Morphology Prior to 1973, Nishinoshima was a small, green island that had no eruptions in the past 10,000 years. The island was merely the tip of an undersea volcano some in height and wide at the base. Its main period of activity was from the
Late Pleistocene to the
Holocene. The volcano takes the form of a
caldera, with the original Nishinoshima and some nearby rocks forming part of the northwest ridge of a caldera about in diameter. The island was originally about long and wide. A 1911 survey determined the caldera was at its deepest. The island's surface is mostly andesitic (consisting of 58–62%
SiO2). The older volcanic body has been displaced by a fault running in the north–northwest–south–east direction. The crust of Nishinoshima is estimated to be 15–21km thick.
1973–1974 eruption On 30May 1973, the crew of a fishing boat Hirukomaru No.2 reported to the Japan Coast Guard that, at around 11:00, white smoke rose east of the island to around in the air every few minutes. The Japan Coast Guard Hydrographic Department went to observe the area using a
YS-11 aircraft that same day and confirmed the presence of a cloudy vent. An aerial inspection the next day said that the eruption was taking place east of the island. The submarine volcanic activity had been thought to have begun in early April. Yellow-green sea water and floating
pumice were seen north of the site. Another crew discovered that two black rocks were rising from the sea. The new island's vent moved in a semicircular arc above the sea surface. In July, the eruption continued to erupt water fountains every few minutes with white smoke rising to 100 m above sea level. An aerial inspection on 14September noted that a new island had formed above the
submarine eruption site, the island formed a
cinder cone around high and across. The new island had a diameter crater that frequently ejected
cinders to a maximum height of . White smoke rose to a height around high. In November, a fissure eruption was noted on the new island with a chain of cinder cones running from southwest to northeast. The most recent cones were formed at the end of the chain, with the original cone being destroyed by the waves. The cones were erupting ash to a height of around . On 21December, the new island grew larger than "old" Nishinoshima. the new island had now developed a central crater that rose to a maximum height of . The cone was continuing to eject
ash and cinders to a height of . By December 2013, the island rose above sea level, with an area of . The island was considered large enough to maintain a presence above sea level for at least several years. By 20 December 2013, the island had grown fast enough that experts predicted that it would probably join up with Nishinoshima before the end of 2013 and cease to be a separate island. On 26December 2013, the
Japanese Coast Guard confirmed that the two islands had joined.
NASA says two cones have formed around the main volcanic vents and stand more than above sea level. The newer portion of the island is now larger than the original Nishinoshima landmass. The merged island is slightly more than across. During July and August 2014, lava flow increased, causing the island to expand rapidly to the east. Between September and December, the lava flow increased further and headed north, almost completely overrunning the pre-existing island, leaving only a small portion of the old island exposed. The volume of the eruption later exceeded 50million m3. On 27December 2014, Japanese authorities said the island had reached nearly in size and is estimated to have risen to about above sea level and that the volcano was still active. The eruption continued throughout the first half of 2015, and the island continued to expand. However, by August, the volcano ceased to erupt smoke and ash, but continued to emit lava. As of 16September 2015, the total area of the island had decreased slightly, but the
fumarolic zone had expanded as "vigorous volcanic activity continued without significant change". Eruptions resumed soon thereafter, but 17November 2015 was the date of the last observed explosive eruption. Fumarolic activity continued, but decreased in amount as no new lava flows were observed. In August 2016, the
Japan Meteorological Agency announced that the alert level for the volcano had been lowered, and that the no-entry zone had been reduced from a radius of to a radius of . This would finally enable landings on the island. Water discoloration was still evident immediately offshore in August, indicating that volcanic activity, while in significant decline, was still present. On 14February 2017, the Japan Meteorological Agency cancelled all alerts for the island, declaring that there was no indication of subsequent eruptions. In October 2016, a team of scientists visited Nishinoshima to conduct research on the island.
2017 and 2018 eruptions On 20April 2017, the Japan Coast Guard confirmed explosive eruptions at the No.7 crater at Nishinoshima and lava flows emerging from the base of the volcano. Satellite imagery from 19April also confirmed high temperatures on the island. Examination of the data revealed that the eruption likely began on 17 April. By 27April, two lava flows had reached the sea, one on the west shore, the other on the south-west shore of the island. By 29June, the flows had created two lobes, one extending 330 meters beyond the western shore, and other 310 to the south-west. The area of the island increased to . By July, surface temperatures began to decrease until they became indistinguishable from the surroundings in August; lava flows also stopped by the end of August. In June 2018, the JMA announced that the no-entry zone had been reduced from a radius of to a radius of . Within a month, however, the zone was returned to after small eruptions were spotted on 12July 2018. On 3October 2018, the Coordinating Committee for Prediction of Volcanic Eruptions of the Japan Meteorological Agency announced that Nishinoshima had likely ceased erupting.
2019–2020 eruption On 6December 2019, the Japan Coast Guard confirmed explosive activity in Nishinoshima has returned, with new lava flows entering the sea by the following day. Due to this eruption, some ocean bottom electromagnetic meters (OBEMs) on the seafloor went missing. One OBEM was later found on Takana beach on
Iriomote Island in February 2021. In January 2020, the Japan Coast Guard observations confirmed that lava was flowing out on the northeast coast. Further activity was observed on 4 February, and the northern extension of the island was expected. On 25June 2020, the explosive-effusive eruption was still continuing, with lava flows on the northeastern slope of the volcano, as well as ash plumes reaching . On 4July, an ash plume reached in height. On 14August, it reached a size of with a diameter over . The 2019–2020 period of eruption changed from mainly andesitic lava outflow to basaltic–andesitic
strombolian eruption with the characteristics of fragmentations of pyroclastic materials.
2021–2023 eruption On 14August 2021, around 6a.m., artificial satellites confirmed an eruption for the first time since late August 2020. The height of the eruption was about . On 15 August, the Japan Coast Guard made observations by aircraft, but no eruption was confirmed. On 1October 2022, satellite images confirmed that Nishinoshima had erupted again and continued to erupt until 12October, when a Japan Coast Guard aircraft confirmed it was no longer erupting. The ash of the eruption reached a height of between on 5–11October. Foggy weather in
Fukuoka Prefecture, Japan, and
Busan, South Korea, occurred from late July to early August 2020 due to the volcanic eruption of the island. The
PM2.5 concentration increased 6.03 times in Fukuoka and 4.32 times in Busan. The Japan Coast Guard reported that during an overflight of Nishinoshima on 25January 2023, an intermittent blackish-gray plumes rising from the central part of the crater was observed with brown discolored water around the island. A small eruption at the central crater was also observed during an overflight done by the Japan Coast Guard on 4October 2023. The ash plumes rose to above sea level. Satellite images from April to September 2023 showed the discoloration of the seawater and the white smoke rising. Overall, in the decade since the latest series of eruptions began, the island increased in size by around 20 times. image of the island. The discolouration of the water around the island is due to volcanic minerals and seafloor sediment. == Ecology ==