So I'm writing a book and need help to better explain this island that sinks, more on that in a moment as I give you the basic build of the planet...

First, its a world on a very tilted angle. the south continent like Antarctica would be is mostly a summer paradise with short 6 hour nights.

The world is mostly similar to earth, 24 hour days, but with 15 month years. The north part of the planet is usually cold and always dark, say except for 2-3 months a year the tilt and wobble allows for daylight to hit to the top of the planet giving it roughly 3-5 hours of daylight per day for those 2-3 months.

So the idea is most continents at the equator of the planet, pretty much all of them, experience the regular cycles of earth, seasons ext ext.

But one continent is divided into two islands, the north and the south, the north sister island being a third of the size of the southern. And every year for at the very least 30-60 days. The sea level falls low enough that the part of the island that is now underwater comes above land & connects both halves. And sink low enough, not for mountains, but at least city's or towns to submerge.

Size of the part that sinks would be about the size of mexico if not double that. And it would make up about 25% of the total island when both ends are connected.

While the rest of the world does not sink or rise that dramatically. only this one island or continent mainly.

I was wondering if there is anyway this could be possible so i'm open to most ideas. My best guess was the ice would melt and freeze in such large proportions at the north it would lower and raise the sea level, but this would effect the world as a whole I would imagine... ... ...

Bonus question would also be, would the south be a tropical paradise? or would it burn into oblivion? I'm sorry for all the information, I just figured it would help gear everyone to an answer that would help make such a fantasy world sound a bit more real but still be this insane world. And I'm just unsure if there is any solution to this at all...

----EDIT---- So far I like the moon idea and even having two of them, It does seem really difficult as many suggest but I am considering putting the two on a horse shoe orbit if that would at all help this situation.

  • Comments are not for extended discussion; this conversation has been moved to chat. – L.Dutch Aug 21 at 18:28
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    Can it be on th back of a giant turtle?... – Salmononius2 Aug 21 at 18:55
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    Polar tilt doesn't work as you describe. Unless something really weird is going on, the poles get the same pattern of sunlight, displaced by half a year (northern January is like southern July). – Anton Sherwood Aug 22 at 0:00
  • Non seasonal daytime differences could be caused by precession, but it would be a hard sell as answered in this question – philn Aug 22 at 1:18
  • There is a movie, about a lake creature that rises and scares a bunch of kids, then one kid, trying to investigate, dissapears, so the start to drain the lake, and its an old tractor, filling with gas, rising up, then once it reaches a certain angle, the air is released and it sinks.. if you had some kind of tether, and way to release the gas, then its just about timing and weight management of the island... – BaneStar007 Aug 22 at 6:57
up vote 53 down vote accepted

Give your planet two moons with different orbits, and once a year for about 30-60 days they align on the sides of the planet perpendicular to the twin islands. The tidal forces draw enough water away to lower the sea in that area, bringing out the island.

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    If you can’t move the island... – Joe Bloggs Aug 21 at 6:45
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    I really really like your idea, highly considering it indeed! Thanks so much for your input! – WolvesEyes Aug 21 at 9:34
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    Note that tides aren't really that simple, and there is no "tidal bulge". But the general idea of two distinct tidal periods coinciding seems at least vaguely plausible. (It's how we get spring and neap tides IRL.) However, I'm having some trouble imagining plausible orbits for your moons that would let them both stay on the same sides of the planet for significantly more than a day. Maybe if you had two large co-orbital moons very close to geostationary orbit... – Ilmari Karonen Aug 21 at 11:27
  • The significant problem that follows from this good initial idea is one of getting two bodies (stars create tides too!) that (a) have slow enough orbits that they are both on the same side of the primary for upwards of a month and (b) both create tidal forcing large enough to create ocean changes that are significant on the scale of terrain features and still (c) represent a stable orbital arrangement. Alas, (a) is going to either move the moon out or reduce the mass of the primary while (b) drive the mass of the moon up, but those two requirements both work against stability. It's tough. – dmckee Aug 22 at 4:21
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    Another option with this would be to have binary planets orbiting each other - more mass. – Paul Aug 22 at 11:10

Although Clay Deitas took my first suggestion, of a shallow island with extreme tides, here's a weaker alternative:

The island is actually a large rock made of an aggregate of pumice/scoria and basalt, trapped within the caldera of an underwater volcano. Periodically, the volcano or geothermal vents give out enough gas that gets trapped under/in the rock, and causes it to float, and drift around slightly in the caldera.

As the volcanic activity reduces again, the gas slowly filtering through the island is no longer being replenished fast enough to keep the water from seeping back in, causing the island to sink again.

The increased geothermal activity could be for a number of reasons, such as an alignment of moons/planets, or passing closer to the sun (see Tidal heating)

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    As an alternative to the gas being trapped: The buoyancy could change in response to the salinity of the water changing. Salt water/fresh water is about 2% difference in density, so you'd need about 100 meters of submerged material for every meter of material that is to rise above the waves (In a column-scenario - in a pyramid-, aka iceberg-scenario you'd need less). – bukwyrm Aug 21 at 9:54
  • Yeah the moons are defiantly my favorite idea,,, but I like this idea to, and just out of curiosity, do you think a meteor hitting the area could cause an eruption and create the caldera of the volcano and the scenario of the volcanic rock rising and lowering the island. If not its fine you can tell me, I know I try to push the limits to far lol. – WolvesEyes Aug 21 at 9:56
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    You could have the island sitting on mangma chamber that periodically fills with magma and rises, and then after a certain period overflows from a nearby volcanic island. It would be convenient as well because people could use the isthmus to cross to the non volcanic island, avoiding the eruption. Probably a culture of revering two deities, one wrathful (who causes volcanoes) and one loving (who saves them by forming a bridge.) Kills two birds with one stone. – Clay Deitas Aug 22 at 4:59
  • @WolvesEyes Forgot to tag you in the above ^ – Clay Deitas Aug 22 at 4:59

The island is sitting on a thick shield of igneous rock, under which a huge deposit of something like anhydrite is located. There is a regular cycle of this mineral being wetted, heated, soured - whatever it takes to make it change volume. The shield of igneous rock between the island and the mineral deposit takes care of leveling out inconsistencies in the raising movement, so the island rising is actually 'the' island, with geography largely intact, instead of 'an' island, just a yearly rising of rubble.

Anhydrite is pretty powerful - when accidentally pierced in drilling operations and thereby freshly coming into contact with water, the whole ground above will be lifted (cm/month usually, but that's what creative license is for). There is no natural way of reclaiming the anhydrite in a timely manner, but there should be lots of chemical reactions that work one way above a certain temperature/pH and the other way below that.

Alternatively it is not a chemical reaction, but simply the volume-change of some mineral or polymer(reasons for huge underground polymer deposits: see Oil, or read the Fifth Elephant) in reaction to: phase change/temperature ('wax motor'), voltage ('piezo crystals'), pH ('proteins'), ...

As for the bonus question: It is what you say it is. If you want truthiness, look at the temperature/rainfall/daylight gradients in the real world, and model your world accordingly. Note that 'tropical' usually is also a function of available water, not just of solar power. Available water in turn is totally dependent on weather patterns that in turn are dependent on the exact layout of landmass vs oceans, height of mountains, vegetation (!)... - Or your world does not have salt water and instead is fresh water all around (you'd need some mechanism to extract the salt - soap?) and any landmass low enough to be near the water table gets to be an oasis.

  • I think it would be tough to beat this answer! – Fattie Aug 21 at 9:35
  • I'm liking your idea as well, and thank you very much for all your input! – WolvesEyes Aug 21 at 9:40
  • Thanks for pointing me at Wax Motors, that's a really cool technology I'd never heard of before! – Ruadhan Aug 21 at 14:54

Continents are usually too large to be plot devices.

You can have island sink and rise if it is located on a lake. This lake can have different depth in dry and wet seasons. Rains pour - lake fills - island sinks. Rains stop - lake eventually empties - island rises.

It can even be a pretty large lake, so treat yourself.

Put a single moon almost at Geosynchronous Orbit. Placed just right, from the point of view of an inhabitant of your world, it'll be in the sky for about half the year, before disappearing for the other half. Instead of daily tides there will be yearly ones, and at low tide your island will be above the surface of the ocean.

  • but that floods other places too. (when moon is away from island) – KGM Aug 21 at 9:13
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    Not necessarily. The difference in low and high tide could be the same on this world as it is on Earth. Afaik, our tide doesn't flood everywhere. Combine this with a very flat hidden island, and it'll work. – Kyyshak Aug 21 at 9:17
  • tide here floods other places,too but u're right, that would be small side effect – KGM Aug 21 at 9:32
  • Like your idea as well, but someone else mentioned one about two moons. Overall, they're seeming like the best solution so far for a flood in a precise location. – WolvesEyes Aug 21 at 9:44
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    @trichoplax, tidal amplification is the result of resonance -- the 17-meter tides in the Bay of Fundy are the result of the bay's 12.4-hour resonant period exactly matching the 12.4-hour tidal cycle. Resonant period increases with size, and you physically can't fit a bay with a 1-year period on an Earth-like planet. With such a slow tidal cycle, dynamic effects will be nonexistent, and you're left with just static tidal forces and small tides. – Mark Aug 21 at 22:26

two tectonic plates frequently crash and then move away from each other again, yearly and naturally.

your planet may be young and huge and therefore have lots of tectonic plate movement

your island is on the border of one of the plates.

why this explanation?

It explains much bigger and higher island emerging from the water than the others do

only such islands get flooded (other such islands maybe existing, too)(some moons gravity may flood other places too)

How does such a fast plate tectonics happen?

our planet orbited around a sun like earth, but then galaxys collided,

and the planet got caught in the gravitational field of a white dwarf.

enter image description here (image source)

it orbits very close and extremely fast around the white dwarf,so the planet gets enough light, but also causing great tidal forces, causing the interior to get extremely hot and move extremely fast, causing fast tectonic plate movement. enter image description here

enter image description here

side effects: earthquakes near tectonic plates borders (picture shows earthquakes most of the times happening at tectonic plate borders on earth, taken from source) enter image description here

  • this is an excellent answer, but honestly it's not necessary to use bold - even though I too like bold ! :) – Fattie Aug 21 at 9:35
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    Interesting idea, but would it not cause tons of earthquakes? – WolvesEyes Aug 21 at 9:43
  • it would, but only at plates borders. – KGM Aug 21 at 9:56
  • Then I could work with that, I'll defiantly consider it. – WolvesEyes Aug 21 at 9:58
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    Tectonics aren't fast enough, and plates have too much momentum. A high-speed plate boundary is moving maybe 100 mm/year; at a converging boundary, this might produce 10mm/year or so of uplift. And plates may be slow, but they've got incredible mass -- getting one to change direction takes millennia, not months. – Mark Aug 21 at 22:20

Water takes an absolutely phenomenal amount of energy to boil; if you're planning to remove a few meters of it from a sea, in a month, your planet will indeed be scorched earth.

If however you really want this to be influenced by the sun, and you're willing to reduce how often this effect happens, then you could have the planet be in a binary star system; which exposes the planet to the full power of both stars once every (say) 10 years, and the rate at which the energy needs to be applied to the surface is much reduced.

Bear in mind that the melting of ice/snow on land must be insignificant; which prevents you from having many large mountains, or a land based pole (we have one, which is why global warming results in sea levels rising... if we didn't, the ice would be displacing the water, and on melting the water level would be the same) - but this could probably be explained by the additional erosion caused by the hurricanes/rain that would inevitably come about from such a large amount of water in the atmosphere.

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