Ata Caldera
{{short description|Mostly-submerged caldera in Kagoshima Prefecture, Japan}}
{{Infobox mountain
| name= Ata Caldera
| native_name={{native name|ja|阿多カルデラ}}
| other_name=
| etymology=
| photo=Ata Caldera Relief Map, SRTM-1 (English).jpg
| photo_caption=Postulated limits of the Ata Caldera have varied over time in the scientific literature. Some recent definitions and evidence would include most of both of the indicated areas in this map.
| photo_size=
| country= Japan
| subdivision1= Kagoshima Prefecture
| subdivision2_type= Region
| subdivision2= Ibusuki, Kagoshima, Kimotsuki District, Tarumizu
| coordinates= {{coord|31.40|N|130.64|E|type:mountain_region:IT_scale:100000|format=dms|display=inline,title}}
| highest=Mount Kaimon
| highest_location={{coord|31|10|48|N|130|31|42|E|type:mountain_scale:100000|format=dms|display=inline}}
| elevation_m=924
| length_km= 25
| length_orientation= NS
| width_km= 15
| width_orientation= EW
| area_km2=
| geology=
| orogeny=
| age= Pleistocene (240,000 years ago) onwards
| type=Caldera
Somma volcano
| map_caption=
| map = Japan
}}
{{nihongo|Ata Caldera|阿多カルデラ|Ata karudera}}, containing the Ata North Caldera, Mount Kaimon and Ikeda Caldera amongst other volcanoes, is a massive, ill defined, mostly submerged volcanic caldera associated with the southern portions of Kagoshima Bay.
Geology
The earliest tephra assigned to the volcano, is the widespread on regional sea bed cores, Ata–Torihama tephra (Ata-Th) at 240,000 years before the present.{{cite journal|last1=Ikehara |first1=Ken |title=Marine tephra in the Japan Sea sediments as a tool for paleoceanography and paleoclimatology |year=2015|journal=Progress in Earth and Planetary Science |volume=2 |issue=36 |pages=1–12 |doi=10.1186/s40645-015-0068-z|bibcode=2015PEPS....2...36I |s2cid=45727649 |url=https://progearthplanetsci.springeropen.com/counter/pdf/10.1186/s40645-015-0068-z.pdf |doi-access=free }}
The caldera contributed to an eruption which has been dated to about 100,000 years before present (range by various techniques mostly fall 100,000 to 109,000) that generated the Ata tephra in southern Japan.{{cite journal|first1=Mitsuru |last1=Okuno|title= Chronological study on widespread tephra and volcanic stratigraphy of the past 100,000 years |journal= The Journal of the Geological Society of Japan |volume=125 |issue=1 |pages=41–53 |date=2019-04-15 |issn=1349-9963 |doi=10.5575/geosoc.2018.0069 |s2cid=146526393 |url=https://www.jstage.jst.go.jp/article/geosoc/125/1/125_2018.0069/_article/-char/en |doi-access=free }} This eruption has been assigned a VEI of 7.5 and generated over {{convert|300|km3|abbr=on}} of tephra.{{cite journal|first1=Victoria C. |last1=Smith|first2= Richard A. |last2=Staff|first3= Simon P.E. |last3=Blockley|first4= Christopher Bronk |last4=Ramsey|first5= Takeshi |last5=Nakagawa|first6= Darren F. |last6=Mark|first7= Keiji |last7=Takemura|first8= Toru |last8=Danhara |title=Identification and correlation of visible tephras in the Lake Suigetsu SG06 sedimentary archive, Japan: chronostratigraphic markers for synchronising of east Asian/west Pacific palaeoclimatic records across the last 150 ka |journal= Quaternary Science Reviews |volume=67|year=2013 | pages=121–137 |issn=0277-3791 |doi=10.1016/j.quascirev.2013.01.026 |bibcode=2013QSRv...67..121S |url=https://www.sciencedirect.com/science/article/pii/S0277379113000413 |url-access=subscription }} This is overlaid in some places in Japan by the more recent Mitake No. 1 (On-Pm1) tephra from an eruption in the Mount Ontake area and K-Tz tephra from the Kikai Caldera. There have been many more lesser eruptions.
=Structure=
Some of the recent literature separates the caldera into a northern almost completely submerged caldera that generated the Ata tephra and Ata ignimbrite, and a southern caldera which includes the recently active Ikeda Caldera and the Kaimondake stratovolcano in the Ibusuki Volcanic Field. {{cite web|access-date=2022-09-19|title=IAVCEI 2013 Scientific Assembly A Guide for Mid-Conference Field Trip |url=https://fdocuments.in/document/2014-1-21-kirishima-sakurajima-and-kaimondake-on-kyushu-and-several-volcanic.html?page=1 }} This southern caldera first had the Ata name but is not believed now to be associated with the vents of the major eruption of 100,000 years ago.{{cite journal|access-date=2022-09-19|last1=Matumoto |first1=Tadaiti|title=Calderas of Kyusyu|url=https://www.oregongeology.org/pubs/ofr/O-66-01.pdf|date=1965-08-28|journal=Transactions of the Luna Geological Field Conference}} High resolution Bouguer gravity imaging of Kyushu is consistent with the larger caldera being the Ata North Caldera but suggests it may be centered near the island of Chiringashima, and that the Ata South Caldera is the smaller in size, overlaps it being centred near Yamagawafukumoto district.{{cite book|last1=Shichi |first1= R. |last2=Yamamoto |first2= A. |last3= Kudo |first3= T. |last4= Murata |first4= Y. |last5=Nawa |first5= Kazunari |last6= Komazawa |first6= Masao |last7= Nakada |first7= M. |last8=Miyamachi |first8= H. |last9=Komuro |first9= Hitoshi |last10= Fukuda|first10= Yoichi |last11= Higashi |first11= T. |last12= Yusa |first12= Y.|title= A Window on the Future of Geodesy |chapter= A Gravity Database of Southwest Japan: Application to Bouguer Gravity Imaging in Kyushu District, Southwest Japan |series= International Association of Geodesy Symposia |year=2005 |volume= 128 |doi= 10.1007/3-540-27432-4_40 |url=https://www.researchgate.net/publication/251134851 |pages=236–241 |isbn= 3-540-24055-1 }}
The National Catalogue of the Active Volcanoes of Japan (JMA, 2013) included features of the Ibusuki Volcanic Field as part of the Ata post-caldera system. By this definition the single caldera may be a rounded triangle about 30km in length and up to 25km in width, although the usual quoted size is smaller.
=Relationships=
Immediately adjacent to the north of the caldera is the Sakurajima volcano in Aira Caldera and further away to the south along what has been termed the Kagoshima Graben is the Kikai Caldera.{{cite journal|last1=Geshi |first1= N. |last2=Yamada |first2= I. |last3= Matsumoto |first3= K. |last4= Nishihara |first4=A. |last5= Miyagi |first5=I. |title= Accumulation of rhyolite magma and triggers for a caldera-forming eruption of the Aira Caldera, Japan |journal= Bulletin of Volcanology |volume=82 |issue= 44 |year=2020 |page= 44 |doi=10.1007/s00445-020-01384-6|bibcode= 2020BVol...82...44G |s2cid= 218652170 |url=https://link.springer.com/article/10.1007/s00445-020-01384-6 |url-access= subscription }} This alignment was first described by Tadaiti Matumoto in the 1940s. The alignment extends all the way north past Mount Kirishima to intersect the Aso Caldera by gravitational anomaly. 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.