Hideo Hoshizumi

Geology and eruptive history of Unzen volcano, Shimabara Peninsula, Kyushu, SW Japan

Abstract During the past 500 thousand years, Unzen volcano, an active composite volcano in the Southwest Japan Arc, has erupted lavas and pyroclastic materials of andesite to dacite composition and has developed a volcanotectonic graben. The volcano can be divided into the Older and the Younger Unzen volcanoes. The exposed rocks of the Older Unzen volcano are composed of thick lava flows and pyroclastic deposits dated around 200–300 ka. Drill cores recovered from the basal part of the Older Unzen volcano are dated at 400–500 ka. The volcanic rocks of the Older Unzen exceed 120 km3 in volume. The Younger Unzen volcano is composed of lava domes and pyroclastic deposits, mostly younger than 100 ka. This younger volcanic edifice comprises Nodake, Myokendake, Fugendake, and Mayuyama volcanoes. Nodake, Myokendake and Fugendake volcanoes are 100–70 ka, 30–20 ka, and

Subaqueous rhyolite block lavas in the Miocene Ushikiri Formation, Shimane Peninsula, SW Japan

Abstract A rhyolite mass of the Miocene Ushikiri Formation in the western part of the Shimane Peninsula, SW Japan, is a small subaqueous edifice about 600 m high and 4 km wide, formed at water depths between 200 and 1000 m. It consists mainly of three relatively flat, lava-flow units 50–300 m in maximum thickness, each of which includes lobes and their polyhedral fragments. The lava lobes are poorly to well vesiculated, glassy to microcrystalline and flow-banded and -folded. Compared with mafic pillows, they are large, having thick, quenched and brecciated, glassy crusts because of their high viscosity, surface tension and thermal conductivity. Their surfaces disintegrate into polyhedral fragments and grade into massive volcanic breccia. The massive volcanic breccia composed of the lobe fragments is poorly sorted and covered with stratified volcanic breccia of the same rock type. The rhyolite lavas commonly bifurcate in a manner similar to mafic pillow lavas. However, they are highly silicic with 1–5 vol.% phenocrysts and have elongated vesicles and flow-folds, implying that they were visco-plastic during flowage. Their surface features are similar to those of subaerial block lava. With respect to rheological and morphological features, they are subaqueous equivalents of block lava.

Co-ignimbrite ash-fall deposits of the 1991 eruptions of Fugen-dake, Unzen Volcano, Japan

Abstract Fugen-dake, the main peak of Unzen Volcano, began a new eruption sequence on November 17, 1990. On May 20, 1991, a new lava dome appeared near the eastern edge of the Fugen-dake summit. Small-scale, 10 4 –10 6 m 3 in volume, Merapi-type block and ash flows were frequently generated from the growing lava dome during May–June, 1991. These pyroclastic flows were accompanied by co-ignimbrite ash plumes that deposited ash-fall deposits downwind of the volcano. Three examples of co-ignimbrite ash-fall deposits from Unzen pyroclastic flows are described. The volume of fall deposits was estimated to be about 30% by volume of the collapsed portions of the dome that formed pyroclastic flows. This proportion is smaller than that described for other larger co-ignimbrite ash-fall deposits from other volcanoes. Grain size distributions of the Unzen co-ignimbrite ash-fall deposits are bi-modal or tri-modal. Most ashes are finer than 4 phi and two modes were observed at around 4–7 phi and 9 phi. They are composed mainly of groundmass fragments. Fractions of another mode at around 2 phi are rich in crystals derived from dome lava. Some of the fine ash component fell as accretionary lapilli from the co-ignimbrite ash cloud indicating either moisture or electrostatic aggregation. We believe that the co-ignimbrite ash of Unzen block and ash flows were formed by the mechanical fracturing of the cooling lava blocks as they collapsed and moved down the slope. These ashes were entrained into the convective plumes generated off the tops of the moving flows.

Paleomagnetism of Unzen volcano: A volcanic record (Senbongi excursion) of the Iceland Basin event and the Brunhes VGP distribution for Japan

A paleomagnetic study was carried out on volcanic rocks from Unzen volcano: samples were collected from a total of 69 sites with 19 sites in pyroclastic flows and 50 sites in lava flows. Ages for the flows were determined either by K-Ar methods or detailed field surveys, and indicate that all of the flows were deposited during the Brunhes chron. After demagnetization 10 pyroclastic and 48 lava flows had stable site-mean directions. One lava flow in the Senbongi area with a K-Ar age of 197±17 ka had an intermediate virtual geomagnetic pole (VGP) at 8.3°N, 21.6°E. The age and VGP position apparently correlate with the Iceland Basin event, suggesting that it is a record of the event on volcanic rocks. Our study added 53 Brunhes-aged VGPs to the overall collection for Japan, increasing it by 40% to a total of 175 VGPs. After excluding all site-mean directions having α95 > 10° and VGP latitude < 50°, the remaining 148 VGPs have a mean pole at 89.7°N, 40.9°E (A95 = 2.2°) showing no significant deviation from the geographic pole. The angular standard deviation (ASD) was calculated as 15.2° ± 1.2° (N = 148), which is compatible with paleosecular variation models from the literature. However, the data set was found to deviate from a Fisher distribution. The actual meaning of the ASD value after removing the intermediate VGPs needs to be reconsidered.