Helping students understand that the scientific community's claims change through time, sometimes radically, allows them to develop a well-grounded sense of the "nature of science." Learning to reason about how scientific claims come to be accepted, and later refuted, is important for understanding the tentative nature of scientific claims (Allchin 2012). The unit described in this article scaffolds students' understanding of plate tectonics and the nature, and history, of geological sciences by engaging them in the process of evaluating competing scientific claims. In the unit students evaluate evidence quality, learn central organizing principles of geology, and develop a more sophisticated understanding of the nature of science in the context of a model-based inquiry unit. Pedagogical techniques include: (1) Making use of historical cases where scientists contended with competing claims; (2) Promoting disciplinary literacy with adapted primary literature; and (3) Using graphical organizers to scaffold student reasoning. The entire unit takes about two weeks (ten class periods of 45--50 minutes) to complete.