Pyroclastic flows melted ice and snow at the summit, forming 4 thick lahars that rushed down the river valleys. As most lahars do, the mudflows began as flows of water, sand, and gravel, and mixed with clay along the way. The lahars were up to wide and six feet deep (2 m) and travelled more than . The lahars destroyed many houses and towns. The town of Armero was completely covered, killing approximately 21,000 people (three-fourths of the population), as well as affecting 13 other villages. The eruption caused an estimated 23,000 deaths, 5,000 injuries, and destroyed more than 5,000 homes. This was the second deadliest volcanic disaster in the 20th century, after the 1902 eruption of Mount Pelee, and the fourth deadliest eruption in recorded history. It is the deadliest lahar in recorded history, and Colombia's worst natural disaster.