a quasi geostrophic diagnosis of the zonal flow associated with cut off lows over south africa and surrounding oceans the zonal flow associated with cut off lows cols comprises two jet streaks of different spatial extents the smaller scale jet streak located north of the cols forms as a result of meridional divergence of vorticity advection and it is quasi stationary relative to the cols it dissipates as the cols do the same the larger scale jet streak gives rise to anticyclonic and equatorward rossby wave breaking rwb as it propagates southeasterly to the base of the ridge south of the col and then northeasterly beyond that point as the jet streak propagates it brings with it the anticyclonic barotropic shear that causes the rossby waves to break its propagation is caused by zonal momentum advection by the zonal flow from jet streak entrance to its exit as it propagates its northwesterly southeasterly orientation changes to one that is more zonal to become south westerly northeasterly at the end of the col life cycle this change in orientation is due to meridional advection of zonal momentum where the meridional flow advects momentum southward northward at the jet streak entrance exit the jet streaks form a split jet structure and the winds between the streaks is decelerated by vorticity advection convergence because the flow and col and rwb life cycle are coupled understanding the dynamics that underlie the changes in the col ambient flow contributes to resolving the outstanding rwb col causality problem

two paradigms of baroclinic wave life cycle behaviour two idealized baroclinic wave 6 life cycles examined here suggest a framework of opposite extremes a in which to view the behaviour of real synoptic scale disturbances in middle latitudes and b in which to interrelate the synoptic and wave theoretic viewpoints using the saturation propagation saturation sps picture of wave 6 life cycle behaviour the two life cycles dented by lc1 and lc2 are higher resolution versions of the simmons hoskins basic and anomalous cases showing strong and weak late decay of eddy kinetic energy eke they illustrate in varying degrees two extreme types of behaviour here disignated anticyclonic and cyclonic and epitomized by strongly contrasting upper air trough behaviour anticyclonic behaviour dominates the late stages of lc1 and is characterized by backward tilted thinning troughs being advected anticyclonically and equatorward as in the commoner cases of planetary scale mid stratospheric rossby wave breaking cyclonic behaviour dominates lc2 and is characterized by forward tilted broadening troughs wrapping themselves up cyclonically and poleward producing major cut off cyclones in high latitudes these morphologies are visualized by upper air maps of potential temperature on the nominal tropopause defined as a constant potential vorticity surface some atmospheric mid latitude disturbances examined here using the same visualization applied to operational analyses show the same two extreme types of trough behaviour together with intermediate cases the sps picture is re examined using eliassen palm and refractive index cross sections it is shown in particular by reference to a wave activity theorem of haynes that the late stages of lc2 can be looked upon as a remarkably clear and morphologically novel large amplitude counterpart of the nonlinear reflection scenario of rossby wave critical layer theory the late stages of lc1 by constrast look more akin to an nonlinear critical layer absorption scenario lc2 exhibits region of largely undular pv contours adjacent to a nonlinear of irreversiby deformen pv contous in the latter region pv rearrangement and hence absorption of rossby wave activity has largely ceased this accounts for the more persistent ekee in the lc2 case

a diagnostic study of jet streaks kinematic signatures and relationship to coherent tropopause disturbances abstract a diagnostic study is conducted of the kinematics and evolution of upper level jet streaks representative of three of the four phases in the shapiro conceptual model of a jet streak progressing through a synoptic scale baroclinic wave over north america the three phases selected for consideration apply to those segments of the wave pattern where jet streaks are relatively straight the 1200 utc 2 december 1991 case trough over ridge considers a strong jet streak located over eastern north america and constitutes the bulk of the study the other two cases which also concern jet streaks over north america are from 0000 utc 11 november 1995 northwesterly flow and 0000 utc 28 october 1995 southwesterly flow kinematic signatures consistent with the classic four quadrant conceptual model for a straight jet streak are evident in all three cases although flow curvature and thermal advection lead to significant departures from this conceptual model the position of the jet streak within the s

processes of formation and evolution of mountain belts on venus orogenic belts in western ishtar terra venus rise up to 11 km in elevation and have been interpreted to be sites of crustal shortening resulting in localized crustal thickening and local changes in thermal gradient airy isostasy models imply crustal thicknesses in excess of 45 km we explore the range of conditions in the mountain ranges linked to variations in crustal thickness and thermal gradient on the basis of the high topographic elevations and evidence for crustal shortening and thickening we interpret the thermal gradients to be less than the values of 15 25 k km estimated for low lying plains regions high venus surface temperatures enhancing crustal detachment and gravity sliding and basalt eclogite phase changes in the deep mountain roots are likely to dominate the topographic evolution of mountain belts on venus as a test of this hypothesis if evidence for early volcanism associated with crustal thickening can be found in the magellan data then low strain rates and high thermal gradients are predicted throughout orogenesis a lack of early volcanism would indicate high strain rates and low thermal gradients

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

climatology of intrusions into the tropical upper troposphere regions of upper tropospheric equatorial westerly winds observed over the pacific and atlantic oceans during northern fall to spring are important for extratropical tropical interactions this paper focuses on one feature of these westerly ducts that has received relatively little attention to date the occurrence of rossby wave breaking events that transport tongues of extratropical air deep into the tropics mix tropical and subtropical air and can affect deep convection a climatology of these intrusion events formed from 20 years of meteorological analyses shows a strong dependence on the basic state flow notably intrusion events are found to occur almost exclusively within westerly ducts with more events in the presence of stronger equatorial westerlies it is also found that there is strong interannual variability in the frequency of pacific events with fewer events during the warm phases of enso consistent with the changes in the basic flow since these intrusion events laterally mix trace constituents and have been linked to tropical convection their spatial and temporal variability may cause related variability in the distribution of trace constituents and tropical convection

the link between cut off lows and rossby wave breaking in the southern hemisphere the climatological link between cut off low col pressure systems that occur from 20 to 50 s and rossby wave breaking rwb in the southern hemisphere sh is examined for 1979 2008 it is shown that cols are associated with either rwb events 89 or with potential vorticity pv intrusions where there is north south advection of high pv air 11 in the vast majority of cols the rwb events occur upstream on or before the day of the col formation the evolution of the pv geopotential heights static stability absolute vorticity and temperature advection fields during the cols are consistent with the formation of high pv anomalies that induce cyclonic circulations as predicted by theory rwb plays a key role in producing the split flow associated with cols in the sh which in turn produces absolute vorticity anomalies by shear curvature vorticity conversion and creates static stability anomalies the cols associated with rwb at 330 k are deeper and more persistent than those associated with 350 k rwb and surface processes differ depending on the isentropic surface on which the associated rwb occurs the seasonality of the rwb and cols are similar and is linked to the seasonal march of the westerly jets copyright 2010 royal meteorological society

thermal evolution of venus abstract the theory of three dimensional and finite amplitude convection in a viscous spherical shell with temperature and pressure dependent physical parameters is developed on the basis of a modified boussinesq fluid assumption the lateral dependences of the variables are resolved through their spherical harmonic representations whereas their radial and time dependences are determined by numerical procedures the theory is then applied to produce thermal evolution models for venus the emphasis is on illustrating the effects of certain physical parameters on the thermal evolution rather than proposing a specific thermal history for the planet the main conclusions achieved in this paper are 1 a significant portion of the present temperature in the mantle and heat flux at the surface of venus is probably owing to the decay of a high temperature established in the planet at the completion of its core formation 2 the effective rayleigh number of the mantle is so high that even the lower order modes of convection cool the planet sufficiently and maintain an almost adiabatic temperature gradient in the convecting region and high temperature gradients in the thermal boundary layers 3 the convection is oscillatory with avalanche type properties which induces oscillatory features to the surface heat flux and the thickness of the crustal layer and 4 a planetary model with a recycling crust cools much faster than those with a permanently buoyant crust the models presented in this paper suggest that venus has been highly convective during its history until 0 5 ga ago the vigorous convection was bringing hot and fresh material from the deep interior to the surface and dragging down the crustal slags floating on the surface in to the mantle the rate of cooling of the planet was so high that its core has solidified in the last 0 5 ga the vigour of convection diminished considerably and the crustal slags developed into a global and permanently buoyant crustal layer the tectonic style on venus has consequently changed from the recycling of crustal plates to hot spot volcanics at the present time the planet is completely solid except in the upper part of its mantle where partial melting may occur

floor fractured impact craters on venus implications for igneous crater modification and local magmatism regional tectonism and volcanism affect crater modification and crater loss on venus but a comparison of venusian craters to lunar floor fractured craters suggests that a third style of more localized crater controlled magmatism also may occur on venus based on lunar models for such magmatism venusian crustal conditions should generally favor crater filling volcanism over crater centered floor fracturing nevertheless three craters on venus strongly resemble extensively modified craters on the moon where deformation can be attributed to failure over large crater centered intrusions models for crater modification over igneous intrusions indicate typical magmatic pressures beneath these three craters of 200 300 bars and intrusion depths of the order of 1 6 km all three craters also share common settings and low elevations whereas craters embayed by regional volcanism preferentially occur at much higher elevations on venus we suggest that the style of igneous crater modification on venus thus may be elevation dependent with crater centered intrusions primarily occurring at low elevations on venus this interpretation is consistent with theoretically predicted variations in magmatic neutral buoyancy depth as a function of atmospheric pressure suggested by other authors

large scale volcanic activity at maat mons can this explain fluctuations in atmospheric chemistry observed by pioneer venus magellan and pioneer venus orbiter pvo found high mountain terrains on venus 2 5 km above 6051 km planetary radius exhibit anomalously low radiothermal emissivity this is thought to result from weathering of primary venus rock which at high altitudes produces a distinctive high dielectric constant mineral assemblage deviations from the nominal altitude emissivity pattern have been used as a crude chronometric tool with which to date venusian landforms this technique indicates maat mons an unusually large shield volcano standing 9 17 km above 6051 km planetary radius at 2 1 n 194 3 e has undergone a recent episode of large scale volcanic activity a deduction also implied morphologically the present paper investigates whether a plinian eruption at maat mons could explain the enhanced concentrations of so2 gas in the upper atmosphere of venus that were detected by the pioneer venus uv spectrometer the results show for a minimum vent radius of 156 m a minimum eruption temperature of 1200 k and a magmatic volatile content of 5 wt a plinian eruption at the summit of maat mons can explain the anomalous concentration of so2 gas this would mean maat mons is an active volcano

the role of nonquasigeostrophic forcing in southern hemisphere blocking onsets abstracta generalized frictionless adiabatic geostrophic zonal wind tendency equation is derived to diagnose the nonquasigeostrophic forcings to blocking onset in the southern hemisphere through case study and composite analysis in general the quasigeostrophic model is capable of representing the key physical processes associated with blocking onset in the troposphere reasonably well in most blocking cases the consideration of nonquasigeostrophic forcings moderately improves the quasigeostrophic representation in a majority of the blocking events selected for this study but not all events this suggests that the nonquasigeostrophic terms could be important in a specific blocking event but not in a composite meaning furthermore the nonquasigeostrophic forcing of geostrophic advection of ageostrophic relative vorticity term is extensively examined in this study this forcing is found to be the leading nonquasigeostrophic forcing term among all nonquasigeostrophic forcings in a composite sense th

a review of the structure and dynamics of upper level frontal zones abstract this article presents a review of upper level fronts with the intent of synthesizing observational and modeling studies into a conceptual and dynamical description of these fronts and their evolution relative to the life cycle of midlatitude baroclinic waves the discussion begins by tracing present day concepts concerning the structure of upper level frontal systems which are based on composite analyses of radiosonde and aircraft data from their origins in the pioneering analyses of upper air data in the 1930s perspectives from scales both smaller and larger than upper level frontal systems are provided respectively by considering the effects of turbulent processes on frontal structure and dynamics and by relating variations in frontal structure to the evolution of the baroclinic waves that provide the dynamical environment for upper level frontogenesis the dynamics of upper level fronts are shown to comprise the interactions between the primary geostrophic and secondary ageostrophic cir

investigating the impact of hurricane harvey and driving on beach dune morphology abstract beach and dune morphology are spatially and temporally variable changing over a broad range of scales simultaneously strong wind waves and storm surge from hurricane harvey substantially eroded the beach and dunes along the texas coast causing significant scarps and berms this paper presents information about how anthropogenic activity such as driving on the beach affected the response and recover of a barrier island with regard to the post storm resiliency along the texas gulf of mexico coast by comparing two adjacent 7 km stretches of coast a driving section and a limited access section a collection of field photos aerial imagery and a september 2016 lidar derived digital elevation model dem dataset provide information on pre storm morphology while field photos taken only 3 days after hurricane harvey made landfall and a structure from motion sfm derived dem and imagery provide qualitative and quantitative information about the post storm morphology while beach and dune erosion in the non driving section was restricted entirely to the beach and incipient dune system the driving section exhibited complex patterns of erosion and deposition along the beach and the entire foredune profile was altered despite the hurricane making landfall north of the study site and closer to the non driving section beach dune erosion and scarping was greatest in the southern section which is accessible to public vehicles results demonstrate that human activity affects the response and recovery of the beach dune system along the gulf side of the island by decreasing alongshore variability in erosion deposition and limiting vegetative and geomorphic recovery

sequential deformation of plains at the margins of alpha regio venus implications for tessera formation abstract the boundaries between the highly deformed tessera terrain and adjacent volcanic plains are primarily those of embayment where the tessera are stratigraphically older than the plains previous studies show that 3 of these boundaries display evidence of tectonic tilting after the emplacement of the plains one of these unusual boundaries is the western margin of alpha regio tessera a zone 100 km in width that separates the plains from the interior structures of alpha this zone is characterized by margin parallel fine scale 1 5 km fractures graben and ridges that truncate and postdate the broad scale 10 30 km ridges and troughs of the interior of alpha the western margin is embayed by several volcanic plains units that are progressively tilted and deformed by graben with closer proximity to alpha regio the earliest deformation of the plains consists of northeast trending graben 1 km in width that are similar in morphology and spacing to graben that deform intratessera plains and plains at the eastern boundary of alpha northwest trending graben then formed over an interval marked by the emplacement of two additional plains units their similarity to northwest trending structures emanating from eve corona and the lada terra rift suggests a possible genetic relationship the tilting of the plains adjacent to western alpha implies relative vertical movement of the margin either uplift of tessera or downwarping of plains subsequent to the formation and relaxation of the interior of alpha regio subsidence of plains at this locale is supported by the presence of a basin to the west of alpha surrounded by a fracture belt contiguous with western alpha thus the fractures and deformation at the western boundary of alpha may be related to the formation of a basin to the west of alpha with some influence from the northernmost extension of the lada terra rift such a basin is not present at a section along the eastern boundary of alpha regio where the origin of tilted plains remains equivocal we conclude that the deformation along the western margin of alpha regio is not directly related to the process of tessera formation but is an example of tessera modification and is consistent with the stratigraphic position of tessera as the oldest unit observed on venus

the global distribution of giant radiating dike swarms on venus implications for the global stress state magellan radar data of venus reveal 163 large radial lineament systems composed of graben fissure and fracture elements on the basis of their structure plan view geometry and volcanic associations at least 72 are interpreted to have formed primarily through subsurface dike swarm eraplacement the remainder through uplift or a combination of these two mechanisms the population of swarms is used to determine regional and global stress orientation the stress configura tion recorded from 330 210oe aphrodite terra is best ex plained by isostatic compensation of existing long wave length topography or coupling between mantle flow and the lithosphere the rest are correlated with concentrations of rifting and volcanism in the beta atla themis region the global stress field on venus is different than that of earth where plate boundary forces dominate from their surroundings where domical topography occurs the ra dius of the associated lineament system is on average 2 5 times greater than that of the dome some 53 of the radiat ing systems are associated with concentric structures ranging from central depression bounding scarps 25 km wide to tec tonic rings 575 km in diameter in 51 of these cases the ra dial pattern originates within but also extends significantly beyond the annulus only 9 of the radiating systems are confined completely leaving 40 located outside but focused upon a central annular structure finally the radial geometry is usually quite pronounced near the center of each structure 52 of the time radial lineaments occupy 270 o of azimuth and 80 exceed 180 o figure 2c in their distal regions how ever only 72 of the radiating systems retain a purely radial geometry as the remaining 28 gradually develop a non radial unidirectional lineament configuration all but seven of the radiating systems exhibit volcanism figure 2d lobate flows emerging from individual linea ments or many such flows strongly correlated with multiple radial lineaments occur for 45 of the systems clusters of small shields occur 72 of the time and in 75 of these in stances distinctive alignment of small shields and or pits along radiating lineaments is seen finally 65 are associ ated with some other form of centralized volcanism including multiple forms of edifice construction limited extrusions and sheet like flows which extend into the surrounding terrain

transitional morphology in west deuteronilus mensae mars implications for modification of the lowland upland boundary abstract west deuteronilus mensae which lies along the lowland upland boundary of mars at 45 latitude 345 longitude exhibits both gradational l a rossbacher 1985 in models in geomorphology woldenberg ed pp 343 372 allen unwin boston and fretted terrain r p sharp 1973 j geophys res 78 4073 4083 boundary types to the west the boundary is gradational and to the east it is defined by fretted terrain the two types of boundaries do not merge with one another within the west deuteronilus mensae region however the gradational boundary materials appear to overlap the fretted terrain by about 500 km contacts between units associated with the gradational boundary and the fretted terrain boundary can both be recognized in the region of overlap the fretted terrain can be identified as much as 300 km north of the gradational boundary in this region lowland units associated with the gradational boundary embay canyons of the fretted terrain in a topographically conformal fashion changes in the fretted terrain across the gradational boundary from uplands to lowlands include a reduction of canyon wall slopes and depths such that the fretted terrain north of the gradational boundary appears mantled but not obscured there are at least two major classes of processes which might explain the lateral overlap 1 erosion of stratified upland terrain and 2 deposition of plains materials onto the sloping upland margin and fretted terrain erosion of stratified upland terrain does not adequately explain the plainward decrease in crater densities across the gradational boundary nor is it consistent with evidence that lowland plains material appears to onlap the sloping upland margin in several places of the possible plains emplacement mechanisms eolian deposition would not produce the sharp apparently topographically conformal gradational unit contacts volcanics plains emplacement would not preserve the complex geometry of the underlying fretted terrain sediment deposition in either a liquid or ice covered sea could produce the draped appearance of the fretted terrain outflow channels along the lowland upland boundary particularly those of the circum chryse and west elysium regions may have flooded the northern lowlands to depths of tens to hundreds of meters the gradational unit contacts may represent the shorelines of such a sea the scale of characteristic morphologies along these contacts may require an unfrozen condition of sufficient duration to allow lacustrine style wave erosion and redistribution of material along the contacts our limited understanding of the martian paleoclimate and h 2 o inventories allows the possibility of clement periods in the past and other geological evidence e g small valley networks and outflow channels strongly suggests an extensive role of liquid water

climatological features of cutoff low systems in the northern hemisphere this study presents the first multidecadal climatology of cutoff low systems in the northern hemisphere the climatology was constructed by using 41 yr 1958 98 of ncep ncar reanalysis data and identifying cutoff lows by means of an objective method based on imposing the three main physical characteristics of the conceptual model of cutoff low the 200 hpa geopotential minimum cutoff circulation and the specific structure of both equivalent thickness and thermal front parameter fields several results were confirmed and climatologically validated 1 the existence of three preferred areas of cutoff low occurrence the first one extends through southern europe and the eastern atlantic coast the second one is the eastern north pacific and the third one is the northern china siberian region extending to the northwestern pacific coast the european area is the most favored region 2 the known seasonal cycle with cutoff lows forming much more frequently in summer than in winter 3 the short lifetime of cutoff lows most cutoff lows lasted 2 3 days and very few lasted more than 5 days and 4 the mobility of the system with few cutoff lows being stationary furthermore the long study period has made it possible i to find a bimodal distribution in the geographical density of cutoff lows for the european sector in all the seasons with the exception of winter a summer displacement to the ocean in the american region and a summer extension to the continent in the asian region and ii to detect northward and westward motion especially in the transitions from the second to third day of occurrence and from the third to fourth day of occurrence the long term cutoff low database built in this study is appropriate to study the interannual variability of cutoff low occurrence and the links between cutoff lows and jet stream systems blocking or major modes of climate variability as well as the global importance of cutoff low in the stratosphere troposphere exchange mechanism which will be the focus of a subsequent paper

a new perspective of the climatological features of upper level cut off lows in the southern hemisphere this study presents a detailed view of the seasonal variability of upper level cut off lows cols in the southern hemisphere the cols are identified and tracked using data from a 36 year period of the european centre for medium range weather forecast reanalysis era interim the objective identification of the cols uses a new approach which is based on 300 hpa relative vorticity minima and three restrictive criteria of the presence of a cold core stratospheric potential vorticity intrusion and cut off cyclonic circulation the highest col activity is in agreement with previous studies located near three main continental areas australia south america and africa with maximum frequencies usually observed in the austral autumn the col mean intensity values show a marked seasonal and spatial variation with maximum minimum values during the austral winter summer a unique feature that has not been observed previously in studies based on the geopotential the link between intensity and lysis is examined and finds that weaker systems are more susceptible to lysis in the vicinity of the andes cordillera associated with the topographic rossby wave lysis and genesis regions are close to each other confirming that cols are quasi stationary systems also cols tend to move eastward and are faster over the higher latitudes the mean growth decay rates coincide with the major genesis and lysis density regions such as the significant decay values across the andes all year as a consequence of using vorticity for the tracking method a longer lifetime of cols is detected than in other studies but this does not affect the total frequency of occurrence comparisons with other studies suggest that the differences in seasonality are due to uncertainties in the reanalyses and the methods used to identify cols

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

relation of major volcanic center concentration on venus to global tectonic patterns global analysis of nasa magellan image data indicates that a major concentration of volcanic centers covering 40 percent of the surface of venus occurs between the beta atla and themis regiones associated with this enhanced concentration are geological characteristics commonly interpreted as rifting and mantle upwelling interconnected low plains in an annulus around this concentration are characterized by crustal shortening and infrequent volcanic centers that may represent sites of mantle return flow and net down welling together these observations suggest the existence of relatively simple largescale patterns of mantle circulation similar to those associated with concentrations of intraplate volcanism on earth

comparative hypsometric analysis of earth and venus the comparison between earth and venus global hypsometric distributions is reinvestigated on the basis of the high resolution data provided by the recent magellan global topographic coverage of venus surface and the detailed etopo 5 topographic database for the earth s surface the study of the cumulative hypsometric curve for both planets reveals that there exists a domain of elevation for which the relationship between the elevation and the square root of cumulative area percentage is linear this domain covers about 80 of the mapped venusian surface and nearly all the terrestrial oceanic surface 55 of the earth s surface and corresponds to an elevation range of about 2000 m for both planets proposing the upper threshold elevation of this domain as a common geophysical reference level it is shown that the terrestrial oceanic peak and the venusian peak coincide very closely in addition the terrestrial and venusian hypsometric distributions for elevations lower than this reference level are quite similar in their amplitude modal distribution and pronounced symmetry the correspondence of the venusian peak with the terrestrial oceanic peak is a striking feature but it does not necessarily imply the occurrence of plate tectonics on venus the present observation is consistent with the existence of a similar thermal isostasy acting at the planetary scale it places an upper bound at 90 10 km for the modal thermal lithosphcric thickness of venus if the venus mantle thermal conditions are earth like this estimate is reduced if hotter conditions occur

insights into the dynamics of planetary interiors obtained through the study of global distribution of volcanoes i empirical calibration on earth abstract the distribution of volcanic features is ultimately controlled by processes taking place beneath the surface of a planet for this reason characterization of volcano distribution at a global scale can be used to obtain insights concerning dynamic aspects of planetary interiors until present studies of this type have focused on volcanic features of a specific type or have concentrated on relatively small regions in this paper the first of a series of three papers we describe the distribution of volcanic features observed over the entire surface of the earth combining an extensive database of submarine and subaerial volcanoes the analysis is based on spatial density contours obtained with the fisher kernel based on an empirical approach that makes no a priori assumptions concerning the number of modes that should characterize the density distribution of volcanism we identified the most significant modes using those modes as a base the relevant distance for the formation of clusters of volcanoes is constrained to be on the order of 100 to 200 km in addition it is noted that the most significant modes lead to the identification of clusters that outline the most important tectonic margins on earth without the need of making any ad hoc assumptions consequently we suggest that this method has the potential of yielding insights about the probable occurrence of tectonic features within other planets

the structure and evolution of lower stratospheric frontal zones part 1 examples in northwesterly and southwesterly flow the structure evolution and dynamics of two lower stratospheric frontal zones are examined from a basic state variables perspective the case studies highlight the asynchronous evolution of the lower stratospheric and upper tropospheric frontal portions of upper level jet front uljf systems as well as some substantial differences in lower stratospheric frontal development that occur in southwesterly and northwesterly flow the evolution of the uljf in northwesterly flow was characterized by an initially intense but weakening lower stratospheric front along with an initially weak but intensifying upper tropospheric front throughout the evolution geostrophic cold air advection in cyclonic shear characterized a substantial portion of the lower stratospheric front this circumstance supported subsidence through the local jet core within the cold upper troposphere weakening the lower stratospheric front via tilting this subsidence extended downward below the jet core where it is suggested to have played a role in the early stages of upper tropospheric frontogenesis in the southwesterly flow case the evolution of the uljf was characterized by a strengthening lower stratospheric front and a weakening upper tropospheric front a deep column of upward vertical motion resulted from the superposition of lower tropospheric ascent associated with convection along a surface cold front and upper tropospheric lower stratospheric utls ascent through the jet core coincident with geostrophic warm air advection in cyclonic shear along large sections of the lower stratospheric front the utls ascent located on the cold edge of the lower stratospheric baroclinicity served to intensify the lower stratospheric frontal zone via tilting the implications of these lower stratospheric frontal processes on the topography of the tropopause and downstream sensible weather are discussed copyright 2012 royal meteorological society

resurfacing history of the northern plains of mars based on geologic mapping of mars global surveyor data 1 geologic mapping of the northern plains of mars based on mars orbiter laser altimeter topography and viking and mars orbiter camera images reveals new insights into geologic processes and events in this region during the hesperian and amazonian periods we propose four successive stages of lowland resurfacing likely related to the activity of near surface volatiles commencing at the highland lowland boundary hlb and progressing to lower topographic levels as follows highest elevations indicated stage 1 upper boundary plains early hesperian 2 0 to 2 9 km stage 2 lower boundary plains and outflow channel dissection late hesperian 2 7 to 4 0 km stage 3 vastitas borealis formation vbf surface late hesperian to early amazonian 3 1 to 4 1 km and stage 4 local chaos zones early amazonian 3 8 to 5 0 km at acidalia mensa stage 2 and 3 levels may be lower 4 4 and 4 8 km respectively contractional ridges form the dominant structure in the plains and developed from near the end of the early hesperian to the early amazonian geomorphic evidence for a northern plains filling ocean during stage 2 is absent because one did not form or its evidence was destroyed by stage 3 resurfacing remnants of possible amazonian dust mantles occur on top of the vbf the north polar layered deposits appear to be made up of an up to kilometer thick lower sequence of sandy layers early to middle amazonian in age overlain by late amazonian ice rich dust layers both units appear to have outliers suggesting that they once were more extensive

rossby wave breaking in the southern hemisphere wintertime upper troposphere abstract the characteristics of rossby wave propagation and breaking in the southern hemisphere upper troposphere during winter are examined although the southern hemisphere subtropical jet is more zonally symmetric than that of the northern hemisphere there are still significant zonal variations in the upper tropospheric flow in particular the flow within a given sector 120 longitude can generally be characterized into one of four different configurations i a single jet ii a broken subtropical jet iii a polar jet at the upstream end of the subtropical jet or iv a polar jet at the downstream end of the subtropical jet using potential vorticity thinking and barotropic wind shear arguments it is argued that the characteristics of the rossby wave propagation and breaking will differ between each flow configuration consistent with these arguments examination of potential vorticity maps and contour advection calculations show differing wave breaking characteristics in particular t

what is real identifying erosion and deposition in context of spatially variable uncertainty abstract the ability to ascribe landscape change to specific processes and events depends on the ability to accurately measure change with confidence many studies simply compute the difference between two digital elevation models dems and do not quantify and propagate vertical uncertainty from multiple sources not accounting for the influence of uncertainty on the results represents a particular problem in situations where the uncertainty of one or both surfaces varies across space and the spatial distribution of uncertainty is not consistent through time using lidar data from 2009 and 2016 for follets island texas usa the purpose of this study is to present a probabilistic approach to dem change analysis that accounts for spatially variable uncertainty surfaces and their impact on the computed dem change the probability that a computed change is not an artifact of the uncertainty in either surface is determined by computing the area where the two uncertainty distributions overlap this overlapping area provides information about the likelihood that the observed change is influenced by uncertainty in both surfaces the described approach is a relatively simple method for propagating uncertainty in one or more datasets however it is not an approach to quantifying uncertainty rather this approach relies on uncertainty from one or more sources already having been quantified given its adaptability to different datasets the change probability cp approach has potential application in identifying areas of real change in disciplines beyond coastal geomorphology

large scale volcanism associated with coronae on venus implications for formation and evolution large scale volcanism in the form of areally extensive flow fields is a previously unrecognized important aspect of the evolution of at least 41 of all coronae on venus the timing and scale of many coronae flow fields is consistent with an origin due to the arrival and pressure release melting of material in the head of a mantle plume or diapir the production of voluminous amounts of volcanism at some coronae is proposed to be the result of larger plume size and or the intersection of mantle upwellings with regions of lithospheric extension and rifting

improving understanding of near term barrier island evolution through multi decadal assessment of morphologic change abstract observed morphodynamic changes over multiple decades were coupled with storm driven run up characteristics at fire island new york to explore the influence of wave processes relative to the impacts of other coastal change drivers on the near term evolution of the barrier island historical topography was generated from digital stereo photogrammetry and compared with more recent lidar surveys to quantify near term decadal morphodynamic changes to the beach and primary dune system between the years 1969 1999 and 2009 notably increased profile volumes were observed along the entirety of the island in 1999 and likely provide the eolian source for the steady dune crest progradation observed over the relatively quiescent decade that followed persistent patterns of erosion and accretion over 10 30 and 40 year intervals are attributable to variations in island morphology human activity and variations in offshore bathymetry and island orientation that influence the wave energy reaching the coast areas of documented long term historical inlet formation and extensive bayside marsh development show substantial landward translation of the dune beach profile over the near term period of this study correlations among areas predicted to overwash observed elevation changes of the dune crestline and observed instances of overwash in undeveloped segments of the barrier island verify that overwash locations can be accurately predicted in undeveloped segments of coast in fact an assessment of 2012 aerial imagery collected after hurricane sandy confirms that overwash occurred at the majority of near term locations persistently predicted to overwash in addition to the storm wave climate factors related to variations within the geologic framework which in turn influence island orientation offshore slope and sediment supply impact island behavior on near term timescales

double tropopause formation in idealized baroclinic life cycles the key role of an initial tropopause inversion layer 1 recent studies have shown that double tropopauses exist in all seasons and at all longitudes in the midlatitudes as of yet the key mechanism responsible for their formation is not known in this study we explore the connection between double tropopauses and midlatitude baroclinic eddies this is investigated in the context of idealized life cycle experiments the key finding of this study is that large areas of double tropopauses form spontaneously at the nonlinear stage of the life cycle evolution provided an extratropical tropopause inversion layer is present in the balanced initial temperature profile we also show that the areas covered with double tropopauses grow as the strength of the initial tropopause inversion layer is increased without such a layer as in canonical examples of baroclinic life cycles much studied in the literature no double tropopause formation occurs in agreement with observations double tropopauses in our life cycle experiments form predominantly in areas of cyclonic flow at upper levels however the air masses that end up between the two tropopauses are found to originate from high latitudes this appears to differ from a recently published case study where the air between double tropopauses was shown to originate partly from low latitudes such a discrepancy suggests that more than one pathway may exist to advect air masses between the two tropopauses

scale dependent behavior of the foredune implications for barrier island response to storms and sea level rise the impact of storm surge on a barrier island tends to be considered from a single cross shore dimension dependent on the relative elevations of the storm surge and dune crest however the foredune is rarely uniform and can exhibit considerable variation in height and width at a range of length scales in this study lidar data from barrier islands in texas and florida are used to explore how shoreline position and dune morphology vary alongshore and to determine how this variability is altered or reinforced by storms and post storm recovery wavelet analysis reveals that a power law can approximate historical shoreline change across all scales but that stormscale shoreline change 10 years and dune height exhibit similar scale dependent variations at swash and surf zone scales 1000 m the in phase nature of the relationship between dune height and storm scale shoreline change indicates that areas of greater storm scale shoreline retreat are associated with areas of smaller dunes it is argued that the decoupling of storm scale and historical shoreline change at swash and surf zone scales is also associated with the alongshore redistribution of sediment and the tendency of shorelines to evolve to a more diffusive or straight pattern with time the wavelet analysis of the data for post storm dune recovery is also characterized by red noise at the smallest scales characteristic of diffusive systems suggesting that it is possible that small scale variations in dune height can be repaired through alongshore recovery and expansion if there is sufficient time between storms however the time required for dune recovery exceeds the time between storms capable of eroding and overwashing the dune correlation between historical shoreline retreat and the variance of the dune at swash and surf zone scales suggests that the persistence of the dune is an important control on transgression through island migration or shoreline retreat with relative sea level rise

mars and earth comparison of cold climate features abstract on earth glacial and periglacial features are common in areas of cold climate on mars the temperature of the present day surface is appropriate for permafrost and the presence of water is suspected from data relating to the outgassing of the planet from remote sensing measurements over the polar caps and elsewhere on the martian surface and from recognition of fluvial morphological features such as channels these observations and the possibility that ice could be in equilibrium with the atmosphere in the high latitudes north and south of 40 latitude suggest that glacial and periglacial features should exist on the planet morphological studies based mainly on viking pictures indicate many features that can be attributed to the action of ice among these features are extensive talus aprons debris avalanches flows that resemble glaciers or rock glaciers ridges that look like moraines various types of patterned ground scalloped scarps and chaotically collapsed terrain that could be attributed to thermokarst processes and landforms that may reflect the interaction of volcanism and ice

a new perspective toward cataloging northern hemisphere rossby wave breaking on the dynamic tropopause abstractrossby wave breaking rwb events are a common feature on the dynamic tropopause and act to modulate synoptic scale jet dynamics these events are characterized on the dynamic tropopause by

corona classification by evolutionary stage abstract the formation of coronae abundant volcano tectonic features on the surface of venus has been modeled as the interaction of mantle diapirs with the lithosphere however the applicability of this model to features of widely different sizes may not be evident from sar images due to the lack of a common scale we use the radius as a scaling parameter for both the distance from the center and relief obviating this problem and mapping the effects from coronae of different radii onto a single scale normalizing profiles for 394 features we find many features classified by e r stoffanet al 1992 j geophys res 97 13 347 13 378 as different types have very similar profiles and relief coronae distribute into three shapes domal circular calderic two shapes not related to coronae radial and volcanic remain variation between corona shapes is gradational and mirrors the evolution of diapirs from initial domal upwellings to calderas we test the implied age progression using impact crater populations within three corona radii despite the inherent uncertainties of this method crater statistics for a size restricted 275 325 km corona set suggest an age progression from domal through circular to calderic the calderic coronae have an associated impact crater population very similar to the global mean though with slightly too few tectonized craters implying that they approximate the average surface age and are no longer active the circular coronae have a low impact crater density and a slightly elevated proportion of tectonized craters and thus are currently active the domal coronae have fewer than the expected number of impact craters of which more are tectonized than expected suggesting that surface modification has begun

hierarchical clustering of pit crater chains on venus composed of a series of circular to elliptical bowl shaped depressions pit crater chains are common on the surface of many of our solar system s terrestrial planets and moons using magellan synth

decoupling processes and scales of shoreline morphodynamics abstract behavior of coastal systems on time scales ranging from single storm events to years and decades is controlled by both small scale sediment transport processes and large scale geologic oceanographic and morphologic processes improved understanding of coastal behavior at multiple time scales is required for refining models that predict potential erosion hazards and for coastal management planning and decision making here we investigate the primary controls on shoreline response along a geologically variable barrier island on time scales resolving extreme storms and decadal variations over a period of nearly one century an empirical orthogonal function analysis is applied to a time series of shoreline positions at fire island ny to identify patterns of shoreline variance along the length of the island we establish that there are separable patterns of shoreline behavior that represent response to oceanographic forcing as well as patterns that are not explained by this forcing the dominant shoreline behavior occurs over large length scales in the form of alternating episodes of shoreline retreat and advance presumably in response to storms cycles two secondary responses include long term response that is correlated to known geologic variations of the island and the other reflects geomorphic patterns with medium length scale our study also includes the response to hurricane sandy and a period of post storm recovery it was expected that the impacts from hurricane sandy would disrupt long term trends and spatial patterns we found that the response to sandy at fire island is not notable or distinguishable from several other large storms of the prior decade

controls on coastal dune morphology shoreline erosion and barrier island response to extreme storms abstract the response of a barrier island to an extreme storm depends in part on the surge elevation relative to the height and extent of the foredunes which can exhibit considerable variability alongshore while it is recognized that alongshore variations in dune height and width direct barrier island response to storm surge the underlying causes of the alongshore variation remain poorly understood this study examines the alongshore variation in dune morphology along a 11 km stretch of santa rosa island in northwest florida and relates the variation in morphology to the response of the island during hurricane ivan and historic and storm related rates of shoreline erosion the morphology of the foredune and backbarrier dunes was characterized before and after hurricane ivan using empirical orthogonal function eof analysis and related through canonical correlation analysis cca the height and extent of the foredune and the presence and relative location of the backbarrier dunes varied alongshore at discrete length scales of 750 1450 and 4550 m that are statistically significant at the 95 confidence level cospectral analysis suggests that the variation in dune morphology is correlated with transverse ridges on the inner shelf the backbarrier cuspate headlands and the historical and storm related trends in shoreline change sections of the coast with little to no dune development before hurricane ivan were observed in the narrowest portions of the island between headlands west of the transverse ridges overwash penetration tended to be larger in these areas and island breaching was common leaving the surface close to the watertable and covered by a lag of shell and gravel in contrast large foredunes and the backbarrier dunes were observed at the widest sections of the island the cuspate headlands and at crest of the transverse ridges due to the large dunes and the presence of the backbarrier dunes these areas experienced less overwash penetration and most of the sediment from the beachface and dunes was deposited within the upper shoreface it is argued that this sediment is returned to the beachface through nearshore bar migration following the storm and that the areas with larger foredunes and backbarrier dunes have smaller rates of historical shoreline erosion compared to areas with smaller dunes and greater transfer of sediment to the washover terrace since the recovery of the dunes will vary depending on the availability of sediment from the washover and beachface it is further argued that the alongshore pattern of dune morphology and the response of the island to the next extreme storm is forced by the transverse ridges and island width through alongshore variations in storm surge and overwash gradients respectively these findings may be particularly important for coastal managers involved in the repair and rebuilding of coastal infrastructure that was damaged or destroyed during hurricane ivan

origin of giant martian polygons extensive areas of the martian northern plains in utopia and acidalia planitiae are characterized by polygonal terrane polygonal terrane consists of material cut by complex troughs defining a pattern resembling mudcracks columnar joints or frost wedge polygons on earth however the martian polygons are orders of magnitude larger than these potential earth analogues leading to severe mechanical difficulties for genetic models based on simple analogy arguments plate bending and finite element models indicate that shrinkage of desiccating sediment or cooling volcanics accompanied by differential compaction over buried topography can account for the stresses responsible for polygon troughs as well as the large size of the polygons although trough widths and depths relate primarily to shrinkage the large scale of the polygonal pattern relates to the spacing between topographic elevations on the surface buried beneath polygonal terrane material geological relationships favor a sedimentary origin for polygonal terrane material but our model is not dependent on the specific genesis our analysis also suggests that the polygons must have formed at a geologically rapid rate

foredunes and blowouts initiation geomorphology and dynamics this paper reviews the initiation dynamics geomorphology and evolution of incipient foredunes established foredunes and blowouts incipient foredunes may be initiated in a variety of ways leading to the formation of ramps terraces or ridges depending on progradation rates vegetation type and cover sediment transport rates and scale of erosional processes vegetation cover morphology growth rates and types combine with aerodynamic processes to determine dune and swale morphology established foredunes are classified into five morpho ecological types and the flow behaviour over two types is detailed a new model of foredune evolutionary paths at various possible time scales is presented blowout initiation dynamics and sediment transport and evolution is examined throughout the review the gaps in present understanding of dune processes and a range of ideas for new research possibilities are provided

calculated effect of vesicles on the thermal properties of cooling basaltic lava flows abstract this paper investigates the effects of porosity in the form of vesicles and bubbles on the transfer of heat within basaltic lava this study was undertaken to provide input for realistic cooling models of basaltic lava flows and to help explain some recent field measurements by jones 1992 1993 these field observations indicate that the surfaces of vesicular pahoehoe flows cool significantly more rapidly than that of dense flows it has been suggested that thermal radiation across vesicles is responsible for this enhanced cooling rate it is shown here that for vesicles in typical pahoehoe flows radiation across vesicles may enhance the effective thermal conductivity and thermal diffusivity but only at high temperatures t 800 c and high vesicularities 40 it is also found that convection of the gas phase within bubbles will not occur unless the cavities are larger than about 1 cm furthermore previous work has demonstrated that porosity greatly reduces the thermal conductivity and thermal inertia of cold lava it is shown here that this should also be true at high temperatures examining only radiation across vesicles severely understates the effect of porosity on the cooling of the surface of lava flows thermal inertia developed to describe diurnal surface temperature variations is the most appropriate thermal parameter in this case thermal inertia is a measure of how quickly surface temperature responds to heating or cooling with low thermal inertia materials responding more quickly it is shown here that porosity greatly reduces thermal inertia at all temperatures and that this provides a more general explanation for the field observation

modification of the quaternary stratigraphic framework of the inner continental shelf by holocene marine transgression an example offshore of fire island new york abstract the inner continental shelf off fire island new york was mapped in 2011 using interferometric sonar and high resolution chirp seismic reflection systems the area mapped is approximately 50 km long by 8 km wide extending from moriches inlet to fire island inlet in water depths ranging from 8 to 32 m the morphology of this inner continental shelf region and modern sediment distribution patterns are determined by erosion of pleistocene glaciofluvial sediments during the ongoing holocene marine transgression much of the shelf is thus an actively forming ravinement surface remnants of a pleistocene outwash lobe define a submerged headland offshore of central fire island east of the submerged headland relatively older pleistocene outwash is exposed over much of the inner continental shelf and covered by asymmetric sorted bedforms interpreted to indicate erosion and westward transport of reworked sediment erosion of the eastern flank of the submerged pleistocene headland over the last 8000 years yielded an abundance of modern sand that was transported westward and reworked into a field of shoreface attached ridges offshore of western fire island west of the submerged headland erosion of pleistocene outwash continues in troughs between the sand ridges resulting in modification of the lower shoreface comparison of the modern sand ridge morphology with the morphology of the underlying ravinement surface suggests that the sand ridges have moved a minimum of 1000 m westward since formation comparison of modern sediment thickness mapped in 1996 1997 and 2011 allows speculation that the nearshore shoreface sedimentary deposit has gained sediment at the expense of deflation of the sand ridges

embayed intermediate volcanoes on venus implications for the evolution of the volcanic plains abstract volcanoes on venus are classified according to size with studies on the stratigraphic position of large volcanoes proposing that most of the large volcanoes postdate the regional volcanic materials some studies regarding intermediate volcanoes proposed that some of these volcanic features could be large volcanoes with embayed flow aprons a situation that would alter the previous stratigraphic considerations about large volcanoes on venus in this work i analyze the global population of embayed intermediate size volcanoes and compare their summits with that of other edifices classified as large volcanoes intermediate size volcanoes are considered embayed when 1 flows from another source clearly overlap the volcano slopes and 2 display scarps related to flank failure processes but with the associated collapse deposits being absent i e interpreted as covered as result of the survey 88 embayed intermediate size volcanoes have been catalogued and integrated into a geographic information system these embayed volcanoes have summit sizes and characteristics similar to large volcanoes and therefore could be interpreted as possible large volcanoes with their flow aprons embayed embayment materials for these volcanoes include all the units present in the history of the volcanic plains and would indicate that this type of central volcanic edifice would occur throughout the geologic history recorded in the venusian plains

insights into the dynamics of planetary interiors obtained through the study of global distribution of volcanoes ii tectonic implications from venus abstract the distribution of volcanic features is ultimately controlled by processes taking place beneath the surface of a planet for this reason characterization of volcano distribution at a global scale can be used to obtain insights concerning dynamic aspects of planetary interiors until present studies of this type commonly have focused on volcanic features of a specific type e g large volcanoes in venus or hot spot volcanism on earth or have concentrated on relatively small regions i e vent distribution within individual volcanic fields but no comparison of extensive databases has been made by using the same tools in both planets in this work the description of the distribution of volcanic features observed over the entire surface of venus is made using the same tool used for earth and is applied to an extensive database the analysis is based on density contours obtained with the fisher kernel as a result several groupings of volcanoes are identified refining the already documented concentration of volcanoes on the bat zone in particular some doughnut like patterns are observed that might be related to the action of mantle plumes the occurrence of such features on earth as well as the existence of a uniform distribution of background volcanism on both planets suggests similarities on their geodynamic behavior that had not been identified previously

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

influence of heating mode on three dimensional mantle convection numerical models of three dimensional thermal convection in highly viscous spherical shells with different combinations of internal and basal heating consistently have upwelling concentrations in the form of cylindrical plumes and downwelling in planar sheets as the proportion of internal heating increases the number of upwelling plumes increases and downwelling sheets become more vigorous and time dependent with any amount of basal heating the entire convective pattern during its evolution is anchored to the upwelling plumes as the proportion of internal heating increases the heat flow carried by the upwelling plumes remains a large fraction of the basal heat flow downwelling sheets carry only a minor fraction approximately 30 of the basal heat flow even when the shell is entirely heated from below but they advect almost all of the internally generated heat the relatively large number of plumes in the earth s mantle inferred from hotspots the possibility that downwelling slabs are vigorous enough to penetrate the lower mantle and the small fraction of terrestrial surface heat flow carried by plumes all suggest that the mantle is predominantly heated from within copyright american geophysical union 1989

ridge belts evidence for regional and local scale deformation on the surface of venus we evaluate models for the formation of the ridge belt fan assemblage on venus through consideration of the orientation spatial distribution topographic expression and wavelengths of observed tectonic surface features we favor a compressional mechanism for long wavelength deformation corresponding to the spacing of ridge belts 300 400 km however short wavelength ridges and grooves 10 20 km that are contained within belts and trend parallel to them are likely to be compressional and extensional and to reflect both regional and local stress fields we hypothesize that large scale ridge spacing early stage mantle downwelling is the source of regional compression responsible for the establishment of the long wavelength of deformation

climatological features of cutoff low systems in the southern hemisphere abstract this study presents the first multidecadal climatology of cutoff low systems in the northern hemisphere the climatology was constructed by using 41 yr 1958 98 of ncep ncar reanalysis data and identifying cutoff lows by means of an objective method based on imposing the three main physical characteristics of the conceptual model of cutoff low the 200 hpa geopotential minimum cutoff circulation and the specific structure of both equivalent thickness and thermal front parameter fields several results were confirmed and climatologically validated 1 the existence of three preferred areas of cutoff low occurrence the first one extends through southern europe and the eastern atlantic coast the second one is the eastern north pacific and the third one is the northern china siberian region extending to the northwestern pacific coast the european area is the most favored region 2 the known seasonal cycle with cutoff lows forming much more frequently in summer than in winter 3 the short

rootless cones on mars indicating the presence of shallow equatorial ground ice in recent times high resolution mars orbiter camera moc images have revealed the existence of clusters of small cones in the cerberus plains marte valles and amazonis planitia mars these cones are similar in both morphology and planar dimensions to the larger of icelandic rootless cones which form due to explosive interactions between surficial lavas and near surface groundwater impact crater size frequency relationships indicate that surfaces upon which the cones sit are no older than 10 ma if martian cones form in the same manner as terrestrial rootless cones then equatorial ground ice or ground water must have been present near the surface in geologically recent times

orogenic belts on venus recent radar observations of venus show evidence for three types of deformational features in the linear mountain belts of ishtar terra linear ridges and troughs oriented parallel to the strike of the mountains and interpreted to be anticlines and synclines broad low arches similar to mare ridge type features seen on the moon and interpreted to represent low angle thrusting and buckling and linear discontinuities that cut across the strike of the ridges and troughs and are interpreted to represent strike slip movement on the basis of the localized concentration of these features their orientations and patterns of distribution and their association with linear mountainous topography we conclude that they mark the location of concentrated horizontal compressional deformation and that akna and freyja montes represent erogenic belts on venus

tessera terrain on venus a survey of the global distribution characteristics and relation to surrounding units from magellan data the tessera terrain on venus comprised of areas of high radar backscatter complex deformation patterns relative to other units and topography standing higher than surrounding plains covers 35 33 106 km2 about 8 of the surface of venus and is nonrandomly distributed being preferentially located at equatorial and higher northern latitudes with a distinct paucity below about 30 s individual tessera occurrences range in area from the lower limits of our measurements about 200 km2 up to the largest tessera ovda with an area of about 8 6 106 km2 or about 2 of the surface area of venus the size frequency distribution of tessera patches is strongly unimodal and skewed toward smaller sizes reflecting the great abundance of small tessera fragments modes of occurrence include 1 large clusters e g aphrodite terra and ishtar terra 2 arc like segments which may extend for thousands of kilometers and are either concave inward toward the major tessera cluster development or away from it 3 areas where tesserae are rare or absent which occur both as low lying plains e g guinevere planitia and as elevated regions e g atla regio tessera terrain has a bimodal elevation frequency distribution with the main peak at about 0 1 km and an additional peak at about 3 km above mean planetary radius in terms of number of occurrences however tesserae do not display a correlation with elevation at the global scale since small tessera patches commonly occupy low lying regions although tessera exhibit a range of gravity signatures many occurrences are interpreted to represent relatively shallow crustal levels of compensation tessera boundaries include type i sinuous embayed dominated by adjacent lava plains embaying tessera massifs 73 of total tesserae boundaries and type ii linear tectonic only a small percentage of the length of all boundary types show no lava embayment and could be interpreted as tectonically active for long periods subsequent to initial tessera formation occurrence of broad slopes of post tessera embayed plains away from tessera boundaries suggests that regional tilting occurred subsequent to final tessera deformation in some places several lines of evidence suggest the possibility that a widespread tessera like basement comprising at least 55 of the surface of venus is buried under a cover of lava plains a few hundred meters to as much as 2 4 km thick a wide variety of deformational structures and patterns is observed within the tessera including those representing extension compression shear and transpression in some cases the apparently complex patterns can be resolved into single event kinematic interpretations involving noncoaxial deformation e g itzpapalotl while in other cases polyphase deformation is more likely e g central ovda and thetis where relations can be determined stratigraphically earliest deformation within the tessera is primarily related to crustal shortening and compression phase i followed by pervasive extensional deformation commonly oriented normal to the strike of phase i features generally along the same principal stress direction phase ii evidence also exists for the contemporaneous formation of these distinctive deformation patterns lava plains within and adjacent to the tessera embay both of these fabrics but sometimes overlap in time with phase ii extensional deformation and with regional tilting tessera terrain as a geologic unit occupies the lowest portion of the stratigraphic column in all areas that we have observed an observation consistent with many other mapping studies we see no evidence for transitional stages between tessera and volcanic rises and or lowlands that might represent a long term sequence of upwelling or downwelling followed by crustal deformation and tessera formation no impact craters deformed by phase i deformation have yet been observed on tessera suggesting that phase i tessera deformation sufficiently intense to eradicate earlier impact craters ceased relatively abruptly somewhat before 300 500 m y ago however the starting time and thus duration of tessera formation is unknown on the basis of the very small number of on tessera craters deformed by phase ii extensional deformation this period probably did not last more than several tens of millions of years after the cessation of phase i little observable deformation of the tessera terrain appears to have taken place in the last several hundred million years during which time the vast volcanic plains were emplaced although tilting of early plains along some tessera margins is observed building on the global synthesis presented here future analyses of individual tessera occurrences will provide the detailed descriptions kinematic interpretations and strain histories necessary to assess and distinguish between the several catastrophic and uniformitarian models for tessera formation

beta regio venus evidence for uplift rifting and volcanism due to a mantle plume abstract geophysical data have led to the interpretation that beta regio a 2000 25000 km wide topographic rise with associated rifting and volcanism formed due to the rise of a hot mantle diapir interpreted to be caused by a mantle plume we have tested this hypothesis through detailed geologic mapping of the v 17 quadrangle which includes a significant part of the beta regio rise and reconnaissance mapping of the remaining parts of this region our analysis documents signatures of an early stage of uplift in the formation of the agrona linea fracture belts before the emplacement of regional plains and their deformation by wrinkle ridging we see evidence that the theia rift associated volcanism occurred during the first part of post regional plains time and cannot exclude that it continued into later time we also see evidence that devana chasma rifting was active during the first and the second parts of post regional plains time these data are consistent with uplift rifting and volcanism associated with a mantle diapir geophysical modeling shows that diapiric upwelling may continue at the present time together these data suggest that the duration of mantle diapir activity was as long as several hundred million years the regional plains north of beta rise and the area east and west of it were little affected by the beta forming plume but the broader area at least 4000 km across whose center northern part includes beta regio could have experienced earlier uplift as morphologically recorded in formation of tessera transitional terrain

crustal analysis of venus from magellan satellite observations at atalanta planitia beta regio and thetis regio we investigated an improved quantitative method for interpreting attributes of the venusian crust from magellan spacecraft imagery and spherical harmonic topographic and gravity models we used spectral correlation theory to compare gravity effects of topography against free air gravity anomalies for three tectonically disparate crustal regions adjusting terrain gravity effects for correlated free air anomalies yields compensated effects that we used to estimate the moho and crustal thickness variations for atalanta planitia the crust may be less than 10 km thick but thickened peripherally at concentric ridge belts and tessera terrain crustal thickness may increase rapidly between the interior of this lowland plain and the peripheral ridge belts for beta regio the crustal thickness may exceed 40 km of the three regions considered the crust of beta regio may be the most poorly compensated because its free air anomalies are largest relative to corresponding terrain gravity effects for thetis regio crustal thicknesses may exceed 50 km the thickened crust of thetis regio may also have been rifted and embayed by plains forming lavas the lavas appear to embay and cover complexly ridged areas except where these units remained above the reach of the plains forming lavas crosscutting relationships which are evident from magellan imagery together with our crustal thickness estimates suggest that the complexly ridged terrain or tesserae may be the oldest crustal units within the three study regions hence these units may represent relict surfaces that predate emplacement of the plains

stratigraphy of small shield volcanoes on venus criteria for determining stratigraphic relationships and assessment of relative age and temporal abundance 1 small volcanic edifices shields with a diameter less than about 20 km are common and sometimes very abundant features on the plains of venus typically they form tight or loose clusters of features known as shield fields small shields are interpreted to be formed due to small scale eruptions through numerous and distinct sources a mode of formation apparently significantly different from the mechanism thought to be responsible for the emplacement of the vast regional plains of venus did the eruption style of small shields occur repeatedly throughout the visible part of the geologic record of venus or was this style more concentrated in a specific epoch or epochs of geologic history do the clusters of shields represent localized development of sources over a thermal anomaly such as a plume or do they represent exposures or kipukas of a more regional unit or units a major step toward answering these questions is an understanding of small shield stratigraphy multiple criteria have been developed to assess the stratigraphic relationships of individual small shields and that of shield fields with the adjacent units in our analysis we expanded and developed the previous criteria and added detailed criteria to describe specific patterns of deformation within shield fields cross cutting and embayment relationships between shield fields and surrounding units we also used secondary characteristics of shield fields such as radar albedo difference changes in shield density and size etc in our study we applied these criteria and analyzed in detail stratigraphic relationships of shield fields in a random sample of features 64 fields and in the global geotraverse along 30 n 77 fields the total number of analyzed shield fields 141 represents about 22 of the general population of these features catalogued by crumpler and aubele 2000 the majority of the fields 98 or 69 predate emplacement of material of vast regional plains with wrinkle ridges fifteen fields 11 appear to be synchronous with regional plains and eleven fields 8 postdate the plains nine fields 6 display ambiguous relationships with regional plains and their relative age is uncertain and eight fields 6 represent unclear cases when fields are covered by crater related materials or by young lava flows or are not in contact with regional plains the results of our study provide evidence for a distinct change of volcanic style from the mode of formation of globally abundant small shields to the mode of emplacement of vast regional plains in many areas on venus this systematic change of volcanic style appears to be inconsistent with the nondirectional or quasi steady state character of the geologic record of venus although individual small shields were formed throughout the majority of the visible geologic history of venus in the syn and postregional plains time the small shields style of volcanism was significantly reduced in abundance the shield fields that predate regional plains do not display a strong tendency to form a single group or a few groups and can be found virtually in all places on venus we interpret this observation to mean that these shield fields were globally distributed before the emplacement of regional plains this interpretation means that the shield fields embayed by regional plains represent exposures of a specific globally widespread unit shield plains psh in contrast shield fields that postdate regional plains occur preferentially in the beta atla themis region on venus well known for its concentration of relatively young volcanic and tectonic activity the spatial association of relatively young fields with the large centers of young volcanism suggests a genetic link of these fields with the formation and development of the large scale volcanic centers the abrupt decrease of the number of shields that postdate the formation of shield plains psh strongly suggests a major change of the style of volcanism following their emplacement as a globally distributed unit

gravity analysis of parga and hecate chasmata implications for rift and corona formation 1 the two largest rift systems on venus parga and hecate chasmata contain one third of all coronae we map variations in elastic thickness and apparent depth of compensation adc in these two regions using the admittance function for gravity and topography we examine the relationship between rifting and coronae by comparing lithospheric structure with corona characteristics including volcanism topographic shape fracture pattern diameter and stratigraphic age at hecate chasmata both adc and elastic thickness correlate with two main rift branches the fracture style and corona characteristics hecate chasma thus appears to be dominated by extensional processes parga chasma shows little correlation between lithospheric properties and the location of coronae or rift segments the adc at hecate and parga chasmata is mostly less than 75 100 km implying that there is not large scale upwelling underlying the rift systems the variations in the corona population sizes between parga chasma which has 131 coronae and hecate chasma with 50 coronae suggest differences in the evolution of the two rift systems the much larger population of coronae at parga contains more small coronae many of which have relatively low volcanism and post date the rift further many of the small coronae occur in chains and have similar diameters as predicted by rayleigh taylor type instabilities these characteristics suggest that the smaller coronae may form via a later secondary decompression melting phase thus hecate chasma may be younger than parga chasma and could experience a secondary stage of corona formation in the future

influence of a spatially complex framework geology on barrier island geomorphology abstract barrier island response and recovery to storms and island transgression with relative sea level rise can be influenced by the framework geology the influence of framework geology on barrier island geomorphology has previously been examined in areas where the framework is rhythmic alongshore or consists of an isolated paleo channel or headland the purpose of this paper is to examine the influence of framework geology on beach and dune geomorphology at padre island national seashore pais texas usa where the framework geology is variable alongshore alongshore beach and dune morphometrics and offshore bathymetric profiles were extracted from a combined topography and bathymetry digital elevation model dem using an automated approach along the 100 km study area and an electromagnetic induction emi survey was used to map the subsurface framework geology wavelet decomposition global wavelet gw and bicoherence analyses were used to test for spatial relationships between and within the extracted alongshore metrics gw trendlines demonstrate that beach and dune morphometrics are structurally controlled hotspots in wavelet coherence plots between framework geology and alongshore island morphometrics indicate that the paleo channels dissecting the island influence beach and dune morphology with large dunes found in the area directly landward of the paleochannels bicoherence analysis of alongshore beach and dune morphometrics indicates that low frequency oscillations due to framework geology interact with higher frequency oscillations with greater small scale variability in the dune line directly landward of the paleo channels these results suggest that the paleo channels of pais non linearly influence beach and dune morphology which in turn alters the response of the island to storms and sea level rise it is argued that an understanding of the framework geology is key to predicting island response to sea level rise and framework geology needs to be included in barrier island models this paper demonstrates that an irregular framework geology influences small scale coastal processes and creates interactions across scales that influence beach and dune morphology and affects barrier island response to storms and sea level rise

novae on venus geology classification and evolution 1 novae are radially fractured centers 100 300 km in diameter and 64 have been identified on venus dense radial fracturing and upraised topography are common and massive amounts of volcanism are seen in some novae are interpreted to be the result of updoming and fracturing of the surface due to interaction of mantle diapirs with the lithosphere and radial fracturing caused by dike emplacement we analyzed the topography and character of deformational and morphological structures of all 64 novae leading to a better understanding of their evolution and relation to diapir dynamics and their age we subdivided novae into four topographic classes 1 upraised 2 annular 3 flat and negative and 4 plateau like novae are located mostly in areas of regional rises and rift zones with a small number in the lowlands about one half the novae studied show association with rifts in some of these the rift troughs surround nova rises and form plateau like novae plateau like novae may predate postdate or form simultaneously with rifts our analysis generally confirms and updates previous models of novae evolution with the following geological factors being important 1 depth of the neutral buoyancy level of the rising mantle diapir 2 rheological characteristics of the overlying lithosphere 3 whether or not the visco plastic material above the diapir undergoes spreading and 4 the nature and influence of regional stress and rift structures these combined factors exert control on the direction of nova evolution and appear to determine the primary characteristics of nova such as topography tectonics and volcanism we found clear evidence for 1 the important role of dike swarm emplacement in the formation of radial patterns of novae and 2 the formation of concentric tectonic features during the relaxation stage of novae novae have multiple evolutionary stages and are long lived structures stratigraphic analysis of all 64 local areas showed a similarity in the sequence of regional geologic units we found that 40 3 of the novae population started to form before emplacement of regional plains with wrinkle ridges and that 11 3 completed their activity before this time 88 7 of the population of novae was active after regional plains formation in contrast to novae coronae activity was greatest before formation of regional plains which may be due to thickening of the lithosphere with time detailed structural analysis shows that novae evolution does not always lead to the formation of corona like features

corona associations and their implications for venus abstract geologic mapping principles were applied to determine genetic relations between coronae and surrounding geomorphologic features within two study areas in order to better understand venusian coronae the study areas contain coronae in a cluster versus a contrasting chain and are 1 directly west of phoebe regio quadrangle v 40 centered at latitude 15 s longitude 250 and 2 west of asteria and beta regiones between latitude 23 n longitude 239 and latitude 43 n longitude 275 results of this research indicate two groups of coronae on venus 1 those that are older and nearly coeval with regional plains and occur globally and 2 those that are younger and occur between beta atla and themis regiones or along extensional rifts elsewhere sometimes showing systematic age progressions mapping relations and earth analogs suggest that older plains coronae may be related to a near global resurfacing event perhaps initiated by a mantle superplume or plumes younger coronae of this study that show age progression may be related to 1 a tectonic junction of connecting rifts resulting from local mantle upwelling and spread of a quasi stationary hotspot plume and 2 localized spread of post plains volcanism we postulate that on venus most of the young post resurfacing coronal plumes may be concentrated within an area defined by the bounds of beta atla and themis regiones

coastal geomorphology of the martian northern plains the paper considers the question of the formation of the outflow channels and valley networks discovered on the martian northern plains during the mariner 9 mission parker and saunders 1987 and parker et al 1987 1989 data are used to describe key features common both in the lower reaches of the outflow channels and within and along the margins of the entire northern plains it is suggested that of the geological processes capable of producing similar morphologies on earth lacustrine or marine deposition and subsequent periglacial modification offer the simplest and most consistent explanation for the suit of features found on mars

venus southern hemisphere geologic character and age of terrains in the themis alpha lada region arecibo high resolution radar images of the southern hemisphere of venus extending to 78 s show that the surface of the themis alpha lada region is characterized by linear deformation zones with volcanoes and corona like features and by regional volcanic deposits primarily plains small shields and large edifices large scale areal deformation is limited to the tessera of alpha regio lada terra in the southern high latitudes contains several large coronae in contrast to ishtar terra in the northern high latitudes the density of craters of possible impact origin is somewhat lower than that observed in the venera 15 and 16 coverage these data extend to 43 percent of the areas of the surface of venus with ages of less than about 1 billion years

hot spot evolution and the global tectonics of venus the global tectonics of venus may be dominated by plumes rising from the mantle and impinging on the lithosphere giving rise to hot spots global sea floor spreading does not take place but direct convective coupling of mantle flow fields to the lithosphere leads to regional scale deformation and may allow lithospheric transport on a limited scale a hot spot evolutionary sequence comprises i a broad domal uplift resulting from a rising mantle plume ii massive partial melting in the plume head and generation of a thickened crust or crustal plateau iii collapse of dynamic topography and iv creep spreading of the crustal plateau crust on venus is produced by gradual vertical differentiation with little recycling rather than by the rapid horizontal creation and consumption characteristic of terrestrial sea floor spreading

three dimensional spherical models of convection in the earth s mantle three dimensional spherical models of mantle convection in the earth reveal that upwelling cylindrical plumes and downwelling planar sheets are the primary features of mantle circulation thus subduction zones and descending sheetlike slabs in the mantle are fundamental characteristics of thermal convection in a spherical shell and are not merely the consequences of the rigidity of the slabs which are cooler than the surrounding mantle cylindrical mantle plumes that cause hotspots such as hawaii are probably the only form of active upwelling and are therefore not just secondary convective currents separate from the large scale mantle circulation active sheetlike upwellings that could be associated with mid ocean ridges did not develop in the model simulations a result that is in agreement with evidence suggesting that ridges are passive phenomena resulting from the tearing of surface plates by the pull of descending slabs

the influence of the lower stratosphere on ridging atlantic ocean anticyclones over south africa abstractthe link between rossby wave breaking and ridging atlantic ocean anticyclones in the south african domain is examined using ncep doe amip ii reanalysis data a simple composite analysis wh

plume channeling as one possible mechanism for the origin of multiple coronae on venus a case study from the helen planitia quadrangle v52 1 coronae are large volcanotectonic features formed in response to plume diapir activity that have played an important role in the geodynamical evolution of venus coronae are classified in terms of their morphology one of the types is the so called multiple coronae models that account for multiple coronae formation include the following emplacement of several diapirs the movement of the lithosphere over a stationary plume and migration of a plume under a stationary lithosphere this work presents the study of a multiple corona located in the helen planitia quadrangle v52 oanuava corona and another corona unnamed formed in the vicinity of oanuava in order to test the viability of these models detailed structural mapping shows a migration of magmatic activity along the structure during corona formation and the development of a set of fractures in response to a local stress field the latter is used as a time marker for the evolution of oanuava and the unnamed coronae we propose that this migration could be explained as the result of a sublithospheric lateral flow and ponding of a mantle plume or diapir into a zone of uplifted and thinned lithosphere plume channeling induced by the interaction of both coronae

pitted cones and domes on mars observations in acidalia planitia and cydonia mensae using moc themis and tes data 1 domes and cones with summit pits located in acidalia planitia and cydonia mensae were studied using moc and themis images and a tes derived thermal inertia map north of 40 5 n latitude the features have a dome like morphology and south of that latitude the morphology is more cone like layering is apparent in the summit craters of fresher looking southern cones and asymmetric aprons were observed in some instances some of the northern domes also display layering in their summit craters but asymmetric aprons were not observed the northern domes can also display multiple summit pits or no summit pits at all and can occur in association with higher albedo pancake features the northern domes are higher in albedo but have apparent thermal inertias that are lower than the surrounding plains the apparent thermal inertia values of the southern cones range from values comparable to the surrounding plains to slightly lower from the tes thermal inertia map we infer that the thermal inertia values of the pitted cones are between those of basaltic fine dust and sand while those of the surrounding plains are closer to that of basaltic sand while a unique interpretation of the origin of the pitted cones is not possible with the available data we do not find compelling evidence to suggest an origin related to either basaltic volcanism or ground ice instead an origin for these features through some combination of mud volcanism and evaporite deposition around geysers and or springs is most consistent with the observations

the driving mechanism of plate tectonics abstract plate velocities reflect the global mantle circulation which carries the heat from the deep part of the earth to the surface three other measurable parameters put strong constraints on the physical processes involved in mantle dynamics the surface topography the geoid and the lateral density heterogeneities mapped throughout the mantle by seismic tomography using these internal loads and the navier stokes equations a first generation of dynamic earth models have been developed in the last five years they have produced satisfactory predictions for the divergence of the surface velocities and for the geoid these predictions are quite sensitive to the assumed radial viscosity variations the best match with observations has therefore helped to discriminate between various possible viscosity structures a new set of models taking the existence of plates into account has been particularly successful in predicting the plate velocities including their vorticity the driving mechanism of plate tectonics is thus put on a firm physical basis and the computed solutions are quantitatively reasonable

venusian highlands geoid to topography ratios and their implications abstract using pioneer venus line of sight gravity data and orbit simulation procedures we have estimated apparent depths of isostatic compensation adcs for twelve venusian highland features asteria atla bell beta ovda phoebe tellus thetis and ulfrun regiones and nokomis gula and sappho montes adcs range from 50 km to 270 km half of the values are less than 100 km using these adcs we estimate geoid to topography ratios gtrs for each area to allow comparison with convection calculations and with terrestrial data for oceanic hot spots swells and plateaus the geoid is estimated in the wavenumber domain from the isostatic formula using the topography and adc for each region in the space domain the gtr is equal to the least squares slope of the linear fit of the geoid to the topography the resulting gtr range is 7 31 m km which is much higher than terrestrial oceanic values 1 to 5 m km the features fall into two distinct groups one with a gtr range of 7 13 m km and one with a range of 19 25 m km the exception is beta regio which has a gtr of 31 m km a model for thermal thinning of a 100 km thick lithosphere fits all values in the lower gtr group to within one standard deviation airy compensation could also be present but cannot fully compensate these features partial dynamic compensation of the lower gtr group in combination with lithospheric mechanisms is also possible but not required to fit the data the upper gtr range 19 25 m km can be fit with an upper mantle constant viscosity convection model the large gtr values are inconsistent with the presence of a low viscosity zone if more than one compensation mechanism is present in the regions in the higher gtr group the gtrs will be underestimated in terms of a dynamic interpretation we thus fit the convection models to the upper end of the gtr range 25 m km rayleigh numbers in the range 10 4 10 6 will produce a gtr of 25 m km when combined with conductive lid thicknesses of 85 150 km the 6 m km range in both of the gtr groups is probably due to varying degrees of crustal and thermal compensation combined with dynamic compensation in the upper gtr group the difference between terrestrial and venusian gtr ranges can be explained largely by the lack of a low viscosity zone on venus

venus coronae craters and chasmata the distributions of venus coronae and craters are related to chasmata which are thought to be extensional zones coronae are almost twice as dense near the chasmata as a random set of the same size of the various types of coronae the radial concentric and multiple are even more highly concentrated near chasmata whereas the concentric caldera type are absent near the chasmata craters to the first order are randomly distributed although when distributions are compared with random sets there is a deficit of about 15 20 craters close to the chasmata the tectonized and embayed craters tend to be near the rift zones and their distribution closely resembles that of the coronae the higher proportion of tectonized and especially embayed craters within 5 10 of the arcs is about the fraction that would be expected from modification of the full set of craters over a width of 100 200 km kolmogorov smirnov statistics are used to compare cumulative distributions for craters and coronae with a large random distribution and to compute probabilities craters and coronae fill disjoint regions that are more connected than regions of randomly assigned points this suggests that the volcano tectonic process creating coronae may be the same one destroying craters

pancam multispectral imaging results from the spirit rover at gusev crater panoramic camera images at gusev crater reveal a rock strewn surface interspersed with high to moderate albedo fine grained deposits occurring in part as drifts or in small circular swales or hollows optically thick coatings of fine grained ferric iron rich dust dominate most bright soil and rock surfaces spectra of some darker rock surfaces and rock regions exposed by brushing or grinding show near infrared spectral signatures consistent with the presence of mafic silicates such as pyroxene or olivine atmospheric observations show a steady decline in dust opacity during the mission and astronomical observations captured solar transits by the martian moons phobos and deimos as well as a view of earth from the martian surface

dark materials in valles marineris indications of the style of volcanism and magmatism on mars rifting on the equatorial canyon system of valles marineris provides a unique view of the interior of the martian crust to depths reaching 7 km exposing several in situ bedrock units which testify to past volcanic and magmatic processes on mars a thick regionally extensive deposit observed in coprates and juventae chasmata is interpreted on the basis of spectral reflectance erosional morphology and tendency for eolian mobilization to be composed of mafic glass possibly an ancient martian analogue of the lunar terra mantling deposits spectral mapping suggests that the dark floor covering materials in the lower canyons are derived from this unit a series of cliffs in the ophir chasma wall rock is interpreted to be exposures of resistant bedrock the spectral signature of this massive and uniform unit most closely resembles that of terrestrial mafic rocks altered to or coated by crystalline hematite application of computer mapping techniques to probable young volcanic materials in the central troughs yields an inferred distribution of volcanic activity consistent with the interpretation of extrusion along faults near the margins of the canyon floors this result supports the hypothesis that the valles originated through tectonic extension

a numerical model study of an intense cutoff low pressure system over south africa abstract investigations of extreme rainfall events in the southern african region are limited by the paucity of the observational network furthermore the lack of full radar coverage for south africa makes quantitative precipitation estimation difficult therefore numerical modeling represents the most effective method for improving the understanding of the mechanisms that contribute to extreme rainfall events in this region with the caveat that accurate validation of model simulations is hampered by the limited observations in the region this paper describes an intense cutoff low event over south africa that led to record rainfall and flash flooding along the south coast of the country and adjoining hinterland analyses from the global forecast system model showed that the cutoff aloft was accompanied by a strong low level jet llj impinging onto the south coast where rainfall was heaviest and that lapse rates were steep in the lower troposphere simulations of the event were carried out using a num

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

sedimentary history and mass flow structures of chryse and acidalia planitiae mars geologic mapping and crater counting in chryse and acidalia planitiae cap reveal five major sedimentary deposits of hesperian to early amazonian age including 1 a mass flow deposited during the early hesperian near deuteronilus mensae northeast of the map region that may have resulted from the carving of kasei valles 3000 km southwest of the exposed part of the deposit 2 knobby plains material consisting of channel likely from simud and tiu valles and possibly ares and shalbatana valles and mass wasting deposits in central and eastern cap 3 material largely from maja and ares valles emplaced in at least western and southern cap outcrops in southern chryse planitia developed thermokarst 4 a thin mass flow covering much of southern chryse planitia that emanated from simud and tiu valles and 5 a thick extensive perhaps 3500 km across mass flow deposit in central and northern cap derived from accumulation and backflow of the preceding thin mass flow or perhaps melting of polar deposits other possible deposits may not be recognizable owing to burial by younger materials or a lack of morphologic signature various associated landforms appear to be consistent with the mass flow interpretations including lobate and linear scarps along deposit edges fractures related to desiccation of thick sediments troughs and ridges near the edges of the deposit indicative of secondary mass movement and deformation pitted domes and fissure fed flows possibly formed by sedimentary mud eruptions and longitudinal channel grooves perhaps formed by roller vortices no convincing evidence for paleoshorelines or stagnant ice sheets is found in cap these findings suggest that mass flow and hyperconcentrated flooding may have been the predominant processes of outflow channel dissection in cap elsewhere in the northern plains similar landforms are prevalent the mass flow interpretation does not require either multiple episodes of extraordinarily high water discharge rates achieved by freeing huge volumes of water from the crust repetitive recycling of immense volumes of water into highland aquifers at the heads of chryse channels or profound climate change mars pathfinder will most likely land on and inspect the surface of the thin mass flow that originated from the canyons of simud and tiu valles

themis regio venus evidence for recent volcanism from virtis data abstract themis regio is interpreted to be a hotspot rise underlain by one or more mantle plumes many volcanic features in the themis region have high emissivity anomalies in venus express visible and infrared thermal imaging spectrometer virtis data other high emissivity anomalies have been found to correspond to volcanic flows interpreted to be relatively recent smrekar et al 2010a science 328 605 608 similarly at themis the majority of the high emissivity anomalies closely correspond to flows associated with coronae and volcanic edifices consistent with the interpretation that they represent recent volcanism these volcanic features also have negative bouguer gravity anomalies indicating low density at depth consistent with hot mantle upwellings leading to pressure release melting in addition several volcanic features at themis have low emissivity anomalies suggestive of more evolved compositions combining geologic mapping data with gravity and emissivity data provide support that themis is an active hotspot swell with associated long lived volcanoes and coronae that are likely associated with small scale upwellings

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

change of stress field in beta atla themis region on venus estimated from surface geometry of dike swarms lava stratigraphy and crater density we estimate the orientations of crustal stress on venus from surface geometry of dike swarms associated with volcanoes the inferred stress field sometimes has various orientations even in the same region using lava stratigraphy volcanoes in the beta atla themis bat region are classified into three groups volcanoes before during and after the formation of corona chains dike swarms in each group show a similar stress orientation between the first two groups the compressional stress orientation had changed from a n s trend to a trend parallel to the strike of the corona chains the stress fields during and after the corona chain formation are similar the crater densities of the two volcano groups are 1 67 0 94 10 6 and 0 86 0 36 10 6 km 2 respectively which indicates a probable age difference 108yr the stress field and volcanic activity before the corona chain formation in the bat are probably associated with adjacent rift activity in beta regio those during the corona chain formation may be attributed to a mantle upwelling centered at atla regio

coronae of parga chasma venus 1 parga chasma is a 10 000 km long fracture and trough system in the southern hemisphere of venus in this study we analyze coronae in the parga region addressing the relationship between corona and rift formation the relationship of volcanism to rifting and coronae and the overall evolution of the rift system our observations are compared with various models with the aim of determining any causative relationships between corona formation volcanism and rifting our statistical analysis shows that coronae are randomly located within this region indicating that corona locations are not significantly controlled by rifting or by lithospheric loading caused by the earlier formation of other coronae our observations suggest that the two processes of corona and rift formation were occurring more or less synchronously with no specific control on each other but that coronae and rifting in this region are genetically related we find a wide variation in the amount of volcanism associated with coronae across the parga region and no evidence for a deep seated plume or multiple plumes beneath parga chasma we suggest that the concentration of volcanic features in the beta atla themis region may actually result from three independent but overlapping broad zones of deformation corona formation and enhanced volcanism associated with the three rifts parga hecate and devana chasmata rather than one large zone of activity covering the whole of the region simultaneously

the timing of giant radiating dike swarm emplacement on venus implications for resurfacing of the planet and its subsequent evolution stratigraphic study of a distributed population of 118 giant radiating dike swarms on venus reveals that within each dike intruded region emplacement of the swarms occurred prior to formation of most impact craters and rifts but subsequent to that of tessera regional plains and most wrinkle ridges the density of impact craters superimposed on the swarm population 1 80 0 57 craters 10 6 km 2 when compared with the average global density 2 01 0 14 craters 10 6 km 2 and the densities reported for other geologic units is consistent with the observed stratigraphy on the basis of these data we conclude that the population of giant radiating swarms formed during or slightly after the waning phases of an interval of widespread volcanic resurfacing the stresses recorded by the dike swarm population when combined with its age and compared with the predictions of several proposed resurfacing models best support the hypothesis that the current surface formed as a result of the catastrophic foundering of a shallow depleted mantle layer formation of the dike swarms through shallow magma stalling is closely linked to and sensitively dependent upon the modern configurations of both long wavelength gravity and topography in addition the surface stresses recorded by the dike swarm population are similarly correlated at a global level with these same long wavelength characteristics we interpret the old age of the dike swarm population to mean that there has been minimal alteration of either the long wavelength topographic expression across most of the planet or the interior processes responsible for such changes since the cratering record was reset at least several hundred million years ago

characteristics of the atlantic storm track eddy activity and its relation with the north atlantic oscillation abstract this study focuses on feedbacks of the high frequency eddy activity onto the quasi stationary circulation particularly with regard to the north atlantic oscillation nao the methodology consists of analyzing ncep ncar reanalysis data and sensitivity runs from a high resolution nonhydrostatic regional model consistent with recent studies results show that the jet displacement characteristic of the nao phenomenon depends strongly on the dynamics of the synoptic scale waves and the way they break positive and negative phases of the nao are closely related to anticyclonic and cyclonic wave breaking respectively indeed the high frequency momentum flux whose sign is directly related to the type of wave breaking is correlated with the nao index over the atlantic the peak of the momentum flux signal precedes that of the nao by a few days suggesting that wave breaking is triggering nao events two examples illustrate the significant impact of single storms in particular those occurring in the e

an automated approach for extracting barrier island morphology from digital elevation models abstract the response and recovery of a barrier island to extreme storms depends on the elevation of the dune base and crest both of which can vary considerably alongshore and through time quantifying the response to and recovery from storms requires that we can first identify and differentiate the dune s from the beach and back barrier which in turn depends on accurate identification and delineation of the dune toe crest and heel the purpose of this paper is to introduce a multi scale automated approach for extracting beach dune dune toe dune crest and dune heel and barrier island morphology the automated approach introduced here extracts the shoreline and back barrier shoreline based on elevation thresholds and extracts the dune toe dune crest and dune heel based on the average relative relief rr across multiple spatial scales of analysis the multi scale automated rr approach to extracting dune toe dune crest and dune heel based upon relative relief is more objective than traditional approaches because every pixel is analyzed across multiple computational scales and the identification of features is based on the calculated rr values the rr approach out performed contemporary approaches and represents a fast objective means to define important beach and dune features for predicting barrier island response to storms the rr method also does not require that the dune toe crest or heel are spatially continuous which is important because dune morphology is likely naturally variable alongshore