There is a need for research-informed instructional approaches that promote school students' deep conceptual understanding of abstract geological concepts. Given that a type of learner-constructed stop-motion animation, 'slowmation', has been shown to offer affordances for learning in science preservice teacher education, we extended its application to middle school students and investigated the impact of the construction process on students' learning about plate tectonics. Drawing upon theoretical notions of knowledge reconstruction, this mixed methods case study explored two research questions that concerned the extent to which the slowmation construction process influenced students' conceptual understanding about plate tectonics and how students' learning was facilitated by the slowmation construction process. The participants were ninth grade students (n = 52) who constructed slowmations in small groups to explain the geological processes that occur at tectonic plate boundaries. Data were generated using a two-tiered multiple-choice test instrument, the GeoQuiz, which we designed and validated, and audio-recordings of students working together as they researched, planned, and constructed their slowmations. A significant improvement was found in students' GeoQuiz scores, from pretest to posttest, which indicates their conceptual understanding improved over the course of the construction process. Analysis of the qualitative data found that students' ideas increased in sophistication through 'teachable moments', wherein students learnt through dialogic teacher-student and student-student exchanges. We assert that such exchanges ought to be viewed as an integral part of the slowmation construction process itself. While the study's findings support existing research that suggests representation-based activities are effective for student learning of geological phenomena, they also raise important questions about how to best engage middle school students in the construction of a slowmation.