**2. Geological setting**

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 region belongs to the Juwangsan volcanic field.

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

**47**

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

The intrusive rocks comprise rhyolitic intrusions relevant to the caldera as well as biotitic granite and felsite dikes irrelevant to the caldera. The rhyolitic intrusions correspond to the volcanic rocks of postcaldera, which can be divided into intracaldera intrusions and ring dikes. The biotitic granite is exposed as small stocks,

In the area, the geology is cut by the Sampo fault and Jayangcheon fault, the strike-slip fault running southwest from northeast and is depressed by a fault run-

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

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

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

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

Volcanic breccias can be subdivided into two lithofacies of disorganized massive

The disorganized massive breccia mostly consists of monolithic blocks of rhyo-

lite and accompanies rare accessory blocks of andesite and welded tuff.

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

whereas the felsite is exposed as linear dikes.

ning west from east (**Figure 1**).

asymmetrical cylindrical caldera [1].

composition, which indicates high silica to low silica [2].

**4. Guamsan Tuff**

volcanic ejecta.

**4.1 Volcanic breccias**

breccia and chaotic massive breccia.

**3. Guamsan caldera**

66.0 km in [2].

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

The intrusive rocks comprise rhyolitic intrusions relevant to the caldera as well as biotitic granite and felsite dikes irrelevant to the caldera. The rhyolitic intrusions correspond to the volcanic rocks of postcaldera, which can be divided into intracaldera intrusions and ring dikes. The biotitic granite is exposed as small stocks, whereas the felsite is exposed as linear dikes.

In the area, the geology is cut by the Sampo fault and Jayangcheon fault, the strike-slip fault running southwest from northeast and is depressed by a fault running west from east (**Figure 1**).
