Aira Caldera

About

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 60 cm (24 in).

Aira caldera is located on Kyushu, the southernmost island of Japan. The supervolcano peaks at 1,117 m (3,665 ft). The colossal Ito eruption forming the Aira Caldera occurred approximately 30,000 years ago. It resulted in tephra and ignimbrite from a vast amount of magma affecting the nearby land. The eruption also aided in the formation of the 200-metre-deep (660 ft) Kagoshima Bay, which formed after sea water entered the area.

Within the Aira caldera, in the northern, inner-most part of Kagoshima Bay, northeast of the currently active Sakurajima is the Wakamiko sub caldera, which has geothermal activity, and is also considered to have formed at the time of the Ito eruption. 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. Additionally, a disaster-prevention system with the world's best high-tech volcanic monitory system was put in place. The caldera is now closely monitored by the Sakurajima Volcano Research Centre, which is a part of the University of Kyoto and Disaster Prevention Research Institute. This ensures the safety of the residents and provides a peaceful coexistence with the people of Kagoshima and the active caldera.

Aira Caldera is almost rectangular in shape related to local faulting and was created in a series of large-scale of pyroclastic surges that contributed to the Shirasu-Daichi pyroclastic plateau, with the last now dated to 29,428 to 30,148 years calibrated before present. The eruption formed a caldera that was 17 km (11 mi) by 23 km (14 mi). The Aira Caldera is one of a series of volcanic complexes in the Kagoshima graben, 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 from a vent near where Sakurajima is now and was quickly followed by an oxidised Tsumaya pyroclastic flow. It is likely that subsequent eruptions in this series were at vents in what has been termed the Wakamiko caldera to the northwest. Basement rock fragments and pumiceous materials from a massive explosion formed the Ito pyroclastic flow, which deposited more than 800 km3 (190 cu mi) of Ito Ignimbrite (known as "Shirasu" locally) and 300 km3 (72 cu mi) of Aira-Tn Tephra in volume. 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 140 km3 (34 cu mi) of magma in a short amount of time. The caldera is known for its gravitational anomalies, which are associated with a funnel-like shape in the strata. The structure of the caldera seemed unique in early work as it was different from the then-typical Valles-type Caldera whose defining characteristics include a Valles-type ring fracture which acts as a channel for such large-scale pyroclastic flows. Such diffuse non-directional pyroclastic flows, overwhelming the local landscape, have now also been described in New Zealand, for example in the Hatepe eruption. 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. Petroleum seeps exist.

Adapted from Wikipedia (CC BY-SA). Photography via Wikimedia Commons.