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?
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$\begingroup$ Could you clarify what you mean by mid-oceanic please? Is it restricted to island built on mid-oceanic ridges, such as St Helena and Ascension, or do hotspot oceanic islands, such as Hawaii and La Reunion, count as well? $\endgroup$– Jean-Marie PrivalApr 21, 2022 at 7:46
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$\begingroup$ Mid-oceanic islands are those located significantly far from mainland/continental coasts, e.g. over 500 miles. They may or, may not be close to mid-oceanic ridges. St Helena & Ascension are both mid-oceanic but while Ascension is relatively close to the Atlantic mid-oceanic ridge St Helena is hundreds of miles away. My question specifically relates to the length of time these mid-oceanic islands were volcanically active, NOT to theories of how such islands formed, e.g. hot spots. $\endgroup$– user8654Apr 21, 2022 at 11:36
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$\begingroup$ Well both are linked, as the formation process of such islands will have an impact on the possible duration of volcanic activity. For example, hotspot islands will be active only until the plate has moved away from the mantle plume enough to create a new island. Which can still be quite a long amount of time. For example, Mauritius has been active for about 10 million years, and now volcanism has shifted to La Reunion, which has already been active for 3 million years. $\endgroup$– Jean-Marie PrivalApr 21, 2022 at 12:36
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1 Answer
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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.
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$\begingroup$ I suspect there are 2 errors in the above: 1. The African plate is thought to move at 21.5mm/a, not 5mm/a. 2. The surface diameter of St Helena (actually 16x8km) seems irrelevant - merely being the section above sea level. The base of St Helena has a diameter of 130Km. If the African plate travels 21.5mm/a, I calculate it would take 6.04Ma to travel 130Km across a theoretical hot spot. That very roughly fits the estimated 7ma period of volcanism. Is this a coincidence or does the base diameter of an extinct volcano indeed have a relationship with its period of activity? $\endgroup$– user8654Apr 22, 2022 at 14:41