Mudflows, or lahars. Lahars are born in a number of ways. In some cases an eruption melts glaciers and snow atop a volcano, creating a torrent of ash-laden water, mud, and stones that roars down the steep flanks, usually following valleys and ravines. The most common trigger for the deadly lahar is intense rain, and when a volcano has emittedenough ash and ejecta, the soaked mixture creates a wall of muddy debris that sluices down existing rivers or streams. Many victims of lahars die from trauma to the head and chest as walls of water and debris sweep over them at speeds of 25 to 35 miles per hour.
Some victims drown and some are asphyxiated.
Tsunamis may have a much longer reach than either lahars or pyroclastic flows and surges. A tsunami can be 100 miles long, travel at 450 miles per hour, and reach heights of 50 to 100 feet when it comes ashore. The tallest tsunami ever recorded, which hit the Ryukyu Islands in Japan in 1971, was 278 feet, roughly the height of a thirty-story building. The fatal combination of a volcano and water poses other kinds of threats. While a volcano itself may be dormant for hundreds of years, its crater lake may hide a volatile fury.