Cruise Report (PDF, 4.8Mb) - NOAA, Ocean Explorer

1.0 NE LAU RESPONSE CRUISE (NELRC) SUMMARY
Joseph Resing and Robert Embley
Summary
This eruption response cruise was aimed at visiting the sites of two recent eruptions in the NE Lau Basin
discovered in November 2008 during an expedition on the R/V. T. G. Thompson. The response expedition went
expecting to find recent eruption deposits at both sites, and hoping to find at least one site still in eruption, which
is what we found (at W. Mata volcano). The cruise subsequently spent 6 days characterizing the volcanic
deposits and associated phenomena, providing fundamental new insights on eruptive phenomena in this
environment, including the first observations of molten lava actively erupting in the deep ocean.
Background and Rationale
The first suspected eruption site was discovered at the southern segment of Northeast Lau spreading center
(NELSC) and the second was at West Mata, a small submarine volcano lying between the NELSC and the frontal
magmatic arc of the Tonga-Kermadec subduction zone. (Figs.1 and 2) Hydrothermal water column plumes found
800-1000 m above the suspected location of the eruption at the NELSC in Nov. 2008 contained high
concentrations of volcanic glass shards and H2 (> 3 uM). The combination of the extreme elevated dissolved H2
concentrations and the presence of sharp edged shards provided unambiguous evidence of the interaction between
molten rock and seawater. Near-bottom temperature anomalies >0.5°C over a small region of the neovolcanic
ridge indicated the likely location of the eruption.
The NELSC event posed the first opportunity to compare the geologic style of back-arc volcanic accretion and
environmental impacts to ones documented over the past two decades on other ridges (Juan de Fuca, Gorda, EPR)
and seamounts (Loihi, NW Rota). Responses to volcanic events on the Juan de Fuca Ridge and East Pacific Rise
spreading centers during the past two decades have resulted in new insights on chemical, biologic and geological
processes, including: (1) volumes and extent of eruptions with major insights on how the ridge is constructed
through time (Chadwick and Embley, 1994; Embley et al., 2000; Fornari et al., 1998); (2) the biological responses
to eruptions (Cowen et al., 2004; Huber et al., 2003; Shank et al., 1998; Tunnicliffe et al., 1997), (3) the chemical
evolution following magmatic perturbations (Butterfield et al., 1997; Lilley et al., 2003; Von Damm et al., 1995);
and (4) the discovery and sampling of the subsurface biosphere beneath the midocean ridge (Cowen et al., 2004;
Delaney et al., 1998; Huber et al., 2003). In addition to these fundamental questions there is also the overarching
question of how submarine volcanism and hydrothermalism works at a variety of geologic settings, depths and
magma composition. The NELSC results could also potentially provide the first data on eruption style (i.e.,
effusive vs. fragmental), the extent and volume of eruptives, and serves as a baseline to compare future eruptions
in the back-arc environment.
In Nov. 2008, the second eruptive site, W. Mata, had an intense plume rising ~175 m with among the highest
values of turbidity, H2 (> 9000 nM), 3 He, oxidation-reduction potential anomaly, and pH anomaly ever measured
in a hydrothermal plume. There was an abundance of large (10-70 uM in length) shards in the plume composed
almost exclusively of Mg-silicates and Ca-Mg-silicates. High acoustic backscatter derived from the Thompson’s
EM300 multibeam sonar system was consistent with very young/current volcanic activity. This was a very
exciting result, because W. Mata would represent only the second site on the planet where we could study an
erupting submarine volcano. Studies of the long term eruption at NW Rota-1 volcano in the Mariana arc have
provided dramatic first insights on the physical volcanology, chemistry and biology of a submarine eruption
(Embley et al., 2006; Chadwick et al., 2008: Resing et al., 2008; Walker et al., 2008; Limen et al., 2006). The W.
Mata site is much deeper and could represent one of several very young eruptive centers in the area. The SW-NE
striking trend of West and East Mata and several ridges to their northwest suggest that recent volcanic activity is
concentrated along tear faults possibly induced by disruption (rifting) of the crust at the northern termination of
the Tonga subduction zone. Boninite lavas (SiO2 >53% with Mg/[Mg+Fe2+] >0.6 have been recovered in
dredges (Falloon et al., 2007) in the proto-rift zone northwest of W. Mata. This entire area is very hydrothermally
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