Aso Caldera
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Aso caldera (also known as Asosan, the Aso Volcano or Mount Aso, although the later term usually is used related to its currently active vents) is a geographical feature of
Kumamoto Prefecture is a prefecture of Japan located on the island of Kyūshū. Kumamoto Prefecture has a population of 1,748,134 () and has a geographic area of . Kumamoto Prefecture borders Fukuoka Prefecture to the north, Ōita Prefecture to the northeast, M ...
, Japan. It stretches 25 kilometers north to south and 18 kilometers east to west. The central core "Aso Gogaku" is the five major mountains in the area. Aso valley (Asodani) runs along the northern base of Mount Aso and Nango valley (Nangodani) along the south. According to research of caldera sediment, lakes used to exist in these valleys. The dried up lake areas have come to be called Old Aso Lake, Kugino Lake, and Aso Valley Lake. The Kikuchi, Shirakawa and Kurokawa rivers now drain the caldera.


Geology

Within the caldera are more than 17 cones. The central "Aso Gogaku" group of volcanoes are Takadake (Mount Taka) at , Nekodake at , Nakadake (Mount Naka) at , Kijimadake at and Eboshidake at . These are higher than the caldera rim itself that towards the south west is high. Other volcanic features include Kishimadake (Mount Kishima), Kusasenrigahama, Komezuka, a scoria cone, Ikenokubo maar, a tuff ring and Takanooban, a lava dome. The currently active Nakadake has seven craters roughly aligned in a north–south direction with most recent eruptions being from the northernmost ''first'' crater of Nakadake which has hydrothermal reservoirs beneath it at about to depth. Presently there is a roughly spherical
magma Magma () is the molten or semi-molten natural material from which all igneous rocks are formed. Magma is found beneath the surface of the Earth, and evidence of magmatism has also been discovered on other terrestrial planets and some natural sa ...
chamber at a depth of , and flattened at located between Kishimadake, Eboshidake, and Nakadake with the magmatic source beneath the center of Aso caldera. Magma migrates diagonally upward from approximately depth through a northward-dipping magma-filled crack propagation zone, then into the shallow magma reservoirs mentioned already and during eruptions into an upright crack intrusion zone at depth. There are many geothermal areas including centrally the Jigoku or "hell" Onsen and Tarutama hot springs and in the northern part, the Uchinomaki hot springs.


Formation

The
caldera A caldera ( ) is a large cauldron-like hollow that forms shortly after the emptying of a magma chamber in a volcano eruption. When large volumes of magma are erupted over a short time, structural support for the rock above the magma chamber is ...
formed from four major
pyroclastic flow A pyroclastic flow (also known as a pyroclastic density current or a pyroclastic cloud) is a fast-moving current of hot gas and volcanic matter (collectively known as tephra) that flows along the ground away from a volcano at average speeds of b ...
events which occurred between 90,000 and 270,000 years ago. The largest of these was the fourth, which reached as far as nearby
Yamaguchi Prefecture is a prefecture of Japan located in the Chūgoku region of Honshu. Yamaguchi Prefecture has a population of 1,377,631 (1 February 2018) and has a geographic area of 6,112 km2 (2,359 sq mi). Yamaguchi Prefecture borders Shimane Prefecture t ...
160 kilometers away. The fourth eruption left a massive pyroclastic plateau which is what remains even today. In 1985 it was discovered that volcanic ash from the fourth eruption covered much of the Japanese islands. The eruptive volume of the fourth eruption is estimated to be dense-rock equivalent (DRE). Ashfall deposit greater than deep was identified on
Hokkaido is Japan's second largest island and comprises the largest and northernmost prefecture, making up its own region. The Tsugaru Strait separates Hokkaidō from Honshu; the two islands are connected by the undersea railway Seikan Tunnel. The lar ...
Island about to the north.


Eruptions

ImageSize = width:220 height:1000 PlotArea = right:50 top:10 left:50 bottom:10 DateFormat = yyyy TimeAxis = orientation:vertical Period = from:450 till:2050 AlignBars = early ScaleMajor = unit:year increment:50 start:450 Colors = id:canvas value:rgb(1,1,0.85) BackgroundColors = canvas:canvas PlotData = width:15 color:white bar:test from:450 till:2050 # PlotData = at:2050 shift:(10,-2) fontsize:XS text:Aso eruptions since written records at:2040 shift:(10,-2) fontsize:XS text:(Nakadake unless noted otherwise) bar:test at:490 mark:(line,pink) bar:test at:553 mark:(line,pink) bar:test at:782 mark:(line,pink) bar:test at:825 mark:(line,pink) bar:test at:840 mark:(line,pink) bar:test at:864 mark:(line,pink) bar:test at:867 mark:(line,pink) bar:test at:1230 mark:(line,pink) bar:test at:1239 mark:(line,pink) bar:test at:1240 mark:(line,pink) bar:test at:1265 mark:(line,pink) bar:test at:1269 mark:(line,pink) bar:test at:1271 mark:(line,pink) bar:test at:1272 mark:(line,pink) bar:test at:1273 mark:(line,pink) bar:test at:1274 mark:(line,red) at:1274 shift:(10,-2) fontsize:XS text:1274 Damage to fields bar:test at:1281 mark:(line,pink) bar:test at:1286 mark:(line,pink) bar:test at:1305 mark:(line,pink) bar:test at:1324 mark:(line,pink) bar:test at:1331 mark:(line,pink) bar:test at:1335 mark:(line,red) at:1335 shift:(10,-2) fontsize:XS text:7 Feb and 26 March 1335 building damage bar:test at:1340 mark:(line,pink) bar:test at:1343 mark:(line,pink) bar:test at:1346 mark:(line,pink) bar:test at:1347 mark:(line,pink) bar:test at:1348 mark:(line,pink) bar:test at:1349 mark:(line,pink) bar:test at:1350 mark:(line,pink) bar:test at:1351 mark:(line,pink) bar:test at:1352 mark:(line,pink) bar:test at:1353 mark:(line,pink) bar:test at:1354 mark:(line,pink) bar:test at:1355 mark:(line,pink) bar:test at:1356 mark:(line,pink) bar:test at:1357 mark:(line,pink) bar:test at:1358 mark:(line,pink) bar:test at:1359 mark:(line,pink) bar:test at:1360 mark:(line,pink) bar:test at:1361 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mark:(line,pink) bar:test at:1584 mark:(line,red) at:1584 shift:(10,-2) fontsize:XS text:1584 agricultural land damage bar:test at:1587 mark:(line,pink) bar:test at:1592 mark:(line,pink) bar:test at:1598 mark:(line,pink) bar:test at:1611 mark:(line,pink) bar:test at:1612 mark:(line,pink) bar:test at:1613 mark:(line,pink) bar:test at:1620 mark:(line,pink) bar:test at:1631 mark:(line,pink) bar:test at:1637 mark:(line,pink) bar:test at:1649 mark:(line,pink) bar:test at:1668 mark:(line,pink) bar:test at:1675 mark:(line,pink) bar:test at:1683 mark:(line,pink) bar:test at:1691 mark:(line,red) bar:test at:1708 mark:(line,pink) bar:test at:1709 mark:(line,pink) bar:test at:1765 mark:(line,red) at:1772 shift:(10,-2) fontsize:XS text:1772 crop damage bar:test at:1772 mark:(line,red) bar:test at:1773 mark:(line,pink) bar:test at:1774 mark:(line,pink) bar:test at:1775 mark:(line,pink) bar:test at:1776 mark:(line,pink) bar:test at:1777 mark:(line,pink) bar:test at:1778 mark:(line,pink) bar:test at:1779 mark:(line,pink) bar:test at:1780 mark:(line,red) bar:test at:1781 mark:(line,pink) bar:test at:1782 mark:(line,pink) bar:test at:1783 mark:(line,pink) bar:test at:1784 mark:(line,pink) bar:test at:1785 mark:(line,pink) bar:test at:1786 mark:(line,pink) bar:test at:1787 mark:(line,pink) bar:test at:1788 mark:(line,pink) bar:test at:1804 mark:(line,pink) bar:test at:1806 mark:(line,pink) bar:test at:1814 mark:(line,pink) bar:test at:1815 mark:(line,red) bar:test at:1816 mark:(line,magenta) at:1816 shift:(10,-2) fontsize:XS text:July 1816 Yunotani Onsen Disaster 1 dead bar:test at:1826 mark:(line,red) bar:test at:1827 mark:(line,red) bar:test at:1828 mark:(line,red) bar:test at:1830 mark:(line,pink) bar:test at:1830 mark:(line,red) bar:test at:1832 mark:(line,pink) bar:test at:1837 mark:(line,pink) bar:test at:1838 mark:(line,pink) bar:test at:1854 mark:(line,magenta) at:1854 shift:(10,-2) fontsize:XS text:26 February 1854 3 killed bar:test at:1856 mark:(line,pink) bar:test at:1872 mark:(line,magenta) at:1872 shift:(10,-2) fontsize:XS text:30 December 1872 Sulfur miners killed bar:test at:1873 mark:(line, pink) bar:test at:1874 mark:(line,pink) bar:test at:1884 mark:(line,pink) bar:test at:1894 mark:(line,red) bar:test at:1897 mark:(line,pink) bar:test at:1906 mark:(line,pink) bar:test at:1907 mark:(line,pink) bar:test at:1908 mark:(line,pink) bar:test at:1910 mark:(line,pink) bar:test at:1911 mark:(line,pink) bar:test at:1912 mark:(line,pink) bar:test at:1916 mark:(line,pink) bar:test at:1918 mark:(line,pink) bar:test at:1919 mark:(line,pink) bar:test at:1920 mark:(line,pink) bar:test at:1923 mark:(line,pink) bar:test at:1926 mark:(line,pink) bar:test at:1927 mark:(line,red) at:1927 shift:(10,-2) fontsize:XS text:1927-1929 crop & livestock damage bar:test at:1928 mark:(line,red) bar:test at:1929 mark:(line,red) bar:test at:1930 mark:(line,pink) bar:test at:1932 mark:(line,red) bar:test at:1933 mark:(line,red) bar:test at:1934 mark:(line,pink) bar:test 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Aso volcano has been active at intervals of approximately 10–20 years with the most active cone in recorded history being Nakadake located in the center of the caldera. It has active since the sixth century. Eruptions occurred from November 2014 to May 2015, with two major phreatomagmatic eruptions on September 14, 2015, and October 8, 2016. Eruptive activity occurred on 20 October 2021. Main eruption ages and eruption volume (DRE ( dense-rock equivalent) is the equivalent magma eruption volume. The volume of erupted products is much higher): * Aso-1: About 266,000 years ago,阿蘇火砕流について
熊本県高等学校教育研究会 地学部会
erupted DRE. * Aso-2: About 141,000 years ago, erupted DRE.阿蘇カルデラ
産総研
* Aso-3: About 130,000 years ago,下山正一
低平地地下における阿蘇3火砕流堆積物(Aso-3)の年代について
佐賀大学低平地防災研究センター編 低平地研究 Vol.10 p.31-38
erupted DRE. * Aso-4: About 90,000 years ago, erupted DRE which was at least VEI 7. * Nojiri pumice about 84,000 years ago erupted DRE. * Kusasenrigahama Pumice from Kusasenrigahama at approximately 30,000 years ago erupted DRE at VEI 5 * Jigoku spa about 8,050 BCE, Ikph2 tephra * Janoo before 5,300 BCE Tephta and scora cone * Nakadake 5,300 to 12,000 BCE tephra falls * near Janoo about 5,300 BCE Akamizu lava flow * Near Ikenokubo about 5,300 BCE tuff ring * Jigoku Onsen 3,000 to 4,400 BCE tephra, Ikph1 tephra * Nakadake 3,000 to 4,400 BCE tephra falls * Nakadake 4,000 to 3,000 BCE tephra falls * Unknown vent near Janoo, 2150 BCE Aso central cone No. 1 pumice, ACP-1 tephra * Kishimadake 2,050 BCE > DME VEI 4, KsS tephra * Nakadake 1,600 to 2,000 BCE tephra falls * Ojodake 1,650 BCE > DME VEI 4, OjS tephra * Komezuka, Kamikomezuka 1,350 BCE tephra DME * Unknown Aso 1270 ± 75 BCE * Western central cones debris avalanche 100 CE to 400 BCE * Nakadake 630 ± 50 BCE tephra falls * Nakadake 440 ± 75 CE N2S tephra fall * See timeline for eruptions in historic written record (KEY-pink if confined to volcano, red tephra fall, magenta human deaths)


Tectonics

There are multiple active faults both under the volcano and adjacent on this area of the Amur Plate. The Okinawa Plate collides with the Amur Plate to the south and the Pacific Plate is subducting under both. The caldera is located where two volcanic lines intersect, being those in the Central Kyushu Rift Valley with volcanoes of
Mount Yufu is a volcano, located on the border of Yufu and Beppu, Ōita, Japan. It is located within Aso Kujū National Park. Outline Mount Yufu is a stratovolcano. It has two peaks, called ‘Higashi-mine’ (East Peak) and ‘Nishi-mine’ (West P ...
, in
Oita Prefecture Oita often refers to: * Ōita Prefecture, Kyushu, Japan * Ōita (city), the capital of the prefecture Oita or Ōita may also refer to: Places * Ōita District, Ōita, a former district in Ōita Prefecture, Japan *Ōita Stadium, a multi-use stad ...
, through the Kuju volcanoes and Aso Caldera, and on to
Mount Unzen is an active volcanic group of several overlapping stratovolcanoes, near the city of Shimabara, Nagasaki on the island of Kyushu, Japan's southernmost main island. In 1792, the collapse of one of its several lava domes triggered a megatsunam ...
and line that runs from Aso Caldera to the Kirishima volcanic group,
Aira Caldera Aira Caldera is a gigantic volcanic caldera that is 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 ...
, Ata Caldera, and on to the Kikai Caldera. The high resolution Bouguer gravity imaging of Kyushu has confirmed the caldera to be piston rather than funnel shaped, as originally proposed, and related to known active faults and in particular the gravity gradient zone of the Aso Caldera is part of the Oita-Kumamoto Tectonic Line (OKTL) gravity gradient zone and this appears to connect with the
Japan Median Tectonic Line , also Median Tectonic Line (MTL), is Japan's longest fault system. The MTL begins near Ibaraki Prefecture, where it connects with the Itoigawa-Shizuoka Tectonic Line (ISTL) and the Fossa Magna. It runs parallel to Japan's volcanic arc, passing ...
s gravity gradient zone, strengthening the evidence that the lines are closely related tectonic features.


Size and other context

Furthermore, in Japan, the caldera forming
Lake Kussharo is a caldera lake in Akan National Park, eastern Hokkaidō, Japan. As with many geographic names in Hokkaidō, the name derives from the Ainu language. It is the largest caldera lake in Japan in terms of surface area, and sixth largest lake ...
, which measures {{{cvt, 26 by 20, km, is larger than Aso caldera leaving it the second largest in Japan. It's not rare to see calderas of this scale; however, to see calderas with an interior stable enough to cultivate land, build highways and lay railroads is quite rare. The caldera contains the city of Aso as well as the town and village of Takamori and Minamiaso.


External links


Asosan
– Japan Meteorological Agency {{in lang, ja
Asosan: National catalogue of the active volcanoes in Japan
– Japan Meteorological Agency volcanic data

– Geological Survey of Japan – maps

– Geological Survey of Japan – volcano data * {{cite gvp, name=Asosan, vn=282110, access-date=2021-06-25
Aso Geopark
ref name="AsoGeoPark" />


References

{{reflist Calderas of Kyushu Pleistocene calderas Active volcanoes Landforms of Kumamoto Prefecture Lists of volcanic eruptions VEI-7 volcanoes Volcanoes of Kyushu