Background: Science teachers represent the critical link to the development of a scientifically literate citizenry who is capable of addressing societal problems and global challenges in ways that positively affect the quality of life. Purpose: We investigated how a long-term, inquiry-based professional development program could be utilized to increase teachers' pedagogical content knowledge of topics related to 8th grade Earth Science. Sample: The sample included 17 eighth-grade science teachers working in Title 1 schools serving students of low socioeconomic status and high populations of English language learners. Design and methods: A mixed-method approach of exploratory design was used in this study. A paired samples t-test for the weighted score of the "Earth Space Science Diagnostic Science Assessment for Middle School Teachers (DTAMS)" was used to investigate changes in teacher knowledge of Earth Science topics. Qualitative data was collected via reflective prompts and thematically analyzed using NVIVO software (Clarke and Braun 2017). Results: DTAMS results showed a statistically significant difference in the scores for the pretest (M = 14.53, SD = 3.83) and posttest (M = 17.01, SD = 3.39) results; t (14) = -2.4, p = 0.03. Eleven themes emerged from the NVIVO analysis that align with and demonstrate the inclusion of four essential components of constructivism (Baviskar, Hartle and Whitney 2009) including prior knowledge (connecting and building blocks); creation of cognitive dissonance (hands on, field trip, pacing, scaffolding, technology, and collaboration/relationship); assessment (informal and formal assessment); and reflection. Conclusion: Based on DTAMS results, the novice teachers gained content knowledge of Earth Science topics. The reflective prompts demonstrated the teachers used constructivism (inquiry-based instruction) during lesson planning and implementation documented with 11 themes associated with the four components of constructivism (Baviskar, Hartle and Whitney 2009).