Super dumb question here.

I wanted to research how to achieve the effect applied in the awesome elevation maps from Pixie here, you see, the way the BORDERS (not shapes) of each color are not smooth, but have some kind of rugosity? Problem is, I have no idea of how that effect is called. I don't mean the overall shape of each color area, I can get interesting irregular shapes with clouds render and posterize, it's the not smoothness of the actual borders that I want to replicate.

English is not my first language, but I don't know how to call it in Spanish either since the keywords I use to look for answers don't come close to what I want to do with the borders of my shapes. So I'm stuck.

Anyone knows how said effect is called and since we're here, how to achieve it?