长白山天池火山气象站期喷发特征研究

Changbaishan Tianchi volcano is one of the most dangerous active volcanoes in China. It had experienced three periods of large-scale eruptions since the Holocene or the Late Pleistocene, i.e. the Tianwenfeng period, the Qixiangzhan period and the Millennium eruption, respectively. The type of Tianwenfeng and Millennium eruptions is commonly accepted to be a typical Plinian eruption. However, there arises a considerable debate about the type of Qixiangzhan eruption as to whether it is effusive or explosive. In high-resolution remote-sensing images, the morphology of the products of Qixiangzhan eruption looks like a lava flow, which erupts from Qixiangzhan parasitic crater, and flows along the northern slope of the volcanic cone about 5.4 km in length and 400-800 m in width. However, the recent research work by the author has revealed that the type of Qixiangzhan eruption may be a small-scale explosive eruption according to the accumulate texture, the phenocryst micro-morphology, and so on. This research project attempts to reveal the type of Qixiangzhan eruption and reconstruct the process of eruption and accumulation by means of interdisciplinary approach, including geologic field observation, micro-texture analysis, petrology, mineralogy and geochemistry. The results of this research work may provide some new insights into the eruption history of Tianchi volcano, and also will be helpful to the prediction and warning of volcanic hazards of Changbaishan Tianchi volcano.In addition, this research is also to provide the empirical for the study on the complex process of volcanic eruption.

长白山天池火山是我国最具潜在喷发危险的一座活火山。气象站期喷发是长白山天池火山全新世或晚更新世以来三次大规模喷发活动中的一次重要事件，此期表现了复杂的喷发特征，如在其喷发类型与过程的研究上至今仍存在着争议与不足。气象站期喷发堆积物在形貌上似一条熔岩流呈蛇形垅状分布于天池火口的北侧，前人通过地貌学和地球化学的一些研究，多认为此期喷发类型为溢流式喷发。而申请者在前期野外堆积结构、岩相学与显微形貌学的研究中，却发现均具有爆炸式喷发的特征，如薄层板状结构、棱角状晶屑等。如何解释这一分歧？本申请拟通过野外地质调查、碎屑物粒度分析、岩石结构、晶屑形态统计与显微形貌等一系列针对性的研究工作，判定气象站期喷发的类型，并进一步恢复其喷发、搬运与堆积的复杂过程，探讨其形成的原因。本研究对完善长白山天池火山全新世或晚更新世以来的喷发序列具有重要的意义，同时也为国内复杂活动过程的火山喷发事件的研究积累经验。

长白山天池火山气象站期喷发是分布于锥体北坡的一套蛇状熔岩，其在喷发类型上存在广泛的争议，有学者认为是溢流式喷发，有学者认为是碎成熔岩喷发，也有学者认为是爆炸式喷发。本项目通过精细的野外地质调查和室内岩相学与岩石矿物成分分析，认为气象站期是一套典型的泡沫熔岩喷发，这是一种少见的强溢流式喷发。判断其是泡沫熔岩喷发的主要依据有（1）岩浆为经历长期分异演化富含挥发份的碱性流纹质岩浆，具有强的潜在泡沫化能力；（2）野外大量的薄层板状结构为泡沫熔岩喷发后大量挥发份（<77%）散失沉降所成，这与其他喷发类型不匹配；（3）岩石的松散结构，岩石主要由大量孔隙，斑晶和浆屑组成，不规则孔隙和颗粒支撑结构说明了岩浆泡沫化破碎后的喷发与沉降过程；（4）完美的蛇形地貌特征显示出强的流动性，对于流纹质岩浆泡沫流是最合理解释。. 本项目通过系列剖面所揭示的堆积层序，反演恢复了气象站期的喷发过程。其喷发过程为（1）岩浆上涌，挥发份出融并在地表下聚集，引起地表的隆起；（2）压力到达极限，熔岩穹丘崩塌并形成块灰流，同时岩浆通道打开；（3）大量泡沫携带着斑晶与浆屑（泡沫熔岩）以幕式脉冲状快速溢出并形成多个舌状熔岩前缘和大量薄层板状堆积；（4）晚期少量泡沫熔岩挤出并沉降堆积在火口周缘，形成一套倾向火口中心的内聚型薄层板状堆积。另外，薄层板状堆积层在挤压，重力失稳与风化等作用下，发生了挠曲、褶皱、断裂与垮塌等改造，形成如今看似凌乱但层理可循的一套蛇状熔岩。. 通过本项目的研究，解决了长期争论的问题，确定了气象站期的喷发类型，恢复了喷发过程，这为理解和预测天池火山小规模喷发活动提供了参考，同时也弥补了天池火山整体喷发活动中的不足。