**4.2 Ash-flow tuffs**

*Forecasting Volcanic Eruptions*

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

*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* 

collapse due to the outward dipping of the fracture zone [1].

**50**

**Figure 4.**

*of the Guamsan tuff.*

Lithofacies in the ash-flow tuffs comprises the graded tuff and lapilli tuff bed, massive tuff bed, and welding-foliated tuff bed.

The graded tuff and lapilli tuff beds exhibit whitish gray to pale gray color and 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 sorting by the difference in final falling velocities.

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

#### **Figure 5.**

*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.*

abundant content and very poor sorting. The lithics exhibit very weak grading, with them accumulated at the base. The thickness ranges from 4 to 25 m in the bed, the boundary of which is distinguished by the accumulation of lithics in a single bed. The lithofacies mostly occur in the lower tuff member (**Figure 4a**) and are mostly plotted into the tuff field. Under a microscope, the tuff has a small amount of plagioclase and orthoclase phenocrysts, as well as extremely rare biotite and opaque minerals. Vitric shards appear intact as skeletal, crescent, and "Y" shapes in the lower part; their outlines sometimes appear by devitrification. Accordingly, they exhibit almost a vitroclastic fabric due to non-welding to partial welding in the lowermost part. However, the lower tuff member produces weak welding foliations by the gradual flattening of pumices and shards with an increase in welding degree, when going up the member. According to the poor sorting, massive bedding, very thick beds, and non-welding, it suggests that the lithofacies were emplaced as ash flows of tephra accumulated from the collapse of relatively high eruption column created by a little huge explosion. The weak grading of lithics signifies the weak fluidization of the ash flow. The abundance in matrix also suggests reduced escapement of volcanic ash from the ash flow due to weak fluidization. Therefore, the lithofacies indicate that they were emplaced by slightly fluidized ash flow from collapse of the high eruption column. However, the increase in welding degree going upward indicates a gradual decrease in the height of the eruption column as well as a gradual increase in the discharge volume.

The welding-foliated tuff bed occurs only in the upper tuff member (**Figure 4c**). The matrix, gray to dark gray in color, corresponds to a vitric tuff that includes a few of plagioclase without quartz grain. Although the tuff on the whole is not sorted and has no stratifications, it is welded so seriously that it shows welding foliation similar to lava together with any features representing high fluidity during its emplacement. Additionally, although the boundary of each flow unit could be determined because of the abundance in lithics at the base, the other boundaries may not be observed as they recede from almost complete welding. Commonly, despite the fact that the ash-flow tuff bed ideally has a surge tuff bed at the base (layer 1) and a fallout tuff bed on the top (layer 3), they are not discovered (**Figure 4c**).

The lithofacies include rare lithics, which are mostly lapilli 2~5 cm in size. Vitric ash and pumices are densely welded and have welding foliations (**Figure 5c**). Pumices exhibit a dark gray color and are extremely flattened by the thick superposition of many ash flows, but they are still difficult to detect due to their small size. They are not easily recognizable as they exhibit aspects similar to lava on fresh surfaces. However, because lithics in the lithofacies are, although small in size, concentrated on the base of flow units, they are used to distinguish the single flow units. Under a microscope, the lithofacies occur as a small amount of plagioclase and alkali feldspars as microphenocrysts and rarely quartz, biotite, and opaque minerals. The pumices are devitrified to be crystalized as micrographic fabrics of silica and feldspars or as mosaic fabrics by vapor-phase crystallization in the core. Therefore, although the lithofacies exhibit welded foliation by dense welding of pumices and shards, they show eutaxitic fabric in the lower part and parataxitic fabric in the middle part; they then transform into the vitrophyric fabric in the upper part, of which is difficult to recognize pyroclastic structure. That is, the tuff represents the vitrophyric fabric similar to obsidian showing no traces of devitrification in spite of the gradual increase in welding degree going upward.

The poor sorting and welding foliation indicate that the lithofacies were emplaced from ash flows. No occurrence of ash cloud-derived fallout tuff in any of the sections may reflect its location proximal to the eruption center or short emplacement time between flow units due to successive eruptions. The abundance in lithics in the base suggests that the lithics were lagged downward by the lateral

**53**

**5. Postcollapse intrusions**

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

movement of ash flows, even during a short time [10]. The sufficiently dense welding to have an indistinct boundary between fiammes and matrix suggests that they were significantly liquefied by almost complete welding under high temperatures. Based on the dense welding, as well as the sparsity or smaller size of phenocrysts in the upper part of the ash-flow tuff, the lithofacies indicate that they were emplaced by ash flows of relatively high temperature. Regarding the severe devitrification, the lithofacies indicate that the cooling period was relatively long by maintaining a longer time under high temperature by the reduction of heat loss, because of thick

accumulation by rapid ash flows from the collapse of low eruption column.

The textural homogeneity, greater thickness, and denser welding reflect the sedimentary facies created by less fluidized non-expanded ash flows that have slow speed and less loss of volcanic ash into ash cloud, because they were originally erupted from hotter magma [11]. Such ash-flow phases also suggest that they originated from nonviolent voluminous eruptions of the type of boiling-over eruptions with continuous pulses. During the eruptions, it is thought that the flooding of repeated ash flows almost not occurring ash cloud had created so voluminous ashflow tuffs that were accumulated inside the caldera. However, the earlier eruptions also produced ash-flow tuffs rich in lithics, because the ash flows occurred along ring fracture forming a caldera. Because the thick ash-flow tuffs helped longer preservation of high temperature in the caldera, they resulted in dense welding as

Fallout tuff consists of pyroclastic rocks and tuffites. If the lithofacies are classified based on grain size and sedimentary structures, the pyroclastic rocks correspond to the planar-bedded tuff, whereas the tuffites comprise planar-bedded

to fine-grained ashes and extends laterally without any variations in thickness (**Figure 5d**). The lithofacies, which are about 1 m in thickness, are intercalated between ash-flow tuffs in medial or distal parts of northern and eastern margins of the caldera (**Figure 4b**). The lithofacies have poor sorting and normal grading that is finely grained upward in a single bed and rarely include accretionary lapilli ranging 5~10 mm in size, so they are easily recognized as a fallout tuff. No cross bedding or erosion tracks are discovered. The lithofacies is a vitric tuff that is mostly composed of vitric shards and scarce crystal grains. Crystal fragments consist of plagioclase, and quartzes are less than 1 mm in size and angular in shape. Evidences that the lithofacies are medium to fine in grain size and thinly intercalated between ash-flow tuffs indicate that the fallout tuff was derived from ash cloud following the ash flows. The fallout phase, derived from ash cloud, lies over the ash-flow tuff. Thus, the fallout tuff appears as a top facies following the normal facies of ashflow tuff. Additionally, the lithofacies occurring in either medial or distal parts of northern and eastern margins of caldera suggests that their crater was located in the southwestern part of the caldera. Because the fallout phase derived from ash cloud becomes abundant as it goes away from the crater, it is almost not observable in the

proximal part near the crater but most dominant in the distal part.

Postcollapse intrusions in the Guamsan caldera are composed of rhyolitic dikes (60.65 Ma in [9]) and plugs, which are exposed as several lithofacies which

Planar-bedded tuff is pale bluish green or gray in color and consists of medium

tuffaceous sandstone and massive tuffaceous mudstone.

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

well as high devitrification.

**4.3 Fallout tuffs**

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

movement of ash flows, even during a short time [10]. The sufficiently dense welding to have an indistinct boundary between fiammes and matrix suggests that they were significantly liquefied by almost complete welding under high temperatures. Based on the dense welding, as well as the sparsity or smaller size of phenocrysts in the upper part of the ash-flow tuff, the lithofacies indicate that they were emplaced by ash flows of relatively high temperature. Regarding the severe devitrification, the lithofacies indicate that the cooling period was relatively long by maintaining a longer time under high temperature by the reduction of heat loss, because of thick accumulation by rapid ash flows from the collapse of low eruption column.

The textural homogeneity, greater thickness, and denser welding reflect the sedimentary facies created by less fluidized non-expanded ash flows that have slow speed and less loss of volcanic ash into ash cloud, because they were originally erupted from hotter magma [11]. Such ash-flow phases also suggest that they originated from nonviolent voluminous eruptions of the type of boiling-over eruptions with continuous pulses. During the eruptions, it is thought that the flooding of repeated ash flows almost not occurring ash cloud had created so voluminous ashflow tuffs that were accumulated inside the caldera. However, the earlier eruptions also produced ash-flow tuffs rich in lithics, because the ash flows occurred along ring fracture forming a caldera. Because the thick ash-flow tuffs helped longer preservation of high temperature in the caldera, they resulted in dense welding as well as high devitrification.

### **4.3 Fallout tuffs**

*Forecasting Volcanic Eruptions*

a gradual increase in the discharge volume.

bed on the top (layer 3), they are not discovered (**Figure 4c**).

the gradual increase in welding degree going upward.

abundant content and very poor sorting. The lithics exhibit very weak grading, with them accumulated at the base. The thickness ranges from 4 to 25 m in the bed, the boundary of which is distinguished by the accumulation of lithics in a single bed. The lithofacies mostly occur in the lower tuff member (**Figure 4a**) and are mostly plotted into the tuff field. Under a microscope, the tuff has a small amount of plagioclase and orthoclase phenocrysts, as well as extremely rare biotite and opaque minerals. Vitric shards appear intact as skeletal, crescent, and "Y" shapes in the lower part; their outlines sometimes appear by devitrification. Accordingly, they exhibit almost a vitroclastic fabric due to non-welding to partial welding in the lowermost part. However, the lower tuff member produces weak welding foliations by the gradual flattening of pumices and shards with an increase in welding degree, when going up the member. According to the poor sorting, massive bedding, very thick beds, and non-welding, it suggests that the lithofacies were emplaced as ash flows of tephra accumulated from the collapse of relatively high eruption column created by a little huge explosion. The weak grading of lithics signifies the weak fluidization of the ash flow. The abundance in matrix also suggests reduced escapement of volcanic ash from the ash flow due to weak fluidization. Therefore, the lithofacies indicate that they were emplaced by slightly fluidized ash flow from collapse of the high eruption column. However, the increase in welding degree going upward indicates a gradual decrease in the height of the eruption column as well as

The welding-foliated tuff bed occurs only in the upper tuff member (**Figure 4c**). The matrix, gray to dark gray in color, corresponds to a vitric tuff that includes a few of plagioclase without quartz grain. Although the tuff on the whole is not sorted and has no stratifications, it is welded so seriously that it shows welding foliation similar to lava together with any features representing high fluidity during its emplacement. Additionally, although the boundary of each flow unit could be determined because of the abundance in lithics at the base, the other boundaries may not be observed as they recede from almost complete welding. Commonly, despite the fact that the ash-flow tuff bed ideally has a surge tuff bed at the base (layer 1) and a fallout tuff

The lithofacies include rare lithics, which are mostly lapilli 2~5 cm in size. Vitric ash and pumices are densely welded and have welding foliations (**Figure 5c**). Pumices exhibit a dark gray color and are extremely flattened by the thick superposition of many ash flows, but they are still difficult to detect due to their small size. They are not easily recognizable as they exhibit aspects similar to lava on fresh surfaces. However, because lithics in the lithofacies are, although small in size, concentrated on the base of flow units, they are used to distinguish the single flow units. Under a microscope, the lithofacies occur as a small amount of plagioclase and alkali feldspars as microphenocrysts and rarely quartz, biotite, and opaque minerals. The pumices are devitrified to be crystalized as micrographic fabrics of silica and feldspars or as mosaic fabrics by vapor-phase crystallization in the core. Therefore, although the lithofacies exhibit welded foliation by dense welding of pumices and shards, they show eutaxitic fabric in the lower part and parataxitic fabric in the middle part; they then transform into the vitrophyric fabric in the upper part, of which is difficult to recognize pyroclastic structure. That is, the tuff represents the vitrophyric fabric similar to obsidian showing no traces of devitrification in spite of

The poor sorting and welding foliation indicate that the lithofacies were emplaced from ash flows. No occurrence of ash cloud-derived fallout tuff in any of the sections may reflect its location proximal to the eruption center or short emplacement time between flow units due to successive eruptions. The abundance in lithics in the base suggests that the lithics were lagged downward by the lateral

**52**

Fallout tuff consists of pyroclastic rocks and tuffites. If the lithofacies are classified based on grain size and sedimentary structures, the pyroclastic rocks correspond to the planar-bedded tuff, whereas the tuffites comprise planar-bedded tuffaceous sandstone and massive tuffaceous mudstone.

Planar-bedded tuff is pale bluish green or gray in color and consists of medium to fine-grained ashes and extends laterally without any variations in thickness (**Figure 5d**). The lithofacies, which are about 1 m in thickness, are intercalated between ash-flow tuffs in medial or distal parts of northern and eastern margins of the caldera (**Figure 4b**). The lithofacies have poor sorting and normal grading that is finely grained upward in a single bed and rarely include accretionary lapilli ranging 5~10 mm in size, so they are easily recognized as a fallout tuff. No cross bedding or erosion tracks are discovered. The lithofacies is a vitric tuff that is mostly composed of vitric shards and scarce crystal grains. Crystal fragments consist of plagioclase, and quartzes are less than 1 mm in size and angular in shape. Evidences that the lithofacies are medium to fine in grain size and thinly intercalated between ash-flow tuffs indicate that the fallout tuff was derived from ash cloud following the ash flows. The fallout phase, derived from ash cloud, lies over the ash-flow tuff. Thus, the fallout tuff appears as a top facies following the normal facies of ashflow tuff. Additionally, the lithofacies occurring in either medial or distal parts of northern and eastern margins of caldera suggests that their crater was located in the southwestern part of the caldera. Because the fallout phase derived from ash cloud becomes abundant as it goes away from the crater, it is almost not observable in the proximal part near the crater but most dominant in the distal part.
