大屯火山群七星山地區年輕爆裂口之研究

Abstract

大屯火山群位在台北盆地北方僅約15公里處，潛藏的火山災害不容忽視，然而其是否可以被定義為活火山尚有爭論。其中，七星山地區有活躍的噴氣、溫泉等後火山活動分布，被視為大屯火山群中火山活動年代較年輕的區域之一。從高解析度的數值高程模型可明顯辨識出在七星火山體東西兩側各有一條斷層帶(裂谷)通過，這兩條斷層帶上存在數個似圓或橢圓形狀的爆裂口，可能代表著較年輕的噴發活動事件。因此，本研究嘗試由地表形態、碎形幾何、野外調查與岩芯分析等研究方法，探討此區域爆裂口的形成機制、年代比較和噴發活動的演變，並進一步提供七星山為活火山的證據。 本研究首先利用1 m x 1 m 光達的數值高程模型，將分布於斷層帶上形狀較完整的爆裂口，依其外圍坡度陡峭的特性分別圈繪出來，並配合在野外調查的結果，將斷層帶重新定義為張裂帶。西側(WFC)和東側張裂帶(EFC)各有8 個和6個的爆裂口構造，其中西側的金露(WFC1)、WFC5 和鴨池爆裂口(DPC)為較完整的橢圓狀；東側則是以EFC2、EFC4、EFC5、EFC6 形狀較為完整，其餘不易區分出橢圓狀外形的陷落地則視為張裂帶的一部分。另外在張裂帶的線性延伸上也存在許多類似的裂隙構造，然而其地形多受下游的溪流侵蝕而破壞原貌。 碎形幾何的結果顯示，盒子計數法的碎形維度結果可能和爆裂口的形成年代順序相對應：WFC2 > EFC2 > EFC4 > DPC > EFC6 > WFC1 > WFC5 > EFC5 > EFC1；三角柱頂面積法反映的碎形特性為坡度、相對高度差的變化程度，可顯示張裂帶的裂隙分布及其延伸的走向；變異曲線法反映的碎形特性為高程起伏頻率和水流密度的變化程度，顯示大尺度構造的張裂帶非流水侵蝕的單一因素可造成。 X 光粉末繞射(XRD)、掃描式電子顯微鏡(SEM)和能量分散式光譜儀(EDS)的結果顯示，鴨池上、中、下3 池岩芯的岩屑和土壤沉積物中皆含有低溫方矽石和低溫石英，且磁鐵礦表面有差異侵蝕現象，指示鴨池過去存在與小油坑相似的酸蝕環境。岩芯中所含大部分的石英表面具有貝殼狀斷口等新鮮破裂面的微構造，指示著石英顆粒因受蒸氣噴發機制造成的撞擊而產生許多新鮮微構造。 綜合所有結果，本研究推測七星山兩側張裂帶和爆裂口的形成時間皆為約6,000年前，西側張裂帶又應稍早於東側張裂帶。年輕的爆裂口相對較晚形成，時間推測可能晚於約4,000年前。由侵蝕速率的因素重新審視爆裂口由早至晚結束爆發的時間序列為：WFC2 > DPC > EFC6 > WFC1 > WFC5 > EFC5。另外，鴨池中池(MP)的岩屑為近500年的一次蒸氣噴發事件直接堆積的產物，以小油坑至WFC5沿線的張裂帶為可能的噴發來源。依據活火山的經驗定義，七星山地區存在近幾千年來至近百年的蒸氣噴發事件，可被定義為活火山。

The Tatun Volcano Group (TVG) is located only 15 km north of the Taipei basin, and whether it is active or not has been in debate for a long time. The Chihsingshan volcano is covered by many gas fumaroles and hot springs and is viewed as a relatively young volcano of the TVG. Furthermore, two apparent fault zones (or rift valleys) with many craters, which pass through the eastern and the western edifice of the Chihsingshan volcano, respectively, can be easily identified by using high-resolution digital elevation model (DEM). Shapes of these craters are nearly circular or elliptic, inferring young eruptive events. Thus, using geomorphology, fractal geometry, field survey and core analysis, we try to understand the formation mechanism, relative age, and evolution of the craters in this area to further provide evidence that the Chihsingshan is an active volcano. This study utilizes 1 m x 1 m LiDAR (Light Detection And Ranging) DEM to investigate the small craters along the fault zones. The boundaries encompassing the crater were depicted by their steep slope, especially the intact ones. With the results of the field survey, the fault zones are redefined as rifting zones. Eight of the west fault craters (WFC) and six of the east fault craters (EFC) have been determined on both sides of the rifting zones. The shapes of WFC1, WFC5, Duck Pond crater (DPC) on the west side and EFC2, EFC4, EFC5, EFC6 on the east side are more intact. The others, which are difficult to recognize from the elliptic shapes, are considered as part of the rifting zone. Numerous fractures exist in the linear extent of the rifting zones, but they are not prominent as the topography has been destroyed by downstream creeks. The results of the fractal geometry show that the fractal dimensions of the box counting method may correspond to the order of the craters formation: WFC2 > EFC2 > EFC4 > DPC > EFC6 > WFC1 > WFC5 > EFC 5 > EFC1. The triangular prism surface area (TPSA) method reveals the variations in slope and elevation differences, which can denote the fracture distribution of the rifting zone and its extensional trend. The variogram method reflects the variations in frequency of topographic relief and water flow density, indicating that the large-scale structure of the rifting zone cannot be caused only by a single factor of natural erosion. The results of X-ray Powder Diffraction (XRD), Scanning Electron Microscope (SEM) and Energy Dispersive Spectrum (EDS) show that the detritus and soil sediments from UP, MP and DP all contain low-cristobalite and low-quartz, and the differential erosion phenomenon on the surface of magnetite, suggesting that DPC has been an acidic alteration environment similar to Siaoyoukeng. Most of the quartz grains in the cores have microstructures of fresh conchoidal fractures and crystalline faces, indicating that these fresh microstructures on the quartz grains were produced by the impact of phreatic or steam eruption mechanism. Based on all the results, our study speculates that the formation time of both the rifting zones and the craters from the Chihsingshan were about 6,000 years ago, and the west side of the rifting zone appeared slightly earlier than the east one. The formation of young craters were relatively late and may have been within 4,000 years. Taking into account the different erosion rate factors, we reassess the time sequence of the craters: WFC2 > DPC > EFC6 > WFC1 > WFC5 > EFC5. The detritus in MP is regarded as a direct product of phreatic eruption in the past 500 years, and the source of the eruption was most likely located between Siaoyoukeng and WFC5 of the rifting zone. According to the empirical definition of active volcanoes, the Chihsingshan has experienced in phreatic eruptions from several thousand years to recent centuries, and can be certainly defined as an active volcano.