These gases are predominately steam, followed in abundance by carbon dioxide, compounds of sulfur and chlorine, and lesser amounts of other gases. While they rarely reach populated areas in lethal concentrations, sulfur dioxide can travel downwind and react with the atmosphere to form acid rain that causes corrosion and a host of other problems. Carbon dioxide is heavier than air and tends to collect in depressions, such as valleys, where it can occur in concentrations lethal enough to cause suffocation of people and animals. Be sure to read about the tragedy of Lake Nios, Cameroon, in your textbook beginning on p. 210. They can travel over 80 kilometers and commonly reach speeds of 35 to 65 kilometers per hour.
Lahars containing a high percentage of rock debris look like fast-moving rivers of concrete.
Close to a volcano, they have the strength to rip huge boulders, trees, and structures from the ground and carry them for great distances. Farther downstream the coarser debris settles to the bottom of the flow, leaving mud to continue on to cover everything it passes. Historically, lahars have been one of the most deadly of the volcanic hazards. The 1985 lahars off Nevado del Ruiz in Colombia killed approximately 23,000 people. A mudline left behind on trees shows depths reached by the mud.