How would a corrosive sea not erode the foundations for islands?

Question

My world consists of a large number of islands laid out in a hexagonal pattern, each one circular and large enough to support a small community. At the center of each island an obelisk sticks out of the ground, ancient and seemingly indestructible.

In between the islands is a sea made of a liquid called kesh. From the island dwellers perspective, there are three main properties of kesh. First, it slowly but fully corrodes anything it touches down to component molecules, and complex molecules into simpler ones. Second, it is of a fixed quantity - you can't make kesh into anything else, but neither is anyone able to make more if it. Third, it's very heavy. Just about anything floats in it, and if it doesn't at first it will when it's been broken down.

The reason the islands are still there is that the obelisks repel kesh from them, up to a certain distance. Legends say that ancient people made the obelisks to fully protect their land back when it was a whole continent, and their power and protection radius has dwindled over time - but no one knows for sure. Anything that falls of the islands eventually dissolves, turns into dust that floats on top, and is moved by waves or wind until it drops down on an island again so all matter eventually circultes. You can move between islands by flying, or by boat if you don't mind constant hull replacements. Water runoff make thin layers on the kesh that tend to quickly evaporate again. Kesh remains liquid at all temperatures.

This is all information that the Islanders know. Now, here's my problem: I want these islands to all be fixed in position, same as any island in the real world. But by this logic, the kesh would eat away at the foundations and bedrock until all the obelisks were independent, and they would start floating around freely with air bubbles around them. How can I set things up so that the islands would remain fixed, without changing anything from the islander perspective?

Bonus points for explaining how the bottom of the sea works. I would prefer for there to be bedrock down there and not magma, but it's not necessary since no one will go there.

I will also mention that there is magic in the setting, though it's a fully separate system from however the kesh and the obelisks work and can only be used by living things. This means I'm not averse to fantastic solutions as long as everything is consistent, but it should be no more complicated than is needed.

Answer 1

Cylinders, not Spheres

There is no explanation for why the obelisks repel the kesh, so its mechanism is up to you - so rather than a simple radius, the obelisks repel kesh in a circle that is of fixed radius but unlimited vertical extent. (Or effectively unlimited.)

So as to prevent the kesh from dissolving its way down to the core of the planet, why not have some magic energy generated by the core of the planet itself that the obelisks channel? So the kesh cannot penetrate beyond a radius of X km from the core, and the obelisks channel that same energy in vast cylinders of power that taper off somewhere outside the atmosphere?

Answer 2

Kesh is a runaway grey swarm and the obelisks have a counteracting grey swarm.

The kesh is a biological, magical, or technological grey swarm, that eats everything it touches. The obelisks are a corrupted version of it, that emits its own swarm that feeds off the kesh within a certain distance. The kesh can't eat the foundations because the obelisks eat it.

The bottom of the ocean is too high pressure for the kesh to function.

Whatever makes it work doesn't work under high pressures. If you go deep enough it fails to work, so it doesn't eat through the planet.

Answer 3

Gold

The obelisks are made of gold, which is very non-reactive. Furthermore, it is as simple as it gets -- it's elemental. You can't break down molecules that don't exist.

Were they of silver or iron or copper, or most other metals, there might be a problem with the surface tarnishing/rusting/acquiring verdigris and the kesh breaking that down in a manner that erodes the metal. But not gold

Answer 4

Foundations already reacted with kesh

Let's put aside answers that the bedrock or things like sand are probably not reacting to kesh. There is a very simple reason you've already given in your opening statement. Stuff reacts to kesh and is reduced by it to a dust. This dust doesn't react with kesh anymore, or if it is, can't be reduced further.

Sure it'll float on top of the ocean at first, but what happens later is more interesting. The dust settles on a beach or is brought together in the ocean. With enough pressure it can start compacting. On the ocean it can get enough weight to start sinking, getting to greater depths with more compacted dust where it'll compact to bigger sizes thanks to the pressure depth. On land it'll just start geological processes that over the years creates more heavily compacted dust and rocks. Both ways can create the core of the planet. So you could even have basically added the kesh ocean to any planet, watch it dissolve into the crust and disappear. Over time the ocean will be pushed back to the surface as heavier things are created by all the processes, leaving a sturdy bedrock and even islands.

When the obelisks are placed, they can stand on solid already reacted with kesh rock. The whole oceans are likely still full of dust and compacted dust not big enough to sink yet, so the obelisks allow for easy placement of the kesh sediment. This can then start changing to things that can be reduced by kesh again.