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Report on Unnamed (Tonga) — November 2001


Bulletin of the Global Volcanism Network, vol. 26, no. 11 (November 2001)
Managing Editor: Richard Wunderman.

Unnamed (Tonga) Possible source for September T-waves and November pumice rafts

Please cite this report as:

Global Volcanism Program, 2001. Report on Unnamed (Tonga) (Wunderman, R., ed.). Bulletin of the Global Volcanism Network, 26:11. Smithsonian Institution. https://doi.org/10.5479/si.GVP.BGVN200111-243091



18.325°S, 174.365°W; summit elev. -40 m

All times are local (unless otherwise noted)

[The following originally appeared as part of a report on Fonualei. Later investigations showed that the seismicity and pumice rafts in question most likely came from an unnamed submarine volcano in the Tonga Islands.]

Seismicity. During 28-29 September 2001 numerous short T-waves were registered by the French Polynesian Seismic Network. The preliminary location of the seismicity was determined to be near the Tonga archipelago at 18.18°S (well constrained) and 174°W (not as well constrained). This spot lies ~40 km W of Fonualei.

The swarm began at 0550 on 28 September and ended at 1113 on 29 September (figure 1). The strongest T-wave was registered at 1229 on 28 September at the PAE seismic station in Tahiti (figure 2). The hydro-acoustic activity was interpreted to be volcanic and explosive and not related to seismicity at the Tonga trench. According to the Laboratoire de Géophysique, the source could be near Fonualei.

Figure (see Caption) Figure 1. A plot showing the overall character of the T-wave swarm inferred to have come from Fonualei during 28-29 September 2001. Basically, the cluster of T waves seen in the main part of the swarm (28 September) consisted of signals with short (15-second) periods. Some of these signals were comparatively strong. T waves seen later in the swarm (1100 on 29 September) had long (120-second) period. Courtesy of Laboratoire de Géophysique.
Figure (see Caption) Figure 2. Seismic trace of the strongest of the T-wave signals attributed to Fonualei during the swarm of 28-29 September 2001. The trace was recorded at 1229 on 28 September at the PAE seismic station in Tahiti (the trace was labeled "PAE CPZ1 (Brut)"). Courtesy of Laboratoire de Géophysique.

Pumice rafts. Roman Leslie, a Ph.D. student at the University of Tasmania visited Fiji (hundreds of kilometers W of Tonga) during 9-25 November 2001. There he observed large (100-m diameter) pumice rafts of gray, aphyric pumice clasts ranging from sand-sized to ~20 cm in diameter. Local residents hadn't seen such large rafts before, but had noticed occasional clasts in recent history.

Leslie initially observed the pumice rafts while on Kadavu island of the Lomaiviti Group while diving on the southern Astrolabe Reef from the 10th-15th. He again saw pumice rafts in the Koro Sea during a flight from Suva to Koro on the 16th. Next, he found them on the coral coast (southern Viti Levu) on the 24th, where samples were collected ~5 km E of Sigatoka.

There he collected pumice samples from the beach at or near the high-tide mark, where they formed discontinuous wave-derived lag deposits of limited thickness, with ~5 m lateral extent. Beach pumice deposits and floating rafts (up to ~150 m in length) were poorly sorted and consisted of brown-grey clasts ranging from ~2 to 100 mm in diameter. Clasts were sub-angular to sub-rounded and appeared to contain small phenocrysts of clinopyroxene and plagioclase. Judging from the approximate color index and mineralogy it seemed that the samples were broadly andesitic.

Whether or not the pumice rafts seen in Fiji during November are related to the activity that registered as T-waves from Tonga during late September is uncertain. The rafts and T-waves may be entirely unrelated in terms of source location, or they may result from a common eruption, perhaps at Fonualei.

Geological Summary. A submarine volcano along the Tofua volcanic arc ~45 km NW of Vava'u Island was first observed in September 2001, ~35 km S of Fonualei and 60 km NE of Late volcano. The site of the eruption is at an approximate bathymetric depth of 300 m. T-phase waves were recorded on 27-28 September 2001, and on the 27th local fishermen observed an ash-rich eruption column that rose above the ocean surface. No eruptive activity was reported after the 28th, but water discoloration was documented the following month. In early November rafts and strandings of dacitic pumice were reported along the coasts of Kadavu and Viti Levu in Fiji. The depth of the summit of the submarine cone following the eruption was determined to be 40 m during a 2007 survey; the crater of the 2001 eruption was open to the E.

Information Contacts: Olivier Hyvernaud; Laboratoire de Géophysique; PO Box 640 Papeete; Tahiti; French Polynesia; Roman Leslie, Centre for Ore Deposit Research, University of Tasmania, GPO Box 252-79, Hobart, TAS 7001, Australia (URL: http://www.utas.edu.au/codes/).