• Korean Journal of Mineralogy and Petrology
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• v.33 no.4
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• pp.451-462
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• 2020

There are 82 volcanoes active during the 48 weeks of 2019 (January 30 to December 31, 2019; USGS data) Approximately 80~90 volcanoes are active on the Earth for a year. More than 91% of these volcanoes are took place in the circum-Pacific volcanic belt, which is commonly called 'Ring of Fire'. This status coincides with the distribution maps of active volcanoes on the earth: about 80 percent on subduction zone of the convergent plate boundaries; 15 percent on divergent plate boundaries; 5 percent on intra-plate zone. Typically five volcanoes are most active during the survey period (48 weeks); Dukono (Halmahera, Indonesia) 48 times, Aira (Kyushu, Japan) 47 times, Ebeko (Paramushir Island, Russsia) 46 times, Merapi (Central Java, Indonesia) 37 times, Krakatau (Indonesia) 33 times. The comparison of volcanic activity between 2018 and 2019 showed no significant difference. It is assumed that volcanic activity remains stable.

• The Journal of the Petrological Society of Korea
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• v.28 no.2
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• pp.53-69
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• 2019

In the Jirisan area of the Yeongnam Massif, Korea, several ductile shear zones are developed within Precambrian gneiss complex (Jirisan metamorphic rock complex). The ductile shear zones have a general NS- and NNE-striking foliation with westward dipping directions. The foliation developed in the shear zones cut the foliation in gneiss complex. The stretching lineations are well developed in the foliated plane of the shear zone, showing ENE-trend with gentle plunging angle to the ESE direction. Within shear zone, several millimetric to centimetric size of porphyroclasts are deformed strongly as a sigmoid form by ductile shearing. The sigmoid patterns of porphyroclasts in the shear zones indicate the dextral shearing. The spatial distribution of ductile shear zone is characterized by the dominant NS- and NNE-striking dextral sense in the central and eastern regions respectively. In the western part, it develops in NE-striking dextral sense which is the general direction of the Honam shear zone. The U-Pb concordant ages obtained from the two samples, the strongly sheared leucocratic gneiss, are $1,868{\pm}3.8Ma$ and $1,867{\pm}4.0Ma$, respectively, which are consistent with the U-Pb ages reported around the study area. We supposed that the ductile shearing in the study area is occurred about 230~220 Ma during late stage of the continental collision around Korea and is preceded by granitic intrusion related to subduction during 260~230 Ma, which are supported by compiling the age data from sheared gneiss, deformed mafic dyke intruded gneiss complex, and non-deformed igneous rocks.

• The Journal of the Petrological Society of Korea
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• v.6 no.1
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• pp.1-18
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• 1997

Andesitic pyroclastics and lava flows are deposited as a part of composite volcanoes by Cretaceous volcanic activity in Geojae Island, off the coast of Korea. The andesitic pyroclastics are composed of tuff breccia and lapilli tuff minor intercalated tuff. Lava flows are divided into dense and porphyritic andesite containing phenocrysts of plagioclase, pyroxene, and/or hornblende. The andesitic rocks represent charactersitcs of carc-alkaline BAR association with basalt, basaltic andesite, andesite, and dacite to rhyolite. Major element variations of the volcanic rocks show that $Al_2O_3$, total FeO, CaO, MgO and $TiO_2$ decrease with increasing $SiO_2$ but $K_2O$ and total alkalis increase, and represent differntiation trend of calc-alkaline rock series. In spider diagram, contents of Sr, K, Rb, Ba, and Th are relatively high, but contents of Nb, P, Ti and Cr are low. These petrochemcial characteristics are similar to those of rocks from island arc or continental margein related to plate subduction. Chondrite-normalized REE patterns of volcanic rocks are paralle to subparallel, with LREE enriched than HREE, and show gradual increase of negative Eu anomaly from basalt to dacite and rhyolite, suggesting comagmatic fractional crystallization with minor effects of assimilation and magma mixing. Andesitic rocks are assumed medium-K orogenic andesites that formed in the tectomagmatic environment of subduction zone under normal continental margin arc.

• The Journal of the Petrological Society of Korea
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• v.25 no.4
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• pp.409-419
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• 2016

The number of volcanoes erupted during the past ten thousand years(Holocene period) on this planet is known to stand around 1,520. Of those volcanoes, the number of active volcanoes during the six-years and seven-month period(January 2010 through the end of July 2016) is totals 209. These findings show that an average of approximate 90 volcanoes erupted every year since 2010. It is also found that over 90 percent of those active volcanoes took place in the circum-Pacific volcanic belt, which is commonly called 'Ring of Fire'. This status coincides with the distribution maps of active volcanoes on the earth: about 80 percent on subduction zone of the convergence of lithospheric plate; 15 percent on spreading zone; 5 percent on intra-plate zone. The period given in this research during 350 weeks, the following three volcanoes showed a frequency of more than 300 times eruption: Kilauea(Hawaii, USA, 338 times), Sheveluch(Kamchatka, Russia, 337 times), and Aira(Kyushu, Japan, 301 times). According to the survey conducted during the given period, there is no conspicuous increase in the frequency of volcano activities. It rather shows that volcanic eruptions took place almost evenly every year.

• Economic and Environmental Geology
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• v.38 no.3 s.172
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• pp.299-304
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• 2005

Geoid undulation and gravity anomaly were calculated from GRACE satellite data on the eastern Asia including Korean peninsula. Geoid undulation varies from -60m in the China to 60m toward the Pacific Ocean across the Korean Peninsula. Calculated gravity anomalies are in the range of -60 and 60 mgal except the subduction zone showing -100 mgal. High positive values are observed at Mt. Baekdu, Kaema highland and Taebaek mountains, and low values at Ulleung, Japan and Yamato basins in the East sea. We removed regional components below the spherical harmonic degree of 10 from gravity anomaly to get the residual anomaly for crust components. Residual gravity anomaly shows high anomalies at the northern mountainous area and Kyungsang basin in the Korean Peninsula. And low anomalies appears at the western Korea bay basin, Kunsan basin, Cheju basin, and Ulleung basin in the marine. Anomalies separated by the spherical harmonic degree as well as the residual anomalies are useful for the study of large crustal structure about geologic scale and depth distribution and for the survey of natural resources.

• The Journal of the Petrological Society of Korea
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• v.26 no.3
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• pp.183-200
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• 2017

Alkali rhyolites in the Cheonmunbong of the Mt. Baekdu stratovolcano show porphyritic texture in the glassy or aphanic groundmass. Major phenocryst is alkali-feldspar, pyroxene, and amphibole, and small amount of microphenocryst is olivine, quartz, opaque mineral (ilmenite). The content of $Fe^{2+}/(Fe^{2+}+Mg^{2+})$ and alkali elements in the mafic minerals is high. Alkali feldspar is classified as sanidine or anorthclase, olivine as fayalite, and pyroxene as ferro-hedenbergite of ferro-augite area. Amphibole belongs to alkali amphibole group, but FeO and $Fe_2O_3$ were not separated, so it is required future studies. Nb(-) anomaly suggesting that slab-derived materials might have played a primary role in the genesis of the rhyolite magma, is not observed. It is noted that they originated in the within plate environment which is not related to subduction zone of the convergent plate boundary. The Mt. Baekdu alkaline rocks are classified into the comendite series. The alkali rhyolites of the summit at Mt. Baekdu shows the disequilibrium mineral assemblages, suggesting that it evolved from thrachytic magma with experience of magma mixing as well as fractional crystallization.

• Economic and Environmental Geology
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• v.49 no.1
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• pp.23-30
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• 2016

Powder samples of natural lawsonite (Ca-lawsonite, $CaAl_2Si_2O_7(OH)_2{\cdot}H_2O$) was studied structurally up to 8 GPa at room temperature using monochromatic synchrotron X-ray powder diffraction and a diamond anvil cell (DAC) with a methanol : ethanol : water (16 : 3 : 1 by volume) mixture solution as a penetrating pressure transmitting medium (PTM). Upon pressure increase, lawsonite does not show any apparent pressure induced expansion (PIE) or phase transition. Pressure-volume data were fitted to a second-order Birch-Murnaghan equation of state using a fixed pressure derivative of 4 leading to a bulk modulus ($B_0$) of 146(6) GPa. This compression is further characterized to be isotropic with calculated linear compressibilities of ${\beta}^a=0.0022GPa^{-1}$, ${\beta}^b=0.0024GPa^{-1}$, and ${\beta}^c=0.0020GPa^{-1}$.

• Geosciences Journal
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• v.22 no.6
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• pp.939-953
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• 2018

We examined the microfabrics of omphacite and garnet in foliated eclogite to determine the influence of the layered structure on seismic observations in subduction zone. The analyzed eclogite, from the Lanterman Range, northern Victoria Land, Antarctica, is characterized by layering in which the modal abundances of garnet and omphacite vary. For garnet, the low aspect ratios, similar angular distribution of long axes relative to the foliation in both layers, uniform grain size distribution, near-random crystallographic preferred orientations (CPOs), and misorientation angle distributions are indicative of passive behavior during deformation. In contrast, omphacite shows relatively high aspect ratios, a low angle between the long axes of crystals and the foliation, a wide grain-size distribution, and distinctive CPOs, suggesting dislocation creep as the main deformation mechanism. The results of fabric analyses are consistent with strain localization into omphacite or omphacite-rich layers rather than garnet or garnet-rich layers. The single-crystal seismic anisotropy of garnet is very weak ($AV_P=0.2%$, $AV_S=0.5-0.6%$), whereas that of omphacite is much stronger ($AV_P=3.7-5.9%$ and $AV_S=2.9-3.8%$). Seismic anisotropy of the omphacite-rich layers shows an increase of 329% for $AV_P$ and 146% for $AV_S$ relative to the garnet-rich layers. Our results demonstrate the importance of the layered structure in strain localization and in the development of the seismic anisotropies of subducting oceanic crust.

• Korean Journal of Mineralogy and Petrology
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• v.35 no.3
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• pp.367-376
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• 2022

This study is a preliminary step to understand the reaction between various liquids and zeolite in the subduction zone environment. Stilbite, NaCa₄(Al₉Si₂₇)O₇₂·28(H₂O), was selected and high pressure study was conducted on compressional behavior by the pressure-transmitting medium (PTM). Water and NaHCO₃ solution that can exist in the subduction zone was used as PTM, and samples were pressurized from ambient to a maximum of 2.5 GPa. Below 1.0 GPa, both experiments show a low linear compressibility in the range of 0.001 to 0.004 GPa^-1 and a high bulk modulus of 220(1) GPa. This is presumably because the structure of the stilbite becomes very dense due to insertion of water molecules or cations into the channel. On the other hand, at 1.0 GPa or higher, the trends of the two experiments are different. In the water run, the linear compressibility of the c-axis is increased to 0.006(1) GPa^-1. In the NaHCO₃ run, the linear compressibility of the b- and c-axis is increased to 0.006(1) GPa^-1. The bulk modulus after 1.0 GPa shows values of 40(1) and 52(7) GPa in water and NaHCO₃ run, respectively, confirming that stilbite becomes more compressible than that before 1.0 GPa. It is caused by the migration of cations and water molecules inside the channel, as the water molecules in the PTM start to freeze and stop to insert toward the channel at 1.0 GPa or more. In the NaHCO₃ run, it is assumed that the distribution of extra-framework species inside the structure is changed by substitution of the Na⁺ cation. It can be expected from tendency of the relative intensity ratio of the (001) and (020) peaks which show a different from that of the water run.

• Economic and Environmental Geology
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• v.57 no.1
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• pp.1-15
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• 2024

The Melanesian region in the western Pacific is dominated by complex plate tectonics, with the largest oceanic plateau, the OntongJava plateau, and a hotspot, the Caroline Islands. To better understand the complex geodynamics of the region, we estimate P- and S-velocity models and 𝛿 (V_P/V_S) model by using relative teleseismic travel times measured at seismometers on land and the seafloor. Our results show high-velocity anomalies in the subduction zones of the Melanesian region to a depth of about 400 km, which is thought to be subducting Solomon Sea, Bismarck, and Australian plates along plate boundaries. Along subduction zones, positive 𝛿 (V_P/V_S) anomalies are found, which may be caused by partial melting due to dehydration. A broad high-velocity anomaly is observed at 600 km depth below the Ontong-Java plateau, with a negative 𝛿 (V_P/V_S) anomaly. This is thought to be a viscous and dry remnant of the Pacific plate that subducted at 45-25 Ma, with a low volume of fluids due to dehydration for a long period in the mantle transition zone. Beneath the Caroline Islands, a strong low-velocity anomaly is obseved to a depth of 800 km and appears to be connected to the underside of the remnant Pacific plate in the mantle transition zone. This suggests that the mantle plume originating in the lower mantle has been redirected due to the interaction with the remnant Pacific plate and has reached its current location. The mantle plume also has a positive 𝛿 (V_P/V_S) anomaly, which is thought to be due to the influence of embedded fluids or partial melting. A high-velocity anomaly, interpreted as an effect of the thick lithosphere beneath the Ontong-Java plateau, is observed down to 300 km depth with a negative 𝛿 (V_P/V_S) anomaly, which likely indicate that little fluid remains in the melt residue accumulated in the lithosphere.