A significant number of studies have examined the features of expert discoursal practices in science and academia, and many have focused on what happens to student writers working their way into the academic community. Less attention has been paid to how a scientific writer's voice continues to change after the Ph.D. dissertation. This study exam- ines the shift in experienced scientific writers' disciplinary voice over the ten-year period following the doctoral dissertation. Using genre analysis triangulated with qualitative methods, a set of indexes that convey field geologists' disciplinary practices and concerns has been identified. Using a measure of standard deviation, the study then compares the use of these indexes by six writers from geology over ten years, and finds that disciplinary voice develops in similar ways. This paper contributes to ongoing discussions about how research on voice is useful for studies on second-language writing. In addition, using tools such as standard deviation allows for a closer analysis of the elusive notion of 'voice'.