laboratory testing of an automatic method for determining normal fault geometry at depth abstract a general kinematic method for determining two dimensional normal fault geometries from a large number of hanging wall horizons has been tested on synthetic data and laboratory models the method assumes that the hanging wall deforms by a combination of arbitrarily inclined bulk simple shear and differential compaction it should be applicable to normal faults which do not change shape as deformation proceeds fault geometry the inclination of simple shear and compaction parameters may all be uniquely determined provided that the geometry of the hanging wall stratigraphy is accurately known prior to and after deformation testing the method on synthetic data has shown that unique solutions can be found solutions obtained using large numbers of hanging wall horizons of laboratory modelled faults are encouraging in each case the method was able to predict a fault geometry close to the fault geometry used in the laboratory model hence bulk simple shear appears to be a good approximation of hanging wall deformation the inclination of simple shear appears to be controlled by the underlying fault geometry rather than the amount of extension or the composition of the hanging wall fill in future it is hoped that a general method similar to this one can be applied routinely to depth converted seismic reflection data

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

high resolution bathymetry of the east pacific rise axial summit trough 9 49 51 n a compilation of alvin scanning sonar and altimetry data from 1991 1995 bathymetric data acquired during six cruises to the east pacific rise crest 9 49 51 n are compiled into a single bathymetric database and used to create high resolution bathymetry maps of two geologically and biologically active areas scanning sonar and alvin altimetry were collected during 23 alvin dives to the p vent bio9 and biomarker 141 areas during 1991 95 bathymetry data were filtered edited and processed as individual dives then combined into a single database for each cruise and statistically evaluated cruise data sets were numerically and visually compared and cross correlated to known bottom features and fixed seafloor reference markers then horizontally shifted to establish a consistent database with 3 m vertical and 5 m horizontal resolutions the compiled and corrected bathymetry was used to produce bathymetric maps of two areas within the axial summit collapse trough a 2 104 m area around p and bio9 vents and a 1 1 10 m area around biomarker 141 the resulting detailed bathymetry maps reveal topography dominated by low relief volcanic constructional features and post eruptive collapse of lobate and sheet lava surfaces

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

seabeam depth changes associated with recent lava flows coaxial segment juan de fuca ridge evidence for multiple eruptions between 1981 1993 after a swarm of earthquakes was detected on the coaxial segment of the juan de fuca ridge in june july 1993 the area was resurveyed with seabeam multibeam sonar to search for depth changes associated with a submarine volcanic eruption quantitative comparison of the 1993 seabeam survey with surveys in 1981 82 and 1991 shows one area of seafloor depth change up to 29 m between 1991 93 exactly where a pristine lava flow was discovered in addition two other depth anomalies up to 37 m and 20 m are identified between 1981 91 evidence that other recent eruptions have occurred along this spreading ridge segment

formation of venusian canali considerations of lava types and their thermal behaviors because liquid water is unstable at present venusian surface conditions the discovery of channels termed canali on venus thousands of kilometers long was not predicted low viscosity lavas that remain fluid for several thousand kilometers are considered to be the canali forming agents possible compositions of venusian canali forming lavas include komatiite and high fe ti lunar type basalts results of analytical and numerical models of these lavas reveal that total cooling is more efficient on venus than on earth suggesting that venusian lavas rapidly form insulating crusts and thus that the canali lavas were essentially tube fed the models also reveal that thermal erosion should be less efficient on venus than on earth suggesting that venusian channels are either the product of mechanical rather than thermal erosion or constructional processes

axial summit trough of the east pacific rice 9 10 n geological characteristics and evolution of the axial zone on fast spreading mid ocean ridge the nature and morphological characteristics of axial summit troughs on fast 90 130 mm yr 1 full spreading rate and superfast spreading 130 mm yr 1 mid ocean ridge crests reflect the time integrated effects of long term magmatic cycles short term volcanic episodicity and the tensional stress regime imposed on young ocean crust two principal types of axial trough morphology have been identified and associated with distinct volcanic and tectonic processes occurring at fast and superfast spreading mid ocean ridge crests 1 narrow axial troughs 300 2000 m wide and 30 100 m deep on the east pacific rise crest are classified as axial summit graben the dimensions of axial summit graben as well as the morphological and structural character of their walls and floors suggest a primary tectonic origin an axial summit graben may contain a nested axial summit collapse trough implying that processes responsible for these endemic features may be linked near bottom side looking sonar and observational data collected using the towed vehicle argo i and submersible alvin have been used to characterize the axial summit trough of the fast spreading east pacific rise between 9 and 10 n a four stage model is presented for the evolution of this axial summit collapse trough as well as for other well studied portions of the east pacific rise crest from 21 n to 20 s we propose that the transition from a narrow surface collapse dominated axial trough to a broader fault bounded graben is controlled by the relative importance of diking volcanism hydrothermal cooling and tectonism along a ridge segment over time periods 104 years

lava lakes on io new perspectives from modeling abstract loki patera 310 w 12 n is io s largest patera at 180 km in diameter its morphology and distinct thermal behavior have led researchers to hypothesize that loki patera may either be an active lava lake that experiences periodic overturn or a shallow depression whose floor is episodically resurfaced with thin flows using results from mathematical models we suggest that a better model for loki s behavior is the terrestrial superfast spreading east pacific rise epr near 17 30 south we propose that like at the southern epr loki patera is underlain by a thin persistent magma lens that feeds thin temporary lava lakes within the patera also like the southern epr overspilling of the volcanic depression is rare with most of the lava volume being emplaced via a subsurface network of lava tubes

multi channel seismic imaging of a crustal magma chamber along the east pacific rise a reflection observed on multi channel seismic profiles along and across the east pacific rise between 8 50 n and 13 30 n is interpreted to arise from the top of a crustal magma chamber located 1 2 2 4 km below the sea floor the magma chamber is quite narrow 4 6 km wide but can be traced as a nearly continuous feature for tens of kilometres along the rise axis

significance of widespread low effusion rate eruptions over the past two million years for delivery of magma to the overlapping spreading centers at 9 n east pacific rise abstract ever since the discovery that the mid ocean ridge axis deepens at most axial discontinuities there has been vigorous debate about whether ridge axis depth and shape reflect magma supply here we report evidence from one of the most thoroughly studied ridge discontinuities on a fast spreading mid ocean ridge that volcanic eruption effusion rates have been low relative to nearby segment centers throughout its two million year history this pattern of low volcanic effusion rates is inferred from the fields of constructional volcanic mounds on the seafloor which are much more abundant in the off axis discordant zone of the east pacific rise epr 9 n overlapping spreading centers osc than on typical epr seafloor detailed seafloor mapping in 2007 shows that these mound fields consist of concentrations of large pillow mounds that formed within the current osc basin and ridge tips rather than off axis pillow lava erupted at approximately an order of magnitude lower effusion rate than lobate or sheet flows primarily builds mounds on the seafloor crustal thickness a proxy for long term magma supply does not decrease in spite of the decreased lava effusion rates implied by widespread pillow eruptions to resolve the problem of achieving the same magma emplacement rate with lower effusion rates without violating our observations by requiring drastically increased eruption frequency or unreasonably long eruption durations we propose that eruptions occur across the entire osc basin the current configuration of mid crustal melt sills within the osc as resolved by multichannel seismic reflection surveys have neither the correct location nor appropriate size to deliver magma to these eruptive vents in the osc basin via vertically propagating dikes instead we propose that magma delivery to eruptions occurs via laterally propagating dikes from robust mid crustal melt sills outside of the overlap zone

pahoehoe and aa in hawaii volumetric flow rate controls the lava structure the historical records of kilauea and mauna loa volcanoes reveal that the rough surfaced variety of basalt lava called aa forms when lava flows at a high volumetric rate 5 10 m3 s and the smooth surfaced variety called pahoehoe forms at a low volumetric rate 5 10 m3 s this relationship is well illustrated by the 1983 1990 and 1969 1974 eruptions of kilauea and the recent eruptions of mauna loa it is also illustrated by the eruptions that produced the remarkable paired flows of mauna loa in which aa formed during an initial short period of high discharge rate associated with high fountaining and was followed by the eruption of pahoehoe over a sustained period at a low discharge rate with little or no fountaining the finest examples of paired lava flows are those of 1859 and 1880 1881 we attribute aa formation to rapid and concentrated flow in open channels there rapid heat loss causes an increase in viscosity to a threshold value that varies depending on the actual flow velocity at which when surface crust is torn by differential flow the underlying lava is unable to move sufficiently fast to heal the tear we attribute pahoehoe formation to the flowage of lava at a low volumetric rate commonly in tubes that minimize heat loss flow units of pahoehoe are small usually 1 m thick move slowly develop a chilled skin and become virtually static before the viscosity has risen to the threshold value we infer that the high discharge rate eruptions that generate aa flows result from the rapid emptying of major or subsidiary magma chambers rapid near surface vesiculation of gas rich magma leads to eruptions with high discharge rates high lava fountains and fast moving channelized flows we also infer that long periods of sustained flow at a low discharge rate which favor pahoehoe result from the development of a free and unimpeded pathway from the deep plumbing system of the volcano and the separation of gases from the magma before eruption achievement of this condition requires one or more episodes of rapid magma excursion through the rift zone to establish a stable magma pathway

the relationship between the geometry of normal faults and that of the sedimentary layers in their hanging walls abstract we derive an analytical expression that relates the shape of a fault in cross section to the shape of the bedding horizons in its hanging wall block the expression assumes that the hanging wall deforms by simple shear and that the footwall remains undeformed throughout although this paper concentrates on normal faults the expression is equally valid and applicable to thrust faults the direction of simple shear in the hanging wall block is arbitrary and has a dramatic effect on the predicted fault or bedding geometry there is no reason to believe that the simple shear occurs on vertical planes as is commonly assumed in graphical approaches to this problem and ignoring the presence of inclined simple shear is likely to lead to considerable underestimates of the amount of extension across normal faults and in the amount of shortening across thrusts similar though more complicated expressions can be obtained when compaction within the hanging wall block is taken into account for a planar normal fault such compaction may result in the development of a hanging wall syncline

crustal fissuring and its relationship to magmatic and hydrothermal processes on the east pacific rise crest 9 12 to 54 n the deep towed argo i optical acoustical vehicle and a geographic information system gis have been used to establish the abundance widths and spatial distribution of fissures as well as the relative age distribution of lavas along the narrow 500 m wide axial zone of the east pacific rise epr from 9o12 to 9o54 n on a second order scale 78 km long wider but less numerous fissures are found in the northern portion of the survey area this changes to narrower more abundant fissures in the south a profile of the cumulative width added by fissures to the axial zone exhibits minima in three areas along strike near 9o49 9o35 and 9o15 n where the most recent eruptions have occurred above sites of magmatic injection from the upper mantle filling and covering older fissures on a fourth order scale 5 15 km long the mean density of fissuring on a given segment is greater where relative axial lava age is greater fissure density also correlates with hydrothermal vent abundance and type increased cracking toward segment tips is observed at the second order scale whereas fourth order segments tend to be more cracked in the middle cracking on a fourth order scale may be driven by the propagation of dikes rather than by the far field plate stresses the above relations constrain the model of haymon et al 1991 in which individual fourth order segments are in different phases of a volcanic hydrothermal tectonic cycle

origin of the sheeted dike complex at superfast spread east pacific rise revealed by deep ocean crust drilling at ocean drilling program hole 1256d 1 superfast spread 22 cm a upper oceanic crust drilled at ocean drilling program site 1256 comprises a thick sequence of extrusive lavas underlain by a thin region of sheeted dikes the sheeted dikes at this site are characterized by the intimate association of in situ hyaloclastic fragmentation and hydrothermal alteration lithostatic and magmastatic pressure estimates reveal that there is no effective level of neutral buoyancy within the extrusive layer to trap dikes in the crust this results in a situation that favors the extrusion rather than intrusion of magma fractures created by magmas rising in dikes were the loci of intense hydrothermal circulation and provided the pathways for subsequent dike intrusions magma rich conditions expected for fast spreading ridge segments give rise to a rapid increase in magmatic pressure which can result in dike intrusion even under only small deviatoric stress this allows for the emplacement of narrow dikes in the upper crust with magmas more likely to extrude and hence the high extrusive intrusive ratio for site 1256

paleointensity applications to timing and extent of eruptive activity 9 10 n east pacific rise placing accurate age constraints on near axis lava flows has become increasingly important given the structural and volcanic complexity of the neovolcanic zone at fast spreading ridges geomagnetic paleointensity of submarine basaltic glass sbg holds promise for placing quantitative age constraints on near axis flows in one of the first extensive tests of paleointensity as a dating tool or temporal marker we present the results of over 550 successful sbg paleointensity estimates from 189 near axis 4 km sites at the east pacific rise 9 10 n paleointensities range from 6 to 53 t and spatially correspond to the pattern expected from known temporal variations in the geomagnetic field samples within and adjacent to the axial summit trough ast have values approximately equal to or slightly higher than the present day samples out to 1 3 km from the ast have values higher than the present day and samples farther off axis have values lower than the present day the on axis samples 500 m from the ast provide a test case for using models of paleofield variation for the past few hundred years as an absolute dating technique results from samples collected near a well documented eruption in 1991 1992 suggest there may be a small negative bias in the paleointensity estimates limiting resolution of the dating technique possible explanations for such a bias include local field anomalies produced by preexisting magnetic terrain anomalously high magnetic unblocking temperatures leading to a small cooling rate bias and or the possibility of a chemical remanence produced by in situ alteration of samples likely to have complicated thermal histories paleointensity remains useful in approximating age differences in young flows and a clear along axis paleointensity contrast near 9 50 n is suggestive of a 150 200 year age difference paleointensity values of off axis samples are generally consistent with rough age interpretations based on side scan data furthermore spatial patterns in the paleointensity suggest extensive off axis flow emplacement may occur infrequently with recurrence intervals of 10 20 kyr results of a stochastic model of lava emplacement show that this can be achieved with a single distribution of flows with flow size linked to time between eruptions

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

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

quantification of extraterrestrial lava flow effusion rates through laboratory simulations we have used carefully controlled laboratory simulations to develop a model which relates lava flow morphology to effusion rate and rheology through comparisons with measured and estimated eruption rates on earth this approach allows us to constrain eruptive styles and compositions of extraterrestrial lava flows by applying this model to lava flows on the moon mars and venus we have determined that all the common flow morphologies domes folds and levees on these planets could have been produced by basalt like or andesite like lavas through either continuous or episodic emplacement the presence of more evolved magma compositions on other planets is not required to explain any of the observed lava flow morphologies

210po 210pb dating of recent volcanic eruptions on the sea floor the globe encircling mid ocean ridge system accounts for most of the earth s volcanism and influences its heat budget morphology and the composition of the oceans it is therefore important to constrain the size frequency and compositional variability of ridge eruptions but this is made difficult by the remoteness of much of the sea floor and the paucity of applicable radioactive chronometers remote monitoring has provided indirect evidence for active ridge volcanism1 3 which in some cases has been strengthened by submersible observations4 5 but it has not been possible to date these eruptions directly here we present a new chronometer based on 210po 210pb radioactive disequilibrium which allows us to date glassy eruption products within a few years of their eruption our first results on lavas from 9 50 n on the east pacific rise confirm that the ridge erupted only months before sample collection and indicate that the eruptions extended over at least one year the chronometer begins with po volatilization during eruption all glasses analysed were 75 100 degassed a crystalline pillow interior was 20 degassed however implying that the flow surface may control po degassing during deep submarine eruptions although the 210po 210pb chronometer is useful for only a short time frame it should prove valuable for dating recent but unobserved submarine eruptions and determining details of eruptive sequences

navigational infrastructure at the east pacific rise 9 50 n area following the 2005 2006 eruption seafloor benchmarks and near bottom multibeam surveys 1 four seafloor benchmarks were deployed with rov jason2 at frequently visited areas along the northern east pacific rise nepr ridge crest near 9 50 n within the ridge2000 epr integrated study site iss bull s eye when used in concert with established deep ocean acoustic positioning techniques these benchmarks provide navigational infrastructure to facilitate the integration of near bottom data at this site by allowing efficient and quantitative coregistration of data and observations collected on multiple dives and over multiple cruises high resolution near bottom multibeam bathymetric surveys also were conducted along and across the ridge crest to provide a morphological and geological context for the benchmark areas we describe the navigation and data processing techniques used to constrain the benchmark positions and outline operational details to effectively use benchmarks at this and other deep ocean sites where multidisciplinary time series studies are conducted the well constrained positions of the benchmarks provide a consistent geospatial framework that can be used to limit navigational uncertainties during seafloor sampling and mapping programs and enable accurate spatial coregistration and integration of observations these data are important to test a range of multidisciplinary hypotheses that seek to link geological chemical and biological processes associated with crustal accretion and energy transfer from the mantle to the hydrosphere at mid ocean ridges

geochronology and geochemistry of lavas from the 1996 north gorda ridge eruption abstract radiometric dating of three north gorda ridge lavas by the 210 po 210 pb method confirms that an eruption occurred during a period of increased seismic activity along the ridge during late february early march 1996 these lavas were collected following detection of enhanced t phase seismicity and subsequent ocean bottom photographs documented the existence of a large pillow mound of fresh appearing lavas 210 po 210 pb dating of these lavas indicates that an eruption coinciding with this seismicity did occur within analytical error and that followup efforts to sample the recent lava flows were successful compositions of the three confirmed young lavas and eleven other samples of this contiguous new flow sequence are distinct from older lavas from this area but are variable at a level outside analytical uncertainty these intraflow variations can not easily be related to a single common parent magma compositional variability within the new flow is compared to that of other recently documented individual flow sequences and this comparison reveals a strong positive correlation of compositional variance with flow volumes spanning a range of 2 orders of magnitude the geochemical heterogeneity in the north gorda new flow probably reflects incomplete mixing of magmas generated from a heterogeneous mantle source or from slightly different melting conditions of a single source the compositional variability range in sample ages up to 6 weeks and range in active seismicity 4 weeks imply that this relatively large flow was erupted over an interval of several weeks

central anomaly magnetization high constraints on the volcanic construction and architecture of seismic layer 2a at a fast spreading mid ocean ridge the epr at 9 30 50 n abstract the central anomaly magnetization high camh is a zone of high crustal magnetization centered on the axis of the east pacific rise epr and many other segments of the global mid ocean ridge mor the camh is thought to reflect the presence of recently emplaced and highly magnetic lavas forward models show that the complicated character of the near bottom camh can be successfully reproduced by the convolution of a lava deposition distribution with a lava magnetization function that describes the variation in lava magnetization intensity with age this lava magnetization function is the product of geomagnetic paleofield intensity which has increased by a factor of 2 over the last 40 kyr and low temperature alteration which decreases the remanence of lava with exposure to seawater the success of the forward modeling justifies the inverse approach deconvolution of the magnetic data for lava distribution and integration of that distribution for magnetic layer thickness this approach is tested on two near bottom magnetic profiles al2767 and al2771 collected using alvin across the epr axis at 9 31 n and 9 50 n our analysis of these data produces an estimate of the relative thickness of the magnetic lava layer which is remarkably consistent with existing multichannel estimates of layer 2a thickness from lines cdp31 and cdp27 the similarity between magnetic layer and seismic layer 2a at the 9 10 n segment of the epr crest provides independent support to the notion that seismic layer 2a in young oceanic crust represents the highly magnetic lava layer and that the velocity gradient at the base of layer 2a is related to the increasing number of higher velocity dikes with depth in the lava dike transition zone the near bottom magnetic anomaly character of the camh is a powerful indicator of the emplacement history of upper crust at mors which allows prediction of the relative thickness and architecture of the extrusive lavas independent of other constraints

a laboratory investigation into the effects of slope on lava flow morphology abstract in an attempt to model the effect of slope on the dynamics of lava flow emplacement four distinct morphologies were repeatedly produced in a series of laboratory simulations where polyethylene glycol peg was extruded at a constant rate beneath cold sucrose solution onto a uniform slope which could be varied from 1 through 60 the lowest extrusion rates and slopes and highest cooling rates produced flows that rapidly crusted over and advanced through bulbous toes or pillows similar to subaerial toey pahoehoe flows and to submarine pillowed flows as extrusion rate and slope increased and cooling rate decreased pillowed flows gave way to rifted flows linear zones of liquid wax separated by plates of solid crust similar to what is observed on the surface of convecting lava lakes then to folded flows with surface crusts buckled transversely to the flow direction and at the highest extrusion rates and slopes and lowest cooling rates to leveed flows which solidified only at their margins a dimensionless parameter primarily controlled by effusion rate cooling rate and flow viscosity quantifies these flow types increasing the underlying slope up to 30 allows the liquid wax to advance further before solidifying with an effect similar to that of increasing the effusion rate for example conditions that produce rifted flows on a 10 slope result in folded flows on a 30 slope for underlying slopes of 40 however this trend reverses slightly owing to increased gravitational forces relative to the strength of the solid wax because of its significant influence on heat advection and the disruption of a solid crust slope must be incorporated into any quantitative attempt to correlate eruption parameters and lava flow morphologies these experiments and subsequent scaling incorporate key physical parameters of both an extrusion and its environment allowing their results to be used to interpret lava flow morphologies on land on the sea floor and on other planets

non explosive lava water interaction in skaelingar iceland and the formation of subaerial lava pillars hollow cylinders of basalt 2 5 m tall and generally 1 m in diameter were generated by non explosive lava water interactions during the emplacement of the laki lava flow in iceland during 1783 1784 we know of only one location within the laki lava flow where these basalt formations occur a valley called skaelingar located at 64 0 n 18 5 w which contains a tributary stream to the skafta river skaelingar was temporarily filled with laki lava when the main body of the lava flow advancing down the skafta river valley became blocked forcing lava to flow upstream into tributary valleys along the north side of the river after the blockage within the skafta river valley was removed the laki lava mostly drained out of these tributary valleys we refer to the remaining vertical hollow basalt pipes as lava pillars because they morphologically resemble subaerial lava trees and submarine lava pillars that have been observed at mid ocean ridges we propose that the subaerial pillars formed as an inflating lava flow advanced slowly over water saturated ground or perhaps into temporarily ponded water causing heated columns of water to rise between adjacent advancing lava lobes the subaerial pillars continued to grow in height and diameter as the lava flow inflated when the lava drained back out of the valley the lava pillars were left standing thus the icelandic subaerial pillars represent a non explosive interaction between lava and water

evidence for flexural shear folding associated with extensional faults the maintainance of bed length and bed thickness within the hanging wall of extensional fault systems requires the development of bed parallel shear four field examples of bed parallel shear related structures are described from a number of positions within the hanging wall and footwall to extensional faults flexural shear folding is a characteristic of both fault bend and fault propagation folding the dominant direction of the over riding shear is directed toward the fault surface in the case of fault bend folds the fault surface may be considered to represent the finite neutral surface of the fold in extensional fault propagation folds the direction of the overriding shear is toward the synclinalfold axial surface the observations suggest that the direction of flexural shear for extensional and contractional folds is opposite for folds of geometrically similar form

mantle upwelling magmatic differentiation and the meaning of axial depth at fast spreading ridges since the fi rst systematic mapping of the east pacifi c rise its axial depth profi le has been used to infer variability in magma supply or mantle temperature here however we show that segment scale rise parallel undulations of both on and off axis seafl oor depth result pri marily from variations in the bulk density of oceanic crust using seismic images of crustal and upper mantle structure combined with gravity data and lava chemistry we demonstrate that segment scale variations in crustal density are caused by magmatic differentiation rise parallel changes in magmatic differentiation are attributed to a skew between the axes of mantle upwelling and plate spreading we conclude that segmentation of axial depth along fast spreading ridges is controlled primarily by the geometry of mantle upwelling

temperature variations at diffuse and focused flow hydrothermal vent sites along the northern east pacific rise in the decade following documented volcanic activity on the east pacific rise near 9 50 n we monitored hydrothermal vent fluid temperature variations in conjunction with approximately yearly vent fluid sampling to better understand the processes and physical conditions that govern the evolution of seafloor hydrothermal systems the temperature of both diffuse flow low temperature and focused flow high temperature vent fluids decreased significantly within several years of eruptions in 1991 and 1992 after mid 1994 focused flow vents generally exhibited periods of relatively stable slowly varying temperatures with occasional high and low temperature excursions lasting days to weeks one such positive temperature excursion was associated with a crustal cracking event another with both positive and negative excursions demonstrated a subsurface connection between adjacent focused flow and diffuse flow vents diffuse flow vents exhibit much greater temperature variability than adjacent higher temperature vents on timescales of a week or less temperatures at a given position within a diffuse flow field often varied by 5 10 c synchronous with near bottom currents dominated by tidal and inertial forcing on timescales of a week and longer diffuse flow temperatures varied slowly and incoherently among different vent fields at diffuse flow vent sites the conceptual model of a thermal boundary layer immediately above the seafloor explains many of the temporal and spatial temperature variations observed within a single vent field the thermal boundary layer is a lens of warm water injected from beneath the seafloor that is mixed and distended by lateral near bottom currents the volume of the boundary layer is delineated by the position of mature communities of sessile e g tubeworms and relatively slow moving organisms e g mussels vertical flow rates of hydrothermal fluids exiting the seafloor at diffuse vents are less than lateral flow rates of near bottom currents 5 10 cm s the presence of a subsurface shallow reservoir of warm hydrothermal fluids can explain differing temperature behaviors of adjacent diffuse flow and focused flow vents at 9 50 n different average temperatures and daily temperature ranges are explained by variable amounts of mixing of hydrothermal fluids with ambient seawater through subsurface conduits that have varying lateral permeability

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

primitive melt replenishment and crystal mush disaggregation in the weeks preceding the 2005 2006 eruption 9 50 n epr abstract the 2005 2006 eruption at 9 50 n east pacific rise provides an exceptional opportunity to investigate the magma plumbing system beneath a well studied ridge system the eruption was preceded by two years of increasingly intense seismicity and occurred from the same location as a previous and well characterized eruption in 1991 1992 here we use the crystal cargo of samples from this eruption to investigate magma reservoir processes in the lead up to the eruption as well as their temporal relationship to the seismicity that preceded it compositional zoning in some plagioclase crystals indicates primitive melt replenishment occurred roughly six weeks or less before the eruption this replenishing event is seen only in the crystals from the central region of the eruption 9 50 9 52 n this is also the area where the most primitive lava compositions are observed and together these observations support models of replenishment being spatially focused the short time between the input of a more primitive melt and eruption onto the seafloor suggests replenishment likely contributed to triggering the eruption rare resorbed plagioclase crystals and glomerocrysts of plagioclase and olivine some of which have rims far from equilibrium with their host melt suggest that disaggregation of a crystal mush occurred within a few days prior to eruption interstitial melt from within this mush zone must have been mixed back into the erupted lava a form of in situ crystallization thus the erupted magmas evolved in a replenished tapped magma reservoir in which at least a part of the crystallization occurred in situ

geometric and experimental models of extensional fault bend folds the geometric models ranged from simple models of listric normal faults with continuous hanging wall deposition to complex models of normal faults with ramps and flats and footwall deposition and or erosion the inclined shear angle i e the angle between the horizontal and the shear direction was 70 in all models withjack and peterson 1993 describe the modeling technique in the experimental models a 4 cm thick layer of either wet clay or dry sand covered a flat metal base consisting of overlapping plates or precut blocks movement of the plates or blocks produced a listric normal fault in the overlying clay or sand layer we added growth layers after every centimeter of displacement withjack and callaway 2000 describe the properties of the wet clay and dry sand

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

variation in fold geometry in the yuso basin northern spain implications for the deformation regime first generation structures in greywackes of the yuso group from the cantabrian mountains of northern spain show a distinct variation in geometry with depth in a regional synclinal structure curavacas and lechada synclines they are easily distinguished from other deformation events in the structurally uppermost level we find flap folds flap folds are recumbent structures with the inverted limb preserved below this level cascade folds are found these structures have a vergence opposite to that of parasitic folds the nomenclature adopted is from harrison and falcon characteristically these structures have shallowly dipping axial surfaces in agreement with the shallow dip of the axial plane regional cleavage in the lowermost structural level upright parasitic folds with a steep cleavage are present the variation in fold geometry is accompanied by a general steepening of the regional cleavage with increasing depth in the absence of overprinting relationships the f1 fold geometries are included in a single deformation event the steepening of the cleavage with depth reflects the change in orientation of the maximum shortening direction from sub vertical in the upper part of the syncline to sub horizontal in the lower part with increasing depth the deformation regime during f changed from bending to buckling the deformation regime on the regional scale however is associated with basement subsidence and passive formation of the regional synclinal structure furthermore the absence of a distinct microfabric for the different f1 folds indicates that on a small scale a similar deformation regime was present we conclude therefore that the scale at which we study a structure only reflects the deformation regime at that particular scale consequently the overall deformation regime cannot be determined from single outcrops or microstructural analysis alone

submarine lineated sheet flows a unique lava morphology formed on subsiding lava ponds lineated sheet flows are flat lying glassy lava flows characterized by a regular surface pattern of parallel grooves or furrows aligned with the flow direction they are unique to the submarine environment we propose that the lineations are developed within the collapsed interiors of partially ponded lobate sheet flows that initially inflate and then drain out during emplacement during lava drainout the original lobate crust founders and a new crust begins to grow on the subsiding lava surface lineated flow texture is created where molten lava emerges laterally from beneath a growing crust the lineations are formed by raking of the emerging lava surface by irregularities on the bottom edge of the crust and are preserved owing to rapid chilling by seawater therefore lineated sheet flows are the product of a specific sequence of events over a short period of time during the course of a deep submarine eruption

rapid emplacement of a mid ocean ridge lava flow on the east pacific rise at 9 46 51 n in april 1991 during a submersible diving expedition to the east pacific rise epr crest at 9 46 51 n a new volcanic eruption on the sea floor was discovered here we report results from numerical modeling of that eruption which indicate that 4 106 6 106 m3 of lava was emplaced in 1 2 h with an average eruption rate of 103 106 m3 s 1 comparable to rates observed in hawaii at kilauea s east rift zone if the rapid emplacement of the 1991 epr lava and its short eruption duration are typical of volcanic events at fast spreading mid ocean ridge crests these characteristics have broad implications for our ability to detect monitor and understand the evolution of magmatic and volcanic processes within the axial zone

rapid ascent and emplacement of basaltic lava during the 2005 06 eruption of the east pacific rise at ca 9 51 n as inferred from co2 contents abstract eruption rates at the mid ocean ridges mors are believed to strongly control the morphology and length of lava flows emplaced along the ridge axis and thus the structure and porosity of the upper oceanic crust eruption rate also represents one of the few tools to gain insight into the driving pressures within sub ridge magmatic systems as eruption rate is inferred to vary systematically along the global mid ocean ridge system understanding of how to assess eruption rate in submarine systems and how it maps to observable features of the ridge axis would provide a powerful tool to understand earth s largest magmatic system eruption rates at mors are poorly constrained however because of a lack of direct observations preventing the duration of an eruption to be quantified this study uses decompression experiments of morb samples and numerical modeling of co 2 degassing to reconstruct the timescales for magma ascent and lava emplacement during the 2005 06 eruption of the east pacific rise at ca 9 51 n samples collected from the lava flow are all supersaturated in dissolved co 2 contents but co 2 decreases with distance from the vent presumably as a consequence of progressive co 2 diffusion into growing bubbles samples collected at the vent contain 10 5 vesicles per cm 3 pieces of these samples were experimentally heated to 1225 c at high pressure and then decompressed at controlled rates results plus those from numerical modeling of diffusive bubble growth indicate that magma rose from the axial magma chamber to the seafloor in 1 h and at a rate of 2 3 km h 1 our modeling as validated by experimental decompression of morb samples with 10 6 vesicles cm 3 also suggests that co 2 degassed from the melt within 10 100 min as the vesicular lava traveled 1 7 km along the seafloor implying a volumetric flow rate on order of 10 3 4 m 3 s 1 given an ascent rate of 0 2 m s 1 the width of a rectangular dike feeding the lava would have been 1 2 m wide morb samples from the pacific ridge are generally more supersaturated in dissolved co 2 than those from slower spreading atlantic and indian ridges our results suggest that pacific morbs ascend to the seafloor faster than atlantic or indian morbs

interplay between faults and lava flows in construction of the upper oceanic crust the east pacific rise crest 9 25 9 58 n the distribution of faults and fault characteristics along the east pacific rise epr crest between 9 25 n and 9 58 n were studied using high resolution side scan sonar data and near bottom bathymetric profiles the resulting analysis shows important variations in the density of deformational features and tectonic strain estimates at young seafloor relative to older sediment covered seafloor of the same spreading age we estimate that the expression of tectonic deformation and associated strain on old seafloor is 5 times greater than that on young seafloor owing to the frequent fault burial by recent lava flows thus the unseen volcanically overprinted tectonic deformation may contribute from 30 to 100 of the 300 m of subsidence required to fully build up the extrusive pile layer 2a many longer lava flows greater than 1 km dam against inward facing fault scarps this limits their length at distances of 1 2 km which are coincident with where the extrusive layer acquires its full thickness more than 2 of plate separation at the epr is accommodated by brittle deformation which consists mainly of inward facing faults 70 faulting at the epr crest occurs within the narrow 4 km wide upper crust that behaves as a brittle lid overlying the axial magma chamber deformation at greater distances off axis up to 40 km is accommodated by flexure of the lithosphere due to thermal subsidence resulting in 50 inward facing faults accommodating 50 of the strain on the basis of observed burial of faults by lava flows and damming of flows by fault scarps we find that the development of layer 2a is strongly controlled by low relief growth faults that form at the ridge crest and its upper flanks in turn those faults have a profound impact on how lava flows are distributed along and across the ridge crest

structural restoration of seismic and geologic sections in extensional regimes seismic sections from extensional basins can be structurally transformed to produce restored seismic sections showing subsurface geology of the past restoration is achieved using a simple and effective algorithm which is suited for use in an interactive seismic interpretation environment seismic data are resampled along paleovertical trajectories which are the deformed equivalents of what were originally closely spaced vertical lines paleoverticals are generated within the present day section as follows 1 origin points are equally spaced along a reference horizon that corresponds to an original depositional surface 2 paleovertical segments are vertical within fault blocks 3 paleoverticals are offset across faults by the reference horizon slip and 4 restor d depth is equated to distance down a paleovertical these rules can be modified to conform with such styles of extensional deformation as vertical simple shear inclined shear or rigid rotation and to account for compaction structures with complex fault patterns readily are restored several examples from the gulf of mexico and offshore west africa show that seismic restoration is useful for 1 unravelling structural and stratigraphic history 2 revealing tectonically masked features and 3 checking interpretations across faults for seismic correlation and structural balance geologic sections can be restored in similar fashion

small scale spatial and temporal variations in mid ocean ridge crest magmatic processes data from a suite of closely spaced lava flows recovered within the axial summit caldera and on the crestal plateau of the east pacific rise around lat 9 31 n indicate that eruptions on this fast spreading part of the mid ocean ridge occur throughout the crestal region and are not restricted to the axis these eruptions contribute to a complex distribution of basalts of various ages and a significant thickening of seismic layer 2a away from the axis in our study area small scale 600 m diversity and nonsystematic distribution of lava types may reflect rapid changes in magma chemistry that occur during crystallization and replenishment in small magma lenses coupled with the effects of frequent low volume eruptions both within and outside of the axial summit caldera

central anomaly magnetization high documentation of crustal accretion along the east pacific rise 9 55 9 25 n near bottom magnetic data collected along the crest of the east pacific rise between 9 55 and 9 25 n identify the central anomaly magnetization high camh a geomagnetic anomaly modulated by crustal accretionary processes over timescales of 104 years a significant decrease in camh amplitude is observed along axis from north to south with the steepest gradient between 9 42 and 9 36 n the source of this variation is neither a systematic change in geochemistry nor varying paleointensity at the time of lava eruption instead magnetic moment models show that it can be accounted for by an observed 50 decrease in seismic layer 2a thickness along axis layer 2a is assumed to be the extrusive volcanic layer and we propose that this composes most of the magnetic source layer along the ridge axis the 9 37 n overlapping spreading center osc is located at the southern end of the steep camh gradient and the 9 42 9 36 n ridge segment is interpreted to be a transition zone in crustal accretion processes with robust magmatism north of 9 42 n and relatively low magmatism at present south of 9 36 n the 9 37 n osc is also the only bathymetric discontinuity associated with a shift in the camh peak which deviates 0 7 km to the west of the axial summit trough indicating southward migration of the osc camh boundaries defined from the maximum gradients lie within or overlie the neovolcanic zone nvz boundaries throughout our survey area implying a systematic relationship between recent volcanic activity and camh source maximum flow distances and minimum lava dip angles are inferred on the basis of the lateral distance between the nvz and camh boundaries lava dip angles average 14 toward the ridge axis which agrees well with previous observations and offers a new method for estimating lava dip angles along fast spreading ridges where volcanic sequences are not exposed

distribution of magma beneath the east pacific rise between the clipperton transform and the 9 17 n deval from forward modeling of common depth point data we have reprocessed seven cross axis common depth point cdp profiles between the clipperton transform and the 9 17 n deval deviation in axial linearity on the east pacific rise epr to understand the relationship between axial magma chamber amc width and seafloor morphology forward modeling of cross axis cdp profiles suggests a segmented amc in which significant variations in width occur across minor rise axis discontinuities e g devals the modeled rise segment widths bounded by the 9 53 n 9 35 n devals the 9 35 n 9 17 n devals and south of the 9 17 n deval were 0 7 km 1 0 1 2 km and 4 15 km respectively transition in amc width across these discontinuities is unclear due to the sparseness of cross axis line spacing however a simple association of devals with decreased magma supply is doubtful the minimum 250 m and maximum 4150 m amc widths are found near the 9 35 n and 9 17 n devals respectively the reprocessing of cdp profiles included repicking stacking velocities to ensure a proper stack of the amc reflector and its associated diffractions imaging postcritical reflections from the base of layer 2a finite difference time migration ray theoretical depth migration and travel time modeling of amc diffraction patterns constraints on amc width were derived from forward modeling techniques based on analytic raytracing velocity models were constructed from seabeam bathymetry and modified expanding spread profile esp velocity depth functions esp velocity models were altered to compensate for off axis thickening of layer 2a as imaged in the cdp reflection data continuous two dimensional velocity models constructed from modified esp velocity depth functions and seabeam bathymetry should account for ray bending at the seafloor basalt interface and any lateral velocity gradients induced by a thickening layer 2a stacked data were time migrated using a finite difference algorithm and extrapolated to depth using ray theoretical depth migration reflector positions were input into our forward modeling scheme to produce a zero offset travel time response of our migrated solution the travel time response was then overlain on the stacked section to ensure an adequate match especially to diffractions generated at the amc edge forward modeling of amc diffraction patterns reveals that original amc width estimates were too large the under migration of the amc reflector resulted from the conversion of stacking to interval velocities without accounting for topographic effects on individual cmp gathers thus resulting in improperly collapsed diffraction hyperbolae ship wandering relative to the amc edge can account for variations in amc reflector amplitude and dropout on the along axis cdp line the continuity of the amc appears unbroken across several ridge axis discontinuities between the clipperton transform and the 9 17 n deval which suggests an amc whose along axis dimension exceeds 75 km reflectivity modeling of cmp gathers suggests that the available data are consistent with a magma chamber comprising only a thin layer of melt overlying a zone of partially solidified crystal mush such a thin layer of melt might inhibit along axis mixing of magmas and thereby account for variations in magma composition along the rise crest this melt lens model for the amc would also produce strong diffraction patterns as imaged in the cdp data

axial melt lenses at oceanic ridges a case study in the oman ophiolite abstract the oman ophiolite a fossil fast spreading oceanic ridge had a melt lens perched on top of the magma chamber wherein the gabbro unit was crystallizing in ophiolites this melt lens is reduced to a horizon that has been identified in the field the gabbros that subsided from the lens horizon and drifted out of the magma chamber while developing a steep magmatic foliation provide insights into the active melt lens processes a gabbro sampled right below the horizon where the lens closed has virtually not subsided and it offers a direct glance at the floor of the lens below the deeper the gabbros are sampled the more they have subsided before drifting out of the magma chamber and the closer they derive from the center of the lens floor cross sections in these gabbros record successive events occurring within the lens in particular anorthosite layers and lenses that are interlayered within the foliated gabbros record the intrusion of a new melt where plagioclase is the first mineral to crystallize on liquidus detailed mapping of anorthosite swarms suggest that melt intrusions are massive lasting less than 4 yr and that their frequency is 100 yr their large volume controls the stability of melt lenses a new melt surge induces rapid and large inflation of the lens melt lens deflation follows that is due to the ongoing process of crustal accretion both in the gabbro unit by continuous subsidence of a thick gabbroic mush from the lens floor and in the lid above the lens roof by sheeted dike intrusions feeding basalt extrusions at the higher frequency of 10 yr

submarine lava flow inflation a model for the formation of lava pillars lava pillars are commonly observed in volcanic terrains at mid ocean ridges and are unique to the submarine volcanic environment they are hollow pipes that extend from the base of a lava flow to remnants of its collapsed upper crust pillars can be as tall as 5 m and have outer diameters typically between 0 5 and 2 m previous models proposed that they form in thick fluid fast moving sheet flows we present a new model for pillar formation based on sea floor observations and on results from laboratory and numerical models in which lava pillars form within initially thin 0 3 m slow moving lobate lava flows as gaps between neighboring lobes water escaping from beneath the flow acts to locally cool and thicken the crust at the gaps pillars attain their full height during subsequent lava flow inflation as the flow thickens pillars that do not continue to channel water are melted and resorbed into the molten flow interior pillars are typically preserved at the margins of ponded lava flows where the flow interior is relatively stagnant and less efficient at remelting pillars and where seawater is more accessible for circulation through the flow

the geometry of listric normal faults and deformation in their hangingwalls abstract in cross sections containing listric extensional faults area balancing techniques for depth to decollement are usually based on either bed length conservation or displacement conservation listric fault geometry may be constructed from a hangingwall roll over profile using the chevron construction this construction based on conservation of heave necessitates a reduction in fault displacement with decreasing fault dip a modification of this construction utilizing conservation of fault displacement predicts a listric fault that detaches at a shallower depth a new construction based on slip lines uses fault perpendicular displacement segments to generate listric fault shape fault propagation strain may be responsible for anomalous hangingwall geometries and these can be predicted by forward modelling using either a modified chevron construction or a slip line construction

balanced cross section construction from seismic sections in areas of extensional tectonics abstract the application of the technique of balanced section construction initially developed for areas of compressional folding and faulting is reviewed with reference to extensional tectonics a number of examples are discussed where these techniques have been successfully applied in the north sea the interpretation of geoseismic sections is considered to be greatly assisted by careful application of geometrical balance and a consideration of strain even in areas of low crustal extensions the nature of seismic sections however places limitations on the validity of balancing which must be borne in mind with such interpretations and wherever possible the balancing of a geoseismic section should be confirmed by complete depth conversion the rapid testing of the integrity of the geoseismic section by attempting to balance the section at the interpretational phase can eliminate many problems as well as allowing the fullest use to be made of the geophysical information

a method for automatically determining normal fault geometry at depth a large number of methods have been described which relate the shape of a normal fault at depth to the shape of a sedimentary horizon in its hanging wall unfortunately the calculated fault geometry is strongly dependent upon the method used here arguments are put forward in favor of a general two dimensional kinematic model which assumes that the hanging wall deforms by a combination of arbitrarily inclined simple shear and differential compaction the method should be applicable to areas where it can be assumed that the fault plane does not change shape as deformation proceeds this assumption may not always be valid especially in the case of large scale basement extending normal faults fault geometry the inclination of simple shear and compaction parameters may all be determined from n beds using a simple inversion scheme based on the general method throughout it is assumed that hanging wall and footwall stratigraphies are accurately known though uncertainty in either could be included as a bounded variable within a formal inversion scheme the most important feature of the model is that it is testable it has first been tested on synthetic data using a range of fault geometries the results of such modeling are encouraging and imply that all unknown parameters including fault geometry can in general be uniquely determined provided that the geometries of two or more beds within the hanging wall are known the greater the number of beds used the greater the degree of confidence in the solution the method has also been applied to published sand box models with surprisingly good results ultimately it is hoped that the method can be applied routinely to depth converted seismic reflection data the preliminary applications presented here suggest that bulk antithetically inclined simple shear combined with differential compaction provide a good approximation for hanging wall deformation in the future the scheme will be generalized to allow for deformation in three dimensions other complexities such as horizontal and vertical variation in the degree of compaction could also be included although at this stage the data do not appear to justify them

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

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

cross section restoration and balancing as aid to seismic interpretation in extensional terranes structural cross sections in extensional terranes can be improved significantly by restoration and balancing principles originally developed for fold and thrust belts in rift basins and passive continental margins use of restoration techniques in analysis of growth faults elongate salt swells and other extensional structures results in geologically valid seismic interpretations sections must be in depth as opposed to two way time and oriented in the direction of material transport and decompaction must accompany restoration hanging wall deformation is characterized by different processes depending on the mechanical properties of the rocks being extended and can be approximated by various geometric and kinematic models antithetic faulting is analogous to antithetic shear combined antithetic and synthetic faulting is approximated by vertical shear domino style extension corresponds to rigid body rotation and flexural slip flow is modeled by bed length balance methods selection of the appropriate algorithm is crucial for proper restoration and balancing applications of these techniques illustrated here using seismic data from the gulf coast and north sea include 1 evaluating and adjusting seismic interpretations 2 projecting listric fault trajectories to depth 3 delineating more accurately the geometry and extent of hydrocarbon reservoirs 4 identifying new exploration leads 5 determining the deformation history of an area and 6 constraining the timing and direction of hydrocarbon migration and entrapment

continuous near bottom gravity measurements made with a bgm 3 gravimeter in dsv alvin on the east pacific rise crest near 9 31 n and 9 50 n a bell bgm 3 gravimeter has been used to collect continuous underway near bottom 3 to 10 m altitude gravity measurements from the deep diving submersible dsv alvin during surveys on the east pacific rise epr crest near 9 31 n and 9 50 n closely spaced 20 to 30 m gravity measurements were made along transects up to 8 km long in both regions repeatability of measurements made at the same location on different dives is 0 3 mgal along track spatial resolution of anomalies is 130 160 m with the limiting factors being precision and sampling rate of the pressure gauge depth data used to calculate vertical accelerations of the submersible the average upper crustal density of the ridge crest determined from the relationship between depth and free water gravity anomalies varies greatly between 9 31 n and 9 50 n average upper crustal densities of 2410 kg m 3 for the 9 50 n area and 2690 kg m 3 for the 9 31 n area were calculated the different densities are not due to differing geometry of the layer 2a 2b boundary or a regional cross axis gravity gradient differences in porosity of the shallow crustal rocks or a difference in the proportion of low density extrusives to higher density dikes and sills within layer 2a in these two areas are the likely causes of the different upper crustal densities bouguer gravity anomalies near the epr axis are primarily small amplitude 0 5 2 mgal are a few hundred meters across and appear to be lineated parallel to the axis larger amplitude bouguer anomalies of up to 4 mgal were found at a few locations across the crestal plateau and are associated with pillow ridges composed of lavas which are clearly younger than the surrounding seafloor these ridges have distinct chemical compositions compared to lavas from the axial summit collapse trough asct at the same latitude probable sources of the 0 5 to 2 mgal anomalies observed on the summit plateau include areas of collapsed and fissured terrain and dike swarms feeding melt through layer 2a to the surface a grid survey of the ridge axis near 9 50 n shows bouguer anomalies lineated along the axis suggesting that dike swarms do contribute to the observed bouguer anomalies the along axis continuity of the gravity anomalies is disrupted at a 75 m offset of the asct suggesting that shallow feeders of lava to the surface may be segmented on a finer scale than the deeper crustal magmatic system this initial study confirms the ability to conduct high resolution near bottom continuous gravity measurements from alvin it also provides important information on how the shallow crustal structure of a fast spreading mid ocean ridge develops and how it varies with the surface morphology

constraints on the formation of submarine lava flows from numerical model calculations abstract to constrain life time and flow rates in a submarine lava tube pillow 3d finite element calculations were carried out for a simplified fixed geometry model tube the principal parameters included into the model are a temperature dependent newtonian viscosity of the lava whereby viscosity is also dependent on composition temperature and crystallinity of the lava cooling by seawater and the release of latent heat of crystallization time dependent model runs suggest the longevity of the tube flow is constrained by the competition of advective heat flow along the tube axis and the conduction of heat towards the margins of the flow in order to keep the lava flowing a minimum critical pressure gradient along the tube is required otherwise the amount of thermal energy brought in by the influx of hot lava is less than the amount lost at the outer boundaries and the tube will solidify providing an exponentially decaying driving pressure at the tube inlet this critical pressure gradient has been calculated for different tube lengths and diameters and is compared to critical pressure values resulting from a more simplistic solution assuming constant viscosity and pure conductive or convective cooling an analysis of the difference between pressure gradients obtained through the numerical and simplified model is used to derive an appropriate power law for the critical pressure gradient in bodies cooled by convection and conduction including the effect of temperature dependent viscosity that gradient scales with the tube radius r as r3 85 this law is valid for tube style lava flows and lava viscosities in the basaltic range

geotectonic setting of hydrothermal activity on the summit of lucky strike seamount 37 17 n mid atlantic ridge we have investigated the relations between volcanic tectonic and hydrothermal activity on lucky strike seamount 37 17 n mid atlantic ridge using a nested survey strategy involving collection of data from different deep sea mapping systems the highly tectonized seamount summit consists of three volcanic cones surrounding a relatively flat depression with a young lava lake in its center hydrothermal activity is focused mainly within the summit depression with most of the vents located proximal to the lava lake isolated active and inactive chimneys and mounds are widespread throughout the summit depression and occur on both volcanic pillow lava and hydrothermal sulfide rubble and hydrothermally cemented breccias substrates the large volume of sulfide rubble together with the nature of the sulfide structures indicates that hydrothermal activity has been episodic but ongoing for a long period of time hundreds to thousands of years on the basis of the distribution of hydrothermal deposits we propose a model of alternation between tectonic and volcanic control on hydrothermalism at lucky strike seamount midsegment melt focusing produces a spatially and temporally stable heat source that sustains focused high temperature hydrothermal activity over long time periods during periods of amagmatic extension active faulting within the summit depression provides multiple near surface fluid flow pathways for discharge of high temperature fluids and widespread deposition of massive sulfides during eruptive events rapid effusion of very hot lava creates a lava lake and hyaloclastite deposits the new sheet flows form a cap on the hydrothermal system and fluid upflow is reorganized discharge of high temperature fluids is restricted to isolated sites with relatively high permeability for example the edges of the lava lake much of the upwelling hydrothermal fluid pools in the subsurface conductively cools and mixes with entrained seawater before discharging as widespread low temperature diffuse flow hyaloclastites become cemented further augmenting the sealing of the system present day activity at lucky strike seamount represents this locally volcanically controlled phase of activity despite the segment as a whole being dominantly amagmatic

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

using submarine lava pillars to record mid ocean ridge eruption dynamics abstract submarine lava pillars are hollow glass lined basaltic cylinders that occur at the axis of the mid ocean ridge and within the summit calderas of some seamounts typically pillars are 1 20 m tall and 0 25 2 0 m in diameter with subhorizontal to horizontal glassy selvages on their exterior walls lava pillars form gradually during a single eruption and are composed of lava emplaced at the eruption onset as well as the last lava remaining after the lava pond has drained on the deep sea floor the surface of a basaltic lava flow quenches to glass within 1 s thereby preserving information about eruption dynamics as well as chemical and physical properties of lava within a single eruption investigation of different lava pillars collected from a single eruption allows us to distinguish surficial lava pond or lava lake geochemical processes from those operating in the magma chamber morphologic major element petrographic and helium analyses were performed on portions of three lava pillars formed during the april 1991 eruption near 9 50 n at the axis of the east pacific rise modeling results indicate that the collected portions of pillars formed in 2 5 h suggesting a total eruption duration of 8 20 h these values are consistent with observed homogeneity in the glass helium concentrations and helium diffusion rates major element compositions of most pillar glasses are homogeneous and identical to the 1991 flow but slight chemical variations measured in the outermost portions of some pillars may reflect post eruptive processes rather than those occurring in subaxial magma bodies because lava pillars are common at mid ocean ridges mors the concepts and techniques we present here may have important application to the study of mor eruptions thereby providing a basis for quantitative comparisons of volcanic eruptions in geographically and tectonically diverse settings more research is needed to thoroughly test the hypotheses presented here

correlation between volcanic and tectonic segmentation of fast spreading ridges evidence from volcanic structures and lava flow morphology on the east pacific rise at 9 10 n 1 combined analyses of volcanic features in dsl 120 sonar data and argo i images along the ridge crest of the east pacific rise 9 09 54 n reveal a consistent decrease in inferred lava effusion rate toward the ends of third order segments the correlation of tectonic segmentation and volcanic style suggests that third order segmentation corresponds to the volcanic segmentation of the ridge along axis changes in volcanic structures from collapse troughs to basaltic lava domes and lava morphology from sheet to pillow flows coincide with the boundaries of morphologically defined third order tectonic segments of the ridge crest visible in shipboard multibeam bathymetry pillow lava flows cover 25 of the surveyed area of the ridge crest and are closely associated with small lava domes that occur primarily at third order segment ends an additional 25 of the surveyed area of the ridge crest is covered by sheet lava flows found in close association with an axial collapse trough the remaining terrain consists of lobate lava flows we interpret the spatial correlations of morphologic structural seismic and petrologic data as evidence that individual volcanic plumbing systems are organized at 20 km spacing along the ridge axis third order segment scale in agreement with the hypothesis that volcanic and tectonic segmentations are correlated for fast spreading ridges we estimate that the longevity of volcanic segments is 104 105 years 1 3 orders of magnitude longer than fourth order segments 102 103 years this implies the present pattern of hydrothermal activity may reorganize tens or hundreds of times while volcanic segmentation remains fairly stable

thickness of a submarine lava flow determined from near bottom magnetic field mapping by autonomous underwater vehicle magnetic field surveys obtained near the seafloor can map the boundaries of recent volcanic eruptions and can provide thickness estimates of these lava flow units independent of bathymetry differencing methods magnetic thickness estimation requires knowledge of the intensity of magnetization of the new lava and surrounding terrain but this can be satisfactorily obtained by representative sampling of the various volcanic units while bathymetry differencing requires pre existing data to assess the thickness of new lava eruptions magnetic surveys can be obtained after an eruption has occurred in this study near bottom magnetic surveys were obtained using an autonomous underwater vehicle auv which operates without a tether or human intervention auv technology offers rapid deployment and an efficient surveying approach for remotely mapping recent lava eruption sites on the seafloor

a record of eruption and intrusion at a fast spreading ridge axis axial summit trough of the east pacific rise at 9 10 n 1 high resolution side scan sonar near bottom multibeam bathymetry and deep sea photo and bathymetry traverses are used to map the axial summit trough ast at the east pacific rise between 9 and 10 n we define three ridge axis morphologic types no ast narrow ast and wide ast which characterize distinct ridge crest domains spanning tens of kilometers along strike near bottom observations modeling of deformation above intruding dikes and comparisons to the geologic and geophysical structure of the ridge crest are used to develop a revised model of ast genesis and evolution this model helps constrain the record of intrusive and extrusive magmatism and styles of lava deposition along the ridge crest at time scales from hundreds to tens of thousands of years the grabens in the narrow ast domain 9 43 53 n are consistent with deformation above the most recent 10 diking events beneath the ridge crest frequent high effusion rate extrusive volcanism in this domain several eruptions every 100 years overprints near axis deformation and maintains a consistent ast width the most recent eruption at the ridge crest occurred in this area and did not significantly modify the physical characteristics of the ast the grabens in the wide ast domain 9 23 43 n originated with similar dimensions to the narrow ast spreading driven primarily by the intrusion of shallow dikes within a narrow axial zone causes the initial graben bounding faults to migrate away from the axis infrequent extrusive volcanism several eruptions every 1000 years fills a portion of the subsidence that accumulates over time but does not significantly modify the width of the ast outside of these domains lower effusion rate constructional volcanism without efficient drain back fills and erases the signature of the ast the relative frequency of intrusive versus extrusive magmatic events controls the morphology of the ridge crest and appears to remain constant over millennial time scales within the domains we have identified however over longer time scales 10 25 ka domain specific intrusive to extrusive ratios do not appear to be fixed in space resulting in a fairly consistent volcanic accretion over the length scale of the second order ridge segment between 9 n and 10 n

a comparison between subaerial and submarine eruptions at kilauea volcano hawaii implications for the thermal viability of lateral feeder dikes eruption styles on the subaerial east rift zone erz of kilauea volcano are reviewed and a classification scheme for the different types of eruption is proposed the various eruption types are produced by differing thermal and driving pressure behaviour in the feeder dikes existing evidence is reviewed and new evidence presented of the types and volumes of eruptions on the puna ridge which is the submarine extension of the erz eruptions on the puna ridge fall into the same five classes as and are of comparable volume to those on the subaerial erz evidence is presented which suggests that feeder dikes for puna ridge eruptions are more thermally viable than those feeding subaerial eruptions and this difference causes long lived large volume eruptions to be more common on the puna ridge than on the subaerial erz this systematic variation in thermal viability may be due to increased dike width for puna ridge dikes or increased pressure gradients driving magma flow lateral dike emplacement is common to many basaltic systems including on other hawaiian volcanoes in iceland and at mid ocean ridges the systematic trend inferred for the erz of kilauea implies that in the other systems large volume eruptions may also be more common at great distances than they are close to the magma centre

extreme events and the morphodynamics of gravel dominated coastal barriers strengthening uncertain ground abstract the long term morphodynamic ordering of gravel dominated coastal systems gdcs many of which serve as coastal defences in northwest europe is dominated by extreme events that generate barrier crest overflow an understanding of this morphodynamic ordering is fraught with several unresolved difficulties these are related to the twin problems of the inadequacy of pertinent morphodynamic parameterisation and of obtaining data from modern shores enabling such parameterisation major uncertainties concern the timing of over crest flow in terms of return period of extreme elevation the intensity and structure of the overflow field antecedent beachface characteristics in response to storms the rate of relative sea level change tidal stage control and barrier resistance to forcing itself determined by a number of unknowns including barrier form and size sediment size and mosaics and barrier resilience while generalised extreme value modelling may provide a means of characterising overwashing return period and its variability exceptional tsunami events are outside the scope of such modelling the characterisation of gdcs morphodynamics in terms of the forcing extreme events will necessitate integrating some or all of these parameters into a single model

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

formation of submarine flat topped volcanic cones in hawai i high resolution bathymetric mapping has shown that submarine flat topped volcanic cones morphologically similar to ones on the deep sea floor and near mid ocean ridges are common on or near submarine rift zones of kilauea kohala or mauna kea mahukona and haleakala volcanoes four flat topped cones on kohala were explored and sampled with the pisces v submersible in october 1998 samples show that flat topped cones on rift zones are constructed of tholeiitic basalt erupted during the shield stage similarly shaped flat topped cones on the northwest submarine flank of ni ihau are apparently formed of alkalic basalt erupted during the rejuvenated stage submarine postshield stage eruptions on hilo ridge mahukona hana ridge and offshore ni ihau form pointed cones of alkalic basalt and hawaiite the shield stage flat topped cones have steep 25 sides remarkably flat horizontal tops basal diameters of 1 3 km and heights 300 m the flat tops commonly have either a low mound or a deep crater in the center the rejuvenated stage flat topped cones have the same shape with steep sides and flat horizontal tops but are much larger with basal diameters up to 5 5 km and heights commonly greater than 200 m the flat tops have a central low mound shallow crater or levees that surrounded lava ponds as large as 1 km across most of the rejuvenated stage flat topped cones formed on slopes 10 and formed adjacent semicircular steps down the flank of ni ihau rather than circular structures all the flat topped cones appear to be monogenetic and formed during steady effusive eruptions lasting years to decades these and other submarine volcanic cones of similar size and shape apparently form as continuously overflowing submarine lava ponds a lava pond surrounded by a levee forms above a sea floor vent as lava continues to flow into the pond the lava flow surface rises and overflows the lowest point on the levee forming elongate pillow lava flows that simultaneously build the rim outward and upward but also dam and fill in the low point on the rim the process repeats at the new lowest point forming a circular structure with a flat horizontal top and steep pillowed margins there is a delicate balance between lava heat supply to the pond and cooling and thickening of the floating crust factors that facilitate construction of such landforms include effusive eruption of lava with low volatile contents moderate to high confining pressure at moderate to great ocean depth long lived steady eruption years to decades moderate effusion rates probably ca 0 1 km3 year and low but not necessarily flat slopes with higher effusion rates sheet flows flood the slope with lower effusion rates pillow mounds form hawaiian shield stage eruptions begin as fissure eruptions if the eruption is too brief it will not consolidate activity at a point and fissure fed flows will form a pond with irregular levees the pond will solidify between eruptive pulses if the eruption is not steady lava that is too volatile rich or that is erupted in too shallow water will produce fragmental and highly vesicular lava that will accumulate to form steep pointed cones as occurs during the post shield stage the steady effusion of lava on land constructs lava shields which are probably the subaerial analogs to submarine flat topped cones but formed under different cooling conditions

geological chemical and biological evidence for recent volcanism at 17 5 s east pacific rise the superfast spreading portion of the east pacific rise at 17 5 s is a magmatically robust section of mid ocean ridge plumes characterized by high volatile metal ratios which has been hypothesized to be indicative of recent magmatism were found between 17 22 s and 17 35 s in november december 1993 dives with the french submersible nautile in december 1993 observed young sheet flows and widespread venting centered on the segment s shoalest point at 17 26 s eight submersible dives made one year later with the japanese submersible shinkai 6500 in september november 1994 found glassy unsedimented lavas a range of diffuse and high temperature vents and several types of biologic communities on the ridge crest beneath the volatile rich 1993 plumes the diffuse vents range from small areas in the youngest lavas characterized by very low temperature flow 10 c and a relatively low macrofaunal diversity to larger areas where the apparent older surface was covered with a dense and diverse biotope including anemones tubeworms mussels and serpulids several inactive vents marked by dead mussel clam beds are present in the southernmost portion of the site in the older lavas submersible mapping indicates that the youngest lava l0 unit erupted for at least 4 km along the strike of the ridge at the southern site and for 5 10 km at the northern site the flow is up to 400 m in width at the southern site and may be as much as 1 2 km in width at the northern site comparison of the observed 1993 fe s h2s heat and 3he heat plume values with time series measurements at sites on the northern east pacific rise and the juan de fuca ridge where the onset of the magmatic event is known implies that the last magmatic event at 17 5 s occurred within several years prior to 1993 continued rapid cooling of the underlying heat source is implied from the significant diminishment in the light scattering intensity and rise height of the overlying plumes recorded during 1994 submersible dives the type and diversity of the vent biota found on the l0 vents also suggests a young system the spatial and temporal scales of the accretion event at the 17 5 s epr site are similar in scale to eruptions on the intermediate rate spreading north cleft segment juan de fuca ridge but the 17 5 s event has more subtle gradients in the effusion rate of the eruptions and thermal and chemical character of the hydrothermal system

faulting and volcanism in the axial valley of the slow spreading center of the mariana back arc basin from wadatsumi side scan sonar images 1 we analyzed in detail the geology of the median valley floor of the mariana basin slow spreading ridge using sea surface geophysical data and a high resolution deep tow side scan sonar survey over one spreading segment analysis of surface magnetic data indicates highly asymmetric accretion with the half spreading rate on the western side of the basin being two to three times larger than on the eastern side surface magnetic and reflectivity data together suggest that asymmetric spreading is accomplished through eastward ridge jumps of 10 km of amplitude deep tow backscatter data indicate along axis variations of the volcanic processes with the emplacement of smooth and hummocky flows at the segment center and end respectively this variation likely relates to changes in the effusion rate due to the deepening or even disappearance of the magma chamber toward the segment end concerning tectonic processes we find a power law distribution of the fractures with an exponent of 1 74 this suggests that within the inner valley floor fracture growth prevails over fracture nucleation and coalescence and that fractures accommodate less than 8 of the strain according to our calculation based on a ratio of 0 02 to 0 03 between the vertical displacement and the length of faults the amount of tectonic strain accommodated in the inner valley floor would consistently be 1 1 3 4 data also show two distinct sets of fractures one trend is parallel to the rift direction at the segment center n160 e and perpendicular to the plate separation direction another set trends 17 oblique to this direction n175 e and is located over the eastern part of the valley in the vicinity of a major bounding fault also trending n175 e that is obliquely to the direction of plate motion we modeled the stress field near a major fault that is oblique to the regional stress field associated with plate separation using a three dimensional boundary element approach we found that the orientation of the predicted fissuring near the oblique fault is locally rotated by 15 due to a flexure of the bending plate close to this fault

magmatic processes and segmentation at a fast spreading mid ocean ridge detailed investigation of an axial discontinuity on the east pacific rise crest at 9 37 n 1 geophysical and petrological boundaries on mid ocean ridges provide ideal locations to study the relationships between magmatic tectonic and hydrothermal processes alvin based observational data and geochemical data for basalts and hydrothermal fluids are used to investigate these relationships at an axial discontinuity on the east pacific rise epr crest between 9 36 n and 9 38 n this ridge crest discontinuity is morphologically expressed by the overlap of an eastern and western axial summit collapse trough asct that delimits the primary volcanic and hydrothermal loci along the ridge crest in this area the ascts overlap by 3 km and are offset in a right lateral sense by 0 45 km near bottom imaging of this area in 1989 and 1991 shows changes in volcanic morphology and increases in hydrothermal and biological activity consistent with the occurrence of a magmatic event during that time interval when combined with the inferred age and structure of the seafloor basalt geochemistry and hydrothermal fluid chemistry these temporal changes suggest active southward propagation of the eastern asct and show that the western asct was unaffected by the recent magmatic event numerous extinct hydrothermal vents and older looking lava flow surfaces suggest waning of magmatic activity in the western asct young looking lava flows within or proximal to the eastern asct have anomalously high mg numbers relative to the regional trend and are chemically similar to lava erupted in 1991 along the 9 46 52 n epr region we propose that the young looking lava flows in the eastern asct are related to the 1991 eruption data show that the 9 37 n axial discontinuity marks a magmatic and hydrothermal boundary along the epr ridge crest and we argue that it be classified as a third order discontinuity this result is consistent with geophysical evidence suggesting fundamental differences in the crust and upper mantle north and south of the 9 37 n discontinuity

basaltic lava domes lava lakes and volcanic segmentation on the southern east pacific rise meter scale dsl 120 sonar mapping and coregistered argo ii photographic observations reveal changes in eruptive style that closely follow the third order structural segmentation of the ridge axis on the southern east pacific rise 17 11 18 37 s near segment ends we observe abundant basaltic lava domes which average 20 m in height and 200 m in basal diameter and have pillow lava as the dominant lava morphology the ubiquity of pillow lava suggests low effusion rate eruptions the abundance of lava domes suggests that the fissure eruptions were of sufficient duration to focus and produce a line of volcanic edifices near segment centers we observe fewer but larger lava domes voluminous drained and collapsed lava lakes and smooth lobate and sheet lava flows with very little pillow lava the abundance of sheet flows suggests that high effusion rate eruptions are common fewer lava domes and large lava lakes suggest that fissure eruptions do not focus to point sources this pattern was observed on eight third order ridge segments suggesting that a fundamental volcanic segmentation of the ridge occurs on this scale the third order segment boundaries also correlate with local maxima in the seismic axial magma chamber reflector depth throughout the study area and decreased across axis width of the region of seismic layer 2a thickening along the one segment where sufficient cross axis seismic lines exist the geochemically defined magmatic segment boundaries in the study area match the locations of our volcanic segment boundaries although rock sampling density is not adequate to constrain the variation across all the third order volcanic segments that we identify these observations suggest that variation in the processes of crustal accretion along axis occurs at a length scale of tens of kilometers on superfast spreading 140 km myr full rate mid ocean ridges

formation of submarine lava channel textures insights from laboratory simulations 1 laboratory simulations using polyethylene glycol peg extruded at a constant rate and temperature into a tank with a uniform basal slope and filled with a cold sucrose solution generate channels that are defined by stationary levees and mobile flow interiors these laboratory channels consistently display the following surface textures in the channel smooth folded lineated and chaotic in the simulations we can observe specific local conditions including flow rate position within the channel and time that combine to develop each texture the textures in peg flows form due to relative differences in shear forces between the peg crust and the underlying liquid wax minimal shear forces form smooth crust whereas folded crust forms when the shear is sufficiently high to cause ductile deformation brittle deformation of solid peg creates a chaotic texture and lineated crust results from shear forces along the channel levee margin we observe similar textures in submarine lava channels with sources at or near the axial summit trough of the east pacific rise between 9 and 10 n we mapped the surface textures of nine submarine lava channels using high resolution digital images collected during camera tows these textural maps along with observations of the formation of similar features in analog flows reveal important information about the mechanisms occurring across the channel during emplacement including relative flow velocity and shear stress

finite difference modelling of sandbox analogues compaction and detachment free deformation abstract the finite difference method for the modelling of hangingwall deformation above faults is applied to a wider range of problems close agreement has been achieved with the overall geometry of hangingwalls in sandbox analogue models of extensional faulting these results give insights into the dynamics involved and zones of faulting can be indicated using stretch rate and shear rate maps the controlling equations are then presented in a more general form which allows the effects of compaction to be included finally the method is shown to be applicable to vertical and horizontal velocities rather than speed and direction and this has been used to model the crust deforming by brittle failure at the top pure shear at the bottom and with a brittle ductile transition in between this system therefore allows modelling of crustal extension without the use of a detachment

geologic history of the summit of axial seamount juan de fuca ridge 1 multibeam 1 m resolution and side scan data collected from an autonomous underwater vehicle and lava samples radiocarbon dated sediment cores and observations of flow contacts collected by remotely operated vehicle were combined to reconstruct the geologic history and flow emplacement processes on axial seamount s summit and upper rift zones the maps show 52 post 410 ce lava flows and 20 precaldera lava flows as old as 31 2 kyr the inferred age of the caldera clastic deposits 1 2 m thick accumulated on the rims postcaldera between 31 ka and 410 ce there are no known lava flows near the summit the oldest postcaldera lava 410 ce is a pillow cone se of the caldera two flows erupted on the w rim between 800 and 1000 ce from 1220 to 1300 ce generally small eruptions of plagioclase phyric depleted mafic lava occurred in the central caldera and on the east rim larger post 1400 ce eruptions produced inflated lobate flows of aphyric less depleted and less mafic lava on the upper rift zones and in the n and s caldera all caldera floor lava flows and most uppermost rift zone flows postdate 1220 ce activity shifted from the central caldera to the upper s rift outside the caldera to the n rift and caldera floor and then to the s caldera and uppermost s rift where two historical eruptions occurred in 1998 and 2011 the average recurrence interval deduced from the flows erupted over the last 800 years is statistically identical to the 13 year interval between historical eruptions

channelized lava flows at the east pacific rise crest 9 10 n the importance of off axis lava transport in developing the architecture of young oceanic crust 1 submarine lava flows are the building blocks of young oceanic crust lava erupted at the ridge axis is transported across the ridge crest in a manner dictated by the rheology of the lava the characteristics of the eruption and the topography it encounters the resulting lava flows can vary dramatically in form and consequently in their impact on the physical characteristics of the seafloor and the architecture of the upper 50 500 m of the oceanic crust we have mapped and measured numerous submarine channelized lava flows at the east pacific rise epr crest 9 10 n that reflect the high effusion rate and high flow velocity end member of lava eruption and transport at mid ocean ridges channel systems composed of identifiable segments 50 1000 m in length extend up to 3 km from the axial summit trough ast and have widths of 10 50 m and depths of 2 3 m samples collected within the channels are n morb with mg indicating eruption from the ast we produce detailed maps of lava surface morphology across the channel surface from mosaics of digital images that show lineated or flat sheets at the channel center bounded by brecciated lava at the channel margins modeled velocity profiles across the channel surface allow us to determine flux through the channels from 0 4 to 4 7 103 m3 s and modeled shear rates help explain the surface morphology variation we suggest that channelized lava flows are a primary mechanism by which lava accumulates in the off axis region 1 3 km and produces the layer 2a thickening that is observed at fast and superfast spreading ridges in addition the rapid high volume flux eruptions necessary to produce channelized flows may act as an indicator of the local magma budget along the epr we find that high concentrations of channelized lava flows correlate with local across axis ridge morphology indicative of an elevated magma budget additionally in locations where channelized flows are located dominantly to the east or west of the ast the ridge crest is asymmetric and layer 2a appears to thicken over a greater distance from the ast toward the side of the ridge crest where the channels are located

submeter bathymetric mapping of volcanic and hydrothermal features on the east pacific rise crest at 9 50 n 1 recent advances in underwater vehicle navigation and sonar technology now permit detailed mapping of complex seafloor bathymetry found at mid ocean ridge crests imagenex 881 675 khz scanning sonar data collected during low altitude 5 m surveys conducted with dsv alvin were used to produce submeter resolution bathymetric maps of five hydrothermal vent areas at the east pacific rise epr ridge2000 integrated study site 9 50 n bull s eye data were collected during 29 dives in 2004 and 2005 and were merged through a grid rectification technique to create high resolution 0 5 m grid composite maps these are the first submeter bathymetric maps generated with a scanning sonar mounted on alvin the composite maps can be used to quantify the dimensions of meter scale volcanic and hydrothermal features within the epr axial summit trough ast including hydrothermal vent structures lava pillars collapse areas the trough walls and primary volcanic fissures existing autonomous benthic explorer abe bathymetry data 675 khz scanning sonar collected at this site provide the broader geologic context necessary to interpret the meter scale features resolved in the composite maps the grid rectification technique we employed can be used to optimize vehicle time by permitting the creation of high resolution bathymetry maps from data collected during multiple coordinated short duration surveys after primary dive objectives are met this method can also be used to colocate future near bottom sonar data sets within the high resolution composite maps enabling quantification of bathymetric changes associated with active volcanic hydrothermal and tectonic processes

distribution of melt along the east pacific rise from 9 30 to 10 n from an amplitude variation with angle of incidence ava technique author posting the author s 2015 this article is posted here by permission of the royal astronomical society for personal use not for redistribution the definitive version was published in geophysical journal international 203 2015 1 21 doi 10 1093 gji ggv251

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

displacement and strain patterns of extensional orogens rock fabrics that result from displacement in extensional orogens provide a means of identifying geometric models responsible for extension of continental crust strain compatibility arguments indicate that a finite extension can be accommodated by displacements across 1 planar nonrotating faults or ductile shear zones 2 shear zones which rotate above a horizontal detachment the domino model or 3 shear zones which rotate as a result of a horizontally oriented pure shear stretching component the plastic model listric normal shear zone geometries may develop as the result of a depth dependent change in the pure shear component in the plastic model or the area loss component in the planar shear zone model within the geometric framework of these various models the effects of superposed simple shear pure shear extension and area change on the rock fabric are investigated these displacement components which may be superposed sequentially or simultaneously determine the state of finite strain associated with a given magnitude of tectonic extension the relationships between displacement and strain are expressed as graphs of foliation dip or strain ratio versus shear strain and as graphs of foliation dip or strain ratio versus tectonic extension the shear strain graphs illustrate the effects of the different displacement components on the rock fabric in a single shear zone whereas the tectonic extension graphs better illustrate these effects on the regional scale the shear strain and tectonic extension graphs can be used by field geologists 1 to determine tectonic extension across a region from measurements of strain ratio or orientation and fluctuation of foliation 2 to distinguish between possible extension models from field data and 3 to evaluate the influence of shear zone attitude and thickness upon the amount of displacement applications of these methods are illustrated for field areas in metamorphic core complexes of arizona and in the basin and range province of the north america cordillera

finite difference modelling of hangingwall deformation abstract modelling of hangingwall deformation during extension or compression may be achieved using a finite difference approach under the assumption of incompressible flow equivalent to area balancing this method enables calculation of a particle velocity field giving the trajectory of any point in the hangingwall during deformation this technique is more flexible than graphical algorithms because it is not restricted to simple shear cases inputs to the algorithm are the bounding fault and the particle displacement directions simple examples are used to illustrate the method

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

geology of the northern cleft segment juan de fuca ridge recent lava flows sea floor spreading and the formation of megaplumes geologic mapping and lava sampling were carried out after the discovery of large bursts of hydrothermal fluids megaplumes over the southern juan de fuca ridge in 1986 and 1987 our investigations of the northernmost section of the cleft segment have discovered 1 semicontinuous low temperature venting and one major high temperature vent site along 17 km of the neovolcanic zone and 2 very glassy lightly sedimented sheet flows and pillow mounds superimposed on older terrain over about 24 km along the northern most part the pillow mounds are documented to have erupted between 1981 and 1987 the occurrence of the megaplumes during this same time period strengthens the hypothesis that megaplumes are caused by sea floor extension events although the basalts from the entire length of the neovolcanic zone of the cleft segment appear to have been derived from the same mantle source a systematic northward increase in mg number along the segment within the neovolcanic zone indicates less shallow level differentiation to the north possibly related to the development of new magma chambers during the recent phase of sea floor spreading that has occurred there

channeled flow analytic solutions laboratory experiments and applications to lava flows although channeled lava flows are common in basaltic volcanism relationships between channel morphology eruption and emplacement parameters and lava properties are not well understood several models have commonly been used to constrain these relations but they have not been well tested on natural or simulated lava flows over a wide range of parameter space here we test the accuracy and assumptions of a moderately simple analytic rectangular channel solution by comparing the behavior of well controlled laboratory polyethylene glycol peg channeled flows to the analytic solution for isothermal steady newtonian flow in a rectangular channel with constant dimensions this analytic solution agrees well with laboratory measurements volumetric effusion rates q m3 s 1 calculated from the analytical model using measured peg flows as input yield ratios of qcalculated qpumped of 0 2 to 3 6 and flow rates calculated from a best fit surface velocity profile to measured velocities give more accurate ratios of 0 8 to 1 2 we find a very weak dependence of solution accuracy on slope attributable to flow front effects within the laboratory flows we subsequently apply the solution to several subaerial and submarine terrestrial flows as well as extraterrestrial channeled flows over a wide range of flow parameters viscosity ranges and flow rates obtained using measured channel dimensions and assumed lava properties are plausible interestingly the resulting extraterrestrial estimates of viscosities and flow rates tend to fall closer to known terrestrial measurements and estimates of channel flow than to previous planetary estimates we therefore suggest that the analytic newtonian rectangular channel flow model is a more appropriate physical model for many channeled terrestrial and planetary flows than the newtonian infinite sheet flow and approximation to bingham channel flow widely used previously