One of the most tragic volcanic events of the 20th century occurred in Colombia in 1985, when an eruption of Nevado del Ruiz generated lahars that swept down river valleys and destroyed communities in its path. With little warning, the volcano began erupting on the afternoon of November 13, 1985. Ash and muddy rain began to fall. Communities were alerted but no formal decision was made to evacuate. Later that evening, a stronger eruption produced pyroclastic flows and surges that mixed with snow