Characteristics Of Pyroclastic-flow Facies In Millennium Eruption And Study On Pyroclastic-flow Hazard Zonation In Tianchi Volcano, Changbai Mountains

Changbaishan Tianchi volcano is the largest composite volcano, which is located in the northeast of China, at the boundary between China and North Korea. In 1215(±15), there was a large plinian eruption in Tianchi volcano which is called millennium eruption. Millennium eruption produced widespread pryolcatic-flow deposits. According to the previous study, there are three problems about the pyroclastic-flow deposits. First, there is no systematic facies about that pyroclastic-flow. Most of the pyroclastic flow deposited in the wild place which is undeveloped. Second, there is no hazard zonation for pyroclasitc-flow which is a major disaster in Changbaishan area. Third, grain-size analysis has not reached to the micron level.In order to answer these problems, some research works have been developed in this paper.1. Main contents and research methodologMain contents for research are:(1) to establish vertical thickness map of pyroclastic-flow in one valley, analysis shape characteristics, and discuss the relationship between shapes of pyroclastic-flow and prime topography before that eruption;(2) to separately establish the stratum characteristics for the proximal, medial and distal pyroclastic-flow, and discuss the transportation of pyroclastic-flow ;(3) to separately establish the litofacies characteristics for the proximal, medial and distal pyroclastic- flow by analyses of micro structure and major element, and discuss the fluid nature of pyroclastic-flow in different stages;(4) to establish the grain-size distribution for the medial and distal pyroclastic-flow, study fine ash distribution, and discuss the relationship between grain-size and pyroclastic- flow transportation;(5) to describe pyroclastic flow stratums at the distal part of the pyroclastic-flow in Baixi, analysis the genetic classification of the stratum, and discuss the transportation and deposition processes.(6).to establish the hazard zonation for pyroclastic-flow which is based on the slide-block model and analyses of DEM&TIN in Tianchi volcano. Research methodologies are :(1) to find a complete pyroclastic-flow in the Tianchi volocano , and determine the object of this study; (2)to study the stratum,litho and geochemistry by analysis of micro structure, major element and Scanning Electron Microscope (SEM); (3) to study the grain-size distribution by sieve instrument (Easy Sieve) and some of them by manual sieve ; (4) to describe the shape of grain-size from the median and distal pyroclastic-flow by analysis of SEM and morphological instrument (Camerasizer);(5) to analyses the fine ash distribution by laser grain-size analyzer.2. Main conclusionsThis paper has studied the shape, strata, litho, geochemistry, grain-size and transportation of ground–surge from pyroclasitc-flow of millennium eruption in Tianchi volcano. The main conclusions which have been got are as follows:(1) The shape of pyroclastic-flow in millennium eruption is strictly affected by the prime topography of Tianchi volcano. About in 0~10 km in the cone, pyroclastic-flow deposited in the valley, and the thickness is at about 1~4m. About in 10~20 km in the shield, pyoclastic-flow deposited in the valley, and the thickness is at about 10~40m. About in 20~42km, pyroclastic-flow deposited in the lava table, and the thickness is less than 10m.(2) The proximal strata of pyroclastic-flow are combined with the eutaxite structures and lava-like structures. The median strata are combined with massive beds,pumice-rich layers,litchis-richer layers and welded zones which are representative of the transportation processes for gravitational differentiation. And columnar joints and block structures are also developed which are representative of the deposition processes for cooling. The distal strata are combined with coarse-tail beds, ground surge beds and climbing beds. There are also some altered beds because of flood. The distal pyroclastic-flow strata have some fluidized characteristics.(3) The proximal ignimbrite is alkaline trachytic welded tuff. The median ignimbrite is also alkaline trachytic welded tuff. And the distal ignimbrite is alkaline rhyolite. The welded tuff is weaker from the proximal to the distal. (4) The proximal and median black pumice is trachyte. And the distal gray pumice is rhyolite.(5) The median diameters of pumice which is less than 64mm from the median and distal strata have an increasing tendency with the distance increasing from the crater. And the median diameters become smaller with the depth increasing from the top of strata. The Sphericity (Spht) has an increasing tendency with the distance increasing from the crater. And the contents of lithoclasts in number percentage decrease with the distance increasing from the crater which reveals gravitational differentiation.(6) With the pumice becoming smaller, there are more angles, richer irregular shapes and simpler transportation. The grain-size distributions of fine ash are similar and have a single peak which is close to the fines.(7) The histograms of ahs cloud and ground surge have a similar characteristic which is ladder-like and close to the fines. And the histograms of pyroclastic- flow have many peaks which are representative of the composite transportation.(8) There was hydration at the distal part of pyroclastic-flow, and the ground surge came from pyroclastic-flow and water interaction in Baixi.(9) According to computation, with 10km column height, the maxim radius of pyroclastic-low hazard zonation is 13.69km; with 20km column height, the maxim radius of pyroclastic-flow hazard zonation is 35.42km; with 30km column height, the maxim radius of pyroclastic-flow hazard zonation is 57.8km. Base on the pyroclasti -flow zonation in millennium eruption and topography of Tianchi volcano, the map of pyroclastic-flow hazard zonation has been established for different column heights which are representative of eruption scale.3. InnovationIn the course of study, four innovations have been discovered as follows:(1)In this paper, through research of shapes, strata, lithofaices, geochemistry, grain-size, transportation and deposition processes, using SEM, Camerasizer and Laser grain-size analyzer, it is firstly proposed the facies of pyoclastic flow in millennium eruption in Tianchi volcano, Changbaishan. (2)It is first to discovery that the grain-size distributions of fine ash are similar and have a single peak which is close to the fines in the pyroclastic-flow strata from median to distal. Compared with the fine ashes in pyroclastic-surge in Longquanlongwan volcano, Longgang volcanic cluster, it is found some differences between pyroclastic-flow and pyroclastic-surge. The grain-size of fine ashes in pyroclastic-surge can reach to 0.1μm, and the grain-size of fine ashes in pyroclastic-flow only reach to 1μm. The pyroclastic-surge which is produced by phreat-magma explosive eruption has more fine fragments than pyroclastic-flow which is produced by magma explosive eruption.(3) Through the analysis of SEM, stratum and grain-size, it is found that there was hydration at the distal part of pyroclastic-flow and the ground surge came from pyroclastic-flow and water interaction in Baixi.(4) In this paper, it is first to use the sliding block model computing the distance of pumice transportation.