At the time of the third age Middle-Earth is indeed spherical. Eru explicitly changed it after the fall of Numenor, putting Valinor out of reach. It was flat be

At the time of the third age Middle-Earth is indeed spherical. Eru explicitly changed it after the fall of Numenor, putting Valinor out of reach. It was flat before. The final war with Morgoth also destroyed a good chunk of western middle earth With such common and devastating divine interventions it is not surprising that plate tectonic is not a good predictor of geography :D. Note that Arda is supposed to be planet Earth , circa ~10000 years ago, but of course the geography only vaguely matches.