Aira Caldera is a gigantic volcanic caldera located on the southern end of Kyushu, Japan. It is believed to have been formed about 30,000 years ago with a succession of pyroclastic surges. It is currently the place of residence to over 900,000 people. The shores of Aira Caldera have rare flora and fauna, including Japanese bay tree and Japanese black pine. The caldera contains the active Mount Sakurajima.
Aira Caldera has an underlying magma chamber that connects with the Kirishima magmatic system. This has enabled magma from the caldera to feed into Sakurajima stratovolcano, causing it to expand over time. Thus, Sakurajima has caused a series of disasters such as the eruption in 1914 which killed 58 people and sank the magma chamber by .
History
Location
Aira caldera is located on Kyushu, the southernmost island of Japan. The supervolcano peaks at .
The colossal Ito eruption forming the Aira Caldera occurred approximately 30,000 years ago.
Aira caldera is surrounded by the major city of Kagoshima which has a population of more than 900,000. Residents do not mind small eruptions because they have measures in place for protection. For example, school students are required to wear hard helmets for protection against falling debris. which has been postulated to extend northward from the undersea Kikai Caldera to the Ata South Caldera, Ata North Caldera (see Ata Caldera), the Aira Caldera associated with Kagoshima Bay, and through past to the Kirishima Volcano Group. The Mount Kirishima group of stratovolcanoes has Shinmoedake as its most famous and active member. This alignment was first noted in the 1940s. The tectonic processes are rather complex in this region, where the Okinawa Plate is colliding with the Amur Plate and the Pacific Plate is subducting under both.
The formation of Aira Caldera started with a Plinian pumice eruption of the Osumi pumice It is likely that subsequent eruptions in this series were at vents in what has been termed the Wakamiko caldera to the northwest. Within the constraints that much of the caldera is under the sea, the reason for the large vent area is because the caldera erupted well over earlier estimates of of magma in a short amount of time.
Rare minerals exist on Kagoshima Bay's seabed with hydrothermal vents including volcanic chimneys. The minerals include stibnite, realgar, orpiment, and pyrite and there are concentrations of mercury, antimony, and arsenic. Some of the hydrothermal fluid-venting chimneys have unique mineralogy, of kerolite and hydrated talc with lesser amounts of dolomite and magnesite, amorphous silica, and stibnite. The depth of the ash fall was over on Kyushu and southern Honshu, approximately of ash on the Kantō Plain (Tokyo) and around for much of Japan.
Volcanic activity
Relationship between Aira and Kirishima magmatic systems
Aira caldera is one of the most active and hazardous calderas in the world. It is connected by the collapse structure of the Kagoshima graben to the Kirishima volcanoes, a group of active volcanoes beyond the north end of Aira caldera. One of these volcanoes, Shinmoedake, has produced two strong magmato-phreatic eruptions, separated by almost 300 years. Minor phreatic activity at Shinmoedake starting in August 2008 was not initially associated with a pattern of ground deformation. Starting in December 2009, active inflation before the outbreak became detectable consistent with, in due course, the accumulation of approximately of magma. A series of sub-plinian events then occurred from 19 to the 31 January 2011. The first phase (eruption climax) was accompanied by a strong co-eruptive deflation.
Aira Caldera may respond to small eruptions that come from a common reservoir. However, not all the volcanic systems are connected all the time as magma pathways open and close. The connection between Aira and Kirishima represents the clearest example of volcano interconnectivity revealed by geodetic monitoring. The inflation of one volcano can enhance the eruption probability of a neighbouring volcano. The subduction of the Philippine Sea Plate beneath the Eurasian Plate is the reason for the active volcanism.