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Small ash emissions were frequently observed from a crater lake, in which the amount of water decreased considerably, at Nakadake, Aso Volcano, southwestern Japan, between April and August 2005. During this period, major ash emitting events, which produced mostly white aggregated ash, occurred on April 14, June 10-12, June 21 (4.2 tons) and July 25 (1.2 tons). Ash emissions at the Nakadake crater in 2005 were classified as three types: gentle release of white aggregated ash from fumaroles inside the crater lake (e.g. daytime of April 14, June 10-12 and July 25); short (ca. 20 seconds) gas-and-ash emission through the crater lake (April 14 20: 41); and emission of black ash from an almost dry vent (June 21). All products from these ash emissions were composed of fine-grained (

Ash emissions occurred from a hot water pool of one of the craters of Nakadake, the only active central cone of Aso Volcano, southwestern Japan, on July 10, 2003 and January 14, 2004. The former ash was dispersed throughout an area extending 14km east-northeast of the vent, whereas the latter ash extended 8km to the east-southeast. Based on isopleth maps, the total weight of ash discharged by each event was 41tons and 32tons, respectively. Both ash-fall deposits were composed of fine-grained (5km). Moreover, the deposit volume obtained from actual data in the proximal area was much larger than the volume extrapolated only from the isopleth data of the distal deposits. These phenomena suggest that the proximal deposits were emplaced mainly by another fine-grain-rich column different from the main ash plume that extended to distal areas, or that some parts of the column detached from the main plume that emplaced the proximal deposits.

The western part of post-caldera central cones of Aso Volcano, southwestern Japan, is characterized by wide chemical variation of lava flows (SiO_2=49-73 wt.%) and complicated topography. Geologic history of the area was reconstructed by stratigraphic relationships between lava flows and airfall tephra layers. Stratigraphy and chronology of lavas and tephra distributed on the western slope of the central cones are as follows: Ayugaerinotaki lava, Matsuhata pumice (MhP), Tochinoki lava (73±10 ka; K-Ar age), Aso central cone pumice 4 (ACP4), Tateno lava, Aso central cone pumice3 (ACP3), Takanoobane lava (51±5 ka), Eboshidake lava, Karisako lava, Akase lava (30 cal ka; calibrated ^14C age), Sawatsuno lava (27±6 ka), Kusasenrigahama pumice (Kpfa; 31 cal ka), Otogase lava and Aso central cone pumice 1 (ACP1; 4cal ka), in ascending order. Between about 70,000 to 50,000 years ago Tochinoki lava, Tateno lava and Takanoobane lava accompanied precursor pumice-fall deposits. A catastrophic eruption occurred in and around the Kusasenrigahama crater at 31 cal ka. The eruption was initiated by the dacite Sawatsuno lava flow and produced the largest plinian pumice-fall deposit (Kpfa; 1.2km^3 in DRE volume). In the western part of post-caldera central cones of Aso Volcano, total discharge volumes of felsic and mafic magmas from about 70,000 to 30,000 years ago were estimated to be about 2.4km^3 and 0.47km^3, respectively.

Two volcaniclastic deposits-a debris-avalanche deposit (NgDA) and a lahar deposit (NgL) were discoverd alone the Nigorikawa River on the western slope of Aso central cones, southwestern Japan. The Nigorikawa debris-avalanche deposit, having a maximum thickness >3 m, contains numerous plastically deformed debris-avalanche blocks (

Aso central cones located within Aso caldera, central Kyushu, southwestern Japan, initiated their activity soon after the formation of the caldera (ca. 90 ka). The cones have produced voluminous airfall tephra layers and lava flows. Most of the tephra layers distributed in and around Aso caldera are andesite to basaltic-andesite scoria-fall and ash-fall deposits. Their stratigraphy is very complicated because it is difficult to distinguish between scoria-fall layers in the field. However, dacite to rhyolite pumice-fall deposits from some central cones interbedded between the tephra layers are very useful to correlate stratigraphic units at separated localities. Therefore, we used the pumice-fall deposits in order to construct the tephrostratigraphy and eruptive history of Aso central cones during the past 90,000 years. Thirty-six pumice-fall deposits were identified including eleven major key beds. In ascending order they are Nojiri pumice (NjP), Ogashiwa pumice (OgP), Yamasaki pumice 5 (YmP5), Sakakura pumice 2 (SsP2) and 1 (SsP1), Aso central cone pumice 6 to 3 (ACP6-ACP3), Kusasenigahama pumice (Kpfa) and Aso central cone pumice 1 (ACP1). Phenocrystic minerals of most pumice are plagioclase, ortho- and clino- pyroxene and magnetite, but NjP, ACP5, ACP3 and ACP1 include biotite, and NjP and SsP2 contains hornblende phenocrysts. On the basis of several 14C ages of buried soils just below pumice (above Kpfa) and stratigraphic position eruption ages for the eleven major pumice are estimated as follows: NjP (85 ka), OgP (80 ka), YmP5 (69 ka), SsP2 (57 ka), SsP1 (56 ka), ACP6 (52 ka), ACP5 (45 ka), ACP4 (40 ka), ACP3 (39 ka), Kpfa (31 ka) and ACP1 (4 ka). During the past 90,000 years Aso central cones produced pumice-fall deposits at an interval of about 2,500 years. Many of the sources of the pumice appear now buried under the present Aso central cones.

Jigoku spa is located west of Ikenokubo basin of Yomineyama volcano, southwestern part of the central cones of Aso volcano. In this basin, "Ikenokubo tuff ring" and two smaller maars were developed. Two layers of ejecta, about 10^5 m3 in bulk volume, formed by phreatic explosions were found in this area. They are very poorly sorted and mainly composed of altered fine-grained ash including lithic fragments. Kikai Akahoya ash erupted about 6,300 years BP (14C) is intercalated between two layers of phreatic ejecta in thick ash layers from Nakadake, the only active central cone. Based on 14C ages of buried Andisols and the succession of the deposits, the eruption ages of the two ejecta are estimated at about 10,000 years BP for older deposit and at about 4,500 years BP for younger one. Distributions of thickness and maximum grain size of the lithic fragments suggest that the both of two phreatic ejecta were exploded not from "Ikenokubo tuff ring" and two maars but from the Jigoku explosion crater in the western area. Ejecta correlative to the "Ikenokubo tuff ring" lies under the older of the phreatic ejecta layers. Consequently, the eruption age of "Ikenokubo tuff ring" is probably older than 10,000 years BP. Frequency of phreatic explosions, larger than 10^5 m3 in volume, is considered to be once in about 5,000 years. This fact is noteworthy for volcanic hazards assessment in the surrounding area of Jigoku spa and adjacent Tarutama spa.

Aso central cones initiated their eruptive activity just after the Aso-4 eruption, about 90,000 yrs BP and have produced large volumes of tephras and lava flows. However, few studies have been made to determine the dates of eruption. We obtained 14C ages of buried Andisols interbedded between tephra layers to determine the eruption ages of Holocene tephras from Aso volcano. The eruption ages are estimated to be 1,600 yrs BP for N2 scoria from Nakadake (N2S), 2,700 yrs BP for Ojodake scoria (OjS), 3,400 yrs BP for Kishimadake scoria (KsS) and 3,700 yrs BP for Aso central cone pumice 1 (ACP-1). In addition, the date of the last lava extrusion from Nakadake is estimated to be 4,800 yrs BP. During the last 6,300 years ash eruption of Nakadake was most active between 3,800 and 3,600 yrs BP. Sub-plinian eruptions of Kishimadake and Ojodake, which accompanied with scoria fall deposits, occurred during the period 3,400 to 2,700 yrs BP.