长白山天池火山千年大喷发的孕育与喷发过程

The Millennium Eruption of Changbaishan-Tianchi volcano, with an eruption magnitude of 7, is considered as one of the largest two eruptions in the past 2000 years on Earth. A lot of geological, petrological and geochemical studies have been performed on the Millennium Eruption, but due to the restrictions of the temporal analysis technics and volcanic theories, the mechanism and eruption processes of this eruption have not been fully understood yet. In the preliminary work, our project team has obtained some new cognition about this eruption. For example, the magmas are comprised of three types: trachyandesite, trachyte and rhyolite, and the eruption processes includes two stages: comenditic and trachytic eruptions. In this project, we will perform geological, petrological and high-precision geochemical studies on the Millennium Eruption. Base on the previous studies and new theories, we aim (1) to re-recognize the types of magma, distribution and depositional features of the eruption products; (2) to reverse the structure of the magma chamber beneath the Tianchi volcano; (3) to explore the mixing and triggering mechanism of the injection process of the new magma from deep crust to the laminated magma chamber; (4) to build the comprehensive model of the Millennium Eruption, including the store conditions of the magma in magma chamber before eruption, the triggering mechanism of this eruption, the transferring processes of magma, the explosive process and the deposition of the eruption products. In addition, we will also focus on the Baguamiao (1668 AD and 1702 AD) and Liuhaojie (1903 AD) eruptions, which were called ‘post-Millennium Eruptions’ and described as ‘rain with ash’ by the historical records. The affiliations of the post-Millennium Eruptions play a key role in understanding the eruption sequence of the Tianchi volcano in Holocene. This project will help us understand the active habits of Tianchi volcano and predict the volcanic disasters in the future. Furthermore, it will make up for the deficiency on the magma system and eruption mechanism for large-scale composite volcanoes in China.

千年大喷发是长白山天池火山最新的一次大规模爆炸式喷发活动，前人研究工作取得了多方面的认识，但由于当时理论和切入点的限制，至今未能达成一致观点和全面认识千年大喷发的活动情况。基于前人研究认识和项目组近年来获得的新线索（如发现多种岩浆的混合，存在碱流质与粗面质两个喷发层序和“后千年喷发”堆积特征等），本项目申请拟借鉴新的火山学理论，通过野外精细深入的地层研究和室内岩相学与地球化学等高精度测试手段，重新认识千年大喷发喷发物的类型和堆积特征，思考两层喷发物间的关系，恢复千年大喷发的喷发与堆积过程，反演天池火山地下岩浆房结构和千年大喷发中新岩浆的注入、混合和触发机制，综合建立千年大喷发孕育和喷发堆积模型，实现对千年大喷发的全面认识和理解。这将提高我国对天池火山及板内大型复合式火山岩浆系统和喷发触发机制的理解，也有助于对天池火山监测资料的分析解释和未来火山灾害的预测，弥补我国此方面研究的不足。

千年大喷发是天池火山最新一次大规模的爆炸式喷发活动，也是地球过去2000年来两次大规模喷发之一，是研究大型火山喷发活动的优质对象。本项目对千年大喷发进行了系统性研究，开展了野外地质调查、岩石学与岩相学、地球化学、数值模拟等工作。野外地质考察获得了喷发物组成、堆积层序及分布特征，认识到喷发由两个阶段组成，第一阶段产生单一巨大的喷发柱，空降浮岩与火山灰主要堆积在火口东南方向，而喷发柱坍塌碎屑物沿沟谷流动冲出沟口堆积在平原区，第二阶段喷发由多个脉冲式中等规模喷发组成，空降浮岩呈现多个韵律堆积在火口东侧，而火山碎屑流分布于近缘沟谷内。岩石学与岩相学观察认为千年大喷发由三种岩浆成分组成，并发生了快速的岩浆混合作用，其中粗安质岩浆的快速注入混合是触发千年大喷发的主要机制。通过地球化学分析与MELTS数值模拟，认为天池火山下方浅部存在碱流质与粗面质上下两个独立岩浆房，且碱流质岩浆由粗面质岩浆的结晶分异作用而形成，而粗面质岩浆房不断接受粗安质岩浆的补给，并进行分异演化形成新的粗面质岩浆。千年大喷发之后，天池火山地下岩浆房继续进行着粗安质岩浆的补给和结晶分异作用，为未来的喷发积蓄新的能量。2002-2005年的天池火山扰动事件便是补给过程中引起的岩浆房波动，表现出了小震群和地表形变。经过对千年大喷发的多手段和多方法的研究，认识到千年大喷发的发育过程、触发机制、喷发过程与成灾范围，同时也认识到现今天池火山地下岩浆房的结构和状态。这些成果将增强对大型火山喷发机制与灾害过程的理解，提升对火山机构、岩浆房结构与岩浆演化过程的认识，并为天池火山的灾害预测、监测数据解释和地方政府防灾减灾提供帮助。