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.

Its land surface has been found to be rising at a rate of 1 meter per year, which has no parallel elsewhere in the world, according to the Japan Meteorological Agency (JMA).

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

The island’s ground temperature is high and small-scale eruptions occur on a fairly regular basis.

GEOLOGICAL HISTORY

Masashi Nagai, a research fellow with the National Research Institute for Earth Science and Disaster Resilience (NIED), explained how the huge crater formed in the northern portion of Iwoto during an aerial inspection of the island aboard The Asahi Shimbun’s “Asuka” aircraft in January.

The crater, a striking feature, measures about 120 meters across. Nagai said it was formed during a Nov. 24 eruption last year.

“It is the biggest crater we have ever confirmed (on the island),” Nagai said.

Iwoto, an elongated, wedge-shaped islet, measures 8.5 km long and 4.5 km wide. Mount Suribachiyama, an approximately 170-meter peak where Japanese forces dug in during the World War II fighting, lies near its southern end. It is where U.S. forces raised 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 around 115 meters above sea level. The plateau is currently home to an air base operated by the Self-Defense Forces.

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

Nagai’s geological studies showed that Iwoto 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.

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

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

The island’s uplift likely began around that time and continues to this day. The rise derives from a phenomenon called “resurgent dome” in which the ground is pushed up by magma accumulating beneath the surface after a caldera has formed.

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

CURRENT STATE

Still, it remains very active today and small-scale steam explosions are a regular occurrence. The island's ground temperature is high and there are many fumaroles, or openings through which hot sulphureous gases and steam emerge, and fumarolic fields.

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

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

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

“The ground on the island is rising from a subsurface accumulation of volcanic fluid, although it is difficult to tell whether the fluid is accounted for by magma or by a mixture of hydrothermal solution and gas,” a JMA official said.

CONSTANT NEED FOR ROAD REPAIRS

Iwoto has no permanent population other than the 300 or so SDF troops stationed on the island. Some U.S. troops also conduct field carrier landing practice there, according to the Defense Ministry.

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

Because of the high geothermy, floors in buildings are erected about 1 meter higher than is typical.

As for the risk of eruption, a Defense Ministry representative said 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 leave, but we refrain from elaborating because the subject has to do with the operation of troops, albeit with regard to volcanic activity,” the official added.