magma extraction from the mantle wedge at convergent margins through dikes a parametric sensitivity analysis 1 we address under what conditions a magma generated by partial melting at 100 km depth in the mantle wedge above a subduction zone can reach the crust in dikes before stalling we also address under what conditions primitive basaltic magma mg 60 can be delivered from this depth to the crust we employ linear elastic fracture mechanics with magma solidification theory and perform a parametric sensitivity analysis all dikes are initiated at a depth of 100 km in the thermal core of the wedge and the moho is fixed at 35 km depth we consider a range of melt solidus temperatures 800 1100c viscosities 10 100 pa s and densities 2400 2700 kg m 3 we also consider a range of host rock fracture toughness values 50 300 mpa m 1 2 and dike lengths 2 5 km and two thermal structures for the mantle wedge 1260 and 1400c at 100 km depth and 760 and 900c at 35 km depth for the given parameter space many dikes can reach the moho in less than a few hundred hours well within the time constraints provided by u series isotope disequilibria studies increasing the temperature in the mantle wedge or increasing the dike length allows additional dikes to propagate to the moho we conclude that some dikes with vertical lengths near their critical lengths and relatively high solidus temperatures will stall in the mantle before reaching the moho and these may be returned by corner flow to depths where they can melt under hydrous conditions thus a chemical signature in arc lavas suggesting partial melting of slab basalts may be partly influenced by these recycled dikes alternatively dikes with lengths well above their critical lengths can easily deliver primitive magmas to the crust particularly if the mantle wedge is relatively hot dike transport remains a viable primary mechanism of magma ascent in convergent tectonic settings but the potential for less rapid mechanisms making an important contribution increases as the mantle temperature at the moho approaches the solidus temperature of the magma

buoyancy driven fracture ascent experiments in layered gelatine laboratory experiments on air filled fracture propagation in solidified homogeneous and layered gelatine have been carried out providing an analogue model for magma filled dikes ascending in the crust the effects of layering on fracture velocity and shape have been analyzed in detail the free surface is found to accelerate approaching fractures layering accelerates or decelerates fractures approaching discontinuities of the elastic parameters depending on the value of the rigidity contrast the shape of fractures are strongly influenced as they pass from one layer to another the observed cross sectional shape when crossing a layer interface and the acceleration with decreasing rigidity can be explained with theoretical models our experiments also reproduce the arrest of fractures in proximity of joints and the formation of sills in the layer below the interface these findings could help in the interpretation of accelerated seismicity and deformation rates observed in volcanic areas

explosive volcanism and the compositions of cores of differentiated asteroids eleven iron meteorite groups show correlations between ni and siderophile trace elements that are predictable by distribution coefficients between liquid and solid metal in fractionally crystallizing metal magmas these meteorites are interpreted to be fragments of the fractionally crystallized cores of eleven differentiated asteroids many of these groups crystallized from s depleted magmas which we propose resulted from removal of the first partial melt the fe nifes cotectic melt by explosive pyroclastic volcanism of the type envisaged by wilson and keil 8 we show that these dense negatively buoyant melts can be driven to asteroidal surfaces due to the presence of excess pressure in the melt and the presence of buoyant bubbles of gas which decrease the density of the melt we also show that in typical asteroidal materials veins will form which grow into dikes and serve as pathways for migration of melt and gas to asteroidal surfaces since cotectic fe nifes melt consists of about 85 wt fes and 15 wt fe ni removal of small volumes of eutectic melts results in major loss of s but only minor loss of fe ni thus leaving sufficient fe ni to form sizeable asteroidal cores

vent geometry and eruption conditions of the mixed rhyolite basalt namshraun lava flow iceland abstract we describe the morphology and circumstances of eruption of the mixed rhyolite basalt lava flow namshraun in the torfajokull vei ivotn area of central iceland the unusual location and exposure of the elongate fissure vent permits its length along strike a total of 275 m and the width of the dyke feeding it up to 10 m to be estimated in the field using analyses of the heat losses during the rise of the mixed magma through the shallow part of its conduit system we are able to refine the absolute minimum dyke width estimate to 1 5 m the lengths of the two main lava flow lobes assuming that their advance was cooling limited imply that the volume effusion rate of the lava varied between 2 7 and 1 5 m 3 s 1 as different parts of the fissure became active prior to its emergence at the surface the magma had at most a small yield strength probably significantly less than 3000 4000 pa and a near newtonian viscosity in the range 1 10 4 to 5 10 6 pa s after its eruption the lava formed flows with marginal levees whose sizes imply a yield strength just less than 30 kpa the lava in the central channels between the levees can be modeled either as a newtonian fluid with a viscosity of between 3 10 7 and 6 10 7 pa s or as a bingham plastic estimates of the plastic viscosity from the two main flow lobes 4 to 6 10 5 and 1 2 10 7 to 1 8 10 7 pa s differ by a very large factor at least 30 and are regarded as unreliable however they lead to a much smaller range of apparent viscosities from 1 5 10 7 to 5 5 10 7 pa s values very similar to the viscosities found when the rheology is assumed to be newtonian if the field estimate of the dyke width is reliable these results imply that the viscosity and yield strength of the magma averaged over the path from its source to the surface had increased by a factor close to 10 by the time that it emerged from the vent alternatively the feeder dike may have been almost twice as wide during the eruption and relaxed to the presently exposed width as the eruption ended the typical advance speeds of the two main flow lobes were less than 4 mm s 1 and their emplacement times were 2 5 and 5 days the implications for the sizes of the conduits feeding other rhyolitic and mixed lavas in central iceland are discussed

u pb zircon geochronology of the paleogene neogene volcanism in the nw anatolia its implications for the late mesozoic cenozoic geodynamic evolution of the aegean abstract the northern aegean region was shaped by subduction obduction collision and post collisional extension processes two areas in this region the rhodope thrace biga peninsula to the west and armutlu almacik nallihan the central sakarya to the east are characterized by extensive eocene to miocene post collisional magmatic associations we suggest that comparison of the cenozoic magmatic events of these two regions may provide insights into the late mesozoic to cenozoic tectonic evolution of the aegean with this aim we present an improved cenozoic stratigraphy of the biga peninsula derived from a new comprehensive set of u pb zircon age data obtained from the eocene to miocene volcanic units in the region the compiled radiometric age data show that calc alkaline volcanic activity occurred at 43 15 ma in the biga peninsula 43 17 ma in the rhodope and thrace regions and 53 38 ma in the armutlu almacik nallihan region which are slightly overlapping we discuss the possible cause for the distinct cenozoic geodynamic evolution of the eastern and western parts of the region and propose that the rhodope thrace and biga regions in the north aegean share the same late mesozoic to cenozoic geodynamic evolution which is consistent with continuous subduction crustal accretion southwestward trench migration and accompanying extension all preceded by the late cretaceous paleocene collision along the vardar suture zone in contrast the armutlu almacik nallihan region was shaped by slab break off and related processes following the late cretaceous paleocene collision along the izmir ankara suture zone the eastern and western parts of the region are presently separated by a northeast southwest trending transfer zone that was likely originally present as a transform fault in the subducted tethys oceanic crust and demonstrates that the regional geodynamic evolution can be strongly influenced by the geographical distribution of geologic features on the subducting plate

surface deformation in volcanic rift zones abstract the principal conduits for magma transport within rift zones of basaltic volcanoes are steeply dipping dikes some of which feed fissure eruptions elastic displacements accompanying a single dike emplacement elevate the flanks of the rift relative to a central depression concomitant normal faulting may transform the depression into a graben thus accentuating the topographic features of the rift if eruption occurs the characteristic ridge trough ridge displacement profile changes to a single ridge centered at the fissure and the erupted lava alters the local topography a well developed rift zone owes its structure and topography to the integrated effects of many magmatic rifting events to investigate this process we compute the elastic displacements and stresses in a homogeneous two dimensional half space driven by a pressurized crack that may breach the surface a derivative graphical method permits one to estimate the three geometric parameters of the dike height inclination and depth to center and the mechanical parameter driving pressure rock stiffness from a smoothly varying displacement profile direct comparison of measured and theoretical profiles may be used to estimate these parameters even if inelastic deformation notably normal faulting creates discontinuities in the profile geological structures open cracks normal faults buckles and thrust faults form because of stresses induced by dike emplacement and fissure eruption theoretical stress states associated with dilation of a pressurized crack are used to interpret the distribution and orientation of these structures and their role in rift formation

volcanic eruptions and intrusions on the asteroid 4 vesta we use basic principles of fluid mechanics to explore the intrusive and eruptive consequences of early large scale silicate melting within the asteroid 4 vesta and relate our findings to the properties of the howardite eucrite diogenite meteorites surface lava flows on vesta would commonly have had widths ranging from a few hundred meters to a few kilometers and lengths lying between a few kilometers and at least several tens of kilometers flow thicknesses would have occupied a much narrower range lying between about 5 and 20 m these thicknesses imply cooling rates and plagioclase crystal growth rates which are extremely similar to those measured in the eucrite meteorites intrusions may have formed at very shallow depths as cooling dikes failed to erupt their contents completely to the surface or at the base of the crust where rising magmas would have been neutrally buoyant as the density structure of vesta s crust evolved deep intrusions would have had lateral extents up to 30 km and thicknesses up to 3 m shallow intrusions would have had vertical extents less than 10 km and widths of 1 m none of these intrusions and surface flows would have been thick enough or have formed frequently enough at a given location to have caused extensive thermal alteration of earlier eruptives lending support to the idea that burial and subsequent long term exposure to the rising geothermal gradient was the main mechanism of thermal metamorphism

mechanical response of the south flank of kilauea volcano hawaii to intrusive events along the rift systems abstract increased earthquake activity and compression of the south flank of kilauea volcano hawaii have been recognized by previous investigators to accompany rift intrusions we further detail the temporal and spatial changes in earthquake rates and ground strain along the south flank induced by six major rift intrusions which occurred between december 1971 and january 1981 the seismic response of the south flank to individual rift intrusions is immediate the increased rate of earthquake activity lasts from 1 to 4 weeks horizontal strain measurements indicate that compression of the south flank usually accompanies rift intrusions and eruptions emplacement of an intrusion at a depth greater than about 4 km such as the june 1982 southwest rift intrusion however results in a slight extension of the subaerial portion of the south flank horizontal strain measurements along the south flank are used to locate the january 1983 east rift intrusion which resulted in eruptive activity the intrusion is modeled as a vertical rectangular sheet with constant displacement perpendicular to the plane of the sheet this model suggests that the intrusive body that compressed the south flank in january 1983 extended from the surface to about 2 4 km depth and was aligned along a strike of n66 e the intrusion is approximately 11 km in length extended beyond the january 1983 eruptive fissures which are 8 km in length and is contained within the 14 km long region of shallow rift earthquakes

clast sizes of ejecta from explosive eruptions on asteroids implications for the fate of the basaltic products of differentiation we use a simple model of the formation growth coalescence and migration of veins of basaltic melt generated by partial melting in chondritic asteroids to deduce the sizes of and pressures within the fluid filled dikes reaching the surfaces of such bodies the gas contents 1000 ppm of mainly co and n2 of the asteroids were high enough that bubbles of free gas trapped in the melt veins gave the basaltic melts significant buoyancy expansion of these gases as a dike opened to the vacuum at the surface led to fragmentation of the melts into liquid droplets which were transported upwards by the accelerating gases to the surface the sizes of these droplets and hence of the pyroclastic glass beads into which they cooled are calculated to lie in the range 30 m to 4 mm this range is essentially independent of the size or gas content of the asteroid parent and only weakly dependent on the internal pressure of the erupting fluid the fate of the pyroclasts however does depend on all of these factors at very low internal pressures significant separation of the gas and liquid in a rising dike may take place and not all of the liquid will be expelled from the dike when it opens to the surface for relatively large 100 km radius asteroids with relatively low 300 ppm gas contents the larger clasts are too heavy to be lifted from the level at which magma fragmentation takes place by the gas flow and so would also remain behind to form basaltic veins the apparent absence of basaltic veins in meteorites then implies both that internal pressures in near surface dikes were generally greater than 0 3 mpa and that low gas contents were not common finally as long as pyroclasts are lofted from the magma fragmentation level they will be accelerated to at least 90 of the final gas speed if this speed exceeds the escape speed from the asteroid as happens readily for high gas contents and small asteroids the pyroclasts will be expelled into space and lost from the meteorite record otherwise low gas contents or large asteroids they will eventually fall back to be incorporated into the surface regolith modifying the chemical and physical properties of meteorites subsequently derived from it

lunar floor fractured craters as magmatic intrusions geometry modes of emplacement associated tectonic and volcanic features and implications for gravity anomalies lunar floor fractured craters are a class of 170 lunar craters with anomalously shallow fractured floors two end member processes have been proposed for the floor formation viscous relaxation and subcrater magmatic intrusion and sill formation recent morphometric analysis with new lunar reconnaissance orbiter laser altimeter lola and image lroc data supports an origin related to shallow magmatic intrusion and uplift we find that the distribution and characteristics of the ffc population correlates strongly with crustal thickness and the predicted frequency distribution of overpressurization values of magmatic dikes for a typical nearside lunar crustal thickness dikes with high overpressurization values favor surface effusive eruptions medium values favor intrusion and sill formation and low values favor formation of solidified dikes concentrated lower in the crust we develop a model for this process make predictions for the morphologic morphometric volcanic and geophysical consequences of the process and then compare these predictions with the population of observed floor fractured craters in our model the process of magmatic intrusion and sill formation begins when a dike propagates vertically towards the surface as the dike encounters the underdense brecciated region beneath the crater the magmatic driving pressure is insufficient to continue vertical propagation but pressure in the stalled dike exceeds the local lithostatic pressure the dike then begins to propagate laterally forming a sill which does not propagate past the crater floor region because increased overburden pressure from the crater wall and rim crest pinch off the dike at this boundary the sill then continues to inflate further raising and fracturing the brittle crater floor when the intrusion diameter to intrusion depth ratio is smaller than a critical value the intrusion assumes a laccolith shape with a domed central region when the ratio exceeds a critical value the intrusion concentrates bending primarily at the periphery resulting in a flat tabular intrusion we predict that this process will result in concentric fractures over the region of greatest bending this location is close to the crater wall in large flat floored craters as observed in the crater humboldt and interior to the crater over the domed floor in smaller craters as observed in the crater vitello a variety of volcanic features are predicted to be associated with the solidification and degassing of the intrusion these include 1 surface lava flows associated with concentric fractures e g in the crater humboldt 2 vents with no associated pyroclastic material from the deflation of under pressurized magmatic foam e g the crater damoiseau and 3 vents with associated pyroclastic deposits from vulcanian eruptions of highly pressurized magmatic foam e g the crater alphonsus the intrusion of basaltic magma beneath the crater is predicted to contribute a positive component to the bouguer gravity anomaly we assess the predicted bouguer anomalies associated with ffcs and outline a process for their future interpretation we conclude that our proposed mechanism serves as a viable formation process for ffcs and accurately predicts numerous morphologic morphometric and geophysical features associated with ffcs these predictions can be further tested using grail gravity recovery and interior laboratory data

inverse modeling for estimating fluid overpressure distributions and stress intensity factors from an arbitrary open fracture geometry abstract we present a new solution for estimating the fluid overpressure driving pressure or net pressure acting on the walls of a fracture with an arbitrary opening displacement in the paper we first present a forward modeling solution using fourier cosine series for the opening displacement of a fracture subject to an overpressure that varies irregularly along the length of a fracture by changing the form of the solution we provide a matrix equation for estimating the fourier coefficients and thereby obtain the overpressure variation from the fracture geometry as numerical tests of this inverse analysis we estimated the overpressure variation from fracture opening displacements given by well known analytical solutions and found that this method can be used for overpressure estimates for a variety of fluid driven fractures we apply our solutions to a mineral vein hosted by gneiss west norway and conclude from the aperture variation that at the time of vein formation the overpressure increased toward the vein tip we also discuss the physical meaning of the fourier coefficients by applying our results to man made hydraulic wing fractures the results indicate that the coefficients of n 0 and n 1 relate to the fluid overpressure and the critical stress intensity factor

magma supply and storage at kilauea volcano hawaii 1956 1983 abstract shallow crustal magma reservoirs beneath the summit of kilauea volcano and within its rift zones are linked in such a way that the magma supply to each can be estimated from the rate of ground deformation at the volcano s summit our model builds on the well documented pattern of summit inflation as magma accumulates in a shallow summit reservoir followed by deflation as magma is discharged to the surface or into the rift zones magma supply to the summit reservoir is thus proportional to summit uplift and supply to the rift zones is proportional to summit subsidence the average proportionality constant is 0 33 10 6 m 3 rad this model yields minimum supply estimates because it does not account for magma which escapes detection by moving passively through the summit reservoir or directly into the rift zones calculations suggest that magma was supplied to kilauea during july 1956 april 1983 at a minimum average rate of 7 2 10 6 m3 month roughly 35 of the net supply was extruded the rest remains stored within the volcano s east rift zone 55 and southwest rift zone 10 periods of relatively rapid supply were associated with the large kapoho eruption in 1960 and the sustained mauna ulu eruptions in 1969 1971 and 1972 1974 bursts of harmonic tremor from the mantle beneath kilauea were also unusually energetic during 1968 1975 suggesting a close link between kilauea s deep magma supply region and shallow storage reservoirs it remains unclear whether pulses in magma supply from depth give rise to corresponding increases in shallow supply or if instead unloading of a delicately balanced magma transport system during large eruptions or intrusions triggers more rapid ascent from a relatively constant mantle source

the role of vegetation and bed level fluctuations in the process of channel narrowing abstract a catastrophic flood ire 1965 on plum creek a perennial sandbed stream in the western great plains removed most of the bottomland vegetation and transformed the single thalweg stream into a wider braided channel following eight years of further widening associated with minor high flows a process of channel narrowing began in 1973 narrowing continues today the history of channel narrowing was reconstructed by counting the annual rings of 129 trees and shrubs along a 5 km reach of plum creek near louviers colorado sixty three of these plants were excavated in order to determine the age and elevation of the germination point the reconstructed record of channel change was verified from historical aerial photographs and then compared to sediment stratigraphy and records of discharge and bed elevation from a streamflow gaging station in the study reach channel narrowing at plum creel occurs in two ways first during periods of high flow sand and fine gravel are delivered to the channel temporarily raising the general bed level subsequently several years of uninterrupted low flows incise a narrower channel second during years of low flow vegetation becomes established on the subaerial part of the present channel bed in both cases surfaces stabilize as a result of vegetation growth and vertical accretion of sediment

modeling the three dimensional structure of macroscopic magma transport systems application to kilauea volcano hawaii we report the results of modeling the three dimensional internal structure of kilauea s magmatic passageways the approach uses a clear plexiglass model containing equally spaced levels upon which well located seismic hypocenters are plotted application of constraining geologic and geophysical criteria to this distributed volume of earthquakes permits the interpretation of seismic structures produced by fracturing in response to locally high fluid pressures four magma transport and storage structures have been identified within and beneath kilauea 1 primary conduit the conduit transporting magma into kilauea s summit storage reservoir rises from the model base 14 6 km to the 6 5 km depth level it is a zone of intense fracturing and inferred intrusion whose horizontal sections are elliptical in planform over its height the average major axis of the component horizontal sections is 3 3 km with an average minor axis of 1 7 km this yields an aspect ratio of 0 52 at the 14 6 km level the strike of the major axis is n67 e during passage from the upper mantle through the oceanic crust this axis rotates in a right handed sense until the strike is n41 w at the 6 5 km level 2 magma chamber complex floor the interval from 6 5 to 5 7 km immediately over the primary conduit is aseismic this suggests differentially high fluid to rock ratios and relatively weak pathways for further vertical transport into higher levels of the storage complex as well as lateral leakage eastward into the mauna ulu staging area for later vertical ascent beneath the upper east rift zone seismicity within the immediately subjacent rocks that form the top of the primary conduit at 6 5 km suggests that this inferred magma rich horizon forms the effective floor of the summit storage complex 3 magma chamber crown intense seismicity over the 1 1 1 9 km depth interval defines an elliptical region in plan view the top of this region has a broad apex with an average major axis of 1 5 km and an average minor axis of 1 1 km producing an aspect ratio of 0 73 this region coincides with the epicentral position of summit vertical displacement maxima during inflationary deformation and is interpreted as a region of intense diking crowning the summit storage complex 4 upper east rift zone pipe from a depth of 5 7 km a cylindrical region of seismicity rises beneath the upper east rift zone to the 1 9 km level where it merges with the subhorizontal upper east rift zone duct this pipe like zone has a mean diameter of 1 4 km and a vertical axis that pierces the surface of the volcano at the intersection of the koae fracture system and the upper east rift zone we suggest this pipe forms a principal connection between the base of the summit storage reservoir and near surface storage compartments and ducts in the upper east rift zone in this role the pipe is suggested to have contributed heavily to mauna ulu s magma supply during 1969 1974 the model of magma transport and storage thus constructed provides a context in which other types of data e g ground deformation and electrical conductivity may be interpreted as well as a means of refining petrological inferences based on lava chemistry

geochemical evidence for invasion of kilauea s plumbing system by mauna loa magma from the beginning of the study of hawaiian volcanism there has been controversy over possible relationships between the neighbouring active volcanoes mauna loa and kilauea1 5 seismic activity thought to reflect upward migration of magma reveals that the magmatic plumbing systems apparently converge at depth to form a broad funnel within the mantle6 although on rare occasions they have erupted concurrently the brief historical eruptive record appears to show that when kilauea is most active mauna loa is in repose and vice versa suggesting that they may be competing for the same magma supply5 7 petrological geochemical and isotope data however require a diametrically opposite conclusion distinct differences in major element trace element and isotope compositions of lavas are regarded as compelling evidence that the two volcanoes have separate magmatic plumbing systems supplied by parental magmas from physically and geochemically distinct mantle sources8 13 here we present preliminary geochemical data which show that in the past 2 000 years kilauea has erupted a spectrum of lava compositions resembling historical kilauea lavas at one end and mauna loa lavas at the other we discuss the cause of this diversity and speculate that magma from mauna loa may have invaded kilauea s high level magmatic plumbing system

magma reservoirs and neutral buoyancy zones on venus implications for the formation and evolution of volcanic landforms we examine the production of magma reservoirs and neutral buoyancy zones nbzs on venus and the implications of their development for the formation and evolution of volcanic landforms the high atmospheric pressure on venus reduces volatile exsolution and generally serves to inhibit the formation of nbzs and shallow magma reservoirs for a range of common terrestrial magma volatile contents magma ascending and erupting near or below mean planetary radius mpr should not stall at shallow magma reservoirs such eruptions would be characterized by relatively high total volumes and effusion rates for the same range of volatile contents at 2 km above mpr about half of the cases result in direct ascent of magma to the surface and half in the production of neutral buoyancy zones in general neutral buoyancy zones and shallow magma reservoirs begin to appear as gas content increases and are nominally shallower on venus than on earth for a fixed volatile content nbzs become deeper with increasing elevation over the range of elevations treated here 1 km to 4 4 km depths differ by a factor of 2 4 these analyses reveal several factors that may help to account for the low height of volcanoes on venus larger primary reservoirs cause the wide dispersal of conduits building edifices models of the position of the shallow nbz reservoir during edifice growth show that for earth the magma chamber center remains at a constant depth below the growing edifice summit thus keeping pace with the increasing elevation while on venus the chamber center becomes deeper relative to the summit of the growing edifice because of the major change in atmospheric pressure as a function of altitude therefore neutral buoyancy zones and magma reservoirs on venus will remain in the prevolcano substrate longer and in many cases may not emerge into the edifice at all the lower rate of vertical migration implies that magma reservoirs would tend to stabilize undergo greater lateral growth and become larger on venus than earth the proportion of the available magma going into production of the edifice relative to that intruded into the substrate is smaller on venus than earth large reservoirs would encourage multiple and more widely dispersed source vents and large volumes for individual eruptions in large reservoirs positively buoyant materials are likely to be produced from differentiation substrate remelting and volatile exsolution nonbuoyant materials exsolving volatiles in a shallow reservoir will need higher gas bubble concentrations to produce eruptions than on earth and when this gas enriched melt emerges at the surface it is more likely to retain its bubbles than to undergo explosive disruption due to the high surface atmospheric pressure therefore there is the potential for the production of a range of erupted lavas that have very high gas bubble concentrations leading to anomalous more viscous rheological properties inhibition of disruption of volatile rich magma for both basaltic and more evolved compositions can lead to the production of 1 more bubble rich vesicular flows characterized by higher viscosity and greater thicknesses the venus equivalent of effusions that would have lost much of their volatiles in the fire fountaining process on earth 2 high volume high discharge rate extremely gas rich basaltic effusions that increase viscosity considerably upon surface extrusion and produce domes whose volumes are considerably higher than their dense rock equivalent and 3 more evolved compositions that are gas rich but do not undergo disruption upon effusion and produce domes and flows of high viscosity the venus equivalent of terrestrial ignimbrites on the basis of these analyses we predict that intrusions and dike emplacement should be common particularly around both shallow and deep magma reservoirs and the tops of dikes should be manifested as radial fractures there should be a range of diameters of circular features associated with the presence of both shallow nbz related and deep plumerelated magma reservoirs large volume lava outpourings should be favored at lower elevations and shallow reservoirs and edifices should be more common at intermediate elevations at the highest elevations magma reservoirs are predicted to be large and relatively deep with low associated edifices direct ascent of magma where neutral buoyancy zones are unlikely low elevations and the propensity for relatively larger magma reservoirs on venus when they do form are both factors which will tend to favor high volume eruptions independent of variations in magma temperature or chemistry individual volcanic features should be interpreted in terms of regional geologic setting and the dynamics of elevation changes commonly associated with various scales of mantle processes and magma emplacement in order to assess global crustal growth processes on venus

mechanisms of magma eruption and emplacement at mt etna between 1989 and 1992 the present eruption sequence of mt etna began during september 1989 with extensive fracturing and culminated in 1991 93 with the largest lava eruption this century1 here we present measurements of gravity and ground deformation which in conjunction with seismic data provide a detailed record of the processes occurring before and during eruption these reflect a complex interplay between magma injection into the paths of lowest compressive stress through the volcano flanks forcing elastic deformation and fracture growth in response to gravitational stresses we find that after the end of the eruptive activity in 1989 fractures in the south southeast flank of the volcano remained unfilled until some months before the 1991 93 eruption these results indicate the utility of microgravity monitoring as the precursory processes that we identify were largely undetected by seismic and ground deformation monitoring

effect of host rock rheology on dyke shape thickness and magma overpressure the size and thickness of dykes is of fundamental importance for volcano dynamics because dykes are the primary path for magma transport and because large numbers of dykes often comprise a major p

variations in tilt rate and harmonic tremor amplitude during the january august 1983 east rift eruptions of kilauea volcano hawaii abstract during january august 1983 a network of telemetered tiltmeters and seismometers recorded detailed temporal changes associated with seven major eruptive phases along the east rift of kilauea volcano hawaii each eruptive phase was accompanied by subsidence of the summit region and followed by reinflation of the summit to approximately the same level before renewal of eruptive activity the cyclic summit tilt pattern and the absence of measurable tilt changes near the eruptive site suggest that conditions in the summit region controlled the timing of the last six eruptive phases the rate of summit subsidence progressively increased from one eruptive phase to the next during the last six phases the amplitude of harmonic tremor increased during the last four phases the increases in subsidence rate and in tremor amplitude suggest that frequent periods of magma movement have reduced the flow resistance of the conduit system between the summit and the rift zone

mechanical energy balance and apparent fracture toughness for dykes in elastoplastic host rock with large scale yielding the dynamics of dyke emplacement are typically modelled by assuming an elastic rheology for the host rock however the resulting stress field predicts significant shear failure in the region surro

growth and persistence of hawaiian volcanic rift zones hawaiian volcanic rift zones are modeled by representing the rifts and adjacent volcano flanks as long ridges with the geometry of flattened triangular prisms the intrusion of dikes along the axis of a rift requires a mechanism to generate the appropriate dike trapping stress field within the prism possible factors that affect the state of stress in the prism include multiple dike intrusion along the ridge axis faulting and gravitational sagging of the topography in extreme models with very steep slopes and high poisson s ratio corresponding to the gelatin models of rift zones by fiske and jackson 1972 results of finite element calculations indicate that gravity induced stresses are sufficient to trap a dike into propagating within the prism and parallel to the rift zone as proposed by fiske and jackson however the mechanism does not work for gently sloping flanks or a more acceptable poisson s ratio of about 0 25 additionally trapping stresses in the gravity loading and density stratification models will not persist after a few dike injection episodes therefore in mature hawaiian rift zones with possibly thousands of dikes additional processes must act to control the stresses that permit continued dike intrusion and rift persistence it is proposed that accommodation to dike emplacement occurs by slip on deep faults possibly of the type proposed for the 1975 kalapana hawaii earthquake as suggested by others for this earthquake the faults could coincide with the contact of the volcano with the seafloor within the weak seafloor sediments such faulting not only provides a means for the flanks to adjust continuously to intrusions but also generates the stress patterns needed to constrain future dikes to propagate along the rift axis other possible faulting mechanisms such as shallow gravity slides and normal faulting of the flanks do not appear to favor rift zone persistence in this model the horizontal stress generated by a standing column of magma at the time of dike emplacement the stresses in the ridges and the fault strength are coupled this results in a feedback between the maximum height that magma can rise along the rift fault friction and fault width through this feedback the slope of the volcano flank is controlled by the fault friction when applied to kilauea volcano the model yields an estimate for the coefficient of fault friction as high as 0 39 assuming normal hydrostatic pore fluid pressure an implication of this model supported by other studies is that rift intrusion and lateral spreading could be major contributors to volcano growth

the heartbeat of the volcano the discovery of episodic activity at prometheus on io the temporal signature of thermal emission from a volcano is a valuable clue to the processes taking place both at and beneath the surface the galileo near infrared mapping spectrometer nims observed the volcano prometheus on the jovian moon io on multiple occasions between 1996 and 2002 the 5 micron m brightness of this volcano shows considerable variation from orbit to orbit prometheus exhibits increases in thermal emission that indicate episodic though non periodic effusive activity in a manner akin to the current pu u o o kupaianaha afterwards referred to as the pu u o o eruption of kilauea hawai i the volume of material erupted during one prometheus eruption episode defined as the interval from minimum thermal emission to peak and back to minimum from 6 november 1996 to 7 may 1997 is estimated to be 0 8 km3 with a peak instantaneous volumetric flux effusion rate of 140 m3 s 1 and an averaged volumetric flux eruption rate of 49 m3 s 1 these quantities are used to model subsurface structure magma storage and magma supply mechanisms and likely magma chamber depth prometheus appears to be supplied by magma from a relatively shallow magma chamber with a roof at a minimum depth of 2 3 km and a maximum depth of 14 km this is a much shallower depth range than sources of supply proposed for explosive possibly ultramafic eruptions at pillan and tvashtar as prometheus type effusive activity is widespread on io shallow magma chambers containing magma of basaltic or near basaltic composition and density may be common this analysis strengthens the analogy between prometheus and pu u o o at least in terms of eruption style even though the style of eruption appears to be similar effusive emplacement of thin insulated compound pahoehoe flows the scale of activity at prometheus greatly exceeds current activity at pu u o o in terms of volume erupted area covered and magma flux whereas the estimated magma chamber at prometheus dwarfs the pu u o o magma chamber it fits within expectations if the pu u o o chamber were scaled for the greater volumetric flux and lower gravity of io recent volumetric eruption rates derived from galileo data for prometheus were considerably smaller than the rate that produced the extensive flows formed in the 17 years between the voyager and galileo missions these smaller eruption rates coupled with the fact that flows are not expanding laterally may mean that the immediate heat source that generates the prometheus plume is simultaneously running out of available volatiles and the thermal energy that drives mobilization of volatiles this raises the question of whether the current prometheus eruption is in its last throes

the role of rift zone storage in controlling the site and timing of eruptions and intrusions of kilauea volcano hawaii repeated eruptive and intrusive activity on the east rift zone of kilauea volcano allows dikes to exist within the rift zone for extended periods of time leakage of magma from the summit storage region into the rift zone can cause inflation of a rift zone dike inflation raises the excess pressure at the dike centre and increases the stress intensity at the dike edges if the inflation is sufficiently large the stress intensity may exceed the fracture toughness of the country rocks and allow vertical or lateral dike emplacement to occur theoretical modelling can be used to predict the conditions under which vertical or lateral dike propagation and hence eruptive or intrusive activity may occur over typical ranges of dike half height and regional tension gradient vertical propagation and hence eruption are favored by high fracture toughness relatively low dike half heights and low regional tension gradients lateral propagation can occur when the fracture toughness is low when dike half height is higher and when the regional tension gradient is high the results of from the model constrain the fracture toughness within the east rift zone to be 100 110 mpa m and the regional tension gradient to be 1000 pa m

thermal evolution and physics of melt extraction on the ureilite parent body we develop a physical model of the thermal history of the ureilite parent body upb that numerically tracks the history of its heating hydration dehydration partial melting and smelting as a function of its formation time and the initial values of its composition formation temperature and water ice content petrologic and chemical data from the main group non polymict ureilite meteorites which sample the interior of the upb between depths corresponding to pressures in the range 3 10 mpa are used to constrain the model we find that to achieve the 30 melting inferred for ureilites from all sampled depths the upb must have had a radius between 80 and 130 km and must have accreted about 0 55 ma after cai formation melting began in the body at 1 ma after cai and the time at which 30 melting was reached varied with depth in the asteroid but was always between 4 5 and 5 8 ma after cai the total rate at which melt was produced in the upb varied from more than 100 m 3 s 1 in the very early stages of melting at 1 ma after cai to 5m 3 s 1 between 2 and 3 ma after cai decreasing to extremely small values as the end of melting was approached beyond 5 ma although the initial period of high melt

lava fountains from the 1999 tvashtar catena fissure eruption on io implications for dike emplacement mechanisms eruption rates and crustal structure the first direct evidence that patterns of basaltic eruptive activity may be very similar on io and earth was seen on november 26 1999 when the solid state imaging system on board the galileo spacecraft obtained high resolution images of a fissure eruption taking place in one of the calderas of the tvashtar catena complex we analyzed the dynamics of the fissure eruption from the morphology of the lava fountains and pyroclastic deposits and derived an estimate of the eruption rate our best estimate of the volume eruption rate per unit length of fissure lies in the range 0 7 to 7 m3 s 1 m 1 and multiplying this rate by the fissure length of 25 km yields a total volume eruption rate of magma of 2 104 to 2 105 m3 s 1 the upper end of the inferred eruption rate is very close to the upper end of the range of eruption rates observed or inferred for fissure eruptions on earth theoretical models of the possible dike systems feeding the fissure show that for eruptions of positively buoyant magma the elastic properties of rocks are such that a magma flux of 6 to 7 m3 s 1 m 1 is the largest value that can ever be realized in such eruptions this limiting value being independent of planetary gravity and thus the same on all silicate bodies the eruption rate inferred for tvashtar can also be achieved by a negatively buoyant magma but in that case the source depth cannot be much greater than a few tens of kilometers we find that some eruptive events are consistent with the effusion of magmas which are buoyant relative to the crust whereas others appear to involve magmas which are not this implies that the distribution of crustal volatiles is laterally very inhomogeneous our analyses suggest that the depths from which magmas make their final ascent to the surface of io are generally of order tens rather than hundreds of kilometers

nature of local magma storage zones and geometry of conduit systems below basaltic eruption sites pu u o o kilauea east rift hawaii example the fluid dynamics of the well documented eruptive episodes at pu u o o kilauea wolfe et at 1987 are used to investigate quantitativeely the size and shape of the shallow conduit system beneath the vent cooling calculations are employed to study the long term survival of conduits and the consequences of multiple dike injection events we find that the subvent conduit must have a planar geometry at depths greater than a few tens to at most a few hundreds of meters with a width of less than a few meters a length of the order of 100 m and a height of the order of 1 kin this structure is clearly the residue of the preeruption dikes although such a feature can be widened somewhat by repeated dike emplacement events there is no evidence to suggest that a larger and much more equant magma reservoir should develop at shallow depth the extensive degassing often occurring after single eruptions or between repeated eruptions previously thought to imply the presence of a large equidimensional shallow magma chamber can be readily explained by the volume of magma in a dikelike subvent planar magma storage zone on the basis of these observations which we infer to apply commonly to basaltic eruption sites we suggest that the terms chamber or reservoir be used with caution because they most often connote a relatively large equant magma body distinct from the feeder dike lying just below the vent we find that the subvent region has the same basic geometric characteristics as the parental dike and we propose that it be referred to as a planar magma storage zone

density structure of io and the migration of magma through its lithosphere we model the density structure of the outer several tens of kilometers of io on the basis of the assumption that condensed volatiles mainly so2 exist at shallow depths in a mafic to ultramafic lithosphere which has a finite porosity due to its accumulation as a mixture of pyroclastic and effusive volcanic deposits which include condensed volatiles the porosity decreases with increasing depth due to pressure driven compaction and temperature driven melting of the volatiles and this controls the bulk density structure of the lithosphere we also model the densities of ascending magmas as a function the types and amounts of volatiles that are available in their source regions the relative densities of the accumulated volcanic deposits and of the magmas rising through them control whether the magmas will erupt at the surface or will stall to form intrusions which can grow into magma reservoirs we find that the depths at which magma reservoirs may exist are strongly influenced by the void space fraction in surface deposits and only weakly controlled by the mass fraction of so2 in the deposits and the mass fraction of so2 in the magmas once nucleated a reservoir has the potential to grow to a very large vertical extent 15 km total height before the magma immediately beneath its roof is again buoyant relative to the surrounding rocks the presence of large volume reservoirs may explain the great extent of the larger lava flow fields seen on io and the longevity of some eruptions the presence of very soluble volatiles such as water in magmas would have virtually no effect on these findings because these volatiles would influence the densities of magmas only very near the surface though they would even in very small amounts then have a dramatic effect on the explosivities of eruptions

triggering basaltic volcanic eruptions by bubble melt separation understanding the processes by which volcanic eruptions are triggered is crucial for volcanic hazard prediction and assessment in intermediate and silicic magmas where water is the dominant volatile phase1 2 second boiling provides an effective eruption trigger1 as the magma cools and crystallizes volatiles are increasingly concentrated in the residual melt where they eventually become saturated and are exsolved thereby raising the magma pressure to the point of eruption1 2 in contrast in basaltic magma chambers in which carbon dioxide is the dominant volatile phase the effect of second boiling is to decrease the chamber pressure but basaltic magmas are also relatively inviscid so volatile bubbles can separate from the turbulently convecting melt to produce a foam layer at the chamber roof3 4 decompression of the rising bubbles can increase the chamber pressure and so trigger an eruption5 6 here we model the complex competition between the processes of magma cooling and bubble ascent we find that in basalt of very low viscosity 1 10pas bubble separation dominates and can increase the reservoir pressure by as much as 10 mpa sufficient to trigger an eruption7 on timescales of 1 10 years comparable to the duration of eruptive episodes at kilauea volcano hawaii4 8