東伊豆単成火山地域の噴火史1:0～32ka

抄録

The eruptive history of the Higashi Izu monogenetic volcano field for the past 32,000 years is revealed by tephrochronology and loess-chronometry. Morphology, color, and size of basaltic tephra grains are widely variable depending on the mode of the eruption; e.g., spinose red scoria are fallout from a strombolian eruption column when a scoria cone is established around the crater; yellowish green lapilli and hard tuff (kata) are products from phreatomagmatic explosions, the former being more magmatic than the latter. In the field, these discriminations are useful not only for identification of each tephra bed, but also for understanding the transition of eruptive styles during one eruptive event. Because vents are closed or sealed at the end of an eruption, an absolutely quiescence occurs between eruptive events in a monogenetic volcano field. This proves the validity of loess-chronometry. Some of the NW-SE or NE-SW trending alignments of volcanoes proved to be created by eruptive fissures; i.e., they are erupted simultaneously. Among them, the 11 km-long Iwanoyama-Iyuzan volcanic chain is the most conspicuous, which was active about 2,000 years ago. Eruptive events more than 10⁹ kg of magma discharge are recognized 13 times during the past 32,000 years, so that the average frequency of eruption in this field is calculated one every 2,500 years. The last is the Iwanoyama-Iyuzan eruption. The discharge rate of magma is 100×l0⁹ kg/ky for the past 32,000 years or 330×10⁹ kg/ky for the past 5,000 years. The rate seems to be accelerated recently, however, it is still an order of magnitude lower than that of a polygenetic volcano such as Izu Oshima. The Kawagodaira eruption of 3,000 years ago is remarkable for two reasons: the largest with 765×10⁹ kg of magma and the first appearance of rhyolite in the field.