upper crustal seismic velocity structure and microearthquake depths at the endeavour segment juan de fuca ridge 1 we present the results of a study to invert microearthquake and explosive shot data from the endeavour segment of the intermediate spreading juan de fuca ridge the average isotropic p wave velocity structure derived from the shot data in the uppermost 1 5 km of the oceanic crust is characterized by an increase with age of 8 from the axis to at least 0 5 ma that is attributed to the sealing of layer 2a porosity by hydrothermal processes superimposed on this variation are axis parallel 2 km wide alternating bands of high and low velocity with a peak to peak variation of 5 12 high and low velocities away from the axis correspond to bathymetric trenches and ridges respectively and are likely due to variations in layer 2a thickness p wave azimuthal anisotropy is present in the data that is best fit with a model of 9 anisotropy at 750 m depth decreasing to 1 at 3 km depth and is likely due to the preferential alignment of vertical cracks and fissures in the along axis direction anisotropy and velocity heterogeneity are coupled anisotropy alone may explain the form but not the magnitude of the axis parallel bands there are strong trade offs between the hypocentral depths of microearthquakes and the p and s wave velocity structures changing the mean hypocentral depth by up to 0 5 km leads to only modest increases in the travel time rms but the resulting velocity models appear more feasible when the earthquakes are forced deeper than when they are forced shallower

volcanic investigations of the puna ridge hawai i relations of lava flow morphologies and underlying slopes abstract previous investigators have demonstrated that submarine lava flow morphologies are the result of a complex interplay between lava rheology effusion rate and underlying slope here we specifically investigate the relation between underlying slope and submarine lava flow morphology along three study sites of the puna ridge kilauea volcano hawai i using electronic still camera esc images and detailed bathymetry collected by argo ii we can compare the lava morphology and the associated underlying slope our results indicate that rubble is most common on the steepest slopes 25 and that pillowed and lobate flows dominate on slopes 20 sheet flows were only seen on relatively shallow slopes 15 these observations disagree with the results obtained from previous laboratory analog experiments suggesting that the laboratory results are directly applicable for lavas emplaced on relatively low

the cleft revealed geologic magnetic and morphologic evidence for construction of upper oceanic crust along the southern juan de fuca ridge 1 the geology and structure of the cleft segment of the southern juan de fuca ridge jdfr have been examined using high resolution mapping systems observations by remotely operated vehicle rov rov mounted magnetometer and the geochemical analysis of recovered lavas bathymetric mapping using multibeam em300 coupled with in situ observations that focused on near axis and flank regions provides a detailed picture of 0 to 400 ka upper crust created at the southern terminus of the jdfr a total of 53 rock cores and 276 precisely located rock or glass samples were collected during three cruises that included sixteen rov dives our observations of the seafloor during these dives suggest that many of the unfaulted and extensive lava flows that comprise and or cap the prominent ridges that flank the axial valley emanate from ridge parallel faults and fissures that formed in the highly tectonized zone that forms the walls of the axial valley the geochemically evolved and heterogeneous nature of these near axis and flank eruptions is consistent with an origin within the cooler distal edges of a crustal magma chamber or mush zone in contrast the most recent axial eruptions are more primitive higher mgo chemically homogeneous lobate sheet and massive flows that generate a distinct magnetic high over the axial valley we suggest that the syntectonic capping volcanics observed off axis were erupted from near axis and flank fissures and created a thickened extrusive layer as suggested by the magnetic and seismic data this model suggests that many of the lavas that comprise the elevated ridges that bound the axial valley of the cleft segment were erupted during the collapse of a magmatic cycle not during the robust phase that established a new magmatic cycle

upper crustal seismic structure of the slow spreading mid atlantic ridge 35 n constraints on volcanic emplacement processes 1 the upper crustal seismic structure of the slow spreading mid atlantic ridge is studied using a genetic algorithm based waveform inversion of multichannel streamer data four single ship multichannel profiles from 35 n are analyzed one in the rift valley and three in the rift mountains along 0 7 1 6 and 1 9 ma crust a layer 2a horizon is continuously imaged along three profiles and is associated with a sharp velocity increase from extrusives to dikes its depth and regularity in the rift valley indicate that most of the extrusive section is built on the inner valley floor through a pattern of deposition and fault bounded uplift into the rift mountains its variability along one line however shows that this process is disrupted during tectonically dominated periods a thickening of layer 2a toward the oceanographer fracture zone may be the result of along axis magma transport the interval of rapid velocity increase at the base of layer 2a thins with age a possible response to enhanced hydrothermal mineralization within the zone of mixed dikes and extrusives transition zone 200 m and off axis layer 2a thicknesses 350 600 m are similar to those at other spreading centers this indicates that equivalent extrusive volumes are produced at all spreading rates along a relatively narrow zone of dike emplacement however differences in on axis layer 2a thickness between this area and fast spreading ridges suggest that the exact pattern of thickening varies between spreading regimes relative to fast spreading ridges the moderate velocity increase with age recorded in the upper crust from 2 3 to 2 7 km s 1 within 2 myr may be due to a less active hydrothermal system and hence slower porosity reduction

seamount resolution in satellite derived bathymetry 1 i compare satellite derived global relief data on 13 seamounts in the northeast pacific ocean to echosounder derived multibeam values from the gulf of alaska seamount expeditions and the west coast seamounts and ridges multibeam surveys for peak height latitude longitude and midsection area i find smith and sandwell s 1997 global sea floor topography relatively accurate for peak geoposition but generally deeper than multibeam by 192 m 132 m cell size alone can explain 50 90 of peak height variation suggesting higher resolution geodetic altimetry could improve the accuracy of these estimates nevertheless midsection areas overlap by 80 11 the altimetric technique clearly resolves the presence and general form of numerous unnamed deep seamounts though predicted seamount peak height estimates for small features may vary from their true depth by up to 50 these findings support high end estimates of global seamount abundance since small seamount features 1200 m relief revealed by satellite may in actuality be quite substantial features 2500 m relief

the role of seamount volcanism in crustal construction at the mid atlantic ridge 24 30 n an analysis of approximately 6000 km2 of sea beam swaths indicates that the floor of the median valley of the mid atlantic ridge mar between 24 30 n is strewn with near circular volcanoes we have identified 481 seamounts in the height range 50 650 m because many of the features which appear to be seamounts do not meet our criteria fully they have not been counted and our numbers are likely a minimum estimate of the population the large abundance of seamounts on the median valley floor indicates that seamount volcanism plays an important role in the accretionary processes at this section of the mar the summit height distribution of the mar seamount population is consistent with the exponential frequency size distribution model for off axis eastern pacific seamounts jordan et al 1983 smith and jordan 1987 the model parameters for the mar population yield an average of about 195 seamounts per one thousand square kilometers and a characteristic height of about 60 m shape statistics compiled from the 481 seamounts give parameters that are exactly the same or cover the same range in values as those obtained from a study of pacific seamounts in the height range 140 3800 m smith 1988 implying a universal control on seamount construction based on the volcanic morphology we identify 18 spreading segments along the ridge all except two contain a prominent axial volcanic ridge many of the seamounts identified are associated with the axial ridges from the sea beam swaths we infer that the ridges are composed of piled up seamounts and hummocky flows and interpret them to be the primary sites of crustal construction this style of volcanism contrasts strongly with that observed at the east pacific rise where seamounts are virtually absent at the spreading axis the construction of seamounts and hummocky flows may be directly related to the fact that the mar is fed by magma chambers that are limited in size and frequency we suggest that small magma pockets with slow eruption rates produce seamounts either from pipes or initial fissure eruptions that collapse to construct a single edifice small magma bodies with somewhat higher eruption rates produce hummocky fissure fed flows using buoyancy arguments the heights of the mar seamounts are related to the depth of the magma bodies we hypothesize that small magma bodies rise buoyantly to the base of the hydrothermally cooled brittle lid where they are trapped and then erupt if this hypothesis is correct the thickness of the brittle lid controls seamount height and the exponential distribution of seamount summit heights implies that the thickness of the brittle hd follows an exponential distribution in time and space with characteristic thickness 1 4 1 9 km if seamounts and hummocky flows are fed each from their own discrete magma body trapped at the brittle ductile transition then the lower crust of the mar will be made up of the products of a multitude of small plutons

using sims to decode noisy stratigraphic δ13c variations in ediacaran carbonates abstract carbonate carbon isotope 13ccarb chemostratigraphy is a valuable tool in precambrian stratigraphic correlation the effectiveness of this tool rests on the assumption that 13ccarb data record global seawater signals however in some cases 13ccarb data exhibit rapid and noisy stratigraphic variations that may have been influenced by authigenic or diagenetic carbonate minerals to assess the contribution of non primary minerals towards bulk carbonate carbon isotope values we acquired sims secondary ion mass spectrometry 13ccarb data electron microscopic data and cl cathodoluminescence microscopic data from two ediacaran successions the lower member ii of the doushantuo formation in south china and the mooifontein member of the zaris formation in southern namibia the doushantuo samples came from a stratigraphic interval with noisy meter scale 13ccarb variations of up to 10 whereas the mooifontein member is characterized by consistent 13ccarb values with limited meter scale variations of less than 4 our data show that the meter scale stratigraphic variations in 13ccarb are also mirrored in the sims data at m mm scales in both doushantuo and mooifontein samples in the doushantuo samples sims 13ccarb values of authigenic calcite vary by up to 10 over m mm scales and can be either higher or lower than those of the co existing dolomite matrix which also appears to be affected by authigenic carbonate on the basis of petrographic observation bulk sample 13ccarb values measured on powders microdrilled from the same sims specimens are within the range of sims 13ccarb values thus we infer that bulk sample 13ccarb values of doushantuo samples represent mixtures of different carbonate components the doushantuo and mooifontein sims 13ccarb data validate the traditional method of using chemostratigraphic consistency to evaluate authigenic diagenetic alteration but given that secondary phases may be 13c enriched relative to matrix carbonate caution is warranted when taking maximum values to approximate primary chemostratigraphic trends

magnetic anomalies at the puna ridge a submarine extension of kilauea volcano implications for lava deposition high resolution magnetic data collected along the axis and the south flank of the puna ridge the submarine extension of the east rift zone of kilauea volcano hawaii aid in the interpretation of magmatic processes crustal magnetization varies along the crest of the puna ridge three prominent magnetization highs are located between the depths of 1500 and 2500 m each of the magnetization highs is associated with distinct volcanic features a flow field on top of fissured terrain a lava plateau and a staircase of lava terraces the crustal magnetization highs can be explained by the presence of these volcanic features taking into account water depth and the size of the volcanic features and making simple assumptions about the rock magnetization no measurements of rock magnetization have been made on puna ridge samples we assume two values 50 and 10 a m based on values from mid ocean ridge basalts and the estimated ages of puna ridge rocks clague et al 1995 the two shallowest magnetization highs are adjacent to a landslide scarp lava flows overtop the scarp suggesting that a dike has been emplaced and eruptions have occurred since the landslide formed a possible scenario is that the collapse of the flank led to dike intrusion and eruption at this location because of a reduction in the horizontal stress within the edifice our observations suggest that flows are starting to fill in the gap caused by the landslide to rebuild this section of the north flank

fine scale seismic structure of young upper crust at 17 20 s on the fast spreading east pacific rise 1 the detailed upper crustal structure of the east pacific rise epr at 17 20 s is examined by applying a genetic algorithm based waveform inversion to five multichannel seismic lines one on axis and two to either side along 42 and 85 kyr old crust on axis a double stepped velocity pattern is recorded beneath 70 100 m of low velocity extrusives 2 1 2 4 km s 1 we define the upper velocity contrast as the base of seismic layer 2a due to its severity and continuity along and across axis the more subdued and intermittent lower velocity step is not observed off axis material between the two high gradient intervals is proposed to represent the pillow dike transition bounded above by a sharp increase in dike fraction with depth and below by an abrupt change in rheology and or deformation extrusive velocities increase quite rapidly in this area with velocities 3 km s 1 common in crust 85 kyr old this plus a rapid 300 400 m thickening of layer 2a observed within 1 4 km of the rise axis indicates that this segment is undergoing focused melt delivery 500 m wide dike intrusion zone and elevated hydrothermal activity these findings demonstrate the ability of single ship multichannel data to record detailed information on the reflectivity and velocity of the upper crust and the ability of the genetic algorithm to efficiently construct accurate seismic models based on this information

offset caldera and crater collapse on juan de fuca ridge flank volcanoes forty three volcanoes located along the flanks of the juan de fuca ridge were selected to study relationships between their morphologies and off axis magmatic processes the volcanoes occur both in chains consisting of up to seven distinct cones and isolated edifices nearly all of the volcanoes are circular truncated cones with steep flanks and large relatively flat summit plateaus in addition most of these volcanoes also have prominent and distinctly offset calderas or craters the most striking characteristic of the volcanoes morphology is that nearly all of their collapse structures are located on the sides of the volcanoes which face the juan de fuca ridge and many are breached with openings toward the ridge a simple model based on these observations accounts for these ridge facing features as plate motion transports a volcano away from its magma source beneath the lithosphere the volcano s magma supply conduits tend to lag behind eventually these conduits are abandoned and ridgeward collapse structures are formed it can be inferred from the model that on average individual volcanoes were active for approximately 50 000 years and that most eruptions took place early in this interval if most of the cone building eruptions occurred during the first thousand years or so associated hydrothermal activity may have temporarily rivaled the present day yearly time averaged hydrothermal output along the entire juan de fuca ridge axis

solidification and morphology of submarine lavas a dependence on extrusion rate the results of recent laboratory experiments with wax extruded beneath relatively cold water may be extrapolated to predict the surface morphology of submarine lavas as a function of the extrusion rate and melt viscosity the experiments with solidifying wax indicated that the surface morphology was controlled by a single parameter the ratio of the time taken for the surface to solidify and a time scale for lateral flow for submarine basalts a solution of the cooling problem which is dominated by conduction in the lava but convective heat transfer in the water and estimates of lava viscosities place this parameter within the empirically determined pillowing regime over a wide range of extrusion rates this result is consistent with the observation that pillow basalts are the most common products of submarine eruptions smoother surfaces corresponding to the various types of submarine sheet flows are predicted for sufficiently rapid extrusions of basaltic magma still higher eruption rates in regions of low topographic relief may produce submarine lava lakes minimum emplacement times can be calculated for submarine volcanic constructs of a single lava flow type

volcanic eruptions on mid ocean ridges new evidence from the superfast spreading east pacific rise 17 19 s uniform sediment cover were recovered from lava that buries older faulted terrain the boundary in lava composition coincides with a change in depth to the top of an axial magma lens seismic reflector consistent with magmas from two separate reservoirs being erupted in the same event chemical compositions from throughout the area indicate that lavas with identical compositions can be emplaced in separate volcanic eruptions within individual segments a comparison of our results to global data on submarine mid ocean ridge eruptions suggests consistent dependencies of erupted volume activated fissure lengths and chemical heterogeneity with spreading rate consistent with expected eruptive characteristics from ridges with contrasting thermal properties and magma reservoir depths index terms 3035 marine geology and geophysics midocean ridge processes 8414 volcanology eruption mechanisms 8439 volcanology physics and chemistry of magma bodies 3655 mineralogy and petrology major element composition keywords lava flow chemical heterogeneity erupted volume lava morphology side scan sonar

flow localization in fissure eruptions during a basaltic fissure eruption heat transfer from the hot magma to the surrounding rock causes a dramatic increase in the magmatic viscosity and solidification at the margins both viscosity contrast and solidification can amplify initial variations in the flow rate and lead to localization of the flow along the strike of the fissure however for typical parameters amplification driven by solidification is slower and significantly weaker than amplification driven by viscosity variations in fact for the parameters examined the amplification due to solidification is so weak that its effect is almost insignificant whereas viscosity variation provides a strong active mechanism for flow localization laboratory experiments illustrate viscous localization and suggest that this mechanism is robust the dependence of viscosity on temperature can cause a small change in the pressure of the magma chamber to lead to a large jump in the flow rate of magma through the fissure

hundreds of small volcanoes on the median valley floor of the mid atlantic ridge at 24 30 n we have identified 481 seamounts from 6 000 km2 of sea beam swaths of the median valley floor of the mid atlantic ridge mar between 24 and 30 n we find that seamount abundances are an order of magnitude larger on average 80 per 1 000 km2 with relief above 50 m than estimated in previous studies along mid ocean ridges in the pacific1 and south atlantic2 suggesting that at this section of the mar seamount volcanism plays an important part in the formation of new oceanic lithosphere the neovolcanic ridge often observed on the inner valley floor3 appears to be constructed totally of individual coalesced seamounts so that our counts represent a minimum estimate the frequency distribution of summit heights of the mar seamounts is consistent with the exponential model describing the heights of off axis seamounts in the eastern pacific4 6 but with different parameters such a distribution of heights may be a fundamental aspect of seamount volcanism related to lithospheric properties and or magmatic plumbing seamount dominated axial volcanism must create a crustal structure very different from classic fissure fed models

mechanical stratigraphy as a factor controlling the development of a sandbox transfer zone a three dimensional analysis abstract in thrust belts fold fault terminations are common features of the structural architecture and can pose complicated problems to unravel in particular when two or more terminations are in close proximity such terminations usually reflect pre existing attributes amongst the many factors lateral variations in the mechanical stratigraphy can control along strike geometry and kinematics of fault related folds a displacement transfer zone was produced in a compressional sandbox model by means of two adjacent mechanically different stratigraphic domains the experiment allowed two discrete chains to develop in the different domains so that a complex structural setting occurred in the connecting area periclinal folds oblique thrust fronts and oblique ramps developed in the resulting transfer zone the interaction between periclines in the transfer zone produced lateral culminations in the folded structures the analysis of displacement across the structural domains revealed that a significant loss of slip along the faults occurred in the relay zone in this area imbricate faulting was partially replaced by layer parallel shortening a linear relationship appears to exist between the bed length of the thrust sheet and the related fault slip

sustained volcanically hosted venting at ultraslow ridges piccard hydrothermal field mid cayman rise at slow spreading mid ocean ridges sustained submarine venting and the deposition of large seafloor massive sulfide deposits have previously been ascribed to tectonically controlled hydrothermal circulation unrelated to young volcanic activity here by contrast we show that the piccard hydrothermal field phf on the ultraslow spreading mid cayman rise represents a site of sustained fluid flow and sulfide formation hosted in a neovolcanic setting the lateral extent and apparent longevity associated with the phf are comparable to some of the largest tectonically hosted vent sites known along the slow spreading mid atlantic ridge if such systems recur along all ultraslow ridges which comprise 20 of the 55 000 km global ridge crest potential implications would include i a higher probability of locating large economically valuable mineral deposits along ultraslow ridges together with ii larger fluxes than previously anticipated of chemicals released from high temperature venting entering the oceans along the atlantic indian ocean sectors of the deep ocean thermohaline conveyor

eruptive hummocks building blocks of the upper ocean crust the spreading axis at many slow spreading mid ocean ridges is marked by an axial volcanic ridge in this study we use a combination of high resolution remote sensing methods to elucidate the detailed nature of volcanoes in such a ridge we find that the hummocks described in previous sidescan sonar studies are dome or cone shaped edifices 5 150 m high with diameters of 30 330 m we estimate they form quickly in single eruptions each of which may produce several hummocks hummock collapse is common and hummocks of all heights are prone to failure collapses generally occur down the regional seafloor slope suggesting control by local topography approximately 33 of hummocks lose 40 of their volume by collapse so 12 of all material erupted on the axial volcanic ridge is rapidly converted to talus the higher porosity of these deposits may increase average upper crustal porosity by several percent contributing 0 5 km s 1 to seismic velocity decrease in the upper oceanic crust and may be one of the dominant mechanisms for increasing porosity in upper slow spreading oceanic crust

seismic properties of layer 2 basalts summary in this paper the physical properties of oceanic layer 2 basalts are examined with the emphasis on micro crack porosity and alteration related changes in physical properties over 160 core samples from some of the more significant deep sea drilling project ocean drilling program holes including 417d 418a 504b and 801c are included in the present investigation overall physical property relationships are discussed in the first half of this study at 200 mpa confining pressure compressionaland shear wave velocities of the sample suite have modal values of 6 1 kms 1 and 3 5 kms 1 respectively a poisson s ratio of 0 28 characterizes the majority of the basalts samples that have experienced extensive low temperature alteration identified through elevated kzo and decreased cao and mgo contents exhibit low velocities and densities and high poisson s ratios the microcrack porosity in the basalt suite is examined using velocity pressure data a pore aspect ratio inversion scheme and scanning electron microscope sem imaging microcracks sealed by alteration minerals such as smectite and calcite or healed by late crystallization are common throughout the sample set rocks from the transition zone and sheeted dyke section of hole 504b are marked by numerous smectite and chlorite filled cracks more significantly the data indicate an increase of low aspectratio open microcracks with increasing depth in samples from several dsdp holes particularly hole 504b these microcracks also observed using the sem are attributed to stress relief as the original drill cores were removed from in situ pressure conditions stress relief micro cracking is apparently more severe in the holocrystalline coarsergrained samples from massive flows than in the phyric basalts with fine grained groundmasses characteristic of pillow units this difference in micro cracking is explained through basic fracture theory

eruption styles of basalt in oceanic spreading ridges and seamounts effect of magma temperature and viscosity basalts erupted along oceanic zones of spreading and on intraplate seamounts at depths where hydroexplosions are not likely follow various styles of eruption quiet eruptions produce a variety of forms including lava lakes and sheet pillow and pavingstone lavas reactive eruptions occur instead when the lava is thermally shattered as in the formation of hyaloclastites the stresses induced in a rapidly cooling lava flow are important in determining the different lava forms with lava lakes and sheet flows resulting from low stresses pillows from higher stresses and hyaloclastites from even higher stresses which cause rupture calculations show that if we consider basaltic lavas of the same composition the intensity of the stresses induced in the cooling lava increases proportionally to the initial viscosity and inversely to the temperature and rate of discharge of the lava at eruption the viscosity is strongly dependent on the crystal liquid ratio of the magma at eruption the temperature and viscosity of the lava at eruption are related ultimately to the thermal regime of the lithosphere below viscosity tends to increase going from fast to slow spreading ridges and to intraplate seamounts within a given ridge segment temperature tends to decrease and viscosity to increase approaching a transform or overlapping boundary thus the model predicts that going from fast to slow spreading ridges i e from east pacific rise to mid atlantic ridge the probability of forming lava lakes and sheet flows decreases and the probability of forming pillow flows increases moreover within a given ridge segment the areal sheet to pillow ratio should decrease as a transform or overlapping boundary is approached sheet flows should be significant only in the earliest stages of the growth of large intraplate seamounts pillows and hyaloclastites in later stages the probability of forming hyaloclastites should be low in spreading ridges higher in intraplate seamounts recent high resolution observations on the east pacific rise the galapagos rift the mid atlantic ridge and the red sea are consistent with the model because they show that 1 the areal ratio of lava lakes and sheet flows over pillows decreases going from fast to slow spreading ridges 2 within a given ridge segment it decreases approaching a transform and 3 hyaloclastites are important on seamounts only

evolution of volcanism and faulting in a segment of the mid atlantic ridge at 25 n 1 we reconstruct the volcanic and tectonic evolution over the last 250 000 years of the median valley floor in the spreading segment of the mid atlantic ridge centered at 25 n in the center of the segment multibeam bathymetry and deep towed side scan images show a large area of smooth textured lava flows more like those of the east pacific rise than those of the mid atlantic ridge hummocky flows more typical of the mid atlantic ridge are found toward the southern end of the segment the presence of the abundant smooth textured flows allows us to interpret the volcanic and tectonic relationships in the segment we construct a geological map using 1 multibeam bathymetry to identify the key volcanic structures and fault scarps and 2 high resolution tobi side scan sonar images to interpret age relationships between features on the basis of overall sediment cover as shown by backscatter brightness bottom photographs across key features on the median valley floor yield detailed information on stratigraphic relationships between volcanic features and faults and allow us to calibrate backscatter brightness in terms of sediment cover and hence of age in this way we derive a history of volcanic activity and deformation in a detailed survey area at the segment center with the most recent flows erupted about 5000 years ago and the youngest smooth flows about 10 000 years ago separated by an episode of faulting using bathymetry and side scan surveys we extrapolate this to the whole of the median valley floor the volcanic activity giving rise to the smooth flows has been continuous for about a quarter of a million years at the segment center over the same period hummocky flows have been continuously erupted at the southern end of the segment electron probe analyses of dredged basalt glasses show that there is a systematic variation in composition with position in the segment basalts from the segment center are all more evolved than those at the southern end of the segment there is however no relation of chemistry with lava type the basalts from the segment center have very nearly the same composition whether they come from hummocky flows or smooth flows the boundary between the smooth flows and hummocky flows has fluctuated with time and migrated rapidly northward over the last few thousand years so that shortly the eruption of smooth flows will probably have ceased the survey shows that flows that are smooth on side scan images are not necessarily sheet flows in this study they uniformly show pillow morphology we conclude that smooth flows were probably erupted at faster eruption rates than hummocky flows

a comparison of volcanic edifices at the reykjanes ridge and the mid atlantic ridge at 24 30 n a detailed analysis of high resolution side scan sonar data shows that the volcanic edifices built within the axial zones of the slow spreading 20 mm yr reykjanes ridge and the slow spreading 25 mm yr mid atlantic ridge are remarkably similar in fact individual seamounts hummocks rounded mounds and hummocky ridges at the reykjanes ridge have nearly identical morphological counterparts at the mid atlantic ridge mar arguing that commonly eruptive conditions are similar at the two ridges the construction of these volcanic edifices typical of slow spreading ridges occurs at the reykjanes ridge despite its proximity to the iceland hot spot and the influence of the hot spot on other characteristics of the ridge axis such as its overall shape an axial high north of about 59 n its shallow axial depths and its thicker crustal section the volume of magma that goes into constructing volcanic edifices at each of 12 segments of the mar and four regions of the reykjanes ridge is calculated using multibeam bathymetry data our results indicate that edifice volume per unit seafloor area equivalent to a uniform thickness over the area of interest is larger at the mar than at the reykjanes ridge on average taking into account spreading rate we then estimate b the average building rate per unit length along the axis which is also larger at the mar we conclude that the same volcanic edifices build the crust at the reykjanes ridge and mar but that their emplacement is more frequent at the mar to explain our results and reconcile them with those of magde and smith 1995 who documented that seamounts are taller and more abundant at the reykjanes ridge than at the mar we consider the controls on diking and eruption within the axial zones of the two ridges we suggest that the combination of slower spreading rate and the oblique geometry of spreading at the reykjanes ridge leads to fewer eruptions which are more widely dispersed across the axial zone than at the mar if this hypothesis is correct the proportion of seamounts modified by subsequent eruptions will be less this might also explain the taller heights of the seamounts if there is less overlap and infilling between edifices

mid atlantic ridge volcanism from deep towed side scan sonar images 25 29 n abstract we present deep towed side scan sonar mosaics of the inner valley floor of eight spreading segments at the slow spreading mid atlantic ridge between 25 and 29 n an analysis of these images which well resolve features a few tens of meters in size confirms that the multitude of small seamounts with diameters between 0 5 and 3 km identified on the inner valley floor from previously collected multibeam bathymetry data are volcanically constructed moreover these images reveal that these volcanoes have distinct surface morphologies not evident in the coarser resolution multibeam bathymetry maps 83 of the seamounts have a hummocky bulbous morphology the other 17 have a smooth morphology in addition to near circular seamounts small 1 2 km long volcanic ridges are abundant in our study regions and are not in general seen in the bathymetry maps we combine these new morphological data with existing models for the construction of the shallow oceanic crust to obtain a better understanding of the melt delivery system that builds the distinctive seafloor topography at the slow spreading mid atlantic ridge

new insights into mid ocean ridge volcanic processes from the 2005 2006 eruption of the east pacific rise 9 46 n 9 56 n digital seafloor imagery collected on 37 camera tows and alvin dives in which we identify 186 contacts between new and old lava are used to create the most detailed map of a mid ocean ridge mor eruption to date lava flows erupted in 2005 2006 at the east pacific rise epr covered an area of 14 6 km2 along 18 km of the epr crest between 9 46 and 9 56 n the 2005 2006 lava is characterized by inflated lobate and sheet morphologies in the flow interiors and pillow forms at terminal flow fronts numerous lava channels 10 50 m wide and 1 5 m deep trending approximately east west served as distributory pathways eruptions were sourced from fissures within the epr axial summit trough as well as fissures located on an off axis fissure mound 600 m east of the epr axis between 9 52 and 9 56 n portions of the lava flow reached as far as 2 km east of the axis near 9 51 2 n using a conservative estimate of 1 5 m for the average flow thickness implies that the 2005 2006 eruptions produced 22 106 m3 of lava 4 5 times larger than estimated volumes of 1991 1992 epr lava flows estimated lava volume for the 2005 2006 eruptions represents 15 of the magma available in the axial magma chamber

quantification of submarine lava flow morphology through analog experiments we developed a technique for determining paleoeffusion rates and emplacement times for submarine lava flows using observations of flow morphology and estimates of eruption volume eruption temperature lava viscosity and preflow topography using laboratory simulations and correlating these results with sea floor observations four submarine lava flow morphologies are considered to be diagnostic of specific effusion rates jumbled folded and lineated sheets and striated pillows we applied this approach to the coaxial flow emplaced in less than 14 days on the juan de fuca ridge in early summer 1993 and we calculated an effusion rate of 100 m3 s giving an emplacement time of 10 days

paving the seafloor volcanic emplacement processes during the 2005 2006 eruptions at the fast spreading east pacific rise 9 50 n 1 the 2005 2006 eruptions near 9 50 n at the east pacific rise epr marked the first observed repeat eruption at a mid ocean ridge and provided a unique opportunity to deduce the emplacement dynamics of submarine lava flows since these new flows were documented in april 2006 a total of 40 deep towed imaging surveys have been conducted with the woods hole oceanographic institution s whoi towcam system more than 60 000 digital color images and high resolution bathymetric profiles of the 2005 2006 flows from the towcam surveys were analyzed for lava flow morphology and for the presence of kipukas collapse features faults and fissures we use these data to quantify the spatial distributions of lava flow surface morphologies and to investigate how they relate to the physical characteristics of the ridge crest such as seafloor slope and inferred dynamics of flow emplacement we conclude that lava effusion rate was the dominant factor controlling the observed morphological variations in the 2005 2006 flows we also show that effusion rates were higher than in previously studied eruptions at this site and varied systematically along the length of the eruptive fissure this is the first well documented study in which variations in seafloor lava morphology can be directly related to a well documented ridge crest eruption where effusion rate varied significantly

mid ocean ridge eruptive vent morphology and substructure evidence for dike widths eruption rates and evolution of eruptions and axial volcanic ridges high resolution side scan sonar data of mid atlantic ridge mar inner valley floor axial volcanic ridges reveal details of their architectural elements we develop quantitative models for basaltic eruptions from dikes and compare the predicted products of these eruptions with the observed morphologic features inhibition of gas exsolution lack of magma disruption and hydrothermal effects combine to decrease the rise speed of magma in submarine dikes and enhance cooling leading to more rapid centralization of eruptions along the widest places in the dike dikes are predicted to initially feed eruptions from fissure vents producing lines of hummocky ridges centralization of activity to several adjacent vents produces chains of hummocky bulbous mounds and can enhance the effusion rate at a single vent to produce small seamounts the widest dikes are predicted to produce smooth flows up to several kilometers in length which should pond in adjacent lows edifice sizes predicted on the basis of volume fluxes and flow lengths implied by the widths of dikes observed on the seafloor in iceland and in ophiolite complexes are in quantitative agreement with the dimensions of observed features each ridge is made up of the products of a variety of these individual dike emplacement and extrusive events involving various dike widths cooling times and eruption durations on the basis of typical mar spreading rates about one dike emplacement event would be expected every 40 years since not all dikes reach the surface the actual mean interval between eruptions will exceed this mar dikes should solidify between average dike emplacement events dike emplacement events are more frequent on the east pacific rise epr and thus new dikes are more likely to reoccupy the sites of incompletely solidified older dikes differences between the morphology of the mar axial volcanic ridge and the sheet flow dominated epr are attributed to on average wider dikes erupting with greater frequency along the epr

building the crust at the mid atlantic ridge the morphology of the sea floor at mid ocean ridge spreading centres provides a key to understanding how ocean crust is constructed images of the axial zone of the slow spreading mid atlantic ridge obtained at a range of spatial scales show that crustal construction is complex and highly variable reflecting an underlying three dimensional variability in magmatic tectonic and hydrothermal processes

structure and ageing of oceanic crust at 14 s on the east pacific rise a wide angle reflection refraction survey was conducted along a flow line on the eastern flank of the east pacific rise at 14 s the data were acquired with a single 32 litre airgun and ocean bottom hydrophones obh and consist of six profiles on 0 5 8 3 myr old seafloor in total 17 obhs provided seismic records of excellent quality to generate 2 d velocity models traveltimes and amplitudes have been calculated using asymptotic ray theory a comparison of the traveltimes and amplitudes of p wave arrivals on all lines revealed important similarities that correlate well with the classical model of oceanic crust the uppermost crust is composed of a 0 4 0 7 km thick low velocity 2 9 4 6 km s 1 surficial layer layer 2a and is bounded at its base by a sharp velocity increase to 5 5 km s 1 at the top of layer 2b overall layer 2 is 1 4 1 8 km thick small seamounts frequently superposed on fast spreading crust are characterized by velocities lower than within the adjacent upper crust velocities 6 0 km s 1 exist throughout the edifices suggesting that the seamounts are dominantly extrusive decreasing amplitudes beyond 8 10 km offset constrain a moderate gradient lower crust layer 3 with velocities of 6 8 7 2 km s 1 crustal thickness is found to be remarkably uniform at 6 11 0 14 km nevertheless the seismic data provide evidence for age dependent variations in both the low velocity surficial layer and the upper mantle we find that velocities in layer 2a increase from 2 9 to 4 3 km s 1 velocities in the middle and lower crust show uniform values while velocities within the uppermost mantle increase gradually from 7 5 km s 1 to 8 0 km s 1 the most viable explanation for increasing velocities in the upper crust is that off axis hydrothermal circulation was accompanied by the precipitation of secondary minerals in open void spaces thereby decreasing the porosity and increasing the velocity low mantle velocities beneath young crust are suggested to be a temperature controlled phenomenon and may indicate the presence of partial melt in the upper mantle

structure and development of an axial volcanic ridge mid atlantic ridge 45 n we describe the most comprehensive and detailed high resolution survey of an axial volcanic ridge avr ever conducted at 45 n on the mid atlantic ridge we use 3 m resolution sidescan sonar deep towed magnetic field measurements video observations from eleven rov dives and two very high resolution bathymetry and magnetic surveys the most recently active avr has high topographic relief high acoustic backscatter high crustal magnetization and little faulting it is sharp crested 25 4 km in extent and 500 m high and is covered by approximately 8000 volcanic hummocks whose detailed nature is revealed for the first time each is an individual volcano 450 m in diameter and 200 m high ranging from steep sided 45 cones to low domes many have suffered significant flank collapse hummocks tend to align in rows parallel to the avr axis parallel to its ne trending spurs or on its lower flanks sub normal to the avr trend these latter are spaced 1 2 km apart and comprise 1 2 km long rows of single volcanoes we infer that their emplacement is controlled by down flank magma transport possibly via lava tubes the avr contains only one large flat topped seamount the flanking median valley floor consists of either older hummocky volcanic terrain or flat lying mostly sediment covered lavas these typically have low relief lobate surfaces inflation and collapse structures and occasional lava tubes and tumuli the avr displays open fissures mostly along its crest there is direct evidence for only a few small faults on the avr though steep outward facing slopes draped by elongate pillows may be small normal faults covered by lava the surrounding median valley floor is heavily fissured normal faults cut it and an older avr the latter displaying significant outward facing faults high crustal magnetization an approximate proxy for crustal age within the brunhes is confined to the active avr magnetic palaeointensity measurements are consistent with ages up to 12 ka for several samples from the active avr and 12 ka for one sample from the median valley floor this is much less than the predicted spreading age implying distribution of melt off axis or episodic avr growth

axial crustal structure of the lau back arc basin from velocity modeling of multichannel seismic data located west of the tonga trench the lau back arc basin is a prime environment for studying the interplay between oceanic spreading systems and an active subduction zone within the basin lie two complementary intermediate rate spreading systems the central lau spreading center clsc and the eastern lau spreading center valu fa ridge elsc vfr that are positioned 170 to 40 km away from the active volcanic arc respectively multichannel seismic mcs images of both systems reveal systematic variations in axial crustal structure primarily related to the proximity of the volcanic arc but also related to spreading rate morphology and petrology upper crustal refraction data selected from the along axis seismic lines collected during the 1999 mcs survey were modeled in both the time versus range t x and intercept time versus slowness p domains to provide validation and detail to the seismic reflection observations the results show that as both the proximity to the arc and the spreading rate decrease southward 1 seismic layer 2a thickens by 0 6 km between the clsc and vfr from 0 4 km to 1 0 km 2 the average depth of the axial magma chamber amc increases from 1 5 km at the clsc to 2 8 km at the southern vfr excluding the northern section of the elsc that shows no continuous amc reflector but does show an isolated melt sill 3 the upper crustal basement velocity decreases from 2 1 km s at the clsc to 1 8 km s at the vfr and 4 the velocities of both layer 2a and 2b decrease between the clsc and northern vfr from 3 2 to 2 5 km s and 5 0 to 3 9 km s respectively along with a broad axial high morphology these features of the clsc thinner layer 2a shallower amc and faster crustal velocities correlate best with the fast spreading east pacific rise and intermediate spreading juan de fuca ridge conversely the structural characteristics of the central elsc vfr have no known counterpart in the global mid ocean ridge system we attribute this primarily to the volatile concentration in the magmas coming from the tonga subduction zone

attenuation differences in layer 2a in intermediate and slow spreading oceanic crust in situ seismic attenuationq 1logs are derived from borehole velocity profiles and reveal sharp boundaries between morphologies of the extrusive volcanic layers in intermediate and slow spreading oceanic crust q 1logs are calculated from the scattering attenuation associated with vertical velocity heterogeneity in ocean drilling program holes 504b and 896a and in hole 395a located in 5 9 7 3 ma crust on the pacific and atlantic plates respectively our results strongly tie crustal properties to seismic measurables and observed geological structures we find that the scattering attenuation can be used to identify the extrusive volcanic sequence because it is closely related to changes in the degree of vertical heterogeneity we interpret a distinct decrease in the q 1log at the transition below the extrusive volcanic layer to correspond with the seismic layer 2a 2b boundary the boundary is located at 465 m depth below the sea floor in both hole 395a and 504b although this is likely to be a coincidence of the sediment thickness at these sites layer 2a is estimated to be approximately 150 m thick in hole 504b and 300 m thick in hole 395a cyclic sequences of high porosity pillows and low porosity massive units in the uppermost 100 m of volcanics in hole 395a result in large velocity heterogeneities which cause 5 times more attenuation in this layer than in hole 504b in hole 896a by contrast fewer pillows more massive flows and a greater volume of carbonate veins decrease the velocity heterogeneity and attenuation significantly over only 1 km distance from hole 504b we conclude that the attenuation in the extrusive volcanics of the ocean crust is largely controlled by variation in local heterogeneity and morphology as well as by subsequent hydrothermal alteration the observed differences inq 1profiles and layer 2a thickness at these sites may be attributed to variations in the volume and duration of volcanic activity at mid ocean spreading centers for these pacific and atlantic ridge segments

constructing the upper crust of the mid atlantic ridge a reinterpretation based on the puna ridge kilauea volcano the volcanic morphology of a number of segments of the slow spreading mid atlantic ridge mar have been reinterpreted based on our understanding of dike emplacement dike propagation and eruption at the east rift zone of kilauea volcano hawaii and its submarine extension the puna ridge the styles of volcanic eruption at the submarine puna ridge are remarkably similar to those of the axial volcanic ridges avrs constructed on the median valley floor of the mar we use this observation to relate volcanic processes occurring at kilauea volcano to the mar we now consider that volcanic features e g seamounts and lava terraces built on the flanks of the avrs are secondary features that are fed from lava tubes or channels not primary features fed directly from an underlying dike we examine simple models of pipe flow and conclude that lava tubes can transport lava down the flanks of submarine rifts to build all of the volcanic features observed there in addition deep water lava tubes are strong enough to withstand the pressures of a few megapascals that the building of a volcanic structure 150 m high at the end of the tube would generate the volumes of individual volcanic terraces and seamounts on the puna ridge and at the mar are large 0 1 1 km3 and similar to the volumes of lava flows that are broadly distributed at the subaerial east rift zone of kilauea this striking difference in the volcanic morphology on a scale of 1 2 km producing terraces and seamounts underwater and low relief flows on land must be related to the enhanced cooling and to the greater mechanical stability of tubes in the submarine environment we suggest that at the mar a crustal magma reservoir most likely located beneath shallow flat sections of the segment provides magma to the rift axis through dikes that propagate laterally tens of kilometers the zone of dike intrusion at least in the neighborhood of the magma body is likely narrower than the width resurfaced by flows yielding a crustal structure that has a rapid vertical transition from lavas to sheeted dikes at segment ends the zone of dike intrusion is likely to be wider giving a resulting structure with a more gradual transition from lavas to dikes