The Tongan island group is made up of 4 main types of islands
- Raised coral islands (Tongatapu, Vava’u) tilted by plate tectonics (eathquakes) so that they have a high side with coastal cliffs but slope gently to a low coastline with indented lagoons and islands. It is notable that Tongatapu and Vava’u slope in opposite directions. ‘Eua is also predominantly a raised coral island but is more complex with three well defined levels, and exposures of much older volcanic lava and tuff.
- Low coral atolls (Ha'afeva, Tungua, Uiha, Lifuka, Foa, and Ha’ano) with generally very gently undulating topography and maximum elevation of 10-15 m in along the eastern liku coast. Nomuka is possibly older than the other atolls, shows evidence of at least two surface levels and is higher and more dissected. There are also sandy cays (small uninhabited islets). Low coral limestone platforms overlain by coral sands (Uoleva is the largest of these).
- Raised volcanic islands (Mango and Nomuka-iki) have a distinctive rolling to hilly topography developed on andesitic tuffs overlain by calcareous tuffs and marls.
- High volcanic islands including Tofua, Kao, Hunga Ha’apai and Hunga Tonga in the Ha’apai group, Funualei north of Vava’u and all of the northern Niua’s group of islands. All of these are formed from basaltic andesites except Niuafo’ou which is geologically distinct being formed from only basalts
Most soils of the Kingdom of Tonga have formed on fine-grained andesitic tephra (ash) which erupted from volcanoes to the west and north, possibly Tofua and Kao, or from now-submarine volcanoes along the western side of the Tonga ridge. This ash is often quite deep, sometimes as deep as 10 m.
The ash varies in two ways. The particle size generally becoming finer towards the east away from its source in the west, and the particle size distribution indicates many sources, including not only the present obvious volcanic vents, but probably a number of other vents now below sea level.
There appear to be two major periods of ash accumulation, but they were not necessarily exactly contemporaneous. So, the older ash in Ha'apai is not necessarily exactly the same as the older ash on ‘Eua, though they are probably roughly of the same age.
