About 23,000 people died when the lahar covered the Armero , making this the most deadly lahar in the history of the world . On November 13 , 1985 , a small eruption produced an enormous lahar that buried and desolated the town of Armero in Tolima Department , causing an estimated 23,000 deaths . This event later became known as the Armero tragedyâ '' the deadliest lahar in recorded history .