pancakelike domes on venus the shape of seven large domes on the plains of venus with volumes between 100 and 1000 km3 is compared with that of an axisymmetric gravity current spreading over a rigid horizontal surface both the altimetric profiles and the horizontal projection of the line of intersection of domes on the synthetic aperture radar images agree well with the theoretical similarity solution for a newtonian fluid but not with the shape calculated for a rigid plastic rheology nor with that for a static model with a strong skin as a viscous current spreads it generates an isotropic strain rate tensor whose magnitude is independent of radius such a flow can account for the randomly oriented cracks that are uniformly distributed on the surface of the domes the stress induced by the flow in the plains material below is obtained and is probably large enough to produce the short radial cracks in the surface of the plains beyond the domes the viscosity of the domes can be estimated from their thermal time constants if spreading is possible only when the fluid is hot and lies between 1014 and 1017 pa s laboratory experiments show that such viscosities correspond to temperatures of 610 to 700 c in dry rhyolitic magmas these temperatures agree with laboratory measurements of the solidus temperature of wet rhyolite these results show that the development of the domes can be understood using simple fluid dynamical ideas and that the magmas involved can be produced by wet melting at depths below 10 km followed by eruption and degassing

a theoretical model of cooling viscous gravity currents with temperature dependent viscosity gravity current theory has applications to any geophysical phenomena involving the spreading of fluid on a horizontal interface many geological gravity currents e g lava flows and mantle plume heads are composed of cooling fluid with temperature dependent viscosity an axisymmetric gravity current theory ac counting for these thermo viscous effects is thus pre sented and explored here unlike isoviscous gravity cur rents huppert 1982 cooling variable viscosity do not conserve shape and can undergo a somewhat ex otic evolution for large viscosity contrasts between cold and hot fluid a constant volume initially hot domed current collapses rapidly into a fiat plateau with a steep edge gravity currents ejected at a constant volume flux from a central conduit also spread with a flattened plateau shape continuously fed currents that have a large hot initial volume develop an out wardly propagating interior plateau regardless of ini tial state continuously fed variable viscosity currents grow primarily by thickening this contrasts signficantly from isoviscous currents which grow almost entirely by spreading

selection of earthquake scaling relationships for seismic hazard analysis a fundamentally important but typically abbreviated component of seismic hazard analysis is the selection of earthquake scaling relationships these are typically regressions of historical earthquake datasets in which magnitude is estimated from parameters such as fault rupture length and area the mix of historical data from different tectonic environments and the different forms of the regression equations can result in large differences in magnitude estimates for a given fault rupture length or area we compile a worldwide set of regressions and make a first order shortlisting of regressions according to their relevance to a range of tectonic regimes plate tectonic setting and fault slip type in existence around the world regression relevance is based largely on the geographical distribution age and quantity quality of earthquake data used to develop them our compilation is limited to regressions of magnitude or seismic moment on fault rupture area or length and our shortlisted regressions show a large magnitude range up to a full magnitude unit for a given rupture length or area across the various tectonic regimes these large differences in magnitude estimates underline the importance of choosing regressions carefully for seismic hazard application in different tectonic environments

estimating locations and magnitudes of earthquakes in southern california from modified mercalli intensities modified mercalli intensity mmi assignments instrumental moment magnitudes m and epicenter locations of thirteen 5 6 m 7 1 training set events in southern california were used to obtain the attenuation relation mmi 1 64 1 41 m 0 00526 h 2 63 log h where h is the hypocentral distance in kilometers and m is moment magnitude intensity magnitudes m i and locations for five 5 9 m 7 3 independent test events were consistent with the instrumental source parameters fourteen historical earthquakes between 1890 and 1927 were then analyzed of particular interest are the m i 7 2 9 february 1890 and m i 6 6 28 may 1892 earthquakes which were previously assumed to have occurred near the southern san jacinto fault a more likely location is in the eastern california shear zone ecsz these events and the 1992 m 7 3 landers and 1999 m 7 1 hector mine events suggest that the ecsz has been seismically active since at least the end of the nineteenth century the earthquake catalog completeness level in the ecsz is m 6 5 at least until the early twentieth century online material figures and table of empirical mmi site corrections

structure and holocene rupture of the morelia fault trans mexican volcanic belt and their significance for seismic hazard assessment abstract the trans mexican volcanic belt is an active volcanic arc being deformed by an intra arc extensional fault network the morelia fault with a length of 14 km and a maximum throw of 260 m is a major normal fault in the central part of this volcanic arc and forms part of a 36 km long fault array slickenlines measured on the primary fault surface indicate north south extensional dip slip with a minor left lateral component in a well exposed cross section in the hanging wall of the morelia fault some few meters from the master fault trace two buried soil layers containing ceramic shards and stone tools yielded radiocarbon ages of a d 420 660 prehispanic classic period chupicuaro culture and a d 1290 1435 postclassic period respectively the underlying bedrock atecuaro ignimbrite is cut in a staircase pattern by three synthetic secondary normal faults that delimit two colluvial wedges three slip events with displacements ranging between 46 and 185 cm average 109 cm can be inferred from this section the latest of these events must have occurred after a d 1290 1435 during the postclassic period or possibly during early novohispanic times the morelia fault which is capable of generating m w 6 7 earthquakes poses a major seismic ground shaking hazard and locally a surface fault rupture hazard within the metropolitan morelia region with a population of more than 900 000 habitants the foci of a sequence of seven small 2007 earthquakes on the morelia fault were located below the center of town at a depth of 11 km and the fault trace crosses urban residential areas moreover the town repeatedly experienced severe ground shaking during historical earthquakes the best documented is the 19 june 1858 earthquake which caused morelia extensive damage corresponding to a macroseismic intensity of viii ix on the modified mercalli scale online material radiocarbon analysis reports local 1894 1904 and 1909 1911 earthquake catalogs

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

non newtonian rheology of igneous melts at high stresses and strain rates experimental results for rhyolite andesite basalt and nephelinite the stress strain rate relationships of four silicate melt compositions high silica rhyolite andesite tholeiitic basalt and nephelinite have been studied using the fiber elongation method measurements were conducted in a stress range of 10 400 mpa and a strain rate range of 10 6 to 10 3 s 1 the stress strain rate relationships for all the melts exhibit newtonian behavior at low strain rates but non newtonian nonlinear stress strain rate behavior at higher strain rates with strain rate increasing faster than the applied stress the decrease in calculated shear viscosity with increasing strain rate precedes brittle failure of the fiber as the applied stress approaches the tensile strength of the melt the decrease in viscosity observed at the high strain rates of the present study ranges from 0 25 to 2 54 log10 pa s the shear relaxation times of these melts have been estimated from the low strain rate newtonian shear viscosity using the maxwell relationship s g non newtonian shear viscosity is observed at strain rates time 1 equivalent to time scales that lie 3 log10 units of time above the calculated relaxation time brittle failure of the fibers occurs 2 log10 units of time above the relaxation time this study illustrates that the occurrence of non newtonian viscous flow in geological melts can be predicted to within a log10 unit of strain rate high silica rhyolite melts involved in ash flow eruptions are expected to undergo a non newtonian phase of deformation immediately prior to brittle failure

on observing the compositional variability of the surface of venus using nightside near infrared thermal radiation 1 there are several windows in the near infrared wavelength range 1 m in which thermal radiation emitted from the planetary surface penetrates through the thick venus atmosphere and clouds in this study we develop an improved method to estimate the surface emissivity on the basis of near infrared thermal emission from the nightside venus which is observed through these windows a simple radiative transfer model demonstrates that multiple reflection of thermal radiation between the atmosphere including clouds and the solid surface has a significant influence on the observed radiance under the condition of venus where reflectivity of overlying atmosphere and clouds is high thus it is necessary to take the effect of the reflection by the planetary surface into account in order to estimate accurately the variation in the surface emissivity on the basis of near infrared observation of venus the net effect of multiple reflection of surface thermal radiation between the atmosphere and the surface is to significantly reduce the spatial contrast in thermal radiation due to surface compositional variation the model calculation demonstrates that despite this effect detection of granitic rocks on the venus surface using near infrared windows is feasible since granitic and basaltic rocks have dramatically different 1 m emissivities granitic rocks are distinguishable from basaltic rocks by ground based telescopic observation a venus orbiter that measures both the near infrared thermal radiation and the surface altitude with great accuracy will provide us with a reliable surface emissivity map of venus which is a very valuable tool to detect granitic i e earth like continental rocks on venus

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

thermal and topographic tests of europa chaos formation models from galileo e15 observations stereo topography of an area near tyre impact crater europa reveals chaos regions characterised by marginal cliffs and domical topography rising to 100 200 m above the background plains the regions contain blocks which have both rotated and tilted we tested two models of chaos formation a hybrid diapir model in which chaos topography is caused by thermal or compositional buoyancy and block motion occurs due to the presence of near surface 1 3 km melt and a melt through model in which chaos regions are caused by melting and refreezing of the ice shell none of the hybrid diapir models tested generate any melt within 1 3 km of the surface owing to the low surface temperature a model of ocean refreezing following melt through gives effective elastic thicknesses and ice shell thicknesses of 0 1 0 3 and 0 5 2 km respectively however for such low shell thicknesses the refreezing model requires implausibly large lateral density contrasts 50 100 kg m 3 to explain the elevation of the centres of the chaos regions although a global equilibrium ice shell thickness of 2k m is possible if europa s mantle resembles that of io it is unclear whether local melt through events are energetically possible thus neither of the models tested here gives a completely satisfactory explanation for the formation of chaos regions we suggest that surface extrusion of warm ice may be an important component of chaos terrain formation and demonstrate that such extrusion is possible for likely ice parameters 2005 elsevier inc all rights reserved

shapes of venusian pancake domes imply episodic emplacement and silicic composition the main evidence available for constraining the composition of the large circular pancake domes on venus is their gross morphology laboratory simulations using polyethylene glycol show that the height to diameter aspect ratios of domes of a given total volume depend critically on whether their extrusion was continuous or episodic with more episodes leading to greater cooling and taller domes thus without observations of their emplacement the compositions of venusian domes cannot be uniquely constrained by their morphology however by considering a population of 51 venusian domes to represent a sampling of many stages during the growth of domes with comparable histories and by plotting aspect ratio versus total volume we find that the shapes of the domes are most consistent with episodic emplacement on earth this mode of dome growth is found almost exclusively in lavas of dacite to rhyolite composition strengthening earlier inferences about the presence of evolved magmas on venus

effects of eruption history and cooling rate on lava dome growth to better understand the factors controlling the shapes of lava domes laboratory simulations measurements from active and prehistoric flows and dimensional analysis were used to explore how effusion history and cooling rate affect the final geometry of a dome fifty experiments were conducted in which a fixed volume of polyethylene glycol wax was injected into a tank of cold sucrose solution either as one continuous event or as a series of shorter pulses separated by repose periods when the wax cooling rates exceeded a critical minimum value the dome aspect ratios height diameter increased steadily with erupted volume over the course of a single experiment and the rate at which height increased with volume depended linearly on the time averaged effusion rate thus the average effusion rate could be estimated from observations of how the dome shape changed with time our experimental results and dimensional analyses were compared with several groups of natural lava flows the recently emplaced mount st helens and soufriere domes which had been carefully monitored while active three sets of prehistoric rhyolite domes that varied in eruptive style and shape and two sets of holocene domes with similar shapes but different compositions geometric measurements suggest that dome morphology can be directly correlated with effusion rate for domes of similar composition from the same locality and that shape alone can be related to a dimensionless number comparing effusion rate and cooling rate extrapolation to the venusian pancake domes suggests that they formed from relatively viscous lavas extruded either episodically or at average effusion rates low enough to allow solidified surface crust to exert a dominating influence on the final morphology

rupture of the pitaycachi fault in the 1887 mw 7 5 sonora mexico earthquake southern basin and range province rupture kinematics and epicenter inferred from rupture branching patterns during the 3 may 1887 mw 7 5 sonora earthquake surface rupture end to end length 101 8 km an array of three north south striking basin and range province faults from north to south pitaycachi teras and otates slipped sequentially along the western margin of the sierra madre occidental plateau this detailed field survey of the 1887 earthquake rupture zone along the pitaycachi fault includes mapping the rupture scarp and measurements of surface deformation the surface rupture has an endpoint to endpoint length of 41 0 km dips 70 w and is characterized by normal left lateral extension the maximum surface offset is 487 cm and the mean offset 260 cm the rupture trace shows a complex pattern of second order segmentation however this segmentation is not expressed in the 1887 along rupture surface offset profile which indicates that the secondary segments are linked at depth into a single coherent fault surface the pitaycachi surface rupture shows a well developed bipolar branching pattern suggesting that the rupture originated in its central part where the polarity of the rupture bifurcations changes most likely the rupture first propagated bilaterally along the pitaycachi fault the southern rupture front likely jumped across a step over to the teras fault and from there across a major relay zone to the otates fault branching probably resulted from the lateral propagation of the rupture after breaching the seismogenic part of the crust given that the much shorter ruptures of the otates and teras segments did not develop branches

the great mexican earthquake of 19 june 1858 expected ground motions and damage in mexico city from a similar future event abstract the description of the great earthquake of 19 june 1858 is unusual damage and high intensities were reported both in the state of michoacan and in mexico city although a coastal epicenter for this earthquake cannot be ruled out the reports agree better with an intermediate depth about 50 km normal faulting event in the subducted cocos plate a careful examination of the reports of this event and other normal faulting events below the mexican altiplano suggests that a likely location is 18 0 n 100 8 w near the epicenter of the 6 june 1964 m 7 3 h 55 km event this location is 220 km sw of the city the magnitude of the earthquake is estimated to be about 7 7 we synthesize expected ground motions in cu a hill zone site in the city from an event similar to that of 1858 using records from the 23 may 1994 earthquake 18 0 n 100 6 w h 50 km m 5 7 as an empirical green9s function and stress parameter of 50 160 and 300 bar the expected peak horizontal acceleration in cu of 160 bar is about 30 gals similar acceleration was recorded in cu during the 1985 michoacan earthquake m 8 0 we compute expected ground motions at many sites in mexico city using empirical transfer functions and random vibration theory and compare these motions and the expected damage in the city with those from the 1985 michoacan earthquake results show that the overall expected damage during the postulated earthquake is and 1 of that during the michoacan earthquake for 160 and 300 bar respectively a greater percentage of low rise construction which constitute about 80 of the total in the city will be damaged during the postulated earthquake than during the michoacan earthquake the expected ground motions for 50 bar are smaller at all periods than those from the michoacan earthquake as the present building code for mexico city contemplates coastal earthquakes of magnitude greater than 8 0 the case of 50 bar is not of interest in this article this preliminary study suggests a need for a more careful evaluation of expected ground motion in the valley of mexico from the postulated earthquake and its impact on the current design spectra of the city

ambient effects on basalt and rhyolite lavas under venusian subaerial and subaqueous conditions both subaerial and subaqueous environments have been used as analog settings for venus volcanism to assess the merits of this the effects of ambient conditions on the physical properties of lava on venus the seafloor and land on earth are evaluated rhyolites on venus and on the surface of earth solidify before basalts do because of their lower eruption temperatures rhyolite crust is thinner than basalt crust at times less than about an hour especially on venus at later times rhyolite crust is thicker because of its lower latent heat relative to basalt the high pressure on the seafloor and venus inhibits the exsolution of volatiles in lavas vesicularity and bulk density are proportional so that lavas of the same composition should be more dense on the seafloor and less dense on land because viscosity depends partly upon the fraction of unvesiculated water in a melt basalts with the same initial volatile abundance will be least viscous on the seafloor and most viscous on land assuming the same preemptive h2o contents molten rhyolites on venus will have viscosities 10 that of rhyolites on land despite lower expected viscosities underwater flows are more buoyant and should have heights like subaerial and venusian lavas of the same composition and extrusive history in cases where the influence of crust is insignificant a volume of rhyolite will have a higher aspect ratio than the same volume of basalt no matter what the environment if flow rheology is dominated by the presence of strong crust aspect ratios differ little among environments or between compositions these analyses support a rhyolitic interpretation for the composition of venusian festooned flows and a basaltic interpretation for the composition of venusian steep sided domes although ambient effects are significant extrusion rate and eruption history must also be considered to explain analogous volcanic landforms on earth and venus

the a d 1567 mw 7 2 ameca jalisco earthquake western trans mexican volcanic belt surface rupture parameters seismogeological effects and macroseismic intensities from historical sources the a d 1567 ameca jalisco earthquake west central mexico is the strongest known historical crustal earthquake in the trans mexican volcanic belt an active volcanic arc being deformed by an intra arc extensional fault network the location of the surface rupture at the base of the sierra de ameca mountain range its length 13 14 leagues 54 59 km and the scarp height 1 2 estados 1 67 3 34 m are inferred from the description of this earthquake in the relacion de ameca of 1579 which contains what is probably the earliest documentation of an earthquake surface rupture in the americas from scaling relations the moment magnitude of this earthquake can be estimated as 7 2 0 3 based on the rupture length the a d 1567 ameca earthquake is likely to have ruptured to the surface along two or all three major segments of the ameca san marcos normal fault system online material excerpts from five historical documents related to the 1567 ameca earthquake

numerical experiments on the movement of a warm core ring with the bottom slope of a western boundary numerical experiments are performed to investigate the effects of the bottom slope of a western boundary on realistic warm core rings using a primitive equation sigma coordinate model warm core rings which initially have baroclinic velocity near a steep bottom slope gradually move northward but those with barotropic velocity do not barotropic velocity on the slope immediately disperses as a topographic rossby wave while baroclinic velocity is retained since the northward movement is caused by an effect very much like the image effect which is originally the effect of a free slip wall the authors term this effect the equivalent image effect the equivalent image effect increases as a slope becomes steeper and bottom friction becomes weaker and vice versa it seems that a steep slope acts like a wall and the bottom friction of a steep slope corresponds to the lateral viscous effect of a wall however the effect of a slope on a warm core ring is different from that of a wall in that ring slope interactions take place at the lower part of the ring because the velocity of a warm core ring is weak in deep layers ring slope interactions are moderate and the leakage of the interior fluid of a ring seen in ring wall interactions is not apparent while the viscous effect is weaker than that of a no slip wall as well these results are applied to the northward movement of kuroshio warm core rings and to warm eddies in the gulf of mexico

the aljibes half graben active extension at the boundary between the trans mexican volcanic belt and the basin and range province mexico the aljibes half graben is located 140 km west northwest of mexico city and is characterized by several parallel 10 km long south dipping and east striking normal faults late miocene basalt flows with 40 ar 39 ar ages of 7 7 0 1 ma and 7 1 0 5 ma are displaced along these faults with reference to this basalt the composite throw and heave of the half graben are 480 m and 270 m respectively and the relative extension is 5 whereas the vertical slip rate along the faults of the aljibes half graben after the deposition of the basalt is 0 07 mm yr the aljibes half graben is located at the northern margin of the trans mexican volcanic belt and perpendicularly intersects northstriking normal faults of the basin and range province the relative sequence of activity of the two stress fields and fault systems cannot be resolved in the study area and the two systems may have been active simultaneously we explain this structural configuration by horizontal migration of the boundary between the two stress provinces with time which requires intermittent permutations between the intermediate and least principal stresses 2 and 3 this is supported by the low stress ratio 2 3 1 3 obtained from modeling the stress tensor associated with the slip along the faults of the aljibes half graben the orientations of the fault planes and slip vectors of the aljibes half graben are practically identical to those of the nearby acambay graben and the venta de bravo fault which caused the acambay earthquake of november 19 1912 m s 6 9 and the maravatio earthquake of february 22 1979 m b 5 3 furthermore the aljibes half graben forms the extrapolated western continuation of the northern master fault of the mezquital graben which ruptured partly in the cardonal earthquake of march 26 1976 m b 5 3 this suggests the aljibes half graben may also be seismically active although no historical earthquake is known to have occurred along this structure further evidence for the likely seismic activity of the aljibes half graben are two east to eastnortheast trending epicenter alignments between the aljibes halfgraben and the mezquital graben and structural indication for stickslip behavior based on the structure of the aljibes half graben we believe the maximum credible earthquake that may occur along the faults of the aljibes half graben is on the order of 2 10 27 dyne cm m 0 6 10 25 dyne cm and 7 5 m w 6 5

the dynamic history of the trans mexican volcanic belt and the mexico subduction zone abstract the trans mexican volcanic belt tmvb is a 1000 km long neogene continental arc showing a large variation in composition and volcanic style and an intra arc extensional tectonics it overlies the rivera and cocos slabs which display marked changes in geometry geophysical studies indicate that lithospheric mantle is very thin or absent beneath the forearc and arc the fluids from the slab are released in a 40 to 100 km wide belt beneath the frontal part of the arc and the lower crust beneath the arc is partially molten east of 101 w the tmvb is built on a precambrian to paleozoic crust with thickness of 50 55 km west of 101 w the tmvb is underlain by jurassic to cenozoic marine and continental arcs with a 35 40 km thick crust the evolution of the tmvb occurred in four stages 1 from 20 to 10 ma the initial andesitic arc moved inland showing progressively drier melting and eventually slab melting suggesting flattening of the subducted slab 2 since 11 ma a pulse of mafic volcanism migrated from west to east reaching the gulf of mexico by 7 ma this mafic lavas marks the lateral propagation of a slab tear triggered by cessation of subduction beneath baja california 3 thereafter the volcanic front started moving trenchward with a marked phase of silicic volcanism between 7 5 and 3 ma local emplacement of small volume intraplate like basalts since 5 ma and development of extensional faulting these features are related to slab rollback enhancing asthenophere flux into the mantle wedge and promoting partial melting of the crust 4 the modern arc consists of a frontal belt dominated by flux and slab melting and a rear belt characterized by more differentiated rocks or by mafic lavas with little or no evidence of subduction fluids but higher asthenosphere fingerprint

large scale phreatomagmatic silicic volcanism a case study from new zealand abstract pyroclastic deposits of the 20 000 year old wairakei formation occur in the region surrounding lake taupo in the central north island of new zealand recent advances in the correlation of both proximal and distal equivalents of the wairakei deposits show that they were the result of one large eruption the ash layers once covered most of new zealand they are also present in deep sea cores from the sw pacific and on the chatham islands the dispersal and grain size parameters of the ash fall members indicate that their vent area was within the taupo volcano tectonic depression which for a long period has contained a large lake the wairakei eruption is interpreted to have been phreatomagmatic throughout each phase of the eruption sequence generated its own characteristic deposits the sequence of events can be summarized as follows 1 eruption and deposition of a white fine grained bedded fall deposit 2 eruption and deposition of a medium grained pumice fall deposit 3 generation of ultra fine wet ash clouds with a fall deposit accumulating largely as aggregates and accretionary lapilli accompanied by base surges 4 formation of water cooled pyroclastic flows and deposition of an extensive ignimbrite 5 return to a dominantly vertical eruption column and deposition of a medium grained bedded ash fall immediately followed by another wet ultra fine fall deposit charged with accretionary lapilli finally 6 an ignimbrite less widespread than that of phase 4 was produced co ignimbrite fall deposits of fine vitric dust and medium grained pumice were laid down over a huge area there may have been a short hiatus in activity between phases 4 and 5

new approaches to inferences for steep sided domes on venus abstract new mathematical approaches for the relaxation and emplacement of viscous lava domes are presented and applied to steep sided domes on venus a similarity solution approach is applied to the governing equation for fluid flow in a cylindrical geometry for two distinct scenarios in the first scenario dome relaxation is explored assuming a constant volume of fluid i e lava has been rapidly emplaced onto the surface cooling of lava is represented by a time variable viscosity and singularities inherent in previous models for dome relaxation have been eliminated at the onset of relaxation bulk dynamic viscosities lie in the range between 10 10 10 16 pa s consistent with basaltic andesite to rhyolitic compositions plausible relaxation times range from 5 to 5000 years depending on initial lava viscosity the first scenario however is only valid during the final stages of dome relaxation and does not consider the time taken for lava to be extruded onto the surface in the second scenario emplacement and growth of a steep sided dome is considered when the volume of lava on the surface increases over time i e time variable volume approach the volumetric flowrate may depend on an arbitrary power of the dome thickness thus embracing newtonian as well as other rheologies for describing terrestrial and planetary mass flows the approach can be used to distinguish between basic flowrate models for fluid emplacement the formalism results in radial expansion of a dome proportional to t 1 2 consistent with the diffusive nature of the governing equation the flow at the front is shown to thicken as the front advances for a constant rate of lava supply emplacement times are intimately correlated with the bulk rheology comparison of the theoretical profiles with the shape of a typical dome on venus indicates that a newtonian bulk rheology is most appropriate consistent with prior studies however results here suggest a bulk dynamic viscosity of 10 12 10 13 pa s and emplacement times of approximately 2 16 years both scenarios investigated give emplacement times significantly less than prior estimates and compositions consistent with basaltic andesite

radar topography of domes on planetary surfaces abstract we investigate the possibility of measuring the heights and morphology of viscously emplaced domes using radar imagery we accurately reproduce the known height and shape of a terrestrial salt dome and estimate the heights of several venusian pancake domes to within a factor of two the terrestrial salt dome is consistent with a bingham flow while the much larger venusian pancake domes are consistent with a newtonian flow applying the same techniques to ganesa macula a potential cryovolcanic dome on titan we estimate a height between 2 0 4 9 km additional factors such as variable roughness and composition might account for some of the discrepancies observed

analysis of venusian steep sided domes utilizing stereo derived topography 1 the morphology and topography of 23 venusian steep sided domes are analyzed to examine possible emplacement mechanisms stereo radargrammetric techniques are used with magellan synthetic aperture radar image data 100 m resolution to generate digital elevation models for the domes 1 km horizontal resolution heights of the domes range from 390 to 1590 m aspect ratios height diameter range from 0 01 to 0 07 and volumes range from 73 to 1790 km 3 dome morphology varies along a continuous trend that is directly related to the aspect ratio domes with a low aspect ratio tend to be sunken in their centers whereas domes with higher aspect ratios have flat to convex surfaces domes with the highest aspect ratios have central peaks these trends are likely due to slight variations in the thickness of the crust and possibly effusion rates episodic emplacement is also inferred to be a significant process for the formation and growth of some of the domes in this sample

observed and simulated kinematic properties of loop current rings two rings shed by the loop current in 1980 and 1982 were observed for several months by satellite tracked drifters to migrate across the gulf of mexico the drifter path data have been inverted to obtain estimates of the paths of the centers of the two rings ring shape and the swirl velocities three drifters were deployed in the 1980 ring and the analysis of that data set establishes the variability of the above kinematic estimates for one ring a comparison of the analysis of data from both rings provides some idea on inter ring variability both rings impacted the mexican continental slope at about 22 8 n 95 5 w after a brief adjustment period both rings reestablished and maintained a vortex character for several months in the slope region while migrating slowly to the north the paths of the centers of the two rings along the slope are virtually identical the same analysis routine was applied to some simulated drifter data obtained from the hurlburt and thompson 1980 gulf of mexico primitive equation model in the midgulf the agreement between the observed rings and the simulated ring is good although the former showed stronger interaction with the continental slope topography and or circulation than was seen in the latter along the slope the model ring kinematic characteristics were in extraordinary agreement with the observations

eruption of crystal mush and the formation of steep sided volcanic domes on venus insight from picritic bodies near marki cyprus abstract steep sided domes are one of the most striking volcanic landforms on venus they may also be key to determining the range of magmatic processes operating on venus as in contrast to all other volcanic landforms they likely represent eruption of viscous lava although there have been various explanations for the presence of high viscosity lavas on a planet dominated by effusive basaltic volcanism it is often assumed that they are silica rich this would necessitate either periodic large scale extensive fractionation of basaltic magma in the venusian crust or a mechanism for re melting an already silica enriched lower crust as such determining the origin of steep sided domes is important in constraining magmatic processes on venus and for understanding geological evolution of stagnant lid regime planets generally here we use observations from the marki region of the troodos ophiolite cyprus to propose an alternative model where steep sided domes form by eruption of crystal mush from the same magmatic systems which fed extensive basaltic terrains with which domes are associated steep sided volcanic landforms near marki represent extrusion of un eruptible extremely olivine rich mush onto the palaeo seafloor following cessation of widespread basaltic volcanism field relations suggest that these bodies formed by localised repeated extrusion of crystal mush fed by extensional faults tapping crustal magma chambers differential stress enabled eruption of viscous non newtonian magmas with crystal contents 50 vol which then built up volcanic edifices on the seafloor a similar much larger scale mechanism can explain many features of steep sided volcanic domes on venus including their intimate relationship with extensive basaltic terrains general morphology and dome spatial and temporal clustering this implies that domes share a common magmatic origin with the venusian basaltic crust rather than representing a discrete magmatic process and that they represent periods of magmatic quiescence it also implies that the contrasting morphology of these domes arises from a fundamental difference in eruptive style from widespread effusive basaltic magmatism to localised extensional fault controlled extrusion of crystal mush if correct this mechanism might also explain formation of steep sided volcanic edifices on other large stagnant lid regime planetary bodies

comparison of the shapes and sizes of seafloor volcanoes on earth and pancake domes on venus the fact that flat topped pancake domes observed on venus resemble flat topped volcanoes on earth s seafloor suggests that volcanic processes on the seafloor and on venus may operate to construct volcanic forms with similar characteristics to test this the shapes and size distributions of terrestrial submarine volcanoes seamounts from several magmatic and tectonic provinces are quantitatively compared with those of venusian pancake domes seamount heights and diameters are highly correlated suggesting that there may be a universal mechanism for their growth the 2014 point sample mean of the height to diameter ratio is d 0 10 0 04 in contrast heights and diameters of 52 pancake domes exhibit no obvious trend and values of d are as much as three orders of magnitude smaller with a 52 point sample mean of d 0 02 0 03 seamounts with basal diameters db 10 km have flatness values ratio of top to bottom diameter that span the entire range between 0 pointed cone and 1 truncated cone larger sized seamounts tend to have small values of flatness and more pointed tops this is not true for pancake domes which show large flatnesses at all sizes additionally seamounts have distinctive profiles with their flank slopes increasing in gradient toward the summit flank slopes of pancake domes however commonly increase in gradient toward the base with respect to sizes the diameter distributions of the different seamount populations are consistent with a model in which the number of seamounts decreases exponentially with increasing diameter the diameter distribution of 126 pancake domes by contrast is more uniform its mean is db 22 13 km this size is at least an order of magnitude larger than the estimated characteristic diameters of the seamount populations based on the above results i conclude that seamounts have significantly different shapes and size distributions than venusian pancake domes that have been identified and measured to date this suggests that one or more of the variables e g magma composition crystallinity magma volume effusion rate cooling rate pre existing topography governing their formation differs in terms of shapes and size distributions the small db 20 km volcanic edifices first identified from venera data are good morphologic matches to seamounts and should be the focus of future work investigating the formation of comparable volcanic features in these two different planetary environments

cryovolcanic emplacement of domes on europa abstract here we explore the hypothesis that certain domes on europa may have been produced by the extrusion of viscous cryolavas a new mathematical method for the emplacement and relaxation of viscous lava domes is presented and applied to putative cryovolcanic domes on europa a similarity solution approach is applied to the governing equation for fluid flow in a cylindrical geometry and dome relaxation is explored assuming a volume of cryolava has been rapidly emplaced onto the surface nonphysical singularities inherent in previous models for dome relaxation have been eliminated and cryolava cooling is represented by a time variable viscosity we find that at the onset of relaxation bulk kinematic viscosities may lie in the range between 103 and 106 m2 s while the actual fluid lava viscosity may be much lower plausible relaxation times to form the domes which are linked to bulk cryolava rheology are found to range from 3 6 days to 7 5 years we find that cooling of the cryolava while dominated by conduction through an icy skin should not prevent fluids from advancing and relaxing to form domes within the timescales considered determining the range of emplacement conditions for putative cryolava domes will shed light on europa s resurfacing history in addition the rheologies and compositions of erupted cryolavas have implications for subsurface cryomagma ascent and local surface stress conditions on europa

motion and evolution of oceanic rings in a numerical model and in observations observed properties of oceanic rings are compared to rings produced in a two gyre wind driven numerical ocean model and in a model of the south atlantic indian ocean their temporal evolution is discussed in terms of structure and translation rate they exhibit substantial similarity in terms of thermocline depth ring size swirl velocities and translations speeds in both observations and numerical model results the propagation speeds are 2 to 5 times faster than that of an equivalent isolated eddy which is of the order of the long rossby wave speed this is attributed to advection by the mean flows furthermore it is observed that the model rings have a coherent structure all the way to the bottom there is strong evidence that this is also the case in real oceanic rings one major difference between observed and modeled rings is in their decay rate the temporal decay of the rings in the models and observations is therefore discussed in relation to the decay mechanisms at work and the frictional parameterization of the model

evidence of recent ruptures in the central faults of the acambay graben central mexico abstract the acambay graben within the central part of the trans mexican volcanic belt is one of the major sources of continental earthquakes in mexico to date the activity and paleoseismological history of the axial faults of the graben are not well constrained we provide morphological structural and sedimentological evidence of the seismogenic nature of two of the axial structures the temascalcingo and the tepuxtepec fault systems faults consist of multiple parallel scarps with en echelon and horse splay patterns fault systems extend for 60 km and displace quaternary to upper miocene volcanic edifices and volcano sedimentary materials surface lengths of individual fault traces range between 3 and 25 km and observed throws reach a minimum of 150 200 m the long term and short term slip rate of the temascalcingo fault system in the studied section presents similar values ranging from 0 06 0 02 minimum long term to 0 12 0 02 mm y 1 maximum value of average short term only the long term slip rate of the tepuxtepec system could be constrained in 0 01 0 02 mm y being a minimum estimate the holocene fault rupture history at two sites provided evidence of six ruptures since 12 500 11 195 bce among which three ruptures should have occurred between 11 847 652 bce and 11 425 465 bce variable single event displacements seds between 6 and 77 are interpreted as the result of fault interdependences and or the interaction with the latest volcanic activity also small displacements triggered by activity on other faults probably contributed to slip variability i e faults display primary and secondary behavior

eddy statistics from lagrangian drifters and hydrography for the northern gulf of mexico slope 1 cyclones and anticyclones on the northern gulf of mexico slope have been statistically characterized using upper layer drifter orbits and repeated hydrographic surveys a total of 243 slope eddy drifter orbits 151 cyclones and 92 anticyclones were analyzed in terms of size mean radius and eccentricity and rotation period orbits in large loop current anticyclones were excluded eddies of both signs scale approximately as the baroclinic radius of deformation with relatively smaller radii on the upper slope where water depths are less than 1000 m the median radius for all eddies was 30 km and median periods were 12 and 13 5 days for cyclones and anticyclones respectively anticyclones also tend to be more circular than cyclones relative vorticity maps were constructed for seven hydrographic surveys which mapped the slope at 4 5 month intervals a total of 26 cyclones and 15 anticyclones had statistics that were consistent with drifter orbit analyses the surveys indicate that at any one time between 2 and 5 cyclones and 1 and 3 anticyclones may be present both drifters and surveys indicate that cyclones have a slight numerical dominance

the unique radar properties of silicic lava domes 1 silicic lava domes exhibit distinct morphologic characteristics at scales of centimeters to kilometers multiparameter radar observations capture the unique geometric signatures of silicic domes in a set of radar scattering properties that are unlike any other natural geologic surfaces backscatter cross section values are among the highest observed on terrestrial lava flows and show only a weak decrease with incidence angle cross polarization backscatter hv shows a unique behavior increasing with increasing wavelength circular polarization ratios are relatively high in the 0 3 0 95 range and increase with increasing wavelength field measurements of boulder size frequency distributions and microtopography indicate that silicic dome surfaces are among the roughest ever measured rms heights at a 1 m lateral scale range from 13 cm to 50 cm rms slopes at 1 m spacing range from 12 to 43 degrees modeling of the scattering behavior suggests it results from a combination of rough surface facet scattering and scattering from block edges that act as a random collection of dipoles the unusual wavelength dependence of the radar parameters appears to result from a higher component of edge scattering at large wavelengths producing for example higher cross polarized backscatter at p band 68 cm steep sided volcanic domes on venus superficially resemble terrestrial silicic domes in plan view gross morphology but few similarities remain when the radar scattering and three dimensional shapes of the features are compared the unique radar scattering properties suggest that such volcanic surfaces can be identified with multiparameter radar observations in future planetary radar missions and in active terrestrial volcanoes where dome development can represent serious hazards

orientational data on the state of stress in northeastern mexico as inferred from stress induced borehole elongations preferential elongation directions are presented from 46 subvertical wells in northeastern mexico from depth intervals ranging from 161 to 4 912 m the measurements document a remarkably consistent west east to northwest southeast direction of the contemporary least horizontal stress in the upper crust of this intraplate region in the regional neotectonic framework the data suggest that mexico north of the trans mexican volcanic belt is being stretched in west east to northwest southeast direction the measurements from the area of the laramide chihuahua and coahuila fold thrust belts define the southern extend of the basin and range rio grande rift stress province of the southwestern united states into northern mexico the least horizontal in situ stress directions recorded in the gulf coastal plain burgos and tampico misantla basins are oblique to the continental margin of the gulf of mexico but subparallel to the direction measured in the area of the basin and range rio grande rift stress province this suggests that the stress distribution in these basins is not mainly the result of gravitational loading as in the gulf coast stress province of the united states but is controlled by the same lithospheric stress system that characterizes the basin and range rio grande rift stress province this is also supported by the north south trending zone of oligocene quaternary alkaline volcanism that crosses the tampico misantla basin furthermore the data indicate that the active deformation of the mexican ridges fold belt east of the study area in the gulf of mexico cannot be caused by a compressional external tectonic load the direction of the least horizontal in situ stress measured in the area of the laramide sierra madre oriental fold thrust belt in hidalgo and san luis potosi states is consistent with that of the other zones however wellbore elongations were recorded less frequently and show a rather large circular variance which suggests that the minimum and maximum horizontal far field stresses are about equal in this area

granite a planetary point of view the telluric planets and the asteroid belt display the same internal structure with a metallic inner core and a silicate outer shell experimental data and petrological evidence in silicate systems show that granite can be produced through all types of magmatic processes on moon 4 4 3 9 ga granite clasts displaying dry mineral assemblages correspond to discrete intrusive events large k ca enrichment and low ree abundances in granite relative to kreep are consistent with silicate liquid immiscibility a process observed in melt inclusions within olivine of lunar basalts on mars black and white rhythmic layers observed on the tharsis rise along the flanks of valles marineris and the peripheral scarps of the tharsis montes giant volcanoes suggest the eruption of possibly felsic pyroclastites samples analysed in the mars pathfinder landing site yield basic to intermediate 45 to 62 wt sio2 compositions though no granites were found so far in the martian snc meteorites a component close to terrestrial continental granitic crust is inferred from trace element and isotope systematics like the martian northern hemisphere venus has suffered extensive volcanic resurfacing whereas folded and faulted areas resemble terrestrial continents the hypothesis of a granitic component is again tantalizing finally extra terrestrial granite is also found as enclaves within iron meteorites and ordinary chondrites granite a major component of the earth s crust can be generated in all geodynamic settings the low density of granite favours continental accretion thus the occurrence or absence of granite and of associated silicic volcanism within the other telluric planets is not a trivial question granite is generally thought to be produced through wet processes lunar evidence shows that dry conditions may apply as well in venus a large planet with high rates of magma production it is speculated that significant volumes of granite can develop

reply to comment on active crustal deformation in the trans mexican volcanic belt as evidenced by historical earthquakes during the last 450 years by g suarez et al

the maximum background earthquake for the basin and range province western north america the maximum background earthquake mbe is the largest earthquake not associated with significant primary surface rupture the mbe is estimated for the basin and range province considering 22 earthquakes from the province and a simple physical model of a circular rupture in the seismogenic zone in the basin and range province the number of historical earthquakes in the magnitude range 6 to 6 6 without significant surface rupture is clearly greater than the two events below magnitude 6 6 with primary surface rupture based on the historical record the mbe for the basin and range province is magnitude 6 6 but given the various uncertainties involved the usage of a magnitude value of 6 5 may be more reasonable for seismic hazard studied

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

volcanism and tectonics on venus we review recent developments in the study of volcanism and tectonics on venus venus s crust is basaltic dry and probably about 30 km thick the mantle convects giving rise to plumes and has a similar composition and mean temperature 1300 c but a higher viscosity 10 20 pa s than that of the earth inferred melt generation rates constrain the lithospheric thickness to between 80 and 200 km the elastic thickness of the lithosphere is about 30 km on average the present day lack of plate tectonics may be due to strong faults and the high viscosity of the mantle most of the differences between earth and venus processes can be explained by the absence of water venus underwent a global resurfacing event 300 600 ma ago the cause and nature of which remains uncertain the present day surface heat flux on venus is about half the likely radiogenic heat generation rate which suggests that venus has been heating up since the resurfacing event

stratigraphic and geographic distribution of steep sided domes on venus preliminary results from regional geological mapping and implications for their origin analysis of the stratigraphic position of 44 steep sided domes in a global band of c1 midrs at 30 n latitude shows that in a region representing over 11 of the surface of venus the vast majority of steep sided domes are closely associated stratigraphically with shield plains psh a unit characterized by a very high abundance of small shields of apparent basaltic extrusive origin previously reported heterogeneities in areal and altitude distribution are now more plausibly interpreted in terms of formation in association with this unit and burial by subsequent regional plains units measurement of abundances of steep sided domes presently exposed in shield plains and extrapolation to areas below younger units suggest that over a thousand of these features may have formed originally interpretation of the mode of formation of the steep sided domes in the context of these new data favors hypotheses for their origin which call on remelting of basaltic crust to produce more silica rich magmas occurrences of some steep sided domes in association with summits of large shields e g tepev mons and sapas mons indicate that other mechanisms of dome formation e g evolution of magma reservoirs have also operated at other times in the history of venus

on the structure and motions of cyclones in the northern gulf of mexico 1 the eddy field on the northern slope of the gulf of mexico has been investigated using high resolution axbt and axcp aircraft surveys and argos tracked drifters a number of vigorous cyclones were mapped their vertical structures showed solid body rotation with radii of 50 75 km with a subsurface maximum 40 cm s 1 at the outer edge at about 150 m flows had little shear in the upper 400 m and only weak currents below depths of 800 to 1000 m cyclones were observed to move on and off the slope under the influence of swirl velocities of larger warm core eddies including loop current lc anticyclones rotational periods range from about 7 to 18 days the mapped eddy fields contained both cyclones and anticyclones with a range of diameters from about 40 to 150 km the presence of many relatively small scale features along the upper slope strongly affects features of the shelf break velocity field that were measured by a moored current meter array the directions and vertical shears of the moored currents were consistent with flows caused by the northern edges of cyclones and anticyclones these eddies can cause persistent events and strong cross isobath fluctuations of the currents counterrotating pairs of eddies on the upper slope seem to be a mechanism for the transport of material on and off the louisiana and east texas shelves coherences between moorings 74 km apart were generally not significant as might be expected when the currents are dominated by small scale eddies however an eof analysis of the shelf break array indicates that about 40 of the along slope current variances could be attributed to westward propagating continental shelf waves

comment on active crustal deformation in the trans mexican volcanic belt as evidenced by historical earthquakes during the last 450 years by g suarez et al

earthquake supercycles and long term fault memory abstract long records often show large earthquakes occurring in supercycles sequences of temporal clusters of seismicity cumulative displacement and cumulative strain release separated by less active intervals supercycles and associated earthquake clusters are only partly characterized via the traditionally used aperiodicity which measures the extent that a sequence differs from perfectly periodic supercycles are not well described by commonly used models of earthquake recurrence in the poisson model the probability of a large earthquake is constant with time so the fault has no memory in a seismic cycle renewal model the probability is quasi periodic dropping to zero after a large earthquake then increasing with time so the probability of a large earthquake depends only on the time since the past one and the fault has only short term memory we describe supercycles with a long term fault memory ltfm model where the probability of a large earthquake reflects the accumulated strain rather than elapsed time the probability increases with accumulated strain and time until an earthquake happens after which it decreases but not necessarily to zero hence the probability of an earthquake can depend on the earthquake history over multiple prior cycles we use ltfm to simulate paleoseismic records from plate boundaries and intraplate areas simulations suggest that over timescales corresponding to the duration of paleoseismic records the distribution of earthquake recurrence times can appear strongly periodic weakly periodic poissonian or bursty thus a given paleoseismic window may not capture long term trends in seismicity this effect is significant for earthquake hazard assessment because whether an earthquake history is assumed to contain clusters can be more important than the probability density function chosen to describe the recurrence times in such cases probability estimates of the next earthquake will depend crucially on whether the cluster is treated as ongoing or over

macroseismic study of shallow earthquakes in the central and eastern parts of the trans mexican volcanic belt mexico we present isoseismal maps for eight shallow earthquakes 1887 pinal de amoles 1912 acambay 1920 jalapa 1950 ixmiquilpan 1976 cardonal 1979 maravatio 1987 actopan and 1989 landa 4 1 mb 6 9 that occurred in the central and eastern parts of the trans mexican volcanic belt based on their intensity distributions we can relate several of these earthquakes to east west striking quaternary normal faults known from geological observations furthermore we derive for these events by empirical curve fitting linear relations between the magnitude calculated from the instrumental record the intensity distribution and the maximum intensity the comparison of these relations with equations from the literature suggests that the area shaken with a certain intensity by an earthquake in the trans mexican volcanic belt is approximately 10 times smaller than the area shaken by a shallow earthquake of the same magnitude in the intracratonic part of north america moreover we use the derived relationships between magnitude isoseismal area and maximum intensity to estimate the intensity magnitude mi of the 1887 pinal de amoles earthquake as 5 3 0 5

formation of submarine flat topped volcanic cones in hawai i high resolution bathymetric mapping has shown that submarine flat topped volcanic cones morphologically similar to ones on the deep sea floor and near mid ocean ridges are common on or near submarine rift zones of kilauea kohala or mauna kea mahukona and haleakala volcanoes four flat topped cones on kohala were explored and sampled with the pisces v submersible in october 1998 samples show that flat topped cones on rift zones are constructed of tholeiitic basalt erupted during the shield stage similarly shaped flat topped cones on the northwest submarine flank of ni ihau are apparently formed of alkalic basalt erupted during the rejuvenated stage submarine postshield stage eruptions on hilo ridge mahukona hana ridge and offshore ni ihau form pointed cones of alkalic basalt and hawaiite the shield stage flat topped cones have steep 25 sides remarkably flat horizontal tops basal diameters of 1 3 km and heights 300 m the flat tops commonly have either a low mound or a deep crater in the center the rejuvenated stage flat topped cones have the same shape with steep sides and flat horizontal tops but are much larger with basal diameters up to 5 5 km and heights commonly greater than 200 m the flat tops have a central low mound shallow crater or levees that surrounded lava ponds as large as 1 km across most of the rejuvenated stage flat topped cones formed on slopes 10 and formed adjacent semicircular steps down the flank of ni ihau rather than circular structures all the flat topped cones appear to be monogenetic and formed during steady effusive eruptions lasting years to decades these and other submarine volcanic cones of similar size and shape apparently form as continuously overflowing submarine lava ponds a lava pond surrounded by a levee forms above a sea floor vent as lava continues to flow into the pond the lava flow surface rises and overflows the lowest point on the levee forming elongate pillow lava flows that simultaneously build the rim outward and upward but also dam and fill in the low point on the rim the process repeats at the new lowest point forming a circular structure with a flat horizontal top and steep pillowed margins there is a delicate balance between lava heat supply to the pond and cooling and thickening of the floating crust factors that facilitate construction of such landforms include effusive eruption of lava with low volatile contents moderate to high confining pressure at moderate to great ocean depth long lived steady eruption years to decades moderate effusion rates probably ca 0 1 km3 year and low but not necessarily flat slopes with higher effusion rates sheet flows flood the slope with lower effusion rates pillow mounds form hawaiian shield stage eruptions begin as fissure eruptions if the eruption is too brief it will not consolidate activity at a point and fissure fed flows will form a pond with irregular levees the pond will solidify between eruptive pulses if the eruption is not steady lava that is too volatile rich or that is erupted in too shallow water will produce fragmental and highly vesicular lava that will accumulate to form steep pointed cones as occurs during the post shield stage the steady effusion of lava on land constructs lava shields which are probably the subaerial analogs to submarine flat topped cones but formed under different cooling conditions

a numerical study of loop current eddy interaction with topography in the western gulf of mexico abstract anticyclones originating from the loop current are known to propagate into the western gulf of mexico their frequency of generation their long lifetimes and satellite data suggest that at any one time one or more eddies may occupy the gulf given the eddy sizes 100 200 km and geometric confinement of the gulf it would appear that there may be significant interactions of individual eddies and or interactions of the eddies with bottom topography these possibilities are explored through the use of a two layer primitive equation model variable parameters in this model study are eddy strength vertical structure lateral friction and initial location relative to topography results indicate that eddy motion is governed by two dynamical regimes depending on its lower layer rotational strength anticyclones with significant lower layer anticyclonic structure develop offshore directed self advective tendencies associated with topographic dispersion which induces asymmetry in the eddy eddies wit

venus surface data extraction from virtis venus express measurements estimation of a quantitative approach nightside emission measurements of the visible and infrared thermal imaging spectrometer virtis on the venus express vex spacecraft were used to estimate the potential for surface data extraction a selection of orbits over the northern hemisphere was performed for footprints that cover different scales of surface elevation variations a correction method was used to remove straylight from the measured spectra that is due to direct sunlight striking the instrument a preliminary radiative transfer calculation technique was applied to simulate venus nightside radiation the basic features of the measured spectra are well reproduced present limitations of the algorithm are discussed the variability of the emission window radiances with respect to cloud opacity and surface elevation is modelled and discussed in direct comparison with the measurements it is demonstrated that a multi spectral analysis in the surface and deep atmosphere window ranges 1 0 2 3 m and the use of radiance ratios are well suited to de cloud the data and to extract surface information from the virtis measurements this method allows a mapping of surface topography and the retrieval of the surface temperature a preliminary topography retrieval at beta regio was performed and compared with magellan radar data differences are possibly due to emissivity variations on the surface

comment on estimation of ground motion in mexico city from a repeat of the m 7 0 acambay earthquake of 1912 by s k singh a iglesias m ordaz x perez campos and l quintanar singh et al 2011 estimate the ground motion in mexico city in case of a recurrence of the m 7 0 acambay crustal earthquake of 1912 an event that ruptured to the surface along several normal faults of a major intra arc lake basin within the trans mexican volcanic belt 100 km northwest of the capital from their assessment they conclude that a repeat of the event would not present a significant hazard to mexico city a conclusion they find confirmed in their assumption that the acambay earthquake was felt there with a modified mercalli intensity mmi of iv in this comment i show that singh et al 2011 severely underestimate the macroseismic intensity of this event in the historical part of mexico city which was more likely an mmi of vii viii consequently their ground motion estimates are unrealistically low based on the observed macroseismic intensity of the 1912 earthquake the linear seismic sources of the acambay graben fault network up to 80 km long pose a significant hazard to the mexico city region especially to its northwestern suburbs located on lake deposits such as tlalnepantla and the cuautitlan area the assumption by singh et al 2011 that the 1912 acambay earthquake was felt in mexico city with an mmi of iv is based on the work by urbina and camacho 1913 who indicate felt intensities in the range of iii iv on the twelve degree 1904 cancani scale musson et al 2010 for five locations in the southern part of the mexico city basin however urbina and camacho 1913 do not provide macroseismic intensity observations for the colonial center of mexico city singh et al 2011 wrongly assume that the 1912 acambay earthquake did not cause damage in mexico city one day after the mainshock el imparcial the leading mexican national newspaper of its time provided a detailed description

on the formation of continental silicic melts in thermochemical mantle convection models implications for early earth abstract important constituents of archean cratons formed in the early and hot history of the earth are tonalite trondhjemite granodiorite ttg plutons and greenstone belts the formation of these granite greenstone terrains is often ascribed to plate tectonic processes buoyancy considerations however do not allow plate tectonics to take place in a significantly hotter earth we therefore propose an alternative mechanism for the coeval and proximate production of ttg plutons and greenstone like crustal successions that is when a locally anomalously thick basaltic crust has been produced by continued addition of extrusive or intrusive basalts due to partial melting of the underlying convecting mantle the transition of a sufficient amount of basalt in the lower crust to eclogite may trigger a resurfacing event in which a complete crustal section of over 1000 km long sinks into the mantle in less than 2 million years pressure release partial melting in the complementary upwelling mantle produces large volumes of basaltic material replacing the original crust partial melting at the base of this newly produced crust may generate felsic melts which are added as intrusives and or extrusives to the generally mafic crustal succession adding to what resembles a greenstone belt partial melting of metabasalt in the sinking crustal section produces a significant volume of ttg melt which is added to the crust directly above the location of subduction presumably in the form of a pluton this scenario is self consistently produced by numerical thermochemical mantle convection models presented in this paper including partial melting of mantle peridotite and crustal meta basalt the metamorphic p t conditions under which partial melting of metabasalt takes place in this scenario are consistent with geochemical trace element data for ttgs which indicate melting under amphibolite rather than eclogite facies other geodynamical settings which we have also investigated including partial melting in small scale delaminations of the lower crust at the base of a anomalously thick crust and due to the influx of a lower mantle diapir fail to reproduce this behavior unequivocally and mostly show melting of metabasalt in the eclogite stability field instead

a review of the origins of subparallel ridges and troughs generalized morphological predictions from terrestrial models the morphologies of ridges and troughs on earth and the processes involved in their formation are reviewed for application to sets of subparallel ridges and troughs on outer planet satellites we consider tension fracturing normal faulting thrust faulting buckle folding strike slip tectonism fissure eruption wall diapirism surface folding of a flow and near surface laccolithic intrusion the landform characteristics and geologic associations predicted of each process are generalized to be independent of planet specific parameters we find that the combinations of landforms derived from each process are unique making each distinguishable in the idealized case morphological parameters most diagnostic of formational process are ridge and trough cross sectional shape termination style and incipient character the characteristics described are useful in constraining the origin of ridge and trough terrains observed in voyager images and will aid in analysis of high resolution images from the galileo and cassini spacecraft

new data on the composition structure and properties of venus rock obtained by venera 13 and venera 14 a study of the composition structure and properties of venusian rocks was carried out by venera 13 and venera 14 the main structural morphological types of provinces of the venus surface were chosen for the study the chemical and mineralogical composition of the rock at the venera 13 landing site shows that it belongs to a group of weakly differentiated melanocratic alkaline gabbroids at the venera 14 landing site the rock is similar to oceanic tholeiitic basalts a most interesting property of the surface rocks is the great number of the strata that were visible in panoramas of the landing sites measurements of their physical and mechanical properties show that venusian rocks have friable weakly cemented porous structures analysis of information from venera 8 9 10 13 and 14 led to the conclusion that the most likely global process responsible for the formation of the present rock is volcanic eruption moreover due to the high gas content of basaltic melts eruptions are of an explosive character leading to the formation of pyroclastic rocks summing up data obtained by venera enabled us to compare venus with the earth mars and the moon to point out some regular global features in the evolution of these bodies and to suggest some new problems for investigation

explosive volcanism on venus transient volcanic explosions as a mechanism for localized pyroclast dispersal it is proposed that transient volcanic explosions of the vulcanian type may provide a mechanism for the generation and dispersal of pyroclastic material on venus the influence of the venusian high atmospheric pressure environment implies that continuous discharge plinian eruptive activity is relatively uncommon the tendency for suppression of exsolution and expansion of magmatic gases favors effusive eruptions however it may be possible for explosive activity to occur in a fashion analogous to vulcanian eruptions on earth as a result of the accumulation of hot pressurized gas under a coherent rock lid the explosion may be initiated by the failure of this retaining caprock causing the catastrophic release of the high pressure gas which expands out of the vent driving the fragmented caprock material ahead of it and displacing the surrounding atmosphere on earth the driving gas may originate either from vaporization of groundwater or from degassing of a stalled magma body in the near surface crust whereas on venus where the presence of crustal stores of volatile compounds is uncertain the latter option only is favored prolonged degassing may lead to an accumulation of gas sufficient to initiate an explosion this paper presents the results of a numerical model describing the explosion process under boundary conditions representing the venusian physical environment this involves treatments of the acceleration of the driving gas caprock and displaced atmospheric gas out of the vent and the subsequent motions and aerodynamic interactions between the atmosphere and the ejected blocks of fragmented caprock in this way predictions of the eruption velocities and of the resulting distribution of large solid ejecta can be obtained for likely conditions on venus deposits of large blocky debris are predicted to range up to a maximum distance of the order of 1 km from the vent on venus compared with distances of several kilometers commonly attained by ejecta from transient explosions on earth more typical blocky deposits may extend for only a few hundred meters which implies that they would not be detected in the magellan radar data however the possible presence of associated pyroclastic flow and fine grained ashfall deposits may constitute aids to the identification of sites of vulcanian eruptions on venus

on lava dome growth with application to the 1979 lava extrusion of the soufriere of st vincent abstract a theoretical analysis is presented for the spread of a viscous liquid flowing under its own hydrostatic pressure on a horizontal surface in order to model lava dome formation two situations are considered in detail the spreading of a constant volume of liquid and the case where the amount of liquid is continually increased experiments with silicone liquids show close agreement with theory the formation of a basaltic andesite lava extrusion in 1979 on the crater floor of the soufriere of st vincent west indies provided the motivation for and an application of the model the extrusion reached a diameter of 868 m and a height of 133 m over a period of 150 days over the first 90 days the growth relationships were consistent with those predicted by theory application of the theory to the soufriere dome suggests an effective viscosity of 2 x 10 12 poise for the basaltic andesite lava the large effective viscosity calculated for the lava may be attributed to the dominant influence of a high viscosity skin which forms at the margins of the flow as it cools after 70 days the rate of growth of the extrusion markedly decreased because a substantial collar of rubble accumulated at the flow front due to this collar the growth of the extrusion ceased after 150 days from approximately two weeks after the initiation of the extrusion the discharge rate of lava decreased approximately linearly with increasing dome height this observation suggests that the lava ascended under a decreasing hydrostatic driving pressure and that extrusion ceased when the lava column reached hydrostatic equilibrium

the spreading of variable viscosity axisymmetric radial gravity currents applications to the emplacement of venusian pancake domes the magellan images of venus have revealed a number of intriguing volcanic features including the steep sided or pancake domes these volcanic domes or flows have morphologies that suggest formation by a single continuous emplacement of lava with a higher viscosity than that of the surrounding basaltic plains numerous investigators have suggested that such high viscosity is due to high silica content leading to the conclusion that the domes are evidence of evolved magmatic products on venus however viscosity depends on crystallinity as well as on silica content high viscosity could therefore also be due to a cooler and therefore higher crystal content lava models of dome emplacement which include both cooling and composition factors are thus necessary in order to determine the ranges of crystallinity and silica content which might lead to the observed gross dome morphologies accordingly in this study domes are modelled as radial viscous gravity currents with an assumed cooling induced viscosity increase to include both effects analytical and numerical results indicate that pancake dome formation is feasible with compositions ranging from basaltic to rhyolitic therefore observations of gross dome morphology alone are insufficient for determining composition and the domes do not necessarily represent strong evidence for evolved magmatism on venus

considerations for effusive cryovolcanism on europa the post galileo perspective 1 cryovolcanic resurfacing is a popular mechanism to explain relatively young surface units on icy satellites of jupiter saturn uranus and neptune prior to the galileo data acquired between 1996 and 2001 europa was thought to have undergone significant cryovolcanic resurfacing facilitated by a global ocean beneath the icy surface however close examination of galileo data at resolutions much better than those of voyager images show that many of the features previously thought to be cryovolcanic are commonly best explained by other formative mechanisms including tectonism and diapirism in this study i present an examination of the characteristics of a variety of europan surface features for which effusive cryovolcanism is a possible origin including apparently lobate flows certain elliptical to circular lenticulae and low lying smooth low albedo surfaces a review of cryovolcanic eruption theory together with galileo data analysis of europan surface geology and composition indicates that cryovolcanism is a viable though not unequivocal explanation for some of these features some constraints on cryomagma properties and lithospheric structure are offered for these cases the presence of small volume low viscosity effusions is supported by observations and modeling some positive relief lenticulae could represent more viscous effusions although diapirism may be a preferable explanation however strong evidence is lacking for cryovolcanic resurfacing on a large scale on the basis of our experience with galileo images of europa and ganymede voyager era inferences for widespread cryovolcanism on icy satellites may be overstated and will need to be carefully reexamined in the light of new data from upcoming spacecraft missions

cryovolcanic features on titan s surface as revealed by the cassini titan radar mapper abstract the cassini titan radar mapper obtained synthetic aperture radar images of titan s surface during four fly bys during the mission s first year these images show that titan s surface is very complex geologically showing evidence of major planetary geologic processes including cryovolcanism this paper discusses the variety of cryovolcanic features identified from sar images their possible origin and their geologic context the features which we identify as cryovolcanic in origin include a large 180 km diameter volcanic construct dome or shield several extensive flows and three calderas which appear to be the source of flows the composition of the cryomagma on titan is still unknown but constraints on rheological properties can be estimated using flow thickness rheological properties of one flow were estimated and appear inconsistent with ammonia water slurries and possibly more consistent with ammonia water methanol slurries the extent of cryovolcanism on titan is still not known as only a small fraction of the surface has been imaged at sufficient resolution energetic considerations suggest that cryovolcanism may have been a dominant process in the resurfacing of titan

putative ice flows on europa geometric patterns and relation to topography collectively constrain material properties and effusion rates europa s surface exhibits numerous small dome like and lobate features some of which have been attributed to fluid emplacement of ice or slush on the surface we perform numerical simulations of non newtonian flows to assess the physical conditions required for these features to result from viscous flows our simulations indicate that the morphology of an ice flow on europa will be at least partially influenced by pre existing topography unless the thickness of the flow exceeds that of the underlying topography by at least an order of magnitude three classes of features can be identified on europa first some possibly most putative flow like features exhibit no influence from the pre existing topography such as ridges although their thicknesses are generally on the same order as those of ridges therefore flow processes probably cannot explain the formation of these features second some observed features show modest influence from the underlying topography these might be explained by ice flows with wide ranges of parameters ice temperatures 230 k effusion rates 10 7 m 3 year 1 and a wide range of grain sizes although surface uplift e g by diapirism and in situ disaggregation provide an equally compelling explanation third several observed features are completely confined by pre existing topographic structures on at least one side these are the best known candidates for flow features on europa if these features resulted from solid ice flows then temperatures 260 k and grain sizes 2 m are required such small grain sizes seem unlikely low viscosity flows such as ice slurries or brines provide a better explanation for these features our results provide theoretical support for the view that many of europa s lobate features have not resulted from solid ice flows

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