Student ontological level is an important consideration for faculty interested in teaching Earth System Science. Interviews with 61 students enrolled in entry-level geology courses at two institutions reveal that very few students exhibit Process or Systems ontologies. Earth Systems Science instruction generally assumes that students are thinking about processes and systems, although this study suggests that students predominantly understand that changes occur on Earth without either 1) a concrete acceptance that these changes result from a cause, or 2) an explanation for these causes beyond scientific terms such as "subduction". This has critical implications for teaching Earth System Science, and suggests that activities that can help college students develop Process and Systems ontologies need to be developed and evaluated.
