**6. Discussion**

Interpretations of many outcrops in the Guamsan caldera area can reconstruct eruption types, caldera collapse, and volcanic processes. Now, the author will discuss the eruption types according to the volcanic processes involved with the caldera collapse. The Guamsan caldera area represents that the volcanic processes occurred in the following order: (1) Pyroclastic flow-forming eruption from column collapse, (2) caldera forming and vent shifting, (3) boiling-over eruption and ashflow phase, (4) postcollapse intracaldera rhyolite intrusions, (5) postcollapse ring rhyolite dike intrusions, and (6) successive rhyodacite intrusion.

#### **6.1 Pyroclastic flow-forming eruption from column collapse**

The internal stratigraphic sequence of Guamsan Tuff can be analyzed so as to infer eruptions and their sequences. For cases of caldera-forming eruptions, the ideal internal sequence would comprise the following: (1) Plinian fallout eruption, (2) pyroclastic flow-forming eruption, and (3) effusive eruption. The initial Plinian eruption would turn into pyroclastic flow-forming eruption from collapsing the eruption column overloaded by the decrease in gas content, widening of the vent radius, etc. Therefore, the transition from fallout eruption to pyroclastic flowforming eruption would be caused by the rapidly increasing discharge rate [14–16] and accompany the ejection of large volume of lithic fragments commonly forming coarse lag breccia [10, 17–19]. The transition connotes an initiation of new vents related to the caldera collapse [20]. However, not all pyroclastic flow-forming eruptions necessarily have an initial Plinian eruption [21, 22]. This may suggest that the vent radius was widened almost simultaneously with the collapse of vent area.

In the Guamsan area, it initiated with the Pelean eruption forming block and ash flows without the Plinian eruption. Evidences supporting an eruption include the block and ash-flow breccia, which possibly correspond to the lowest part of the Guamsan Tuff. The breccia that belongs to the pyroclastic breccia got accumulated on the slope of crater by pyroclastic flow. The mechanism forming the pyroclastic flow is dominated by the collapse of lava dome or eruption column. The fact that the lithofacies mainly consist of rhyolite blocks suggests that the Pelean eruption may have occurred by the temporary collapse of active lava dome, from which lava fragments flowed on a cone slope as block and ash flows. In addition, it turned into the eruptions forming strongly fluidized pyroclastic flow due to widening vent by collapse of the vent area. The eruption was probably a single vent phase erupted from a vent.

**57**

*Eruption Types and Processes in the Guamsan Caldera, Korea*

was created by the collapse of the eruption column (**Figure 6a**).

falling from the ash clouds detached from pyroclastic flows.

The ash-flow tuffs and fallout tuffs in the lower member were supplied from the central vent (**Figure 6a**). With the gradually increasing discharge volume of magma, the top of the magma chamber began sinking down like a ring shape. Here, the collapse occurred off northeastward from the central vent. The pressure in the magma chamber decreased gradually as the tapping of magma continued, and then the collapse seemed to occur, when the underpressure in the magma chamber exceeded the strength of overlying rocks. The roof collapse may recover lithostatic pressure at the top of the magma chamber. In this way, the role of central vent was rapidly reduced simultaneously with the initial collapse of caldera. The following

**6.2 Forming caldera and vent transmitting**

The flow mechanism of pyroclastic flow represents partially fluidized flow [24–26]; the mechanism of transportation and sedimentation is similar to that of debris flow with slight differences. That is, while big clasts are transported by finegrained matrix and water in the debris flow, big lithic fragments are transported by fine ashes and gas in the pyroclastic flow. Most pyroclastic flows are laminar flows in their body, although they are turbulent flows in their head. Pyroclastic flow can be distinguished into the three types of non-expanded flow, expanded flow, and segregating flow based on fluidization behaviors [11]. The ash-flow tuffs, occurring in the lower tuff member of Guamsan Tuff, seem to have been mostly emplaced by the expanded flow. The normal grading of lithic fragments and inverse grading of pumices in the lower ash-flow tuffs are dominated by settling velocity that depends on their density difference from matrix. The lithic fragments will occur normal grading according to each settling velocity because of the higher density of lithic fragments than matrix, whereas the pumices will occur the inverse grading according to floating over because of the lower density than matrix. Such grading appears more distinctly in accordance with the increase in density difference from matrix, when becoming greater in expansion degree of pyroclastic flow. The flow process of gas included during fluidization causes grading and sorting of fine-grained ashes. According to the expansion degree, the highly expanded flow would detach a large volume of fine-grained ashes from the flow to ash cloud, whereas the less expanded flow would drift upward only a small volume. Because the ash-flow tuffs in the lower tuff member mostly belong to the category of expanded flow, they represent not only the normal grading of lithic fragments but also planar-bedded tuffs by

The lower member of Guamsan Tuff, overlying this breccia, consists of disorganized massive breccia, graded bedding tuff, lapilli tuff, block tuff, and sheet bedding tuff in lithofacies; the first three lithofacies correspond to pyroclastic flow rocks, while the last one belongs to fallout tuff. Because the fallout tuff mostly consists of ashes less than 1 mm in diameter, is less than 1 m thick, and is overlying each ash-flow tuff unit, it corresponds to the ash cloud-derived fallout phase rather than the Plinian fall phase. Thus, a pair of ash-flow tuff and fallout tuff represents the sedimentary phases of a flow unit and an ash cloud fallout unit that follow a pyroclastic flow. According to Sparks et al. [23], the flow unit corresponds to a main body of pyroclastic flow called layer 2, whereas the ash cloud fallout unit refers to layer 3. Here, the ground surge unit, called layer 1, is not recognized at the base. The ash-flow tuff without fallout tuff might be attributable to the successive pyroclastic flow that did not leave sufficient time to deposit fallout ash from ash cloud or to the rapid denudation of ash cloud-fallout deposits by fast violent pyroclastic flow near the crater. However, the pyroclastic flow-forming eruption successively occurred after the temporary Pelean eruption. Here, the mechanism forming pyroclastic flow

*DOI: http://dx.doi.org/10.5772/intechopen.84647*

#### *Eruption Types and Processes in the Guamsan Caldera, Korea DOI: http://dx.doi.org/10.5772/intechopen.84647*

*Forecasting Volcanic Eruptions*

**6. Discussion**

textures may also imply the sequential continuous intrusions of magma. However, the textural changes southwestward from the northern part of the caldera suggest

The gradual compositional changes from the northern to southwestern parts of the caldera suggest in the sequential successive intrusion of magma that more silicic top of magma chamber first injected into the ring fracture zone, and then less silicic part of magma remaining down there injected into the zone sequentially. Therefore, the relationship between the two intrusions was the relationship between liquid and liquid. That is, the textural changes imply that rhyolite magma injected in the ring fracture was successively intruded by rhyodacite magma. The rhyolite magma was more evolved, with residual melt remaining sufficiently in the top of the magma chamber below the caldera block. First, the rhyolite magma was tapped in order to rapidly excavate fusible melt from the magma chamber, and then, second, the rhyodacite magma down there was successively tapped from the chamber [2].

Interpretations of many outcrops in the Guamsan caldera area can reconstruct

The internal stratigraphic sequence of Guamsan Tuff can be analyzed so as to infer eruptions and their sequences. For cases of caldera-forming eruptions, the ideal internal sequence would comprise the following: (1) Plinian fallout eruption, (2) pyroclastic flow-forming eruption, and (3) effusive eruption. The initial Plinian eruption would turn into pyroclastic flow-forming eruption from collapsing the eruption column overloaded by the decrease in gas content, widening of the vent radius, etc. Therefore, the transition from fallout eruption to pyroclastic flowforming eruption would be caused by the rapidly increasing discharge rate [14–16] and accompany the ejection of large volume of lithic fragments commonly forming coarse lag breccia [10, 17–19]. The transition connotes an initiation of new vents related to the caldera collapse [20]. However, not all pyroclastic flow-forming eruptions necessarily have an initial Plinian eruption [21, 22]. This may suggest that the vent radius was widened almost simultaneously with the collapse of vent area. In the Guamsan area, it initiated with the Pelean eruption forming block and ash flows without the Plinian eruption. Evidences supporting an eruption include the block and ash-flow breccia, which possibly correspond to the lowest part of the Guamsan Tuff. The breccia that belongs to the pyroclastic breccia got accumulated on the slope of crater by pyroclastic flow. The mechanism forming the pyroclastic flow is dominated by the collapse of lava dome or eruption column. The fact that the lithofacies mainly consist of rhyolite blocks suggests that the Pelean eruption may have occurred by the temporary collapse of active lava dome, from which lava fragments flowed on a cone slope as block and ash flows. In addition, it turned into the eruptions forming strongly fluidized pyroclastic flow due to widening vent by collapse of the vent area. The eruption was probably a single vent phase erupted from a vent.

eruption types, caldera collapse, and volcanic processes. Now, the author will discuss the eruption types according to the volcanic processes involved with the caldera collapse. The Guamsan caldera area represents that the volcanic processes occurred in the following order: (1) Pyroclastic flow-forming eruption from column collapse, (2) caldera forming and vent shifting, (3) boiling-over eruption and ashflow phase, (4) postcollapse intracaldera rhyolite intrusions, (5) postcollapse ring

rhyolite dike intrusions, and (6) successive rhyodacite intrusion.

**6.1 Pyroclastic flow-forming eruption from column collapse**

that the erosion degree increased while going further southwestward.

**56**

The lower member of Guamsan Tuff, overlying this breccia, consists of disorganized massive breccia, graded bedding tuff, lapilli tuff, block tuff, and sheet bedding tuff in lithofacies; the first three lithofacies correspond to pyroclastic flow rocks, while the last one belongs to fallout tuff. Because the fallout tuff mostly consists of ashes less than 1 mm in diameter, is less than 1 m thick, and is overlying each ash-flow tuff unit, it corresponds to the ash cloud-derived fallout phase rather than the Plinian fall phase. Thus, a pair of ash-flow tuff and fallout tuff represents the sedimentary phases of a flow unit and an ash cloud fallout unit that follow a pyroclastic flow. According to Sparks et al. [23], the flow unit corresponds to a main body of pyroclastic flow called layer 2, whereas the ash cloud fallout unit refers to layer 3. Here, the ground surge unit, called layer 1, is not recognized at the base. The ash-flow tuff without fallout tuff might be attributable to the successive pyroclastic flow that did not leave sufficient time to deposit fallout ash from ash cloud or to the rapid denudation of ash cloud-fallout deposits by fast violent pyroclastic flow near the crater. However, the pyroclastic flow-forming eruption successively occurred after the temporary Pelean eruption. Here, the mechanism forming pyroclastic flow was created by the collapse of the eruption column (**Figure 6a**).

The flow mechanism of pyroclastic flow represents partially fluidized flow [24–26]; the mechanism of transportation and sedimentation is similar to that of debris flow with slight differences. That is, while big clasts are transported by finegrained matrix and water in the debris flow, big lithic fragments are transported by fine ashes and gas in the pyroclastic flow. Most pyroclastic flows are laminar flows in their body, although they are turbulent flows in their head. Pyroclastic flow can be distinguished into the three types of non-expanded flow, expanded flow, and segregating flow based on fluidization behaviors [11]. The ash-flow tuffs, occurring in the lower tuff member of Guamsan Tuff, seem to have been mostly emplaced by the expanded flow. The normal grading of lithic fragments and inverse grading of pumices in the lower ash-flow tuffs are dominated by settling velocity that depends on their density difference from matrix. The lithic fragments will occur normal grading according to each settling velocity because of the higher density of lithic fragments than matrix, whereas the pumices will occur the inverse grading according to floating over because of the lower density than matrix. Such grading appears more distinctly in accordance with the increase in density difference from matrix, when becoming greater in expansion degree of pyroclastic flow. The flow process of gas included during fluidization causes grading and sorting of fine-grained ashes. According to the expansion degree, the highly expanded flow would detach a large volume of fine-grained ashes from the flow to ash cloud, whereas the less expanded flow would drift upward only a small volume. Because the ash-flow tuffs in the lower tuff member mostly belong to the category of expanded flow, they represent not only the normal grading of lithic fragments but also planar-bedded tuffs by falling from the ash clouds detached from pyroclastic flows.

### **6.2 Forming caldera and vent transmitting**

The ash-flow tuffs and fallout tuffs in the lower member were supplied from the central vent (**Figure 6a**). With the gradually increasing discharge volume of magma, the top of the magma chamber began sinking down like a ring shape. Here, the collapse occurred off northeastward from the central vent. The pressure in the magma chamber decreased gradually as the tapping of magma continued, and then the collapse seemed to occur, when the underpressure in the magma chamber exceeded the strength of overlying rocks. The roof collapse may recover lithostatic pressure at the top of the magma chamber. In this way, the role of central vent was rapidly reduced simultaneously with the initial collapse of caldera. The following

#### **Figure 6.**

*Schematic sections, pictorially explaining the eruption types and volcanic processes in the Guamsan caldera. (a) Pyroclastic flow-forming eruptions through a central vent; (b) caldera collapse along ring fractures and boiling-over eruptions through multiple ring fissure vents; (c) effusive eruptions of rhyolitic magma through multiple vents along the intracaldera and ring fractures; (d) renewed rhyodacitic volcanism along the southwestern ring fracture; (e) present erosion surface.*

**59**

the eruption column.

*Eruption Types and Processes in the Guamsan Caldera, Korea*

ash flows were produced by erupting from many vents along the ring fracture zones, and then the caldera was collapsed further (**Figure 6b**). The pyroclastic flow-forming eruption with two-staged caldera-forming processes was already

The intercalation and lateral change of breccias in the Guamsan Tuff demonstrate that the collapse of Guamsan caldera is related with pyroclastic flow-forming eruption. This is directly evident in that the chaotic massive breccia is intruded as a wedge shape at the base of upper member of Guamsan Tuff. The breccia is a caldera-forming breccia produced by debris flow moving on steep caldera wall following the caldera collapse; this suggests the vent shift that connotes the fissure eruption. Thus, the appearance of breccia not only indicates the beginning with caldera collapse but also implies the closing of a single central vent as well as the initiation of multiple ring fissure vents. In addition, the Guamsan Tuff is nonexistent in extracaldera, whereas it is still over 850 m in thickness in intracaldera. Such thickness of the intracaldera tuffs is roughly three times thicker than the thickness of tuffs from extracaldera outflow, which is similar to cases of other tuffs from elsewhere around the world [12]. That is, this supports the fast accumulation of ash-flow tuff inside the caldera due to continuous eruption during the caldera collapse as opposed to collapsing the caldera after termination of the pyroclastic flow-forming eruption. Partial existence of intrusive tuff in northern ring dike can be counted as further evidence as well. In addition, although the lower member of Guamsan Tuff inside the caldera hardly exhibits welded zone but represents vitroclastic fabric, the upper member of Guamsan Tuff that exhibits welded foliation is dominant in eutaxitic and parataxitic fabrics and undergoing the devitrification process, leaving coarse-grained crystals. This is seemingly attributable to the high temperature that lasted for a long time by rapid accumulation of ash-flow tuff

Thus, it is thought that the development of multiple ring fractures by caldera collapse during pyroclastic flow-forming eruption from central vent could serve as a natural momentum shifting to multiple vents along the ring fractures concurrently

Some volcanologists have suggested from the two-staged models that the pyroclastic flow-forming eruptions converted into ring fracture vents from a single vent related with a caldera [17, 18], occasionally into the eruption from ring fractures [18, 27]. It seems to be the direct main cause that the shifting vents into ring fractures were transformed into the boiling-over eruption from the pyroclastic flow-forming eruption by column collapse. The conversion into boiling-over eruption was attributable to the rapidly increasing discharge of magma; the increase in discharge was possible from sinking the caldera block into magma chamber along the outward dipping of ring fracture zones. That is, because of the increasing number of vents together with outward dipping of ring fracture zones, the conduits were naturally gradually widened, and instantaneous discharge rate was rapidly increased as the subsidence occurred. Accordingly, the eruption at this moment did not form high eruption column but turned into the boiling-over eruption that produced a large volume of ash flow (**Figure 6b**). The discharge rate of magma at this moment was much higher than the discharge rate necessary for maintaining

In addition, the naturally widening conduits along subsidence were not necessary for eroding the conduit walls. Accordingly, the lithic fragments rapidly decrease in volume and are much smaller in size within the upper ash-flow tuffs as

*DOI: http://dx.doi.org/10.5772/intechopen.84647*

proposed by Druitt and Sparks [20].

inside the caldera.

with the closing of a single vent.

**6.3 Boiling-over eruption and ash-flow eruption**

*Forecasting Volcanic Eruptions*

**58**

**Figure 6.**

*southwestern ring fracture; (e) present erosion surface.*

*Schematic sections, pictorially explaining the eruption types and volcanic processes in the Guamsan caldera. (a) Pyroclastic flow-forming eruptions through a central vent; (b) caldera collapse along ring fractures and boiling-over eruptions through multiple ring fissure vents; (c) effusive eruptions of rhyolitic magma through multiple vents along the intracaldera and ring fractures; (d) renewed rhyodacitic volcanism along the* 

ash flows were produced by erupting from many vents along the ring fracture zones, and then the caldera was collapsed further (**Figure 6b**). The pyroclastic flow-forming eruption with two-staged caldera-forming processes was already proposed by Druitt and Sparks [20].

The intercalation and lateral change of breccias in the Guamsan Tuff demonstrate that the collapse of Guamsan caldera is related with pyroclastic flow-forming eruption. This is directly evident in that the chaotic massive breccia is intruded as a wedge shape at the base of upper member of Guamsan Tuff. The breccia is a caldera-forming breccia produced by debris flow moving on steep caldera wall following the caldera collapse; this suggests the vent shift that connotes the fissure eruption. Thus, the appearance of breccia not only indicates the beginning with caldera collapse but also implies the closing of a single central vent as well as the initiation of multiple ring fissure vents. In addition, the Guamsan Tuff is nonexistent in extracaldera, whereas it is still over 850 m in thickness in intracaldera. Such thickness of the intracaldera tuffs is roughly three times thicker than the thickness of tuffs from extracaldera outflow, which is similar to cases of other tuffs from elsewhere around the world [12]. That is, this supports the fast accumulation of ash-flow tuff inside the caldera due to continuous eruption during the caldera collapse as opposed to collapsing the caldera after termination of the pyroclastic flow-forming eruption. Partial existence of intrusive tuff in northern ring dike can be counted as further evidence as well. In addition, although the lower member of Guamsan Tuff inside the caldera hardly exhibits welded zone but represents vitroclastic fabric, the upper member of Guamsan Tuff that exhibits welded foliation is dominant in eutaxitic and parataxitic fabrics and undergoing the devitrification process, leaving coarse-grained crystals. This is seemingly attributable to the high temperature that lasted for a long time by rapid accumulation of ash-flow tuff inside the caldera.

Thus, it is thought that the development of multiple ring fractures by caldera collapse during pyroclastic flow-forming eruption from central vent could serve as a natural momentum shifting to multiple vents along the ring fractures concurrently with the closing of a single vent.

#### **6.3 Boiling-over eruption and ash-flow eruption**

Some volcanologists have suggested from the two-staged models that the pyroclastic flow-forming eruptions converted into ring fracture vents from a single vent related with a caldera [17, 18], occasionally into the eruption from ring fractures [18, 27]. It seems to be the direct main cause that the shifting vents into ring fractures were transformed into the boiling-over eruption from the pyroclastic flow-forming eruption by column collapse. The conversion into boiling-over eruption was attributable to the rapidly increasing discharge of magma; the increase in discharge was possible from sinking the caldera block into magma chamber along the outward dipping of ring fracture zones. That is, because of the increasing number of vents together with outward dipping of ring fracture zones, the conduits were naturally gradually widened, and instantaneous discharge rate was rapidly increased as the subsidence occurred. Accordingly, the eruption at this moment did not form high eruption column but turned into the boiling-over eruption that produced a large volume of ash flow (**Figure 6b**). The discharge rate of magma at this moment was much higher than the discharge rate necessary for maintaining the eruption column.

In addition, the naturally widening conduits along subsidence were not necessary for eroding the conduit walls. Accordingly, the lithic fragments rapidly decrease in volume and are much smaller in size within the upper ash-flow tuffs as 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 accumulate fallout tuff.

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 increasing eruption volume.
