Mudflows and lahars are special forms of debris flows
that are mainly made of the smallest mud and silt particles.
Extremely heavy rain, or a sudden thaw can trigger these
types of flows. In the case of lahars, a sudden thaw of
mountain snow due to a volcanic eruption can send a
torrent of mud, ash, and hot water down the slope of
he volcano and over neighboring towns.

As can be seen from this photograph of the unfortunate
town of Armero, a lahar can overtake a town far from
a volcano. This lahar rushed down stream and river valleys
into the town and killed over 23,000 people. They had no
warning. The town was quickly buried by mud that later,
as rescuers attempted to find victims, dried and hardened like cement.