Facies variations in the pyroclastic density currents (PDCs) produced by the Millennium Eruption of the Changbaishan Tianchi volcano, NE China

The Plinian Millennium Eruption (ME) of the Changbaishan Tianchi volcano in 946 AD generated a total tephra volume of 100 ~ 172km3 produced by a large scale of ME-I eruption and a small scale of ME-II eruption. This study examines the facies variations in the pyroclastic density currents (PDCs) emplaces that were produced by the ME based on observations of the depositional units, vertical sequences in 53 outcrops. Grain-size and composition analysis using standard dry sieving techniques and microscope observations of thin sections in the laboratory provided detailed information for the PDCs emplacement. The ME PDCs deposits of the Changbaishan Tianchi volcano can be divided into two facies: canyon filling PDCs facies and fan-shaped PDCs facies. The canyon filling PDCs facies are mainly developed in the circumference area of the volcanic cone (8 ~23km from the crater and slope between 15° to 60°) which are mainly restricted by the volcanic valleys, and can further be divided into massive canyon filling and layer canyon filling sub-facies. In contrast, fan-shaped PDCs facies are mainly distributed in the areas between 23 ~45km from the crater which present relatively gentle slope between 5° to 15°. In this area, the fan-shaped PDCs were not restricted by the topography and generally formed fan-shaped emplacements with a large aspect ratio. Fan-shaped PDCs can also be divided into fan head and fan body sub-facies. The vertical stacked multiple PDCs units occurred in both layer canyon filling and fan head sub-facies, which can be attributed to the separation and proliferation of flow units in the process of PDCs transport. According to the maximum distribution range and the thickness of the ME PDCs, in case of another Plinian eruption like ME, the area within at least 45km from the eruption center has a huge risk of PDCs disaster. This study is helpful to understand the spatial distribution of the PDCs emplacement in the Changbaishan Tianchi volcano, in order to mitigate the potential disastrous consequences of a future similar volcanic eruption.