2005 Salt Lake City Annual Meeting (October 16–19, 2005)

Paper No. 10
Presentation Time: 10:40 AM


MARSTON, Thomas M., BROWN, Francis H. and NASH, Barbara P., Department of Geology and Geophysics, University of Utah, 135 S. 1460 E., Rm 719, Salt Lake City, UT 84112-0111, georockman@att.net

There are numerous historical reports of pumice washing ashore along the coast of eastern Africa within a year of the climactic eruption of Krakatau in Indonesia on August 26-27, 1883. Today, pumice occurs along the swash line of beaches in Kenya. Samples were collected from two beaches at Malindi and from a beach at Takwa on Manda Island. The pumice fragments are generally rounded to subrounded and range from 4–12 cm in diameter. Pumice consists predominantly of vesicular glass with phenocrysts of plagioclase, orthopyroxene, clinopyroxene, titanomagnetite and ilmenite. Pyroxenes and Fe-Ti oxides commonly contain inclusions of glass and apatite. Microprobe analyses of pumice glass (n= 60) are remarkably uniform and yield (wt. %): SiO2 = 72.3; TiO2 = 0.54; Al2O3 = 14.3; FeO*= 2.55; MnO = 0.12; MgO = 0.59; CaO = 2.16; Na2O = 3.82; K2O = 2.61; Cl = 0.24; Total = 99.3. The glass (other than Na2O) and mineral compositions are nearly identical to those of glass and minerals reported in pumice from the August 1883 eruption of Krakatau (Manderville et al., J. Volc. Geotherm. Res., 1996). Kenyan plagioclase (An 42-46) lie at the mean for Krakatau plagioclase (An 46-50). Magnesium numbers [Mg/(Mg+Fe)] for Kenyan orthopyroxene and clinopyroxene (0.71-0.73 and 0.76-0.78, respectively) are virtually identical to Krakatau Mg#'s (0.70-0.71 and 0.75-0.78).

These data confirm a prior report, based on bulk analyses of pumice, that Krakatau was the probable source for some specimens in a suite of compositionally diverse drift pumice found on East African shores from Kenya to Durban. A Krakatau source for the Kenyan beach pumice is also consistent with historical accounts of the encounter of pumice by ships in the Indian Ocean and the arrival of pumice on African shores within a year of the climactic eruption. The occurrence in Kenya is made possible by westward transport in the South Equatorial Current coupled with northern transport by the East African Coastal Current that is particularly strong during the July-August monsoon. Based on drift rates determined from encounters with pumice recorded in ships' logs and accounts of its arrival in Madagascar and Zanzibar, Krakatau pumice would have washed onto Kenyan beaches in latest July or early August 1884, eleven months after the climactic eruption.