Nishinoshima (Ogasawara)

Nishi-no-shima (西之島, "western island") is a volcanic island 940 km south of Tokyo,^[2] that is part of the Volcano Islands arc. The island was enlarged in 1974 after fresh eruptions created a new section of the island. Another eruption that began in November 2013 further enlarged the island. Its maximum height was estimated at 110 meters on 27 December 2014.^[3] It has also been known as Rosario Island.^[4]

Morphology

Nishinoshima before 1973

Prior to 1974, Nishinoshima formed a small, green island which had no eruptions in the past 10,000 years. The island was merely the tip of an undersea volcano some 3000 m in height and 30 km wide at the base.^[5] Its main period of activity was from the Late Pleistocene to the Holocene.^[6] 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 1 km in diameter.^[7] The island was originally about 650 m long and 200 m wide.^[8] A 1911 survey determined the caldera was 107 meters at its deepest.^[5] The volcano has many large, submarine, satellite cones to the south, west and northeast. The southern cone rises to within 196 m of the surface, around 9 km SSE of Nishinoshima.^[5]

Eruptions

Prior to 1973, no eruptions had ever been recorded at Nishinoshima.

1973–1974 Eruption

Nishinoshima in 1978

On 30 May 1973, the crew of a passing ship noted that, at around 11:00, white smoke rose east of the island to around 100 m in the air every few minutes. An aerial inspection the next day said that the eruption was taking place 400 m east of the island. A whirlpool was also seen. Yellow-green sea water and floating pumice were seen 5 km north of the site. Another crew discovered that two black rocks were rising from the sea.

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 14 September noted that a new island had formed above the submarine eruption site, the island formed a cinder cone around 40 m high and 120 m across. The new island had a 70 m diameter crater that frequently ejected cinders to a maximum height of 300 m. White smoke rose to a height around 1,500 m 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 300 m.

In December, the new island grew larger than "old" Nishinoshima. The island was 700 m long and 250 m wide; the new island had now developed a central crater that rose to a maximum height of 40 m. The cone was continuing to eject ash and cinders to a height of 100 m. A small craterlet on the new island was also emitting lava flows.

By February 1974, the new island had developed two cinder cones at the western end of the new island, but only the eastern cinder cone was erupting. Observations on 11 January noted that the active eastern cinder cone was emitting a lava flow.

In March, the new island was still erupting; by then, the island had five cinder cones; one cinder cone was erupting red hot lava. Lava flows were still erupting from the new island's flanks. On March 1, the five cinder cones were quiet, but a large lava flow may have still been effusing from the island's flanks. After that, the eruption ended; wave movements then joined the new and old islands.

2013–2015 eruption

New island on 8 December 2013

In November 2013, an eruption created a new, small island south-southeast off the shore of the volcano.^{[9][10][11][12]} By December 2013, the island rose 20–25 m (66–82 ft) above sea level, with an area of 56,000 square meters (13.8 acres). The island was considered large enough to maintain a presence above sea level for at least several years.^[13][14] 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.^[15] On 26 December 2013, the Japanese Coast Guard confirmed that the two islands had joined.^[16]

The island was not given an official name but was mentioned in Japanese reports as "new island": atarashii shima (新しい島) or shintō (新島).^[17] Government officials said the island would be named "after it is stable and it is clear it will remain".^[18] As the island has merged with Nishinoshima, "there is little possibility it will be named as a separate entity."^[19]

The United States space agency, NASA, says two cones have formed around the main volcanic vents and stand more than 60 m above sea level.^[20] The newer portion of the island is now larger than the original Nishinoshima landmass. The merged island is slightly more than 1 km (3,280 ft) 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.^[21] On 27 December 2014, Japanese authorities said the island had reached nearly 2.3 square kilometres in size and is estimated to rise to about 110 meters above the sea level and that the volcano was still active.^[3]

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.^[22] As of September 16, 2015, the total area of the island had decreased slightly, but the fumarolic zone had expanded as "vigorous volcanic activity continued without significant change".^[23]

References

External links

• Nishinoshima, Ogasawara by Geospatial Information Authority of Japan (in Japanese)
• Nishinoshima eruption observed by LANDSAT 8, Geospatial Information Authority of Japan
• Global Volcanism Program
• Nishinoshima, Kaiiki Kazan Database, Japan Coast Guard (in Japanese)
• New Japanese Island Forming in Pacific Ocean: Photos, 22 November 2013
• New island at NASA's Earth Observatory website