The island of St Helena in the South Atlantic was volcanically active for 5 million years, or maybe longer. Is this period of time unusual - which other mid-oceanic islands were active for as long as this?
There is a series of volcanic islands in the Atlantic.
Besides St. Helena there are for example Bermuda, Madeira, The Canary Islands, Ascension, Tristan da Cunha, The Azores, and Iceland, which is the largest island on the ridge and is active at least for the past 16-18 Ma.
On St. Helena Abdel-Monem & Gast, (1967) and Baker et al. ( 1967) report K-Ar ages of 7-14 Ma for the NE and SW volcanoes.
Data from Chaffey et al. (1989) report the smaller interval of 7-9 Ma.
An interval of activity of several Ma appears not unusual. If hotspot volcanism is the origin, the activity interval is related to the size of island and the relative speed of plate motion across the hotspot. A typical speed is 5 mm/a, corresponding to 5km/Ma. For an island with 20 km diameter this corresponds to about 4 Ma. Again, 5 Ma seems to be a reasonable time interval.