Forecasting the onset, evolution and end of volcanic eruptions relies on interpretation of monitoring data—particularly seismic signals, such as persistent volcanic tremor—in relation to causative magmatic processes. Petrology helps establish such links retrospectively but typically lacks the required temporal resolution to directly relate to geophysical data. Here we report major and volatile element compositions of glass from volcanic ash continuously sampled throughout the 2021 Tajogaite eruption of Cumbre Vieja volcano, La Palma, Canary Islands. The data reveal the evolving chemistry of melts supplied from depth at a daily temporal resolution. Erupted melt compositions become progressively more primitive until the tenth week of activity, but a sharp reversal of this trend then marks the decline of mantle magma supply and a precursory signal to the eruption end. We find that melt SiO2 content is positively correlated with the amplitude of narrow-band volcanic tremor. Tremor characteristics, inferences from simulations and model calculations point to melt viscosity-controlled degassing dynamics generating variations in tremor amplitude. Our results show promise for a monitoring and forecasting tool capable of quickly identifying rejuvenated and waning phases of volcanic eruptions and illustrate how subtle changes in melt composition may translate to large shifts in geophysical signals.