Colossal low-temperature upturn in the heat capacity of volcanic glasses

•Discovery of a unique anomaly in Cp/T³ versus temperature in volcanic glasses.•Identification of its magnetic origin through rigorous examination and magnetic field testing.•Uncovering complex magnetic interplay to explain the observed phenomenon. Here we explore the low-temperature heat capacity (Cp) of two natural endmember volcanic glasses, unveiling an unprecedented anomalous upturn in Cp/T³ versus temperature (T) dependencies. This distinctive behavior, previously unseen at this scale, obscures the accurate boson peak position, posing a significant analytical challenge. Through rigorous examination, we established the magnetic origin of this anomaly, proved by subjecting the samples to a magnetic field up to 9 Tesla. The investigations uncovered a nuanced interplay between magnetic models, as we sought to provide an explanation for describing this captivating phenomenon.