• Journal of Magnetics
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• v.6 no.1
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• pp.5-8
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• 2001

Cheju island is composed of volcanic tuff. The soil in Cheju island has principally sprung from basalt and partially made up of trachyte, and andesite. Also ancient relics, plain coarse pottery kilns and pantiles kilns are homogeneously distributed all over the Cheju island. In this study samples of plain coarse pottery and pantiles from five regions of Cheju island have been examined through X-ray fluorescence spectrometer and Mossbauer spectroscopy. It is thought that these samples be partially formed from neutral volcanic rock like trachyte and the valence state of iron is almost $Fe^{3+}$. Also the magnetic hyperfine field of goethite contained in these samples is less than synthetic goethite. This result shows the degradation of magnetic order caused by the partial substitution of diamagnetic positive ion $Fe^{3+}$ by $Al^{3+}$ in the goethite lattice.

• Economic and Environmental Geology
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• v.19 no.1
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• pp.25-33
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• 1986

The geology of the Jeju island is characterized by the occurrence of thick voluminous basaltic lava flows and minor trachytic lavas. The land form can be divided topographically into the lava plateau, the shield-shaped Halla volcanic edifice and the parasitic cones whose number is more than 360, which is closely related to the cyclic volcanism of the island. Especially, the trachytic lavas seem to be the latest differentiation products of the cyclic activities and largely classified into two groups (Backlokdam group and Sanbangsan group) on the basis of the occurrence and the petrography. The paleomagnetism and the radiometric age on the two groups of trachyte suggest a lower and an upper time limits of the volcanic stratigraphy of the island. An average age of the trachyte of the Sanbangsan group is $0.733{\pm}0.056m.y.$, and is correlated to a horizon a little lower than the boundary (0.69m.y.) between the Brunches normal epoch and Matuyama reversed epoch. An average age of $0.025{\pm}0.008m.y.$ determined from the Backlokdam trachyte may be corrected to anyone horizon of the Laschamp, or the Lake Biwa, or the Lake Mono excursion. The two groups of trachytes are distributed with the latitude difference of 9'($0.15^{\circ}$), and with age difference of about 0.71 m.y. Assuming that the two trachyte groups were erupted from the same stationary hot spot, the lithosphere comprising the Jeju island is considered to have moved southward with a rate of about 2.3 cm/year.

• The Journal of Engineering Geology
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• v.25 no.4
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• pp.499-510
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• 2015

Artificial accelerated weathering test evaluated rocks from near the circuit road of Ulleungdo island, approximately 120 km from east of the Korean Peninsula. The tests subjected rock specimens to conditions based on the climate of the island. The specimens (such as basaltic breccia, trachyte, volcanic breccia) were preliminarily classified using a TAS diagram (XRF data) and based on the constituent minerals (XRD data); they were further classified by weathering degree according to their absorption ratios. During the artificial accelerated weathering, the absorption ratio of most of the specimens increased, but the point-load strength did not decrease in most cases, except for the volcanic breccia. The greater initial absorption ratio of trachyte rock specimen in comparison with the other specimens led to a greater increase of its absorption ratio during the artificial accelerated weathering test. The volcanic breccia specimens showed the greatest increase of absorption ratio and the biggest reduction ratio of the point- load strength during the tests. These results could aid prediction of the weathering rate of rocks in Ulleungdo island subjected to weathering processes; trachyte which appears to accelerate with time, and volcanic breccia whose mechanical strength can largely decrease in a relative short period of time. Proper measures therefore appear necessary for the prevention of natural disaster such as rock fall and landslide around the circuit road.

• The Journal of the Petrological Society of Korea
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• v.19 no.2
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• pp.141-156
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• 2010

Dokdo Island, Korea, is located in the East Sea belonging to back arc basin. In this study we examined petrology and geochemistry of massive tuffaceous breccia (MTB) from Dongdo (Eastern islet) and Seodo(Western islet), the two largest islands of Dokdo. Field studies and chemical analysis distinguish the MTB in Dongdo and Seodo. The Dongdo MTB (DMTB) is exposed up to 50 m on the ocean cliff and it has dominant basalt and trachybasalt with moderate amount of trachyte and scoria. On the other hand, Seodo MTB (SMTB), which is preserved between trachyte dike and trachyandesite, is composed of roughly equal amounts of basalt, trachybasalt and trachyte. The location of the islets were related to the source vent having in contact with underlying trachyte lava and differential pyroclastic deposits made them different characteristics. According to trace element analysis of trachytic volcanic clasts, the Ba concentration ranges from 66 to 103 ppm and Sr varies from 44 to 56 ppm in DMTB. However, Br and Sr in SMTB correspondingly showed relatively wide ranges: Br 785-1259 ppm and Sr 466-1230 ppm. These differential trends between DMTB and SMTB, along with the difference in P and Ti, indicate that the crystallization of alkali feldspar, feldspathoid, biotite, apatite and titanium took place differently. Nevertheless, DMTB and SMTB are similar in REE patterns and they are correspondingly characterized by high LREE, low HREE and similar $(La/Yb)_N$ values with 23.9-40.2 in DMTB and 27.4-32.9 in SMTB. These patterns suggest that Dongdo and Seodo might be originated from coeval magma suites. Dokdo island shows high concentrations of Ba, K and Rb. These signatures mark a result attributed to the mantle upwelling because the magma derived from the asthenosphere was metasomatized with subduction-related fluids.

• The Journal of Engineering Geology
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• v.21 no.2
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• pp.125-131
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• 2011

The discharge characteristics of the Seodo Mulgol Spring-the only groundwater-producing area in Dokdo-were evaluated by measurements of discharge rate and electrical conductivity (EC) on five occasions. The Seodo Mulgol Spring is fed by rainfall in upstream areas of the Mulgol cave, and the rainwater of the area moves down along cooling joints developed in trachyandesite II and trachyte, finally discharging at the Mulgol cave. The discharge rate of the Seodo Mulgol Spring varied from 1.12 to 7.02 $m^3/d$ during the study period and EC varied from 2,650 to 3,390 ${\mu}S/cm$, showing a sharp increase during heavy rainfall. The observed variations in discharge rate and EC at the Seodo Mulgol Spring are attributed to the relatively short distance between the recharge area and the Mulgol cave, and to the rapid movement of groundwater through columnar joints developed in trachyandesite II and trachyte. Additional discharge measurements, combined with precise rainfall data, are required at Dokdo to elucidate the discharge characteristics of the Seodo Mulgol Spring.

• Economic and Environmental Geology
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• v.19 no.2
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• pp.153-163
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• 1986

Paleomagnetic and geological studies of volcanic and sedimentary rocks of Jeju Island have been carried out to determine the position of virtual geomagnetic pole(VGP), and to estimate the geological sequence and their age. As a result of paleomagnetic studies, the reversal polarities are measured in the Sanbangsan trachyte and Hwasun formation, and the normal are the rest. In case of normal polarity, the mean values of declination and inclination are $2.3^{\circ}$ and $48.4^{\circ}$, respectively, and the average value of VGP is $85.4^{\circ}N$ and $79.9^{\circ}W$. The locations of VGP's are coincident with those obtained from world-wide Plio-Pleistocene rocks. The Hwasun formation and Seongsan formation which have been known to be sedimented in the similar time in the 2nd-stage of volcanic eruption, possess reversal and normal polarities, respectively. This fact brings about the result that two formations should be separated in a sense of geological sequence. Consequently, the geological sequence of the 2nd-stage of volcanic eruption is Pyoseonri basalt-Seoguipo hawaiite-Hwasun formation-Seongsan formation-Jungmun hawaiite-Sanbangsan trachyte. Referring to the paleomagnetic studies and the previous and present geological studies, Seoguipo formation corresponds to the Gauss normal epoch, the 2nd-stage of volcanic eruption to Matuyama reversed epoch, and the 3rd-, 4th-, and 5th-stages to Brunhes normal epoch. Therefore, the Seoguipo formation is mostly sedimented during late Pliocene and/or presumably extended to the early Pleistocene. The rocks of the 2nd- to 5th-stage are formed later than this.

• Ocean and Polar Research
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• v.24 no.4
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• pp.465-482
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• 2002

Petrological, geochemical, and geochronological studies of Dokdo volcanic rocks, East Sea, have been carried out to understand their petrogenesis. Dokdo volcanic activity is divided into three stages according to occurrences and eruption ages of rocks. The second-stage activity is accompanied by large volume of pyroclastics and lavas of intermediate composition, and occupies most of the East and West islets. K-Ar biotite and whole-rock ages indicate that Dokdo volcanic activity occurred during late Pliocene and became systematically younger toward later stages: namely, 2.7-2.4 Ma for the first-stage trachyte, 2.4-2.3Ma for the second-stage trachyandesite and 2.2-2.1 Ma for the last-stage trachyte and dikes. Dokdo volcanic rocks are of intermediate to felsic compostions, and have OIB-like alkaline nature. The geochemical similarities between Dokdo and Ulleungdo volcanic rocks suggest that they were formed from the same mantle plume. However, considering the difference of eruption ages between Dokdo (2.7-2.1 Ma) and Ulleungdo (1.4-0.01 Ma) volcanic rocks, the former seems to have been formed by earlier hot spot activity.

• Korean Journal of Mineralogy and Petrology
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• v.33 no.2
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• pp.87-97
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• 2020

The compositions of the phenocrystic feldspars of the Sanbangsan trachyte range from labradorite(An_53.6) to andesine(An_35.4), and of the microphenocrysts and laths range from andesine(An_31.2) to oligoclase(An_18.7). Mantled feldspar which forms a thin rim around the phenocrysts and microphenocrysts, is anorthoclase(Or_20.5An_9.4) to sanidine(Or_49.2An_1.4). Phenocrystic plagioclase, which shows a distinct zonal structure, represents an oscillatory zoning in which the An content of the zone repeatedly increases or decreases between andesine (An_39.3) and labradorite (An_51.3) from the core toward the rim, and the rim of the phenocrysts is surrounded by alkali feldspar(Or_31.9-39.4Ab_63.2-57.0An_4.9-3.7), showing the antirapakivi texture. Microphenocryst which does not represent the antirapakivi texture, shows the normal zoning with a decreasing An content (An_36.4→An_25.6) as it moves outward from the center of a crystal. As a result of X-ray mapping of K, Ca, and Na elements for the feldspar phenocrysts representing the typical zonal structure, shows the oscillatory zoning that six zones show the distinctive compositional differences, and the rims are mantled by alkali feldspar to indicate the antirapakivi texture. The groundmass is composed of K-enriched, Ca-poor alkali feldspar. The antirapakivi texture of feldspar which appears in Sanbangsan trachyte, may have been formed in mixing systems as a result of the juxtaposition of near liquidus melt, rich in alkali feldspar components(trachytic magma), with plagioclase phenocrysts and microphenocrysts already crystallized in a more mafic system.

• Economic and Environmental Geology
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• v.57 no.2
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• pp.205-217
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• 2024

This study evaluated the physical characteristics and quality of volcanic rocks distributed in the Jeju Island-Ulleung Island area as aggregate resources. The main rocks in the Jeju Island area include conglomerate, volcanic rock, and volcanic rock. Conglomerate is composed of yellow-red or gray heterogeneous sedimentary rock, conglomerate, and encapsulated conglomerate in a state between lavas. Volcanic rocks are classified according to their chemical composition into basalt, trachybasalt, basaltic trachytic andesite, trachytic andesite, and trachyte. By stratigraphy, from bottom to top, Seogwipo Formation, trachyte andesite, trachybasalt (I), basalt (I), trachybasalt (II), basalt (II), trachybasalt (III, IV), trachyte, trachybasalt (V, VI), basalt (III), and trachybasalt (VII, VIII). The bedrock of the Ulleung Island is composed of basalt, trachyte, trachytic basalt, and trachytic andesite, and some phonolite and tuffaceous clastic volcanic sedimentary rock. Aggregate quality evaluation factors of these rocks included soundness, resistance to abrasion, absorption rate, absolute dry density and alkali aggregate reactivity. Most volcanic rock quality results in the study area were found to satisfy aggregate quality standards, and differences in physical properties and quality were observed depending on the area. Resistance to abrasion and absolute dry density have similar distribution ranges, but Ulleung Island showed better soundness and Jeju Island showed better absorption rate. Overall, Jeju Island showed better quality as aggregate. In addition, the alkaline aggregate reactivity test results showed that harmless aggregates existed in both area, but Ulleungdo volcanic rock was found to be more advantageous than Jeju Island volcanic rock. Aggregate quality testing is typically performed simply for each gravel, but even similar rocks can vary depending on their geological origin and mineral composition. Therefore, when evaluating and analyzing aggregate resources, it will be possible to use them more efficiently if the petrological-mineralological research is performed together.

• Economic and Environmental Geology
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• v.27 no.2
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• pp.209-212
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• 1994

Cheju Island was formed by volcanic activity probably related to the inferred geodynamics in the early Quaternary times. Paleostress analysis, in spite of a few fault slip data collected near Sanbangsan trachyte dome (dated 0.87 Ma) represents an extentional tectonic event with the direction ENE-WSW. Joint anayses in the vicinity of Seahwa reveal three extensional tectonic events of directions NW-SE, NE-SW and ENE-WSW. Especially the extensional event with the direction ENE-WSW affected the whole Cheju area during the most recent time.