Northeast view of Heisei Shinzan, formed in the 1990–1995 eruption, with the endogenous part at top. Taken from above the Taruki Forest Park.
Geological Period
Holocene
Main geological interest
Volcanology
Geomorphology and active geological processes
Location
Shimabara Peninsula, Nagasaki Prefecture, Japan
32°45’41”N, 130°17’55”E
Northeast view of Heisei Shinzan, formed in the 1990–1995 eruption, with the endogenous part at top. Taken from above the Taruki Forest Park.
Scientifically best-documented dome growth during the Hensei eruption (1990-1995) at Unzen Volcano.
The Heisei Era eruption that occurred at Mount Unzen, one of the Decade Volcanoes of the International Decade for Natural Disaster Reduction (IDNDR), has been thoroughly studied, and it has become one of the best-documented lava dome eruptions. This eruption was unlike the explosive eruption of Pinatubo, which erupted at about the same time; however, the compositions of their magmas were similar. This contradiction led to a challenging project by the International Continental Scientific Drilling Program (ICDP) to excavate the path followed by the magma immediately after the eruption of Mount Unzen (Sakuma et al., 2007).
The dome surface structure of Heisei Shinzan, showing the endogenous mound behind the exogenous part (foreground) that grew earlier.
- Geological description
Heisei Shinzan is a dacite lava dome that formed during the Heisei Era eruption (1990–1995) of Mount Unzen. The dome first grew exogenously and then endogenously, indicating a decreasing effusion rate (Nakada et al., 1995; 1999; Umakoshi et al., 2011). Its formation was accompanied by thousands of pyroclastic density currents due to partial collapses of the lava dome (Nakada and Fujii, 1993). Pyroclastic flows ran down the slope in the growth direction of the dome and resulted in the dome’s advance over thick talus and a wide deposition of pyroclastic materials. Furthermore, lahars were repeatedly generated during the subsequent rainy seasons. On the outcrops, volcanic ash layers serve as timestamps that reveal the chronology of the eruption. Forty-three people (including media personnel, local residents, and volcanologists Katia and Maurice Krafft and Harry Glicken) were killed by pyroclastic surges on June 3, 1991. The surge in the largest pyroclastic-flow event during this eruption burnt down an elementary school. These events, along with the June 1991 eruption of Mount Pinatubo in the Philippines, triggered a reconsideration of volcanic disaster preparedness. Heisei Shinzan was designated as a national natural monument and now access is graned only for scientific research and monitoring.
- Scientific research and tradition
Geophysical, geochemical, and geological observations were conducted during the eruption, and the observational data and timestamped ejecta have been studied. The drilling project revealed the mechanism of eruption, and its cores are still used for research (Yilmaz et al., 2021). Volcanic observations have been conducted since the eruption, including the monitoring of dome stability.
Section of pyroclastic deposits at Taruki Forest Park, northeast of the dome, telling the story of the eruption. The scale bar is 50 centimeters.
- Reference
Nakada, S. et al. (1995) ‘Endogenous growth of dacite dome at Unzen volcano (Japan), 1993–1994’, Geology, 23(2), pp. 157–160. Available at: https://doi.org/10.1130/0091-7613(1995)023<0157:EGODDA>2.3.CO;2.
Nakada, S. and Fujii, T. (1993) ‘Preliminary report on the activity at Unzen Volcano (Japan), November 1990-November 1991: Dacite lava domes and pyroclastic flows’, Journal of Volcanology and Geothermal Research, 54(3), pp. 319–333. Available at: https://doi.org/10.1016/0377-0273(93)90070-8.
Nakada, S., Shimizu, H. and Ohta, K. (1999) ‘Overview of the 1990–1995 eruption at Unzen Volcano’, Journal of Volcanology and Geothermal Research, 89(1), pp. 1–22. Available at: https://doi.org/10.1016/S0377-0273(98)00118-8.
Sakuma, S. et al. (2008) ‘Drilling and logging results of USDP-4 — Penetration into the volcanic conduit of Unzen Volcano, Japan’, Journal of Volcanology and Geothermal Research, 175(1), pp. 1–12. Available at: https://doi.org/10.1016/j.jvolgeores.2008.03.039.
Umakoshi, K., Itasaka, N. and Shimizu, H. (2011) ‘High-frequency earthquake swarm associated with the May 1991 dome extrusion at Unzen Volcano, Japan’, Journal of Volcanology and Geothermal Research, 206(3), pp. 70–79. Available at: https://doi.org/10.1016/j.jvolgeores.2011.07.004.
Yilmaz, T.I. et al. (2021) ‘Rapid alteration of fractured volcanic conduits beneath Mt Unzen’, Bulletin of Volcanology, 83(5), p. 34. Available at: https://doi.org/10.1007/s00445-021-01450-7.
- Author(s)
Setsuya Nakada.
The University of Tokyo (presently NIED). Japan.
Marekazu Ohno.
Unzen Volcanic Area UNESCO Global Geopark (presently, Mt. Chokai & Tobishima Island National Geopark). Japan.
