Here is the map of the elemental planes.
Once you go beyond the borders in any direction you get to the part where it is just pure that element, and if you get past that you end up in the Elemental Chaos where it all randomly mixes together.
Mordenkainen's tome of Foes describes this exactly.
Basically if a Demon Lord gets enough of a beachhead into the World and is allowed to run rampant with their followers for too long, the world will become irrevocably changed and eventually become a layer of the Abyss.
The classic adventure...