How long are oceanic volcanos active and how does that tie in with the hot spot theory?

The South Atlantic island of St Helena has volcanic-origin rock aged between 7-14 million years old. Almost all accounts of the island's origin state it was formed from a hot spot. Hot spot convincingly explain the formation of the linear line of Hawaii volcanos, each of which were created over a period of a million years or less for as long as they were over the hot spot in the earth's crust. However, St Helena is on the African plate which is moving at about 20 mm per annum, so during the 7 million years it was active, it will have moved about 140 Km from its starting point. That does not sound like a single hot spot to me, more like a long series of hot spots or a "hot streak". Therefore, I would like to know if any other mid-oceanic volcanoes other than St Helena were active for a long period of time and what is the explanation for their longevity. The hot spot idea does not seem to work for such volcanoes.
Question asked by: ianbruce

Asked on: 20 Mar 2009

The length of activity depends on a number of factors, and so there is no answer to the question other than it depends.
By: knowitall
Replied at: 05 Apr 2009
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