International Workshop on the mechanism of Phreatic Eruption

The September 14, 2015 phreatomagmatic eruption ofNakadake first crater, Aso Volcano
Yasuo Miyabuchi1, Yoshiyuki Iizuka2, Chihoko Hara3, Akihiko Yokoo4 and Takahiro Ohkura4
1 CWMD, Kumamoto University, 2 Institute of Earth Sciences, Academia Sinica, 3 Nishikokubu Elementary School,
4 Aso Volcanological Laboratory, Kyoto University
E-mail: miyabuchi(at)gmail.com

Nakadake Volcano, which is the only active central cone inside the Aso caldera, is one of
the most active volcanoes in Japan. The active crater (first crater) of Nakadake is occupied
by a hot, hyperacidic (pH = 0.43) crater lake during its calm periods (Miyabuchi and Terada,2009; Ohsawa et al., 2010). During active periods, volcanic activity of Nakadake first crateris characterized by continuous fallout of black sandy ash from a dark eruption plume (Ono etal., 1995). In more active periods, strombolian eruptions have scattered red-hot scoriaceous
clasts around the vent. Moreover, phreatic or phreatomagmatic eruptions occurred in
September 1979 and April 1990, ejecting coarse lithic blocks and generating small
low-temperature pyroclastic density currents around the crater (Ono et al., 1982; Ikebe et al.,2008).
Following the November 2014-May 2015 magmatic activity (Miyabuchi and Hara, 2019), an
explosive eruption occurred at Nakadake first crater on September 14, 2015. The sequenceand causes of the eruption were reconstructed from the distribution, textures, grain-size,component and chemical characteristics of the related deposits, and video record(Miyabuchi et al., 2018). The eruptive deposits are divided into ballistics, pyroclastic densitycurrent and ash-fall deposits. A large number of ballistic clasts (mostly < 10 cm in diameter;maximum size 1.6 m) are scattered within about 500 m from the center of the crater. Almost
half of the ballistics appear as fresh and unaltered basaltic andesite rocks interpreted to bederived from a fresh batch of magma, while the rest is weakly to highly altered clasts. A
relatively thin ash derived from pyroclastic density currents covered an area of 2.3 km2 with
the SE-trending main axis and two minor axes to the NE and NW. The pyroclastic density
current deposit (maximum thickness < 10 cm even at the crater rim) is wholly fine grained,
containing no block-sized clasts. Based on the isopach map, the mass of the pyroclastic
density current deposit was estimated at approximately 5.2 × 104 tons. The ash-fall deposit
is finer grained and clearly distributed to about 8 km west of the source crater. The mass of
the ash-fall deposit was calculated at about 2.7 × 104 tons. Adding the mass of thepyroclastic density current deposit, the total discharged mass of the September 14, 2015

International Workshop on the mechanism of Phreatic Eruption

eruption was 7.9 × 104 tons. The September 14 pyroclastic density current and ash-falldeposits consist of glass shards (ca. 30%), crystals (20-30%) and lithic (40-50%) grains.
Most glass shards are unaltered poorly crystallized pale brown glasses which probably
resulted from quenching of juvenile magma. This suggests that the September 14, 2015
event at the Nakadake first crater was a phreatomagmatic eruption although the eruption is
one order of magnitude smaller in eruptive volume than the September 6, 1979 and April 20,
1990 eruptions. These events highlight the potential hazard from phreatic orphreatomagmatic eruptions at Nakadake first crater, and provide useful information that willassist in preventing or mitigating future disasters at other similar volcanoes worldwide.
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