Download What is unique about the West Mata submarine volcano?

West Mata Volcano
University of Iceland
Faculty of Earth Sciences,
Volcanology
Erica Massey
April 2013.
More than 75% of the Earth’s volcanic eruptions occur under water, sight unseen.
Less than 5% of the deep sea has been studied.
The Lau Basin is considered the most geologically active areas of sea floor on the
planet. In May 2009, scientists observed volcanic activity in the NE Lau Basin and for
the first time, at West Mata, pillow lavas, explosive violent activity and almost
continuous large blocks of lava shedding from a deep submarine volcano were
witnessed. The first and deeper vent on the SW area of West Mata volcano, named
Hades, exploded both effusively and explosively with occasional gas bubbles 1 m in
diameter. About 100 m away is another shallower vent close to the summit,
Prometheus, with continuous eruptive activity of fire fountains. Both vents release
sulfurous gas and bright orange lava rather than CO2 gas, common at other
hydrothermal vents. Volatile-rich fluids found here fuel chemosynthetic biological
activity, such as shrimp, crabs and microbial mats.
What is unique about the West
Mata submarine volcano?
-Deepest submarine volcano eruption observed
and filmed.
-Previous to the observation of West Mata’s
eruption, boninite had been found only near
extinct volcanoes more than one million years old.
Therefore, West Mata is the only active volcano
observed that is erupting unusually primitive
boninite lavas.
-West Mata may provide scientists with better
understanding of early subduction stages and
processes at island and back-arcs zones.
Figure 1. The Mata volcanoes.
Figure 2. a, West Mata region; and b,
volcano vents, lavas and dives completed.
Location and Tectonics of West Mata Submarine Volcano
The West Mata volcano is located at 15° 05.68’S; 173° 44.95’W, in the NE Lau Basin within a triangle of
Samoa, Tonga and Fiji in an unusual location between the Tafua arc and the subducting Tonga Trench. The
summit of West Mata is 1174 m and its base is 3000 m below the ocean’s surface, whereas the Tonga
Trench to the NE is 11,000 m deep. This submarine volcano and its neighbors are elongated with two rift
zones situated in the same orientation as what is thought to be tears in the ocean crust underneath them.
To the west, the NELSC (Northeast Lau Spreading Center) differs as a back-arc spreading center.
West Mata is one of nine elongate volcanoes that overlap each other in a southeast to northwest en
echelon progression from near the Tonga Arc to the east–west strike-slip portion of the Tonga Trench.
These volcanoes seem to form on small tears in the oceanic crust above a much larger tear in the
subducting Pacific plate. This rapidly extending basin resides above the subducting slab, reproducing
conditions invoked for nascent subduction zones where arc volcanoes and backarc ridges form.
Boninite: West Mata’s Unique Lava Rock
The West Mata volcano produces pillow lavas and both pyroclastic and phreatic events
occuring at its two vents, named Prometheus and Hades. Boninite is a primitive andesite
composed of mafic extrusive rock that is derived from metasomatised mantle melting and
likely fractional crystallization. It contains high concentrations of Mg and SiO2, specifically > 8%
MgO, > 52% SiO2 and < 0.5% TiO2 and is formed in fore-arc environments of primitive island
arcs close to a trench. It is found in early stages of subduction, and since the Earth’s older crust
has mostly been subducted, boninite lava has only previously been found in ancient deposits
over one million years old.
The lowest pH measured above the plume was a very acidic pH of 1.4. It is depleted of nonfluid mobile heavy REE (rare earth elements) such as Nb, Ta and HF, yet enriched in fluidmobile elements, i.e. Rb, Ba and K. Whole-rock samples at West Mata submarine volcano
contained 10-40% vesicles and 5-20mm thick glass rich quench rinds with high concentration of
pyroxene and olivine crystals. The boninite lava lacks plagioclase phenocrysts and was found
to have a high concentration of Ni and Cr, indicating an unusually high concentration of MgO
andesite parental magma. Chemical analysis showed it to be a high Ca-boninite with high
levels of 3He, CO2, H2, particulate S, dissolved SO2 and S. Seawater reacts with SO2 to create
acid and precipitation of sulphur as a white particle:
Figure 3: Whole rock composition for
samples from the West Mata eruption
and regional data from the northern Lau
Basin and the Tonga Arc.
a. MgO vs. SiO2. The red box defines
the field for boninites (that is,
SiO2>52% and MgO>8%).
b. CaO vs. Al2O3. The red line
indicates a CaO:Al2O3 ratio of 0.75,
demarking the high- and low-Ca
boninites fields.
Expeditions & Exploration of NE Lau Basin
The NOAA Vents program has ongoing expeditions to the NE Lau Basin since November 2008 through May 2009, May 2010, December 2010 and most
recently in 2012. Vehicles utilized for exploration of the West Mata and nearby volcanoes:
Figure 4. a. the acoustic glider path from launch to recovery.
b. the yellow points are the positions when the glider surfaced.
Figure 5. Remotely operated vehicle: Jason-2 ROV.
Jason-4 ROV collected rocks, hot spring water and biological specimens whereas the underwater vehicle, D. Allan B, created high-resolution maps of the
West Mata eruption site. An acoustic glider operated remotely from 5000 miles away, records volcano sounds, detecting temperature, conductivity and
turbidity anomalies associated with hydrothermal and eruptive plumes.
Fig. 6. a, Discovery of the eruption at Hades Vent. Field of view (FOV) is ~4 m across. b. Active degassing and explosive clast formation from combined pyro- and hyaloclastic processes at Prometheus Vent. White particles are primarily elemental sulphur. FOV is approximately 3 m across. c, Magma bubble and active degassing with
degassed lava flowing downhill from the vent, forming pillow flows at Hades Vent. FOV is ~3.5 m. d, Quenched lava collected from an active flow. The active pillow is
~0.3 m wide. Inset is the quenched sample being stored on the ROV. e, Pillow lava extrusion. f, Double magma bubble emerging from the vent before breaking apart at
Hades Vent. The base of the bubble is ~0.5–0.8 m. Most of the observed bubbles ranged in size from 0.25–1 m in diameter with occasional larger bubbles. g, Microbial flock
near diffuse venting between Prometheus and Hades vents. h. Colony of shrimp near diffuse venting. Warm water being collected. The two red dots are 0.1m apart. (2011
Resing).