Abstract We investigate phase transitions, glass transition, and dynamic behavior in the hyperquenched 69SiO 2 –31Al 2 O 3 (mol%) glass ( SA glass). Upon reheating, the SA glass exhibits a series of thermal responses. Subsequent to the sub‐ T g enthalpy release, the glass undergoes a large jump in isobaric heat capacity (Δ C p ) during glass transition, implying the fragile nature of the SA glass. The mullite starts to form before the end of glass transition, indicating that the SA glass is extremely unstable against crystallization. After the mullite formation, the remaining glass phase exhibits an increased T g and a suppressed Δ C p . The formation of cristobalite at 1553 K indicates the dominance of silica in the remaining glass matrix. The cristobalite gradually re‐melts as the isothermal heat‐treatment temperature is raised from 1823 to 1853 K, which is well below the melting point of cristobalite, while the amount of the mullite remains unchanged.