Formation processes of the 1909 Tarumai and the 1944 Usu lava domesin Hokkaido, Japan

The formation of the two particular lava domes in Hokkaido, Japan is described and interpreted mainly from
geophysical viewpoints. The 1909 eruption of Tarumai volcano was not violent but produced a lava dome over
four days. The growth rate of the dome is discussed under the assumption that the lava flow was viscous and
plastic fluid during its effusion. By Hagen-Poiseuille’s Law, the length of the conduit of the lava dome is rather
ambiguously determined as a function of viscosity of the magma and diameter of the conduit. The 1944 Usu
dome extruded as a parasitic cone of Usu volcano, not in the crater, but in a flat cornfield at the foot of the volcano.
From the beginning to the end for more than 17 months, seismometric and geodetic observations of the
dome activity were carried out by several pioneering geophysicists. Utilizing their data, pseudo growth curves
of the dome at each stage can be drawn. The lava ascended rather uniformly, causing uplift of the ground surface
until half-solidified lava reached the surface six months after the deformation began. Thereafter, the lava
dome added lateral displacements and finally achieved its onion structure. These two lava domes are of contrasting
character, one is andesitic and formed quickly while the other is dacitic and formed slowly, but both of
them behaved as viscous and plastic flows during effusion. It is concluded that both the lava domes formed by
uplift of magma forced to flow through the conduits, analogous to squeezing toothpaste out of a tube.