• The Journal of the Petrological Society of Korea
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• v.7 no.3
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• pp.147-160
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• 1998

The Samrangjin Caldera, a trapdoor-type, formed by the voluminous eruption of the silicic ash-flows of the Samrangjin Tuff which is above 630m thick at the northern inside of the caldera and thinnerly 80m at the southern inside. The caldera volcanism eviscerated the magma chamber by a series of explosive eruptions during which silicic magma was ejected to form the Samrangjin Tuff. The explosive eruptions began with phreatoplinian eruption, progressed through small plinian eruption and transmitted with ash-flow eruption. During the ash-flow eruption, contemporaneous collapse of the roof of the chamber resulted in the formation of the Samrangjin caldera, a subcircular depression subsiding above 550m deep. During postcaldera volcanism after the collapse, flow-banded rhyolite was emplaced as cental plug along the central vent and ring dikes along the caldera margins. Subsequently rhyodacite porphyry and dacite porphyry were emplaced along the inner side of the ring dike. After their emplacement, residual magma was emplaced as a hornblende biotite granite stock into the southwestern caldera margin. In the northeastern part, the eastern dikes were cut final intrusions of granodioritic to granitic composition along the fault zone of $^{\circ}$W trend.

• 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.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.

• 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.