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Axial Seamount is a basaltic hot spot volcano with a summit caldera at a depth of ~1500 m below sea level, superimposed on the Juan de Fuca spreading ridge, giving it a robust and continuous magma ...

Axial Seamount is the most active submarine volcano in the NE Pacific Ocean, and is monitored by instruments connected to a cabled observatory (the US Ocean Observatories Initiative Cabled Array), supplemented by autonomous battery-powered instruments on the seafloor at ~1500 m depth. Axial Seamount is a basaltic hot spot volcano superimposed on the Juan de Fuca spreading ridge, giving it a robust and apparently continuous magma supply. It has had three effusive eruptions in the last 23 years in 1998, 2011, and 2015. Deformation measurements have been conducted at Axial Seamount since the late 1980’s with bottom pressure recorders (BPRs) that can detect vertical movements of the seafloor with a resolution of ~1 cm. This monitoring has produced a long-term time-series including co-eruption rapid deflation events of 2.5-3.2 meters, separated by continuous gradual inter-eruption inflation at rates that have varied between 15-80 cm/yr. The overall pattern appears to be inflation-predictable, with eruptions repeatedly triggered at or near a critical level of inflation. Using this pattern, the 2015 eruption was successfully forecast within a one-year time window, 7 months in advance. As of January 2021, Axial Seamount has re-inflated ~2.1 m (~83%) of the 2.54 m it deflated during the 2015 eruption, but the rate of inflation has been decreasing since then. Our current eruption forecast window is between 2022-2026, based on the latest rate of inflation. Modeling of the seafloor deformation data along with other recent results from ocean bottom seismometers and multichannel seismic surveys inform our interpretation of the subsurface structure of the volcano and the geometry and depth of the shallow magma storage system.

Axial Seamount is the most active submarine volcano in the NE Pacific Ocean, and is monitored by instruments connected to a cabled observatory (the US Ocean Observatories Initiative Cabled Array), ...

Arc volcanoes are important to our understanding of submarine volcanism because at some sites frequent eruptions cause them to grow and collapse on human timescales. This makes it possible to document volcanic processes. Active submarine eruptions have been observed at the summit of NW Rota-1 in the Mariana Arc. We use remotely operated vehicle videography and repeat high-resolution bathymetric surveys to construct geologic maps of the summit of NW Rota-1 in 2009 and 2010 and relate them to the geologic evolution of the summit area over a 10 year period (2004–2014). We find that 2009 and 2010 were characterized by different eruptive styles, which affected the type and distribution of eruptive deposits at the summit. Year 2009 was characterized by ultraslow extrusion and autobrecciation of lava at a single eruptive vent, producing a large cone of blocky lava debris. In 2010, higher-energy explosive eruptions occurred at multiple closely spaced vents, producing a thin blanket of pebble-sized tephra overlying lava flow outcrops. A landslide that occurred between 2009 and 2010 had a major effect on lithofacies distribution by removing the debris cone and other unconsolidated deposits, revealing steep massive flow cliffs. This relatively rapid alternation between construction and destruction forms one end of a seamount growth and mass wasting spectrum. Intraplate seamounts, which tend to grow larger than arc volcanoes, experience collapse events that are orders of magnitude larger and much less frequent than those occurring at subduction zone settings. Our results highlight the interrelated cyclicity of eruptive activity and mass wasting at submarine arc volcanoes.

Volcano monitoring goes into the deep Axial Seamount is a large and active submarine volcano along the Juan de Fuca midocean ridge off the coast of the western United States. Eruptions in 1998 and 2011 were followed by periods of magma recharge, making it an ideal location to include in the Ocean Observatories Initiative Cabled Array. Wilcock et al. present real-time seismic data from the most recent eruption in April 2015 that allow the tracking of magma before and during eruption. Nooner and Chadwick show that eruptions are predictable on the basis of deformation data. As magma pools underneath it, Axial Seamount inflates and erupts when the inflation hits a threshold. Both studies elucidate the dynamics of submarine volcanoes, which vastly outnumber their aboveground counterparts. Science , this issue p. 1395 , p. 1399

West Mata is a 1200 m deep submarine volcano where explosive boninite eruptions were observed in 2009. The acoustic signatures from the volcano's summit eruptive vents Hades and Prometheus were recorded with an in situ (~25 m range) hydrophone during ROV dives in May 2009 and with local (~5 km range) moored hydrophones between December 2009 and August 2011. The sensors recorded low frequency (1–40 Hz), short duration explosions consistent with magma bubble bursts from Hades, and broadband, 1–5 min duration signals associated with episodes of fragmentation degassing from Prometheus. Long-term eruptive degassing signals, recorded through May 2010, preceded a several month period of declining activity. Degassing episodes were not recorded acoustically after early 2011, although quieter effusive eruption activity may have continued. Synchronous optical measurements of turbidity made between December 2009 and April 2010 indicate that turbidity maxima resulted from occasional south flank slope failures triggered by the collapse of accumulated debris during eruption intervals.

NW Rota-1 is a submarine volcano in the Mariana volcanic arc located ∼100 km north of Guam. Underwater explosive eruptions driven by magmatic gases were first witnessed there in 2004 and continued until at least 2010. During a March 2010 expedition, visual observations documented continuous but variable eruptive activity at multiple vents at ∼560 m depth. Some vents released CO2 bubbles passively and continuously, while others released CO2 during stronger but intermittent explosive bursts. Plumes of CO2 bubbles in the water column over the volcano were imaged by an EM122 (12 kHz) multibeam sonar system. Throughout the 2010 expedition numerous passes were made over the eruptive vents with the ship to document the temporal variability of the bubble plumes and relate them to the eruptive activity on the seafloor, as recorded by an in situ hydrophone and visual observations. Analysis of the EM122 midwater data set shows: (1) bubble plumes were present on every pass over the summit and they rose 200–400 m above the vents but dissolved before they reached the ocean surface, (2) bubble plume deflection direction and distance correlate well with ocean current direction and velocity determined from the ship's acoustic doppler current profiler, (3) bubble plume heights and volumes were variable over time and correlate with eruptive intensity as measured by the in situ hydrophone. This study shows that midwater multibeam sonar data can be used to characterize the level of eruptive activity and its temporal variability at a shallow submarine volcano with robust CO2 output.

We present multiple lines of evidence for years to decade-long changes in the location and character of volcanic activity at West Mata seamount in the NE Lau basin over a 16 year period, and a hiatus in summit eruptions from early 2011 to at least September 2012. Boninite lava and pyroclasts were observed erupting from its summit in 2009, and hydroacoustic data from a succession of hydrophones moored nearby show near-continuous eruptive activity from January 2009 to early 2011. Successive differencing of seven multibeam bathymetric surveys of the volcano made in the 1996–2012 period reveals a pattern of extended constructional volcanism on the summit and northwest flank punctuated by eruptions along the volcano's WSW rift zone (WSWRZ). Away from the summit, the volumetrically largest eruption during the observational period occurred between May 2010 and November 2011 at ∼2920 m depth near the base of the WSWRZ. The (nearly) equally long ENE rift zone did not experience any volcanic activity during the 1996–2012 period. The cessation of summit volcanism recorded on the moored hydrophone was accompanied or followed by the formation of a small summit crater and a landslide on the eastern flank. Water column sensors, analysis of gas samples in the overlying hydrothermal plume and dives with a remotely operated vehicle in September 2012 confirmed that the summit eruption had ceased. Based on the historical eruption rates calculated using the bathymetric differencing technique, the volcano could be as young as several thousand years.

Submarine landslides are an important process in volcano growth yet are rarely observed and poorly understood. We show that landslides occur frequently in association with the eruption of West Mata volcano in the NE Lau Basin. These events are identifiable in hydroacoustic data recorded between ~5 and 20 km from the volcano and may be recognized in spectrograms by the weak and strong powers at specific frequencies generated by multipathing of sound waves. The summation of direct and surface-reflected arrivals causes interference patterns in the spectrum that change with time as the landslide propagates. Observed frequencies are consistent with propagation down the volcano's north flank in an area known to have experienced mass wasting in the past. These data allow us to estimate the distance traveled by West Mata landslides and show that they travel at average speeds of ~10–25 m/s.

[1] We combine high-resolution bathymetry acquired using the Autonomous Underwater Vehicle ABE with digital seafloor imagery collected using the remotely operated vehicle ROPOS across the axial valley of the Southern Explorer Ridge (SER) to infer the recent volcanic and tectonic processes. The SER is an intermediate spreading ridge located in the northeast Pacific. It hosts the Magic Mountain hydrothermal vent. We reconstruct the unfaulted seafloor terrain at SER based on calculations of the vertical displacement field and fault parameters. The vertical changes between the initial and the restored topographies reflect the integrated effects of volcanism and tectonism on relief-forming processes over the last 11,000–14,000 years. The restored topography indicates that the axial morphology evolved from a smooth constructional dome >500 m in diameter, to a fault-bounded graben, ~500 m wide and 30–70 m deep. This evolution has been accompanied by changes in eruptive rate, with deposition of voluminous lobate and sheet flows when the SER had a domed morphology, and limited-extent low-effusion rate pillow eruptions during graben development. Most of the faults shaping the present axial valley postdate the construction of the dome. Our study supports a model of cyclic volcanism at the SER with periods of effusive eruptions flooding the axial rift, centered on the broad plateau at the summit of the ridge, followed by a decrease in eruptive activity and a subsequent dominance of tectonic processes, with minor low-effusion rate eruptions confined to the axial graben. The asymmetric shape of the axial graben supports an increasing role of extensional processes, with a component of simple shear in the spreading processes.

For volcanoes at submarine rift zones, a direct link between seismicity, seafloor deformation and magma intrusion has not been demonstrated. Recordings from ocean-bottom hydrophones and bottom-pressure recorders map an increasing rate of seismicity at Axial Seamount, northeast Pacific, over several years before its eruption in April 2011.

The Axial Seamount submarine volcano exhibits an inflation–deflation cycle comparable to similar volcanoes on land. Measurements of ocean bottom pressure document the entire inflation–deflation cycle between eruptions at Axial Seamount in 1998 and 2011, and imply that the timing of submarine eruptions could be more predictable than that of their subaerial counterparts.

At frequently active submarine volcanoes, it is difficult to distinguish between new and pre-existing lava flows. A combination of high-resolution bathymetric surveys taken before and after an eruption at Axial Seamount in 2011 allows detailed mapping of the 2011 lava flows, and highlights the tendency of new flows to mimic older ones.

NW Rota-1 is a submarine volcano in the Mariana volcanic arc that is notable as the site where underwater explosive eruptions were first witnessed in A.D. 2004. After years of continuous low-level eruptive activity, a major landslide occurred at NW Rota-1 in August 2009, triggered by an unusually large eruption that produced 10 times the acoustic energy of the background level of activity. An anomalous earthquake swarm preceded the eruption, suggesting that the sequence started with a magmatic intrusion and associated faulting beneath the volcano. We quantify the size and extent of the landslide using bathymetric resurveys and interpret the timing of events using data from an in situ hydrophone. This is the first instrumental documentation of an earthquake-eruption-landslide sequence at a submarine volcano, and illustrates the close interaction between magmatic activity and mass wasting events in the growth of undersea arc volcanoes.

The May 2005 eruption of Fernandina volcano, Galapagos, occurred along circumferential fissures parallel to the caldera rim and fed lava flows down the steep southwestern slope of the volcano for several weeks. This was the first circumferential dike intrusion ever observed by both InSAR and GPS measurements and thus provides an opportunity to determine the subsurface geometry of these enigmatic structures that are common on Galapagos volcanoes but are rare elsewhere. Pre- and post- eruption ground deformation between 2002 and 2006 can be modeled by the inflation of two separate magma reservoirs beneath the caldera: a shallow sill at ~1 km depth and a deeper point-source at ~5 km depth, and we infer that this system also existed at the time of the 2005 eruption. The co-eruption deformation is dominated by uplift near the 2005 eruptive fissures, superimposed on a broad subsidence centered on the caldera. Modeling of the co-eruption deformation was performed by including various combinations of planar dislocations to simulate the 2005 circumferential dike intrusion. We found that a single planar dike could not match both the InSAR and GPS data. Our best-fit model includes three planar dikes connected along hinge lines to simulate a curved concave shell that is steeply dipping (~45–60°) toward the caldera at the surface and more gently dipping (~12–14°) at depth where it connects to the horizontal sub-caldera sill. The shallow sill is underlain by the deep point source. The geometry of this modeled magmatic system is consistent with the petrology of Fernandina lavas, which suggest that circumferential eruptions tap the shallowest parts of the system, whereas radial eruptions are fed from deeper levels. The recent history of eruptions at Fernandina is also consistent with the idea that circumferential and radial intrusions are sometimes in a stress-feedback relationship and alternate in time with one another.

Sierra Negra volcano, the most voluminous shield volcano in the Galápagos archipelago and one of the largest basaltic calderas in the world, erupted on October 22, 2005 after more than [Formula: see text] of quiescence. GPS and satellite radar interferometry (InSAR) monitoring of the deformation of the caldera floor in the months prior to the eruption documented extraordinary inflation rates [Formula: see text]. The total amount of uplift recorded since monitoring began in 1992 approached [Formula: see text] at the center of the caldera over the eight days of the eruption the caldera floor deflated a maximum of 5 m and subsquently renewed its inflation, but at a decelerating rate. To gain insight into the nature of the subsurface mass/density changes associated with the deformation, gravity measurements performed in 2005, 2006, and 2007 are compared to previous measurements from 2001-2002 when the volcano underwent a period of minor deflation and magma withdrawal.The residual gravity decrease between 2001-2002 and 2005 is among the largest ever recorded atan active volcano (−950 μGal) and suggests that inflation was accompanied by a relative density decrease in the magmatic system. Forward modeling of the residual gravity data in 4D (from 2002 to 2005) gives an estimate of the amount of vesiculation in the shallow sill required to explain the observed gravity variations. Geochemical constraints from melt inclusion and satellite remote-sensing data allow us to estimate the pre-eruptive gas content of the magma and place constraints on the thickness of the gas-rich sill necessary to produce the gravity anomalies observed. Results suggest that reasonable sill thicknesses [Formula: see text] and bubble contents (10–50 volume %) can explain the large decrease in residual gravity prior to eruption. Following the eruption (2006 and 2007), the deformation and gravity patterns suggest re-equilibration of the pressure regime in the shallow magma system via a renewed influx of relatively gas-poor magma into the shallow parts of the system.

International audience; We present preliminary results of a morphological previous termstudynext term of the summit of the Southern Explorer Ridge (SER). The SER is an inflated intermediate-rate spreading center located in the northeast Pacific off the West coast of Canada, that hosts a large hydrothermal vent complex known as “Magic Mountain”. A previous termquantitativenext term assessment of faulting on the axial summit graben floor close to the ridge summit is accomplished through the analysis of high-resolution, near-bottom, bathymetric data. These data were acquired using a multibeam system mounted on an autonomous underwater vehicle operated a few tens meters above the seafloor. Structural mapping reveals numerous subvertical fissures and normal faults that nucleated from tension fissures. The ratio between the length and the maximum scarp height of normal faults is not constant contrary to what is generally observed on subaerial faults, highlighting the probable importance of fault segment linkage and fault growth processes within the relatively thin brittle layer. Populations of small faults exhibit an exponential size-frequency distribution that reflects the importance of linkage in the fault growth history and the relatively large amount of tectonic strain (3.7 to 18.4%) accommodated by the normal faults. We propose that the 500 to 600-m wide and not, vert, similar 60-m deep asymmetric axial summit graben of the SER formed due to magma chamber deflation as well as normal faulting that initiated on the present eastern border of the graben. We find a well-defined geographic distribution in the types of lava flows, which indicates a general decrease of the eruption rate through time. We also find that the “Magic Mountain” hydrothermal field is located in the vicinity of the large eastern axial summit graben bounding fault whose dimensions suggest it may reach to the brittle ductile-transition depth. This fault likely has provided an efficient physical pathway for fluids from the subsurface to the seafloor for a significant period of time thus allowing the hydrothermal system to grow and mature.

GPS and gravity networks were established on Sierra Negra and Fernandina volcanoes in 2000 and remeasured in 2001 and 2002. After a decade in which the caldera inflated by over 2.5 m, Sierra Negra's caldera began to subside between 2000 and 2002, at a rate of about 9 cm/a. Neither inflation nor deflation was accompanied by eruption. Simple elastic modeling indicates that the deformation at Sierra Negra can be attributed to volumetric contraction of a 2.1 km-deep sill beneath the caldera, caused by either movement of magma out of the sill or loss of bubbles through degassing. Precise gravity measurements support but do not prove the former mechanism. Fernandina volcano is in a phase of slow refilling of the summit reservoir after the 1995 eruption, causing lateral expansion of the caldera rim by about 3 cm/a. Our estimate for the depth of the shallowest part of the Fernandina magma chamber is 1 to 2 km. Neither volcano shows evidence of flank spreading.

In July 2000, an array of instruments called acoustic extensometers was deployed at the Cleft segment of the southern Juan de Fuca Ridge, a seafloor observatory site selected by the National Science Foundation RIDGE Program. These instruments are designed to precisely measure horizontal deformation across the axis of a mid-ocean ridge in order to detect and quantify seafloor spreading events. The instruments were deployed in semipermanent seafloor benchmarks in a linear array that is 1.2-km long and spans the floor of the axial valley. The instruments make daily measurements of distance to their neighbors in the array by recording the round trip travel time of 100-kHz acoustic pulses, and simultaneous temperature measurements are used to correct the ranges for sound speed variations. The instruments are expected to have lifetimes of at least five years. In addition, precise pressure measurements have been made at each benchmark with a remotely operated vehicle in order to monitor for vertical deformation across the array. Preliminary results show that the resolution of the acoustic measurements is /spl plusmn/1-2 cm and that no abrupt deformation events occurred during the first year.

When submarine volcanoes erupt, several processes can create sounds in the ocean, mostly at low frequencies

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

The 1993 seismic swarm and volcanic eruption on the CoAxial segment of the Juan de Fuca Ridge was the first verified mid-ocean ridge accretion event monitored by the National Oceanic and Atmospheric Administration/U.S. Navy Sound Surveillance System (SOSUS). Ambiguity in the location of the initial seismicity resulted in an uncertainty as to whether the dike intruded from the summit and north rift zone of the adjacent Axial Volcano or whether it arose locally within the CoAxial segment. However, analyses of multibeam, side-scan sonar, towed camera, submersible, and geochemical data show that the CoAxial segment is morphologically, structurally, and geochemically distinct from the north rift zone of Axial Volcano. There is no geologic or geochemical evidence that dike injections from Axial Volcano have extended north of 46°18'N in the past, whereas the 1993 eruption site is at 46°31'N. Furthermore, all seafloor manifestations of the 1993 dike injection lie along the central neovolcanic zone of the CoAxial segment. Analyses of repeat SeaBeam surveys combined with seafloor observations show that two other eruptions of approximately the same volume as the 1993 eruption occurred along the CoAxial segment in the 1981–1991 interval. These three diking events may have relieved decades of accumulated stress over ∼35 km or more of this segment. The spatial pattern and hydrothermal history of the 1993 event is consistent with a dike with a significant lateral component of injection.

Acoustic extensometer instruments capable of making precise daily measurements of horizontal distance were deployed across the north rift zone of Axial Volcano in June 1996 and were in place when a submarine eruption began on Axial's south rift zone in January 1998. The instruments recorded a gradual 9-cm extension over a 405-m baseline leading up to the eruption, and then an abrupt, 4-cm contraction at the time of the eruption. An elastic point-source deformation model shows that deflation of Axial's summit can explain both the 4-cm distance decrease at the extensometer array and a 3.2-m subsidence measured by another instrument in the caldera, if the pressure source is located at a depth of 3.8 km below the center of the caldera. The 9-cm distance increase may represent pre-eruption spreading across the rift zone.

Lava flows erupted during the January/February 1998 seismic swarm at Axial Volcano on the Juan de Fuca Ridge have been identified by differencing of pre- and post-event multibeam bathymetric surveys and by seafloor observations. A sheet flow more than 3 km in length and 500–800 m wide erupted from the uppermost south rift is the site of a robust hydrothermal system. 1998 lavas occur over about 9 km of the upper south rift zone, or about 20% of the along-axis length of the seismicity event (∼50 km). The estimated volume of lava erupted is 18-76 ×106 m³ for the extrusion and 100-×106 m³ for the intrusion. The total volume is consistent with the volume from modeling of seafloor strain measurements recorded during the event.

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

Grabens have been mapped immediately adjacent to recently erupted submarine lava flows on the Cleft and CoAxial segments of the Juan de Fuca Ridge (JdFR). The grabens are 10–100 m wide and 5–15 m deep and are located uprift and/or downrift of the eruptive vents that fed the flows. We interpret that these structures formed (or were reactivated) directly over the dike that fed the eruptions as it was intruding toward the surface. These graben structures are primary conduits for diffuse hydrothermal venting on the seafloor during the cooling of newly intruded dikes. The axial summit “graben” or “caldera” on the East Pacific Rise (EPR) in some locations has similar dimensions to the grabens observed on the JdFR, and we interpret that it too may be a dike-induced graben structure (although often buried by subsequent eruptions where it is narrowest). These grabens on the JdFR and the EPR are distinctly narrower and deeper than grabens that formed during well-documented dike intrusions on slow spreading rifts on land (Iceland and Afar). Mechanical modeling suggests that narrow grabens would form when a dike is at shallow depth where it imposes a high stress perturbation on the ambient stress state to cause faulting. Therefore the narrow grabens on the JdFR and EPR imply that a relatively high level of horizontal compressive stress typically exists perpendicular to the ridges, and this must be overcome by high dike-induced perturbations to cause faulting. This is probably because gradual plate spreading rarely gets enough time to lower the compressive stress significantly due to the high frequency of dike intrusion events relative to the plate spreading rate. The dikes that do intrude in this environment must have relatively high internal magma pressure. Therefore the size and character of dike-induced grabens that form at the surface on intermediate to fast spreading ridges reflect the fact that volcanism dominates over tectonism in regulating the local stress state where a robust magma supply is available.

The morphological, structural, and volcanic characteristics of the neovolcanic zone at mid-ocean ridges (MOR) vary strongly with spreading rate. At fast-spreading ridges, the neovolcanic zone is narrow, has low-relief both across and along strike, is dominated by the products of fluid, fissure-fed eruptions, and exhibits morphologic and magmatic continuity along axis. At slow-spreading ridges, the neovolcanic zone is wider, has greater relief, is characterized by many discrete point-source constructs, and exhibits less morphologic and magmatic continuity along axis compared to fast-spreading ridges. Intermediate-spreading ridges typically have characteristics that vary in time and space between these two extremes. Lava flow morphology also varies markedly with spreading rate - sheet flows are dominant on fast-spreading ridges whereas pillow lavas are dominant at slow-spreading ridges.

The East Blanco Depression (EBD), a pull-apart basin within the western Blanco Transform Fault Zone (BTFZ), was the site of an intense earthquake T-wave swarm that began at 1317Z on January 9, 1994. Although technically generated earthquakes occur frequently along the BTFZ, this swarm was unusual in that it was preceded and accompanied by periodic, low-frequency, long-duration acoustic signals, that originated from near the swarm epicenters. These tremor-like signals were very similar in character to acoustic energy produced by a shallow-submarine eruption near Socorro Island, a seamount several hundred km west of Baja, California. The ∼69 earthquakes and ∼400 tremor-like events at the EBD occurred sporadically, with two periods of peak activity occurring between January 5–16 and 27–31. The swarm-like character of the earthquakes and the similarity of the tremor activity to the Socorro eruption indicated that the EBD was undergoing an intrusion or eruption episode. On January 27, six CTD/rosette casts were conducted at the site. Water samples from two of the stations yielded anomalous ³He concentrations, with maxima at ∼2800 m depth over the main basin. In June 1994 two camera tows within the basin yielded evidence of pillow-lava volcanism and hydrothermal deposits, but no conclusive evidence of a recent seafloor eruption. In September 1994, deployments of the U.S. Navy's Advanced Tethered Vehicle resulted in the discovery of an active hydrothermal mound on the flanks of a pillow-lava volcano. The hydrothermal mound consists of Fe-rich hydrothermal precipitate and bacterial mats. Temperatures to 60°C were measured 30 cm below the surface. This is the first discovery of active hydrothermal vents along an oceanic fracture zone. Although no conclusive evidence of volcanic activity associated with the T-wave event swarm was found during these response efforts, the EBD has been the site of recent seafloor eruptions.

A distinctive and unusual pattern of eruptive fissures is observed on the active volcanoes of the Galapagos islands, reflecting circumferential dike intrusion near the calderas and radial dike intrusion on the volcano flanks. Elastic finite-element models were used to investigate how a stress field could be produced and maintained to promote both circumferential and radial dike emplacement. Modeling results show that magma reservoirs of Galapagos volcanoes are probably diapiric, because this shape promotes both circumferential and radial intrusions, but magma pressure alone cannot create the observed pattern of dikes. Loading by volcano growth and magma reservoir pressure could produce a stress field of suitable orientation but insufficient magnitude. The intrusion of circumferential dikes could alter the stress field in a way that promotes future radial diking, and vice versa. Faulting or slumping within the calderas or on the volcano flanks in response to repeated intrusions could also create a stress field conducive to continued intrusion.

Between June 26 and July 10, 1993, swarms of “T-wave” events occurred over a 40-km portion of the CoAxial segment on the northern Juan de Fuca Ridge. A rapid response utilizing a CTD/rosette/chemical scanner and a remotely operated vehicle occurred in the month following the T-wave swarms. The pattern of T-wave events and water-column anomalies (including several event plumes) are remarkably coincident. The only known eruptive area is at the northern swarm area, where a very fresh pillow lava ridge was discovered, mapped, and sampled with the remotely operated vehicle ROPOS. A vent area about 22 km south of the lava flow was emitting large quantities of bacterially generated floccular material. The temporal pattern of T-wave events and the coincidence between the T-wave swarms, the young lava flows, and hydrothermal plumes suggests that there is a close analogy between this activity and lateral dike injections such as have been closely monitored at Icelandic central volcanoes.

After a swarm of earthquakes was detected on the CoAxial segment of the Juan de Fuca Ridge in June-July 1993, the area was resurveyed with SeaBeam multibeam sonar to search for depth changes associated with a submarine volcanic eruption. Quantitative comparison of the 1993 SeaBeam survey with surveys in 1981/82 and 1991 shows one area of seafloor depth change (up to 29 m) between 1991–93 exactly where a pristine lava flow was discovered. In addition, two other depth anomalies (up to 37 m and 20 m) are identified between 1981–91, evidence that other recent eruptions have occurred along this spreading ridge segment.

The northern portion of the Cleft segment, which is the southernmost segment of the Juan de Fuca Ridge, is the site of a seafloor spreading episode during the mid-1980s that was originally discovered by the occurrence of anomalous hydrothermal bursts (megaplumes) and later documented by seafloor mapping of new pillow mounds (NPM) that were erupted. Several field seasons of investigations using sidescan sonar, a deep-tow camera system, and the submersible Alvin reveal that about 30 km of the ridge crest is hydrothermally active and/or has experienced recent volcanic and tectonic activity associated with this episode. The most intense hydrothermal activity within this area and all the known high-temperature vents lie along a fissure from which a young sheet flow (YSF) erupted. Extinct chimneys located within 100–200 m on either side of the fissure system represent an older (>100 years) and probably less intense, hydrothermal regime. The bathymetry and the morphology of the YSF suggest that this eruption occurred over a 1–2 km section of the fissure system that forms its eastern boundary and that it flowed to the south. Fields of lava pillars concentrated at the margins of the YSF where lava probably formed when the lava stagnated near the edges of the flow. A comparison of sidescan data sets collected in 1982 and 1987 implies that the YSF was erupted at least 7 months prior to the NPM, consistent with analysis of bottom photographs that suggests that the eruptions of the YSF and NPM were only separated by a few years. The low hydrothermal flux over the NPM relative to the YSF suggests a rapidly cooled underlying heat source beneath the former. We propose that the NPM were erupted from a dike or dikes injected laterally to the north from a magma body lying beneath the YSF. Recent evidence of a decrease in the intensity of the overlying hydrothermal plumes suggests that the system is continuing to cool down.

A series of lava flows with a total volume of 0.05 km3 were erupted in the mid-1980s along 17 km of the northern Cleft segment of the Juan de Fuca Ridge. Observations from camera tows and submersible dives show that the new flows are all similar in appearance and consist entirely of pillow lava with a mixture of smooth and striated surface textures, suggesting a relatively uniform eruption rate approaching 1 m3/s at point source vents. The flows vary in size from small patches to large steep-sided ridges and were probably erupted from a dike intruded along the ridge axis because they are aligned along a linear fissure/graben system. Observations at north Cleft show that the physical appearance of new flows changes more rapidly than previously realized and that earlier qualitative dating of young lavas based on sediment cover and glassy surface texture were probably overestimates by an order of magnitude. Sediment accumulation on the lavas is quite variable and locally surprisingly substantial, mainly due to hydrothermal deposits that formed while the lava flows were cooling. Biological vent communities photographed on the new flows in 1989 show that vent animals can colonize new vent sites rapidly but that warm water was still venting only in a few places. Nonvent animals are much slower to colonize the new flows and rates of colonization observed at north Cleft may be useful for making improved age estimates of young (

Our understanding of submarine volcanic eruptions has improved substantially in the past decade owing to the recent ability to remotely detect such events and to then respond rapidly with synoptic surveys and sampling at the eruption site. But these data are necessarily limited to observations after the event. In contrast, the 1998 eruption of Axial volcano on the Juan de Fuca ridge was monitored by in situ sea-floor instruments. One of these instruments, which measured bottom pressure as a proxy for vertical deformation of the sea floor, was overrun and entrapped by the 1998 lava flow. The instrument survived-being insulated from the molten lava by the solidified crust-and was later recovered. The data serendipitously recorded by this instrument reveal the duration, character and effusion rate of a sheet flow eruption on a mid-ocean ridge, and document over three metres of lava-flow inflation and subsequent drain-back. After the brief two-hour eruption, the instrument also measured gradual subsidence of 1.4 metres over the next several days, reflecting deflation of the entire volcano summit as magma moved into the adjacent rift zone. These findings are consistent with our understanding of submarine lava effusion, as previously inferred from seafloor observations, terrestrial analogues, and laboratory simulations.

[1] Since 2000, ambient seawater pressure has been precisely measured at five seafloor benchmarks inside the summit caldera at Axial Seamount to monitor volcanic inflation, using a remotely operated vehicle to deploy a mobile pressure recorder (MPR) in campaign-style surveys. Additionally, seawater pressure has been measured at the caldera center with multiyear deployments of continuously recording bottom pressure recorders (BPRs). These pressure data (converted to depth) are currently the only measurements of volcanic inflation at a submarine volcano. We show new data spanning 2004 to 2007 documenting steady inflation of 12.7 ± 0.4 cm/a at the caldera center. The spatial pattern of uplift is consistent with magma storage in a shallow reservoir underlying the caldera at a depth of ∼3.5 km, and the current uplift rate implies that magma is being supplied to the volcano at a rate of ∼7.5 × 106 m3/a. However, the supply rate immediately after the last eruption in 1998 was significantly higher, and the temporal pattern of uplift at Axial caldera appears to be governed by at least two processes occurring at very different time scales. We interpret the high uplift rates immediately following the 1998 eruption as either due to influx from one or more small satellite magma bodies or as the result of viscoelastic relaxation and/or poroelastic behavior of the crust surrounding the shallow magma chamber, and we present a numerical model which supports the latter interpretation. In contrast, we interpret the current lower uplift rate as due to a steady longterm magma supply from the mantle. This two component uplift pattern has not been observed on land volcanoes, suggesting that magma supply/storage processes beneath this ridge axis volcano differ from volcanoes on land (including Iceland). To reconstruct the uplift history at Axial, we fit the combined MPR and BPR data to two possible uplift scenarios, with which we forecast that the next eruption at Axial is likely to occur by about 2020, when most of the ∼3 m of deflation that occurred during the 1998 eruption will have been recovered.

During 14–16 September 1988, a large intracaldera avalanche and an eruption of basaltic tephra and lava at Fernandina volcano, Galapagos, produced the most profound changes within the caldera since its collapse in 1968. A swarm of eight earthquakes (m b 4.7–5.5) occurred in a 14 h period on 24 February 1988 at Fernandina, and two more earthquakes of this size followed on 15 April and 20 May, respectively. On 14 September 1988, another earthquake (m b 4.6) preceded a complex series of events. A debris avalanche was generated by the failure of a fault-bounded segment of the east caldera wall, approximately 2 km long and 300 m wide. The avalanche deposit is up to 250 m thick and has an approximate volume of 0.9 km3. The avalanche rapidly displaced a preexisting lake from the southeast end of the caldera floor to the northwest end, where the water washed up against the lower part of the caldera wall, then gradually seeped into the avalanche deposit and was completely gone by mid-January 1989. An eruption began in the caldera within about 1–2 h of the earthquake, producing a vigorous tephra plume for about 12 h, then lava flows during the next two days. The eruption ended late on 16 September. Most of the eruptive activity was from vents on the caldera floor near the base of the new avalanche scar. Unequivocal relative timing of events is difficult to determine, but seismic records suggest that the avalanche may have occurred 1.6 h after the earthquake, and field relations show that lava was clearly erupted after the avalanche was emplaced. The most likely sequence of events seems to be that the 1988 feeder dike intruded upward into the east caldera wall, dislocated the unstable wall block, and triggered the avalanche. The avalanche immediately exposed the newly emplaced dike and initiated the eruption. The exact cause of the earthquakes is unknown.

Maps of the eruptive vents on the active shield volcanoes of Fernandina and Isabela islands, Galapagos, made from aerial photographs, display a distinctive pattern that consists of circumferential eruptive fissures around the summit calderas and radial fissures lower on the flanks. On some volcano flanks either circumferential or radial eruptions have been dominant in recent time. The location of circumferential vents outside the calderas is independent of caldera-related normal faults. The eruptive fissures are the surface expression of dike emplacement, and the dike orientations are interpreted to be controlled by the state of stress in the volcano. Very few subaerial volcanoes display a pattern of fissures similar to that of the Galapagos volcanoes. Some seamounts and shield volcanoes on Mars morphologically resemble the Galapagos volcanoes, but more specific evidence is needed to determine if they also share common structure and eruptive style.

[1] Repeated multibeam bathymetric surveys at Monowai Cone, a shallow submarine basaltic volcano and part of the Monowai Volcanic Center in the northern Kermadec arc, were conducted in 1998, 2004, and 2007. These surveys document dramatic depth changes at the volcano including negative changes up to −176 m from two sector collapses and positive changes up to +138 m from volcanic reconstruction near the summit and debris avalanche deposits downslope of the slide scars. One sector collapse occurred on the SE slope between 1998 and 2004 with a volume of ∼0.09 km3, and another occurred on the SW slope between 2004 and 2007 with a volume of ∼0.04 km3. The volume of positive depth change due to addition of volcanic material by eruption is of the same order: ∼0.05 km3 between 1998 and 2004 and ∼0.06 km3 between 2004 and 2007. During these time intervals, monitoring by the Polynesian Seismic Network detected frequent T wave swarms at Monowai, indicative of explosive eruptive activity every few months. An unusual T wave swarm on 24 May 2002 was previously interpreted as the collapse event between the 1998 and 2004 surveys, but no similarly anomalous T waves were detected between 2004 and 2007, probably because the Polynesian Seismic Network stations were acoustically shadowed from the second slide event. We interpret that the sector collapses on Monowai are caused by the unstable loading of fragmental erupted material on the summit and steep upper slopes of the volcano (>20°). Moreover, there appears to be a cyclic pattern in which recurrent eruptions oversteepen the cone and periodically lead to collapse events that transport volcaniclastic material downslope to the lower apron of the volcano. Volumetric rate calculations suggest that these two processes may be more or less in equilibrium. The repeated collapses at Monowai are relatively modest in volume (involving only 0.1–0.5% of the edifice volume), have occurred much more frequently than is estimated for larger debris avalanches at subaerial volcanoes, and may be characteristic of how persistently active shallow submarine arc volcanoes grow.

Monowai submarine volcano is one of the three most historically active volcanoes of the Kermadec arc. Repeat multibeam surveys of Monowai Cone from September 1998 and September 2004 and T wave data recorded by the Reseau Sismique Polynesien network for the same period document the collapse and subsequent regrowth of the cone within this 6-a period. Grid differencing of the two bathymetric data sets, acquired 6 a apart, reveals that a landslide ∼2230 m long occurred between the surveys, within which a postcollapse cone and talus ridge (∼0.023 km3 in volume) subsequently formed. The volume of this collapse, minus postcollapse construction, is ∼0.085 km3. We interpret an unusual, strong-amplitude T wave event on 24 May 2002 as recording “hot landsliding”, where the 100- to 160-m-thick collapse has “unroofed” the uppermost parts of the vent conduit, with the subsequent explosive interaction, and cooling, of hot magma and volcaniclastic rubble with ambient seawater. This interpretation is consistent with the lack of emergent events, sharp onset, and large amplitude of the 24 May 2002 T waves. The subsequent >2500 T wave events, between November 2002 and September 2004, occurred in swarms with emerging and waning activity and with typical explosive volcanic acoustic signatures, which are interpreted as recording the regrowth of an ∼90-m-high cone back to a near-1998 elevation, at an average rate of 47 m a−1. This study provides (1) a lower bound for frequency-magnitude relationships of landsliding for submarine arc volcanoes and (2) estimates of 0.013 km3 a−1 of submarine cone growth during eruptive cycles.

[1] Extraordinary video and hydrophone observations of a submarine explosive eruption were made with a remotely operated vehicle in April 2006 at a depth of 550–560 m on NW Rota-1 volcano in the Mariana arc. The observed eruption evolved from effusive to explosive, while the eruption rate increased from near zero to 10–100 m3/h. During the peak in activity, cyclic explosive bursts 2–6 min long were separated by shorter non-eruptive pauses lasting 10–100 s. The size of the ejecta increased with the vigor of the explosions. A portable hydrophone deployed near the vent recorded sounds correlated with the explosive bursts; the highest amplitudes were ∼50 dB higher than ambient noise at frequencies between 10 and 50 Hz. The acoustic data allow us to quantify the durations, amplitudes, and evolution of the eruptive events over time. The low eruption rate, high gas/lava ratio, and rhythmic eruptive behavior at NW Rota-1 are most consistent with a Strombolian eruptive style. We interpret that the eruption was primarily driven by the venting of magmatic gases, which was also the primary source of the sound recorded during the explosive bursts. The rhythmic nature of the bursts can be explained by partial gas segregation in the conduit and upward migration in a transitional regime between bubbly flow and fully developed slug flow. The strongest explosive bursts were accompanied by flashes of red glow and oscillating eruption plumes in the vent, apparently caused by magma-seawater interaction and rapid steam formation and condensation. This is the first time submarine explosive eruptions have been witnessed with simultaneous near-field acoustic recordings.

Pressure measurements made on the seafloor at depths between 1500 and 1700 m at Axial Seamount, an active submarine volcano on the Juan de Fuca Ridge in the northeast Pacific Ocean, show evidence that it has been inflating since its 1998 eruption. Data from continuously recording bottom pressure sensors at the center of Axial’s caldera suggest that the rate of inflation was highest in the months right after the eruption (20 cm/month) and has since declined to a steady rate of ~15 cm/ year. Independent campaign-style pressure measurements made each year since 2000 at an array of seafloor benchmarks with a mobile pressure recorder mounted on a remotely operated vehicle also indicate uplift is occurring in the caldera at a rate up to 22F1.3 cm/year relative to a point outside the caldera. The repeatability of the campaign-style pressure measurements progressively improved each year from F15 cm in 2000 to F0.9 cm in 2004, as errors were eliminated and the technique was refined. Assuming that the uplift has been continuous since the 1998 eruption, these observations suggest that the center of the caldera has re-inflated about 1.5F0.1 m, thus recovering almost 50% of the 3.2 m of subsidence that was measured during the 1998 eruption. This rate of inflation can be used to calculate a magma supply rate of 1410 6 m 3 /year. If this rate of inflation continues, it also suggests a recurrence interval of ~16 years between eruptions at Axial, assuming that it will be ready to erupt again when it has re-inflated to 1998 levels. D 2005 Elsevier B.V. All rights reserved.

[1] Lava pillars are hollow, vertical chimneys of solid basaltic lava that are common features within the collapsed interiors of submarine sheet flows on intermediate and fast spreading mid-ocean ridges. They are morphologically similar to lava trees that form on land when lava overruns forested areas, but the sides of lava pillars are covered with distinctive, evenly spaced, thin, horizontal lava crusts, referred to hereafter as “lava shelves.” Lava stalactites up to 5 cm long on the undersides of these shelves are evidence that cavities filled with a hot vapor phase existed temporarily beneath each crust. During the submarine eruption of Axial Volcano in 1998 on the Juan de Fuca Ridge a monitoring instrument, called VSM2, became embedded in the upper crust of a lava flow that produced 3- to 5-m-high lava pillars. A pressure sensor in the instrument showed that the 1998 lobate sheet flow inflated 3.5 m and then drained out again in only 2.5 hours. These data provide the first quantitative constraints on the timescale of lava pillar formation and the rates of submarine lava flow inflation and drainback. They also allow comparisons to lava flow inflation rates observed on land, to theoretical models of crust formation on submarine lava, and to previous models of pillar formation. A new model is presented for the rhythmic formation of alternating lava crusts and vapor cavities to explain how stacks of lava shelves are formed on the sides of lava pillars during continuous lava drainback. Each vapor cavity is created between a stranded crust and the subsiding lava surface. A hot vapor phase forms within each cavity as seawater is syringed through tiny cracks in the stranded crust above. Eventually, the subsiding lava causes the crust above to fail, quenching the hot cavity and forming the next lava crust. During the 1998 eruption at Axial Volcano, this process repeated itself about every 2 min during the 81-min-long drainback phase of the eruption, based on the thickness and spacing of the lava shelves. The VSM2 data show that lava pillars are formed during short-lived eruptions in which inflation and drainback follow each other in rapid succession and that pillars record physical evidence that can be used to interpret the dynamics of seafloor eruptions.