Abstract
We have investigated rock magnetic properties and remanent magnetization directions of samples collected from a lava dome of Tomuro Volcano, an andesitic mid-Pleistocene volcano located on the Japan Sea side of central Honshu. Detailed thermal demagnetization experiments and rock magnetic investigations including analyses of isothermal remanent magnetization and anhysteretic remanent magnetization specify the mineralogy, concentration, and grain size of magnetic minerals in andesite samples. Magnetite and hematite are identified as magnetic carriers. However, each mineral concentration differs between bluish andesite (so-called Ao-Tomuroishi) and reddish one (Aka-Tomuroishi); the latter contains more hematite and less magnetite than the former. Magnetic granulometry shows that magnetite in reddish andesite is finer than that in bluish one. At the Mt. Tomuro andesite dome, bluish andesite constitutes the majority of the dome whereas reddish andesite occurs as highly oxidized parts at the dome surface. Based on these observations, we conclude that in reddish andesite, high-temperature oxidation of magnetite grains caused their partial change to hematite and, as a result, reduced their effective grain sizes. Inconsistent site-mean directions were determined at three sites, although they all have northerly directions of normal polarity. This inconsistency would be explained by assuming deformation of lava at the dome surface below blocking temperatures of magnetite and hematite.
