Report on Unnamed (Tonga) — 14 August-20 August 2019
Smithsonian / US Geological Survey Weekly Volcanic Activity Report, 14 August-20 August 2019
Managing Editor: Sally Kuhn Sennert
Please cite this report as:
Global Volcanism Program, 2019. Report on Unnamed (Tonga). In: Sennert, S K (ed.), Weekly Volcanic Activity Report, 14 August-20 August 2019. Smithsonian Institution and US Geological Survey.
18.325°S, 174.365°W; summit elev. -40 m
All times are local (unless otherwise noted)
A submarine eruption in early August from an unnamed seamount about 50 km NW of Vava’u in Tonga created extensive areas of pumice rafts. Sailing crews encountered the pumice starting on 9 August about 40 km NNW of Late Island. Rachel Mackie described a pronounced odor of sulfur, and eventually being surrounded as far as she could see, with the surface pumice layer being 30 cm deep and containing pieces up to 80 cm in diameter. Sentinel-2 satellite imagery from 11 August showed the raft, averaging about 2.5 km across but strung out for 35 km NE-SW, located 70 km WNW of Late Island; another band of scattered rafts extended directly back from the main mass towards Late for 50 km. The raft continued drifting, and was at least another 60 km SW from its previous location as of 16 August. Eruptive activity at this location was previously reported in September-October 2001.
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.