The age and compositional zonation in crystals from the Youngest Toba Tuff record the prelude to Earth's largest Quaternary eruption. We used allanite crystals to date and decipher this zoning and found that the crystals retain a record of at least 150,000 years of magma storage and evolution. The dominant subvolcanic magma was relatively homogeneous and thermally stagnant for ∼110,000 years. In the 35,000 years before eruption, the diversity of melts increased substantially as the system grew in size before erupting 75,000 years ago.
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