Original Article

Econ. Environ. Geol. 2017; 50(2): 105-116

Published online April 30, 2017

https://doi.org/10.9719/EEG.2017.50.2.105

© THE KOREAN SOCIETY OF ECONOMIC AND ENVIRONMENTAL GEOLOGY

Occurrence and Genesis of Obsidian in Gombawi Welded Tuff, Ulleung Island, Korea

Ji Hyeon Im1 and Chang Oh Choo2*

1Korea Institute of Geoscience and Mineral Resources, Daejeon 305-350, Republic of Korea
2Department of Geology, Kyungpook National University, Daegu 702-701, Republic of Korea

Correspondence to : mineralogy@hanmail.net

Received: February 20, 2017; Revised: March 27, 2017; Accepted: March 28, 2017

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License
(http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction
in any medium, provided original work is properly cited.

Abstract

The purpose of this study is to provide the information on genesis of obsidian occurring in the southwestern part of Ulleung Island, Korea, and to discuss its implications for volcanic activity through volcanological and mineralogical properties of obsidian. Obsidian occurs locally at the lower part of the Gombawi welded tuff, showing various complex textures and flow banding. Though obsidian is mostly homogeneous, it is closely associated with alkali feldspar phenocrysts, reddish tuff, and greyish trachyte fragments. The obsidian occurs as wavy, lenticular blocks or lamination composed of fragments. Cooling fractures developed on obsidian glass are characterized by perlitic cracks, orbicular or spherical cracks, indicating that obsidian rapidly quenched to form an amorphous silica-rich phase. It is evident that hydration took place preferentially at the outer rim relative to the core of obsidian, forming alteration rinds. The glassy matrix of obsidian includes euhedral alkali feldspars, diopside, biotite, ilmenite, and iron oxides. Microlites in glassy obsidian are composed mainly of alkali feldspars and ilmenite. Quantitative analysis by EPMA on the obsidian glass part shows trachytic composition with high iron content of 3 wt.%. Accordingly, obsidian formed with complex textures under a rapid cooling condition on surface ground, with slight rheomorphism. Such results might be induced by collapse of lava dome or caldera, which produced the block-and-ash flow deposit and the transportation into valley while keeping high temperatures.

Keywords obsidian, Ulleung Island, welded tuff, cooling fracture, block-and-ash flow

울릉도 곰바위용결응회암 내 흑요암의 산출특징과 성인

임지현1 ·추창오2*

1한국지질자원연구원, 2경북대학교 지질학과

요 약

Article

Original Article

Econ. Environ. Geol. 2017; 50(2): 105-116

Published online April 30, 2017 https://doi.org/10.9719/EEG.2017.50.2.105

Copyright © THE KOREAN SOCIETY OF ECONOMIC AND ENVIRONMENTAL GEOLOGY.

Occurrence and Genesis of Obsidian in Gombawi Welded Tuff, Ulleung Island, Korea

Ji Hyeon Im1 and Chang Oh Choo2*

1Korea Institute of Geoscience and Mineral Resources, Daejeon 305-350, Republic of Korea
2Department of Geology, Kyungpook National University, Daegu 702-701, Republic of Korea

Correspondence to:mineralogy@hanmail.net

Received: February 20, 2017; Revised: March 27, 2017; Accepted: March 28, 2017

This is an Open Access article distributed under the terms of the Creative Commons Attribution Non-Commercial License
(http://creativecommons.org/licenses/by-nc/3.0) which permits unrestricted non-commercial use, distribution, and reproduction
in any medium, provided original work is properly cited.

Abstract

The purpose of this study is to provide the information on genesis of obsidian occurring in the southwestern part of Ulleung Island, Korea, and to discuss its implications for volcanic activity through volcanological and mineralogical properties of obsidian. Obsidian occurs locally at the lower part of the Gombawi welded tuff, showing various complex textures and flow banding. Though obsidian is mostly homogeneous, it is closely associated with alkali feldspar phenocrysts, reddish tuff, and greyish trachyte fragments. The obsidian occurs as wavy, lenticular blocks or lamination composed of fragments. Cooling fractures developed on obsidian glass are characterized by perlitic cracks, orbicular or spherical cracks, indicating that obsidian rapidly quenched to form an amorphous silica-rich phase. It is evident that hydration took place preferentially at the outer rim relative to the core of obsidian, forming alteration rinds. The glassy matrix of obsidian includes euhedral alkali feldspars, diopside, biotite, ilmenite, and iron oxides. Microlites in glassy obsidian are composed mainly of alkali feldspars and ilmenite. Quantitative analysis by EPMA on the obsidian glass part shows trachytic composition with high iron content of 3 wt.%. Accordingly, obsidian formed with complex textures under a rapid cooling condition on surface ground, with slight rheomorphism. Such results might be induced by collapse of lava dome or caldera, which produced the block-and-ash flow deposit and the transportation into valley while keeping high temperatures.

Keywords obsidian, Ulleung Island, welded tuff, cooling fracture, block-and-ash flow

울릉도 곰바위용결응회암 내 흑요암의 산출특징과 성인

임지현1 ·추창오2*

1한국지질자원연구원, 2경북대학교 지질학과

Received: February 20, 2017; Revised: March 27, 2017; Accepted: March 28, 2017

요 약

    KSEEG
    Apr 30, 2024 Vol.57 No.2, pp. 107~280

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