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So, suppose we have a very large island. Exactly how big it is doesn't matter too much, but it must be big enough to support a small country.

This is a volcanic island, and the volcanoes on this island erupt regularly and predictably. In this way, lava flows periodically cover parts of the island, enriching the soil with deep-earth minerals and nutrients. Suppose this happens once a decade or so.

The people living here know roughly when the volcanoes will erupt, and are able to respond to eruptions by moving to a safe location beforehand, and then immediately farming on the newly cooled volcanic ground to produce extremely fertile farmland.

How plausible is this scenario? What mechanism keeps the volcano erupting consistently, and how do the locals know when a given part of the island is about to be covered in lava? How do they know where the safe spots are to wait out the eruption? are there any other details that might prevent this, like toxic gasses?

The answer should only use known natural processes. No magic.

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    $\begingroup$ Fair enough -- but don't just tell me! Edit your question so everyone else knows too. You might also add the tag "reality check". $\endgroup$
    – elemtilas
    Commented Feb 11, 2021 at 22:13
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    $\begingroup$ BTW, Lava isn't nutrient-rich. It's rock (mostly hard rock at that). It's the ash layers and ejecta that enrich the soil. It can also take quite a number of years to decades for the soil to recover since you are burying the rich organics. If you make your pralayas (apocalyptic renewals) too frequent, no one will be able to live there. Perhaps once a century would likely be enough. phys.org/news/2016-03-benefits-volcanoes.html $\endgroup$
    – DWKraus
    Commented Feb 11, 2021 at 22:28
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    $\begingroup$ you can have regular volcanic eruptions they won't be predictable but they will be anticipated. As DWK pointed out volcanic rock is only good soil once it has eroded which takes a long time. $\endgroup$
    – John
    Commented Feb 12, 2021 at 0:01
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    $\begingroup$ The "then immediately farming on the newly cooled volcanic ground" is not realistic. Volcanic ground does make for really excellent fertilized soil, but it requires minimum 20 years, more like 150 years of weathering to be useable. $\endgroup$
    – PcMan
    Commented Feb 12, 2021 at 5:28
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    $\begingroup$ This sounds to me a lot like Hawaii. It was an independent island nation built on a series of (somewhat) active volcanoes. Kilauea, for example, erupts every few years. However, the eruptions don't have the perfectly predictable clockwork period that you seem to be looking for. $\endgroup$
    – Jafego
    Commented Feb 13, 2021 at 5:40

2 Answers 2

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Orbital mechanics

Every N days, the planet has considerable force applied by some external process. For example a conjunction of the sun and 2 moons such that peak tides and peak stress is applied to the planet.

This stress isn't enough to rip the planet apart, its direct force is barely detectable without tools, but it is enough to trigger low-intensity earthquakes, and fracture the solid rock at the top of the volcano.

Your people figured out enough astronomy to know that when "these 3 curves intersect - bad things happen.", and can evacuate accordingly.

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Make it a geyser. Like Waimangu Geyser but with a period of 365 days instead of 1.5. That would take a remarkably large underground chamber (illustration), one about 200 times larger volume than Waimangu so maybe 6 times larger in every direction. Depending on your world you might be able to propose some unusual geology that could support such an underground structure.

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