Mt.Unzen, Japan, 1792 Shelton smith

Mtn. Unzen eruption of 1792 volcanic eruption of Mount Unzen, western Kyushu, Japan, that led to a destructive landslide and a tsunami. The death toll from the disaster is estimated at some 15,000 people, making it the most deadly volcanic eruption in Japan’s history.

Mount Unzen actually consists of a group of composite volcanoes located on Japan’s Shimabara Peninsula east of Nagasaki. The area was the site of a major volcanic eruption in 1792. After an initial eruption, a large earthquake triggered a landslide from the Mayuyama peak, a 4,000-year-old lava dome rising above the city of Shimabara. The massive landslide swept through the city and eventually reached the Ariake Sea, where it set off a tsunami. The wave surge devastated nearby areas, causing further widespread damage and death. Most of the estimated 15,000 deaths caused by the event are believed to have resulted from the landslide and the tsunami. The scar created from the Mayuyama landslide remains visible today

The 1792 eruption served as a reminder to the Japanese people of Earth’s unpredictability. A series of earthquakes and small eruptions at Mount Unzen in the early 1990s escalated fears of another disaster, but that catastrophe never materialized. Still, anxiety has remained over the Mount Unzen volcano because of the dense population nearby and its history of catastrophic events.

Unzen is a large complex volcano made of several adjacent and overlapping lava domes. The volcano covers much of the Shimabara Peninsula and is east of the city of Nagasaki. In 1792, collapse of the Mayu-yama lava dome created an avalanche and tsunami that killed an estimated 14,524 people. Most of the people were killed by the tsunami.

After the 1792 eruption Unzen was dormant for 198 years. Photograph copyrighted and provided by Steve O'Meara Volcano Watch International.

Spaceborne Imaging Radar-C/X-band Synthetic Aperture Radar (SIR-C/X-SAR) image of Unzen taken January 26, 1995. The image shows an area 25.7 miles by 20.3 miles (41.5 kilometers by 32.8 kilometers) North is toward the upper left of the image. Image courtesy of NASA/JPL.

The 1990 eruption was preceded by a swam of earthquakes that began in November 1989. The earthquakes gradually migrated towards the summit of Unzen over time. Tremor was first noted four months before the first eruption.

A phreatic eruption on November 17, 1990, marked the onset of the most recent eruption of Unzen. Beginning in late January 1991 continuous tremor was noted beneath the volcano. The eruption resumed on February 12, 1991. It became stronger with time and the composition of the ash particles indicated that fresh magma was being erupted. A volcanic dome made of dacite lava began to grow on May 20, 1991. The dome continued to grow for four more years. Photograph copyrighted and provided by Steve O'Meara of Volcano Watch International.

During this time pyroclastic flows were frequently generated by the collapses of lava blocks from margins of the dome. During 1991-1994, approximately ten thousand pyroclastic flows were counted on Unzen.

The next three photos show the last nuee ardente erupted from Unzen during intense activity on June 3-4, 1991. Photographs by Mike Lyvers, June 4, 1991.

Unzen is out of the photo to the right. Scale is hard to appreciate in these photos. The second two photos are wide-angle. The pine forest in the middle ground was completely flattened by the eruption. Photographs by Mike Lyvers, June 4, 1991.

Landslides also generated large pyroclastic flows that traveled as far as 3.4 miles (5.5 km) from the dome. On June 3, 1991, one of these pyroclastic flows killed 43 people, including noted volcanologists Maurice and Katia Krafft and Harry Glicken.

The next four photos show a smaller pyroclastic flow descending Unzen, December 30, 1991.

Photographs by Mike Lyvers.

More than 2,000 buildings were destroyed by pyroclastic- and debris-flows by the summer of 1993. Debris flow frequently occurred during the rainy season. The number of evacuated people was approximately 12,000 in the summer of 1991. The number of evacuated people was reduced to 3,000 by the end of 1993. Photograph copyrighted and provided by Steve O'Meara of Volcano Watch International

Funnel cloud generated by heat from hot pyroclastic flow deposits after the eruption. This photo is looking down on an area (once a suburb) where 44 people died, June 3, 1991. Photograph by Mike Lyvers.Generation of pyroclastic flows and major deformation of the dome had stopped in February 1995.

In mid-February 1996, Unzen generated several small block-and-ash flows. These flows were caused by collapse of part of the volcanic dome. The flows traveled about 0.6 mile (1 km) from their source. Cooling of the lava dome or seasonal temperature changes may have triggered the collapses.

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