Abstract
The 946 A.D. Millennium Eruption (ME) of Tianchi volcano produced masses of pyroclastic density current (PDC) deposits that cover an area that extends roughly 50 km from the crater. In order to understand PDC dynamic processes, this study examines the proximal to distal variations in the PDC facies by analysing the topography, stratigraphy, granulometry and petrography. Topography of Tianchi volcano has different slopes, including the cone (>6°), lava shield (2–5°) and lava plateau (20 km) are less than 10 m thick, with coarse-tail layers, ground surge layers, climbing layers, and carbonized woods in the plateau. The degree of PDC welding decreases with the distance from the crater. From the proximal to the distal strata, the emplacement temperature of the PDCs decreased from 740 °C to 280 °C. The medial layers indicate a gravitational differentiation effect and weakened transportation of the PDCs, which led to the formation of thick beds. The distal PDCs demonstrate fluidisation characteristics and a density far lower than those of the proximal and medial strata, which exhibit clear stratification. Grain size data (<64 mm) show that the median diameters decreases with increasing distance from the crater. Rhyolitic and trachytic pumice are found in the PDCs and are assumed to be associated with the composition of the magma chamber. Magmatic mixing structures were confirmed by microscope observations. The volume estimation of PDCs in China is 6.9806 km3. The PDCs exhibit characteristics of high temperatures, large volumes, flowing over long distances, and having the ability to climb hills. The PDCs were the most hazardous volcanic phenomena and can provide a reference for future potential PDC hazards around the Tianchi volcano.
Highlights
Large-scale eruptions, characterized by volcanic explosivity index (VEI) ≥ 7, always produce masses of pyroclastic density currents (PDCs), which are one of the most hazardous eruptive phenomena owing to their complex emplacement processes
To better understand emplacement processes, we investigate the PDCs emplaced during the Millennium Eruption (ME) in the Chinese territory via five aspects, i.e., the topography of Tianchi volcano, its stratigraphic, granulometric, and microscopic characteristics, and whole-rock composition, so as to establish the facies model of the PDCs during the ME
The characteristics of the PDC deposits during the ME are closely related to the original geomorphology of Tianchi volcano
Summary
Large-scale eruptions, characterized by volcanic explosivity index (VEI) ≥ 7, always produce masses of pyroclastic density currents (PDCs), which are one of the most hazardous eruptive phenomena owing to their complex emplacement processes. These PDC deposits are always inhomogeneous in their distributions, thicknesses, stratigraphic structures, and temperatures from the eruptive center to the distal (Branney and Kokelaar, 2002). Tianchi volcano ( known as Changbaishan or Baitoushan) is a giant intraplate stratovolcano (Wei et al, 1997) that is located on the border between China and North Korea and the HunchunYalu River tectonic fault zone (Figure 1A; Liu et al, 1998). The Tianchi caldera lake has stored nearly 2 billion cubic meters of water and has a gap in its northern reaches, which are located in the Chinese territory (Figure 1B)
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