Mount Aso volcano in the center of the Japanese island of Kyushu exploded at noon last Wednesday, October 20. A huge cloud of smoke and ash rose to a height of more than three kilometers, and large stones – volcanic bombs – flew a kilometer from the crater.
Seconds after the explosion, a pyroclastic flow descended from the volcano. No one was hurt, but the sight was impressive. We look into it and try to understand what exactly we saw.
Here is the main video of the day before yesterday, captured by the webcam of the Japanese media company PKK:
What you see is an explosive, or explosive, eruption. And those who happened to be in the neighborhood with it were not lucky because the magma under their feet turned out to be too viscous and saturated with water. When magma rises to the surface from the magma chamber, pressure drops, water boils, and the explosion blows cubic kilometers of ash into the atmosphere. Just six thousand kilometers southwest of Japan, in the Hawaiian Islands, volcanoes behave much more calmly: during an eruption, liquid magma simply flows like water down the slopes, forming shield volcanoes.
The explosion did not lead to casualties and destruction. Volcanologists and residents of Kumamoto Prefecture are well aware of the nature of their neighbor: over the past 500 years, this volcano has erupted more than 150 times. At the same time, the volcano has become a tourist attraction.
A national park has been created around it, there are observation platforms, and until 2016 the crater could be reached by cable car. Aso has its own museum, which is located next door in the city of the same name. During the eruptions, access to the park is closed. To protect those who, at the time of the eruption, still end up in the park, 13 reinforced concrete shelters were built, each of which can accommodate up to 30 people.
Aso has been working for a very long time. According to volcanologists, it has been active for about 300 thousand years. Since that time, there have been four powerful explosive eruptions, the last of which was about 90 thousand years ago. So there was a 20-kilometer caldera. At the same time, overheated “avalanches” of ash and gas, pyroclastic flows, these explosions covered most of the island of Kyushu. Since then, 17 cones have grown in the caldera, producing a total of 18 cubic kilometers of tephra. The four largest of them received their own names, one of them – Nakadake – the only active one.
Nakadake formed about 22-21 thousand years ago and produced eruptions throughout the Holocene. Its explosion in 553 was the first documented eruption in Japan, after which it woke up more than 170 times. Seismic tomography found a volcanic magma chamber with a volume of 100 cubic kilometers at a depth of six kilometers.
Over the past decades, Nakadake erupted in the second half of the 1970s, in 1988-1990 and, after almost 20 years of silence, in 2014-2016. Since 2007, Nakadake has been constantly monitored by employees of the Fukuoka Meteorological Observatory: they measure the concentration of volcanic gas and conduct television observation of the crater. A lot of devices were installed in the vicinity of Aso – cameras, sensors that react to surface movements, acoustic sensors.
The data of recent decades indicate that each time the eruption took place approximately according to the same scenario: first, the lake in the crater dried out, then a mud eruption occurred, followed by a phreatic eruption (that is, with the release of gases and steam), then phreato-magmatic, and finally, an explosion occurs according to the Strombolian type. This is what the beginning of the eruption in November 2014 looked like from the same parking lot:
The first signs that an eruption will take place appeared in the spring. The Japan Meteorological Agency (JMA) seismic network recorded increased activity in May 2021. The movement of magma in the crust caused low-frequency vibrations, volcanic tremors. The shaking continued for three days, from 2 to 4 May. Sulfur dioxide emissions in this short period reached 200 tons per day. The population was asked to stay at least a kilometer from Nakadake.
On October 12, characteristic peaks appeared on the seismograms. On October 14, an ash plume already appeared over the Nakadake crater, ash began to fall out in several neighboring areas. Small eruptions also occurred the next day, then during October 18 and 19, the activity of the volcano increased markedly, the trail was also visible on satellite images.
Mt. Aso #volcano, |#Japan on October 18, 2021 – you can see it puts out plenty of ash on a regular basis, based on the downwind, fresh ash trail. Sentinel-2 #satellite. Impressive eruption after this last night, October 20. pic.twitter.com/H4SXOrZVYV— AI6YR (@ai6yrham) October 20, 2021
Finally, an explosive eruption occurred on October 20 at 11:43 Japanese Standard Time. In terms of power, this burst of activity of the volcano has already exceeded all last year’s events, but it is still just beginning. So far, the Japan Meteorological Agency has set a hazard level of “three” on a five-point scale for this eruption. This means that one should refrain from approaching the volcano, new descents of pyroclastic flows are possible – at a distance of up to two kilometers.
Researchers cannot predict the duration of the Nakadake eruption and the amount of ash and rock it will eject. However, in historical time, Nakadake did not throw more than 0.01 cubic kilometers of ash into the atmosphere at a time, and 90 percent of its eruptions were estimated on the scale of volcanic activity (VEI) no more than 1-2 units (for comparison: the Pinatubo eruption in 1991 gained 6 points). The last eruption of Aso volcano with VEI-3 was in 2016, and before that – in 1906.
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