Ash flows as defined by Ross and Smith (1961) ". . . are turbulent mixture of gas and pyroclastic materials of high temperature, ejected explosively from a crater or fissures that travel swiftly down the slopes of a volcano or along the ground surface. The solid mate rials in an ash flow, although unsorted is dominantly of particles of ash size (less than 4 mm. in diameter) but generally contains different amounts of lapilli and blocks."