澎湖柱狀玄武岩地景調查

Columnar Basalt on the Penghu Islands

王鑫(Shin Wang) ； 吳芳豪(Fang-Hao Wu) ； 馮治華(Jyhhwa Ferng) ； 王永賢(Yeong-Shyan Wang)

《臺灣省立博物館年刊》 33卷 (1990/12) Pp. 17-54

https://doi.org/10.6548/ATMB.199012_33.0002

摘要

澎湖群島位於我國大陸與台灣本島之間，在台灣海峽的中央。在澎湖群島上，可以發現垂直的、傾斜的、彎曲的、放射狀的和瓶頸狀的柱狀玄武岩。根據地質學者的研究，澎湖群島玄武岩質熔岩的噴發，可能與中新世中期到晚期南中國海地殼的擴張有關，噴發的時間大約在一千萬年前。玄武岩質熔岩噴發至地表後，隨著熔岩流的冷卻和凝固，柱狀節理便在其中生成。而後，堅硬的玄武岩岩體不斷地受海水侵蝕，便露出各種形態的玄武岩柱，並形成陡峭的海崖。這些柱狀玄武岩不僅在地質學上深具意義，同時也創造了高品質的景觀，值得保護。本研究根據航照判讀及野外調查，確定澎湖群島各島嶼柱狀玄武岩的分布及出露位置。並在現場對柱狀玄武岩體（出露高度、柱狀節理、板狀節理、表面光滑度）、地形、植被、水體、人為活動等諸項地景組成元素加以描述、記錄、評分。最後得知:小白沙嶼、鳥嶼、錠鉤嶼、雞善嶼、員貝嶼、西吉嶼、七美嶼、貓嶼等島嶼的柱狀玄武岩發育最為良好。

關鍵字

地形 ； 景觀 ； 玄武岩 ； 柱狀節理

並列摘要

Basaltic rocks with well-developed columnar joints are widely exposed on the Penghu Islands, which are situated in the Taiwan Strait between Taiwan proper and the China mainland. Recently geologists have proposed an age of 12-13 Ma for the eruption of the basaltic lava on the Penghu Islands. Tectonically, the eruption was related to the opening of the South China Sea during the middle and late Miocene. These basaltic rocks not only represent a tectonic event of great importance but also create a high-quality landscape which deserves strict protection. It is known that, when basaltic lava erupted onto the surface, cooling and solidification followed together with the development of shrinkage cracks. Because of the relatively homogeneous nature of the basaltic lava, shrinkage takes place around many equally spaced centers and results in a polygonal pattern of cracks, ideally hexagonal. The prodnction of the columnar joints of basaltic lava is analogous with the formation of the pattern of mud cracks on the floor of a drying-up pond. Columnar joints may curve in response to variation of the temperature gradient. The changing orientation and curvature of some of the cloumns reflect the uneven cooling rates of the lava flow. Vertical, inclined, curved, radial and bottle-necked columns are all displayed.

並列關鍵字

Geomorphology ； landscape ； basalt ； columnar joint