There are many coarse to very coarse boulders, which include a tsunami boulder caused by the AD 1854 Ansei-Tokai earthquake, on a wave-cut bench at Shimoda, Shizuoka, Central Japan. This area experienced storm surge during 2017 typhoon 21 (Lan). Significant wave height of 14.65 m was observed during the typhoon at Irozaki located 10 km southwest of the study area. Comparison of locations of these boulders before and after the typhoon shows that the storm surge could not move coarse to very coarse boulders. This observation supports the interpretation that the tsunami boulder was rolled by a tsunami associated with the AD 1854 Ansei-Tokai earthquake.
We found emerged cluster of calcareous tubes of Pomatoleios kraussii on a very coarse boulder, 3.0 × 2.7 × 1.0 m in size and estimated to >7 ton, on beach at Iruma, Minami-izu, Shizuoka, Central Japan. The cluster is distributed at 0.8–1.4 m above mean sea-level. Radiocarbon dating of the fossils indicates that the boulder was emplaced after AD 1950. Based on the occurrence of the fossils and disaster record of the 1974 Izu Peninsula earthquake, the boulder may be rolled by a storm surge associated with typhoon Tip at 1979.
Recent studies of minor elements (CaO, Al2O3, MnO, NiO, P2O5 and H2O) in igneous olivines are reviewed. CaO content is used to distinguish between igneous and peridotitic olivines because CaO contents of igneous and peridotitic olivines are typically higher and lower than ~0.1 wt.%, respectively. However, recent studies suggest that olivine-melt Ca partition coefficient decreases as melt H2O content increases, and as a result, arc magmas sometimes crystallize olivines with CaO content comparable to those of peridotitic olivines. Therefore, it is not straightforward to distinguish between igneous and peridotitic olivines in arc magmas based on only CaO content. Aluminum content of olivine in equilibrium with spinel increases as temperature increases; the relation is formulated as Al-in-olivine geothermometer, which enables us to estimate crystallization temperatures of igneous olivines. Nickel contents and Mn/Fe ratios of olivines are used to identify the source lithology of their parental magmas; Ni content and Mn/Fe ratio are thought to increase and decrease with increasing a proportion of pyroxenite-derived component in the parental melt, respectively. However, recent experimental and petrological studies contest the validity of this notion. Phosphorus concentration mapping is used to clarify crystal growth kinetics and diffusion processes of igneous olivines. The results suggest that dendritic growth is a common growth mechanism of igneous olivines. Hydrogen contents in igneous olivines are 10–100 times lower than those of peridotite-derived olivine xenocrysts. Hydrogen diffusion is significantly rapid in olivine, and therefore H2O contents of olivine-hosted melt inclusions reequilibrate with the olivine-hosting magmas in short time. Minor elements in igneous olivines offer constraints on pre-eruptive magmatic conditions and crystallization process, but extracting information about physicochemical conditions and source lithology of their parental melts is not straightforward.
We have examined the surface geology and tectonic landforms of the Hoshiyama Hills and its surrounding areas in the Fujikawa-kako Fault Zone (FKFZ), central Japan. The FKFZ is generally regarded as a collision zone between the Honshu Arc, mainly composed of the pre-Neogene accretionary and the Neogene collisional complexes of South Fossa Magna region, and the Neogene Izu-Bonin Volcanic Arc on the Philippine Sea Plate. This zone is believed to be the most active and dangerous area of Japan in association with violent earthquakes. One of the keys to understand the activities of this zone since the Pleistocene is to reveal the geologic structures of the Hills, as well as the characteristics of the Omiya and Iriyamase Faults along the northeastern and southeastern margin of this Hills, respectively. Our surface geological survey reveals that the upper Lower-lower Middle Pleistocene Ihara Group, main constituent of the basement of the Hoshiyama Hills, has complicated structures including several-hundreds meters scale steeply-dipping beds without distinctive preferred orientations. The structures also include chevron-shaped anticlines and flat synclines, suggesting that the E-W horizontal shortening due to fault-related foldings was the main cause of their formations. The flexure-landform associated with the Omiya Fault clearly suggests that the Fault, previously believed to be a high-angled normal fault dipping toward NE, is a reverse fault dipping toward SW. However, the landform around the Iriyamase Fault, also believed to be a NW dipping reverse fault, show no evidence of its existence. Finally, we have summarized the tectonic and volcanic events in and around the FKFZ since about 1 Ma. These results suggest that the Quaternary tectonics and seismic activities of the FKFZ should be necessary to re-evaluate based not only on the surface geological and landform data but also on the subsurface geological structures now being poorly known.