Nishinoshima (Ogasawara)

Nishi-no-shima (Japanese: 西之島, "western island") is a volcanic island located around 940 km (584 mi) south-southeast of Tokyo,[2] that is part of the Volcano Islands arc.

Nishinoshima
Native name:
西之島
Nishinoshima in 1 December 2018
Nishinoshima
Nishinoshima (Japan)
Geography
LocationPacific Ocean
Coordinates27°14′49″N 140°52′28″E
ArchipelagoVolcano Islands
Total islands1
Area4.1[1] km2 (1.6 sq mi)
Highest elevation200 m (700 ft)
Administration
PrefectureTokyo
SubprefectureOgasawara Subprefecture
VillageOgasawara
Demographics
Population0

Originally, the above-water part of the ridge of an underwater caldera, Nishinoshima 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 and attracted worldwide attention.[3][4][5] A volcanic cone soon formed, rising to an estimated height of 142 m (466 ft) by July 2016.[2][6] The eruptions ceased by November 2015, though emissions of volcanic gases continued for several months afterwards.

As of 2016, the island was about 2.7 km2 (1.0 sq mi) in size and showed evidence of the return of various plants and animal species.[7]

There were further eruptions in 2017,[8] 2018, and 2019, with the latest eruption continuing as of July 2020.[9] As of 14 August 2020, it is about 4.1 km2 (1.6 sq mi) and over 2 km (1.2 mi) in diameter.[1]

Etymology

The island was named "Jena Island" when it was discovered by the Spanish ship Rosario in 1702.[10] That was the island's name until 1904 when the Japanese term "Nishinoshima" (literally, "West Island") was made the official name.[11]

When a new island was formed in the 1973 eruption, that was called Nishinoshima Shintō (西之島新島, literally, "New Nishinoshima Island"), but due to erosion and shifting sands, that island joined the main island and ceased to be considered a separate entity.[11]

During the early stages of the 2013 eruption, a new volcanic island was formed about southeast from the original Nishinoshima. 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).[12] Government officials said the island would be named "after it is stable and it is clear it will remain".[13] As the island has now merged with Nishinoshima, "there is little possibility it will be named as a separate entity."[14]

Geology

Morphology
Nishinoshima before 1973

Prior to 1974, Nishinoshima was 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 3,000 m (9,800 ft) in height and 30 km (19 mi) wide at the base.[15] Its main period of activity was from the Late Pleistocene to the Holocene.[16] 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 (0.6 mi) in diameter.[17] The island was originally about 650 m (710 yd) long and 200 m (220 yd) wide.[11] A 1911 survey determined the caldera was 107 m (351 ft) at its deepest.[15] The volcano has many large, submarine, satellite cones to the south, west, and northeast. The southern cone rises to within 196 m (643 ft) of the surface, around 9 km (6 mi) SSE of Nishinoshima.[15]

1973–1974 Eruption
Nishinoshima in 1978 aerial photo. The upper part of the bay is the old island, and the lower part is the new island formed by the 1973 eruption. Later, the bay was filled with sediment due to sedimentation, forming a trapezoidal coastline just before the eruption in 2013.

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 (330 ft) in the air every few minutes. An aerial inspection the next day said that the eruption was taking place 400 m (440 yd) east of the island. A whirlpool was also seen. Yellow-green sea water and floating pumice were seen 5 km (3 mi) 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 (130 ft) high and 120 m (130 yd) across. The new island had a 70 m (77 yd) diameter crater that frequently ejected cinders to a maximum height of 300 m (980 ft). White smoke rose to a height around 1,500 m (4,900 ft) 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 (980 ft).

In December, the new island grew larger than "old" Nishinoshima. The island was 700 m (770 yd) long and 250 m (270 yd) wide; the new island had now developed a central crater that rose to a maximum height of 40 m (130 ft). The cone was continuing to eject ash and cinders to a height of 100 m (330 ft). 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 1 March, 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
Nishinoshima as of 8 December 2013.
Nishinoshima as of 24 December. The land from the eruption grew rapidly in less than two weeks.
Nishinoshima as of 2014 July 4. It becomes even larger, and the relatively white old island is slightly visible.
Nishinoshima as of 2015 March 1 (All are satellite images of NASA).

In November 2013, an eruption created a new small island south-southeast off the shore of the original island.[4][5][18][19] 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.[20][21] 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.[22] On 26 December 2013, the Japanese Coast Guard confirmed that the two islands had joined.[23]

The United States space agency, NASA, says two cones have formed around the main volcanic vents and stand more than 60 m (200 ft) above sea level.[3] The newer portion of the island is now larger than the original Nishinoshima landmass. The merged island is slightly more than 1 km (0.6 mi) 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.[24] On 27 December 2014, Japanese authorities said the island had reached nearly 2.3 km2 (570 acres) in size and is estimated to have risen to about 110 m (360 ft) above the sea level and that the volcano was still active.[6] A pyroclastic cone formed around the vents which further rose the island to about 135 m (442 ft) by 23 February 2015.[2][25]

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.[26] As of 16 September 2015, the total area of the island had decreased slightly, but the fumarolic zone had expanded as "vigorous volcanic activity continued without significant change".[27] Eruptions resumed soon thereafter,[28] but 17 November 2015, was the date of the last observed explosive eruption.[29] Fumarolic activity continued, but decreased in amount as no new lava flows were observed.[29] Decreases in temperatures were also recorded, and subsidence was observed near the summit of the volcano.[29] 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 2 km (1.2 mi) to a radius of 500 m (1,600 ft).[29][30] 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.[31] On 14 February 2017, the Japan Meteorological Agency cancelled all alerts for the island, declaring that there was no indication of subsequent eruptions.[32]

In October 2016, a team of scientists visited Nishinoshima to conduct research on the island.[7][33] Apart from documenting the island's ecology and geology, monitoring equipment was also installed for future volcanic activity.[34]

Wyss Yim, a retired professor of Earth Sciences, has theorised that the Nishinoshima eruption caused the North Pacific Blob, a mass of warm surficial water off the Pacific Coast of North America.[35][36]

2017 and 2018 eruptions

On 20 April 2017, the Japan Coast Guard confirmed explosive eruptions at the No. 7 crater at Nishinoshima and lava flows emerging from the base of volcano. Satellite imagery from 19 April also confirmed high temperatures on the island.[37] Examination of the data revealed that the eruption likely began on 17 April.[38] By 27 April, two lava flows had reached the sea, one on the west shore, the other on the south-west shore of the island.[39] By 29 June, 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 2.9 km2 (1.1 sq mi).[40] 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.[38] On 3 October, the Coordinating Committee for Prediction of Volcanic Eruptions of the Japan Meteorological Agency announced that Nishinoshima had likely ceased erupting.[38] In June 2018, the JMA announced that the no-entry zone had been reduced from a radius of 2 km (1.2 mi) to a radius of 500 m (1,600 ft).[41] Within a month, however, the zone was returned to 1500m after small eruptions were spotted on 12 July 2018.[42] These eruptions quickly subsided, and the no-entry zone was reduced to 500m on 31 October 2018.[43]

2019-2020 eruption
Nishinoshima as of 26 January 2020. Infrared data (red) are superimposed on a natural-color image. The remote sensing data were acquired by the Operational Land Imager (OLI) and Thermal Infrared Sensor (TIRS) aboard Landsat 8.

On 6 December 2019, the Japan Coast Guard confirmed explosive activity in Nishinoshima has returned, with new lava flows entering the sea by the following day.[6] The conditions were enough that the Meteorological Agency issued a warning to passing ships to stay clear of Nishinoshima.[44]

In January 2020, the Japan Coast Guard observations confirmed that lava was flowing out on the northeast coast.[45] Further activity was observed on 4 February, and the northern extension of the island was expected.[46]

As of 25 June 2020, the explosive-effusive eruption was still continuing, with lava flows on the northeastern slope of the volcano, as well as ash plumes reaching 2600 meters (8500 feet).[47]

On 4 July, an ash plume reached 8300 meters (27200 ft) in height.[48] On 14 August 2020, it reached a size of 4.1 km2 (1.6 sq mi) with a diameter over 2 km (1.2 mi).[1]

Ecology

Since the 2013 eruption, biologists have expressed great interest in how Nishinoshima would be colonized by plants and wildlife. It is expected that organic matter, mainly deposited by nesting birds in the form of feathers or excrement, would enrich the island surface for plants to grow.[49] To prevent interference of the colonization process, researchers were also advised not to bring external species to Nishinoshima.

In October 2016, a team of scientists visited Nishinoshima to examine its environment and geologic makeup for the first time up-close.[7][33] Their studies have discovered that both plant and animal life continue to exist on the remaining part of the old Nishinoshima island that had not been covered by lava. An accompanying video crew from Nippon TV captured masked boobies laying eggs and a pod of dolphins (either Indo-Pacific, Tursiops aduncus, or common bottlenose, T. truncatus) swimming off the coast.[33] Gannets, bramblings and earwigs were also discovered living on the island by researchers.[50] Several cetaceans have been known to live around the island prior to the eruption such as spinner dolphins and short-finned pilot whales,[51] and above mentioned bottlenose dolphin. Humpback whales had been seen before the eruption,[52] and their recent returns have been confirmed as well.[53] In terms of flora, plants that were present on the original island like goosegrass and purslane were found to be growing again on the "old" section of Nishinoshima.[50]

Important Bird Area

The island and its surrounding waters has been recognised as an Important Bird Area (IBA) by BirdLife International because it supports a population of greater crested terns.[54]

See also

References
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