How Shinmoedake erupted
On Friday last week, a lava dome about 50 meters in diameter was confirmed in the Shinmoedake crater. Shinmoedake is one of more than 20 volcanic peaks in the Kirishima mountain range. The magma is viscid, and is likened to the one at Fugendake peak in the Unzen mountain range in Nagasaki Prefecture, whose destructive effect of the pyroclastic eruption has been noted and studied in all textbooks.
Today’s terrestrial TV broadcast mentioned that rocks as large as a car have been spotted being ejected from the crater’s blasts.
Below are a few links to some most spectacular Youtube video footages of the eruptions:
1) Pyroclastic eruption, cloud, ash and rock missives; 2) Pyroclastic cloud, electrical lightning-like charges 3)
Recent eruptions similar to those 300 yrs ago / Lava dome confirmed; experts warn more violent blasts could be in offing at Shinmoedake peak
Kyoichi Sasazawa and Takashi Ito / Yomiuri Shimbun Staff Writers
How Shinmoedake erupted
[Click on photo to see full view]
Volcanic experts have sounded an ominous warning about the recent eruptions on Shinmoedake peak, saying they closely resemble highly destructive blasts that occurred there nearly 300 years ago.
One of more than 20 volcanic peaks in the Kirishima mountain range on the Kagoshima-Miyazaki prefectural border, Shinmoedake is believed to have formed between 7,300 and 25,000 years ago.
Most of the recorded eruptions on the Kirishima range have occurred at Shinmoedake and Ohachi peaks.
Small eruptions caused by phreatic explosions were observed on Shinmoedake from March to May last year. Phreatic explosions occur when the heat of rising magma causes underground water to boil and steam pressure rises.
According to experts, however, the eruptions that have taken place since Wednesday are explosive eruptions characteristic of phreatomagmatic explosions, which are caused when magma and underground water directly interact.
Traces of small pyroclastic flows going down 500 to 600 meters have also been observed on the southwestern side of the volcano.
The magma at Shinmoedake is relatively viscid, according to the experts, as it contains a large amount of silica, a main ingredient of volcanic ash. When the volcano erupts, a great amount of ash is also ejected.
On Friday, a lava dome about 50 meters in diameter was confirmed in the crater of the 1,421-meter peak during observations from the air by the University of Tokyo’s Earthquake Research Institute. The dome formed as magma rose to the crater and stopped there.
“It’s already reached the ‘magma-eruption’ stage, in which magma directly erupts from the volcano,” said Associate Prof. Ryusuke Imura of Kagoshima University, an expert on volcanic topography. He has been conducting on-site research near the volcano.
The magma also is viscid at Fugendake peak in the Unzen mountain range in Nagasaki Prefecture, where large pyroclastic flows were observed 20 years ago. A large lava dome was observed in the crater at that time, and was the source of pyroclastic flows for a long time.
A lava dome sometimes grows larger and larger due to a continuous supply of magma, although there are exceptions. For example, on Mt. Showa Shinzan in Sobetsucho, Hokkaido, lava domes cooled as soon as they formed, raising the height of the peak from 1944 to 1945.
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Edo period eruptions
Large eruptions took place at Shinmoedake in 1716 and 1717, during the Kyoho era of the Edo period (1603-1867).
Analysis of the volcanic material deposited in the soil by the different eruptions has found that they changed from phreatic to phreatomagmatic explosions and then to magmatic eruptions.
According to Imura, the Edo eruptions were 10 times bigger than the current ones, and also involved pyroclastic and mud flows. Although there was no lava flow, intermittent eruptions continued for about 18 months, resulting in the deaths of six people, Imura said.
Ash from the eruptions traveled as far as Hachijojima island in Tokyo’s Izu Island chain, about 850 kilometers from Shinmoedake.
According to analysis by the National Institute of Advanced Industrial Science and Technology on pumice stones from the eruptions through Thursday, the magma is very similar to that ejected in the 1716-17 eruptions.
“The eruption process is quite similar to the major eruptions of the Kyoho era, so more violent eruptions could take place,” Imura said.
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‘Mountain swelling’
Meanwhile, the Geospational Information Authority of Japan, which has been observing Shinmoedake’s crustal movement via the Global Positioning System, said the volcano body began swelling, which indicates an accumulation of magma, in December 2009.
During a nine-month period from May 2010, about 6 million cubic meters of magma accumulated in a reservoir about six kilometers underground and about 10 kilometers west-northwest of the Shinmoedake crater. During the same period, about 1 million cubic meters of magma accumulated in a chamber about three kilometers underground just beneath the crater.
Generally, a mountain body swells when magma accumulates underground, causing the distance between observation points to become longer. When magma is released through eruptions, the mountain body will contract and observation points move closer together.
There are exceptions, however. Eruptions have continued to take place on Sakurajima in Kagoshima, for example, but the mountain’s body is swelling because the amount of magma accumulating underground is larger than the volume released through the eruptions.
Although no data is available yet on how much magma was ejected from Shinmoedake, distances between observation points have already changed from expanding to shrinking.
Originally 23 kilometers, the distance between two particular points in the Kirishima mountain system expanded by four centimeters during the approximately one year from December 2009 to just before the eruptions. However, the same distance shrank by one centimeter in three days from Wednesday.
“According to the observation results, we can say the amount of magma has fallen,” said Tetsuro Imakiire, a senior technical official at the Geospational Information Authority of Japan.
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