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Some islands can survive for very long. Madagascar, for example, has been around for 80-100 million years and is likely to remain isolated for hundreds of millions of years more.

Volcanic islands, however, are generally much more short-live, due to the fact that the same spot rarely has access to magma for long. The oldest one I know of is Iceland, which has existed for about 13 million years.

However, is it possible for an island - ideally on Earth - to somehow remain a) above sea level and b) isolated from the mainland continents for a period on the scale of, let's say, 100 million years? If so, what conditions would be necessary for it to happen?

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By being an island arc volcano.

You seem to be referring to hotspot volcanoes, which are fed by abnormally hot regions of the Earth's mantle. As the continental plates drift by overhead, the area fed by the hotspot changes over time, and so you end up with an island chain in which the older, dormant islands have eroded to almost nothing, but the islands still being fed by the hotspot (and therefore, still volcanically active) are much larger.

Hawaii is a good example of an island chain produced by a moving hotspot. You can clearly see the movement of the hotspot over time, from Kure (formed ~28 mya and now completely eroded, leaving just an atoll) to Hawaii, where it now approximately resides.

enter image description here

Island arcs, however, are chains of volcanoes that form along subduction zones: places where two continental plates meet, and one is forced beneath the other. This feeds volcanic activity, and as long as the plates continue to subduct, this activity will continue.

The Aleutian Islands, at the boundary of the Pacific and North American plates, are an example of an island arc. They are thought to have formed around 50-55 mya, and are still volcanically active. Just as you can trace the path of the Hawaii hotspot by looking at the islands, you can trace the Pacific/North American plate boundary by looking at the Aleutians:

enter image description here

So all you need is for your island to be located at a subduction zone that's been around for 100 million years or more. Tectonics will take care of the rest.

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    $\begingroup$ I would also suggest that a tectonic plate that is stationary relative to a volcanic hotspot could produce an isolated, but long-lasting, volcanic island. I don't know the feasibility of this, especially given that OP specifies "ideally on Earth", so I'm not sure it belongs in an answer of its own. $\endgroup$ – MrSpudtastic Nov 1 at 14:24
  • $\begingroup$ Thanks! All the island arcs I've seen are more or less connected to continents. Is it possible to have one which is ~1,000 km or more away from other landmasses? $\endgroup$ – SealBoi Nov 1 at 14:54
  • $\begingroup$ @SealBoi Your best bet would be the Kermadec-Tonga subduction zone, east of Australia. Tonga's located there, as the name implies, and is ~1,800 km from the nearest major landmass (New Zealand). $\endgroup$ – F1Krazy Nov 1 at 19:44
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    $\begingroup$ +1 but NB: the magmas that form the volcanoes cited by OP are of a different type (basaltic) from the ones typically associated with subduction zones (andesitic out rhyolitic). The resulting volcanoes typically look and behave differently. This may be a problem if, for example, OP needs a lava lake or frequent pahoehoe lava flows. sciencelearn.org.nz/resources/650-types-of-volcanic-rock $\endgroup$ – id est laborum Nov 2 at 3:56
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    $\begingroup$ @idestlaborum Those features aren't that important to me, my main objective was having an island which rose up out of the ocean rather than breaking off a continent. $\endgroup$ – SealBoi Nov 2 at 10:10
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You are looking for a Microcontinent. Microcontinent is basically a shard of a larger continental plate that is drifting separately from its parent. There may be active volcanoes on this landmass (like in New Zealand) or not (like in Madagascar). The landmass can be large (like Madagascar) or small (like Seychelles). In either case this is not a hot spot volcanic island - I don't know if that fits your criteria.

Mirocontinent (unlike a hot spot volcanic island) can remain dry land almost indefinitely, because it is made up of think continental plate rather than deformed oceanic plate. Microcontinents do tend to sink, though, but, unlike for volcanic islands, this fate is not mandatory.

Microcontinents can remain separated from large continents for tens of millions of years. This period of separation is limited mostly by the randomness of continental drift process. For example, India had been separated from Gondwana some 100 million years ago, but then some 50 millions ago it collided with Asia, and remained connected ever since.

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The example of Madagascar which you stated is also the answer to your question. Size matters. Once the supply if new lava stops, no surface will be added.

Considering that erosion and crust sinking slowly reduce the surface of the island, the larger the island the longer it can survive under the wear of times.

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    $\begingroup$ I think you might have it the wrong way around here. The reason the oldest volcanic islands are the biggest (Iceland being a prime example) is because they've had access to magma for the longest, so they keep expanding. $\endgroup$ – SealBoi Nov 1 at 13:59
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On that timescale, a series of pole shifts could help your volcanic island avoid erosion. If soon after the volcano stopped providing new magma, the poles shifted such that the island became completely covered by ice year round, and if that ice were thick enough and heavy enough to remain motionless across the millennia, then the rock underneath might remain un-eroded.

Then a second pole shift, say a million years ago, could bring the island back out from under the ice more recently, making it available for your use.

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    $\begingroup$ Ice caps don't just sit there motionless. The ice flows down and out, and that means it erodes the bedrock. $\endgroup$ – Keith Morrison Nov 1 at 14:50
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    $\begingroup$ @KeithMorrison, Thanks for that info. It invalidates my answer, but that is fair trade for learning something new. $\endgroup$ – Henry Taylor Nov 1 at 14:58

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