博士 / 國立臺灣大學 / 地質科學研究所 / 97 / 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.
| Identifer | oai:union.ndltd.org:TW/097NTU05138004 |
| Date | January 2008 |
| Creators | Yueh-Ping Ku, 古月萍 |
| Contributors | , 陳中華, 宋聖榮 |
| Source Sets | National Digital Library of Theses and Dissertations in Taiwan |
| Language | en_US |
| Detected Language | English |
| Type | 學位論文 ; thesis |
| Format | 241 |
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