**Abstract**

The Guamsan caldera is associated with the Guamsan Tuff and rhyolitic intrusions. The Guamsan Tuff consists of dominant ash-flow tuffs with some volcanic breccias and fallout tuffs. The breccias comprise block and ash-flow breccia near a vent and caldera-collapse breccia near a ring fracture. The lower member of the ash-flow tuffs is produced from pyroclastic flow-forming eruptions with any ash-cloud falls on the flow units, whereas the upper member is formed by many ash-flow from boiling-over eruptions. The rhyolitic intrusions are divided into intracaldera plug and ring dikes. The volcanic activities in the caldera exhibit the volcanic processes along a caldera cycle together with eruption types during 63.77–60.1 Ma. The activities began with pelean eruption that occurred with block and ash-flows from lava dome collapse, progressed through expanded pyroclastic flows and ash-cloud falls by pyroclastic flow-forming eruptions from a single central vent, and transmitted with non-expanded ash-flows from boiling-over eruptions along multiple ring fissure vents. Then the caldera collapse induced any translations into multiple ring fissure vents from an earlier single central vent. The boiling-over eruptions were followed by effusive eruptions along which rhyolitic magma was injected as a small plug and ring dikes with some lava domes on the surface.

**Keywords:** Guamsan caldera, Guamsan tuff, ring dikes, pyroclastic flow-forming eruptions, boiling-over eruptions, effusive eruptions
