• 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.

• The Journal of the Petrological Society of Korea
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• v.22 no.1
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• pp.49-62
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• 2013

The Dongmakgol Tuff is divided into 8 lithofacies based on their grain size and depositional structures: massive tuff breccia(TBm), welded tuff and lapilli tuff(LTw), rheomorphic tuff and lapilli tuff(LTr), massive lapilli tuff(LTm), stratified lapilli tuff(LTs), gradedly bedded lapilli tuff(LTg), crudely bedded lapilli tuff(LTb) and massive fine tuff(Tm). They can be divided into 3 pyroclastic rock group based on their constituents of the lithofacies. The lower group(LI) is composed of LTm, LTw and LTr, which are interpreted to have resulted from emplacement of voluminous pyroclastic flows due to ignimbrite-form eruption to boiling-over eruption. The middle group(LT+MI) consists of LTs, LTg and LTm associated with Tm in the lower part, and of LTm, LTw and LTr in the middle and upper parts; these suggest that started with deposition of pyroclastic surges from phreatoplinian eruption by poor eternal water, passed through emplacement of pyroclastic flows from ignimbrite-form eruption and ended with deposition of voluminous pyroclastic flows from boiling-over eruption. The upper group(lUT+uUT+UI) is composed of LTs, LTg and Tm in the lowermost, TBm, LTb, LTb and Tm in the lower part, and LTm and LTw in the middle and upper part, suggesting that began with deposition of surges from phreatoplinian eruption, passed through deposition of pumice- and ash-fallouts from plinian eruption and transformed into emplacement of pyroclastic flows due to boiling-over eruption. As result, eruptive processes in the Dongmakgol Tuff approximately began with phreatoplinian or/and plinian eruption, transformed into ignimbrite-forming eruption and proceeded into boiling-over eruption in each volcanism, but proceeded presumably without phreatoplinian or plinian eruption in the earlier stage of 1st volcanism.

• The Journal of the Petrological Society of Korea
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• v.21 no.3
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• pp.367-383
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• 2012

The Gyeongsang Arc is the most notable of the Korea Arc that is composed of several volcanic arcs trending to NE-SW direction in the Korean peninsula. The Hayang Group has many volcanogenic interbeds of lava flows by alkaline or calc-alkaline basaltic volcanisms during early Cretaceous. Late Cretaceous calc-alkaline andesitic and rhyolitic volcanisms reconstructed the Gyeongsang Arc that consist of thick volcanic strata on the Hayang Group in The Gyeongsang Basin. The volcanisms characterize first eruptions of basaltic and andesitic lavas with small pyroclastics, and continue later eruptions of dacitic and rhyolitic ash-fall and voluminous ash-flow with some calderas and then domes and dykes. During the Early Cretaceous (about 120 Ma), oblique subduction of the Izanagi plate to NNW from N direction results in sinistral strike-slip faults to open a pull-apart basin in back-arc area of the Gyeongsang Arc, in which erupted lava flows from generation of magma by a decrease in lithostatic pressure. Therefore the Gyeongsang Basin is interpreted into back-arc basin reconstructed by a continental rifting. Arc volcanism began in about 100 Ma with exaggeration of the back-arc basin in the Gyeongsang, and then changed violently to construct volcanic arcs. During the Late Cretaceous (about 90 Ma), orthogonal subduction of the Izanagi plate to NW from NNW direction ceased development of the basin to prolong violent volcanisms.