evidence of active ground deformation on the mid ocean ridge axial seamount juan de fuca ridge april june 1988 since september 1987 a precision bottom pressure recorder bpr has been deployed within the summit caldera of axial seamount on the central juan de fuca ridge the instrument is capable of measuring pressure to 1 mbar resolution and recording these measurements at 64 samples per hour for up to 15 months after removal of oscillatory signals due to tidal and oceanographic effects and linear trends caused by sensor drift any significant change in the pressure record should indicate a change of depth associated with vertical ground movement in subaerial volcanic systems such ground movements commonly indicate active inflation or deflation of underlying magma bodies and are frequently coincident with rift eruptions axial seamount was selected for this pilot study for three primary reasons 1 its tectonic setting 2 the presence of a well formed summit caldera and 3 observational evidence of geologically recent volcanic activity results from the first 9 months of the bpr deployment revealed a significant change in pressure which is interpreted to represent a 15 cm subsidence of the caldera floor during two 2 to 3 week periods in april june 1988 also during these periods an anomalous decline in temperature at the site was recorded that is correlated with an apparent increase in current velocity at the axial seamount hydrothermal emissions study ashes vent field suggesting vigorous advection of cold water into the caldera concurrent oceanographic data from geosat and from current meter arrays do not indicate any large scale oceanographic phenomena capable of generating these simultaneous events one mechanism to explain simultaneous ground subsidence and temperature decline at the caldera center and increased bottom current at the caldera margin is the generation of a buoyant parcel of heated water in response to the intrusion or the eruption of magma associated with volcanic deflation such a parcel must have been sufficiently buoyant and proximal to induce horizontal entrainment currents capable of producing the bottom water conditions that were observed at both sites similar volcanic events also may have generated large midwater plumes that have been described previously along the southern juan de fuca ridge

the january 1998 earthquake swarm at axial volcano juan de fuca ridge hydroacoustic evidence of seafloor volcanic activity on january 25 1998 at 11 33z an intense swarm of earthquakes began within the summit caldera of axial volcano central juan de fuca ridge jdfr the earthquake swarm was detected using the u s navy hydrophones in the northeast pacific ocean the earthquake swarm lasted 11 days and produced 8247 detected 1037 located earthquakes during the first two days of the swarm earthquake activity migrated from the summit caldera 50 km along the south rift zone of axial volcano at rates of 0 92 0 13 and 0 23 0 09 m s 1 earthquake epicenter migration is characteristic of a lateral magma dike injection the largest three swarm earthquakes occurred within the caldera their timing and mechanisms are consistent with adjustment of the caldera floor as magma is removed from beneath the summit earthquakes with short t wave rise times shallow focal depths cluster at two spots along the summit and south rift zone and are considered possible seafloor eruption sites in situ ground deformation and hydrothermal plume monitors and later shipboard observations confirmed the occurrence of a seafloor volcanic eruption at the volcano s summit

the juan de fuca ridge hot spot propagating rift system new tectonic geochemical and magnetic data underway geophysics deep tow camera work and preliminary geochemical and magnetic studies from a 1980 cruise to the juan de fuca ridge allow the following interpretations 1 the central third between 45 5 n and 46 5 n of the ridge is now actively spreading from a zone which is 10 to 20 km west of the axis bisecting the brunhes matuyama magnetic reversal boundary 2 the apparent terminus of surficial igneous activity at the northern tip of the active cobb propagator has been identified by dredging and camera work the tip appears curved or offset slightly to the west does not correspond to the northernmost tip of the central magnetic anomaly elvers et al 1974 and spreading has been initiated beneath sediments farther north and 3 iron and titanium in fresh glasses from the active volcanic zone exhibit a progressive enrichment northward then a pronounced reversal to more normal concentrations in the vicinity of the cobb offset

flow hydrography turbulent mixing and dissipation at fieberling guyot examined with a primitive equation model 1 the extensive set of measurements previously taken at fieberling guyot provides a backdrop for model results on flow hydrography turbulent mixing and derivative variables a z level primitive equation model driven at subinertial and superinertial tidal frequencies allows the nonlinear interaction of free and topographically trapped motions to jointly determine model outcomes model residual flows mean hydrographic features topographic amplification of oscillatory currents turbulent dissipation and vertical diffusivity kv profiles are all examined in the light of the observations azimuthal residual flows are toroidal above the summit with maximum speed 20 cm s 1 diurnal flow over the summit is intensified by a factor 20 downward vertical flow as a time mean w t occurs from 150 m above the summit apex to summit rim depth with radially inward flow above the apex and radial outward flow below upward w t rings the summit at larger r the existence of the observed cold dome is shown to depend on w t in the summit circulation cell vertical current shear is concentrated in a 150 m thick cap above the summit accounting for a domed distribution of and kv with maxima of 10 7 w kg 1 and 30 cm2 s 1 respectively profiles most like the observations of kv are obtained with a subgrid scale parameterization in which the turbulent prandtl number pr depends on the shear richardson number ri

the june july 1993 seismo acoustic event at coaxial segment juan de fuca ridge evidence for a lateral dike injection the coaxial segment juan de fuca ridge was the site of an intense swarm of earthquakes that began at 21 43 gmt on june 26 1993 julian day 177 the swarm started near 46 15 n and migrated northward over the next 40 hours to 46 36 n where the majority of 676 events occurred during the following 3 weeks of activity the earthquakes propagated nne at a velocity of 0 3 0 1 m s 1 from the southern to northern swarm sites the activity went undetected by land based seismic networks along the oregon and washington coasts suggesting that the earthquakes were all m 4 0 the character of this earthquake swarm is very similar to dike injections observed at krafla and kilauea volcanoes the earthquake activity and subsequent migration probably represent a lateral dike injection into faults and fissures comprising the coaxial segment the reservoir acting as the dike s source likely resides beneath or to the south of the initial southern swarm of earthquakes the large magma supply at axial volcano cannot be ruled out as the source for the coaxial dike even though axial volcano exhibited no earthquake activity related to the coaxial swarm the t wave earthquake swarm reported here is the first deep ocean observation of volcanic seismicity associated with what most likely was a mid ocean ridge dike injection event

long term seismicity and ground deformation at axial volcano juan de fuca ridge seismicity along the juan de fuca ridge jdfr has been monitored since august 1991 using u s navy hydrophones in the northeast pacific ocean during this time the focus of jdfr seismicity has been axial volcano where a total of 9055 earthquakes have been detected 1669 located this activity occurred within 48 discrete swarms along the summit caldera and flank rift zones from august 1991 to may 1996 axial volcano exhibited only one swarm with 50 earthquakes from may 1996 to november 1997 swarm activity increased significantly with five swarms of 50 earthquakes activity culminated at the volcano in january 1998 with an 11 day 8247 event swarm that was the result of a major eruption along the summit and southern flank seafloor deformation sensors indicated pre eruption swarms in october 1995 may 1996 and june 1996 coincided with subsidence of the caldera floor the 1996 1997 swarm activity increase also coincided with horizontal extension of the north rift zone consistent with pre eruption deformation of the volcano following the 1998 eruption swarm activity dramatically decreased and no earthquakes have been detected since april 1998 this is the longest aseismic period 1 yr observed at the volcano since acoustic monitoring began suggesting the january 1998 swarm marked the end of an eruptive cycle at axial volcano

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

the quantum event of oceanic crustal accretion impacts of diking at mid ocean ridges seafloor diking eruptive events represent the irreducible quantum events of upper oceanic crustal accretion they record events by which a large portion of the oceanic crust has formed through geological history since 1993 the u s navy s real time sound surveillance system has allowed location of ongoing acoustic signatures of dike emplacement and basalt eruptions at ridge crests in the northeast pacific these diking eruptive events trigger a sequence of related rapidly evolving physical chemical and biological processes magmatic volatiles released during these events may provide nutrients for communities of subseafloor microorganisms some of which thrive in high temperature anaerobic environments many of the organisms identified from these systems are archaea if microorganisms can thrive in the water saturated pores and cracks within deep volcanically active portions of our planet other hydrothermally active planets may harbor similar life forms

evidence of a recent magma dike intrusion at the slow spreading lucky strike segment mid atlantic ridge 1 mid ocean ridge volcanic activity is the fundamental process for creation of ocean crust yet the dynamics of magma emplacement along the slow spreading mid atlantic ridge mar are largely unknown we present acoustical seismological and biological evidence of a magmatic dike intrusion at the lucky strike segment the first detected from the deeper sections 1500 m of the mar the dike caused the largest teleseismic earthquake swarm recorded at lucky strike in 20 years of seismic monitoring and one of the largest ever recorded on the northern mar hydrophone records indicate that the rate of earthquake activity decays in a nontectonic manner and that the onset of the swarm was accompanied by 30 min of broadband 3 hz intrusion tremor suggesting a volcanic origin two submersible investigations of high temperature vents located at the summit of lucky strike seamount 3 months and 1 year after the swarm showed a significant increase in microbial activity and diffuse venting this magmatic episode may represent one form of volcanism along the mar where highly focused pockets of magma are intruded sporadically into the shallow ocean crust beneath long lived discrete volcanic structures recharging preexisting seafloor hydrothermal vents and ecosystems index terms 3035 marine geology and geophysics midocean ridge processes 7280 seismology volcano seismology 8419 8149 tectonophysics planetary tectonics 5475 4259 oceanography general ocean acoustics 9325 information related to geographic region atlantic ocean keywords mid atlantic ridge earthquake hydroacoustic

high resolution studies of the summit of axial volcano sea beam bathymetry sea marc i side scan 30 khz towed camera and submersible data from axial volcano juan de fuca ridge jdfr have been integrated to produce detailed maps of the volcano s summit region in order to elucidate its volcanic and tectonic evolution the proximity of axial volcano the youngest edifice of the cobb eickelberg seamount chain to the jdfr superposes a hotspot derived radial stress pattern onto the linear stress field of a mid ocean ridge spreading center the morphology of the summit of axial volcano reflects the differing eruptive styles produced from this superposition long rift zones begin at the edges of the caldera and extend as much as 75 km to the north and south axial volcano has a distinct summit plateau similar to those found on many other submarine volcanos the process by which summit plateaus are constructed is illustrated by the lava flows originating from the south rift zone which have partially filled the southern half of the caldera and have smoothed the flanks the summit plateau contains penetrative ne sw fracturing that is approximately parallel to the jdfr the unusual rectangular caldera is 3 8 km in size and its trend is about 160 three possible mechanisms are discussed for the caldera s formation 1 canary island type trapdoor formation 2 overlapping spreading center interaction and 3 magma source migration although none of these models is fully compatible with the present data base the overlapping spreading center model is preferred postcaldera volcanism probably began with the formation of the central caldera eruptive complex which was characterized by relatively low volume high viscosity flows the most recent volcanism has apparently taken place along the northern and southern rift zones which have erupted more fluid lavas resulting in areally extensive flows covering up to 60 of the caldera floor extensive low temperature venting and areas of high temperature venting near the northern and southern rift zones and caldera walls suggest a shallow magmatic source beneath the summit of axial volcano

microbial biomass in the hydrothermal plumes associated with the 1998 axial volcano eruption the axial response team art 1 documented greatly intensified chronic style hydrothermal plumes at axial volcano following a 1998 eruptive event significantly higher numbers of bacteria were found in the plume versus background stations and depths due largely to 5 samples with high numbers to 1 8 105 ml in far field stations highest ratios 0 20 of metal depositing capsuled bacteria to total bacteria were found in the near field over caldera plume an unusual capsule form fecap that dominated the near field plume capsule populations was present in all plume samples but were absent in background samples multi cell filaments metal encrusted sheathed clusters and matrix enmeshed colonies all uncommon in the water column were also present in plume samples

recent rift related volcanism in afar ethiopia abstract rift zones are the most common magmatic environment on earth however opportunities to observe active rifting are rare and consequently the volcanological characteristics of rift systems are not well understood an ongoing phase of magmatic rifting along a section of the red sea system in afar ethiopia presents an exceptional opportunity to constrain relationships between volcanism and crustal growth here by integrating analyses of satellite images i e modis omi aster and ali with field observations we characterise two recent august 2007 and june 2009 basaltic fissure eruptions in afar and evaluate the role and significance of volcanism in the rifting process both events were brief 36 72 h and erupted 4 4 18 106 m3 of lava from a fissure system 4 6 5 km in length data from the spaceborne ozone monitoring instrument omi suggests total so2 emissions for each eruption of 26 5 kt 2007 and 34 7 kt 2009 consistent with complete degassing of the erupted magma volumes using geodetic models for the intrusive activity in afar we estimate the partitioning of magma between intrusive and extrusive components up to july 2009 to be 180 1 comparing the first order volcanic characteristics and the intrusive extrusive volume balance for the afar volcanism with data from the 1975 1984 krafla rifting cycle iceland suggests that the volcanic flux in afar will rise significantly over the next few years as the stresses are increasingly relieved by dyking and subsequent dykes are able to propagate more easily to the surface as a consequence basaltic fissure eruptions in this section of the afar rift will become of increasing large magnitude as the rifting event matures over the next 5 10 yr using available models of magmatic rifting we forecast the likely size and location of future eruptions in afar

circulation hydrography and transport over the summit of axial seamount a deep volcano in the northeast pacific a numerical model of ocean flow hydrography and transport is used to extrapolate observations of currents and hydrography and infer patterns of material flux in the deep ocean around axial seamount a destination node of nsf s ocean observatories initiative s cabled array using an inverse method the model is made to approximate measured deep ocean flow around this site during a 35 day time period in the year 2002 the model is then used to extract month long mean patterns and examine smaller scale spatial and temporal variability around axial like prior observations model month long mean currents flow anti cyclonically around the seamount s summit in toroidal form with maximum speeds at 1500m depth of 10 11 cm s as a time mean the temperature salinity anomaly distribution takes the form of a cold briny dome above the summit passive tracer material continually released at the location of the ashes vent field exits the caldera primarily through its southern open end before filling the caldera once outside the caldera the tracer circles the summit in clockwise fashion fractionally re entering the caldera over lower walls at its north end while gradually bleeding southwestward during the modeled time period into the ambient ocean a second tracer release experiment using a source of only two day duration inside and near the casm vent field at the northern end of the caldera suggests a residence time of the fluid at that locale of 8 9 days

anomalous helium and heat signatures associated with the 1998 axial volcano event juan de fuca ridge water column plumes sampled before and after the 1998 eruptive event on axial volcano juan de fuca ridge showed a complex pattern in he and heat very different from that observed during previous magma injection events plumes detected in 1997 before the event had relatively high he heat consistent with the volatile rich character of the known hydrothermal vents within the summit caldera a response cruise in february 1998 immediately after the event failed to detect any event plumes but instead found an intense steady state type plume with high temperature signals moderate he enrichments and low he heat ratios this plume was present over the caldera and extended a considerable distance off axis in contrast all of the plumes detected during follow up cruises in july september 1998 had moderate temperature signals high he concentrations and elevated he heat ratios comparison with previous events suggests that the low he heat plume sampled in february was either the remnant of an event plume which had already migrated off axis or an event plume like discharge which was smeared out by strong currents it is likely that the high he heat plumes observed in july september were derived from the injected dike or from the new lava flow

1998 eruption of axial volcano multibeam anomalies and sea floor observations lava flows erupted during the january february 1998 seismic swarm at axial volcano on the juan de fuca ridge have been identified by differencing of pre and post event multibeam bathymetric surveys and by seafloor observations a sheet flow more than 3 km in length and 500 800 m wide erupted from the uppermost south rift is the site of a robust hydrothermal system 1998 lavas occur over about 9 km of the upper south rift zone or about 20 of the along axis length of the seismicity event 50 km the estimated volume of lava erupted is 18 76 106 m for the extrusion and 100 106 m for the intrusion the total volume is consistent with the volume from modeling of seafloor strain measurements recorded during the event

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

model estimates of m2 internal tide generation over mid atlantic ridge topography abstract the conversion of barotropic to baroclinic m2 tidal energy is examined for a section of the mid atlantic ridge in the brazil basin using a primitive equation model model runs are made with different horizontal smoothing 1 5 6 and 15 km applied to a 192 km 183 km section of multibeam bathymetry to characterize the influence of topographic resolution on the model conversion rates in all model simulations barotropic to baroclinic conversion is highest over near and supercritical slopes on the flanks of abyssal hills and discordant zones from these generation sites internal tides propagate upward and downward as tidal beams the most energetic internal tide mode generated is mode 2 consistent with the dominant length scales of the topographic slope spectrum 50 km the topographic smoothing significantly affects the model conversion amplitudes with the domain averaged conversion rate from the 1 5 km run 15 1 mw m 2 4 and 19 higher than for the 6 km 14 5 mw m 2 and 15 km runs 12

evolution of submarine eruptive activity during the 2011 2012 el hierro event as documented by hydroacoustic images and remotely operated vehicle observations submarine volcanic eruptions are frequent and important events yet they are rarely observed here we relate bathymetric and hydroacoustic images from the 2011 to 2012 el hierro eruption with surface observations and deposits imaged and sampled by rov as a result of the shallow submarine eruption a new volcano named tagoro grew from 375 to 89 m depth the eruption consisted of two main phases of edifice construction intercalated with collapse events hydroacoustic images show that the eruptions ranged from explosive to effusive with variable plume types and resulting deposits even over short time intervals at the base of the edifice rov observations show large accumulations of lava balloons changing in size and type downslope coinciding with the area where floating lava balloon fallout was observed peaks in eruption intensity during explosive phases generated vigorous bubbling at the surface extensive ash vesicular lapilli and formed high density currents which together with periods of edifice gravitational collapse produced extensive deep volcaniclastic aprons secondary cones developed in the last stages and show evidence for effusive activity with lava ponds and lava flows that cover deposits of stacked lava balloons chaotic masses of heterometric boulders around the summit of the principal cone are related to progressive sealing of the vent with decreasing or variable magma supply hornitos represent the final eruptive activity with hydrothermal alteration and bacterial mats at the summit our study documents the distinct evolution of a submarine volcano and highlights the range of deposit types that may form and be rapidly destroyed in such eruptions

evidence for deformation associated with the 1998 eruption of axial volcano juan de fuca ridge from acoustic extensometer measurements acoustic extensometer instruments capable of making precise daily measurements of horizontal distance were deployed across the north rift zone of axial volcano in june 1996 and were in place when a submarine eruption began on axial s south rift zone in january 1998 the instruments recorded a gradual 9 cm extension over a 405 m baseline leading up to the eruption and then an abrupt 4 cm contraction at the time of the eruption an elastic point source deformation model shows that deflation of axial s summit can explain both the 4 cm distance decrease at the extensometer array and a 3 2 m subsidence measured by another instrument in the caldera if the pressure source is located at a depth of 3 8 km below the center of the caldera the 9 cm distance increase may represent pre eruption spreading across the rift zone

quantitative constraints on the growth of submarine lava pillars from a monitoring instrument that was caught in a lava flow 1 lava pillars are hollow vertical chimneys of solid basaltic lava that are common features within the collapsed interiors of submarine sheet flows on intermediate and fast spreading mid ocean ridges they are morphologically similar to lava trees that form on land when lava overruns forested areas but the sides of lava pillars are covered with distinctive evenly spaced thin horizontal lava crusts referred to hereafter as lava shelves lava stalactites up to 5 cm long on the undersides of these shelves are evidence that cavities filled with a hot vapor phase existed temporarily beneath each crust during the submarine eruption of axial volcano in 1998 on the juan de fuca ridge a monitoring instrument called vsm2 became embedded in the upper crust of a lava flow that produced 3 to 5 m high lava pillars a pressure sensor in the instrument showed that the 1998 lobate sheet flow inflated 3 5 m and then drained out again in only 2 5 hours these data provide the first quantitative constraints on the timescale of lava pillar formation and the rates of submarine lava flow inflation and drainback they also allow comparisons to lava flow inflation rates observed on land to theoretical models of crust formation on submarine lava and to previous models of pillar formation a new model is presented for the rhythmic formation of alternating lava crusts and vapor cavities to explain how stacks of lava shelves are formed on the sides of lava pillars during continuous lava drainback each vapor cavity is created between a stranded crust and the subsiding lava surface a hot vapor phase forms within each cavity as seawater is syringed through tiny cracks in the stranded crust above eventually the subsiding lava causes the crust above to fail quenching the hot cavity and forming the next lava crust during the 1998 eruption at axial volcano this process repeated itself about every 2 min during the 81 min long drainback phase of the eruption based on the thickness and spacing of the lava shelves the vsm2 data show that lava pillars are formed during short lived eruptions in which inflation and drainback follow each other in rapid succession and that pillars record physical evidence that can be used to interpret the dynamics of seafloor eruptions

high resolution bathymetric surveys using scanning sonars lava flow morphology hydrothermal vents and geologic structure at recent eruption sites on the juan de fuca ridge the coaxial and cleft segments of the juan de fuca ridge have isolated chronic high temperature and focused hydrothermal vent sites both segments also have experienced recent volcanic eruptions which produced extensive ephemeral low temperature and diffuse hydrothermal venting to study the geologic setting of these sites high resolution bathymetric surveys at eight locations on the coaxial and cleft segments were collected between 1993 and 1999 two 675 khz scanning sonar systems were used mesotech on the submersible alvin and imagenex on the remotely operated vehicle jason the bathymetry from these surveys can be gridded at a scale of 2 4 m and contoured at 1 m and thus can resolve many fine scale features on the seafloor that are indistinguishable in multibeam bathymetry collected at the sea surface bathymetric data at this resolution are particularly useful for identifying geologic features related to diking faulting and lava flow emplacement for example the high resolution bathymetric maps show that submarine fissure eruptions that form pillow lavas last long enough to become localized and to produce point source constructs along their length and their extrusion rate is low enough that no significant drainback occurs in contrast lobate sheet flows are formed by short lived high effusion rate eruptions in which no localization of output occurs along the eruptive fissure and inflation is quickly followed by drainback resulting in extensive collapse features however if the process of submarine lava flow inflation occurs at a slower rate and over a longer period of time it can create lava rises up to 25 m high with distinctive structure and morphology the scanning sonar data also show that fissures and grabens have formed or reactivated where dikes approach the surface adjacent to recent eruptive sites the fine scale bathymetry establishes that all the hydrothermal vent sites studied at the coaxial and cleft segments are located along prominent volcanic or tectonic extensional structures which provide the physical pathway for fluids from the subsurface to the seafloor furthermore the fine scale morphology of recent lava flows can be used as a qualitative indication of eruption duration

paleointensity record in zero age submarine basalt glasses testing a new dating technique for recent morbs abstract thellier thellier paleointensity experiments were conducted on a collection of glasses from three very recent submarine axial flows two were erupted along the juan de fuca ridge at around 46 n and one along the east pacific rise south at around 18 s the within sample dispersion of paleointensity results from the animal farm flow epr south is very low and leads to a well defined mean value of 35 6 1 t 95 error on the mean based on 11 glass chips from four independent samples today s geomagnetic field intensity in the area is 31 2 t comparing animal farm results with published field model reference curves developed for the past 400 yr suggests an eruptive date estimated between 1880 a d and 1950 a d taking into account different sources of errors this is consistent with qualitative evidence for the age of this flow and constitutes the first precise demonstration of using paleointensity as a dating tool for very recent mid ocean ridge basalts morbs however in the juan de fuca area results show a more erratic pattern with samples varying by up to 30 higher and lower from the expected value of about 55 t the dispersion is attributed to the large local crustal magnetic anomalies in this area that can lead to inconsistent intensity values over the same unit local magnetic anomalies should thus always be checked when doing paleointensity on morb samples which should also be distributed as widely as possible in a flow unit when no significant magnetic anomalies are detected the paleointensity dating tool is anticipated to be especially efficient to investigate the volcanic cyclicity along the epr axis during the last several hundred years

minimum speed limit for ocean ridge magmatism from 210pb 226ra 230th disequilibria although 70 per cent of global crustal magmatism occurs at mid ocean ridges where the heat budget controls crustal structure hydrothermal activity and a vibrant biosphere the tempo of magmatic inputs in these regions remains poorly understood such timescales can be assessed however with natural radioactive decay chain nuclides because chemical disruption to secular equilibrium systems initiates parent daughter disequilibria which re equilibrate by the shorter half life in a pair here we use 210pb 226ra 230th radioactive disequilibria and other geochemical attributes in oceanic basalts less than 20 years old to infer that melts of the earth s mantle can be transported accumulated and erupted in a few decades this implies that magmatic conditions can fluctuate rapidly at ridge volcanoes 210pb deficits of up to 15 per cent relative to 226ra occur in normal mid ocean ridge basalts with the largest deficits in the most magnesium rich lavas the 22 year half life of 210pb requires very recent fractionation of these two uranium series nuclides relationships between 210pb deficits 226ra 230th activity ratios and compatible trace element ratios preclude crustal magma differentiation or daughter isotope degassing as the main causes for the signal a mantle melting model can simulate observed disequilibria but preservation requires a subsequent mechanism to transport melt rapidly the likelihood of magmatic disequilibria occurring before melt enters shallow crustal magma bodies also limits differentiation and heat replenishment timescales to decades at the localities studied

post eruption succession of macrofaunal communities at diffuse flow hydrothermal vents on axial volcano juan de fuca ridge northeast pacific abstract hydrothermal vents harbor dense aggregations of invertebrate fauna supported by chemosynthesis severe tectonic events and volcanic eruptions frequently destroy vent communities and initiate primary succession at new vents on ridge crest submarine lava flows an eruption on axial volcano 1500 m depth a seamount on the juan de fuca ridge jdfr in the northeast pacific ocean occurred in january 1998 which created new substratum and vents this study examines the development of the macrofaunal vent assemblages associated with tubeworms ridgeia piscesae at eight diffuse flow vents over the following 3 years biological collections by suction of lava surfaces also characterized pre tubeworm assemblages coupled fluid sampling showed an overall decrease in temperature sulphide and sulphide to heat ratios over 3 years as well as large spatial variability across the new vents we examined collections of pre eruption diffuse flow vent assemblages at axial volcano to compare the stages of new community development to mature vents mature vent assemblages are characterized by two major community types dominated by limpets lepetodrilus fucensis and alvinellid polychaetes paralvinella pandorae and or p palmiformis the following post eruption succession patterns emerged first r piscesae tubeworms took up to 3 years to establish aggregations at the new vents and the majority of pre tubeworm assemblages were dominated by grazing polychaetes second species colonized quickly and by 30 months after habitat creation 60 of axial s species pool had arrived at the new vents abundance at mature vents predicted colonization success with some notable exceptions third shifts in species dominance occurred rapidly and by year 3 new vent assemblages resembled mature pre eruption communities in general tubeworm assemblages were dominated by alvinellid polychaetes p pandorae and p palmiformis in the first 2 years post eruption with limpets l fucensis becoming more numerous in year 3 fourth successional trajectories corresponded to four habitat variables the timing of r piscease recruitment vent age maximum vent fluid sulphide to heat ratios and maximum vent fluid temperatures the two mature community types dominance by limpets or alvinellid polychaetes seem to relate in part to vent differences in fluid properties with limpets more prevalent than alvinellids at diffuse vents with lower maximum sulphide to heat ratios and temperatures we present a general model of post eruption succession for juan de fuca ridge diffuse flow vents the model incorporates potential abiotic and biotic drivers of community development and identifies six successional stages a pre tubeworm assemblage four tubeworm assemblage states determined by the relative dominance of alvinellid polychaetes and the limpet and a final senescent stage when venting wanes future eruptions will allow for this model to be tested through direct observation and experimentation

in situ ground deformation measurements from the summit of axial volcano during the 1998 volcanic episode since september 1987 seafloor instrumentation monitoring ground deformation has been maintained on the summit of axial volcano on the central juan de fuca ridge during a january 1998 volcanic episode pressure records from the central caldera showed a significant increase in pressure indicating caldera subsidence of over 3 m this observed subsidence is interpreted to represent volcanic deflation as magma is erupted and or injected into the volcanic flanks temperature anomalies of nearly 0 45 c were also observed a comparison of the subsidence record with hydroacoustic recordings indicates a continuous volcanic tremor signal coincident with the subsidence event the time coincidence of subsidence of the summit caldera with migration of epicenters into the volcanic rift zones confirms that during the 1998 episode the volcanic flanks were supplied by lateral dike injection from a central magma body rather than locally derived magma sources

the co chondrites major recent antarctic finds their thermal and radiation history and describing the metamorphic history of members of the class abstract thermoluminescence tl properties of 29 co chondrites from the miller range mil and five chondrites from the dominion range dom have been measured mil has a relatively strong natural tl signal 19 6 14 7 krad while some of the dom samples have a very weak natural tl signal

establishing a new era of submarine volcanic observatories cabling axial seamount and the endeavour segment of the juan de fuca ridge abstract at least 70 of the volcanism on earth occurs along the 65 000 km network of mid ocean ridge mor spreading centers within these dynamic environments the highest fluxes of heat chemicals and biological material from the lithosphere to the hydrosphere occur during volcanic eruptions however because underwater volcanoes are difficult and expensive to access researchers are rarely if ever in the right place at the right time to characterize these events therefore our knowledge is limited about the linkages among hydrothermal chemical and biological processes during seafloor formation and crustal evolution to make significant advancements in understanding the evolution of mor environments the united states and canada have invested in the first plate scale submarine cabled observatory linked through the global internet spanning the juan de fuca tectonic plate these two networks include 1700 km of cable and 14 subsea terminals that provide 8 10 kw power and 10 gbs communications to hundreds of instruments on the seafloor and throughout the overlying water column resulting in a 24 7 365 presence in the oceans data and imagery are available in real to near real time the initial experimental sites for monitoring volcanic processes include the mor volcanoes called axial seamount and the endeavour segment that are located on the juan de fuca ridge axial a hot spot influenced seamount is the most robust volcano along the ridge rising nearly 1400 m above the surrounding seafloor and it has erupted twice in the last 15 years in contrast the endeavour segment is characterized by more subdued topography with a well defined axial rift and it hosts one of the most intensely venting hydrothermal systems known a non eruptive spreading event lasting 6 years was documented at endeavour between 1999 and 2005 hydrothermal venting intensity chemistry and temperature as well as associated biological communities at both sites were significantly perturbed by the magmatic and intrusive events this paper presents the similarities and differences between the axial and endeavour volcanic systems and identifies reasons why they are ideal candidates for comparative studies the u s has made a 25 year commitment for sustained observations using the cabled infrastructure the highly expandable nature of submarine optical networking will allow for the future addition of novel experiments that utilize ever evolving advancements in computer sciences robotics genomics and sensor miniaturization comprehensive modeling of the myriad processes involved will continue to assimilate and integrate growing databases yielding a new understanding of integrated processes that create the seafloor in the global ocean basins

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

vertical deformation monitoring at axial seamount since its 1998 eruption using deep sea pressure sensors pressure measurements made on the seafloor at depths between 1500 and 1700 m at axial seamount an active submarine volcano on the juan de fuca ridge in the northeast pacific ocean show evidence that it has been inflating since its 1998 eruption data from continuously recording bottom pressure sensors at the center of axial s caldera suggest that the rate of inflation was highest in the months right after the eruption 20 cm month and has since declined to a steady rate of 15 cm year independent campaign style pressure measurements made each year since 2000 at an array of seafloor benchmarks with a mobile pressure recorder mounted on a remotely operated vehicle also indicate uplift is occurring in the caldera at a rate up to 22f1 3 cm year relative to a point outside the caldera the repeatability of the campaign style pressure measurements progressively improved each year from f15 cm in 2000 to f0 9 cm in 2004 as errors were eliminated and the technique was refined assuming that the uplift has been continuous since the 1998 eruption these observations suggest that the center of the caldera has re inflated about 1 5f0 1 m thus recovering almost 50 of the 3 2 m of subsidence that was measured during the 1998 eruption this rate of inflation can be used to calculate a magma supply rate of 1410 6 m 3 year if this rate of inflation continues it also suggests a recurrence interval of 16 years between eruptions at axial assuming that it will be ready to erupt again when it has re inflated to 1998 levels d 2005 elsevier b v all rights reserved

ocean currents at axial volcano a northeastern pacific seamount 1 axial volcano is a magmatically active seamount that straddles the juan de fuca ridge jdfr with a summit depth of 1400 m the volcano intercepts deep ocean flows at a height well above that of the linear ridge adjoining it to the north and south multiyear yearlong observations of currents at axial show that mean flow circles the seamount in an anticyclonic sense consistent with dynamical constraints with means as large as 6 6 cm s 1 spectra of currents indicate prominent tidal inertial and in some cases weather band period 3 to 7 day peaks as has been seen elsewhere along the jdfr ridge for k1 o1 and s2 tidal currents at the seamount amplitudes are several times larger than the corresponding background amplitudes and rotation around the tidal current ellipses is anticyclonic m2 tidal currents show none of the same properties energy at the inertial frequency decreases by more than an order of magnitude from 100 m above to 400 m below the volcano s summit in contrast weather band wb period energy increases tenfold over the same depth range with wb energy all but unobservable at summit depth the presence of wb oscillations makes axial unlike other seamounts where long observations have been made the current meter observations also provide a picture of seasonal trends as expected during the period summer 1996 to spring 1997 largest amounts of wb energy are found during autumn and winter in contrast wb motion at axial volcano during winter 2000 was summer like in intensity a linear baroclinic model of forced damped azimuthal mode one oscillatory flow around an idealized axisymmetric seamount shows that amplification of diurnal flow should occur principally at the seamount summit while amplification of the wb should occur as the observations show on the seamount flanks model and measured phase relationships for temperature and current time series pairs circumscribing axial support the view that azimuthal mode one oscillations are prominent over the 0 8 to 6 day range for forcing period tc the model indicates that the principal topographic resonance as indicated by kinetic energy ke amplification occurs for a tc greater than 1 and less than 2 days with only mild dependence in that range on background stratification and seamount shape additional but weaker topographic resonances can occur at longer periods in model results but as tc becomes slightly larger than the frictional timescale topographic resonances no longer appear high dissipation and thus short frictional timescales reported by others in regions of rough sloping topography make topographic resonances at axial at periods greater than a few days somewhat unlikely the relatively broad wb spectral peaks at some axial sites are consequently more likely the result of spectral peaks in forcing than of topographic resonance

evidence for iron and sulfur enrichments in hydrothermal plumes at axial volcano following the january february 1998 eruption in response to 12 days of seismic activity at axial volcano juan de fuca ridge in january february 1998 the nsf noaa sponsored axial response team conducted three response cruises in february august and august september to map and sample hydrothermal plumes over the region vertical profiles of particulate fe and s over the eruption site show high concentrations from about 1400 m to the bottom chemical and scanning electron microscope analysis of the february plume samples revealed anhydrite fe ferrihydrites elemental sulfur and angular glassy basalt shards up to 190 m in the longest dimension many of these shards had halite coatings which is consistent with subseafloor basalt seawater reactions at temperatures 450 c at 1500 m depth in august and september cruises the basalt shards were no longer present in the hydrothermal plumes instead the plumes were predominantly composed of fe ferrihydrites elemental sulfur and sulfur filaments a unique feature of this data set is the high concentrations of elemental sulfur in the lower 60 m of the water column the sulfur results are suggestive of a significant enrichment of the bacterial biomass in the water column over the eruption site with time within the axial caldera approximately 10 20 of the total sulfur is present as sulfur filaments these post eruption particle compositional changes have strong similarities to the results from the 1993 coaxial eruption on the juan de fuca ridge

contamination of basaltic lava by seawater evidence found in a lava pillar from axial seamount juan de fuca ridge a lava pillar formed during the 1998 eruption at axial seamount exhibits compositional and textural evidence for contamination by seawater under magmatic conditions glass immediately adjacent to anastomosing microfractures within 1 cm of the inner pillar wall is oxidized and significantly enriched in na and cl and depleted in fe and k with respect to that in glassy selvages from the unaffected outer pillar wall the affected glass contains up to 1 wt cl and is enriched by 2 wt na2o relative to unaffected glass consistent with a nearly 1 1 molar incorporation of nacl glass bordering the cl enriched glass in the inner pillar wall is depleted in na but enriched in k the presence of tiny 10 m grains of cu fe sulfides and fe sulfides as well as elemental ni ag and au in the na depleted k enriched glass of the inner pillar wall implies significant reduction of this glass presumably by hydrogen generated during seawater contamination and oxidation of lava adjacent to microfractures we interpret the compositional anomalies we see in the glass of the interior pillar wall as caused by rapid incorporation of seawater into the still molten lava during pillar growth probably on the time scale of seconds to minutes only one of seven examined lava pillars shows this effect and we interpret that seawater has to be trapped in contact with molten lava inside the lava pillar in this case to produce the effects we see thus under the right conditions seawater contamination of lavas during submarine eruptions is one means by which the oceanic crust can sequester cl during its global flux cycle however since very few recent lava flows have been examined in similar detail the global significance of this process in effecting earth s cl budget remains uncertain

episodic melt transport at mid ocean ridges inferred from magnetotelluric sounding oceanic crust is generated at mid ocean ridges by decompression melting of upwelling mantle at depths of between 50 and 120 km geodynamic and geochemical models of upwelling melt extraction and melt emplacement into crustal magma reservoirs present a variety of possible migration geometries most of which assume steady state or near steady state processes here we present results from marine magnetotelluric mt measurements carried out as part of the ramesses experiment on the slow spreading reykjanes ridge which support a model of melt extraction and migration that is episodic rather than steady state

the 1998 eruption of axial seamount new insights on submarine lava flow emplacement from high resolution mapping 1 axial seamount an active submarine volcano on the juan de fuca ridge at 46 n 130 w erupted in january 1998 along 11 km of its upper south rift zone we use ship based multibeam sonar high resolution 1 m bathymetry sidescan sonar imagery and submersible dive observations to map four separate 1998 lava flows that were fed from 11 eruptive fissures these new mapping results give an eruption volume of 31 106 m3 70 of which was in the northern most flow 23 in the southern most flow and 7 in two smaller flows in between we introduce the concept of map scale submarine lava flow morphology observed at a scale of hundreds of meters as revealed by the high resolution bathymetry and an interpretive model in which two map scale morphologies are produced by high effusion rate eruptions inflated lobate flows are formed near eruptive vents and where they drain downslope more than 0 5 1 0 km they transition to inflated pillow flows these two morphologies are observed on the 1998 lava flows at axial a third map scale flow morphology that was not produced during this eruption pillow mounds is formed by low effusion rate eruptions in which pillow lava piles up directly over the eruptive vents axial seamount erupted again in april 2011 and there are remarkable similarities between the 1998 and 2011 eruptions particularly the locations of eruptive vents and lava flow morphologies because the 2011 eruption reused most of the same eruptive fissures 58 of the area of the 1998 lava flows is now covered by 2011 lava

microearthquake patterns following the 1998 eruption of axial volcano juan de fuca ridge mechanical relaxation and thermal strain 1 ocean bottom seismic networks deployed following the 1998 eruption of axial seamount reveal an evolving pattern of microearthquake activity associated with subsurface magmatism and thermal strain seismicity rates decay steadily over 15 months of observation february 8 1998 to april 30 1999 consistent with a trend toward thermal and mechanical equilibrium in the shallow crust after the magmatic event immediately after the eruption seismicity rates were high for about 60 days in the southeast corner of the caldera where lava flows from the 1998 eruption were mapped a small burst of seismic activity was observed on the southeast shoulder of the volcano from 100 to 150 days after the eruption these events which are characterized by slip on nearly vertical faults in the shallow crust extend about 6 km from the southeast corner of the caldera and overlie a mid crustal low velocity zone after this episode seismicity rates remain low until the end of the observation period 455 days after the eruption shallow 0 7 km depth events consistent with thermal contraction and volume changes of 2 10 3 m3 in 5 m3 sources are observed in individual clusters beneath hydrothermal vents within the 1998 lava flow at the southeast edge of the caldera microearthquakes observed during the last 70 days of observation are distributed around the central caldera most likely representing small amounts of subsidence on caldera faults during the final stages of equilibration following melt withdrawal associated with the 1998 eruption