PETROGRAPHY AND GEOCHEMISTRY OF THE LAVA FLOW FROM THE JULY TO OCTOBER 2006 ERUPTION OF MAYON VOLCANO, PHILIPPINES

Mayon Volcano is one of the most active volcanoes in the Philippine archipelago. This stratovolcano has erupted 48 times since its first recorded eruption in 1616. The activity of Mayon include phreatic, strombolian, vulcanian and plinian types of eruption bringing various hazards to almost 900,000 people living in the three cities and five towns surrounding the volcano. Mayon has consistently erupted basalt, basaltic andesite and andesite lava and pyroclasts with SiO₂ contents from 51 to 59 weight percent (Newhall, 1979).

The most recent eruption of Mayon occurred on 07 August 2006. This eruption was characterized by both explosive and non-explosive activities. The explosive phase of the eruption was characterized by series of ash explosions which sometimes triggered the collapse of unstable portions of the lava dome, producing small pyroclastic flows that stop in the middle slopes. The non-explosive phase included the quiet effusion of lava flow in the southwestern flank of the volcano. The continuous extrusion of lava went on for two and a half months, ending on 01 October 2006 (PHIVOLCS Bulletins). Field mapping and sampling of the 2006 lava flow was done concurrent with its deposition from 20 July to 20 September.

Petrographic examination of the samples revealed that the 2006 lava is medium grained and porphyritic. Plagioclase, orthopyroxene, clinopyroxene and opaque minerals made up the phenocryst assemblage of the lava. The groundmass is fine grained but appears crystalline. It is composed of acicular plagioclase, pyroxene crystallites and opaque minerals. Flow textures are apparent. Vesicles are irregular and vein-like. The lava is medium K basaltic andesite. Major element compositions are similar to lavas from the 2000 eruption. The mineralogy and crystal growth patterns can be used to elucidate the physical and chemical processes that occurred in the magma reservoir prior to eruption. Particularly when combined with the major and trace element analyses, the study of erupted crystals allow us to gain a more complete picture of the recent history of magmatism at Mayon, including processes such as assimilation, fractional crystallization and magma mixing.