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Roger Weller, geology instructor
by Melissa Pratt
Sakurajima Volcano, Japan
Figure 1: City of Kagoshima overlooking Sakurajima
The Aira Caldera sits in the northern half of Kagoshima Bay. It was formed about 22,000 years ago and is about 12 miles across. Beginning 13,000 years ago new activity began in the caldera, forming Sakurajima. The volcano sits about 5 miles south of the center of the caldera. It began as an island, but a major eruption in 1914 caused it to join with the Osumi Peninsula. It is less than 10 miles away from the city of Kagoshima, which frequently has to deal with ash fall from the volcano.
Figure 2: Aerial view of Sakurajima
The eruption of 1914 began on January 11. It started as a very explosive eruption with pyroclastic flows, but after a large earthquake on January 13th, it began generating a large lava flow, which continued for months. People had already been evacuated because of pre-eruption earthquakes. This lava flow caused the island to grow, connecting it to the mainland by a narrow isthmus. Parts of Kagoshima bay had also become quite a bit shallower and the tides became higher. In the final stages of the eruption the center of the Aira Caldera subsided by about 24 inches and the city of Kagoshima was covered in ash. While this eruption caused significant changes to the topography of the area, the largest historical eruption from Sakurajima was in 1471-76.
Figure 3: 1914 eruption
Sakurajima has three main peaks: Kita-dake, Naka-dake and Minami-dake. Kita-dake is the north peak, Naka-dake is the middle peak and Minami-dake is the south peak. Minami-dake is also where most of Sakurajima’s explosions occur. It is one of Japan’s most active volcanos, with, on average, between 100 and 200 explosions a year. The first recorded explosion was in 708. The last big eruption was on February 5th 2016. Entry to the area was banned and an already existing no-go zone was increased to a 1.5 mile radius. The volcanic alert level was increased from a 2 to a 3, out of 5. It was the first explosion since September of the previous year. It was a dramatic explosion, but only average compared to explosions in the past. Between the 15th and 19th of August, 2016 there were a series of small scale explosions at Sakurajima. Before 1955 Sakurajima only erupted every few years. However, since 1955 activity at the volcano increased significantly, with ongoing explosions and pyroclastic flows. Eruption plumes usually rise from half a mile to 1.8 miles high, with bigger explosions occurring occasionally.
Figure 4: Sakurajima at sunset
Despite the frequent eruptions, Sakurajima boasts a number of tourist attractions. One such attraction is the Sakurajima Volcano Special. This is a five day tour to observe Sakurajima’s volcano activity from up close. The first day is for arrival at Kagoshima and travelling to a hotel at the base of Sakurajima. The second through fourth days are spent observing the volcano and traveling to various sites around the volcano and visiting various nearby attractions. The last day is for travelling back to Kagoshima and is the end of the tour. Although, visitors can extend their visit and sight see around the area on their own.
Another interesting attraction is the Kurokami Shrine Gate. Before the explosion of 1914 the gate stood about 9.8 feet tall. After the eruption only about the top 3 feet were left uncovered by ash. Villagers began to dig it out, but the village chief suggested leaving it as it was to preserve the memory of the eruption. The Kurokami Observation Point is only a little ways from the Shrine Gate. The observation point has a slightly raised platform in front flat terrain just before the volcano rises. From here visitors can see the Showa Crater and have a good view of some of the small eruptions.
Figure 5: Kurokami Shrine Gate