This paper introduces the special issues on “Determination of the Construction of an Outcrop Database to Reveal Eruptive History”, which will be published in the three issues. Eruptive history is not only a basic subject in volcanology but is also focused on fundamental information that can be used to reduce and prevent volcanic disasters. Although outcrops are a primary source in geoscience, including volcanology, they are easily covered by artificial material and thick vegetation, or can disappear as a result of erosion. An outcrop database would be useful tool for storing and reusing such basic information. However, it would be difficult to construct such a database because of the cost and labor. The purpose of the special issues was to collect a wide range of case studies, from geological descriptions of outcrops to the eruptive history of volcanoes. Review papers covering informatics approaches for various applications will also be included. We expect that this series of issues will contribute to the construction of a new outcrop database.

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We performed systematic sampling and description of the Adatara-Dake tephra outcrop on the summit of Adatara volcano, Fukushima Prefecture, Japan in order to reveal the detailed eruption sequence and temporal evolution of the magma system of this volcano that erupted 120ka BP. Even though there is no recognizable eruption hiatus represented by a paleosoil layer, pyroclastic fall characteristics at the outcrop permit to divide Adatara-Dake tephra into 19 layers: A to R; from bottom to top. The earlier layers (A to L) are characterized by successive pumice fall deposits, intercalated by thin volcanic sand layers. The later layers (M to R) are rich in scoria fall and are partly welded, with agglutinate found in layers O and R. Representative clasts from each layer were analyzed to determine the grain size distribution, componentry, modal composition and whole rock chemistry. Layer M can be correlated petrologically and petrographically with the lower part of Yugawa pyroclastic flow deposit on the eastern foot and the Motoyama pyroclastic flow deposit on the western flank of the volcano. Similarly, the upper part of Yugawa pyroclastic flow correlate with layer N, meanwhile the upper part of Motoyama pyroclastic flow correlate with layers O or R. There is an increase in the lithic fraction with decreasing sorting in layer M, which we interpret to indicate increasing enlargement of vent during the phase of this layer. The scoria/pumice volume ratio also increases remarkably in layer M, suggesting that vent enlargement and sudden increase of mafic magma during phase M caused the column collapse that generated the Yugawa and Motoyama pyroclastic flows. The mafic magma composition changed after layer O, with the FeO^*/MgO ratios becoming less than 2.1. After this change in chemistry in phases O to R, pyroclastic flows were continuously generated and agglutinated deposits were formed at the summit. This suggests that the eruption style of the final phase abruptly changed to relatively low column height.

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Kuju volcano is located along the northeastern side of the Beppu-Shimabara Graben in Kyushu Island. This volcano consists of small strato-volcanoes and lava domes. This paper reports on stratigraphy of tephra deposits by phreatic eruptions above Danbaru scoria (ca. 5-6 cal kBP). The six fallout deposits, which are phreatic origin, are identified by the field survey around the proximal area and the detailed ¹⁴C dating. The six fallout deposits are named as Kuju phreatic deposit 6 to 1 in ascending order. The eruption ages are cal BP 3610-3714, 2887-3065, 2356-2700, cal AD 214-387, 893-1152 and 1283-1393. The eruption interval is about 500±200 years. The most voluminous phreatic eruption in Kuju volcano is Kj-ph6, and it's volume is ca. 6.1×10⁶m³. The argillaceous pyroclastic deposit, which was deposited by the surface failure around hydrothermal field, and related lahar and reworked deposits are recognized. These deposits were formed in cal AD 892 to 1152.

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We made a detailed geological study to understand the eruptive history of the Ikeda volcano, which includes the Ikeda caldera, maars and lava domes. The pre-caldera activity began at about 20 cal kBP with the Iwamoto ash fall deposit. The Senta lava was also effused before the Kikai-Akahoya tephra (7.3 cal kBP). The caldera-forming eruption began at 6.4 cal kBP with a phreatic explosion that produced the Ikezaki ash fall and surge deposits. This was soon followed by the magmatic eruptions that produced the Osagari and Mizusako scoria fall deposits and the Ikeda pumice fall deposit. During the climactic stage, the Ikeda ignimbrite was erupted and covered portions of the coastal area. Immediately after the caldera-forming event, four maars were formed along the fissure vent southeast of the caldera. The Yamagawa maar, which is the largest and is located at the southeastern end of the fissure vent, erupted a pumiceous base surge (the Yamagawa base surge), while the other maars ejected small amounts of accessory or accidental materials. During the late stage of the Ikeda eruption, a phreatomagmatic eruption occurred at the bottom of the caldera floor, which formed the widespread Ikedako ash fall deposit. The central lava dome was formed during the late stage of this eruption. After the Ikedako ash fall, secondary explosions of the Ikeda ignimbrite occurred mainly along the coastal area, generating small base surge deposits. About 2,000 years after the caldera-forming eruption at 4.8 cal kBP, new magmatic activity began on the margin of the caldera rim, and formed the Nabeshimadake lava dome.

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This paper introduces a practical use of outcrop data in determining the correlation, stratigraphy and distribution of large-scale pyroclastic flow deposits (PFL). The studied area is the Akan and Kutcharo volcanic zone in Eastern Hokkaido, Japan, which have had a long and complex history of more than 20 caldera-forming eruptions during the Quaternary. A database of the stratigraphy and glass chemistry for the more than 20 PFL can be established by studying a sufficient number of representative outcrops. We found representative outcrops where stratigraphic relationships between several PFL can be observed at the same time. We analyzed glass chemistry of juvenile pumices (＞10 clasts) of the PFL. The database enables to identify all exposed PFL in this area, thus allowing us to draw detailed maps of the distribution for each PFL. The database can be also used for correlation and chrono-stratigraphic determination of reworked volcanic deposits, such as terrigenous marine deposits in Kushiro region, located on the plains at the foot of Akan and Kutcharo volcanoes.

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The Kirishima volcanic group is located in southern Kyushu and is comprised of more than 20 volcanic edifices. The Kirishima volcanic group has produced many tephra layers and lava flows over the past 30,000 years, but few type localities have been established. Unfortunately, one type locality for tephra layers from Kirishima volcanic group, described by Imura (1996) in Kamamuta settlement, was destroyed by road construction. An alternative tephra locality near the destroyed one was found in the Kamamuta, and it is proposed a new type locality in this letter. Additionally, it is described type localities of Kirishima tephra layers dating from 20,000 to 7,300 cal BP that were generated from the western Kirishima volcanic group, including Koshikidake, Karakunidake and Ebinokogen in the Daio settlement.

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A great phreatic explosion at Bandai volcano on 15 July 1888 was accompanied by a huge rock avalanche that flowed northward and strong windblasts and mudflows that flowed southeastward. This extraordinary eruption occurred at the dawn of modern Japanese civilization during the middle Meiji era when the Imperial University of Tokyo began to introduce to their curriculum for systematic studies of scientific theory developed in the western world. Although more than 120 years have passed since the eruption, no firm conclusions have been reached about the mechanism of the 1888 phreatic explosion at Bandai. This is partly because statements and interpretations by Sekiya and Kikuchi (1890) were based on circumstantial evidence from their own observations and those of local inhabitants, and partly because in their 1890 paper, Sekiya and Kikuchi chose to disregard scenarios for the eruption other than the one they published in Japanese. However, after a critical review of their paper and related reports written in Japanese, we found evidence that Prof. Sekiya (the first Japanese seismologist) and Associate Prof. Kikuchi (a young and spirited geologist) had different opinions on the mechanism of the 1888 eruption. Therefore, they faced several dilemmas when preparing their paper for publication. For example, they disagreed on whether the explosive source was beneath Kobandai-san (the collapsed edifice after the 1888 eruption) or beneath Numanotaira (the old crater). Prof. Sekiya appears to have overruled his junior colleague: their published interpretation of events was based on Humboldt’s (1849) outdated views on the causes of earthquakes and volcanic eruptions. Associate Prof. Kukuchi favored the more modern theory of volcanism of Scrope (1862), but consented reluctantly to Prof. Sekiya’s interpretation of the mechanism of the 1888 eruption. Here, we provide a detailed discussion of how the dilemma faced by these scientists developed and reveal that they were deeply affected by mid-19th century controversies in geological theory as espoused in the western world. We must take into account that Japanese geologists of that era generally accepted the western world’s theories about the causes of earthquakes and volcanic eruptions, some of which are now seen to be fanciful.

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