The Pacaya is one of the many volcanoes forminb the Coastal Range of Guatemala resulting from the subduction of the Pacific Plate under the American Plates. During its January 1987 eruption, the Pacaya produced large bombs, many of thes having a pseudo-pumice core of dacitic composition. A potassium-argon dating of this pumice yields an age of 25 M.a. The most probable origin of the cores is to be found in the volcano-sedimentary basement of the Pacaya, rather than regarding them as transformed diatomites.