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Volcanoes of Tonga | John Seach
Tonga is located on a subduction zone at the junction of the Pacific and Australian tectonic plates.
The Tongan Islands comprise a double island chain, and lie at the northern end of an active island arc extending NNE from north island of New Zealand.
The eastern chain are limestone covered and the western island chain contains active volcanoes. A parallel offshore trench lies 95-160 km to the east. The seismic zone lies at a depth of approximately 100 km beneath the active Tongan volcanoes.
Further reading
Bryan, W.B., Stice, G.D. and Ewart, A., 1972. Geology, petrography, and geochemistry of the volcanic islands of Tonga. Journal of geophysical research, 77(8), pp.1566-1585.