Bull. Volcanol. Soc. Japan
Vol. 60 (2015) No. 1, pp. 17-33

ArticleGeology and Evolution of the Nakajima Islands (Toya Caldera, Hokkaido,Japan) Inferred from Aerial Laser Mapping and Geological Field Surveys
Yoshihiko GOTO＊, Satoru MATSUZUKA＊＊, Seiji KAMEYAMA＊＊ and Tohru DANHARA＊＊＊(Received June 12, 2014 ; Accepted January 10, 2015)
Aerial high-resolution laser-scanner mapping and geological field surveys were performed over the Nakajima
Islands within Toya caldera, southwestern Hokkaido, Japan, to study the evolutional history of the islands. The laserscanner mapping survey covered the entirety of the Nakajima Islands, an area of 3×3 km. A three-dimensional digital
map produced from the laser-scanning data revealed detailed topographic features of the islands. Geological fieldsurveys were carried out over the whole area of the Nakajima Islands, to determine how their topography relates to thegeological and lithological features. These surveys suggest that the islands consist of a tuff cone, eight lava domes, anda cryptodome. The tuff cone has a low profile and wide crater and consists of dacitic pyroclastic deposits, suggestingthat the cone was produced by explosive eruptions resulting from the interaction of dacitic magma and groundwater.
The lava domes are conical or pancake-shaped and composed of dacitic to andesitic lavas, suggesting that the domes
formed by extrusions of high-viscosity, dacitic to andesitic magmas. The cryptodome consists of coherent dacite
overlain by mudstone and sandstone, suggesting that the dome formed by the uplift of caldera-floor deposits following
the intrusion of high-viscosity dacitic magma. Mudstones and sandstones are present along the northeastern andsouthwestern parts of the islands, implying that the caldera floor was uplifted during or prior to the volcanism, forming
a bulge (small resurgent dome) at the center of the caldera. We infer that the Nakajima Islands have evolved from
caldera resurgence related to the ascent of voluminous dacitic to andesitic magma, followed by subsequent formation ofmultiple dacitic to andesitic domes and phreatomagmatic eruptions on the resurgent dome.
Key words : laser-scanner mapping, geology, Nakajima Islands, Toya caldera, evolutional history

1．Introduction
LIDAR (light detection and ranging) is a powerful tool
for those studying the morphological features of volcanoes(e.g., Chiba et al., 2007a, 2007b ; Hunter et al., 2003 ; Pesci
et al., 2007). Three-dimensional digital mapping based onhigh-resolution laser-scanning data provides invaluable
information on the distribution and morphology of craters,lavas, domes, and pyroclastic deposits, as well as reworkeddeposits. Laser-scanner mapping is particularly useful forsurveying topographic features in thickly vegetated areas,
for which ‘tree-removing’ data filtering can be used toreveal the topography of the ground surface (Chiba et al.,2007b ; Goto et al., 2011).
We conducted aerial laser-scanner mapping and geological field surveys over the Nakajima Islands within
Toya caldera, southwestern Hokkaido, Japan (Fig. 1), tostudy the morphological and geological features of theislands. The Nakajima Islands are a silicic volcanic complex
produced by post-caldera volcanism (Katsui, 1990 ; Ota,＊

College of Environmental Technology, Graduate Schoolof Engineering, Muroran Institute of Technology, Mizumotocho 27-1, Muroran, Hokkaido 050-8585, Japan.
＊＊Tanaka Consultant Co. Ltd, Shinkai-cho 2-1-3, Tomakomai,
Hokkaido 053-0052, Japan.

1956 ; Soya et al., 2007 ; Ui et al., 2013 ; Yokoyama et al.,1973), but their detailed geology remains unknown
because it is obscured by the thick vegetation cover (Fig.
2). Three-dimensional digital mapping based on the laserscanning data allowed us to study the detailed topographic
features of the islands, and use them as the basis forunderstanding their geology. This paper describes thetopographic and geological features of the Nakajima
Islands and discusses the evolutional history of the islands.
2．Nakajima Islands
The Nakajima Islands are located in the central part ofToya caldera (Fig. 1). The caldera covers an area of 10×11 km, and was formed by violent silicic eruptions atapproximately 110 ka (Ganzawa et al., 2007 ; Machida andArai, 2003 ; Okumura and Sangawa, 1984 ; Takashima etal., 1992 ; Yokoyama et al., 1973). The caldera-forming
eruptions generated large-volume, rhyolitic, pyroclastic
fall (Machida and Arai, 2003), and flow (Yokoyama et al.,＊＊＊

Kyoto Fission-Track Co. Ltd, Minamitajiri-cho, Omiya,Kita-ku, Kyoto 603-8832, Japan.

Corresponding author : Yoshihiko Gotoe-mail : ygoto@mmm.muroran-it.ac.jp