steep sided domes on venus characteristics geologic setting and eruption conditions from magellan data a survey of more than 95 percent of the venus surface reveals 145 steep sided domes which can be subdivided into a variety of morphologic forms the most common being shaped like inverted bowls or flat topped domes results of a preliminary analysis of the distribution and geologic setting of the domes are presented the relation of the domes to analogous terrestrial features is examined and possible models for their mode of emplacement are outlined

magellan mission summary magellan started mapping the planet venus on september 15 1990 and after one cycle one venus day or 243 earth days had mapped 84 of the planet s surface this returned an image data volume greater than all past planetary missions combined spacecraft problems were experienced in flight changes in operational procedures and reprogramming of onboard computers minimized the amount of mapping data lost magellan data processing is the largest planetary image processing challenge to date compilation of global maps of tectonic and volcanic features as well as impact craters and related phenomena and surface processes related to wind weathering and mass wasting has begun the magellan project is now in an extended mission phase with plans for additional cycles out to 1995 the magellan project will fill in mapping gaps obtain a global gravity data set between mid september 1992 and may 1993 acquire images at different view angles and look for changes on the surface from one cycle to another caused by surface activity such as volcanism faulting or wind activity

venus tectonics an overview of magellan observations the nearly global radar imaging and altimetry measurements of the surface of venus obtained by the magellan spacecraft have revealed that deformational features of a wide variety of styles and spatial scales are nearly ubiquitous on the planet many areas of venus record a superposition of different episodes of deformation and volcanism this deformation is manifested both in areally distributed strain of modest magnitude such as families of graben and wrinkle ridges at a few to a few tens of kilometers spacing in many plains regions as well as in zones of concentrated lithospheric extension and shortening the common coherence of strain patterns over hundreds of kilometers implies that even many local features reflect a crustal response to mantle dynamic processes ridge belts and mountain belts which have characteristic widths and spacings of hundreds of kilometers represent successive degrees of lithospheric shortening and crustal thickening the mountain belts of venus as on earth show widespread evidence for lateral extension both during and following active crustal compression venus displays two principal geometrical variations on lithospheric extension the quasi circular coronae 75 2600 km diameter and broad rises with linear rift zones having dimensions of hundreds to thousands of kilometers both are sites of significant volcanic flux but horizontal displacements may be limited to only a few tens of kilometers few large offset strike slip faults have been observed but limited local horizontal shear is accommodated across many zones of crustal stretching or shortening several large scale tectonic features have extremely steep topographic slopes in excess of 20 30 over a 10 km horizontal scale because of the tendency for such slopes to relax by ductile flow in the middle to lower crust such regions are likely to be tectonically active in general the preserved record of global tectonics of venus does not resemble oceanic plate tectonics on earth wherein large rigid plates are separated by narrow zones of deformation along plate boundaries rather tectonic strain on venus typically involves deformation distributed across broad zones tens to a few hundred kilometers wide separated by comparatively undeformed blocks having dimensions of hundreds of kilometers these characteristics are shared with actively deforming continental regions on earth the styles and scales of tectonic deformation on venus may be consequences of three differences from the earth 1 the absence of a hydrological cycle and significant erosion dictates that multiple episodes of deformation are typically well preserved 2 a high surface temperature and thus a significantly shallower onset of ductile behavior in the middle to lower crust gives rise to a rich spectrum of smaller scale deformational features 3 a strong coupling of mantle convection to the upper mantle portion of the lithosphere probably because venus lacks a mantle low viscosity zone leads to crustal stress fields that are coherent over large distances the lack of a global system of tectonic plates on venus is likely a combined consequence of a generally lesser strength and more limited horizontal mobility of the lithosphere than on earth

flexural ridges trenches and outer rises around coronae on venus high resolution altimetry collected by the magellan spacecraft reveals trench and outer rise topographic signatures around major coronae e g eithinoha heng o artemis and latona in addition magellan synthetic aperature radar images show circumferential fractures in areas where the plates are curved downward both observations suggest that the lithosphere around coronae is flexed downward by the weight of the overriding coronal rim or by the negative buoyancy of subducted lithosphere we have modelled the trench and outer rise topography as a thin elastic plate subjected to a line load and bending moment beneath the corona rim the approach was tested at northern freyja montes where the best fit elastic thickness is 18 km in agreement with previously published results the elastic thicknesses determined by modelling numerous profiles at eithinoha heng o artemis and latona are 15 40 37 and 35 km respectively at eithinoha artemis and latona where the plates appear to be yielding the maximum bending moments and elastic thicknesses are similar to those found at the middle america mariana and aleutian trenches on earth respectively estimates of effective elastic thickness and plate curvature are used with a yield strength envelope model of the lithosphere to estimate lithospheric temperature gradients at heng o artemis and latona temperature gradients are less than 10 k km which correspond to conductive heat losses of less than one half the expected average planetary value we propose two scenarios for the creation of the ridge trench and outer rise topography differential thermal subsidence and lithospheric subduction the topography of heng o is well matched by the differential thermal subsidence model however at artemis and latona the amplitudes of the trench and outer rise signatures are a factor of 5 too large to be explained by thermal subsidence alone in these cases we favor the lithospheric subduction model wherein the lithosphere outboard of the corona perimeter subducts rolls back and the corona diameter increases

viscoelastic relaxation of topographic highs on venus to produce coronae coronae on venus are believed to result from the gravitationally driven relaxation of topography that was originally raised by mantle diapirs we examine this relaxation using a viscoelastic finite element code and show that an initially plateau shaped load will evolve to the characteristic corona topography of central raised bowl annular rim and surrounding moat stresses induced by the relaxation are consistent with the development of concentric extensional fracturing common on the outer margins of corona moats however relaxation is not expected to produce the concentric faulting often observed on the annular rim the relaxation timescale is shorter than the diapir cooling timescale so loss of thermal support controls the rate at which topography is reduced the final corona shape is supported by buoyancy and flexural stresses and will persist through geologic time development of lower flatter central bowls and narrower and more pronounced annular rims and moats enhanced by thicker crusts higher thermal gradients and crustal thinning over the diapir

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

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

insights into corona formation through statistical analyses 1 statistical analysis of an expanded database of coronae on venus indicates that the populations of type 1 with fracture annuli and 2 without fracture annuli corona diameters are statistically indistinguishable and therefore we have no basis for assuming different formation mechanisms analysis of the topography and diameters of coronae shows that coronae that are depressions rimmed depressions and domes tend to be significantly smaller than those that are plateaus rimmed plateaus or domes with surrounding rims this is consistent with the model of smrekar and stofan 1997 and inconsistent with predictions of the spreading drop model of koch and manga 1996 the diameter range for domes the initial stage of corona formation provides a broad constraint on the buoyancy of corona forming plumes coronae are only slightly more likely to be topographically raised than depressions with type 1 coronae most frequently occurring as rimmed depressions and type 2 coronae most frequently occurring with flat interiors and raised rims most type 1 coronae are located along chasmata systems or fracture belts while type 2 coronae are found predominantly as isolated features in the plains coronae at hot spot rises tend to be significantly larger than coronae in other settings consistent with a hotter upper mantle at hot spot rises and their active state

a stratigraphic study of small volcano clusters on venus recently proposed global stratigraphy and geologic history models have led to two end member volcanic evolution models for venus a directional model that proposes venus has progressed through a series of stages each characterized by a particular style of volcanic activity a t basilevsky and j w head 1997 j geophys res 103 8531 8544 and a nondirectional model that proposes a more earth like type of volcanic activity with different eruption styles occurring globally on a local scale throughout the recent history of the planet j e guest and e r stofan 1999 icarus139 55 66 the former infers that most small volcanoes d 20 km define a global unit referred to as the shield plains that predates emplacement of the widespread plains with wrinkle ridges or regional plains to test this hypothesis cross cutting and superposition relations were used to determine the stratigraphic positions of small volcano clusters in seven quadrangles 11 of the surface the seven quadrangles included 179 clusters containing over 22 000 constructs three materials were defined for each cluster 1 the constructs 2 the material immediately surrounding the constructs called the cluster material and 3 the regional plains where possible relative ages were established between the constructs cluster material and the regional plains there are six possible temporal relations between the constructs and regional plains emplacement 1 the constructs postdate regional plains emplacement 2 some constructs postdate regional plains emplacement and other constructs predate regional plains deformation but their ages relative to regional plains emplacement are ambiguous 3 the constructs predate regional plains emplacement 4 some constructs predate regional plains emplacement and other constructs predate regional plains deformation but their ages relative to regional plains emplacement are ambiguous 5 the constructs both predate and postdate regional plains emplacement and 6 the ages of the constructs relative to regional plains emplacement are ambiguous an age relative to regional plains emplacement could not be determined for almost half of the clusters studied of the clusters for which a relative age could be determined almost half postdate regional plains emplacement and an additional third contain at least some constructs that postdate regional plains emplacement less than a tenth of the clusters predate regional plains emplacement these observations indicate that many small constructs postdate the emplacement of the regional plains and therefore are consistent with the nondirectional volcanic evolution model in which small constructs represent a global process occurring locally throughout the recent history of venus

investigation on the distribution characteristics of ground cracks in the chengchao iron mine china in this study a comprehensive assessment of the distribution characteristics of ground cracks in the chengchao iron mine was conducted by field investigation and monitoring in general ground cracks are initially distributed around collapse pits and subsequently extend outwards centered on the mined out area greater ground deformation leads to more developed ground cracks ground cracks can be divided into cut through crack deep crack and shallow crack based on their downward extension depths furthermore the formation of ground cracks is controlled by both underground mining and geological structures ground cracks can be divided into mining induced crack and tectonic crack based on their main influencing factors moreover many types of ground cracks are caused by different failure modes including tensile failure shear failure and mixed failure ground cracks can be divided into vertical shear crack vertical shear and horizontal tensile crack horizontal tensile crack horizontal shear crack and mixed crack based on their formation mechanisms in particular the movement line and the break line defined as e 2 mm m and 6 mm m respectively can be reasonably regarded as disturbance boundary and crack boundary in a metal mine the disturbance zone caused by underground mining can be divided into collapse zone break zone and movement zone based on the size of ground deformation and the characteristics of ground cracks increases in ground crack width and length exert different degrees of stress release on the rock masses outside and inside the crack thus increasing their deformation difference in addition the extension of ground cracks is related to the ground topography and the strength and integrity of the rock masses and soil cracks on hilltop are more developed than those on flat land and cracks will initially appear in rock mass and soil rather than on a concrete structure due to the lower strain resistance of the rock mass and soil however concrete structures are more sensitive to cracking than rock mass and soil because of the better integrity of concrete structures

coronae on venus morphology classification and distribution abstract two major types of circular features with no direct morphologic analog on the terrestrial planets were identified in venera 15 16 radar images of venus coronae and corona like features coronae are irregular to circular structures characterized by a complex interior zone and maximum widths of 170 690 km corona like features resemble coronae but lack an annulus of ridges twenty one structures that fit the definition of coronae have been identified in venera 15 16 coverage along with eleven corona like features each group has been subdivided into three classes based on general morphology symmetrical coronae are characterized by circular to oval form with interiors characterized by flows domes and tectonic ridges asymmetrical coronae have similar morphology but have only a partial annulus or are asymmetrical about a central axis subdued coronae are dominated by smooth plains with flooded and embayed annuli double corona like structures are composed of two linked rings and are found along ridge belts in the region east of atalanta planitia irregular corona like features are surrounded by a rim rather than an annulus as are the double structures and are very subdued in appearance heart shaped corona like features are rounded at one end and form a wedge at the other end we interpret the classes of coronae and corona like features to have a similar mode of origin with differences in morphology due to the stage of evolution and or varying amounts and significance of geologic processes i e tectonism volcanism gravitational relaxation that occur during their evolution corona evolution is characterized by early uplift and volcanism followed by annulus formation formation of a peripheral trough may occur in the middle to late stages of corona evolution followed by extensive volcanism and lowering of topographic relief both coronae and corona like features are interpreted to have formed as a result of uplift and volcanism caused by a thermal anomaly at depth followed by gravitational relaxation

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

corona structures on venus models of origin coronae on venus are circular to elongate structures with maximum widths of 150 1000 km characterized by annuli of concentric ridges surrounding complex interiors the features have raised topography relative to the surroundings they are associated with volcanic activity and most are partially surrounded by a peripheral trough variations in morphology between individual coronae are due to differences in their stage of evolution and or differences in the relative significance of the geologic processes that occur in each stage we examine models for three processes that may be involved in corona origin and evolution 1 a hotspot or rising mantle diapir model 2 a sinking mantle diapir model and 3 gravitational relaxation of topography rising mantle diapirs are caused by heating at depth e g hotspot while sinking mantle diapirs may result from cooling or a phase change causing increased density and negative buoyancy at the base of the lithosphere the hotspot model is most consistent with the major characteristics of coronae with gravitational relaxation occurring as a modificational process the sinking mantle diapir would produce dominant central compression that has not been observed at coronae however higher resolution image and altimetry data from magellan can be used to distinguish more fully between the two models coronae in various states of formation and degradation can be identified in the venera 15 16 data suggesting that the process may be continuing today

coronae of mnemosyne regio morphology and origin abstract coronae are circular to elliptical structures 150 600 km across characterized in venera 15 16 and arecibo radar images by an annulus of concentric ridges the interiors of the structures are geologically complex displaying volcanic flows domes and radial concentric and or obliquely trending ridges and are generally distinct from the exterior the mnemosyne regio area to the west of ishtar terra contains seven coronae the largest cluster of coronae identified on venus on the basis of the morphologic and topographic characteristics of the coronae in the mnemosyne region we assess several possible models of corona origin using cross cutting and stratigraphic relationships we have been able to identify a sequence of features and events for this group of coronae and have determined that a variety of processes contribute to the origin and evolution of coronae we find that several processes associated with diapirism or hotspots and gravitational relaxation of high topography are most consistent with observed corona topography and morphology and are thus likely to have been significant in the formation and evolution of coronae on the basis of these observations and stratigraphic relationships the following sequence of events is outlined for the formation and evolution of coronae in this region 1 regional volcanic and tectonic activity 2 localized uplift and volcanism caused by diapirism or hotspots 3 annulus and trough formation related to gravitational relaxation accompanied by further volcanic activity 4 continued local volcanic flooding of the corona annulus and trough differences in morphology between coronae reflect variations in amount and significance of these geologic processes coronae in other regions exhibit the same overall characteristics as the mnemosyne coronae and support a similar origin for the majority of coronae

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

magellan radar performance and data products the magellan venus orbiter carries only one scientific instrument a 12 6 centimeter wavelength radar system shared among three data taking modes the synthetic aperture mode images radar echoes from the venus surface at a resolution of between 120 and 300 meters depending on spacecraft altitude in the altimetric mode relative height measurement accuracies may approach 5 meters depending on the terrain9s roughness although orbital uncertainties place a floor of about 50 meters on the absolute uncertainty in areas of extremely rough topography accuracy is limited by the inherent line of sight radar resolution of about 88 meters the maximum elevation observed to date corresponding to a planetary radius of 6062 kilometers lies within maxwell mons when used as a thermal emission radiometer the system can determine surface emissivities to an absolute accuracy of about 0 02 mosaicked and archival digital data products will be released in compact disk cdrom format

post seismic relaxation following the great 2004 sumatra andaman earthquake on a compressible self gravitating earth summary the mw 9 0 2004 december 26 sumatra andaman and mw 8 7 2005 march 28 nias earthquakes which collectively ruptured approximately 1800 km of the andaman and sunda subduction zones are expected to be followed by vigorous viscoelastic relaxation involving both the upper and lower mantle because of these large spatial dimensions it is desirable to fully account for gravitational coupling effects in the relaxation process we present a stable method of computing relaxation of a spherically stratified compressible and self gravitating viscoelastic earth following an impulsive moment release event the solution is cast in terms of a spherical harmonic expansion of viscoelastic normal modes for simple layered viscoelastic models which include a low viscosity oceanic asthenosphere we predict substantial post seismic effects over a region several 100s of km wide surrounding the eastern indian ocean we compare observed gps time series from ten regional sites mostly in thailand and indonesia beginning in 2004 december with synthetic time series that include the coseismic and post seismic effects of the 2004 december 26 and 2005 march 28 earthquakes a viscosity structure involving a biviscous burgers body rheology in the asthenosphere explains the pattern and amplitude of post seismic offsets remarkably well

magellan observations of alpha regio implications for formation of complex ridged terrains on venus magellan images of alpha regio reveal previously undetected structures and details of the morphology of this region of complex ridged terrain we examine the complex ridged terrain of alpha regio using morphology and crosscutting relationships between structures to derive a sequence of tectonic events structures include broad 10 20 km wide linear and arcuate ridges fine scale 3 km wide ridges linear disruption zones ldzs up to several kilometers wide and numerous grabens 5 km wide and associated scarps and troughs based on their morphology we interpret the broad and fine scale ridges as compressional structures possibly folds ldzs appear to be due to small amounts of lateral shear which most commonly disrupts the older ridge fabric graben and associated structures are interpreted as extensional features they crosscut ridges and ldzs and thus appear to be the youngest structures in alpha regio this sequence of events and information on the orientation of these various structures are compared to the predictions of two models for the formation of complex ridged terrain and highlands on venus in general a hotspot model and a coldspot model the presence of compressional features along and parallel to the margins of alpha regio and the lack of any high elevation ring of extensional features are more consistent with a coldspot or roughly axisymmetric mantle downwelling mantle downwelling appears to be the most likely mode of formation of the upland of alpha regio and is likely to be important in other highland regions such as ovda and thetis regiones which are also dominated by complex ridged terrain

the geology and geomorphology of the venus surface as revealed by the radar images obtained by veneras 15 and 16 a region by region condensed description of almost all of the area that was radar photographed by veneras 15 and 16 is presented using some generalizations the diversity of terrain was reduced to a discrete set from which a geological morphological map was constructed the predominant type of terrain of the studied area is a plain that was tentatively subdivided into five morphological types ridge and band patchy rolling plain dome and butte plain smooth plain and high smooth plain stratigraphically the ridge and band plains are the oldest and the smooth plains are the youngest the stratigraphic position of the other types is yet to be determined large sections of the plains show similarities to the mare type basaltic plains of the moon mercury and mars other types of terrain are combinations of ridges and grooves in various patterns linear parallel orthogonal diagonal or chevron like and chaotic in some places the ridge and groove terrain is stratigraphically below the plain material but in other places it appears to be plain material that has been subsequently deformed near the eastern and western boundaries of ishtar terra large several hundred kilometers in diameter ring like features can be seen that are named coronae or ovoids evidence of tectonic deformation and the presence of flow like patterns support their designation as volcano tectonic features beta regio seems to be an uplifted plain showing evidence of rifting and volcanism all types of terrain are sparesely peppered with craters of obvious impact morphology their average density gives the plain an age range of 0 5 to 1 109 years the fact that many impact craters are still in the pristine state indicates a very low rate of surface reworking at least for the last 0 5 to 1 109 years no evidence for water erosion sedimentation processes has been found the tectonic activity of venus has no equivalent on the moon mercury or even mars and can be compared only with that of the earth intensive horizontal deformation previously known only on earth occurs on venus but in a characteristic venusian style

styles of tectonic deformations on venus analysis of venera 15 and 16 data analysis of the radar images obtained by veneras 15 and 16 leads to the conclusion that the ridge and groove structures on the surface of venus are the result of tectonic deformation although the mechanism of such deformation cannot yet be unequivocally deduced several styles of deformation can be described areal deformation occurs where horizontal stresses have operated over large areas shear deformation appears in bands showing differential longitudinal deformation transversal stresses operating over long and relatively narrow areas have produced belt deformation circular deformation is related to a specific locus of the stresses the absence of densely cratered areas indicates that the terrains were deformed after the period of heavy bombardment the origin of the stresses could be drag of the lithosphere by asthenospheric currents or gravity induced spreading of surface material over upwellings a general conclusion is that in the surveyed area of venus neither terrestrial plate tectonics nor lunar highland type terrain exists

explosive volcanism on mercury analysis of vent and deposit morphology and modes of eruption abstract the messenger mission revealed for the first time conclusive evidence of explosive volcanism on mercury several previous works have cataloged the appearance and location of explosive volcanism on the planet using a variety of identifying characteristics including vent presence and deposit color as seen in multispectral image mosaics we present here a comprehensive catalog of vents of likely volcanic origin our classification scheme emphasizes vent morphology we have analyzed the morphologies of all vents in our catalog and recognize three main morphologies simple vent pit vent and vent with mound the majority of vents we identify are located within impact craters the spatial distribution of vents does not correlate with the locations of volcanic smooth plains deposits in contrast to the moon nor do vents correlate with the locations of large impact basins except for the caloris and tolstoj basins using the degradation state of the vent host crater as a proxy for maximum age we suggest that vent formation has been active through the mansurian and into the kuiperian periods although the majority of vents were likely formed much earlier in mercurian history the morphologies and locations of vents are used to investigate a set of plausible formation geometries we find that the most likely and most prevalent formation geometry is that of a dike stalled at depth which then explosively vents to the surface we compare the vent and deposit size of mercurian pyroclastic deposits with localized and regional lunar pyroclastic deposits and find a range of possible eruption energies and corresponding variations in eruption style localized lunar pyroclastic deposits and the majority of mercurian pyroclastic deposits show evidence for eruption that is consistent with the magmatic foam at the top of a dike reaching a critical gas volume fraction a subset of mercurian vents including the prominent copland rachmaninoff vent to the northeast of the rachmaninoff basin indicates eruption at enhanced gas volume fractions this subset of vents shows a similar eruptive behavior to the lunar orientale dark mantle ring deposit suggesting that the dikes that formed these vents and deposits on mercury underwent some form of additional volatile build up either through crustal volatile incorporation or magma convection within the dike there also exists a population of mercurian vents that no longer retain a visible associated pyroclastic deposit we hypothesize that the visible signature of the pyroclastic deposit has been lost through space weathering and regolith mixing processes together these results provide a comprehensive analysis of explosive volcanism on mercury and inform continued research on the thermal history of mercury and magma composition and evolution

the morphology and evolution of coronae on venus coronae on venus are large circular to ovoidal surface features that have distinctive tectonic volcanic and topographic expressions they range in diameter from less than 200 km to at least 1000 km new data from the magellan spacecraft have shown coronae to be among the dominant tectonic forms on the planet and have revealed their morphology in unprecedented detail typical coronae are dominated by concentric tectonic features and have a raised rim a central region higher than the surounding plains but in many instances lower than the rim and commonly a peripheral depression or moat some coronae also show significant amounts of radial tectonic structure and in most cases this predates the concentric features in addition there are other features on venus recognized for the first time in magellan data that consist of domical rises with intense radial tectonic patterns and little or no concentric structure all of these features commonly are associated with moderate to large quantities of volcanism in fact some radially fractured domes have undergone so much volcanism that volcanic construction appears to have played a significant role in establishing their topography we explore a model of corona formation that links these forms into a genetic sequence the model begins with the ascent of a mantle diapir upward mantle flow driven by its ascent forces the lithosphere above the diapir upward producing a gentle dome with a radiating pattern of extensional fractures as the diapir impinges on the underside of the lithosphere it flattens and spreads transforming the uplift to a more flat topped shape in this flattened near surface configuration the diapir can cool rapidly with the resultant loss of buoyancy the raised plateau can relax to form a central sag a raised rim and a depressed moat concentric tectonic features develop primarily during the latter stages of corona formation and hence are best preserved on mature coronae volcanism takes place during all phases of the uplift and may diminish as the relaxation occurs our analyses to date suggest that this scenario is broadly consistent with many of the coronae on venus however there is enormous diversity in corona morphology and features are present that require substantial deviations from this simple model in particular some circular depressions appear corona like in synthetic aperature radar images but may in fact be large calderas some of the variations observed in corona morphology may ultimately be interpretable in terms of variations in the behavior of individual diapirs and in the local properties of the venusian lithosphere

geophysical models for the formation and evolution of coronae on venus coronae are large circular features on venus characterized by an annulus of concentric tectonic features interior fracturing volcanism and generally upraised topography they are suggested to form over sites of mantle upwelling and modified by subsequent gravitational relaxation we examine this proposition using two geophysical models to determine whether and under what conditions these mechanisms can produce the topography and tectonics exhibited by coronae in the magellan altimetry data and radar images our results show that mantle diapirism can produce the domical topography of novae which may be coronae in the earliest stage of formation the model stresses induced at the surface by a mantle diapir imply the formation of radially oriented extensional fracturing as observed in novae the dimensions of novae indicate that the diapirs responsible for them are smaller than about 100 km in radius and that the elastic lithosphere is less than 32 km thick diapirs that have reached the top of the mantle are expected to spread and flatten producing plateaulike rather than domical topography we model a flattened diapir at the top of the mantle and show that it will result in plateaulike uplift the volume of the flattened model diapir is similar to that of the spherical diapirs derived for novae we model gravitational relaxation of isostatically uncompensated plateaus and show that they relax to the topographic forms associated with coronae and that the model stresses are consistent with the development of the annulus of tectonic features around coronae

the spatial distribution of coronae and related features on venus coronae are large quasi circular geologic features that are common on venus they appear to be the surface tectonic and volcanic expressions of mantle diapirs that have impinged on the underside of the venusian lithosphere we have investigated the spatial distribution of 335 coronae and related features identified in magellan radar data it is more clustered than a poisson distribution with a statistical certainty of more than 99 it is dominated by a single large cluster centered near the equator at about 245 longitude the features are preferentially found at elevation and geoid values close to the planetary mean with a paucity at both the highest and lowest levels of topography and geoid some coronae appear aligned in quasi linear chains we attribute the clustering of coronae and related features to preferential formation of these features above regions of broad scale mantle upwelling and suggest that a major mantle upwelling underlies the one large cluster we suggest that coronae are rare at the lowest elevations because these may be regions of mantle downwelling the shortage of coronae at the highest elevations may result both from obscuration by other intense tectonism there and from suppression there of their formation by an unusually thick crust corona chains may be produced by enhanced passive mantle uplift below failed or incipient rifts

the interaction of plume heads with compositional discontinuities in the earth s mantle the effects of compositional discontinuities of density and viscosity in the earth s mantle on the ascent of mantle plume heads is studied using a boundary integral numerical technique three specific problems are considered 1 a plume head rising away from a deformable interface 2 a plume head passing through an interface and 3 a plume head approaching the surface of the earth for the case of a plume attached to a free surface the calculated time dependent plume shapes are compared with experimental results two principal modes of plume head deformation are observed plume head elongation or the formation of a cavity inside the plume head the inferred structure of mantle plumes namely a large plume head with a long tail is characteristic of plumes attached to their source region and also of buoyant material moving away from an interface and of buoyant material moving through an interface from a high to low viscosity region as a rising plume head approaches the upper mantle most of the lower mantle will quickly drain from the gap between the plume head and the upper mantle if the plume head enters the upper mantle if the plume head moves from a high to low viscosity region the plume head becomes significantly elongated and for the viscosity contrasts thought to exist in the earth could extend from the 670 km discontinuity to the surface plume heads that are extended owing to a viscosity decrease in the upper mantle have a cylindrical geometry the dynamic surface topography induced by plume heads is bell shaped when the top of the plume head is at depths greater than about 0 1 plume head radii as the plume head approaches the surface and spreads the dynamic topography becomes plateau shaped the largest stresses are produced in the early stages of plume spreading when the plume head is still nearly spherical and the surface expression of these stresses is likely to be dominated by radial extension as the plume spreads compressional stresses on the surface are produced beyond the edges of the plume consequently extensional features will be produced above the plume head and may be surrounded by a ring of compressional features

venus tectonics initial analysis from magellan radar imaging and altimetry data from the magellan mission have revealed a diversity of deformational features at a variety of spatial scales on the venus surface the plains record a superposition of different episodes of deformation and volcanism strain is both areally distributed and concentrated into zones of extension and shortening the common coherence of strain patterns over hundreds of kilometers implies that many features in the plains reflect a crustal response to mantle dynamic processes ridge belts and mountain belts represent successive degrees of lithospheric shortening and crustal thickening the mountain belts also show widespread evidence for extension and collapse both during and following crustal compression venus displays two geometrical patterns of concentrated lithospheric extension quasi circular coronae and broad rises with linear rift zones both are sites of significant volcanism no long large offset strike slip faults have been observed although limited local horizontal shear is accommodated across many zones of crustal shortening in general tectonic features on venus are unlike those in earth s oceanic regions in that strain typically is distributed across broad zones that are one to a few hundred kilometers wide and separated by stronger and less deformed blocks hundreds of kilometers in width as in actively deforming continental regions on earth

venus volcanism initial analysis from magellan data magellan images confirm that volcanism is widespread and has been fimdamentally important in the formation and evolution of the crust of venus high resolution imaging data reveal evidence for intrusion dike formation and cryptodomes and extrusion a wide range of lava flows also observed are thousands of small shield volcanoes larger edifices up to several hundred kilometers in diameter massive outpourings of lavas and local pyroclastic deposits although most features are consistent with basaltic compositions a number of large pancake like domes are morphologically similar to rhyolite dacite domes on earth flows and sinuous channels with lengths of many hundreds of kilometers suggest that extremely high effusion rates or very fluid magmas perhaps komatiites may be present volcanism is evident in various tectonic settings coronae linear extensional and compressional zones mountain belts upland rises highland plateaus and tesserae volcanic resurfacing rates appear to be low less than 2 km 3 yr but the significance of dike formation and intrusions and the mode of crustal formation and loss remain to be established