• Economic and Environmental Geology
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• v.36 no.6
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• pp.527-536
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• 2003

The Dokdo volcano in the south-central part of the East Sea is classified into 8 rock units. The units and sequence suggest that the Dok Island forms a small stratovolcano constructed from at least 3 times eruption cycles above the sea level and proceeded with transformation of a few different eruption styles during each cycle. Reconstruction of the volcanic form, from the geologic structures and spatial lithofacies changes, suggests that the island is remnants of the southwestern caldera rim of the stratovolcano whose central part lies several hundred meters to the northeast. The subaqueous volcano shows abig guyot, which looks like a shield volcano, that represents gentle slope at 90-175 m deep and relatively steep one in 200∼2,000m, and 25∼30km wise base on sea floor. Therefore the total Dokdo volcano represents a multiple volcano that stratovolcano with small caldera overlies the big guyot.

• Economic and Environmental Geology
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• v.47 no.6
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• pp.601-610
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• 2014

Udo is the site of a short-lived monogenetic submarine volcanic vent which builts basaltic clastic deposits below sea level, by early Surtseyan-type explosive eruption and later effusive lava eruptions. Mineral assemblage, major and trace element chemistry indicate that the Someori sub-alkaline lava flows were related to the WPTB(within plate tholeiitic basalt) rather than IAT(island-arc tholeiite) and that the geotectonic regime of Korean Peninsula went away from the subduction zone after the Later Miocene.

• The Journal of the Petrological Society of Korea
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• v.2 no.2
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• pp.156-166
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• 1993

The volcanic stratigraphy and geochemistry of the Cretaceous volcanic rocks in the southern part of the Pusan showed that the volcanic rocks of the study area consist of alternating pyroclastic rocks and andesitic lavas, apparently constituting a thick volcanic sequence of a stratovolcano. The andesitic rocks contain augite, plagioclase, and hornblende as phenocrysts. Matrix minerals are augite, magnetite, hornblende, apatite. Mafic minerals, such as chlorite, epidote, sericite, and iron oxides occur as alteration products. Dacitic volcanic breccia and rhyolitic welded ash-flow tuff locally overlie the andesitic rocks. The rocks reported in the previous studies as andesitic breccia and andesite plot in the field of basalt, basaltic andesite, andesite, dacite and rhyolite, based on their chemical compositions. The volcanic rocks of the study area belong to the calc-alkaline series, and the andesitic rocks which are predominant in the area plot to the field of orogenic andesite.

• Economic and Environmental Geology
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• v.54 no.2
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• pp.187-197
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• 2021

The Hyeongjeseom (Islet) is an erosional remnant of volcano which is located about 2 km northeast of sea shore of the Songaksan tuff ring, and is composed of volcaniclastic deposit, agglomerate and scoria deposit, ponded lava, aa lava flows, reworked deposit and beach deposit in ascending order from the base. The volcano is formed by volcaniclastic deposits and lava flows that recorded a transition from initial phreatomagmatic to magmatic explosions followed by lava effusion. It is interpreted that the outcropped volcaniclastic deposit may be a remaining portion of outer ring of a tuff cone. A bomb and a ponded lava yield geochemically basaltic trachyandesite compositions (SiO2 51.3 wt%, Na2O+K2O 6.0 wt%) and belong to olivine basalt with scarce (<5 %) phenocrysts of olivine, petrographically. By incremental heating Ar-Ar dating method, the plateau age of lava flow in the Heongjesom is 9.2±3.6(2σ) ka, implying that the volcanism of Heongjeseom may have occurred earlier than the Songaksan tuff ring which erupted ca. 3.7 ka. It still remains a task to find a volcano which matches with a historical record of volcanic activity that occurred a thousand years ago.

• 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.

• The Journal of the Petrological Society of Korea
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• v.11 no.2
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• pp.74-89
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• 2002

Rock units, relating with the Guamsan caldera, are composed of Guamsan Tuff and rhyolitic intrusions. The Guamsan Tuff consists almost entirely of ash-flow tuffs with some volcanic breccias and fallout tuffs. The volcanic breccia comprises block and ash-flow breccias of near-vent facies and caldera-collapse breccia near the ring fracture. The lower ash-flow tuffs are of an expanded pyroclastic flow phase from the pyroclastic flow-forming eruption with an ash-cloud fall phase of the fallout tuffs on the flow units, but the upper ones are of a non-expanded ash-flow phase from the boiling-over eruption. The rhyolitic intrusions are divided into intracaldera intrusions and ring dikes that are subdivided into inner, intermediate and outer dikes. We compile the volcanic processes along a single cycle of cadela development from the eruptive phases in the Guamsan area. The explosive eruptions began with block and ash-flow phases from collapse of glowing lava dome caused by Pelean eruption, progressed through expanded pyroclastic flow phases and ash-cloud fallout phases during high column collapse of pyroclastic flow-forming eruption from a single central vent. This was followed by non-expanded ash-flow phases due to boiling-over eruption from multiple ring fissure vents. The caldera collapse induced the translation into ring-fissure vents from a single central vent in the earlier eruption. After the boiling-over eruption, there followed an effusive phase in which rhyolitic magma was injected and erupted to be progressively emplaced as small plugs/dikes and ring dikes with many lava domes on the surface. Finally rhyodacitic magma was on emplaced as a series of dikes along the junction of both outer and intermediate dikes on the southwestern side of the caldela.

• 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.

• Economic and Environmental Geology
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• v.56 no.4
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• pp.385-396
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• 2023

The Mesozoic Bansong Group, distributed along the NE-SW thrust fault zone of the Okcheon Fold Belt in the Danyang-Yeongwol-Jeongseon areas, contains important information on the two Mosozoic orogenic cycles in the Koran Peninsula, the Permian-Triassic Songrim Orogeny and the Jurassic Daebo Orogeny. This study aims to review previous studies on the stratigraphy, depositional period, and basin evolution of the Bansong Group and to suggest future research directions. The perspective on the implication of the Bansong Group in the context of the tectonic evolution of the Korean Peninsula is largely divided into two points of view. The traditional view assumes that it was deposited as a product of the post-collisional Songrim Orogeny and then subsequently deformed by the Daebo Orogeny. This interpretation is based on the stratigraphic, paleontologic, and structural geologic research carried out in the Danyang Coalfield area. On the other hand, recent research regards the Bansong Group as a product of syn-orogenic sedimentation during the Daebo Orogeny. This alternative view is based on the zircon U-Pb ages of pyroclastic rocks distributed in the Yeongwol area and their structural position. However, both models cannot comprehensively explain the paleontological and geochronological data derived from Bansong Group sediments. This suggests the need for a new basin evolution model integrated from multidisciplinary data obtained through sedimentology, structural geology, geochronology, petrology, and geochemistry studies.

• Economic and Environmental Geology
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• v.26 no.1
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• pp.55-65
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• 1993

Udo Island, some 3 km off the coast of Sungsan Peninsula at the eastern promontory of Cheju Island, occurs in such a regular pattern on the sequences which reprent an excellent example of an eruptive cycle. The island comprises a horseshoe-shaped tuff cone, a nested cinder cone on the crater floor, and a lava delta which extends over northwest from the moat between two cones. The volcanic sequences suggest volcanic processes that start with emergent Surtseyan eruption, progress through Strombolian eruption and end with lava effusion followed by reworking of smooth tephra on the tuff cone. Eruptive environment and hydrology of vent area in the Udo tuff cone are poorly constrained because the stratigraphic units under the tuff cone are unknown. It is thoughl, however, that the tuff cone could be mainly emergent because the present cone deposits show no evidence of marine reworking, and standing body of sea water could play a great role. The emergent volcano is characterized by distinctive steam-explosivity that results primarily from a bulk interaction between rapidly ascending magma and a highly mobile slurry. The sea water gets into the vent by flooding accross or through the top or breach of tephra cone. Udo tuff cone was constructed from Surtseyan eruption which went into with tephra finger jetting activities in the early stage, late interspersed with continuous uprush activities and proceeded to only continuous uprush activities in the last. When the enclosure of the vent by a long-lived tephra barrier would prevent the flooding and thus allow the vent to dry out, the Surtseyan eruption ceased to transmit into Strombolian activities, which constructed a cinder cone on the crater floor of the tuff cone. The Strombolian eruption ceased when magma in the conduit gradually became depleted in gas. In the case of Udo, the last magmatic activity was Hawaiian-type (and/or fountain) which accumulated basalt lava delta. And then the loose tephra of the tuff cone reworked over the moat lava and the northeastern flank.