mechanisms by which gulf of guinea and eastern north atlantic sea surface temperature anomalies can influence african rainfall abstract the sensitivity of precipitation over west africa to sea surface temperature anomalies sstas in the gulf of guinea and the eastern north atlantic is studied using a gcm results from nine perpetual july simulations with various imposed sstas are presented and analyzed to reveal associations between the precipitation and sst fields via large scale circulation and atmospheric moisture anomalies rainfall increases over the guinean coast and decreases over the congo basin when warm sstas are present in the gulf of guinea these precipitation perturbations are related to the forcing of a kelvin and a rossby wave the former is associated with a weakening of the walker circulation while the latter strengthens the west african monsoon rainfall over west africa is less sensitive to cold sstas than to warm anomalies three contributing factors are identified as follows 1 latitude of the sst forcing 2 background flow and 3 nonlinearity of the clausius clapeyron equation no more than a 20 effec

large topographic rises on venus implications for mantle upwelling topographic rises on venus have been identified that are interpreted to be the surface manifestation of mantle upwellings these features are classified into groups based on their dominant morphology atla and beta regiones are classified as rift dominated dione western eistla bell and imdr regiones as volcano dominated and themis eastern eistla and central eistla regiones as corona dominated at several topographic rises geologic indicators were identified that may provide evidence of uplifted topography e g volcanic flow features trending upslope we assessed the minimum contribution of volcanic construction to the topography of each rise which in general represents less than 5 of the volume of the rise similar to the volumes of edifices at terrestrial hotspot swells the total melt volume at each rise is approximated to be 104 106 km3 the variations in morphology topography and gravity signatures at topographic rises are not interpreted to indicate variations in stage of evolution of a mantle upwelling instead the morphologic variations between the three classes of topographic rises are interpreted to indicate the varying influences of lithospheric structure plume characteristics and regional tectonic environment within each class variations in topography gravity and amount of volcanism may be indicative of differing stages of evolution the similarity between swell and volcanic volumes for terrestrial and venusian hotspots implies comparable time integrated plume strengths for individual upwellings on the two planets

how does venus lose heat the tectonics and volcanism of the terrestrial planets are controlled by the loss of heat from the planetary interior on the earth about 70 of the heat flow through the mantle is attributed to the subduction of cold lithosphere in order to understand the tectonic and volcanic processes on venus it is necessary to understand how heat is transported through its mantle in this paper three alternative end member hypotheses are considered the first is the steady state loss of heat through the mantle to the surface in analogy to the earth however without plate tectonics and subduction on venus a steady state requires either a very high plume flux or very rapid rates of lithospheric delamination the required plume flux would be equivalent to about 80 plumes with the strength of the hawaiian plume the required delamination flux implies a 50 delamination of the entire venus lithosphere every 10 m y neither appears possible so that it is concluded that venus cannot transport heat through its mantle to its surface on a steady state basis the second hypothesis is that there has been a strong upward concentration of the heat producing elements into the crust of venus the heat generated is then lost by conduction surface measurements of the concentrations of the heat producing elements place constraints on this model if everything is favorable this hypothesis might be marginally acceptable but it is considered to be highly unlikely the third hypothesis is that heat is lost by episodic global subduction events followed by long periods of surface quiescence the near random distribution of craters suggests that the last subduction event occurred about 500 ma this model implies a thick thermal lithosphere 300 km at the present time which is consistent with a variety of surface observations lava lakes on the earth are considered as analogies to plate tectonics they also exhibit episodic subduction events

discrete element simulations of gravitational volcanic deformation 1 deformation structures and geometries 1 we have carried out two dimensional particle dynamics simulations of granular piles subject to frictional coulomb failure criteria to gain a first order understanding of different modes of gravitational deformation within volcanoes under uniform basal and internal strength conditions granular piles grow self similarly developing distinctive stratigraphies morphologies and structures piles constructed upon cohesive substrates exhibit particle avalanching forming outward dipping strata and angle of repose slopes systematic decreases in basal strength lead to progressively deeper and steeper internal detachment faults and slip along a basal decollement landslide forms grade from shallow slumps to deep seated landslide and finally to axial subsidence and outward flank displacements or volcanic spreading surface slopes decrease and develop concave up morphologies with decreasing decollement strength depositional layers tilt progressively inward spatial variations in basal strength cause lateral transitions in pile structure stratigraphy and morphology this approximation of volcanoes as coulomb granular piles reproduces the richness of deformational structures and surface morphologies in many volcanic settings the gentle slopes of hawaiian volcanoes and olympus mons on mars suggest weak basal decollements that enable volcanic spreading high angle normal faults favored above weak decollements are interpreted in both settings and may explain catastrophic sector collapse in hawaii and broad aureole deposits surrounding olympus mons in contrast steeper slopes and shallow detachment faults predominate in the canary islands thought to lack a weak decollement favoring smaller more frequent slope failures than predicted for hawaii the numerical results provide a useful predictive tool for interpreting dynamic behavior and associated geologic hazards of active volcanoes

monitoring groundwater storage changes in complex basement aquifers an evaluation of the grace satellites over east africa although the use of the gravity recovery and climate experiment grace satellites to monitor groundwater storage changes has become commonplace our evaluation suggests that careful processing of the grace data is necessary to extract a representative signal especially in regions with significant surface water storage i e lakes reservoirs in our study we use cautiously processed datasets including grace lake altimetry and model soil moisture to reduce scaling factor bias and compare grace derived groundwater storage changes to in situ groundwater observations over parts of east africa over the period 2007 2010 a strong correlation between in situ groundwater storage change and grace groundwater estimates spearman s 0 6 is found piecewise trend analyses for the grace groundwater estimates reveal significant negative storage changes that are attributed to groundwater use and climate variability further analysis comparing groundwater and satellite precipitation datasets permits identification of regional groundwater characterization for example our results identify potentially permeable and or shallow groundwater systems underlying tanzania and deep and or less permeable groundwater systems underlying the upper nile basin regional groundwater behaviors in the semi arid regions of northern kenya are attributed to hydraulic connections to recharge zones outside the sub basin boundary our results prove the utility of applying grace in monitoring groundwater resources in hydrologically complex regions that are under sampled and where policies limit data accessibility this article is protected by copyright all rights reserved

seismic stratigraphy of the hawaiian flexural moat approximately 4 400 km of single channel seismic reflection data were collected over the northern hawaiian flexural moat between hawaii and kauai these data include eight cross moat profiles and three 600 km long moat parallel lines the thick 2 km sedimentary section filling the hawaiian flexural moat is composed primarily of the products of large scale mass wasting from massive slope failures on the adjacent hawaiian islands the observed stratigraphy can be described by four main lithostratigraphic units 1 a basal unit of relatively constant thickness that we interpret as pelagic sediment predating the formation of the flexural moat 2 a thick wedge of lens shaped units onlapping the flexural arch each with highly chaotic internal reflectivity that we interpret as buried landslide deposits 3 a sequence of highly reflective continuous horizons that offlap the flexural arch and are tilted down toward the islands and 4 a ponded unit confined to the deepest part of the moat representing the youngest sediments transported to the moat this distinctive stratigraphy onlap of the flexural arch in the lower moat section landslide unit offlap migrating back toward the islands in the upper moat section offlapping unit and ponding of the youngest sediments in the deepest parts of the moat ponded unit can be explained in terms of the competing effects of sediment influx to the moat from large scale mass wasting and distributed subsidence due to the progressive loading of a purely elastic plate by each successive volcano the role of large scale mass wasting in the sedimentation history of the hawaiian flexural moat is expected to characterize the moats of other large subaerial oceanic volcanoes

extratropical impacts on atlantic tropical cyclone activity abstractwith warm sea surface temperature sst anomalies in the tropical atlantic and cold sst anomalies in the east pacific the unusually quiet hurricane season in 2013 was a surprise to the hurricane community the authors analyses suggest that the substantially suppressed atlantic tropical cyclone tc activity in august 2013 can be attributed to frequent breaking of midlatitude rossby waves which led to the equatorward intrusion of cold and dry extratropical air the resultant mid to upper tropospheric dryness and strong vertical wind shear hindered tc development using the empirical orthogonal function analysis the active rossby wave breaking in august 2013 was found to be associated with a recurrent mode of the midlatitude jet stream over the north atlantic which represents the variability of the intensity and zonal extent of the jet this mode is significantly correlated with atlantic hurricane frequency the correlation coefficient is comparable to the correlation of atlantic hurricane fre

evolution of the relationship between near global and atlantic sst modes and the rainy season in west africa statistical analyses and sensitivity experiments monthly sea surface temperature anomalies ssta at near global scale 60 n 40 s and may to october rainfall amounts in west africa 16 n 5 n 16 w 16 e are first used to investigate the seasonal and interannual evolutions of their relationship it is shown that west african rainfall variability is associated with two types of oceanic changes 1 a large scale evolution involving the two largest ssta leading eigenmodes 16 of the total variance with stronger loadings in the equatorial and southern oceans related to the long term multiannual component of rainfall variability mainly expressed in the sudan sahel region and 2 a regional and seasonally coupled evolution of the meridional thermal gradient in the tropical atlantic due to the linear combination of the two largest ssta modes in the atlantic 11 with strong inverse loadings over the northern and southern tropics which is associated with the interannual and quasi decadal components of regional rainfall in west africa linear regression and discriminant analyses provide evidence that the main july september rainfall anomalies in sudan sahel can be detected with rather good skills using the leading april june or synchronous july september values of the four main oceanic modes in particular the driest conditions over sahel more marked since the beginning of the 1970s are specifically linked to the warm phases of the two global modes and to cold warm anomalies in the northern southern tropical atlantic idealized but realistic ssta patterns obtained from some basic linear combinations of the four main oceanic modes appear sufficient to generate quickly from mid july to the end of august significant west african rainfall anomalies in model experiments consistent with the statistical results the recent negative impact on west african rainfall exerted by the global oceanic forcing is primarily due to the generation of subsidence anomalies in the mid troposphere over west africa when an idealized north to south ssta gradient is added in the tropical atlantic strong north to south height gradients in the middle levels appear these limit the northward excursion of the rainbelt in west africa the sahelian area experiences drier conditions due to the additive effect subsidence anomalies latitudinal blocking while over the guinea regions wet conditions do not significantly increase since the subsidence anomalies and the blocking effect act here in opposite ways

magma ascent pathways associated with large mountains on io abstract while jupiter s moon io is the most volcanically active body in the solar system the largest mountains seen on io are created by tectonic forces rather than volcanic construction pervasive compression primarily brought about by subsidence induced by sustained volcanic resurfacing creates the mountains but at the same time inhibits magma ascent in vertical conduits dikes we superpose stress solutions for subsidence along with thermal stress both from the crustal conveyor belt process of resurfacing in io s lithosphere with stresses from io mountain sized loads in a shallow spherical shell solution in order to evaluate magma ascent pathways we use stress orientation least compressive stress horizontal and stress gradient compression decreasing upwards criteria to identify ascent pathways through the lithosphere there are several configurations for which viable ascent paths transit nearly the entire lithosphere arriving at the base of the mountain where magma can be transported through thrust faults or perhaps thermally eroded flank sections the latter is consistent with observations of some io paterae in close contact with mountains

the influence of the regional hadley and walker circulations on precipitation patterns over africa in el nino la nina and neutral years this study focuses on the differential impacts of the positive el nino negative la nina and neutral phases of the el nino southern oscillation enso on precipitation over africa during djf and jja evaluated through changes in the regional hadley and walker circulations identification of the hadley and walker cells was done using stream function mass transport calculations of era interim reanalysis data from 1979 to 2014 analysis of the spatial pattern of precipitation anomalies shows that during djf el nino la nina negatively positively impacts precipitation over the african continent during jja el nino la nina influences precipitation variability over the sahel region producing positive negative anomalies negative precipitation anomalies associated with el nino djf over southern africa are linked to a strengthening in subsidence of the descending branch of the regional hadley cell and during jja the negative precipitation anomalies over the sahel are associated with a weakening of the ascending branch of the regional hadley cell during la nina events in djf there is a tendency toward increased convection in southern africa associated with a stronger ascending branch and weaker descending branch of the regional hadley cell during la nina events in jja positive precipitation anomalies over the sahel are associated with an intensification of the ascending branch of the regional hadley cell north of the equator

flow field stratigraphy surrounding sekmet mons volcano kawelu planitia venus i detailed mapping has revealed several stratigraphic components among lobate plains in the vicinity of sekmet mons volcano 44 5 n lat 240 5 long located on the northern lowland plains of kawelu planitia on venus volcanic effusion events produced discrete lobate plains throughout the area superposition of flow margins between adjacent lobate plains components indicate that the effusive activity generally progressed from southwest to northeast across the sekmet mons area leading to a cumulative total of 1 5 million km 2 covered by adjacent lobate plains components source areas for the effusion responsible for the lobate plains components occur primarily along fracture zones or from concentrations of low volcanic domes shield fields instead of from single constructs magellan radar backscatter values from a mixture of both radar bright and radar dark flow components within the flow fields are well below values typical of clinkery a a lava flows on earth but they are consistent with values from terrestrial pahoehoe flows detailed mapping of strenia fluctus centered on 41 5 n lat 251 0 long one of the latest lobate plains units in the area does not show a systematic trend among effusive centers that contributed to the generation of this flow field instead the strenia fluctus flow field is a complex mixture of flows that emanated from a shield field along mist chasma and that flowed down a very gentle 0 1 regional slope to the east a 4 km diameter cone north of strenia fluctus was the source of a flow complex traceable for more than 200 km over a slope of only 0 03 individual flows within strenia fluctus are composed of intermixed lobes similar to relationships observed on the distal portion of the 75 km long carrizozo basalt flow in new mexico which also displays abundant inflation features and a predominant pahoehoe surface texture if the venusian lobate plains consist of pahoehoe flows comparable to the carrizozo flow they were most likely emplaced at modest effusion rates 50 to 500 m 3 s and even with inflation the flows are likely 15 m in thickness under these conditions all of the lobate plains surrounding sekmet mons could have been emplaced in 14 000 to 1400 earth years assuming continuous effusion with only one source vent active at a time at similar rates all of the lowland plains in the northern hemisphere of venus could have been resurfaced by 15 m thick flows through continuous single vent effusion in 1 4 million to 140 thousand earth years

impact craters and venus resurfacing history venusian impact crater size frequency distributions locations and preservation states were analyzed to reconstruct the history of resurfacing by tectonism and volcanism an atmospheric transit model for meteoroids demonstrates that for craters larger than about 30 km the size frequency distribution is close to the atmosphere free case with this result and assuming that the surface records a crater production population a catastrophic resurfacing model crm an age of cessation of rapid resurfacing of 500 ma is obtained crater locations are widely dispersed across venus and the hypothesis that they are completely spatially random csr cannot be rejected however craters that show embayment by plains materials or modification by throughgoing faults i e tectonized are preferentially found in areas with relatively few craters overall the primary region where these modified craters are found is the aphrodite volcanotectonic zone extending from ovda regio on the west to the region east of atla regio these results together with the appearance of plains material on most crater floors and evidence for complex volcanic stratigraphy imply that a range of surface ages are recorded by the impact crater population e g the aphrodite zone is relatively young an end member model equilibrium resurfacing model erm was developed to quantify resurfacing scenarios in the erm venus has been resurfacing at an average rate of approximately 1 km2 yr 1 however the crm and erm are idealized end member representations of possible resurfacing histories for both models the resurfacing rate can be expressed as the product of resurfacing patch area a normalized by planetary surface area and the frequency of resurfacing events numerical simulations of resurfacing showed that there are two solution branches that satisfy the csr constraint a 0 1 74 diameter circle the former range corresponds to resurfacing diameters smaller than the average intercrater distance whereas the latter is associated with large infrequent events resurfacing 10 of the planet every 50 ma to 100 every 500 ma the observed fraction of embayed and tectonized craters further constrains values of a and only values near 0 0003 are admissible the resurfacing model that best fits all of the statistical and geological constraints has resurfacing with small patches that occurs in any given geological episode in only a limited number of regions on the planet

surface modification of venus as inferred from magellan observations of plains in sedna planitia clear stratigraphic relations can be discerned among volcanic flow units young flows exhibit sar specific cross section values similar to fresh terrestrial basalt flows whereas older flows exhibit backscatter signatures similar to degraded terrestrial basalt flows total degradation of 1 m depth over 0 6 b y is inferred for the sedna area from radar signatures impact crater abundances and ejecta superposition relations with respect to volcanic flow units analyses of parabolic ejecta deposits associated with the crater stuart imply that the material is typically centimeters in thickness a relatively small fraction 10 of venusian impact craters exhibit prominent parabolic ejecta deposits these craters are interpreted to be relatively young and parabolic deposits are interpreted to be dispersed by aeolian activity over at least tens of millions of years the inferred dispersal rate 10 3 m yr is too low to produce the degradation of flows at sedna planitia and it is concluded that the dominant flow modification process is in situ weathering in addition elevation dependent weathering is inferred in western ovda regio where plains above 6054 km radius have enhanced reflection coefficients as compared to adjacent plains at lower elevations the inferred rate of generation of high reflection coefficient materials is no more than 10 2 m yr based on the inability of aeolian activity to cover high reflectivity surfaces with normal reflection coefficient materials and the ubiquitous nature of high reflectivity surfaces at high elevations surface modification rates on venus are orders of magnitude lower than on earth venusian rates are also much lower than the inferred rate of aeolian dispersal of friable materials on mars but are comparable to the estimated rate of weathering and erosion of martian bedrock low surface modification rates imply that it will be possible to determine regional scale age variations on venus based on the degree of preservation of volcanic landforms and microwave signatures

new constraints on volcano tectonic evolution of large volcanic edifices on venus from stereo topography derived strain estimates radial fracture systems at large volcanoes on venus offer opportunities to unravel the tectonic and magmatic evolution of such edifices however the relatively low spatial resolution 10 20 km of altimetry data for venus has hampered previous efforts at quantitative analysis of fault systems recently released stereo derived topography enables kilometer scale strain estimates to be made at tectonic features the new data resolve fault valleys grabens that cut the summit regions of two large volcanoes showing them to be deeper than previously assumed the grabens formed after most edifice building was complete we demonstrate that inflation of oblate sill like magma chambers is consistent with the magnitudes and distributions of strain at these volcanoes and estimate chamber depth radius and overpressure

importance of the indian ocean for simulating rainfall anomalies over eastern and southern africa the relative contributions of the indian ocean and pacific ocean sea surface temperatures ssts to the rainfall variability over eastern central and southern africa during the austral spring summer are examined the variability of african rainfall is statistically related to both oceans but the variability in the two oceans is also related to separate the effects of the indian and pacific oceans a suite of numerical model simulations is presented goga the atmosphere is forced by observed ssts globally ioga the atmosphere is forced by observed ssts only in the indian ocean basin and poga the atmosphere is forced by observed ssts only in the tropical pacific basin while the sst variability of the tropical pacific exerts some influence over the african region it is the atmospheric response to the indian ocean variability that is essential for simulating the correct rainfall response over eastern central and southern africa analyses of the dynamical response s seen in the numerical experiments and in the observations indicate that the pacific and indian oceans have a competing influence over the indian ocean african region this competition is related to the influence of the two oceans on the walker circulation and the consequences of that variability on low level fluxes of moisture over central and southern africa finally given the high correlation found between sst variability in the indian and pacific oceans with the pacific leading by 3 months we speculate on an approach to long lead dynamical climate prediction over central east and southern africa

bell regio venus integration of remote sensing data and terrestrial analogs for geologic analysis the geology and surface morphology of bell regio 18 42 n 32 58 e are investigated using a combination of magellan venera and analogous terrestrial data the properties of surface units are compared to either direct terrestrial analog measurements or to the behaviors predicted by theoretical models five major volcanic sources are identified from geologic mapping tepev mons nefertiti corona a large shield volcano east of tepev and two small edifices southeast of tepev the volcano api mons lies northeast of the main bell uplift the oldest volcanic units are associated with an extensive low shield volcano east of tepev mons and a small edifice southeast of tepev the annular flow apron of tepev mons formed next with volcanism at a second small edifice on the southeast flank of tepev mons producing the youngest flow units comparisons between magellan data terrrestrial radar images and field topography profiles suggest that only three units resemble terrestrial a a flows the remainder are consistent with smoother pahoehoe type surfaces this suggests that most of the flow units were erupted at relatively low volume effusion rates 100 m3 s over long periods of time or had very low viscosities one flow unit has a knobby texture which resembles large tumuli or ridge structures much of the tectonic deformation in the area is in the form of fractures circumferential to the edifices and isolated blocks of tessera terrain there are no rift zones such as those which occur at beta regio atla regio or western eistla regio tepev mons is characterized by very steep slopes up to 40 along the east flank a relatively flat summit and two large 11 and 31 km calderas though covered in places by high dielectric material the tepev mons summit area is relatively smooth with the exception of annular rough deposits which ring the two calderas these calderas are quite shallow on the order of a few hundred meters at most with no discernible difference in radius between individual altimeter footprints inside and outside the larger eastern feature magellan and venera data suggest that the eastern caldera is covered by a layer of unconsolidated material ejecta from miriam crater has produced a halo with a dielectric constant of 6 7 surrounding nefertiti corona and potanina crater ejecta is the likely source material for radar dark wind streaks south of tepev mons a low dielectric e 2 3 triangular region on the south flank of tepev mons may be a pyroclastic or crater ejecta deposit which has been spread westward by the wind this region has undergone a wide variety of volcanic processes with a major shift in eruptive style from the older low relief eastern volcanic center to the steep slopes and large summit calderas characteristic of tepev mons and the two smaller edifices

central eistla regio origin and relative age of topographic rise large topographic rises on venus pose a number of fundamental questions concerning the mantle processes responsible for them eistla regio a 7500 km long elevated region extending wnw from the west end of aphrodite terra is a particularly interesting example the sappho patera quadrangle 0 25 n 0 30 e includes all of central eistla regio a region dominated by two interesting apparently hybrid structures irnini and anala montes with characteristics of both shield volcanoes and coronae mapping and tectonic analysis within this region indicate that this part of the rise experienced progressive deformation extending from prior to emplacement of the regional plains to after emplacement of young lavas that is for most of the stratigraphically defined history of venus estimates of the relative roles of uplift and volcanic construction in the evolution of irnini and anala montes are critically dependent on whether they are primarily anomalous coronae that have been uplifted or whether they are true shield volcanoes that have evolved into coronae studies elsewhere as well as most models of corona evolution favor the first alternative the geology of central eistla regio favors the second suggesting that local thinning of the lithosphere allowed shield volcanoes to evolve into coronae because these hybrid edifices are not typical of either most coronae or most shield volcanoes on venus they deserve detailed modeling for now the relative roles of uplift and volcanic construction in central eistla regio cannot be even approximately estimated

atmospheric hydrospheric mechanisms of climate anomalies in the western equatorial indian ocean atmosphere ocean mechanisms of rainfall anomalies at the coast of eastern africa are studied using long term ship observations in the indian ocean surface current measurements subsurface casts upper air analyses by the european centre for medium range weather forecasts rain gauge series in eastern africa and india and an index of the southern oscillation so the high so phase being defined by anomalously high low pressure at tahiti darwin the causalities of precipitation anomalies at the coast of eastern africa differ for the two rainy seasons centered on april may and october november and only the latter is strongly related to the so in the high so phase april may pressure is low over the entire indian ocean domain whereas in october november pressure is high in the west and low in the east concomitantly surface waters are anomalously cold in the west and strong westerlies sweep the equatorial zone of the indian ocean eastern african rainfall anomalies are related to the so through a combination of cooperative mechanisms that function most effectively in the boreal autumn rainy season of eastern africa 1 equatorial westerly winds are conducive to lower tropospheric divergence over equatorial east africa and in the high so phase these are accelerated especially in october november owing to the anomalous eastward pressure gradient 2 the equatorial westerly winds drive the eastward equatorial jet in the upper hydrosphere which entails cold water upwelling in the western extremity of the basin where sea surface temperature further hydrostatically affects the zonal pressure gradient and thus feeds back into the equatorial westerly winds 3 in addition cold water anomalies in the western indian ocean most pronounced in october november during the high so phase also suppress convection 4 in the high so phase the indian summer monsoon tends to be strong leaving behind an anomalously cold western indian ocean which in turn feeds into mechanisms 1 to 3 the eastward equatorial jet thus has a role to play in feedback mechanisms contributing to the anomalies of the boreal autumn rains at the coast of eastern africa

regional topographic rises on venus geology of western eistla regio and comparison to beta regio and atla regio regional topographic rises in the equatorial region of venus western eistla regio beta regio and atla regio form a unique class of structures which on the basis of geologic mapping and geophysical data are interpreted to be sites of mantle upwelling or plumes the geologically least complex of these regions is western eistla regio an elongated domical rise that contains the volcanoes sif and gula montes sif mons is a large volcano diameter 100 km whose local stratigraphy indicates that it has undergone a history marked by initial radial fracturing widespread outpouring of lava additional fracturing along its northern flank and extrusion of lava that is either extremely fluid or was erupted at a high rate of effusion sif contains a 40 km diameter summit caldera within which are nested smaller calderas and pit structures theoretical considerations suggest multiple levels of magma emplacement within the volcano with the primary reservoir residing in the crust below the edifice gula mons is made up of multiple volcanic source regions and is associated with guor linea a zone of lithospheric stretching faulting and rifting the geologically complex highland of beta regio is a tectonic junction with three arms of devana chasma intersecting at theia mons magellan data reveal 1 the details of rifting and volcanism concentrated at theia mons 2 that rhea mons is highly deformed and possesses characteristics of tesserae and whose origin as a volcano is placed in question 3 that tesserae are a major unit arrayed in a pattern around theia from east to west starting on the eastern flank of beta and continuing westward to the northern part of asteria regio 4 that the northern periphery of the highland contains coronae connected by fractures forming chains and 5 the presence of a deformed crater within devana chasma split and extended in an east west direction indicating that part of the rift has undergone as much as 20 30 extension like beta atla regio is a complex area of converging rifts five separate arms that are centered on the volcano ozza mons the magellan images show that volcanism and rifting have occurred contemporaneously at ozza with lava flows both covering and being crosscut by faults a second volcano maat mons contributes to the infilling of the rift dali chasma and does not appear to be deformed by faulting two of the rifts at atla contain chains of coronae suggesting that upwelling along linear zones of extension has occurred beta atla and western eistla are similar in that they are broad regional rises are associated with major rift zones have large apparent depths of isostatic compensation and contain large volcanoes their associations with features mapped as coronae and tesserae are quite variable suggesting that the detailed characteristics of the individual highlands are most likely linked to each areas local geologic environment

extension and uplift at alba patera mars insights from mola observations and loading models high resolution mars orbiter laser altimeter mola profiles of the large shield volcano alba patera call for a critical reexamination of models for the growth and evolution of this volcanic edifice an extensive system of graben cuts across alba patera extending from ceraunius fossae to the south to tantalus fossae to the northeast in the vicinity of the edifice the otherwise generally north south orientations of these graben are deflected to circumferential directions mola topography reveals that the circumferential graben are located well up the flanks of alba patera both the type extensional and location midflank of faulting on alba patera are inconsistent with the state of stress predicted for purely surface loaded flexure models to constrain the conditions governing the evolution of alba patera we employ finite element models for the volcano and lithospheric stress field under a combination of loading mechanisms buoyant sublithospheric loads representing underplated magma low density mantle residuum or dynamic support from mantle convection and intralithospheric displacements representing sills trapped by horizontal compressive stresses in the upper lithosphere can generate the observed midflank slope breaks and circumferential extensional fault zones at radial distances comparable to that of the subsurface load s characteristic radial extent both mechanisms moderate stresses in the upper lithosphere in favor of continued magma ascent however bottom loading requires a disk like load geometry and an elastic lithosphere thickness te sufficiently small to allow the load s shape to be apparent in the surface deformation te 32 km such low te is inconsistent with mars global surveyor mgs gravity and topography data in contrast intralithospheric sill complexes can produce the observed topography and faulting at te values consistent with those inferred from gravity and topography 50 te 100 km for these conditions principal stress orientations on the lower flanks are also consistent with the orientations of lower flank graben provided that faulting was induced by dike intrusion or tensile failure rather than shear failure

mylitta fluctus venus rift related centralized volcanism and the emplacement of large volume flow units mylitta fluctus is a volcanic flow field that covers approximately 300 000 km2 in southern lavinia planitia the flows are typically radar bright with uniform surface textures central channels are common maximum flow lengths range from 400 to 1000 km flow widths range 30 100 km in the medial and distal portions of the flow field the total volume of the flow field based on estimates of flow thickness is of the order of 2 104 km3 the flow field is composed of six smaller flow fields that are interpreted to represent major eruption events in the evolution of mylitta an asymmetric shield volcano 200 km in radius with a central caldera has been identified as the single major source it is located along a possible rift zone at the northern edge of lada terra no evidence for fissure fed eruptions is observed although eruptions may have occurred along fissures within the rift before the main vent centralized mylitta is similar in scale to many terrestrial flood basalt provinces although it is lower in total estimated volume and is distinguished by the presence of a central source edifice the origin of mylitta is proposed to be linked to regional extension and possible hotspot activity in a manner similar to that suggested for the origin of terrestrial flood basalts the lower apparent volume of mylitta relative to terrestrial flood basalts suggests that the amount of material upwelling from the mantle may have been lower in this case or that the formation of the flow field has not yet finished detailed studies of the morphology and distribution of flow fields similar to mylitta should yield insight into the variation and evolution of hotspot related volcanism and the formation of possible flood basalts on venus

hotspot evolution and venusian tectonic style because hotspots represent an important manifestation of heat loss on venus their geological evolution is of fundamental importance for any attempt to understand venusian tectonics eistla regio is a 7500 km long moderately elevated region inferred to overlie one or more large mantle upwellings or hotspots it also contains many shield volcanoes and coronae believed due to the rise of thermal plumes in the mantle central eistla regio includes two large volcanoes sappho and anala and several coronae in close proximity detailed mapping in this region results in two conclusions of tectonic significance 1 sappho and anala occur near the intersection of two major extensional deformation zones and 2 the coronae are older than the large volcanoes several of the coronae occur as a chain along guor linea one of the major extensional deformation zones stratigraphic relationships indicate that the coronae began forming very soon after the emplacement of the widespread regional plains materials thus central eistla regio was the site of a swarm of plumes that first formed coronae and then later formed shield volcanoes the expected result of such a swarm would be thermal thinning of the elastic lithosphere with time however model results geological observations and gravity data suggest that the change from coronae to shield volcanoes was accompanied by a thickening of the lithosphere with time this thickening is interpreted to be the result of global cooling of the lithosphere following the most recent episode of near global resurfacing the global cooling must have occurred faster than local heating of the lithosphere due to the impingement of thermal plumes

characteristics of summer stationary waves in the northern hemisphere abstract summer stationary waves in the northern hemisphere are separated by a midlatitude transition zone into the subtropical monsoon regime with a vertical phase reversal and the subarctic regime with a vertically uniform structure the dynamics and maintenance mechanism of the subtropical stationary waves have been investigated in the context of monsoon circulation depicted in terms of streamfunction with 40 yr ecmwf re analysis era 40 the dynamic characteristics of stationary waves in the transition zone and the subarctic region are thus the focus of this study the dynamics and maintenance mechanism of these waves were explored with the streamfunction budget and the velocity potential maintenance equations stationary waves across the transition region consist of anticyclonic shear zones over the north pacific and north atlantic and a cyclonic shear zone in east eurasia these transition elements are linked to subtropical oceanic anticyclones and continental thermal lows at high latitudes a th

zonal circulations over the equatorial indian ocean abstract the ncep ncar 1958 97 upper air dataset and surface observations have been analyzed for evidence of zonal vertical circulations along the indian ocean equator and their role in climatic variability the long term mean upper tropospheric circulation is dominated in boreal winter by divergent outflow from the southern indian ocean northwestward into southern asia and in summer from southern asia southwestward into the southern hemisphere in boreal autumn only divergent easterlies blow from indonesia along the equator into an upper tropospheric convergence band over east africa and only then a closed zonal vertical circulation cell materializes along the indian ocean equator between the centers of ascending motion over indonesia and of subsidence over equatorial east africa and featuring westerlies in the lower layers the boreal autumn zonal vertical circulation varies interannually a regime of intense circulation features accelerated equatorial surface westerlies enhanced subsidence and d

filling of flexural moats around large volcanoes on venus implications for volcano structure and global magmatic flux the absence of topographic moats and concentric normal faulting around large volcanoes on venus is attributed to filling of the annular flexural depression by lava flows from the central edifice large volcanoes on venus are characterized by prominent approximately conical edifices surrounded by relatively flat flow aprons the surfaces of both of these constructional components consist dominantly of radially oriented flows from analytic plate flexure models we generate a synthetic stratigraphy for venus volcanoes from which we calculate the volume of material filling the flexural moats to the level of the flow apron the total volume of volcano associated extrusive lavas including moat fill can be an order of magnitude greater than the volume of the edifice alone extended to all large volcanoes on venus this procedure yields estimates for magmatic flux on venus comparable to the present terrestrial intraplate extrusive flux but this scenario is nonetheless consistent with observations of areally limited resurfacing since the most recent global resurfacing event the absence of flexure related tectonic features around most large volcanoes by the arguments advanced here is attributable to masking by apron flows and low stresses in the apron moat fill on venus a volcanic edifice and associated moat filling material constitute a single structurally coherent unit large volcanoes on venus thus appear to be structurally distinct from those on earth and mars

interaction of the atlantic equatorial cold tongue and the african monsoon the seasonal cycle of equatorial atlantic sea surface temperature sst is characterized by a rapid cooling from april to july coinciding with the onset of the west african summer monsoon and followed by a slow warming that lasts 3 times longer two ensemble simulations are carried out with an atmospheric general circulation model to investigate the mechanisms for the wind changes that cause this rapid oceanic cooling and its feedback onto the african monsoon in the control simulation sst is globally prescribed in its full climatological seasonal cycle while in the second simulation equatorial atlantic sst is held constant in time from 15 april onward comparison of these simulations indicates that the equatorial cooling exerts a significant influence on the african monsoon intensifying the southerly winds in the gulf of guinea and pushing the continental rainband inland away from the guinean coast the intensification of the cross equatorial southerlies associated with the onset of the african monsoon in turn triggers the oceanic cooling in the east the equatorial easterlies are also important for the seasonal cooling by inducing local upwelling and raising the thermocline in the east three mechanisms are identified for the easterly wind acceleration in the equatorial atlantic in boreal summer first the monsoon rainfall distribution is such that it induces zonal sea level pressure gradients and easterly anomalies in the eastern atlantic second the strong cross equatorial southerlies advect the easterly momentum from the south into the equator finally zonal pressure gradients associated with the equatorial ocean cooling accelerate surface easterly winds in the middle and western atlantic this interaction of equatorial sst and zonal wind causes their westward copropagation analogous to that in the equatorial pacific

a physical basis for the interannual variability of rainfall in the sahel a major factor in rainfall variability over sahelian west africa is the latitudinal location of the tropical rainbelt when it is displaced abnormally far northward the sahel experiences a wet year an anomalous southward displacement results in drought in this paper we examine the question of what controls the location during the boreal summer hypothesizing that inertial instability plays a role an analysis of surface pressure and temperature fields wind fields divergence and vertical motion show that the criteria for inertial instability are satisfied in wet augusts but not in dry ones the key determinant appears to be the surface pressure gradient between the continent and the equatorial atlantic when this is large inertial instability results in the development of a low level westerly jet the presence of this jet enhances the horizontal and vertical shear and displaces the african easterly jet northwestward associated with this situation is strong vertical motion over the sahel and subsidence over the guinea coast producing dry conditions over the latter the result is a rainfall dipole one of two major modes of variability over west africa important factors include sea surface temperatures ssts in the equatorial atlantic and pressure in the south atlantic the first of these factors suggests a link with the atlantic nino mode of tropical atlantic variability while the second suggests a possible link with the pacific and the extratropical south atlantic overall our study relates the well known sst influence on sahel rainfall to atmospheric dynamics over the continent copyright 2007 royal meteorological society

localization of gravity and topography constraints on the tectonics and mantle dynamics of venus summary we develop a method for spatio spectral localization of harmonic data on a sphere and use it to interpret recent high resolution global estimates of the gravity and topography of venus in the context of geodynamical models our approach applies equally to the simple spatial windowing of harmonic data and to variable length scale analyses which are analogous to a wavelet transform in the cartesian domain using the variable length scale approach we calculate the localized rms amplitudes of gravity and topography as well as the spectral admittance between the two fields as functions of position and wavelength the observed admittances over 10 per cent of the surface of venus highland plateaus and tessera regions are consistent with isostatic compensation of topography by variations in crustal thickness while admittances over the remaining 90 per cent of the surface rises plains and lowlands indicate that longwavelength topography is dominantly the result of vertical convective tractions at the base of the lithosphere the global average crustal thickness is less than 30 km but can reach values as large as 40 km beneath tesserae and highland plateaus we also note that an earth like radial viscosity structure cannot be rejected by the gravity and topography data and that without a mechanical model of the lithosphere admittance values cannot constrain the thickness of the thermal boundary layer of venus modelling the lithosphere as a thin elastic plate indicates that at the time of formation of relief in highland plateaus and tesserae the effective elastic plate thickness k was less than 20 km estimates of t at highland rises are consistently less than 30 km our inability to find regions with t 30 km is inconsistent with predictions made by a class of catastrophic resurfacing models

aridity over a semiarid zone in northern china and responses to the east asian summer monsoon in this study the variability of the aridity over a semiarid zone in northern china from 1961 to 2013 and its association with the northernmost margin of the east asian summer monsoon easm and its intensity were investigated using composite analysis correlative analysis and unrotated and rotated empirical orthogonal function analyses the results indicate that a nonsignificant drying trend 0 03 10 years occurs over the entire eastern semiarid zone accompanied by a southward retreat in the northernmost margin of the easm 0 24 degrees 10 years and that drying 0 11 10 years and southward retreat 0 28 degrees 10 years are most obvious on the loess plateau the response of the annual variation of aridity to changes in the northernmost margin of the easm is also compared with its responses to changes in the intensity of the easm the results indicate that it is more related to the former especially over the loess plateau furthermore an abnormal aridity event is likely to occur over the loess plateau in a year when the easm margin advances to its northernmost latitude abnormally ahead of time and retreats abnormally ahead of time compared to the mean state

regional fracture patterns around volcanoes possible evidence for volcanic spreading on venus abstract magellan data show that the surface of venus is dominated by volcanic landforms including large flow fields and a wide range of volcanic edifices that occur in different magmatic and tectonic environments this study presents the results from a comprehensive survey of volcano rift interaction in the bat region and its surroundings we carried out structural mapping of examples where interaction between volcanoes and regional fractures results in a deflection of the fractures around the volcanic features and discuss the nature of the local volcano related stress fields that might be responsible for the observed variations of the regional fracture systems we propose that the deflection of the regional fractures around these venusian volcanoes might be related to volcanic spreading a process recognized as of great importance in the tectonic evolution of volcanoes on earth and mars but not previously described on venus

seismic stratigraphy of the flexural moat flanking the hawaiian islands the elastic thickness of oceanic lithosphere based on flexure studies in the region of surface loads is two to three times smaller than its seismic thickness1 thus as a load ages the thickness of the mechanically supportive part of the lithosphere must decrease from its short term seismic thickness to its long term mechanical thickness extrapolation of data from experimental rock mechanics suggests that thinning occurs by brittle failure in the uppermost part of the lithosphere and thermally activated creep in its lower part2 modelling shows that for load ages less than about 1 myr flexural moats change shape and the peripheral bulges migrate in towards the load3 these transient changes in flexure are best manifest we believe in the stratigraphical sequences that infill flexural moats the largest thicknesses of sediments occur in moats flanking orogenic belts4 5 however the stratigraphical record in these basins is complicated by the effects of compaction sub aerial erosion and sea level changes and by the fact that the loading history extends over very long periods of time here we describe a multichannel seismic reflection profile across a flexural moat flanking the volcanic load of oahu in the hawaiian islands the advantages of oahu are that it is located in the interior of a plate the load is 2 7 myr old and formed within 1 myr6 and the flexural moat was not significantly affected by sea level changes and erosion we show that the observed stratigraphical patterns cannot be explained by changes in material influx load distribution and rapid relaxation of the lithosphere to its long term mechanical thickness rather the observed patterns suggest a prolonged relaxation 1 myr due to either local or regional re heating of the lithosphere this re heating has important implications for the relationship between elastic thickness and age of the lithosphere at the time of loadings

formation and evolution of volcanic edifices on the dione regio rise venus mapping of dione regio venus a regional highland 1200 by 2700 km in size with mean elevation of 6052 km located in the southern hemisphere to the nw of lavinia planitia reveals a region characterized by early tectonically dominated structure and later widespread volcanism and centralized edifice construction small patches of complexly deformed terrain tessera and larger regions of plains characterized by a single direction of deformation tectonized plains preserved as inliers are embayed by more recent widespread volcanic plains smaller scale plains resurfacing superposed these plains following a pervasive compressional event that lead to distributed wrinkle ridge formation four major volcanoes dominate the topographically irregular highland each with similar early stage radar dark lava flow deposits but significantly different late stage eruptions these late stage differences are manifested as dark diffuse deposits possibly pyroclastic in origin and construction of a single steep sided dome at ushas mons a small shield volcano field at nepthys mons and bright diffuse deposits possible pyroclastic deposits with summit dome and pit formation at innini and hathor montes the presence at all of these volcanoes of radial lineaments and graben interpreted to be related to near surface lateral dyke emplacement and late stage summit constructional features indicate that shallow magma chambers occur at each edifice variations in the evolutionary history of and eruptions from these chambers may explain the differences observed in the deposits at these major edifices it is also possible however that fundamental differences in the magma source region was a major factor in determining the appearance of the deposits of the several models considered for the formation of the region secondary upwellings from a single large mantle plume or the near contemporaneous upwelling of multiple smaller plumes best explain the observed characteristics at the rise a comparison to other volcanic highlands suggests that there are at least two classes of volcanic rises on venus an areally and topographically large type with significant volcanism and rift zone formation like beta regio and a smaller scale type with extensive volcanism but moderate rifting like western eistla regio these two classes reflect a spectrum in size and vigor of mantle upwellings the dione rise is most like western eistla regio in terms of areal extent topographic elevation and geologic complexity

circulation anomalies associated with tropical temperate troughs in southern africa and the south west indian ocean daily rainfall variability over southern africa sa and the southwest indian ocean swio during the austral summer months has recently been described objectively for the first time using newly derived satellite products the principle mode of variability in all months is a dipole structure with bands of rainfall orientated northwest to southeast across the region these represent the location of cloud bands associated with tropical temperate troughs ttt this study objectively identifies major ttt events during november to february and on the basis of composites off ncep reanalysis data describes the associated atmospheric structure the two phases of the rainfall dipole are associated with markedly contrasting circulation patterns there are also pronounced intra seasonal variations in early summer the position of the temperate trough and ttt cloud band alternates between the swio and southwest atlantic in late summer the major ttt axis lies preferentially over the swio associated with an eastward displacement in the indian ocean high in all months positive events in which the ttt cloud band lies primarily over the swio are associated with large scale moisture flux anomalies in which convergent fluxes form a pronounced poleward flux along the cloud band this suggests that ttt events are a major mechanism of poleward transfer of energy and momentum moisture transport occurs along three principle paths 1 the northern or central indian ocean where anomalous fluxes extend eastward to the maritime continent 2 south equatorial africa and the equatorial atlantic 3 from the south within a cyclonic flow around the tropical temperate trough the relative importance of 2 is greatest in late summer thus synoptic scale ttt events over sa swio often result from large scale planetary circulation patterns hovmoeller plots show that ttt development coincides with enhanced tropical convection between 10 30 e itself exhibiting periodicity of around 5 days and often with convergence of eastward and westward propagating convection around 40 e harmonic analysis of 200 hpa geopotential anomalies show that ttt features are forced by a specific zonally asymmetric wave pattern with wave 5 dominant or significant in all months except february when quasi stationary waves 1 2 and 3 dominate these findings illustrate the importance of tropical and extratropical dynamics in understanding ttt events finally it is suggested that in november januar ttt rainfall over sa swio may be in phase with similar rainfall dipole structures observed in the south pacific and south atlantic convergence zones

simulated impacts of the south atlantic ocean dipole on summer precipitation at the guinea coast an intermediate complexity atmospheric general circulation model has been used to investigate the influence of the south atlantic ocean sao dipole saod on summer precipitation over the guinea coast of west africa two ensemble integrations in which idealized but realistic saod type sea surface temperature sst anomaly is prescribed only in the sao and then globally are performed and inter compared consistently above below the average precipitation is simulated over the guinea coast during the positive negative phase of the saod comparison of the two set of experiments reveal that in its active years the saod is a dominant mechanism that shapes the spatial character of summer precipitation at the guinea coast the global sst variability merely slightly moderate its effects during the saod cool sst anomaly in the extra tropical sao off the brazil uruguay argentina coast gives rise to suppressed convection and mass divergence in turn the subsidence tends to amplify the sub tropical arm of anomalous hadley type circulation and consequently large scale convection and mass flux convergence in the equatorial atlantic ocean gulf of guinea region bordering on the coastal fringes of west africa precipitation is therefore increased at the guinea coast

growth and persistence of hawaiian volcanic rift zones hawaiian volcanic rift zones are modeled by representing the rifts and adjacent volcano flanks as long ridges with the geometry of flattened triangular prisms the intrusion of dikes along the axis of a rift requires a mechanism to generate the appropriate dike trapping stress field within the prism possible factors that affect the state of stress in the prism include multiple dike intrusion along the ridge axis faulting and gravitational sagging of the topography in extreme models with very steep slopes and high poisson s ratio corresponding to the gelatin models of rift zones by fiske and jackson 1972 results of finite element calculations indicate that gravity induced stresses are sufficient to trap a dike into propagating within the prism and parallel to the rift zone as proposed by fiske and jackson however the mechanism does not work for gently sloping flanks or a more acceptable poisson s ratio of about 0 25 additionally trapping stresses in the gravity loading and density stratification models will not persist after a few dike injection episodes therefore in mature hawaiian rift zones with possibly thousands of dikes additional processes must act to control the stresses that permit continued dike intrusion and rift persistence it is proposed that accommodation to dike emplacement occurs by slip on deep faults possibly of the type proposed for the 1975 kalapana hawaii earthquake as suggested by others for this earthquake the faults could coincide with the contact of the volcano with the seafloor within the weak seafloor sediments such faulting not only provides a means for the flanks to adjust continuously to intrusions but also generates the stress patterns needed to constrain future dikes to propagate along the rift axis other possible faulting mechanisms such as shallow gravity slides and normal faulting of the flanks do not appear to favor rift zone persistence in this model the horizontal stress generated by a standing column of magma at the time of dike emplacement the stresses in the ridges and the fault strength are coupled this results in a feedback between the maximum height that magma can rise along the rift fault friction and fault width through this feedback the slope of the volcano flank is controlled by the fault friction when applied to kilauea volcano the model yields an estimate for the coefficient of fault friction as high as 0 39 assuming normal hydrostatic pore fluid pressure an implication of this model supported by other studies is that rift intrusion and lateral spreading could be major contributors to volcano growth

an investigation of interannual rainfall variability in africa abstract interannual variations in african rainfall are examined using rotated principal component analysis pca applied to anomalies from the annual mean as well as seasonal anomalies the rotated pca loading patterns suggest several preferred continental scale rainfall anomaly patterns the dominant features of year to year variations in african rainfall appear to be the high spatial coherence of rainfall anomalies over large portions of the continent in addition several dipole regions that is adjacent regions which tend to experience rainfall anomalies of opposite sign are found and discussed one dipole region the sub saharan region appears to have a relationship with a characteristic atlantic sea surface temperature anomaly pattern during boreal summer during austral summer the tendency for a large scale dipole pattern in southeast africa is apparent as is an association of this pattern with the warm and cold phases of the el nin southern oscillation enso phenomenon normalized rainfal

chemical dieferentiation of a convecting planetary interior consequences for a one plate planet such as venus partial melting to generate the crust of a planet creates compostionally buoyant residual mantle in the absence of mantle flow associated with plate tectonics this buoyant refractory layer may collect at the top of the mantle with important implications for the evolution of the interior and surface in this study models of the thermal and chemical evolution of a planetary interior demonstrate the possible consequences of a chemically buoyant depleted mantle layer as the depleted layer thickens the melting temperature at the top of the underlying convecting mantle also increases and the degree of partial melting of mantle added to the depleted layer decreases as less depleted mantle with less positive compositional buoyancy is added negative thermal buoyancy of the layer eventually exceeds its positive compositional buoyancy the depleted layer then sinks into and mixes with the convecting interior the top of the convecting mantle then moves to a shallower depth larger degrees of melting resume and a new depleted layer accumulates this accumulation and instability of the depleted layer occurs repeatedly over a substantial portion of the planet s evolution with a period of 300 500 myr on venus the population of impacts craters is indistinguishable from a random distribution over the surface and give a surface age of about 500 myr we speculate that the mechanism described above may explain this episodic global resurfacing of venus

magma reservoirs and neutral buoyancy zones on venus implications for the formation and evolution of volcanic landforms we examine the production of magma reservoirs and neutral buoyancy zones nbzs on venus and the implications of their development for the formation and evolution of volcanic landforms the high atmospheric pressure on venus reduces volatile exsolution and generally serves to inhibit the formation of nbzs and shallow magma reservoirs for a range of common terrestrial magma volatile contents magma ascending and erupting near or below mean planetary radius mpr should not stall at shallow magma reservoirs such eruptions would be characterized by relatively high total volumes and effusion rates for the same range of volatile contents at 2 km above mpr about half of the cases result in direct ascent of magma to the surface and half in the production of neutral buoyancy zones in general neutral buoyancy zones and shallow magma reservoirs begin to appear as gas content increases and are nominally shallower on venus than on earth for a fixed volatile content nbzs become deeper with increasing elevation over the range of elevations treated here 1 km to 4 4 km depths differ by a factor of 2 4 these analyses reveal several factors that may help to account for the low height of volcanoes on venus larger primary reservoirs cause the wide dispersal of conduits building edifices models of the position of the shallow nbz reservoir during edifice growth show that for earth the magma chamber center remains at a constant depth below the growing edifice summit thus keeping pace with the increasing elevation while on venus the chamber center becomes deeper relative to the summit of the growing edifice because of the major change in atmospheric pressure as a function of altitude therefore neutral buoyancy zones and magma reservoirs on venus will remain in the prevolcano substrate longer and in many cases may not emerge into the edifice at all the lower rate of vertical migration implies that magma reservoirs would tend to stabilize undergo greater lateral growth and become larger on venus than earth the proportion of the available magma going into production of the edifice relative to that intruded into the substrate is smaller on venus than earth large reservoirs would encourage multiple and more widely dispersed source vents and large volumes for individual eruptions in large reservoirs positively buoyant materials are likely to be produced from differentiation substrate remelting and volatile exsolution nonbuoyant materials exsolving volatiles in a shallow reservoir will need higher gas bubble concentrations to produce eruptions than on earth and when this gas enriched melt emerges at the surface it is more likely to retain its bubbles than to undergo explosive disruption due to the high surface atmospheric pressure therefore there is the potential for the production of a range of erupted lavas that have very high gas bubble concentrations leading to anomalous more viscous rheological properties inhibition of disruption of volatile rich magma for both basaltic and more evolved compositions can lead to the production of 1 more bubble rich vesicular flows characterized by higher viscosity and greater thicknesses the venus equivalent of effusions that would have lost much of their volatiles in the fire fountaining process on earth 2 high volume high discharge rate extremely gas rich basaltic effusions that increase viscosity considerably upon surface extrusion and produce domes whose volumes are considerably higher than their dense rock equivalent and 3 more evolved compositions that are gas rich but do not undergo disruption upon effusion and produce domes and flows of high viscosity the venus equivalent of terrestrial ignimbrites on the basis of these analyses we predict that intrusions and dike emplacement should be common particularly around both shallow and deep magma reservoirs and the tops of dikes should be manifested as radial fractures there should be a range of diameters of circular features associated with the presence of both shallow nbz related and deep plumerelated magma reservoirs large volume lava outpourings should be favored at lower elevations and shallow reservoirs and edifices should be more common at intermediate elevations at the highest elevations magma reservoirs are predicted to be large and relatively deep with low associated edifices direct ascent of magma where neutral buoyancy zones are unlikely low elevations and the propensity for relatively larger magma reservoirs on venus when they do form are both factors which will tend to favor high volume eruptions independent of variations in magma temperature or chemistry individual volcanic features should be interpreted in terms of regional geologic setting and the dynamics of elevation changes commonly associated with various scales of mantle processes and magma emplacement in order to assess global crustal growth processes on venus

a southern hemisphere wave response to enso with implications for southern africa precipitation ensemble gcm simulations with an imposed idealized warming of the eastern pacific ocean reveal two wave anomalies in the southern hemisphere one in the eastern and one in the western hemisphere both are statistically significant at the 99 confidence level application of a steady state linear model and a rossby wave source analysis is used to diagnose the causes of the waves the western hemisphere wave is forced by the advection and stretching of planetary vorticity by the divergent flow from the southern hemisphere component of the central pacific twin anticyclones that straddle the equator during warm events the eastern hemisphere wave is a result of the northeastward shift of the south indian convergence zone sicz that in turn is forced from the upper troposphere by convergence to the north an upper level convergence maximum over the equatorial indian ocean induces divergence to the south encouraging vertical motion and precipitation to the northeast of the sicz s normal position the resulting anomalous upper level convergence in the climatological position of the sicz as well as the anomalous vorticity flux convergence by the transients associated with an equatorward shift of the storm track behind the sicz force the eastern hemisphere rossby wave since a shift of the sicz is a fairly robust observed consequence of enso events these results suggest the mechanism by which drought conditions develop over southern africa at the height of many warm events seasonal prediction capabilities in this region can be improved by monitoring and understanding the details and consequences of the adjustment of the walker circulation near the equator outside of the pacific ocean basin

crustal structure flexure and subsidence history of the hawaiian islands seismic reflection profile data have been used to estimate the long term 106 years mechanical properties of the oceanic lithosphere underlying the hawaiian islands the data show prominent reflectors associated with the top of the oceanic crust reflector 2 the m discontinuity and the base of the crust reflector 3 the two way travel time to the reflectors has been depth converted using velocity depth functions derived from seismic refraction expanding spread profile data and compared to calculated depths based on simple three dimensional elastic plate models a good fit to the reflector depths was obtained for a model with an effective elastic thickness of the lithosphere te of 40 km the main discrepancies with the refraction data occur beneath oahu and molokai where the predicted depth of the base of the flexed crust is too shallow and beneath the southeast flank of hawaii where it is too deep we attribute the thickened crust beneath oahu to ponding of magma at the base of the crust following the last phases of tholeiitic shield building and the thinned crust beneath the flanks of hawaii to crustal melting beneath the youngest part of the chain the effective te of 40 km is higher than expected for 80 m y oceanic lithosphere possibly due to the effects of subsurface loads that act at or near the base of the flexed crust according to free air gravity anomaly and bathymetry data te does not change significantly along the island chain retaining a similar value for hawaii as it does for oahu we have used this result to estimate the pattern of subsidence and uplift that would be expected along the hawaiian islands because of flexural loading of hawaii the observed depth of the haleakala wave cut terrace northeast of maui can be generally explained by the flexure model the main discrepancies between observed and the predicted depths occur northwest of hawaii where they suggest a somewhat higher value of te possibly due to incomplete adjustment of the lithosphere we suggest that the addition of loads to the end of a volcanic chain generated as a plate moves over a mantle hotspot is accommodated by progressive flexing downstream causing subsidence of preexisting volcanos that lie in the flexural moat and uplift of islands on the flanking bulge the progressive flexure model predicts a characteristic pattern of offlap and onlap in the regions flanking the hawaiian islands that is generally similar to stratigraphic relationships observed along previously published seismic profiles of the moat a progressive flexure model may also apply to the case of a continental plate moving over a mantle hotspot and could explain the observations of 1 a clinoform pattern of stratigraphy in some flood basalt provinces and 2 a seaward dipping reflector sequence at some passive continental margins

the gravitational signature of martian volcanoes by modeling the elastic flexure of the martian lithosphere under imposed loads we provide a systematic study of the old and low relief volcanoes 3 2 ga 0 5 to 7 4 km and the younger and larger prominent constructs within the tharsis and elysium provinces 3 2 ga 0 5 to 7 4 km and the younger and larger constructs clustered in the tharsis and elysium provinces 3 ga 5 8 to 21 9 km the observed topography is used to define the load and to derive a simulated gravity field that is compared to the observed gravity the bulk density of the lava that composes all volcanoes is found to be 3 200 kg m 3 which is representative of iron rich basalts as sampled by the martian basaltic meteorites the elastic part of the lithosphere that is the part maintaining loads over geologic time is found to have been thin when the small old volcanoes formed which implies that the lithosphere was hot early in geologic history conversely larger volcanoes are found to have been emplaced on a cold and thicker elastic lithosphere

an analysis of west african dynamics using a linearized gcm abstract this study utilizes a linear primitive equation spherical model to study the development and propagation of easterly wave disturbances over west africa perturbations are started from an initial disturbance consisting of a barotropic vortex and the governing equations are integrated forward in time the perturbations are introduced into basic states corresponding to the observed dynamical and thermodynamical characteristics of two wet years in the sahel and two dry years the model simulations show consistent contrasts in wave activity between the wet and dry years the waves are markedly stronger in the wet years and show a barotropic structure throughout the troposphere the waves tend to extend throughout the troposphere to the level of the tropical easterly jet tej in the wet years but not in the dry years the upper tropospheric shear which is stronger in wet years appears to be a key factor in wave development this shear is dependent on the intensity of the tej suggesting that the t

evidence for active hotspots on venus from analysis of magellan gravity data abstract the 500 myr average crater retention age for venus has raised questions about the present day level of tectonic activity in this study we examine the relationship between the gravity and topography of four large volcanic swells beta atla bell and western eistla regiones for clues about their stage of evolution the magellan line of sight gravity data are inverted using a point mass model of the anomalous mass to solve for the local vertical gravity field spectral admittance calculated from both the local gravity inversions and a spherical harmonic model is compared to three models of compensation local compensation a flexural model with local and regional compensation of surface and subsurface loads and a hotspot model of compensation that includes top loading by volcanoes and subsurface loading due to a deep low density mass anomaly the coherence is also calculated in each region but yields an elastic thickness estimate only at bell regio in all models the long wavelengths are compensated locally the long wavelength estimates of the deep compensation depth for bell atla western eistla and beta regiones are 125 175 200 and 225 km respectively with an error of approximately 35 km the flexural model is rejected primarily because it gives values of effective elastic thickness of approximately 100 km which is at least a factor of two larger than estimates of effective elastic thickness obtained from the coherence spectra at bell regio and from other studies that model the topographic expression of flexure at bell regio and in other tectonic settings globally assuming a crustal thickness of 30 km the fit to the hotspot model at atla regio gives an effective elastic thickness of 30 5 km atla regio is interpreted as an active hotspot because of the deep compensation depth and a strong subsurface loading signature at bell regio effective elastic thickness is 30 5 km at short wavelengths and 50 5 km at long wavelengths using a crustal thickness of 10 km the 30 km value is interpreted as the effective elastic thickness at the time when the volcanoes are emplaced the 50 km value is believed to reflect the present day effective elastic thickness bell regio has a relatively shallow compensation depth 125 km and a weak bottom to top loading ratio determined from the coherence spectra these results may indicate a relatively old possibly inactive plume the data at beta and western eistla regiones are of relatively poor quality and do not permit detailed interpretations these areas are interpreted as active hotspots because of their large compensation depths greater than typical of the rest of the planet

mean age of rifting and volcanism on venus deduced from impact crater densities unlike the extensively cratered highlands of the moon and mars the surface of venus does not preserve a record of heavy bombard ment from the early history of the solar system1 3 those craters that are found on venus appear to be statistically indistinguishable from a random spatial population and rarely show modification by folds faults and lava flows1 3 although the volcanic and tec tonic history of venus is still much debated2 5 there is mounting evidence for near global resurfacing 300 500 myr ago1 2 6 moreover it has recently been noted that the density of impact craters on large volcanic structures is less than the average crater density of the planet suggestive of significant activity after the resurfacing event7 it is not clear however whether these features represent late remnants of the global event or continuing volcanism and tectonism of a still active planet to address this question we have used the regional variations in crater density to date volcan oes rifts and coronae which based on stratigraphic evidence clearly post date the main resurfacing event8 11 the calculated mean ages of 70 125 myr exclude the possibility that the majority of these features represent the final stages of the global event

the drying tendency of shallow meridional circulations in monsoons shallow meridional overturning circulations are superimposed on the deep circulations that produce precipitation in nearly all monsoon regions and these shallow circulations transport subtropical mid tropospheric dry air into the tropical monsoon precipitation maxima here horizontal moisture advection produced by shallow meridional circulations is characterized in the monsoon regions of west africa south asia australia and southern africa during local summer horizontal flow in the upper and lower branches of the shallow meridional circulations consistently dries and moistens air respectively in the continental precipitation maxima of each region the peak drying by horizontal advection occurs at a lower altitude than peak winds in the upper branch of the shallow circulations consistent with the small scale height of water vapor advection of time mean moisture by time mean wind dominates horizontal moisture advection in south asia and west africa while most horizontal moisture advection in australia and southern africa is produced by transient eddies much of the transient eddy advection can be accurately represented as a first order horizontal diffusion with a constant globally uniform diffusivity these results suggest that horizontal moisture advection in theoretical and conceptual models of seasonal mean monsoons can be adequately represented in terms of time mean winds plus a simple horizontal moisture diffusion finally interannual variations in the summer mean regional averages of monsoon precipitation and horizontal advective drying in the lower free troposphere are shown to be negatively correlated in most regions consistent with the hypothesis that advective drying by shallow meridional circulations inhibits monsoon precipitation

dynamic basis of volcanic spreading the established models of the structure and dynamics of a typical hawaiian volcano lead inevitably to a paradox finite element viscoelastic calculations using tecton melosh and raefsky 1980 1981 predict that a compressional stress field should characterize their upper flanks instead extension is observed the paradox is solved by postulating that volcanic evolution is determined by feedback processes related to growth and spreading of the volcanic edifice many examples of the process of spreading are documented in the geologic literature on volcanoes of a very broad size range and very different geologic settings the process is similar across many orders of magnitude in size the deformation of the substratum related to magma intrusion during lava dome building events is at the lower end of the scale the spreading of seafloor at oceanic ridges is at the higher end of the scale volcanoes may evolve from small constructs into huge shield volcanoes and perhaps into ocean plates key elements for volcanic spreading are 1 the existence of a weak basal layer and 2 a sufficiently high mass and magma influx to drive the process as the mass of a volcano grows it passes through five phases which may overlap repeat or be omitted building compressing thrusting intruding and spreading during the building phase the mass of a volcano does not significantly affect the stress field of the substratum primitive magmas are likely to be erupted as the mass increases the volcano subsides and becomes characterized by a compressional stress field which inhibits any further intrusion of magma into the volcanic edifice magma may differentiate in the crust allowing for large caldera forming eruptions to occur in the next phase thrusting begins on a decollement usually located at the foot of the edifice while the stress field on the upper slopes of the volcano remains compressional the stress at the base close to the center becomes extensional allowing the magma to intrude and differentiate at the base of the edifice a basal intrusive complex begins to grow explosive eruptions may be expected finally spreading of this complex creates an extensional stress field on the upper flanks of the volcano confining compression to the lower slopes of the edifice the extension allows primitive magma to erupt again at the summit the process of spreading seems to have a fundamental influence on the structural and magmatic evolution of volcanoes at every scale this process results from coupling and feedback between the gravitational and thermal fields thus studying this process may improve our understanding of the relation between structural dynamics and magma evolution and of the origin and evolution of volcanic seismicity and eruptions

modification of the southern african rainfall variability enso relationship since the late 1960s analysis of 149 raingauge series 1946 1988 shows a weak positive correlation between late summer rainfalls january march in tropical southern africa and the southern oscillation index soi the correlation coefficients have been unstable since world war ii they were close to zero before 1970 and significant thereafter before 1970 southern african late summer rainfalls were more specifically correlated with regional patterns of sea surface temperature sst mainly over the southwestern indian ocean after 1970 teleconnections with near global sst anomaly patterns i e over the central pacific and indian oceans dominate the regional connections the increase in the sensitivity of the southern african rainfall to the global so related circulation anomalies is simultaneous with the correlation between soi and more extensive sst anomalies particularly over the southern indian ocean this feature is part of longer term decadal global sst variability as inferred from statistical analyses numerical experiments using the meteo france general circulation model arpege climat are performed to test the impact of the observed sst warming in the southern indian and extratropical oceans during el nino southern oscillation enso events on southern african rainfall simulated results show that enso events which occurred in the relatively cold background of the pre 1970 period in the southern oceans had a little effect on southern africa climatic conditions and atmospheric circulation by contrast more recent enso events with warmer sst over the southern oceans lead to a climatic bipolar pattern between continental southern african and the western indian ocean which is characterized by reduced enhanced deep convection and rainfall over the subcontinent the western indian ocean a weaker subtropical high pressure belt in the southwestern indian ocean is also simulated along with a reduced penetration of the moist southern indian ocean trade winds over the southern african plateau these results are consistent with the strong droughts observed over all southern africa during enso events since 1970

thick plate flexure summary we derive analytical expressions for the displacements and stresses due to loading of a floating uniform elastic plate of arbitrary thickness by a plane or axisymmetric harmonic load the solution is exact except for assumptions of small strains and linear boundary conditions and gravitation within the plate is neglected for typical earth parameters its predictions are comparable to those of the usual thin plate theory frequently assumed in studies of lithospheric flexure gravity and regional isostasy even for a very thick lithosphere which may exist in some regions of mars the thin plate theory is a better approximation to the thick plate solution than the elastic half space limit except for short wavelength loads

state of stress faulting and eruption characteristics of large volcanoes on mars the formation of a large volcano loads the underlying lithospheric plate and can lead to lithospheric flexure and faulting in turn lithospheric deformation affects the stress field beneath and within the volcanic edifice and can influence magma transport modeling the interaction of these processes is crucial to an understanding of the history of eruption characteristics and tectonic deformation of large volcanoes we develop models of time dependent stress and deformation for the tharsis volcanoes on mars by means of a finite element code we calculate stresses and displacements due to a volcano shaped load emplaced on an elastic plate overlying a viscoelastic mantle models variously incorporate growth of the volcanic load with time and a detachment between volcano and lithosphere the models illustrate the manner in which time dependent stresses induced by lithospheric plate flexure beneath the volcanic load may affect eruption histories and the derived stress fields can be related to tectonic features on and surrounding martian volcanoes as a result of flexure there are three regions where stresses become sufficiently large to cause failure by faulting according to the mohr coulomb criterion at the surface of the plate just outward of the volcano near the base of the elastic lithosphere beneath the center of the volcano and on the upper flanks of the volcano early in its growth history normal faulting is the dominant mode of failure predicted for the first region consistent with circumferential graben observed around the tharsis montes and with the scarp at the base of olympus mons interpreted as a large offset listric normal fault normal faulting mostly radially oriented is predicted for the second region failure in the third region is predicted to consist of thrust faulting circumferentially oriented on the upper and middle flanks and radially oriented on the lower flanks in models simulating a growing volcano this portion of the edifice is subsequently covered by later units which exhibit lower stresses and are not predicted to fail this volume of early failure remains the most highly stressed area in the edifice concentric terraces interpreted by some workers as thrust faults on the upper flanks of olympus mons may correspond to the predicted circumferential thrust features if the most recent increments of volcano growth were relatively large or in the presence of local material property or stress field variations for volcanoes detached from the plate predicted failure in the edifice takes the form of radial normal faulting near the volcano base the addition of a local extensional stress arising from the regional topographic slope yields a pattern of predicted faulting which closely matches that observed on the tharsis montes including the development of radial rifts on the lower volcano flanks to the northeast and southwest and the asymmetric formation of circumferential flank graben this stress state is also consistent with an interpretation of the aureole deposits of olympus mons as the result of gravity sliding along a basal detachment our models also suggest an explanation for the lack of strike slip features predicted by previously published flexural models around the tharsis volcanoes for a given load increment the first mode of near surface failure for most of the area immediately outward of the load is circumferential normal faulting and graben formation as the volcano grows and the flexural response to the increasing load proceeds the predicted failure mode in a portion of this annular region surrounding the volcano changes to strike slip faulting because normal faulting has been predicted to have taken place earlier however it is likely that release of later stresses will occur by reactivation and growth of these normal faults and graben rather than by the formation of new strike slip faults

small volcanic edifices and volcanism in the plains of venus the most widespread terrain type on venus consists of volcanic lowland plains several styles of volcanism are represented in the plains the most extensive volcanic units consist of flood lavas the largest of which have volumes of the order of thousands of cubic kilometers as with terrestrial flood lavas they are inferred to have erupted at high effusion rates they show a range of radar backscatter characteristics indicating different surface textures and ages small edifices on the plains occur mainly in clusters associated with fracture belts the majority are shield volcanoes that may be up to a few tens of kilometers across but are generally 10 km or less in diameter volcanic cones have the same size range volcanic domes have diameters up to several tens of kilometers and volumes of the order of 100 km3 these are interpreted as being constructed of lava erupted with a relatively high effective viscosity and thus possibly composed of more silicic lava for many domes the flanks were unstable during and afte eruption and suffered gravity sliding that produced steep scalloped outer margins because of the high atmospheric pressures on venus explosive activity is less likely to occur than on earth however n a few plains areas there is evidence of pyroclastic deposits surrounding craters indicating that volatile contents in some of the magmas may be high in comparison to earth the clusters of small volcanic edifices are considered to be analogous to plains volcanism similar to that of the snake river plain of idaho there may also be analogues with terrestrial volcanic clusters associated with mid oceanic ridges

comparison of flat topped stellate seamounts on earth s seafloor with stellate domes on venus using side scan sonar and magellan synthetic aperture radar abstract the diameters of many domes on venus imaged by the magellan synthetic aperture radar sar are an order of magnitude larger than terrestrial subaerial domes however surveys of the seafloor using sonar imaging systems such as gloria geological long range inclined asdic and seabeam have revealed flat topped volcanic features with similar diameters volumes and slope angles suggested analogies between domes on venus and seamounts on the northern east pacific rise and off hawaii have been tested by d k smith j volcanol geotherm res 73 1996 47 64 using comparisons of size and shape and significant differences identified gloria images of flat topped seamounts near hawaii show them to have stellate planforms over half of the total population of 375 domes on venus have been modified and many have stellate planforms we have focused on the similarities in stellate planforms between flat topped seamounts and modified domes on venus we have used gloria sonar and magellan radar data to constrain the formation and evolution of both flat topped seamounts on the ocean floor and volcanic domes on venus to see if the processes that formed them are similar we propose that flat topped seamounts on the hawaiian ridge are built on sediments overlying the oceanic crust and that available evidence supports three processes that can form stellate margins 1 radial dike intrusions related to flank rift zones 2 dike intrusions which cause slope failures and 3 gravitational slope failures unrelated to intrusions for venus where low erosion rates and the absence of a sediment layer results in lavas being emplaced directly onto the crust the available evidence supports stellate margins originating by gravitational slope failures unrelated to diking though intrusions cannot be ruled out with existing data

tracking rainfall in the northern mediterranean borderlands during sapropel deposition the role of mid latitude precipitation in the hydrological forcing leading to the deposition of sapropels in the mediterranean sea remains unclear the new gdec 4 2 borehole east corsica margin northern tyrrhenian sea provides the first precisely dated evidence for enhanced rainfall in the western mediterranean during warm intervals of interglacial periods over the last 547 kyr comparison of gdec 4 2 proxy records with pollen sequences and speleothems from the central and eastern mediterranean reveals that these pluvial events were regional in character and occurred probably in response to the intensification of the mediterranean storm track along the northern mediterranean borderlands in autumn winter our dataset suggests that the timing of maxima of the mediterranean autumn winter storm track precipitation coincide with that of the north african summer monsoon and sapropel deposition besides highlighting a close coupling between mid and low latitude hydrological changes our findings suggest that during warm intervals of interglacial periods the reduced sea surface water salinities together with the high flux of nutrient and organic matter produced by the monsoonal nile and wadi systems floods were maintained throughout the winter by the mediterranean rainfall this provides an important additional constraint on the hydrological perturbation causing sapropel formation

tectonic evolution of bell regio venus regional stress lithospheric flexure and edifice stresses in order to understand the relationship between volcanic and tectonic processes and the stress state in the lithosphere of venus we analyzed the stress environments and resulting tectonic features associated with the major volcanic edifices in bell regio using magellan synthetic aperture radar sar images and altimeter measurements of topography the major volcanoes of bell regio tepev mons and nyx mons exhibit tectonic characteristics that are unique relative to other volcanic edifices on venus the most prominent distinction is the lack of large rift zones within the overall highland uplift which characterize many other highland rises on venus also previous studies have determined that many large venus volcanoes exhibit radial tectonic structures on their flanks but generally lack the circumferential graben which surround volcanoes on earth and mars tepev and nyx montes exhibit both the radial tectonic features associated with other venusian edifices and numerous concentric graben nyx mons implies a more distributed magmatic system by its broad shape radial chains of pit craters and expansive flow fields whereas tepev mons is a more centralized volcanic system with limited associated long flows we investigate the regional stresses associated with bell regio and structural features believed to be a consequence of lithospheric flexure due to volcanic loading modeling both nyx mons and tepev mons as axisymmetric loads with gaussian mass distributions on an elastic plate the relationship between the tectonic features surrounding tepev mons and stresses associated with magma chamber inflation are also examined through finite element analysis using topography data to model the shape of the volcano we determine that a horizontally ellipsoidal or tabular reservoir at a range of depths from approximately 20 to 40 km can satisfy the locations of graben formation observed in magellan images these results imply a shift in volcanic style within bell regio from an early phase of broad low shield formation to later steep sided more centralized edifice development such changes are consistent with an increase in the thickness of the lithosphere over time

rifts and large volcanoes on venus global assessment of their age relations with regional plains we report on the results of synoptic global mapping of rifts and large volcanoes on venus and detailed mapping in six areas illustrating the relationships among these features rifts and large volcanoes were subdivided into old transitional for volcanoes only and young features the geologic event separating old features from young ones is the formation of wrinkle ridges on the regional plains global mapping showed that the postplains rifts dominate in the population of rifts both in terms of number of mapped segments and in area the postplains volcanoes are more abundant in terms of number and area occupied than those that are transitional in age and these in turn dominate over the old ones these data show that in post regional plains time rifting and formation of large volcanoes are significantly more important processes than in the immediately preceding period the time period during which regional plains were emplaced and old and partly transitional large volcanoes formed was much shorter than the postplains time therefore the mean rate of formation of large volcanoes during this first period was significantly higher than in the subsequent time similarly the relatively short time duration of the plains emplacement time indicates that the mean rate of old rifting was higher than in more recent time this study shows prominent changes with time in the absolute rates and roles of rifting and large volcano formation this supports the directional model of the geologic evolution of venus during the last several hundred million years as opposed to the nondirectional model which predicts a more random occurrence of geologic processes throughout history

elastic thickness of the lithosphere in the marquesas and society islands analysis of sea surface gravity data along six profiles collected during the crossgrain 2 and ew9103 expeditions to the marquesas islands demonstrates that the elastic plate thickness t e is remarkably uniform with a value of 18 2 km over the entire chain given the 50 ma age of the marquesan lithosphere at the time of loading this t e value agrees with the empirical relationship between elastic plate thickness and plate age for hotspot volcanoes in the north pacific atlantic and indian oceans using more limited gravity data from the society chain we find that the best fitting elastic plate thickness there is resolvably higher 23 2 km in both cases our values for t e are larger than those derived from analysis of satellite altimetry profiles we attribute the low te values derived from satellite altimetry to inaccuracies in available gridded bathymetric databases for this part of the world and the assumption of a high value for the density of the seafloor although our shipboard data sample only two french polynesian island chains the results suggest that the amount of elastic plate thinning over the south pacific superswell may be less than previously proposed thus eliminating conflict between estimates of the thermal structure of the superswell lithosphere as derived from elastic plate thickness and that derived from heat flow depth anomalies and love wave dispersion

coordinated abrupt weakening of the eurasian and north african monsoons in the 1960s and links to extratropical north atlantic cooling abstractprevious modeling and paleoclimate studies have suggested that cooling originating from the extratropical north atlantic can abruptly weaken the eurasian and north african monsoons the climatic signature includes a widespread cooling over the eurasian and north african continents and an associated increase to surface pressure it is explored whether such coordinated changes are similarly exhibited in the observed twentieth century climate in particular with the well documented shift of sahel rainfall during the 1960s surface temperature sea level pressure and precipitation changes are analyzed using combined principal component analysis cpca the leading mode exhibits a monotonic shift in the 1960s and the transition is associated with a relative cooling and pressure increase over the interior eurasia and north africa and rainfall reduction over the sahel south asia and east asia the local circulation changes suggest that the rainfall shift results from the regional response of the sum

impact crater densities on volcanoes and coronae on venus implications for volcanic resurfacing the density of impact craters on large volcanoes on venus is half the average crater density for the planet the crater density on some classes of coronae is not significantly different from the global average density but coronae with extensive associated volcanic deposits have lower crater densities these results are inconsistent with both single age and steady state models for global resurfacing and suggest that volcanoes and coronae with associated volcanism have been active on venus over the last 500 million years

venus topography and kilometer scale slopes during the first 8 months of the magellan mission the radar altimeter has made some three million measurements of the surface of venus covering the latitude range from 85 n to 80 s methods involving range correlation doppler filtering multi burst summation and range migration are used to focus the observations and to achieve high surface resolution results are presented as maps of the global distribution of topography meter scale roughness and power reflection coefficient the results are similar to those reported in previous experiments surface heights exhibit a unimodal distribution with more than 80 of the surface lying within 1 km of the 6051 84 km mean radius but the higher resolution of the magellan altimeter has disclosed several surprisingly steep features e g the southwest face of the maxwell montes the southern face of the danu montes and the chasmata to the east of thetis regio where average kilometer scale slopes of greater than 30 are not uncommon this conclusion is corroborated by close inspection of synthetic aperture radar imagery

the south indian convergence zone and interannual rainfall variability over southern africa abstract the south indian convergence zone sicz is identified in this paper as a region of enhanced precipitation extending off the southeast coast of southern africa during austral summer unlike the south pacific convergence zone the sicz is a land based convergence zone lbcz with position and intensity at least partially determined by surface conditions over southern africa an idealized gcm simulation is used to explore the basic dynamics of lbczs in the tropics and subtropics output from a realistic gcm simulation and the national centers for environmental prediction national center for atmospheric research 40 year reanalysis are analyzed to apply this basic dynamical framework to the case of the sicz in contrast to the intertropical convergence zone where column moisture convergence is primarily due to meridional wind convergence in the moist environment precipitation within the sicz and the lbczs in general is also supported by zonal wind convergence moisture convergence by transient eddy

geology and distribution of impact craters on venus what are they telling us magellan has revealed an ensemble of impact craters on venus that is unique in many important ways we have compiled a data base describing the 842 craters on 89 of venus surface mapped through orbit 2578 the craters range in diameter from 1 5 to 280 km we have studied the distribution size density morphology geology and associated surface properties of these craters both in the aggregate and for some craters in greater detail we find that 1 the spatial distribution of craters is highly uniform 2 the size density distribution of large craters diameters 35 km is similar to the young crater populations on other terrestrial planets but at a much lower density that indicates an average age of only about 0 5 ga based on the estimated population of venus crossing asteroids 3 unlike the case on other planets the density of small craters diameters 35 km declines rapidly with decreasing diameters because of atmospheric filtering 4 the spectrum of crater modification differs greatly from that on other planets 62 of all craters are pristine only 4 are embayed by lavas and the remainder are affected by tectonism but none are severely and progressively depleted as extrapolated from the size density distribution of larger craters 5 large craters have a progression of morphologies generally similar to those on other planets but small craters are typically irregular or multiple rather than bowl shaped 6 diffuse radar bright or dark features surround some craters and 367 similar diffuse splotches with no central crater are observed and 7 other crater features unique to venus include radar bright or dark parabolic arcs opening westward and extensive outflows originating in crater ejecta the first three of these observations are entirely unexpected we interpret them as indicating that the planet s cratering record was erased by a global resurfacing event or events the latest ending about 0 5 ga after which volcanic activity declined but did not cease entirely since the last resurfacing event a maximum of 10 of the planet has been resurfaced and only about 4 of the craters have been obliterated convective thermal evolution models support this interpretation arkani hamed and toksoz 1984 observations 3 7 confirm quantitatively the expectation that the dense atmosphere of venus has strongly affected the production of craters large impactors have been relatively unaffected intermediate sized ones have been fragmented and have produced overlapping or multiple craters a narrow size range has produced shock induced splotches but no craters and the smallest bodies have had no observable effect on the surface the number of craters eliminated by the atmospheric filter is enormous about 98 of the craters between 2 and 35 km in diameter that magellan might have observed on a hypothetical airless venus unique crater related features such as parabolas and outflow deposits demonstrate the roles of venus high atmospheric density and temperature in modifying the crater formation process finally heavily fractured craters and lava embayed craters are found to have higher than average densities along the major fracture belts and rifted uplands connecting aphrodite terra and atla beta themis and phoebe regiones these craters thus provide physical evidence for recent volcanic and tectonic activity at a low level

melt production during granulite facies anatexis experimental data from primitive metasedimentary protoliths we conducted fluid absent partial melting experiments at 0 5 and 1 0 gpa in the temperature range 750 to 1000 c to investigate the influence of bulk rock mg 100mg mg fe and the effects of additional tio2 on the granulite grade anatectic evolution of relatively magnesian metapelites and metagreywackes in these experiments melting began between 780 and 830 c by the incongruent breakdown of biotite to produce quartz saturated granulite facies residual mineral assemblages in equilibrium with h2o undersaturated granitic melt the glass quenched melt compositions produced in this study vary little generally the glasses have compositions similar to those of many natural strongly peraluminous leucogranites the solidus temperatures in both rock types increase with increasing mg but are unaffected by the presence or absence of a tio2 component at 0 5 gpa the metapelites melted at temperatures up to 50 c lower than the equivalent metagreywackes but at 1 gpa there was no discernible difference this study suggests that the fluid absent solidus has a steep positive dp dt slope in metapelites and steep negative dp dt slope in metagreywackes the pattern of melt production with increasing temperature is strongly controlled by the upper limit of biotite stability in tio2 free compositions this was found to increase by 15 to 20 c in the metapelites and by 30 to 40 c in the metagreywackes as a function of increasing mg from 49 to 81 the presence of a tio2 component increases the upper limit of biotite stability by 50 c in the metapelites and by 80 c in the metagreywackes over that observed in the equivalent tio2 free compositions in consequence in the tio2 free samples large pulses of melt up to 35 wt are produced over narrow temperature ranges as little as 15 c in these experiments between 830 and 875 c in the tio2 bearing samples the major pulse of melt production occurs more gradually between 830 and 900 c

venus resurfacing rates constraints provided by 3 d monte carlo simulations the range of surface ages on venus and the rate of recent volcanism are two major questions that can be addressed through an analysis of magellan images we report on a 3 d monte carlo resurfacing model that seeks to explain the resurfacing history of venus by simulating the evolving surface under the competing processes of impact cratering and volcanism by incorporating the observed size distribution of volcanic forms on venus and the assumptions of constant volcanic and impactor fluxes we seek a solution that is consistent with the observed impact crater population in the monte carlo simulations a volcanic flux of 0 37 km3 yr operating for 550 m y on initially craterless surfaces yields surfaces that are statistically similar to venus it is shown that the statistics of the impact cratering record are inconsistent with an equilibrium resurfacing model

structure of the mobile south flank of kilauea volcano hawaii how do huge landslides occur in hawaii we can begin to address this question by examining an incipient landslide structure on the mobile south flank of kilauea volcano between 1970 and 1989 this part of the south flank moved in a southeast direction seaward by 4 m on its eastern end and by 10 m on its western end parallel to the western boundary of a large oifshore topographic bench we show that abrupt decreases in cumulative flank movement correspond to abrupt decreases in hypocenter concentration most of the earthquakes occur in a 7 to 10 km deep band beneath the south flank and relative relocation of a subset of these events shows that they are derived from a single horizontal plane which appears to be a decollement focal mechanisms for this deep seismicity are dominated by thrust fault mechanisms with the overlying block moving seaward a comparison of south flank bathymetry with decollement structures on other volcanoes suggests that the distal end of this decollement coincides with the distal end of the offshore topographic bench 30 50 km from shore and 5 km below sea level the fault system forming the hilina pali is connected with this decollement so that slip on the hilina pali s normal faults coincides with slip of the decollement and with reverse faulting of the distal end of the bench this interpretation is supported by studies of the events associated with the m7 2 kalapana earthquake in 1975 when the hilina pali is connected to the outer bench with a decollement inferred from seismicity a wedge 8 km thick adjacent to kilauea s magma system and 1 2 km thick along the edge of the outer bench an area over 3000 km2 is defined the thickness adjacent to the magma system decreases eastward along kilauea s east rift zone and southwestward from kilauea caldera whereas the thickness of the lip of the outer bench decreases only eastward geologic observations and geophysical studies of the subaerial flank reveal an extensive magma system beneath the koae fault system as well as beneath kilauea caldera and its east rift zone we infer that the boundary of the proximal end of the mobile flank is the southern boundary of this magma system and that this boundary coincides with the southern boundaries of both the koae fault system and kilauea s east rift zone

growth of large volcanoes on venus mechanical models and implications for structural evolution the structure tectonics and evolution of large volcanoes on venus as revealed by data from the magellan mission appear to be distinct from those on earth and mars to determine the conditions and processes that account for these differences we model the evolution of stress and deformation in growing volcanic edifices on venus using the finite element method large volcanoes on venus are characterized by topographically prominent conical edifices surrounded by relatively flat flow aprons the surfaces of both edifice and apron consist dominantly of radially oriented flows tectonic features usually the surface expression of shallowly intruded dikes also have predominantly radial orientations features similar to hawaiian style linear rift zones or large scale flank failure common on large volcanoes on earth and mars are absent by comparing predictions of faulting for models with detached and welded basal boundary conditions with the observed tectonic features we determine that a welded condition is more likely horizontal compressive stresses transmitted into edifices across welded basal boundaries are inversely proportional to elastic lithosphere thickness te contravening stress increments from magma chamber expansion differential thermal contraction or buoyant loading from beneath the crust or lithosphere are required to reorient principal stresses in the edifice so that magma ascent and radial dike formation are favored incremental volcano growth results in stress distributions that decrease with height in the edifice dike intrusion is easiest in the uppermost youngest layers consistent with the observed shallow radial dikes on venus volcanoes small values of te greatly inhibit the formation of large shields in that conical edifice topography cannot be supported and large contravening stresses are required for further growth large volcano formation is much more likely at large te where more moderate contravening stresses are sufficient for growth the near surface manifestation of mantle upwelling on venus may thus depend on te in that the response to upwellings beneath thin or nonexistent lithosphere is likely to be dominantly ductile leading to the formation of coronae but a thick elastic lithosphere is required to support the growth of the largest volcanoes the transitional value of te between these modes of evolution likely depends on the horizontal scale of the upwelling

near surface zonal flow and east african precipitation receipt during austral summer abstract this paper examines the influence of moist atlantic air masses on east african precipitation receipt during periods of westerly outbreaks across east africa the influence of the indian ocean on the interannual variability of east african rainfall is widely known but there is also a less well known association between precipitation receipt and the inflow of moist atlantic air atlantic air masses advected inland around the northern margins of the climatological mean heat low over southwestern africa are thought to be uplifted over the high elevations encountered inland and may produce rainfall over east africa after cooling to saturation during austral summer december january february djf atlantic air masses converge with easterly trade wind and monsoon flows from the indian ocean in a complex meridionally oriented convergence zone 0 20 s 20 40 e an index of near surface zonal flow during the austral summer period djf is created for this convergence zone and is correlated with vari

prodigious submarine landslides on the hawaiian ridge the extensive area covered by major submarine mass wasting deposits on or near the hawaiian ridge has been delimited by systematic mapping of the hawaiian exclusive economic zone using the side looking sonar system gloria these surveys show that slumps and debris avalanche deposits are exposed over about 100 000 km2 of the ridge and adjacent seafloor from kauai to hawaii covering an area more than 5 times the land area of the islands some of the individual debris avalanches are more than 200 km long and about 5000 km3 in volume ranking them among the largest on earth the slope failures that produce these deposits begin early in the history of individual volcanoes when they are small submarine seamounts culminate near the end of subaerial shield building and apparently continue long after dormancy consequently landslide debris is an important element in the internal structure of the volcanoes the dynamic behavior of the volcanoes can be modulated by slope failure and the structural features of the landslides are related to elements of the volcanoes including rift zones and fault systems the landslides are of two general types slumps and debris avalanches the slumps are slow moving wide up to 110 km and thick about 10 km with transverse blocky ridges and steep toes the debris avalanches are fast moving long up to 230 km compared to width and thinner 0 05 2 km they commonly have a well defined amphitheater at their head and hummocky terrain in the lower part oceanic disturbance caused by rapid emplacement of debris avalanches may have produced high level wave deposits such as the 365 m elevation hulopoe gravel on lanai that are found on several islands most present day submarine canyons were originally carved subaerially in the upper parts of debris avalanches subaerial canyon cutting was apparently promoted by the recently steepened and stripped slopes of the landslide amphitheaters

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

the marquesas archipelagic apron seismic stratigraphy and implications for volcano growth mass wasting and crustal underplating multichannel seismic lines sonobuoy and gravity data across the marquesas islands are used to study volcano growth island mass wasting and crustal underplating at island chains with overfilled moats the marquesas bathymetry reflects the changing thickness of the sedimentary infill rather than the basement topography the moat contains two major regions of differing seismic stratigraphy 1 the moat edges where a unit of continuous layered reflectors is present containing minor lenses of chaotic diffractors and 2 the central moat where the deep moat basins are overfilled by an acoustically opaque unit of discontinuous reflectors of up to 2 km thickness in places capped by a ponded unit plate flexure models require broad underplating of the crust by low density crustal material at the marquesas islands to explain the depth to volcanic basement and gravity observations the seismic velocities and seismic stratigraphy as well as the general structure of the islands and surrounding seafloor indicate the apron is mostly debris from island mass wasting reflectors of the outermost moat generally onlap the flexural arch in the lower section and offlap and overfill it in the upper section in the central moat reflectors change shape from concave up in the lower section to convex in the upper section three dimensional diffusion models of sedimentation which incorporate a time dependent seafloor deflection from progressive island loading and vary sediment influx as islands are formed and mass waste suggest that three main factors make the moat stratigraphy at the marquesas different from hawaii 1 the marquesas moat is overfilled while the hawaiian moat is underfilled 2 sediment diffusivities are lower at the marquesas and 3 the marquesas islands are separated by deep sedimentary basins in contrast to hawaii where islands are separated by a shallow ridge the lower sediment diffusivity at the marquesas may reflect a larger proportion of blocky massive material in the central marquesas moat or alternatively a change in the dominant process of sediment transport while there is similar sediment supply for a given along moat distance at both the marquesas and hawaii the underfilled moat at hawaii is apparently a consequence of greater moat volumes due to the larger size of the hawaiian volcanoes and possibly variations in underplating that load the plate the difference in sediment edifice ratios is likely related to the larger eruption rates at hawaii and different styles of volcano construction between hawaii and the marquesas

the frequency area distribution of volcanic units on venus implications for planetary resurfacing the areas of volcanic units on venus have been measured on the 1 5000000 geological maps published by nasa usgs these data were used to obtain a frequency area distribution the cumulative frequency area distribution of 1544 specific occurrence of units cover six orders of magnitude from the largest unit 30 106 km2 to the smallest 20 km2 the probability distribution function has been calculated the medium and large volcanic units correlate well with a power law fractal relation for the dependence of frequency on area with a slope of 1 83 there are fewer small units than the expected values provided by the power law relation our measurements cover 21 02 of the planetary surface 3 59 of the study area was found to be tessera terrain and is excluded from this study of volcanism the measurements were restricted to areas where geological maps have been published the analysis was performed on two independent areas of the planet with a complete coverage of published maps in both areas the largest volcanic unit covers a significant portion of the surface 58 75 and 63 64 respectively for the total measured volcanic units excluding tessera these two largest units that could correspond to the same unit or not cover the 61 18 and they are stratigraphically superimposed on older volcanic units which cover 3 37 of the area the remaining area 35 45 is occupied by younger volcanic units stratigraphically superimposed on the large volcanic unit s these results are based on the independent mapping of a large number of geologists with different ideas about the geodynamical evolution of venus and different criteria for geological mapping despite this fact the presence of these very large units is incompatible with the equilibrium resurfacing models because their generation at different ages would destroy the crater randomness our frequency area distribution of the mapped volcanic units supports a catastrophic resurfacing due to the emplacement of the largest unit s followed by a decay of volcanism our data for the frequency area distribution of volcanic units provide new support for catastrophic resurfacing models it is difficult to make our observations compatible with equilibrium steady state resurfacing models

interdecadal changes in the major modes of asian australian monsoon variability strengthening relationship with enso since the late 1970s abstract the present paper develops an integral view of the year to year variability across the entire asian australian monsoon a am system which covers one third of the global tropics between 40 and 160 e using season reliant empirical orthogonal function s eof analysis the authors identified two major modes of variability for the period 1956 2004 the first exhibits a prominent biennial tendency and concurs with the turnabout of el nino southern oscillation enso providing a new perspective of the seasonally evolving spatiotemporal structure for tropospheric biennial oscillation the second mode leads enso by one year the remote el nino forcing the monsoon warm pool ocean interaction and the influence of the annual cycle are three fundamental factors for understanding the behavior of the first mode the monsoon ocean interaction is characterized by a positive feedback between the off equatorial convectively coupled rossby waves and the underlying sea surface temperature sst dipole anoma

the geological evolution of atai mons venus a volcano corona hybrid atai mons venus is classified as a large volcano yet displays characteristics that are typical of coronae coronae are widespread volcano tectonic features that are apparently unique to venus detailed analysis of atai mons using photogeological and geophysical techniques illustrates that it has experienced a complex history and may still be active we show that processes found at both coronae and large volcanoes have occurred throughout the history of atai mons with no definite temporal preference it does not have an evolution that can be described simply as the evolution of one feature into another these results suggest that large volcanoes and coronae are more intimately linked than previously thought and that detailed mapping and gravity surveys are reliable ways of establishing the evolution of hybrid features

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

a theoretical model of hotspot volcanism control on volcanic spacing and patterns via magma dynamics and lithospheric stresses many linear island chains are thought to be the result of the steady motion of the lithospheric plates over stationary hotspots the occurrence of discrete nearly regularly spaced volcanoes rather than continuous ridges are assumed to be caused by the interaction of flexural stresses due to the volcanic loads with magma percolation a parameter study is performed on a simple model that incorporates these effects in addition to dike wall erosion it is demonstrated that the strength of the erosional feedback determines whether the model generates discrete volcanoes or a continuous ridge the intervolcanic spacing depends not only on the elastic thickness of the lithosphere but also on the magma pressure at the base of the lithosphere the size of the eruptive region of the individual volcanoes is controlled by the elastic response of the lithosphere to magma overpressurization if an initial off axis edifice is introduced the model is able to preserve this asymmetry and produce an alternating series of volcanoes a small initial perturbation grows over time resulting in double lines or wider patterns depending on the width of the magma source region single lines of volcanoes therefore indicate very narrow magma source regions

the global resurfacing of venus the impact cratering record on venus is unique among the terrestrial planets fully 84 of the craters are in pristine condition and only 12 are fractured remarkably only 2 5 of the craters and crater related features are embayed by lava although intense volcanism and tectonism have affected the entire planet furthermore the spatial and hypsometric distribution of the craters is consistent with a completely random one including stochastic variations monte carlo simulations of equilibrium resurfacing models result in a minimum of 17 times more embayed craters than observed or unobserved nonrandom crater distributions for resurfacing areas between 0 03 and 100 of the planet s surface these models also are not consistent with the number and nonrandom distribution of volcanoes and the nonrandom distribution of embayed and heavily fractured craters the constraints imposed by the cratering record strongly indicate that venus experienced a global resurfacing event about 300 m y ago followed by a dramatic reduction of volcanism and tectonism this global resurfacing event ended abruptly 10 m y the present crater population has accumulated since then and remains largely intact thermal history models suggest that similar global resurfacing events probably occurred episodically in the past the tesserae statistically have the same average surface age crater retention age as the rest of the planet but they probably represent older rock units deformed by earlier episodes of global resurfacing although they largely survived the latest global resurfacing event their surfaces were severely deformed by it monte carlo simulations indicate that only about 4 6 of the planet has been volcanically resurfaced since the global event and that the lava production rate has been no more than 0 01 0 15 km3 yr during this time this rate is significantly less than the current rate of intraplate volcanism on earth 0 33 0 5 km3 yr most of venus recent volcanism occurs in the beta atla themis region and most of the recent tectonism is associated with the major global scale tectonic disruption zones that lie within and connect the equatorial highlands the approximately 33 of the planet s surface bounded by latitudes 30 n and 30 s longitudes 60 and 300 e contains twice as many heavily fractured craters and 1 4 times more lava embayed craters as the planetary average this region includes most of the major tectonic belts in the equatorial region because the craters are indistinguishable from a statistically random distribution both spatially and hypsometrically this concentration of strongly fractured and embayed craters is considered indicative of a continuing low level of limited extension and volcanic activity in this region over the past 300 m y however these craters are simply fractured and or embayed and very few have been subjected to complete tectonic disruption complete burial and subsequent removal from the surface as was the case during the global resurfacing event we show that neither the present level and style of geologic activity nor anything less than global resurfacing could have produced the observed cratering record the effects of recent geologic activity are much less than those of the earlier global resurfacing event when the record of all the early heavy bombardment and much of the later light bombardment was erased from the surface by massive volcanism and tectonic activity episodic regional resurfacing events that had global effects also occurred on earth e g the mid cretaceous superplume and probably on mars on mars they may have triggered the catastrophic releases of water that formed the outflow channels

the influence of pacific sst variability on the precipitation over southern africa this study is an analysis of agcm model results to understand the dynamics of the response of precipitation over southern africa sa to anomalies in the sea surface temperature sst over the pacific ocean the pattern of interannual precipitation anomaly over sa and its temporal variations are quite similar in both the ensemble mean of the control where agcm is forced with observed ssts in all ocean basins and experimental runs where agcm is forced with seasonally varying climatological sst over the pacific ocean however the amplitude of the variability is found to be relatively reduced in the experimental runs this is shown to be a result of the modulation of the walker circulation by the variability of pacific ocean sst the regional teleconnection pattern between the dominant mode of sa precipitation variability and sst anomalies over the eastern indian ocean is also influenced by the variations in pacific sst the nature of the teleconnection between sa precipitation and eastern indian sst is apparent only when the pacific sst variability is excluded this is corroborated from observations as well

estimating lithospheric properties at atla regio venus abstract magellan spherical harmonic gravity and topography models are used to estimate lithospheric properties at atla regio venus a proposed hotspot with dynamic support from mantle plume s global spherical harmonic and local representations of the gravity field share common properties in the atla region in terms of their spectral behavior over a wavelength band from 2100 to 700 km the estimated free air admittance spectrum displays a rather featureless long wavelength portion followed by a sharp rise at wavelengths shorter than about 1000 km this sharp rise requires significant flexural support of short wavelength structures the bouguer coherence also displays a sharp drop in this wavelength band indicating a finite flexural rigidity of the lithosphere a simple model for lithospheric loading from above and below is introduced d w forsyth 1985 j geophys res 90 12 623 12 632 with four parameters f the ratio of bottom loading to top loading z m crustal thickness z l depth to bottom loading source and t e elastic lithosphere thickness no model is found that fits simultaneously the estimated free air admittance and bouguer coherence across the entire spectrum a dual mode compensation model is introduced in which the shorter wavelengths 1000 km might be explained best by a predominance of top loading by the large shield volcanoes maat mons ozza mons and sapas mons and the longer wavelengths 1500 km might be explained best by a deep depth of compensation possibly representing bottom loading by a dynamic source a monte carlo inversion technique is introduced to thoroughly search out the four space of the model parameters and to examine parameter correlations in the solutions it is shown that simultaneous inversion on admittance and bouguer coherence in the short wavelength band constrains the elastic thickness to a range of approximately 40 to 50 km introduction of the free air coherence as an observable in the inversion narrows the estimates of the other three parameters f 0 10 0 01 z m 30 13 km z l 7 5 km and provides a solution t e 45 3 km in the long wavelength band z l is found to be about 150 km and this is the only parameter that can be estimated robustly crustal thickness z m is unbounded and the solutions are degenerate in that there is no discrimination between top and bottom loading acceptable elastic thicknesses range from 0 to 140 km the elastic thickness results from both wavelength bands are corroborated in the spatial domain by examining residual standard deviations for a range of t e values the results found here are consistent with the results of smrekar 1994 for wavelengths longer than 600 km values of z l are consistent with other analyses that exclude wavelengths longer than about 2 000 km inclusion of wavelengths longer than about 3000 km leads to compensation depths of approximately 200 km moment curvature relationships based on yield strength envelopes are used to constrain temperature gradient average curvature is obtained from the flexurally deformed moho surface by fourier transform techniques surface temperature gradients are obtained in the range 7 10 k km 1 depending on the strain rate adopted the relationship between elastic thickness and temperature gradient is calibrated by comparing elastic thickness estimates of two east african hotspots with measured heat flow the excess heat flow at the east african hotspots over the african continental mean if applicable to venus can be used to show that the globally averaged heat flow on venus must be much lower than the earth scaled value predicts a thick 350 km ambient thermal lithosphere as proposed by a b kucinskas and d l turcotte in this issue and interpretable from the long wavelength depth of compensation at atla regio provides an acceptable background temperature gradient when it is added to the excess heat flow or temperature gradient the predicted heat flow at atla regio is consistent with the moment curvature results the thermal lithosphere beneath atla regio may be about 100 km thick and this answer can be obtained by two independent methods from the ambient thermal lithosphere thickness and simple elevation scaling result dependent on long gravity wavelengths and from the elastic thickness estimate converted to temperature gradient result dependent on short gravity wavelengths either venus is considerably deficient in heat sources relative to earth or the thermal lithosphere is overthickened in response to an earlier episode of significant heat loss from the planet