On November 26, 1846, Cerro Azul erupted. This was the first report of activity at the volcano, and no trace of fumaroles, adjacent vents, or pre-eruptive activity exists. Most descriptions of the eruption come from the backcountry herdsmen (arrieros). One, who was camped in a valley approximately east of Quizapu, heard a great noise and a cloud of ash emanate from the mountain in the late afternoon. No precursor activity was reported, and the herdsman claimed that there were no earthquakes during the late afternoon eruption.