What evidence do geologists study to understand the relative size, severity, or frequency in time of past volcanic eruptions (and other geologic hazards)?

Simply multiply pi by the diameter

Magma forms by partial melting of upper mantle and crust. partial melt means that only a fraction of the available material forms a melt, and that the remainder stays solid. the partial melt rises because of its lower density and ascends through he crust. this is so conclusive mean the answer would be magma igneous rocks or balistic magma.