Sequence and eruptive style of the 1783 eruption of Asama Volcano, central Japan: a case study of an andesitic explosive eruption generating fountain-fed lava flow, pumice fall, scoria flow and forming a cone

Abstract

The 3-month long eruption of Asama volcano in 1783 produced andesitic pumice falls, pyroclastic flows, lava flows, and constructed a cone. It is divided into six episodes on the basis of waxing and waning inferred from records made during the eruption. Episodes 1 to 4 were intermittent Vulcanian or Plinian eruptions, which generated several pumice fall deposits. The frequency and intensity of the eruption increased dramatically in episode 5, which started on 2 August, and culminated in a final phase that began on the night of 4 August, lasting for 15 h. This climactic phase is further divided into two subphases. The first subphase is characterized by generation of a pumice fall, whereas the second one is characterized by abundant pyroclastic flows. Stratigraphic relationships suggest that rapid growth of a cone and the generation of lava flows occurred simultaneously with the generation of both pumice falls and pyroclastic flows. The volumes of the ejecta during the first and second subphases are 0.21 km³ (DRE) and 0.27 km³ (DRE), respectively. The proportions of the different eruptive products are lava: cone: pumice fall=84:11:5 in the first subphase and lava: cone: pyroclastic flow=42:2:56 in the second subphase. The lava flows in this eruption consist of three flow units (L1, L2, and L3) and they characteristically possess abundant broken phenocrysts, and show extensive "welding" texture. These features, as well as ghost pyroclastic textures on the surface, indicate that the lava was a fountain-fed clastogenic lava. A high discharge rate for the lava flow (up to 10⁶ kg/s) may also suggest that the lava was initially explosively ejected from the conduit. The petrology of the juvenile materials indicates binary mixing of an andesitic magma and a crystal-rich dacitic magma. The mixing ratio changed with time; the dacitic component is dominant in the pyroclasts of the first subphase of the climactic phase, while the proportion of the andesitic component increases in the pyroclasts of the second subphase. The compositions of the lava flows vary from one flow unit to another; L1 and L3 have almost identical compositions to those of pyroclasts of the first and second subphases, respectively, while L2 has an intermediate composition, suggesting that the pyroclasts of the first and second subphases were the source of the lava flows, and were partly homogenized during flow. The complex features of this eruption can be explained by rapid deposition of coarse pyroclasts near the vent and the subsequent flowage of clastogenic lavas which were accompanied by a high eruption plume generating pumice falls and/or pyroclastic flows.

Appendix

General geology

Asama volcano (36°4′N, 138°1′E) consists of three volcanic edifices (Aramaki 1963): "Kurofu volcano", "Hotokeiwa volcano", and "Maekake volcano" from oldest to youngest (Fig. 1). Kurofu volcano (100–20 ka) is an andesitic composite volcano, the upper half of which was lost by sector collapse ca. 22,000 years b.p. Remnants of Kurofu volcano forms a somma outside the present Asama volcano. Hotokeiwa volcano (20–10 ka) consists of thick dacitic to rhyolitic lava flows, which are associated with numerous pumice falls and pyroclastic flows. The youngest Maekake volcano (10 ka–present), is an andesitic composite volcano that is still active. The activity of Maekake volcano started after 10,000 years b.p. (Takemoto 2000). About ten major sub-Plinian to Plinian eruptions are postulated based on the pumice fall deposits, and the most recent one occurred in 1783 a.d. (Aramaki 1963). Individual eruptions generated products on the order of 10⁻¹ km³ (DRE), which consist of pumice falls, pyroclastic flows, and lava flows. Small-scale Vulcanian explosions occurred intermittently between the major eruptions. Since the 1783 eruption, thousands of small-scale Vulcanian explosions have been recorded at the summit crater. The repose intervals of the explosions vary from less than a day to several tens of years. The explosions were frequent from 1927 to 1961 (up to several hundred times per year), whereas they have occurred rarely since 1982. The last one occurred in 2003. Each explosion ejected bombs and blocks (order of 10⁴ ton or less) near the crater and rarely deposited ash and lapilli onto the flanks of the volcano (e.g., Minakami 1942; Aramaki and Hayakawa 1982).