Recognition and Significance of Pumice in Marine Pyroclastic Rocks

Pumice is abundant in many ancient sequences of marine pyroclastic rocks and is regarded as important evidence that contemporaneous, or nearly contemporaneous, volcanic activity was the source of at least some of the fragmental debris. The pumice in many such sequences of rocks, however, is easily overlooked, chiefly because most marine pyroclastic rocks have been altered or metamorphosed to varying degrees, masking or obliterating the delicate cellular structures of the original pumiceous material. With care, however, and with the knowledge that pumice-rich rocks commonly occur toward the top of thick, vertically graded beds of lapilli tuff and tuff breccia, the elusive pumiceous fraction of most sequences of rocks can generally be recognized. Hand-lens examination of wetted specimens in the field will usually reveal the wispy and ragged outlines of some of the pumice that is present. More detail can be seen in thick sections and in conventional thin sections of pumiceous rocks. In general, the pumice in more altered and metamorphosed rocks can be seen by careful examination of hand specimens or thick sections; the pumice in relatively unaltered rocks can best be seen in thin section. Examples of pumice-rich rocks from the Precambrian of Arizona, the Cretaceous of Puerto Rico, and the Tertiary of Japan are described and illustrated.