Large magnitude (≥7) explosive eruptions (≥ 40km3 DRE) can cause devastating global impacts due to long distance volcanic ash transport and climate disruption caused by volcanic sulphate aerosols. Yet both our understanding of the conditions that lead to large eruptions and the long-term impacts of these events are limited. I will explore ways in which re-examination of the c.7700 yr B.P. eruption of Mount Mazama, which produced Crater Lake, Oregon, can provide insight into both questions. Questions of eruption onset are addressed by combining field studies, petrologic and textural analysis; eruption impacts are addressed by field studies, modelling of ash transport and deposition, and aggregation of studies that address impacts on global climate, regional landscape evolution, vegetation studies and human migration.