Inquiry-based science instruction has been advocated for decades. However, the implementation of inquiry teaching and learning in everyday classrooms is rather limited due to various factors. Previous research investigated implementations of inquiry-based teaching at the activity level with self-reported methods. However, little is known about how inquiry-based teaching is enacted at the discursive level in daily classrooms. The purpose of the study is to explore how a science teacher orchestrates discourse to engage students in inquiry in regular classroom practices in Taiwan where the educational systems pose significant challenges to inquiry-based teaching. Specifically, the study used a two-dimensional framework to characterize the teacher's discursive modes and to document their changes over time from the videos of a semester-long, grade 10 Earth Science course. Three types of discursive modes were identified according to the characteristics presented in the cognitive and guidance dimensions: epistemic-thinking provoking mode, analogic reasoning mode, and cognitive-elaboration mode. The epistemic-thinking provoking mode and the analogic reasoning mode exemplify two forms of guided inquiry engaging students in epistemic thinking and scientific reasoning. The decrease in time spent on inquiry-oriented discursive modes and an increase in time spent on the authoritative, non-interactive mode reflected the pressure and obstacles imposed by the content-heavy curriculum and educational environment. The limitations of the epistemic talk in this Earth Science classroom call attention to disciplinary differences in scientific inquiry.