The volcanic islands of Take-shima and Satsuma Iwo-jima (pictured here) are the subaerial parts of the northern edge of the Kikai caldera. Credit: The Yomiuri Shimbun/AP
A giant submarine caldera in the vicinity of Japan, it houses a lava dome of 8 trillion litres of molten rock.
The dome, which is 6.2 miles (10 kilometers) wide and 1,968 feet (600 metres) long, is a solid rock now, and it doesn’t presage an approaching eruption. However, it does add a new wrinkle to the history of the Kikai caldera, a large depression that formed during a massive volcanic super-eruption about 6,300 or 7,300 years ago (in the wide range has to do with the various methods of dating the eruption). That eruption sent heated pyroclastic flow of 50 miles (80 km) over the sea and the spreading axis to 620 miles (1000 km), said Yoshi Tatsumi, the author of a new study on the caldera’s inner workings, published today (Feb. 9) in the journal Scientific Reports.
The system is still active, and it is a relatively high risk for volcanic activity, said Erik Klemetti, a volcanologist in Denison University, who was not involved in the study. The volcano also blew the top in super-eruptions95,000 years ago, and about 140,000 years ago. Occasionally burps ash and steam even in the modern day, with the last recorded eruption occurring between 2013 and 2014. [The 11 Largest Volcanic Eruptions in History]
But because the caldera is hidden underwater world, it is hard to keep track of her activities. Tatsumi and his colleagues performed multiple remotely operated vehicles diving on the caldera floor, to the south of Kyushu Island in the Japanese archipelago. They used sonar to map the caldera the ground, and let the small explosive charges in the seabed to create seismic waves that they can record and use to image the surface. The team also collected data on the water column chemistry and took rock samples from the looming dome in the middle of the caldera.
The findings showed that the dome was in fact made up of lava, in particular, a form of lava called rhyolite — about 8 trillion gallons (32 cubic kilometres). This dome could have formed always since the last eruption, Klemetti said, so it is not clear how new it is. However, Tatsumi and his colleagues found that the chemical composition is different than the lava ejected from the caldera during the last super-eruption. This finding suggests that a new magma system formed after the eruption, Tatsumi told Live Science.
“The post-caldera activity, at least [at] this caldera, is considered the preparatory phase towards the next super-eruption, not as the calming-down stage of the previous super – eruption,” he said.
That does not mean that an eruption is imminent, but that the volcanic system that lies at the base of the caldera is changing and evolving in the course of the centuries, the researchers reported. It is interesting to see that the lava dome apparently come from a different part of the magmatic system (underground chambers of molten rock) than the last super-eruption of the lavas, Klemetti said. [50 Amazing Facts About Volcanoes]
The best way to be sure that the dome has a separate origin would be to test the minerals in the lavas and to find out when they formed, or before the caldera-forming super-eruption, around the same time or after, Klemetti said. Tatsumi and his team plan to look deeper beneath the caldera. Given the vast size of the lava dome, would there be a large magma reservoir beneath the surface, Tatsumi said. The team plans for the use of surface imaging to look for the reservoir and describe it as it exists.
Original article on Live Science.