Specks of earthy green, brown and white dot the sea just south of the mainland Japanese island of Kyushu, some of which have a rather recent volcanic history. Compared to the gigantic volcanic scar that they sit on the rim of, though, such tiny little territories vanish into insignificance.
This geological crucible has had a spectacularly violent past stretching back 140,000 years, and its paroxysm 7,300 years ago – the Akahoya eruption – unleashed more than 500 cubic kilometers (120 cubic miles) of material before leaving behind the 19-kilometer (12-mile) caldera we can see today.
There’s now an increasing amount of evidence suggesting that a lava dome exists within the caldera. This viscous mound, comprised of thick rhyolitic lava and taking up around 32 cubic kilometers (7.7 cubic miles) of space, was likely formed after the Akahoya eruption, but it’s far more than a silent remnant of a bygone era.
Judging by the fact that it’s associated with an active hydrothermal system, including high-temperature hot springs and fairly profuse gas bubbling, this lava dome is “still active”, according to the Kobe University-led team.
The researchers reached their conclusions after conducting three recent surveys of the caldera aboard the ship Fukae Maru. A suite of experiments were carried out using a variety of techniques, from underwater robots to artificial earthquakes to peek at Kikai's buried structure.
There’s a lot about the dome that remains enigmatic, and the timing and nature of its formation still require further investigation. In any case, the lava dome doesn’t mean that a supereruption is imminent; the two volcanic phenomena are rather different kettles of fish.
Previous work has suggested that there’s a roughly one-in-100 chance of such an event occurring in the next century or so anywhere along the Japanese archipelago. There’s actually no evidence to suggest that a supereruption is around the corner, at Kikai or anywhere else in the region. A lava dome is a curiosity, not a sign of impending doom.
The point here is that this somewhat restless lava dome suggests that the magmatic source beneath Kikai isn’t cold and dead, but active enough to cause such an extrusion in the first place at some point in the recent past.
Fascinatingly, the dome’s geochemistry appears to be different from the material generated by the last supereruption at Kikai. In fact, the extrusion’s chemical signature is similar to that of volcanic products found on nearby islands, including Showa Iwo-jima, a deserted volcanic islet that burst forth from the waves in the mid-1930s.
This suggests that the magma source that formed Show Iwo-jima is the same one responsible for generating this lava dome, and both “may tap a magma system that has evolved both chemically and physically since the 7,000-year-old supereruption.”
The lava dome is just one piece of the jigsaw puzzle of what the team refer to as the “whole caldera cycle”. The motivation of papers like this is to improve our understanding of Kikai’s past in order to better understand its future, especially if that future involves a repeat of the Akahoya event.
University College London’s Professor Bill McGuire, a geophysicist and geological hazards expert who wasn't involved in the study, told IFLScience that he debates the team’s use of the word “supereruption”. This technically requires an ejection of at least twice as much volcanic material.
Still, the Akahoya event was “huge,” and “something on this scale today would be… more than three times the size of the 1815 Tambora blast. The impact on the global climate would be significant, probably with a few degrees of cooling for several years and major harvest failures.”
So where does this lava dome fit in on this cycle, and what does it tell us about the next supereruption? These are questions the team hope to answer when their next voyage takes place this March.