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Abstract After more than three months of lava dome extrusion, La Soufrière (St. Vincent) transitioned to a series of explosive eruptions in April 2021. Here we present a time-series petrologic analysis of the phenocryst and microlite populations during the first ∼48 hours of explosivity to constrain ascent conditions and processes that drove changes in behavior. Primary eruptive products were crystal-rich (45-50 vol%) basaltic andesites with similar phenocryst phase assemblages and compositions. The change in eruptive style is consistent with overpresurization as a consequence of second boiling from anhydrous microlite crystallization. The microlites display variation between the explosive phases, with two populations: 1) “inherited” - normally zoned high-An plagioclase (>An 70 ) + olivine (Fo 62-79 ) + clinopyroxene + titanomagnetite, inferred to have crystallized at depths >15 km and high water pressures; 2) “juvenile” - unzoned plagioclase (An 45-65 ) + clinopyroxene + orthopyroxene + intermediate pyroxene (Wo 12-38 ) + titanomagnetite, inferred to have crystallized upon ascent due to decompression and degassing. Scoria from the first explosions featured extensive groundmass crystallization and a significant “inherited” microlite population. Later explosions had a more abundant “juvenile” microlite population and lower crystallinity, consistent with more rapid ascent from depth, initiated by decompression following initial blasts and destruction of the lava dome. Supplementary material at https://doi.org/10.6084/m9.figshare.c.6534864