Weathering of Basaltic Tephra on the Island of Hawaii

Three basaltic tephra deposits of Kilauea were studied in order to determine the nature and amount of weathering as a function of age and climate. One of these deposits, the Pahala Ash, is a regional tephra blanket 10,000 to 17,000 yrs old that is weathering under present rainfall ranging from about 25 to 635 cm/yr. The Keanakakoi Formation and Uwekahuna Ash, which contain the other deposits investigated, are less than 10,000 yrs old, and are present only in the vicinity of Kilauea caldera, where rainfall is from 115 to 255 cm/yr. Basaltic glass in the Pahala Ash is extensively weathered, whereas that in the Keanakakoi Formation generally is little altered. The principal weathering products of glass are palagonite and clay. Montmorillonite and calcite are weathering products of the Pahala Ash in the drier areas; gibbsite has formed at rainier sites. Plagioclase, pyroxene, and olivine are weathered only in the Pahala Ash. Chemical changes on weathering of the tephra deposits vary as a function of climate. From comparative analysis of unaltered and altered solids, SiO2 and Na + K + Ca + Mg seem to be lost under heaviest rainfall in average proportions as high as 2:1; in the driest areas they have been leached in the ratio of about 1:3. In an area of intermediate rainfall, SiO2 and Na + K + Ca + Mg are lost in an average ratio of 3:2, as shown by chemical composition of both solids and interstitial waters. However, an unresolved discrepancy occurs between the rate of leaching, as calculated from solids and that calculated from solution data, when an age of 10,000 yrs is taken for the upper part of the Pahala Ash. Ion-activity products calculated from the solution analyses provide a framework for elucidation of the weathering processes and products.