catastrophic debris avalanche deposit of socompa volcano northern chile between 10 000 and 500 yr ago the socompa volcano in northern chile experienced a catastrophic collapse of a 70 deg sector of the original cone causing a debris avalanche that descended nearly 3000 m vertically and traveled more than 35 km from the volcano the deposits cover some 490 sq km and have a minimum volume of 15 cu km parts of the original cone slumped in a nearly coherent form and are now preserved as large blocks more than 400 m high the primary avalanche traveled northwestward over sloping ground before coming to rest transiently forming a prominent marginal ridge and then slid away northeastward to form a secondary flow overriding much of the primary avalanche deposit abundant prismatic jointed dacite blocks within the debris avalanche deposit and a thin fine grained pumiceous deposit beneath it suggest that the collapse was triggered by magmatic activity and may have been accompanied by a violent lateral blast collapse was followed by eruption of pumiceous pyroclastic flows and extrusion of voluminous dacite domes

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

lessons from venus for understanding mantle plumes on earth abstract mantle plumes are important in the magmatic and tectonic history for both earth and venus the expression of plumes is distinctive on venus and complementary to that on earth therefore a cross comparison is useful for better understanding plume magmatism on both planets in contrast to the earth venus has no observed record of plate tectonics a low degree of surface erosion and an apparently short duration for the formation of the present planetary surface the absence of plate tectonics indicates that all magmatism is intraplate and is generated beneath a stagnant lithospheric lid a low degree of surface erosion preserves the surface structures and short wavelength topography the short duration of preserved magmatic activity suggests a global resurfacing event magmatic elements include a individual volcanoes with diameters ranging up to 1000 km which represent hotspots b annular structures termed coronae with diameters averaging 300 km but ranging up to 2600 km and which appear to lack terrestrial i e earth analogues c radiating graben fissure systems extending up to 2000 km in radius some of which are purely uplift related while others mark the plumbing system dyke swarms of volcanic systems d lava flow fields of scale comparable to terrestrial flood basalts large igneous provinces lips and e regions of small shield volcanoes representing shallow source melting there are several hierarchies of magmatic events on venus ordered in terms of increasing scale and significance 1 isolated coronae volcanoes flow fields and radiating graben systems 2a individual and small clusters of volcanoes and coronae associated with topographic swells geoid highs and triple junction rifting these are most clearly indicative of terrestrial type plumes originating from the deep mantle 2b coronae distributed along rifts chasmata these are the clearest examples of melt generation associated with rifting 3 regional concentration of activity in the beta atla themis bat region this is the closest example of a plume cluster event sometimes termed a superplume event and 4 global volcanic resurfacing of the volcanic plains no terrestrial analogue is confirmed although the global burst of terrestrial plume activity in the neoarchean is a possible analogue

role of olivine cumulates in destabilizing the flanks of hawaiian volcanoes the south flank of kilauea volcano is unstable and has the structure of a huge landslide it is one of at least 17 enormous catastrophic landslides shed from the hawaiian islands mechanisms previously proposed for movement of the south flank invoke slip of the volcanic pile over seafloor sediments slip on a low friction decollement alone cannot explain why the thickest and widest sector of the flank moves more rapidly than the rest or why this section contains a 300 km3 aseismic volume above the seismically defined decollement it is proposed that this aseismic volume adjacent to the caldera in the direction of flank slip consists of olivine cumulates that creep outward pushing the south flank seawards average primary kilauea tholeiitic magma contains about 16 5 wt mgo compared with an average 10 wt mgo for erupted subaerial and submarine basalts this difference requires fractionation of 17 wt 14 vol olivine phenocrysts that accumulate near the base of the magma reservoir where they form cumulates submarine erupted kilauea lavas contain abundant deformed olivine xenocrysts derived from these cumulates deformed dunite formed during the tholeiitic shield stage is also erupted as xenoliths in subsequent alkalic lavas the deformation structures in olivine xenocrysts suggest that the cumulus olivine was densely packed probably with as little as 5 10 vol intercumulus liquid before entrainment of the xenocrysts the olivine cumulates were at magmatic temperatures 1100 c when the xenocrysts were entrained olivine at 1100 c has a rheology similar to ice and the olivine cumulates should flow down and away from the summit of the volcano flow of the olivine cumulates places constant pressure on the unbuttressed seaward flank leading to an extensional region that localizes deep intrusions behind the flank these intrusions add to the seaward push this mechanism ties the source of gravitational instability to the caldera complex and deep rift systems and therefore limits catastrophic sector failure of hawaiian volcanoes to their active growth phase when the core of olivine cumulates is still hot enough to flow

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

development of large volcanoes on venus constraints from sif gula and kunapipi montes abstract sif gula and kunapipi montes are large volcanoes on venus they have differing morphologies reflecting their different eruptive histories and plumbing systems all have a steeper upper cone surrounded by more shallowly sloping flanks interpreted as resulting from histories of more frequent shorter lower effusion rate summit eruptions accompanied by infrequent more voluminous flank eruptions all three volcanoes have multiple flow field types digitate sheet and fan which probably reflect variations in effusion rate eruption duration and vent geometry sif mons summit has a large caldera gula mons has summit calderas at either end of a summit rift and the summit of kunapipi mons is composed of two volcanic centers on a summit plateau the summit morphologies of sif gula and kunapipi montes are interpreted to reflect differences in magma chamber location over time and differing internal plumbing we find no positive evidence that neutral buoyancy has governed magma chamber position instead magma chambers at the three volcanoes appear to have changed position over time with lithospheric stresses as well as neutral buoyancy affecting chamber location all three volcanoes have had episodes of diking at their summits followed by episodes of extrusion we interpret this to be caused by variations in either magma supply rate or magma volatile content or both over time fractures on the flows of the volcano indicate that dike intrusion has occurred throughout the visible history of the volcanoes indicating that the local stress field within the volcano favored intrusion as well as extrusion over much of its history the overall shape and history of the venusian volcanoes is similar to the histories of terrestrial volcanoes however some aspects of these volcanoes including the rift at gula the large size and very long flows differ significantly from terrestrial volcanoes

venus geology geochemistry and geophysics edited by v l barsukov a t basilevsky v p volkov and v n zharkov university of arizona press 1992 421p us 75 00 isbn 0 8165 1222 1

axial seamount an active ridge axis volcano on the central juan de fuca ridge axial seamount some of the manuscripts in this special section refer to the edifice with the more precise name of axial volcano a large ridge axis volcano is located on the central segment of the juan de fuca ridge approximately 250 nautical miles west of the washington oregon british columbia coast currently both volcanically and hydrothermally active axial lies directly at the intersection of the cobb eickelberg seamount chain and the juan de fuca ridge figure 1 the volcanic activity associated with the seamount formation strongly interacts with and is affected by the normal seafloor spreading processes at the intersection because of this unique geologic setting its proximity to west coast ports and oceanographie institutions and its shallow depth axial has become the focus of a large number of scientific investigations over the past decade

venus rock composition at the vega 2 landing site the vega 2 lander determined the composition of rock in the area of the northern slope of the high mountain massif aphrodite terra which had never previously been analyzed the rock was analyzed by x ray fluorescence analysis employing instrumentation similar to that on venera 13 and 14 however our experience with those probes made it possible to introduce some changes in the design of the device and in the program of its operation as a result the reliability of the device was improved at the same time the analytical characteristics of the instrument were improved the range of the elements determined was expanded and the sensitivity was increased the x ray fluorescence spectrometer operated on the venusian surface in a preprogrammed mode the abundances of the major rock forming elements from mg to fe inclusively and also upper limits for some heavier rare elements were determined comparison of these data with analyses of terrestrial rocks enables us with certain reservations to suggest that the analyzed venusian sample is olivine gabbro norite

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

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

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

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

styles of volcano induced deformation numerical models of substratum flexure spreading and extrusion the gravitational deformation of volcanoes is largely controlled by ductile layers of substrata using numerical finite element modelling we investigate the role of ductile layer thickness and viscosity on such deformation to characterise the deformation we introduce two dimensionless ratios a volcano radius ductile layer thickness and b viscosity of ductile substratum failure strength of volcano we find that the volcanic edifice spreads laterally when underlain by thin ductile layers a 1 while thicker ductile layers lead to inward flexure a 1 the deformation style is related to the switch from predominantly horizontal to vertical flow in the ductile layer with increasing thickness increasing a structures produced by lateral spreading include concentric thrust belts around the volcano base and radial normal faulting in the cone itself in contrast flexure on thick ductile substrata leads to concentric normal faults around the base and compression in the cone in addition we show that lower viscosities in the ductile layer low b lead to faster rates of movement and also affect the deformation style considering a thin ductile layer if viscosity is high compared to the failure strength of the volcano high b then deformation is coupled and spreading is produced however if the viscosity is low low b substratum is effectively decoupled from the volcano and extrudes from underneath it in this latter case evidence is likely to be found for basement compression but corresponding spreading features in the volcano will be absent as the cone is subject to a compressive stress regime similar to that produced by flexure at volcanoes where basement extrusion is operating high volcano stresses and outward substratum movement may combine to produce catastrophic sector collapse an analysis of deformation features at a volcano can provide information about the type of basement below it a useful tool for remote sensing and planetary geology also knowledge of substratum geology can be used to predict styles of deformation operating at volcanoes where features have not yet become well developed or are obscured

stress and flexural modeling of the martian lithospheric response to alba patera abstract the martian volcano alba patera is surrounded by a system of graben which appears to be the result of an extensional stress applied to the region after the construction of the volcano these faults provide a map of the stress field that created them we used this information as well as the recent goddard spherical harmonic gravity model gmm 1 and a spherical harmonic representation of the mars digital elevation model to constrain models of 1 the lithospheric flexure under the load of the volcano 2 the gravitational anomaly due to the volcano and 3 the magnitude of the extensional stress in the region we found that elastic lithospheric thicknesses between 10 and 50 km resulted in statistically acceptable fits to the gravity and topography data we also found that a 1 5 2 kilobar extensional regional stress in conjunction with the axisymmetric stress field of the volcanic load produced a stress field that is consistent with the observed trends of the graben

a gravitational spreading origin for the socompa debris avalanche abstract socompa volcano arguably provides the world s best exposed example of a sector collapse derived debris avalanche deposit new observations lead us to re interpret the origin of the sector collapse we show that it was triggered by failure of active thrust anticlines in sediments and ignimbrites underlying the volcano the thrust anticlines were a result of gravitational spreading of substrata under the volcano load about 80 of the resulting avalanche deposit is composed of substrata formerly residing under the volcano and in the anticlines the collapse scar can be traced up to 5 km from the edifice truncating two spreading related anticlines which collapsed in the event outcrops near the volcano preserve evidence of edifice material being carried along on top of mobilised substrata on the north side of the scar the avalanche motion was initially at right angles to the failure edge structural relations indicate that immediately prior to collapse the substrata disintegrated became effectively liquidised and were ejected from beneath the edifice catastrophic mobilisation of substrata probably resulted from breakdown of ignimbrite clasts and cement it may have occurred through progressive rock fracture by high shear strain during spreading material ejected from under socompa formed a layer on which volcanic edifice debris was transported this interpretation of events explains the puzzling observation that avalanche units with the lowest gravitational potential energy moved the furthest it can also account for avalanche motion normal to the collapse scar walls ignimbrites and other rock types probably capable of similar behaviour underlie many other volcanoes identification of spreading at other sites could therefore be a first step towards assessment of the potential for this style of catastrophic sector collapse

formation of low potential vorticity over the deep pacific abstract low potential vorticity extends over the deep waters of the north pacific and possibly the bottom waters of the north atlantic isopycnic model integrations are conducted to investigate how these potential vorticity distributions are controlled first for an idealized double hemisphere and second for the pacific with realistic topography dense water is released from a southern high latitude source and circulates over the domain with diapycnic mixing gradually reducing its stratification the potential vorticity contrast is large over the southern hemisphere but weak over the northern hemisphere where the meridional changes in planetary vorticity and layer thickness oppose each other including an active eddy field inhibits the grounding of dense water which increases the potential vorticity contrast in the overlying layer incorporating realistic topography leads to the dense fluid spreading via deep channels with tight recirculations and jets bifurcating the experiments suggest that ex

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

comprehensive analysis of traveltime data collected through gps acoustic observation of seafloor crustal movements we have developed a comprehensive inversion scheme to analyze traveltime data collected through gps acoustic observations on a campaign basis our method uses a quantity called the nadir total delay ntd which is analogous to the zenith total delay ztd in gnss analyses to represent the time variation of the sound speed the observation equation using the ntd is an approximated formulation we examine its applicable scope using numerical experiments and demonstrate that the approximation holds well enough in practice traveltime data from all our observation campaigns were utilized together for inversion calculation to determine 1 the positions of individual precision acoustic transponders pxps at the time of a particular reference campaign 2 the displacement from these positions of a rigid array at the time of each campaign and 3 the time variation of the ntd during each campaign we applied this method to actual data to fix the array geometry more precisely also we have made a first step in detecting vertical motions as well as horizontal motions though further enhancement of the accuracy will be required in performing linear regression of array displacements obtained from the inversion their error covariance has a significant effect on the resulting velocities when the covariance is taken into account nearly the same velocities are obtained no matter what campaign is taken as reference in the inversion

tectonic patterns and regional stresses near venusian coronae abstract many venusian coronae exhibit distinctive patterns of faults in particular we find the tectonic patterns of one such corona similar to the striking patterns of the grabens surrounding alba patera on mars pursuing this similarity we perform a stress analysis analogous to one that successfully explained the alba patera pattern and in so doing determined a value for the local regional stress local stress fields of venusian coronae are assumed to be created by either a gaussian axially symmetric upwarp or a cylindrical load due to accumulation of volcanic materials on an elastic lithosphere the local coronal stresses are combined with varying amounts of a uniform regional stress field for different elastic lithospheric thicknesses from the magnitudes and orientations of the resultant principal stresses the predicted patterns of surface tectonic features i e graben and thrust fault chains are plotted these patterns are compared to those of coronae on magellan images matching the patterns thus determines the regional stress and elastic lithospheric thickness about the coronae comparison of predicted to observed tectonic patterns for three specific coronae leads to estimated regional stresses of approximately 0 1 0 6 kbar and effective elastic lithospheric thicknesses consistent with 10 5 km although one area may have had a lithosphere as thin as 2 km during corona formation