Interesting things lead me to investigating various volcanoes. One recent paper described the connection between massive volcanic eruptions that inject dust and aerosols into the upper atmosphere and the inability to see the moon during a lunar eclipse. Typically, the more dust, the more difficult it is to see the moon. There is even a scientific measurement of the luminosity of the moon during a lunar eclipse, the Danjon Scale of Lunar Eclipse Brightness.
Eclipse prediction was one of the early goals of ancient astronomers. Those that got it right were rewarded. Those that didn’t were occasionally executed. Eclipses are connected to something called the Saros Cycle, the interval between times when the sun, earth, and moon line up for eclipses. This happens once every 18 years, 10 days (11 in leap years) and 8 hours. Ancient Mesopotamian astronomers (astrologers back then) had figured out that cycle as long as 2,700 years ago in 700 BC.
A properly predicted eclipse is generally a Good Thing, especially in the distant past. The darkest lunar eclipses (moons) since 1600 AD have been linked to massive volcanic eruptions – 1601 (1600 Huaynaputina), 1642 (1641 Parker), 1816 (1815 Tambora), 1884 (1883 Krakatau), 1913 (1912 Katmai – Novarupta), 1983 (1983 El Chichon), and 1992 (1991 Pinatubo).
There were seven lunar eclipses observed in Europe 1110 – 1120. There are 17 documented observations of these eclipses. The darkest of these took place in 1110. The appearance of this particular eclipse was sufficiently striking that it triggered the most detailed written accounts between 500 – 1800 AD. It was as dark or darker than eclipses connected to the 1257 Samalas and 1883 Krakatau eruptions. Interestingly, there were no reports of atmospheric effects that are normally attributed to volcanic dust in the upper atmosphere – dimming of the sun, red twilight glows, reddish solar halos 1108 – 1110.
So, now we have a mystery, and it is time to take a look at ice cores and tree rings.
A 2020 paper by Guillet, et al, Climatic and societal impacts of a “forgotten” cluster of volcanic eruptions in 1108 – 1110 CE described the detective story associated with tying a previously unexplained very dark lunar eclipse with the volcanic eruption that caused it.
Analysis of ice core data from Greenland and Antarctica originally concluded there was a single volcanic event 1108 – 1109 somewhere in the tropics. Upon further review, there appear to be multiple, closely spaced large volcanic eruptions (dust and SO2 aerosols). The first peak was in mid-1108, a second peak in late 1110 that persisted until late 1112 – early 1113.
From this, researchers suggested three possible scenarios. First was a tropical eruption in 1108, an almost simultaneous high latitude Northern Hemisphere eruption (Asama in 1108?), and a third high latitude Northern Hemisphere eruption in late 1109 – 1110. Second was a number of large eruptions in mid to high latitude Northern Hemisphere 1108 – 1113 (Asama is one potential source), and a tropical eruption in late 1109. Final scenario would assume no tropical eruptions, rather a number of large eruptions in mid to high latitudes (Asama being one) 1108 – 1113, and a large Southern Hemisphere late 1108 – 1109.
The only known eruption from this period took place in Oct 1108 and is well documented. It was assigned a VEI 5 explosivity, the largest Holocene eruption of Asama.
Mount Asama
Mount Asama (Asama-yama, Asamayama, Gunma) is one of Honshu’s most active and dangerous volcanoes. It sits on the intersection of the Izu – Bonin – Mariana Arc impinging from the S and the NE Japan Arc, meaning it has a prolific deep magma source. It overlooks the resort town of Karuizawa, 140 km NW of Tokyo. Historic records of its activity stretch over the last 1,300 years.
It is an andesitic / basaltic andesitic / dacite stratovolcano topping out at 2,568 m. It has produced several major Plinian eruptions, the largest of these in 1108 and the most recent in 1783. The most recent eruption was in 2019.
The volcanic complex has been active for the last 100,000 years. Over that time, the center of the activity has migrated from W to E. The currently active Asama is at the eastern end of the complex and started forming some 24,000 years ago.
The oldest edifice is the andesitic Kurofu stratovolcano. The second is the Hotokeiwa dacitic stratovolcano (10,000 – 20,000 years old). Hotokeiwa is built of pumice fall deposits and lava flows (13,000 – 11,000 years old), the Kosama lava dome (20,000 years ago), and pyroclastic flow deposits. Newest structure is the Maekake volcano, some 10,000 years ago. Activity at Maekake began and the Kamayama crater at the summit is the currently active part of the volcano. The size and shape of the Kamayama crater changes eruption by eruption and is currently around 500 m in diameter.
Early activity at Asama built a cone-shaped stratovolcano, Kurofu (Kurohu, Kurohu-yama), some 2,000 m tall. The eastern portion of the cone was destroyed by faulting and a large-scale steam eruption.
Second stage of activity built the Hotokeiwa shield volcano. Lava flows during this construction are exposed on the SSE side of the present active crater. Kosama (Ko-asama-yama) is a parasitic dacite lava dome on the E side of Asama topping out at 1,655 m. It erupted dacites similar to those that built the original stratovolcano.
The third stage produced two large volume ignimbrites that spread over wide areas to the N and S of the shield volcano.
The most recent stage built andesitie stratovolcanoes. Recent eruptions produced thick lava flows, pyroclastic flows and pyroclastic ejecta. The outer crater of the cone was formed by a partial flank collapse following a pyroclastic flow. There is a parasitic andesite lava dome, Sekison-zan, on the S side of this cone. It is younger than Kurohu-yama but older than the youngest cone.
The more recent stratovolcano has two craters. The western rim of the outer crater is Maekake (Maekake-yama), 2,493 m. The outer crater is elliptical, 1,200 x 900 m. It is 2 km E from Kurofu. Kama-yama is the most recent cone in the center of the outer crater. It tops out at 2,550 m. It is the currently active portion of the volcano, with all recorded eruptions from this crater. Hotoke-iwa is a parasitic shield volcano to the SE of the outer crater of the younger stratovolcano.
Asama has erupted 121 times in historic times. Most of these have been Vulcanian in explosivity. Total volume of the edifice is around 56 km3.
Seismic observations of Asama have been done for most of the last century. Early days of these observations led to identified volcano – tectonic earthquakes, low frequency earthquakes, explosive earthquakes, and harmonic tremors. These started being classified before 1960. The modern monitoring network was greatly expanded after 2000, with the number of seismometers tripling since then. Today, there are 9 continuous GPS sites within 20 km of the summit.
Eruptions
The most recent eruptive sequence started Aug. 7, 2019, sending a plume around 2 km into the atmosphere. The volcanic alert level was raised for the next two weeks and subsequently lowered. A second eruption took place on Aug. 25. The previous eruption was in June 2015. Eruptions over the last few decades have become somewhat less explosive, though the volcano is still easily capable of VEI 1 – 2 sized eruptions.
The 2004 VEI 2 eruption was the first observed with the new instrumentation. The eruption included over 500 volcanic earthquakes Jan 2004 – Oct 2005. Observations suggested a dike intrusion in the western flank of Osama 1.3 km below the surface. Magma ascended gradually to about 1 km above sea level by July 2004. It increased seismic activity around the vent. Magma migration was relatively slow from the deep chamber to the shallower portions of the vent. That migration was thought to be stopped for a time by a plug at the top of the vent. After the Sept 1 eruption, magma migration became much easier, leading to more volcano-tectonic earthquakes. The eruption sequence started on Sept 1 with a Vulcanian eruption of VEI 2. The plume was ejected 6 – 8 km above sea level. It was followed by Strombolian eruptions and then multiple Vulcanian eruptions 23 Sept – 9 Dec. Erupted magma is similar to magmas erupted over the last 10,000 years. The eruption sequence emplaced a lave dome. Magma composition became increasingly juvenile over the eruption sequence.
There was a 4-year period of relative quiet before eruptions resumed in 2008. This was accompanied by N-S extension starting July 2007 with yet another dike intrusion W of the summit.
The most recent major eruption (VEI 4) took place in 1783. Eruptions began on May 9. The first major eruption was on July 17. Volcanic ash dusted Tokyo on July 28. Volcanic earthquakes were felt in Tokyo. The eruption sequence peaked Aug 2 and continued for the next 3 days. It produced volcanic thunderstorms, pyroclastic flows, massive ashfall and lahars. There was a massive lava flow down the N slope, followed shortly by a partial flank collapse of the N flank. The flank collapse and associated lahar on Aug 5 destroyed Kanbara, some 12 km N. Eruptions mostly stopped by Aug 6. Lahars reached the Pacific coast by Aug 9. This eruption produced 0.51 km3 DRE.
At the time of the eruption, Kanbara was an important transport center in the region. The eruption sequence in early August is called the Great Tenmei eruption. The lava flow, flank collapse and associated lahars to the N buried Kanbara, killing 463 of its 556 inhabitants. The surviving building in Kanbara was a small stone temple on a hill overlooking the village. The survivors took refuge in the temple, at the top of a stairway of 112 stone steps. The last 10 steps of the stairway were not buried.
Lahars and subsequent flooding to the S into the Agatsuma River reached the Pacific Ocean some 200 km away a few days later. Lahars and debris from the eruption blocked the Azuma River for a time. Water building behind that temporary dam cleared leading to the subsequent flooding downstream. Over 1,600 were killed in the eruption, mostly by the avalanche, lava flow, lahars and subsequent downstream flooding. The eruptive plume reportedly blocked sunlight over northern Japan for the next several months. This led to a temperature drop which was blamed for the Tenmei famine, one of three major famines during the period. 1.4 million of a total population of 25 million perished. The Onioshidashi Park on the N flank of the volcano features volcanic rock – lava flows and debris flow deposits – from the 1783 eruption. The word Onioshidashi means “rocks pushed by the devil.” The volcano was relatively quiet for a century after the 1783 eruption.
The 1108 VEI 5 eruption has been named the Tennin 1 eruption. It was the most massive of Asama’s hundreds of eruptions in historic time. Ash injected into the upper atmosphere is suspected of blocking out the lunar eclipse in 1110. It was recently connected to extreme weather and a severe famine in Europe. This connection comes from a Swiss research report. Previous research tried to tie European climate anomalies of torrential rain and consecutive cold summers to a 1104 eruption of Hekla in Iceland. Problem is that the increased volcanic sulfate levels in Greenland ice cores took place 1108 – 1113, which did not match up with the Hekla eruption. The research concluded that the sulfates in the ice core were deposited by the 1108 Asama eruption. Not only did the researchers investigate ice cores, but they also took a look at Japanese historical records. The 1108 eruption was twice as powerful as the 1783 eruption.
The oldest written description of an Asama eruption, the Chuyuki, describes the 1108 eruption. It was written in Kyoto, 300 km SW of Asama. The eruption sequence started on Aug 29. It produced a heavy ashfall (lower half of the B scoria) 50 km E of Asama. The climatic eruption producing the Oiwake pyroclastic flow took place late Aug – early Sept. This part of the eruption was not captured by the Chuyuki. There was a pause of around 20 days when the eruption resumed. Sept 25 – Oct 11 a drum-like sound and red glow in the E were observed in Kyoto (upper half of B scoria).
Eruptive products from the 1108 eruption show decreasing amounts of volatiles in the erupted magma. Initial eruptive phase was a widespread scoria fall. This was followed by multiple intermediate-type pyroclastic flow units, the Oiwake pyroclastic flow. The final eruption product was the volatile poor Kamino-butai lava flow. One of the differences between this and the 1783 eruption was that the 1783 eruption did not have the pumice flow stage, though it did produce the Agatsuma pyroclastic flow. Total output from the 1108 eruption was 0.62 km3 DRE. There was closely spaced VEI 4 in 1128, ejecting 0.28 DRE and the Daiji 3 pumice.
There have been at least 9 VEI 4 eruptions mainly from Maekake over the last 13,000 years.
Tectonics
Asama is located on Honshu, 140 km N of Tokyo. This is a tectonically complex region. Primary tectonic event is the subduction of the Pacific Plate under the Okhotsk Plate. To the west, we have the Amur Plate portion of the Eurasian Plate. South of Asama, we have the subduction of the Philippine Sea Plate beneath the Eurasian Plate. The Boso Triple junction lies offshore to the east, at the end of the Sagami Trough. The Sagami Trough is a 340 km long trough where the Philippine Sea Plate subducts beneath the Okhotsk Plate. The southern arm of the Boso Triple Junction is the Izu-Bonin Arc, the Izu-Bonin Trough where the Pacific Plate subducts beneath Philippine Sea Plate to the SW. The northern arm of the triple junction is the Japan Trench where the Pacific Plate subducts beneath the Okhotsk Plate to the west. Location of this triple junction has been relatively stable over the last 10 Ma.
The western end of the Sagami Trough ends roughly between Mount Fuji and Tokyo, an area of intense volcanic activity and complex tectonics. West of this region is the boundary between the Okhotsk and Amur (Eurasian) plates. Just to make sure everyone is sufficiently complex, the Izu-Bonin arc is subducting beneath Honshu in the vicinity of Tokyo Bay.
This complex tectonic region supplies significant magma to feed volcanic arcs in northern Honshu, Izu-Bonin and southern Honshu. It is also the source of multiple megathrust earthquakes.
Basaltic magmas in the arc are thought to be generated by upwelling of mantle diapirs from the partly molten zone along the subducting slab beneath the volcanic front and back-arc side. Upper depth of these bodies is estimated to be 100 km. The diapirs seem to correspond with a group of volcanoes in central Japan. Diapirs are estimated to be around 50 km in diameter.
Conclusions
Mount Asama is a vigorous andesitic volcano with what appears to be an ample magma supply. While it has been over 200 years since the last significant (VEI 4+) eruption, it has been quite active in historic times. It is close enough to Tokyo to do some real damage should that happen, though Fuji and Hakone may be more immediate problems. This is a dangerous volcano.
Additional information
Nippon.com – Mount Asama: A volcano with a deadly past, Sept 4, 2019
Japan Travel – Mount Asama (Gunma)
Magma supply path beneath Mt Asama volcano, Japan, Takeo et al, 2006
Active Volcanoes of Japan – Asama Volcano
Magma pathway and its structural controls of Asama Volcano, Aoki, et al, 2013
Volcanic eruptions and hazards of Asama written in historical records, Hayakawa and Nakajima
The Asahi Shimbun – Mt Asamayama eruption in 1108 may have led to famine in Europe, Jun 20, 2020
Textures of the eruptive products of Asama-Maekake volcano from the 12th Century, Yasui, 2017
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