What geologist studying volcanic activity is collecting rocks that formed from hot molten magma?

A geologist studying volcanic activity who collects rocks formed from hot molten magma is typically referred to as a petrologist. They focus on understanding the origin, composition, and structure of igneous rocks, which are formed from the cooling and solidification of magma. By analyzing these rocks, petrologists can gain insights into the processes occurring within the Earth's mantle and crust, as well as the dynamics of volcanic eruptions.