Tolkien wrote on The Silmarillion about Tol Eressea, an island that the vala Ulmo (a god-like figure) used to ferry the elves back and forth across the sea into the Western lands. José Saramago also wrote a novel (The Stone Raft), about the Iberian Peninsula breaking off the rest of Europe.

Trouble is, I have asked a question where I was answered that breaking two continents apart very suddendly would be so catastrophic that people would very hardly survive, especially on the coast.

I would like to write about a portion of land breaking off from the main continent and becoming an island. On top of that portion of land, there would be a city with people on it (their technological level would be ancient or medieval).

Now, I don't care about the damages on the city. But I would like that a substantial number of people would survive the event. And then that those people would be carried on top of that island from the continent of origin to another continent on the span of a lifetime

My question is two-fold:

  1. Is it possible for such an event to occur without the intervention of a god-like Ulmo figure? (Note: I want a geological explanation: No human intervention and no "we thought we were on an island but it was actually the shell of a giant turtle the whole time" kind of twist)

  2. If there is no other way to explain it except for the intervention of a god-like Ulmo figure... how would the people fare on that island? Would the ferrying cause massive earthquakes and tsunamis throughout the entire journey that would kill all my travelers? Is there any way to avoid this? (Edit: For clarification, the divine intervention would be limited to physically moving the island around, not protecting the city).

(Edit: I wouldn't like answers about moving land bridges, but true islands)

  • $\begingroup$ Spoiler alert. You might also see This 19th century novel (Gutenberg link) suggesting another mechanism would could still be useful to you. $\endgroup$
    – Chris H
    Dec 12, 2016 at 8:59
  • $\begingroup$ Then there's the Spider Monkey Island approach proposed in Hugh Lofting's Voyages of Doctor Dolittle $\endgroup$
    – sq33G
    Dec 12, 2016 at 11:15
  • $\begingroup$ Maybe it’s like San Serriffe? There was also a floating island in Doctor Doolittle but I don’t think the movie explained it (I’ve not read the original stories). $\endgroup$
    – JDługosz
    Dec 12, 2016 at 11:25
  • $\begingroup$ Not forgetting Noah's Island. $\endgroup$
    – OrangeDog
    Dec 12, 2016 at 13:40

12 Answers 12


There are two options here that I can see:

1) Continental drift on this world is very, very fast. On Earth, you're looking in the order of centimetres per year, or metres per lifetime. For your story, you want somewhere around 100,000 - 1,000,000 times faster than that - 100K times faster would mean that the continents would be hundreds of km apart in the time it takes for a baby to die of old age (assuming a long life-span; halve it if everyone tends to die by 40) and 1M would give you thousands of km in that time (so trans-oceanic distances). If it's just continental drift carrying them apart, then no need for a catastrophic event - however, if the plates are moving in the order of 100m-1km per month, then it's hardly going to be unexpected either.

2) Your bit of land isn't much like any rock on Earth. The simplest option here is that through some strange set of geological circumstances, a bit of land is composed of rock that is less dense than water. If the two aren't very well attached (note that they're not continental plates here) then perhaps the light part could literally float away. Perhaps sea level rise causes the whole slab of rock to float up, whereupon currents gradually carry it away (maybe it moves slowly because most of the time it sits on the sea bed, and only moves at very high tides). Moving and stopping along the sea-bed would probably cause some earthquake-like issues, but nothing that couldn't be built around.

  • 21
    $\begingroup$ 2) There are rocks that float - volcanic pumice. So your island is a big chunk of pumice with soil on top. Alternatively, there are floating islands that are mats of vegetation. If you want to get into alternate biology, the island is grown by coral-like creatures who incorporate gas-filled lift cells into their construction. $\endgroup$
    – jamesqf
    Dec 11, 2016 at 19:00
  • 2
    $\begingroup$ Indeed - by 'not like any rock on Earth' I meant more that (as far as I know) there aren't any huge chunks of pumice large enough to build towns on. Hadn't thought of the vegetation angle though, that's a good idea. $\endgroup$ Dec 11, 2016 at 19:03
  • 5
    $\begingroup$ ...and, as we will see as soon as Beaker resurfaces, floating islands constructed of our new super light stone are the only practical solution to the overpopulation problem. This is Dr. Bunsen Honeydew saying, be sure to tune in next week when Muppet Labs will present another geophysical breakthrough -- super dense water! $\endgroup$ Dec 11, 2016 at 19:58
  • 1
    $\begingroup$ MattBowyer: Your option no. 1 is not what I'm looking for, because, as you said: "if the plates are moving in the order of 100m-1km per month, then it's hardly going to be unexpected either". Your option no. 2, OTOH, I could use. In fact, as soon as you mentioned it, an island of pumice rock came to my mind (for that, I thank @jamesqf also). Now, what would it take to be able to have such an amount of pumice as to make a floating island? $\endgroup$ Dec 11, 2016 at 20:49
  • 3
    $\begingroup$ @MatthieuM. : Not just continental collisions. Imagine you're constantly rubbing two plates of rock together at 1.3 m/day under the sea. Thats one heck of a lot of frictional heat. A world like that is never going to stop shaking. Or boiling. $\endgroup$
    – Joe Bloggs
    Dec 12, 2016 at 10:12

I am surprised nobody has mentioned those yet...


The main disavantage is that, obviously, it cannot sustain agriculture, but a population based on hunting and fishing and whaling could be based in an ice shore.

And some of them are large, with over 11,000 square kilometers and last for several years.

Although in Earth the bigger icebergs are produced from the very unhospitalable Antartida, some of considerable size (up to 260 sq km) may appear in more "comfortable" places like Greenland, and I understand from your question that your setting won't be Earth.

  • $\begingroup$ I liked this answer very much, but unfortunately I was picturing putting my island on an equatorial / tropical zone. $\endgroup$ Dec 12, 2016 at 20:21
  • $\begingroup$ Some reason you can't have land on top of an iceberg? How about an unusually low density and very thick permafrost layer? $\endgroup$
    – Samuel
    Dec 12, 2016 at 20:55
  • $\begingroup$ @Samuel You can put land on an iceberg, but it would be still frozen. Nothing would grow on that land. $\endgroup$
    – SJuan76
    Dec 12, 2016 at 21:01
  • $\begingroup$ @SJuan76 Things certainly grow where there is permafrost. Not really well, granted, but feasibly enough to scrape a living. No? Especially, since it's now an island, when supplemented with fish and other sea nutrition. $\endgroup$
    – Samuel
    Dec 12, 2016 at 21:04
  • $\begingroup$ @PedroGabriel How about this Iceburg rams,super ocean current, your island, and it starts sinking. People move on to the iceburg to live. $\endgroup$
    – cybernard
    Dec 12, 2016 at 23:00

Back in the last ice age there was a volcano near a lake that itself was near the ocean. The area was lashed by frequent storms dropping very heavy rain.

The rain tended to wash the pumice from the area down into the lake. By some handwavium process this fused into a solid mass of pumice.

Civilization comes along, people discover this area on the seacoast made of pumice--you can dig your home into it rather than actually build one. Cheap housing, people flock to it.

What nobody realizes is that once the ice age ended and the ocean level rose to normal the terrain of the ancient lake and surround is now below sea level. The great mass of pumice is actually buoyant but stuck in the mud. The constant tugging of the tides has been slowly wiggling it loose for millenia, one day it finally comes loose. The whole city bobs up (nasty earthquake for the inhabitants but not all that lethal), the area was flat enough that the bottom clears the terrain, ocean currents take over and off it goes.

Surviving an ocean crossing before running out of food and water will be problematic, though!


I missed the obvious

There are literal "floating islands", both man-made and natural.

Image via Wikipedia


Just to put numbers on one potential answer...

Volcanic pumice floats, having a density of around a quarter that of water. An enthusiastic world-builder might suggest "well, the city was built on pumice".

The interweb reckons 60-120 tons per house. Let's be optimistic and say 50 metric tonnes on average, just to pick a sensible number. That doesn't include footings though. Pumice is incredibly fragile, so houses built on it will probably need a good metre or so of pad to sit on. Concrete weighs around 2.4 tonnes per cubic metre. If your house footprint is around 75m^2, that's another 180 tonnes per house. So just the housing takes 23 million tonnes. Now think about roads, which be the same kind of construction as house footings. Suppose we have 100km of roads in total (which is probably a very low estimate for a reasonable-sized city), all of them an average of 8m wide, then we need roughly another 2 million tonnes of concrete for the roads.

Let's allow each house another 75m^2 for gardens. Then the city has a total surface area of around 16 million m^2, meaning the city exerts an average downward pressure of around 1.5 tonnes per m^2.

Now we need this to float. On pumice which has a density of 0.25g/cm^3 (1/4 tonne/m^3), floating on water which has a density of 1g/cm^3 (1 tonne/m^3). If Pv is the pumice volume in m^3,

Pv = 1.5 + 0.25*Pv = 2m^3

So a 2m depth of "solid" pumice, across the whole city, would actually float it. As far as Mythbusters goes, this myth certainly at least rates "plausible".

Interestingly, I started out with the intention of disproving this idea. But when I started putting numbers to it, the result has certainly surprised me!

Of course, the resulting floating city would be unstable as hell and prone to break up. Still, there are ways and means. If you supposed the city's geology had a granite (or other solid rock) base layer, maybe 3m deep, then this would be heavier than the city on top of it. If you can make the pumice layer thicker (maybe 5m), add another layer of rock on top of that (maybe 1m), and then add piles to tie the two layers together (these might be added as part of building houses), then the city has a granite "keel" to keep it upright, and a nice reinforced structure to stop the pumice taking all the stress and cracking up.

Of course it's vastly unlikely. But by the numbers, it doesn't look like there's anything which would theoretically stop this happening.

  • $\begingroup$ Good to know! Thank you for these calculations. $\endgroup$ Dec 12, 2016 at 20:12

the problem is the undefined limits of god-like. How much does physics matter?

Assuming: A. they are just applying force (that is to say the island physically moves and does not disappear and reappear at the new location)

B. it is done quickly (aka not on a geologic timescale)

C. it is a normal island, as indicated in the question.

given those assumptions then your island dwellers are screwed. friction alone may remelt the rock of the island. Even if it doesn't you will see richter scale 10+ earthquakes everywhere. not even tree's will be left standing. the displacement of water will create tsunami on epic scales. soils will liquify and pour into the ocean, taking almost everything with them. People just don't realize how much energy is involved in moving tectonic plates.

As for how to achieve it, I have no idea, that is going to depend on the capabilities of the entity doing it. But it will be entirely supernatural, I know of no natural mechanism that exists to achieve it within those assumptions.

  • $\begingroup$ Thank you for your comment... I've edited my post so as to clarify the limits of divine intervention. $\endgroup$ Dec 11, 2016 at 20:51
  • 1
    $\begingroup$ @PedroGabriel That clarification isn't a clarification on the subject $\endgroup$
    – Yakk
    Dec 12, 2016 at 11:30

The mountain cannot come to Mohammed

For centuries, villagers have traveled half a day westward through the low plains to reach the capital city of their kingdom. To the east lies a treacherous sea where the brave, lucky few that have returned describe a week-long trip through a watery hell, but the folks on the other side seem nice enough.

A record-breaking thunderstorm leaves the villagers scared and soggy but generally secure. A band of strangers rides in from the east just as reports of natural disasters to the west hit the gossip mill.

Centuries later astronomers would refer to the event as Near Miss 2893'5, signifying the 5th time Object 2893 had crossed Earth's orbit close enough for potentially observable effects.

Detractors insist this theory relies on unconfirmed conjecture of Object 2893's mass, and the real cause of the sub-continental flooding and emergence of new land bridges was the result of a breech of a local volcanic-formed lake system.


All of the methods I can think of that would cause some portion of land to rapidly become an island without involving massive tectonic or volcanic upheavals or meteoric impacts involve something like a flood, storm, or river outbreak partitioning off the end of a peninsula. But none of those would actually move the land, just remove connecting lowlands to leave it separated. So, that leaves us with divine intervention. As to how the people onboard would fare, it really depends on how fast they're going and how the deity is moving them.

Let's look at the speed first. You ask for movement across an ocean within a human lifetime. For the sake of having some numbers to play with, let's say we need to move 2000 miles (roughly the width of the Atlantic between Brazil and Western Africa) in 50 years. That's ~40 miles a year, ~580 ft/day, or ~24 ft/hr.

All things considered, that's not actually very fast, assuming it's a steady movement. Last month, New Zealand suffered a magnitude 7.8 earthquake. During the earthquake, a portion of one of the faults moved 33 feet in the space of two minutes. That's about forty times faster than the 0.4 ft/min we need to maintain for our journey, which tells me that we're not quite working at a "catastrophic earthquake" level of earth-movement, unless it's moving in fits and starts as opposed to maintaining a constant rate of movement. This leads to questions about our motive force.

If the deity moving the island is physically shoving a mountain across the bottom of the ocean, with all the grinding and friction that implies, that is going to build up a lot of frictional heat, and also effectively cause a continuous low-level earthquake on the island. If it's not able to maintain a steady movement, but rather moves it in shorter bursts between rest periods, that will dramatically increase the earthquake magnitude involved. But it's changes in the movement (acceleration) that are felt more than the rate of movement. Once the island is up to speed and moving steadily, the ride will smooth out laterally, even if there's some up-and-down bumps due to unevenness on the seabed. It's stopping and starting (or otherwise changing speed) that will cause the strongest shocks.

However, you already have a deity moving the island. For the purposes of your story, it can also soften/liquefy the base of the island as part of moving it, so that the island basically "skates" across the ocean floor rather than grinds. This would dramatically reduce friction and heat produced, and also reduce the rumbling/shaking felt on the surface. Picture an ice cube sliding across a tabletop for what I'm talking about. As a "natural" process, this would slowly eat up the base of the mountain/island and cause it to sink, so divine intervention would be needed to explain why that didn't happen.

At the rate of motion implied, there isn't any tsunami danger. Assuming the motion is steady, real-world currents already move much faster than the island is--if the current around the island is flowing in the direction of the island's movement, it would actually be pushing the island forward. (Heck, this could actually be part of the story -- the deity of the earth loosens the island so that it can move, and the deity of the ocean pushes it where it needed to go with its current.)

As for the inhabitants, the people on the island are probably due for a rough time, since they'll basically be dealing with a constant mid-level earthquake for fifty years. It's not ideal, but mankind is adaptable, and they'd make it work. Tents or single-level wooden structures that can flex with the motion would rapidly become the dominant architectural style. Stone walls would be kept low (e.g. boundary walls) and would need to be carefully engineered if used, and stone or heavy-timbered roofs would probably be considered a bad long-term idea, even if they were strongly built to begin with.

Water travel around the island shouldn't be an issue; like I said earlier, the rate of motion of the water around the island is going to dominate the currents around it unless it'd be otherwise completely calm, so navigation around the island won't be harder than around a normal island. The exception is that long-distance voyages wouldn't be recommended, unless you're certain that you know what course the island is moving in. Shorter trips up to a week or two shouldn't be a big problem as long as the weather remains clear -- the island's moving less than a mile a week, so as long as there's any reasonable promontory on it, it should be visible on a clear day from your departure point.

Honestly, what might be the bigger issue is the population relative to the size of the island. It's entirely possible that the resources of a small island could be depleted before the journey is complete. Fresh water especially could become a problem, if groundwater is disrupted and not replaced with sufficient quantities of rainwater. Again, divine intervention may be required. I'd suggest looking at modern-day Pacific islands to get an idea of what size island can support what size population long-term.

TL:DR - Try to do it via real-world methods and destroy your planet and everyone on it. Get a deity to move your island at a slow, steady, even rate, and you should be fine. Your sustainable population is probably about the same as any normal island, assuming a reliable water supply.

  • $\begingroup$ Upvote for the liquification of the ocean floor and the ice cube sliding across the table. Which takes care of the earthquake issue. It would be quite windy probably, but then, islands generally are. $\endgroup$ Dec 12, 2016 at 11:05
  • $\begingroup$ No windier than islands normally are. My answer postulates moving about a tenth of a mile over the course of a whole day. The faintest breezes are already moving several orders of magnitude faster than that. $\endgroup$
    – Salda007
    Dec 12, 2016 at 12:07
  • $\begingroup$ Perfect. Now, about the liquification -- I'm thinking that the heavier the island, the easier that would be. Some googling leads me to believe that water is the key -- water saturated rock and / or sand -- the very definition of an ocean floor. One wonders that not more islands skate across the ocean. $\endgroup$ Dec 12, 2016 at 12:47
  • 1
    $\begingroup$ Up-voted because of your thoughts regarding inertia and the living habits of the islanders. $\endgroup$ Dec 12, 2016 at 20:40

This only requires two conditions for it basically to work. (1) The land between the mainland and what will become the island is low lying terrain. (2) There is a sea-level rise due to the end of the Ice Age.

The seas simply overrun and submerge the lower land between the continent and the higher region that will become the island.

Elements of this answer have also been suggested by Loren Petchel and Patrick Trentin. However, this answer was devised before I saw their answer and comment respectively, but they did get in first and more power to them.

This answer provides a plausible geological mechanism for a geographical change of this kind. Locally the islanders wouldn't experience a catastrophic change, the rising waters could take place over months or even years. They could simply progressively adapt to their changing circumstances in becoming islanders.

The divergence from events in our world is the establishment of an ancient or medieval civilization in late stages or the end of an Ice Age.

EDIT: Salda007 pointed out I had missed the part about moving the island from one continent to another with the span of a lifetime.

Can this be done by natural forces? Absolutely not, unless you want the mortals living on the island to perish.

Therefore, this intercontinental movement must be due to a godlike super-being. The island is levitated off its base, but the below the waterline, and then set in motion to cross the ocean. Godlike super-beings are well known for being sensible about islands from one location to another.

A levitated island will not experience any earthquakes or other seismic events. Its passage across the ocean will be gentle and stable. No sea sickness for the islanders. No shaking, rattling or rolling of their city too.

If the island moved too quickly, erosion would soon reduce the island to almost nothing. This is not good for any mortal islanders. So by moving the island slowly and in gentle manner the erosion is no more than of tidal motion and the island will be comparatively intact when it arrives at its new continent. The city on the island doesn't need any protection and the god can concentrate of moving the island physically.

Once the island reaches its destination, the god will need to prepare a platform for the island's base to settle upon and the island can be lowered to the sea-floor. The islanders and their city will have experienced the journey of, quite literally, a lifetime.

Remember to always sail with Deity Movement Islands! See the sea at the leisurely pace of your lifetime.

  • 1
    $\begingroup$ You missed the part of the question where they ask to have the island travel from one continent to another, which a change in sea level wouldn't accomplish.. (FWIW, I started my answer along your lines, too, before I re-read the question.) $\endgroup$
    – Salda007
    Dec 12, 2016 at 9:10
  • $\begingroup$ @Salda007 Yep! You're right. How annoying and after coming up with what was a sensible answer to find it was hot down by the second part of the question. $\endgroup$
    – a4android
    Dec 12, 2016 at 9:23

You have suggested either a scientific explanation or a supernatural one, and have some good answers regarding each path.

There is a third path: art.

How did Saramango explain things? Perhaps not at all satisfactorily from a hard-science point of view, yet presumably the novel works anyway.

In the story "The Distance of the Moon," Italo Calvino adopted the (true) premise that the Moon is gradually moving away from the Earth (spiraling outward in its orbit), and wrote a story about things that happened when the Moon was so close that people could climb from the Earth onto the Moon. Even a moment's though will tell you this is nonsensical in several ways (the cataclysms predicted upon your island's separation from the mainland are nothing compared to what would happen from the Moon passing so close to the Earth, for one thing) yet it is a good story.

Another writer about a hundred years ago observed that many tall buildings were being built on the lower tip of Manhattan, and wrote a story in which part of that island broke off and sank into the sea under the weight of those buildings. Anyone with a slight knowledge of the geology of New York at the time could have said this was impossible, yet the story was pleasing enough to be published. (I read this story a few decades ago, so I do not recall clearly who wrote it. It is possible that the story I'm thinking of is "The Tilting Island" by T. J. Vivian and G. J. Bennett, anthologized in Science Fiction by Gaslight by Samuel Moskowitz, but I have not found an actual copy of the book to confirm this.)

So whether your villagers can survive the trip without explicit divine intervention is a matter of how you write the story.

  • $\begingroup$ Thank you for your input. I certainly know that both Tolkien and Saramago wrote their novels without much regard for plausibility and noone blames them for it. But we're on a worldbuilding Q&A site and I thought it would be interesting to see if we could find a fresh angle on something that had already been done, So, instead of having readers say "Woah, a floating island that carries people around" they would say "Woah, a plausible floating island that carries people around". But still, up-voted for art's sake. $\endgroup$ Dec 12, 2016 at 20:15
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    $\begingroup$ @PedroGabriel Thanks for the compliment! It did occur to me (after posting) that while some details of this answer may be of interest, it avoids answering at least one essential part of the question, as your comment points out. $\endgroup$
    – David K
    Dec 12, 2016 at 20:31

Assumptions: (1) the distance to be traveled is relatively short, (2) the sea is relatively shallow, (3) a volcanic fault lies approximately along the direction of travel, and (4) a series of wildly improbable coincidences are acceptable. The scenario envisioned is that of an island crawling amoeba-like across the sea...

The people are settled on a long low peninsula extending in the direction of another continent visible on the horizon. The fishing is good and they build a city on a hill at the very tip of the peninsula.

There are a series of earthquakes causing the ground that connects them to the mainland to suddenly subside beneath the waves, but they stay put as their city is unaffected and they are happy to be separated from their warlike neighbors. Following this, for a decade the land beneath the seafloor begins to gradually bulge upward like the lava dome in mount St Helens until it emerges from the sea first as a marsh and then dry land. Earthquakes on the hill where the city is built prompt the people to abandon it and move to villages on the new land.

The hill where the original city was built turns out to be a volcano and begins to erupt. Meanwhile some distance offshore another volcano erupts and emerges from the sea. The people live peacefully on the land between these volcanoes for some years as they both grow. The sea volcano produces copious lava that spreads into the sea and eventually connects it to the island where they live; another long tongue of lava spreads in the other direction. The sea-volcano dies down, but eruptions on the far side continue to raise land along the fault line as lava spills out in a chain of cinder cones. These deposit soil-like ash atop the lava. After another ten years the island is now a long linear reef anchored by two volcanoes, with more cinder cone islands further out.

But the volcano under the old city erupts more violently and the land continues to rise, and suddenly it blows its top like Santorini, leaving only a few rocks sticking out of the sea where the city once was. All that remains is a single temple atop a sea-stack, which the people take as a sign they will survive and be blessed on the islands.

Encouraged, the people build a new city on the far side of the now dormant sea-volcano. Thick ash has filled in a hollow between the volcano and a cinder cone and it has hardened into tufa, into which they dig cave-dwellings. Some grasses and shrubs even grow here on the surface of the fertile ash. Most of the rest of the island is bare rock.

The volcanoes further out to sea continue to grow, little cinder cones connected by sandbars like a pearl necklace made of embers and ash. The people explore but don't settle yet. Over the next 10 years the nearer cinder cones fizzle out and grow cold, and grasses and shrubs colonize the fine ash on their slopes. The land that rose a generation ago is now sinking into the sea again, as the lava chamber that pushed it up has emptied. Thirty years after being separated from the mainland, the land is completely submerged and the people have all migrated to the new city in the tufa. Their economy is almost all fishing, and there is little fuel except driftwood, but it's a living.

The next event is a sudden earthquake. It turns out the fault is actually along a submarine cliff. A great submarine landslide takes 3/4 the sea-volcano into the depths, causing a tsunami that ravages faraway kingdoms. Incredibly the people's town is unaffected, now perched on the edge of a brand new cliff. Again divine favor is acknowledged. Prudent, the people begin to look for a new place to build, and find a location far away among the cinder cones, where the waves have filled a low lying area with sand and driftwood. A new town is built here and the people slowly migrate here, abandoning tufa-town.

Within 20 years the remainder of the sea volcano has slidden into the abyss, along with tufa-town and several of the nearer cinder cones. Sand-town is well established and seems safe from the pattern of landslides, but the waves are eroding the cinder cones and it is apparent they will not last more than a few more decades. It is found that the furthest cinder cones lie near the mouth of a great river delta on the far continent. After several unusually stormy years, silt extends the delta until it connects with the chain of cinder cones, while the islands are eroding with alarming speed. The delta is new land and unclaimed, and seems tectonically stable, so the people begin migrating there where they build new villages.

As the last of the cinder codes are washed away by the sea, the people have completed a land-transition across the sea to a new continent in the span of some 50 years, leaving virtually no trace of the moving land bridge behind them.

  • $\begingroup$ Thank you for your answer. But it seems you're describing a moving land bridge, instead of a true island. That's not exactly what I was searching for, because, as you said, it implies that the distance to be traveled is short (when it is not, I want them to cross an ocean). Also, this would mean the people would have to constantly rebuild the city, which I didn't want to happen. $\endgroup$ Dec 13, 2016 at 21:27

Just thinking about the physics, an object in water is buoyed up by a force equal to the weight of the volume of water it displaces.

Weights for various materials

  • Dry sand coarse - 1.6 tonnes per cubic metre.
  • Topsoil (some moisture) - 1.44 tonnes per cubic metre.
  • Ballast - 1.76 tonnes per cubic metre.
  • Gravel - 1.92 tonnes per cubic metre.
  • Water - 1 ton per cubic metre

If you want the island to have any kind of topography above sea-level it will need at least 2x the volume of open airspace beneath the surface for each square metre of total land mass (that exists both above and below sea-level).

If the island is a technological wonder of engineering (rather than a natural phenomenon), the weight of the vessel's construction materials and machinery (for propulsion and life support) would also need to be factored in, especially whatever is required for an internal structure strong enough to bear the weight of the land on top. Add a bunch of military and transportation hardware to supply the island and keep the pirates at bay and you're really cranking up the weight.

If we are talking about a highly advanced construction there is current, real-world research that's producing materials which are as light as Styrofoam and as strong as titanium but the costs and energy requirements for construction of a floating island using these materials would be vast - there would need to be a very good reason for having to build it.


In modern ship design there is at least as much ship beneath the water-line as above (and that's without covering them in topsoil). So there would either be a large volume of open areas beneath the surface of your island, or a huge amount of expanding foam filler ;)

Since it has come up a few times; A cubic metre of pumice stone weighs 0.64 tonnes so you would need roughly 1.5x more to achieve the same effect as a cubic metre of air void. Let's say roughly approximately 3cu/m pumice per cu/m of land mass. That's a lot of rock beneath the surface and it would need to be perfectly impermeable.

Apart from that, the biggest problem will be scraping all the barnacles off ;)

  • $\begingroup$ Incidentally, my favourite floating island is Laputa. $\endgroup$
    – Bob
    Dec 13, 2016 at 15:19
  • $\begingroup$ Sorry I realise this doesn't answer the question as such, but it helps imagine how deep the fault would need to be. $\endgroup$
    – Bob
    Dec 13, 2016 at 15:25
  • $\begingroup$ How about if there was a massive oilfield under the land mass that was extracted and pumped full of water, then some ancient buried seeds germinated and rapidly grew huge root systems in the water eventually pushing the land apart (a bit like a tree root under asphalt). $\endgroup$
    – Bob
    Dec 13, 2016 at 15:28

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