The silicic dome complex of Santiaguito, Guatemala, has exhibited continuous extrusive activity for 90years. Despite its longevity, remarkably little is known about the magmatic plumbing system beneath Santiaguito. Here, we use petrological analyses of lava samples to define this plumbing system, from storage in the lower to mid-crust through to extrusion onto the surface. Magmatic storage conditions are constrained using amphibole and plagioclase phenocrysts; ascent processes are examined using the breakdown rims of amphibole phenocrysts and the texture and composition of groundmass, while shallow processes are revealed by the alteration of titanomagnetites and matrix glass. Santiaguito magmas contain amphiboles that formed from ~24km to ~12km beneath the surface, with temperatures of ~940 to ~980°C, and f O2 of NNO +0.4 to NNO +1.2. Amphibole breakdown rims suggest that during the final phases of ascent, magma may rise from ~12km (the limit of amphibole stability) relatively rapidly (~27 to ~84mh -1). We infer from the texture of the groundmass that melt rigidifies prior to extrusion - a finding that may have important consequences for conduit dynamics. © 2012 Elsevier B.V.