Decay comes with dense ore worlds and become more unstable the more times you add that symbol. I did a world with 30 dense ore symbols it crumbled in just a few minutes, however there was more ore than stone on the world.
If you're going to exploit worlds like this I would highly suggest a couple different strategies.
Use EE tools like destruction catalyst to get as much as you can and link book back each time. Alchemical bags are great for carrying back tons of goodies.
Build a platform that doesn't connect to any blocks below and set up a quarry. From what I've seen unconnected platforms will not decay. This is a safe method, but slow to set up. Its useful if you don't have any EE tools. Works better on low decay worlds I would suspect.
Keep moving in a straight line. Only loaded chunks decay. If you can move fast enough you won't have to worry about decay.