Should instructors assume that students possess conceptual knowledge of plate tectonics when they reach a second college geoscience course? Five cohorts in a historical geology course over 5 y--a total of 149 students--completed an in-class assignment in which they drew sketches of plate boundaries with required annotations. Analysis of the sketches revealed that most students lack an explanatory mental model that links the locations of earthquakes, volcanoes, and magma generation to plate-boundary processes and hold a pervasive alternative conception of Earth's interior structure that does not distinguish between compositional and rheological boundaries. Students who drew sketches that illustrated the most alternative conceptions also scored lower on a beginning-of-the-course administration of the Geoscience Concept Inventory, showing that conceptual understanding of plate tectonics correlates with overall conceptual geoscience knowledge obtained during previous course experiences. In addition, students tracking to the historical geology course via an introductory physical geology course showed stronger conceptual understanding of plate tectonics than those choosing an Earth-system science prerequisite, with those students previously enrolled in both courses illustrating the fewest alternative conceptions.