Constraining magmatic and eruptive processes is key to understanding how volcanoes operate. However, reconstructing eruptive and pre-eruptive processes requires the ability to see through any post-eruptive modification of the deposit. The well-preserved Grey’s Landing ignimbrite from the central Snake River Plain provides an opportunity to systematically investigate the post-eruptive processes occurring through a single deposit sheet. Despite overall compositional homogeneity in both bulk and glass compositions, the Grey’s Landing ignimbrite does preserve differences in the abundance of Li in plagioclase crystals which are strongly associated with the host lithology. Li abundances in plagioclase from the quickly cooled upper and basal vitrophyres are typically low (average 5 ppm, n = 262) while plagioclase from the microcrystalline interior of the deposit has higher Li contents (average 33 ppm, n = 773). Given that no other trace elemental parameter in plagioclase varies, we interpret the variability in Li to reflect a post-depositional process. Groundmass crystallisation of a rhyolite like Grey’s Landing requires ∼50 % crystallisation of sanidine and variable amounts of a silica-rich phase (quartz, tridymite, cristobalite) and plagioclase to satisfy mass balance. We suggest the low affinity of Li for sanidine causes migration of groundmass Li into plagioclase during crystallisation. Even within the microcrystalline interior of the deposit, the morphology of the groundmass varies. The more marginal, finer-grained regions are dominated by cristobalite as the SiO2-rich phase while tridymite and quartz are additionally found in the more slowly cooled, coarser-grained portions of thick sections of the ignimbrite. Numerical models of cooling and crystallisation tested against field observations indicate that the groundmass crystallisation occurred relatively rapidly following emplacement (a maximum of a few years where the ignimbrite is thickest). These numerical models also illustrate that the time at which the potential for rheomorphism ceases (either the ignimbrite is a solid glass, or >50 % crystalline) is diachronous across the deposit.