what speeds can lahars reach?
Although lahars contain at least 40% (by weight) volcanic ash and rock fragmentsâmaking them dense and viscous like wet concreteâthey actually flow faster than clear-water streams. These mudflows can rush down the flanks of a volcano at speeds as great as 40 miles (65 kilometers) per hour and can travel more than 50 miles (80 kilometers). Lahars that contain the most debris (as much as 90%) move the fastest and are the most destructive.