This study examines the implementation of teaching strategies by graduate teaching assistants (GTAs) in inquiry-based introductory geology labs at a large research university. We assess the degree of inquiry present in each Physical Geology lab and compare and contrast the instructional practices of new and experienced GTAs teaching these labs. We demonstrate that GTAs are able to teach these labs consistently but use different instructional strategies. Further, we found that the incorporation of particular teaching strategies was related to prior GTA experience. Experienced GTAs teach in a more reformed, student-centered manner than new GTAs. Teaching practices were assessed through direct classroom observation and the application of the Reformed Teaching Observation Protocol. The lessons learned from this project can be used to inform other science departments seeking to effectively incorporate inquiry based labs that encourage effective teaching practices from GTAs.