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[SVC31-14] Eruptive sequence and eruption style in the 12th Century eruption of the Asama-Maekake volcano
Keywords:Asama Volcano, Eruption style, 12th century eruption, fountain collapse
The 1783 eruption was the latest large-scale eruption in the Asama-Maekake volcano. Since it has many historical records, when reconstructing the eruption sequence, it is possible to include a time axis in the stratigraphy from the analysis of old documents. On the other hand, regarding the eruptions in the 12th Century, historical records do not help for reconstructing the eruptive sequence because only one record says that there was a large-scale eruption generating serious disasters. Characteristically the 12th Century eruption has many eruption units of pyroclastic fall and pyroclastic density current deposits. Since the eruptive products of numerous eruption units are distributed in almost all directions around the crater, it is extremely difficult to understand the overall stratigraphical relationships. However, a lot of geological facts obtained help estimate the eruption sequence and style of the 12th Century eruption. In particular, pyroclastic density current (PDC) deposits known as intermediate type by Aramaki (1963) are widely distributed, and numerous flow units are recognized, including lobe topography in the surface. Examination of the stratigraphic relationship between these and pyroclastic fall deposits (As-B) suggests that many eruptions continued over a long period in the early 12th Century, and the eruption history was much more complex than that of the 1783 eruption. Blocks contained in the pyroclastic density currents (Oiwake) are characterized by a composite block consisting of multiple blocks. The presence of ash fall layers called the Red ash is also distinctive. The Red ash is composed of more than 60 fall units. Each layer of the Red ash exhibits a bimodal grain size distribution consisting of coarse angular lithic fragments and fine brown glass shards. The glass shards are indistinguishable from the matrix ash of Oiwake pyroclastic density current deposits and are believed to be co-ignimbrite ash. As an interpretation, we considered an eruption style that repeated the cycle of Vulcanian explosions and fountain collapses to generate pyroclastic density currents and called this eruption style "Asama-style eruption". After the Phase 1 Vulcanian explosion, it causes vesiculation due to sudden decompression and magma fragmentation. For some reason, magma fragments begin to coalesce within the misty flow within the conduit. In Phase 2, fountain collapse, coarse composite blocks with a high terminal velocity selectively deposit inside the crater and separate from the pyroclastic density current. During the short rest period of Phase 3, the blocks that fill the crater will weld together, forming a kind of plug. As new magma rises, it builds up pressure under this plug, causing another explosion (Phase 1). Since there are over 60 layers of the Red ash, it is considered that the same cycle has been repeated over and over again. Based on this image, if ash-kagura volcanic ash derived from pyroclastic flow was deposited immediately after the deposition of a coarse lithic fragment layer due to a Vulcanian eruption, then the bimodal grain size distribution of the Red ash. It is also consistent with the fact that coarse lithic fragments and fine brown glass particles are intermixed within individual layers of red ash.
Finally, we will consider the differences between the 18th Century eruptions and the 12th Century eruptions. In particular, one point that differs from the 18th Century eruption is the existence of the Red ash. Compared to 1783, the total amount of magma erupted was large, the eruption style and sequence were different, and complex activity appears to have continued for a long time. In the 18th Century eruptions, most pyroclastic density currents occurred within half a day. In contrast, in the 12th Century, pyroclastic density currents with large vulcanian eruptions appear to have occurred intermittently over long periods of several years or more. It is also worth noting that this was followed by an eruption of As-B' with a different magma composition.
Finally, we will consider the differences between the 18th Century eruptions and the 12th Century eruptions. In particular, one point that differs from the 18th Century eruption is the existence of the Red ash. Compared to 1783, the total amount of magma erupted was large, the eruption style and sequence were different, and complex activity appears to have continued for a long time. In the 18th Century eruptions, most pyroclastic density currents occurred within half a day. In contrast, in the 12th Century, pyroclastic density currents with large vulcanian eruptions appear to have occurred intermittently over long periods of several years or more. It is also worth noting that this was followed by an eruption of As-B' with a different magma composition.