Many thanks to Mike Ross (@eruptionchaser) who pointed out Sakurajima on March, 29, 2015 with the following comment:
Heads-up for Sakurajima everyone.
Average 450 eruptions a year. So far in 2015, over 300 eruptions.
31 eruptions on March 27th alone. I’ve been watching the webcam:
Deformation/swelling/inflation has been reported.
This is potentially shaping up to be something biggish. Raw data (in Japanese) from JMA: http://www.data.jma.go.jp/svd/vois/data/tokyo/STOCK/volinfo/gensho.html
(This report from my own observations and from a contact in Japan)
Webcams can be found here:
Sakurajima is a composite stratovolcano that occupies a peninsula sitting on the Aira Caldera in Kyushu, Japan. It sits within 10 km of Kagoshima, capital city of Kagoshima Prefecture. Kagoshima is populated with over 680,000 people.
The volcano has three peaks, one of which is active, and tops out at just over 1,100 m above the bay. It has been in near continuous eruption since 1955 with hundreds of explosions yearly.
Eruptions are typically explosive, with significant ash and debris. Most of them are Strombolian, VEI 1 in intensity, sending plumes some 4 – 5 km into the air. There are regular ash advisories issued by the Japanese Meteorological Agency notifying residents and aviation of ash hazards.
There appear to be two magma sources, one an andesitic (dacite) source near the center of the Aira Caldera, and the basaltic source much deeper.
The current eruption series started off with a bang in 1955 with a Vulcanian VEI 3 eruption. Before that time, Sakurajima erupted every few years, with increasing frequency in more recent years.
The largest historic eruptions of Sakurajima took place in 1471 – 76 and 1914. The earlier eruption was a VEI 5 while the 1914 eruption was a VEI 4. The 1914 eruption connected Sakurajima Island to the mainland via a peninsula built of ash and lava.
Sakurajima is the active expression of the Aira Caldera, which measures some 17 x 23 km in diameter. It was formed around 22,000 years ago in a VEI 7 eruption that deposited some 400 km3 of tephra and pyroclastic flows on the region. Ash from this eruption has been found over 1,000 km from the vent.
The eruption took place in two phases. It began with a Plinian phase that ejected some 100 km3 of tephra. It was closely followed by a massive series of pyroclastic flows that deposited the rest of the 400 km3 total. The vent for both eruptions was at the current site of Sakurajima Volcano. There were at least three additional caldera forming eruptions out of Aira over the last 300,000 years.
The most recent caldera forming eruption produced primarily rhyolite. The cause appears to be a large rhyolitic magma body that was destabilized by an intrusion of basalt from underneath. We saw this sort of thing with the Katmai – Novarupta eruption of 1912. This contrasts with earlier andesitic / andesitic-basaltic eruptions out of Aira. Current activity at Sakurajima is also andesitic in nature.
Volcanic activity in Kyushu is primarily driven by the subduction of the Philippine Plate under the Amurian Plate. There is a spreading center beginning in central Kyushu extending southeast into the Pacific Ocean. The interaction of the spreading center and the subduction of the Philippine Plate has been sufficient to provide enough melt and crustal weakness to trigger what in my mind has been a caldera outbreak in southern Kyushu and south into the Pacific.
Cbus wrote an article about the five calderas of Kyushu in 2013. It is an excellent introduction to caldera activity on and near Kyushu. You can find it here. https://volcanocafe.wordpress.com/2013/07/22/southern-japan-calderas/
If Mike and his contacts are correct, things may be on the verge of getting more than a little interesting at Sakurajima. This is a very dangerous volcano which has gone caldera several times in the not so recent past. There is no reason to believe it is finished.
http://www.jma.go.jp/jma/ (Japanese language)