This paper describes how plate tectonics and rock genesis, two topics that are typically addressed separately in secondary Earth science classes, can be taught together as an integrated system. We define the TecRocks Reasoning Framework, developed to support student reasoning about rock formation situated in the context of plate tectonics. We also explain how we leveraged the framework in the designs of a new curriculum, interactive computer simulation, and assessment instrument. We show how the instrument was used to evaluate the curriculum, which included the computer simulation. We analyzed pretest and post-test data of 319 students taught by 10 teachers in diverse middle and high schools in the United States. Our analysis allowed us to discern different levels of reasoning in student explanations. On average, students made a significant pretest to post-test gain (p < 0.001) with a large effect size (Cohen's d = 1.03) after using the curriculum. All student groups made significant gains regardless of their gender, English language status, race, or school level. However, the amount of gain significantly differed by school level, p < 0.001. The middle school students as a group made a larger pretest to post-test gain (d = 1.42) than the high school students (d = 0.66).