火山 第 60 巻 ( 2015)
第 2 号 211-240 頁

論 説

降下火砕堆積物からみた浅間前掛火山の大規模噴火
安 井 真 也＊（2014 年 5 月 26 日受付，2015 年 4 月 19 日受理）

Large-scale Eruptions Inferred from the Pyroclastic Fall
Deposits of the Asama-Maekake Volcano
Maya YASUI＊
Geological research on the large-scale eruptions of the Asama-Maekake volcano was carried out by investigating
pyroclastic fall deposits such as A (1783 AD), B(1128 AD), and B (1108 AD). These deposits are mainly composed ofpumice layers. In the case of the well-studied 1783 eruption, the most voluminous fall unit A-21 is classified as
subplinian from its estimated weight. Ash and lapilli layers composed of lithic fragments also characterize thepyroclastic fall deposits. The particles of these layers are massive and angular to subangular in shape. They are alsosimilar to the particles produced in the recent small-scale eruptions (e.g., 2004 eruption). Most of the recent eruptions,typically vulcanian eruptions, have mainly generated pyroclasts originating from solidified lava in a shallow level of theconduit. The pyroclastic fall deposits of the large-scale eruptions consist of pumice layers and lithic fragment layers,
suggesting that intermittent vulcanian and subplinian eruptions occurred in the course of the large-scale eruptions asabove.
Descriptions and isopach maps of the pyroclastic fall deposits were made as detailed as possible in this study. The
distributions of some fall units of the lithic fragment layers of pyroclastic fall deposits B and B were mappable. These
isopach maps show elongated distributions, suggesting the strong effect of wind on dispersal. These lithic fragment
layers are composed of coarser grains than those of the recent small-scale eruptions preserved in the ashy soil at alllocalities. These findings indicate that large-scale vulcanian eruptions occurred in the course of the 12th century
eruptions.
At present, the 1783 eruption is the only example in the history of the Asama-Maekake volcano for which thetemporal variations in the eruptive style and eruptive volume can be discussed with high reliability. Detailed
reconstruction of the 1783 eruptive sequence was found to be possible by comparison between the stratigraphy of theeruptive products and information in old documents. The large-scale subplinian eruption that occurred after theintermittent eruptions is considered to be associated with the large-scale clastogenic lava flows owing to vigorous
fountaining. On the other hand, little information is available on eruptions before 1783 because of the limited exposureand the availability of few old documents. Although the reconstruction reliability for the eruptions in the 12th century ispoor, these eruptions might have occurred with a different sequence from those of the 1783 eruption. The eruptions in
the 12th century were characterized by intermittent large-scale vulcanian eruptions after a major pyroclastic eruption inwhich subplinian pumice falls and pyroclastic flows were generated. This is a major difference from the 1783 eruption.
Furthermore, little information, such as the distribution and stratigraphy of the eruptive products, is available foreruptions predating the 12th century owing to the lack of exposure. For instance, although isopach maps of the
pyroclastic fall deposits can be prepared, the preparation of an accurate map is difficult for older deposits. Consequently,
in the case of the Asama-Maekake volcano, it is not easy to predict the eruptive sequence of future large-scale eruptions
on the basis of past eruptions.
Key words : Asama-Maekake volcano, pyroclastic fall deposit, isopach map, eruptive sequence, eruptive style＊〒156-8550 東京都世田谷区桜上水 3-25-40
日本大学文理学部地球システム科学科
Department of Geosystem Sciences, College of Humanities
and Sciences, Nihon University, 3-25-40, Sakurajosui,

Setagaya-ku, Tokyo 156-8550, Japan.
e-mail : yasui.maya@nihon-u.ac.jp