The effects of two contained H. E. explosions in volcanic tuff were examined by mining directly into the explosion sites. Ore explosion (516 lb of TNT) increased its initial shot chamber volume of about 9 cu ft by a factor of sbout 5 and produced in addition some 126 cu ft of broken rock. Around this explosion, only natural joints in one direction were filled with carbon to a maximum distance of 42 ft, and no new fractures in other directions were developed. The other explosion (973 lb of TNT) expanded its 17 cu ft chamber to 10 times this initial volume and led to rock breakage, mostly by subsequent roof collapse, of 345 cu ft. Because this shot vented on firing, very little of the carbon-carrying gases entered joints, and fractures caused by the explosion are almost absent. The features characteristic of these two explosions were compared to an earlier 1000-lb explosion in salt in which, by contrast, numerous radial carbon-filled cracks were produced, and the less expanded chambers survived without collapse. For the explosions in tuff it was concluded that joints exercised a primary role in locating the surfaces of fracture failure, early venting inhibits development of carbon-markedmore » fractures, and the medium undergoes greater expansion and more readily collapses after the shot than does salt. (auth) « less