This paper explored the integration of science and language instruction during a unit on plate tectonics in a 7th grade transitional bilingual classroom. As not many studies have explored engaging bilingual learners in the Next Generation Science Standards practices of argumentation and explanation, we sought to analyze emergent bilinguals' construction and communication of scientific arguments and explanations in oral and written forms. We collected both classroom video data and student written work. We first analyzed student work associated with the unit's culminating task as evidence of the students' developing proficiency with science ideas, cognizant of what emergent bilinguals at varying levels of language proficiency are able to do with respect to producing the language functions of explain and argue. For video data, we created event maps of all classroom interaction and then narrowed our focus to key moments of interaction to shed light on the relationships among classroom instruction, written work, and writing reflected in the culminating task. With respect to argumentation, findings indicated that students demonstrated success in constructing claims throughout the unit and on the culminating task and were able to provide some type of evidence to support their claims. This was evident when students could tap into all their linguistic repertoires, using home language and new language to make sense of science. Although most students were still developing to provide sufficient evidence and incorporate scientific reasoning, the use of both home and new language allowed students to articulate initial understandings about scientific concepts such as convection currents and plate boundaries. Students who used home language were also able to provide more extended explanations, moving from using language immediately tied to the environment to attempting to use scientific discourse to explain phenomena. We recommend that teachers celebrate emerging skills and build on the rich linguistic and experiential resources emergent bilinguals bring to optimize participation in the practices of science and facilitate scientific understanding.