It’s a common elemental theme we see across stories, fantasies, etc. It seems like it is an element that should be part of that pantheon but maybe there is a scientific explanation of why it isn’t? Or is it simply, they did not have enough understanding of it at the time those 4 were suggested.
Always appreciate someone who needs out about ATLA. I enjoyed the read and I guess I agree with you on the lava thing. Putting in context of the water benders being able to change the phases of water it makes more sense.
Man I wish I could rewatch ATLA for the first time again.