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Report on Unzendake (Japan) — January 1992


Bulletin of the Global Volcanism Network, vol. 17, no. 1 (January 1992)
Managing Editor: Lindsay McClelland.

Unzendake (Japan) Lava dome growth continues; dome collapses spawn pyroclastic flows

Please cite this report as:

Global Volcanism Program, 1992. Report on Unzendake (Japan) (McClelland, L., ed.). Bulletin of the Global Volcanism Network, 17:1. Smithsonian Institution. https://doi.org/10.5479/si.GVP.BGVN199201-282100



32.761°N, 130.299°E; summit elev. 1483 m

All times are local (unless otherwise noted)

Dome growth continued through early February, mainly endogenously, and pyroclastic flows were generated by collapse from the edges of domes 5 (formed in late November) and 6 (appeared 3 December). Talus from the domes continued to bury the SE slope of the dome complex.

Magma was supplied primarily to the head of dome 6 (370 m long, 180 m wide, and 50 m thick), causing the dome to thicken (to 80 m), especially over the vent area, as it was shortened by collapse. Magma intrusion in dome 6 caused uplift of older dome materials behind the vent, where a small graben had formed earlier. The peak of the resulting cryptodome surpassed the summit of dome 5, previously the highest point in the dome complex (1,375 m elevation). Cryptodome growth pushed dome 5 NE, and caused its partial collapse northeastward and eastward. Rockfalls from dome 5 eroded the surface of dome 4, and began to bury it with talus. A group of parallel to radial cracks developed on the surface of dome 4, forming a central groove that divided it into N and S parts by early February. The N part appeared to be pushed eastward by the movement of dome 5 and the cryptodome.

Pyroclastic flows traveled mainly SE and E from dome 6 and the cryptodome, with ash clouds from the larger flows rising to 2 km. Some small pyroclastic flows and rockfalls occurred NE and E of domes 4 and 5. A total of 308 flow events were recorded seismically in January (compared to 395 in December); activity increased somewhat in early February. Seismicity beneath the dome complex continued at high levels in January, when the highest monthly total (5,337 earthquakes) since the start of dome growth was recorded.

Two blocks at the front of dome 6 (estimated block volume, 3-5 x 105 m3) collapsed on 2 February at 1738, forming a larger-than-normal pyroclastic flow. Associated ash decreased visibility to 30 m in Fukae (4 km ESE) immediately after the flow, and nearby roads were briefly closed. Despite initial estimates from TV-monitor footage and eyewitness accounts that the pyroclastic flow traveled 3 km, the area 2.5-3.0 km from the volcano did not appear devastated and lacked noticeable pyroclastic-flow deposits when viewed during a helicopter overflight 2 days later. The speed of the pyroclastic flows was estimated at about 65 km/hour using a TV monitor. Compared to pre-October pyroclastic flows, more recent ones were less energetic, with lower velocities and a narrower area of devastation. However, the quantity of co-ignimbrite ash had not changed noticeably.

A second series of pyroclastic flows, lasting about 30 minutes, developed on 12 February when a pile of blocks (~ 5 x 105 m3) collapsed on the S margin of dome 6 after being pushed southward by lava emerging from the vent. Small horseshoe-shaped depressions, up to 100 m wide, were created by the rockfalls. The pyroclastic flows all descended along the same path, eroding a narrow v-shaped groove in the accumulated talus, which terminated at the top of a 100-m cliff. The flows spread at the base of the cliff, depositing material on the gentler slopes. Some of the southward-moving flows were deflected by the N face of Iwatoko hill (roughly 1.5 km S of the vent) ~70° towards the E and into the Akamatsu valley. The pyroclastic flows traveled about another 500 m, to a total of 2.2 km from their source.

Geological Summary. The massive Unzendake volcanic complex comprises much of the Shimabara Peninsula east of the city of Nagasaki. An E-W graben, 30-40 km long, extends across the peninsula. Three large stratovolcanoes with complex structures, Kinugasa on the north, Fugen-dake at the east-center, and Kusenbu on the south, form topographic highs on the broad peninsula. Fugendake and Mayuyama volcanoes in the east-central portion of the andesitic-to-dacitic volcanic complex have been active during the Holocene. The Mayuyama lava dome complex, located along the eastern coast west of Shimabara City, formed about 4000 years ago and was the source of a devastating 1792 CE debris avalanche and tsunami. Historical eruptive activity has been restricted to the summit and flanks of Fugendake. The latest activity during 1990-95 formed a lava dome at the summit, accompanied by pyroclastic flows that caused fatalities and damaged populated areas near Shimabara City.

Information Contacts: S. Nakada, Kyushu Univ; JMA.