**5.1 Intracaldera intrusions**

The intracaldera intrusions are intruding the Guamsan Tuff in the moat between the center and margin of caldera to form shapes of irregular circular plugs and straight dikes (**Figure 1**). In particular, they are annually distributed to form a circular ring shape along the caldera moat. Their exposure area is much wider in circular plugs than in straight dikes. The intrusions mostly consist of flow-banded rhyolite and rare spherulitic rhyolite in lithofacies. The rhyolite is mostly reddish gray in color and glassy and rarely contains plagioclase phenocrysts. Further, it develops flow foliations, especially spherulitic structure in the northern plug. The flow foliations have strikes almost parallel to intrusive contacts as well as steep dips ranging from 40 to 75°.

The lithofacies and occurrence patterns reflect that they are a vent region widened during the eruption of lava from residual magma rising through existing vents or fissures created due to crumpling by the collapse of the Guamsan caldera. In particular, the development of spherulitic structure in northern plugs is remarkable in contact with the fallout tuff. Therefore, the structure is to have been radially crystallized into vapor-phase crystallization by the moisture effect within the fallout tuff when the rhyolite intruded the tuff.
