Spatial reasoning ability is key to an individual's understanding of geology and geosciences more broadly. One reason why is because spatial ability allows individuals to use multiple external representations (MERs) to understand information that they cannot normally interact with directly, such as the Earth. This article presents results from two studies that investigate the relationship between students' understanding of plate tectonics and their use of MERs along with spatial ability more broadly. Students' spatial ability was measured using the Perspective Taking Spatial Orientation Test, Water Level Task, and Purdue Spatial Visualization Tests: Visualization of Rotations. Students' understanding of plate tectonics was measured using a variable representation assessment. This type of assessment lets students actively use different MERs as answer options. The results of the two studies suggest that spatial ability did not have a direct relationship with students' scores on the plate tectonics assessment. Instead, students' understanding of plate tectonics may be better understood through their prior knowledge of the subject and their use of MERs. Specifically, their collective increased attention in using one particular type of representation. More research should be conducted to further understand what scenarios spatial ability aids students' understanding of plate tectonics and other areas of geosciences.