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Report on Gemini-Oscostar (Vanuatu) — February 1996


Bulletin of the Global Volcanism Network, vol. 21, no. 2 (February 1996)
Managing Editor: Richard Wunderman.

Gemini-Oscostar (Vanuatu) Submarine eruption; the first recorded historical activity

Please cite this report as:

Global Volcanism Program, 1996. Report on Gemini-Oscostar (Vanuatu) (Wunderman, R., ed.). Bulletin of the Global Volcanism Network, 21:2. Smithsonian Institution. https://doi.org/10.5479/si.GVP.BGVN199602-258001



20.957°S, 170.13°E; summit elev. -33 m

All times are local (unless otherwise noted)

A submarine eruption was observed in the southern New Hebrides island arc (Vanuatu), an area without previously documented historical activity. The activity was first observed by the merchant ship OSCO STAR cruising in this area on 18 February around 1800. It was described as "continual steam and frequent vertical bursts of very dark water." Observations during a New Caledonia Coast Guard flight on 19 February revealed a white zone within a steaming black patch. A similar flight on 22 February enabled a television crew from RFO New Caledonia to take videotape footage for the local news. Observers on that flight noted that the white zone, from which steam was rising, had a diameter of ~400 m. This zone was located inside a wider ellipse, brown-ochre in color, elongated ~4 km down-current. Every 3-9 minutes an explosion sent black products ~20 m above sea level. After each explosion, the diameter of the white area diminished drastically, rising again during the next explosion. The black products were diluted to form the brown-ochre zone. This activity was probably similar to that documented on 18 February.

Located ~100 km S of Anatom Island, about halfway between Yasur Volcano (Tanna Island) and Matthew Island, the Eastern Gemini seamount is one of several seamounts along the southern submarine extension of the New Hebrides island arc. Several basalt samples and one andesite dredged from this seamount in 1989 (Monzier and others, 1993) were described as glassy, vesicular, and extremely fresh (Bargibant and others, 1989). Because all of the samples were devoid of marine animal traces, the activity was described as very recent. The nearby Western Gemini seamount is located near 21.0°S, 170.05°E, at a depth of 30 m below sea level. Well-developed marine life around its summit suggests that its activity is older.

References. Monzier, M., Danyushevsky, L.V., Crawford, A.J., Bellon, H., and Cotten, J., 1993, High-Mg andesites from the southern termination of the New Hebrides island arc (SW Pacific): Journal of Volcanology and Geothermal Research, v. 57, p. 193-217.

Bargibant and others, 1989, ORSTOM Noumea Earth Sciences Report, no. 12, 13 p. (unpublished).

Geological Summary. The submarine Gemini-Oscostar volcanic field lies near the southern end of the Vanuatu (or New Hebrides) Volcanic Arc, ~100 km S of Aneityum Island. Water discoloration and bursts of very dark water were observed from Oscostar seamount (also known as Eastern Gemini) from a passing ship on 18 February 1996. Overflights as late as 22 February noted periodic explosions that ejected black material to about 20 m above sea level. It consists of an elongated NNE-SSW-trending ridge of submarine volcanoes with satellitic cones. Several basaltic samples and one andesitic rock dredged in 1989 were described as glassy, vesicular, and extremely fresh.

Information Contacts: Bernard Pelletier, Centre ORSTOM de Noumea, BP A5, Noumea, New Caledonia; Michel Lardy, ORSTOM, BP 76, Port Vila, Vanuatu; Michel Monzier and Claude Robin, ORSTOM, AP 17-11-6596 CCI, Quito, Ecuador; Jean-Philippe Eissen, Centre ORSTOM de Brest, BP 70, 29280 Plouzane, France.