Search

Deep sea photographs were collected for several camera-tow transects along and across the axis at the East Pacific Rise crest between 9°49′ and 9°52′ N, covering terrain out to 2 km from the ridge axis. The objective of the surveys was to utilize fine-scale morphology and imagery of seafloor volcanic terrain to aid in interpreting eruptive history and lava emplacement processes along this fast-spreading mid-ocean ridge. The area surveyed corresponds to the region over which seismic layer 2A, believed to correspond to the extrusive oceanic layer, attains full thickness (Christeson et al., 1994a, b, 1996; Hooft et al., 1996; Carbotte et al., 1997). The photographic data are used to identify the different eruptive styles occurring along the ridge crest, map the distribution of the different morphologies, constrain the relative proportions of the three main morphologies and discuss the implications of these results. Morphologic distributions of lava for the area investigated are 66% lobate lava, 20% sheet lava, 10% pillow lava, and 4% transitional morphologies between the other three main types. There are variations in inferred relative lava ages among the different morphological types that do not conform to a simple increase in age versus distance relationship from the spreading axis, suggesting a model in which off-axis transport and volcanism contribute to the accumulation of the extrusive layer. Analysis of the data suggests this ridge crest has experienced three distinctly different types of volcanic emplacement processes: (1) axial summit eruptions within a ∼1 km wide zone centered on the axial summit collapse trough (ASCT); (2) off-axis transport of lava erupted at or near the ASCT through channelized surface flows; and (3) off-axis eruptions and local constructional volcanism at distances of ∼0.5-1.5 km from the axis. Major element analyses of basaltic glasses from lavas collected by Alvin, rock corer and dredging in this area indicate that the most recent magmatic event associated with the present ASCT erupted relatively homogeneous and mafic (>8.25 weight percent wt.% MgO) basalts compared to older, off-axis lavas which tend to be more chemically evolved (Perfit and Chadwick, 1998; Perfit and Fornari, unpublished data). The more primitive lavas have a more extensive distribution within and east of the ASCT. More evolved basalts (MgO ∼500 m from the ASCT suggests that regions of off-axis volcanism may be sourced from older and cooler sections of the axial magma lens. Analysis of these data suggests that this portion of the EPR has not experienced large scale volcanic overprinting in the past ∼30 ka. The predominance of lobate flows (66%) throughout much of the crestal region, and subtle variations in sediment cover and apparent age between flows, suggest that eruptive volumes and effusion rates of individual eruptions have been similar over much of the last 30 ka and that most of the eruptions have been small, probably similar in volume to the 1991 EPR flow which had an estimated volume of ∼1×106 m3 (Gregg et al., 1996).

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.

Deep ocean science is poised to enter a new millennium characterized by cooperation among scientists of many different disciplines who are seeking to gain an understanding of the complex linkages between physical, chemical, biological, and geological processes occurring at and beneath the ocean floor in the world oceans. This multidisciplinary imperative has been spurred by unprecedented advances in understanding the complexities and interdependence of these phenomena made possible through research that used deep submergence vehicles over the past two decades. Marine scientists of all disciplines are forecasting that the next decade will see even greater linkage between oceanographic disciplines. The need to understand the temporal dimension of the processes being studied will sustain continued use of deep ocean submersibles and utilization of newly developed, remotely operated vehicles (ROVs) and autonomous underwater vehicles (AUVs) for conducting time-series and observatory-based research in the deep ocean and at the seafloor. These approaches will enable marine scientists to achieve a greater understanding of global processes and of climate change and geochemical mass balance. These same approaches will enable them to grapple with intriguing problems concerning the interrelated processes of crustal generation, evolution and transport of geochemical fluids in the crust and into the oceans, and origins and proliferation of life both on Earth and beyond.

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.

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

The April 1991 discovery of newly formed hydrothermal vents in areas of recent volcanic eruption between 9°45′N and 9°52′N on the East Pacific Rise provided a unique opportunity to follow temporal changes in biological community structure from the “birth” of numerous deep-sea hydrothermal vents. In March l992, DSV Alvin was used to deploy an on-bottom observatory, the Biologic–Geologic Transect, to monitor faunal succession along a 1.37 km segment of the axial summit caldera between 9°49.61′N and 9°50.36′N (depth ∼2520 m). Photo- and videographic documentation of megafaunal colonization and chemical analyses of diffuse hydrothermal fluids associated with many of these developing communities within the Transect were performed in March 1992, December 1993, October 1994, and November 1995. Photographic and chemical time-series analyses revealed the following sequence of events in low-temperature venting areas. (1) Immediately following the 1991 eruption, hydrogen sulfide and iron concentrations in diffuse fluids were extremely high (>1 mmol kg-1) and microbially derived material blanketed active areas of venting in the form of thick microbial mats. (2) Mobile vent fauna (e.g. amphipods, copepods, octopods, and galatheid and brachyuran crabs) and non-vent fauna (e.g. nematocarcinid shrimp) proliferated in response to this increased biological production. (3) Within 1 yr of the eruption, areal coverage of microbial mat was reduced by ∼60% and individuals of the vestimentiferan tube worm Tevnia jerichonana settled gregariously in areas where diffuse flow was most intense. (4) Two years after the eruption, maximum levels of H2S decreased by almost half (from 1.90 to 0.97 mmol kg-1) and dense thickets of the vestimentiferan Riftia pachyptila dominated vent openings previously inhabited by Tevnia jerichonana. (5) Three years after the eruption, maximum hydrogen sulfide levels declined further to 0.88 mmol kg-1 and mussels (Bathymodiolus thermophilus) were observed on basaltic substrates. (6) Four years after the eruption, galatheid crabs and serpulid polychaetes increased in abundance and were observed close to active vent openings as maximum hydrogen levels decreased to 0.72 mmol kg-1. Also by this time mussels had colonized on to tubes of Riftia pachyptila. (7) Between 3 and 5 yr after the eruption, there was a 2- to 3-fold increase in the number of species in the faunal assemblages. In the absence of additional volcanic/tectonic disturbance, we predict that mytilid and vesicomyid bivalves will gradually replace vestimentiferans as the dominant megafauna 5–10 yr following the eruption. We also anticipate that the abundance of suspension feeders will decline during this period while the abundance of carnivores will increase. We hypothesize that the above series of events (1–7) represents a general sequence of biological successional changes that will occur at newly formed low-temperature deep-sea hydrothermal vents along the northern East Pacific Rise and contiguous ridge axes. Megafaunal colonization at deep-sea hydrothermal vents is considered to be the consequence of an intimate interaction of the life-history strategies of individual species, physical oceanographic processes, and the dynamic hydrothermal environment. Our observations indicate that the successful sequential colonization of dominant megafaunal vent species, from Tevnia jerichonana to Riftia pachyptila to Bathymodiolus thermophilus, also may be strongly influenced by temporal changes in geochemical conditions. Additional evidence demonstrating the close link between diffuse vent flux, fluid geochemistry, and faunal succession included the rapid death of several newly formed biological assemblages coincident with abrupt changes in the geochemical composition of the venting fluid and the local refocusing or cessation of vent flow. These correlations suggest that future models of faunal succession at hydrothermal vents along intermediate to fast-spreading mid-ocean ridges should consider not only the interplay of species-specific life-history strategies, community productivity, and physical oceanographic processes, but also the influence of changing geochemical conditions on the sequential colonization of megafaunal species.

Two-thirds of Earth's surface is formed at mid-ocean ridges, yet sea-floor spreading events are poorly understood because they occur far beneath the ocean surface. At 9 degrees 50'N on the East Pacific Rise, ocean-bottom seismometers recently recorded the microearthquake character of a mid-ocean ridge eruption, including precursory activity. A gradual ramp-up in activity rates since seismic monitoring began at this site in October 2003 suggests that eruptions may be forecast in the fast-spreading environment. The pattern culminates in an intense but brief (approximately 6-hour) inferred diking event on 22 January 2006, followed by rapid tapering to markedly decreased levels of seismicity.

International audience; The history of emplacement, tectonic evolution, and dismemberment of a central volcano within the rift valley of the slow spreading Mid-Atlantic Ridge at the Lucky Strike Segment is deduced using near-bottom sidescan sonar imagery and visual observations. Volcano emplacement is rapid (

From 1965 to 1985, deep submergence technology and science relied heavily on human-occupied submersibles. DSV Alvin has played a significant role, both as a testbed for technical development and for conducting basic research in the water column and on the deep seafloor. Human presence is extremely valuable and still essential for many facets of deep submergence science. However, in order to permit routine access to the deep ocean for repeated measurements over long periods of time, a new class of remotely operated vehicles was needed.

Bathymetry, side-looking sonar and magnetics data from the northern East Pacific Rise have been analyzed to determine the tectonic history of the Clipperton Fracture Zone (CFZ) and the Siqueiros Fracture Zone (SFZ) over the last 2–3 m.y. Results of tectonic reconstructions indicate a series of counterclockwise changes in spreading direction at ∼2.5 Ma (1–2°), ∼1.5 Ma (1–2°), and ∼0.5 Ma (4–5°). Along the right-stepping Clipperton Transform, the most recent change in spreading direction has resulted in fracture zone normal compression, which we propose has created a median ridge and transform-parallel troughs along the active transform fault. Volcanic intersection highs located near the ridge-transform intersections (RTI) of the CFZ are interpreted to be the result of fracture-zone-normal extension that has created pathways for magma emplacement into/onto the older lithosphere across from the RTI. Along the left-stepping SFZ, we propose the changes in spreading direction have generated extension across the transform and have resulted in the formation of intratransform spreading centers and flexural transverse ridges. Tectonic reconstructions indicate a single Euler pole is unable to describe the motion of the Pacific-Cocos plate boundary between the CFZ and SFZ during periods of spreading direction change since ∼3 Ma. Transtensional transforms will adjust to a new spreading direction almost immediately, while transpressional transforms will experience compression for a period of time comparable to half the age offset of the compressional transform.

Meter scale seafloor topography and sidescan backscatter imagery of volcanic terrain along the axis of the fast-spreading northern East Pacific Rise (EPR) near 9° 50'N, coupled with visual and photographic observations provide data that constrain spatial relationships between hydrothermal vents and primary volcanic features and processes along the EPR axis. High-temperature (>350°C) hydrothermal vents are present in several areas within the EPR axial trough where recent eruptions have been focused and where drainback of lava into the primary eruptive fissure occurred. Chaotic collapse crusts and draped sheet lava surfaces along the margin of eruptive fissures typify sites where drainback primarily occurred. These areas are also coincident with ∼10-30 m-wide channels that serve to transport lava across the crestal plateau. Regions of diffuse hydrothermal flow at low temperatures (

We present new Seabeam and SeaMARC I data for small (

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.

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 (

[1] Anomalous volcanism and tectonics between near-ridge mantle plumes and mid-ocean ridges provide important insights into the mechanics of plume-lithosphere interaction. We present new observations and analysis of multibeam, side scan sonar, sub-bottom chirp, and total magnetic field data collected during the R/V Melville FLAMINGO cruise (MV1007; May–June, 2010) to the Northern Galapagos Volcanic Province (NGVP), the region between the Galapagos Archipelago and the Galapagos Spreading Center (GSC) on the Nazca Plate, and to the region east of the Galapagos Transform Fault (GTF) on the Cocos Plate. The NGVP exhibits pervasive off-axis volcanism related to the nearby Galapagos hot spot, which has dominated the tectonic evolution of the region. Observations indicate that ∼94% of the excess volcanism in our survey area occurs on the Nazca Plate in three volcanic lineaments. Identified faults in the NGVP are consistent with normal ridge spreading except for those within a ∼60 km wide swath of transform-oblique faults centered on the GTF. These transform-oblique faults are sub-parallel to the elongation direction of larger lineament volcanoes, suggesting that lineament formation is influenced by the lithospheric stress field. We evaluate current models for lineament formation using existing and new observations as well as numerical models of mantle upwelling and melting. The data support a model where the lithospheric stress field controls the location of volcanism along the lineaments while several processes likely supply melt to these eruptions. Synthetic magnetic models and an inversion for crustal magnetization are used to determine the tectonic history of the study area. Results are consistent with creation of the GTF by two southward ridge jumps, part of a series of jumps that have maintained a plume-ridge separation distance of 145 km to 215 km since ∼5 Ma.

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 (

The recent R/V Maurice Ewing EW9105 Hydrosweep survey of the East Pacific Rise (EPR) and adjacent flanks between 7°S and 9°S provides an excellent opportunity to explore the causal relationship between the ridge and the abyssal hills which form on its flanks. These data cover 100% of the flanking abyssal hills to 115 km on either side of the axis. We apply the methodology of Goff and Jordan (1988) for estimating statistical characteristics of abyssal hill morphology (rms height, characteristic lengths and widths, plan view aspect ratio, azimuthal orientation, and fractal dimension). Principal observations include the following: (1) the rms height of abyssal hill morphology is negatively correlated with the width of the 5- to 20-km-wide crestal high, consistent with the observations of Goff (1991) for northern EPR abyssal hill morphology; (2) the characteristic abyssal hill width displays no systematic variation with position relative to ridge segmentation within the EW9105 survey area, in contrast with observations of Goff (1991) for northern EPR abyssal hill morphology in which characteristic widths tend to be smallest at segment ends and largest toward the middle of segments; (3) abyssal hill rms heights and characteristic widths are very large just north of a counterclockwise rotating “nannoplate”, suggesting that the overlap region is being pushed northward in response to microplate-style tectonics; and (4) within the 7°12′S–8°38′S segment, abyssal hill lineaments are generally parallel to the ridge axis, while south of this area, abyssal hill lineaments rotate with a larger “radius of curvature” than does the EPR axis approaching the EPR-Wilkes ridge-transform intersection.

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.

[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.

The sub-seabed disposal (SSD) of high level radioactive waste (HLRW) program achieved considerable success in identifying promising ocean floor sites that could, with proper engineering and scientific evaluation, be technically feasible. The program also conducted a suite of preliminary scientific baseline studies that would inform both modeling and subsequent in situ experiments. A renewed interest in SSD options is especially relevant in light of significant technological and engineering advances in methods for understanding deep-ocean chemical and biological processes, geophysical monitoring, seafloor instrumentation, deep-ocean drilling and ocean floor observatories. This chapter presents summary of historical data related to science programs involved in SSD up until 1987, key research that remains to be accomplished and how 21st century oceanographic techniques and seafloor observatory programs provide significant opportunities for testing viability of SSD for HLRW. The synergy and links between technology development and social change will continue well into this century and beyond. Keywords: high level radioactive waste (HLRW); in situ experiments; oceanographic techniques; sub-seabed disposal (SSD)

SeaMARC II and Sea Beam bathymetric data are combined to create a chart of the East Pacific Rise (EPR) from 8°N to 18°N reaching at least 1 Ma onto the rise flanks in most places. Based on these data as well as SeaMARC II side scan sonar mosaics we offer the following observations and conclusions. The EPR is segmented by ridge axis discontinuities such that the average segment lengths in the area are 360 km for first-order segments, 140 km for second-order segments, 52 km for third-order segments, and 13 km for fourth-order segments. All three first-order discontinuities are transform faults. Where the rise axis is a bathymetric high, second-order discontinuities are overlapping spreading centers (OSCs), usually with a distinctive 3:1 overlap to offset ratio. The off-axis discordant zones created by the OSCs are V-shaped in plan view indicating along axis migration at rates of 40–100 mm yr−1. The discordant zones consist of discrete abandoned ridge tips and overlap basins within a broad wake of anomalously deep bathymetry and high crustal magnetization. The discordant zones indicate that OSCs have commenced at different times and have migrated in different directions. This rules out any linkage between OSCs and a hot spot reference frame. The spacing of abandoned ridges indicates a recurrence interval for ridge abandonment of 20,000–200,000 yrs for OSCs with an average interval of approximately 100,000 yrs. Where the rise axis is a bathymetric low, the only second-order discontinuity mapped is a right-stepping jog in the axial rift valley. The discordant zone consists of a V-shaped wake of elongated deeps and interlocking ridges, similar to the wakes of second-order discontinuities on slow-spreading ridges. At the second-order segment level, long segments tend to lengthen at the expense of neighboring shorter segments. This can be understood if segments can be approximated by cracks, because the propagation force at a crack tip is directly proportional to crack length.

[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 (

A recent SeaMARC II survey along the Hanks and crest of the East Pacific Rise between 13° and 15° N (Edwards et al., 1988) provides us with a unique opportunity to compare stochastic models of the seafloor against bathymetric data. Stochastic models can be used to quantitatively characterize intermediate to small-scale (

[1] 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, http://www.marine-geo.org). The mosaic-generating and viewing software is available through the Computer Vision and Robotics Group Web page at the University of Girona (http://eia.udg.es/∼rafa/mosaicviewer.html).

[1] As part of a rapid response cruise in May 2006, we surveyed water column hydrothermal plumes and bottom conditions on the East Pacific Rise between 9°46.0′N and 9°57.6′N, where recent seafloor volcanic activity was suspected. Real-time measurements included temperature, light transmission, and salinity. Samples of the plume waters were analyzed for methane, manganese, helium concentrations, and the δ13C of methane. These data allow us to examine the effects of the 2005–2006 volcanic eruption(s) on plume chemistry. Methane and manganese are sensitive tracers of hydrothermal plumes, and both were present in high concentrations. Methane reached 347 nM in upper plume samples (250 m above seafloor) and exceeded 1085 nM in a near-bottom sample. Mn reached 54 nM in the upper plume and 98 nM in near-bottom samples. The concentrations of methane and Mn were higher than measurements made after a volcanic eruption in the same area in 1991, but the ratio of CH4/Mn, at 6.7, is slightly lower, though still well above the ratios measured in chronic plumes. High concentrations of methane in near-bottom samples were associated with areas of microbial mats and diffuse venting documented in seafloor imagery. The isotopic composition of the methane carbon shows evidence of active microbial oxidation; however, neither the fractionation factor nor the source of the eruption-associated methane can be determined with any certainty. Considerable scatter in the isotopic data is due to diverse sources for the methane as well as fractionation as methane is consumed. One sample at +21‰ versus Peedee belemnite standard is among the most enriched methane carbon values reported in a hydrothermal plume to date.

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.

[1] New multibeam bathymetric and side-scan sonar data from the southwestern edge of the Galapagos 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 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 Galapagos 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 Galapagos 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.

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.

[1] 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 Galapagos 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 Galapagos 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 Galapagos 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.

[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.

Interaction between the hydrothermal system and the axial magma chamber at a mid-ocean ridge spreading centre takes place in a boundary layer of crust that separates circulating sea water from basaltic melt1. The nature of heat flow through this region is critical because it determines the pressure–temperature conditions of the water–rock interaction and regulates the total heat flux through the system2. Here we combine seismic, thermal and chemical time-series data from high-temperature vents on the East Pacific Rise axis at 9° 50.2′ N to link a microearthquake swarm with changes measured in vent fluids. Four days after the earthquake swarm opened fractures near the base of the circulation system, a sudden increase in fluid temperature in the overlying ‘Bio9’ black-smoker vent was observed. Temperatures peaked at the vent 11 days after the swarm and gradually declined back to just above pre-swarm levels (365 °C) over the next 70 days. These observations are consistent with the Bio9 hydrothermal system tapping a previously isolated region of crust, and an upflow fluid residence time of 4 days, compared to previous lower-resolution estimates of 3 years or less3.

New multibeam and side-scan sonar surveys of Fernandina volcano and the geochemistry of lavas provide clues to the structural and magmatic development of Galapagos 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 Galapagos 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.

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 (

[1] 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 × 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.

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.

[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.

[1] Mid-ocean ridge volcanic activity is the fundamental process for creation of ocean crust, yet the dynamics of magma emplacement along the slow spreading Mid-Atlantic Ridge (MAR) are largely unknown. We present acoustical, seismological, and biological evidence of a magmatic dike intrusion at the Lucky Strike segment, the first detected from the deeper sections (>1500 m) of the MAR. The dike caused the largest teleseismic earthquake swarm recorded at Lucky Strike in >20 years of seismic monitoring, and one of the largest ever recorded on the northern MAR. Hydrophone records indicate that the rate of earthquake activity decays in a nontectonic manner and that the onset of the swarm was accompanied by 30 min of broadband (>3 Hz) intrusion tremor, suggesting a volcanic origin. Two submersible investigations of high-temperature vents located at the summit of Lucky Strike Seamount 3 months and 1 year after the swarm showed a significant increase in microbial activity and diffuse venting. This magmatic episode may represent one form of volcanism along the MAR, where highly focused pockets of magma are intruded sporadically into the shallow ocean crust beneath long-lived, discrete volcanic structures recharging preexisting seafloor hydrothermal vents and ecosystems. INDEX TERMS: 3035 Marine Geology and Geophysics: Midocean ridge processes; 7280 Seismology: Volcano seismology (8419); 8149 Tectonophysics: Planetary tectonics (5475); 4259 Oceanography: General: Ocean acoustics; 9325 Information Related to Geographic Region: Atlantic Ocean; KEYWORDS: Mid-Atlantic Ridge, earthquake, hydroacoustic

[1] Despite its circular coastline and calderas, Genovesa Island, located between the central Galapagos Platform and the Galapagos Spreading Center, is crosscut by both eruptive and noneruptive fissures trending NE-SW. The 075° bearing of the fissures parallels that of Genovesa Ridge, a 55 km long volcanic rift zone that is the most prominent submarine rift in the Galapagos and constitutes the majority of the volume of the Genovesa magmatic complex. Genovesa Ridge was the focus of detailed multibeam and side-scan sonar surveys during the Revelle/Drift04 cruise in 2001. The ridge consists of three left stepping en echelon segments; the abundances of lava flows, volcanic terraces, and eruptive cones are all consistent with constructive volcanic processes. The nonlinear arrangement of eruptive vents and the ridge's en echelon structure indicate that it did not form over a single dike. Major and trace element compositions of Genovesa Ridge glasses are modeled by fractional crystallization along the same liquid line of descent as the island lavas, but some of the glasses exhibit higher Mg # than material sampled from the island. Most of the submarine and the subaerial lavas have accumulated plagioclase. Incompatible trace element abundances of dredged Genovesa Ridge rocks are lower than the island's lavas, but ratios of the elements are similar in the two settings, which suggests that the island and ridge lavas are derived from nearly identical mantle sources. Glass inclusions in plagioclase phenocrysts from the ridge are compositionally diverse, with both higher and lower MgO than the matrix glass, indicative of homogenization at shallow levels. The structural and geochemical observations are best reconciled if Genovesa Ridge did not form in response to injection of magma laterally from a hot spot-supplied central volcano, like Kilauea's Puna Ridge. Instead, Genovesa Ridge and its western extension are the result of passive upwelling directed by far-field tectonic stresses that are generated by tension across the 91°W transform. The proximity of the plume causes magmatism in the extensional zones where it would not ordinarily occur.

[1] Collapse pits and an associated suite of collapse-related features that form in submarine lava flows are ubiquitous on the global mid-ocean ridge crest. Collapse pits, the lava tube systems they expose, and lenses of talus created by the collapse process combine to produce a permeable region in the shallow ocean crust and are thought to contribute significantly to the 100–300 m thick low velocity zone observed at intermediate to fast-spreading mid-ocean ridges. This horizon of low-density, high-porosity material is likely to be an important aquifer for the transfer of hydrothermal fluids in the upper ocean crust. In a recent survey of the East Pacific Rise at 9°37′N, we used photographs, video and observations from the submersible Alvin, and DSL-120A side scan data to determine that 13% of the 720,000 m2 of seafloor imaged had foundered to form collapse pits. In 98% of the images collapse pits occurred in lobate flows, and the rest in sheet flows. On the basis of our observations and analyses of collapse features, and incorporating data from previous models for collapse formation plus laboratory and theoretical models of basalt lava behavior in the deep ocean, we develop a detailed multistage physical model for collapse formation in the deep ocean. In our model, lava extruded on the seafloor traps pockets of seawater beneath the flow that are instantly vaporized to a briny steam. The seawater is transformed to vapor at temperatures above 480°C with a 20 times expansion in volume. Bubbles of vapor rise through the lava and concentrate below the chilled upper crust of the lava flow, creating gas-filled cavities at magmatic temperatures. Fluid lava from the cavity roofs drips into the vapor pockets to create delicate drip and septa structures, a process that may be enhanced by water vapor diffusing into the magma and reducing its melting point. As the vapor pocket cools, the pressure within it drops, causing a pressure gradient to develop across the upper crust. The pressure gradient often causes the roof crust to collapse during cooling, though vapor pocket geometry may be such that the roof remains intact during subsidence of the underlying lava. Alternatively, drainaway of the molten lava may cause collapse in locations where inflated lava roof crusts are not supported from below by bounding walls or lava pillars. Post-eruption seismicity, lava movement, or hydrovolcanic explosions may cause continued collapse of the lava carapace after the eruption.

Lava erupts into cold sea water on the ocean floor at mid-ocean ridges (at depths of 2,500 m and greater), and the resulting flows make up the upper part of the global oceanic crust. Interactions between heated sea water and molten basaltic lava could exert significant control on the dynamics of lava flows and on their chemistry. But it has been thought that heating sea water at pressures of several hundred bars cannot produce significant amounts of vapour and that a thick crust of chilled glass on the exterior of lava flows minimizes the interaction of lava with sea water. Here we present evidence to the contrary, and show that bubbles of vaporized sea water often rise through the base of lava flows and collect beneath the chilled upper crust. These bubbles of steam at magmatic temperatures may interact both chemically and physically with flowing lava, which could influence our understanding of deep-sea volcanic processes and oceanic crustal construction more generally. We infer that vapour formation plays an important role in creating the collapse features that characterize much of the upper oceanic crust and may accordingly contribute to the measured low seismic velocities in this layer.

RECENT detailed surveys of the East Pacific Rise have revealed the complexity of the volcanic and magmatic processes occurring along and across fast-spreading ocean ridge crests1–7. In parallel with geological and geochemical investigations, it is now possible to investigate the temporal and spatial pattern of volcanism at ocean ridges by dating young basalts using mass spectrometric uranium-series disequilibria methods8–11. Here we use 238U-230Th and235U-231Pa ages for basalts to quantify the spatial extent of young volcanism and crustal accretion at 9°31′ N on the East Pacific Rise. Most of the ages are younger than would be expected based on off-axis distance and spreading rate. We infer from these anomalously young ages that most of the dated basalts on the crestal plateau were erupted 0.5–2 km outside the axial summit caldera, with some volcanism occurring as far as 4 km off-axis. Melts erupted outside the axial summit caldera can have crustal residence times and magmatic supply systems that differ from those of axial lavas.

[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.

[1] The global mid-ocean ridge (MOR) is one of Earth's most fundamental geologic structures. Active volcanic, tectonic, hydrothermal, and biological processes occurring at the MOR affect nearly every attribute of the world's oceans and oceanic crust. For the past quarter century the overarching goal of the U.S. National Science Foundation (NSF)–funded multidisciplinary Ridge 2000 program (http://www.ridge2000.org/) and its predecessor, Ridge Interdisciplinary Global Experiments (RIDGE), here collectively called “R2K,” has been to use observations, experiments, and models to answer fundamental questions about oceanic spreading center processes. Since its inception, R2K has worked to develop a holistic understanding of MORs. There are multiple interrelated consequences of oceanic crust generation at MORs, including transfer of material and energy from the mantle to the crust and ocean; impacts on marine ecology; and temporal, spatial, and rate-dependent interactions between biological and geological processes. Consequently, a diverse yet tightly knit community of collaborating scientists, including geologists, chemists, geophysical modelers, microbiologists, and oceanographers, has developed under R2K programs. This research community has spanned multiple generations of investigative effort, requiring it to confront transformations in communications technology, tools for use and access of data, and attitudes about cooperative approaches to scientific discovery. The tools and approaches R2K has used to enhance cross-disciplinary understanding of complex problems are adaptable to other multidisciplinary research efforts.

[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.

[1] Vailulu'u Seamount is identified as an active volcano marking the current location of the Samoan hotspot. This seamount is located 45 km east of Ta'u Island, Samoa, at 169°03.5′W, 14°12.9′S. Vailulu'u defines the easternmost edge of the Samoan Swell, rising from the 5000-m ocean floor to a summit depth of 590 m and marked by a 400-m-deep and 2-km-wide summit crater. Its broad western rift and stellate morphology brand it as a juvenile progeny of Ta'u. Seven dredges, ranging from the summit to the SE Rift zone at 4200 m, recovered only alkali basalts and picrites. Isotopically, the volcano is strongly EM2 in character and clearly of Samoan pedigree (87Sr/86Sr: 0.7052–0.7067; 143Nd/144Nd: 0.51267–0.51277; 206Pb/204Pb: 19.19–19.40). The 210Po-210Pb data on two summit basalts indicate ages younger than 50 years; all of the recovered rocks are extremely fresh and veneered with glass. An earthquake swarm in early 1995 may attest to a recent eruption cycle. A detailed nephelometry survey of the water column shows clear evidence for hydrothermal plume activity in the summit crater. The water inside the crater is very turbid (nephelometric turbidity unit (NTU) values up to 1.4), and a halo of “smog” several hundred meters thick encircles and extends away from the summit for at least 7 km. The turbid waters are highly enriched in manganese (up to 7.3 nmol/kg), providing further evidence of hydrothermal activity. Vailulu'u is similar to Loihi (Hawaii) in being an active volcanic construct at the eastern end of a hotspot chain; it differs importantly from the Hawaiian model in its total lack of tholeiitic basalt compositions.

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.