This article presents a theoretical model of the process by which students construct and elaborate explanations of scientific phenomena using visual representations. The model describes progress in the underlying conceptual processes in students' explanations as a reorganization of fine-grained knowledge elements based on the Knowledge in Pieces perspective. The core case study involved a pair of fifth-grade students who generated visual representations to explain the phases of the moon and collaboratively elaborated and improved their representations and explanations. The model describes the process of developing explanations as iterations of temporarily stable stages of coherence. The progression from one temporary coherent structure to the next is described as the increase of "Resolution" and/or "Range" of the explanation. "Resolution" and "Range" are newly defined theoretical constructs. The model accounts for the continuity in the students' developing understanding and highlights the productive nature of their intuitive knowledge resources. (Contains 7 footnotes and 18 figures.)