Mount Aso, Characteristics And Origins Of One Of The Largest Volcanoes In Japan
Mount Aso is one of Japan's largest volcanoes and last erupted on October 20, 2021. But what are its characteristics?
Mount Aso is one of Japan's largest active volcanoes. Located on the island of Kyushi (the southernmost of the country) in the prefecture of Kumamoto , the mountain reaches a height of 1592 meters above sea level. Passed to the news for the eruption of October 20, 2021 , let's see its characteristics in more detail.
Characteristics of Monte Aso
Mount Aso is a stratovolcano located in southern Japan, about 8 km from the city of Aso, with a population of about 26,000 . One of its most interesting features is its caldera, one of the largest in the world : we are talking about 25 km wide in the NS direction and 18 km in the EW direction – this means that its circumference is about 120 km ! It is believed to have formed during four explosive eruptions between 90,000 and 270,000 years ago., which gave rise to abundant pyroclastic flows and volcanic ash that poured over much of the region. Nowadays the caldera is surrounded by five peaks: Nekodake, Takadake, Nakadake, Eboshidake, Kishimadake. Of these, only Nakadake (located in the center of Mount Aso) is actually an active volcano .
The volcano normally alternates between calmer and more turbulent phases. In the calmer ones inside the crater a green / blue volcanic lake forms from which small plumes of smoke usually rise . However, as volcanic activity increases, the lake begins to boil and dry up, until the actual eruption - the last of which took place on October 20, 2021 . Precisely following this last eruptive event, Mount Aso is one of the 47 most monitored volcanoes in the country (out of a total of 110 active volcanoes present in Japan).
How was Mount Aso formed?
Volcanism in Japan is well known: in fact, the country boasts the beauty of 111 active volcanoes , in proportion about 10 times more than the Italian ones! But why are there so many volcanoes? Simple! Japan is located at the point of contact between four tectonic plates: the Pacific, the Philippine, the Eurasian (represented in part by the blue and green lines).
In the specific case of Mount Aso (visible in the image below inside the black circle) its formation is linked by the subduction of the Philippine plate below the Eurasian one. More generally, we can say that the volcano is located along the famous Pacific "Ring of Fire", that is, the area along which 3/4 of all volcanoes on the planet are concentrated.
According to the Geological Survey of Japan , Mount Aso was formed about 300,000 years ago . Today on its summit there is a caldera measuring 25 x 18 km, i.e. a depression that is generated following the collapse of the upper portion of the volcanic edifice. This collapse was probably caused by a strong eruption which occurred around 70-80 thousand years ago . A whole series of volcanic cones have developed inside this caldera and one of these, the Nakadake, is the one involved in today's eruptions, October 20, 2021.
What do we know about past volcano eruptions? The first documented eruption dates back - according to the JMA databases - to the year 854 while the last eruptive cycle opened in 2019. Also in that case the crater involved was the Nakadake and the eruption lasted from 7 October 2019 to 20 May 2020, with sporadic events until the end of June. Fortunately it was a modest eruption, without the emission of large volcanic blocks or pyroclastic flows on its slopes.