由火山活動後期噴硫氣現象之盛衰談大臺北地區火山再噴發的可能性評估

Exploring the Potentiality of Volcanic Reactivation in Metropolitan Taipei Area, Northern Taiwan Based on the Transition of its Geographic Landscape and fumaroles Activity

莊文星(Wen-Shing Juang) ； 張郇生(Shyun-Sheng Chang) ； 陳汝勤(Ju-Chin Chen)

《臺灣鑛業》 65卷1期 (2013/03) Pp. 1-18

摘要

2011年3月11日發生芮氏9.0級日本東北宮城大地震，引發海嘯及造成核電廠反應爐爆炸輻射外洩，造成全球的恐慌。在臺灣北部有核一、核二與核四等3座核能發電廠，而大臺北地區火山活動與山腳斷層等活動斷層之存在經常造成民眾心理的威脅。近期間平面媒體常大幅報導龜山島與大油坑猶如日本的富士山為活火山，潛存著極大自然災害的隱憂與危機。憑藉著臺灣地形保持完整的火山錐與火山口，如大屯火山群紗帽山、面天山、向天山、烘爐山、磺嘴山以及離島彭佳嶼，鉀氬放射性同位素定年的年齡均在10萬年以上，而且噴硫氣現象均已停止，實難推斷為活火山。有噴硫氣的龜山島，在日據時代大正12（1923）年日日新新聞就報導龜山島海岸邊有陸地與海底噴硫黃與水蒸汽之濃煙曾引起魚群大量死亡，這是早期的文字記載。而於1954與1968年更有龜山島頭部噴硫氣與水蒸氣之照片。仔細觀察，濃郁噴氣現象的盛衰史，由興盛到不存在，是漸進式的，而非為偶發事件，係由地表高處朝海面下逐漸消失，最後於1993年陸地上已完全消逝，而海面下的地熱溫泉亦逐漸減緩。況且，這些年來，地溫梯度亦顯示有逐漸下滑的趨勢。反觀臺灣大屯火山群的大油坑、小油坑、中山樓、冷水坑、馬槽、硫磺谷、紗帽谷、死磺子坪、地獄谷以及焿子坪等地熱現象均有萎縮逐漸不活躍，其中，尤其以冷水坑為最，其硫氣孔的噴出物已轉換為毛礬而非以硫黃為主，且其溫度已由攝氏120度降至室溫而名為冷水坑。此外，小油坑舊期噴氣口之遺跡亦十分醒目。由火山活動末期噴硫氣噴水蒸汽的逐漸消長即可看出一些端倪。因此臺灣北部大地構造若無重大改變，則大屯火山區之火山與龜山島，百年內再爆炸噴發的可能性不大。但為防範未然，加緊火山學與微震調查研究並設置火山監測作業系統有其必要性，但如無充分詳實的觀測結果，也不必急於宣示及過分喧染造成民眾不必要的恐慌。

關鍵字

休眠火山 ； 地貌 ； 噴氣孔 ； 火山口 ； 大屯火山群 ； 龜山島 ； 彭佳嶼

並列摘要

Tsunamis and subsequent radiation released by the explosion of the nuclear reactors caused by the magnitude 9.0 Tohoku earthquake on March 11, 2011 in Japan have frightened people across the world. There are three (No.1, No.2 and No.4) Nuclear Power Plants in northern Taiwan; moreover, the volcanic activities in the metropolitan Taipei area and fault activities of the Sanjiao fault have consistently worried people. Recently, Kueishantao (KST) and Dayoukeng were reported by the mass media as an active volcano and the metropolitan Taipei area was particularly vulnerable to natural disasters. In northern Taiwan, the volcanic landforms of volcanic bodies and craters are preserved quite well. Their Potassium-Argon dating are older than 0.1 Ma and gas fumaroles are ceased, such as Mt. Shamao, Mt. Miantian, Mt. Siangtian, Mt. Honglu and Mt. Huangtsui in Tatun volcanic cluster, and the outlying Island Pengjia Islet. Volcanic activity belong to Pleistocene (not Holocene) potentially inactive volcanoes. Kueishantao (KST) with fumaroles, according to the news in Japan Taisho era 1923, a strong emission of steam and sulfur had induced death of large quantity of fish. This is a historical record of the phenomenon of heat flow at KST. In 1954 and 1968, moreover, the truth that KST had a significant phenomenon of emission of steam and sulfur was illustrated by photographs. On closer examination the history of rise and decline of the fumaroles is proved to be a gradual process, rather than an accidental event. The emission faded away from the high mountains above the sea towards the level under the sea. And ultimately in 1993 it was completely disappeared on the earth's surface and the hot spring under the sea is gradually getting languor as well. Nevertheless, for many years, the geothermal gradient also seems to decline gradually. Based on the Tatun Volcanic Group variation of fumaroles from strong (Dayoukeng, Xiaoyoukeng, Gengziping, Dahuangzui) to medium (Diregu, Sihuangziping, Baya, Yangmingshan including Jhongshanlou and Houshan), weak to extinct (Macao, Chigu, Lingssheikeng) to show the post volcanic activity of the Tatun geothermal area in northern Taiwan, the fumarolic stage a later or decadent type of volcanic activity may not be a symptom of volcano reactivity. In the Tatun Volcanic Groups: Lengshueikeng, Macau their geothermal heat flow, emission of steam, water vapor, sulfur gas and fumaroles phenomenon are declined and gradually become inactive. Lenghsueiken cave has hair like sulfate mineral from volcanic fumaroles called harz alunogen (hair salts) its formation temperature may be lower than 120℃. All phenomena indicate that geothermal gradient is decreased gradually and the probability of volcanic re-eruption within a hundred even a thousand years is low, except the tectonic structures have changed greatly.

並列關鍵字

dormant volcano ； geomorphology ； fumaroles ； crater ； Tatun ； Kueishantao ； Pengchiahsu