Approximately 7300 years ago, a volcano off the coast of Japan’s Kyushu island erupted in what remains the largest known event of the Holocene, our current geological epoch.
In a recent study, scientists elucidated how this volcano’s massive magma chamber is incrementally refilling, a finding that could potentially shed light on the eruption cycles of this and similar volcanoes, thereby aiding humanity’s efforts toward earlier and more precise forecasting of future activity. The research outcomes were published in the journal Communications Earth & Environment.
The Kikai Caldera volcano, during its Akahoya eruption 7300 years ago, expelled roughly 160 cubic kilometers (38 cubic miles) of dense rock, an amount exceeding eleven times that ejected by Novarupta in 1912, and thirty-two times that of Pinatubo in 1991.
This colossal blast scattered debris across an area spanning 4500 square kilometers and generated pyroclastic flows reaching up to 150 km from the epicenter. Ashfall (tephra) blanketed extensive regions across Japan and the Korean Peninsula.
Since that time, the volcano has remained quiescent regarding such scale, though it is still active, responsible only for isolated, minor ejections in recent decades.
Prior investigations had detected signs of renewed volcanic unrest beneath the Kikai caldera, indicative of a growing lava dome and sparking concerns about the possibility of a subsequent eruption.
Despite a scarcity of evidence and the absence of written records, it is theorized that the Akahoya eruption devastated the Jōmon people, who inhabited what is now Japan from around 14,000 to 300 BCE.
Much has changed over the past seven millennia, and given the current regional population density, another eruption—even one relatively contained—could now inflict significantly more devastating consequences.
Aside from Kikai, notable calderas (huge, shallow craters left after eruptions) include the Yellowstone volcano in North America, whose last caldera-forming eruption occurred about 640,000 years ago, and Indonesia’s Toba, site of the largest volcanic blast in recorded history around 74,000 years ago.
These powerful volcanoes are known to awaken and erupt following protracted quiescent periods, though the mechanics governing these long-term cycles remain largely obscure, complicating efforts to predict their next calamitous episode.
“To grasp how giant caldera eruptions occur, we must comprehend how such immense volumes of magma can accumulate,” states co-author Nobukazu Seama from Kobe University in Japan.
The Kikai Caldera is presently mostly submerged, which restricts physical access but simultaneously preserves relics of past eruptions and facilitates contemporary research into these phenomena.
“The underwater setting allows us to undertake systematic, large-scale surveys,” Seama notes.
Seama and his colleagues from Kobe University and the Japan Agency for Marine-Earth Science and Technology deployed research vessels to investigate the area, utilizing an airgun system alongside several dozen ocean-bottom seismometers.
The researchers generated seismic pulses using the airguns and subsequently employed the seismometers to measure how those pulses propagated through the crust, yielding valuable insights into the subsurface structure.
This methodology led to the detection of a substantial magma chamber that evidently fed the Akahoya eruption.
“Based on its size and location, it’s evident that this is the same magma reservoir that supplied the prior eruption,” Seama affirms.
However, the magma currently inside does not appear to be residual material; chemical analysis reveals its composition differs from the Akahoya ejecta. Prior studies also suggest a new lava dome has been building within the caldera over the last 3,900 years.
“This implies that the magma presently residing within the reservoir beneath the lava dome is very likely newly introduced magma,” Seama explains.
Based on the collected data, the researchers propose a novel overarching model for magma chamber replenishment beneath giant calderas, offering insights into Kikai and other volcanoes worldwide.
“This magma re-injection model is consistent with the presence of large, shallow magma reservoirs beneath other giant calderas, such as Yellowstone and Toba,” Seama states.
“We aim to refine the techniques that proved so useful in this study to achieve a deeper understanding of these re-injection processes,” Seama adds. “Our ultimate objective is to better track the critical indicators preceding future colossal eruptions.”
About the Author
