VOLCANIC ACTIVITIES OF TATUN VOLCANIC GROUP IN LAST 20 KYR, NORTHERN TAIWAN

Quaternary andesitic Tatun Volcanic Group (TVG), near metropolitan Taipei, has been considered potentially active. More than 20 well-preserved volcanic edifices, multiple hot springs and fumaroles up to 120^oC with magmatic He isotopic signatures, as well as shallow volcano-tectonic earthquakes registering by local seismic network were observed. Moreover, layers of volcanic ash discovered in sediments of Taipei basin indicated the Holocene eruptions of the TVG. Surrounded by densely populated areas (>6 million people) as well as by multiple industrial factories including two nuclear power plants, the TVG poses major threat to life and property.

The physical and chemical volcanology of the TVG, with focus on pyroclastic stratigraphy and eruption dynamics has been investigated. Explosive eruptions of TVG were identified from weak-to-mild, but rather diverse with deposition of fallout tephras, base surges and pyroclastic flows. Many of the eruptions have evidences of water/magma interaction: poor sorting, wide span of vesicularity of juvenile material, abundant xenoliths of country rocks, associated base surges as well as ballistics of bread-crust and cauliflower type. The only found completely pyroclastic edifice of the group is represented by tuff ring ~ 500 m in diameter and 40 m high. The lahars were formed soon after the eruptions resulting in massive rapid redeposition of the pyroclastic material.Radiocarbon dating has shown that the youngest eruptive period of the group occurred ~ 10,000-15,000BP, and was associated mostly with activity of Mt. Cising.

Our data allowed to conclude that volcanoes of TVG had multiple eruptions at the Pleistocene – Holocene boundary and possibly younger. The obtained results provided basis for reassessment of volcanic hazards of this area.