magmatic effects of the lunar late heavy bombardment abstract large volumes of mare basalts are present on the surface of the moon located preferentially in large impact basins mechanisms relating impact basins and mare basalt eruptions have previously been suggested lunar impacts removed low density material that may have inhibited eruption and created cracks for fluid flow icarus 139 1999 246 and lunar basins have long been described as catchments for magma e g rev geophys space phys 18 1980 107 and references therein we present a new model for melt creation under near side lunar basins that is triggered by the impacts themselves magma can be produced in two stages first crater excavation depressurizes underlying material such that it may melt in situ second the cratered lithosphere rises isostatically warping isotherms at the lithosphere asthenosphere boundary which may initiate convection in which adiabatic melting can occur the first stage produces by far the largest volume of melt but convective melting can continue for up to 350 ma we propose that giant impacts account for a large portion of the volume and longevity of mare basalt volcanism as well as for several compositional groups including high alumina high titanium kreep rich and picritic magmas

explosive volcanic eruptions on mercury eruption conditions magma volatile content and implications for interior volatile abundances images obtained by the messenger spacecraft have revealed evidence for pyroclastic volcanism on mercury because of the importance of this inference for understanding the interior volatile inventory of mercury we focus on one of the best examples determined to date a shield volcano like feature just inside the southwestern rim of the caloris impact basin characterized by a near central irregularly shaped depression surrounded by a bright deposit interpreted to have a pyroclastic origin this candidate pyroclastic deposit has a mean radius of 24 km greater in size than the third largest lunar pyroclastic deposit when scaled to lunar gravity conditions from the extent of the candidate pyroclastic deposit we characterize the eruption parameters of the event that emplaced it including vent speed and candidate volatile content the minimum vent speed is 300 m s and the volatile content required to emplace the pyroclasts to this distance is hundreds to several thousands of parts per million ppm of the volatiles typically associated with pyroclastic eruptions on other bodies e g co co2 h2o so2 h2s for comparison measurements of the exsolution of volatiles h2o co2 s from basaltic eruptive episodes at kilauea volcano hawaii indicate values of 1300 6500 ppm for the terrestrial mantle source evidence for the presence of significant amounts of volatiles in partial melts derived from the interior of mercury is an unexpected result and provides a new constraint on models for the planet s formation and early evolution

pit floor craters on mercury evidence of near surface igneous activity abstract high resolution images obtained during the first mercury surface space environment geochemistry and ranging messenger flyby of mercury show evidence for igneous modification of the surface through the formation of pit craters on the floors of several impact craters pit craters are rimless steep sided depressions that are inferred to have formed by non impact processes among the arguments in favor of this interpretation are that the features lack evident rims exhibit no observable ejecta are irregularly shaped e g subcircular elliptical or arcuate with the long axis of the pit crater often concentric to the rim of the host impact crater and are superposed on smooth plains deposits within the craters pit craters documented to date do not exhibit signs of extrusive flows impact craters hosting pit craters termed pit floor craters vary in size from 55 to 120 km in diameter while the associated pit craters range in maximum horizontal dimension from 20 to almost 40 km pit crater radius and host crater radius are not well correlated for pit floor craters on mercury in contrast to the situation for otherwise similar features on mars on the basis of morphology structural association relative age and proximity to smooth plains for pit craters on mercury we test mechanisms of pit crater formation previously proposed for such features on other planetary bodies e g earth moon mars and venus we conclude that pit craters on mercury formed through collapse into an underlying drained magma chamber pit floor craters thus provide evidence for near surface magmatic activity on mercury and extend the range of evidence for magmatism beyond such surface expressions as smooth plains and pyroclastic deposits

basaltic magma reservoirs factors controlling their rupture characteristics and evolution the manner in which magma enters and leaves a magma reservoir is a fundamental aspect of magmatic activity a quantitative model is developed to evaluate the variation of stress acting on the wall of a magma reservoir as a function of depth factors assessed include the size of the reservoir its depth of burial the strength of the country rocks and the variation with depth of the densities of the magma and country rocks it is shown that small magma reservoirs with halfheights less than 1 km centered at levels of neutral buoyancy can grow in all directions with nearly equal ease by injecting dikes into their surroundings at the sites of wall ruptures in larger reservoirs with half heights greater than 2 km however the stress required to cause wall failure is much less at the depth corresponding to the center of the reservoir than it is near the top or bottom thus larger reservoirs are likely to grow predominantly sideways by lateral dike injection in these cases vertical growth will not be important unless significant accumulation of low density material can occur in the upper part of the reservoir the formation of a low density layer due to gas exsolution or chemical differentiation of the magma can facilitate failure of the roof and upward dike migration from a mature reservoir as the reservoir grows and evolves through time the increasing magma pressure head at the depth corresponding to the center of the reservoir means that the amount of gas exsolution or chemical differentiation required to allow the occurrence of vertical dike emplacement also increases ultimately a vertical reservoir size is reached beyond which the excess stress required to cause lateral dike injection is so much less than that required to cause vertical injection that essentially no further vertical growth of the reservoir will occur the reservoir will thus develop a more laterally elongate shape with time the final aspect ratio will then depend mainly on the supply rate of magma to the reservoir from the mantle

a vaporization model for iron silicate fractionation in the mercury protoplanet abstract a study has been carried out of the vaporization of a totally molten silicate magma of chondritic composition heated into the range 2500 3500 k the motivation for this was to determine the changes in the composition of the mantle that would occur in the mercury protoplanet should that body have been subjected to the high temperature phase in the evolution of the primitive solar nebula but the results are of more general interest we have used both ideal and nonideal descriptions of the magma chemistry an empirical model based on ideal mixing of complex components developed by hastie and co workers was used to describe the nonideal magma comparison of our results of the nonideal magma calculations with experimental work on vaporization of chondritic material shows generally good agreement we find that vaporization of about 70 80 of the original amount of silicate from a chondritic planet is required to produce an iron rich body with a mean uncompressed density equal to that deduced for mercury 5 3 g cm3 at this point the silicate is depleted in the alkalis feo and sio2 and enriched in cao mgo al2o3 and tio2 relative to chondritic material we also predict the production of unique trace element abundance patterns which are depleted in easily oxidized elements relative to other trace elements of similar volatilities

mercury s global contraction much greater than earlier estimates observations of compressional structures on mercury have fallen short of accommodating the global contraction that is required owing to cooling of the planet s interior mapping of folds and faults across mercury s surface using messenger spacecraft images reveals deformation consistent with a planet that has contracted radially as much as seven kilometres over its history

hollows on mercury messenger evidence for geologically recent volatile related activity high resolution images of mercury s surface from orbit reveal that many bright deposits within impact craters exhibit fresh appearing irregular shallow rimless depressions the depressions or hollows range from tens of meters to a few kilometers across and many have high reflectance interiors and halos the host rocks which are associated with crater central peaks peak rings floors and walls are interpreted to have been excavated from depth by the crater forming process the most likely formation mechanisms for the hollows involve recent loss of volatiles through some combination of sublimation space weathering outgassing or pyroclastic volcanism these features support the inference that mercury s interior contains higher abundances of volatile materials than predicted by most scenarios for the formation of the solar system s innermost planet

the effect of the caloris impact on the mantle dynamics and volcanism of mercury 1 the messenger spacecraft has imaged more than 95 of the surface of mercury images of the caloris impact basins along with several smaller impact structures reveal evidence for volcanic plains within and exterior to the basin that are younger than the basin rim and might be associated with the long term aftermath of caloris s formation the broad influence of caloris on the surface of mercury indicates that it might also affect heat flow within the mantle and thereby the core dynamics here we investigate possible links between the caloris impact on mercury thermochemical evolution of the mantle and the volcanism within and surrounding the basins a finite element model of thermochemical convection in a spherical shell is used to explore the consequences of the formation of large impacts in the mercurian mantle while the impact cannot have formed the younger melts directly the thermal impulse from such a large impact can alter the underlying mantle dynamics producing subsequent volcanism far from the impact site we further find that despite the relatively thin mercurian mantle the expected heating from the caloris impact cannot penetrate to the core and should thus have minimal effects on a dynamo this is largely due to the high expected velocity of impactors at mercury and thus relatively small size of the caloris forming impactor

the apparent absence of kilometer sized pyroclastic volcanoes on mercury are we looking right spacecraft data reveal that volcanism was active on mercury evidence of large volume effusive and smaller scale explosive eruptions has been detected however only large 15 km volcanic features or vents have been found so far despite abundant high resolution imagery on other volcanic planets the size of volcanoes is anticorrelated with their frequency small volcanoes are much more numerous than large ones here we present results of a numerical model that predicts the shapes of ballistically emplaced volcanic edifices and hence can explain the lack of kilometer sized constructional explosive volcanoes on the surface of mercury we find that due to the absence of the atmosphere particles are spread on this planet over a larger area than is typical for earth or mars erupted volumes are likely insufficient to build edifices with slope angles that enable their easy recognition with currently available data or that could survive destruction by subsequent impact bombardment

evolution of the interior of mercury influenced by coupled magmatism mantle convection system and heat flux from the core to discuss mantle evolution in mercury i present two dimensional numerical models of magmatism in a convecting mantle thermal compositional and magmatic buoyancy drives convection of temperature dependent viscosity fluid in a rectangular box placed on the top of the core that is modeled as a heat bath of uniform temperature magmatism occurs as a permeable flow of basaltic magma generated by decompression melting through a matrix widespread magmatism caused by high initial temperature of the mantle and the core makes the mantle compositionally stratified within the first several hundred million years of the 4 5 gyr calculated history the stratified structure persists for 4 5 gyr when the reference mantle viscosity at 1573 k is higher than around 1020 pa s the planet thermally contracts by an amount comparable to the one suggested for mercury over the past 4 gyr mantle upwelling however generates magma only for the first 0 1 0 3 gyr at lower mantle viscosity in contrast a positive feedback between magmatism and mantle upwelling operates to cause episodic magmatism that continues for the first 0 3 0 8 gyr convective current stirs the mantle and eventually dissolves its stratified structure to enhance heat flow from the core and temporarily resurrect magmatism depending on the core size these models however predict larger contraction of the planet coupling between magmatism and mantle convection plays key roles in mantle evolution and the difficulty in numerically reproducing the history of magmatism of mercury without causing too large radial contraction of the planet warrants further exploration of this coupling

characterization of previously unidentified lunar pyroclastic deposits using lunar reconnaissance orbiter camera data 1 we used a lunar reconnaissance orbiter camera lroc global monochrome wide angle camera wac mosaic to conduct a survey of the moon to search for previously unidentified pyroclastic deposits promising locations were examined in detail using lroc multispectral wac mosaics high resolution lroc narrow angle camera nac images and clementine multispectral ultraviolet visible or uvvis data out of 47 potential deposits chosen for closer examination 12 were selected as probable newly identified pyroclastic deposits potential pyroclastic deposits were generally found in settings similar to previously identified deposits including areas within or near mare deposits adjacent to highlands within floor fractured craters and along fissures in mare deposits however a significant new finding is the discovery of localized pyroclastic deposits within floor fractured craters anderson e and f on the lunar farside isolated from other known similar deposits our search confirms that most major regional and localized low albedo pyroclastic deposits have been identified on the moon down to 100 m pix resolution and that additional newly identified deposits are likely to be either isolated small deposits or additional portions of discontinuous patchy deposits

prolonged eruptive history of a compound volcano on mercury volcanic and tectonic implications a 27 13 km rimless depression 100 km inside the southwest rim of the caloris basin is revealed by high resolution orbital imaging under a variety of illuminations to consist of at least nine overlapping volcanic vents each individually up to 8 km in diameter it is thus a compound volcano indicative of localised migration of the site of the active vent the vent floors are at a least 1 km below their brinks but lack the flat shape characteristically produced by piston like subsidence of a caldera floor or by flooding of a crater bottom by a lava lake they bear a closer resemblance to volcanic craters sculpted by explosive eruptions and or modified by collapse into void spaces created by magma withdrawal back down into a conduit this complex of overlapping vents is at the summit of a subtle edifice at least 100 km across with flank slopes of about only 0 2 degrees after correction for the regional slope this is consistent with previous interpretation as a locus of pyroclastic eruptions construction of the edifice could have been contributed to by effusion of very low viscosity lava but high resolution images show that the vent facing rim of a nearby impact crater is not heavily embayed as previously supposed on the basis of lower resolution flyby imaging contrasts in morphology sharpness versus blurredness of the texture and different densities of superposed sub km impact craters inside each vent are consistent with but do not prove substantial differences in the age of the most recent activity at each vent this suggests a long duration of episodic magmagenesis at a restricted locus the age range cannot be quantified but could be of the order of a billion years if each vent was fed from the same point source geometric considerations suggest a source depth of at least 50 km however the migration of the active vent may be partly controlled by a deep seated fault that is radial to the caloris basin other rimless depressions in this part of the caloris basin fall on or close to radial lines suggesting that elements of the pantheon fossae radial fracture system that dominates the surface of the central portion of the caloris basin may continue at depth almost as far as the basin rim 2013 elsevier b v

calderas and caldera structures a review abstract calderas are important features in all volcanic environments and are commonly the sites of geothermal activity and mineralisation yet it is only in the last 25 years that a thorough three dimensional study of calderas has been carried out utilising studies of eroded calderas geophysical analysis of their structures and analogue modelling of caldera formation as more data has become available on calderas their individuality has become apparent a distinction between caldera caldera complex cauldron and ring structure is necessary and new definitions are given in this paper descriptions of calderas based on dominant composition of eruptives basaltic peralkaline andesitic dacitic rhyolitic can be used and characteristics of each broad group are given styles of eruption may be effusive or explosive with the former dominant in basaltic calderas and the latter dominant in andesitic dacitic rhyolitic and peralkaline calderas four end member collapse styles occur plate or piston piecemeal trapdoor and downsag but many calderas have multiple styles features of so called funnel and chaotic calderas proposed in the literature can be explained by other collapse styles and the terms are considered unnecessary ground deformation comprises subsidence or collapse essential characteristics of a caldera and uplifting doming and fracturing due to tumescence and or resurgence frequent but not essential collapse may occur on pre existing structures such as regional faults or on faults created during the caldera formation and the shape of the collapse area will be influenced by depth size and shape of the magma chamber the final morphology of a caldera will depend on how the caldera floor breaks up whether collapse takes place in one event or multiple events whether vertical movement is spasmodic or continuous throughout the eruptive sequence and whether blocks subside uniformly or chaotically at one or more collapse centres a meaningful description of any caldera should therefore include number of collapse events presence or absence of resurgence caldera floor coherency caldera floor collapse geometry and dominant composition of eruptives

volcanism on mercury evidence from the first messenger flyby the origin of plains on mercury whether by volcanic flooding or impact ejecta ponding has been controversial since the mariner 10 flybys 1974 75 high resolution images down to 150 meters per pixel obtained during the first messenger flyby show evidence for volcanic vents around the caloris basin inner margin and demonstrate that plains were emplaced sequentially inside and adjacent to numerous large impact craters to thicknesses in excess of several kilometers radial graben and a floor fractured crater may indicate intrusive activity these observations coupled with additional evidence from color images and impact crater size frequency distributions support a volcanic origin for several regions of plains and substantiate the important role of volcanism in the geological history of mercury

mechanisms of explosive volcanism on mercury implications from its global distribution and morphology the identification of widespread pyroclastic vents and deposits on mercury has important implications for the planet s bulk volatile content and thermal evolution however the significance of pyroclastic volcanism for mercury depends on the mechanisms by which the eruptions occurred using images acquired by the mercury surface space environment geochemistry and ranging spacecraft we have identified 150 sites where endogenic pits are surrounded by a relatively bright and red diffuse edged spectral anomaly a configuration previously used to identify sites of explosive volcanism we find that these sites cluster at the margins of impact basins and along regional tectonic structural trends locally pits and deposits are usually associated with zones of weakness within impact craters and or with the surface expressions of individual thrust faults additionally we use images and stereo derived topographic data to show that pyroclastic deposits are dispersed up to 130 km from their source vent and commonly have either no relief or low circumpit relief within a wider thinner deposit these eruptions were therefore likely driven by a relatively high concentration of volatiles consistent with volatile concentration in a shallow magma chamber prior to eruption the colocation of sites of explosive volcanism with near surface faults and crater related fractures is likely a result of such structures acting as conduits for volatile and or magma release from shallow reservoirs with volatile overpressure in these reservoirs a key trigger for eruption in at least some cases our findings suggest that widespread long lived explosive volcanism on mercury has been facilitated by the interplay between impact cratering tectonic structures and magmatic fractionation

large impact basins on mercury global distribution characteristics and modification history from messenger orbital data 1 the formation of large impact basins diameter d 300 km was an important process in the early geological evolution of mercury and influenced the planet s topography stratigraphy and crustal structure we catalog and characterize this basin population on mercury from global observations by the messenger spacecraft and we use the new data to evaluate basins suggested on the basis of the mariner 10 flybys forty six certain or probable impact basins are recognized a few additional basins that may have been degraded to the point of ambiguity are plausible on the basis of new data but are classified as uncertain the spatial density of large basins d 500 km on mercury is lower than that on the moon morphological characteristics of basins on mercury suggest that on average they are more degraded than lunar basins these observations are consistent with more efficient modification degradation and obliteration of the largest basins on mercury than on the moon this distinction may be a result of differences in the basin formation process producing fewer rings relaxation of topography after basin formation subduing relief or rates of volcanism burying basin rings and interiors during the period of heavy bombardment on mercury from those on the moon

the distribution and origin of smooth plains on mercury 1 orbital images from the messenger spacecraft show that 27 of mercury s surface is covered by smooth plains the majority 65 of which are interpreted to be volcanic in origin most smooth plains share the spectral characteristics of mercury s northern smooth plains suggesting they also share their magnesian alkali basalt like composition a smaller fraction of smooth plains interpreted to be volcanic in nature have a lower reflectance and shallower spectral slope suggesting more ultramafic compositions an inference that implies high temperatures and high degrees of partial melting in magma source regions persisted through most of the duration of smooth plains formation the knobby and hummocky plains surrounding the caloris basin known as odin type plains occupy an additional 2 of mercury s surface the morphology of these plains and their color and stratigraphic relationships suggest that they formed as caloris ejecta although such an origin is in conflict with a straightforward interpretation of crater size frequency distributions if some fraction is volcanic this added area would substantially increase the abundance of relatively young effusive deposits inferred to have more mafic compositions smooth plains are widespread on mercury but they are more heavily concentrated in the north and in the hemisphere surrounding caloris no simple relationship between plains distribution and crustal thickness or radioactive element distribution is observed a likely volcanic origin for some older terrain on mercury suggests that the uneven distribution of smooth plains may indicate differences in the emplacement age of large scale volcanic deposits rather than differences in crustal formational process

spectroscopic properties of explosive volcanism within the caloris basin with messenger observations volcanism on mercury has been indisputably identified at various locations on the surface by means of both effusive and explosive volcanism its characterization is crucial to understand the evolution of the planet in particular the thermal evolution of the mantle and the volatile content of the planet this analysis presents a detailed view of the pyroclastic deposits of the caloris basin observations from the mercury atmospheric and surface composition spectrometer mascs are used to understand the spectral characteristics of the pyroclastic deposits both in the visible and near infrared additional calibration steps are proposed to reconcile the difference of absolute reflectance between the visible vis and near infrared nir detectors these calibration steps allow the use of the full spectral range of the mascs instrument pyroclastic deposits exhibit a redder spectral slope in the vis and nir this spectral slope diminishes toward the edge of the deposits to match that of mercury s average surface spectral properties in the ultraviolet uv also change as a function of distance to the vent only the uv properties unambiguously separate the pyroclastic deposits from mercury s average spectra the spectral variations are consistent with a lower iron content of the pyroclastic deposits with respect to the average surface of mercury similar to what has been proposed for pyrolcastic deposits on the lunar surface nonetheless given the limited illumination conditions diversity of the mascs instrument other causes such as grain size space weathering and bulk composition could also be accounted for the spectral variations variability of the pyroclastic deposits properties within the entire basin are potentially identified between the three main clusters and could be related to space weathering of deposits of different ages

long lived explosive volcanism on mercury the duration and timing of volcanic activity on mercury are key indicators of the thermal evolution of the planet and provide a valuable comparative example for other terrestrial bodies the majority of effusive volcanism on mercury appears to have occurred early in the planet s geological history 4 1 3 55 ga but there is also evidence for explosive volcanism here we present evidence that explosive volcanism occurred from at least 3 9 ga until less than a billion years ago and so was substantially more long lived than large scale lava plains formation this indicates that thermal conditions within mercury have allowed partial melting of silicates through the majority of its geological history and that the overall duration of volcanism on mercury is similar to that of the moon despite the different physical structure geological history and composition of the two bodies

recharge mode and mineralization of groundwater in a semi arid region sidi bouzid plain central tunisia groundwater is the most important source of water supply in sidi bouzid plain located in central tunisia proper understanding of the geochemical evolution of groundwater is important for sustainable development of water resources in this region a hydrogeochemical survey was conducted on the mio plio quaternary aquifer system using stable isotopes radiocarbon tritium and major elements in order to evaluate the groundwater chemistry patterns and the main mineralization processes occurring in this system the chemical data indicate that dissolution of evaporate minerals and evaporation are the main processes controlling groundwater mineralization the isotopic data show that groundwater in the study area is a mixture of recent shallow waters located upstream and along wadi al fakka bed and paleowaters located towards plain limits and discharge areas low 3h and 14c contents are observed in major part of the plain indicating that recharge of the aquifer occurs mainly through direct infiltration at wadi al fakka while there is no evidence of significant recharge in major part of the plain and mountains piedmonts

mercury s hollows constraints on formation and composition from analysis of geological setting and spectral reflectance 1 landforms unique to mercury hollows are shallow flat floored irregular depressions notable for their relatively high reflectance and characteristic color here we document the range of geological settings in which hollows occur most are associated with impact structures simple bowl shaped craters to multiring basins and ranging from kuiperian to calorian in age hollows are found in the low reflectance material global color unit and in low reflectance blue plains but they appear to be absent from high reflectance red plains hollows may occur preferentially on equator or hot pole facing slopes implying that their formation is linked to solar heating evidence suggests that hollows form because of loss of volatile material we describe hypotheses for the origin of the volatiles and for how such loss proceeds intense space weathering and solar heating are likely contributors to the loss of volatiles contact heating by melts could promote the formation of hollows in some locations lunar ina type depressions differ from hollows on mercury in a number of characteristics so it is unclear if they represent a good analog we also use messenger multispectral images to characterize a variety of surfaces on mercury including hollows within a framework defined by laboratory spectra for analog minerals and lunar samples data from messenger s x ray spectrometer indicate that the planet s surface contains up to 4 sulfur we conclude that nanophase or microphase sulfide minerals could contribute to the low reflectance of the low reflectance material relative to average surface material hollows may owe their relatively high reflectance to destruction of the darkening agent sulfides the presence of alteration minerals and or physical differences in particle size texture or scattering behavior

the evolution of mercury s crust a global perspective from messenger mapping the distribution and extent of major terrain types on a planet s surface helps to constrain the origin and evolution of its crust together messenger and mariner 10 observations of mercury now provide a near global look at the planet revealing lateral and vertical heterogeneities in the color and thus composition of mercury s crust smooth plains cover approximately 40 of the surface and evidence for the volcanic origin of large expanses of plains suggests that a substantial portion of the crust originated volcanically a low reflectance relatively blue component affects at least 15 of the surface and is concentrated in crater and basin ejecta its spectral characteristics and likely origin at depth are consistent with its apparent excavation from a lower crust or upper mantle enriched in iron and titanium bearing oxides

hollows on mercury materials and mechanisms involved in their formation recent images of the surface of mercury have revealed an unusual and intriguing landform sub kilometre scale shallow flat floored steep sided rimless depressions typically surrounded by bright deposits and generally occurring in impact craters these hollows appear to form by the loss of a moderately volatile substance from the planet s surface and their fresh morphology and lack of superposed craters suggest that this process has continued until relatively recently and may be on going hypotheses to explain the volatile loss have included sublimation and space weathering and it has been suggested that hollow forming volatiles are endogenic and are exposed at the surface during impact cratering however detailed verification of these hypotheses has hitherto been lacking in this study we have conducted a comprehensive survey of all messenger images obtained up to the end of its fourth solar day in orbit in order to identify hollowed areas we have studied how their location relates to both exogenic processes insolation impact cratering and solar wind and endogenic processes explosive volcanism and flood lavas on local and regional scales we find that there is a weak correlation between hollow formation and insolation intensity suggesting formation may occur by an insolation related process such as sublimation the vast majority of hollow formation is in localised or regional low reflectance material within impact craters suggesting that this low reflectance material is a volatile bearing unit present below the surface that becomes exposed as a result of impacts in many cases hollow occurrence is consistent with formation in volatile bearing material exhumed and exposed during crater formation while in other cases volatiles may have accessed the surface later through re exposure and possibly in association with explosive volcanism hollows occur at the surface of thick flood lavas only where a lower reflectance substrate has been exhumed from beneath them indicating that this form of flood volcanism on mercury lacks significant concentrations of hollow forming volatiles

using hydrochemical and isotopic data to determine sources of recharge and groundwater evolution in an arid region a case study in the upper middle reaches of the shule river basin northwestern china understanding sources of recharge and mechanisms for hydrogeochemical evolution of groundwater in the shule river basin an arid inland river basin in northwestern china is essential for successful water resource management sources of water and associated groundwater recharge processes were investigated using hydrogeological hydrogeochemical and isotopic methods the study area was divided into three parts changma the shule valley and the beishan area the hydrogeochemical and isotopic analysis results show that groundwater in changma mainly originated from precipitation in the qilian mountains lateral flow from changma is the primary recharge mechanism while direct recharge by infiltration of precipitation can be neglected in the shule valley minerals within the aquifer material e g halite calcite dolomite and gypsum dissolve into water that recharges the groundwater system therefore strong linear relationships were found between na and cl and between ca2 mg2 and so42 hco3 with stoichiometry ratios of approximately 1 1 in both cases groundwater quality in the region is generally good with low salinity and hardness local precipitation is the only source of recharge for groundwater in the beishan area low groundwater velocity and dry climatic conditions suggest that dissolution of minerals from the aquifer material controls hydrogeochemical evolution water rarely travels from the beishan area to the shule valley therefore almost all the groundwater in the shule valley originates from changma global climate change and loss of glaciers in the qilian mountains may lead to a reduction of recharge therefore the groundwater resources of the shule valley must be managed to sustain water supply

the spatial distribution of mercury s pyroclastic activity and the relation to lithospheric weaknesses abstract mercury s surface preserves a rich history of volcanism impact cratering and tectonic deformation geological observations show that the earliest evidence of thrust faulting that was induced by the secular cooling and resulting global contraction of the planet coincided with the waning stages of effusive volcanism but that explosive volcanism continued beyond this point stresses from global contraction however would have precluded efficient vertical magma ascent sites of pyroclastic activity manifest as irregular depressions surrounded by diffuse spectrally distinct halos spatially coincide with lithospheric discontinuities such as faults or those associated with impact craters the vast majority of explosive vents are situated on the floors rims central peaks or peak rings of impact structures a substantial portion of such vents is also proximal to thrust faults they are most spatially concentrated at or within 20 km of faults with ever fewer vents progressively farther from tectonic structures we statistically evaluated the spatial distribution of sites of pyroclastic activity with respect to faults and impact craters by generating sets of random point locations of equal count to those volcanic sites computing their spatial relationship to the mapped faults and craters and comparing them to our observations we find that although the observed proximity of vents to faults is indistinguishable from a random distribution their spatial association with impact craters is non random to examine the interrelatedness of several geospatial relationships of lithospheric weaknesses and pyroclastic activity we performed a principal component analysis that tested correlations between vent size the presence of vents within a crater the diameters and degradation states of those craters and vent distance from mapped faults which help tie together interpretations of magma volumes and eruption energies repeated utilization of magma pathways and durations of eruptive events in the geological context of global contraction results reveal a predominance of small sized vents indicative of short lived low volume pyroclastic activity that are consistent with suppressed volcanism after the onset of global contraction greater size ranges of vents are found in large impact craters and when faults are nearby which points to denser fracture networks facilitating magma ascent

possible mantle origin of olivine around lunar impact basins detected by selene the composition structure and evolution of the moon s mantle is poorly constrained a global survey of the moon s surface using the spectral profiler onboard the lunar explorer selene kaguya identifies a number of exposures of olivine in concentric regions around lunar craters with a possible mantle origin

earthquakes injection wells and the perry nuclear power plant cleveland ohio on january 31 1986 an earthquake of richter magnitude 4 9 occurred in northeastern ohio some 17 0 km south of the perry nuclear power plant pnpp and 12 0 km south of the calhio injection wells accelerometers on site at the pnpp recorded accelerations as high as 0 19 to 0 23 g many instruments tripped due to high amplitude vibrations microearthquake networks have recorded 16 microearthquakes within 5 0 km of the injection wells with focal depths ranging from 1 0 to 3 0 km a hydrological model of an anisotropic reservoir 7 2 km wide and 18 4 km long indicates a pressure buildup of 5 3 mpa at the epicenter and 11 8 mpa at the injection well the assumption of an anisotropic reservoir is consistent with available geophysical and geologic data a pressure increase of 11 8 mpa based on stress ratio estimates in crustal rocks in the region is more than sufficient to induce failure to a depth of 5 0 km furthermore brittle faults and extensive fracture permeability within the basement rocks would allow for the migration of pressure transients to hypocentral distances the indicated pressure buildup of 5 3 mpa at the epicenter may have been sufficient to trigger the january 31 1986 earthquake

an integrated model of kimberlite ascent and eruption diatremes are carrot shaped bodies forming the upper parts of very deep magmatic intrusions of kimberlite rock these unusual enigmatic and complex features are famous as the source of diamonds here we present a new model of kimberlite ascent and eruption emphasizing the extremely unsteady nature of this process to resolve many of the seemingly contradictory characteristics of kimberlites and diatremes dyke initiation in a deep co2 rich source region in the mantle leads to rapid propagation of the dyke tip below which co2 fluid collects with a zone of magmatic foam beneath when the tip breaks the surface of the ground gas release causes a depressurization wave to travel into the magma this wave implodes the dyke walls fragments the magma and creates a ringing fluidization wave together these processes form the diatreme catastrophic magma chilling seals the dyke no precursor to the eruption is felt at the surface and the processes are complete in about an hour

sulfur solubility in reduced mafic silicate melts implications for the speciation and distribution of sulfur on mercury abstract chemical data from the messenger spacecraft revealed that surface rocks on mercury are unusually enriched in sulfur compared to samples from other terrestrial planets in order to understand the speciation and distribution of sulfur on mercury we performed high temperature 1200 1750 c low to high pressure 1 bar to 4 gpa experiments on compositions representative of mercurian lavas and on the silicate composition of an enstatite chondrite we equilibrated silicate melts with sulfide and metallic melts under highly reducing conditions iw 1 5 to iw 9 4 iw iron wustite oxygen fugacity buffer under these oxygen fugacity conditions sulfur dissolves in the silicate melt as s 2 and forms complexes with fe 2 mg 2 and ca 2 the sulfur concentration in silicate melts at sulfide saturation scss increases with increasing reducing conditions from 10 wt s at iw 8 and with increasing temperature metallic melts have a low sulfur content which decreases from 3 wt at iw 2 to 0 wt at iw 9 we developed an empirical parameterization to predict scss in mercurian magmas as a function of oxygen fugacity f o 2 temperature pressure and silicate melt composition scss being not strictly a redox reaction our expression is fully valid for magmatic systems containing a metal phase using physical constraints of the mercurian mantle and magmas as well as our experimental results we suggest that basalts on mercury were free of sulfide globules when they erupted the high sulfur contents revealed by messenger result from the high sulfur solubility in silicate melt at reducing conditions we make the realistic assumption that the oxygen fugacity of mantle rocks was set during equilibration of the magma ocean with the core and or that the mantle contains a minor metal phase and combine our parameterization of scss with chemical data from messenger to constrain the oxygen fugacity of mercury s interior to iw 5 4 0 4 we also calculate that the mantle of mercury contains 7 11 wt s and that the metallic core of the planet has little sulfur

time dependent friction of granite implications for precursory slip on faults frictional forces were measured during sliding between saw cut cylinders of barre granite deformed in a servo controlled triaxial loading machine two different effects occur depending on the type of loading when steady shortening of a stably sliding sample is halted suddenly slip continues at a diminishing rate which depends on the logarithm of the initial sliding velocity thus the frictional strength of the surface is decreasing with time when shortening at a constant rate resumes after the holding period however the frictional resistance temporarily rises to a peak proportional to the logarithm of time of no driving showing a well known increase in friction with time the subsequent decrease in resistance from this peak is continuous the observed frictional resistance of a surface thus results from the interaction of several processes and is not a constant in general but depends on the immediate and preceding deformation when shortening is resumed after a period of holding slip rate across the friction surface temporarily accelerates above the driving rate the slip rate can then either decrease to the driving rate and remain stable or accelerate further and lead to stick slip motion at stress levels below the peak value there is no apparent difference in the frictional resistance until the unloading rate of the loading machine is exceeded between slip episodes which settle stably to the constant driving rate and those which become unstable and result in stick slip the stick slip failure criterion is not a peak stress but rather loss of strength more rapidly that the unloading rate of the loading machine the evidence that accelerating motion before stick slip is due to the time variable strength of rock suggests that accelerating slip will occur before earthquakes unfortunately for easy observation experiments indicate that slip need not be uniform over a fault surface also to be established is whether slip will be sufficiently large in the earth to be detectable nevertheless a plausible mechanism for generating observable phenomena prior to earthquakes is suggested by the laboratory observations

the tectonics of mercury the view after messenger s first flyby article i nfo during its first flyby of mercury messenger imaged many tectonic landforms most of which are contractional in nature and include lobate scarps high relief ridges and wrinkle ridges wrinkle ridges are found on the smooth plains that partially fill the interior and surround the exterior of the caloris basin and also on smooth plains that fill the interiors of smaller impact basins and larger craters messenger revealed a radial graben complex pantheon fossae nearlyco centered with the caloris basin pantheon fossae and a network of mostly basin concentric graben intheouterportionsofthecalorisbasininteriorformapatternofextensionnotseenelsewhereonmercury the first clear example of extensional faults outside of the caloris basin has been documented on smooth plains inside the peak ring of the relatively young raditladi basin a map of the distribution of tectonic landforms imaged by messenger and mariner 10 shows that lobate scarps are the most widespread type of feature estimates of the horizontal shortening associated with lobate scarps that crosscut and overthrust small impact craters imaged by messenger range from 1 to 3 km previously unrecognized lobate scarps detected in areas imaged by mariner 10 indicate thatearlierestimatesof contractional strainarelow anewestimate of theaverage arealcontractionalstrain 0 06 accommodated byscarps is at leastonethirdgreater thancomparableprevious estimates andcorresponds to a decrease in mercury s radius of at least 0 8 km since the end of heavy impact bombardment of the inner solar system these figures are lower bounds because of the likelihood that not all lobate scarps have been identified even in areas imaged to date crosscutting and embayment relations revealed by messenger suggest that lobate scarps formed before the end of smooth plains emplacement and continued to be active after the emplacement of the youngest smooth plains deposits relatively recent activity on lobate scarps may be the expression of slow but continuous radial contraction that accompanied cooling of mercury s interior and the growth of the planet s solid inner core published by elsevier b v

deep generation of magmatic gas on the moon and implications for pyroclastic eruptions 1 lunar pyroclastic beads are interpreted to represent primitive magmas derived from great depths and rapidly erupted to the surface in explosive events however a detailed mechanism for gas generation at great depth and rapid magma transport to the surface has not yet been described furthermore the pyroclastic beads are not petrogenetically related to basalts erupted near the sampling sites we propose a model in which these conundrums are resolved through gas build up in a low pressure micro environment near the tip of a magma filled crack dike propagating rapidly from the magma source depth to the surface the gas rich region consists of a free gas cavity overlying a foam extending vertically for 20 km eruption of the foam results in the widespread emplacement of unfractionated pyroclastic beads subsequent ascent of the underlying gas free picritic magma is unlikely to occur perhaps accounting for the lack of sampled eruptive equivalents

investigating sources of mercury s crustal magnetic field further mapping of messenger magnetometer data 106 low altitude passes of magnetometer data from the last two months of the messenger mission have been applied to produce a map of the crustal magnetic field at a constant altitude of 40 km covering latitudes of 35 75 n and longitudes of 270 90 e some anomalies correlate significantly with impact basins craters e g rustaveli and vyasa while other basins craters have no obvious anomalies a possible interpretation that is consistent with lunar evidence is that some impactors delivered more ferromagnetic fe ni metal to the interior subsurfaces and ejecta fields of the craters basins that they produced the amount of metallic iron that could plausibly be delivered is limited by the diameter and mass of an impactor that would yield a crater with observed diameters e g 200 km for rustaveli this in turn limits the maximum amplitude of anomalies that could be induced by impactor added iron in the present day mercury global field to relatively low values it is therefore concluded that if impactor added iron is the source of the observed crater associated anomalies then they must be almost entirely a consequence of ancient remanent magnetization a broad magnetic anomaly occurs over the northern rise a topographically high region with an associated strong free air gravity anomaly a possible interpretation of the latter anomaly is that an early major impact preconditioned the region for a later mantle uplift event

thrust faults and the global contraction of mercury 1 lobate scarps on mercury are the surface expression of thrust faults and are generally thought to be the result of global contraction our analysis indicates that these thrust faults are not uniformly distributed and exhibit non random patterns on the imaged hemisphere the greatest area normalized cumulative length of the faults occurs in the southern hemisphere below 50 s the thrust faults in this region dip to the north indicating a broad zone where the crust has been displaced to the south widespread thrust faulting may not have initiated before the emplacement of calorian smooth plains well after the emplacement of pre tolstojan intercrater plains our observations suggest that regional scale tectonic stresses on mercury dominated over stresses resulting from global contraction

lunar mare volcanism stratigraphy eruption conditions and the evolution of secondary crusts abstract lunar mare basalt deposits cover 17 of the lunar surface occur preferentially in topographic lows on the nearside and have a total volume estimated at 1 107 km3 analysis of returned samples and photogeologic and remote sensing studies shows that mare volcanism began prior to the end of heavy bombardment the period of cryptomare formation in pre nectarian times and continued until the copernican period a total duration approaching 3 5 4 ga stratigraphic analyses show that the flux was not constant but peaked in early lunar history during the imbrian period which spans the period 3 85 3 2 ga average volcanic output rate during this peak period was about 10 2 km 3 a very low relative to the present global terrestrial volcanic output rate and comparable to the present local output rates for individual volcanoes such as vesuvius and kilauea hawaii volcanic landforms indicate that many eruptions were of high volume and long duration some eruptions associated with sinuous rules may have lasted of the order of a year and emplaced 103 km3 of lava representing the equivalent in one year of about 70 000 years at the average flux shallow magma chambers were uncommon the nearsidefarside mare deposit asymmetry can be readily explained by differences in crustal thickness magma ascending from the mantle or from a buoyancy trap at the base of the crust should preferentially extrude to the surface on the nearside but should generally stall and cool in dikes in the farside crust extruding only in the deepest basins the occurrence of farside maria within craters whose diameter is generally near to or less than the thickness of the crust may be accounted for by the difference between local and regional compensation dikes that establish pathways to the surface on the nearside should have very high volumes comparable to the volumes associated with many observed flows and sinuous rilles an abundance of dikes should exist in the lower crust of the moon many more than those feeding surface eruptions the upper limit is 37 50 of the crust by volume the presence and abundance of such dike swarms have important implications for the interpretation of the average composition of the lunar crust and the composition of basin and crater ejecta the interplay between thermal contraction and differentiation leads to net cooling and ultimate contraction of the outer portions of the moon resulting in a regional horizontal compressive stress acting on the lunar crust in addition overall cooling and deepening of sources require the production of ever larger volumes of magma in order to reach the surface with time stresses became high enough so that few dikes could open to the surface causing eruptive activity to be severely diminished in the eratosthenian and perhaps to cease in the copernican period lower stress levels are required to terminate eruptive activity on the lunar farside consistent with the imbrian age of the farside maria and the nearside location of the youngest maria lunar mare deposits provide an example of the transition from primary crusts to secondary crusts and illustrate the significance of several factors in the evolution of secondary crusts such as crustal density variations in crustal thickness presence of impact basins state and magnitude of stress in the lithosphere and general thermal evolution these factors are also responsible for the extremely low volcanic flux even during periods of peak extrusion

phase equilibria trigger for explosive volcanic eruptions 1 a mechanism for explosive volcanic eruptions based on multicomponent phase equilibria modelling of four explosive volcanic systems is proposed in each system either isochoric or isobaric crystallization where either crystals or crystals and exsolved fluid are chemically fractionated from melt leads inevitably to near solidus dynamical instability culminating in violent explosive eruption driving this instability is a dramatic increase in the volume fraction of fluid bubbles in the magma exceeding the limit for magma fragmentation this phenomenon is independent of magma decompression country rock may be weak allowing magma to remain in lithostatic equilibrium with its host rock or strong leading to divergence of magma and lithostatic pressure bubbles may be retained or expelled during crystallization instability is the inevitable outcome of crystallization at shallow levels in the crust for all four systems regardless of the mechanical state of host country rock we speculate that this phase equilibria mechanism driving explosive eruptions has general significance

compositional analyses of lunar pyroclastic deposits abstract the 5 band clementine uvvis data at 100 m pixel were used to examine the compositions of 75 large and small lunar pyroclastic deposits lpds and these were compared to representative lunar maria and highlands deposits results show that the albedo spectral color and inferred composition of most lpds are similar to those of low titanium mature lunar maria these lpds may have consisted largely of fragmented basalt with substantial components of iron bearing mafic minerals pyroxenes olivine and smaller amounts if any of volcanic glass several smaller lpds also show substantial highland components three classes of very large deposits can be distinguished from most lpds and from each other on the basis of crystallinity and possible titanium content of their pyroclastic components one class has spectral properties that are dominated by high titanium crystallized black beads e g taurus littrow a second consists of a mixture of high titanium glasses and beads with a higher glass bead ratio sulpicius gallus than that of taurus littrow and a third has a significant component of quenched iron bearing volcanic glasses aristarchus with possible moderate titanium contents although areally extensive these three classes of very large pyroclastic deposits compose only 20 of the 75 deposits studied 27 and eruption of such materials was thus likely to have been less frequent on the moon

the eco geomorphological roles of rocky deep crevices for water supply on arid zone mountain slopes case study mehriz yazd iran rocky slope landscapes are widely distributed on arid land mountains where there are many restoration and forestation project sites this study has surveyed the role of deep crevices and stone layers for supplying water to establish trees and shrubs on arid lands rocky slopes the research was concentrated on the layers of limestone granite and andesite in central iran thirty transects each 25 m long were randomly laid out throughout the studied lithologic units correlation of vegetation cover percentage and abundance of trees were studied along with crack dimensions including width depth and length categorized into three classes large medium and low sizes along those transects the amount of ec ph organic materials caso4 caco3 soil texture n p k and soil moisture content in the cracks were also measured the results showed significant differences p 0 05 among crack properties vegetation cover and life forms in the study units also the vegetation cover and life forms have positive significant relationships with increase of crevice width as well as length and depth of each lithologic unit the findings demonstrated that the inherent potential of limestone rocky slopes with deep layers is higher than other rocky units for restoration and afforestation in this arid region

groundwater recharge in semi arid carbonate aquifers under intensive use the estepa range aquifers seville southern spain quantifying groundwater recharge in carbonate aquifers located in semi arid regions and subjected to intensive groundwater use is no easy task one reason is that there are very few available methods suitable for application under such climatic conditions and moreover some of the methods that might be applied were originally designed with reference to non carbonate aquifers in addition it is necessary to take into account the fact that in any given aquifer groundwater recharge is modified by the groundwater exploitation here we focus on four methods selected to assess their suitability for estimating groundwater recharge in carbonate aquifers affected by intensive exploitation the methods were applied to the estepa range aquifers of seville southern spain which are subjected to different degrees of exploitation two conventional methods were used chloride mass balance and daily soil water balance these results were compared with the results obtained by means of two non conventional methods designed for application to the carbonate aquifers of southern spain the aplis and eras methods the results of the different methods are analogous comparable to those obtained in nearby non exploited carbonate aquifers confirming their suitability for use with carbonate aquifers in either natural or exploited regimes in a semi arid climate

constructional volcanic edifices on mercury candidates and hypotheses of formation mercury a planet with a predominantly volcanic crust has perplexingly few if any constructional volcanic edifices despite their common occurrence on other solar system bodies with volcanic histories using image and topographical data from the mercury surface space environment geochemistry and ranging messenger spacecraft we describe two small 15 km diameter prominences with shallow summit depressions associated with volcanically flooded impact features we offer both volcanic and impact related interpretations for their formation and then compare these landforms with volcanic features on earth and the moon though we cannot definitively conclude that these landforms are volcanic the paucity of constructional volcanic edifices on mercury is intriguing in itself we suggest that this lack is because volcanic eruptions with sufficiently low eruption volumes rates and flow lengths suitable for edifice construction were highly spatiotemporally restricted during mercury s geological history we suggest that volcanic edifices may preferentially occur in association with late stage post impact effusive volcanic deposits the esa jaxa bepicolombo mission to mercury will be able to investigate further our candidate volcanic edifices search for other as yet unrecognized edifices beneath the detection limits of messenger data and test our hypothesis that edifice construction is favored by late stage low volume effusive eruptions

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

characterisation of a fractured reservoir using microearthquakes induced by hydraulic injections abstract microseismic activity is induced by a number of engineering activities which involve fluid injection or extraction in naturally fractured rock masses evidence collected during the last 15 years of hot dry rock hdr geothermal energy research has clearly indicated the very strong positive correlation between induced microearthquake locations and fluid flow paths within the fractured rock mass microseismic monitoring is now used regularly in hdr research as the only effective means of mapping hydraulic stimulations and monitoring hydromechanical processes during operation of the fractured reservoir many of the techniques that have been developed through hdr research are directly applicable to the hydrocarbon industry for the monitoring of hydraulic fracturing treatments waterflood operations and reservoir depletion this paper provides a brief introduction to the hydromechanical interactions that result in induced microseismicity and presents case studies from three european hdr sites where the monitoring of microseismic activity has proved vital in targeting production wells mapping the progress of stimulation and waterfloods and understanding the hydromechanical behaviour of the fractured system

the partial volatilization of mercury during recent years my research on the primitive solar nebular has followed two main themes 1 very early in the development of the nebula conditions probably favored the occurence of major gaseous instabilities leading to the formation of giant gaseous protoplanets but the rapid rise of the external temperature soon evaporated the envelopes of these protoplanets possibly leaving behind precipitated solids which formed the cores and mantles of the terrestrial planets 2 models of the nebula indicate a later stage when conditions in the inner solar system became very hot at the position of mercury the temperature was probably in the range 2500 3500 k this leads to the hypothesis that the original protomercury was a body substantially more massive than the present planet and of normal composition but that when it was immersed in the high temperature field of the dissipating solar nebula most of the rocky mantle was vaporized and mixed into the solar nebula gases and carried away by them this hypothesis is investigated in the present paper for simplicity the vaporization of a mantle composed of enstatite mgsio3 was computed for a planet with 2 25 the mass of mercury at a temperature of 3000 k it is argued that the mantle could probably be largely removed in the available time of 3 104 years subsequent accretion would restore some magnesium silicates to the mantle of the planet

the relationship between microseismicity and high pore pressures during hydraulic stimulation experiments in low permeability granitic rocks several hundred microearthquakes with local magnitudes ranging between 4 and 2 were recorded during hydraulic stimulation experiments at the hot dry rock demonstration site fenton hill new mexico these events are probably caused by shear failure induced by high pore fluid pressures since the event locations seem to cluster in a narrow band near the hydraulic fracture we were able to use micro seismic techniques to locate the hydraulic fracture and monitor its growth we calculate the minimum pore pressure increase necessary to induce rock failure using a simple model of slippage on preexisting fractures by comparing a pore pressure distribution calculated using a one dimensional diffusion model with the distribution of event foci we demonstrate that high pore pressures are a probable cause for these microearthquakes stress drops for these microearthquakes calculated from spectral parameters increased by a factor of 3 during the course of the experiment the stress drop increase correlates with a migration of seismic activity away from the injection well bore which suggests relatively low in situ shear stresses near the well bore caused by strain release in this region during previous pressurizations

explosive volcanism in complex impact craters on mercury and the moon influence of tectonic regime on depth of magmatic intrusion vents and deposits attributed to explosive volcanism occur within numerous impact craters on both the moon and mercury given the similarities between the two bodies it is probable that similar processes control this spatial association on both however the precise morphology and localization of the activity differs on the two bodies indicating that the nature of structures beneath impact craters and or volcanic activity may also be different to explore this we analyze sites of explosive volcanism within complex impact craters on the moon and mercury comparing the scale and localization of volcanic activity and evidence for post formation modification of the host crater we show that the scale of vents and deposits is consistently greater on mercury than on the moon indicating greater eruption energy powered by a higher concentration of volatiles additionally while the floors of lunar craters hosting explosive volcanism are commonly fractured those on mercury are not the most probable explanation for these differences is that the state of regional compression acting on mercury s crust through most of the planet s history results in deeper magma storage beneath craters on mercury than on the moon the probable role of the regional stress regime in dictating the depth of intrusion on mercury suggests that it may also play a role in the depth of sub crater intrusion on the moon and on other planetary bodies examples on the moon and also on mars commonly occur at locations where flexural extension may facilitate shallower intrusion than would be driven by the buoyancy of the magma alone

hydrochemical and isotopic study of groundwater in wadi el hechim garaa hamra basin central tunisia hydrochemical and isotopic study of miocene and mio plio quaternary m p q aquifers in wadi el hechim garaa hamra basin central tunisia was undertaken in order to investigate recharge mode and processes leading to mineralization of groundwater as well as interaction between both systems the results revealed striking differences between the two aquifer systems while the miocene aquifer contains recently recharged waters with generally low mineralization around 0 5 g l 1 stemming mainly from dissolution of carbonate minerals the m p q aquifer reveals tds values reaching 3 g l 1 controlled mainly by dissolution of evaporitic minerals isotopic data indicate that the miocene aquifer contains water recharged in past several decades bomb tritium and bomb radiocarbon detected the m p q system appears to be much slower with time scales of groundwater flow possibly reaching some thousands of years sharp discontinuity of hydrochemical and isotope characteristic of groundwater observed across the major tectonic fault separating the miocene and m p q aquifers supports the idea of very limited if any hydraulic interconnection between both studied systems this in turn calls for revision of existing conceptual models of groundwater flow in the region postulating significant groundwater fluxes crossing the fault in the direction of m p q aquifer and adjacent aquifers in the wadi al fakka plain

global resurfacing of mercury 4 0 4 1 billion years ago by heavy bombardment and volcanism analysis of craters on mercury s oldest most heavily cratered terrains shows that they were formed 4 0 4 1 billion years ago and that the planet s previous geological history was erased most probably by voluminous volcanism which may have been triggered by heavy asteroidal bombardment at that time the most heavily cratered terrains on mercury exhibit a lower density of craters smaller than about 100 km in diameter than on the moon a deficit that has been attributed to resurfacing by formation of ancient intercrater plains simone marchi et al used a crater areal density map based on data from the messenger spacecraft the colour coded foreground on cover with a global surface mosaic in the background to locate the oldest surfaces on mercury and interpret the crater populations in the framework of a recent lunar crater chronology they conclude that the oldest surfaces were emplaced just after the start of the late heavy bombardment 4 0 to 4 1 billion years ago the large impact basins not previously dated yield a similar surface age this agreement implies that resurfacing was global and due to volcanism perhaps aided by heavy bombardment as previously suggested the most heavily cratered terrains on mercury have been estimated to be about 4 billion years gyr old1 2 3 4 but this was based on images of only about 45 per cent of the surface even older regions could have existed in the unobserved portion these terrains have a lower density of craters less than 100 km in diameter than does the moon1 3 5 an observation attributed to preferential resurfacing on mercury here we report global crater statistics of mercury s most heavily cratered terrains on the entire surface applying a recent model for early lunar crater chronology6and an updated dynamical extrapolation to mercury7 we find that the oldest surfaces were emplaced just after the start of the late heavy bombardment lhb about 4 0 4 1 gyr ago mercury s global record of large impact basins8 which has hitherto not been dated yields a similar surface age this agreement implies that resurfacing was global and was due to volcanism as previously suggested1 5 this activity ended during the tail of the lhb within about 300 400 million years after the emplacement of the oldest terrains on mercury these findings suggest that persistent volcanism could have been aided by the surge of basin scale impacts during this bombardment

a history of hot dry rock geothermal energy systems abstract this is a short history so far as it can now be assembled of early speculations and observations concerning the existence and origin of natural heat in the earth s crust of some of the many methods proposed to extract and use it and of recent investigations designed to develop and demonstrate such methods it is probably only the preface to a much longer history still to be written of mankind s beneficial use of this one of earth s oldest most plentiful most widespread and secure energy sources

a unique volcanic field in tharsis mars pyroclastic cones as evidence for explosive eruptions based on theoretical grounds explosive basaltic volcanism should be common on mars yet the available morphological evidence is sparse we test this hypothesis by investigating a unique unnamed volcanic field north of the shield volcanoes biblis patera and ulysses patera on mars where we observe several small conical edifices and associated lava flows twenty nine volcanic cones are identified and the morphometry of many of these edifices is determined using established morphometric parameters such as basal width crater width height slope and their respective ratios their morphology morphometry and a comparison to terrestrial analogues suggest that they are martian equivalents of terrestrial pyroclastic cones the most common volcanoes on earth the cones are tentatively interpreted as monogenetic volcanoes according to absolute model age determinations they formed in the amazonian period our results indicate that these pyroclastic cones were formed by explosive activity the cone field is superposed on an old elevated window of fractured crust which survived flooding by younger lava flows it seems possible that a more explosive eruption style was common in the past and that wide spread effusive plain style volcanism in the late amazonian has buried much of its morphological evidence in tharsis

maar diatreme volcanoes a review maar diatreme volcanoes are produced by explosive eruptions that cut deeply into the country rock a maar is the crater cut into the ground and surrounded by an ejecta ring while the diatreme structure continues downward and encloses diatreme and root zone deposits here we attempt an evenhanded review of maar diatreme volcanology that extends from mafic to kimberlitic varieties and from historical maar eruptions to deeply eroded or mined diatreme structures we conclude that maar diatreme eruptions are episodic ejecta rings provide invaluable insight into eruption processes and sequence but are incomplete records of diatreme formation deposits within the diatreme structure include in varying proportions lower unbedded deposits sometimes typified by subvertical contacts among domains of debris emplaced sequentially and upper bedded deposits formed by sedimentation on surfaces open to the atmosphere a basal root zone comprises the transition from coherent magmatic feeder dike to clastic deposits formed by fragmentation of magma and enclosing country rock root zones are irregular in form and the clastic deposits are typically intruded by contorted dikes irregular root zone like chaotic breccias cut by contorted dikes are also present within diatreme deposits where they represent intra diatreme fragmentation zones and record changes in the location of the explosion locus during eruption

modification of caloris ejecta blocks by long lived mass wasting a volatile driven process abstract the caloris basin is the largest well preserved impact basin on mercury as such caloris ejecta afford us an opportunity to study material from mercury s deep interior with remote sensing we have made observations of the geomorphology colour distribution and flank slopes of the circum caloris knobs our observations suggest that these circum caloris knobs are modified ejecta blocks from the caloris impact high resolution messenger images show that knobs are conical and relatively uncratered compared with the surrounding plains which implies the knobs have undergone resurfacing we have observed material that has sloughed off knobs superposing impact craters that demonstrably postdate the caloris impact which requires some knob modification to have been more recent we have observed hollows depressions in mercury s surface generally believed to have been caused by volatile loss on and closely associated with several knobs which indicates that many knobs contain volatile material and that knob modification could extend into mercury s recent past our measurements show that knob flanks typically have slopes of 21 which is steep for a mound of unconsolidated material that was originally emplaced 3 8 ga the conical shape of knobs their steep slopes the dearth of superposed craters on knobs and knob superposition relationships with other landforms suggest that caloris ejecta blocks of arbitrary original shape were modified into their present shapes by long lived mass wasting mass wasting must have dominated over impact gardening which would have produced domal morphologies only we suggest that mass wasting was probably driven by volatile loss in a manner analogous to terrestrial landforms called molards if the circum caloris knobs are analogous to molards then they represent a landform and a process hitherto undocumented on mercury with implications for the volatile content of the planet s interior these knobs therefore are prime targets for bepicolombo which could search for fresh failures and volatile exposures in the knobs

topography of the northern hemisphere of mercury from messenger laser altimetry laser altimetry by the messenger spacecraft has yielded a topographic model of the northern hemisphere of mercury the dynamic range of elevations is considerably smaller than those of mars or the moon the most prominent feature is an extensive lowland at high northern latitudes that hosts the volcanic northern plains within this lowland is a broad topographic rise that experienced uplift after plains emplacement the interior of the 1500 km diameter caloris impact basin has been modified so that part of the basin floor now stands higher than the rim the elevated portion of the floor of caloris appears to be part of a quasi linear rise that extends for approximately half the planetary circumference at mid latitudes collectively these features imply that long wavelength changes to mercury s topography occurred after the earliest phases of the planet s geological history

determination of the origins and recharge rates of the sfax aquifer system southeastern tunisia using isotope tracers the origin of groundwater in the sfax aquifer system was studied using environmental isotopic tracers 18o 2h and 3h in total 164 water samples were analyzed for stable isotopes 73 collected from the sfax shallow aquifer 63 from the sfax middle aquifer and 28 from the sfax deep aquifer recent recharge of the groundwater in the sfax aquifer was identified using tritium concentrations in 82 groundwater samples from different depths the isotopic ratios of the shallow aquifer range from 5 55 to 1 59 for 18o and from 38 38 to 14 19 for 2h and the isotopic ratios in the middle aquifer range from 6 86 to 2 97 for 18o and from 44 18 to 22 38 for 2h the deep aquifer exhibited markedly lower isotopic values ranging from 6 70 to 5 70 for 18o and from 42 40 to 38 89 for 2h the mixing proportions inferred from stable isotopic mass balance calculations suggest that the deep aquifer contributes significantly to the middle aquifer through geologic structures and may reach 100 in the menzel chaker region the isotopic mass balance model also indicates that the middle groundwater aquifer may contribute up to 100 of the shallow plio quaternary aquifer particularly in the western and northeastern parts of the study area between bir ali ben khalifa and djebeniana the tritium data support the existence of recent recharge the tritium and stable isotope data clearly indicate the presence of mixing processes especially in the northwestern and coastal portions of the study area a conceptual model is established explaining the pressure differences that generate vertical leakage which is a reasonable mechanism for flow between the aquifers

localization of bedding plane slip and backthrust faults above blind thrust faults keys to wrinkle ridge structure a mechanically based model for wrinkle ridge development is developed that combines wrinkle ridge morphologies regional topographic offsets suggestive of subsurface thrust faults and folding of near surface layers this model provides explicit relationships between observed morphologic elements characteristic of wrinkle ridges and plausible mechanisms in the subsurface a key component that is absent in previous qualitative fault based scenarios for these structures as developed in this paper wrinkle ridges are the surface expression of anticlines that grow above a blind thrust fault as a result of both flexural slip folding of near surface strata and the nucleation and growth of echelon arrays of backthrust faults calculations of displacements both horizontal shortening and vertical uplift and coulomb stress change related to slip along blind thrust faults in the model demonstrate physically important spatial inhomogeneities in these quantities with revealing and useful implications 1 the ratio of shortening due to folding at the surface to the shortening due to faulting at depth is characteristically small for coupled wrinkle ridge blind thrust fault systems and decreases with increasing fault depth the depth of the blind thrust fault s upper tip thus profoundly influences the surface strains 2 folding and uplifted topography forming the topographic ridge are produced above the area of the slipping blind thrust fault plane horizontal and vertical deformation at the surface extend over several ridge widths or a total of at least 50 km for a 10 km wide ridge implying that topographic profiles and geologic studies must extend sufficiently far from the wrinkle ridge to fully characterize the surface deformation 3 calculations of coulomb stress changes suggest that fault slip can localize both bedding plane slip in overlying strata and new backthrust faults that propagate upward to become wrinkles on the trailing side of the ridge initiation of bedding plane slip in association with slip along the blind thrust fault likely determines whether the resulting surface structure becomes a wrinkle ridge or a lobate scarp wrinkle ridge spacing may also be related to stress changes associated with slip along the underlying blind thrust fault

global inventory and characterization of pyroclastic deposits on mercury new insights into pyroclastic activity from messenger orbital data we present new observations of pyroclastic deposits on the surface of mercury from data acquired during the orbital phase of the mercury surface space environment geochemistry and ranging messenger mission the global analysis of pyroclastic deposits brings the total number of such identified features from 40 to 51 some 90 of pyroclastic deposits are found within impact craters the locations of most pyroclastic deposits appear to be unrelated to regional smooth plains deposits except some deposits cluster around the margins of smooth plains similar to the relation between many lunar pyroclastic deposits and lunar maria a survey of the degradation state of the impact craters that host pyroclastic deposits suggests that pyroclastic activity occurred on mercury over a prolonged interval measurements of surface reflectance by messenger indicate that the pyroclastic deposits are spectrally distinct from their surrounding terrain with higher reflectance values redder i e steeper spectral slopes and a downturn at wavelengths shorter than 400 nm i e in the near ultraviolet region of the spectrum three possible causes for these distinctive characteristics include differences in transition metal content physical properties e g grain size or degree of space weathering from average surface material on mercury the strength of the near ultraviolet downturn varies among spectra of pyroclastic deposits and is correlated with reflectance at visible wavelengths we suggest that this interdeposit variability in reflectance spectra is the result of either variable amounts of mixing of the pyroclastic deposits with underlying material or inherent differences in chemical and physical properties among pyroclastic deposits

mid infrared spectroscopy of impactites from the nordlinger ries impact crater abstract this study is part of an effort to build a mid infrared database 7 14 m of spectra for mertis mercury radiometer and thermal infrared spectrometer an instrument onboard of the esa jaxa bepicolombo space probe to be launched to mercury in 2017 mercury was exposed to abundant impacts throughout its history this study of terrestrial impactites can provide estimates of the effects of shock metamorphism on the mid infrared spectral properties of planetary materials in this study we focus on the nordlinger ries crater in southern germany a well preserved and easily accessible impact crater with abundant suevite impactites suevite and melt glass bulk samples from otting and aumuhle as well as red suevite from polsingen were characterized and their reflectance spectra in mid infrared range obtained in addition in situ mid infrared spectra were made from glasses and matrix areas in thin sections the results show similar but distinguishable spectra for both bulk suevite and melt glass samples as well as in situ measurements impact melt glass from aumuhle and otting have spectra dominated by a reststrahlen band at 9 3 9 6 m bulk melt rock from polsingen and bulk suevite and fine grained matrix have their strongest band between 9 4 and 9 6 m there are also features between 8 5 and 9 m and 12 5 12 8 m associated with crystalline phases there is evidence of weathering products in the fine grained matrix such as smectites mercury endured many impacts with impactors of all sizes over its history so spectral characteristics observed for impactites formed only in a single impact like in the ries impact event can be expected to be very common on planetary bodies exposed to many more impacts in their past we conclude that in mid infrared remote sensing data the surface of mercury can be expected to be dominated by features of amorphous materials

impacts do not initiate volcanic eruptions eruptions close to the crater many papers on meteorite impact suggest that large impacts can induce volcanic eruptions through decompression melting of the underlying rocks we perform numerical simulations of the impact of an asteroid with a diameter of 20 km striking at 15 km s 1 into a target with a near surface temperature gradient of 13 k km 1 cold case or 30 k km 1 hot case the impact creates a 250 300 km diameter crater with 10 000 km3 of impact melt however the crater collapses almost flat and the pressure field returns almost to the initial lithostat even an impact this large cannot raise mantle material above the peridotite solidus by decompression statistical considerations also suggest that impacts cannot be the common initiator of large igneous provinces any time in post heavy bombardment earth history