Analysis of volcanic glass and radiocarbon dating were applied to shallow marine sediment of 6 cores obtained from the Tokushima and Nobi plains, central Japan. Six basal horizons of Kikai-Akahoya (K-Ah) ash and two ones of Amagi-Kawagodaira (Kg) ash were identified from these cores based on the contents of volcanic glass, morphology of glass shards and these colors. The known ages of these tephras, K-Ah: 7165-7303 cal BP and Kg: 3137-3160 cal BP, were compared with the radiocarbon ages measured around these horizons. Seven pairs of radiocarbon ages from plants and shells were consistent with the tephra eruption ages. However, other 1 pair was 500-1000 years older than the age of tephra because the age was obtained from organic sediment. Integrated chronology by tephras and radiocarbon dating will enable to determine us high-resolution interpretation of shallow marine sediments.
Kaimondake volcano, located in southwest Japan, first erupted ca. 4.4 cal kBP and produced 12 eruptive episodes (Km1 to Km12, in ascending order) until AD885. As radiocarbon dating is a useful tool for establishing a chronostratigraphic framework, this study determined five radiocarbon dates of charcoal fragments to check the stratigraphy of tephra layers exposed at sea cliffs surrounding the volcano. Km12 a pyroclastic flow deposits (AD 874) are distributed at the western foot of the volcano (Hanasezaki to Tanosaki). Three dates (1310±40 BP, 1235±40 BP, and 1045±40 BP) obtained from the deposits are nearly consistent with ancient documents pertaining to Km12. The remaining part of the sea cliff (Tanosaki to Kawajiri) consists of tephra layers and lavas of the Km10 (3rd Century) and Km11 (7th Century) eruptions. Two dates (1690±35 BP and 1705±45 BP) obtained from an ash fall deposit distributed from Kurose to Kaimonzaki suggest that this deposit was a product of the Km10 eruption.
We investigated geology of the Matsunodai debris avalanche deposit (MDA) and its adjacent area in Kuju Volcanic Group, Kyushu, Japan. Based on the eruptive history, the source of the MDA should be inferred to be the Mimata-Gairinzan lava. Volume of the collapsed part of the lava is calculated to be ca. 0.3 km3 . On the other hand, volume of the MDA is estimated as ca. 0.2 km3 , which is almost consistent with collapsed volume. The MDA can be divided into three terraces on the surface. Debris avalanche rushed down 6 km to the north, and spilled over the gentle slope at least 40 m above the lower land. We could not find any evidence for high-temperature deposition from previous reports on rock-magnetism of the MDA.
There are two significant outcrops that can be observed at Tarukidaichi (Taruki Height) located around 2.5 km NW of Heiseishinzan lava dome generated by the 1990-1995 eruption of Unzen Volcano. This paper discussed the dates and time of eruptions based from the observations on the pyroclastic deposits exposed at these outcrops. Pyroclastic deposits observed at Tarukidaichi can be divided into 5 layers based on their occurrences. Layer 1 corresponds to mainly ash fall deposits resulting from pyroclastic flows that occurred from May 20, 1991 to early September 1991. Layer 2 is composed of pyroclastic surge deposits that are accompanied by large-scale pyroclastic flows that happened on September 15, 1991. Layer 3 is composed of deposits caused from the series of pyroclastic surges and ash falls that occurred from January to March 1992. Layer 4 is ash falls accompanied with pyroclastic flows, pyroclastic surges and their reworked deposits from March 1992 to middle June 1993. Layer 5 is composed of deposits from pyroclastic flows that occurred on June 23, 1993. These outcrops are very useful for students who want to learn volcanic geology. These sites should be conserved and protected through effective collaboration between the scientific community and local residents.
Two photo databases of middle to late Pleistocene tephras from Hakone and its adjacent volcanoes are open to the public on the website of the Kanagawa Prefectural Museum of Natural History (KPMNH). They are representative tephra-photo databases on websites in Japan. One is the main tephra archive of approximately 1,000 outcrop photos, and it has been opened since 2008. The other is an archive of hidden and destroyed outcrops containing less than 400 past geological photos, and it has been opened since 2010. The museum is located at the foot of the Hakone volcano, where is important area for Quaternary tephra-stratigraphic studies in Japan. However, a number of tephra outcrops cannot be observed because of artificial cover with concrete related to land development from urbanization. It is therefore urgently necessary to construct a database of tephra outcrops. The photo database is useful not only for the comparative study of tephras, including tephra identification, but also for reconstruction of the eruption history of the Hakone volcano. For constructing such databases, considerable work over a long time is required since a volcano has produced a large amount of tephra layers in every direction. Many photos of newly found outcrops and outcrops that have disappeared need to be collected. For collecting photos of new outcrops, local museums including KPMNH play an important role for collecting new data published in academic journals to enrich the tephra databases. On the other hand, collecting photos of outcrops that have disappeared is generally difficult. Cooperation with a large number of geologists is one possible solution. The preservation and practical use of geologists' field data can also enrich tephra databases. Academic societies will play an important role in the collection of geologists’ data.
Kanagawa Prefectural Museum of Natural History has reposited surface peel and moulage specimens collected from real outcrops, mostly of Quaternary volcanic ejecta of Hakone Volcano and related sedimentary layers, as well as other specimens of natural history collections. They are useful for observing tephra and understanding geological processes in laboratories, and are therefore suitable for exhibition and education in museums. Surface peel specimens have the same meaning as natural outcrops. Their clearer colors and textures than the real outcrops could help researchers to recognize unknown tephras or sedimentary structures. Nowadays it is more important to keep those peel specimens as geological evidences, because of disappearance of real outcrops related to road construction, land development or concrete spraying on outcrops. Substantial collection of such peel specimens provides information on geological textures and structures, for example about volcanic fall and flow units, turbidite sequence, load cast, slump, liquification, small-scale deformation structures such as fault and joint, etc., and can be repeatedly examined in related sciences. Collection building and database of such specimens are also meaningful for natural history researches.
Outcrops provide fundamental information for reconstructing eruptive history. Such information is important not only for geological studies, but also for outreach activity for reducing volcanic hazards. Nevertheless, it is difficult to construct an outcrop database. As a result, much outcrop information is unavailable to researchers. This paper describes the basic concept of general database systems and reviews the structure of conventional volcano databases in terms of information engineering. Based on these reviews, we clarify some issues with the conventional approach and present our new framework to solve them. To construct an outcrop database for volcanic geology, we propose a new framework that can manage various kinds of data with manually added tags, keywords or key-phrases, and automatically added tags, such as global positioning system information. This framework enables researchers and non-expert users, such as the public, to register new data easily because the database does not require a fixed input format or limit data notation on the entry into the database. It means that database users can contribute to constructing the database. Furthermore, our framework can deal with various types of data as stored data; thus, the database can handle many kinds of research material, ranging from raw data (primary data) to arranged data and research papers (more than secondary data). The framework also provides an application programming interface for easy construction of Web database applications. We also introduce several database applications based on our framework and supporting tools for researchers and non-experts. One of them is a web-based database application to manage research materials, such as photos and documents, for geologists. This application enables users to manage the material files using tags. Another is a Web-based database application called “Geo-Log”, which is used to collect and share outcrop information. Our framework can build a new outcrop database for volcanic geology as a knowledge infrastructure for collecting various research data from a wide range of users and apply the data to multi-purpose use, as a research material database for experts, education for reducing disasters, and as an information guide for Geopark visitors.
Morphometrical studies on small volcanic edifices such as scoria cones have been extensively conducted. Nevertheless, some morphometric parameters have been difficult to obtain. For instance, it is often difficult to measure a slope angle of volcano edifice because DEM (Digital Elevation Model) with high spatial resolution are not always available. We used the K-GPS (Kinematic Global Positioning System) for high resolution measurement of the slope angle of Omuroyama, a typical young scoria cone in Eastern Izu, Japan, assuming no detailed DEM available in the region. We show the existence of constant slope of 32° from the base to the top. The result of the K-GPS measurement is consistent with the existing DEM of this region. We report the measuring system is simple and viable for summary survey of slope angle where no DEMs are available.
Akita-Komagatake volcano straddles the boundary between Akita- and Iwate-prefectures in Japan. Medake, one of its central cones, erupted in 1970-71. The condition of this volcano was monitored by geophysical observations from a few years after the end of the eruption to 2013. Geo-temperature, total magnetic intensity and gravity were repeatedly measured at fixed stations distributed mainly in and around Medake. As the temperature of the 1970-crater decreased after the end of the eruption, the geothermal activity was intensified in the surrounding areas. This geothermal activity was at its peak around the 1977-79 period and decreased gradually to about 1995, and then the geothermal zones almost diminished to a very small area. The activity, however, re-started to intensify in about 2006, and has been extending to the eastern side of Medake to date (2014). The maximum temperature at 1 m depth in the thermal zones is about 95℃, which corresponds to the boiling point of water at the elevation of the site (about 1500 m). Changes of the total magnetic intensity and the gravity were conformable with that of the thermal activity. The gravity increased and decreased with diminishing and intensifying of the geothermal activity, respectively. Model analyses suggest that these variations are due to the change of underground temperature nearly below the boiling point. The change of the magnetic field is considered to be caused by the variation of remnant magnetization, while thermal expansion affects gravity. The related behavior of pore water may also enhance the variation of gravity in some degree.
Volcanic deposits from Tsumaya pyroclastic flow and Osumi pumice fall, which composed the first half of Aira pyroclastic eruption occurred about 29,000 ago, were studied to clarify magma plumbing system of the eruption. Both volcanic deposits contain phenocrysts (15-20 vol.%) of quartz, plagioclase, orthopyroxene, magnetite and ilmenite. In addition, a very small amount of amphibole is observed in Osumi pumice fall deposits. The chemical composition of the phenocrysts and their melt inclusions were analyzed by EPMA. The core compositions of the phenocryst plagioclase and orthopyroxene cluster around An42 and Mg#=48, respectively. Although some cores of plagioclase and orthopyroxene phenocrysts show remnants of magma mixing, the rim compositions having essentially the same composition as the clustered core composition suggest that the magma erupted from an approximately homogeneous magma reservoir. Application of geothermo- and geobarometers (Fe-Ti oxides, orthopyroxene-liquid, amphibole) reveals that the condition of magma storage was 790-850℃, 110 MPa, and FMQ to 0.6 orders of magnitude above it. Estimated water contents of unleaked melt inclusions hosted in quartz, plagioclase and orthopyroxene determined by reflection FT-IR technique were 3.1-5.4 wt.% with an average of 4.5 wt.%. Coexistence of bubbles in some melt inclusions suggests that the magma was water-saturated prior to eruption. Phenocrysts content calculated by MELTS program agreed with the observed one when the given pressure was 80-110 MPa. In summary, all estimates indicate that the pressure at the top of the magma chamber is probably around 100 MPa. This pressure is equivalent to a depth of 4-5 km. The suggested depth is quite shallower than the previously considered depth of magma chamber, i.e. 8-10 km, which was responsible for the Aira pyroclastic eruption.
Shinmoedake volcano, one of the eruption centers of the Kirishima volcano group in SW Japan, exhibited three subplinian eruptions in January 2011. This series of eruptions were well observed, and almost all the relationships between the eruption times and pyroclastic fall deposits have already been reported. However, stratigraphic boundary of the first and the second subplinian eruptions remained unclear. In order to distinguish the deposits into two sections corresponding to these two subplinian eruptions, we carried out grain size analysis with higher time resolution. We collected samples at 3 localities, Takachiho-gawara (Tg: 2.7 km SE), Miike elementary school (Mk: 7.9 km SE), and Natsuo elementary school (Nt: 11.3 km SE). Our measurements of stratigraphic Mdφ variations show two clear peaks at Tg (Mdφ: -3.4〜-1.2) and Mk (Mdφ: -1.0〜-1.5), but only one gentle peak at Nt (Mdφ: -0.7〜+1.5). At all localities, Mdφ peaks appear at almost the same stratigraphic position, and the farther the distance from the vent is, the finer the peak Mdφ values is. Assuming that a single plinian eruption makes a single Mdφ peak through the deposit, we inferred that our analyzed deposits correspond to the first two subplinian eruptions (the thickness ratio of the first and the second subplinian deposits is 7:3〜6:4). We found that dominant grain sizes vary with time from fine at the initial stage, coarse at the climax, and fine during the final stages of a single subplinian eruption deposit. Pyroclastic fall deposits were subjected to the compaction effect for one year after deposition.