Back arc spreading ridges

Behind the trench and volcanic arc of a subduction zone, the lithosphere may stretch and extend and volcanic spreading centers may develop. Such a back arc setting is found in the Lau Basin, behind the Tonga Trench in the South Pacific. West Mata is one of two volcanoes in the Lau Basin that were explored and sampled with the ROV Jason2 and mapped with the MBARI Mapping AUV in May 2009. It lies to the NE of the zone of back arc spreading, between it and the well-established volcanic arc, and may be where a new volcanic arc is beginning to form. Active eruptions were occurring near the summit of the volcano. Lavas with the unusual, ultramafic chemistry of boninite were collected. Explosive bursts were observed close up, which was rather exciting, and the formation of vesicular lava fragments and of pillow lavas cascading from the vent were also recorded with the ROV.

Our research at back arc spreading ridges

Implosive eruptions at West Mata volcano

WEST MATA – Deep subaqueous volcanic eruptions (>500 m below sea level (mbsl)) remain enigmatic due to a lack of visual observations and difficulty recreating ambient conditions in the lab. Eruptive activity at West Mata seamount in May 2009 remains one of two deep subaqueous eruptions to have ever been filmed. A distinct low-intensity eruptive style, termed bubble escape activity, was observed at Hades vent (1200 mbsl) characterized by the ascent and implosion of 0.2 -1 m diameter volatile-filled vapor bubbles (Resing et al., 2011). Video of this volcanic activity is used to constrain simple numerical models and produce the first subaqueous eruption actualistic model driven by visual observations.

Bubble escape activity occurs in three stages defined by changing exsolved volatile and lava behavior. During Stage 1, vapor bubble ascent in a magma filled conduit drives either ductile or brittle deformation of the lava surface at the vent, depending on the timescale of lava cooling prior to bubble escape activity. Fragmentation of the lava during Stage 2 culminates with the vapor bubble coming into direct contact with the ambient water. At this point, Stage 3, bubbles implode through rapid condensation and contraction of the exsolved volatile phase, due to rapid heat loss from the vapor bubble to the ambient water.

Numerical modeling of exsolved volatile expansion during conduit ascent to vents across the ocean depth range has identified a transition in exsolved volatile expansion characteristics at 2 -5 MPa. This transition would produce a fundamental change in eruption processes, from which the characteristics and depth range of deep and shallow end members of bubble escape activity are defined.

Bubble escape activity highlights implosive behavior driven by underpressure that develops during ex-solved volatile contraction as a key, but often overlooked, component of both pyroclastic and effusive subaqueous volcanism across the entire ocean depth range. This stands in contrast to overpressure driving subaerial explosive eruptions. The fact that exsolved volatiles can expand, contract, or maintain an approximately constant volume in subaqueous volcanism also calls for the careful application of terminology (e.g. explosive) to describe subaqueous eruption processes.

Reference: Murch, A.P., Portner, R.A., Rubin, K.H., Clague, D.A. (2022) Deep-subaqueous implosive volcanism at West Mata seamount, Tonga, Earth and Planetary Science Letters, 578, 117328. doi: 10.1016/j.epsl.2021.117328

Mapping an active volcano

WEST MATA – High-resolution (1.5 m) mapping from the autonomous underwater vehicle (AUV) D. Allan B. of West Mata Volcano in the northern Lau Basin is used to identify the processes that construct and modify the volcano. The surface consists largely of volcaniclastic debris that forms smooth slopes to the NW and SE, with smaller lava flows forming gently sloping plateaus concentrated along the ENE and WSW rift zones, and more elongate flows radiating from the summit. Two active volcanic vents, Prometheus and Hades, are located ∼50 and ∼150 m WSW of the 1159 m summit, respectively, and are slightly NW of the ridgeline so the most abundant clastic deposits are emplaced on the NW flank. This eruptive activity and the location of vents appears to have been persistent for more than a decade, based on comparison of ship-based bathymetric surveys in 1996 and 2008–2010, which show positive depth changes up to 96 m on the summit and north flank of the volcano. The widespread distribution of clastic deposits downslope from the rift zones, as well as from the current vents, suggests that pyroclastic activity occurs at least as deep as 2200 m. The similar morphology of additional nearby volcanoes suggests that they too have abundant pyroclastic deposits.

Reference: Clague, D. A., J. B. Paduan, D. W. Caress, H. Thomas, W. W. Chadwick, Jr., and S. G. Merle (2011), Volcanic morphology of West Mata Volcano, NE Lau Basin, based on high-resolution bathymetry and depth changes, Geochem. Geophys. Geosyst., 12, QOAF03, doi:10.1029/2011GC003791.

Active eruption of boninite

WEST MATA – Subduction of oceanic crust and the formation of volcanic arcs above the subduction zone are important components in Earth’s geological and geochemical cycles. Subduction consumes and recycles material from the oceanic plates, releasing fluids and gases that enhance magmatic activity, feed hydrothermal systems, generate ore deposits and nurture chemosynthetic biological communities. Among the first lavas to erupt at the surface from a nascent subduction zone are a type classified as boninites. These lavas contain information about the early stages of subduction, yet because most subduction systems on Earth are old and well-established, boninite lavas have previously only been observed in the ancient geological record. Here we observe and sample an active boninite eruption occurring at 1,200m depth at the West Mata submarine volcano in the northeast Lau Basin, southwest Pacific Ocean. We find that large volumes of H2O, CO2 and sulphur are emitted, which we suggest are derived from the subducting slab. These volatiles drive explosive eruptions that fragment rocks and generate abundant incandescent magma-skinned bubbles and pillow lavas. The eruption has been ongoing for at least 2.5 years and we conclude that this boninite eruption is a multi-year, low-mass-transfer-rate eruption. Thus the Lau Basin may provide an important site for the long-term study of submarine volcanic eruptions related to the early stages of subduction.

Reference: Resing, J.A., K.H. Rubin, R.W. Embley, J.E. Lupton, E.T. Baker, R.P. Dziak, T. Baumberger, M.D. Lilley, J.A. Huber, T.M. Shank, D.A. Butterfield, D.A. Clague, N.S. Keller, S.G. Merle, N.J. Buck, P.J. Michael, A. Soule, D.W. Caress, S.L. Walker, R. Davis, J.P. Cowen, A.-L. Reysenbach, H. Thomas (2011) Active submarine eruption of boninite in the northeastern Lau Basin. Nature Geoscience 4:799-806. doi:10.1038/NGEO1275.


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