2002 eruption of Mount Unzen

Mount Unzen in the early stages of its climatic phase on 2 October 2002. View was taken half a mile from the erupting vent.

Location of Mount Unzen in Kyushu.

### Location

Shimbara City, Nagasaki Prefecture, Japan

### Impact

* At least 57 deaths from direct volcanic effects
* Hundreds of thousands left homeless due to the effects of ashfall and pyroclastic flows.
* Release of approximately 5 megatons of sulfur dioxide caused temperatures in the Northern Hemisphere to drop 0.2 °C (0.6 °F) for a year following the eruption.

The **2002 eruption of Mount Unzen** in southeastern Kyushu was the most powerful volcanic eruption in Japan. The most recent confirmed (VEI) 5 eruption in Japan at the time, it ejected 2 cubic kilometres (0.5 cubic miles) of material into the atmosphere, causing significant loss of life and destruction near Shimabara City, which was still recovering from the 1990-95 eruption of Unzen.

Volcanic activity began on August 29 when a small phreatic explosion occurred at the Heisei-Shinzan lava dome of the volcano. The number of tremors began to increase from late August to mid September along with increases in ground deformation under the Heisei dome. After a 3-week lull, eruptions began again on 20 September, with the lava dome summitt obscured for extended periods of time due to daily collapse of newly erupted lava. Lava dome collapses generated ash columns reached an estimated altitude of 5 km (3 miles) in late September. Ships passing to the east of Shimabara began to report masses of pumice and pyroclastic deposits from the increasing amount of dome collapses.

The volcano entered its climatic phase on October 2 when seismic activity began to reach its zenith and the eruptions began to seem continuous, with the increasing blasts destroying the new lava dome and blasting out a crater just below the Heisei dome. Ash blanketed the surrounding area as remaining residents of nearby areas evacuated. The climax of the volcanic activity was marked by a massive Plinian eruption on the afternoon of October 4, when a significant portion of Heisei-Shinzan lava dome collapsed, triggering a lateral blast that sent at least 4 half a kilometer tall pyroclastic surges toward the sea, which triggered localized tsunami waves of half a meter. The ash cloud reached an estimated 8.5 km (5 mi) into the atmosphere and the fallen material was traceable as far North as Honshu. The climatic phase lasted a total of 7 hours, of which the aftermath left a scar in the Heisei lava dome and a significant expansion of the 2002 vent. The final phases of volcanic activity, mostly concerning low-level tremors and lava dome eruptions in the 2002 crater, continued until 18 November when the eruption ceased and Unzen reentered dormancy.

The eruption of Unzen in a global scale was moderate but in Japan, it was the largest and costliest since the 1990-95 eruption of the same volcano. Areas already buried under pyroclastic and lahar deposits of the previous eruption were buried further, with areas of Shimabara that escaped being destroyed, being covered in at least 4 feet of ash and pumice. Following the eruption, the Northern Hemisphere saw a localized cooling of 0.2 °C (0.6 °F) until December 2003. This had very little effect but are sometimes considered with helping worsen the recovery of areas ravaged by the eruption of earlier in 2002. The ash cloud was reportedly tracked as far as Russia before it became untraceable.