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.