In 1595, a lahar flowed down along the valleys of the River Guali and the River Lagunillas, killing 636 people. In 1845, another massive lahar flooded the upper valley of the River Lagunillas, killing over 1,000 people. It continued for downstream before spreading across a plain in the lower valley floor. It is believed both of these formed from melting of the snow and ice that cover the summit, as did the original eruption. 