Lava dome growth continued through mid-May, accompanied by frequent avalanches and pyroclastic flows produced by partial collapse of the lava dome complex (800 m E-W, 650 m N-S, and 350 m high by mid-April). The youngest dome (7), which first appeared on 6 April, grew slightly faster than material was removed by collapse along the leading edge. Its viscous lava formed "banana peel-like" structures with several radial lobes. These structures had typically been observed on the surfaces of newly extruded lava that overrode older domes (dome 3 over dome 2, and dome 5 over dome 4). By mid-May, dome 7 was about 200 m long, 110 m wide, and 90 m high, and had the highest extrusive vent of the dome complex.

The cryptodome that formed among domes 3, 4, 6, and 7 continued to swell, and small reddish oxidized pieces, probably from its interior, were exposed on the surface. Intrusion and erosion rates were balanced, keeping the peak shape and height nearly stable. Gullies developed around the cryptodome, extensively eroding its ENE side, where rockfalls frequently occurred. The rockfalls commonly traveled E and NE, and rarely N, producing reddish to pink-colored ash clouds. Many cracks appeared on the head of dome 6, pushed by the swelling cryptodome. The foot of dome 6 reappeared from the talus deposits, while dome 4 (E of the cryptodome) was buried by talus. Dome 2 was similarly buried in mid-December 1991. The E part of dome 3 was also pushed, and covered by reddish lava blocks from the cryptodome.

The daily number of seismically recorded pyroclastic flows ranged from 4 to 28, almost unchanged from previous months, totaling 322 in April . . . . Flows originating at dome 7 traveled down the SE flank of the dome complex toward Iwatoko-yama and the Akamatsu River valley. Pyroclastic deposits buried the gentle slope along the Akamatsu River valley. April's longest flow extended 3.5 km E from the dome. Although ash clouds (generally 500-1,000 m high, maximum 1,500 m) extended to > 500 m beyond the main flow deposits, they had only minor effect, neither burning nor toppling the trees that they passed. Geologists suggested that this may indicate a decrease in the auto-explosivity of the lava blocks.

On 22 April, repeated lava-block falls from the toe and sides of dome 7 generated multiple pyroclastic flows that cascaded down the steep SE slope made of pre-1990 volcanics, forming a gully 50 m deep, 100 m wide, and up to 400 m long. By mid-May, the gully had been buried by rockfall talus.

From mid-April to mid-May, blue gas was emitted continuously from dome 3, and sporadically from the heads of domes 6 and 7, and the cryptodome. Ash emission was weak and infrequent. Small earthquakes continued to occur beneath and within the dome complex, fluctuating between 20 and 200/day during April and early May, and totaling 3,053 in April (down from ~ 4,000-6,000 in prior months). Roughly 7,600 people remained evacuated as of early May.

Analyses of lava samples collected from pyroclastic-flow deposits generated by collapse of domes 6 and 7 indicate compositions similar to those of the other domes; ~ 65 weight % SiO₂, with ~ 20 volume % of plagioclase, hornblende, and biotite phenocrysts. The samples had specific gravities of 2.1-2.2, also similar to the other domes, implying that vesicularity of the dome lavas has remained nearly constant throughout the 1991-92 eruption. Bombs erupted directly from the magma conduit on 8 and 11 June 1991 had specific gravities of 0.8-2.6.

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.