Yes it is theory, but very well supported theory, unlike global warming. All non-experimental geology is theory based on circumstantial evidence. The evidence is heavily in favor of plate tectonic theory.
Are you a geosynclinalist?
Theory and hard science are too often confused.
Interestingly enough, it was the son of Charles Darwin who came up with the “theory” that the moon was moving away form the earth an why.
He was wrong about why and how but today we know for a fact that the moon is moving away from the earth.
Dude! You can't just ask someone that on a public forum.