Explosive volcanic eruptions may be triggered by processes in the conduit, including P-T variations, volatile loss, and crystal growth. These unobserved processes may be inferred via textural and chemical examination of eruptive products. Juvenile clasts were examined from vulcanian explosions of the Soufrière Hills volcano. Here we show that secondary ion mass spectrometry (SIMS) depth profiling into plagioclase phenocrysts provides a nearly continuous record of magma ascent from chamber to surface via high-resolution (~10 nm) chemical profiles of decompression-induced crystal growth. Changes in lithium concentration allow quantification of crystal growth rates, which range from 2 × 10−8 mm/s to 7 × 10−8 mm/s. Anorthite variations suggest conduit temperature increases of 50 to 100 °C, assuming contemporaneous overpressures approaching 20 MPa. These data demonstrate a new means of constraining conduit conditions preceding explosive volcanic eruptions.