Low-level eruptions at Satsuma-Iojima volcano, Kikai caldera, Japan

The Satsuma-Iojima volcano, located within the Kikai caldera in Japan, is showing signs of increased activity since September 1, 2024.

• Kikai is the source of one of the world’s largest Holocene eruptions about 6 300 years ago when rhyolitic pyroclastic flows traveled across the sea for a total distance of 100 km (62 miles) to southern Kyushu, and ashfall reached the northern Japanese island of Hokkaido. The eruption devastated southern and central Kyushu, which remained uninhabited for several centuries.

The first eruption took place on September 1 at 02:28 UTC, with ash rising to an altitude of 1 830 m (6 000 feet) — about 1 100 m (3 600 feet) above the crater. The volcanic ash from this eruption was not identifiable from satellite data at the time.

A second eruption occurred on September 3 at 01:08 UTC, sending ash to a height of 1 520 m (5 000 feet) above sea level. Similar to the first eruption, the ash wasn’t detectable via satellite.

Kikai Caldera, situated just south of the Ryukyu Islands of Japan, is a mostly submarine caldera spanning approximately 19 km (11.8 miles) in diameter. Satsuma Iwo Jima, also known as Satsuma-Iwojima and Tokara Iojima, is located on the northwest rim of the caldera.

The island’s highest peak, Iodake, is part of this volcanic structure. The last significant eruption period at Kikai was on October 6, 2020, marked by an explosion and thermal anomalies in the crater. More recent observations have noted intermittent thermal activity and gas-and-steam plumes.

Between May 2021 and April 2023, the Japan Meteorological Agency (JMA) and various satellite data reported low-level volcanic activity, including white gas and steam plumes, nighttime incandescence, seismic activity, and discolored water. Throughout this period, the alert level remained at 2 on a 5-level scale, according to JMA.

Activity during the reporting period was generally low, with occasional white gas-and-steam emissions rising 200 to 1 400 m (656 – 4 593 feet) above the Iodake crater. Nighttime incandescence was observed at the Iodake crater using a high-sensitivity surveillance camera.

Volcanic earthquakes were frequently detected each month, and sulfur dioxide levels were measured by the University of Tokyo Graduate School of Science, Kyoto University Disaster Prevention Research Institute, Mishima Village, and JMA.

Recent activity has been observed at multiple eruption centers within Kikai Caldera, a mostly submerged structure near the northern end of the Ryukyu Islands.

This caldera was the site of one of the world’s largest Holocene eruptions — approximately 6 300 years ago. During that eruption (VEI 7), rhyolitic pyroclastic flows traveled across the sea for 100 km (62 miles) to southern Kyushu, with ashfall reaching as far as Hokkaido.

The eruption devastated southern and central Kyushu, leaving the region uninhabited for several centuries.

Subsequent eruptions within the caldera have led to the formation of the Iodake lava dome, the Inamuradake scoria cone, and submarine lava domes. Recorded eruptions have occurred at or near Satsuma-Iojima, a small island forming part of the northwest caldera rim.

Showa-Iojima lava dome, a small island located 2 km (1.2 miles) east of Satsuma-Iojima, was formed during submarine eruptions in 1934 and 1935. In recent decades, mild-to-moderate explosive eruptions have occurred from Iodake, a rhyolitic lava dome at the eastern end of Satsuma-Iojima.

References:

¹ Volcanic Ash Advisory – Tokyo VAAC -September 1 and 3, 2024

² Kikai – Geological Summary – GVP– Accessed on September 3, 2024