We explored students' conceptions of plate tectonics using a combined qualitative and quantitative approach consisting of multiple-choice ConcepTest questions, questionnaires, and interviews. When shown schematic images illustrating plate tectonics, half of the students were unable to determine the correct number of tectonic plates. These students appeared to have the most difficulty determining whether or not to count a divergent boundary as a plate boundary, but additional difficulties include confusion between continent-ocean boundaries (shorelines) and plate boundaries, and failure to see the larger picture as a result of focusing on individual boundaries. We propose that the underlying causes for these difficulties stem from the tendency for students to construct their understanding of plate tectonics based on inappropriately applied prior knowledge. For example, when viewing a divergent boundary, many students activate two lines of prior knowledge: (1) if entities are the same (such as ocean plates on both sides of a divergent boundary) then they are not considered separate; and (2) if there is no obvious break (which is not seen on diagrams of divergent boundaries), then they are also not considered separate. The application of both of these lines of prior knowledge results in students concluding the two sides of a divergent boundary are the same plate. Retention of these alternative concepts prevents conceptual change from occurring during the period of instruction and results in students not recognizing divergent boundaries as plate boundaries, leading them to incorrectly count the number of plates.