So, vaguely, I remember from earth science that when submarine volcanoes erupt they eventually form archipelagos. My question actually has several parts;

  • Starting with the first eruption to break surface, how long does it take for those islands to form and become habitable? i.e has vegetation, and sources of food, drinkable water.

  • Without the use of modern technology, is there a way to know that the eruption occurred?

The antagonist in my current WIP is the deity of mountains and volcanoes, she creates these islands so she can have a place to do her scheming and such out of sight of the other deities. The eruption occurs in the middle of the ocean so the water is fairly deep. Saltwater if that makes any difference.

  • Is this a quick or slow process, does she have the time to make the islands habitable before getting caught?

  • How would the water itself be affected by the eruption? The aquatic animals?

    I know initially the water will be hot and the fishies will most likely not love that-- but after cooling etc. how does the pH change, mineral content, current, plant life, animal species change?

  • 1
    $\begingroup$ Without even googling, I know it's gonna be a number of years with a lot of zeroes behind it. I would imagine at least on the order of tens of thousands, possibly millions or more. Your deity had better be a long-term planner (unless she can speed up the process with her magic)! $\endgroup$
    – Qami
    Sep 28, 2018 at 15:32
  • $\begingroup$ I figured as much, but how long is that really in the eyes of the gods? haha. $\endgroup$ Sep 28, 2018 at 15:34
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    $\begingroup$ EarthScience.SE would know... $\endgroup$
    – RonJohn
    Sep 28, 2018 at 15:42
  • $\begingroup$ "How long" certainly depends on how deep the water is. "How would the water itself be affected by the eruption?" Maybe... it would get hot. The aquatic animals probably wouldn't like that, and swim away. $\endgroup$
    – RonJohn
    Sep 28, 2018 at 15:44
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    $\begingroup$ Something else to include, how much influence will the deity have on the vegetation and animals? It has to get there from somewhere, and the middle of the ocean isn't going to have many sources of land animals and plants, but if the deity brings seeds and animals over in secret, it could greatly speed the process up. $\endgroup$
    – Ryan
    Sep 28, 2018 at 18:40

4 Answers 4


Let's do some math, using Sherwood's Surtsey example. We could use the Hawaii example instead, but I'm assuming your deity would prefer to build islands at Surtsey-speed instead of Hawaii's leisurely pace:

It was roughly one day between the start of the Surtsey eruptions, and when the new land broke the surface of the water. Let's call it a rise of 130m/day.

But! New land doesn't just rise vertically. Most undersea eruptions form cinders, with an average angle of repose of 35 degrees. So let's transform Surtsey's vertical rise into a volume. We know the height and the angle of repose, let's use that to get the radius of our land-cone:
r = h * tan(x)
where x is 90 degrees minus our angle of repose:
r = 130 * tan(90-35) = 130 * tan(55) = 186m

From there, we can get the volume of volcanic matter that was erupted that day:
volume = pi * r2 * h / 3 = 4709750 m3 / day

Now let's assume the source of your islands erupt at the same rate. However, the depth they need to rise from is a little more than Surtsey. The abyssal plain lies at a depth of 3000-6000m, so let's go with a depth of 4500m. Your island's required time is:

r = 4500 * tan(55) = 6427m
v = pi * 64272 * 4500 / 3 = 194651489600 m3
days = v / rate = 194651489600 / 4709750 = 41329 days = 113 years

So, you could get your islands in a little over a century, provided you keep the volcanic activity at a relatively frenetic pace the entire time.

As for when life arrives, I'd give it less than a year. Life is pretty impressive when it comes to colonizing new land.


Google Surtsey.


This was an undersea volcano that formed off the coast of iceland. National Geographic had a series of articles on it.

The water was shallow(130 m), so the island didn't have to make a huge thickness of land to get above the surface.

Before breaking the surface, there was a lot of bubbles, floating pumice, ash clouds, but the interval from the start of the eruption to breaking surface was days.

First plants were there within a year. At this point the island is covered in green. Nearest land is 18 km away.

  • $\begingroup$ this was very helpful thank you. Do you think this process was so quick because the water was shallow? How would it be different in the middle of the sea? $\endgroup$ Sep 28, 2018 at 16:33
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    $\begingroup$ Building the island is quick. Keeping the island is hard -- barring further eruptions, Surtsey has a remaining life expectancy of a century or so before it erodes completely. $\endgroup$
    – Mark
    Sep 28, 2018 at 21:50
  • $\begingroup$ Build time should go up with roughly the cube of the water depth. The abyssal plain is about 4000 meters, 30 times as deep. Assuming the erruption didn't stop it would take roughly 30^3 or 27,000 times as long for it to break the surface. $\endgroup$ Sep 30, 2018 at 14:18

Hawaii, as a case study: Millions of years

From https://dhrititimelineofplatetectonics.weebly.com/formation-of-hawaii.html :

This is how the islands first became to form 40 million years ago even though some were able to start developing 70 million years ago.


The oldest island of Hawaii is Kauai which formed 5.1 million years ago, then Niihau which formed 4.9 million years ago, then Oahu, Hawaii's third largest island formed 3 and a half million years ago, followed by Molokai which was formed 1 and a half million years, then Maui, Hawaii's second largest island formed a million years ago and lastly The Big Island of Hawaii which formed half a million years ago and is the youngest of the Hawaiian chain. Another volcano named Lo'ihi is being created currently and is expected to emerge from the seas in 10,000 to 100,000 years time as it is still 1000m below the ocean's surface and become an island.

From this quote, we can see that from beginning (eruptions on the sea floor beginning) all the way to having an island, it takes millions of years. Individual islands can form concurrently, however, and emerge next to each other within the space of tens or hundreds of thousands of years.

  • 2
    $\begingroup$ Lots of useful information, but I feel that the question is about a duration of a specific step in the process (volcano to an island with vegetation), so the age of a younger parts of Big Island (100,000-300,000 years) should be more to the point. $\endgroup$
    – Alexander
    Sep 28, 2018 at 19:34

Rangitoto Island in New Zealand is an example of forming and being covered in vegetation in 600 years. From Wikipedia,

Rangitoto was formed by a series of eruptions commencing at least 6000 years ago.[7] The most recent eruptions occurred between 550 and 600 years ago in two episodes, 10 to 50 years apart, and are thought to have lasted for several years during the later shield-forming episode. The first recent episode erupted most of the volcanic ash that mantles neighbouring Motutapu Island, and produced the lower, northern scoria cone. The second episode built most of Rangitoto


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