Students hold a surprising number of ideas about the Earth's structure and process. This paper begins with a discussion on the nature of understanding in the conceptually confined domain of geosciences. There then follows a report on a study of the ideas about a range of concepts relating to "crystals", "volcanoes", "rocks", and the "Earth", held by eighth-grade students (13-14 years) in one middle school. Such patterns, described here as "alternative frameworks", can be used to inform our understanding of students' learning in earth science. If these alternative frameworks are not taken into consideration, they can represent "critical barriers" to learning in this domain in addition to other barriers identified in this research. The aim of this paper is to relate the students' alternative frameworks, the "critical barriers" that have been spotted and the possibilities of overcoming them. Several different recruitment strategies were used to collect data in order to get to know the students' alternative frameworks. The methodology of this study is based on two researches: a test of the Q-Sort and a paper-pencil test. Based on the results, some suggestions to help teachers and students avoid critical barriers that may be difficult to overcome later in their geological education are presented.