(Fig. 2.39) On March 19,1982, an explosive eruption at Mount St. Helens (Washington) resulted in a lahar (the dark deposit on the snow) flowing from the crater into the North Fork Toutle River valley.
Credit: USGS, Thomas J. Casadevall
Lahars are mudflows formed by the mixing of volcanic particles and water which often cause a lot of environmental and economical damage.
The direct impact of a lahar's turbulent flow front or from the boulders and logs carried by the lahar can easily crush, abrade, or shear off just about anything at ground level in the path of a lahar.
The force of a lahar is so big that buildings and valuable land may become partially or completely buried by one or more cement-like layers of rock debris (even if not crushed or carried away).
People caught in the path of a lahar have a high risk of death from severe crush injuries, drowning or asphyxiation. If a volcano known to produce lahars looks likely to erupt soon, the best thing to do is to evacuate the people involved.