My research focuses on how long it takes to assemble the magma bodies under volcanoes prior to eruption. I work on very large volcanoes that give rise to caldera-forming eruptions. Caldera-forming eruptions are large explosive eruptions that leave a crater, or caldera, in their wake. Although caldera-forming eruptions are infrequent, they can be catastrophic. To improve hazard mitigation planning and response to such large eruptions, we must better understand how the magma bodies feeding these eruptions develop in the crust, specifically where magma is stored, in what volumes, under what conditions, and for what duration. I address these questions by looking at minerals preserved in volcanic rocks. Volcanic minerals can record information such as pressure and temperature of storage, the composition of the magma, and the age of the minerals. This information can elucidate where minerals are stored in the magma reservoir prior to eruption, which further helps us to understand which parts of the reservoir are active and when. This work will help build toward the community-wide goal of eventually being able to forecast volcanic eruptions.