Tonga sits on the eastern edge of the Indo-Australian Plate, which is forced up as the Pacific Plate pushes under it at the Tonga Trench. This long oceanic valley running 2,000 km from Tonga to New Zealand is one of the lowest segments of the ocean floor, in places over 10 km deep.
Tonga is on the circum-Pacific Ring of Fire, which extends from New Zealand to Samoa, then jogs over to Vanuatu and the Solomons. Where Tongatapu (a raised atoll), Lifuka (a low coral island), and Vava'u (another uplifted atoll) are today, towering volcanoes once belched fire and brimstone. When they sank, coral polyps gradually built up the islands.
Study the map of Tonga and you'll distinguish four great atolls in a line 350 km long. The two central atolls (Ha'apai) are now largely submerged, with Lifuka and Nomuka the largest remaining islands of each. As Ha'apai sank under the weight of new volcanoes such as Kao and Tofua, the outermost groups, Vava'u and Tongatapu, tilted toward the center, creating cliffs on their outer edges and half-submerged islands facing in.
Tonga is moving east-southeast at the rate of 20 millimeters a year and the crack in the earth's crust that originally built Tonga has shifted northwest.
Thus, the active volcanoes of today are in a line 50 km west of Ha'apai-Vava'u: Fonuafo'ou, Tofua, Lateiki, Late, Fonualei, and Niuafo'ou have all erupted during the last 200 years.