• Debris flow composed of a slurry of ash, rock, mud, and water. (Both fluid and solid stresses important) 
• Triggered by rapid melting of snow/ice during an eruption, or any other volcanic process that combines water with loose materials
• Flows long distances (up to hundreds of km) from a volcano
• Can transport large boulders on low slopes, extremely destructive

Nevado del Ruiz, Colombia,

~25,000 people killed