**6.5 Intrusion of postcollapse ring rhyolite dikes**

*Forecasting Volcanic Eruptions*

accumulate fallout tuff.

ing eruption volume.

a plug or dikes.

compared to the lower ash-flow tuff. It is very difficult to distinguish boundaries between each flow unit because of the fewer lithic fragments in the upper ash-flow tuffs, which are densely welded or even almost completely liquefied and devitrified enough to form eutaxitic and parataxitic fabrics. If it would form highly welded tuffs, it should be slow and less fluidized non-expansive ash flows, and it should be thickly accumulated during a short period, by the low column collapse derived from high-temperature magma, because heat loss could be prevented through the production and flooding of high-temperature ash flows. In order to reduce the height of the eruption column in this way, it is possible for magma to have a higher discharge rate, lower gas diffusion rate, and less gas content during eruption. Under such conditions, the ash flows may not spread far away and diminish with the loss of vitric ashes. Such pyroclastic flow-forming eruptions also indicate that they originated from nonviolent voluminous eruption, which supports that they were boiling-over eruptions that successively occurred without near explosiveness. The eruptions could produce voluminous ash-flow tuffs by flooding repetitive ash flows that do not almost produce ash cloud, whereas they did not have chances to

The ring fissure eruption which caused such phenomena converted into effusive

eruption together with the injections of magma into fissures by exhausting its explosive force and continuously collapsing caldera block due to the rapidly increas-

The boiling-over eruptions through ring fractures were also converted into non-explosive activities of magma by an exhaustion of volatile materials due to eruptions of voluminous ash flows. With the caldera formation, the residual magma were intruded into fissures of inverse wedge-like shape formed in the existing vent or in moat, and effusive eruptions successively occurred through the fissures (**Figure 6c**). The evidence for this is that the rhyolites occur in the moat as modes of

This also suggests that the distribution pattern of these rhyolites is dominant in fissures made in the moat along the caldera collapse. Thus, it is thought that the rhyolites are products of residual magma that filled fissures in the moat by the pressure imposed to the magma chamber after caldera collapse. Though volcanic domes formed at this time had already disappeared by erosion and denudation, it is presumable that they may have connected to the magma chamber as roots of

The following points of evidence suggest that they are intrusive roots of volcanoes after the collapse of caldera. Flow foliations in the rhyolite plugs dip almost vertically inside the caldera, whereas they dip toward the center outside the caldera. The intrusion of these rhyolites up to the high level of the Guamsan Tuff through fissures in the moat suggests that these are the roots of volcanoes after the collapse of caldera occurred at the higher level. The contacts of the rhyolites with Guamsan Tuff without cooling margin and the chemical composition of rhyolites similar to Guamsan Tuff also indicate that the intrusion of rhyolites was that the volcanic event occurred directly following the eruption of ash flows. That is, the intrusions can be regarded as underground residuals of volcanic activity occurring along the intracaldera fissures. At that time, the residual magma was probably very viscous and could not flow far away, fill only fissures up or at the upmost, and form small domes on the surface. Because they contain almost no phenocryst and develop flow foliation, they would perhaps have formed small

**6.4 Intrusion of postcollapse intracaldera rhyolites**

postcollapse volcano with dome shape.

**60**

lava domes on the ground.

Following the formation of the Guamsan caldera together with the ash flow-forming eruptions along the ring fracture, the residual magma formed ring rhyolite dikes by gradual injection into ring faults (**Figure 6c**). The injection of magma into ring fault, which is driven by the overpressure of magma causing its buoyance, can form dikes without any erosion of wall rocks [28]. The dikes are considered to be intrusion roots of several volcanoes created along the ring fracture after the caldera collapse. It is supposed that the volcanoes would have formed rhyolite domes aligned along the ring fracture on the ground. This is because the flow foliations in the ring dikes dip sub-vertically inside the caldera, whereas they dip inward outside the caldera.

At the same time as the intrusion of intracaldera rhyolites, the residual magma began with rising into ring fracture zones by the force accumulated therein. At that time, the residual magma was injected mainly into ring fissure vents, which were the passage of later ash-flow tuffs, and then formed the ring rhyolite dikes (**Figure 6c**). These are also a series of small postcollapse volcanoes that formed several rhyolite domes.

The rhyolites in ring dikes are generally glassy or microcrystalline in terms of crystallinity without cooling margins and distinct dragging in contact with adjacent rock body, implying that the dikes intruded the fissures along the ring fracture zones. That is, the ring dikes indicate that it is closely involved in the collapse of Guamsan caldera. Comparing the fact that the inner dikes are dominantly glassy with flow-banded to stony feature, the outer dikes are dominantly with stony to porphyritic texture and no cooling margins at the contact; this should be explained as a consequence of temporary pulsational intrusions, considering that magma chamber may be cooled down inward from the outside. However, sequential successive intrusion should be explained simultaneously, because the lithological relations in the outer and intermediate ring dikes are gradational. Such a relationship is strongly supported by the gradual compositional changes explained in the following.

#### **6.6 Successive intrusion of rhyodacite**

Following the intrusion of ring rhyolite dikes, the sequential successive intrusions of rhyodacite occurred in the junction site of outer and intermediate ring dikes in southwest part of the caldera (**Figure 6d**). The reason for this is that both dikes have the gradational lithology in the contact. Besides, the rhyodacites perhaps were intruded from the more crystallized inner part of the magma chamber which was cooled down inwardly from its margin and top. This is because, as compared to the rhyolite dike, which is mostly glassy to microcrystalline, the rhyodacite at the junction part exhibits a coarser texture that is porphyritic and microcrystalline.

The gradual change from rhyolite to rhyodacite in the outer ring dike suggests that the more silicic top of magma chamber was earlier injected along the ring fracture zone and following on it the more mafic magma below it was successively intruded. The emplacement of rhyodacite in the ring dike was possible by discharging the rhyodacite magma following after exhaustion of effective melt by tapping the rhyolite magma [2]. By accounting for the patterns and locations of the ring dikes, the dikes can be judged to be the products of magma rising along the junction part of two ring fracture zones during the final stage of Guamsan volcanic activities; thus, they are also regarded as the roots of postcollapse volcanoes connected to the magma chamber.

The intrusion of this rhyodacite resulted in slow crystallization after rising magma emplaced through the junction part of ring dikes; this implies the closure of the Guamsan volcanic activities. Therefore, this spatiotemporal view corresponds to a final intrusion along the ring fracture zone of southwestern caldera in Guamsan magmatic system, but they now display their root zones due to deep erosions during long time (**Figure 6e**).
