Photo/Illutration A volcanic eruption off the coast of Iwoto island on July 13 spews pieces of rocks and volcanic ash to the height of about 20 meters, covering an around 40-meter wide area. (Provided by the National Research Institute for Earth Science and Disaster Resilience)

For possibly the first time in 1,000 years, volcanic eruptions spewing magma are thought to have occurred off the coast of Iwoto island, the Japan Meteorological Agency announced on Aug. 8.

“The eruptions have been small, but we will continue our observations,” a JMA official said. 

Iwoto island, part of the Ogasawara island chain and also known as Iwojima, is an active volcanic island.

The island's ground surface has shown abnormal swelling of up to one meter in one year as a result of volcanic activity.

However, eruptions have normally involved water vapor, not magma.

There is currently an Air Self-Defense Force base on Iwoto. However, although ASDF members and construction workers are temporarily posted on the island, there are no permanent residents there.

The Ogasawara island chain lies about 1,200 kilometers south of central Tokyo.

The eruptions started on July 11 around 900 meters off the south coast of Iwoto island.

Officials of the National Research Institute for Earth Science and Disaster Resilience (NIED) visited the site between July 12 and July 15 and observed volcanic eruptions every five minutes, which created water columns tens of meters high and spewed black smoke.

The officials also observed many rocks that had washed ashore on the coast of Iwoto island.

There were small cavities inside the rocks.

The officials believe they are cooled lava, formed after magma has erupted, quickly cooled and solidified.

The temperature on the inside of some of the rocks was as high as 120 degrees, according to the officials.

Volcanic eruptions were still occurring when JMA officials visited the site at the end of July.

They observed volcanic eruptions every several minutes and columns of black water that were spewing 20-30 meters high.

Water vapor or mud eruptions are normally observed on Iwoto island. Water vapor eruptions can occur when underground water erupts after getting heated by magma.

However, magma eruptions have not been observed in the area since the end of the 19th century, when data collection started.

A huge amount of pumice believed to have come from Iwoto island was washed ashore along the coast of Okinawa around 1,300 years ago, according to Masashi Nagai, researcher at the NIED and an expert on volcanic geology.

Magma eruptions back then eventually formed Mount Suribachi on the island.

Pumice is thought to have spewed out during these large volcanic eruptions.

Although there were many unknown things about volcanic eruptions on the remote island in ancient days, Nagai believes the recent magma eruptions were the first instances since the eruptions of around 1,300 years ago.

“The upper part of magma, which has caused active crustal movements, could have erupted," Nagai said about the recent eruptions. "It’s unknown whether more magma eruptions will occur in the future.”

However, JMA officials said the July eruptions “were small, with no significant change in the occurrence of earthquakes or crustal movement.”

Thus, the agency announced the eruptions as part of its regular monthly reports on volcanic activity across Japan rather than making a one-off report just on them.

Iwoto island is one of 50 volcanoes the agency observes 24 hours a day.