Abstract
  Geology of Izu-Oshima Volcano and its surrounding submarine area

 Izu-Oshima Volcano is a frontal arc basaltic stratovolcano that began its activity around 40,000 to 30,000 years ago. These earliest eruptions produced deposits that cover three or four older volcanoes and are contained within the Senzu Group, composed of ejecta from phreatomagmatic eruptions, and the overlying Older Oshima stage, composed of pyroclastic fall deposits and lava flows. Since 17,000 years ago, about 100 major eruptions have been recorded by tephra fall deposits. At the summit of Izu-Oshima Volcano there is a caldera approximately 4 km in diameter. Explosive activity has occurred frequently from this summit area and as lateral eruptions. There are more than 80 flank volcanoes most of which are aligned in a northwest-southeast direction. Most of these features are scoria cones, but tuff rings and maars, such as the Habuminato crater, have formed near the coast due to phreatomagmatic eruptions. The current caldera topography was created by a large phreatic eruption at the summit about 1,700 years ago. Volcanic deposits from the caldera formation and post-caldera activity belong to the Younger Oshima stage, which contains recognizable products from 12 major eruption periods. The caldera-forming eruption of the Younger Oshima stage, known as S2, began with an eruption at the summit, followed by debris flows, and lateral eruptions in multiple locations, then a large-scale phreatic eruption at the summit and associated pyroclastic flows that covered the entire island. Many eruptions after the formation of the caldera began with a high-volume explosive magma eruption and lava effusion, then end with ash emission, but there are some exceptions.

 The most recent magmatic eruption, in 1986, began with lava fountaining at the central cone, Mt. Mihara crater. Initially the eruption was confined to the Mt. Mihara crater, but on the sixth day after the eruption began, seismic activity began in the northwestern part of the island, and a lateral eruption suddenly began. Lava flows from the craters outside the caldera flowed down toward residential areas, and as the possibility of an eruption in the southeastern part of the island increased, all residents evacuated the island. One year after the 1986 eruption, the lava lake inside the Mt. Mihara crater drained with accompanying explosive activity, and the pit crater reappeared. After the eruption, the volcano continued to expand slowly but has stagnated since 2018. At the time of writing, no ground deformation anomalies were observed.

  Izu-Oshima Volcano is flanked by numerous, subparallel NW-SE-trending chains of submarine knolls and ridges extending up to 22 km to the NW and the SE from the summit of Mt. Mihara. To the NW of Izu-Oshima island, there is a series of small knolls that generally have heights of 50–100 m and basal diameters of around 300–1,000 m. To the SE of the island, each knoll has a relative height of 70–250 m and a basal diameter between 0.5–1.6 km, and a chain of the knolls forms the Habu Spur.

 Submersible surveys revealed that these knolls and ridges are fissures that erupted basaltic spatter and lava flows. The geochemical composition of erupted magma is consistent along the length of each ridge, with minor variations due to crystal fractionation and some plagioclase accumulation. Compositions of the ridges are also very similar to lavas from the subaerial flank volcanoes that can be traced down the flanks of Izu-Oshima Volcano. This implies that pairs of subaerial cones and submarine ridges represent the locus of magma transport events away from the storage system beneath Izu-Oshima.

 The subparallel ridges are observed to transect a series of seamounts known as the Higashi-Izu-oki submarine volcanoes, which are dispersed across the area between the volcanic front and the Izu Peninsula. Nishi-Chigasaki Knoll, one of the Izu-Tobu volcanoes with a relative height of around 450 m and a basal diameter of 3 km, occurs in the area where the NW-SE trending submarine ridges occur. Three to four NW-SE trending ridges can be recognized transecting the NW and SE slopes of this knoll, implying that the ridges are younger.

 Seismic reflection profiles to the NW of Izu-Oshima Island indicates that the submarine volcanic ridges were formed where a series of normal faults dissect the basement and the entire sequence of overlying sediments. Since these faults displace the uppermost sediment layers, the normal faults are likely to be still active around Izu-Oshima Volcano, i.e, the area is under extension.

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