COUNTERACTING STUDENT MISCONCEPTIONS REGARDING FORMATION OF SEDIMENTARY ROCKS

Students have numerous misconceptions regarding rock-forming processes, particularly those involved in forming sedimentary rocks (e.g., Kusnick 2002; Kortz and Murray 2009). Misconceptions include: 1) accretionary processes that form clasts within environments of deposition (e.g., Kusnick’s “growing pebbles”), 2) lithification results from drying, and 3) pressure is required to lithify sediments. These misconceptions may arise from the “hidden” nature of diagenetic processes. In the absence of direct experiences, students call upon vaguely related experiences and construct naïve explanations. We have designed a physical model to counteract some of the most common student misconceptions regarding lithification and the formation of sedimentary rocks.

Our physical model is constructed from readily available, inexpensive, safe materials. A supersaturated solution of sodium acetate trihydrate (see MSDS) is used in combination with aquarium gravel. Within minutes, the sediment is lithified with a cement of clearly visible sodium acetate crystals. This model directly confronts students’ accretionary, drying and pressure misconceptions, as these processes are clearly not present in the formation of the synthetic rock. If instructors frame their use of the model as a small-scale representation of larger domains, the model also has the potential to help students rectify misconceptions of rocks (hand specimens) as clasts. Our model may be used as a student-centered activity or a teacher-led demonstration.

All models have advantages and disadvantages (Gilbert and Ireton 2003). The key advantage of our model is that it directly confronts the most common and significant misconceptions regarding lithification. The rapid formation of the synthetic rock may reinforce misconceptions regarding time scales of geologic processes and relatively small size of the synthetic rocks may reinforce the “rock as clast” misconception. However, we feel that, if used properly, the advantages the model provides in dispelling the most significant misconceptions outweigh any disadvantages.