This article examines the role of gesture in the shared meaning-making processes of 6th-grade students studying plate tectonics using a data visualization tool; specifically, a geographic information system. Students' verbal and gestural characterizations of key concepts of plate motions (i.e., "subduction", "rift", and "buckling") were transcribed and coded across episodes of small-group work during the course of the unit, tracking emergent concepts and their subsequent refinements. The emergence histories of these concepts showed that they were initiated in gesture before they were conveyed in speech. Once they appeared in speech, speech and gesture figured prominently in the further elaboration, modification, and refinement of the key concepts. Gestures introduced by 1 member of the group were picked up and used by peers. Gestures were found to afford joint attention to concepts and negotiation of meaning in the group (e.g., through manual manipulation, imitation, and correction of gestures). These interactions appeared to advance the group's shared understanding of plate tectonics, and to mediate individual student learning. The findings suggest that explicit attention to gesture in instruction and assessment may impact the development of domain understandings in science investigations. (Contains 3 tables, 9 figures and 2 footnotes.)