**5.2 Ring dikes**

Ring dikes occur along the caldera margin, and they can be categorized into the inner, intermediate, and outer ring dikes.

In terms of chemical composition, the dikes are mostly rhyolitic, whereas the southwestern ring dike is rhyodacitic in [2]. This indicates that the dikes are, chemically and mineralogically, similar to the Guamsan Tuff; this suggests that the ring dikes have a closely spatiotemporal relation with the caldera-forming eruption [13].

1.**Inner ring dike:** The dike is a combination of flow-banded rhyolite in the inner margin with stony rhyolite in the outer margin. The flow-banded rhyolite makes a steep slope in the inner margin of the ring dike. On the whole, the flow foliation is developed as closely spaced intervals and exhibits reddish gray to pale red colors. The flow-banded rhyolite is poor in phenocryst and alternates with very thin glassy bands and microcrystalline bands. The absence in phenocrysts seems to be the consequence of filter-pressing that passed only liquid except for crystals when magma was injected through fracture by compression in magma chamber. The glassy and microcrystalline textures indicate that the rhyolite was rapidly cooled because of the loss of volatile materials during magma rising instead of heat loss into conduit wall.

**55**

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

line or cryptocrystalline feldspar, quartz, biotite, etc.

resulting from caldera collapse.

Stony rhyolite is frequently called felsite. It has a pale pink to pale gray color and indistinct or no flow foliation and rarely contains very tiny phenocrysts of quartz and feldspars. Under a microscope, quartz phenocrysts are rarely bipyramid in shape or embayed by resorption. The groundmass exhibits an intergranular texture by crystallization into microcrystalline to cryptocrystalline grain size. Therefore, according to the crystallinity, the earlier emplacement of

inner side was followed by sequential intrusions after caldera collapse.

2.**Outer and intermediate dikes:** The outer ring dike intrudes along contact with the caldera and has contact with sedimentary rocks, Jugjang Volcanics, Naeyeonsan Tuff, and Muposan Tuff. The intermediate ring dike branches off inward from the southeastern part of the outer ring dike and then joins together at the south part; two small branches extend from the inner margin. The lithofacies exhibit whitish gray, pale gray, pale green, and pale pink colors, and they only show flow foliation in narrow dikes. They lack spherulite texture but represent stony or porphyritic textures. Thus, the outer ring dike shows stony rhyolite in the western and northern parts, porphyritic rhyolite in the eastern part, and porphyritic rhyodacite in the southern part, in which both are gradual. Further, the dike includes intrusive tuff in the northern part. In addition, the intermediate ring dike mostly consists of stony rhyolite.

Stony rhyolite contains microphenocrysts of plagioclase and alkali feldspars and accompanies tiny amounts of quartz and opaques. The phenocrysts are below 1 mm in diameter and are very small in content but also vary significantly depending on the locations. Occasionally, the rhyolite gradually converts into porphyritic rhyolite. The groundmass is mostly of quartzofeldspathic and occasionally contains biotite or opaques. Porphyritic rhyolite contains phenocrysts of quartz, alkali feldspars, and plagioclase and rarely accompanies opaques. Quartz phenocrysts, up to 3 mm across, show embayed outline by significant resorption and contain rarely microcrystalline inclusions and occasionally bipyramid crystals. Some plagioclases constitute glomerophenocrysts. The groundmass shows an intergranular texture consisting of microcrystalline crystals. It is difficult to determine the boundary of such lithofacies because of the gradational change between porphyritic rhyodacite and stony rhyolite. Porphyritic rhyodacite is dominant in plagioclase phenocryst and rarely accompanies alkali feldspars, hornblende, biotite, and opaques. Phenocrysts locally show very significant variations in content. The plagioclase phenocrysts are euhedral in shape and show a faint zonal structure. The groundmass consists of microcrystal-

On the whole, the ring dikes show textural changes such as flow-banded, porphyritic, and stony structures in going toward the outer ring dike from the caldera center and also exhibit gradually compositional variation from rhyolite to rhyodacite in passing through southwest from the central part. Because all these constitute the ring dikes, this suggests that they intruded through ring faults or fractures

It is thought that the emplacement timings may be locally different between rhyolites and various lithofacies. The textural changes from the caldera center to the outer ring dike suggest that the injection speed of magma was faster in the inner ring dike than in the outer ring dike, and the cooling rate was so slow that magma could crystallize to coarser grains owing to an intrusion of magma through wider passage in outer ring dike. Alternately, the changes between flow-banded textures and others may imply for temporal pulsational intrusions that the earlier injection of much hotter magma into the inner ring fault was followed by the injection of less hot magma into the outer ring fault, and changes between stony and porphyritic

*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*

formed [12].

**5.1 Intracaldera intrusions**

ranging from 40 to 75°.

**5.2 Ring dikes**

fallout tuff when the rhyolite intruded the tuff.

inner, intermediate, and outer ring dikes.

are presumably connected to the identical magma chamber beneath the caldera. Based on differences in igneous structures and chemical composition, the lithofacies can be roughly subdivided into flow-banded rhyolite, porphyritic rhyolite, porphyritic rhyodacite, and stony rhyolite. In the field, a single rock body shows change in lithofacies within a few 100 m or a few km. In such cases, the erosion level in the Guamsan caldera area may nearly be approaching the original roof of the intrusions. Thus, according to positions related to the caldera-forming eruption, the intrusions can be categorized into intracaldera intrusions and ring dikes (**Figure 1**). The intrusions into the higher level of the Guamsan Tuff can demonstrate that they are the roots of postcollapse volcano, because the Guamsan Tuff has been of the final products and placed at a higher level as the caldera was

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

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

Ring dikes occur along the caldera margin, and they can be categorized into the

1.**Inner ring dike:** The dike is a combination of flow-banded rhyolite in the inner margin with stony rhyolite in the outer margin. The flow-banded rhyolite makes a steep slope in the inner margin of the ring dike. On the whole, the flow foliation is developed as closely spaced intervals and exhibits reddish gray to pale red colors. The flow-banded rhyolite is poor in phenocryst and alternates with very thin glassy bands and microcrystalline bands. The absence in phenocrysts seems to be the consequence of filter-pressing that passed only liquid except for crystals when magma was injected through fracture by compression in magma chamber. The glassy and microcrystalline textures indicate that the rhyolite was rapidly cooled because of the loss of volatile materials during

magma rising instead of heat loss into conduit wall.

In terms of chemical composition, the dikes are mostly rhyolitic, whereas the southwestern ring dike is rhyodacitic in [2]. This indicates that the dikes are, chemically and mineralogically, similar to the Guamsan Tuff; this suggests that the ring dikes have a closely spatiotemporal relation with the caldera-forming eruption [13].

**54**

Stony rhyolite is frequently called felsite. It has a pale pink to pale gray color and indistinct or no flow foliation and rarely contains very tiny phenocrysts of quartz and feldspars. Under a microscope, quartz phenocrysts are rarely bipyramid in shape or embayed by resorption. The groundmass exhibits an intergranular texture by crystallization into microcrystalline to cryptocrystalline grain size. Therefore, according to the crystallinity, the earlier emplacement of inner side was followed by sequential intrusions after caldera collapse.

2.**Outer and intermediate dikes:** The outer ring dike intrudes along contact with the caldera and has contact with sedimentary rocks, Jugjang Volcanics, Naeyeonsan Tuff, and Muposan Tuff. The intermediate ring dike branches off inward from the southeastern part of the outer ring dike and then joins together at the south part; two small branches extend from the inner margin. The lithofacies exhibit whitish gray, pale gray, pale green, and pale pink colors, and they only show flow foliation in narrow dikes. They lack spherulite texture but represent stony or porphyritic textures. Thus, the outer ring dike shows stony rhyolite in the western and northern parts, porphyritic rhyolite in the eastern part, and porphyritic rhyodacite in the southern part, in which both are gradual. Further, the dike includes intrusive tuff in the northern part. In addition, the intermediate ring dike mostly consists of stony rhyolite.

Stony rhyolite contains microphenocrysts of plagioclase and alkali feldspars and accompanies tiny amounts of quartz and opaques. The phenocrysts are below 1 mm in diameter and are very small in content but also vary significantly depending on the locations. Occasionally, the rhyolite gradually converts into porphyritic rhyolite. The groundmass is mostly of quartzofeldspathic and occasionally contains biotite or opaques.

Porphyritic rhyolite contains phenocrysts of quartz, alkali feldspars, and plagioclase and rarely accompanies opaques. Quartz phenocrysts, up to 3 mm across, show embayed outline by significant resorption and contain rarely microcrystalline inclusions and occasionally bipyramid crystals. Some plagioclases constitute glomerophenocrysts. The groundmass shows an intergranular texture consisting of microcrystalline crystals. It is difficult to determine the boundary of such lithofacies because of the gradational change between porphyritic rhyodacite and stony rhyolite.

Porphyritic rhyodacite is dominant in plagioclase phenocryst and rarely accompanies alkali feldspars, hornblende, biotite, and opaques. Phenocrysts locally show very significant variations in content. The plagioclase phenocrysts are euhedral in shape and show a faint zonal structure. The groundmass consists of microcrystalline or cryptocrystalline feldspar, quartz, biotite, etc.

On the whole, the ring dikes show textural changes such as flow-banded, porphyritic, and stony structures in going toward the outer ring dike from the caldera center and also exhibit gradually compositional variation from rhyolite to rhyodacite in passing through southwest from the central part. Because all these constitute the ring dikes, this suggests that they intruded through ring faults or fractures resulting from caldera collapse.

It is thought that the emplacement timings may be locally different between rhyolites and various lithofacies. The textural changes from the caldera center to the outer ring dike suggest that the injection speed of magma was faster in the inner ring dike than in the outer ring dike, and the cooling rate was so slow that magma could crystallize to coarser grains owing to an intrusion of magma through wider passage in outer ring dike. Alternately, the changes between flow-banded textures and others may imply for temporal pulsational intrusions that the earlier injection of much hotter magma into the inner ring fault was followed by the injection of less hot magma into the outer ring fault, and changes between stony and porphyritic

textures may also imply the sequential continuous intrusions of magma. However, the textural changes southwestward from the northern part of the caldera suggest that the erosion degree increased while going further southwestward.

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].
