Report on Unnamed (Tonga) — January 2002
Bulletin of the Global Volcanism Network, vol. 27, no. 1 (January 2002)
Managing Editor: Richard Wunderman.
Unnamed (Tonga) Submarine center identified S of Fonualei may be the source of T-waves and pumice
Please cite this report as:
Global Volcanism Program, 2002. Report on Unnamed (Tonga). In: Wunderman, R. (ed.), Bulletin of the Global Volcanism Network, 27:1. Smithsonian Institution. https://doi.org/10.5479/si.GVP.BGVN200201-243091.
18.325°S, 174.365°W; summit elev. -40 m
All times are local (unless otherwise noted)
The following was largely condensed from a report by Paul Taylor submitted to the Tongan government (Taylor, 2002). Our previous report on the topic appeared under the heading "Fonualei" (BGVN 26:11). The bulk of that report described T-wave signals on 28-29 September 2001 traced to near Fonualei and fresh pumice found along beaches in Fiji (hundreds of kilometers W of Tonga) during 9-25 November 2001. The T-wave signals and pumice sightings both relate to the activity discussed here.
During September through early November 2001, submarine volcanic activity was observed ~33 km S of Fonualei (figure 3). This same spot lies ~30 km NW of the Vava'u Group of the Tongan islands. This volcanic center lacked prior historical activity, although Taylor and Ewart (1997) indicated that a number of submarine structures were present between Late and Fonualei islands.
Form, structure, and depth. Although no details are available concerning the form and structure of this eruptive site, it is likely to be the summit of a submarine stratovolcano that rises from a NNE-SSW trending topographic high. A shoal has not been reported at the site during historical times. No surveys of this area have been conducted; however, its bathymetry suggests that several submarine structures rise from a depth of about 1 km to probably within 200-300 m of the surface. No shoal or island was observed when the site was visited by the Tonga Defense Services during early and mid-October 2001.
Volcanic activity. The activity appears to have been submarine and explosive in character. Known reports relating to this eruption are given in table 1. A plot of the seismic activity from stations in the Cook Islands and French Polynesia during 28-29 September 2001 were provided in Figure 1 of BGVN 26:11.
|27-28 Sep 2001||T-phase seismic waves from a probable volcanic source recorded in French Polynesia. Approximate coordinates of 18.39°S; 174.6°W, are located near the Vava'u Group.|
|27 Sep 2001||1800 - Reports of submarine activity were received from near Vava'u. A local fisherman experienced "an abnormal disturbance from the deep ocean." Shortly after an ash-rich eruption column rose from the sea at 18.325°S, 174.365°W.|
|28 Sep 2001||1300 - An "island" was reported to have formed during the explosive activity with an ash-rich eruption column still being produced. The "island" was estimated to be about 2 miles [~ 3 km] long. The sea was "highly disturbed and silky" at this time.|
|01 Oct 2001||0930 - Royal Tongan Airlines flights 801 and 802 reported that activity above the surface had ceased. A huge underwater bank, about 1.5 miles [2.4 km] across, was observed at 18.358°S, 174.346°W, [3.8 km SW] of the initial location. The water was reported as "boiling bubbles of seawater oozing out from the area to the sea surface".|
|03 Oct 2001||A Tonga Defense Services patrol boat visited the area, but due to heavy seas observations were restricted. The surface of the sea in the region was discolored a "dark whitish color". The discolored area was estimated to be 3 miles [~ 5 km] long (N-S direction) and 1.5-2 miles[2.4-3 km] wide. Near the reported location, the sea appeared to contain a mixture of whitish and yellow-brownish substances although no pumice was observed floating on the surface. A local Notice to Mariners (NTM 15/01) was issued, warning shipping to stay away from the area.|
|09 Oct 2001||1600 - A Tonga Defense Services aircraft flew over the site and reported that an area of discolored water was present. No eruption column or pumice was observed and the island reported earlier was not present.|
|26 Oct 2001||A Tonga Defense Services patrol boat visited the site and observed an area of discolored water 300 m long (NE-SW direction) centered on a position of 18.303°S, 174.377°W, [a spot 2.7 km NE of the initial position]. The discoloration was light-brownish in the center and light greenish toward the outside. The charted depth of the shoal at this location was 298 meters. No depth was recorded by the boat's echo sounder and no attempt was made to take a sounding over the discolored water.|
|early Nov 2001||Pumice strandings were reported along the coast of Kadavu and on the S coast of Viti Levu, Fiji. Rafts reported to be over 100 m in diameter with pumice fragments ranging in size from under 1 cm to ~20 cm.|
Comments. As noted above, the charted depth prior to the eruption was ~200-300 m and the syn-eruptive depth was not determined. Further, Taylor learned that post-eruptive depths had not been taken at the site. He goes on to state, "The initial activity was the result of submarine explosions, producing what was reported as 'an island' and an eruption column." In his report, Taylor concluded that the island was essentially a floating pumice raft and ". . . was more likely the effect of gases and pyroclastic material produced by the explosions breaking the surface, which appeared land-like. An eruption column of predominantly volcanic gas, steam, and pyroclastic material was then ejected above the surface."
Taylor (2002) goes on to discuss relevant volcanic hazards. Regarding approaching the volcano, he recommended that access be prohibited within 2 km, access restricted within the interval 2 to 4 km, and extreme care be taken when approaching or within the interval 4 to 5 km.
References. Taylor, P.W., 2002, Volcanic hazards assessment following the September-October 2001 eruption of a previously unrecognized submarine volcano W of Vava'u, kingdom of Tonga: Australian Volcanological Investigations, AVI Occasional Report No. 02/01
Taylor, P.W., 1999, A volcanic hazards assessment following the January 1999 eruption of Submarine Volcano III Tofua Volcanic Arc, Kingdom of Tonga: Australian Volcanological Investigations, AVI Occasional Report No. 99/01.
Taylor, P.W., and Ewart, A., 1997, The Tofua Volcanic Arc, Tonga, SW Pacific: A review of historic volcanic activity: Australian Volcanological Investigations, AVI Occasional Report No. 97/01.
Geologic Background. A submarine volcano along the Tofua volcanic arc was first observed in September 2001. The newly discovered volcano lies NW of the island of Vava'u about 35 km S of Fonualei and 60 km NE of Late volcano. The site of the eruption is along a NNE-SSW-trending submarine plateau with 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 sea surface. No eruptive activity was reported after the 28th, but water discoloration was documented during the following month. In early November rafts and strandings of dacitic pumice were reported along the coast of Kadavu and Viti Levu in the Fiji Islands. The depth of the summit of the submarine cone following the eruption determined to be 40 m during a 2007 survey; the crater of the 2001 eruption was breached to the E.
Information Contacts: Paul Taylor, Australian Volcanological Investigations, PO Box 291, Pymble NSW 2073, Australia; Olivier Hyvernaud, Laboratoire de Geophysique, Papeete Tahiti, French Polynesia; Dan Shackelford, 3124 E. Yorba Linda Blvd., Apt. H-33, Fullerton, CA 92831-2324, USA.