Wow…that was quick! I had barely arrived at Unzen-dake ..or Mount Unzen as it is also known…before Jesper “Volcano-maniac” Sandberg found me. This was a fun adventure though, and I need to get to know the volcanoes of Japan more.
Unzen is a complex of several overlapping stratovolcanoes on the island of Kyushu in the south of Japan. Its summit is called Fugen-dake and, until around a quarter century ago, was the highest point of the volcanic complex. That honour now belongs to Heisei Shinzan, the huge lava dome that began growing on Hugen-dake’s east flank during the volcano’s most recent period of eruptive activity.
Lava domes are characteristic of Unzen, with its thick but gas-poor dacite lava. Though dacite lava contains less silica than rhyolite, it can be even more viscous and thus more conducive to highly explosive eruptions and pyroclastic flows.
One such lava dome on the lower flanks of Unzen was the cause of Japan’s most deadly volcano-related disaster. In 1792, Mayu-yama, a lava dome located near the foot of Unzen close to the Ariake Bay, partially collapsed as the result of a series of earthquakes that following an eruption. Large pieces of the dome tumbled into the bay and caused a megatsunami with waves as high as 100 meters. Around 15,000 people were killed in the catastrophe.
The volcano remained quiet after that disaster, but rumbled into life again in 1989 with a series of earthquakes that gradually occurred closer and closer to the summit. Evacuations were promptly begun. Some phreatic eruptions had took place in November of 1990, but one of Unzen’s most violent single eruptions happened the following June, when a dome collapse triggered an enormous pyroclastic flow that swept 4.5 kilometres down the slopes and claimed the lives of 43 assorted scientists, journalists and other personnel who had assembled in what they believed to be a safe location to observe the volcano’s activity. Among those 43 were Katia and Maurice Krafft, perhaps two of the most renowned and beloved individuals in the field of volcanology.
During the next few years, the volcano generated an estimated 10,000 pyroclastic flows of lesser degree, destroying around 2,000 houses. Activity gradually diminished until the eruption was officially declared over in 1995. However, the area was still plagued by lahars for some time after.
Due to its explosivity and proximity to densely populated areas, Unzen was designated a Decade Volcano in 1991. In 1999, an exploratory drilling project was begun at Unzen. Scientists hoped to actually drill into the magma conduit and possibly gain insights into why magma always follows the same conduits even though those conduits have solidified, as well as how the magma can become so gas-poor even though there is no apparent opening through which the magma can degas. The borehole reached its target in 2004 to discover the rock was only registering a temperature of 155*C, much lower than anticipated…possibly due to hydrothermal cooling over the 10-year period since eruptions ceased.
Speaking of hydrothermal, Unzen also counts among its features several thermals that feed hot springs around the volcano. One such springs is called Obama, and has been turned into a popular destination for locals as well as tourists. The Obama hot spring and footbath has seated and walk-through footbaths, steaming pots for cooking fish and vegetables, and even special footbaths for your pets..all free of charge to guests.
Jesper picks up five points for finding me first. Everybody be sure to check back here Monday nights around 11pm New York time (Early Tuesday morning if you live east of Iceland) for a new adventure at a new volcano.