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Tahiti is near the southeast end of the Society Island chain. Two main volcanoes, Tahiti-nui and Tahiti-iti, make the island. They are both deeply eroded basaltic volcanoes. The island is about 18 miles (30 km) wide and 33 miles (55 km) long. The total volume of the coalesced volcanoes is 60,000 cubic km. Only about 2% of the volcanoes are above sea level. Plutonic rocks, that crystallized 2-3 km beneath the summits of the volcanoes, are exposed in the eroded centers of each volcano. There is more than one mile (about 2 km) of topographic relief on the island. This photograph shows Tahiti_nui.
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Tahiti-nui (center of photo) began to form about 1.7 million years ago. The main shield building phase lasted from 1.3 to 0.6 million years ago. Late stage, or capping stage, volcanism began about 700,000 years ago and ended 300,000 years ago. Tahiti-iti is to the left (east) of Tahiti-nui. About 900,000 years ago, as eruptions at Tahiti-nui waned, activity shifted to the southeast to Tahiti-iti.
Tahiti-nui and Tahiti-iti are extinct volcanoes.
Source of Information:
Duncan, R.A., Fisk, M., White, W.M., Nielsen, R.L., 1994, Tahiti:
Geochemical evolution of a French Polynesian volcano: J. Volcanol. Geotherm.
Res., v. 99, p. 24,321-24,357.
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