The Japan Meteorological Agency (JMA) reports that for the first time in 1 000 years, volcanic eruptions spewing magma are believed to have taken place just off the southern coast of Ioto volcano, also known as Iwoto, Iwojima and Iojima.
Small eruptions started on July 11, 2022, some 900 m (2 950 feet) away from the coast. From July 12 to 15, they occurred every 5 minutes, and continued through the rest of the month, creating water columns up to 30 m (98 feet) high.1
Analysis of the rocks that washed ashore on the island showed small cavities inside the rocks, which, officials believe, are cooled lava. In addition, the temperature inside some of the rocks was as high as 120 °C (3.93 °F), JMA said.2
Masashi Nagai, a researcher at the National Research Institute for Earth Science and Disaster Resilience (NIED) who specializes in volcanic geology, thinks the most recent magma eruptions were the first since those that occurred about 1 300 years ago.
Earlier this year, the land surface of Ioto island was found to be rising at a rate of 1 m (3.3 feet) per year, which has no parallel elsewhere in the world, JMA said in April 2022.3
Nagai’s geological studies showed that Ioto was a large stratovolcano between several hundred thousand years and 100 000 years ago or later. Even then, it might have been a volcanic island with its peak rising above the sea surface.
A large-scale eruption of the volcano occurred about 100 000 years ago, leaving a 10 km (6 miles) wide caldera in its center.
Another large-scale eruption occurred around 2 700 years ago and produced vast volumes of submerged lava and pyroclastic flows. Several further eruptions happened beyond the caldera until around 800 years ago, when Mount Suribachiyama was formed.
It is likely that this is the time when the island began to grow and it hasn’t stopped since.
Ioto (changed from Iwo-jima in 2007) in the central Volcano Islands portion of the Izu-Marianas arc lies within a 9-km-wide (5.6 miles) submarine caldera. Ioto, Iwo-jima, and Iojima are among many transliterations of the name.
The volcano is also known as Ogasawara-Iojima to distinguish it from several other “Sulfur Island” volcanoes in Japan.
The triangular, low-elevation, 8-km-long (5 miles) island narrows toward its SW tip and has produced trachyandesitic and trachytic rocks that are more alkalic than those of other Izu-Marianas arc volcanoes.
The island has undergone dramatic uplift for at least the past 700 years accompanying resurgent doming of the caldera. A shoreline landed upon by Captain Cook’s surveying crew in 1779 is now 40 m (131 feet) above sea level.
The Motoyama plateau on the NE half of the island consists of submarine tuffs overlain by coral deposits and forms the island’s high point. Many fumaroles are oriented along a NE-SW zone cutting through Motoyama.
Numerous historical phreatic eruptions, many from vents on the west and NW sides of the island, have accompanied the remarkable uplift.4
1 Ioto (Iwojima) volcanic activity commentary material for July 2022 – JMA
2 Eruption of magma off Iwoto island seen as 1st in 1,000 years – The Asahi Shimbun – August 9, 2022
3 Volcanic Iwoto island rising at unmatched rate of 1 meter a year – The Asahi Shimbun – April 9, 2022
4 Ioto – Geological summary – GVP
Featured image credit: Copernicus EU/Sentinel-2, The Watchers. Acquired on July 15, 2022
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