The volcanic island of Iwoto is rising at an unprecedented rate of 1 meter per year

Video footage taken from the Asahi Shimbun’s “Asuka” plane in January shows Iwoto Island where the land surface is rising at an unprecedented rate of 1 meter per year. (Kazuhiro Ichikawa)

The volcanic island formerly known as Iwojima that was the site of one of the most famous battles of the Pacific War is making history again, but in strictly geological terms.

According to the Japan Meteorological Agency (JMA), its land surface is rising at a rate of 1 meter per year, which has no equivalent anywhere else in the world.

Now called Iwoto, the piece of rock that lies about 1,200 kilometers south of Tokyo is designated as one of Japan’s 111 active volcanoes.

The ground temperature of the island is high and small-scale eruptions occur quite regularly.


National Institute for Earth Science and Disaster Resilience (NIED) Researcher Masashi Nagai explained how the huge crater formed in the northern part of Iwoto during an aerial inspection from the island aboard the Asahi Shimbun’s “Asuka” plane in January. .

The crater, a striking feature, is about 120 meters in diameter. Nagai said it formed in an eruption on November 24 last year.

“It’s the biggest crater we’ve ever confirmed (on the island),” Nagai said.

Iwoto, an elongated wedge-shaped islet, is 8.5 km long and 4.5 km wide. Near its southern end is Mount Suribachiyama, a roughly 170-meter peak where Japanese forces sank during World War II fighting. This is where US forces hoisted a Stars and Stripes flag, which was captured in an iconic photograph, during the Battle of Iwojima in 1945.

The central and northern parts of the island are covered by a plateau centered on Mount Motoyama, which rises to about 115 meters above sea level. The plateau is currently home to an airbase operated by the Self-Defense Forces.

Iwoto, including its submerged part, is just a big volcano just like Mount Fuji.

Nagai’s geological studies have shown that Iwoto was a large stratovolcano between several hundred thousand years ago and 100,000 years or later. Even then, it could have been a volcanic island with its summit rising above the surface of the sea.

Iwoto suffered a large-scale eruption around 100,000 years ago or later, which created a caldera about 10 km in diameter in the central part of the volcano.

Another large-scale eruption about 2,700 years ago generated massive amounts of lava and pyroclastic flows underwater. Several other eruption events occurred outside the caldera until about 800 years ago, when Mount Suribachiyama took shape.

The uprising of the island probably began around this time and continues to this day. The uplift comes from a phenomenon called a “resurgent dome” in which the ground is pushed upward by magma that accumulates below the surface after a caldera forms.

Iwoto for the most part is now a flat volcanic island because its peak which overlooks the ocean was eroded by waves as the islet rose.


Yet it remains very active today and small scale steam explosions occur regularly. The ground temperature of the island is high and there are many fumaroles or openings through which escape hot sulphurous gases and steam, and fumarolic fields.

Iwoto is characterized by high lifting speeds. Surveys from the Geospatial Information Authority of Japan (GSI) show that the ground is rising at a rate of about 1 meter per year.

The rate of increase, however, has been variable. Measurements taken by NIED show that the northern part of Iwoto Island continued to sink between 2003 and 2006 before starting to rise again. There was a strong uplift of more than 2 meters between 2011 and 2012.

Iwoto is one of 50 volcanoes in Japan that the JMA monitors around the clock.

“The floor of the island rises from an underground accumulation of volcanic fluid, although it is unclear whether the fluid is due to magma or a mixture of hydrothermal solution and gas,” said a JMA official.


Iwoto has no permanent population other than the approximately 300 SDF soldiers stationed on the island. Some US troops also conduct carrier landing drills there, according to the Defense Department.

The heaving of the ground generates cracks and steps on the track and the roads of the island, which means that there is a constant need for repairs. Minor damage can be repaired on site, but swollen paving and other work requires the intervention of construction workers.

Due to the high geothermal energy, the floors of the buildings are erected about 1 meter higher than usual.

As for the risk of an eruption, a representative of the Ministry of Defense said that adequate preparations had been made for such an event.

“We have a set of guidelines to deal with the matter, including at what stage we are supposed to go, but we refrain from elaborating because the subject has to do with the functioning of the troops, although in terms of volcanic activity,” the official said. added.