reekoscience.com
Volcanic ash and volcanic gas - Reeko's Mad Scientist Lab
Volcanic ash is made up of fragments less than 1/5 inch (0.5 centimeter) in diameter. Most volcanic ash falls to the surface and becomes welded together as rock called volcanic tuff. Sometimes, volcanic ash combines with water in a stream and forms a boiling mudflow. Mudflows may reach speeds of 60 miles (97 kilometers) per hour and can be highly destructive.