**3. Guamsan caldera**

*Forecasting Volcanic Eruptions*

intrusions and ring dikes (**Figure 1**). The lithofacies and sequences of these extrusive rocks and intrusive rocks are sufficient to interpret eruption types and recon-

*Generalized geological map in and around the Guamsan caldera in the northeastern Gyeongsang basin. RI, ring intrusions (RI, inner ring dike; RI2, intermediate ring dike; RI3, outer ring dike); CI, intracaldera* 

Eruption types and volcanic processes before and after caldera collapse will be discussed in this study. The ultimate results reveal that the volcanic activities in Gyeongsang Basin are not only related with caldera volcanisms but are also significant for understanding the characteristics of the igneous processes. Further, the consequences will contribute to the understanding of other volcanisms and

The Gyeongsang basin has a broad distribution of volcanic rocks which are products of the Late Cretaceous to Early Paleogene calc-alkalic volcanism in the subduction zone along the Eurasian continental margin [3–6]. These volcanic rocks are mostly distributed in the Yucheon subbasin and are also found in the region between southeastern Yeongyang subbasin and mid-eastern Euiseong subbasin; this

The volcanic rocks occurring in the field mostly consist of extrusive rocks accompanied by small amounts of intrusive rocks. The extrusive rocks are placed on sedimentary rocks of the Hayang Group and can be roughly categorized into lower basic to intermediate volcanic rocks and upper acidic volcanic rocks. The former extrusive rocks comprise the stratigraphic units of Daejeonsa Basalt, Ipbong Andesite, and Jukjang Volcanics. The latter extrusive rocks consist of Jipum Volcanics (68.5 Ma in [7]), Juwangsan Tuff, Naeyeonsan Tuff, Neogudong Formation, Muposan Tuff

struct volcanic processes in the Guamsan caldera.

*intrusions; F1, Sampo fault; F2, Jayangcheon fault; EW, trending fault.*

region belongs to the Juwangsan volcanic field.

**2. Geological setting**

**Figure 1.**

processes in the calderas as well as their comparative effects.

(67.08 Ma in [8]), and Guamsan Tuff (63.77 ∼ 60.1 Ma in [9]).

**46**

The Guamsan caldera is bound along the structural line as determined by the outer ring dike. The caldera is approximately 9.2 km in maximum diameter and 8.0 km in minimum diameter [1]; the resulting internal area is then approximately 66.0 km in [2].

The intracaldera Guamsan Tuff has a contact with the underlying Muposan Tuff, with the ring dike intervening between two units, suggesting that the former unit has been subsided as compared to the latter unit. This can be counted as direct evidence of the subsidence by the collapse of a caldera. The welding foliation and bedding in the volcanic and sedimentary rocks generally represent a basin structure like a bowl shape, which has steep to gentle dips inwardly from the caldera margin. The structure suggests a direct subsidence from the collapse of the caldera. The caldera block shows an asymmetrical feature that was collapsed to 900 m along the northern margin, whereas it was collapsed to 300 m along the southern margin [1].

Therefore, the caldera was formed by down-sagging and ring-faulting. Based on comprehensive integration of these evidences, the caldera is classified as one of the asymmetrical cylindrical caldera [1].

### **4. Guamsan Tuff**

Guamsan Tuff refers to a stratigraphic unit composed of volcanic breccias, ashflow tuffs, fallout tuffs, and tuffites derived from the Guamsan caldera (**Figure 2**). The stratigraphic unit mostly consists of ash-flow tuffs that are only distributed inside the caldera (**Figure 1**). Though it was thick tuffs accumulated from the radial spreading of voluminous ash flow erupted from the crater hidden inside the caldera, it now remains only inside the caldera, due to prolonged erosion and denudation. The remaining body exposes its cross sections of the lower member (63.77 Ma in [9]) to upper member (60.1 Ma in [9]), because it has not only deep valleys that have been made by erosion but is also inclined northward by an igneous intrusion in the southern outer part of the caldera. The tuffs range 72~78% SiO2 in composition, which indicates high silica to low silica [2].

The author describes lithofacies and mutual relations in the Guamsan Tuff and reconstructs the evolution processes of complex volcanic events from the volcanic ejecta.

#### **4.1 Volcanic breccias**

Volcanic breccias can be subdivided into two lithofacies of disorganized massive breccia and chaotic massive breccia.

The disorganized massive breccia mostly consists of monolithic blocks of rhyolite and accompanies rare accessory blocks of andesite and welded tuff.

#### **Figure 2.**

*Typical columnar section of the Guamsan tuff, which exhibits variations in stratigraphic units that comprise volcanic breccias, fallout tuffs, and tuffites in entire ash-flow tuffs.*

The blocks are typically 5~15 cm in diameter, rarely reaching over 1 m, and are subround to subangular in shape (**Figure 3a**). The matrix consists of pale gray to gray ash that supports the blocks. The lithofacies exhibit the vertically thick lenticular form with laterally poor continuity. Typically, the lithofacies may be very thick in topographic depression and very thin on topographic high. Except for the blocks, it resembles lapilli tuff without internal stratification and grading. The lithofacies occur as basal breccia in the southwestern side of the outer ring dike (**Figure 4a**), from which the breccia gradually becomes grading into lapilli tuff in going northeastward. Along the emplaced site, it suggests that the breccia could be significantly different in terms of the mechanism forming the lithofacies. This can be supported by the fact that the trend is significantly different from those of other rock units in granulometric classification diagram.

**49**

**Figure 3.**

*thick tuff and lapilli tuff bed.*

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

The breccia embraces the southwestern side of the ring dike (**Figure 1**) and overlies the Naeyeonsan Tuff, cutting the Jugjang Volcanics. The lithofacies are correlated to the lowermost part of the Guamsan Tuff, based on sulfide alteration, stratigraphic relation, and lithologic correlation [1]. Therefore, the breccia may be considered as pyroclastic rocks accumulated from pyroclastic flows on a cone slope. The mechanism forming the pyroclastic flow may be dominated by the collapse of lava dome, because of the rhyolitic monolithic blocks. Namely, the pyroclastic flow-forming eruption type may be of the block and ash-flow phase that flowed along the slope from the collapse of active lava dome. Here, such lithofacies may indicate that it would be a near-vent facies, suggesting the initiation of volcanism in the Guamsan area. Chaotic massive breccia, while it cannot be expressed on a geological map, is intercalated as a wedge shape between ash-flow tuffs in the lower part of the upper tuff member. The lithofacies consist of many blocks of rhyolite and welded tuff, which range 10~20 cm in diameter and occasionally over 1 m (**Figure 3b**). Based on granulometry, the rock is classified as tuff breccia, of which the trend is very different from those of other rock units. The boundary surface of the lithofacies can be investigated due to the fact that it is deeply eroded enough to expose the base. Though the base is irregular, the flat top is discontinuing laterally to be connected to normal lapilli tuffs. The lithofacies have very irregularly chaotic internal structures, which are cut by small faults (**Figure 3b**). The lithofacies, which occur sporadically in a valley adjacent to the northern ring fracture zone, are intercalated stratigraphically in the middle part of the Guamsan Tuff (**Figure 4b**). These facts suggest that the lithofacies had been produced by gravitational sliding of blocks or debris flow from caldera wall along the ring fracture zone. Therefore, many blocks around 1 m in diameter are observed around the collapse zone. Therefore, the breccia is considered as a caldera-collapse breccia, which is classified as the debris-flow phase by the caldera collapse. In addition, the lithofacies are thought to suggest a vent transition that indicates fissure eruption. If the eruption

*Photographs in the Guamsan tuff. (a) Disorganized massive breccia in a near-vent facies; (b) chaotic massive breccia near the northern caldera margin; (c) graded lapilli tuff; (d) eutaxitic fabric in the middle part of the* 

*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*

#### **Figure 3.**

*Forecasting Volcanic Eruptions*

**48**

**Figure 2.**

granulometric classification diagram.

*volcanic breccias, fallout tuffs, and tuffites in entire ash-flow tuffs.*

The blocks are typically 5~15 cm in diameter, rarely reaching over 1 m, and are subround to subangular in shape (**Figure 3a**). The matrix consists of pale gray to gray ash that supports the blocks. The lithofacies exhibit the vertically thick lenticular form with laterally poor continuity. Typically, the lithofacies may be very thick in topographic depression and very thin on topographic high. Except for the blocks, it resembles lapilli tuff without internal stratification and grading. The lithofacies occur as basal breccia in the southwestern side of the outer ring dike (**Figure 4a**), from which the breccia gradually becomes grading into lapilli tuff in going northeastward. Along the emplaced site, it suggests that the breccia could be significantly different in terms of the mechanism forming the lithofacies. This can be supported by the fact that the trend is significantly different from those of other rock units in

*Typical columnar section of the Guamsan tuff, which exhibits variations in stratigraphic units that comprise* 

*Photographs in the Guamsan tuff. (a) Disorganized massive breccia in a near-vent facies; (b) chaotic massive breccia near the northern caldera margin; (c) graded lapilli tuff; (d) eutaxitic fabric in the middle part of the thick tuff and lapilli tuff bed.*

The breccia embraces the southwestern side of the ring dike (**Figure 1**) and overlies the Naeyeonsan Tuff, cutting the Jugjang Volcanics. The lithofacies are correlated to the lowermost part of the Guamsan Tuff, based on sulfide alteration, stratigraphic relation, and lithologic correlation [1]. Therefore, the breccia may be considered as pyroclastic rocks accumulated from pyroclastic flows on a cone slope. The mechanism forming the pyroclastic flow may be dominated by the collapse of lava dome, because of the rhyolitic monolithic blocks. Namely, the pyroclastic flow-forming eruption type may be of the block and ash-flow phase that flowed along the slope from the collapse of active lava dome. Here, such lithofacies may indicate that it would be a near-vent facies, suggesting the initiation of volcanism in the Guamsan area.

Chaotic massive breccia, while it cannot be expressed on a geological map, is intercalated as a wedge shape between ash-flow tuffs in the lower part of the upper tuff member. The lithofacies consist of many blocks of rhyolite and welded tuff, which range 10~20 cm in diameter and occasionally over 1 m (**Figure 3b**). Based on granulometry, the rock is classified as tuff breccia, of which the trend is very different from those of other rock units. The boundary surface of the lithofacies can be investigated due to the fact that it is deeply eroded enough to expose the base. Though the base is irregular, the flat top is discontinuing laterally to be connected to normal lapilli tuffs. The lithofacies have very irregularly chaotic internal structures, which are cut by small faults (**Figure 3b**). The lithofacies, which occur sporadically in a valley adjacent to the northern ring fracture zone, are intercalated stratigraphically in the middle part of the Guamsan Tuff (**Figure 4b**). These facts suggest that the lithofacies had been produced by gravitational sliding of blocks or debris flow from caldera wall along the ring fracture zone. Therefore, many blocks around 1 m in diameter are observed around the collapse zone. Therefore, the breccia is considered as a caldera-collapse breccia, which is classified as the debris-flow phase by the caldera collapse. In addition, the lithofacies are thought to suggest a vent transition that indicates fissure eruption. If the eruption

was initiated along the fissure, then it may have left many lithic fragments from the erosion of the vent walls. However, the content of lithic fragment decreases rapidly as it goes upward; this is a result of widening the conduit without erosion along caldera collapse due to the outward dipping of the fracture zone [1].

#### **Figure 4.**

*Stratigraphic sections displaying variations in lithofacies of the Guamsan tuff (GT), underlain by the Muposan tuff (MT). (a) Lower member in the proximal zone; (b) lower member in the distal zone; (c) upper member of the Guamsan tuff.*

**51**

**Figure 5.**

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

massive tuff bed, and welding-foliated tuff bed.

sorting by the difference in final falling velocities.

Lithofacies in the ash-flow tuffs comprises the graded tuff and lapilli tuff bed,

The graded tuff and lapilli tuff beds exhibit whitish gray to pale gray color and

Massive tuff bed is a lithic-rich vitric tuff that consists of lithics with various colors and a small amount of pumices. The matrix usually exhibits a whitish gray, pale gray, or pale bluish green color, whereas the lithics exhibit a dark gray, pale brown, or dark bluish green color; the pumices are mostly tinged with whitish gray color (**Figure 5b**). The matrix shows a massive appearance because it has very

*Photographs in the Guamsan tuff. (a) Lithic-rich zone in the lower part of the thick tuff and lapilli tuff bed; (b) pale gray massive tuff; (c) dark gray welding-foliated tuff in the upper member; (d) planar-bedded tuff* 

*beds intercalated between ash-flow tuff beds in the lower member.*

are classified as lithic-rich vitric tuff consisting of lithics and pumice lapilli in the volcanic ash matrix. On the whole, the lithic lapilli are rich in the lower part, whereas pumice lapilli are more or less rich in the upper part. Therefore, lithics show a normal grading that gradually decrease upward in grain size within a single bed (**Figure 3c**). The matrix is composed of coarse ash, which is relatively richer than lapilli in the upper part. The lithofacies range from 2 to 20 cm in thickness and show laterally well extensibility. They represent eutaxitic fabric due to slightly welding in a thick single bed (**Figure 3d**), with lithics accumulated in its base (**Figure 5a**). The rock facies dominantly appear in the lower tuff member, whereas they occur in the lower part of the upper tuff member (**Figure 4a, b**). The grading in the lithofacies suggests a column collapse phase that was derived from the collapse of the high eruption column produced by a huge explosion. Then, collapsed tephra creates strongly fluidized pyroclastic flow that is supported by rising fluids in the same way as water vapor, etc., and dense lithics in flowing result in gradual

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

**4.2 Ash-flow tuffs**
