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Even the tallest mountains will be ground to dust with time. Given, of course, nothing sustains them (like tectonic activity which prevents mount Everest from eroding away too quickly, even making it taller each year).

Let time pass and the elements will erode away anything that isn't supplied materials quicker than it is being ground down.

And so the question arises: What could prevent floating islands from becoming floating clouds of dust or flat layers of floating dirt?

My best guess at the moment are wind currents taking matter (dust and particles) from the surface up to the top of the islands, so they settle as sedimentary layers over the island's surface.

This would mean the system grinding down the islands is the same keeping them from dissapearing or losing all geographical features. Which... I'm not too sure about.

In my world the islands are locked to certain points of a magic spherical layer that rotates over the planet's surface around its axis.

A good answer will provide a solution for the question that allows for (more or less) consistent renewal of lost material.

All suggestions are accepted and appreciated. Make those creative juices flow!

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    $\begingroup$ Does this first edit solve those issues properly? @JBH $\endgroup$ – El Nitromante Jul 19 at 18:55
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    $\begingroup$ Floating in water, or floating in the air? $\endgroup$ – nick012000 Jul 20 at 3:57
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    $\begingroup$ Lava + Water = Cobblestone Generator! In all seriousness, nothing in permanent in nature. $\endgroup$ – Aron Jul 20 at 4:15
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    $\begingroup$ Handwavium destined be fixed with handwavium - what else one would expect from it. $\endgroup$ – MolbOrg Jul 20 at 5:37
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    $\begingroup$ I may be mistaken, but you seem to be ignoring that fact that most NON-floating islands DO NOT have volcanic or tectonic activity to increase their size. Or, at least not frequently in terms of human lifespans. Yet, they manage to maintain their surface area! By your reasoning, how is this possible? I think you are missing some critical thinking here, but who knows! $\endgroup$ – Mark G B 2 days ago

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Nothing. Eventually - over many thousands of years - a floating island will indeed be diminished until it's nothing but dust.

However, individual grains of floating island don't themselves float forever because they aren't magically powerful enough. Instead, they gradually sink back to earth and are deposited in sediments and then compressed under great pressure, becoming new metamorphic rocks. In this process, the density of magical particles rises enough that the new rock would float... except that it's still underground. Every once in awhile, the erosion of the surface layer of rock will advance enough that the inner, floating layer escapes and becomes a new floating island.

Through this cycle, though individual islands are born and diminish, the overall amount of rock in the sky at any one time remains relatively constant.

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    $\begingroup$ +1, this is what I would have suggested. It's interesting to think about what the island birthing process would be like. Maybe there are geologically active regions where, in addition to earthquakes and volcanoes, there is a risk that a new island will burst up from the ground. Or alternatively, perhaps it's a more peaceful process, in which in a few places in the world there are hourglass-shaped mountains or unnaturally huge overhangs, whose peaks will eventually break free and float up into the sky. $\endgroup$ – Nathaniel Jul 20 at 8:26
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    $\begingroup$ We have floating islands, we call them "icebergs" $\endgroup$ – Jasen 2 days ago
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Vegetation

Thanks to animals and plants you can grow your island. Roots can overhang or pierce the island, getting nutrition out of the water and deposit it on the island in one way or the other. Even just sucking the ground water inside the island can help attract nutrients. Vegetation is crucial to hold things together and prevent erosion as well.

Creatures will live there, able to get more out of the water and deposit there. It'll be a large nutrient rich environment that has a possibility to grow.

An example is boats. Even though they don't hold nutritional value, they do offer purchase for some creatures. Barnacles can thrive, essentially growing the boat. It is such a mechanism that has the potential for a growing floating island.

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  • $\begingroup$ Organic weathering breaks down rocks too, so this may not be a complete answer to the problem. $\endgroup$ – rek Jul 20 at 0:01
  • $\begingroup$ @rek it does. I can imagine many more reasons why the island would shrink again. The idea is only to give the island a chance to grow. That is why I say it's a possibility, not a given. $\endgroup$ – Trioxidane Jul 20 at 7:19
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    $\begingroup$ In some stories, a floating "island" turns out to be nothing but vegetation--A giant floating mat of living plant matter. $\endgroup$ – Solomon Slow Jul 20 at 17:55
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    $\begingroup$ I wanted to write something similar, but point out that vegetation CREATES new soil, making the island floor thicker. $\endgroup$ – Fels 2 days ago
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    $\begingroup$ I second Fels' comment. Vegetation literally turns air and rain into soil, which is exactly what you need. Most of the carbon in trees is from the air, NOT the soil below. $\endgroup$ – Wayfaring Stranger 2 days ago
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Where on Earth did THAT come from?

A magical floating island BY DEFINITION is violating the laws of physics. But we want some reasonable plausible ways that these still exist. I will give some different options, some of which may or may not fit.

First, we need to know how the islands are created in the first place, and why they don't fall. I'll assume island creation is intentional (someone makes island when they want one) or natural (a process causes them to arise). If the creation of new islands exceeds erosion, problem solved. So creation is either rare or non-existent, or the question is moot.

  • Island generation: If the islands follow a sort of track/ley line around the world, with all islands eventually going full-course, the "track" can eventually end up going inside the planet at some point. It sounds like the islands follow a sort-of almost LaGrange point, with the matter of the islands carried along in it. The oldest islands crash (if there's anything left of them) into the ground somewhere (possibly building a mountain) and when the track emerges on the other side, it rips a chunk of rock and surface out with it (the basis of a new island)

Second, the island needs to be supported by all of its mass, not a point source, or else it would fall apart. So what IS or ISN'T island needs clear definitions. Further, the islands (to be perpetual) must be able to gain mass that might be lost by various means. I'll assume that there is a kind of gravity pulling island matter (IM) together. Any matter touching IM gradually becomes IM, and any matter lost to the island gradually becomes normal. Islands will thus tend to become somewhat spherical. Still-living matter cannot be IM, or else you'd have some VERY strange effects like people and trees growing upside-down (you may still have some very odd effects when birds swallow stones, or dead things convert to IM). Locally, the force creating IM must be sufficient so that you are not constantly losing some matter at the bottom where gravity and IM force are at maximum competition. How you handle water is up to you. If it sticks, then your islands will rapidly drown out and be balls of dirty water, so I'll assume it doesn't. If it doesn't, your islands will likely be arid to desert-like

Loss is not inevitable if the force is sufficient. Like dust collecting to form an asteroid, this could be spontaneous. So a vacated point would gradually trap all the dust that blew there. But your question seems to assume loss of material, so how do we replace it?

  • Birds and bats and guano: Your islands will be havens for birds and bats. But especially if they are dry, they'll likely be only mediocre food sources. So birds and bats build nests, dwell in caves, and fly down to the surface or mid-air to eat seeds, bugs etc. If the island is dry, they'll likely bring nesting material from elsewhere. All these seeds, guano, and nesting material will become IM over time, building the mass of the island.
  • IM plants: Perhaps plants CAN become IM. They grow best on the top, but IM lichens, moss, etc. grows all over. As these plant grow, they fix carbon and nitrogen from the atmosphere, adding to the mass of the island. Vine-like masses may bind soil to keep it in place. The plants may have cactus-like abilities to trap water so it can't drain away
  • Lime scale: At some point in their travels, your islands touch down briefly in the water. This is also a great way non-magical people and animals can be introduced to your islands. Your islands, being dry but allowing water to leave, soak up large quantities of water. Then they slowly dry out, leaving behind all the dirt, salts and minerals that were present in the water. So like hard water buildup, the islands harden from all those minerals and gain mass.
  • People (broadly defined): In a magical world, floating islands are likely prime real estate, especially if you can fly, and aren't simply a traveler who hopped onto an island in the ocean that suddenly took off. But these islands don't have a lot of wood, or hard rock, or other building materials on them. So people bring cut stone, wood, food, loot (if the islands are wandering pirate bases...) and so on. People die and are buried. You could have thousands of years of ruins, graves, and midden pits littering the island, which might be more ruin than island.
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Magical Stasis

The floating Isle can't erode since the spell binded everything together. You can see the wind taking part of the dirt and some flowers away, but at the same time you see how little rocks move slowly towards their original position and little petals in the wind attach themselves after dancing free for a few moments.

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Ice.

The islands are sky glaciers, in large part (or in whole) composed of ice. When it is cold the islands grow from rain and snow deposited on them. In warm weather they shrink. If it were sunny and cold, water might melt on the sunny top and refreeze on the bottom, forming long icicles.

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Just look at what prevents erosion of islands in the ocean! Wind and waves may not be the same but similar methods can be used to prevent them. Vegetation is very good at preventing erosion, and so are coral reefs if you want to make a version of those to grow on your islands. But that's just the tip of the iceberg especially if you're including artificial methods.

You can also continually build up an island. Perhaps it is home to a volcano or it somehow picks up more rock from another source.

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The Magic Binds Them

In the same way as a planet would turn to a dust cloud without gravity, the islands would wear away to dust without magic. The magic provides a force that slowly pulls loose dust and clumps of dirt back to the existing islands.

If the magic provided a small degree of attraction between every particle of the dirt and stone, you would have smaller islands slowly form, drifting until they get captured by the larger islands and eventually merge.

Alternatively, it could be an extension of how your magical sphere holds the islands up. The sphere pulls all dirt and stone within it slowly to the fixed points, providing a mechanism for the islands to grow. The new material is pulled in and compacted, while water cuts canyons that eventually create isthmus and peninsula. These grow until the force of the magic pulling them in exceeds their compressive strength and they snap and "fall" into the rest of the island, creating interesting and unusual terrain.

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Islands are like icebergs - calving from (depending on your preference) from either solidified magical lava spouted by magical volcanoes (my preference) or magical ice fields.

Of course magical material fells off al the time, and drifts down when not bind tightly to other magical pieces. So it spreads all over the planet, making areas with more material more magical. Magical dunes? Magical beaches? Floating magical sandbars? Your choice.

People mine the magical dust. It cannot float but it can make wearer lighter.

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Volcanoes

Each floating island - or at least each one that survives erosion - has a regular supply of new material through a volcano on the island.

If islands can float, there is no reason to believe that matter could not spontaneously form in or be magically transported to the volcanoes, maybe even from the place the island originally was created on the ground?! (Ground based creation is just an assumption, of course...)

Edit: This idea was already a side note in an earlier answer given by @StarSeeker, which I just stumbled upon.

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If you consider erosion, you need to consider how these islands came to existence in the first place. Obsessing over "how do we keep material" while ignoring "how did that happen in the first place" is just pointless - especially as you can ignore erosion in a typical span of a story because it is a very slow process. It can be even a plot point: scientists: "yes, after all those studies we found that erosion is indeed taking place and will ruin our island in few hundred years." people: "These is enough time so let's party." 300 years later: some people: "oh, now we have only a decade left, we need to find a way to slow this down and/or get off the island before our city falls of the edge!" Other people: "why stop the party now, we are all going to die anyway lets enjoy while it lasts."

If you found nothing wrong with the plot point, well... Erosion is much much much slower than that. You CAN'T use it as a such plot point (unless magic floating islands erode much faster for some reason). In our world, you would be looking at 300 MILLION years instead.

Ok, with that clarified, what could be those island generation methods:

If islands appear when there is enough floaty magicky stuff on Earth together to violently erupt and come to the sky, maybe that is all you need to have islands "regenerate". This is Cadence's answer.

If islands are "living" (either trees or coral reefs), you have your regeneration also built in. Many other answers along that way.

If islands appeared because that fine dust clumped together (like space stuff), then that could be your method of renewal too: You can have those islands collide and merge. Each individual piece of magic rock that floats attracts others (think gravity just stronger) and is perhaps attracted to special magical places as well so you have more islands at some places and less at others. Wind erosion will be taking pieces of those islands away. But every now and then, that dust will by chance clump together to create a small flat island. Then two islands will bump into each other, creating a larger island and a new mountain range. Because all magic dust always floats you aren't losing island-making-material, it just ceases to be a part of an island for a while - before it finds other dust and re-assembles.

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The same force that's holding them up

Let's say that there's some sort of gravitational anomaly that's holding the islands up. Let's also say that it's uneven, thus causing the material to rest in clumps. That means that any dust that blows past will tend to settle on the island, at a similar rate to the dust being blown off. If the island gets a bit too big, it's likely that material on the edges will get near the boundary, and fall away. But generally, the anomaly will hold it together.

Think of it as a sand bar. It's being continually eroded, but also deposited, due to the peculiar local currents and eddies. Hence it flows and moves, but maintains its size. Or like the Lagrange points that hold the trojan asteroids together for example, only much more localised.

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Floating islands are made of floating dust that gets progressively liberated from seabed.

Indeed they don't erode, they grow.

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