SUMMARY
The formation of the two particular lava domes in Hokkaido, Japan is described and interpreted mainly fromgeophysical viewpoints. The 1909 eruption of Tarumai volcano was not violent but produced a lava dome overfour days. The growth rate of the dome is discussed under the assumption that the lava flow was viscous andplastic fluid during its effusion. By Hagen-Poiseuille?s Law, the length of the conduit of the lava dome is ratherambiguously determined as a function of viscosity of the magma and diameter of the conduit. The 1944 Usudome 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 thedome activity were carried out by several pioneering geophysicists. Utilizing their data, pseudo growth curvesof the dome at each stage can be drawn. The lava ascended rather uniformly, causing uplift of the ground surfaceuntil half-solidified lava reached the surface six months after the deformation began. Thereafter, the lavadome added lateral displacements and finally achieved its onion structure. These two lava domes are of contrastingcharacter, one is andesitic and formed quickly while the other is dacitic and formed slowly, but both ofthem behaved as viscous and plastic flows during effusion. It is concluded that both the lava domes formed byuplift of magma forced to flow through the conduits, analogous to squeezing toothpaste out of a tube.