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  • 學位論文

西太平洋菲律賓附近邊緣海第四紀海洋火山灰: 大噴發之特性、事件間隔、以及隱示

Quaternary Marine Tephra from Western Pacific Marginal Seas Adjacent to Philippines: Characteristics, Episodicity and Implications of Large Explosive Eruptions

指導教授 : 陳中華 宋聖榮
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摘要


火山爆發之際,大量火山灰經由火山雲柱帶往高空、在大氣中擴散、降沉,而堆積至地面或海盆。菲律賓地區擁有超過四十座以上的全新世火山,噴發物相互重疊覆蓋、不易調查;高溫多雨的陸域環境,風化侵蝕強烈,更增加火山岩保存的困難性;反之,周圍海盆則提供一個較安靜的沉積環境,可以依序保存成層堆積的火山灰、紀錄陸域地區的大型火山爆發事件。國際海洋古全球變遷研究的海盆採樣技術、以及已發表之海洋有孔蟲氧同位素地層或是生物地層、火山灰資料,為海盆保存的火山灰層序,提供極佳的年代解析。本研究以南中國海、西菲律賓海、塞勒比斯海盆採取之三口岩芯 (MD97-2142、MD97-2143、MD01-2387)、已發表之火山灰層為材料,分析火山玻璃的主要元素含量、鍶同位素組成,結合堆積年代的計算、以及陸上火山岩相關研究資料,試圖探討菲律賓第四紀爆發性火山活動及其特性。 菲律賓呂宋島兩側海盆岩芯 (MD97-2142、MD97-2143) 資料顯示,多數火山灰層,其玻璃顆粒含有較低的SiO2含量 (55.1-71.5 wt.%),被分類為高鉀至鉀玄岩系列 (SiO2=56.0-69.7 wt.%;K2O=2.0-5.1 wt.%;87Sr/86Sr=0.7040-07048)、以及鈣鹼至高鉀系列 (SiO2=55.1-71.5 wt.%;K2O=1.1-3.6 wt.%;87Sr/86Sr=0.7046-0.7053),分別對比於西南呂宋島 (Macolod Corridor) 東北部、以及西南部的火山岩 (其分布與馬尼拉海溝延伸方向垂直)。海盆火山灰層序顯示,西南呂宋島的第四紀大型火山爆發集中在兩個活動時期 (1,977-1,355 ka、478-0 ka)。兩期的火山活動區域不同,在歷經較安靜時期 (1,355-478 ka) 後,活動區呈現向西南遷移的現象。結合陸上火山岩地球化學資料推測、以及已發表之地體構造環境可能變化模式,進一步認為岩芯紀錄裡的較安靜活動時期 (1,355-478 ka),可能對應著西南呂宋島下方隱沒板塊傾斜角度調整的時期;此調整時期,對應著兩個活動區之間、單成火山活動的開始。岩芯紀錄的兩個活動期,則為隱沒板塊傾斜角度較穩定之兩個階段,因而較能穩定的供應岩漿、形成岩漿庫,大量的岩漿也較能規律地經由大型火山爆發釋放;兩個活動期分別約每156±52 kyrs和28±17 kyrs發生一次大型火山爆發;最年輕的爆發紀錄在6 ka。 位於呂宋島中部Mt. Pinatubo之Inararo Tuff已被認定為Modern Pinatubo最老且最劇烈的噴發事件產物,但其噴發年代卻未能明確。菲律賓呂宋島西側海盆岩芯 (MD97-2142) 共記錄著22層成層火山灰層,其中火山灰層Layer D (81 ka) 之黑雲母化學成分 (FeO=14.55±0.89 wt.%; MgO=16.96±0.90 wt.%)、玻璃顆粒組成 (SiO2=77.30±1.47 wt.%;K2O=2.91±0.36 wt.%)、玻璃鍶同位素 (87Sr/86Sr=0.7042),可對比於Inararo Tuff。因此,透過海盆岩芯記錄,認為Inararo Tuff之噴發年代,極可能在81 ka。 菲律賓民答那峨島西側岩芯 (MD01-2387) 之火山灰層的玻璃鍶同位素組成 (87Sr/86Sr=0.7038-0.7042) ,以及火山灰層的晶體富集特性顯示,民答那峨島至摩鹿加海碰撞地區 (由北至南,被分為完成碰撞帶、轉換帶、以及未碰撞帶) 的火山群,為火山灰來源區。海盆火山灰層序顯示,338 ka以來,這群火山頻繁發生的大型火山爆發,有減少活動的趨勢;顯示 (1) 自240 ka以來,西北側 (完成碰撞帶西區、轉換帶西區) 火山群,明顯減少火山爆發發生次數;(2) 自151 ka以來,民答那峨島中部 (碰撞後火山活動區) 釋放鉀玄岩質岩漿的火山群,停止發生大型火山爆發。海盆火山灰層紀錄,呼應陸上火山岩的研究資料,證明在過去5 Ma 弧-弧碰撞所促使的第四紀火山群活動,確實已於局部區域、不同時間,逐漸減少、甚至停止活動,這現象已被認為可能與碰撞地區南邊的摩鹿加海持續閉合、北邊的完成碰撞帶範圍持續向南擴展有關。 此外,利用過去40年大氣風場資料模擬運算之火山爆發、顆粒沉降結果,結合五十萬年來呂宋島西、東側岩芯 (MD97-2142、MD97-2143) 火山灰的分布、粒徑資料發現,高層風以東風為主的西南呂宋島,釋放的火山灰,極易向西側擴散、沉降,形成呂宋島西、東兩側岩芯火山灰層的不對稱分布 (18層:2層)。東側岩芯記錄的火山灰粒徑資料推測,在301 ka (MIS 8 或 MIS 9/8;1ψ) 以及172 ka (MIS 6;3ψ) 時,其季節風場強度與現今平均強度可能有所不同 (低層平流層風速減小、對流層風速為增大或減小),但其風場強度有可能未超出過去40年風場強度的變異範圍。這樣的古風場推測,顯示過去五十萬年來,西南呂宋島平均每28±17 kyrs發生一次的大型火山爆發,其雲柱高度 ≧30~40 km之發生次數並不多;而東側岩芯記錄到的兩次噴發事件,正符合雲柱高度≧30~40 km之條件。 本研究成果,除了顯示菲律賓的第四紀爆發性火山活動具有明顯時、空特性之外,亦能為第四紀地體動力模式提供較高解析的年代參數、陸上大型火山爆發事件發生年代亦可被訂定,為西南呂宋島的爆發性火山作用災害預測提供過去噴發雲柱高度與出現頻率之參考,並能為西北太平洋低緯地區過去五十萬年來可能的高空風場強度提供可能性的推測;另外,文中提出的顆粒粒徑數值模擬呈現方式,希望將來仍有所應用與發展。

並列摘要


Strong explosive eruptions eject tephra particles into the air, where they fall almost synchronically onto onland surfaces and deep-sea basins that retain tephra particles accumulated over long periods, protecting them from high erosion. In the low latitude western Pacific Ring of Fire the history of Quaternary strong explosive eruptions has not been thoroughly documented. This study uses the tephrostratigraphies of three deep-sea giant piston cores from the South China Sea, West Philippine Sea, and Celebes Sea (MD97-2142, MD97-2143 and MD01-2387) surrounding the Philippines archipelago to reconstruct the activities and behaviors of Quaternary explosive eruptions in the low latitude western Pacific Ring of Fire, that have been explored in a limited way by onland investigations. The tephrostratigraphies are reported with major element contents and Sr isotopic ratios of glass particles, as well as the estimated ages for tephra layers, based on published marine isotope stratigraphies or biostratigraphy as well as tephra correlation. These reports further infer the feature of Quaternary volcanic activity in specific source regions, the history of Quaternary geodynamic adjustment and paleo-wind intensities of this region. Evidence from deep-sea sediment sequences can improve and clarify the understanding for characteristics of explosive eruptions, geodynamic evolution, and wind systems over this region during the Quaternary. In Cores MD97-2142 and MD97-2143, High-K to Shoshonitic series glass particles, containing 56.0-69.7 wt.% SiO2, 2.0-5.1 wt.% K2O and 0.7040-07048 for 87Sr/86Sr, and Calc-alkaline to High-K series glass particles, containing 55.1-71.5 wt.% SiO2, 1.1-3.6 wt.% K2O and 0.7046-0.7053 of 87Sr/86Sr, are traced back to Quaternary active volcanic areas, respectively, in the northeastern and southwestern Macolod Corridor of southwestern Luzon Island. In the tephrostratigraphies, these two types of glass particles, that were densely recorded, respectively, in the periods of 1,977-1,355 ka and 478-0 ka, indicate the active volcanic area for strong eruptions in southwestern Luzon Island migrated southwestward. The periods of 1,977-1,355 ka and 478-0 ka, with cyclic strong explosive eruptions every 156±52 kyrs and 28±17 kyrs, respectively, appear to define two periods for the subducted slab with relatively stable geometry that lies beneath southwestern Luzon Island. By contrast, the period of 1,355-478 ka, characterized by an absence of deep-sea tephra layers and accompanied by the commencement of the activity of small monogenetic volcanoes located in the central Macolod Corridor, may define a possible period for the steepening of the slab beneath southwestern Luzon Island. In addition to these two types of glass particles, one tephra layer at Core MD97-2142, with 14.55±0.89 wt.% FeO and 16.96±0.90 wt.% MgO in its biotite crystals and 77.30±1.47 wt.% SiO2, 2.91±0.36 wt.% K2O and 0.7042 for 87Sr/86Sr in its glass particles, is well correlated to the onland Inararo Tuff. This tuff formation is recognized as the product of the oldest, largest known eruption of Modern Pinatubo. However, its age had been uncertain. Through the sediment record at Site MD97-2142, the age of this eruption event was determined to be 81 ka. In Core MD01-2387, the Sr isotopic composition of glass particles (<0.7043) and crystal-rich characteristic of some of the tephra layers indicate that the Quaternary active volcanic area of the Mindanao-Molucca Sea collision zone was the source area for the tephra particles. Tephrostratigraphy recorded that there has been a decline trend in explosive eruptions in past 338 kyrs. Eruptions in the northwestern collision zone have declined significantly after 240 ka, while eruptions in Central Mindanao, releasing glass particles with shoshonitic composition, became extinct at 151 ka. The deep-sea record appears to show that the Quaternary volcanic activity of Mindanao-Molucca Sea collision zone, started by arc-arc collision in 5 Ma, has regionally declined and extinguished. This feature of changing volcanic activity has been interpreted by the southward extension of the area that has been defined as a complete-collision zone. Tephra fallouts from volcanic centers of southwestern Luzon Island, simulated by a published tephra sedimentation model with a new expression method, would be easily dispersed westward to form asymmetric deposit patterns from eruptive centers because of the dominant easterlies above this region. A modern monthly average wind system over 40 years applied to the model may explain why in the past 500 kyrs tephra layers were asymmetrically recorded in eastern and western sites: only two layers at Site MD97-2143, but 18 layers at Site MD97-2142. According to the observed maximum clast size in the two layers at Core MD97-2143: 1ψ at 301 ka (MIS 9/8 or MIS 8) and 3ψat 172 ka (MIS 6), estimated paleo-wind intensities above this region may differ from the modern monthly averages, but they may not appear to go beyond the range of modern wind intensities. This assessment of wind intensities implies that the volcanic plume heights of explosive eruptions in this region, taking place every 28±17 kyrs, were rarely higher than 30-40 km. The strongest eruption was at 301 ka; the youngest strong explosive eruption occurred at 6 ka. The results of this study not only reconstruct the temporal and spatial relationships of Quaternary explosive eruptions in the low latitude western Pacific Ring of Fire but also provide a higher time resolution for the evolution of the Quaternary geodynamic environment. Moreover, eruption plumes, eruption ages, and repose times for onland strong explosive eruptions are determined, aiding in the hazard prediction of strong explosive eruptions. Further, additional data on the regional wind intensities above the low latitude southwestern Pacific over the past 500 kyrs was gathered and evaluated. The author also hopes the new expression of simulated tephra fallouts from the model used in this study may be applied to other regions in the future.

參考文獻


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