A volcanic breccia is a rock composed predominantly of angular fragments resulting from brecciation or emplacement due to volcanic action: it may, or may not, have a matrix. Four criteria have commonly been used in their classification: (1) cause of brecciation, (2) environment of deposition or emplacement, (3) location (in relation to geologic forms), and (4) rock type. Cause of brecciation has been used in cases where intrusion-breccia formed as the result of intrusion of magma into wall-rock (Fig. 1). Where there has been movement of the brecciated material and the cause of brecciation is more difficult to establish, terms such as intrusive breccia indicate environment of development. Location, as reflected by such terms as vent-breccia and contact-breccia, can be avoided in most instances by using the appropriate term to describe the process of formation, e.g., vent-breccia is normally a pyroclastic breccia. Rock...