Eruptive History Of The Changbaishan Tianchi Volcano,China/DPRK, Since The Late Pleistocene

Changbaishan Tianchi volcano(or Baegdusan, Paektusan in North Korean; Baitoushan in Japanese), located on the border of China and North Korea and as one of three caldera of Changbaishan volcano system(the others are Taibao caldera in North Korea and Wangtiane caldera in China), is a potentially hazardous intraplate stratovolcano, about 1,200 km west of the west Pacific subduction zone.Volcano unrest at Tianchi, expressed by swarms of volcanic tremors and small earthquakes during the period 2002 and 2006, and the famous “Millennium” eruption(ME), occurred in 946±3 AD and registered to be 7 on the Volcanic Explosivity Index(VEI) scale, attracted much attention from the government and scientists in China and other countries.The historically active polygenetic Tianchi volcano has undergone three major evolutionary stages:(1) an early basaltic shield-forming stage(2Ma-1Ma);(2) followed by a trachytic composite cone-forming stage(1Ma-0.04Ma); and(3) the latest explosive stage, during which comendite(rhyolite) and trachyte magma was erupted. The study of this thesis focuses on the eruption sequence of the latest explosive stage.Because the understanding of the latest highly explosive activities of utmost importance in hazards assessments of possible future eruptions, research on eruptive history has been focused on the explosive stage since the first studies on Changbaishan volcano. We have collected all published data on events of this stage and their inferred ages. There exist considerable uncertainty and debates about subdivisions of the products of the entire eruption sequence because the characteristic of products always varies according to the processes and environment of eruption, transport and deposit. Such inconsistencies prompted us to undertake additional studies, in hope of obtaining more consistent subdivisions of the volcanic succession.For previous results of the eruption sequences, we partially agree with Liu(2000) for purposes of comparison with other sequences. The author has recognized four major units of eruptive products around Tianchi volcano from old to young. They are described briefly as follows:(1) Products of the Tianwenfeng eruption(TWF) are found on the rim of caldera and in the drill hole at Yuanchi. The age of this eruption has not been well determined and was estimated to be ~5 ka based on the 2040±90a BPradiocarbon age of upper sediments between the TWF and ME deposits;(2) The clastogenic lava of Qixiangzhan(QXZ) exhibits good flow morphology on the northern slope of the volcanic cone. The age of the QXZ was estimated about 4 ka based on the dating by the Electron Spin Resonance method(ESR) and Thermoluminescene(TL);(3) The well-known “Millennium” eruption(ME) was determined to be 1215±15 AD as a large explosive eruption;(4) The product of Baguamiao eruption(BGM) is the black trachytic pumice and limited to the internal part and rim of the caldera. This eruption was considered as a phreatomagmatic eruption in 1668 AD or 1702 AD according to poor historical records. Collectively, these results indicate that all eruptions of Tianchi volcano during the explosive stage took place within the Holocene. In actuality however, the understanding of the scale,age, and detailed distribution of products for each eruption is still insufficient and poorly constrained.In this thesis, we have conducted detailed field investigations and petrographic, geochemical, and geochronologic analyses to provide data sufficiently to classify the eruption events and determine their ages.Main research contents and methods1) Field geological investigation.We have carried out detailed field investigations around Tianchi volcano and observed more than100 geological sites, and then selected several well-exposed geological sections, which show the depositional sequences that record the distinctive volcanic eruption events and their ages. From the depositional sequences, we found that the well-stratified weathering product deposit between the lapilli fall deposits of the TWF and ME provides supplementary evidence for the intermittent nature of the eruption activity, and also found that the black BGM trachytic lies directly on the comendite tephra of the ME without a break suggesting the continuous eruption of these two stages. In addition, The Baitoushan-Tomakomai tephra(B-Tm) and Baitoushan-Japan Sea tephra(B-J) exhibited in the drill holes in Japan Sea were considered as the distal fallout of large eruptions from Tianchi volcano.2) The studies of petrography and geochemistry.The petrographic study focuses mainly on the content, size and morphology of phenocrysts as well as the shape of vesicle walls. Geochemical data includes the major- and trace- element of glass by using Electron Probe Microanalyzer(EPMA) and Laser Ablation Inductively Coupled Plasma MassSpectrometer(LA-ICP-MS). These characteristics were used to provide critical evidence for demonstrating whether the volcanic products distributed in different regions originated from the same eruptive event. For example, the phenocrysts of the BGM were severely eroded as represented by large numbers of irregular erosion pits on their surface, so the pumice at the lakeside of Tianchi, the lapilli fall deposits at top of Yuanchi and the pyroclastic flow deposits in valley around Tianchi are considered to the same eruption due to the phenocrysts of these deposits with the same characteristic. In addition, the composition of the B-J shards in Japan Sea corresponds exactly to that of the TWF eruption of Tianchi volcano. It confirms that the products of the TWF and the B-J are from the same eruption. The compositions of the B-Tm as a thin-layer in Japan Sea are composed of a high-SiO2 part,corresponding to the ME, and a low-SiO2 part corresponding to the BGM. These relationships indicate further that the B-Tm tephras come from both the ME and BGM eruptions, and reveal also that the ME and BGM represent a continuous eruptive event without a significant break, while allowing change in magma composition.3) GeochronologyWe have carried out a series of dating studies, including radiocarbon, 40Ar/39 Ar, and Optically Stimulated Luminescence(OSL) techniques. Additionally, we also considered the published age data obtained from zircon dating and tephrochronology, in an attempt to arrive at the most reliable eruption age. Finally, the newly determined age for the ME and BGM eruptions is 946±3 AD, obtained from the dating of a 264 year old tree trunk by using high-precision 14 C wiggle-match method; the result of the QXZ is about 11.0±0.5 ka based on the incrementally heating of single crystal with the 40Ar/39 Ar dating;With respect to the eruption age of the TWF, it can be bracketed to 41-62 ka based on the combined dating results of 14 C, OSL and 40Ar/39 Ar, and was finally considered to be 50.6 ka which is the age of the corresponding B-J tephra in Japan Sea.The discussion for hot issueAt first, the relationship between the ME and QXZ need to be resolved. In previous studies, the ME and BGM were considered as two separate eruption events: the ME erupted ~1 ka ago producing comendite and the BGM erupted in 1668 or 1702 producing trachyte. However, diverse evidence now available demonstrates that the ME and BGM are better considered as different phases of one eruptioninvolving magma mixing. The main evidence is as follows:(1) the BGM lapilli falls directly overlies the grey comendite tephra of the ME without depositional hiatus;(2) abundant mingled trachyte-comednite pumices can be found in pyroclastic flow of the BGM and ME;(3) bimodal populations of trachytic and comenditic glass shards of ‘B-Tm’, a thin tephra layer from Japan Sea,correspond well to comendite and trachyte from the ME and BGM deposits;(4) if the BGM was attributed to the year 1668 or 1702 and as VEI ~5-7 eruption based on the historical description, this event should be widely detected the products and clearly documented in ancient Chinese historical records. However, no eruptive products of this supposed eruption have been found and only obscure anecdotal information was deduced so far. Therefore, we suggest that the ME and BGM should be combined into one eruption in 946±3 AD, initiating as a major plinian eruption of comendite and then transiting to trachyte in its final phases.Secondly, timing of the formation of Tianchi caldera has long been the focus of discussion in studies of Changbaishan volcano. Previous studies have considered that the explosion of the ME formed the caldera and crater lake about one thousand years ago. However, this study suggests that Tianchi caldera formed during the TWF. The main evidence is as follows:(1) the strata of the TWF directly overlie the trachytic residual cone after explosive eruption around the caldera edge and on basalts of shield-forming stage. These stratigraphic relations suggest that the TWF is the first major,and largest, explosive eruption in the eruptive history of Tianchi volcano. It seems more reasonable to infer that the caldera was formed during the first huge explosive event;(2) the existence of abundant breccia in the TWF indicates that the TWF eruption blasted away the trachytic cone and formed the Tianchi caldera;(3) the lacustrine sediments underlie the ME deposit was observed within the caldera,and this stratigraphic relation indicates that the caldera and its lake had existed and started deposition before the ME. To review the eruption history, the TWF is the only large explosive event before the ME. In brief, we conclude that the Tianchi caldera was formed during the TWF, and the time of caldera formation is inferred to be about 50.6 ka. However, there is basically no doubt that the shape of caldera was modified by the ME eruption in 946±3 AD.Main innovations of this thesis(1) The begin of the explosive stage of Changbaishan Tianchi volcano is at 50.6 ka, which is themiddle of Late Pleistocene. Three times explosive eruptions have taken place from the middle of Late Pleistocene. The TWF eruption is about 50.6 ka as a first large explosive eruption. The QXZ eruption is about 11.0±0.5 ka with a low-intensity explosive eruption and formed clastogenic lava flow. The new ME eruption with the comenditic and trachytic stages is considered to be 946±3 AD.2) The Tianchi caldera was formed during the TWF, and the time of caldera formation is inferred to be about 50.6 ka. Furthermore, the ME eruption have modified the shape of caldera and extended to southwest on size in 946±3 AD.3) In previous studies, the ME and BGM were considered as two separate eruption events. After this study, we suggest that the ME and BGM should be combined into one eruption in 946±3 AD,initiating as a major plinian eruption of comendite and then transiting to trachyte in its final phases.ConclusionBased on the new information presented in this thesis, we can conclude that three eruptive episodes—rather than four as previously believed— have occurred at Changbaishan Tianchi volcano since the middle of late Pleistocene. Although the recent new findings have improved our understanding of the behavior of this volcanic center, this study also underscores the need for, and the direction of, additional integrated field and laboratory investigations required to quantify this revised eruptive history. Should Tianchi reactivate in the future, it could pose significant volcanic hazards for Northeastern Asia, and, depending on the size of its next eruption, also for the world.