A geological synopsis of the three main islands of the Tristan da Cunha group is given and their rate of growth by volcanic processes, and their subsequent erosion, are examined in the light of recent radiometric age data. These data, though few as yet, suggest that each island has a 'life cycle' consisting of a brief constructional volcanic phase lasting less than 1,000,000 years, followed by an erosional phase of some 20,000,000 years. The possibility that other oceanic islands have had comparable histories is discussed.

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