we have plenty of evidence of the movement of plates. we know where subduction zones are. what does it take to prove a theory if not repeatable tests/observations?
> we can't directly observe what's going on the earth's crust over a period of millions of years
Depends what you mean by “observe”. The parallel lines of reversing magnetic polarity that are embedded in the sea floor on either side of the great rifts are observations that demand explanation.
but we can't "prove" plate tectonics, because we can't directly observe what's going on the earth's crust over a period of millions of years
in scientific nomenclature, a theory is a very robust thing indeed
vs. the vernacular, where it isn't, e.g. "I have a theory that my cat vomits behind the couch after I give him ice-cream"