This essay explores how students' misconceptions about writing might be transformed into accurate threshold concepts of writing through disciplinary writing experiences. Through an activity analysis of a geology major and students' writing in that program, I demonstrate that these students' conceptions of writing changed through their legitimate peripheral participation in geological activity. Students' learning in the major situated writing within the activity of professional geological communities, and they recognized both how writing constructs and circulates knowledge within their discipline and their need for writing to enable participation in those communities. Their example suggests that WID programs attend to conceptual change and legitimate peripheral participation as essential mechanisms for creating transformative writing experiences that enable student learning.