seismic evidence for variations in axial magma chamber properties along the southern juan de fuca ridge abstract multichannel seismic data collected along the cleft segment on the southern juan de fuca ridge show that this intermediate spreading center is underlain by a mid crustal reflector interpreted as the top of an axial magma chamber amc the amc reflection is present along most of the segment and deepens gently from 2 0 km near the southern end of the segment beneath the ridge cleft observatory site to 2 3 km at the northern end beneath the site of the mid 1980s submarine eruption we analyzed the one dimensional seismic structure of the amc at two locations with contrasting lava chemistry beneath two different hydrothermal vent fields at the northern site waveform modeling in the time intercept slowness p domain indicates that the amc is 100 m thick and it is characterized by a decrease in p wave velocity from 6 km s to 3 7 km s in contrast the p wave velocity within the shallower 100 m thick amc at the southern site is higher 5 0 km s the decrease in seismic velocity within the amc indicates that it is partially molten and that it is not a cracking front as previously suggested for other intermediate spreading segments the data show a coherent seismic phase interpreted as the p to s wave conversion at the amc pamcs stacking of this event shows that the pamcs is only present along the northern part of the segment our results thus suggest along axis variations in the crystallinity of the amc the amc along cleft varies from a high crystal content

evolution of a brine saturated layer at the base of a ridge crest hydrothermal system vent fluids with chlorinities significantly greater than seawater are indicative of mixing between hydrothermal seawater and brines that have formed as a result of supercritical phase separation we relate the evolution of a brine saturated layer to magmatic and tectonic processes at the base of a ridge crest hydrothermal system a conductive heat balance indicates that under steady state conditions such a brine saturated layer is typically 1 10 m thick and contains 107 to 108 kg of brine with 6 10 times the chlorinity of seawater simple mixing and fractionation models indicate that the depletion and replenishment of brine saturated layers may occur on a timescale of years to centuries a timescale of years is consistent with temporal changes in chlorinity of fluids at vents in areas of recent known dike injection in these systems brines may be undergoing depletion by mixing with liquid phase seawater vents stably emitting high chlorinity fluids on timescales of decades may represent older systems in which brines are being depleted by diffusion into an overlying liquid phase hydrothermal system

long submarine lava flows observations and results from numerical modeling long 100 km lava flows are relatively common on mars and venus and have been identified on the moon but they are rarely documented on earth however although 75 of the earth s surface is covered by water only a small percentage of the ocean floor has been investigated at a resolution sufficient to unequivocally identify the boundaries of long submarine lava flows even so basaltic lava flows as long as 110 km have been identified on the deep 1500 m seafloor near hawaii and the east pacific rise ambient conditions on the deep ocean floor may favor the development of long lava flows for the following reasons first high pressures 15 mpa keep volatiles dissolved in basaltic lavas preventing viscosity increases associated with exsolution and vesiculation second seawater rapidly quenches the surface of submarine basalt flows so that an insulating glass layer 1 5 cm thick encases submarine flows within seconds after their emplacement this glass rind effectively insulates the molten flow interior from additional heat loss making submarine basalt flows behave as well insulated subaerial tube fed flows thus for identical basalt flows emplaced on the deep seafloor and subaerially a submarine flow could advance farther before stopping results of numerical modeling indicate that thin 1 m submarine basalt flows behave similarly to identical subaerial flows but thicker submarine flows may advance significantly farther than their subaerial counterparts

detection of changes in ridge crest morphology using repeated multibeam sonar surveys a quantitative analysis method is developed to objectively identify significant bathymetric changes between multibeam sonar surveys of the same seafloor area the technique involves mathematical gridding of bathymetric surveys from different years co registration of grids based on prominent features covered in both surveys calculation of slope weighted depth difference grids and determination of the boundaries of statistically significant differences the resulting difference grids are used in interpreting geological processes and calculating the volume of new material added to the seafloor the method is applied to data from the southern juan de fuca ridge where a seafloor volcanic eruption of 0 05 km3 is inferred to have occurred between 1983 and 1987 camera tow data and submersible observations provide ground truth which confirms the validity of the quantitative analysis method and indicates empirically that the detection limit of the method allows new seafloor landforms measuring 1 105 m2 in area or about 200 300 m in diameter and 5 15 m in thickness to be detected through repeated sea beam bathymetric surveys

temporal and spatial variability of hydrothermal manganese and iron at cleft segment juan de fuca ridge a unique data set for hydrothermal mn and fe was collected at cleft segment on the juan de fuca ridge between 1983 and 1991 the data set includes observations of focused and diffuse venting fluids and neutrally buoyant plumes formed by chronic and episodic venting manganese heat and iron heat ratios for plumes from the north end of the cleft segment were combined with independently determined estimates of plume heat flux to yield annually averaged chronic venting fluxes for mn of 0 36 0 17 mol s 1 and for fe of 0 61 0 34 mol s 1 over 6 years of plume measurements at north cleft segment observed episodic hydrothermal discharge accounted for 15 of the total vented mn and 35 of vented fe the chronic fluxes for mn and fe at a second venting center located at the south end of the cleft segment were estimated to be approximately equal to the fluxes at north cleft segment chronic plumes at north cleft segment are mixtures of focused and diffuse discharge that contribute heat mn and fe in variable proportions similar examination of south cleft segment data strongly suggests the presence of an as yet unobserved venting source relatively depleted in mn and fe but contributing substantially to the overall heat temporal and spatial variations in the concentrations of mn and fe and in mn heat and fe heat ratios for focused seafloor vents were difficult to resolve within complex chronic plumes manganese heat and iron heat ratios of megaplumes suggest they may have derived from reservoirs of diffuse fluids while smaller event plumes may have formed by different processes and have properties similar to chronic plumes the accurate assessment of segment scale hydrothermal fluxes of mn and fe requires coordinated measurements of representative seafloor sources and the neutrally buoyant plume that integrates all seafloor discharge

seismic structure and anisotropy of the juan de fuca ridge at 45 n a seismic refraction experiment was conducted with air guns and ocean bottom seismometers on the juan de fuca ridge at 45 n at the northern cleft segment and at the overlapping rift zone between the cleft and vance segments these data determine the average velocity structure of the upper crust and map the thickness variability of the shallow low velocity layer which we interpret as the extrusive volcanic layer the experiment is unique because a large number of travel times were measured along ray paths oriented at all azimuths within a small 20 km by 35 km area these travel times provide evidence for compressional velocity anisotropy in the upper several hundred meters of oceanic crust presumed to be caused by ridge parallel fracturing compressional velocities are 3 35 km s in the ridge strike direction and 2 25 km s across strike travel time residuals are simultaneously inverted for anisotropy as well as lateral thickness variations in the low velocity layer extrusive layer thickness ranges from approximately 200 m to 550 m with an average of 350 m the zone of the thinnest low velocity layer is within the northern cleft segment axial valley in a region of significant hydrothermal activity layer thickness variability is greatest near the cleft vance overlapping rift zone where changes of 300 m occur over as little as several kilometers laterally these low velocity layer thickness changes may correspond to fault block rotations in an episodic spreading system where the low side of each fault block accumulates more extrusive volcanics

cyclic eruptions and sector collapses at monowai submarine volcano kermadec arc 1998 2007 1 repeated multibeam bathymetric surveys at monowai cone a shallow submarine basaltic volcano and part of the monowai volcanic center in the northern kermadec arc were conducted in 1998 2004 and 2007 these surveys document dramatic depth changes at the volcano including negative changes up to 176 m from two sector collapses and positive changes up to 138 m from volcanic reconstruction near the summit and debris avalanche deposits downslope of the slide scars one sector collapse occurred on the se slope between 1998 and 2004 with a volume of 0 09 km3 and another occurred on the sw slope between 2004 and 2007 with a volume of 0 04 km3 the volume of positive depth change due to addition of volcanic material by eruption is of the same order 0 05 km3 between 1998 and 2004 and 0 06 km3 between 2004 and 2007 during these time intervals monitoring by the polynesian seismic network detected frequent t wave swarms at monowai indicative of explosive eruptive activity every few months an unusual t wave swarm on 24 may 2002 was previously interpreted as the collapse event between the 1998 and 2004 surveys but no similarly anomalous t waves were detected between 2004 and 2007 probably because the polynesian seismic network stations were acoustically shadowed from the second slide event we interpret that the sector collapses on monowai are caused by the unstable loading of fragmental erupted material on the summit and steep upper slopes of the volcano 20 moreover there appears to be a cyclic pattern in which recurrent eruptions oversteepen the cone and periodically lead to collapse events that transport volcaniclastic material downslope to the lower apron of the volcano volumetric rate calculations suggest that these two processes may be more or less in equilibrium the repeated collapses at monowai are relatively modest in volume involving only 0 1 0 5 of the edifice volume have occurred much more frequently than is estimated for larger debris avalanches at subaerial volcanoes and may be characteristic of how persistently active shallow submarine arc volcanoes grow

evolution of seismic layer 2b across the juan de fuca ridge from hydrophone streamer 2 d traveltime tomography how oceanic crust evolves has important implications for understanding both subduction earthquake hazards and energy and mass exchange between the earth s interior and the oceans although considerable work has been done characterizing the evolution of seismic layer 2a there has been little analysis of the processes that affect layer 2b after formation here we present high resolution 2 d tomographic models of seismic layer 2b along 300 km long multichannel seismic transects crossing the endeavour northern symmetric and cleft segments of the juan de fuca ridge these models show that seismic layer 2b evolves rapidly following a different course than layer 2a the upper layer 2b velocities increase on average by 0 8 km s and reach a generally constant velocity of 5 2 0 3 km s within the first 0 5 myr after crustal formation this suggests that the strongest impact on layer 2b evolution may be that of mineral precipitation due to active hydrothermal circulation centered about the ridge crest and driven by the heat from the axial magma chamber variations in upper layer 2b velocity with age at time scales 0 5 ma show correlation about the ridge axis indicating that in the long term crustal accretion processes affect both sides of the ridge axis in a similar way below the 0 5 ma threshold differences in 2b velocity are likely imprinted during crustal formation or early crustal evolution layer 2b velocities at propagator wakes 5 0 0 2 km s where enhanced faulting and cracking are expected and at areas that coincide with extensional or transtensional faulting are on average slightly slower than in normal mature upper layer 2b analysis of the layer 2b velocities from areas where the hydrothermal patterns are known shows that the locations of current and paleohydrothermal discharge and recharge zones are marked by reduced and increased upper layer 2b velocities respectively additionally the distance between present up flow and down flow zones is related to the amount of sediment cover because as sediment cover increases and basement outcrops become covered direct pathways from the igneous basement through the seafloor are cut off forcing convective cells to find alternate paths

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

lava flows from a mid 1980s submarine eruption on the cleft segment juan de fuca ridge a series of lava flows with a total volume of 0 05 km3 were erupted in the mid 1980s along 17 km of the northern cleft segment of the juan de fuca ridge observations from camera tows and submersible dives show that the new flows are all similar in appearance and consist entirely of pillow lava with a mixture of smooth and striated surface textures suggesting a relatively uniform eruption rate approaching 1 m3 s at point source vents the flows vary in size from small patches to large steep sided ridges and were probably erupted from a dike intruded along the ridge axis because they are aligned along a linear fissure graben system observations at north cleft show that the physical appearance of new flows changes more rapidly than previously realized and that earlier qualitative dating of young lavas based on sediment cover and glassy surface texture were probably overestimates by an order of magnitude sediment accumulation on the lavas is quite variable and locally surprisingly substantial mainly due to hydrothermal deposits that formed while the lava flows were cooling biological vent communities photographed on the new flows in 1989 show that vent animals can colonize new vent sites rapidly but that warm water was still venting only in a few places nonvent animals are much slower to colonize the new flows and rates of colonization observed at north cleft may be useful for making improved age estimates of young 10 years lava flows elsewhere the north cleft eruption represents about 2 of the estimated average annual volcanic output along the global mid ocean ridge implying that many other submarine eruptions are occurring undetected

long term monitoring of hydrothermal heat flux using moored temperature sensors cleft segment juan de fuca ridge heat flux from submarine vent fields can vary gradually on interannual scales or nearly instantaneously in response to volcano tectonic events in the underlying crust neither case is well documented because measurements of vent field scale heat flux are scarce we report here a new approach to hydrothermal plume monitoring sensitive to both progressive changes and hydrothermal events from june 1991 to may 1992 we moored 35 self contained temperature sensors and six current meters on seven moorings located in and around the plume from a vent field on the cleft segment juan de fuca ridge the hydrothermal plume was identified by a local temperature anomaly of 0 01 to 0 03 c in the lowermost 200 m of the water column plume heat flux defined as the net advection of this temperature anomaly averaged about 250 mw during the deployment this flux is less than previous estimates and thus supports speculation that the heat flux is declining after a sudden reinvigoration triggered by a seafloor rifting event in 1986

a 6 year time series of hydrothermal plumes over the cleft segment of the juan de fuca ridge the causal links between magmatic tectonic processes and hydrothermal venting along the mid ocean ridge axis will be difficult to discern without broadband observations of the temporal variability of hydrothermal activity i report here on temporal variability in the intensity and distribution of chronic and event plumes generated by continuous and episodic hydrothermal discharge at two vent fields on the cleft segment of the juan de fuca ridge hydrothermal temperature and light attenuation c anomalies were comprehensively mapped between 1 and 4 times per year during nine cruises from 1986 to 1991 intensity of and c in the chronic plume overlying the north cleft vent field showed more variability between years than within a given year both and c were exceptionally high in 1986 fell to moderate levels during 1988 to 1990 and declined to minimum values in 1991 the chronic plume over the south cleft vent field varied similarly superimposed on the chronic plumes were brief and intense releases of hydrothermal fluid that created event plumes distinguishable by their symmetry and exceptional rise height event plumes at the north cleft field ranged from the 1986 megaplume with a rise height exceeding 1000 m to smaller events in 1987 and 1989 with rise heights of 400 m chronic hydrothermal heat 660 277 1 mw and mass 204 86 1 g s 1 fluxes from the north cleft field were determined by assuming these fluxes were equivalent to the long term net advection of a mean cross section of the vertically integrated and c in the plume this 6 year history of hydrothermal plumes indicates that episodic discharge has been an important component of the recent hydrothermal flux and that the aggregate heat and mass fluxes declined significantly though not consistently between 1986 and 1991

an assessment of the effect of chemical scavenging within submarine hydrothermal plumes upon ocean geochemistry profiles of 210pb over the endeavour and north cleft segments of the juan de fuca ridge are used to model a time scale for the scavenging by hydrothermal plumes of reactive elements in seawater the hydrothermal plumes above these ridge segments are sites of intense scavenging removal of 210pb at endeavour the total 210pb activities within the plume are as low as 8 dpm 100 l and dissolved activities are as low as 3 dpm 100 l at the north cleft which is characterized by higher particulate fe concentrations the total 210pb activities are 4 5 dpm 100 l the dissolved activities are 1 2 dpm 100 l and the 210pb activities are deficient with respect to the activity of the 210po daughter these are perhaps the lowest 210pb activities ever measured in the deep sea the large gradient of 210pb between the plume and surrounding deep water suggests that scavenging is focused into the plumes through horizontal transport the implication therefore is that this process might impact the ocean on a scale larger than that local to the ridge crest by coupling published measurements of particle flux from endeavour with 210pb activities on particles trapped at that site the total volume of seawater stripped of 210pb per year for that site was calculated to be 7 4 1012 l y globally the extrapolated volume flux of seawater stripped of reactive constituents is 5 7 1015 l y such that the entire ocean is processed in this manner in 2 4 105 y the geochemical cycle of elements with ocean residence times much shorter than this e g pb and th will not be greatly affected by hydrothermal scavenging on the other hand this process holds significance for the geochemistry of other elements scavenged by hydrothermal plumes such as p and v whose ocean residence times are 104 y

hydrothermal plumes along the east pacific rise 8 40 to 11 50 n plume distribution and relationship to the apparent magmatic budget abstract the interactions between hydrothermal circulation and large scale geological and geophysical characteristics of the mid ocean ridge cannot be ascertained without large scale views of the pattern of hydrothermal venting such multi ridge segment surveys of venting are accomplished most efficiently by mapping the distribution and intensity of hydrothermal plumes in november 1991 we mapped hydrothermal temperature and light attenuation c anomalies above the east pacific rise epr continuously from 8 40 to 11 50 n a fast spreading ridge crest portion bisected by the clipperton transform fault plume distributions show a precise correlation with the distribution of active vents where video coverage of the axial caldera is exhaustive 9 09 54 n elsewhere in the study area the sketchy knowledge of vent locations gleaned from scattered camera tows predicts only poorly the large scale hydrothermal pattern revealed by our plume studies plumes were most intense between 9 42 and 9 54 n directly over a march april 1991 seafloor eruption these plumes had exceptionally high c ratios compared to the rest of the study area we suggest that the phase separated gas rich vent fluids discharging here fertilize an abundant population of bacteria hydrothermal plume distributions define three categories intense and continuous 8 48 8 58 n 9 29 10 01 n and 11 05 11 27 n weak and discontinuous 8 58 9 29 n and negligible the location of each category is virtually congruent with areas that are respectively magmatically robust magmatically weak and magmatically starved as inferred from previous measurements of axial bathymetric undulations cross axis inflation and magma chamber depth and continuity this congruency implies a fine scale spatial and temporal connection between magmatic fluctuations and hydrothermal venting we thus speculate that at least along this fast spreading section of the epr cyclic replenishment eruption and freezing of the thin axial melt lens exerts greater control over hydrothermal venting than the more enduring zones of crystal mush and hot rock we found intense and continuous plumes along 33 of the surveyed ridge crest an observation implying that any point on the ridge is on average hydrothermally active one third of the time combining this result with the 20 plume coverage found along the medium rate juan de fuca ridge suggests that superfast 150 mm yr spreading ridges should support vigorous venting along 50 of their length if spreading rate and along axis plume coverage are linearly related

repeat bathymetric surveys at 1 metre resolution of lava flows erupted at axial seamount in april 2011 at frequently active submarine volcanoes it is difficult to distinguish between new and pre existing lava flows a combination of high resolution bathymetric surveys taken before and after an eruption at axial seamount in 2011 allows detailed mapping of the 2011 lava flows and highlights the tendency of new flows to mimic older ones

hydrothermal venting and the apparent magmatic budget of the juan de fuca ridge the fact that vigorous hydrothermal discharge occurs on ridge segments with a variety of spreading rates and morphological characteristics makes it difficult to discern the large scale geological controls on the distribution of hydrothermal venting along the mid ocean ridge we undertook to simplify this problem by mapping the distribution of hydrothermally produced thermal and light attenuation anomalies along the crest of the juan de fuca ridge a medium rate spreading center with segments of markedly differing morphology hydrothermal anomalies were detected by towing a high precision conductivity temperature depth transmissometer ctdt package in a sawtooth pattern through the lowermost 1000 m of the water column above each segment these surveys conducted between 1985 and 1989 have produced the first quantitative and continuous visualizations of hydrothermal plumes along a multisegment portion of the mid ocean ridge hydrothermal discharge is strongest on those segments or portions of segments where the apparent magmatic budget is highest as indicated by the degree of along axis inflation and other morphological characteristics this correspondence implies that magma is episodically delivered in small and spatially discontinuous bodies of melt rather than by the constant resupply of a steady state magma chamber we propose that the probability of discharge occurring at any point on the mid ocean ridge is proportional to the spreading rate this probability scaled by the observation that significant plumes presently emanate from 20 of the juan de fuca ridge is expected to range from 5 on the slowest spreading ridge to 50 on the fastest spreading ridge

processes associated with ocean crustal formation the juan de fuca ridge recent geophysical geological and geochemical studies on the juan de fuca ridge have been gathered into a special collection of manuscripts that demonstrate the interdisciplinary nature of the crustal formation and evolutionary processes on the ridge axis these new studies include long term sea floor and water column measurements that indicate real time changes due to volcanic activity and related hydrothermal flux sea floor measurements of seismic activity detecting swarms of earthquakes near the axial zone and submersible observations of a 2 km along strike spacing of large scale hydrothermal fields detailed sampling of axial basalts show that in spite of abundant evidence of excess volcanic activity the juan de fuca ridge is not a traditional hot spot on a larger scale geophysical surveys of the ridge flanks show that the upper crust can vary in thickness by a factor of two that crustal porosity systematically decreases with age and the topographic forcing is the dominant control of off axis hydrothermal circulation finally the juan de fuca ridge has long been a testing ground for innovative survey techniques and this issue includes reports on the development of acoustic backscatter as a quantitative measure of basement roughness and as an indicator of the presence of lithified sediments overlying zones of fluid upwelling

reconstructing lava flow emplacement processes at the hot spot affected galapagos spreading center 95 w and 92 w 1 volcanic eruptions at mid ocean ridges mors control the permeability internal structure and architecture of oceanic crust thus establishing the foundation for the evolution of the ocean basins to better understand the emplacement of submarine lava flows at mors we have integrated submersible based geologic mapping with remote sensing techniques to characterize the lava flow morphology within previously mapped lava flow fields produced during single eruptive episodes at the galapagos spreading center gsc detailed attributes describing the surface geometry and texture of the lava flows have been extracted from high resolution sonar data and combined with georeferenced visual observations from submersible dives and camera tows based on signatures contained in these data a fuzzy logic based classification algorithm categorized lava flow morphology as pillows lobates or sheets the resulting digital thematic maps offer an unprecedented view of gsc lava morphology collectively covering 77 km2 of ridge axis terrain at a resolution of 2 m 2 m error assessments with independent visual reference data indicate approximately 90 agreement comparable to subaerial classification studies the digital lava morphology maps enable quantitative spatially comprehensive measurements of the abundance and distribution of lava morphologies over large areas of seafloor and within individual eruptive units a comparison of lava flow fields mapped at lower and higher magma supply settings 95 and 92 w respectively indicates that effusion rates increase along with magma supply and independent of spreading rate at the gsc although a complete range of eruptive behavior exists at each setting

acoustic detection of a seafloor spreading episode on the juan de fuca ridge using military hydrophone arrays until recently no practical method has been available to continuously monitor seismicity of seafloor spreading centers the availability of the u s navy s sound surveillance system sosus for environmental research has allowed the continuous monitoring of low level seismicity of the juan de fuca ridge in the northeast pacific on june 22 1993 noaa installed a prototype system at u s naval facility whidbey island to allow real time acoustic monitoring of the juan de fuca ridge using sosus on june 26 2145 gmt a burst of low level seismic activity with accompanying harmonic tremor was observed and subsequently located near 46 15 n 129 53 w on the spreading axis of the juan de fuca ridge over the following 2 days the activity migrated to the nne along the spreading axis with the final locus of activity near 46 31 5 n 129 35 w the nature of the seismicity was interpreted to represent a lateral dike injection with the possibility of eruption on the seafloor based on this interpretation a response effort was initiated by u s and canadian research vessels and both warm water plumes and fresh lavas were subsequently identified at the site

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

evidence for volcanic eruption on the southern juan de fuca ridge between 1981 and 1987 the formation of new ocean crust at mid ocean ridges is known to be a discontinuous process in both space and time but little is known about the frequency and duration of eruptions along an active ridge segment here we present evidence from sea beam surveys and underwater photography for the eruption of lavas along a segment of the juan de fuca ridge between 1981 and 1987 although previous studies have inferred volcanic activity on ridges in areas where recent seismicity or young lava flows have been observed1 4 none has yet had direct evidence to date such a recent submarine eruption the temporal coincidence between this eruptive episode and the megaplumes huge sudden emissions of hot mineral laden water observed over this part of the ridge5 6 in 1986 and 1987 supports previous suggestions5 10 that megaplumes are caused by sea floor spreading events

compositions growth mechanisms and temporal relations of hydrothermal sulfide sulfate silica chimneys at the northern cleft segment juan de fuca ridge three active hydrothermal vents forming sulfide mounds and chimneys monolith fountain and pipe organ and more widely distributed inactive chimneys are spatially related to a system of discontinuous fissures and young sheet flow lavas at the northern cleft segment juan de fuca ridge the formation of zoned tubular curich chimneys type i on the monolith sulfide mound is related to focused flow of high temperature to 328 c fluid bulbous chimneys type ii or beehives at the monolith and fountain vents are products of diffuse high temperature to 315 c discharge a broader zone of vigorous mixing between the hydrothermal fluid and seawater results in quench crystallization of anhydrite rich shells columnar zn sulfide rich chimneys with narrow channelways type iii are constructed where focused and relatively low temperature 261 c fluid vents directly from the basalt substrate the bulk chemistry low cu high pb ag and sio2 contents mineralogy pyrite marcasite wurtzite amorphous silica anglesite colloform and filamentous textures and oxygen isotope characteristics of inactive type iv chimneys indicate a low temperature 250 c origin involving diffuse and sluggish flow patterns and conductive cooling seafloor observations and 210pb data indicate that 1 type iv chimneys are products of an earlier period of hydrothermal activity that ended no more than 60 years ago but prior to the sheet flow eruption 2 the high temperature monolith and fountain vents are manifestations of the same heating event shallow emplacement of magma that led to the sheet flow eruption and recent megaplumes and 3 the pipe organ vent is in a very youthful stage of development and chimney deposition postdates the sheet flow eruption

sims microanalysis of the strelley pool formation cherts and the implications for the secular temporal oxygen isotope trend of cherts abstract the significance of oxygen isotope ratios in archean chert has long been debated cherts from the c 3 4 ga strelley pool formation spf pilbara craton western australia host some of the oldest stromatolite and microfossil evidence for life but the genesis and timing of silica cements has been unclear field relations petrography and a combination of laser fluorination and in situ sims measurements of 18o in quartz show that bedded cherts of the spf were originally precipitated as carbonates and were later widely replaced by quartz three localities were studied and analyzed for 18o qz in chert 1 camel creek foliated metamorphosed bedded cherts and meter scale black chert veins 2 unconformity ridge and abdp8 drill core stromatolitic and bedded chert overlying basal detrital quartz sandstone and 3 the trendall locality bedded stromatolitic chert replacing original dolomite low temperature hydrothermal quartz and mm to decimeter scale chert quartz veins laser fluorination mm scale values of 18o qz range from 14 2 to 18 2 vsmow at camel creek 9 3 to 18 9 at unconformity ridge and 13 7 to 25 7 at trendall values of 18o qz in cm to decimeter scale hydrothermal chert veins cutting bedded carbonates at trendall range from ca 15 to 16 whereas bedded cherts are 17 to 26 these laser data include the highest 18o values reported for cherts in paleoarchean sediments and are up to 4 higher than the upper limit of 22 reported in other studies in apparent contrast to the long standing secular temporal trend which shows such high 18o only in younger chert however analysis by laser fluorination at the 1 mm scale cannot resolve microtextures seen petrographically in contrast in situ sims analyses can resolve petrographic microtextures and show 18o qz at 10 m scale have an even greater range of 7 31 in bedded cherts at the trendall locality up to 9 above the secular temporal trend textures observed optically at the trendall locality were classified as microquartz mesoquartz chalcedony megaquartz veins and cavity megaquartz sem cl imaging shows two generations of meso and megaquartz bright cl with well developed growth zoning and dark cl with massive or mottled texture microquartz is the earliest textural generation of quartz and has a maximum 18o qz of 22 by sims dark cl mesoquartz has similar 18o to microquartz and is interpreted to also be early bright zoned cl mesoquartz which formed post archean has even higher 18o up to 29 vein megaquartz crosscuts most quartz generations and has a restricted range of 18o mostly from 16 to 19 chalcedony pseudomorphs rhombic cavities and fractures lines the edges of veins and has similar 18o to veins 16 19 late cavity megaquartz is bright and zoned by cl grows into late open cavities and has the highest 18o qz values reported from the pilbara up to 31 3 thus the highest 18o quartz cements at the trendall locality are the youngest and may be related to weathering early silicification and the formation of microquartz chalcedony and low 18o mesoquartz occurred during low temperature hydrothermal activity in the archean none of the spf quartz examined is interpreted to have formed as a direct precipitate from paleoarchean seawater thus values of 18o qz do not record either water chemistry or temperature of archean oceans in situ sims analysis shows that high 18o qz values above 22 are only found in late forming cavity megaquartz and bright zoned cl mesoquartz at the trendall locality the spf results from our sample suite demonstrate the ability to resolve complex history using detailed petrography and sims analysis similar studies may show equal complexity of 18o qz data for other localities that are interpreted to show secular temporal trends for chert the apparent increase of 18o qz through time may reflect differences in diagenesis and or an inherent and previously unrecognized sampling bias that compares fundamentally different populations of quartz such as archean hydrothermal chert from volcanic greenstone belts with unrelated phanerozoic biogenic quartz

plate motion at the ridge transform boundary of the south cleft segment of the juan de fuca ridge from gps acoustic data 1 we measure the present day plate velocity of the juan de fuca ridge 25 km off axis to be 63 6 3 6 mm a at s67 2 e 7 9 degrees 1 relative to the pacific plate pa this velocity was derived from gps acoustic gpsa measurements in 2000 2001 2002 and 2003 that observed the position of a seafloor array 44 43 n 130 03 w 2900 m depth with a repeatability of 4 6 mm three transient events at the juan de fuca ridge and blanco transform account for 10 of this motion in viscoelastic modeling suggesting that the observed gpsa pa velocity is due primarily to steady state plate dynamics subtracting the modeled transient motion gives a velocity of 57 3 3 9 mm a at s72 9 e 12 1 degrees 1 which is consistent at the 95 confidence level with the velocity calculated from the wilson 1993 0 0 78 ma euler pole therefore this site is interpreted to be in a region of continuous full rate plate motion a robust result of this study which holds with and without correcting for transient motions these results provide direct geodetic evidence that spreading occurs predominantly within 25 km of the axis at this intermediate spreading rate ridge previously reported geodetic monitoring across the 1 km wide axial valley from 1994 1999 and 2000 2003 shows no significant extension chadwell et al 1999 hildebrand et al 1999 chadwick and stapp 2002 w w chadwick personal communication 2006 and seismic monitoring shows no activity this suggests the crust between 0 5 and 25 km off axis accommodates 26 mm of aseismic deformation each year through some combination of near axis fault motion and elastic strain accumulation

detection and response to volcanic hydrothermal events on the mid ocean ridge the mid ocean ridge is by far the most continuous volcanic system on the planet but since it lies beneath more than 2 km of seawater a thorough study of its thermal and chemical interaction with the ocean remains very difficult until 1990 the end of the period covered by the last iugg report literally all of our understanding of the dynamics of magmatic accretion on the mid ocean ridge was based on inferences from 1 geochemical geophysical and morphologic studies of long sections of the ridge using a variety of instruments mounted on ships hulls or towed on long cables near the seafloor and 2 submersible based mapping and sampling of small sections of the ridge particularly hydrothermally active ones these types of studies have continued to flourish parmentier 1995 humprhis 1995 but the past 4 years have also seen the first near real time observations of accretion events on two pacific spreading centers and the development of technology and techniques that can monitor the occurrence and extent of such events over a selected portion of the ridge system these monitoring capabilities allow interdisciplinary studies of seafloor hydrothermal systems so that the thermal and chemical effects of hydrothermal venting on the oceans can be assessed

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

helium isotope variations in juan de fuca ridge basalts we have measured he 4he ratios and he and ne concentrations on a suite of 24 basalt glasses from the neovolcanic zone of the juan de fuca ridge jdfr from 44 6 n near the blanco transform up to 48 0 n on the endeavour segment the helium isotope ratios exhibit a clear geographic variation with relatively constant values of 7 8 ra along the southern jdfr increasing to a maximum of 8 8 ra at 46 9 n on the cobb segment and then dropping to values of 8 0 ra at the cobb offset ratios along the endeavour segment further north are somewhat higher averaging 8 2 ra basalts dredged from axial seamount have he 4he 7 9 8 4 ra indicating that the seamount does not have a geochemical or isotopic signature distinct from other portions of the jdfr this confirms that while axial seamount is the locus of excess magma generation it is devoid of any hotspot or ocean island basalt geochemical signature for the whole sample set he 4he ratios show a negative correlation with 87sr 86sr we attribute the geographical variations in he 4he to broad scale heterogeneity in the mantle source region beneath the jdfr beginning at axial seamount and further north he 4he shows a positive correlation with fe8 0 suggesting that melting dynamics in the underlying mantle may exert some control on the helium isotope ratios along part of our survey area

comment on geochemistry of the 2 51 ga old rampur group pelites western himalayas implications for their provenance and weathering by bhat and gosh 2001 precambrian research 108 1 16

eruptive modes and hiatus of volcanism at west mata seamount ne lau basin 1996 2012 we present multiple lines of evidence for years to decade long changes in the location and character of volcanic activity at west mata seamount in the ne lau basin over a 16 year period and a hiatus in summit eruptions from early 2011 to at least september 2012 boninite lava and pyroclasts were observed erupting from its summit in 2009 and hydroacoustic data from a succession of hydrophones moored nearby show near continuous eruptive activity from january 2009 to early 2011 successive differencing of seven multibeam bathymetric surveys of the volcano made in the 1996 2012 period reveals a pattern of extended constructional volcanism on the summit and northwest flank punctuated by eruptions along the volcano s wsw rift zone wswrz away from the summit the volumetrically largest eruption during the observational period occurred between may 2010 and november 2011 at 2920 m depth near the base of the wswrz the nearly equally long ene rift zone did not experience any volcanic activity during the 1996 2012 period the cessation of summit volcanism recorded on the moored hydrophone was accompanied or followed by the formation of a small summit crater and a landslide on the eastern flank water column sensors analysis of gas samples in the overlying hydrothermal plume and dives with a remotely operated vehicle in september 2012 confirmed that the summit eruption had ceased based on the historical eruption rates calculated using the bathymetric differencing technique the volcano could be as young as several thousand years

petrology and geochemistry of basalts from the southern juan de fuca ridge controls on the spatial and temporal evolution of mid ocean ridge basalt three morphologically distinct regions within the neovolcanic zone of the cleft segment of the southern juan de fuca ridge were investigated and sampled in detail using the dsrv alvin additional along strike and off axis samples were recovered by dredge the southernmost region the southern cleft site is characterized by a 3 km wide axial valley floored by ponded sheet flows and bisected by a 30 to 50 m wide cleft farther north at the young sheet flow site the ridge axis is characterized by a distinct 500 m wide inner graben that is largely covered by distinctly younger looking sheet flows the northernmost of the three regions is defined by a linear series of discontinuous constructional pillow mounds that extend the trend of the cleft segment well into the zone of overlap with the neighboring vance segment the pillowed lavas at the young pillow mound site represent the most recent episode of volcanism along the cleft segment strong correlations exist between degrees of fractionation relative ages of lavas and latitude lavas are progressively younger looking and more mafic to the north the compositional range of mid ocean ridge basalts from the neovolcanic zone can generally be accounted for by 35 40 low pressure fractional crystallization of relatively primitive but not primary depleted n type melts scatter of the geochemical data about calculated liquid lines of descent is probably the result of mixing of magmas with slightly different parental compositions generated from small scale mantle heterogeneities furthermore the chemical variability may be the result of mixing of very depleted and more enriched sources or melts that are present beneath the southern juan de fuca ridge the more primitive nature of the young pillow mound basalts and their slightly different chemical characteristics indicates they cannot be simply related to the older southernmost lavas by along axis flow in a continuous axial magma chamber or conduit rather the data suggest lavas were derived from discrete magma chambers or lenses each in a different stage of evolution the youngest events may be associated with a new influx of magma into the northern part of the segment and subsequent northward diking and propagation to form the new pillow mounds the oldest stage 40 additional crystallization of the most mafic composition is associated with focused hydrothermal activity and tectonic extension whereas the youngest events are characterized by ridge inflation diking and dispersed hydrothermal activity geochemical and tectonomagmatic features observed along the cleft segment are similar to those recently documented along the east pacific rise from 9 30 n 10 n suggesting the scales processes and stages of magmagenesis are similar along medium to fast spreading ridge segments

volcanic eruption of the mid ocean ridge along the east pacific rise crest at 9 45 52 n direct submersible observations of seafloor phenomena associated with an eruption event in april 1991 in april 1991 we witnessed from the submersible alvin a suite of previously undocumented seafloor phenomena accompanying an in progress eruption of the mid ocean ridge on the east pacific rise crest at 9 45 n 52 n the volume of the eruption could not be precisely determined although comparison of pre and post eruption seabeam bathymetry indicate that any changes in ridge crest morphology resulting from the eruption were 10 m high effects of the eruption included 1 increased abundance and redistribution of hydrothermal vents disappearance of numerous vent communities and changes in characteristics of vent fauna and mineral deposits within the eruption area since december 1989 2 murkiness of bottom waters up to tens of meters above the seafloor due to high densities of suspended mineral and biogenic particulates 3 destruction of a vent community by lava flows mass wasting and possible hydrovolcanic explosion at a site known as tubeworm barbecue in the axial summit caldera asc at 9 50 6 n 4 near critical temperatures of hydrothermal vent fluids ranging up to 403 c 5 temporal variations over a 2 week interval in both temperatures and chemical isotopic compositions of hydrothermal fluids 6 unusual compositions of end member vent fluids with ph values ranging to a record low of 2 5 salinities ranging as low as 0 3 wt nacl one twelfth that of seawater and dissolved gases reaching high concentrations 65 mmol l for both co2 and h2s 7 venting at temperatures above 380 c of visually detectable white vapor that transformed to plumes of gray smoke a few centimeters above vent orifices 8 disorganized venting of both high temperature fluids black and gray smoke and large volumes of cooler diffuse hydrothermal fluids directly from the basaltic seafloor rather than from hydrothermal mineral constructions 9 rapid and extensive growth of flocculent white bacterial mats species unknown on and under the seafloor in areas experiencing widespread venting of diffuse hydrothermal fluid and 10 subseafloor downslope migration of magma normal to the ridge axis in a network of small scale 1 5 m diameter lava tubes and channels to distances at least 100 200 m outside the asc we suggest that in april 1991 intrusion of dikes in the eruption area to 200 m beneath the asc floor resulted in phase separation of fluids near the tops of the dikes and a large flux of vapor rich hydrothermal fluids through the overlying rubbly cavernous lavas low salinities and gas rich compositions of hydrothermal fluids sampled in the eruption area are appropriate for a vapor phase in a seawater system undergoing subcritical liquid vapor phase separation boiling and phase segregation hydrothermal fluids streamed directly from fissures and pits that may have been loci of lava drainback and or hydrovolcanic explosions these fissures and pits were lined with white mats of a unique fast growing bacteria that was the only life associated with the brand new vents the prolific bacteria which covered thousands of square meters on the ridge crest and were also abundant in subseafloor voids may thrive on high levels of gases in the vapor rich hydrothermal fluids initially escaping the hydrothermal system white bacterial particulates swept from the seafloor by hydrothermal vents swirled in an unprecedented biogenic blizzard up to 50 m above the bottom the bacterial proliferation of april 1991 is likely to be a transient bloom that will be checked quickly either by decline of dissolved gas concentrations in the fluids as rapid heat loss brings about cessation of boiling and or by grazing as other organisms are re established in the biologically devastated area

a detailed comparison of repeated bathymetric surveys along a 300 km long section of the southern east pacific rise little is known about the frequency magnitude relationship for volcano tectonic events over long sections of fast spreading ridges a technique that can both detect and characterize the size shape and approximate time of a volcanic or tectonic event is to survey an area of seafloor on multiple occasions using multibeam sonar systems we have performed a detailed comparison of repeated bathymetric surveys of a 300 km long section of the southern east pacific rise with the objective of detecting and characterizing any changes in seafloor morphology associated with eruptions and perhaps tectonic deformation repeat bathymetry data were collected as part of three cruises that surveyed the axial high between 16 s and 18 40 s latitude in late 1995 in late 1996 and in mid 1997 during the latter two surveys the way points of the 1995 survey were repeated with the same survey vessel thus closely approximating the acquisition geometry of the first survey we have developed new methods for removing systematic errors and suppressing random noise in the bathymetry data which reduce errors between surveys to a root mean square value of 8 m or with areas 0 2 km 2 and depth differences 4 m we estimate that any seafloor depth changes that occurred between the survey times must have dimensions less than these values or they would have been detected for comparison lava flows detected along the juan de fuca and gorda ridges using similar techniques are 10 75 m in thickness and have estimated total areas of 0 52 2 8 km 2

preeruptive flow focussing in dikes feeding historical pillow ridges on the juan de fuca and gorda ridges linear hummocky pillow mound volcanism dominates at slow and intermediate spreading rate mid ocean ridges volcanic hummocks are thought to be formed by low effusion rates or as a result of flow focussing during effusive fissure style eruptions in which the initial dike intercepts the seafloor and erupts along its entire length in this study high resolution autonomous underwater vehicle auv bathymetry is used to accurately map the extents of four historical fissure eruptions of the juan de fuca and gorda ridges on the north gorda north cleft and coaxial ridge segments the four mapped eruptions take the form of pillow mounds which are similar in both lithology and dimension to hummocks on the mid atlantic ridge pillow mounds may be isolated or coalesce to form composite mounds aligned as ridges or as clustered groups in three of the four mapped sites the eruptions were discontinuous along their lengths with pillow mounds and composite mounds commonly separated by areas of older seafloor this style of discontinuous eruption is inconsistent with typical en echelon fissure eruptions and is probably due to a mildly overpressured fingering dike intersecting the seafloor along parts of its length

chemical and mineralogical influences on concentrations of trace metals in hydrothermal fluids abstract concentrations of trace metals in hydrothermal solutions from the southern juan de fuca ridge sjfr and the tag hydrothermal field on the mid atlantic ridge are influenced by the combined effects of source rock composition brine seawater mixing and complex interplay among various metals and minerals deposited and remineralized in vent mounds and chimneys copper mo and co show predicted sharp decreases in concentration for vent fluids as temperatures decrease below 350 c however concentrations of cu in 363 c fluids at the tag site are well above values determined from seawater basalt interactions due to subseafloor refining processes and remineralization of chalcopyrite concentrations of zn cd pb as and ga do not show any temperature dependence between 214 to 363 c however zn values do correlate well with cl on an area by area basis due to subseafloor mixing of brine with altered seawater zinc levels also correlate well with concentrations of cd pb and as for all vent fluid samples as well as with basalt and zn rich sulfides implying some continuity in behavior from source rock to vent fluid to sulfide minerals gallium values correlate with zn and cl levels for the sjfr but are high relative to zn at the tag site due to subseafloor refining processes concentrations of tl follow cl k and rb consistent with the behavior predicted for tl overall the cl and zn rich fluids from the sjfr are characteristic of a brine dominated system formed during recent volcanism in contrast high temperature fluids from the tag area are representative of a system that has evolved over the past 100 000 yr with high concentrations of cu co and ga that result from higher temperatures and complex refining processes within a large vent mound

a 220 km2 recently erupted lava field on the east pacific rise near lat 8 s seamarc ii records show a 220 km2 area of unusually high reflectivity on the east pacific rise epr near lat 8 s which we interpret to be a recently erupted lava field this is the only segment of the epr exhibiting high reflectivity over such a large area in our continuous seamarc ii coverage from lat 8 n to 18 n and 3 s to 23 s the lavas appear to issue from the axial neovolcanic zone between lat 8 15 and 8 17 s the lava field extends up to 18km off axis and inundates scarps up to 100 m high on the basis of measurements of scarp heights just outside areas where they are buried we estimate the average thickness of the field to be 70 20 m thus the volume of the lava field is 15 4 km3 in comparison the average annual volcanic budget for the earth is approximately 4 5 km3 and the largest historic basaltic eruption from the icelandic laki fissure in 1783 was approximately 12 3 km3 when did the eruption occur comparative studies of acoustic reflectivity and photographed bottom characteristics elsewhere along the epr suggest that the lava field has little or no sediment cover and is very young this is also one of the only areas on the epr where earthquakes occur that do not appear to be caused by transform faulting if the earthquakes are related to the lava field then major eruptions may have occurred in 1964 1965 and 1969 a large helium plume near lat 13 s may be caused in part by eruptions of the lat 8 s lava field this is consistent with estimates of currents at 2 3 km depth which are such that a plume generated at lat 8 s would eventually appear off axis at lat 13 s our observations suggest that this spreading segment which is bounded by small ridge axis discontinuities at lat 8 05 s and 8 47 s may have been the most volcanically active segment along the epr during the past 25 yr

spectra and magnitudes of t waves from the 1993 earthquake swarm on the juan de fuca ridge a swarm of earthquakes on the crest of the juan de fuca ridge was detected in june and july 1993 by a network of hydrophones the activity migrated 60 km along the crest suggesting a lateral dike injection and the possibility of a volcanic eruption subsequent geologic and oceanographic investigations confirmed that an eruption had taken place examination of the individual acoustic arrivals shows changes in the character of the signal that are consistent with an injection of magma a reduction in the rise time of the wave packet and a proportional increase in high frequency energy was observed and is interpreted to result from a shoaling of the earthquake source region second the source magnitudes were largest at the onset of the swarm and became smaller over time also consistent with shoaling of the dike the appearance of the t wave arrivals changed significantly 5 days after the beginning of the swarm potentially indicating the onset of a surface eruption

hydrothermal processes at mid ocean ridges submarine hydrothermal systems are an integral component of crustal construction along the global system of mid ocean ridges thermally induced circulation of seawater through the permeable parts of the crust and upper mantle has been estimated to account for 34 of the predicted global oceanic heat flux which in turn comprises close to 25 of the total heat flux of the earth stein and stein 1994 and references therein discharge of hydrothermal fluids is manifest along mid ocean ridges as high temperature 200 400 c focused and lower temperature 200 c diffuse fluid flow off axis fluid flow may extend out to the crustal sealing age of 65 10 million years and may be responsible for more than 70 of the hydrothermal heat flux stein and stein 1994

the submarine volcanic system of the errigoiti formation albian santonian of the basque cantabrian basin northern spain stratigraphic framework facies and sequences abstract the errigoiti formation located in the central domain of the basque cantabrian basin is an albian to santonian bathyal submarine volcanic system of alkali basaltic character these volcanics occurred in an extensional geodynamic context generated by the drifting of the iberian plate with respect to the european plate stratigraphic analyses reveal the facies distribution and depositional architecture of a longitudinal cross section of the errigoiti volcanic system four facies groups were distinguished sheet lavas sl derived from low viscosity lava flows at high magma discharge rate pillow lavas pl derived from intermediate viscosity lava flows at moderate discharge rate pillow breccias pb derived either from in situ fragmentation of lavas pb1 or rock falling from pillow lava slopes pb2 and stratified volcaniclastites vc derived from volcaniclast resedimentation through gravity flows either by early remobilization vc1 or post volcanic reworking vc2 the facies groups occur in a predictable repetitive succession essentially from base to top an sl pl pb vc stacking pattern this is interpreted to reflect an upward waning in the magma discharge rate five major volcanic sequences make up the errigoiti formation each comprises packages tens to hundreds of meters thick deposited during short time spans of volcanic activity separated by long time intervals of volcanic inactivity consequently boundaries between sequences are associated with marked hiatuses reworked volcaniclasts and or turbidite pelagic sedimentation analysis of lateral and vertical facies evolution enabled us to propose a depositional model consisting of three depositional environments from proximal to distal settings these are amalgamated pillow volcanoes scattered pillow volcanoes and volcanic fringe basically responding to a proximal to distal pl pb pl vc and sl vc evolution of facies associations contemporaneous fossiliferous turbidite pelagic sequences that surround the volcanic succession allow paleobathymetry to be estimated on the basis of biofacies sites with the highest rate of volcanic emission correspond to an average depth of 900 m sites with thinner volcanic pile accumulated at a minimum depth of 1100 m the facies sequence established in this work and the processes involved in its formation are comparable with the sequences and processes described in other submarine basic volcanic systems consequently the depositional model for the errigoiti formation can perhaps serve as a reference model for this type of volcanic system