Students do not have a good understanding of how rocks form. Instead, they have many non-scientific alternative conceptions to explain different aspects of rock formation. Using 10 interviews and nearly 200 questionnaires filled out by students at four different colleges, we identified many alternative conceptions students have about rock formation. We then used themes within those alternative conceptions to identify the underlying conceptual barriers that cause them. Conceptual barriers are deeply-held conceptions that prevent students from understanding scientific explanations. One conceptual barrier can cause many alternative conceptions, and alternative conceptions can be the result of more than one conceptual barrier. The seven conceptual barriers identified in the study that prevent students from understanding rock formation are Deep Time, Changing Earth, Large Spatial Scale, Bedrock, Materials, Atomic Scale, and Pressure. Because of these conceptual barriers, students cannot form scientifically correct mental models of how rocks form, resulting in alternative conceptions, so the conceptual barriers need to be overcome before students truly learn the scientific explanations of how rocks form. The results of this study can be applied to other areas of geology in addition to rock formation.