• The Journal of the Petrological Society of Korea
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• v.1 no.2
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• pp.91-103
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• 1992

Eruptive mechanisms and processes at Udo tuff cone can be inferred from indicative characters of products, bedforms and lithofacies, and ring faults. In terms of bedforms and lithofa-cies in particular, massive lapilli tuff beds and chaotic lapilli tuff beds are derived from subaerial falls of aggregated tephra of wet tephra finger jets, occurring dominantly at the lower sequences of proximal part at the tuff cone. Crudely stratified lapilli tuff are derived from subaerial falls of slightly aggregated tephra of less wet tephra finger jets, whereas reversely graded lapilli tuff beds are from slightly disaggregated subaerial falls of continuous uprush. Both beds frequently occur in the middle sequences at proximal and near medial part of the tuff cone. Block and lapilli tephra lenses, ash-coated lapilli tephra beds(lenses) and thin-bedded tuff beds are derived from extremely disaggregated subaerial falls of dry tephra in the continuous uprush, frequently occurring at the upper sequences of medial part at the tuff cone. Udo tuff cone is a basaltic volcano emergent through the sea water surface while water could flood across or into the vent area. Emergence of the tuff cone was from the type-Surtseyan eruption characterized by earlier tephra finger jets and later continuous uprush columns of tephra with copious volumes of steam. Explosions began when boiling of wter produced a bubble column reducing the hydrostatic pres-sure, allowing exsolution of gases from the magma. This expansion of magma into a vesiculating froth fragmented the magma and permitted mixing of magma and water so that a more vigorous generation of steam could proceed. Tephra finger jetting explosions continued to build the crater rims, then remove water from the vent that their deposits flowed like slsurries until the continuous uprush explosion ensued. Continuous uprush explosions were associated with most rapid accumula-tion of tephra. The increasing volume rate led to partial removal of water from the vent area by the newly tephra ring so that more vigorous activity could be attended by a reducing water supply. This might restrain surplus of cold water entering the vent and thus enhance the vigour of the eruption by allowing optimal heat exchange. Eventually the crater became so deep and unsuported that piecemeal sliding, or massive subsidence on indipping ring faults, filled and closed the vent, and the cycle of explosions and collapse began anew.

• Economic and Environmental Geology
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• v.21 no.4
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• pp.335-348
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• 1988

The author aimed to describe the volcano-stratigraphy and petrology of the volcanic mass in the Koheung peninsula, South Cheolla province. The volcanic mass is composed of the volcanics and intrusives of late Cretaceous which extruded the Pre-cambrian metamorphic(Jirisan gneiss complex) and the early Cretaceous sedimentary(Duwon Formation) basement. The volcanic pile consists of, in ascending order, Bibongsan andesite, Koheung tuff and breccia, and Palyeongsan welded tuff, and are intruded by ring intrusives( intrusive breccia, andesite porphyry, intrusive rhyolite and fine-grained quartz-diorite) and central pluton(diorite, quartz monzodiorite, biotite granite and micrographic granite). Bibongsan andesite mainly consists of andesite tuff and lava. Koheung tuff consists of alternation of fine tuff, coarse tuff and lapilli tuff, and Palyeongsan welded tuff which overlies Koheung tuff, comprises K-feldspar and quartz phenocrysts, elongated brown fiamme, lithic fragments in matrix of devitrified brown glass shards, and mainly consists of rhyodacite to rhyolite vitric ash-flow tuff. The results of petrochemical studies of the igneous rocks suggest that the rocks were a serial differentiational products of fractional crystallization of calc-alkaline magma series. This study reveals that the volcanic mass in this area is inferred to the remnant of the resurgent cauldron, measuring 30 by 25 km in diameter. The cauldron block was lowered at least 1,000 m by ring fault displacement.

• Economic and Environmental Geology
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• v.54 no.1
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• pp.35-48
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• 2021

The Gumi basin, situated in the mid-southeastern Yeongnam Massif, has the Cretaceous stratigraphy that is divided into Gumi Formation, andesitic rocks (Yeongamsan Tuff, Busangni Andesite), rhyolitic rocks (Obongni Tuff, Doseongul Rhyolite, Geumosan Tuff) and Intrusives (ring dikes, other dikes) in ascending order. The Geumosan Tuff is composed mostly of many ash-flow tuffs which are associated with Geumosan caldera along with the ring dikes. The caldera is outlined by ring faults and dikes and has about 3.5 × 5.6 km in diameters. The intracaldera volcanics show a downsag structure that is dipped inward in their flow and welding foliations. The caldera block represent an asymmetric subsidence, which drops 350 m in the northern margin and 600 m in the southern one. Based on these data, the Geumosan caldera is geometrically classified as an asymmetric piston subsidence caldera that suggests a single caldera cycle. The caldera reflects the piston subsidence of the caldera block bounded by the outward-dipping ring faults following a voluminous eruption of magma from the chamber. The downsag in the caldera block refers to the downsagging during the initial subsidence at the same time as the full development of the bound fault. In the ring fissures following the sagging, magma was injected due to the overpressure of magma chamber caused by subsidence.

• The Journal of the Petrological Society of Korea
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• v.8 no.3
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• pp.171-182
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• 1999

The Myeonbongsan caldera, 10.2X8.0 km, developed within older sequences of sedimentary formations and intermediate composition volcanis in the southern Cheongsong area. Volcanic rocks in the caldera block include lower intermediate volcanics, middle tuffaceous sequences and upper silicic ones. The silicic volcanics, which is named Myeonbongsan Tuff, are composed of crystal-rich ash-flow tuff(300 m) , bedded tuff(30 m) and pumice-rich ash-flow tuff(700 m) in ascending order. Several intrusions dominate the early sequences within the caldera. The caldera collapsed in a trapdoor type when silicic ash-flow tuffs erupted fro major vent area in the caldera. Normal faulting along a ring fault system except the southwestern part dropped the tuffs down to the northrase with a maximum displacement of about 820 m. The Myeonbongsan Tuff is just about 1,030 m thick inside the northeastern caldera, with its base not exposed, and southwestward thinning down. Rhyolitic plug and ring dikes are emplaced along the central vent and the caldera margins, and the ring dikes are cut by plutonic stocks in the southeastern and northwestern parts. The caldera volcanism eviscerated the magma chamber by a series of explosive eruptions during which silicic magma was erupted to form the Myeonbongsan Tuff. Following the last ash-flow eruption, collapse of the chamber roof resulted in the formation of the Myeonbongsan caldera, a subcircular trapdoor-type depression subsiding about 820 m deep. After the collapse, stony to flow-banded rhyolites were emplaced as circular plugs and ring dikes along the central vent and the caldera margins respectively. Finally after the intrusions, another plutons were emplaced as stocks outside the caldera.

• The Journal of the Petrological Society of Korea
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• v.28 no.4
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• pp.237-249
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• 2019

The volcanic rocks around the Jayang caldera are classified in an order such as Jukjang Volcanics, Doil Rhyolite, Unjusan Tuff and Rhyolite intrusions. By the SHRIMP U-Pb zircon datings from zircons, eruption ages of the Unjusan Tuff are constrained as 66.65±0.96 Ma in the intracaldera, and 66.08±0.62 Ma in the extracaldera outflow, and intrusion age of the ring dike is investigated as 60.74±0.66 Ma. The age data indicate that the caldera was collapsed between 66.08 Ma and 60.74 Ma, just before the dike intruded after the explosive eruption of the Unjusan Tuff. The Jayang caldera shows the composite igneous process of a perfect volcanic cycle passing from ash-flow tuffs through caldera collapse into ring dikes in the Jayang area.

• Economic and Environmental Geology
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• v.50 no.6
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• pp.467-476
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• 2017

Volcanic rocks related to the Guamsan cadera, which find in the southeastern Cheongsong, are divided into Volcanic breccia, Guamsan Tuff and Post-collapse intrusions. We determined their eruption, intrusion and caldera-forming timings based on SHRIMP U-Pb zircon dating. The dating results yield earlier eruption age of $63.77{\pm}0.94Ma$ from the lower ash-flow tuff and an later eruption age of $60.1{\pm}1.8Ma$ from the upper ash-flow tuff of the Guamsam Tuff, and intrusion age of $60.65{\pm}0.95Ma$ from the rhyolite ring dyke of the Post-collapse intrusions. The age data suggest that the Guamsan caldera is formed in 60.65~60.1 Ma between eruption of the upper ash-flow tuff and intrusion of the rhyolite ring dyke. The Guamsan cadera exhibits the volcanic processes of a perfect igneous cycle passing from ash-flow eruptions through caldera collapse to ring intrusions during 63.77~60.1 Ma.

• The Journal of the Petrological Society of Korea
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• v.25 no.2
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• pp.143-149
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• 2016

Eruptive volumes of three monogenetic volcanoes (Songaksan tuff ring, Biyangdo scoria cone, and Ilchulbong tuff cone) with the youngest eruption age are calculated using the model, applied to Auckland Volcanic Field in New Zealand, to investigate the volcanic eruption scale and to evaluate volcanic hazard of Jeju Island. Calculated eruptive volumes of the volcanoes are $24,987,557m^3$, $9,652,025m^3$, and $11,911,534m^3$, respectively, and the volumes include crater infill, tuff ring (tuff cone), scoria cone, and lava flow. Volcanic explosivity indices of Songaksan tuff ring, Biyangdo scoria cone, and Ilchulbong tuff cone are estimated based on the eruptive volumes to be 3, 2, and 3 respectively, and eruption type is Strombolian to Surtseyan. It is assumed that the amount of emitted sulfur dioxide gas is $2-8{\times}10^3kt/y$ according to the correlation between volcanic explosivity index and volcanic sulfur dioxide index. Recent age dating researches reveal evidences of several volcanic activities during the last 10,000 years indicating the possible volcanic eruption in Jeju Island in the near future. Therefore, it is necessary for appropriate researches regarding volcanic eruption of the island to be accomplished. In addition, establishment of the evaluation and preparation system for volcanic hazard based on the researches is required.

• The Journal of the Petrological Society of Korea
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• v.6 no.2
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• pp.96-110
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• 1997

The Wondong Caldera, formed by the voluminous eruption of the rhyolitic ashflows of the Wondong Tuff which is about 1,550 m thick at the intracaldera and 550 m at the outflow, is a resurgent caldera which shows a dome structure on the central exposure of the caldera. The Wondong caldera volcanism eviscerated the magma chamber by a series of explosive eruptions during which rhyolitic magma was ejected, as small fallouts and voluminous ash-flows, to form the Wondong Tuff. The explosive eruptions began with ash-falls, progressed through pumice-falls and transmitted ash-flows. During the ash-flow phase the initial central vent eruption transmitted into late ring-fissure eruption which accompanied with caldera collapse. Contemporaneous collapse of the roop of the chamber resulted in the formation of the Wondong Caldera, a subcircular depression subsiding about 1,930 deep. Following the collapse, quartz porphyry was intruded as ring dykes along the ring fracture near the southwestern caldera rim. Subsequently the central part of the caldera floor began to be uplifted into a circular resurgent dome by the rising of residual magma. Concurrent with the resurgent doming, the volcaniclastic sediments of Hwajeri Formation were accumulated in the caldera moat and then rhyodacite lava erupted from the initial central resurgent dome and another ash-flow tuff from the northern ring fracture. After the sedimentation, the find-grained granodiorite was intruded as an arc along the eastern ring fracture of the caldera. Finally in the central part, the resurgent magma was emplaced as a hornblende biotite granite stock that formed the central dome.

• Journal of The Geomorphological Association of Korea
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• v.18 no.3
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• pp.1-10
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• 2011

Sanbangsan and Yongmuri Beach as geosites are very wide areas, and tourists have accessed them via various courses; hence the need to double the number of geosite signs and disperse them at each point. The signs should read "Sanbangsan lava dome," "Yongmuri tuff ring," and "Relationship between the Sanbangsan lava dome and Yongmuri tuff ring." The contents of the sign should be systemic, simple, and clear because tourists have to read it in a short time. The viewpoint of the research area can be largely divided into Sanbangsan View, Yongmuri Beach View, Sanbangsan, and Yongmuri Beach View. Three of each viewpoint (total of 9 viewpoints) can be placed. In the geosite of Sanbangsan and Yongmuri Beach are 5 sub-theme views including the boundary sheet of Sanbangsan and Yongmuri tuff ring, xenolith basalt, marine pothole, tafoni, and crossed sedimentary layer. These sub-theme views are important in understating not only the geosite but the overall geopark as well, so they should be developed and utilized aggressively.

• The Journal of the Petrological Society of Korea
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• v.7 no.3
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• pp.161-176
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• 1998

There are exposed Samrangjin Tuff and intracaldera intrusions, of which rhyolitic rocks emplaced as postcollapsed central and ring intrusions within the Samrangjin caldera, and fine-grained granodiorite and biotite granite as regional tectonic intrusions nearby. The Samrangjin Tuff and the rhyolitic rocks are of a single Samrangjin magmatic system. Flow-banded rhyolite among rhyolitic rocks was emplaced in the outer part of the ring intrusions, rhyodacite in the inner part of the eastern ring, and porphyritic dacite and dacite porphyry in the inner part of the northwestern ring. Totally the Samrangjin Tuff and the rhyolitic rocks range from rhyolite to dacite in chemical composition. The Rb-Sr isotopic data of the Samrangjin Tuff and the rhyolitic rocks yield an age of $80.8{\pm}1.5(2{\sigma})$ Ma with the initial $^{87}Sr/^{86}Sr$ ratio of $0.70521{\pm}0.00010(2{\sigma})$. The continuous compositional zonations generally define a large stratified magma system in the postcollapse magma chamber. The Sr isotopic data suggest that the compositional zonations might have resulted from the fractional crystallization of a parental dacitic magma.