A project undertaken at Australian Volcanological Investigations, Pymble, NSW, and supervised by P W Taylor
The Tofua Volcanic Arc (TVA) is an active volcanic feature within the Kingdom of Tonga. The TVA, consisting of a series of several dozen active, dormant and extinct, subaerial and submarine volcanic centres (Figure 1), has formed on a NNE-SSW trending submarine ridge, between latitudes 14.5oS and 26oS. Between 18oS and 21oS, the TVA is located only 40-50 km west of the densely populated Tongatapu, Ha’apai and Vava’u Groups.
The Ha’apai island group consists of 62 islands with a total land area of around 110 km2, scattered over an area of 10,000 km2 or ocean. The islands, 17 of that are inhabited, range in size from less than 1 hectare up to 46.5 km2 (Figure 2). An estimated 8,500 Tongans live in 30 villages on the inhabited islands, with their livelihood from subsistence agriculture and fishing. Many of the uninhabited islands and atolls also provide a valuable agricultural and fishing resource
Figure 1. Locality map for the Tofua Volcanic Arc and the Tonga Ridge. Symbols – (J) volcanoes with a record of historic eruptions, (H) volcanic islands with no record of historic eruptions, assumed to be either dormant or extinct; (K) submarine highs, many of which may be of volcanic origin.
Figure 2. Locality map of the Ha’apai Group and the Central Tofua Volcanic Arc. The Ha’apai Group is separated from the volcanic arc by the Tofua Trough
Figure 3. The upper 0.5 metres of core 1 from Lotafoa swamp, Foa, Ha’apai. Note that a layer of dark brown to black peat overlies a layer of grey scoreacious ash and lapilli.
Figure 4. Preliminary volcanic hazards map for the Central Ha’apai Group
Figure 5. Tofua and Kao volcanoes as viewed from near Ha’afeva Island, Ha’apai, 30 kms to the east.
Figure 6. Upper 50 cms of the Folaha Swamp core, with a 7 cm ash-rich layer underlying 30 cms of black-brown peaty material.
Figure 7. Shows the mantling effect of the ash layer over the existing ground surface at the time of deposition.
Figure 8. Possible source volcanoes to the NW and SW of Tongatapu Island.
Figure 9. Mr Taylor explaining features present in a core recovered from the Folaha swamp to Sisi Tonga’onevai of the Ministry of Lands Survey and Natural Resources.
Figure 10. Two class groups involved in discussions on volcanic hazard types during the workshop.
The central part of the TVA to the west of the Ha’apai Group includes the currently and recently active volcanic centres of Home Reef, Metis Shoal (Lateiki), Tofua and Falcon Island (Fonuafo’ou), all of which have experienced frequent periods of activity during the last 100 years. Numerous other submarine volcanoes along this section of the arc may also have erupted, with activity going unreported.
Although recent eruptions that have occurred from the centres have been relatively minor events, there is evidence on a number of the islands of the Ha’apai Group that large magnitude eruptive events have occurred and have had a significant affect on the environment. Recent exploratory coring of swamp sediments (Figure 3) has revealed that at least 5 and up to 10 large eruptions have impacted the Ha’apai group in the last c. 6500 years. A preliminary assessment of the impacts of these events on the communities in the Ha’apai Group was carried out (Figure 4). Probable sources for the deposits may include Tofua and Koa volcanoes (Figure 5).
A preliminary survey of Tongatapu was also carried out with, several sites being investigated. Swamp sediments (Figure 6) and quarry outcrops (Figure 7) have provided a record of at least 2 large eruptions that have occurred during the last c. 1-2,000 years. These eruptions would have had a significant effect on the communities in the Tongatapu Group. Probable sources for the deposits include Hunga Tonga and Hunga Ha’apai and several other submarine volcanoes to the NW of Tongatapau (Figure 8).
Through a period of field and laboratory work it was possible to provide a wealth of new data to enable a volcanic hazards assessment of the impacts of future activity on the Ha’apai Group to be conducted. It was also possible, as part of the project, to provide capacity-building training for local scientists in the recognition and assessment of volcanic hazards in the field (Figure 9) and in the classroom (Figure 10). Due to logistical considerations, this aspect of the project was conducted on the island of Tongatapu.