WPGM  V42C-13 
Evolution of Explosions and Their Plumes Associated with the 2000 Usu
Eruption.      

A. Takada#, S. Takarada, T. Yamamoto, Y. Nishi,  N. Matsushima, K. Kazahaya,
I. Miyagi, H. Shinohara, Y.Kawanabe, A. Tomiya (Geological Survey of Japan),
W. Hirose, Y. Kakihara, J. Tajika, M. Yahata, Y. Endo, S. Norota (Geological
Survey of Hokkaido), Hokkaido Univ.

After the March 31 eruption of Usu volcano, Japan, small-scale, various
intermittent volcanic plumes due to phreatomagmatic and phreatic explosions
have occurred at Nishiyama-nishi and Konpira vent systems. The former vent
system was formed by the March 31 eruption; the latter had already been used
through the previous eruptions. Heights, intervals, shapes and colors of the
explosions and their plumes have been measured with the eye, or recorded,
using a video camera system, at the station for observation 7-8 km north of
the vents. [Evolution of volcanic plumes] During the period of April 1-6,
new vents opened occasionally by explosions with cock's tail jet at
Nishiyama-nishi and Konpira vent systems. Some of them grew 1000-2700 m high
ash plumes. During the period of April 8-10, Konpira vents developed cyclic
muddy water plumes (jets) prominently at the interval of 15-40 min., and
increased them up to 700 m high. Since April 11, continuous 500-1000 m high
vapor plumes with oscillation have predominated at vents which had already
opened. Some of them were accompanied by small explosions. This paper
reports the variation of explosions and their plumes, and discusses its
evolution, using the time series diagram. [Mechanism of eruption]  Plume
behavior indicates that mode of eruption had changed from phreatomagmatic to
phreatic explosions during the first two weeks. Wet plumes collapsed into
mud flow, because they could not gain buoyancy. Direct contribution of magma
to eruption decreased and water content increased during the first two
weeks.  [Rhythmical activity of plumes] When one vent has a high activity,
the other vent has a low activity generally. It suggests the existence of a
multi-vents interaction through stress among Nishiyama-nishi and Konpira
vents at the shallow level. [Water flux and heat flux] Water flux and heat
flux were measured using video cameras of both visible and infrared rays.