Mudflows are caused by conditions favorable to soft, wet mud sliding downhill instead of staying put. Earthquakes, heavy rains, hurricanes, volcanic eruptions, tsunamis and even nearby explosions have been known to trigger mudflows.
A mudflow is a damaging river of thick mud similar to a mudslide but with less viscosity and more speed due to its higher liquid content. Because of its mass, it can uproot trees, destroy homes and do immense damage to communities and nature. Unlike floods, mud does not subside. The damage from a mudflow is often irreparable or lasts for years.
Volcanic mudflows, called lahars, are a little different from other mudflows. Exploring Earth explains that in a lahar, melted snow, ash, steam, pyroclastic material and mud blend together into a fast-moving and dangerous superheated mudflow. When it stops, the mud sets like concrete. Leftover ash on the volcano can be transformed into a mudflow by any strong rainstorm, sweeping down the slope over the smooth old surface of the original mudflow with terrifying speed.
Mudflow-prone areas can be stabilized by planting trees and deep-rooted shrubs on them. Over time, the roots create a living barrier, holding formerly unstable land in place. Pameno, a disaster preparation and recovery magazine, suggests building retaining walls and finding ways to divert runoff water away from homes and other endangered areas.