Abstract
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.
Highlights
The 1909 lava dome of Tarumai volcano, Hokkaido is a typical example of an andesite lava dome formed within its crater
Features of lava dome formation change with the viscosity of the magma, which increases with increasing silica content
Dome formation is likely for dacites and rather rare for basalts
Summary
The 1909 lava dome of Tarumai volcano, Hokkaido is a typical example of an andesite lava dome formed within its crater. The dome, measuring 1.5 × 107 m3 in volume, was built probably in four days This was typically formed by squeezing of magma or forced extrusion of magma through a narrow opening. A neighbor of Tarumai volcano, formed a lava dome during 13 months in 1944 and 1945 This dome is unique in that it was formed at the base of a volcano, on flat ground, not within a crater. The magma pushed up the ground forming a mound and a half-solidified lava extruded from the mound and grew to a lava dome measuring 2.2 × 107 m3 in volume above the mound. The results of precise levels carried out at the foot of the dome enable us to depict its pseudo growth curves, which indicate forced extrusion of half-solidified magma to form the lava dome. The sequential process of each dome first will be introduced and followed by some quantitative interpretations of their formation processes
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