Tavui Volcano: neighbour of Rabaul and likely source of the Middle Holocene penultimate major eruption in the Rabaul area

Abstract

A major, geologically youthful, submarine caldera volcano, Tavui, was discovered in the Rabaul area of Papua New Guinea in 1986. Tavui Volcano has lateral dimensions of 9 to 10 km, slightly smaller than those of Rabaul Volcano, but Tavui’s caldera is much deeper than the nested caldera complex at Rabaul, and in general, its escarpment walls are very steep. The two caldera systems are essentially silicic and are separated by a zone of dominantly basalt-andesite stratovolcanoes. Rock samples dredged from Tavui have low- to medium-K contents, in contrast to the medium- to high-K rocks of Rabaul, indicating that the two systems have evolved separately and that the chemical, and perhaps physical, conditions within these neighbouring systems are different. Tavui is the likely source of the 6.9 ka BP Raluan Ignimbrite, the penultimate major eruption deposit in the Rabaul area. The Raluan Ignimbrite is rhyolitic and has geochemical characteristics incompatible with those of products from Rabaul Volcano. On the other hand, there is a close match between the geochemistry of the Raluan Ignimbrite and that of rhyolitic samples dredged from Tavui Caldera. The much older (≈79 ka) Tokudukudu Ignimbrite, which is also rhyolitic, is slightly more K-rich than both the Raluan Ignimbrite and rhyolites dredged from Tavui Caldera, but in general, its geochemical characteristics are similar to those of Tavui rhyolites and, therefore, is considered to be a possible product of Tavui. The recognition that Tavui was the likely source of the penultimate major eruption and of at least one other significant eruption in the Rabaul area markedly changes the perceptions of local volcanic hazard. In addition, Tavui’s potential for generation of tsunami is acknowledged, not just in association with volcanic eruptions but also from earthquake-related and possibly spontaneous collapse of parts of the steep caldera walls. The presence of Tavui greatly increases the net geologic hazard in the Rabaul area.