Kikai Volcano Eruptions

Ash clouds with plume heights ranging from 1.2-1.5 km were produced by small explosions from Iwo-dake between March - September 2004.

Phases of weak intermittent explosive activity occurred between Oct 2000-Dec 2001, 24 May-5 June 2002, and again 7 June - 22 September 2003.
The height of ash plumes never exceeded 2-3 km and the eruption caused no damage, only minor ash fall in nearby areas.

Small explosive activity occurred at Iwo-dake some time between late-April and early-May 1998. A 5 cm ash deposit was observed by Japanese geologists at the crater of Iwo-dake lava dome.
Residents of Tokara-Iwo-Jima saw ash eruptions in August and October 1998.

Following a quiet of 333 year, the first activity since 1935 occurred on 18 January 1988 and consisted in small phreatic explosions ejecting ash to 400-500 m above the summit crater of Iwo-dake.
Residents did not feel any seismic activity preceding the eruption. A high-temperature fumaraole at the summit was measured 788°C, more or less unchanged since the last survey in 1985.
Source: GVP monthly reports

A submarine eruption occurred 2 km east of Tokara-Iwo-Jima between 17 September 1934 and 20 September 1935.
The eruption started from vents at a water depth of 200-300 m. "Smoke" and floating pumice were seen in September 1934 and detonations could be heard. By November, a new island started to appear, but was destroyed by explosions on 30 December 1934.
A second island formed in January 1935, reaching 300 m length, 150 width and a height of 20 m when measured on 20 January. It had a crater of 50 m diameter and a viscous lava flow was observed on the western side. A 3rd, smaller island was born on 20 February. Its activity was surtseyan, frequent phreatomagmatic eruptions at intervals of less than a few minutes, building up a pyroclastic cone. When observed on 26 March, it was 22-23 m high. The other island had entered a magmatic phase and a lava dome had appeared reaching 50-60 m height.
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Sources:
- Peter Hedervari (1986) "CATALOG OF SUBMARINE VOLCANOES AND HYDROLOGICAL PHENOMENA ASSOCIATED WITH VOLCANIC EVENTS", World Data Center A for Solid Earth Geophysics, Report SE-42, Georgiana Observatory, Budapest
- Kano, K. (2003) "Subaqueous Pumice Eruptions and Their Products: A Review", in White et al (eds.) "Explosive Subaqueous Volcanism", American Geophysical Union monograph 140, pp 231-244

This ultra-plinian eruption of Kikai around 4350 BC was one of the largest Holocene eruptions on the planet. It ejected about 150 cubic kilometers of rhyolithic magma, i.e. 3 times more than the largest historic eruption, the 1815 Tambora eruption!
The eruption produced massive rhyolithic pyroclastic flows that traveled across the sea for 100 km and reached southern Kyushu. Ash layers from this eruption are found all over Japan until the northernmost island Hokkaido and form an important geologic marker.
The eruption devastated southern and central Kyushu, which afterwards remained uninhabited for several centuries. The present-day caldera resulted at least partly from collapse after the eruption.