Ash by the 2000 eruption

Bulk composition of ash

by the 2000 eruption (April 23, 2000)

We analized ash deposits of the 2000 eruption by XRF analyzer. The samples are collected by T. Kaneko (VRC, ERI) on 4/8 and 9.

Fig. 1

The samples were collected at NE, N, SE and ESE of Mt. Usu. As each ash cloud during 3/31-4/2 had characteristic disperse direction , we can distinguish one from the others. On the basis of photo-analysis by GSJ using ASTER images, collected samples are correlated to ash clouds of 3/31NE, 3/31N, 4/1SE, and 4/2ESE, respectively (Fig. 1).

Fig. 2

The ash samples of the 2000 eruption are plotted on the line between basaltic somma lavas (black triangle) and essential pumices* (orange square; Hokkaido Univ.). The 3/31NE ash, which is emitted by most vigorous explosion so far, is nearest to the essential composition. The 3/31N and 4/2ESE ash are plotted rather near to basaltic lavas which constitute the surrounding rock of the vents. The fact indicates that the 3/31NE ash cloud contained more essential magma than those of 3/31N and 4/2ESE. This is concistent with the vigorous clouds and the report by GSJ that shows a half of the ejecta is magmatic component. Although volcanic glasses and pumices by Toya Caldera formation and historical eruptions are also plotted on the same line, the temporal change of these components is unlikely. Thus the result may show that periodical sampling and XRF analysis of ash deposit enables estimating contribution of magma to total ash.

*: Tomiya-san's page (GSJ; Jananese only)

**: Usu page (presented by Miyagi-san, GSJ; Jananese only)

Morphology of the ash (April 15, 2000)

collecter: T. Kaneko (VRC, ERI)

Each photo is 2 mm * 2.6 mm. Distribution maps are from Hokkaido branch of GSJ. The word "washed" means the sample is washed by water to remove fine component in grain size.

#13:07- March 31, 2000 fallout ash

Fig. 1 (GSJ Hokkaido)

Photo 1-1 (not washed)

Photo 1-2 (washed)

Photo 1-3 (washed)

NE (ca. 9, 11, and 12 km from the N craters)