Fornari Daniel J.

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Daniel J.

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  • Article
    Submarine Fernandina : magmatism at the leading edge of the Galapagos hot spot
    (American Geophysical Union, 2006-12-19) Geist, Dennis J. ; Fornari, Daniel J. ; Kurz, Mark D. ; Harpp, Karen S. ; Soule, Samuel A. ; Perfit, Michael R. ; Koleszar, Alison M.
    New multibeam and side-scan sonar surveys of Fernandina volcano and the geochemistry of lavas provide clues to the structural and magmatic development of Galápagos volcanoes. Submarine Fernandina has three well-developed rift zones, whereas the subaerial edifice has circumferential fissures associated with a large summit caldera and diffuse radial fissures on the lower slopes. Rift zone development is controlled by changes in deviatoric stresses with increasing distance from the caldera. Large lava flows are present on the gently sloping and deep seafloor west of Fernandina. Fernandina's submarine lavas are petrographically more diverse than the subaerial suite and include picrites. Most submarine glasses are similar in composition to aphyric subaerially erupted lavas, however. These rocks are termed the “normal” series and are believed to result from cooling and crystallization in the subcaldera magma system, which buffers the magmas both thermally and chemically. These normal-series magmas are extruded laterally through the flanks of the volcano, where they scavenge and disaggregate olivine-gabbro mush to produce picritic lavas. A suite of lavas recovered from the terminus of the SW submarine rift and terraces to the south comprises evolved basalts and icelandites with MgO = 3.1 to 5.0 wt.%. This “evolved series” is believed to form by fractional crystallization at 3 to 5 kb, involving extensive crystallization of clinopyroxene and titanomagnetite in addition to plagioclase. “High-K” lavas were recovered from the southwest rift and are attributed to hybridization between normal-series basalt and evolved-series magma. The geochemical and structural findings are used to develop an evolutionary model for the construction of the Galápagos Platform and better understand the petrogenesis of the erupted lavas. The earliest stage is represented by the deep-water lava flows, which over time construct a broad submarine platform. The deep-water lavas originate from the subcaldera plumbing system of the adjacent volcano. After construction of the platform, eruptions focus to a point source, building an island with rift zones extending away from the adjacent, buttressing volcanoes. Most rift zone magmas intrude laterally from the subcaldera magma chamber, although a few evolve by crystallization in the upper mantle and deep crust.
  • Article
    Geochemistry of lavas from the 2005–2006 eruption at the East Pacific Rise, 9°46′N–9°56′N : implications for ridge crest plumbing and decadal changes in magma chamber compositions
    (American Geophysical Union, 2010-05-12) Goss, Adam R. ; Perfit, Michael R. ; Ridley, W. Ian ; Rubin, Kenneth H. ; Kamenov, George D. ; Soule, Samuel A. ; Fundis, A. ; Fornari, Daniel J.
    Detailed mapping, sampling, and geochemical analyses of lava flows erupted from an ∼18 km long section of the northern East Pacific Rise (EPR) from 9°46′N to 9°56′N during 2005–2006 provide unique data pertaining to the short-term thermochemical changes in a mid-ocean ridge magmatic system. The 2005–2006 lavas are typical normal mid-oceanic ridge basalt with strongly depleted incompatible trace element patterns with marked negative Sr and Eu/Eu* anomalies and are slightly more evolved than lavas erupted in 1991–1992 at the same location on the EPR. Spatial geochemical differences show that lavas from the northern and southern limits of the 2005–2006 eruption are more evolved than those erupted in the central portion of the fissure system. Similar spatial patterns observed in 1991–1992 lavas suggest geochemical gradients are preserved over decadal time scales. Products of northern axial and off-axis fissure eruptions are consistent with the eruption of cooler, more fractionated lavas that also record a parental melt component not observed in the main suite of 2005–2006 lavas. Radiogenic isotopic ratios for 2005–2006 lavas fall within larger isotopic fields defined for young axial lavas from 9°N to 10°N EPR, including those from the 1991–1992 eruption. Geochemical data from the 2005–2006 eruption are consistent with an invariable mantle source over the spatial extent of the eruption and petrogenetic processes (e.g., fractional crystallization and magma mixing) operating within the crystal mush zone and axial magma chamber (AMC) before and during the 13 year repose period. Geochemical modeling suggests that the 2005–2006 lavas represent differentiated residual liquids from the 1991–1992 eruption that were modified by melts added from deeper within the crust and that the eruption was not initiated by the injection of hotter, more primitive basalt directly into the AMC. Rather, the eruption was driven by AMC pressurization from persistent or episodic addition of more evolved magma from the crystal mush zone into the overlying subridge AMC during the period between the two eruptions. Heat balance calculations of a hydrothermally cooled AMC support this model and show that continual addition of melt from the mush zone was required to maintain a sizable AMC over this time interval.
  • Article
    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
    (American Geophysical Union, 2005-08-18) Soule, Samuel A. ; Fornari, Daniel J. ; Perfit, Michael R.
    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.
  • Article
    Central Anomaly Magnetization High documentation of crustal accretion along the East Pacific Rise (9°55′–9°25′N)
    (American Geophysical Union, 2008-04-09) Williams, Clare M. ; Tivey, Maurice A. ; Schouten, Hans A. ; Fornari, Daniel J.
    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.
  • Article
    Globally aligned photomosaic of the Lucky Strike hydrothermal vent field (Mid-Atlantic Ridge, 37°18.5′N) : release of georeferenced data, mosaic construction, and viewing software
    (American Geophysical Union, 2008-12-05) Escartin, Javier E. ; Garcia, Rafael ; Delaunoy, O. ; Ferrer, J. ; Gracias, Nuno ; Elibol, A. ; Cufi, X. ; Neumann, L. ; Fornari, Daniel J. ; Humphris, Susan E. ; Renard, J.
    We present a georeferenced photomosaic of the Lucky Strike hydrothermal vent field (Mid-Atlantic Ridge, 37°18′N). The photomosaic was generated from digital photographs acquired using the ARGO II seafloor imaging system during the 1996 LUSTRE cruise, which surveyed a ∼1 km2 zone and provided a coverage of ∼20% of the seafloor. The photomosaic has a pixel resolution of 15 mm and encloses the areas with known active hydrothermal venting. The final mosaic is generated after an optimization that includes the automatic detection of the same benthic features across different images (feature-matching), followed by a global alignment of images based on the vehicle navigation. We also provide software to construct mosaics from large sets of images for which georeferencing information exists (location, attitude, and altitude per image), to visualize them, and to extract data. Georeferencing information can be provided by the raw navigation data (collected during the survey) or result from the optimization obtained from image matching. Mosaics based solely on navigation can be readily generated by any user but the optimization and global alignment of the mosaic requires a case-by-case approach for which no universally software is available. The Lucky Strike photomosaics (optimized and navigated-only) are publicly available through the Marine Geoscience Data System (MGDS, The mosaic-generating and viewing software is available through the Computer Vision and Robotics Group Web page at the University of Girona (∼rafa/mosaicviewer.html).
  • Article
    Submarine volcanic morphology of the western Galapagos based on EM300 bathymetry and MR1 side-scan sonar
    (American Geophysical Union, 2007-03-21) Glass, Jennifer B. ; Fornari, Daniel J. ; Hall, Hillary F. ; Cougan, Allison A. ; Berkenbosch, Heidi A. ; Holmes, Mark L. ; White, Scott M. ; De La Torre, Giorgio
    A compilation of high-resolution EM300 multibeam bathymetric and existing MR1 side-scan sonar data was used to investigate the volcanic morphology of the flanks of the western Galápagos Islands. The data portray an assortment of constructional volcanic features on the shallow to deep submarine flanks of Fernandina, Isabela, and Santiago Islands, including rift zones and groups of cones that are considered to be the primary elements in constructing the archipelagic apron. Ten submarine rift zones were mapped, ranging in length from 5 to 20 km, comparable in length to western Canary Island rift zones but significantly shorter than Hawaiian submarine rift zones. A detailed analysis of the northwestern Fernandina submarine rift, including calculated magnetization from a surface-towed magnetic study, suggests that the most recent volcanism has focused at the shallow end of the rift. Small submarine volcanic cones with various morphologies (e.g., pointed, cratered, and occasionally breached) are common in the submarine western Galápagos both on rift zones and on the island flanks where no rifts are present. At depths greater than ∼3000 m, large lava flow fields in regions of low bathymetric relief have been previously identified as a common seafloor feature in the western Galápagos by Geist et al. (2006); however, their source(s) remained enigmatic. The new EM300 data show that a number of the deep lava flows originate from small cones along the mid-lower portion of the NW submarine rift of Fernandina, suggesting that the deep flows owe their origin, at least in part, to submarine rift zone volcanism.
  • Article
    Paleointensity applications to timing and extent of eruptive activity, 9°–10°N East Pacific Rise
    (American Geophysical Union, 2006-06-08) Bowles, Julie A. ; Gee, Jeffrey S. ; Kent, Dennis V. ; Perfit, Michael R. ; Soule, Samuel A. ; Fornari, Daniel J.
    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 μT 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.
  • Article
    Seafloor photo-geology and sonar terrain modeling at the 9°N overlapping spreading center, East Pacific Rise
    (John Wiley & Sons, 2013-12-20) Klein, Emily M. ; White, Scott M. ; Nunnery, James Andrew ; Mason-Stack, Jessica L. ; Wanless, V. Dorsey ; Perfit, Michael R. ; Waters, Christopher L. ; Sims, Kenneth W. W. ; Fornari, Daniel J. ; Zaino, Anne J. ; Ridley, W. Ian
    A fundamental goal in the study of mid-ocean ridges is to understand the relationship between the distribution of melt at depth and seafloor features. Building on geophysical information on subsurface melt at the 9°N overlapping spreading center on the East Pacific Rise, we use terrain modeling (DSL-120A side scan and bathymetry), photo-geology (Jason II and WHOI TowCam), and geochemical data to explore this relationship. Terrain modeling identified four distinct geomorphic provinces with common seafloor characteristics that correspond well to changes in subsurface melt distribution. Visual observations were used to interpret terrain modeling results and to establish a relative seafloor age scale, calibrated with radiometric age dates, to identify areas of recent volcanism. On the east limb, recent eruptions in the north are localized over the margins of the 4 km wide asymmetric melt sill, forming a prominent off-axis pillow ridge. Along the southern east limb, recent eruptions occur along a neovolcanic ridge that hugs the overlap basin and lies several kilometers west of the plunging melt sill. Our results suggest that long-term southward migration of the east limb occurs through a series of diking events with a net southward propagation direction. Examining sites of recent eruptions in the context of geophysical data on melt distribution in the crust and upper mantle suggests melt may follow complex paths from depth to the surface. Overall, our findings emphasize the value of integrating information obtained from photo-geology, terrain modeling, lava geochemistry and petrography, and geophysics to constrain the nature of melt delivery at mid-ocean ridges.
  • Article
    The largest deep-ocean silicic volcanic eruption of the past century
    (American Association for the Advancement of Science, 2018-01-10) Carey, Rebecca ; Soule, Samuel A. ; Manga, Michael ; White, James D. L. ; McPhie, Jocelyn ; Wysoczanski, Richard ; Jutzeler, Martin ; Tani, Kenichiro ; Yoerger, Dana R. ; Fornari, Daniel J. ; Caratori Tontini, Fabio ; Houghton, Bruce ; Mitchell, Samuel ; Ikegami, Fumihiko ; Conway, Chris E. ; Murch, Arran ; Fauria, Kristen ; Jones, Meghan ; Cahalan, Ryan ; McKenzie, Warren
    The 2012 submarine eruption of Havre volcano in the Kermadec arc, New Zealand, is the largest deep-ocean eruption in history and one of very few recorded submarine eruptions involving rhyolite magma. It was recognized from a gigantic 400-km2 pumice raft seen in satellite imagery, but the complexity of this event was concealed beneath the sea surface. Mapping, observations, and sampling by submersibles have provided an exceptionally high fidelity record of the seafloor products, which included lava sourced from 14 vents at water depths of 900 to 1220 m, and fragmental deposits including giant pumice clasts up to 9 m in diameter. Most (>75%) of the total erupted volume was partitioned into the pumice raft and transported far from the volcano. The geological record on submarine volcanic edifices in volcanic arcs does not faithfully archive eruption size or magma production.
  • Article
    Construction of the Galapagos platform by large submarine volcanic terraces
    (American Geophysical Union, 2008-03-19) Geist, Dennis J. ; Diefenbach, Bridget A. ; Fornari, Daniel J. ; Kurz, Mark D. ; Harpp, Karen S. ; Blusztajn, Jerzy S.
    New multibeam bathymetric and side-scan sonar data from the southwestern edge of the Galápagos platform reveal the presence of ∼60 large, stepped submarine terraces between depths of 800 m and 3500 m. These terraces are unique features, as none are known from any other archipelago that share this geomorphic form or size. The terraces slope seaward at <2° and are surrounded by escarpments that average ∼300 m in height with average slopes of 24°. The stepped morphology, fine-scale features, and sinuous planform continuity of terrace edges indicate that each terrace results from a sequence of major submarine volcanic eruptions, similar in extent to young deep-water (>3000 m) lava flow fields west of Fernandina and Isabela Islands. The terraces are formed of thick sequences of lava flows that coalesce to form the foundation of the Galápagos platform, on which the subaerial central volcanoes are built. The compositions of basalts dredged from the submarine terraces indicate that most lavas are chemically similar to subaerial lavas erupted from Sierra Negra volcano on southern Isabela Island. There are no regular major element, trace element, or isotopic variations in the submarine lavas as a function of depth, relative stratigraphic position, or geographic location along the southwest margin of the platform. We hypothesize that magma supply at the western edge of the Galápagos hot spot, which is influenced by both plume and mid-ocean ridge magmatic processes, leads to episodic eruption of large lava flows. These large lava flows coalesce to form the archipelagic apron upon which the island volcanoes are built.
  • Article
    Discovery of active off-axis hydrothermal vents at 9° 54’N East Pacific Rise
    (National Academy of Sciences, 2022-07-21) McDermott, Jill M. ; Parnell-Turner, Ross ; Barreyre, Thibaut ; Herrera, Santiago ; Downing, Connor C. ; Pittoors, Nicole C. ; Pehr, Kelden ; Vohsen, Samuel A. ; Dowd, William S. ; Wu, Jyun-Nai ; Marjanovic, Milena ; Fornari, Daniel J.
    Comprehensive knowledge of the distribution of active hydrothermal vent fields along midocean ridges is essential to understanding global chemical and heat fluxes and endemic faunal distributions. However, current knowledge is biased by a historical preference for on-axis surveys. A scarcity of high-resolution bathymetric surveys in off-axis regions limits vent identification, which implies that the number of vents may be underestimated. Here, we present the discovery of an active, high-temperature, off-axis hydrothermal field on a fast-spreading ridge. The vent field is located 750 m east of the East Pacific Rise axis and ∼7 km north of on-axis vents at 9° 50′N, which are situated in a 50- to 100-m-wide trough. This site is currently the largest vent field known on the East Pacific Rise between 9 and 10° N. Its proximity to a normal fault suggests that hydrothermal fluid pathways are tectonically controlled. Geochemical evidence reveals deep fluid circulation to depths only 160 m above the axial magma lens. Relative to on-axis vents at 9° 50′N, these off-axis fluids attain higher temperatures and pressures. This tectonically controlled vent field may therefore exhibit greater stability in fluid composition, in contrast to more dynamic, dike-controlled, on-axis vents. The location of this site indicates that high-temperature convective circulation cells extend to greater distances off axis than previously realized. Thorough high-resolution mapping is necessary to understand the distribution, frequency, and physical controls on active off-axis vent fields so that their contribution to global heat and chemical fluxes and role in metacommunity dynamics can be determined.
  • Article
    Crustal magnetization and the subseafloor structure of the ASHES vent field, Axial Seamount, Juan de Fuca Ridge : implications for the investigation of hydrothermal sites
    (John Wiley & Sons, 2016-06-24) Tontini, F. Caratori ; Crone, Timothy J. ; de Ronde, Cornel E. J. ; Fornari, Daniel J. ; Kinsey, James C. ; Mittelstaedt, Eric ; Tivey, Maurice A.
    High-resolution geophysical data have been collected using the Autonomous Underwater Vehicle (AUV) Sentry over the ASHES (Axial Seamount Hydrothermal Emission Study) high-temperature (~348°C) vent field at Axial Seamount, on the Juan de Fuca Ridge. Multiple surveys were performed on a 3-D grid at different altitudes above the seafloor, providing an unprecedented view of magnetic data resolution as a function of altitude above the seafloor. Magnetic data derived near the seafloor show that the ASHES field is characterized by a zone of low magnetization, which can be explained by hydrothermal alteration of the host volcanic rocks. Surface manifestations of hydrothermal activity at the ASHES vent field are likely controlled by a combination of local faults and fractures and different lava morphologies near the seafloor. Three-dimensional inversion of the magnetic data provides evidence of a vertical, pipe-like upflow zone of the hydrothermal fluids with a vertical extent of ~100 m.
  • Article
    Submeter bathymetric mapping of volcanic and hydrothermal features on the East Pacific Rise crest at 9°50′N
    (American Geophysical Union, 2007-01-19) Ferrini, Vicki L. ; Fornari, Daniel J. ; Shank, Timothy M. ; Kinsey, James C. ; Tivey, Maurice A. ; Soule, Samuel A. ; Carbotte, Suzanne M. ; Whitcomb, Louis L. ; Yoerger, Dana R. ; Howland, Jonathan C.
    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.
  • Article
    A photographic and acoustic transect across two deep-water seafloor mounds, Mississippi Canyon, northern Gulf of Mexico
    (Elsevier B.V., 2008-05-21) Hart, Patrick E. ; Hutchinson, Deborah R. ; Gardner, Joan ; Carney, Robert S. ; Fornari, Daniel J.
    In the northern Gulf of Mexico, a series of seafloor mounds lie along the floor of the Mississippi Canyon in Atwater Valley lease blocks 13 and 14. The mounds, one of which was drilled by the Chevron Joint Industry Project on Methane Hydrates in 2005, are interpreted to be vent-related features that may contain significant accumulations of gas hydrate adjacent to gas and fluid migration pathways. The mounds are located not, vert, similar150 km south of Louisiana at not, vert, similar1300 m water depth. New side-scan sonar data, multibeam bathymetry, and near-bottom photography along a 4 km northwest–southeast transect crossing two of the mounds (labeled D and F) reveal the mounds' detailed morphology and surficial characteristics. Mound D, not, vert, similar250 m in diameter and 7–10 m in height, has exposures of authigenic carbonates and appears to result from a seafloor vent of slow-to-moderate flux. Mound F, which is not, vert, similar400 m in diameter and 10–15 m high, is covered on its southwest flank by extruded mud flows, a characteristic associated with moderate-to-rapid flux. Chemosynthetic communities visible on the bottom photographs are restricted to bacterial mats on both mounds and mussels at Mound D. No indications of surficial gas hydrates are evident on the bottom photograph
  • Article
    Formation of submarine lava channel textures : insights from laboratory simulations
    (American Geophysical Union, 2006-03-28) Garry, W. Brent ; Gregg, Tracy K. P. ; Soule, Samuel A. ; Fornari, Daniel J.
    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.
  • Article
    Temperature variations at diffuse and focused flow hydrothermal vent sites along the northern East Pacific Rise
    (American Geophysical Union, 2006-03-03) Scheirer, Daniel S. ; Shank, Timothy M. ; Fornari, Daniel J.
    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.
  • Article
    Extreme heterogeneity in mid-ocean ridge mantle revealed in lavas from the 8 degrees 20 ' N near-axis seamount chain
    (American Geophysical Union, 2020-12-14) Anderson, Molly ; Wanless, V. Dorsey ; Perfit, Michael R. ; Conrad, Ethan ; Gregg, Patricia M. ; Fornari, Daniel J. ; Ridley, W. Ian
    Lavas that have erupted at near‐axis seamounts provide windows into mid‐ocean ridge mantle heterogeneity and melting systematics which are not easily observed on‐axis at fast‐spreading centers. Beneath ridges, most heterogeneity is obscured as magmas aggregate toward the ridge, where they efficiently mix and homogenize during transit and within shallow magma chambers prior to eruption. To understand the deeper magmatic processes contributing to oceanic crustal formation, we examine the compositions of lavas erupted along a chain of near‐axis seamounts and volcanic ridges perpendicular to the East Pacific Rise. We assess the chemistry of near‐ridge mantle using a ∼200 km‐long chain at ∼8°20′N. High‐resolution bathymetric maps are used with geochemical analyses of ∼300 basalts to evaluate the petrogenesis of lavas and the heterogeneity of mantle feeding these near‐axis eruptions. Major and trace element concentrations and radiogenic isotope ratios are highly variable on <1 km scales, and reveal a continuum of depleted, normal, and enriched basalts spanning the full range of ridge and seamount compositions in the northeast Pacific. There is no systematic compositional variability along the chain. Modeling suggests that depleted mid‐ocean ridge basalt (DMORB) lavas are produced by ∼5%–15% melting of a depleted mid‐ocean ridge (MOR) mantle. Normal mid‐ocean ridge basalts (NMORB) form from 5% to 15% melting of a slightly enriched MOR mantle. Enriched mid‐ocean ridge basalts (EMORB) range from <1% melting of 10% enriched mantle to >15% melting of 100% enriched mantle. The presence of all three lava types along the seamount chain, and on a single seamount closest to the ridge axis, confirms that the sub‐ridge mantle is much more heterogeneous than is commonly observed on‐axis and heterogeneity exists over small spatial scales.
  • Article
    Lava geochemistry as a probe into crustal formation at the East Pacific Rise
    (The Oceanography Society, 2012-03) Perfit, Michael R. ; Wanless, V. Dorsey ; Ridley, W. Ian ; Klein, Emily M. ; Smith, Matthew C. ; Goss, Adam R. ; Hinds, Jillian S. ; Kutza, Scott W. ; Fornari, Daniel J.
    Basalt lavas comprise the greatest volume of volcanic rocks on Earth, and most of them erupt along the world's mid-ocean ridges (MORs). These MOR basalts (MORBs) are generally thought to be relatively homogeneous in composition over large segments of the global ridge system (e.g., Klein, 2005). However, detailed sampling of two different regions on the northern East Pacific Rise (EPR) and extensive analysis of the samples show that fine-scale mapping and sampling of the ridge axis can reveal significant variations in lava chemistry on both small spatial and short temporal scales. The two most intensely sampled sites within the EPR Integrated Study Site (ISS) lie on and off axis between 9°17'N and 10°N, and from a wide region centered around 9°N where two segments of the EPR overlap (see Fornari et al., 2012, Figure 3, in this issue). The chemical composition of erupted lavas, similar to the genotype of an organism, can be used by igneous petrologists to trace the evolution of magmas from the mantle to the seafloor. The extensive and detailed geochemical studies at the EPR highlight how a thorough understanding of the variability in lava compositions on small spatial scales (i.e., between lava flows) and large spatial scales (i.e., from segment center to segment end and including discontinuities in the ridge crest) can be used in combination with seafloor photography, lava morphology, and bathymetry to provide insights into the magmatic system that drives volcanism and influences hydrothermal chemistry and biology at a fast-spreading MOR.
  • Article
    Submarine radial vents on Mauna Loa Volcano, Hawai`i
    (American Geophysical Union, 2006-05-02) Wanless, V. Dorsey ; Garcia, Michael O. ; Trusdell, F. A. ; Rhodes, J. M. ; Norman, M. D. ; Weis, Dominique ; Fornari, Daniel J. ; Kurz, Mark D. ; Guillou, Herve
    A 2002 multibeam sonar survey of Mauna Loa’s western flank revealed ten submarine radial vents and three submarine lava flows. Only one submarine radial vent was known previously. The ages of these vents are constrained by eyewitness accounts, geologic relationships, Mn-Fe coatings, and geochemical stratigraphy; they range from 128 years B.P. to possibly 47 ka. Eight of the radial vents produced degassed lavas despite eruption in water depths sufficient to inhibit sulfur degassing. These vents formed truncated cones and short lava flows. Two vents produced undegassed lavas that created ‘‘irregular’’ cones and longer lava flows. Compositionally and isotopically, the submarine radial vent lavas are typical of Mauna Loa lavas, except two cones that erupted alkalic lavas. He-Sr isotopes for the radial vent lavas follow Mauna Loa’s evolutionary trend. The compositional and isotopic heterogeneity of these lavas indicates most had distinct parental magmas. Bathymetry and acoustic backscatter results, along with photography and sampling during four JASON2 dives, are used to produce a detailed geologic map to evaluate Mauna Loa’s submarine geologic history. The new map shows that the 1877 submarine eruption was much larger than previously thought, resulting in a 10% increase for recent volcanism. Furthermore, although alkalic lavas were found at two radial vents, there is no systematic increase in alkalinity among these or other Mauna Loa lavas as expected for a dying volcano. These results refute an interpretation that Mauna Loa’s volcanism is waning. The submarine radial vents and flows cover 29 km2 of seafloor and comprise a total volume of ~2 x 109 m3 of lava, reinforcing the idea that submarine lava eruptions are important in the growth of oceanic island volcanoes even after they emerged above sea level.
  • Article
    Hydrothermal discharge during submarine eruptions : the importance of detection, response, and new technology
    (The Oceanography Society, 2012-03) Baker, Edward T. ; Chadwick, William W. ; Cowen, James P. ; Dziak, Robert P. ; Rubin, Kenneth H. ; Fornari, Daniel J.
    Submarine volcanic eruptions and intrusions construct new oceanic crust and build long chains of volcanic islands and vast submarine plateaus. Magmatic events are a primary agent for the transfer of heat, chemicals, and even microbes from the crust to the ocean, but the processes that control these transfers are poorly understood. The 1980s discovery that mid-ocean ridge eruptions are often associated with brief releases of immense volumes of hot fluids ("event plumes") spurred interest in methods for detecting the onset of eruptions or intrusions and for rapidly organizing seagoing response efforts. Since then, some 35 magmatic events have been recognized and responded to on mid-ocean ridges and at seamounts in both volcanic arc and intraplate settings. Field responses at mid-ocean ridges have found that event plumes occur over a wide range of eruption styles and sizes, and thus may be a common consequence of ridge eruptions. The source(s) of event plume fluids are still debated. Eruptions detected at ridges generally have high effusion rates and short durations (hours to days), whereas field responses at arc volcanic cones have found eruptions with very low effusion rates and durations on the scale of years. New approaches to the study of submarine magmatic events include the development of autonomous vehicles for detection and response, and the establishment of permanent seafloor observatories at likely future eruption sites.