Notes on the 1965 Taal Volcanic eruptions

Neutron activation analyses were performed on samples of olivine-bearing basalt, of volcanic bomb, of volcanic ashes, of residues of water from the old crater lake, of residues of bubbling waters taken near the cinder cone, and of residue of rainwater collected at Parc, Diliman, Quezon City, all of which were volcanic products of the 1965 Taal eruption. The analysis was performed to determine the approximate ratios of important minor and trace elements in the volcanic products, in order to better understand the geochemical processes involved in the 1965 eruption of the Taal Volcano. The radionuclides of Cu, Fe, Na and Mn were positively identified in all the volcanic samples that were irradiated. Scandium was present only in the ash samples. The aproximate Cu/Na ratios for the volcanic ashes, volcanic bomb, and basalt are similar; they ranged from 0.171 to 0.200. The Cu/Na values obtained for the water samples varied from 0.253 to 2.010. The results of fast neutron activation show that the Si, Al, Fe, Mg, and O percentages of the different ash layers are similar. Based on the present work and upon past analyses, it is suggested that the solidification index for Taal is about 10. The eruptivity of Taal Volcano seems to be related to the rift in the crust, rate of migration of the magmatic volatiles, and pressure of the volatile fraction in the magma reservoir. All these factors are probably operating jointly and effectively, making it possible for the Taal Volcano to possess strongly predictable eruptivity, in the light of its past eruption history and its periodic cycle of eruption. It is suggested that the Taal and the Mayon Volcanoes are related from a regional and structural point of view. This observation is reflected in the striking similarity of their respective periodic cycle of eruptions.