Izu-Oshima is a volcano island about 30 km east of the Izu Peninsula. The island is about 15 km long, elongated in the NW-SE direction. The elevation of the summit, Miharayama, which is the post-caldera central cone, is 764 m a.s.l. The Izu-Oshima volcano itself is younger than 40 thousand years ago. The younger stages of its history started around 1500 year BP after the formation of the summit caldera with 3-4 km diameter. More than 70 parasitic cones and the central cone arranged in the zone as narrow as 2 km wide, trending in NW-SE. The detailed and thoughtful volcanological knowledge about this island was acquired by the late Prof. Kazuaki Nakamura, who passed away after seeing the 1986 eruption with his own eyes. One of his works shows clearly that relatively large eruptions had occurred each 150 year at this volcano, although small eruptions, including the 1986 eruption, had also occurred each 35 year recently.

Eruption started at the central crater of the Miharayama at 17:25 (JST), November 15, 1986, making lava fountains and lava lake in the crater. Lava began to overflow and flow down the slope of the Miharayama on November 18, and reached and spread on the caldera floor. The activity in the central crater had declined once until 20 November. The activity suddenly resumed with large fissure eruptions on the caldera floor at 16:15, November 21. The lava fountain was as high as 1,600 m. Large amounts of scoria deposited within the island. The fissure reached the length of 1 km. The central crater was re-activated soon after the fissure eruptions. At 17:45, eruption also started outside caldera. A lava flow descended toward the town but stopped before reaching the residential area. The activity had declined after the final bursting at 09:43, November 23. Total volume of products of this eruption was estimated as 7,900 ton.


Photo 1
Lava fountain at the summit crater, and the surface of a lava lake which filled up the crater with the depth of more than 200 m. Photo taken by Osamu Oshima around 08:00 hr (JST), November 18, 1986.


Photo 2
Beginning of the fissure eruptions on caldera floor, viewed from the north side of caldera wall. Photo taken by Shin Shishikura, 16:15 hr, November 21, 1986.


Photo 3
Stereo-aerial photos before the 1986 eruption; the 1950-51 lava flows and the landscape of Miharayama central crater. North is left. Photo taken by Asahi Koyo Co. Ltd., June 5, 1985, "Oshima" C2-5, 6, original with 1/18,000 scale.


Photo 4
Stereo-aerial photos after the 1986 eruption; the 1986 lavas and craters (fissure vents). Craters outside the caldera is situated in the left end. The Miharayama central crater was filled with lava. North is left. Photo by Pasco Co. Ltd., December 24,1986, "Oshima" C2-2013, 2014, original with 1/14,000 scale.

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Link to Postcards of Izu-Oshima Volcano 1986 Eruption