localized subcritical convective cells in temperature dependent viscosity fluids abstract numerical simulations of infinite prandtl number convection in the stagnant lid regime of temperature dependent viscosity convection demonstrate the existence of spatially localized stable convective cells below the critical rayleigh number subcritical convection these solutions are in stark contrast to the usual supercritical convective planforms where convective cells form in the entire layer the isolated cell has a shape of an axisymmetric dome with an upwelling at the center and thus appears as a very weak plume formation of these structures requires subcritical conditions and a localized initial temperature perturbation but does not require any spatial heterogeneity in the material properties or the heat flux when several localized plumes form they tend to attract to each other and form stable clusters this type of subcritical convection may play a role in the formation and longevity of localized features on planetary bodies including the crustal dichotomy and tharsis region on mars and the asymmetric pattern of volcanism on mercury

seismic evidence for overpressured subducted oceanic crust and megathrust fault sealing water and hydrous minerals play a key role in geodynamic processes at subduction zones however there are few seismological constraints on the presence of water within the subducting plate audet et al model converted teleseismic waves to constrain the seismological properties of subducted oceanic crust from the cascadia continental margin to its intersection with the forearc mantle their observations suggest that water is pervasively present in fluid form at high pore pressures indicating that the megathrust is a low permeability boundary these results may hold important implications for our understanding of seismogenesis subduction zone structure and the mechanism of episodic tremor and slip this study models converted teleseismic waves to constrain the seismological properties of subducted oceanic crust from the cascadia continental margin to its intersection with the forearc mantle the observations suggest that water is pervasively present in fluid form at high pore pressures indicating that the megathrust is a low permeability boundary these results may hold important implications for our understanding of seismogenesis subduction zone structure and the mechanism of episodic tremor and slip water and hydrous minerals play a key part in geodynamic processes at subduction zones1 2 3 by weakening the plate boundary aiding slip and permitting subduction and indeed plate tectonics to occur4 the seismological signature of water within the forearc mantle wedge is evident in anomalies with low seismic shear velocity marking serpentinization5 6 7 however seismological observations bearing on the presence of water within the subducting plate itself are less well documented here we use converted teleseismic waves to obtain observations of anomalously high poisson s ratios within the subducted oceanic crust from the cascadia continental margin to its intersection with forearc mantle on the basis of pressure temperature and compositional considerations the elevated poisson s ratios indicate that water is pervasively present in fluid form at pore pressures near lithostatic values combined with observations of a strong negative velocity contrast at the top of the oceanic crust our results imply that the megathrust is a low permeability boundary the transition from a low to high permeability plate interface downdip into the mantle wedge is explained by hydrofracturing of the seal by volume changes across the interface caused by the onset of crustal eclogitization and mantle serpentinization these results may have important implications for our understanding of seismogenesis subduction zone structure and the mechanism of episodic tremor and slip

forty years of ttg research abstract ttgs tonalite trondhjemite granodiorite are one of the archetypical lithologies of archaean cratons since their original description in the 1970s they have been the subject of many studies and discussions relating to archaean geology in this paper we review the ideas concepts and arguments brought forward in these 40 years and try to address some open questions both old and new the late 1960s and the 1970s mark the appearance of grey gneisses ttg in the scientific literature during this period most work was focused on the identification and description of this suite and the recognition that it is a typical archaean lithology ttgs were already recognised as generated by melting of mafic rocks this was corroborated during the next decade when detailed geochemical ttg studies allowed us to constrain their petrogenesis melting of garnet bearing metamafic rocks and to conclude that they must have been generated by archaean geodynamic processes distinct from their modern counterparts however the geodynamic debate raged for the following 30 years as many distinct tectonic scenarios can be imagined all resulting in the melting of mafic rocks in the garnet stability field the 1990s were dominated by experimental petrology work a wealth of independent studies demonstrated that melting of amphibolites as well as of mafic eclogites can give rise to ttg liquids whether amphibolitic or eclogitic conditions are more likely is still an ongoing debate from 1990s onwards one of the key questions became the comparison with modern adakites as originally defined these arc lavas are reasonably close equivalents to archaean ttgs pending issues largely revolve around definitions as the name ttg has now been applied to most archaean plutonic rocks whether sodic or potassic irrespective of their hree contents this leads to a large range of petrogenetic and tectonic scenarios a fair number of which may well have operated concurrently but are applicable only to some of the rocks lumped together in the ever broadening ttg bin

debris jets in continental phreatomagmatic volcanoes a field study of their subterranean deposits in the coombs hills vent complex antarctica abstract the ferrar large igneous province of antarctica contains significant mafic volcaniclastic deposits some of which are interpreted to fill large vent complexes such a complex was re examined at coombs hills to map individual steep sided cross cutting bodies in detail and we found several contrasting types two of which are interpreted to have filled subterranean passageways forcefully opened from below into existing non consolidated debris these transient conduits were opened because of the propagation of debris jets upward moving streams of volcaniclastic debris steam magmatic gases liquid water droplets following explosive magma aquifer interaction some debris jets probably remained wholly subterranean whereas others made it to the surface but the studied outcrops do not allow us to differentiate between these cases the pipes filled with country rock rich lapilli tuff or tuff breccia are interpreted to have formed following phreatomagmatic explosions occurring near the walls or floor of the vent complex causing fragmentation of both magma and abundant country rock material in contrast some of the cross cutting zones filled with basalt rich tuff breccia or lapilli tuff could have been generated following explosions taking place within pre existing basalt bearing debris well away from the complex walls or floor we infer that once focused jets were formed they did not incorporate significant amounts of existing debris while travelling through them instead incorporation of fragments from the granular host took place near explosion sites other basalt rich tuff breccia zones accompanied by domains of in situ peperite and coherent basalt pods are inferred to have originated by less violent processes

resurfacing history of europa from pole to pole geological mapping abstract we produced geologic maps from two regional mosaics of galileo images across the leading and trailing hemispheres of europa in order to investigate the temporal distribution of units in the visible geologic record five principal terrain types were identified plains bands ridges chaos and crater materials which are interpreted to result from 1 tectonic fracturing and lineament building 2 cryovolcanic reworking of surface units with possible emplacement of sub surface materials and 3 impact cratering the geologic histories of both mapped areas are essentially similar and reflect some common trends tectonic resurfacing dominates the early geologic record with the formation of background plains by intricate superposition of lineaments the opening of wide bands with infilling of inter plate gaps and the buildup of ridges and ridge complexes along prominent fractures in the ice it also appears that lineaments are narrower and more widely spaced with time the lack of impact craters overprinted by lineaments indicate that the degree of tectonic resurfacing decreased rapidly after ridged plains formation in contrast the degree of cryovolcanic resurfacing appears to increase with time as chaos formation dominates the later parts of the geologic record these trends and the transition from tectonic to cryovolcanic dominated resurfacing could be attributed to the gradual thickening of europa s cryosphere during the visible geologic history that comprises the last 2 or 30 80 myr of europa s history an originally thin brittle ice shell could be pervasively fractured or melted through by tidal and endogenic processes the degree of fracturing and plate displacements decreased with time in a thickening shell and lineaments became narrower and more widely spaced formation of chaos regions could have occurred where the thickness threshold for solid state convection was exceeded and can be aided by preferential tidal heating of more ductile ice in a long term context it is not clear at this point whether this inferred thickening trend would reflect a drastic change in the thermal evolution of the satellite or cyclic or irregular episodes of tectonic and cryovolcanic activity

some remarks on the models of plate tectonics on terrestrial planets from the view point of mineral physics abstract some mineral physics related issues are reviewed that are closely related to the operation of plate tectonics on terrestrial planets with different mass plate tectonic style of convection would occur when the surface layer has only modest strength relative to the stress generated by mantle convection so that it can deform and subduct into the mantle both the stress on the lithosphere generated by mantle convection and the resistance of the lithosphere for subduction depend on the relevant materials properties a review is presented on the scaling relationships between relevant physical properties and planet mass and on the strength of the lithosphere it is shown that if physically plausible scaling is made both for the relevant materials properties and the macroscopic energy balance a large earth like planet may not necessarily have plate tectonics in addition to the internal processes the surface conditions such as the surface temperature may also play an important role via its effects on the thickness of thermal or chemical lithosphere making it difficult for plate tectonics to operate on small planets therefore in this model plate tectonics would operate on planets with modest size like earth but the validity of this conclusion hinges on the characterization of i the influence of pressure dependent properties on the vigor of convection and of ii the resistance for subduction in particular the processes determining the resistance for subduction have an important influence on the operation of plate tectonics key issues are highlighted that require further studies including the influence of depth dependent properties on convection and the formulation of the resistance of the lithosphere for subduction

improved gravity field of the moon from lunar prospector an improved gravity model from doppler tracking of the lunar prospector lp spacecraft reveals three new large mass concentrations mascons on the nearside of the moon beneath the impact basins mare humboltianum mendel ryberg and schiller zucchius where the latter basin has no visible mare fill although there is no direct measurement of the lunar farside gravity lp partially resolves four mascons in the large farside basins of hertzsprung coulomb sarton freundlich sharonov and mare moscoviense the center of each of these basins contains a gravity maximum relative to the surrounding basin the improved normalized polar moment of inertia 0 3932 0 0002 is consistent with an iron core with a radius of 220 to 450 kilometers

a new model for the emplacement of columbia river basalts as large inflated pahoehoe lava flow fields extensive flows of the columbia river basalt crb group in washington oregon and idaho are dominantly inflated compound pahoehoe sheet lavas early studies recognized that crb lavas are compound pahoehoe flows with textures suggesting low flow velocities but it was thought that the great thickness and extent of the major flows required very rapid emplacement as turbulent floods of lava over a period of days or weeks however small volume 1 km compound pahoehoe flows on kilauea hawai i demonstrate that such flows can thicken by at least an order of magnitude through gradual inflation and the same mechanism has been proposed for larger 10 20 km3 pahoehoe flows in iceland the vertical distribution of vesicles and other morphologic features within crb lava flows indicate that they grew similarly by inflation small pahoehoe lobes at the base and top of many crb pahoehoe lava flows indicate emplacement in a gradual piecemeal manner rather than as a single flood we propose that each thick crb sheet flow was active for months to years and that each group of flows produced by a single eruption a flow field was emplaced slowly over many years

mantle convection with a brittle lithosphere thoughts on the global tectonic styles of the earth and venus summary plates are an integral part of the convection system in the fluid mantle but plate boundaries are the product of brittle faulting and plate motions are strongly influenced by the existence of such faults the conditions for plate tectonics are studied by considering brittle behaviour using byerlee s law to limit the maximum stress in the lithosphere in a mantle convection model with temperature dependent viscosity when the yield stress is high convection is confined below a thick stagnant lithosphere at low yield stress brittle deformation mobilizes the lithosphere which becomes a part of the overall circulation surface deformation occurs in localized regions close to upwellings and downwellings in the system at intermediate levels of the yield stress there is a cycling between these two states thick lithosphere episodically mobilizes and collapses into the interior before reforming the mobile lid regime resembles convection of a fluid with temperature dependent viscosity and the boundary layer scalings are found to be analogous this regime has a well defined nusselt number rayleigh number relationship which is in good agreement with scaling theory the surface velocity is nearly independent of the yield stress indicating that the plate motion is resisted by viscous stresses in the mantle analysis suggests that mobilization of the earth s lithosphere can occur if the friction coeycient in the lithosphere is less than 0 03 0 13 lower than laboratory values but consistent with seismic field studies on venus the friction coeycient may be high as a result of the dry conditions and brittle mobilization of the lithosphere would then be episodic and catastrophic

chaos on io a model for formation of mountain blocks by crustal heating melting and tilting mountains are distributed across the surface of io the fiercely tidally heated moon of jupiter the large crustal thicknesses implied by their great heights can be reconciled with io s high heat flow if most of the heat escapes directly via volcanic eruptions the heat pipe model but the origin of the mountains has remained obscure recent images show that many of io s mountains are tilted blocks undergoing tectonic collapse and we propose here that the volcanic heat pipe and continuous terrain burial model naturally leads to such unstable topography that is burial 1 generates horizontal tensile stresses as the volcanic crustal stack is loaded 2 creates large horizontal compressive confining stresses as io s crust subsides moves to a smaller effective radius and importantly 3 allows for potentially large horizontal compressive thermal stresses as the base of the crust reheats owing to fluctuations in the efficiency of the volcanic heat piping faulting associated with these stresses may raise mountain scarps directly or in concert with thermal uplift due to the crustal reheating continued crustal heating and melting then lead to mountain collapse all over 1 m y to a few million years our model predicts that regions of active mountain formation and volcanic activity on io should be anticorrelated which is observed moreover substantial tidal heating and disruption of planetary crust are seen elsewhere in the jupiter system in the chaos terrains of europa there may be stronger commonalities between the two inner jovian moons and early earth than previously realized

extrapolations of secular trends in magmatic intensity and mantle cooling implications for future evolution of plate tectonics abstract the beginning of plate tectonics on earth remains the subject of fundamental debate also future evolution of plate tectonics has not yet been addressed adequately in the literature here i develop models to extrapolate secular trends of plate tectonics closely associated with mantle potential temperature intensity of magmatic activities to estimate the lifetime of plate tectonics first a fractal model is utilized to analyze global igneous and detrital zircon u pb age datasets to characterize nonlinear intensity and periodicity of peaks in magmatic activities associated with deep seated avalanche type events slab break off lithospheric root detachments and mantle plumes the results show descending trends of both nonlinear intensity and duration of peaks in magmatism observed from 3 ga to the present day suggesting a general trend of mantle cooling this relation when extrapolated yields 1 45 gyr of time for when nonlinear intensity of major magmatic activity would vanish further it is demonstrated by an independent polynomial model fitted to the relation between mantle potential temperature and future age of the earth that this result conforms to the time estimated for when mantle potential temperature is reduced to mantle solidus temperature

the evolution of hadean eoarchaean geodynamics abstract geodynamic modelling of hadean eoarchaean tectonics typically requires higher rates of internal heat production and higher mantle temperatures in models that possess temperature dependent viscosity and a yield criterion in such models under hadean conditions for a wide range of geodynamic configurations and modelling approaches subduction has a propensity to fail this has led to the suggestion that the predominant tectonic regime in the hadean was stagnant lid convection with intermittent recycling events various lines of evidence support this suggestion from i the long mixing time of mantle isotopic anomalies or compositional heterogeneities such as 142nd 182w and platinum group elements to ii the long residence time of the hadean protolith to the jack hills zircons and iii thermal evolution models which typically require lower heat flux in the past to avoid the archaean thermal catastrophe the framework provided by stagnant lid or episodic overturn convection iv provides an explanation for the formation of early archaean ttgs and greenstones and v explains the interleaving arc plume sequence observed in many archaean terranes suggesting subduction initiation events may have been common increasing their preservation potential implications include a low magnetic field strength in the hadean which is consistent with emerging paleointensity data from these times

estimates of europa s ice shell thickness from elastically supported topography 1 the thickness of europa s solid ice shell is uncertain and has important implications for europa s habitability and thermal history and the design of future spacecraft missions here we obtain an estimate of the ice shell thickness from observations of a plateau sw of cilix impact crater stereo topographic profiles suggest that the plateau is flexurally supported with an effective elastic thickness te of 6 2 5 km for a conductive temperature profile this te value implies a solid ice shell thickness of 15 9 20 km if the shell is convecting this estimate is a lower bound combined with independent estimates we infer a probable shell thickness of 25 km the shell thickness is likely to be uniform over the entire satellite

compositionally diverse magmas erupted close together in space and time within a karoo flood basalt crater complex geochemical data and mapping from a karoo flood basalt crater complex reveals new information about the ascent and eruption of magma batches during the earliest phases of flood basalt volcanism flood basalt eruptions at sterkspruit south africa began with emplacement of thin lava flows before abruptly switching to explosive phreatomagmatic and magmatic activity that formed a nest of craters spatter and tuff rings and cones that collectively comprise a crater complex 40 km2 filled by 9 18 km3 of volcaniclastic debris rising magma flux rates combined with reduced access of magma to external water led to effusion of thick karoo flood basalts burying the crater complex beneath the 1 5 km thick lesotho lava pile geochemical data is consistent with flood basalt effusion from local dikes and some lava flows likely shared or re occupied vent sites active during explosive eruptions at sterkspruit flood basalt magmas involved in sterkspruit eruptions were chemically heterogenous this study documents the rapid perhaps simultaneous eruption of three chemically distinct basaltic magmas which cannot be simply related to one another from one vent site within the sterkspruit crater complex stratigraphic and map relationships indicate that eruption of the same three magma types took place from closely spaced vents over a short time during formation of the bulk of the crater complex two magma types recognized there have not been recognized in the karoo province before the variable composition of flood basalts at sterkspruit argues that magma batches in flood basalt fields may be small 0 5 1 km3 and not simply related to one another this implies in turn that heterogeneities in the magma source region may be close to each other in time and space and that eruptions of chemically distinct magmas may take place over short intervals of space and time without significant hybridisation in flood basalt fields

planetary science on earth as it is on mars the small spheres of haematite nicknamed blueberries that litter the mars landing site of nasa s rover opportunity might have an analogue on earth formed from groundwater in southern utah

convection scaling and subduction on earth and super earths super earths are the smallest class of discovered extra solar planets owing to their relatively small mass some might resemble earth and perhaps be habitable because of the connection to habitability through thermal evolution we investigate the tectonic regime of massive terrestrial planets two independent studies o neill c lenardic a oct 2007 geological consequences of super sized earths grl 34 l19204 valencia d o connell r j sasselov d d nov 2007a inevitability of plate tectonics on super earths apjl 670 l45 l48 have reached opposing conclusions about the likelihood of plate tectonics here we offer possible reasons for the discordant findings and address three key aspects of sustaining plate tectonics deformation on faults negative buoyancy and energy dissipation during subduction we show that in general the ratio of driving force to plate resistance increases with planetary mass this is a consequence of increasing convective stresses thinning plates and similar plate structure we conclude that even though the strength of dry and wet of faults increases for massive terrestrial planets the convective stress increases even more allowing deformation to take place also despite shorter timescales for plate cooling rocky super earths achieve negative buoyancy at subduction zones finally by investigating the effects of energy dissipation during subduction we find that massive terrestrial planets dissipate less energy during subduction and hence provide a positive feedback to sustain active lid tectonics in conclusion rocky super earths have more favorable conditions than earth for the subduction of plates and hence for sustaining plate tectonics

computer simulations of california tectonics confirm very low strength of major faults recent improvements in the technique of modeling fault networks with thin plate finite elements yield full convergence of the solutions without any compromise in the representation of the frictional rheology of the upper crust or the transition to dislocation creep in the lower crust we apply these techniques to model the california region incorporating all faults with estimated slip rate over 1 mm yr as well as variations in elevation heat flow and crustal thickness velocity boundary conditions on the model sides are based on the nuvel 1 plate model and an approximation of deformation in the great basin the frictional and dislocation creep rheologic constants of the crust are calibrated to reproduce the observed variations in the maximum depth of seismicity which occurs at model temperatures of 350 410 c this leaves two free parameters the time averaged coefficient of friction on faults and the apparent activation energy for creep in the lower crust these parameters are systematically varied in three sets of 10 81 and 64 experiments respectively the predictions of each model are tested against three published data sets a set of 79 geologic limits on average slip rate on faults a set of 221 principal stress directions and a set of 841 secular rates of geodetic baselines both trilateration and vlbi very long baseline interferometry the patterns of model scores indicate that the time averaged friction coefficient of major faults is only 0 17 0 25 or only 20 30 of the value 0 85 that is assumed for the friction in the intervening blocks the laser trilateration data taken alone would indicate a preference for a high stress model but this model is one with very little fault slip and seismicity which can be rejected on other grounds in the final three parameter set of 64 models we test whether the anomalous weakness of faults is uniform or proportional to net slip with slip dependent weakening there is a general reduction in prediction errors which supports the hypothesis the geologic data and geodetic data agree in implying that fault weakness that is friction of 0 17 is general with an additional 30 slip dependent weakening of the san andreas to friction of 0 12 or 14 of normal the stress data however are fit best if all weakness is slip dependent effectively only the san andreas is weak thus the hypothesis that all weakening is slip dependent should not be rejected the best models predict all available data with rms root mean square mismatch of as little as 3 mm yr 6 of the relative plate velocity so their predictions may be useful for seismic hazard estimation at least along faults where no data are available this study extends the finding of very low friction from subduction zones where it was previously documented to a dominantly strike slip system in which underthrusting of wet sediment is not widespread return of previously subducted water from the mantle and its chemical or physical binding in fine grained gouge are possible explanations for the very low friction

pacific plate motion change at the time of the hawaiian emperor bend constrains the viscosity of earth s asthenosphere important constraints on asthenospheric viscosity come primarily from modeling the glacial rebound of the past 20 kyr but remain somewhat loose because of the intrinsic resolving power of these models we obtain narrower bounds by building on the notion that the asthenosphere also controls the ability to change plate motions over myr we focus on the pacific kinematic change at the time of the hawaiian emperor bend event which is linked to the coeval inception of subduction in the western pacific we sample plausible asthenospheric viscosity and thickness values by requiring the rate at which torque varied to generate the observed kinematics consistent with the nature of subduction initiation uncertainties on the bend event duration and the occurrence of pacific hot spots drift do not hamper our results that suggest that the asthenosphere viscous response to vertical shear over kyr is consistent with that to horizontal shear over myr

effective elastic thickness and heat flux estimates on ganymede 1 we identify sites of apparent flexural uplift at rift zone boundaries on ganymede using galileo stereo derived topography the estimated effective elastic thickness te is 0 9 1 7 km for a nominal young s modulus of 1 gpa using a viscoelastic model of the ice crust we find that the temperature defining the base of the elastic layer is 185 k for likely strain rates the inferred local heat flux during deformation is less than 245 mw m 2 and probably close to 100 mw m 2 the stresses required to cause fault motion are around 1 mpa both the high heat flux and the high stresses are consistent with estimates of these quantities during an episode of transient tidal heating in ganymede s past

friction of ice the frictional strength of ice seems to be well below that for all other rocks triaxial testing of cylinders of pure water ice containing a 45 inclined sawcut at temperatures of 77 t 115 k and confining pressures 0 1 p 250 mpa reveals the frictional laws 0 20 n 8 3 mpa for p 10 mpa and 0 55 n 1 0 mpa for p 5 mpa where and n are the shear and normal stresses on the sawcut surface friction is independent of t and over the one order of magnitude tested of average sliding velocity the sliding behavior is invariably stick slip examination of sliding surfaces shows only minor gouge development a curious arrangement of densely packed short vertical fractures occurs primarily in samples with anomalously low strength these results apply to models of near surface tectonic activity on jupiter s three large icy moons europa callisto and ganymede they indicate for example that a global expansion on ganymede of 3 linear percent will cause extensional movement on preexisting faults at depths to 7 3 km

tidal dissipation surface heat flow and figure of viscoelastic models of io abstract the distribution of hot spots on io s surface and io s figure constrain models of io s interior voyager observations indicate that most hot spots and volcanic plumes are close to the equator recently however it has been proposed that a large low albedo feature in the south polar region may be the source of 40 of the surface heat flow we calculate spatial distributions of the dissipation rate surface heat flow and the figures of three and four layer maxwell rheology models of io by using the correspondence principle the models are required to satisfy the observed surface heat flow constraint and are consistent with the composition of a dehydrated c2 chondrite they consist of an inviscid fe fes core and viscoelastic silicate mantle and lithosphere layers a comparison of the calculated equilibrium figure with the observed figure of io supports the chosen core radius of 980 km in addition the four layer model has a partially molten asthenosphere a few tens of kilometers thick the preferred three layer model has a mantle shear modulus of 10 10 pa and a viscosity of 2 10 16 pa s consistent with rock properties the tidal dissipation rate for this model is largest along the polar axis integration of the dissipation rate over radius gives a surface heat flow with maxima at the poles and minima near the sub jovian and anti jovian points in contrast the dissipation rate in a molten or partially molten incompressible asthenosphere has maxima near the equator and is close to zero at the poles the asthenosphere dissipation rate is mainly a function of asthenosphere viscosity and thickness the thinner the asthenosphere the smaller is the required viscosity a 50 km thick asthenosphere requires a viscosity of about 10 8 pa s a strong south polar hot spot if it can be confirmed by infrared observation favors dissipation in the mantle but does not exclude dissipation in a partially molten asthenosphere equatorial hot spots and the concentration of prometheus type plumes near the equator favor asthenosphere heating but do not rule out deep mantle dissipation both asthenosphere and deep mantle heating may contribute to io s surface heat flow the properties of magma transport through the lithosphere and the planform of mantle convection might exert a control on the surface expression of heating at greater depth

growth of the hemispheric dichotomy and the cessation of plate tectonics on mars 1 although mars is currently not tectonically active it may have experienced plate tectonics early in its history the southern hemisphere of mars possesses a thick crust which probably renders the lithosphere positively buoyant in this paper we present numerical and scaling arguments which show that if the area of positively buoyant lithosphere grows beyond a critical fraction 50 for mars plate tectonics will stop heat transfer through the buoyant lithosphere is inefficient which causes mean mantle temperatures to increase as the surface area of buoyant lithosphere increases the resulting reduction in mantle viscosity reduces shear stresses if these shear stresses drop below the yield strength of the lithosphere plate motions will cease and the planet will behave as a one plate system thus the end of plate tectonics on mars is a natural consequence of the growth of the southern highlands similar arguments for the earth predict that it should operate in the plate tectonic regime now but that it may have experienced stagnant lid convection in the past index terms 5475 planetology solid surface planets tectonics 8149 5430 planetology solid surface planets interiors 8147 5418 planetology solid surface planets heat flow 5455 planetology solid surface planets origin and evolution keywords hemispheric dichotomy mantle convection thermal evolution rheology

accretion mode of oceanic ridges governed by axial mechanical strength oceanic spreading ridges exhibit structural changes as a function of spreading rate mantle temperature and the balance of tectonic and magmatic accretion the role that these or other processes have in governing the overall shape of oceanic ridges is unclear here we use laboratory experiments to simulate ridge spreading in colloidal aqueous dispersions whose rheology evolves from purely viscous to elastic and brittle when placed in contact with a saline water solution we find that ridge shape becomes increasingly linear with spreading rate until reaching a minimum tortuosity this behaviour is predicted by the axial failure parameter f a dimensionless number describing the balance of brittle and plastic failure of axial lithosphere slow spreading fault dominated and fast spreading fluid intrusion dominated ridges on earth and in the laboratory are separated by the same critical f value suggesting that the axial failure mode governs ridge geometry values of f can also be calculated for different mantle temperatures and applied to other planets or the early earth for higher mantle temperatures during the archaean our results preclude the predicted formation of large tectonic plates at high spreading velocity the shape of mid ocean ridges is influenced by lithospheric mechanical strength according to laboratory simulations of diverging plates the results imply that large tectonic plates probably could not have formed on a younger hotter earth

effects of serpentinization on the lithospheric strength and the style of normal faulting at slow spreading ridges rock deformation experiments indicate that serpentinization can strongly influence the strength and tectonics of the oceanic lithosphere strength versus depth profiles calculated for conditions appropriate for slow spreading ridges indicate that the presence of serpentinite can reduce the integrated strength of the lithosphere by up to 30 results from flexural fault models indicate that if serpentinization is isolated to fault zones strain localization should be enhanced providing an explanation for the variations in the style of normal faulting along slow spreading ridge segments at segment centers where serpentinites are scarce deformation is accommodated on closely spaced faults with small throws at the segment ends where serpentinites are most abundant faults are widely spaced and have large throws q 1997 elsevier science b v

constraints on plate tectonics initiation from scaling laws for single cell convection abstract the earth is the only planet known to have plate tectonics while other planets are covered with a stagnant lid on the earth the initiation of subduction which is thought to be the fundamental process for plate tectonics initiation is caused not only by the negative buoyancy of the lithosphere but also by the forces from plate motions however for planets which do not have plate tectonics the very first episode of lithospheric failure has to be caused by forces other than plate motions sublithospheric convection has been proposed as a possible mechanism that provides lithospheric instability through inducing stresses in the lithosphere and lithospheric failure can occur when the yield stress is below a critical value we test the applicability of scaling laws for the critical yield stress obtained in single cell convection simulations to strongly time dependent multi cell systems we show that with an appropriate choice of characteristic aspect ratio for the convective system the scaling laws from single cell simulations can be used to evaluate the conditions on the terrestrial planets in the inner solar system for plate tectonics to exist in agreement with previous studies the estimated values for critical yield stress and coefficient of friction are much lower than the expected values for the earth s lithosphere

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

thin skinned deformation of sedimentary rocks in valles marineris mars deformation of sedimentary rocks is widespread within valles marineris characterized by both plastic and brittle deformation identified in candor melas and ius chasmata we identified four deformation styles using hirise and ctx images kilometer scale convolute folds detached slabs folded strata and pull apart structures convolute folds are detached rounded slabs of material with alternating dark and light toned strata and a fold wavelength of about 1 km the detached slabs are isolated rounded blocks of material but they exhibit only highly localized evidence of stratification folded strata are composed of continuously folded layers that are not detached pull apart structures are composed of stratified rock that has broken off into small irregularly shaped pieces showing evidence of brittle deformation some areas exhibit multiple styles of deformation and grade from one type of deformation into another the deformed rocks are observed over thousands of kilometers are limited to discrete stratigraphic intervals and occur over a wide range in elevations all deformation styles appear to be of likely thin skinned origin crism reflectance spectra show that some of the deformed sediments contain a component of monohydrated and polyhydrated sulfates several mechanisms could be responsible for the deformation of sedimentary rocks in valles marineris such as subaerial or subaqueous gravitational slumping or sliding and soft sediment deformation where the latter could include impact induced or seismically induced liquefaction these mechanisms are evaluated based on their expected pattern scale and areal extent of deformation deformation produced from slow subaerial or subaqueous landsliding and liquefaction is consistent with the deformation observed in valles marineris

interior structure and seasonal mass redistribution of mars from radio tracking of mars pathfinder doppler and range measurements to the mars pathfinder lander made using its radio communications system have been combined with similar measurements from the viking landers to estimate improved values of the precession of mars pole of rotation and the variation in mars rotation rate the observed precession of 7576 35 milliarc seconds of angle per year implies a dense core and constrains possible models of interior composition the estimated annual variation in rotation is in good agreement with a model of seasonal mass exchange of carbon dioxide between the atmosphere and ice caps

rothenberg scoria cone east eifel a complex strombolian and phreatomagmatic volcano rothenberg scoria cone eifel formed by an alternation of three strombolian and three phreatomagmatic eruptive phases eruptions took place from up to six vents on a 600 m long fissure building an early tuff ring and then two coalescing scoria cones strombolian volcanism dominated volumetrically as the supply of external water was severely limited magma water interaction only occurred during the opening stages of eruption at any vent when discharge rates were low and the fragmentation surface was below the water table the phreatomagmatic deposits consist of relatively well sorted fall beds and only minor surge deposits they contain juvenile clasts with a wide range of vesicularity and grain size implying considerable heterogeneity in the assemblage of material ejected by the phreatomagmatic explosions the transition from phreatomagmatic to strombolian eruption at any vent was rapid and irreversible and strombolian volcanism persisted even when eruption rates are inferred to have waned at the close of each eruptive phase as by then the fragmentation surfaces were high in the growing cones and water was denied access to the magma the strombolian deposits are relatively homogenous consisting of alternating coarser and finer grained well sorted fall beds erupted during periods of open vent eruption and partial blockage of the vent respectively the intervals of strombolian eruption were always a delicate balance between discharge of freely degassing magma and processes such as ponding of degassed magma in the vent collapse of the growing cones and repeated recycling of clasts through the vent clear evidence of the instability of the rothenberg cones is preserved in numerous unconformities within deposits of the inner walls of the cones the close of strombolian phases was probably marked by a decreasing supply of magma to the vents accompanied by ponding and stagnation of lava in the craters

a novel technique for measuring lg attenuation results from eastern canada between 1 to 10 hz spectral amplitudes of regionally recorded lg waves are studied in detail between 0 6 and 10 hz using vertical component velocity seismograms of the eastern canada telemetered network stations and a supplementary seismic research observatory type station at glen almond gac quebec we find that the site responses vary among these stations by more than a factor of 3 within the frequency range of interest furthermore they are found to be strongly frequency dependent consequently it is essential that they be taken into consideration in studies of lg wave attenuation and lg source spectra of regionally recorded seismic events we present a new method of measuring interstation surface wave attenuation which is closely related to the conventional two station method while retaining all the desirable features of the conventional two station method the new technique which we will call the reversed two station method allows simple direct one parameter determination of the lg wave attenuation from sparse spectral data in a manner unaffected by station site effects and associated instrument error the reversed two station method is successfully tested over weakly attenuating short 53 to 210 km interstation paths in eastern canada a difficult experimental condition by normal standards the lg attenuation coefficient 0 6 to 10 hz in eastern canada is found to be frequency dependent and of the form f 0 0008 f 0 81 km 1 at higher frequencies the lg attenuation appears to be essentially frequency independent this latter finding is preliminarily interpreted as evidence that regionally recorded lg waves in the canadian shield are as in the case of lg waves propagating through the structurally complex appalachian province contaminated by the high frequency coda of sn waves the lg contamination over the shield paths becomes severe starting at 14 hz twice the frequency above which the lg signal propagating over the appalachian province becomes completely dominated by non lg arrivals

a window for plate tectonics in terrestrial planet evolution abstract the tectonic regime of a planet depends critically on the contributions of basal and internal heating to the planetary mantle and how these evolve through time we use viscoplastic mantle convection simulations with evolving core mantle boundary temperatures and radiogenic heat decay to explore how these factors affect tectonic regime over the lifetime of a planet the simulations demonstrate i hot mantle conditions coming out of a magma ocean phase of evolution can produce a hot stagnant lid regime whilst a cooler post magma ocean mantle may begin in a plate tectonic regime ii planets may evolve from an initial hot stagnant lid condition through an episodic regime lasting 1 3 gyr into a plate tectonic regime and finally into a cold senescent stagnant lid regime after 10 gyr of evolution as heat production and basal temperatures wane and iii the thermal state of the post magma ocean mantle which effectively sets the initial conditions for the sub solidus mantle convection phase of planetary evolution is one of the most sensitive parameters affecting planetary evolution systems with exactly the same physical parameters may exhibit completely different tectonics depending on the initial state employed estimates of the early earth s temperatures suggest earth may have begun in a hot stagnant lid mode evolving into an episodic regime throughout most of the archaean before finally passing into a plate tectonic regime the implication of these results is that for many cases plate tectonics may be a phase in planetary evolution between hot and cold stagnant states rather than an end member

the evil twin of agenor tectonic convergence on europa the dilemma of the surface area budget on europa is resolved by identification of sites of crustal convergence which have balanced the continual and common creation of new surface along dilational bands and pull aparts convergence bands are characterized by a distinctive albeit subdued morphology the prominent unusual lineament agenor is one of several examples we also find diametrically opposite agenor a similar bright linear feature surrounded by markings that allow reconstruction which shows it to be a convergence feature until recently identification of convergence sites was difficult because these features are subtle and do not exhibit structures like the himalayas or plate subduction familiar from convergence of thick solid crusts on terrestrial planets

heat transport efficiency for stagnant lid convection with dislocation viscosity application to mars and venus mantle convection on mars and venus is likely to occur in the regime known as stagnant lid convection we perform thermal boundary layer analyses as well as finite element simulations of stagnant lid convection with non newtonian viscosity which is believed to be more appropriate for the lithosphere and upper mantle and discuss one particular application of the results the efficiency of heat transport on the terrestrial planets as in the case of newtonian viscosity the efficiency of heat transfer in the stagnant lid regime is extremely low compared to plate tectonics for example in the absence of plate tectonics the mantle temperature on earth which is already close to the solidus would be about 700 1500 k higher for the present day value of the surface heat flux for venus the critical heat flux which can be removed without widespread melting is only 10 20 m w m2 for mars it is 15 30 m w m2 therefore there are no doubts that in the absence of mobile plates the mantle temperature would significantly exceed solidus during planetary evolution it is hypothesized that this could cause one or a combination of two possible processes 1 differentiation of radiogenic isotopes into the crust during early planetary magmatism and 2 initiation of some kind of plate tectonics as a result of plate weakening due to melting

faulting within the pacific plate at the mariana trench implications for plate interface coupling and subduction of hydrous minerals we investigate faulting within the incoming pacific plate at the mariana subduction trench to understand stresses within the bending plate regional stresses acting upon the plate interface and the extent of possible faulting induced mantle serpentinization we determine accurate depths by inverting teleseismic p and sh waveforms for earthquakes occurring during 1990 2011 with global centroid moment tensor gcmt solutions for earthquakes with mw 5 0 we determine centroid depths and source time functions and refine the fault parameters results from central mariana indicate that all earthquakes are extensional and occur at centroid depths down to 11 km below the moho at the southern mariana trench extensional earthquakes continue to 5 km below the moho one compressional earthquake at 34 km below the seafloor suggests stronger plate interface coupling here in addition we model the stress distribution within the pacific plate along two bathymetric profiles extending seaward from the mariana subduction trench axis to better understand whether our earthquake depth solutions match modeled scenarios for plate bending under applied external forces results from our flexure models match the locations of extensional and compressional earthquakes and suggest that the pacific plate at southern mariana is experiencing larger compressional stresses possibly due to greater interplate coupling additionally we conclude that if extensional faulting promotes the infiltration of water into the subducting plate mantle then the top 5 15 km of the pacific plate mantle are partially serpentinized and a higher percentage of serpentinization is located near the central mariana trench where extensional events extend deeper

fabric development as the key for forming ductile shear zones and enabling plate tectonics abstract lithospheric deformation on earth is localized under both brittle and ductile deformation conditions as high temperature ductile rheologies are fundamentally strain rate hardening the formation of localized ductile shear zones must involve a structural or rheological change or a change in deformation conditions such as an increase in temperature in this contribution i develop a localization potential that quantifies the weakening associated with these changes the localization potential corresponds to the increase in strain rate resulting from that change under constant stress conditions i provide analytical expressions for the localization potential associated with a temperature increase grain size reduction an increase in water fugacity melt content or the abundance of a weak mineral phase i show that these processes cannot localize deformation from a mantle convection scale 103 km to a ductile shear zone scale 1 km to achieve this is it necessary to invoke a structural transition whereby the weak phase in a rock forms interconnected layers this process is efficient only if one phase is much weaker than the others or if the weakest phase has a highly non linear rheology micas melt and fine grained aggregates unless dry rheologies are used have the necessary characteristics as none of these phases is expected to be present in the dry lithosphere of venus this concept can explain why venus unlike the earth does not display a global network of plate boundaries the diffuse plate boundary in the central indian ocean may be as yet non localized because serpentinization has not reached the ductile levels of the lithosphere

multistage evolution and variable motion history of agenor linea europa abstract europa the second galilean moon of jupiter is composed of a silicate mantle and an ice shell which overlies a supposed subsurface ocean the surface of europa is scarred with fractures varying in morphology dimensions and geometry we focus on agenor linea a 1500 km bright band that extends across europa s antijovian to trailing southern hemisphere agenor linea is morphologically a band like strike slip fault however it experienced at least three evolutionary growth phases marked by three zones of varying albedo each with a different geological history structures within the band material and kinematic indicators are consistent with formation through oblique left lateral dilation followed by normal dilation and right lateral strike slip movement or combinations thereof these motions produced a cumulative maximum right lateral offset of 29 5 km and a maximum band width of 34 km we interpret agenor linea to have formed primarily in response to the combined effects of nonsynchronous rotation stress and diurnal tidal flexing if so its orientation is optimal for right lateral oblique opening in the current global stress field consistent with its most recent kinematic behavior a small amount of offset of relatively young tension fractures that postdate cryospheric disruption by lenticulae suggest the possibility of ongoing activity along agenor in contrast to a previously published model in which strike slip duplexing was the primary process of band formation this study shows that initial dilation of three distinct morphological zones under disparate stress orientations preceded strike slip motion that resulted in both localized and distributed deformation features within the zones across the width of the band

experimental tests of simple models for the dynamics of diffuse oceanic plate boundaries summary diffuse plate boundaries which are zones of deformation hundreds to thousands of kilometres wide occur in both continental and oceanic lithosphere here we build on our prior work in which we use analytical and numerical models to investigate the dynamics of diffuse oceanic plate boundaries assuming that the vertically averaged rheology of deforming oceanic lithosphere is characterized by a power law rheology that is n where 1 n is strain rate and is deviatoric stress a major conclusion of this prior work is that the pole of relative rotation of plates adjoining a diffuse oceanic plate boundary is predicted to be confined to the diffuse oceanic plate boundary especially if n 3 here we use laboratory experiments to test the general validity of this conclusion in these analogue experiments deforming oceanic lithosphere is modelled alternately with a newtonian fluid n 1 and a power law fluid n 6 10 the experimental results are consistent with the analytical and numerical models in particular the experimental results confirm that for a given applied force the pole of rotation is more strongly confined to the laboratory analogue of a diffuse oceanic plate boundary for a power law fluid than it is for a newtonian fluid for forces applied far enough from the centre of the analogue diffuse plate boundary however results for newtonian and power law fluids are similar

generation of felsic crust in the archean a geodynamic modeling perspective abstract as a consequence of secular cooling of the earth there is generally no modern analog to assist in understanding the tectonic style that may have operated in the archean higher mantle temperatures and higher radiogenic heat production in the archean earth would have impacted the thickness and composition of the crust for this reason well constrained numerical modeling based on the fragmentary evidence preserved in the geological record is the most appropriate tool to evaluate hypotheses of archean crust formation the main lithology of archean gray gneiss complexes is the sodic tonalite trondhjemite granodiorite ttg suite melting of hydrated basalt at garnet amphibolite granulite or eclogite facies conditions is considered to be the dominant process that generated the archean ttgs taking into account geochemical signatures of possible mantle contributions to some ttgs models proposed for the formation of archean crust include subduction melting at the bottom of thickened continental crust and fractional crystallization of mantle derived melts under water saturated conditions we evaluated these hypotheses using a 2d coupled petrological thermomechanical tectono magmatic numerical model with initial conditions appropriate to the eoarchean mesoarchean based on the result of our experiments we identify three tectonic processes by which intermediate to felsic melts may be generated from hydrated primitive basaltic crust 1 delamination and dripping of the lower mafic crust into the mantle 2 local thickening of the crust and 3 small scale crustal overturns in the context of a stagnant deformable lid tectono magmatic geodynamic regime that is terminated by short lived subduction we identify two distinct types of continental crust the first type is a pristine granite greenstone like crust with dome and keel geometry formed over delaminating upwelling mantle which is mostly subjected to vertical tectonics processes by contrast the second type is a reworked accreted crust comprising strongly deformed granite greenstone and subduction related sequences and subjected to both strong horizontal shortening and vertical tectonics processes thus our study has identified a possible spatial and temporal transition from pristine granite greenstone like crust with dome and keel geometry to reworked accreted crust forming more felsic gneiss terranes in the archean we suggest that the contemporaneity of the proposed mechanisms can explain the variety and complexity of the archean geological record

three regimes of mantle convection with non newtonian viscosity and stagnant lid convection on the terrestrial planets numerical simulations of convection with strongly temperature dependent viscosity suggest that non newtonian viscosity convection dislocation creep passes through three convective regimes similar to those observed for newtonian viscosity convection diffusion creep the small viscosity contrast regime the transitional regime and the stagnant lid regime for realistic viscosity contrasts mantle convection is in the stagnant lid regime characterized by formation of a very viscous slowly creeping lid on top of an actively convecting mantle this explains the tectonic style observed on the terrestrial planets and the moon on the other hand this eliminates the possibility that the plates on earth could be mobile due to non newtonian viscosity the nature of the mobility of lithospheric plates on earth has yet to be explained

thermal and metamorphic environment of subduction zone episodic tremor and slip 1 episodic tremor and slip ets have been detected in the cascadia and southwest japan subduction zones where the subducting crust is relatively warm because of the young incoming lithosphere 20 ma and modest plate convergence rates 40 60 mm a in the southwest japan subduction zone low frequency earthquakes occur on the plate interface at depths of 30 35 km beneath shikoku where finite element thermal models predict temperatures of 425 c in the subducting oceanic crust and at depths of 30 40 km beneath the kii peninsula where predicted temperatures are 325 c warmer temperatures of 575 c are predicted at ets depths beneath southern vancouver island in the cascadia subduction zone but here tremor also occurs within the overlying fore arc crust where temperatures are lower in the southwest japan and cascadia subduction zones subducting oceanic crust passes through the blueschist greenschist and amphibolite metamorphic facies where mineral dehydration reactions are complex the different temperatures predicted for the two subduction zones suggest that ets does not coincide with a specific temperature or metamorphic reaction several lines of evidence indicate that a free h2o rich fluid is present at least transiently in subducting oceanic crust and fluids released by prograde metamorphic dehydration reactions may help trigger or enable ets within the subducting plate less clear is the role h2o may play in tremor observed in the cascadia fore arc crust where free h2o may exist locally in faults and fractures but retrograde hydration reactions are expected to consume h2o

mountains and calderas on io possible implications for lithosphere structure and magma generation abstract the combination of voyager images and newly acquired galileo images with low illumination and resolutions ranging from 2 to 6 km pixel now allows determination of the global distribution of mountains and volcanic centers on io the mountains generally do not have characteristics typical of terrestrial volcanic landforms they are evenly distributed across the surface and show no obvious correlation with known hot spots or plumes relative elevations determined by shadow measurements and stereoscopy indicate that mountains in the newly imaged area range in elevation up to at least 7 6 km the origin of the mountains remains uncertain some appear to be multitiered volcanic constructs others enclosing the partial remains of large circular depressions appear to be remnants of old volcanoes yet others show extensive tectonic disruption volcanic centers also appear to be distributed evenly across the surface except for an apparently somewhat lower density at high latitudes the low latitudes have one volcanic center per 7 10 4 km 2 and on average the centers are spaced roughly 250 km apart the global distribution of high mountains suggests that the lithosphere over most of io is thick although the thickness cannot be calculated the previously suggested 30 km appears reasonable as a lower limit the high rates of resurfacing combined with the likely dissipation of most of the tidal energy in the asthenosphere and underlying mantle implies a very low temperature gradient in the upper part of the lithosphere and steep gradients in the lower lithosphere the slow rate of separation of melt from host rock in the magma source regions as a consequence of the low gravity on io coupled with the high rate of magma production will likely result in larger melt fractions than is typical for source regions on earth the variety of volcanic landforms suggests that volcanic products with a range of compositions are deposited on the surface this mixture will be carried downward through the lithosphere as a consequence of the 0 5 1 5 cm yr resurfacing rates during descent the more volatile components will tend to be driven off early but complete or near complete melting at the base of the lithosphere may result in rehomogenization of the silicate mixture that remains

effects of lithospheric properties on the formation of type 2 coronae on venus 1 over 500 coronae occur on venus a recent survey defined type 2 coronae as those with 50 complete fracture annuli this paper synthesizes the results from a statistical study of the size topographic morphology and geologic setting of the entire corona population and a study of the gravity and topography of the 32 type 2 coronae resolved in the gravity data using the estimated lithospheric properties loading signature and geologic characteristics we examine the factors controlling corona morphology size and fracture completeness type 1 versus type 2 we find no difference between lithospheric properties in the plains and fractures belts nearly half of the type 2 coronae studied are isostatically compensated and probably inactive this implies that the final stage of a corona may have a wide range of topographic morphologies elastic thickness has no correlation with diameter and therefore does not limit corona size the ratio of crustal thickness to plume diameter does not control morphology as a function of size as predicted by a spreading drop model however rim only coronae which are predicted to form via isostatic rebound of crust thickened by delamination are supported by a density interface isostasy and delamination of the lower lithosphere are likely to have a greater effect on corona formation and possibly in understanding the evolution of the venusian lithosphere than variations in elastic thickness the role of isostasy in the formation of type 2 coronae suggests that surface fractures may be reduced due to low strain rates

strike slip motion and double ridge formation on europa layer thickness of 2 km and shear velocity of 6 10 7 ms 1 such a velocity is appropriate for diurnal 85 hour tidal motion the local increase in temperature may cause 100 m uplift around the shear zone through thermal buoyancy the stresses required to produce velocities of order 10 7 ms 1 are similar to estimates for present day tidal stresses on europa 10 4 10 5 pa brittle layer thicknesses 2 km are unlikely to persist at active shear zones because of the effect of shear heating shear velocities greater than or equal to 10 6 ms 1 will give rise to melting at shallow depths the removal of material by downwards percolation of meltwater may cause surface collapse along the shear zone inward motion leading to compression may also result the combination of thermally or compression induced uplift and melt related collapse may be responsible for the pervasive double ridges seen on europa s surface index terms 5475 planetology solid surface planets tectonics 8149 6218 planetology solar system objects jovian satellites 8120 tectonophysics dynamics of lithosphere and mantle general 8160 tectonophysics rheology general keywords ice shear zone melting viscosity brittle ductile transition

maar diatreme phreatomagmatism at hopi buttes navajo nation arizona usa remnants of an extensive maar diatreme volcanic field are magnificently exposed at various depths of erosion in the hopi buttes volcanic field of northeastern arizona field and petrographic studies of both the maar and diatreme elements of a selection of volcanoes within the field show that 1 lower sections of the maar rim sequences are typically rich in sandy mudrock derived from the pre eruptive mio pliocene bidahochi formation and the muddy bidahochi sediment was soft and wet at the time of maar eruptions 2 beds higher within the rims contain generally increased proportions of sandstone clasts from the triassic wingate formation in the diatremes late emplaced breccia has deeper seated lithics than more marginal breccia emplaced earlier and 3 many vents are topped by megacryst enriched scoria and spatter and deep seated xenoliths are known only from upper diatreme and craterfilling tephra these observations show that 1 eruptions at hopi buttes involved interaction of magma with unconsolidated mudrock at shallow levels and the phreatomagmatic processes that provided the bulk of the energy involved in the violent eruptions were driven by the interaction of magma and wet sediment 2 the locus of explosive activity migrated downward as eruptions progressed and 3 the closing stage of many eruptions was characterized by rapid magma rise and relative depletion of water

sulfate rich eolian and wet interdune deposits erebus crater meridiani planum mars this study investigates three bedrock exposures at erebus crater an 300 m diameter crater approximately 4 km south of endurance crater on mars these outcrops called olympia payson and yavapai provide additional evidence in support of the dune interdune model proposed for the formation of the deposits at the opportunity landing site in meridiani planum there is evidence for greater involvement of liquid water in the olympia outcrop exposures than was observed in eagle or endurance craters the olympia outcrop likely formed in a wet interdune and sand sheet environment the facies observed within the payson outcrop which is likely stratigraphically above the olympia outcrop indicate that it was deposited in a damp wet interdune sand sheet and eolian dune environment the yavapai outcrop which likely stratigraphically overlies the payson outcrop indicates that it was deposited in primarily a sand sheet environment and also potentially in an eolian dune environment these three outcrop exposures may indicate an overall drying upward trend spanning the stratigraphic section from its base at the olympia outcrop to its top at the yavapai outcrop this contrasts with the wetting upward trend seen in endurance and eagle craters thus the series of outcrops seen at meridiani by opportunity may constitute a full climatic cycle evolving from dry to wet to dry conditions

strength of slightly serpentinized peridotites implications for the tectonics of oceanic lithosphere we deformed cores of peridotite with 10 15 lizardite and chrysotile serpentine to determine the influence of serpentine content on the strength and the style of deformation the strength the pressure dependence of strength and the nominally nondilatant mode of brittle deformation of slightly serpentinized peridotites are comparable to those of pure serpentinites these results indicate that deformation is accommodated primarily by serpentine while olivine despite being the more abundant component remains nominally undeformed on the basis of these data and previous work we determine that the transition from a strong dilatant dunite rheology to a weak nondilatant serpentinite rheology is not a linear function of the degree of serpentinization instead an abrupt transition in strength is observed at low degrees of serpentinization the pressure of the transition from localized to distributed deformation also decreases abruptly from 1000 mpa to 150 350 mpa the change in rheological behavior occurs at a serpentine content of 10 15 or less which corresponds to published compressional seismic velocity of 7 8 7 5 km s at a pressure of 200 mpa the seismic velocity of the oceanic lithosphere particularly of that formed at slow spreading ridges can thus provide constraints on its mechanical properties at depth because slightly serpentinized peridotites have a rheology similar to that of pure serpentinite significant lithospheric weakening may occur after the onset of alteration near or at the ridge axis

physical volcanology of a large crater complex formed during the initial stages of karoo flood basalt volcanism sterkspruit eastern cape south africa abstract the sterkspruit complex exposed at the base of the karoo flood basalts in south africa is a complex of mostly massive volcaniclastic rocks dikes and lava that fills a crater 45 km 2 cut into country rock and is surrounded by a blanket of more distal bedded volcaniclastic rocks the moderately to steeply inward dipping margins of the crater are delineated by country rock breccia that grades into rocks within the complex intra complex rocks are dominated by massive polymict lapilli tuff and tuff breccia 150 m thick with intercalated lavas and minor bedded volcaniclastic rocks pillow lava 50 m thick and thinly bedded inferred lacustrine deposits cap the complex succession and are in turn covered by subaerial karoo flood basalts beyond the crater a succession of bedded finer grained volcaniclastic rocks 100 60 m thick rests on country rock and thins distally over at least 5 km magma emplaced over a broad area through a network of feeder dikes and sheets created many small vents that grew into their neighbours through lateral quarrying and vent migration and whose deposits overlap both within and outside the complex an abrupt end to eruptions that formed volcaniclastic deposits is recorded by flooding of the lowest part of the crater by first water then lava then subsequently by inundation of the complex and the surrounding landscape by voluminous effusion of flood basalt formation of the sterkspruit complex and many others like it in south africa confirms that the opening phases of karoo flood basalt volcanism were partly explosive the abundant volcaniclastic products at sterkspruit and in other flood basalt provinces result from eruptions that disperse ash and volcanic aerosols differently than effusive eruptions of lava and thus may have important implications for climate change and landscape development associated with the initial stages of flood basalt provinces

martian plate tectonics the northern lowlands of mars may have been produced by plate tectonics preexisting old thick highland crust was subducted while seafloor spreading produced thin lowland crust during late noachian and early hesperian time in the preferred reconstruction a breakup margin extended north of cimmeria terra between daedalia planum and isidis planitia where the highland lowland transition is relatively simple south dipping subduction occurred beneath arabia terra and east dipping subduction beneath thatsis montes and tempe terra lineations associated with gordii dotsum are attributed to ridge parallel structures while phelegra montes and scandia colles are interpreted as transform parallel structures or ridge fault fault triple junction tracks other than for these few features there is little topographic roughness in the lowlands seafloor spreading if it occurred must have been relatively rapid quantitative estimates of spreading rate are obtained by considering the physics of seafloor spreading in the lower c 0 4 g gravity of mars the absence of vertical scarps from age differences across fracture zones and the smooth axial topography to the first order the height of vertical scarps across fracture zones does not involve gravity crustal thickness at a given potential temperature in the mantle source region scales inversely with gravity thus the velocity of the rough smooth transition for axial topography also scales inversely with gravity plate reorganizations where young crust becomes difficult to subduct are another constraint on spreading age possible plate reorganizations for example the end of spreading through alba patera occur when the ridge axis is far from the trench that is rapid plate motions are inferred to have placed young oceanic crust far from the ridge axis the preferred full spreading rate 900 from the plate pole is 80 mm yr 1 plate tectonics if it occurred dominated the thermal and stress history of the planet a geochemical implication is that the lower gravity of mars allows deeper hydrothermal circulation through cracks and hence more hydration of oceanic crust so that more water is easily subducted than on the earth age and structural relationships from photogeology as well as median wavelength gravity anomalies across the now dead breakup and subduction margins are the data most likely to test and modify hypotheses about mars plate tectonics

sulphur release from flood lava eruptions in the veidivotn grimsvotn and katla volcanic systems iceland book description humans have long marvelled at and feared the odorous and colourful manifestations of volcanic emissions and in some cases have harnessed them for their economic value the degassing process responsible for these phenomena is now understood to be one of the key factors influencing the timing and nature of volcanic eruptions moreover the surface emissions of these volatiles can have profound effects on the atmospheric and terrestrial environment and climate even more fundamental are the relationships between the history of planetary outgassing differentiation of the earth s interior chemistry of the atmosphere and hydrosphere and the origin and evolution of life this book provides a compilation of 23 papers that investigate the behaviour of volatiles in magma the feedbacks between degassing and magma dynamics and the composition flux and environmental atmospheric and climatic impacts of volcanic gas emissions

strain localization in homogeneous granite near the brittle ductile transition a case study of the kellyland fault zone maine usa abstract we present a detailed case study of a strike slip shear zone that cut homogeneous granite near the brittle ductile transition this zone contains three distinct strain facies 1 a 2 3 km wide belt of foliated granite 2 a 100 300 m wide belt of small localized shear zones and 3 a 200 400 m wide belt of ultramylonite that includes both homogeneous and local pinstripe ultramylonites the foliated granite formed via dislocation creep of quartz and minor dissolution precipitation creep and dislocation creep of feldspar under amphibolite facies conditions shortly after granite crystallization the localized shear zones and homogeneous ultramylonites formed directly from pseudotachylyte and cataclasite and they deformed by granular flow grain boundary sliding coupled with dissolution precipitation creep these deformation features reveal a three phase rheologic evolution in granite near the brittle ductile transition phase 1 is recorded by the foliated granite and the rheology was governed by dislocation creep of quartz phase 2 was a transient period of brittle deformation and represents a temporal strength maxima in the fault zone phase 3 was a long lived period of ductile deformation and the rheology was governed by the granular flow mechanism strain weakening in this zone was a direct result of grain size reduction and phase mixing during transient brittle deformation

volcanism in the solar system the myriad bodies that occur in the solar system show a wide range of physical properties exploration by spacecraft during the past four decades has shown that volcanism a major mechanism by which internal heat is transported to the surface is common on many of these bodies

the initial cooling of pahoehoe flow lobes in this paper we describe a new thermal model for the initial cooling of pahoehoe lava flows the accurate modeling of this initial cooling is important for understanding the formation of the distinctive surface textures on pahoehoe lava flows as well as being the first step in modeling such key pahoehoe emplacement processes as lava flow inflation and lava tube formation this model is constructed from the physical phenomena observed to control the initial cooling of pahoehoe flows and is not an empirical fit to field data we find that the only significant processes are a heat loss by thermal radiation b heat loss by atmospheric convection c heat transport within the flow by conduction with temperature and porosity dependent thermal properties and d the release of latent heat during crystallization the numerical model is better able to reproduce field measurements made in hawai i between 1989 and 1993 than other published thermal models by adjusting one parameter at a time the effect of each of the input parameters on the cooling rate was determined we show that a the surfaces of porous flows cool more quickly than the surfaces of dense flows b the surface cooling is very sensitive to the efficiency of atmospheric convective cooling and c changes in the glass forming tendency of the lava may have observable petrographic and thermal signatures these model results provide a quantitative explanation for the recently observed relationship between the surface cooling rate of pahoehoe lobes and the porosity of those lobes jones 1992 1993 the predicted sensitivity of cooling to atmospheric convection suggests a simple field experiment for verification and the model provides a tool to begin studies of the dynamic crystallization of real lavas future versions of the model can also be made applicable to extraterrestrial submarine silicic and pyroclastic flows

capelinhos 1957 1958 faial azores deposits formed by an emergent surtseyan eruption the 1957 1958 eruption of capelinhos faial island azores involved three periods of surtseyan hydromagmatic activity two in 1957 and one in 1958 deposits from this eruption are exposed both in sea cliffs cut into the flanks of the tuff cone and more distally 1 km from the vent five lithofacies are identified lithofacies i is composed of even thickness beds with laterally continuous internal stratigraphy and is interpreted to have been formed by fallout lithofacies ii consists of beds with internally discontinuous lenses and has sand wave structures that increase in abundance toward the outer margins of the tuff cone this lithofacies is interpreted as having been deposited from pyroclastic surges lithofacies iii is composed of mantle bedded deposits with laterally discontinuous internal stratigraphy this lithofacies is interpreted to have been formed by hybrid processes where fallout of tephra occurred simultaneously with pyroclastic surges in the outer flanks of the tuff cone lithofacies iii grades laterally into fallout beds of lithofacies i lithofacies iv consists of alternating beds of coarse ash aggregates and non aggregated fine ash and is particularly well developed in distal regions some of this facies was formed by fallout alternating beds also occur plastered against obstacles up to 2 km from the vent indicating an origin from wet pyroclastic surges the orientation of plastered tephra indicates that the surges were deflected by topography as they decelerated the distinction between surge and fallout in distal regions is uncertain because wind drifted fallout and decelerating surge clouds can generate similar deposits lithofacies v consists of scoria lapilli beds interpreted to be fallout from hawaiian style fire fountaining in the later stages of the eruption juvenile pyroclasts within hydromagmatic deposits are predominantly poorly vesicular 25 60 of clasts 30 vesicles however on both micro and macroscopic scales there is a wide range in clast vesicularity up to 70 vesicles indicating that although fragmentation was predominantly hydromagmatic vesiculation and magmatic volatile driven fragmentation operated simultaneously

formation of clastogenic lava flows during fissure eruption and scoria cone collapse the 1986 eruption of izu oshima volcano eastern japan the 1986 eruption of b fissure at izu oshima volcano japan produced among other products one andesite and two basaltic andesite lava flows locally the three flows resemble vent effused holocrystalline blocky or aa lava however remnant clast outlines can be identified at most localities indicating that the flows were spatter fed or clastogenic the basaltic andesite flows are interpreted to have formed by two main processes a reconstitution of fountain generated spatter around vent areas by syn depositional agglutination and coalescence followed by extensional non particulate flow and b syn eruptive collapse of a rapidly built spatter and scoria cone by rotational slip and extensional sliding these processes produced two morphologically distinct lobes in both flows by a earlier non particulate flow of agglutinate and coalesced spatter which formed a thin lobe of rubbly aa lava ca 5 m thick with characteristic open extension cracks revealing a homogeneous holocrystalline interior and b later scoria cone collapse which created a larger lobe of irregular thickness 20 m made of large detached blocks of scoria cone interpreted to have been rafted along on a flow of coalesced spatter the source regions of these lava flows are characterized by horseshoe shaped scarps 30 m high with meso blocks ca 30 m in diameter of bedded scoria at the base one lava flow has a secondary lateral collapse zone with lower ca 7 m scarps backward tilted meso blocks are interpreted to be the product of rotational slip and forward tilted blocks the result of simple toppling squeeze ups of coalesced spatter along the leading edge of the meso blocks indicate that coalescence occurred in the basal part of the scoria cone this low viscosity coalesced spatter acted as a lubricating layer along which basal failure of the scoria cone occurred rotational sliding gave way to extensional translational sliding as the slide mass spread out onto the present caldera floor squeeze ups concentrated at the distal margin indicate that the extensional regime changed to one of compression probably as a result of cooling of the flow front sliding material piled up behind the slowing flow front and coalesced spatter was squeezed up from the interior of the flow through fractures and between rafted blocks the andesite flow although morphologically similar to the other two flows has a slightly different chemical composition which corresponds to the earliest stage of the eruption it is a much smaller lava flow emitted from the base of the scoria cone 2 days after the eruption had ceased this lava is interpreted to have been formed by post depositional coalescence of spatter under the influence of the in situ cooling rate and load pressure of the deposit extrusion occurred through the lower part of the scoria cone and subsequent non particulate flow of coalesced material produced a blocky and aa lava flow the mechanisms of formation of the lava flows described may be more common during explosive eruptions of mafic magma than previously envisaged

conditions for the onset of plate tectonics on terrestrial planets and moons abstract plate tectonics on earth is driven by the subduction and stirring of dense oceanic lithosphere into the underlying mantle for such a regime to exist on any planet stresses associated with mantle convection must exceed the strength of the lithosphere this condition is sufficiently restrictive that plate tectonics currently operates only on earth and mantle convection in most terrestrial planets and moons is probably in a stagnant lid regime convective stresses on the lithosphere depend on the viscosity and velocity of underlying cold downwellings the lithospheric yield stress is controlled by its friction coefficient and elastic thickness the depth to the brittle ductile transition or bdt both convective stresses and the plate s yield strength depend critically on the size thermal state and cooling history of a planet accordingly here we use numerical simulations and scaling theory to identify conditions in which mantle convection leads to lithospheric failure for a range of conditions relevant to the terrestrial planets whereas earth is expected to be in a plate tectonic regime over its full thermal evolution the moon and mercury are expected to have always remained in a stagnant lid regime venus io and europa currently fall on the transition between the two regimes which is consistent with an episodic style of mantle convection for venus a tectonic component to deformation on io and the resurfacing history and lithospheric evolution of europa our results suggest that venus may have been in a plate tectonic regime in the past while stagnant now it is plausible that mars may have also been in an active lid regime depending on whether there was liquid water on the surface

rootless cones on mars a consequence of lava ground ice interaction fields of small cratered cones on mars are interpreted to have formed by rootless eruptions due to explosive interaction of lava with ground ice contained within the regolith beneath the flow melting and vaporization of the ice and subsequent explosive expansion of the vapour act to excavate the lava and construct a rootless cone around the explosion site similar features are found in iceland where flowing lava encountered water saturated substrates the martian cones have basal diameters of c 30 1000 m and are located predominantly in the northern volcanic plains high resolution mars orbiter camera images offer significant improvements over viking data for interpretation of cone origins a new model of the dynamics of cone formation indicates that very modest amounts of water ice are required to initiate and sustain the explosive interactions that produced the observed features this is consistent with the likely low availability of water ice in the martian regolith the scarcity of impact craters on many of the host lava flows indicates very young ages suggesting that ground ice was present as recently as less than 10 l00 ma and may persist today rootless cones therefore act as a spatial and temporal probe of the distribution of ground ice on mars which is of key significance in understanding the evolution of the martian climate the location of water in liquid or solid form is of great importance to future robotic and human exploration strategies and to the search for extraterrestrial life

europa s differentiated internal structure inferences from four galileo encounters radio doppler data from four encounters of the galileo spacecraft with the jovian moon europa have been used to refine models of europa s interior europa is most likely differentiated into a metallic core surrounded by a rock mantle and a water ice liquid outer shell but the data cannot eliminate the possibility of a uniform mixture of dense silicate and metal beneath the water ice liquid shell the size of a metallic core is uncertain because of its unknown composition but it could be as large as about 50 percent of europa s radius the thickness of europa s outer shell of water ice liquid must lie in the range of about 80 to 170 kilometers

elastic layer thickening with age of the oceanic lithosphere a tool for prediction of the age of volcanoes or oceanic crust summary to first order the estimated thickness te of the elastic part of the oceanic lithosphere increases linearly with the square root of the age of the lithosphere at the time of loading t in order to quantify this relationship in the particular case of volcanic loading a synthesis of te estimates reported both in this paper and in previous studies is conducted excluding the anomalously low estimates from the south central pacific the values are very consistent for the three main oceans and follow the empirical relationship te km 2 7 0 15 t ma the relationship is used to predict the age of volcanoes when the age of the crust is known for the trindade chain in the south atlantic ocean and to predict the age of the crust when the age of the volcanoes is known for the pacific plate east of the tonga trench the age estimates are in good agreement with the structural setting

a climate induced transition in the tectonic style of a terrestrial planet abstract we explore the possibility that an increase in the surface temperature of a terrestrial planet due to an enhanced concentration of atmospheric greenhouse gasses and or increased solar luminosity could initiate a transition from an active lid mode of mantle convection e g plate tectonics to an episodic or stagnant lid mode i e single plate planet a scaling theory is developed to estimate the required temperature change as a function of the temperature dependence of mantle viscosity and the yield stress of the lithosphere the theory relies on the assumptions that convective stresses scale with mantle viscosity and that a planet will adjust to surface temperature changes so as to maintain a surface heat flow that balances internal heat production the theory is tested against a suite of numerical simulations of mantle convection the comparisons are favorable the combined theory and numerics suggest that if the yield stress for the earths lithosphere is 30 35 mpa then a surface temperature change of 60 120 could shut down an active lid mode of convection assuming present day conditions lower values are predicted for higher yield stresses and for earlier times in the earth geologic evolution

lithospheric flexure and thermal anomalies plots of effective elastic thickness te of the oceanic lithosphere as a function of age when elastically deformed show a general increase in te with the square root of age although there is much scatter some of the scatter can be eliminated by correcting for the effect of finite yield strength in rocks which makes te for more sharply bent plates underestimate the true depth tm to the rheological boundary at the base of the high strength mechanical lithosphere after converting effective values te to true thicknesses tm it appears that the mechanical lithosphere is always thinner beneath islands and seamounts compared to lithosphere of similar age flexed at subduction zones from several other lines of evidence such as depth anomalies seamount subsidence and heat flow data it has been suggested that the thermal structure in plates subjected to hot spot type volcanism is reset to values appropriate for much younger lithosphere the reduction in mechanical plate thickness beneath hot spot volcanoes is also consistent with this concept of thermal rejuvenation since the strength of rocks decreases exponentially with rising temperature this dependence implies that variations in mechanical plate thickness can be used to map temperature anomalies in the lithosphere since the mechanical plate thickness depends principally upon the temperature at one depth specifically the depth at which the temperature reaches 550 600 c depending on strain rate flexural data are particularly sensitive indicators of the vertical structure of lithospheric reheating compared to quantities dependent on depth integrals of temperature such as swell height and subsidence the thermal rejuvenation hypothesis predicts that the flexural profile should mimic viscoelastic relaxation in the first 10 ma after loading if initially reheating is confined to depths close to and below the 550 c isotherm the flexural data combined with depth anomaly information for hawaii point to extremely rapid reheating localized in the lower lithosphere rejuvenation in the society islands on the other hand is compatible with slower methods of heat injection involving the entire lithospheric column

initiation of subduction by small scale convection 1 the possibility of initiation of subduction by sublithospheric small scale convection is investigated with the help of fowler s theory numerical constraints on the lithospheric stresses generated by convection and direct numerical simulations of subduction the derived scaling laws show that the critical yield stress for initiation of subduction by small scale convection is of the order of 30 mpa this value is comparable with the critical yield stress required for initiation of subduction by plate forces and with the estimates of stresses in the subduction zone this implies that small scale convection can play an important role in the initiation of subduction on the present day earth this also implies that small scale convection can initiate subduction in the absence of already occurring plate tectonics and thus can be a trigger mechanism for plate tectonics on the early earth and other terrestrial planets the critical yield stress for initiation of subduction by either mechanism is at least one order of magnitude smaller than the estimates of the lithospheric strength suggested by laboratory experiments on rock deformation such a low lithospheric strength is most likely caused by a relatively high abundance of water in the interior of the earth since the critical yield stress depends only weakly on planetary parameters water seems to be the key condition required for plate tectonics to occur on a terrestrial planet

an episodic hypothesis for venusian tectonics it is suggested that episodic plate tectonics occurs on venus episodes of rapid plate tectonics are separated by periods of surface quiescence for the last 500 200 m y it is postulated that the surface of venus has been a single rigid plate that has been thickening due to conductive cooling a near uniform surface age is consistent with observed crater densities and the relatively small number of craters modified by surface tectonics or embayed by lava flows a lithosphere that has conductively thickened for some 500 m y has a thickness of about 300 km nearly an order of magnitude greater than the thickness associated with steady state conductive heat loss such a thick lithosphere can support the high topography and associated gravity anomalies on venus as well as the unrelaxed craters studies of lithospheric flexure at coronae are also consistent with a thick elastic lithosphere incipient subduction associated with large coronae may represent the onset of a new episode of rapid plate tectonics on the earth 75 90 of mantle heat transport is attributed to the creation of new oceanic lithosphere at ocean ridges this process is not operative on venus this paper suggests that episodic plate tectonics on venus constitutes the primary mechanism for mantle heat transport on that planet

evolution of the quaternary melitite nephelinite herchenberg volcano east eifel the quaternary herchenberg composite tephra cone east eifel fr germany with an original bulk volume of 1 17 107 m3 dre of 8 2 106 m3 and dimensions of ca 900 600 90 m length width height erupted in three main stages a initial eruptions along a nw trending 500 m long fissure were dominantly vulcanian in the northwest and strombolian in the southeast removal of the unstable underlying 20 m thick tertiary clays resulted in major collapse and repeated lateral caving of the crater the northwestern lower cone 1 lc1 was constructed by alternating vulcanian and strombolian eruptions b cone building mainly strombolian eruptions resulted in two major scoria cones beginning initially in the northwest cone 1 and terminating in the southeast cones 2 and 3 following a period of simultaneous activity of cones 1 and 2 lapilli deposits are subdivided by thin phreatomagmatic marker beds rich in tertiary clays in the early stages and devonian clasts in the later stages three dikes intruded radially into the flanks of cone 1 c the eruption and deposition of fine grained uppermost layers phreatomagmatic tuffs accretionary lapilli and strombolian fallout lapilli presumably from the northwestern center cone 1 terminated the activity of herchenberg volcano the herchenberg volcano is distinguished from most strombolian scoria cones in the eifel by 1 small volume of agglutinates in central craters 2 scarcity of scoria bomb breccias 3 well bedded tephra deposits even in the proximal facies 4 moderate fragmentation of tephra small proportions of both ash and coarse lapilli bomb size fraction 5 abundance of dense ellipsoidal juvenile lapilli and 6 characteristic depositional cycles in the early eruptive stages beginning with laterally emplaced fine grained xenolith rich tephra and ending with fallout scoria lapilli herchenberg tephra is distinguished from maar deposits by 1 paucity of xenoliths 2 higher depositional temperatures 3 coarser grain size and thicker bedding 4 absence of glassy quenched clasts except in the initial stages and late phreatomagmatic marker beds and 5 predominance of strombolian cone building activity the characteristics of herchenberg deposits are interpreted as due to a high proportion of magmatic volatiles dominantly co2 relative to low viscosity magma during most of the eruptive activity

constraints of the topography gravity and volcanism on venusian mantle dynamics and generation of plate tectonics abstract venus s mantle convection model was studied in a three dimensional spherical shell domain with depth and temperature dependent viscosity numerical results show that key observations of venus including the number of major hotspot volcanic systems spectral patterns of the surface topography and geoid at long and intermediate wavelengths can be explained in models that have a spinel to post spinel endothermic phase change of 3 5 mpa k clapeyron slope and averaged mantle viscosity of 2 10 21 pa s i e convective rayleigh number of 1 8 10 7 our models with the endothermic phase change show relatively weak time dependence suggesting that the phase change may not be the primary cause for catastrophic resurfacing on venus our calculations also show that venus cannot have a weak asthenosphere that is similar to that on the earth in order to match the observations thus supporting a key role of asthenosphere in producing plate tectonics

immense vent complex marks flood basalt eruption in a wet failed rift coombs hills antarctica large lava effusions can have impressive explosive antecedents although our picture of flood basalt is overwhelmingly effusive phreatomagmatic eruptions have preceded quiet effusion of some flood basalts and reflect the same influence of vent architecture and hydrology on eruptive style as seen for small volume eruptions the scale of phreatomagmatic deposits associated with flood basalts can be huge at coombs hills a vast but otherwise typical phreatomagmatic vent complex is exposed over more than 25 km 2 and its features are interpreted to reflect processes of tephra jet eruptions with diatreme development similar vent complexes are probably the source of laharic deposits reported elsewhere in the transantarctic mountains and in the karoo province of south africa

variations in planetary convection via the effect of climate on damage abstract a new model for the generation of plate tectonics suggests an important interaction between a planet s climate and its lithospheric damage behavior and thus provides a simple explanation for the tectonic difference between earth and venus we propose that high surface temperatures will lead to higher healing rates e g grain growth in the lithosphere that will act to suppress localization plate boundary formation and subduction this leads to episodic or stagnant lid convection on venus because of its hotter climate in contrast earth s cooler climate promotes damage and plate boundary formation the damage rheology presented in this paper attempts to describe the evolution of grain size by allowing for grain reduction via deformational work input and grain growth via surface tension driven coarsening we explore the interaction of damage and healing in two dimensional numerical convection simulations we also develop a simple drip instability model to test the hypothesis that the competition between damage and healing controls convective and plate tectonic style by modulating episodicity at subduction zones at small values of damage f a or large values of healing k a the lithosphere remains strong enough to resist subduction on time scales of billions of years at intermediate values of f a and k a the lithosphere may become mobilized and allow for short bursts of tectonic behavior followed by periods of quiescence at large small values of f a k a the fineness is increased so that the viscosity of the plate boundary is reduced to allow for continuous unimpeded subduction of lithosphere and plate like deformation the results suggest the feasibility of our proposed hypothesis that the interplay of climate and damage control the mode of tectonics on a planet

shallow seated controls on styles of explosive basaltic volcanism a case study from new zealand abstract the pyroclastic deposits of many basaltic volcanic centres show abrupt transitions between contrasting eruptive styles e g hawaiian versus strombolian or dry magmatic versus wet phreatomagmatic these transitions are controlled dominantly by variations in degassing patterns magma ascent rates and degrees of interaction with external water we use crater hill a 29 ka explosive effusive monogenetic centre in the auckland volcanic field new zealand as a case study of the transitions between these end member eruptive styles the crater hill eruption took place from at least 4 vents spaced along a nne trending 600 m long fissure that is contained entirely within a tuff ring generated during the earliest eruption phases early explosive phases at crater hill were characterised by eruption from multiple unstable and short lived vents later dominantly extrusive volcanism took place from a more stable point source most of the crater hill pyroclastic deposits were formed in 3 phreatomagmatic p and 4 dry magmatic m episodes forming in turn the outer tuff ring and maar crater p1 m1 p2 and scoria cone 1 m2 m4 this activity was followed by formation of a lava shield and scoria cone 2 purely wet activity is represented by the bulk of p1 and p2 and purely dry activity by much of m2 m4 however m1 and parts of m2 and m4 show evidence for simultaneous eruptions of differing style from adjacent vents and rapid variations in the extent and timing of magma water interaction at each vent the nature of the wall rock lithics and these rapid variations in inferred water magma ratios imply interaction was occurring mostly at depths of 80 m and the vesicularity patterns in juvenile clasts from these and the p beds imply that rapid degassing occurred at these shallow levels we suggest that abrupt transitions between eruptive styles in time and space at crater hill were linked to changes in the local magma supply rate and patterns and vigour of degassing during the final metres of ascent