonon conductivity component and a superadiabatic temperature gradient in the deep lower mantle finally the initial value of the core mantle boundary temperature can be inferred to wield a strong influence on the subsequent mantle thermal evolution in this model with both variable thermal conductivity and viscosity we may conjecture that other rheological and conductivity complexities such as grain size dependence of mantle properties would also have an impact on the current state of the mantle resulting from the primordial thermal condition

delayed cooling of the earth s mantle due to variable thermal conductivity and the formation of a low conductivity zone abstract the thermal conductivity of mantle materials decreases with higher temperatures and increases with greater pressure due to phonon mechanism but increases with temperature for radiative transfer this trade off attribute allows for the formation of a low conductivity zone lcz within the top thermal boundary layer in mantle convection we have studied with a two dimensional 2 d cartesian model the potential of this low conductivity zone in retarding secular cooling of the mantle for a longer period of time than models with constant thermal conductivity using a recently proposed model for thermal conductivity and an adiabatic boundary condition for the bottom of the mantle we have carried out a set of numerical experiments within the framework of the extended boussinesq approximation for constant viscosity depth dependent thermal expansivity and variable thermal conductivity with surface rayleigh numbers between around 106 and around 107 we have employed internal heating between two and four times the chondritic level and half life time values of 2 5 ga and 5 ga the cooling rate of the mantle can be decreased by the feedback interaction between mantle conductivity and internal heating which gives rise to slower sinking cold currents the retardation time increases with the strength of radiogenic heating and can be as long as a couple of billion years for high initial heating rates four times the chondritic value however increasing the radiative contribution of the conductivity speeds up the cooling process the less the radiative contribution is the more mantle cooling would be retarded in the course of adiabatic cooling we find the paradoxical situation in which the effective rayleigh number of the mantle can actually increase with time we suggest then that because of the possibilities for the formation of low conductivity zones adjacent to the thermal boundary layers which bring forth unexpected consequences mantle thermal conductivity should be considered as a fundamentally important factor in constraining the thermal evolution of both the core and mantle

rheological structure and deformation of subducted slabs in the mantle transition zone implications for mantle circulation and deep earthquakes abstract rheological structure of subducted slabs of oceanic lithosphere in the mantle transition zone is investigated based on mineral physics observations incorporating grain size stress temperature and pressure dependence of rheology it is shown that the rheological structure of slabs depends strongly on subduction parameters through temperature that controls the grain size of spinel ringwoodite and the magnitude of forces acting on a slab we use a theoretical model of grain size evolution associated with the olivine spinel transformation plastic flow laws of olivine and spinel combined with a thermal model of subducting slab in which the effect of latent heat release is incorporated three deformation mechanisms for olivine and spinel diffusional creep power law dislocation creep and the peierls mechanism are considered due to the large variation in temperature stress and grain size a subducting slab is shown to have a complicated rheological structure which varies both laterally and with depth a cold slab in the deep transition zone is characterized by a weak fine grained spinel region surrounded by narrow but strong regions the flexural rigidity and the curvature of a slab are calculated using a new formulation in which the effects of stress dependent rheology is incorporated in a self consistent fashion although uncertainties in both the transformation kinetics and the rheology of high pressure phases are still large the general trend of dependence of slab flexural rigidity and the curvature on the subduction parameters is well constrained slabs with very low thermal parameters warm slabs are weak but slabs with large thermal parameters cold slabs are also weak due to small spinel grain size and large external force bending moment slabs with intermediate thermal parameters will have a relatively large flexural rigidity and could penetrate into the lower mantle without much deformation thus the 660 km discontinuity may work as a rheological filter for mantle convection this prediction provides a natural explanation for a paradoxical observation that significant deformation of slabs is observed exclusively in the western pacific where temperatures of the slabs are considered to be low our slab rheology models also have important implications for deep earthquakes overall rheological weakening of slabs in the deep transition zone results in high rates of deformation under relatively low temperatures providing a favorable environment for thermal runaway instability adiabatic shear instability our model predicts heterogeneous energy dissipation as a result of heterogeneous rheology energy dissipation in deep cold slabs is concentrated in high strength regions surrounding a weak fine grained spinel core the regions of high energy dissipation are prone to thermal runaway instability and are likely to be seismogenic the width of this seismogenic region is well constrained by our model and is predicted to be 40 60 km which is in excellent agreement with seismological observations other features of deep earthquakes including low seismic efficiency and low aftershock activities can also be explained by the thermal instability model

the impact of the atmosphere on the eyjafjallajokull 2010 eruption plume 1 the eruption of eyjafjallajokull volcano in 2010 lasted 39 days 14 april 23 may the eruption had two explosive phases separated by a phase with lava formation and reduced explosive activity during the explosive phases there were episodes of strong winds that advected ash to the south and southeast leading to widespread disruptions in air traffic the height of the eruption plume was monitored with a weather radar and with web cameras mounted with a view of the volcano three different types of the impact of the ambient atmosphere on the eruption plume are described first the weather situation throughout the eruption has been analyzed the frequency of northerly wind component is found to be unusually high or 71 in comparison to 49 on average in spring secondly during the effusive phase of the eruption diurnal variation was observed in the plume altitude and there is evidence that suggest that nocturnal inversions may have played a role limiting the rise of the weak plume thirdly images from a web camera were analyzed and the rise of individual cloud heads associated with explosions at the volcano vent mapped the velocity profiles obtained largely agree with conceptual models of volcanic plumes

dynamical consequences in the lower mantle with the post perovskite phase change and strongly depth dependent thermodynamic and transport properties article i nfo we have carried out numerical simulations of large aspect ratio 2 d mantle convection with the deep phase change from perovskite pv to post perovskite ppv using the extended boussinesq approximation for a fluid with temperature and pressure dependent viscosity we have investigated the effects of various ppv phase parameters on the convective planform heat transport and mean temperature and viscosity profiles since ppv is expected to have a relatively weak rheology with respect to pv and a large thermal conductivity we have assumed that the transition from pv to ppv is accompanied by both a reduction in viscosity by 1 to 2 orders of magnitude and by an increase in thermal conductivity by a factor of 2 furthermore we have analyzed the combined effects of a strongly decreasing thermal expansivity in pv and steeply increasing thermal conductivity according to recent evidence from high pressure experiments and first principle calculations as long as the thermal expansivity and conductivity are constant ppv exerts a small but noticeable effect on mantle convection it destabilizes the d layer causes focusing of the heat flux peaks and an increase of the average mantle temperature and of the temporal and spatial frequency of upwellings when the latest depth dependent thermal expansivity and conductivity models are introduced the effects of ppv are dramatic on the one hand without ppv we obtain a very sluggish convective regime characterized by a relatively cool mantle dominated by large downwellings that tend to stagnate beneath the transition zone with ppv on the other hand we observe an extremely significant increase of the average mantle temperature due to the formation of large sized and vigorous upwellings that in some cases tend to cluster thus forming superplumes if a very large thermal conductivity at the core mantle boundary is assumed k 20 wk 1 m 1 we obtain a quasi steady state regime characterized by large and stable plumes with long lifetimes the combination of strongly depth dependent expansivity and conductivity is a viable mechanism for the formation of long wavelength long lived thermal anomalies in the deep mantle even if a low viscosity ppv atop the core mantle boundary is included

geomorphology of subglacial volcanoes in the azas plateau the tuva republic russia abstract the azas plateau volcanic field is located in the tuva republic of the russian federation the compositions of the azas plateau volcanics include low viscosity trachybasalt and basanite volcanic edifices of middle late pleistocene age are widely distributed in the southeastern part of the volcanic field there are subglacial volcanoes among the volcanic edifices and their formation coincided with extensive ancient glaciations in the region the dominant subglacial volcanoes in the area are tuyas the general shapes of the tuyas flat tops with steep sides are due to eruptions into meltwater lakes and confinement of ice walls and effusive subaerial eruptions of basaltic lavas there are also non flat topped subglacial volcanoes and at least one subaerial volcanic edifice in the area degradation appears to have modified the primary shapes of the tuya edifices and such processes seem to include failures of over steepened slopes gully formation due to stream runoff and debris flows cirque valley glaciation and modification by rock glaciers the estimated thicknesses of the ice sheets covering the subglacial volcanoes during their eruptions range 300 600 m on average

ice volcano interactions during the 2010 eyjafjallajokull eruption as revealed by airborne imaging radar 1 during the eruption of the ice covered eyjafjallajokull volcano a series of images from an airborne synthetic aperture radar sar were obtained by the icelandic coast guard cloud obscured the summit from view during the first three days of the eruption making the weather independent sar a valuable monitoring resource radar images revealed the development of ice cauldrons in a 200 m thick ice cover within the summit caldera as well as the formation of cauldrons to the immediate south of the caldera additionally radar images were used to document the subglacial and supraglacial passage of floodwater to the north and south of the eruption site the eruption breached the ice surface about four hours after its onset at about 01 30 utc on 14 april 2010 the first sar images obtained between 08 55 and 10 42 utc show signs of limited supraglacial drainage from the eruption site floodwater began to drain from the ice cap almost 5 5 h after the beginning of the eruption implying storage of meltwater at the eruption site due to initially constricted subglacial drainage from the caldera heat transfer rates from magma to ice during early stages of cauldron formation were about 1 mw m 2 in the radial direction and about 4 mw m 2 vertically meltwater release was characterized by accumulation and drainage with most of the volcanic material in the ice cauldrons being drained in hyperconcentrated floods after the third day of the eruption meltwater generation at the eruption site diminished due to an insulating lag of tephra

morphostructural study and type of volcanism of submarine volcanoes over the pitcairn hot spot in the south pacific undersea volcanoes found at about 80 km southeast of the island of pitcairn are believed to be the manifestation of a hot spot activity located near 129 30 w 25 10 s along the strike of the mururoa gambier pitcairn volcanic alignment hydrothermal activities and recent volcanic flows were observed on the two largest 20 km in basal diameter and shallowest 60 m and 450 m depth volcanoes more than twenty other smaller volcanic edifices 0 25 made up of ancient morb type tholeiitic rocks are inferred to be inherited from the epr axial regions the shallow volcanic activity which occurred on the two largest edifices are classified as 1 reactive eruptions with hydromagmatic activities giving rise to volcanic ejecta bombs xenoliths and ash and 2 quiet eruptions which formed pillows and or lobated lavas and large massive flows intrusives dyke and sill were observed cutting through the volcanic ejecta near the summit 500 m depth of the seamounts the general structural orientations of the rift zones recognized from the bathymetry of individual volcanoes n160 180 n80 n30 and n120 are comparable to those observed in the society and austral hot spot regions these orientations were inferred as corresponding to the structural discontinuities of the ancient oceanic crust and to the regional stress field of the pacific plate rock samples from the pitcairn region consist of alkali basalts basanites trachyandesites and trachytes which are closer in chemical composition to some of the volcanics from the society rather than to those from the austral hot spot regions

the relative importance of supraglacial versus subglacial meltwater escape in basaltic subglacial tuya eruptions an important unresolved conundrum the hydraulic behaviour of meltwater during subglacial basaltic eruptions in temperate ice is of paramount importance in understanding the eruptive processes lithofacies and architecture of the edifices formed hydraulics also determines the timing location and volume of meltwater discharge which may be sudden and catastrophic and via subglacial and or supraglacial routes increasing our knowledge of meltwater hydraulics is therefore important for understanding predicting and mitigating the impact of meltwater release on vulnerable human communities new observations about eruption related meltwater hydraulics are presented for well exposed glaciovolcanic lava fed deltas on james ross island antarctica and accounts of historical eruptions are also re examined to identify the major meltwater discharge routes the study provides the first conceptual model for how meltwater escapes supraglacially in the absence of a crevassed layer which will dominate any meltwater flow overflowing may be initiated by enhanced rates of seepage despite the intrinsically low hydraulic conductivities of snow and firn once overflowing is established the rate of spillway incision is a likely overriding control on the evolution of the system and whether the discharge is unstable fast or stable slower the james ross island sequences demonstrate that meltwater discharge is highly dynamic and may have involved both subglacial and supraglacial escape subglacial discharge probably occurs throughout basaltic tuya eruptions but some periods may be dominated by concurrent overflowing it is still unclear if overflowing systems are sufficiently stable to enable the growth of laterally extensive glaciovolcanic lava fed deltas

constraints on the dynamics of subglacial basalt eruptions from geological and geochemical observations at kverkfjoll ne iceland the kverkfjoll area ne iceland is characterised by subglacial basalt pillow lavas erupted under thick ice during the last major glaciation in iceland the water contents of slightly vesiculated glassy rims of pillows in six localities range from 0 85 0 03 to 1 04 0 03 wt the water content measurements allow the ice thickness to be estimated at between 1 2 and 1 6 km with the range reflecting the uncertainty in the co2 and water contents of the melt the upper estimates agree with other observations and models that the ice thickness in the centre of iceland was 1 5 2 0 km at the time of the last glacial maximum many of the pillows in the kverkfjoll area are characterised by vesiculated cores 40 60 vesicles surrounded by a thick outer zone of moderately vesicular basalt 15 20 vesicles the core contains 1 mm diameter spherical vesicles distributed uniformly this observation suggests a sudden decompression and vesiculation of the still molten core followed by rapid cooling the cores are attributed to a jokulhlaup in which melt water created by the eruption is suddenly released reducing the environmental pressure mass balance and solubility relationships for water allow a pressure decrease to be calculated from the observed change of vesicularity of between 4 4 and 4 7 mpa depressurization equivalent to a drop in the water level in the range 440 470 m consideration of the thickness of solid crust around the molten cores at the time of the jokulhlaup indicates an interval of 1 3 days between pillow emplacement and the jokulhlaup upper limits for ice melting rates of order 10 3 m s are indicated this interpretation suggests that jokulhlaups can reactivate eruptions

rhyolitic volcano ice interactions in iceland iceland contains an abundance and diversity of rhyolitic edifices produced during volcano ice interactions glaciovolcanism that is unmatched by any other volcanic province with an estimated 350 km3 erupted during the past 0 8 ma from at least fifteen volcanic systems this review summarises research undertaken during the past decade and provides a new summary of the distribution of rhyolitic glaciovolcanic rocks erupted during the past 0 8 ma descriptions of effusion dominated edifices focus on the two best studied edifices the c 0 6 km3 and 570 m high prestahnukur edifice and the smaller volume 0 1 km3 drape of blahnukur both show little or no evidence of magmatic fragmentation i e driven by volatile exsolution but both show abundant evidence of quench fragmentation and meltback of ice walls a particular feature of prestahnukur is its sheet like lava bodies which are interpreted as sills that intruded the ice edifice interface rhyolitic tuyas are products of sustained eruptions into thick ice and form distinctive steep sided edifices with flat or broad dome like tops they are 300 700 m high with volumes up to 2 km3 though 0 5 1 0 km3 is more typical initial vigorous phreatomagmatic eruptions within well drained ice vaults build steep sided piles of tephra confined by ice walls gradual upwards increases in inflated clasts point to the decreasing involvement of meltwater and the increasing ability of the magma to vesiculate as the eruption progresses and terminates non explosive degassing produces lava caps up to 300 m thick oraefajokull is a stratovolcano illustrating complex volcano ice interactions generated when rhyolitic lavas erupt onto steeper slopes and encounter ice of variable thickness and properties e g fracturing eruptive units record dramatic ice thickness fluctuations of up to 500 m in valley glaciers between eruptions which emphasises that useful palaeoenvironmental information can be gleaned from detailed studies of volcano ice interactions considerable value is added when palaeoenvironmental information is combined with ar ar dating for example ar ar dating of two rhyolitic tuyas at torfajokull reveal that they erupted prior to the last glacial maximum when temperatures were at least 4 c cooler than the present day ar ar dating of prestahnukur suggests it erupted during the last interglacial glacial transition into over 700 m of ice which corroborates studies arguing for rapid accumulation of land based ice during global cooling

some dynamical characteristics of microseism time series summary microseism time series are non stationary non linear and stochastic and these characteristics can be reproduced by a forced non linear damped oscillator in the present study we show that such an oscillator is also able to explain other widely observed features such as the variation for a given seismic station of the frequency of the secondary peak the variation of the frequency of the primary peak for different seismic stations relative to the same source the variations of amplitude of the power spectrum for stormy days with respect to quiet days and the incoherent propagation of microseisms numerical simulations with the proposed phenomenological model suggest i that the main spectral peak may be due to a competitive process between the resonant response of the medium and an external harmonic force longuet higgins model ii that the secondary peak may be generated by the process associated with the activity of the coastal waves or as a subharmonic of the resonant frequency and iii that the large amplitude variations between quiet and stormy days refers in fact to variations in the source storm distance from a general point of view we can say microseism activity can be interpreted as the resonant response of the earth to atmospheric cyclonic storms coupled with the oceans microseisms are worldwide phenomena usually understood as strong background noise temporally and spatially varying which strongly influences the detection of transient wave arrivals as described by aki richards 1980 two maxima of the power spectrum at about 0 07 hz the primary peak and 0 14 hz the secondary peak are typical features of almost all the recordings at seismic stations the primary peak is usually less intense than that the secondary one and as proposed first by wiechert in 1904 has been attributed to the direct impact of ocean waves on nearby coasts since it roughly coincides with the primary peak of ocean wave oscillations the most intensive peak was interpreted by longuethiggins 1950 as the result of fluctuations of pressure caused by standing waves along the seabed the existence of the two above mentioned peaks although the primary peak may be absent can be considered as a basic feature of seismic noise which may suffer some shift in the frequency location of the two peaks from place to place although preserving an approximate relation 2 1 and strong amplitude variations up to two orders of magnitude due either to the location of the seismic station in quiet or noise places or if in the same place to the presence or absence of storm waves at sea the above observations can be summarized by saying that the shape of the power spectrum is preserved

the interaction between the post perovskite phase change and a thermo chemical boundary layer near the core mantle boundary numerical simulations of thermo chemical mantle convection with the newly discovered post perovskite ppv phase change are used to characterize its dynamical consequences for the dq region which has frequently been interpreted as being thermo chemical in origin a strong lateral variation in the appearance of the ppv region is observed with high cmb temperatures leading to bdouble crossingq of the phase boundary in average or cold regions or no crossing in hot regions lower cmb temperatures result in a single crossing everywhere but with highly variable depth the strongly exothermic nature of the post perovskite phase change induces an anti correlation between regions containing a thick ppv layer and piles of chemically dense material formed by the segregation of the crustal component from subducted slabs the ppv phase change destabilizes chemically dense material allowing subducted slabs to easily penetrate into a dense layer forming isolated patches of the post perovskite phase whereas the lateral spectrum of thermal anomalies in the deep mantle is dominated by long wavelengths the lateral spectrum of composition is relatively flat and is the dominant contributor to seismic velocity variations at all wavelengths except the very longest ones d 2005 elsevier b v all rights reserved

effects of low viscosity post perovskite on the stability and structure of primordial reservoirs in the lower mantle we performed numerical experiments of thermochemical convection in 3 d spherical geometry to investigate the effects of a low viscosity of post perovskite ppv on the stability and structure of primordial reservoirs of dense material in the lower mantle of the earth our results show that weak ppv 1000 viscosity reduction in regions containing ppv strongly increases the core mantle boundary cmb heat flux the volume averaged mantle temperature with weak ppv is slightly higher than that with regular ppv except in the lowermost mantle this is because weak ppv weakens the base of the cold downwellings allowing cold slabs to spread more easily and broadly along the cmb the stability and size of the dense reservoirs is not substantially altered by weak ppv in the weak ppv case slabs spreading along the cmb slightly decreases the stability of dense reservoirs i e the amount of dense material entrained upward is slightly larger than in the regular ppv case i e viscosity of ppv identical to that of perovskite furthermore the topography and steepness of these reservoirs slightly increase however as in the regular ppv case the dense reservoirs are maintained over periods of time comparable to the age of the earth

convection with internal heat sources and thermal turbulence in the earth s mantle summary the influence of internal heat sources on mantle convection is investigated using numerical calculations of 2 d thermal convection in an infinite prandtl number incompressible fluid the geometry is a cylindrical annulus with inner and outer radii in proportion to the whole mantle time dependent calculations are made starting from random initial conditions with rayleigh numbers rat based on boundary temperature difference and ra based on internal heat production in the range 1dsrats lo7 and osra 524rat at fixed rat increasing ra results in transitions in flow structure from steady cells to a pattern of stationary cells with time variable amplitude and finally to thermally turbulent convection with a non stationary cell count for rat 10 and ra rat approximately the travelling plumes disrupt the large scale circulation producing turbulent convection at ra lo7 the flow is fully developed thermal turbulence and for ra 0 consists of a rapidly fluctuating irregular flow driven by transient rising and sinking sheets of buoyant fluid large fluctuations in total kinetic energy occur in this regime with periodicities ranging from 40 to 1400myr the transition to thermal turbulence occurs in these calculations at rayleigh numbers well below the value estimated for subsolidus convection in the mantle suggesting thermally turbulent convection may occur in the mantle a consequence of internal heat sources thermal turbulence offers an explanation for long term fluctuations in the rate of subduction sea floor spreading and global volcanic activity

implications of post perovskite transport properties for core mantle dynamics abstract recent evidence on perovskite to post perovskite phase change in the lowermost mantle suggests that post perovskite piles or lens should be present in the relatively cold downwelling areas while the roots of the hot upwelling plumes consist of perovskite post perovskite is often believed to be deformed predominantly by the dislocation creep and there are some hints that the activation parameters of the dislocation creep in post perovskite induce lower viscosity than that of perovskite at the same pressure and temperature conditions that can even result in a viscosity paradox in the lowermost mantle viscosity in cold downwellings transformed to post perovskite might be lower than the viscosity of warm perovskite plumes such a viscosity structure was indeed recently suggested by the geodynamical inversions of the geoid rheologically weak areas at the base of the subducted slabs may have important consequences for the slab deformation in the d we investigate the dynamics of the cold slabs transformed to post perovskite in the lowermost mantle by means of the simulations of thermal convection in a 2d cartesian model of the lower mantle with composite rheology including diffusion creep and dislocation creep different creep parameters are used for the perovskite lower mantle and for the post perovskite lens or layer respectively while perovskite is considered to be deformed via linear diffusion creep post perovskite is modeled either as a constant viscosity material with viscosity ranging from 1020 pa s to 1023 pa s or it is deformed purely by the dislocation creep the presence of the rheologically distinct post perovskite not only strongly influences the slab deformation above the core mantle boundary cmb but also results in different dynamic regimes of the cmb region characterised by e g the different length scales of the upwelling plumes depending on the creep parameters of post perovskite further the presence of very low viscosity lens or a layer above the cmb strongly enhances the cmb heat flux in the cold slab areas according to the amount of post perovskite present in the d and the rheology of post perovskite the heat flux across the cmb can vary considerably with time which may have important implications for the core mantle coupling and the character of the earth s geomagnetic field